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EnergySpectrum - Issue 1, Vol. 5, 2010
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Specialised Transformer Use for Active Power Flow Control in
the Electric Power System
Michal Kolcun1), Daniel Hlubeň2), Alexander Mészáros 3), Ľubomír Beňa 4), Martin Duch 5)
Technical University of Kosice, Faculty of Electrical Engineering and Informatics, Department of Electric Power
Engineering, Mäsiarska 74, 041 20 Kosice, Slovak republic, http://web.tuke.sk/fei-kee/kee-s.html
1)
tel: +421 55 602 3550, email: [email protected]
2)
tel: +421 55 602 3559, email: [email protected]
3)
tel: +421 55 602 3562, email: [email protected]
4)
tel: +421 55 602 3561, email: [email protected]
5)
Slovenská elektrizačná prenosová sústava, Obchodná 2, Žilina, Slovenská Republika,
www.sepsas.sk, email: [email protected]
ABSTRACT
The article deals with possibilities of the specialized
transformer (PST – phase shifting transformer) to power
flows control in interconnected electrical power system. The
main goal is to find out an influence of installation of PST
on international lines and losses of the transmission system
(TS) of the Slovak Republic.
Keywords: Power flow, HVDC, PST, FACTS, transmission system
1 INTRODUCTION
Transmission system of Slovak republic is a part of Union
for the Co-ordination of Transmission of Electricity (UCTE).
The operation and dispatch control of electric power
systems requires resources for power flow control in
networks but periodic labours by the absence of these
facilities. In electrical power system of west Europe and USA
development of the systems requires using active elements
for the power load flow controlling in network before now.
2 TRENDS IN THE ELECTRIC POWER SYSTEMS COOPERATION
With reference to interconnected transmission networks
operation, the actual tendencies of the electric power systems cooperation have the following character:
ƒ increasing electric energy transits on big distances in
case of the participation of more electric power systems in transit,
ƒ increasing capacities of the electric energy international exchanges in term of exportation from the
sources, as well as in term of powers transit,
ƒ increasing operating exploitation of the transmission
elements, mainly international lines of interconnected
electric power systems. This advanced form of the using of elements causes less reserves in case of the line
surcharge. In background, the international lines were
used mainly to increase the operation electric power
ISSN: 1214-7044
system dependability in given area. Nowadays is overrides big business using, at what networks and their
interconnections were not conceptually constructed,
ƒ increasing differences between physical and business
electric energy flows with the negative consequence to
loss. These differences are still more expanding in last
years,
ƒ it is exists relatively big unstableness and time
changes of the transmission size. These processes are
not possible to well predict,
ƒ the networks operation is often adapted (by nonstandard solutions too) to the business events,
ƒ on these conditions, in any cases during the operation
are beginning to detect networks bottlenecks, which
can to be limiting factor for the desired business
changes. Sequentially, these situations can to cause
the risk of the fail and breaking of the electric energy
supply in areas.
The classic solution of the networks development (networks bottlenecks elimination), relative with the reinforcing
and building of the new lines, is no wear as sufficient and
quickly in continuity with the problems to obtain of the new
corridors and in connection with environmental problems.
Therefore, there are often sought the ways, which would to
enable at least regionally to affect the negative functioning of
the therein before presented processes for the transmission
networks operation [2].
3 THE TOOLS FOR INFLUENCING OF POWER FLOW
IN THE SLOVAK TRANSMISSION SYSTEM
The tools for the power flow control in the Slovak
transmission system are the following:
ƒ changing of the sources operation,
ƒ controlling of the consumption,
ƒ electrical network reconfiguration,
ƒ earmarking of the supply area,
ƒ earmarking of the source operation.
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EnergySpectrum - Issue 1, Vol. 5, 2010
However, these possibilities are insufficient from the point
of view of increasing demands.
For the reasons mentioned in chapter 1, in last date it is
increasing the need for the technical application facilities,
which can effectively affect largeness and direction of the
transmitted flows in networks. In west-European systems and
USA are already in the operation some facilities, which can
by its using greatly to serve for the dispatching and business
needs of the networks operators [1], [2].
Faster development of The market with FACTS and
HVDC equipment for load flow control is expected in the
future, as a result of the deregulation and liberalization in
the power industry. The market in the HVDC field is further
progressing fast. A large number of high power long
distance transmission schemes using either overhead lines
or submarine cables, as well as back-to-back projects have
been put into operation or they are in the stage of
installation [5], [6].
PST
4 SPECIALIZED TOOLS FOR POWER FLOW
CONTROL IN ELECTRIC POWER SYSTEM
HVDC and FACTS
HVDC (High Voltage Direct Current) system was
commercially first used 50 years ago. Since then a growing
number of transmission systems have been constructed
around the world. HVDC system has a number of properties
that makes it different from AC transmission.
The most important are:
ƒ the two stations that are not synchronized or does not
even have the same frequency, can be connected to
together via HVDC system
ƒ power can be transmitted over very long distances
without compensation of the reactive power,
ƒ only two conductors are needed (or even one conductor if the ground or the sea is used as return) for
HVDC compared to three conductors of alternating
current transmission system [3].
Converter stations HVDC are very good controllable and
able to change quickly amount and direction of transmitted
power flow with influence on AC network.
FACTS (Flexible AC Transmission Systems), based on
power electronics have been developed to improve the
performance of long distance AC transmission. Later, the
technology has been extended to the devices which can also
control power flow. world-wide are available Excellent
operating experiences. FACTS technology became mature
and reliable.
Theoretically, the PST can be considered a sinusoidal AC
voltage source with controllable amplitude and phase angle.
Function of PST can be described through the current
distribution over parallel lines, see Fig. . The „natural”
current distribution depends on the impedance of the lines.
This distribution may be rather inefficient, if Zline1 and Zline2
are extremely different. the introduction of an additional
voltage source generates a circulating current , which
equalizes the currents.
Fig. 2 Current distribution over the parallel lines without
and with PST
Because the main part of the line impedance (on high
voltage levels) is inductive reactance, inserting a voltage in
phase with or opposite to the line voltage (changing the
magnitude of the voltage) will have an impact mainly on the
reactive part of currents (reactive power flows). The boost
voltage with a phase angle perpendicular to the line voltage
(creating a phase shift) influences mainly the real part of
currents (real power flows) [4].
VFT
Fig. 1 Simplified model of the line between two nodes of
electrical network
P12 =
U1 ⋅ U 2
⋅ sin (ϑ1 − ϑ2 ) (W;V,V, Ω)
X 12
(1)
The main idea of FACTS can be explained by the basic
equation for transmission (1). Power transmitted between
two nodes in the systems (Fig. 1) depends on voltages at
both ends of the interconnection, the impedance of the line
and the angle difference between both systems. Different
FACTS devices can actively influence one or more of these
parameters and control the power flow through the
interconnection.
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One of the possible technical elements, which might be
the cheaper equivalent of HVDC in the future, is Variable
Frequency Transformer. This device, when compared with
HVDC back-to-back provides unparalleled flexibility for
utilities and transmission system developers to create viable
economic business models to meet the everchanging energy
markets. The VFT provides a simple and controlled path
between electrical grids, while retaining many of the
inherent virtues of an AC interconnection. This permits
power exchanges that could not previously be
accomplished, due to technical constraints such as
asynchronous boundaries or congested systems. The low
grid interaction of the VFT, in terms of harmonics, control
interactions, and impact on nearby generators, allows the
installation and operation to be decoupled from other grid
issues.
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EnergySpectrum - Issue 1, Vol. 5, 2010
The VFT system, based on a combination of hydrogenerator and transformer technologies, consists of a rotary
transformer, for continuously controllable phase shift at any
angle. This with a drive system and control that adjusts the
angle and speed of the rotary transformer to regulate the
power flow through the VFT. Smooth power control comes
from regulating the torque through the drive system.
Rotational speed is dictated by the difference in grid
frequencies and generally will be below 3 rpm.
Power transfer through the rotary transformer is a
function of the torque applied to the rotor. If torque is
applied in one direction, then power flows from the stator
winding to the rotor winding. If torque is applied in the
opposite direction, then power flows from the rotor winding
to the stator winding. If no torque is applied, then no power
flows through the rotary transformer. Regardless of power
flow, the rotor inherently orients itself to follow the phase
angle difference imposed by the two asynchronous systems,
and will rotate continuously if the grids are at different
frequencies.
However, if the power grid on one side experiences a
disturbance that causes a frequency excursion, the VFT will
rotate at a speed proportional to the difference in frequency
between the two power grids. VFT is designed to
continuously regulate power flow with drifting frequencies
on both grids. Reactive power flow through the VFT
follows conventional ac-circuit rules. It is determined by
the series impedance of the rotary transformer and the
difference in magnitude of voltages on the two sides.
Unlike power-electronic alternatives, the VFT produces no
harmonics and cannot cause undesirable interactions with
neighbouring generators or other equipment on the grid.
[11].
The power losses before PST consideration in Slovak
transmission system are following:
- power losses on lines 220 kV: 2,731 MW
- power losses on lines 400 kV: 19,770 MW
- power losses on transformers: 0,724 MW
Total power losses: 23,225 MW
Fig. 4 The regulation effect of PST added into line 404
5 APPLICATION OF PST ON SLOVAK
TRANSMISSION SYSTEM
In order to simulate power flows in the Transmission
system of the Slovak Republic, it is necessary to have a
model of the whole transmission system of UCTE. In this
simulation the model of UCTE (29th November 2007 at
12.30 o’clock) was used to simulate power flows on
boundary lines. This model contains 5784 nodes (from this
982 generators, 3262 loads), 7798 lines, 1102 transformers
[7].
Transformer PST was added separately into each of
international 400 kV lines of Slovak transmission system
(Fig. 3). After individual simulation, the regulation effect
and losses was calculated, see Fig. 4 - Fig. 17.
Fig. 5 Power losses in Slovak transmission system (PST in
line 404)
Fig. 6 The regulation effect of PST added into line 424
Fig. 3 International400 kV lines of Slovak TS
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EnergySpectrum - Issue 1, Vol. 5, 2010
Fig. 7 Power losses in Slovak transmission system (PST in
line 424)
Fig. 8 The regulation effect of PST added into line 440
Fig. 9 Power losses in Slovak transmission system (PST in
line 440)
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Fig. 10 The regulation effect of PST added into line 448
Fig. 11 Power losses in Slovak transmission system (PST in
line 448)
Fig. 12 The regulation effect of PST added into line 449
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EnergySpectrum - Issue 1, Vol. 5, 2010
Fig. 13 Power losses in Slovak transmission system (PST in
line 449)
Fig. 14 The regulation effect of PST added into line 477,
line 478 switched off
Fig. 15 Power losses in Slovak transmission system (PST in
line 477, line 478 switched off)
Fig. 16 The regulation effect of PST added into line 497
Fig. 17 Power losses in Slovak transmission system (PST in
line 497)
As we can see:
ƒ PST added into 404 affects mostly the power transmissions in profile SK-PL,
ƒ PST added into 424 affects mostly the power transmissions in profile SK-HU,
ƒ PST added into 440 affects mostly the power transmissions in profile SK-HU,
ƒ PST added into 448 affects mostly the power transmissions in profile SK-CZ,
ƒ PST added into 449 affects mostly the power transmissions in profile SK-UA,
ƒ PST added into 477 (or 478) affects mostly the power
transmissions in profile SK-CZ,
ƒ PST added into 497 affects mostly the power transmissions in profile SK-HU.
The putting of PST into transmission line and the angle
changing between primary and secondary voltage causes the
increasing of the power losses in whole interconnected
electric power system [7].
6 CONCLUSION
In case that power should be transmitted through a
meshed system, undesired load flow occur which loads
other parts of the system. This can lead to bottlenecks in the
system. In such cases specialised devices (FACTS, HVDC,
PST, VFT) could help to improve the situation. The article
analyses influence of PST installation on international lines
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EnergySpectrum - Issue 1, Vol. 5, 2010
and losses of the transmission system of the Slovak
Republic.
ACKNOWLEDGEMENTS
This work was supported by Scientific Grant Agency of
the Ministry of Education of Slovak Republic and the
Slovak Academy of Sciences under the contract No.
1/4072/07 and by Slovak Research and Development
Agency under the contract No. APVV-0385-07 and No. SKBG-0010-08.
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