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Topic E: Power System Dynamic Performance
Copyright © IFAC Power Generation Distribution and Protection, Pretoria, South Africa, 1980.
TOPIC E POWER SYSTEM DYNAMIC PERFORMANCE
Session E I-Subsynchronous Resonance
SUB SYNCHRONOUS RESONANCE WITH REFERENCE TO KOEBERG AND THE SERIES-COMPENSATED TRANSMISSION SYSTEM by M F Hadingham (South Africa)
6. No concentration on the shaft itself where the problems are more or less independent of the series compensation. 7. The advantage of a combined analog digital model compared with a comprehensive computer program representing the whole system.
J Werakso (South Africa)
Has any of the large auxiliary MV motors installed at Koeberg a natural frequency which could make it susceptible to the sub synchronous resonance effect: has any such a case been considered?
8. Plan for measuring on the actual shaft after commissioning of the turbo-generator unit.
Author's reply
9. A package of realistic counter-measures should be selected if necessary and economically justified.
Motors are not as susceptible to SSR problems as turbine generators because they do not have complicated shaft systems and thus do not have the many torsional resonant frequencies characteristic of a turbine generator. For this reason no consideration was given to motors in the SSR studies.
Authors' reply The authors agree that the laboratory system does not accurately represent the Koeberg turbo-generator system, but then the laboratory system was constructed in the first place to corroborate the computer calculations of the Koeberg system, in which it succeeded very well.
Comment by M S Baldwin (U S A) With regard to the question about probable SSR with other smaller generating units in the Koeberg area, other generating units in the same plant or close electrical proximity to the subject generator will, if in service, reduce the probability of SSR. In one proposed installation in the USA four otherwise identical generator units were to be designed with slightly different torsional natural frequencies.
The damping matrix D in equation Eq.(6) is diagonal as was stated in the paper, but was not shown in full since it has been published in the Transactions of the SAIEE Vol 70, Part 11, page 278. We do not agree that the effect of series capacitors has been excluded since most of the results shown in the paper in fact consider the effect of different values of series capacitor compensation. The paper did not report on the use of supplementary exciter control signals to stabilise the sub synchronous resonance because this forms the subject of a separate paper which is in the process of publication. We agree with the last question that a package of counter-measures should really be selected since it is quite clear that no single measure such as filters, auxiliary exciter control, spark gaps across capacitors etc. will in itself be successful. The sub synchronous resonance problem could, of course, be avoided if one were able to concentrate on changing the mechanical shafting system by, for example, choosing different inertias and stiffnesses, but by
N Fahlen (Sweden) Please comment on the following: 1.
Simplified scheme.
2.
Damping matrix excluded.
3.
Series capacitors operations excluded.
4.
Exact analog modelling not possible.
5. Supplementary exciter control not considered.
ACPG _ W
537
538
Topic E
the time this problem was fully appreciated on this particular system it was too late to persuade the manufacturer to change his design. The main reason for an analog model such as the one described in this paper is to corroborate the digital computer calculations. This is the first report to appear in the world technical press which describes laboratory measurements of sub synchronous resonance and which compares measured values with predicted ones. In general the results show that by not mathematically representing the damping sufficiently accurately, the predicted values are in fact failing to safety since they are forecasting instability at lower values of compensation than was experienced on the laboratory system. A further advantage of such a laboratory system not mentioned in this paper is its suitability for investigating the use of special auxiliary excitation control signals and any other controllers which one would like to implement on such a system. For example, if one used state variable feedback designed according to the theory of optimal control it shows that in theory certain gains could be used to stabilise the system, but once these are implemented one finds that due to practical problems, limitations, and noise, these theoretically designed controllers do not perform all that successfully. Such problems can only be appreciated on a real physical system and could not be experimented with on a large turbo-generator in a power station but only on a laboratory scale. We agree that it would be interesting and useful if it were possible to take measurements on the shafting of the turbo-generator unit at Koeberg after commissioning. However, such a decision rests with the Electricity Supply Commission, South Africa. CENTRALISED VERSUS DECENTRALISED DAMPING OF LOW-FREQUENCY OSCILLATIONS IN A LARGE POWER NETWORK by C Brasca, G Guardabassi, A Locate11i and N Schiavoni (Italy)
a single operating condition e.g. peak load. From the designer's point of view it may be desirable for each operating condition. He can then choose a set of gain settings that satisfy the range of operating conditions. Can such a range of feedback gain settings be obtained from the author's method or must it be done by a total and error approach? Can the method be adapted to include more than one type of stabilising signal on a selected machine? Authors' reply The design method proposed in the paper is not intended to supply a range of feedback gains which are satisfactory under a given set of ~ifferent operating conditions of the network. However, for the particular class of control problems considered here, the changes in the parameters of the process model can be computed in a rather simple way - for instance through a load-flow analysis which has to be periodically performed anyway in connection with the solution of other problems, such as optimal despatching. As a result, in view of the not too burdensome computational effort which is required by the optimisation procedure suggested here, there is some evidence that an acceptably complex adaptation algorithm for the controller parameters can be set up. From another point of view, moreover, it is conceivable to consider the changes in the operating conditions of the network as raising a tolerance issue concerning the process parameters. This could be conveniently tackled by suitably modifying the optimisation procedure adopted here according to an algorithm like the one proposed by Po1ak and Sangiovanni Vincente1li (1978).
As for the second question, the answer is definitely "Yes" as the controller can be designed in such a way as to allow any prespecified set of the process inputs (control variables of the various generators) to depend upon any prespecified set of the process outputs (measured variables) according to any selected information pattern.
M J Gibbard (Australia)
Reference:
The paper makes an interesting and useful contribution to the co-ordinated damping of system oscillations using stabilising signals. The example relates to the derivation of feedback gain on settings for
Po1ak, E. and Sangiovanni Vincente11i (1978). On optimisation algorithms for engineering design problems with distributed constraints, tolerances and tuning Proc. Joint Automatic Control Conference.
Discussions
539
Session E 2-Analysis of Power System Dynamic Performance
DYNAMIC INTERACTION OF ELECTRICAL POWER PLANTS, LOADS AND TRANSMISSION NETWORK by E Welfonder and F Heilemann (Germany)
Authors' reply
M Scott (South Africa)
The power system simulation program has been implemented on the Control Data 6600 computer in the computing centre of the University of Stuttgart.
Could the authors please give some details on the implementation of the simulation and the machine on which it was implemented?
For the application considered in the paper a storage of 36 K was required. The ratio of simulation to real time was about 5.
TOPIC E POWER SYSTEM DYNAMIC PERFORMANCE
Session E I-Subsynchronous Resonance
SUB SYNCHRONOUS RESONANCE WITH REFERENCE TO KOEBERG AND THE SERIES-COMPENSATED TRANSMISSION SYSTEM by M F Hadingham (South Africa)
6. No concentration on the shaft itself where the problems are more or less independent of the series compensation. 7. The advantage of a combined analog digital model compared with a comprehensive computer program representing the whole system.
J Werakso (South Africa)
Has any of the large auxiliary MV motors installed at Koeberg a natural frequency which could make it susceptible to the sub synchronous resonance effect: has any such a case been considered?
8. Plan for measuring on the actual shaft after commissioning of the turbo-generator unit.
Author's reply
9. A package of realistic counter-measures should be selected if necessary and economically justified.
Motors are not as susceptible to SSR problems as turbine generators because they do not have complicated shaft systems and thus do not have the many torsional resonant frequencies characteristic of a turbine generator. For this reason no consideration was given to motors in the SSR studies.
Authors' reply The authors agree that the laboratory system does not accurately represent the Koeberg turbo-generator system, but then the laboratory system was constructed in the first place to corroborate the computer calculations of the Koeberg system, in which it succeeded very well.
Comment by M S Baldwin (U S A) With regard to the question about probable SSR with other smaller generating units in the Koeberg area, other generating units in the same plant or close electrical proximity to the subject generator will, if in service, reduce the probability of SSR. In one proposed installation in the USA four otherwise identical generator units were to be designed with slightly different torsional natural frequencies.
The damping matrix D in equation Eq.(6) is diagonal as was stated in the paper, but was not shown in full since it has been published in the Transactions of the SAIEE Vol 70, Part 11, page 278. We do not agree that the effect of series capacitors has been excluded since most of the results shown in the paper in fact consider the effect of different values of series capacitor compensation. The paper did not report on the use of supplementary exciter control signals to stabilise the sub synchronous resonance because this forms the subject of a separate paper which is in the process of publication. We agree with the last question that a package of counter-measures should really be selected since it is quite clear that no single measure such as filters, auxiliary exciter control, spark gaps across capacitors etc. will in itself be successful. The sub synchronous resonance problem could, of course, be avoided if one were able to concentrate on changing the mechanical shafting system by, for example, choosing different inertias and stiffnesses, but by
N Fahlen (Sweden) Please comment on the following: 1.
Simplified scheme.
2.
Damping matrix excluded.
3.
Series capacitors operations excluded.
4.
Exact analog modelling not possible.
5. Supplementary exciter control not considered.
ACPG _ W
537
538
Topic E
the time this problem was fully appreciated on this particular system it was too late to persuade the manufacturer to change his design. The main reason for an analog model such as the one described in this paper is to corroborate the digital computer calculations. This is the first report to appear in the world technical press which describes laboratory measurements of sub synchronous resonance and which compares measured values with predicted ones. In general the results show that by not mathematically representing the damping sufficiently accurately, the predicted values are in fact failing to safety since they are forecasting instability at lower values of compensation than was experienced on the laboratory system. A further advantage of such a laboratory system not mentioned in this paper is its suitability for investigating the use of special auxiliary excitation control signals and any other controllers which one would like to implement on such a system. For example, if one used state variable feedback designed according to the theory of optimal control it shows that in theory certain gains could be used to stabilise the system, but once these are implemented one finds that due to practical problems, limitations, and noise, these theoretically designed controllers do not perform all that successfully. Such problems can only be appreciated on a real physical system and could not be experimented with on a large turbo-generator in a power station but only on a laboratory scale. We agree that it would be interesting and useful if it were possible to take measurements on the shafting of the turbo-generator unit at Koeberg after commissioning. However, such a decision rests with the Electricity Supply Commission, South Africa. CENTRALISED VERSUS DECENTRALISED DAMPING OF LOW-FREQUENCY OSCILLATIONS IN A LARGE POWER NETWORK by C Brasca, G Guardabassi, A Locate11i and N Schiavoni (Italy)
a single operating condition e.g. peak load. From the designer's point of view it may be desirable for each operating condition. He can then choose a set of gain settings that satisfy the range of operating conditions. Can such a range of feedback gain settings be obtained from the author's method or must it be done by a total and error approach? Can the method be adapted to include more than one type of stabilising signal on a selected machine? Authors' reply The design method proposed in the paper is not intended to supply a range of feedback gains which are satisfactory under a given set of ~ifferent operating conditions of the network. However, for the particular class of control problems considered here, the changes in the parameters of the process model can be computed in a rather simple way - for instance through a load-flow analysis which has to be periodically performed anyway in connection with the solution of other problems, such as optimal despatching. As a result, in view of the not too burdensome computational effort which is required by the optimisation procedure suggested here, there is some evidence that an acceptably complex adaptation algorithm for the controller parameters can be set up. From another point of view, moreover, it is conceivable to consider the changes in the operating conditions of the network as raising a tolerance issue concerning the process parameters. This could be conveniently tackled by suitably modifying the optimisation procedure adopted here according to an algorithm like the one proposed by Po1ak and Sangiovanni Vincente1li (1978).
As for the second question, the answer is definitely "Yes" as the controller can be designed in such a way as to allow any prespecified set of the process inputs (control variables of the various generators) to depend upon any prespecified set of the process outputs (measured variables) according to any selected information pattern.
M J Gibbard (Australia)
Reference:
The paper makes an interesting and useful contribution to the co-ordinated damping of system oscillations using stabilising signals. The example relates to the derivation of feedback gain on settings for
Po1ak, E. and Sangiovanni Vincente11i (1978). On optimisation algorithms for engineering design problems with distributed constraints, tolerances and tuning Proc. Joint Automatic Control Conference.
Discussions
539
Session E 2-Analysis of Power System Dynamic Performance
DYNAMIC INTERACTION OF ELECTRICAL POWER PLANTS, LOADS AND TRANSMISSION NETWORK by E Welfonder and F Heilemann (Germany)
Authors' reply
M Scott (South Africa)
The power system simulation program has been implemented on the Control Data 6600 computer in the computing centre of the University of Stuttgart.
Could the authors please give some details on the implementation of the simulation and the machine on which it was implemented?
For the application considered in the paper a storage of 36 K was required. The ratio of simulation to real time was about 5.