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Control Measures to Ensure Dynamic Stability of the Cahora Bassa Scheme and the Parallel Hvac System

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Control Measures to Ensure Dynamic Stability of the Cahora Bassa Scheme and the Parallel Hvac System 146 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 Germany Canada Portugal South Africa 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 thyristor 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 Cahora Bassa - \ 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) ESKOM 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.
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