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CONTROL MEASURES TO ENSURE DYNAMIC STABILITY OF THE CAWORA BASSA SCHEME AND THE PARALLEL HVAC SYSTEM

W. Bayer, K. Habur, D.Povh D.A Jacobson J.M.G. Guedes D. Marshall Canada

INTRODUCTION operation with an open busbar coupler at Songo. Im- proved stability is achieved by utilizing DC power to The paper discusses parallel operation of a 1500 km rapidly control the voltage angle difference between bipolar HVDC link and a relatively weak parallel 400 / Songo and Apollo. This permits system operation 330 kV AC link. Stable operation can be ensured by with voltage angles of the order of 60 deg. use of switched braking resistor and control of the voltage angle difference between sending and Disturbances from the DC side such as commutation receiving end. During critical contingencies dynamic failures at Apollo and DC line faults affect the AC sys- splitting of the HVDC and AC linkat the sending end is tem (Zimbabwe). The operational requirements dur- necessary with respect to stability. ing contingencies are:

split point - no loss of .rotor angle stability normally closed - \ filters damping of power oscillations in the AC link - limitation of transient voltage drops ( except during short-circuit ) - infrequent switching operations ( busbar splitting ) at Songo due to disturbances These requirements are not fulfilled unless special countermeasures are carried out. A combination of the following countermeasures will meet the above requirements: use of a thyristor switched braking resistor (480 MW) I I use of a breaker switched braking resistor I I / (240 MW) I// control of voltage angle difference by braking / 0 to Mogambiqu resistor and DC power modulation dynamic busbar splitting ( only in exceptional I/I cases ) Bindura line tripping (only in exceptional cases) tripping of generators ( only in exceptional network cases ). Fig. 1 : Principle Network Configuration Voltage angle control requires synchronous angle measurement at Songo and Apollo which can be veri- TASK DESCRIPTION AND SUMMAFW fied by use of the GPS (global positioning system). OF RESULTS Cahora Bassasystemoperates in angle control mode The study deals with the stabilization of the planned or in frequency control mode when the AC link is inter- Cahora Bassa - Zimbabwe - South Africa intercon- rupted. Automatic change over of control modesand nection. The 220 kV busbars at Songo are coupled resynchronization after interruption of the AC link and the AC and DC systems between Songo and ( Cahora Bassa - Zimbabwe - South Africa ) is pro- South Africa are operated in parallel. This offers tech- vided. nical and economical advantages in operation. Maxi- Design considerations for the controls are robust- mum generation at Cahora Bassa is 2000 MW. ness, transparency and insensitivity against failure of The AC/ DC interconnectioncan improvethe stability equipment or loss of relevant control parameters. of the Zimbabwe network substantially compared to

'AC and DC Power Transmission', 29 April-3 May 1996. Conference Publication No. 423,O IEE, 1996 147

BASIC REQUIREMENTS FOR STABILIZATION Fig. 2 shows a selection of relevant GMPC output OF THE AC LINK variables. The most relevant equipment and controls related to network stabilization are described in the The DC transmission is subjected to a relatively large following sections. number of disturbances ( commutation failures and DC line faults with or without successful restart ). More than 1 fault per day cannot be excluded accor- VOLTAGE ANGLE DIFFERENCE AS ding to former operation experience. CONTROL CRITERICIN The AC link on the other hand is weak because of the The control criterion has the following advantages: long distance. The power change is of the order of 7 MW per degree change of voltage angle. Therefore - Independence on transients in the Zimbabwe net- the AC link can absorb only asmall quantity of the load work and its interconnection to other systems deficit caused by faults on the DC transmission. - Controlled bumpless reconnection of the AC link Braking resistors are therefore necessary to keep the after line tripping due to line faults power balance at Songo. - Reliableoperationat highvoltageanglesof 60deg Apart from stabilizing the AC link, the braking resis- and above tors prevent frequent generator tripping which is un- - Possibility to interconnect other weak ties bet- desirable with respect to the service life of the genera- ween Songo and ESKOM (through MoGambique) tors. as traced in Fig.1 in1 the eastern area. In addition to sufficient braking power a suitable con- trolcriterion is required. The power in the AC linkdoes Power oscillations develop between Songo and ES- not reflect adequately the deviation of the Songo- KOM via the Zimbabwe network. The oscillations are Apollo angle because of transients between the Zim- damped by measures i3t Songo ( modulation of bra- babwe and the ESKOM networks ( Fig. 1 ). Therefore king resistor power and DC power ). the measured angle difference between Songo and Other oscillations which develop mainly between Apollo is used as control criterion. The voltage angle Zimbabwe and ESKOM are also reflected to Songo. between Songo and Bindura which has the advan- However, the possibilities to damp these oscillations tage that it can be calculated from locally measured by measures at Songo are limited as Songo is located variables at Songo is no adequate substitute for the far away from the areas where the oscillations occur. measured angle Songo - Apollo. These oscillations can be damped by adequate mea- sures in the Zimbabwe network. GRID MASTER POWER CONTROLLER ( GMPC ) After multiphase faults on the single line part of the AC link the interconnectionbetween Songo and ESKOM The control functions for power balance and grid sta- via AC is interrupted. ,Automatic high-speed reclo- bilization are combined in the GMPC. sure could be applied, however, only within certain limits of the angle difference across the split point. Bus-AC Bus-DC filters regional load Moreover there is the risk of unsuccessful reclosure. With theanglecontrolan automaticdelayedreclosure can be carried out in a controlled way such that the re- closure is bumpfree. llnsuccessful reclosure does not worsen the situatioin with respect to rotor angle stability. The interconnectioncan beoperatedat relatively high voltageangles between SongoandApollo. The maxi- mum steady state angle in the studies was 54 deg. However, this is not a limit with respect to rotor angle 1 turbine power reference stability. The limitation is only caused by the transient 2 DC power reference DC voltage deviation in the Zimbabwe network which 3 TSBR power reference should not exceed 10 ”/. Rotor angle stability could 4 BSBRonloff 5 busbar splitting command be achieved at much higher angles as the angle con- trol provides always correct control signals also be- yond 90 deg. From both the Songo and the ESKOM networks tie lines also exist into the hrloqambique network. These lines could not be interconnected up to now as they would not develop sufficient synchronizing power to keep Cahora Bassa and ESKOM networks in syn- 4 48 chronism. Wfih angk control between Songo and dability can alsobe ed for weaklie lines. nding the voltage o is 50 - 90 ms including measurement and data processing.

BRAKING RESISTORS ( BR ) The BR serve a double purpose. Stability Control Voltage Angte Difference Songo-Apollo BR are well suited for keeping the angle at Songo within acceptable limits. However, the stabilizing task during operation with AC interconnection requires faster reaction and better controllability than the task to keep the frequency in isolated operation within the given limits. Voltage Deviation from Prefault Wue The examined worst case condition in the network studies is: operation with all 5 generators ( 2000 ~ l000MW ~ IOOOMW MW), 4 + 2 DC bridges on the 2 poles ( 2 bridges out of service), fault on the DC line with 4 bridges and un- I successful restart attempts. This requires a total of OL HVDC Power on Healthy Pole 1 720 MW BR capacity. Different types of BR have been examined. The se- lected solution was a total braking resistor power of 720 MW where at least 2/3 or 480 MW are thyristor switched. With respect to stability there was no rele- 0 10s 0 10 s vant difference between the effect of either a thyristor Total Power Output of Braking Resistors controlled braking resistor ( TCBR ) which is control- led continuously or a thyristor switched braking resi- Continuous Control 7 x 68.6 MW Steps stor (TSBR) which iscontrolled in steps of approx. 70 MW ( reference Fig. 3 ). Fig. 3 : Comparison of Continuous or The TCBR or TSBR is used whenever the generated Stepwise Thyristor Control of Braking Resistors power exceeds the capability of the DC poles and the AC link to pick up load as the number of insertions is not restricted. On the other hand insertion of the Prevention of Generator Tripping Breaker Switched Braking Resistor (BSBR) is In former operation the planned AC link Songo -Zim- delayed such that the BSBR is not inserted during the babwe - ESKOM asshown in Fig.1 did not exist. Ca- relatively frequent commutation failures with normal hora Bassa was an isolatedpower plant with indepen- restart of DC power. However, the delay of insertion is dent frequency. adisadvantage in cases with delayed power restart as Frequent DC line faults with unsuccessful or delayed this cannot be predicted. Here fast insertion of the power restart resulted in frequent generator tripping braking resistors is important as the angle deviation as the overload capacity of the DC poles was (apart of during a contingency is proportional to the double in- short transients) limited to 105% I, and the transient- tegral ( over time ) of the power imbalance between ly available frequency range is limited to 50 -t-1 Hz. generated power (turbine power) and load ( genera- Because of the negative effect of frequent generator tor power ). Therefore a compromise between suffi- tripping on the service life of the generators applica- ciently fast insertion on one hand but not too frequent tion of 240 MW BR was already considered in former insertions on the other hand is strived for. The final isolated operation. setting is left to operational experience. POWER CONTROL An alternative where the total braking resistor power is under thyristor control would ( except redundancy Fast Control aspects ) offer the following advantages: Power Balance Control. Rapid power balance ad- - faster insertion justments are necessary following severe distur- - no limitation of number of insertions ( no breaker bances( i.e lossof DCpole) inordertomaintainstabi- switching ) My. The required power insertion iscalculated quickly 149 and is providedby any available DC overload and bra- on the Matimba line the Bindura line provides the king resistors. The control is in principle a temporary power balance and the DC power is adapted accord- feedforward correction. Feedback controls are pro- ingly. The total load in the interconnectedsystem and vided by the power modulationcontrols for permanent the generation at Cahora Bassa remain unchanged. fine tune power adjustments. Onlythedistributionof IpowerflowfromCahoraBassa to the grid via AC and DC links is changed. Power Modulation Control. The power modulation control adds to the output of the power balance con- There are other possible AC line trippings within the trol transient components which stabilize the voltage Zimbabwe network whlich limit the power the system angle at Songo ( PD controller ). The P-component can absorb from Songo. Such faults are detected by a provides the synchronizing power, the D-component logic which provides braking resistor power and provides damping of power oscillations. ramps it down to zero after elapse of a few seconds. In steady state operation the output of the power bal- In case of transient line faults the voltage angle at ance and the power modulation controls is zero. Songo remains within preset ( acceptable ) limits and The total transient power requirement is passedto the the steady state power flow to Bindura keeps the pre- load joint control where the load is distributed to the fault value. Reference1 lower diagrams of Fig. 4. HVDC poles and the braking resistors. 180 deg reference angle reduced The fast control is responsiblefor stabilization during -;-- --..._ measured angle contingencies. 0 deg Slow Control 600 MW measured AC power The load demand from the AC and the DC links is checked against the power transfer capability of the F--- 0 MW links and the available generation. Then the refer- Permanent Fault, AC Power Reduced ence values are defined. The required total genera- tion is distributed to the individual generators. Individ- I 180 deg- ual upper limits can be set. This allowsfor example to reference angle constant - ..-I..-. avoid working points of the turbines where cavitation Odeg occurs. Moreover, any generator can be taken out of k- the joint control and be operated manually. 600 MW In case of sudden changes of load or generation ( e.g. measured AC power trip of a generator or DC line fault without power re- OMW start ) the new reference values are calculated and E-O 20 s adjusted under consideration of the maximum rate- Transient Fault, AC Power not Reduced of-change capability of the turbine power. If the total power demand is higher than the genera- Fig. 4 : AC Power Flow in the Bindura Line tion capability the AC load has priority. after a Fault in the Zimbabwe Net- The control of the power in the AC link is performed by work slow integral control of the angle reference value of In case of a permanent line trip the angle exceeds a the PD controller. The maximum steady state angle is set limit. As a consequence the power in the Bindura limited to avalueset by the operator. If the Zimbabwe line will be reduced to i2 value which the Zimbabwe network cannot (for any reason) absorb the required network can absorb. Reference upper diagrams of power within acceptable angle limits the power flow in Fig. 4. the Bindura line is reduced accordingly. DYNAMIC ISOLATION' OF THE AC AND DC NET- FAULTS IN THE AC LINK WORKS During Iphase faults in the Bindura line ( with rapid The bus coupler at Songo is normally closed ( Fig. 1 ). single phase auto-reclosure ) 250 MW braking resi- However, if the stabilizing measures at Songo are not stor power are inserted for about 1 s until the faulted sufficient during contingencies the AC and DC net- phase is reclosed. 1phase faults on other lines do not worksaresplit. Forthetaultscomingfromthe DCside require control intervention at Songo. ( commutation failures and DC line faults ) the bus After 3phase faults ( without high-speed reclosure ) coupler is the suitable split point. These faults are ex- on the Bindura line 480 MW braking resistor power pected to be the more frequent and critical ones. For are inserted. Then the turbine power is set back by faultscoming from the AC side ( Bindura line and lines the amount of the lost line load and the braking power within the Zimbabwe network ) the line breaker at is removed accordingly. The signal for insertion of Songo is the preferred split point. braking power originates from the line protection. The number of dynamic network splittings shall be After 3phase faults ( without high-speed reclosure ) limited to the order of 1C) per year for operational rea- 150 sons. The task is to avoid unnecessary splitting but to 2 different interconnection procedures are consid- split as fast as possible if necessary. Different strate- ered. gies have been examined including a prediction me- If the Matimba linewastrippedthe lnsukamini line end thod where the development of a disturbance is con- is closed after the transients have died away. The di- tinuously assessed for the future ( approx. 100 ms ) sturbance by a possibly unsuccessful reclosure does and where the assessment is continuously corrected not add to the first disturbance where the line has for the past. been tripped. However, under consideration of all aspects, espe- If the line reclosure is successful a paralleling device cially robustness, a relatively simple method has at Matimba is prepared to reconnect the AC link if the been used. It is based on an assessment of the gen- criteria (frequency, angle) are fulfilled. The intercon- erated power (turbine power) and the capability of the necting control at Songo provides these criteria by network to absorb load. The difference is accumu- controlling the frequency at Songo to the ESKOM fre- lated over the time. If a set limit ( MWs ) is reached quency minus 50 mHz for example. Then the paral- busbar splitting is executed. leling conditions at Matimba are fulfilled latest after 20 An angle dependent splitting criterion serves as back- s. The course of paralleling is bumpfree. UP. Two signals are exchanged between Songo and ES- Other contingencies where busbar splitting is ex- KOM control centre for this purpose. ecuted are: (1) A release signal from Songo to Matimba indi- cates that the controls at Songo are ready to change (1) Multiphasefault son 220 kV linesclose to Songo from frequency control ( in isolated operation ) to which are not cleared in the first protection zone. The angle control ( in parallel AC / DC operation ). Recon- criterion is the positive sequence voltage at Songo. nection of the AC link while Cahora Bassa stays in fre- (2) Loss of the filters. Immediate busbar splitting quency control must be avoided as it woutd result in separates the AC loads from the DC poles which are loss of stability. the source of uncompensated harmonics now. (2) Closing of the AC link at Matimba is indicated (3) Complex disturbances and / or loss of relevant by a signal sent to the Songo controls. Consequently control functions. the control mode changes from interconnecting to For the dynamic isolation of AC and DC networks angle control and the busbar splitting logic which was 2-cycle breakers are considered. blocked during isolated operation (frequency control) is reactivated. CONTROL MODES TELECOMMUNICATION REQUIREMENTS Angle or Frequency Control FOR STABILITY CONTROL The normalcontrol mode in parallel AC/ DC operation The stability controls at Songo are based on locally is angle control. available control variables as far as possible. Howev- In isolated operation ( no AC / DC interconnection ) er, the following variables and signalsfrom remote lo- the frequency at Songo is controlled to 50 Hz with re- cations are required at Songo: spect to the small steady state frequency band of the - Voltage angle from Apollo ( 1500 km distance ) filtersof 50f 0.5 Hz. The controller is of the PID type. with a time stamp. The arriving angle is combined The switching state of the AC link ( parallel to DC or at Songo with the according local angle measure- not ) is derived from the switching states of the bus ment in order to get the angle difference Songo - coupler at Songo, the Bindura line and the Matimba Apollo. line. The first 2 states are available as local criteria. - Matimba line switching state. The last one must be signalled from ESKOM control From Songo to ESKOM control centre a release sig- centre to Songo. nal is transmitted which indicates that the Matimba line can be closed for AC / DC parallel operation. Interconnecting Control Interconnecting control is an auxiliary control mode BACKUP FUNCTIONS FOR for transition from Cahora Bassa island operation ( STABILITY CONTROL with frequency control ) to interconnected operation Songo - Zimbabwe - ESKOM ( with angle control ). Stability of the AC link is maintained by control de- The purpose of the interconnecting control mode is vices. Therefore backup functions are provided in or- - to make the interconnecting procedure easier derto minimize the riskof instability asfaraspossible. for the operator Theangle from Apolloto Songo istransmitted on2dif- - to reduce the risk of malfunction ( loss of rotor ferent paths ( 2 DC lines ). Plausibility controls are angle stability ). provided. 151

However, during commutation failures both paths In a number of study cases the generator connected may be distorted by flash over in the earth wire which to Bus-AC ( reference Fig. 1 ) wassubjected toslowly servesascarrier. In this case theprefault angle differ- increasing power oscillations after the busbars are ence is frozen and the frequency at Songo is used for split. After busbar splitting it is no longer possible to power oscillation damping. damp these power oscillations by modulation of DC If the indication of the switching state of the Matimba and / or braking resistor power as these are con- lineis uncertain,thestate is interpretedasclosed.As- nected to Bus-DC. suming that the real state is open, this would result in However, as the oscillations develop relatively slowly, erroneous operation in angle control which does not a reduction of the turbine power of the generator on affect stability. Bus-AC will result in damping of the power oscilla- tions. The opposite, erroneous operation in frequency con- trol (assuming open AC link) while the link is in fact closed would result in instability and must therefore be prevented. This may happen if during the inter- GENERAT0 R TRIP P IIN G connection procedurethe line-on-signal from Matim- ba does not arrive at Songo. As stability should not Generatortripping is the last remedy to keep the pow- dependon a switching statesignal receivedfrom a re- er balance. It should occur only in rare contingencies mote location a backup function is provided. ( e.g. if a braking resistlor is not available). The num- Before the AC link becomes unstablethe power in the ber of generators to trip is calculated from the power Bindura line will decrease in a characteristic way. This balance control. If more than 1 generator should be is used as a local criterion to change the control mode tripped, only 1 generator is tripped after elapse of 190 immediately from interconnectionto angle. ms and the busbars are split instead of tripping more generators. The rest of generators selected for trip- It is not practical to check the controls for any fault in ping are prepared for frequency dependent tripping the network, any disturbance of equipment and any where the tripping frequencies are set dynamically combination thereof. such that the upper frequency limit for tripping of the Adequate controls have therefore been provided and filters is not violated. This procedure was found as a checked only for contingencies with a relatively high compromise of the following requirements: probability of occurrence or where application of suit- - minimize generator tripping able remedies is easy. - prevention againtst spurious tripping In all other cases where there may be a hazard to sta- - selective tripping bility preventive busbar splitting is executed. This - avoid filter tripping eliminatesstability problems betweenAC and DC link - avoid frequent busbar splitting. presupposed that the splitting occurs in time. The number of such events is assumed to be small.