使用VSC—HVDC来提高电压的稳定性

Improvement of V oltage Stability by Using
VSC-HVdc
torre,Student Member,IEEE and M.Ghandhari,Member,IEEE
Abstract—This paper analyzes and compares the stability of a power system when either a new ac transmission line or a dc link based on VSCs is connected in the grid.The location of the new transmission line is determined by the restrictions in the transfer of power.From the controllability point of view,this is not the most suitable location for a VSC-HVdc to provide damping. However the voltage support capability of the VSC-HVdc can be exploited to keep the system from losing synchronism due to voltage collapse.
Index Terms—Transfer capability,voltage stability,VSC-HVdc.
I.I NTRODUCTION
T HE improvement of the stability margins in power system when using VSC-HVdc has made of this technology an important option among grid owners when there is a need for increasing the transmission capacity of the network.
The capability of rapidly control both active and reactive power independent of each other allows VSC-HVdc to en-hance transient stability,increase damping of electromechan-ical oscillations and improve voltage stability.Furthermore, when connected in synchronous systems,VSC-HVdc provides highflexibility in the control of the powerflow in the net-work[1],[2].
V oltage collapse usually occurs following a large distur-bance in heavily stressed power systems,which results in increased reactive power and hence leads to a voltage drop. Restrictions in the power system to provide reactive power together with the control of mechanism that try to restore the voltage magnitude at operating conditions often lead to voltage instability.Embedded VSC-HVdc provides counter-measures for both transient and long term voltage instability mechanism[3].
The objective of this paper is to make an analysis of the stability of a power system when it needs to be expanded. Two alternatives to increase the transmission capacity are considered and compared:1)a new ac transmission line and 2)a new dc transmission line based on VSCs.
II.C ONTROL OF THE VSC-HV DC
Since a VSC-HVdc independently control the active and reactive power,each converter has two control loops.In the active power control one converter is set to control the active power itself and the other converter is set to control the dc voltage.In the reactive power control,each converter can be The authors are with Royal Institute of Technology,Department of Electric Power Systems,Sweden.Emails:torre@ee.kth.se and mehrdad.ghandhari@ee.kth.se.Contact and additional information can be found in the web page:http://www.eps.ee.kth.se set to control either the ac voltage or the reactive power.
Fig.1shows o block diagram for one converter.The other
converter has the same control.In thefigure,the inputs ΔP andΔQ are intended for supplementary control.In this paper a supplementary control for Power Oscillation Damping
(POD)and active powerflow control is included.There is no
supplementary control for modulation of reactive power.
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3
3
Fig.1.Control VSC-HVdc
A.POD
The control strategy used in this paper has specific ap-
plication for VSC-HVdc systems[4].Although the main
application of this control strategy is for enhancing transient
stability,in this paper it is used for POD.
ΔP P OD=k t(f i−f j)(1) where i,j are the nodes of connection of the VSC-HVdc,k t is a positive gain and f i,f j are the frequencies measured at node i and j.
B.Voltage Support
For voltage support,both VSCs are switched to U ac control
(Fig.1).The initial conditions(pre-fault conditions)of the
voltage at the nodes of connection of the converters are set as U acref
C.Power Flow
The VSC-HVdc controls the powerflow in the system based on either an order sent by the operator of the substation or
information obtained directly from the system.In the latter, the VSC-HVdc changes the transfer of either active power or reactive power or both in post-disturbance conditions in order to reach a higher margin of rmation from the system might be obtained from voltage measurements at remote nodes or at generator terminals or measurements of power in neighbors transmission lines.In this paper,power flow control is only carried out when the system is in steady state.In this analysis the magnitude voltage is used in the input of the powerflow.
input=(U des−U ac)(2) where U des is the desired voltage magnitude and U ac is the actual voltage magnitude.
III.N ORDIC32A-C IGR´E T EST S YSTEM
The Nordic32A is characterized by high hydro generation and low load consumption in the north;thermal power gen-eration and high demand of electric energy in the center;and few thermal units and low load consumption in the south west. An equivalent of the interconnection with a external grid is represented by two generators in the the external part.The major transfer of power is from the north part to central area where the main centers of load are located.Fig.2shows a single line diagram of the system.A detail description of the system can be found in[5].
The transfer of power from north area to central part is 3260MW in peak load conditions.Under higher demand of power,the power system is capable of transferring100MW more,approximately.It is not possible to transfer a higher amount of power.
In this paper an analysis of voltage stability is performed when an increase of power of300MW from north to center area is needed.For such purpose,a new transmission line is connected between nodes4032and4044.Two alternatives are considered:1)an ac transmission line(denoted as ac option) and2)a dc transmission line based on VSC(denoted as dc option).
This location4032-4044is not the most appropriate for the VSC-HVdc to increase damping in the existing interarea mode between the external and south west areas.The farther from the the north nodes the VSC-HVdc is connected the lower is the residue of the oscillatory mode[6].However,the VSC-HVdc might still be capable of increasing the margin of stability of the system by controlling the reactive power of the VSCs.
IV.S IMULATIONS AND R ESULTS
A.VSC-HVdc Model
The model of VSC-HVdc used in simulations is a model available in the software of simulation SIMPOW[7].The VSC connected at node4032is set to control the active power. Consequently,VSC at node4044is set to control the dc voltage.As described in section II-B the converters are set to control the voltage magnitudes at node4032and node4044. The ratings of the VSC-HVdc are based on a converter of the type M9[8].The rated dc voltage is±350kV,and the capacity of each VSC is550MV A.
Fig.2.Nordic32A Power System
B.Fault Cases
The following fault cases compare the results when a new ac transmission line is connected in the power system and when a dc-link is connected instead of the ac line.When the HVdc is connected in system,the voltage magnitude at node4032, voltage magnitude at node4044and the transfer of active power in the new transmission line are kept as close as possible to the initial conditions when the ac line is connected.
The Nordic32A is a robust system and basically fulfill the n-1criterion.In order to make a comparison under highly stressed conditions,the fault cases considered in the simulations correspond mostly to n-2condition.
1)Case1:The transmission line4021-4032is discon-nected.After60s the transmission line4042-4043is also disconnected.
ac option:Fig.3shows the voltage profile at node4044. The system is stable,although thefinal magnitude of the voltage is low.At t=21s,the exciters of generators4031 and4042become to be limited.As a result the injection of reactive power drops.At t=82s the exciter of generator4051 is also limited and more reactive power is lost in the system. The system begins to experience large power oscillations. At t=121s,exciters in generators4031and4042are not limited anymore.The voltage recovers and oscillations are damped.However these two generator exceed again the current
TORRE et al.:IMPROVEMENT OF VOLTAGE STABILITY BY USING VSC-HVDC3
limits and the exciters are limited.V oltage magnitude again drops.At t=182s exciter in generator is not limited anymore. The system begins to settle down andfinally reaches a new equilibrium point.
dc option:At t=21s exciters in generators4031and4042 begin to be limited.At that time the VSCs inject more reactive power into the system.This injection of power makes that no more exciters are limited in the system.At t=61s,when the the second transmission line is disconnected,VSCs inject as much reactive power as the limits of the converters allow it. Even after the second disturbance,no new exciters are limited in the system.At t=121s,exciters in generators4031and 4042are not limited anymore.This allows the generation of more reactive power and the voltage at node4044increases somewhat.However,the current in the generator is too close to the limits and the higher production of reactive power makes the exciters in generator4031and generator4042to be limited again at t=145s and t=160s,respectively.Nevertheless, thefinal voltage magnitude was higher than the ac option. Furthermore only two exciters became limited in the power system.
Fig.3.V oltage at node4044.Case1
Fig.4shows the rotor angle oscillations in generator4063. After thefirst disturbance larger oscillations(compared to ac option)appear in the system.The cause for these larger oscillations is the voltage control in the VSCs.POD control helps little providing damping.
Fig.4.Rotor angle generator4063.Case1
2)Case2:The generator connected at node4062is discon-
nected.This fault represents a lost of535MW and40Mvar.
ac option:The system losses synchronism.Fig.5shows the voltage profile at node4044.After the disconnection of
the generator the voltage is recovering,but at t=21s exciters
of generators4031and4042begin to be limited,which
causes a lost of reactive power and the voltage drops.20s
later,exciter in generator4021also becomes limited.At this
time generators in south part begin to accelerate dramatically.
Finally generator1043losses synchronism.
dc option:The system remains in synchronism.When the generator is disconnected,the VSCs inject the maximum allowed reactive power.At t=21s the exciter in generator4031 is limited.8s later exciter in generator4042is also limited.By this time the voltage is still close to the initial value.However at t=44s,exciter in generator4021becomes limited and the generation of reactive power drops.This makes the voltage to fall to0.96p.u.VSC were already operating at their limits. At t=121s,t=129s and t=144s exciters in generators4031, 4042and4021,respectively,are not limited.Since voltages are low,generators inject more reactive power,whichfinally leads those
three exciters to become limited again.This behavior becomes periodic.
Fig.5.V oltage at node4044.Case2
3)Case3:The transmission line4041-4061is discon-nected.After60s the transmission line4032-4044is also disconnected.
ac option:Fig.6shows the voltage profile at node4044. This fault case leads the system to voltage collapse.Exciter in generator4042is thefirst exciter being limited(t=36s). V oltage at node4044is not affected yet.At t=56s exciter in generator4031becomes limited.V oltage is lightly affected. 20s later the second transmission line is disconnected exciter in generator4021is limited.The reduction in reactive power generation makes the voltage to drop to0.9p.u.The power system seems to reach a new equilibrium point.However at t=102exciter in generator4047becomes limited and the systems collapses due to lack of reactive power support.
dc option:The voltage support from both VSCs keeps the system from collapsing and losing synchronism.After the first disturbance the VSCs keep the voltage in the reference value.No exciters are limited in the system.When the second transmission line is disconnected generators4031and4042
4IEEE T&D ASIA-SIEF2009,SEOUL,KOREA,OCTOBER26-30,2009
drops injection of reactive power at t=81s due to limitation in their exciters.At t=181s the limitation in these two exciters ceases.However due to low voltage at the nodes,generators inject more reactive power,the current in the stator becomes high and exciters are limited again at t=181s.This behavior becomes periodic.
Fig.6.V oltage at node4044.Case3
In this fault a change in the powerflow is done.The input in eq.(2)is used in the powerflow.U des is set to1p.u. and U ac is the measured voltage at node4044.Fig.7shows the simulation results.Due to limits in the VSC the voltage does not reach1p.u.magnitude,but increases to0.978p.u. Exciters in generator4031and4042are still limited,but it can be seen in Fig.8that the generation of reactive power decreases compared to no powerflow control.Obviously,in the case of higher rating in the VSCs,the converters will be capable of producing more reactive power,the generator will not have the need for generating reactive power,the current of the stator will not reach the limits and the periodic behavior
will not happen.
Fig.7.V oltage at node4044.Case3.Powerflow control
V.C ONCLUSIONS
A comparison between a new ac line and a new dc link based on VSC-HVdc when increasing the transmission capac-ity in the test Nordic32A power system has been analyzed. Simulation results showed that VSC-HVdc notably improved the stability of the system compared to a new ac line.V oltage
Fig.8.Reactive power in generator4042.Case3.Powerflow control stability was greatly enhanced and kept the system from collapsing due to the lack of reactive power.Furthermore VSC-HVdc allowed generators to reduce the level of stress in exciter and increase the margin of operation.These new conditions were obtained by changing the powerflow in the system based on voltage measurements.However,due to the location of the VSC-HVdc its controllability index was low and therefore there was little contribution in damping.
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B IOGRAPHIES
H´e ctor torre received the M.Sc.degree in Electrical Engineering from Royal Institute of Technology,Stockholm,Sweden,in2002.He was employed by Interconexi´o n El´e ctrica S.A.-ISA-,Colombia,in the area of design of substations for9years.He is currently Ph.D.student at the Royal Institute of Technology(KTH).
Mehrdad Ghandhari received the M.Sc.,Tech.Lic.and Ph.D.degrees in Electrical Engineering from Royal Institute of Technology,Stockholm, Sweden,in1995,1997,and2000,respectively.He is currently Assistant Professor at the Royal Institute of Technology(KTH).
Lennart S¨o der(M91)was born in Solna,Sweden in1956.He received his M.Sc.and Ph.D.degrees in Electrical Engineering from the Royal Institute of Technology,Stockholm,Sweden in1982and1988respectively.He is currently a professor in Electric Power Systems at the Royal Institute of Technology.He also works with projects concerning deregulated electricity markets,distribution systems,risk analysis and integration of wind power.。