Table Of ContentUCTE-IPS/UPS Interconnection:
Wind Energy Emergency Control Issues
A. Arestova*, A. Grobovoy**
* Power System Emergency Control Laboratory, Novosibirsk, Russia,
(Tel: +7 9232467198; e-mail: [email protected]).
** Power System Emergency Control Laboratory, Novosibirsk, Russia,
(Tel: +7 9039362937; e-mail: [email protected])
Abstract: This paper concentrates on the possible directions of examination in the frame of one of the
projects fulfilled under FP7 program umbrella. In 2009 the project “Intelligent Coordination of Operation
and Emergency Control of European Union and Russian Power Grids” has been started to develop the
prototype of the large power system SCADA/EMS. The project could become the basis for development
of the main principles for the control system of possible UCTE-IPS/UPS interconnection that could be
occurring in the near future. The paper ascertains the potential of attracting both back-to-back converter
stations and wind power plants to increase the efficiency of functioning of the power system emergency
control structure. The possible ways of improving of UCTE-IPS/UPS interconnection with the use of the
recently obtained results of the UCTE-IPS/UPS feasibility study are discussed.
Keywords: Wind Farm, SPS complex, SIPS, UCTE-IPS/UPS interconnection.
1. INTRODUCTION
1.1 The result of past investigations
The impending coupling of the Union for the Co-ordination
of Transmission of Electricity (UCTE) and power grids of
the Commonwealth of Independent States (CIS) including
the Baltic States power systems (IPS/UPS) provoke special
attention both of European and Russian bodies. For instance,
the study of feasibility of synchronous interconnection of
UCTE and IPS/UPS was undertaken by the System
Operators (SO) of these power grids during 2005-2008. The
examined questions consisted in the following: 1) if
synchronous interconnection is practicable, 2) what have to
be undertaken by UCTE and IPS/UPS in order to realize the
interconnection, 3) what could be the cost of this
interconnection realization.
In spite of the fact that the project has mainly been intended
Fig. 1. Possible ways for UCTE – IPC/UPC interconnection
for examination of synchronous coupling of the power grids,
with the use of the DC and AC technologies: gray, brown,
the predictable conclusion about necessity of Back-to-Back
and green colours symbolize 750 kV, 400 kV, and 230 kV
(BtB) converter stations utilization in the UCTE-IPS/UPS
transmission lines
interface has been drawn. This circumstance was conducted
because of possibility of low frequency oscillations
appearance after happening of the strong disturbances either 1.2 BtB and WPP potential
in UCTE or IPS/UPS. That is known that such oscillations
The recommendations regarding appropriateness of other
are difficult for damping and can pose a threat for reliability
technologies examination for the power grids coupling were
of UCTE-IPS/UPS interconnection. At the same time the
formulated in the project conclusion. In addition, as a
specific actions have to be used to prevent power system
possible variant shown in Fig. 1, at least three BtB converter
instability.
stations with the rated power of 600 MW were recommended
for exploration of the coupling. In the conditions of the
unexpected increase of the use of wind energy in UCTE, one
This work was supported in part by Russian Federal Agency
can assumed that the probable emergency control system of
of Scientist and Innovations under Contract 02.527.11.0004.
UCTE-IPS/UPS interface has to be based on the potential of 2. IMPLEMENTATION OF WEEC IN EMERGENCY
these BtB converter stations. In one's part, the wind power CONTROL SYSTEM
plants (WPPs) located in UCTE could be attracted to the
power grids emergency control. In that case the potential of
2.1. Interaction BtB and WEEC control centers
WPP and BtB converter stations could be integrated into unit
in order to improve the UCTE-IPS/UPS reliability. The
Possible relations which could be realized between WEEC
figure has been adopted from Arestova et al. (2009a, b) and
and BtB control centers are illustrated again by Fig. 1. The
supplemented with generalized abstract pictograms of
white and blue arrows symbolize the data and control signal
numerous WPPs in UCTE. Taking into account the specific
exchange between the control centers located at the BtB
operational requirements to the off-shore WPPs such as
converter station and WPP. This configuration is only one of
rather large capacity and heightened level of diagnostic, off-
numerous possible structures. In different cases these
shore wind farm could play a considerable part in UCTE-
structure have to be modified in accordance with the
IPS/UPS emergency control. In order to underscore this
technical requirements to the project. However, two
circumstance one of three pictograms is represented by the
elementary structures can be picked out for analysis. The
symbolic sketch of maritime waves. The role of WPPs in
structures are based on the concept of centralized and
Wind Emergency Control System (WEEC) will be intensified
decentralized decision making regarding the types and size of
in proportion as the energy storage devices will be put in
the control actions needed for the power system stability safe
operation in the schemes of wind turbines. In that case, the
keeping.
off-shore wind farms could be used as the object for
realization of control actions like generator rejection. The ways of realization of the power system emergency
control in UCTE and IPS/UPS are different. We can adapt
only the Russian experience of power system emergency
1.3 Objectives of the investigations of 2009-2012
control to possible UCTE-IPS/UPS emergency control
structure. Integration of WEEC system and the grid system
It is significant that Russian Federal Agency of Science and
protection scheme could be analogous with structures shown
Innovations in couple with European Commission (EC) have
in Fig. 2. If we will be guided by the dominated concept of
started one more collaborative project in the frame of the FP7
Russian power grid's SO, the first structure (A) has to be
program ENERGY named as "Innovative operational and
signed as the pre-eminent one. It supposes the collecting and
monitoring tools for large power systems". There are 13
processing of power system information in centralized
official bodies from 10 different European countries and 6
System Protection Schemes (SPS) control centers and further
legal entities from Russia participate in the project. Three
dissemination either of control actions or the sets for
prerequisites were determined in the project requirements
preparing the control actions at the local SPS devices.
specification for providing secure operation of the impending
interconnection 1) optimal choice of network interconnection
Taking into account the intricate structure of mutual relations
and extension technologies, 2) the strategies of effective
between the owners of WPPs and UCTE SOs one can
control and monitoring systems, and 3) well defined
suppose that the second configuration (B) of WEEC can have
protection functions that ensure operation of all partner
the right to existence too. In our opinion this structure can
networks in critical cases.
become ever more attractive in comparison with the first one.
According as the productivity of communication and
In fact, the modern state High Voltage Direct Current
computer techniques will increase, great number of the
(HVDC) and Flexible Alternating Current Transmission
traditional SPS functions can be sent to the low levels of
Systems (FACTS) technologies allow realizing appreciably
control systems. This concept adjusts with “Smart Grid” idea
more effective ways for UCTE-IPS/UPS interconnection. It is
very well. Thus, we believe that it could be used in the future
VSC-HVDC technology based on voltage source converters
emergency control systems including UCTE-IPS/UPS one.
(VSC).
For one’s part, VSC-HVDC technology used in meshed
alternating current (AC) transmission networks in couple
with Wide Area Measurement/Control Systems
(WAMS/WACS) technologies could allow to put into
practice a new generation of large power grid control system
[Pan et al. (2008)]. Possible utilization of these technologies,
with reference to UCTE-IPS/UPS interconnections, will be
the subject of the project of 2009-2012.
In the project it seem expedient to discuss possible ways of
integration of System Protection Schemes (SPS) into the
structures of UCTE and IPS/UPS control system, as well as
Wind Farms emergency control into System Integrity
Protection Schemes [Begovic et al. (2007)]. Fig. 2. Possible relations between SPS components:
Fig. 3. Probable wind power plant structure in Chinese power grid
During the use of the off-shore WPPs for realization of coupled with China power grid, as was described in
generator rejection for prevention of power system instability [Arestova (2009,c)], has been used. A hypothetical WPP that
an obvious question is how one can forecast capacity may be used in the north China power grid was examined. Its
available for realization these control actions. The question structure is presented in Fig.3. The scheme of the series
arises because of a stochastic form of wind generation and dynamic braking with reference to partly or full-rated
uncertainty with available size of capacity assigned for converters is adduced there too.
emergency control. There is an obvious answer this question:
the WPP controller that could realize the function of the 2.3. Conditions and results of simulation
devices designed to select a quantity and number of wind
turbines is needed. The same approach has been realized in
The sequences of the events under simulating were the
the frame of SPS complexes at some Russian hydro power
following:
plants. It is obvious that the approach can be used for WEEC,
especially because the structures of WPPs conform to the 1 sec – phase-to-phase fault in high level network;
requirement of control actions optimization. Serial-parallel 1.05 sec – dynamic braking insertion;
chains of wind turbines at the WPPs, which associate to a 1.15 sec – fault clearance;
connection point, allow decreasing the superfluity of control 1.2 sec – dynamic braking switching off;
actions. 2 sec – reclosing of transmission line.
The model of DFIG takes into accounts the action of the
2.2. Series dynamic braking for DFIG stator protection device whose algorithm is below. The wind
turbine is opened when the voltage drops below a threshold
A variety of types of the wind turbines determines possible (0.75 p.u.) and remains under that threshold for certain time
structures for emergency control. One should remember that (0.08 sec). The wind turbine disconnection is performed after
the fixed-speed wind turbines with fixed pitch [Soder et al. the delay of 1 sec. The model of the wind turbine equipped
(2005), Knudsen et al. (2005)] can use the so called series with DFIG is the well-known model described including the
dynamic braking for a fault right-through, just as it was standard model library of EUROSTAG software. In
discussed in Causebrook (2007), while other type of turbines, EUROSTAG, the double fed induction generator is modeled
with partly or full-rated converters, can be used for by an induction machine with the rotor connected to a voltage
prevention of instability appearance in post-emergency power source and an injector. The rotor voltage is controlled by the
system conditions. regulator, and assigned to the induction machine. The power
electronics converter connected to the network at the machine
Also, the series dynamic braking resistors in couple with
terminal is modeled by the injector [EUROSTAG – Package
automatic voltage regulator could be used to improve the
Documentation (2008)].
conditions of the wind turbine fault ride-through and to
prevent the disconnection the DFIG type turbines because of Fig. 4 illustrates the results of comparing the process of fault
voltage drops at the buses. To prove this guess the test bed ride-through for DFIG turbines with and without using of the
network of the Russian Far East interconnected power system dynamic braking resistors.
Fig. 5. Time periods of WEEC at the same WPP
which will be defined by special conditions. For instance, if
one can assume that the WPPs contain some different types
of the wind turbines, and shirt circuit appearance inside
WPPs is occur, the strategy of emergency control could be
the following: 1) during shirt circuit the fixed-speed wind
turbines have to be undergone effecting of series dynamic
braking. After the shirt circuit clearing and the dynamic
braking is opened, the next stage of control is beginning, 2)
the variable-speed wind turbines with double-fed induction
generators as well as the variable-speed wind turbines with
fully-rated converters can be used for damping of power
oscillations. Fig. 5 shows this principle.
2.5. SPS Coordination of power grid and WEEC
With reference to WEEC system the centralized and
decentralized principles of control can be considered. Both
the centralized and decentralized SPS complexes have to be
coordinated either by centralized SPS system or system
operator in order to contribute the power system stability.
Many obstacles can arise under development of these
structures. That stipulated by market conditions. As is well
Fig. 4. DFIG response during a fault without (blue line) and known the largest part of wind energy belongs to private
with (red line) using of dynamic braking investors. To involve the investors to development of power
system emergency control, it is need regional, national and
The commutation at the moment of time of 1.22 sec is European legislation. The last grid codes regarding WPPs
conditioned by the stator opening due to the action of the using are already promote this process [Eriksen et al. (2005)].
protection device. Fig. 4a is a good example to demonstrate
achieving the threshold on voltage. It can be observed the A mix of two technologies such as BtB and Wind Energy is
moments of the DFIG turbine tripping on the Fig. 4b. Both distinctive feature of the power system emergency control
the disconnection, conditioned by the action of DFIG stator approach that could be realized in UCTE-IPS/UPS
protection, and reconnection, stipulated by the rotor speed interconnection. Thus, WPPs SPS can become by
normalization, is illustrated by blue curve. The insertion of sophisticated control system with own Wide Area Detection
dynamic braking resistors of the DFIG construction is System (WADS) and own control action dissemination
executed for 0.05 sec after the fault identification, in turn system. This principle is explained by Fig. 6.
shunting of dynamic braking is fulfilled at 0.05 sec after fault
Because of complexity of regional and UCTE WEECs the its
the clearing. The rate power of dynamic braking is 30 percent
structure could be design, for instance, similar to Russian
of unit active power. The speed reference in disconnection
Power grid emergency control system, where Power system
phase, determined by the wind turbine regulator, is 1 p.u.
SPS complexes are replaced by wind farm SPS. Depending
while the speed dead band for resynchronization acceptation
on power system conditions and type of power system
is 0.1 Hz.
disturbances the control action for wind farm SPS can be
formed by Coordinating SPS System, Centralized and Nodal
2.4. Series dynamic braking and WEEC SPS complexes or wind farm SPS. The strategy of emergency
control has to be corrected with according as political,
The series dynamic braking could be realized with the use of economical and technical conditions.
both a local and mutual information about power system
dynamics. In the second case the dynamic braking can be There are many variants to attract the WPPs owners to
used both for keeping wind turbine with connection of power participate in WEEC. It will be subject of the future
system and damping of low frequency oscillations. Here is investigations, but just now one can assume that the
one of the possible schemes. There are many other variants equipment of the WPPs has to satisfy to the modern grid
system disturbances, only AC transmission line can be
Coordinating opened, in order to prevent cascading outages. However, this
SPS System method intends certain coordination of different SPS
complexes. Fig. 7, borrowed from Grobovoy (2005), offers
two possible SPS strategies which assume that the objective
Centralized of the centralized SPS complexes is to maintain electricity
SPS change in UCTE-IPS/UPS market in all possible emergency
complex situations. In that case, the centralized SPS complexes or
remote (local) SPS devices can be situated on one side of the
intersection but its influence has to be applied to another side.
Nodal SPS Nodal SPS
The centralized SPS systems have to be used in order to
complex complex
correct the set points of remote SPS devices, nodal and wind
farms SPS complexes. The set points have to be transferred
with the use of telecommunications means. Another way of
Wind Farm the coordination consists in transferring to the centralized
SPS SPS complexes the information regarding power system
network models for correcting control actions by the
Fig. 6. Possible wind farm SPS structure centralized SPS complexes without assistance. In that case,
both power network model and the decision tables can be
used for forming control actions. It is needed to note that the
coordination centers shown in Fig. 7 could be located in other
codes [De Almeida et al. (2006), Erlich et al. (2005), Qiao et places, for example, at the Ukraine and Switzerland. It should
al. (2008)]. Perhaps, it needs to plan more widely using of be the subject of WAMS/WACS structures designing in the
wind turbines with full-rated converters. The cost spent now near future. The positions of the centralized and coordinating
can be compensated in the near future. In one’s turn, the SPS systems on the map have been chosen roughly. Its
double-fed induction generators as well as the wind turbines accurate location has to be determined after detail study and
with fully rated converters could be used as traditional attainment of agreement on the principles of emergency
control actions such as generator rejection. control for the UCTE-IPS/UPS interconnection.
The positions of the centralized SPS complexes in Fig.7 are
3. EMERGENCY CONTROL COORDINATION
suggested taking into account the existing and possible power
transfer corridors that will be formed by the 750 kV
3.1 Two strategies for SPS coordination
transmission lines. The bidirectional white arrows with
orange color border mean the transfer of special information
Hence, the interface of UCTE-IPS/UPS can be formed on the contents which depend on the principle of the centralized SPS
base both of VSC-HVDC and AC transmission ties. Under
complexes – either behavior prediction or behavior-
Fig. 7. Two possible strategies for SPS coordination in UCTE-IPS/UPS interconnection
assumption. Two directions of the arrows have to emphasize stations and WPPs at the UCTE-IPS/UPS interface has to
an ability of the centralized SPS complexes to fulfill the become the subject of a wide international discussion. To
power system Dynamic Security Assessment (DSA) and to develop the effective defense plan against UCTE-IPS/UPS
transfer the information in the coordinating SPS system. The possible blackouts the market principles have to become the
white arrow with blue color borders means the transfer of the base for SPS coordination. Harmonization of interests of
information required for DSA for the UCTE-IPS/UPS power different owners of BtB converter stations and WPPs
grid at the UCTE Transmission System Operators (TSO) and participants is indispensable.
the IPS/UPS TSO separately. The black arrows symbolize the
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