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TRANSACTIONS
ON ELECTRICAL ENGINEERING
CONTENTS
Zach, P., Hradílek, Z.: Heat Pumps Earth-Water . . . . . . . . .
1– 5
Priščáková, Z., Rábová, I.: Solar Energy as a Primary Source of
Energy for a Cloud Server . . . . . . . . . . . . . . . . . . .
6– 9
Bejvl, M., Šimek, P., Škramlík, J., Valouch, V.: Control Techniques
of Grid Connected PWM Rectifiers under Unbalanced Input
Voltage Conditions . . . . . . . . . . . . . . . . . . . . . . .
10 – 21
Bilik, P., Petvaldsky, P., Kaspirek, M.: Flickermeter Comparison
Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22 – 25
Medveď, D.: Utilising of EMTP ATP for Modelling of
Decentralized Power Sources Connection . . . . . . . . . . .
26 – 29
Beňa, Ľ., Jakubčák, R., Kolcun, M.: Use of Specialized Devices to
Power Flow Control in Power Systems . . . . . . . . . . . . .
30 – 33
Vol. 2 (2013)
No.
1
ERGO NOMEN
pp.
1 - 33
TRANSACTIONS ON ELECTRICAL ENGINEERING
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International scientific journal of electrical engineering
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Prof. DOLEZEL Ivo, The Academy of Sciences of the Czech Republic, Czech Republic
Prof. DUDRIK Jaroslav, Technical University of Kosice, Slovakia
Prof. NAGY Istvan, Budapest University of Technology, Hungary
Prof. NOVAK Jaroslav, University of Pardubice, Czech Republic
Prof. ORLOWSKA-KOWALSKA Teresa, Wroclaw University of Technology, Poland
Prof. PEROUTKA Zdenek, University of West Bohemia, Czech Republic
Prof. PONICK Bernd, Leibniz University of Hannover, Germany
Prof. RICHTER Ales, Technical University of Liberec, Czech Republic
Prof. RYVKIN Sergey, Russian Academy of Sciences, Russia
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Prof. WEISS Helmut, University of Leoben, Austria
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©2013 ERGO NOMEN, o.p.s. All right reserved.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
1
Heat Pumps Earth-Water
Petr Zach 1), Zdeněk Hradílek 2)
VSB - Technical University of Ostrava, Faculty of Electrical Engineering and Computer Science,
Department of Electrical Power Engineering, 17. listopadu 15, 708 33 Ostrava - Poruba, Czech Republic
1)
e-mail: [email protected]
2)
e-mail: [email protected]
Abstract—The article deals with heating of large
properties using heat pumps of earth-water type. An
example of use of geothermal energy can be seen in the
heating systems installed in the Assembly Hall of VSB Technical University Ostrava. The source of heat comprises
10 heat pumps connected within cascade pattern with the
total installed capacity of 700 kW. The primary source of
energy for heat pumps is formed by the set of 100 boreholes
to the depth of 140 m.
Keywords— Heat pumps, geothermal energy
I. INTRODUCTION
Geothermal energy belongs to sources of renewable
alternate energy. This is a flow of geothermal heat from
the depth of the Earth since its very creation. The centre of
the Earth stores sufficient amount of geothermal energy,
its sources can be reached yet within the top layer of the
Earth crust. Geothermal heat also includes the heat
element supplied to the Earth upon creation of our solar
system. The Sun is a source of all energy received by the
Earth from space. However, this heat only warms the
atmosphere, water surfaces and a thin surface layer of
continental rocks. Solar radiation supplies thermal energy
into the Earth and prevents rapid escape of heat from
internal parts of the Earth.
II. GEOTHERMAL ENERGY
The thermal field of the Earth is characterised by
several basic terms: geothermal gradient, temperature
gradient, heat flow and heat conductivity of rocks. The
geothermal gradient refers to the number of meters of
descent below the surface to achieve a temperature
increase by 1 °C. However, the neutral zone close to
surface must be accounted for, as the temperature in this
area is affected by external impacts. The average value of
geothermal gradient is 33 m, what means that the
temperature increase will be equal to 3 °C for every
hundred metres travelled to the depth. The temperature
gradient is a vertical gradient in the Earth crust. It is
expressed in degrees of Celsius per one meter of depth. Its
value fluctuates between 0.01 and 0.1 °C per meter of
depth. Temperature data from depths greater than any
levels reached so far are calculated on geophysical
models. Heat flow (W) expresses the amount of heat Q
passing through a unit of area per one unit of time t.
P=
dQ
dt
(1)
The value of heat flow allows partial derivation of the
temperature increase rate with the depth reached. As far as
utilisation of geothermal energy is concerned, potential
options can be found mainly in cases with high heat flow
values. The heat flow density (mW·m-2) is expressed as
passing of specified heat output through the area of 1 m2.
q=
dP
d 2Q
=
dS dS ⋅ dt
(2)
The heat flow density on the Earth surface ranges
within 30 and 120 mW·m-2. The mean value calculated
from several tens of thousands of measurements is equal
to 70 mW·m-2. Heat conductivity (W·m-1·K-1) of rocks
determines the ability to conduct heat and it depends on
rock types in the Earth crust.
Sources of geothermal energy can be divided by
temperature to low-, medium- and high temperature ones.
The temperature of low-temperature sources remains
below 150 °C. These are used for heating in residential
developments, industrial processes and heat pumps. The
temperature range for medium temperature sources is
within 150 and 200 °C. These are used for direct heating
and power production. The level reached by high
temperature sources is above 200 °C. These can be used
for power production.
III. HEATING OF LARGE PROPERTY USING HEAT PUMPS
An example of heating in a large property might be the
building with Assembly Hall of VSB - Technical
University in Ostrava. The building with Assembly Hall
and the Information Technologies Centre is heated using
bedrock source heat pumps. The source of low-potential
heat used comprises of ground boreholes. The heat pumps
provide for 82 - 85 % of the heat supply for the building in
an average year. The bivalent, i.e. auxiliary, source of heat
is represented by the exchange station of the centralized
heat supply. The total built-up area amounts to 3,917 m2.
The heat source for this system comprises of 10 heat
pumps manufactured by the IVT company from Sweden,
with the total capacity of 700 kW based on the pattern of
110 boreholes drilled to the depth of 140 m. These
boreholes are located within the parking area at the
Assembly Hall building and library of VSB-TUO. The
boreholes fittings include four-pipe equipment with two
loops of 32 mm polyethylene pipeline of nominal
diameter 32 mm with a special connection mount. The
boreholes have been fitted with approximately 70,000 m
of polyethylene piping. The boreholes have been injected
with concrete mixture. The primary circuit is filled with an
anti-freeze heat carrying liquid with the total volume of
18,000 litres. The system of 110 boreholes converges into
five collector shafts, which means 22 boreholes per 1
shaft. Every shaft is then linked with a separate pipe,
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
mounted with a separate circulating pump. The heat loss
incurred within the Assembly Hall, at the outdoor
temperature of -15 °C, amounts to approx. 1,200 kW. The
heating system includes the under-floor heating, heating
bodies and air conditioning unit. The individual heating
systems have been designed for low thermal gradient. The
production of domestic hot water is provided by the plate
exchanger within hot water accumulation magazines.
2
TABLE I.
PERFORMANCE PARAMETERS OF IVT HEAT PUMPS
Temperature
(°C)
Heating output
(kW)
Power input
(kW)
Heating factor
(-)
0/35
67,8
16,7
4,06
0/50
69,8
22,3
3,13
Fig. 1. The Assembly Hall of VSB-TUO
IV. ASSESSMENT OF MEASURED DATA
The heat pumps are controlled by the ProCop software.
The software provides automatic monitoring of all values
within 10-minute intervals and saves results into the
database.
Data used for assessment have been collected over the
heating season in the period of 2011/2012. The heating
seasons subject to assessment within the period of
2011/2012 started on 1st October and ended on 30th April
respectively.
Table II shows the source of heating season with
energy supplied by heat pumps and energy supplied into
air-conditioning system and the energy supplied into
central heating, the energy consumer by heat pumps and
outside temperature. Further data include resultant heating
factor calculated for the heating season.
TABLE II.
ENERGY SUPPLIER AND CONSUMER DURING THE HEATING SEASON OF
2011/2012
Heating season
2011/2012
Energy supplied by heat pumps (GJ)
1445,97
Energy supplied into the air conditioning system (GJ)
305,84
Energy supplied into the central heating (GJ)
1140,12
Energy consumed by heat pumps (kWh)
143178
Average outdoor temperature (°C)
4,2
Heating factor of heat pumps (-)
2,8
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
3
Fig. 2. Heat Pumps Connection Diagram
Fig. 3. Connection Diagram for Energy Supplied from Heat Pumps
A. Assessment of Energy Produced and Consumed by
Heat Pumps
The Graph 1 shows course of energy produced and
consumed by heat pumps during the heating season. The
energy produced by the heat pumps is supplied into the
central heating and air-conditioning systems. The graph
shows the dependency of the energy produced by the heat
pumps on the outdoor temperature, as the decreasing
outdoor temperature rises the heat loss of the building.
The volume of the consumed energy is determined by the
sum of energy for heat pumps compressor drive,
circulating pumps in the primary circuit and the control
power supply.
B. Assessment of Heating Factor of Heat Pumps
The Graph 2 shows the course of heating factor of heat
pumps during the heating season. This graph can be used
to define the dependency of heating factor on the outdoor
temperature. The outdoor temperature is expressed in
daily, minimum and maximum values. When the outdoor
temperature achieves positive values, the heating factor
experiences an imbalance, which is caused by activation
of the heat pumps and circulating pumps. With the
outdoor temperature at negative levels, the heating factor
experiences a slight drop, yet heat pumps still have
enough energy from boreholes.
C. Assessment of Temperature Courses in Individual
Borehole Circuits
Graph 3 shows courses of temperature in particular
borehole circuits and outdoor temperature during the
heating season. The heat pumps use the system of 110
boreholes drilled to the depth of 140 m. The boreholes
form a drilling field extended within the parking area at
the Assembly Hall building and library. The boreholes are
distributed in 10x10 m pattern under the parking area.
These boreholes are linked into 5 circuits. One circuit is
then made of 22 boreholes. If we extract heat from a
specific circuit, the temperature in the boreholes will be
reduced over a certain period of time. The extracted circuit
will be disconnected; its boreholes will be restored, while
the system makes use of another circuit in the mean time.
The duration of specific circuit utilisation is equal to 7
days.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
Graph 1. Courses of Produced and Consumed Energy by the Heat Pumps
Graph 2. Course of Heating Factor of the Heat Pumps
Graph 3. Courses of Temperature in Individual Borehole Circuits
4
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
Fig. 4. Heat Pumps, 10 units
5
V. CONCLUSION
Heating in large properties can be ensured by means of
heat pumps. Heat pumps supply up to three times the
volume of heat compared to the amount of power they
consume from the mains. Heating with heat pumps system
is set for fully automatic operation with control options
depending on utilisation of the building. The data
measured on heat pumps relate to the heating season of
2011/2012. The data was used as base to assess the course
of heating season in the building. The total energy
supplied by heat pumps amounted to 1,445.97 GJ, out of
which 305.84 GJ were supplied into the air-conditioning
and 1,140.12 GJ into the central heating system
respectively. The total amount of electric power consumer
by heat pumps reached 143,178 kWh. The heating factor
of the heat pumps during the heating season resulted in
2,8. The low heating factor is due to asynchronous
utilisation of the building. The coldest day in heating
season subject to assessment was 3th February 2012,
when the temperature reached -15.2 °C and the heat pump
system was still sufficient for heating in the building.
ACKNOWLEDGEMENT
This work was supported by the Czech Science
Foundation (102/09/1842), by the Ministry of Education,
Youth and Sports of the Czech Republic (SP2012/188)
and by the project ENET (Research and Development for
Innovations Operational Programme (CZ.1.05/2.1.00/
03.0069).
REFERENCES
Fig. 5. Circulation Pump in the Primary Circuit, 5 units
[1] Hradílek, Z., Zach, P.: Heat pumps, renewable energy source for
the assembly hall at VSB-TU Ostrava, conference Proceedings EPE
2010, Brno.
[2] Hradílek, Z., Zach, P.: Heat pumps, renewable energy source for
the assembly hall at VSB-TU Ostrava - Evaluation of the heating
season, conference Proceedings EPE 2011, Kouty nad Desnou.
[3] Zach, P., Hradílek, Z.: Energy Issues in Heating of Large Buildings,
conference Proceedings EPE 2012, Brno.
[4] Heat Pumps IVT, online http://www.cerpadla-ivt.cz/
[5] Monitoring system Procop, online http://www.alfamik.cz/
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
6
Solar Energy as a Primary Source of Energy
for a Cloud Server
Zuzana Priščáková 1), Ivana Rábová 2)
Mendel University in Brno, Faculty of Business and Economics, Department of Informatics, Zemědělská 1,
613 00 Brno, Czech Republic, ui.pefka.mendelu.cz
1) [email protected]
2) [email protected]
Abstract — Cloud Computing is a modern innovative
technology for solution of a problem with data storage, data
processing, company infrastructure building and so on.
Many companies worry over the changes by the
implementation of this solution because these changes could
have a negative impact on the company, or, in the case of
establishment of a new companies, this worry results from
an unfamiliar environment. The aim of this paper is to offer
and scientifically confirm a proposal of an accessibility
solution of cloud by implementing of solar energy as a
primary source.
Keywords — Cloud Computing, data accessibility, solar
energy.
I. INTRODUCTION
Cloud Computing belongs to modern trends in the field
of information technology. Under the term cloud we can
really imagine a cloud full of data. Every day we use the
technology of cloud without realising that our data are not
saved in a particular place but in abstraction. The aim of
this technology is to offer services, applications saved in
cloud providers´ servers. The only condition of data
accessibility is internet access. We can access data
whenever and wherever. The access may be also enabled
by a web browser. As we have already mentioned, cloud
is a business model with disposal of disc capacity and
high performance and server computing capacity with the
use of virtualisation. Gartner has defined cloud computing
as a style of computing where IT is scalable and flexible
with support of delivering as a service with the help of
information technology [1].
Cloud Computing can be created either by a computer,
when the virtualisation is gained, or by implementation to
a server. The latter, i.e., the implementation to a server, is
always chosen in the business environment. After
installation and configuration of a chosen cloud
computing model relevant applications are created for
company purposes, employee accounts eventually, or the
cloud will represent the data storage of the company.
Increase of performance effectiveness is one of the
possibilities after implementation of a cloud solution.
A cloud solution may carry some negatives like
problems with data storage, security, or with data access.
Security in cloud comes out its feature – multiplicity.
Through the net we can connect to a relevant server where
the user application may be found. Thanks to the
application we get to data. Safety conditions are important
by the work with data. Data integration, secrecy, and
accessibility belong here. Data integrity ensures that data
are not duplicated. Authentication codes, which are
assigned after data coding, are used by integrity. By
coding by the means of keys, the principles of
cryptography are used. The last condition is accessibility.
This term denotes realisation of server connection to a
source of energy. The current solution is dependence on
the mains. By power failure data may be partially or
completely lost, and data access and the access to the
application saved in cloud may be denied. It is a small
problem with a great impact on our work.
The use of alternative sources of energy is one of the
accessibility solutions. The Sun is one of the most
important sources of energy. Solar energy originates
deeply in the core of the Sun. The temperature
(15 000 000 °C; 27 000 000 °F) and the pressure (340
times higher than that at the sea level) are so intense that
nuclear fusion reactions occur [2]. The Sun belongs to
inexhaustible energy sources and it belongs to the group
of energy sources with no negative impact to the
environment.
When solar radiation passes through the Earth´s
atmosphere, its intensity decreases gradually. Three kinds
of radiation hit the Earth (direct, diffuse, and reflected –
Fig.1). Direct radiation gains a lower rate of luminosity
than other radiations. Diffuse radiation originates from the
distraction and reflection from the Earth´s surface.
Fig. 1: Types of solar radiation
The solar electric system serves to collect radiation. The
higher costs by its implementation will be returned when
the system is used for a certain period, i.e., energetic
amortization. Regarding the long term durability of solar
panels and lower energetic costs for their production,
solar panels are considered to be a source with long
energetic returnability. Unfortunately, solar systems
cannot work without an additional source since in the
conditions of Central Europe they are not able to generate
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
their whole consumption in an economically effective way
[3].
When the intensity of luminosity is lower during the
day (at the night), the solar panel does not generate
energy, therefore it is necessary to use a secondary energy
source. In this case, the use of another alternative low-cost
energy source (water energy, wind energy) would by
complicated regarding the external forcing of the climate
in the given locality. A solution could be found in the use
of energy produced and stored in accumulators during the
day, eventually in a combination of solar energy and
connection to the mains.
II. BASIC FACTS
When calculating the amount of solar energy reaching
1 m2 of the Earth, we rely on essentials of physics. The
Earth revolves around the Sun in an elliptical path. The
Sun is right in a focus of the Earth´s path. Since the
trajectory of the Earth revolving has the shape of an
ellipse, it is necessary to count on the change of the
distance between the Earth and the Sun. When passing
through the atmosphere, intensity of sun rays is getting
lower. A total luminous flux is called luminosity L [4].
We can calculate the radiation intensity on the basis of the
relation between luminosity and the distance of the Earth
from the focus. It is important to notice that within the
relation to the Sun, the Earth can be in two positions.
Perihelion (P) is the position when the Earth is nearest to
the Sun. Aphelion (A) is the position when the Earth is
farthest from the Sun. The days when the Earth is in these
two positions are called solstices. The distance which
changes depending on the trajectory could be conveyed in
a calculation of eccentricity (e) with respect to the
constants mentioned so far [5].
To adapt the model to the real conditions, it is necessary
to count on the angle formed by the connection of a
random point in the trajectory of the Earth with the Sun
and by the connection of the Sun and the Earth when it is
in perihelion. This angle is marked with a Greek sign φ
in Fig.2.
Fig. 2: The trajectory of the Earth around the Sun
A direct sun ray passes through the atmosphere when
hitting the Earth´s surface. Clouds bend the solar radiation
and that leads to light scattering. Refraction is described
by Snell´s law [6]. In two different media the ratio of the
sine of the angle of incidence to sine of the angle of
refraction is called relative refractive index. Two types of
refraction are known depending on the density of the
media. If the ray of light travels from the medium with an
optically lower density to the medium with an optically
greater density, light will refract away from the
perpendicular. Otherwise, light will refract towards the
7
perpendicular. When calculating diffuse radiation on the
Earth, we have to consider the aspect that rays of light hit
a surface that is heterogeneous. The Earth´s surface
consists of land, oceans, and water areas. When reaching
land, sun rays reflect from mountains and one part of
diffuse radiation originates this way. Oceans and water
areas evaporate, vapor originates and this phenomenon is
related to diffuse resistance regarding the medium. The air
has the lowest diffuse resistance.
It is very important to include also the angle of solar
panel tilt by diffuse radiation. In the vertical position the
collector is tilted. The radiation reaching the area of the
panel is just partial. The most energy is generated by a
horizontal position of the collector. The angle of tilt is null
so the solar panel area is fully available for collecting
diffuse radiation.
III. SUGGESTION FOR SOLUTION
Calculations are based on the rate of the Sun´s
luminosity L = 3, 842 × 10 26 W [7]. The intensity of solar
radiation is marked with a sign I 0 . When we want to
express the intensity mathematically, it is necessary to
include a relation between the positions of the Earth and
the Sun, marked as the distance r, to the equation. The rate
of solar radiation intensity is in direct proportion to the
Sun´s luminosity and in inverse proportion to the change
of the distance between the Sun and the Earth.
I0 ( r ) =
L
4πr 2
(1)
The distance is considered as a constant in this equation
but this is not correct. The Earth revolves around the Sun
in an elliptical path. In terms of this ellipse, the Earth
comes to the points of equinox and the ellipse has two
focuses. The distance between the focus and the centre of
the ellipse is defined by the rate of ellipse eccentricity ε .
This may be conveyed by a simple equation
ε = a 2 − b 2 (2). When the change of the distance
between the Sun and the centre of the ellipse is conveyed,
it is possible to convey the distance between the Earth and
the Sun. Here it is necessary to include also the distance
by equinox marked as r0 . This distance is in direct
proportion to the distance between the Earth and the Sun
regarding the angle φ . The angle φ is in inverse
proportion to the sidereal year, i.e., the time taken by the
Earth to orbit the Sun once with respect to the fixed stars.
The time taken from the moment when the Earth crosses
the point of equinox to the given point on its trajectory is
related to the angle φ and it is marked by a letter t,
resulting in the relation:
r ( φ) =
r0
1 + ε cos φ
(3)
The dependence of eccentricity on the distance is
visible in this relation. Let´s expand the formula for
eccentricity by application of knowledge about two
focuses of an ellipse. In this case, these focuses are called
perihelion and aphelion. After converting the equation, we
get two equations to calculate the distance in both focuses
of the ellipse.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
Let´s go back to the beginning and let´s apply given
formulas to the basic formula for the calculation of the
Sun´s intensity.
r0
,
1− ε
r
= 0
1− ε
rapohelium =
rperihelium
(4)
L
L
L
2
=
1 + ε cos φ ) =
(1 + 2ε cos φ ) (5)
2
2 (
4πr
4πr0
4πr02
Besides direct radiation, a solar collector is hit by
diffuse radiation. When the collector is placed
horizontally, this radiation is in direct proportion to direct
radiation. It is necessary to include external impacts of
scattering, so the constant of the diffuse factor µ has to be
used.
I d = I0µ
(6)
The constant of the diffuse factor can be calculated
from the relation:
µ = 0, 095 + 0, 04 sin ( 360 / 365 ) . ( t − 100 )  (7)
When these two types of radiation are applied, the
relation for calculation of total solar radiation during
a clear day in W/m2 is
I = I0 + Id
(8)
It is possible to calculate total solar radiation reaching
1 m2 from the formula but it is not possible to determine
what area a solar collector in a horizontal position should
have to generate the amount of energy that is necessary
for consumption of a server per day. To determine the
maximum energy consumption of a server in conditions of
a medium-sized company, an IT company was addressed.
On the basis of its experience the load 2.5 kW per day was
determined.
In the case of horizontal position of the collector it is
very important to realise that the tilt is null. Sun rays reach
the area of the collector directly. The calculation of the
area of the collector depends on the particular kind of
solar collectors. The effective area of the absorber, marked
as Sa, and the useful power output of the collector are
important parameters. The constant Sa is defined by
a producer. The useful power output is sometimes defined
but it is possible to convey it by energetic effectiveness η
and by the amount of energy reaching the collector, i.e., I.
Qu = I.η
S=
(9)
A total area of collectors S can be calculated as a ratio
of the load per day to the useful power output of the
collector.
Qz
Qu
(10)
Besides the area of collectors, it is necessary to
calculate the number of collectors, marked as n.
n=
The given relation is valid when a solar collector is
placed in a horizontal level since it is the most effective
way of solar radiation utilisation.
I0 ( r ) =
8
S
Sa
11)
It is obvious by the load per day that more collectors
will be needed for the realisation. The number depends on
collector power. In this case it is not specified as the use
of a solar power system but as the use of an island solar
power system and maybe the term a net solar power plant
would be suitable. As a suggestion of solution we offer the
use of solar power plant with 14 solar panels with
efficiency 230 Wp and one photovoltaic inverter.
A project by a certificated expert, permission, and licences
are needed to use the plant. A switchboard with protection
is necessary to connect the plant to the grid.
IV. CONCLUSION
The use of the technology of virtualisation and the
cloud solution creation for companies is an up-to-date
solution of the work with data. Despite of advantages,
cloud computing has brought also troubles that have not
been solved yet. Data security is considered as one of the
basic troubles. Data integration, data secrecy, and
accessibility belong here. Since this innovative technology
is dependent on a source of energy, its accessibility
depends on electricity. A modern solution of cloud
computing implementation on a server is influenced by
the connection of a server to the mains. In the case of
power failure the access to the system is not possible, or
data may be lost or stolen. The use of alternative sources
of energy instead of the mains is one of possible solutions
of this problem and solar energy, which is often used
today, offers such possibilities.
Solar energy is considered as one of the most ecological
solutions of power dependence. The source of this energy
is the Sun. The Earth revolves around the Sun in an
elliptical path and the Sun is in one of two focuses of this
ellipse. Perihelion, the nearest, and aphelion, the farthest,
are two most interesting points on the trajectory of the
Earth around the Sun. The distance between the Earth and
the Sun affects the intensity of solar radiation that reaches
the Earth´s surface. Before reaching the Earth, sun rays
pass through the atmosphere so the intensity of radiation is
lower. Besides direct sun rays, diffuse rays reach the
Earth. These rays are reflected from the Earth´s surface,
clouds, or water vapour above the water areas and oceans.
Light scattering occurs on the basis of light refraction,
described by the Snell´s law. When the intensity of sun
rays reaching the Earth is being calculated, it is necessary
to consider the angle between the Earth and the Sun. Total
solar radiation reaching the Earth may be calculated as
addition of all types of solar radiation. Solar collectors
serve to utilise solar radiation. The purpose is to collect as
many sun rays as possible and transform them to solar
energy. Solar radiation collecting is most effective when
the solar collector is in a horizontal position. Rays hit the
absorbing area of the collector each sun hour available per
day. A large number of collectors are needed to generate
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
electricity necessary for a correct work of a server. A total
area of collectors may be calculated with a respect to the
useful power output of the collector and the effective
absorbing area. The number of collectors depends on the
total area of collectors. When a larger number of
collectors are used, we call it an island solar electric
system. However, this term is not correct in this case.
A solar power plant originates when solar energy is used
as an energy source. We offer the use of 14 collectors with
power 230 Wp as one possibility. Before the realisation, it
is necessary to consider not only the expenses for solar
techniques but also licences, a project, and confirmations
for the work of the plant. Initial investment will be
returned in a few years and regarding the long term
durability of the solar power plant, this solution becomes
interesting for medium-sized companies, eg. various
daughter companies like Kongsberg Automotive (Vráble,
Slovak Republic), Lear Corporation (Vyškov, Czech
Republic), Tyco Electronics (Kuřim, Czech Republic).
9
REFERENCES
[1] Gartner, IT Glossary – defining in IT industry, 2012. [Online].
Available:
http://www.gartner.com/it-glossary/cloud-computing/
[Accessed: 12 Oct. 2012].
[2] C. J. Hamilton, Views of the solar system, 2012. [Online].
Available: http://www.solarviews.com/eng/sun.htm [Accessed:
5 Oct. 2012].
[3] I. Iliaš, K. Guschlbauer-Hronek, B. Benesch, G. Bayer, Slnko k
službám: Možnosti využívania slnečnej energie, Energetické
centrum Bratislava, Phare, 2006.
[4] M. S. Keil, Gradient representations and the perception of
luminosity, Vision Research, December 2007.
[5] L.A. McFadden, P. Weissman, T. Johnson, Encyclopedia of the
Solar System (Second Edition), New York: Academic Press, 2007.
[6] R.J. Schechter, Snell´s law: Optimum pathway analysis, Survey of
Ophthalmology, June 2010.
[7] S. Gil, M. Mayochi, L. J. Pellizza, Experimental estimation of the
luminosity of the Sun, Física re-Creativa, 10 March 2009. [Online].
Available:
http://www.fisicarecreativa.com/papers_sg/papers_sgil/Docencia/lu
minosity_sun_2k6.pdf [Accessed: 15 Oct. 2012].
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
10
Control Techniques of Grid Connected PWM
Rectifiers under Unbalanced Input Voltage
Conditions
Martin Bejvl 1), Petr Šimek 2), Jiří Škramlík 3), Viktor Valouch 4)
Institute of Thermomechanics AS CR, v. v. i., Department of Electrical Engineering and Electrophysics,
Dolejškova 1402/5, 182 00 Praha 8, Czech Republic, www.it.cas.cz
1)
[email protected], 2) [email protected], 3) [email protected], 4) [email protected]
Abstract — Current-controlled voltage source
converters are widely used in grid-connected applications,
for example for ac drives with indirect frequency
converters. The structure and parameters of the PLL are
developed and proposed in order to cope with the grid
containing both the positive and the negative sequence
components, and minimize the wrong frequency transients
during phase angle steps and also in the start-up stage. The
DSC (Delayed Signal Cancellation) technique was realised.
There is also necessary to compensate the negative sequence
component in the grid voltage. The negative sequence
component of the grid voltage causes ripples of the dc
voltage in the DC link. Several sophisticated topologies of
converter current controller were developed, simulated and
tested for this purpose. Results of simulation and
experimental tests are provided to evaluate different current
control schemas and phase locked loop techniques.
Keywords — Phase Locked Loop (PLL), DSC, Current
Controller, PWM rectifier.
I. INTRODUCTION
The ac/dc rectifiers are often used in many applications,
especially in electric drives with ac motors. The rectifier
works at these drives as an input part of the indirect
frequency inverter.
In an ideal situation, the 3-phase grid voltage is harmonic,
shifted by 120° phase-by-phase, and the voltage and
current are in phase. The track of the space vector of the
3-phase voltage transformed to αβ coordinates has a form
of the circle.
In a real situation, the 3-phase grid voltage is distorted by
a negative sequence component, high-order harmonics
and by a shift between voltages and currents in the
phases, which produces a reactive power and also ripples
in the voltage on the capacitors in the DC link. The track
of the space vector of the 3-phase voltage transformed to
αβ coordinates has a deformed form.
This deformed track of the voltage space vector, with
harmonics neglected, consists of the positive and negative
superposed components. The first component is equal
with the ideal circle shape of the space vector track and is
running by frequency 50 Hz. This is so called positive
sequence component. The second component of the space
vector is running in the opposite direction with the
frequency 50 Hz and represents the negative sequence
component of the grid voltage. The second component is
called negative sequence component.
Now, our object is an analysis of the positive and
negative sequence component control of the ac current in
order to eliminate reactive power, high-order harmonics
and the ripples of the voltage in the dc link. The current
also consists of the positive and the negative sequence
components similarly like the voltage. Both components
of the current are controlled separately and
simultaneously. The positive sequence component of the
current is controlled in the positive coordinates, which
looks like the dc control. The negative sequence
component of the current is controlled in the negative
coordinates, which looks like the dc control too. Both of
these controls are realized simultaneously by separate PI
controllers. This system is very effective because it
permits the complete control by one chip and one
program without additionally SW or HW devices.
The necessary condition of a reliable operation of an
electronic power converter connected to the grid is an
effective synchronization of the control system, usually to
the positive sequence component of the grid voltage. We
present a several possibilities of the synchronization by
Phase Locked Loop (PLL) strategies in the text below.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
11
Fig. 1: Control schema of rectifier with standard dual current controller; reproduced from [1]
From many methods published recently only those were
selected for an analysis which are suitable for three phase
systems and do not need tuning of many parameters (like
e.g. those with varying switching periods, with adaptive
parameter tuning, with many low- or high-pass filters,
etc.). These selected PLL techniques have been more or
less verified by simulations or experiments and
recommended to use not only by their authors too.
can cause dc voltage ripples. It means that the control
system based on calculation from
ea , eb , ec is not
usable for an extremely unbalanced grid.
B. Nonlinear dual current controller
A new method, which calculates current references
idqp∗
pn
idqn∗ with help of the output voltage vdq
, was proposed in
II. CURRENT CONTROLLER: INVESTIGATED AND TESTED
METHODS
A. Standard dual current controller
displays the schema of the control algorithm, which is
considered to be a standard dual current controller [1]-[2].
This control system is based on calculation of the
reference
currents
with
help
of
the
grid
voltages
ea , eb , ec . For the exactly working control
system it is necessary to compute the current references
from voltages
va , v b , vc , thus from voltages at the ac
poles of the converter, because v a , v b , v c , together with
phase currents, are responsible for an active power that
[3].
The
output
voltage
pn
vdq
is
expressed
by
pn
pn
vdq
= edq
± ωLiqdpn ,
where
pn
edq
is measured. The disadvantage of this method
consists in necessity to solve nonlinear equations, what is
very difficult and complicated.
C. Dual current controller with feedback from
PWM
This method will avoid the nonlinearity of the previous
one [4]. The nonlinearity is transformed to linearity by
use an additional feedback path for pole voltages
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
Phase error
Input v1(t)
v2(t)
12
Phase
detector
ϑe
Loop filter
(PI Controler)
ω
VCO
Fig. 2: Basic structure of closed loop PLL
(va , v b , vc ) . So, the algorithm for calculation of the
which is dissipated in filters, are used:
current references becomes simpler. The four sequence
In the first case (case 1), the dc side of the converter
supplies oscillating power to the filter, it means that the
oscillating powers are set to zero on the grid side, i.e.
components of the pole voltages
vdp , v qp , vdn , vqn , are
extracted from the space vector PWM block and fed back
to the algorithm for calculation of the current references.
Ps2in = Pc2in = 0 .
A disadvantage of the methods is that oscillating powers
that have been dissipated in the input filtering inductors
are neglected.
In the second case (case 2), the oscillating power flows
from the grid into the filter, i.e. the oscillating powers are
forced to zero at the converter terminals
and
Pcin2 = - ∆Pc2
Psin2 = - ∆Ps2 .
D. Dual current controller with calculation of
dissipated power
This method [5] comes from calculation of the power,
which is dissipated in the input inductors. The method
calculates the current references with the help of the
measured voltage e. The difference is considered between
the power at the grid side and power at the ac side of the
converter. So the regard to losses at the inductors is
assumed. But the equations for the power are still
nonlinear. The authors avoided the nonlinearity by giving
the previous patterns of the calculated reference current
to the equations for calculating the actual current
references.
So nonlinearity (multiplying unknown components of the
current with unknown components of the current) is
replaced by linearity. This is the difference between this
work and the nonlinear controller.
Two different methods for calculation of the power,
Input vabc(t)
III.
GRID SYNCHRONISATION
The PLL unit can be described by the basic structure
shown in the block diagram in Fig. 2 This PLL structure
comprises a phase detector (PD), a loop filter (LF), and a
voltage controlled oscillator (VCO). Each part of which
can be implemented in several different forms.
The block diagram in Fig. 2 shows a basic structure of the
closed loop synchronization block. The input (reference)
signal is compared with the internal (estimated) signal in
the phase detector (PD), called also phase comparator.
The phase error being the output of this block is used as
the input signal for the frequency controller (PI).
Frequency from this controller is used for the control of
the Voltage Controlled Oscillator (VCO). The output of
the VCO is compared with the input signal in the PD
block. When the input and internal signal are in the same
vsd=Vs
abc
abc→dq
dq
vsq→0
ω
PI
ϑcont
∫ dt
Fig. 3: Block structure of the SFR-PLL
ϑ
ϑ
to 0..2π
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
angular frequency and phase angle, the error signal is
zero and PLL is in steady state.
13
Voltage Vd, Vq
10000
Vq
5000
A. SFR (Synchronous Frame Reference)
A basic closed loop technique is the SFR-PLL
(Synchronous Frame Reference PLL) where the reference
frame is synchronized by the vector of the grid threephase voltage. It is the most extended technique used for
frequency and phase angle detection in three phase
systems.
The input voltage is transformed to the synchronous
reference frame. One of two synchronous components is
required to be zero (in Fig. 3 vsq). If the first component is
(nearly) equal zero the second component represent
magnitude Vs of the grid voltage vector and the angle ϑ
represents the instantaneous phase angle of this vector.
Vd
0
-5000
1.4
1.5
1.6
1.7
1.8
1
1.9
2
2.1
2.2
2.3
Vabc
4
x 10
Va
Vb
0
Vc
-1
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
error of angle (in degrees)
1
Source
SFR
0.5
0
-0.5
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
frequency
52
Under ideal grid conditions (without harmonic distortion
and negative sequence component) a high bandwidth of
the SFR-PLL feedback loop yields a fast and precise
detection of the phase and amplitude of the grid voltage
vector. In case when utility is distorted with high order
harmonics, the SFR-PLL can operate satisfactorily only if
this bandwidth is reduced.
Fig. 4 shows typical waveforms obtained by using the
SFR PLL. The upper graph shows the detected voltages
in the synchronous frame (vd, vq). The lower graph shows
the instantaneous angle error (top), and the instantaneous
detected frequency (middle). Here, the blue lines sign the
theoretically correct values. The bottom graphs in both
diagrams show the phase voltages. Between 1.5 and 2.5 s
the voltage negative sequence component of the
magnitude 10 % was added. The negative sequence
component is transferred as a second order harmonic in
the transformed voltages, and manifests itself in the
detected frequency and phase waveforms.
B. DDSFR (Double Decoupled Synchronous Frame
Reference)
In [11] the method for decoupling the positive and
negative sequence voltage components was described.
This method uses the filtered negative and positive
sequence components to eliminate (decouple) them in the
input signal. A decoupling cell is used for the
cancellation of the coupling effect between signals in the
two different frames.
Block diagram of the DDSFR-PLL is shown in Fig. 5.
The decoupling cell for decoupling signals between
Source
SFR
51
50
49
1.4
1.5
1.6
1.7
1.8
1
1.9
V
4
x 10
2
2.1
2.2
2.3
abc
V
V
0
V
-1
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
a
b
c
2.3
Fig. 4: SFR-PLL under unbalanced grid conditions. The negative
sequence component of magnitude 10 % was added between
1.5 and 2.0 s
positive and negative sequence components can be
expressed as
v*d +  vd +   cos2ϑ sin2ϑ vd − 
 * +  = v +  − 
 
vq   q  −sin2ϑ cos2ϑ vq− 
(1)
v*d −  vd −   cos ( −2ϑ) sin ( −2ϑ)  vd + 
 
 *  =  −
vq−  vq−  −sin ( −2ϑ) cos ( −2ϑ) vq+ 
The low pass filters at the outputs of the decoupling block
stop (or attenuate) the transformed negative sequence
component (it is transferred as a component with double
frequency). All the frequencies at the input that are
greater than this frequency (i.e. harmonics) are attenuated
too. But only the negative sequence component is
decoupled and quite suppressed. The PLL filter is situated
outside of the control loop. Thus, the loop bandwidth of
the PLL is not reduced.
The simulation of this type of PLL was performed with
the recommended LPF type (Butterworth of the 4th order)
and cut-off frequency value. The Fig. 6 shows transient
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
abc
dq+
vabc
v*d +
vdq +
abc
dq−
14
v*q +
Decoupling
v*d −
cells
vdq −
v*q −
LPF
LPF
Vs = vd
vd +
vq +
PI
LPF
LPF
ω
1
s
ϑ
vd −
vq −
Fig. 5: Block diagram of the DDSFR-PLL strategy
response after applying an unsymmetrical voltage
component.
Positive sequence
10000
V q+
5000
V d+
0
-5000
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
Negative sequence
C. DSC (Delayed Signal Cancellations)
The DSC (Delayed Signal Cancellation) strategy
represents an effective and robust method for the
detection of the positive and negative sequence
components of the voltage in the three-phase system. The
positive sequence vector appears as a dc component in
the synchronous reference frame.
500
V q0
V d-
-500
-1000
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
V abc
4
1
x 10
Va
0.5
Vb
0
+
vαβ
( t ) = 0.5 ( vαβ ( t ) + jvαβ ( t − T4 ) )
Vc
-0.5
-1
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
The voltage vector in a stationary α-β reference frame is
delayed. This technique is based on a combination of the
positive- and negative-sequence components with a time
delay of one quarter of a period (5 ms at 50 Hz).
2.3
(2)
−
vαβ
( t ) = 0.5 ( vαβ ( t ) − jvαβ ( t − T4 ) )
error of angle (in degrees)
0.6
Source
DDSFR
0.4
0.2
D. PLL based on the pq theory [14]
0
-0.2
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
frequency
Source
DDSFR
50.65
50.6
The phase detector operation is based on the product of
the space vectors v1(t) and v2(t). Its output can be
obtained from
50.55
50.5
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
x 10
e
j ( Φ1 −Φ 2 )
(3)
where the asterisk (*) denotes the complex conjugate
quantity. Alternatively, the phase error signal can be
expressed in rectangular form as
Va
0.5
Vb
0
Vc
-0.5
-1
1.4
j (ω1 −ω2 )
V abc
4
1
vd ( t ) = v1 ( t ) ⋅ v2 ( t ) = V1V2 e
*
50.45
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
Fig. 6: Detection of positive and negative sequence in the
synchronous frames (dq+ and dq-) with DDSFR-PLL
vd ( t ) = ( v1α v2α + v1β v2 β ) + j ( v1β v2α − v1α v2 β ) (4)
It is evident from
(4) that the real and imaginary
components of vd(t) have, respectively, the same form as
the p and q power components from Akagi’s
instantaneous power theory.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
fiα
vabc
abc
cos(ϑ)
vα
PI
controller
fp
αβ
15
vβ
fiβ
ϑ
ω
∫ dt
sin(ϑ)
Fig. 7: PLL based on the pq theory
Two different approaches can be adopted: if the real part
of vd(t) is used as the feedback error signal, then we have
the so called p-type PLL system. Using the imaginary
part of vd(t) yields the so called q-PLL.
(stationary α-β and synchronous d-q). The extracted
positive sequence voltage component is the input to an
SFR-PLL for detecting the frequency and the angle of the
desired voltage vector.
Now considering that ω1=ω2 = ω0 (i.e. the p-type PLL is
nearly locked at the centre frequency), then the phase
error signal given by (3) can be simplified to
It is possible to obtain the symmetrical phasor
components (s+, s- and s0) of a three-phase signal from
vd = VV
1 2con (ϑ1 − ϑ2 )
 S%+ 
1 1∠120° 1∠ − 120°   S%a 
%  1
 % 
 S −  = 3 1 1∠ − 120° 1∠120°   S b 
 S%0 
1
  S%c 
1
1
 
(5)
For small deviations it can be approximated linearly by
vd ( t ) ≈ K dϑe ( t )
(6)
(6)
The negative and positive components can be extracted in
the abc system by
where ϑe is the phase error and Kd = V1V2.
Block diagram is showed in the Fig. 7. The resulting
quantity can also be interpreted as being analogous to the
instantaneous real power according to Akagi’s pq theory.
E. Simple mathematical transformations for
canceling some harmonics [15]
This is the next method for elimination of the negative
sequence component and most harmonics. The method is
based on different ways of obtaining the positive and
negative sequence components of the grid voltage, by
applying the symmetric component theory to the voltage
vector transformed into different reference frames
vabc
αβ
S%a− 
1∠60° 1∠− 60° S%a 
 −1
 %−  1 
 
−1
1∠60°  S%b  (8)
Sb  = 3 1∠− 60°
S%c− 
 1∠60° 1∠− 60°
−1  S%c 
 
 S%a+ 
1∠− 60° 1∠60°  S%a 
 −1
 %+  1 
 
−1
1∠− 60° S%b  (9)
 Sb  = 3  1∠60°
 S%c+ 
1∠− 60° 1∠60°
−1  S%c 
 
The operations (8) and (9) can be written in different
ways for obtaining the same response if the phase signals
contain only the fundamental frequency components.
vq
αβ
abc
Cαβ
Dαβ
dq
(7)
Aαβ
Bαβ
Fig. 8: Block diagram of the PLL which uses the mathematical
transformations
MDC
ϑ
ω
PI
∫ dt
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
16
However, the harmonics presented in the original signals
are differently modified by each of such operations.
the corresponding values in degrees. The negative
advance is used to indicate delayed instantaneous signal.
The Eq. (8) can be rewritten as (in the instantaneous
values)
The formulas (10) - Chyba! Nenalezen zdroj odkazů.
can be used to improve the operation under harmonics of
the SFR-PLL after the transformation to the stationary αβ frame [15] (Fig. 8). Most of the harmonics has been
suppressed (all odd except 12n+1, (n is an integer)) or
  sa 
 sa− 
 sa 60 
 sa −60  
1  
 −




 sb  = − 3  A1  sb  + A2  sb 60  + A3  sb − 60   (10)
 s 
 sc− 
 sc 60 
 sc − 60  
 




  c
AB
1
0.8

 sa− 
 sa 
 sa −90  

1
 −
 


 sb  = 3  − B1  sb  + B2  sb −90  

 sc− 
 sc 
 sc −90  
 
 



0.6
(11)
(11)
0.4
0.2
0
-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
The Eq. (9) can be rewritten as (in the instantaneous
values)
0 1
2
3
4
5
6 7
8
9 10 11 12 13 14 15
Fig. 9: Gain of the operation A and B in cascade
  sa 
 sa+ 
 sa −60 
 sa 60  
1  
 +


 
 sb  = − 3  C1  sb  + C2  sb −60  + C3  sb 60   (12)
 s 
 sc+ 
 sc −60 
 sc 60  
 


 
  c
CD
1
0.8
0.6
0.4
0.2

 sa+ 
 sa 
 sa 90  

1
 +
 


 sb  = 3  − D1  sb  + D2  sb 90  

 sc+ 
 sc 
 sc 90  
 
 



0
-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2
3 4 5 6 7 8
9 10 11 12 13 14 15
(13)
Fig. 10: Gain of the operation C and D in cascade
attenuated.
where the matrixes
 −1 0 0 
A1 = C1 =  0 −1 0 
 0 0 −1
0 1 0 
A2 = C2 = 0 0 1 
1 0 0 
0 0 1 
A3 = C3 = 1 0 0 
0 1 0 
1
 −1 12
2 

1
1 
B1 = D1 =  2 −1 2 
1
 12
−1
2
 0

B2 = D2 =  − 23
 3
 2
3
2
0
−
3
2


3
2 

0 
−
Due to the synchronous d-q frame transformation the
harmonic order of a component in the input signal is
decreased. Then, even harmonics become odd and vice
versa. The signals in the synchronous d-q reference frame
go through the operation A and B in cascade. The gain of
this operation is shown in Fig. 9. Then the odd harmonics
(even harmonics in the original signal) are eliminated.
The gain of the operation A and B in the cascade is
shown in Fig. 10.
The eleventh negative and thirteenth positive (and all
other harmonics 12n+1) get through without attenuation,
as well as the fundamental harmonic. This harmonic
cannot be suppressed by use of this method.
3
2
(14)
The subscripts 60 and 90 are used to indicate
instantaneous signals advanced from the original ones by
F. PLL with moving average value
The integral of the voltage of any harmonic frequency in
the full period of the fundamental frequency is zero. This
mathematical knowledge is used in the Frequency
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
Input vabc(t)
17
vsd=Vs
abc
abc→dq
dq
vsq
AVG
ω
PI
∫ dt
ϑ
Fig. 11: Block structure of the SFR-PLL
Fig. 12: Dip response under single current controller in comparison to standard dual current controller; reproduced from [1]
Variable Average PLL (FVAVG-PLL). This technique is
capable to suppress all harmonics components (Fig. 11).
IV.
SIMULATION AND EXPERIMENTAL RESULTS
A. Standard dual current controller
The 50 % dip has been simulated in order to test the
effectiveness of the dc controller under the condition of
an unbalanced voltage [1]. The current of the dc-link load
is constant 0.5 p.u. Fig. 12 displays response of the output
dc voltage to the described dip, where the single current
controller and standard dual current controller have been
used. The dip starts at 20ms and ends at 180ms. It is
obvious that the dc voltage ripple is almost zero under the
dual controller. For the single current controller system,
the dc-voltage ripple oscillates with twice the grid
frequency and the amplitude of the ripple is 4 %. So,
possibilities of the single current controller system are
limited in the presence of negative sequence components.
Fig. 13: Simulation waveforms for the single controller under
unbalance; reproduced from [3]
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
18
B. Nonlinear dual current controller
This type of the dual current controller was tested under
Fig. 16: Simulation waveforms of the dual current controller with
feedback from PWM controller with a compensation for the 15 %
one phase unbalance; reproduced from [4]
Fig. 14: Simulation waveforms for the nonlinear dual current
controller under unbalance; reproduced from [3]
conditions of 15 % decrease of the voltage amplitude in
one phase [5]. This type of unbalance appears often in a
weak ac system.
The simulation results are displayed in Figs. 13, 14. The
figures display waveforms of the dc voltage vdc, phase
current ia and grid voltage ea.
Fig. 13 displays results if the simple single PI controller
without any compensation of the unbalance in the control
program is used. Fig. 14 displays results of the dual
current controller.
Fig. 17: Grid current (top) and dc voltage (bottom) with single current
controller; reproduced from [5]
It is obvious that the output voltage vdc has a strong ripple
in its waveform in Fig. 13. The output voltage vdc has any
ripple in its waveform in Fig. 14.
C. Dual current controller with feedback from
PWM
The dual current controller with the feedback from PWM
was tested under conditions of 15 % decrease of the
voltage amplitude in one phase [4].
Fig. 15: Simulation waveforms of the single controller without a
compensation for the 15 % one phase unbalance; reproduced
from [4]
Fig. 15 and Fig. 16 display simulation the waveforms of
the line voltage wave form in phase a (ea), line current in
phase a (ia) and output dc voltage (vdc). Fig. 15 displays
results of the conventional single PI controller use
without unbalance compensation. Fig. 16 displays results
of the dual current controller use. We can see that the
dual current controller provides much lower ripple of the
vdc than the conventional controller.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
19
1
x 10
4
v alpha, sine, cosine
v alpha
sine
cosine
0
-1
0.3
0.31
0.32
0.33
0.34
0.35
theta
0.36
0.37
0.38
0.39
10
0.4
theta
5
0
0.3
1
0.31
x 10
0.32
0.33
4
0.34
0.35
0.36
v d, v q, v+ d, v+ q
0.37
0.38
0.39
0.4
vd
v+ d
vq
v+ q
0
-1
0.3
0.31
0.32
0.33
0.34
0.35
0.36
Time(s)
0.37
0.38
0.39
0.4
Fig. 20: Waveforms of DSC-PLL after unbalance is added
to input voltage (vd, vq – input voltages, v+d, v+q –
detected positive sequence)
Fig.18: Grid current (top) and dc voltage (bottom) with dual current
controller with calculation of dissipated power in method 1; reproduced
from [5]
It is obvious that the use of this algorithm results in
quality current references. On the other side, the dc side
supplies the oscillating powers and therefore the dc ripple
is about to 20 %.
Fig. 19 displays waveforms of the grid currents and
output dc voltage for the dual current controller algorithm
of the method 2. With the oscillating powers supplied by
the grid, the dc voltage is smoothed out, apart from the
transients at the beginning and end of the dip.
The currents are almost the same in magnitude in both
cases, but they are more smoothed in the case 2.
Fig. 19: Grid current (top) and dc voltage (bottom) with dual
current controller with calculation of dissipated power in
method 2; reproduced from [5]
D. Dual current controller with calclatiton of
dissipated power
Several simulations were realized in order to test the
controller [5]. There was a 40 % dip in the phases B and
C.
Fig. 17 displays waveforms of the grid currents and
output dc voltage for the single current controller. The
unbalance of the grid voltage causes the ripple in the
output dc voltage of about 6.5 %.
Fig. 18 displays waveforms of the grid currents and
output dc voltage for the dual current controller
algorithm. The dual current controller algorithm of the
method 1 was used here.
Fig. 21: Measured time responses of DSC-PLL after changing grid
voltage in unbalanced form: a) upper part: grid voltage component va
(1000 V/div), sinΘ, cosΘ; b) middle part: angle Θ (π/div); c) lower part:
grid voltage components vd, vq (250 V/div) – time scale 10 ms/div
E. Phase Locked Loop strategies
All proposed PLL strategies were simulated with
different parameters and by several voltage distortions.
Fig. 20 shows the response of the DSC-PLL to the
unsymmetrical voltage (the magnitude of the voltage in
the phase a was rapidly decreased). In Fig. 20, lower part,
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
20
Frequency
Frequency
50.4
50.15
Source
DSC
DDSFR
50.2
Source
FVAVG
SFR
50.1
50.05
50
50
49.8
49.95
49.6
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
1.4
1.5
1.6
Angle Error (in Degrees)
1
0.5
0.5
0
0
-0.5
-0.5
1.5
1.6
1.7
4
1
1.8
1.9
2
2.1
-1
1.4
2.2
1.5
1.6
1.7
4
Voltage
x 10
1.8
1.9
2
2.1
2.2
2.1
2.2
Angle Error (in Degrees)
1
-1
1.4
1.7
1
1.8
1.9
2
Voltage
x 10
Va
0.5
Vb
Vc
0
-0.5
-1
1.4
Va
0.5
Vb
Vc
0
-0.5
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
-1
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
Fig.22: Waveforms obtained by applying DSC-PLL, DDSFR-PLL, FVAVG-PLL, SFR-PLL strategies when unbalance is added to input
voltage
two pairs of the d, q grid voltage components are
+
d
depicted: vd, vq of the real grid voltage and v ,
+
q
v of the
extracted positive symmetrical voltage sequence
component only, which are used in the DSC-SPLL
algorithm to synchronize the controller to the grid.
The DSC-PLL method was adopted as the most suitable
for a practical realisation.
Fig. 21 presents captured time responses of the DSC-PLL
variables under the same conditions as those in Fig. 20.
High frequency disturbances at the instant of a voltage
system change result from the switch process in the
programmable power source. Nevertheless, even by this
dramatic voltage change, the synchronization was
reached in circa half of the fundamental period (that is,
+
the components v d ,
v q+
V. CONCLUSION
It has been introduced that the unbalanced input voltage
causes wrong work of the rectifier. So the unbalance must
be compensated. These compensations are really very
important for a good function of the PWM rectifier. The
compensation method must be tested in different nonstandard states and also in transient processes. Therefore,
a lot of efforts have been made to propose a control
schema, which would make possible to compensate the
input voltage unbalance reliably.
In the second part of the paper an overview of different
synchronizations techniques suitable for the three phase
PWM rectifiers were presented. Most of the techniques
were simulated too. The DSC-PLL technique was
realized in the control system with the DSP.
were again constant).
Fig. 22 shows the responses of different PLL’s by an
unsymmetrical input voltage. The DSC-PLL, DDSFRPLL, FVAVG-PLL, SFR-PLL strategies were simulated
and compared. The responses of the estimated frequency,
angle error, and three-phase voltages are presented. The
negative sequence component was added between
1.5 and 2.0 s.
ACKNOWLEDGEMENT
With institutional support RVO//:61388998.
Financial support of the Ministry of Industry and Trade of
the Czech Republic, through grant number FR-TI1/330, is
highly acknowledged.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
21
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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Yongsuh Suh, Thomas Lipo: “A control scheme of improved
transient response for PWM AC/DC converter under
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No. 5, October 1999.
Giuseppe Saccomando, Jan Svensson: “Transient operation of
grid-connected voltage source converter under unbalanced
voltage conditions”. Industry applications conference, 2001.
36th IAS annual meeting. Conference record of the 2001 IEEE,
vol.4, 30th September – 4th October 2001. Pages 2419 - 2424.
Yongsuh Suh, Valentin Tijeras, Thomas Lipo: “A nonlinear
control of the instantaneous power in dq synchronous frame for
PWM AC/DC converter under generalized unbalanced
operating conditions”. Conference record of the industry
applications conference, 2002. 37th IAS annual meeting, vol. 2,
13th – 18th October 2002. Pages 1189 – 1196.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Yongsuh Suh, Thomas Lipo: “A control scheme of improved
transient response for PWM AC/DC converter under
generalized unbalanced operating conditions”. 2004 35th annual
IEEE Power electronics specialists conference.
Fainan Magueed, Ambra Sanino, Jan Svensson: “Transient
performance of voltage source converter under unbalanced
voltage dips”. IEEE 35th annual power specialists conference
(PESC´04). Aachen, Germany. 20-25 June 2004. Pages: 1163 –
1168.
Rodriguez, Pedro Pou, Josep Bergas, Joan Candela, J. Ignacio
Burgos, Rolando P. Boroyevich, Dushan "Decoupled Double
Synchronous Reference Frame PLL for Power Converters
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conditions," in Proc. of IEEE-IAS Annual Meeting, vol. 4, Sept.
2001, pp. 2419-2424.
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The contribution was presented on the conference ELEN 2012, PRAGUE, CZECH REPUBLIC
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
22
Flickermeter Comparison Test
Petr BILIK 1), Petr PETVALDSKY 2), Martin KASPIREK 3)
1)
VSB - Technical University of Ostrava/Faculty of Electrical Engineering and Computer Science FEECS, Ostrava,
Czech Republic, e-mail: [email protected]
2)
VSB - Technical University of Ostrava/Faculty of Electrical Engineering and Computer Science FEECS, Ostrava,
Czech Republic, e-mail: [email protected]
3)
E.ON Czech Republic/Grid Assets Management Electric, Ceske Budejovice, Czech Republic, e-mail:
[email protected]
Abstract—The paper describes the test of flicker
evaluation made by eleven different types of power quality
analyzers. The standard IEC 61000-4-15Ed.2 (Functional
and design specification of flickermeter) issued on August
2010 specifies performance testing. Existing flickermeters
from different manufacturers may provide different results
when processing non-uniform voltage fluctuations. The
flickermeters response to voltage varying signals with
envelope shape typical for sawmill, heat pump, granulator
was tested. Voltage fluctuation caused by operating of this
electrical equipment was measured in the real low voltage
distribution network by means of the power quality
analyzer Topas 1000.
One-period records of voltage
fluctuation were available for the analysis. These were
simulated on the programmable power voltage source
HP6834B in the university lab.
Otherwise the choice of the PQ analyzer type, in case of
unsatisfactory PQ result, has resulted in the need for
different technical measures.
II. TECHNICAL EQIUPMENT
Keywords—Flicker, power quality analyzer, flickermeter,
voltage fluctuation
I. INTRODUCTION
Liberalization of the electricity market brings
considerable pressure to introduce penalties for
inconvenient voltage quality parameters. These penalties
should be the nature of the rebate payments for electricity,
in case of poor voltage quality. In case of detection of
unsatisfactory voltage quality because of the customer
complaints, it is necessary to take some corrective action.
If the correction action is not done in specified time limit
this can be another reason for DNO penalties. It has been
shown that the most common parameter in the LV
distribution network negatively affecting the voltage
quality within the meaning of EN 50160 [2] is the level of
flicker. Correct evaluation of this parameter is crucial for
DNO’s in terms of:
• The voltage quality complaint settlement on
customer side
• The choice of technical solutions in the
suppression of flicker
• The amount of investment funds deposited into
the distribution network in order to suppress
flicker
From the above mentioned information follow the
demands for
Power Quality analyzers, which are
intended to verify compliance with the requirements of
EN 50160 [2]. When different types of PQ analyzers are
measuring on the same site in the same measurement
period, then identical results should be obtained.
Fig. 1. Programmable power source HP6834B, AGILENT
oscilloscope, National Instruments cDAQ chassis with NI9225 module
All the tests described below were conducted at the
Department of Measurement and Control, Faculty of
Electrical Engineering and Computer Science, VSBTechnical University of Ostrava. Voltage changes were
simulated on the programmable power source Agilent
HP6834B. The source is a high-performance device, with
a rating up to 3kW, so it is not a problem to provide
power supply to the tested devices from it. An application
for controlling the HP6834B source was developed in
LabVIEW, allowing controlling the source via a GPIB
interface. A combination of the source and a control PC
can be provided for a broad range of voltage changes. The
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
generated progress was verified by an independent
measuring application created in LabVIEW, using the
following hardware: cDAQ chassis, which can be
connected to a PC via a USB port, and NI9225 module
that offers three galvanic separated voltage inputs with a
range of 300V. The whole assembly, together with the
AGILENT oscilloscope, is shown in Fig. 1.
III. TEST NUMBER 1
Test was realized in February 2010. Six power quality
analyzers were tested. Publication of the results of test 1
inspired a vivid discussion among professionals and
further interest in testing other devices. The test results
allowed one of the manufacturers to detect an error in the
firmware: only one device, declared by the manufacturer
as a class B analyzer under EN 61000-4-30 [3], showed
very different results for flicker evaluation. Distribution
system operators, consultancy companies and producers of
PQ analyzers expressed interest in verifying devices used,
traded or manufactured by the companies. This is why
another test was undertaken, with some additional tests
(unlike the previous test) under the latest IEC 61000-4-15
Ed.2 [1].
23
The following devices were tested:
• Topas 2000,(S/N TN92562BA, Fluke)
• Fluke 1744, (S/N Y621466CA, Fluke)
• MEG30, (S/N 60034, new FW, Mega)
• MEG30, (S/N 60036, Mega)
• ENA 450, (S/N EVAV090011, ELCOM)
• ENA 330, (S/N EVAV070002, ELCOM)
• ION 7600, (S/N PL0107A08001, Schneider)
• ION 7650, (S/N.PJ-0605A029-01, Schneider)
• MI 2292, (S/N 09510131, Metrel)
• PMD-A, (S/N 40229256-PMDA01, Qwave)
• SMPQ44, (no S/N provided, KMB)
• Simon PQ, (no S/N provided, KMB)
The MEG30 S/N 36 analyzer is still furnished with the
original firmware version. The test results allowed the
manufacturer to identify an error in the firmware and the
MEG30 No. 34 analyzer contains a new firmware version
with a corrected algorithm for the flicker evaluation.
Furthermore, the progress of voltage changes typical for
some appliances is described. The test is not defined by
the IEC standard and is called an "operating test".
A. Granulator
A granulator in operation exhibits fluctuating load,
mostly without idle run. The operation of the granulator
and the consequent power consumption result in an everchanging RMS network voltage without delays where
voltage changes would not occur. Therefore, it is possible
to except the flicker progress without major changes.
Fig. 2. Picture of the tested PQ analyzers
IV. TEST NUMBER 2
The test was performed in September 2010 and
included 11 different PQ analyzers, see Fig. 3.
Fig. 4. Detail of Urms changes in the granulator connection point, voltage
changes Up-p 11V, change interval 2.4s
Fig. 3. Picture of the tested PQ analyzers
B. Sawmill
A sawmill is operated in the mode in which the operator
handles the processed materials, i.e. it runs without any
load for some time, followed by an interval for cutting, i.e.
causing irregular network load, see Fig. 5. Irregular
network load causes a changing level of RMS voltage
value and, depending on the ratio of idle run and operation
under load a changing flicker value can be expected.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
24
E. Results of Performed Tests
The results are summarized in the single Table I. The
legend is as follows: SM (Saw Mill), GR (Granulator), HP
(Heat Pump), SV (Signalling Voltages), SV+h (Signalling
Voltages and harmonics).The table clearly shows that the
evaluated results vary. The Meg30 S/N 36 instrument was
operated with the original firmware with an erroneously
evaluating flicker, while the Meg30 S/N 34 instrument is
already modified. Another remarkable result is that of the
Fluke 1744 instrument shows zero for low flicker values.
TABLE I.
COMPARISON OF PST FLICKER BY TEST NO 2
Flicker Pst
Fig. 5. Detail of Urms changes in the sawmill connection point - voltage
change interval T = 220 ms, voltage changes Up-p 9V
C. Heat Pump
The heat pump causes decreases, typical for rapid
voltage changes. If the heat pump is switched on, there is
a high current peak caused by the switching on of a cage
inductor motor. When the motor starts, the current peak
fades and the motor runs at the nominal current. After
some time, the heat pump is switched off, current drops to
zero and the voltage gets back to the original value. The
described process causes irregular network load, with a
specifically long time when the pump is on and off
(during the simulation, the intervals were 15 and 15
minutes). The irregular network load causes infrequent
changes of the RMS voltage value and a highly varying
flicker value can be expected.
Fig. 6. Detail of Urms changes at the time of actuation of an actual heat
pump, change duration 100ms, change depth 16V
D. Other Signals
Constant sinusoidal signal with the presence of
cyclically repeated telegram Signalling Voltages at the
frequency of 216.66 Hz and 9 % amplitude.
Constant sinusoidal signal with the presence of
cyclically repeated telegrams Signalling Voltages at the
frequency of 216.66 Hz and 2 % amplitude, harmonic
voltage: 5 h. 6 % Un, 15 h 0.5 % Un.
SM
GR
HP
SV
SV+h
Topas 2000
4,05
2,31
0,88
0,61
0,15
Fluke 1744
4,18
2,23
0,90
0,63
0,00
MEG30 #34
4,50
2,05
0,93
0,66
0,13
MEG30 #36
2,71
2,33
0,21
0,56
0,12
ENA330
3,93
2,24
0,85
0,58
0,13
ENA450
3,92
2,24
0,86
0,59
0,13
Metrel
4,25
2,34
0,90
0,78
0,17
Qwave
3,90
2,26
0,86
0,59
0,13
ION 7650
4,00
2,35
0,89
0,73
0,15
ION 7600
4,05
2,31
0,88
0,61
0,15
SMPQ 44
4,01
2,29
0,89
1,11
0,26
Simon PQ
4,01
2,29
0,88
1,09
0,25
F. Tests according to the Standard IEC 61000-4-15
The testing of functionality of the flickermeter based
on the standard [1] is performed for instantaneous flicker
Pinst and short-term flicker Pst. None of the devices is in
the test records Pinst. For time reasons, the tests were not
performed for all of the prescribed test conditions. The
sequences of the tests were selected to utilize the time for
test performance as much as possible. The following tests
were performed.
Test 6.1 of standardized response of the flickermeter
for rectangular voltage fluctuations. It is performed on the
basis of standardized response tables and the test
measures output instantaneous flicker Pinst. Pinst, max
shall be equal to 1.00, with a ± 8 % tolerance. Since the
test signal prescribed in the standard was generated for 70
minutes, it is possible to use the equivalent value Pst for
verifying Pinst in this case. Reading from the FLUKE
61000A calibrator was used to find the equivalent value
Pst, where the calibrator shows the value Pst on its
display for the selected type, depth and frequency of
modulation. Results of above described test are
summarized in the Table II.
Test 6.2 of the classifier in block 5 of the flickermeter.
Rectangular modulation is used for the test as stipulated
in the prescribed table. The Pst output is tested; the output
shall be equal to 1.00, with a ± 5 % tolerance. Results of
this test are summarized in the Table III.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
TABLE II.
COMPARISON OF FLICKER PST BY TEST 6.1
8,8Hz,
0,196%,
Pst=0.74
Test 6.1, Pinst=1
18Hz,
25Hz,
0,446%,
0,746%,
Pst=0.56 Pst=0.49
33,33Hz,
1,671%,
Pst=0.70
Topas 2000
0,77
0,71
0,69
0,72
Fluke 1744
0,73
0,67
0,64
0,64
MEG30 #34
0,79
0,73
0,71
0,62
MEG30 #36
0,71
0,56
0,39
0,20
ENA330
0,72
0,66
0,64
0,65
ENA450
0,72
0,66
0,64
0,65
Metrel
0,75
0,66
0,57
0,64
Qwave
0,72
0,72
0,67
0,69
ION 7650
0,72
0,65
0,58
0,72
ION 7600
0,77
0,71
0,69
0,72
SMPQ 44
0,76
0,70
0,67
0,69
Simon PQ
0,75
0,71
0,65
0,68
TABLE III.
COMPARISON OF FLICKER PST BY TEST 6.2
1 CPM
2,715%
Test 6.2, Pst=1
2 CPM
7 CPM
2,191% 1,450%
39 CPM
0,894%
25
G. Evaluation
The results of the tests carried out on the basis of a
definition in the standard specified in this chapter are not
aimed to substitute a calibration protocol from an
accredited laboratory. The chapter only describes what
was tested and presents the results. However, it is
necessary to draw the readers' attention to some of the
results evaluated by the instrument MEG30 with the
original firmware, which had been used for a long time
until June 2010, when the manufacturer corrected the
firmware.
V. CONCLUSION
The information above shows that it is necessary to test
the PQ monitoring devices and evaluate the results. It is
obvious from the results of the tests mentioned above,
prescribed in the standard or reflecting the conditions
commonly occurring in practice that in spite of many
uncertainties in the definition of a flickermeter [1], the
different types of devices from different manufacturers
yielded quite identical results. In the whole test authors
focused on the short-time flicker Pst. The reason is, that
instant the flicker P(t) data are not provided by any
instrument involved in the test. The relation between the
long-time flicker Plt and the short-time flicker Pst is
clearly defined in [1] by the exact formula, anyway there
can be differences among particular manufacturer. The
field of flickermeter testing or more broadly, the field of
PQ analyzer testing, is important for practice. The
Department of Measurement and Control, VSB-Technical
University of Ostrava will continue testing other
parameters of PQ analyzers.
Topas 2000
0,98
0,97
0,98
1,00
Fluke 1744
0,99
0,98
0,96
0,97
MEG30 #34
0,98
0,96
0,97
0,97
MEG30 #36
0,53
0,62
0,78
1,00
ENA330
0,95
0,94
0,95
0,97
ENA450
0,94
0,94
0,95
0,97
REFERENCES
[1] IEC 61000-4-15 Ed.2.0. Electromagnetic compatibility (EMC): Part
4-15: Testing and measurement techniques – Flickermeter –
Functional
design
specifications.
Geneva:
International
Electrotechnical Commission, 2010. 88 p. ISBN 978-2-88912-0765
[2] EN 50160 Ed.3 Voltage characteristics of electricity supplied by
public distribution systems. Brussels: Eurepean Committee for
Electrotechnical Standardization, 2010. 20 p.
[3] IEC 61000-4-30 Ed.2.0 Electromagnetic compatibility (EMC): Part
4-30: Testing and measurement techniques – Power quality
measurement methods. Geneva: International Electrotechnical
Commission, 2008. 134 p. ISBN 2-8318-1002-0.
Metrel
0,97
0,98
1,00
1,01
Qwave
0,96
0,95
0,96
0,98
ION 7650
0,98
0,98
0,98
0,99
ION 7600
0,98
0,97
0,98
1,00
SMPQ 44
0,97
0,97
0,97
0,98
Simon PQ
0,97
0,96
0,97
0,98
ACKNOWLEDGMENT
This research has been carried out under the financial
support of the project, VSB-TU Ostrava FEECS, under
Project SGS SP2011/161 and under support of project
CZ.3.22/2.3.00/09.01525 of Ministry for Regional
Development.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
26
Utilising of EMTP ATP for Modelling of
Decentralized Power Sources Connection
Dušan Medveď
Technical University of Košice, Department of Electric Power Engineering, Mäsiarska 74, Košice, Slovak Republic,
e-mail: [email protected]
Abstract—The ensuring of the quality of electric power is
one of the actual tasks that could be solved in the field of
electric power supplying. For that reason it is necessary to
monitor the particular electric quantities, such as voltage
and current fluctuations, overvoltages, voltage drops,
voltage harmonic distortion, asymmetry phase voltages,
frequency variation and fluctuation and others that cause
e.g. power losses, devices failure, and so on.
This article deals with some typical failures in a power
system that unfavourably cause considerable electric losses
or undesirable damages. The prevention of potential failure
leads to decreasing of power losses. It is possible to solve the
various power networks using computer modelling. One of
the suitable computer software is the Electromagnetic
Transient Program (EMTP ATP). This software is mostly
intended for solving and modelling transient phenomena
and this feature was also utilized during the modelling. In
this software there were modelled various connections of
decentralized power sources and determined the maximum
power and node voltage that can be connected in particular
part of the power grid. In this article there are also
indicated another various failures that caused significant
power losses (short-circuits, overvoltages caused by
connection and disconnection of a part of the grid,
atmospheric overvoltages).
Keywords—power system, computer modelling, EMTP ATP,
decentralized sources, SimPowerSystem, electric losses.
I. INTRODUCTION
A. Reconnection problems of decentralized power energy
sources to distribution grid
Reconnection problems of wind power plant:
• convention sources must be „on“ and prepared, in
the case of wind power plants outage;
• dependence on actual meteorological situation;
• relatively small power of wind power plants;
• they are not possible to operate when the wind
velocity is above 30 m⋅s–1 or below 3 m⋅s–1.
Reconnection problems of solar power plants:
• convention sources must be „on“ and prepared, in
the case of solar power plants outage;
• problems with the season variations of sunlight (in
December is 7-times weaker than in July);
• difference between night and day is very
significant.
Reconnection problems of water power plants:
• they generate electric power only when the water
flows is in allowable range.
B. EMTP ATP (Electromagnetic Transient Program)
•
Generally, there is possible to model the power
system network of 250 nodes, 300 linear branches,
40 switchers, 50 sources, ...
•
Circuits can be assembled from various electric
component of the power system:
Components with the lumped parameters R, L, C;
Components with the mutual coupling
(transformers, overhead lines, ...);
Multiphase transmission lines with lumped or
distributed parameters, that can be frequencydependent;
Nonlinear components R, L, C;
Switchers with variable switching conditions,
that are determined for simulation of protection
relays, spark gaps, diodes, thyristors and other
changes of the net connection;
Voltage and current sources of various
frequencies. Besides of standard mathematical
functions, there is possible to define also sources
as a function of time;
Model of three-phase synchronous machine with
rotor, exciting winding, damping winding;
Models of universal motor for simulation of
three-phase
induction
motor,
one-phase
alternating motor and direct current motor;
Components of controlling system and sense
points.
II. CHOOSING OF SUITABLE MODEL OF POWER SYSTEM FOR
CONNECTION OF DECENTRALIZED SOURCES
There was chosen a part of the radial network on the
eastern side of Slovakia for choosing a suitable model of
the power system for determination of electric losses of
the connected decentralized sources. The power system is
on 22 kV voltage level and it is supplied from 110 kV
lines through the transformer. The chosen network with
particular parameters of components was modelled in
program EMTP ATP.
Parameters of power system
The input parameters of particular devices were entered
according to obtained data (length, diameter, material of
overhead lines, transformers parameters, and so on).
Individual loads were set in respect of relevant actual
connected appliances.
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
27
Fig. 2. Power of the first source, when it operates alone (0 ÷ 0,5 s), with
the second one (0,5 ÷1 s) and consequently with the third one (1 ÷ 2 s)
Fig. 1. Schema of electric power network for simulations in EMTP-ATP
A. Reconnection of power sources
• There were reconnected various sources in
different locations of the power system
• The first source was connected from the beginning
of the simulation, the second one was connected at
0,5 s and the third one at 1 s
• All parameters of components in the power system
were inserted as the card data of given components
• Consequently, there were changed voltages and
powers of the connected sources
• The measured data (voltages, currents, ...) were
recorded and evaluated in various nodes of the
network
• The maximum possible connected power was
calculated and tested with permitted difference of
voltages (quality of voltages must agree with
conditions of ± 2 % from the nominal voltage in
grid) (see conditions in ref. [5])
• The results were evaluated for the phase L1 (A),
because the loads were almost symmetrical
The maximum immediate power measured in the
closest distances from the sources:
• Power of the source 1 (single) = 2,2643 MW
• Power of the sources 1 and 2 = 3,5280 MW =
2,2541 MW + 1,2739 MW
• Power of the sources 1 and 2 and 3 = 3,5653 MW
= 2,1458 MW + 1,2621 MW + 0,1574 MW
III. TRANSIENT SIMULATIONS OF CHOSEN ELECTRIC
SOURCES IN THE POWER SYSTEM MODEL IN EMTP-ATP
V
BCT
11
Y
V
BCT
AGA
5
U1
TR5
BCT
V
Y
BCT
12
V
V
LCC
BCT
I
M1
AAO
LCC
V
0
AA
AB
LCC
LCC
BCT
4
V
13
BCT
Y
15
Y
Y
TR2
00
TR12
AEA
2
Y
TR0
22 kV
V
BCT
Y
V
110kV
TR11
LCC
Y
TR13
TR4
AC
AD
AE
LCC
LCC
LCC
TR15
V
V
01
AF
AG
AH
AI
AJ
AK
LCC
LCC
LCC
LCC
LCC
LCC
0k
M3
I
V
BCT
Y
1
I
TR1
LCC
V
BCT
ADA
V
BCT
14
V
B CT
Y
3
17
16
Y
TR14
Y
TR3
V
BCT
Y
TRnz
M2
1.002 k m
AFA
AFB
AFC
AFD
LCC
LCC
LCC
LCC
TR16
V
BCT
10
Y
TR10
BCT
V
9
Y
V
BCT
TR9
8
Y
V
BCT
TR8
7
Y
V
BCT
TR7
6
Y
TR6
Fig. 3. Complemented schema (in EMTP-ATP version 5) for the
simulation of transient phenomena in M1, M2, M3 – places of failure
event; measuring places
90
[kV]
TABLE I.
SIMULATION OF RECONNECTION OF TWO SOURCES WITH THE VARIOUS
PARAMETERS AND THE MAXIMUM VOLTAGE OF 391 V (2ND SOURCE)
AND 333 V (3RD SOURCE)
VN
1 [V]
1+2 [V]
1+2+3[V]
NN
1 [V]
1+2 [V]
1+2+3[V]
node
X0003
17933
18090
18092
X0016
319,48
322,9
322,96
node
X0040
17815
18105
18115
X0058
317,91
323,98
324,23
node
X0125
17723
18089
18111
X0132
317,95
324,55
324,92
node
X0069
17709
18094
18110
X0164
317,75
324,35
332,04
Node
X0071
17703
18088
18104
X0188
318,51
325,13
325,58
node
X0067
17744
18129
18145
X0076
322,7
332,85
333,12
node
X0162
17710
18075
18102
X0096
318,81
325,78
326,05
60
30
Node
0
-30
X0116
318,49
325,45
325,71
Maximum voltages, that are possible to reach with the
respecting of ± 2 % voltage variation in every node:
• Source 1: Um1 = 89815 V
• Source 2: Um2 = 391 V
• Source 3: Um3 = 333 V
-60
-90
0,040
0,062
(f ile 2f skrat_M1.pl4; x-v ar t) v :X0080A
0,084
v :X0080B
0,106
0,128
[s]
0,150
v :X0080C
Fig. 4. Voltage characteristics before two-phase short-circuit
(overvoltage after short circuit elimination) in location M1, during the
short circuit and after short circuit, measured in location M1 (see Fig. 3)
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
TABLE II.
TWO-PHASE SHORT CIRCUIT – MEASURED RESULTS
Location M1
Steady state
Overvoltage after shortcircuit interruption
Peak current
during shortcircuit
Mutual distances
[km]
U [V]
U [V]
ip [A]
M1
0
17309
86268
3122,3
M2
4,110
16969
71133
197,08
M3
8,121
16701
69032
36,822
Location M1
Steady state
Overvoltage after shortcircuit interruption
Peak current
during shortcircuit
Measured
place
Measured
place
main line AA-AL 8121 m long. The sections have their
circuit breakers that are marked S17-S20. These circuit
breakers can switch off the whole section in the case of
maintenance or revision of this section.
Fig. 6. Model schema of the chosen part of the power system in the
Matlab/SimPowerSystems
Mutual distances
[km]
U [V]
U [V]
ip [A]
M1
– 4,110
17309
167630
2536,6
M2
0
16969
178690
2697,5
M3
4,011
16701
163660
36,975
Location M1
Steady state
Overvoltage after shortcircuit interruption
Peak current
during shortcircuit
Mutual distances
[km]
U [V]
U [V]
ip [A]
M1
7,466
17309
69409
1881,2
M2
4,011
16969
67746
1854
M3
0
16701
98249
1938
Measured
place
28
V. THREE-PHASE SHORT-CIRCUIT ON OVERHEAD LINE
In this type of fault the metal connecting of all three
phases occurred. The part of the network is in Fig. 7,
where 3-phase short-circuit occurred. In this case, the load
no. 17 was disconnected. The three-phase short-circuit
occurred before the switcher no. 8 at the time of t1 = 0,04 s
and the failure was removed at the time of t2 = 0,1 s.
20
[kV]
15
Fig. 7. Part of the power network with the highlighting of the fault place
(three-phase short-circuit)
10
5
0
-5
-10
-15
-20
0,09
(file M1.pl4; x-var t) v:M2A
0,10
v:M2B
0,11
0,12
0,13
0,14
[s]
0,15
v:M2C
Fig. 5. Voltage characteristics during the phase interruption in location
M1, measured in location M2 (see Fig. 3)
IV. COMPARISON OF SIMULATION ALGORITHM TO MATLAB
/ SIMPOWERSYSTEM
According to the schema in EMTP ATP in Fig. 3 there
was created in Matlab/SimPowerSystem the model of the
network (Fig 6).
Fig. 6 shows the model of the power system, it is the
inefficiently grounded network. In the schema, there are
situated particular branches, respectively loads. The
presented chosen part of the schema (section) is in the
As one can see from the resulting characteristics in
Fig. 8a, there occurred the overvoltage in all three phases.
After short-circuit creation, in the phase L3, one can read
at the time tk1 = 0,0401 s the magnitude of the overvoltage
at the level U3k,max = 32996 V. After removing the failure
(at the time of 0,1 s) then occurs to consequential
overvoltage rising with the highest magnitude in phase L2
at the level U2K,max = 54327 V (at the time of
tk2 = 0,1017 s), in the negative half period. To eliminate
the overvoltages and shock currents (impulse character) it
is recommended, as in this case, to use surge overvoltages.
During the short-circuit, there can be observed the rapid
current rising from the value I = 89,02 A to value of peak
short-circuit current in the phase L3, ip3 = 4977 A (at the
time of t1 = 0,049 s) (Fig. 8b). These impulse values of
current can be very dangerous for the equipment in the
network. For dimensioning of equipment and setting of
overcurrent protections it is necessary to ensure the correct
setting of the particular protection. After incorrect setting
Transactions on Electrical Engineering, Vol. 2 (2013), No. 1
of the protection there can occur dynamic and thermal
effects of short-circuit, that can damage the equipment.
a)
29
VI. CONCLUSION
• By use of the EMTP-ATP and SimPowerSystem it
is possible relatively quickly consider the
connectivity of new power source (voltage change,
short-circuit ratio, overvoltage, ...) and determine
the power losses on particular devices;
• there were confirmed the theoretical assumptions
that the most important points with the highest
quantity change are the closest branches to
investigated node, i.e.:
- the highest increase of the voltage magnitude is
in the node, where the new source is connected,
- the highest increase of the short-circuit current is
also in the node of the source connection,
• if there were connected 3 sources, the voltage in
the power system was increased to permitted
maximum voltage, and then it is possible to
connect another load to the grid without significant
complications,
• a similar procedure can be used for various small
power systems.
ACKNOWLEDGMENT
This article is the result of the Project implementation:
Research centre for efficient integration of the renewable energy
sources, ITMS: 26220220064 supported by the Research &
Development Operational Programme funded by the ERDF.
b)
Fig. 8. Voltage (a) and current (b) characteristics in 3-phase short-circuit
A similar procedure can be applied to other types of
faults as it is shown in Fig. 9.
a)
b)
Fig. 9. Characteristics of the current in 2-phase metal short-circuit (a)
and 2-phase earth-fault (b)
We support research activities in Slovakia / Project is cofinanced from EU funds.
REFERENCES
[1] Medveď, D.: Electric losses modeling of decentralized power
sources connection using EMTP ATP. In: ELEN 2010, ČVUT
Praha, 2010, p. 1-9. ISBN 978-80-254-8089-2.
[2] Medveď, D., Nemergut, L.: Modelovanie prechodných javov
využitím nástroja MatlabSimPowerSystem. In: Electrical
Engineering and Informatics 3: proceeding of the Faculty of
Electrical Engineering and Informatics of the Technical University
of Košice. Košice: FEI TU, 2012 s. 745-748. ISBN 978-80-5530890-6.
[3] Szathmáry, P., Kanálik, M., Rusnák, J., Hvizdoš, M.: Nepriaznivé
vplyvy nesymetrie napätia na elektrické zariadenia a možnosti ich
eliminácie. In: AT&P journal. č. 2 (2010), s. 51-53. ISSN 1336233X.
[4] Kolcun, M.: Electric power system operation control. In:
Efektywność w sektorze dystrybucji energii elektrycznej: aspekty
techniczne. - Bydgoszcz: Wydawnictwo Tekst, 2009. pp. 113-134.
ISBN 978-83-7208-022-6.
[5] Východoslovenská distribučná a.s.: Pripojenie energetického
zariadenia na výrobu elektriny do distribučnej sústavy [online]. [cit
2012-12-12]. Dostupné na internete: < http://www.vsds.sk/wps/
portal/vsd/domov/vyrobcovia/pripojenie-zdroja >.
[6] Medveď, D., Hvizdoš, M.: Modelovanie v prostredí EMTP – ATP.
1. vyd. Košice: TU 2011. 74 s. ISBN 978-80-553-0776-3.
[7] Medveď, D.: Modelovanie prechodných dejov pri pripojovaní
rozptýlených zdrojov energie v prostredí EMTP ATP. In:
Elektroenergetika, Vol. 3, No. 7, 2010, p. 15-18. ISSN 1337-6756.
[8] Hvizdoš, M.: Modelovanie prevádzkových a poruchových stavov
v elektrizačnej sústave. In: E2006/10 – Simulace a dynamické
modelování systémů a procesů v elektrizační soustavě, EGÚ Praha,
2006, p. 1-16.
[9] Medveď, D.: Modelovanie v elektroenergetike – Zbierka príkladov
1. [1. vyd.] - Košice : TU, 2012. 204 s. ISBN 978-80-553-1188-3.
Transactions on ElectricalEngineering, Vol. 2 (2013), No. 1
30
Use of Specialized Devices to Power Flow
Control in Power Systems
Beňa Ľubomír1), Jakubčák Roman2), Kolcun Michal3)
1)
Technical university in Košice/Department of electrical power engineering, Košice, Slovakia, [email protected]
2)
Technical university in Košice/Department of electrical power engineering, Košice, Slovakia,
[email protected]
3)
Technical university in Košice/Department of electrical power engineering, Košice, Slovakia, [email protected]
Abstract—This article discusses specialized devices for
the power flow control in power systems. Today it is an
actual topic because of permanent increasing power flow in
the transmission lines. This trend can lead to overloading of
transmission lines and can endanger the security of electric
energy supply.
Keywords— power flow control, FACTS, phase shift
transformer.
I. INTRODUCTION
Most of the world electric power systems are widely
interconnected. These connections include also
international connection. This is done for economical
reason, to reduce the cost of electricity and to improve
reliability of the power supply. Today we are witnessing a
continuous increase of electricity demand. This trend
along with market liberalization causes problems in
management of power systems. These problems can lead
to overloading of transmission lines and in the worst
scenario lead to disconnection of these lines. The tools for
the power flow control in the Slovak transmission system
are insufficient from the point of view of increasing
demand. For this reason we need specialized devices that
are able to control the power flow in transmission lines.
Between these devices belong FACTS (Flexible
Alternating Current Transmission System) and PST
(Phase Shift Transformer). These devices provide the
ability to manage the power flow in power systems and
are able to prevent creation of power system transmission
lines overloading [4], [6].
possible in the case when the off lines are
available. If a line is overloaded it can be turn off.
As the result of this action is redistribution of the
power flow in the power system but this action can
lead to overloading of another transmission line
and in the worst scenario to outage of the
transmission line.
The above mentioned way offers insufficient options to
the power flow control and for this reason we are looking
for specialized devices for the power flow control in a
power system.
III. SECIALIZED DEVICES FOR POWER FLOW CONTROL
The need for greater control of the power flow requires
use of specialized equipment, which includes FACTS
devices and specialized transformer PST.
FACTS devices–According the IEEE FACTS devices
are: Alternating Current Transmission Systems
incorporating power electronics-based and other static
controllers to enhance controllability and power transfer
capability. They open new opportunities for controlling
the power flow and enhancing the capability of present as
well as new lines. Controlled parameters are current,
voltage, line impedance and phase angle.
The FACTS devices for control the active power flow
can include, among others:
A. TCSC
The basic Thyristor-Controlled Series Capacitor is
shown in Fig. 1.
II. THE CURRENT MEANS FOR CONTROLLING THE ACTIVE
POWER FLOW
Present tools for the power flow control in the Slovak
transmission system are:
1. Change of the source operation – The distribution
of power plants and support services generally
remove a transmission element overloading.
Usability is practicable during normal operating
situations as well as in emergency critical states of
the power system when a state of emergency is
declared.
2. Consumption control – In this case power is
adjusted in distribution networks supplied from the
transmission system. It depends mainly on the
possibilities of the distribution network.
3. Electrical network reconfiguration–For example,
the restriction during maintenance work and
switching temporarily disabled elements. This is
Fig. 1. Thyristor-Controlled Series Capacitor
It consists of the series compensating capacitor shunted
by a Thyristor-Controlled Reactor (TCR). In a practical
Transactions on ElectricalEngineering, Vol. 2 (2013), No. 1
TCSC implementation, several such basic compensators
may be connected in series to obtain the desire voltage
rating and operating characteristics. The basic idea behind
the TCSC is to provide a continuously variable capacitor
by means of partially cancelling the effective
compensating capacitance by the TCR. The TCR at the
fundamental system frequency is continuously variable
reactive impedance, controllable by the delay angle α, the
steady-state impedance of the TCSC is a parallel LC
circuit, consisting of a fixed capacitive impedance XC, and
a variable inductive impedance XC(α) [1].
The active power transported over a transmission line is
given by the following equation:
(1)
31
The behaviour of an SSSC can be similar to a
controllable series capacitor and controllable series
reactor. The basic difference is that the voltage injected by
SSSC is not related to the line current and can be
independently controlled. The SSSC is effective for both
low and high loading [3].
The application of SSSC:
- Power flow control – The SSSC can be used both
for increasing and decreasing the power flow
through the transmission line.
- Voltage and angle stability enhancement –The
SSSC gives a better possibility for damping
electromechanical oscillations.
The SSSC can be represented as series connected
voltage source.
P is the transmitted active power, U1 and U2 are the
voltages at the beginning and end of line, Xline is the line
reactance, the angles δ1 and δ2 are the voltage angles at the
line beginning and end . After addiction TCSC:
(2)
Fig. 3. Representation of a series connected voltage source
∆U, ∆X and ∆δ are changes of the voltage, line
reactance and phase angle due to insertion of TCSC into
the power system. From above mentioned equations it is
clearly seen that we can control the power flow through
the line by a change of the line impedance..
B. SSSC
The SSSC (Static Synchronous Series Compensator) is
a series connected synchronous voltage source that can
vary the effective impedance of a transmission line by
injecting a voltage containing an appropriate phase angle
in relation to the line current [2].
In the principle, an SSSC is capable to interchange
active and reactive energy in the power system. The
injected voltage could be controlled in magnitude and
phase if sufficient energy source is provided. For the
reactive power compensation function, only the magnitude
of the voltage is controllable since the vector of the
inserted voltage is perpendicular to the line current. The
SSSC can be smoothly controlled at any leading or
lagging value within the operating range of voltage source
converter (VSC) [3].
Fig. 2. Basic configuration of an SSSC
Phase shift transformer (PST)– Phase shift transformer
consists of two units that create phase shift. PST is a
device for controlling the power flow through specific
lines in a complex power transmission network. The basic
function of PST is to change the effective phase
displacement between the input voltage and the output
voltage of a transmission line, thus controlling the amount
of active power that can flow in the line [3].
Fig. 4. Phase shift transformer
The PST consists of:
- exciting transformer – it provides input voltage to
the phase shifter.
- boosting transformer – it injects a controlled
voltage in series in the system.
PST’s have many different forms. They can be
classified into four classes:
1. Direct PST’s – based on one three phase core. The
phase shift is obtained by connecting the windings
in an appropriate manner.
2. Indirect PST’s – based on a construction with two
separate transformers, one variable tap excited to
regulate the amplitude of the quadrature voltage
Transactions on ElectricalEngineering, Vol. 2 (2013), No. 1
32
and one series transformer to inject the quadrature
voltage in the right phase.
3. Asymmetrical PST’s – create an output voltage
with an altered phase angle and amplitude
compared to the input voltage.
4. Symmetrical PST’s – create an output voltage with
an altered phase angle compared to the input
voltage, but with the same amplitude [4].
Conventional PST includes mechanical tap changer. In
TCPST (Thyristor Controlled Phase Shift Transformer)
classical mechanical switches in conventional PST were
replaced by thyristors. They are able to emulate
mechanical switches. Advantage is that they offer
continuously phase angle control .
Fig. 6, Influence of PST on active power flows (PST is installed in line
V449)
IV. USING PST FOR ACTIVE POWER FLOW CONTROL
According to data from international exchanges of
electricity, the most loaded interstate profile transmission
system of the Slovak Republic is the profile of SK-HU.
According to SED Žilina, power flow on the profile is
between approximately 400 – 1800 MW.
The main affects on the SK-HU profile are :
1. Deficit of electricity production in southern part of
Europe.
2. Export opportunities in Slovakia and neighbouring
countries especially East Germany, Poland and the
Czech Republic.
3. The actual configuration of the interconnected
system. Export from Slovakia passes the profile
SK-HU. In the case of Poland, the power flow
passes through Poland-Czech-Slovakia-Hungarian
profile. Energy export from the Czech Republic
passes through Czech-Slovakia-Hungarian profile
and also Czech-Austrian profile.
For these reasons the SK-HU profile is potentially the
most suitable for PST deployment . Further is mentioned a
depth analysis of PST deployment on this profile. The
impact of PST on the active and reactive power flow
control is shown in the following figures.
Fig. 7. Influence of PST on reactive power flows (PST is installed in
line V448)
Fig. 8. Influence of PST on reactive power flows (PST is installed in
line V449)
Fig. 5. Influence of PST on active power flows (PST is installed in line
V448)
As we can see the profiles SK-HU and SK-CZ are
particularly affected if PST is installed in the line V448.
The profiles SK-HU and SK-UA are particularly affected
if PST is installed in the line V449. The SK-HU profile
control has no effect on the SK-PL profile.
V. CONCLUSION
The article dealt with the analysis of the active power
flow control on the common international lines of SK-HU
using PST. This specialized device for the power flow
control can be used for operational management of the
loading lines, in fault conditions, but also for solving
business cases.
The control has no affect only on the active power
flow, but also on the active power losses in the power
system. In some power systems due to control the active
power losses can increase in other cases they can decrease.
These losses have to be covered by increasing production.
Transactions on ElectricalEngineering, Vol. 2 (2013), No. 1
ACKNOWLEDGMENT
This work was supported by Scientific Grant Agency of
the Ministry of Education of the Slovak Republic and the
Slovak Academy of Sciences under the contract No.
1/0166/10 and by the Slovak Research and Development
Agency under the contract No. APVV-0385-07 and No.
SK-BG-0010-08.
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TRANSACTIONS ON ELECTRICAL ENGINEERING VOL. 2, NO. 1 HAS BEEN PUBLISHED ON 29TH OF MARCH 2013