Download Power Quality issues and its improvement in wind

MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
116
Power Quality Issues and its Improvement
in Wind Energy Generation Interface to
Grid System
Sharad W. Mohod
Dept. of Electronic Engineering,
Prof. Ram Meghe Institute of Technology & Research
Badnera-Amravati, India
email:[email protected]
Mohan V. Aware
Dept.of Electrical Engineering,
Visvesvaraya National Institute of Technology,
Nagpur, India
email:[email protected]
Abstract— Injection of the wind power into an electric grid
affects the power quality. The performance of the wind
turbine and their power quality are determined on the basis
of measurements, and according to the guideline specified
in International Electro-technical Commission , IEC
61400-21.The influence of the wind turbine in the grid
system concerning the power quality measurements arethe active power, reactive power, variation of voltage,
flicker, harmonics, and electrical behavior of switching
operation and these are measured according to national /
international guidelines. The paper study demonstrates the
power quality problem due to installation of wind turbine
with the grid. The pulse width modulation (PWM) inverter
scheme for the grid connected wind energy generation for
power quality improvement is simulated using MATLAB/
SIMULINK in power system block set. Due to
improvement in technologies, the wind turbine is expected
to support the grid and therefore the wind turbine has to
control the reactive power over a wide range, and also to
deliver the reactive power, in case of voltage drop and
can remain connected during short-term voltage drop.
Hence to ensure its fulfillments, the power quality measure
is specified in IEC 61400-21.The scheme for improvement
in power quality has been presented in the paper.
be of large capacity, up to 2 MW, feeding into distribution
network with high source impedance, particularly with
customers connected in close proximity [1]. In case of fixedspeed wind turbine operation, all the fluctuation in the wind
speed are transmitted as fluctuations in the mechanical torque,
electrical power on the grid and leads to large voltage
fluctuations. Thus the network needs to manage, the excessive
voltage transients which are to be avoided. Today in the
variable-speed wind turbine designs, the uses of power
electronic converters are mostly used. Thus the issue of
harmonic distortion of the network voltage should be
considered. During the normal operation, wind turbine
produces a continuous variable output power. These power
variations are mainly caused by the effect of turbulence, wind
shear, and tower-shadow and of control system in the power
system. These effect leads to periodic power variation at the
frequency with which the blade passes through the tower, which
are superimposed on slow variation caused by changes in the
wind speed. There may be a high frequency power variation
caused by the dynamics of the turbine. Today, the use of
variable speed wind turbine operation has got the advantage,
the fast power variations are not transmitted and can be made
smooth. Thus the power quality issues can be viewed with
respect to the wind generation and the transmission and
distribution network can cause the variation in voltage to which
the wind turbine is connected, such as voltage sag, swells, etc.
However the wind generator introduces disturbances into the
distribution network. The connection of power converter into
the power system may inject the harmonic current into the
grid and cause a reduction in power quality. Today the PWM
inverter control technology has been developed and it can
technically manage to control the power level associates with
the commercial wind turbines. The wind turbine and their
quality are assessed according to the national and international
guidelines, and so it is important to evaluate the grid connection
with wind generating system [2]-[4].
KEYWORDS—Component power quality, Wind Energy Generating
System (WEGS)
I. INTRODUCTION
The power quality is used to describe how closely the
electrical power delivered to customers corresponding to the
appropriate standard and so operate their end-use equipment
correctly. It is an essential customer-focused measure and is
greatly affected by the operation of a distribution and
transmission network. The growing importance of power
quality is due to the widespread use of power electronic
equipments such as information technology, power electronic
converter, programmable logic controller and energy-efficient
lighting. These loads are sensitive in nature and simultaneously
it is a major cause and major victim of power quality problems.
Due to this non-linear nature of such load, causes the
disturbance in the waveform. This issue of power quality is of
great importance to the wind turbine. The individual units can
II. MOTIVATION FOR POWER QUALITY CONCERN
(1) The lasted equipments in the power system are with
microprocessor based control and power electronic
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
117
devices, are more sensitive to power quality than used
Causes—Mainly due to the opening and automatic re-closure
in past.
of protection devices.
(2) To increase overall efficiency in the system, use of
Consequences—Tripping of protection devices, stoppage
adjustable speed motor, power factor correction are of sensitive equipments like personal computer, programmable
result in increase of harmonics level in power system. logic control system.
(3) Deregulation of utilities, distributed generations have
increase the power quality problem
(4) Awareness of end user for interruption, switching
transients.
(5) Globalization of industry around the world.
III. POWER QUALITY ISSUES AND ITS CONSEQUENCES
A. VOLTAGE VARIATION
The voltage variation mainly results from the wind velocity
and generator torque. The voltage variation is directly related
to real and reactive power variations. The wind generating
system equipped with an asynchronous generator consumes
the reactive power and can cause additional negative problem
for the grid. Switching the wind turbine generator ON and
OFF also varies the voltages. The voltage variation is
commonly classified as short duration and long duration
voltage variation.
(d) Long duration voltage variation
It is total interruption of electrical supply for a duration
greater than 1-2 seconds.
Causes- Equipment failure in power system, failure of
protection equipments.
B. FLICKER
Voltage flicker describes dynamic variations in the network
voltages caused by wind turbine or by varying loads. Thus the
power fluctuation from wind turbine occurs during continuous
operation. The amplitude of voltage fluctuation depends on
grid strength, network impedance, phase-angle and power
factor of the wind turbines. It is defined as a fluctuation of
voltage in a frequency 10-35 Hz. The IEC 61400-4-15 specifies
a flicker meter that can be used to measure flicker directly.
The flicker coefficient gives a normalized dimensionless
measure of flicker, independent of network situation and
(a) Voltage Sag /Voltage Dips
independent of short circuit apparent power of the grid. It gives
It is the decreased of the nominal voltage level between 10% ratio of short circuit power and generated rated apparent power,
to 90% of the nominal rms voltage, at the power frequency, which is necessary to achieve a long term flicker level. (P ) ,
lt
for the duration of 0.5 cycle to 1min. According to the European as the given equation (1).
Standard EN 50160,the dips with depths of 10% to 15% are
commonly due to the switching loads, where as large dips may
S
(1)
C ( K , Va ) Plt K
caused by fault.
Sn
Causes—Start-up of wind turbines, Fault on the transmission/
Where, C(Yk , V a)-flicker coefficient depends on grid
distribution network, Fault in consumer installation, connection
impedance
angle Yk and the average wind velocity Va.
of heavy loads, start up of large motors.
SK - Short-circuit power of grid at point of common coupling.
Consequences—Malfunction of equipments namely
microprocessor based control system, programmable logic
Sn- Apparent power of wind turbine at rated power.
controller, adjustable speed drives, that may lead to a process
P lt- Long term flicker emission.
stoppage, tripping of contractors, relays trip for voltage
sensitive load and loss of efficiency in electric machine.
The flicker standards are generally used to characterize the
transient voltage variations .The short flicker is evaluated over
(b) Voltage Swells
a 10 min period and long term flicker is evaluated over 2
It is momentary increase of voltage at power frequency, with hours period.
Causes: Fluctuation of active and reactive power of wind
duration of more than one cycle and typically less than few
turbine, i.e. yaw error, wind shear, wind turbulence or
seconds.
Causes—Start/stop of heavy loads, fault on the system, badly fluctuation in control system, switching operations in wind
turbine. In fixed speed wind turbine each time a rotor blade
regulated transformer during off peak hours.
passes through the tower, the power output of the turbine
Consequences-Flickering of light and screen, Damage of
reduces .This effect cause’s periodical power fluctuation with
sensitive equipments.
a frequency of about 1 Hz, where as in variable speed turbine
power fluctuation are smoothed .Flickers are produced by arc
(C) SHORT INTERRUPTIONS
furnace, arc lamps, capacitor switching.
<
Consequences—Degradation of power quality, damage to
It is total interruption of electrical supply for a duration from
sensitive equipments.
few milliseconds to one or two seconds.
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
C. HARMONICS
It results from the operation of power electronic converters.
The harmonic voltage and current should be limited to the
acceptable level at the point of wind turbine connection to the
network. The emission of harmonic current during the
continuous operation of wind turbine with power converter
has to be stated. The relative harmonic current limit is stated
in the Table 1.
118
system. The disadvantages of self excitation are the safety
aspect and balance between real and reactive power.
Causes—If the sensitive equipment is connected to the
generator during the self excitation, the equipment may be a
subject to over load, under voltage and over frequency
operation [5]-[7].
IV. GRID CODE FOR WIND POWER SYSTEM
Table 1.
The arrangement of the technical requirements within grid
code is required in the grid connected power system. The
specific grid code are now being imposed for wind energy
3-4
1.5-3
1-2.5
Admissible harmonic (Ih / I i) 5-6
generating system, taking into account the development of
where Ih is the total harmonic current of hth order caused by wind power plant capabilities. The typical requirement for the
the consumer and Ii is the rms current corresponding to the wind generator in the system as follows:
consumer agreed power.
• Control of reactive power-these requirements contributes
to voltage control on the network.
The IEC-61400-21 does not require measurement of
harmonic emission as till now no damage to equipment is
• Control of active power.
reported. Thyristor based converter are expected to emit
• Protective devices.
harmonic current that may influence the harmonic voltage.
• Power quality.
European Standard EN50160 includes the limit. To ensure the
It is important that these requirements are often specified at
harmonic voltage within limit, each source of harmonic current
can allow only a limited contribution, as per the IEC-61400- the point of common coupling (PCC) between the wind turbine
36 guideline. Modern variable-speed wind turbine uses a and the electricity network (grid).
voltage source converter and normally switching is made by
insulated bipolar transistor at several KHz to synthesize a sine V. GRID CONNECTED W IND GENERATION INTERFACE
wave and eliminates the lower order harmonics (<19th).The
TO UNDERSTSND POWER QUALITY
rapid switching gives a large reduction in lower order harmonic
current compared to the line commutated converter, but the
Wind generation interface system is connected to grid system
output current will have high frequency current and can be with voltages on each side of the impedance shown in Fig. 1.
easily filter-out.
Harmonic number
5
7
11
13
D. WIND TURBINE LOCATION IN THE POWER SYSTEM
The way of connecting the wind generating system into the
power system highly influences the power quality. As a rule,
the impact on power quality at the consumers terminal is
located close to the load is higher, than connected far away
from the load. When the wind generating system is connected
to a medium voltage transmission line , the distance between
the wind generating station and point of common coupling is
small, such system are economical as compared to other
location. Thus the operation and its influence on power system
depends on the structure of the adjoining power network.
Fig. 1. Grid connected wind generator interface to power
system.
E. SELF EXCITATION OF WIND TURBINE GENERATING
SYSTEM
In the power system,the power is transmitted using three
phase power that is as symmetrical as possible. The line-to-
The self excitation of wind turbine generating system
(WTGS) with an asynchronous generator takes place after
disconnection of WTGS with local load. The risk of self
excitation arises especially when WTGS is equipped with
compensating capacitor. The capacitor connected to induction
generator provides reactive power compensation. However the
voltage and frequency are determined by the balancing of the
line voltage is 3 times larger than phase voltage and total
three phase power is constant. The voltage drop over the
impedance can be written as in (2)
V1 V2
3IZ
(2)
Where V1 -rms voltage ,z- impedance of transmission line,
transformer in the feeding grid.
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
At the point of common connection (PCC),wind farm and
local load is also connected. The short circuit power SK ,in wind
connection is shown in (3)
SK
V12
Z
(3)
119
(3) Magnetic Synthesizers
(4) On line UPS
(5) Flywheel Energy storage system
(6) Superconducting Magnetic Energy storage device.
VII. A STUDY OF WIND ENERGY GENERATION
The change in wind power production will cause changes
INTERFACE WITH-STATCOM
in the current through the impedance Z. These current changes
cause the changes in the voltage V2 . In practice, connections
The proposed system for STATCOM—interface with wind
with network with short circuit ratio < 2.5 are avoided, as this energy generation system as shown in Fig. 2 . The system is
give rise to voltage fluctuations, called as weak grid.
simulated in MATLAB/SIMULINK in power system block
The impedance Z = R + JX at the fundamental frequency. set. The system parameter for given system is given Table I.
Generally the impedance in presence of harmonics become as
Table 2: System Parameters
shown in (4)
R jhX L
(4)
Where h is the harmonic order,the inductive reactance
changes linearly with frequency.
Z (h)
The combination of wind power production and load are
represented as P + JQ, where P is the active power and Q is
the reactive power. The reactive power is depend on the phase
shift between voltage and current, such as shown in (5)
I tan 1 Q P
S.N. Parameters
Ratings
1
Grid Voltage
3-phase, 415V, 50 Hz
2
Induction Motor/Generator 3.35 kVA, 415V, 50 Hz, P = 4,
Speed = 1440 rpm,
Rs = 0.01W, Rr = 0.015Ù,
Ls = 0.06H, Lr = 0.06H
3
Line Series Inductance
0.05mH
4
Inverter Parameters
DC Link Voltage = 800V, DC
link Capacitance = 100 ìF.
Switching frequency = 2 kHz,
5
IGBT Rating
Collector Voltage = 1200V,
Forward Current = 50A,
Gate voltage = 20V,
Power dissipation = 310W
6
Load Parameter
Non-linear Load 25kW.
(5)
The reactive power in the wind has an impact on voltage V 2
The
impact is also depend on local load and on the feeding
,
grid impedance.
Today the wind generation equipped with induction
generator, consume reactive power and reduces the voltage
V2 at PCC. It is necessary to manage reactive power for both
customer and utility providers. The reactive power consumes,
transmission and generation resources, incur real power loss
on transmission system. Thus it is possible to control the
reactive generation or consumption so as to maintain power
quality at PCC.
VI. GENERALISED MITIGATION TECHNIQUES
The different technologies are available based on the specific
requirement of the system as are:
(1) Reactive power compensation technologies:
(a) Synchronous condenser
(b) Static VAR compensator (SVR)
Fig. 2. Wind energy generator interface with STATCOM
(c) Static synchronous compensator (STATCOM)
(d) Static synchronous series compensator (SSSC)
The proposed system consists of the following main modules
[7]-[11].
(e) Dynamic voltage restore (DVR)
(f) Unified power flow controller (UPFC)
A. Wind turbine model
(g) Interline power flow controller (IPFC)
(h) Active Filters
(2) Constant Voltage transformer
The implemented model of wind turbine does not include
mechanical dynamics and the detailed electrical model of
induction machine . The uniform wind speed is considered so
as to generate the same power.
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
120
B. Induction generator
The power quality is observed at PCC, so that the source
voltage and the source current are in-phase quantity. The fast
Induction generator is connected to the distribution network,
response of STATCOM is observed in the grid system at t=0.7
it needs an external reactive source connected to its stator
sec. as shown in Fig. 5. From the technological point of view,
winding to provide an output voltage control. It is sufficient
the most effective location to install STATCOM is just directly
that it works at a speed above synchronous speed.
at PCC bus.
C. STATCOM
The STATCOM is a three-phase voltage source inverter
having the capacitance on its DC link. It is connected at the
point of common coupling (PCC) through input inductance /
transformer inductance. The STATCOM injects a compensating current of variable magnitude and frequency component
at the bus, point of common coupling. shown as in Fig. 3.
Fig. 5. Supply Voltage and Current at PCC.
Fig. 3. VSC based STATCOM
Three phase source current and three phase load current on
the load side shows that the grid current is affected due to the
effects of non-linear load, thus purity of waveform may be
lost on both sides in the system. The simulated distorted voltage
and distorted current with non linear load, is shown in Fig. 6.
VIII. SIMULATION PERFORMANCE
A. Dynamic performance of the system
The performance of the induction generator is carried out
by making a change in the torque from motoring mode to
generator mode in the simulation. The wind variation will
change the mechanical torque and the machine speed,
therefore, the torque on machine changes. The induction
machine starts as motor mode and is taken to a generating
mode at t=0.4 sec. The rotor speed, electrical torque, threephase generated current and voltage are shown in Fig. 4.
Fig. 6. (a) Load Voltage (b) Load Current (c) FFT of load
current
The performance of the system is measured by switching
the STATCOM at time t=0.7 sec in the system and how the
STATCOM responds to the step change command for increase
in additional load, when applied at time t=1.0 sec. in the system.
The source current, load current and injected current from
STATCOM and generated current of induction generator is
Fig. 4. (a) Rotor Speed (b) Electric Torque (c) Generated
Current (d) Generated three phase’s voltage of Induction shown, when controller STATCOM is in an OFF condition
and performance is shown in Fig. 7.
Machine
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
121
The DC link voltage regulates the source current in the grid
system, so the DC link voltage is maintained constant across
the capacitor as shown in Fig. 10.The average DC capacitor
voltage of about 760 Volt, is dynamically controlled and does
not change due to the step change command in the load.
Therefore the control topology is validated.
Fig. 7 (a) Source Current (b) Load Current (c) Inverter Injected
Current (d) Wind energy generator (Induction generator)
current.
Fig. 10. DC Link Voltage across the capacitor
IX. CONCLUSION
The controller STATCOM is made ON at time t=0.7 sec.,
The increase awareness of power quality issues of the
without change in any other load condition parameter and consumer and electric utility for standardization and evaluation
performance is shown in Fig. 8.
of performance is an important aspect in the wind generating
system. The wind turbine generating system describes the
electrical performance of the system, as it reflects on grid and
influences the power quality. The International guidelines IEC
61400-21 sets the requirement and grid code for the power
quality measurements. The paper simulates the scheme in
MATLAB/SIMULINK for maintaining the power quality in
such a way that it can cancel out the reactive and harmonic
parts of the load current and maintain the source voltage and
current in-phase at the point of common coupling in the grid
system. Thus it fulfills the power quality norm on the grid as
per the IEC61400-21 standard.
Fig. 8. (a) Source Current (b) Load Current (c) Inverter Injected
Current (d) Wind energy generator (Induction generator)
X. REFERENCES
current.
As the STATCOM in operation, it injected the current into [1]
the power system and start mitigate for reactive demand, as
well as harmonic current. The transient takes almost one
fundamental period, until the source current resembles [2]
sinusoidal waveform. The controller response for additional
increase in load is shown in Fig. 9.
Tande, J.O.Q., Applying power quality characteristic of wind
turbine for assessing impact on voltage quality’, Wind Energy,
pp. 5, 37-2002
[3]
Z. Chen, E. Spooner, ‘Grid Power Quality with Variable Speed
Wind Turbines’, IEEE Trans on Energy Conversion, Vol. 16,
No .2, pp. 148-154, June 2001.
[4]
Z.Chang, E. Spooner,” Grid power quality with variable speed
wind turbine”, IEEE Trans. on Energy Conversion, Vol. 16,
No. 2, June 2003.
[5]
F. Blaabjerg, Z. Chen, S.B. Kjaer, “Power Electronics as
efficient interface in dispersed power generation System.”
IEEE Trans. On Power Electron. Vol. 19, No. 5, pp. 11841194., Sept. 2004.
[6]
Sung-Hun, Seong R. Lee, Hooman Dehbonei, Chemmangot
V. Nayar,” Application of Voltage-and Current–Controlled
Voltage Source Inverters for Distributed Generation System.,”
IEEE Trans. On Energy Conversion, Vol.21, No.3, pp. 782788, Sept. 2006
Fig. 9. (a) Source Current (b) Load Current (c) Inverter
Injected Current (d) Wind energy generator (Induction
generator) current.
Thiringer, T. Petrup P. ‘Power Quality Impact of Sea Located
Hybrid Wind Park’ IEEE Tran. on Energy Conversion, pp.
23-127, 2001.
MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp 116-122
ISSN 2230-7656 © MIT Publications
[7]
[8]
Juan Manel Carrasco, ‘Power Electronic System for Grid
improvement,” IEEE, System Journal, Vol. 2, issue 3, pp. 346Integration of Renewable Energy Source: A Survey’, IEEE
352, Sept. 2010.
Trans on Industrial Electronics, Vol. 53, No. 4, pp. 1002- [10] Chong Han, Alex Q. Huang, Mesut Baran, Subhashish
1014, August 2006.
Bhattacharya, Wayne Litzenberger, Loren Anderson,
“STATCOM Impact Study on the Integration of a Large Wind
S.W. Mohod, M.V. Aware, “Grid Power Quality with Variable
Farm into a Weak Loop Power System.”, IEEE Trans. On
Speed Wind Energy Conversion.”proceeding of IEEE Int.
Energy Conv., Vol. 23, No. 1, March 2008, pp. 226-232.
Conf. on Power Electronic Drives and Energy System
(PEDES), Delhi, 5A-21, Dec. 2006.
[9]
122
S.W. Mohod and M.V. Aware, “A STATCOM control scheme
for grid connected wind energy system for power quality
[11] P. Siemes, H.J. Haubrich, H. Vennegeerts, ‘Concepts for the
improved integration of wind power into German
interconnection System’, IET Review, Vol. 2, No.1,
pp. 26-33, Power generation 2008.