METHOD AND DEVICE FOR REDUCING VOLTAGE FLUCTUATIONS IN A SUPPLY NETWORK

Abstract

Voltage fluctuations in a supply network are intended to be reduced efficiently and cost-effectively. According to the method, a current flowing into a load is measured and a corresponding current measurement signal is obtained. The voltage fluctuations are reduced with the aid of a TCR, which constitutes a thyristor-controlled reactance, and a VSC, which constitutes a voltage source converter. The current measurement signal or a corresponding variable is divided into a first portion and a second portion depending on a predefined absolute limit value. The TCR is controlled on the basis of the first portion and the VSC is controlled on the basis of the second portion. Alternatively, the TCR can be controlled with the load current measurement signal and the VSC can be controlled with a sum of the load current measurement signal and a TCR current measurement signal.

Claims

1-16. (canceled)

17. A method for reducing voltage fluctuations in a supply network which are caused by operating a load from the supply network, the method comprising: measuring a current between the load and the supply network to thereby acquire a current measurement signal; reducing the voltage fluctuations with the aid of a thyristor-controlled reactance (TCR), and reducing the voltage fluctuations with the aid of a voltage source converter (VSC); dividing the current measurement signal or a variable corresponding thereto into a first portion and a second portion based on a predefined absolute limit value; controlling the TCR on a basis of the first portion of the current measurement signal; and controlling the VSC on a basis of the second portion of the current measurement signal.

18. The method according to claim 17, wherein the predefined absolute limit value represents a cut-off frequency.

19. The method according to claim 18, wherein all frequencies of the first portion lie below the cut-off frequency and all frequencies of the second portion lie above the cut-off frequency.

20. The method according to claim 18, wherein the cut-off frequency lies between 0 and 8 Hz.

21. The method according to claim 20, wherein the cut-off frequency lies between 1 and 5 Hz.

22. The method according to claim 17, wherein the predefined limit value represents an intensity of the current or a power.

23. The method according to claim 22, which comprises forming the first portion with components of the current signal or of the variable corresponding thereto which are above the predefined limit value.

24. A compensation device for reducing voltage fluctuations in a supply network which are caused by operating a load from the supply network, the compensation device comprising: a measuring device for measuring a current between the load and the supply network for acquiring a corresponding current measurement signal; a thyristor-controlled reactance (TCR) for reducing the voltage fluctuations; a voltage source converter (VSC) for reducing the voltage fluctuations; a splitter device for dividing the current measurement signal or a variable corresponding thereto into a first portion and a second portion on a basis of a predefined absolute limit value; a first control device for controlling said TCR on a basis of the first portion of the current measurement signal; and a second control device for controlling said VSC on a basis of the second portion of the current measurement signal.

25. The compensation device according to claim 24, wherein said splitter device comprises a frequency splitter, and the predefined absolute limit value is a cut-off frequency of said frequency splitter.

26. The compensation device according to claim 24, wherein said splitter device comprises a limiter configured to use the predefined limit value to limit a control value for said VSC.

27. A method for reducing voltage fluctuations in a supply network which are caused by operating a load from the supply network, the method comprising: measuring a current between the load and the supply network to acquire a corresponding first current measurement signal; reducing the voltage fluctuations with the aid of a thyristor-controlled reactance (TCR); reducing the voltage fluctuations with the aid of a voltage source converter (VSC); measuring a current between the TCR and the supply network to acquire a corresponding second current measurement signal; controlling the TCR on a basis of the first current measurement signal; and controlling the VSC on a basis of the first and second current measurement signals.

28. The method according to claim 27, which comprises controlling the VSC on a basis of a sum of the first and second current measurement signals.

29. The method according to claim 27, which further comprises measuring a voltage of the supply network and also reducing the voltage fluctuations on a basis of the measured voltage.

30. A compensation device for reducing voltage fluctuations in a supply network which are caused by operating a load from the supply network, the compensation device comprising: a first measuring device for measuring a current between the load and the supply network to acquire a corresponding first current measurement signal; a thyristor-controlled reactance (TCR) for reducing the voltage fluctuations; a voltage source converter (VSC) for reducing the voltage fluctuations; a second measuring device for measuring a current between the TCR and the supply network to acquire a corresponding second current measurement signal; a first control device for controlling the TCR on a basis of the first current measurement signal; and a second control device for controlling the VSC on a basis of the first and second current measurement signals.

31. The compensation device according to claim 30, which further comprises an adder connected upstream of said second control device, and wherein the VSC is controlled on a basis of a sum of the first and second current measurement signals.

32. The compensation device according to claim 30, which further comprises a filter circuit connected in the supply network, said filter circuit being connected to interact with said TCR and said VSC in order to reduce the voltage fluctuations.

33. The compensation device according to claim 32, wherein said filter circuit is a passive filter, acting capacitively and being tuned to said TCR and said VSC.

Description

[0061] The present invention is now explained in more detail using the accompanying drawings, in which:

[0062] FIG 1 shows a supply system having a disruptive load, compensation means and other loads;

[0063] FIG. 2 shows probability distributions for voltage fluctuations above a predetermined voltage fluctuation;

[0064] FIG. 3 shows a control system having a combination of a TCR and a VSC;

[0065] FIG. 4 shows control system having a TCR for reducing voltage fluctuations and a VSC for reducing harmonic disturbances;

[0066] FIG. 5 shows a circuit diagram of control of the TCR and the VSC according to the invention by dividing the activity of the TCR and the VSC on the basis of the frequency of the voltage fluctuations;

[0067] FIG. 6 shows a graph of probabilities of voltage fluctuations during control operations according to the invention;

[0068] FIG. 7 shows an alternative embodiment to FIG. 5 with a limiter;

[0069] FIG. 8 shows a further development of the exemplary embodiment from FIG. 7 with a further limiter; and

[0070] FIG. 9 shows an alternative embodiment to FIG. 5 with TCR current measurement.

[0071] The exemplary embodiments described in more detail below are preferred embodiments of the present invention. For the description of these embodiments, reference is additionally made to the above explanations with respect to FIGS. 1 to 4. Only the differences are highlighted in more detail below.

[0072] The present invention provides a plurality of different methods for operating the TCR and the VSC (or the SVC and the STATCOM) in a coordinated manner. These methods relate to situations in which the load requirements are above the compensation ability of a single STATCOM and are below the compensation ability of two STATCOMs. These methods are naturally not restricted to these situations alone.

[0073] According to the first example according to the invention which is represented, in principle, in FIG. 5, the compensation requirement is divided into a first, slow part lt, which is to be compensated for by the TCR 8, and a remaining, second, fast part st, which is to be compensated for by the VSC 10. The division is carried out by a splitter device 22, 23 which comprises a low-pass filter (TP) 22 here. The signal at the output of the low-pass filter 22 has only the slow or low-frequency portions lt of the current measurement signal. The signal lt is used as the input signal for the TCR control unit 17 which therefore uses only the slow portions of the current measurement signal, in addition to the voltage measurement signal from the voltmeter 19 (cf. descriptions with respect to FIGS. 3 and 4), to control the TCR 8. The slow portion lt at the output of the low-pass filter 22 is also supplied to the subtractor 23 which subtracts this slow portion from the entire current measurement signal, thus resulting in the fast portion st. This fast portion st is used to control the VSC control unit (VCO) 18 which likewise receives the voltage measurement signal from the voltmeter 19. The VSC control unit 18 then controls the VSC 10 on the basis of the fast portion st of the current measurement signal.

[0074] The cut-off frequency of the splitter device is preferably such that current or voltage fluctuations of 9 Hz and above are represented in the second portion, namely the fast portion st. Accordingly, the cut-off frequency could be 5 Hz, for example.

[0075] During operation of the compensation device illustrated in FIG. 5 and during the corresponding method, the fast responsiveness of the VSC 10 is therefore used to compensate for the critical 9 Hz load fluctuations. Accordingly, the TCR 8 is used only for slow load fluctuations. This means that, on account of its slow triggering and extinction behavior, the TCR 8 is used only to compensate for slow fluctuations in the reactive power.

[0076] An estimation of the performance of the method and of the compensation device according to FIG. 5 with the frequency division can be gathered from FIG. 6. The probability of a voltage fluctuation in a fictitious system being greater than a particular U and of no compensation taking place is revealed by the curve 12, as in FIG. 2. If compensation now takes place according to the above method with frequency splitting, that is to say using a TCR, a VSC, a frequency splitter and a passive filter circuit, the estimated probability is in the region 24. Accordingly, an estimated bandwidth 25 results for the new method. In principle, it would naturally be desirable if the same performance as that of two STATCOMs were achieved by the combined operation of the SVC and the STATCOM. The probability of voltage fluctuations in the event of compensation using two STATCOMs results from curve 26. According to this graph from FIG. 6, although the probabilities of voltage fluctuations with the compensation according to the invention are considerably below those in systems without compensation, they are still slightly above the probabilities during compensation using two STATCOMs.

[0077] The above probability curves and, in particular, the band 24 and the bandwidth 25 also approximately apply to the second method described below and the corresponding second compensation device in the exemplary embodiments according to FIGS. 7 and 8.

[0078] With respect to the description of FIGS. 7 and 8, reference is again made to the description of all preceding figures where the same elements are mentioned. The following description concentrates only on the differences. In the second method, the VSC 10 compensates for the reactive load in the normal situation. During these phases, the TCR 8 opposes the capacitively acting passive filter circuit 15 with a constant inductive power. The TCR 8 provides compensation only when the voltage source converter VSC 10 reaches its output limit.

[0079] Specifically, this is achieved in the compensation device according to FIG. 7 by virtue of the fact that the current measurement signal from the ammeter 16 is made available to the VSC control unit 18. On the basis of said current measurement signal and on the basis of the voltage measurement signal from the voltmeter 19, the VSC control unit 18 generates a control signal which is supplied to a splitter device 27, 28. The splitter device comprises a limiter 27 here. The control signal from the VSC control unit 18 represents the current measurement signal or a corresponding variable. If the current measurement signal is therefore high, this is accordingly represented in the control signal from the VSC control unit 18. The limiter 27 provides limitation and outputs a limit value predefined by it when the value of the control signal exceeds this limit value. The VSC 10 is then controlled only using the limit value and runs at the intended maximum power.

[0080] The splitter device also comprises a subtractor 28 which is supplied with the control signal from the VSC control unit (VCO) 18 and with the output value from the limiter 27. If the control value from the VSC control unit 18 is above the limit value, the difference between the two signals is positive and this difference value is supplied to the TCR control unit (TCO) 17 for further control of the TCR 8 also on the basis of the voltage measurement signal from the voltmeter 19. In contrast, if the value of the control signal from the VSC control unit 18 is less than the limit value of the limiter 27, the limiter 27 is virtually ineffective and controls the VSC 10 using the signal from the VSC control unit 18. The output signal from the subtractor 28 then has the value 0 since the input signal and the output signal of the limiter 27 are the same. Accordingly, the TCR 8 is controlled such that it does not compensate for any reactive power. In this case, the reactive power is therefore compensated for completely by the VSC 10.

[0081] In one preferred exemplary embodiment, the capacitively acting passive filter circuit 15 is designed in such a manner that it can completely counteract the inductive output power of the TCR 8 and the VSC 10, the constant inductive power of the TCR 8 being considered for the normal situation. If the VSC 10 is now at its capacitive limit, the TCR 8 reduces its inductive power, which eases the capacitive requirement imposed on the VSC 10. The interaction between the VSC 10 and the TCR 8 and vice versa is fluid. In this second method, the VSC 10 keeps the voltage fluctuations low most of the time, and the design of the TCR 8 or the SVC can be accordingly low.

[0082] In order to increase the reliability of the control system for reducing the voltage fluctuations, the system from FIG. 7 can be optimized with a further limiter, as illustrated in FIG. 8. Reference is generally made here again to the description of the system from FIG. 7. In particular, an additional VSC control unit 18 is provided in the exemplary embodiment from FIG. 8 and also receives, as the input signal, the current measurement signal from the ammeter 16 and the voltage measurement signal from the voltmeter 19, like the VSC control unit 18. The output signal from VSC control unit 18 is again supplied to the limiter 27 here and the possibly limited signal is used to control the VSC 10. The parallel VSC control unit 18 is now provided for the purpose of compensating for the peak reactive powers, the output signal from which control unit is supplied both to a second limiter 27 and to a subtractor 28. The output signal from the limiter 27 is subtracted from the output signal from the VSC control unit 18 in the subtractor 28, and the resulting difference signal is used to control the TCR control unit 17 or the TCR 8. A power is therefore retrieved from the TCR 8 only when the value of the control signal from the VSC control unit 18 is above the limit value of the limiter 27. The limit values of the two limiters 27 and 27 are preferably the same, but need not be. In this case too, the VSC 10 therefore manages the base load, while the TCR 8 assumes the peak load. However, the TCR and the VSC are controlled here by separate control units, as result of which the reliability can be increased.

[0083] According to the invention, the coordination of the operation of the SVC and the STATCOM is therefore optimized to the effect that the highest performance is achieved. at the lowest costs. The compensation performance of a large STATCOM is better than that of a conventional SVC system. Solutions which are based only on STATCOMs, however, are much more expensive. If a performance between one STATCOM and two STATCOMs is sufficient, the most cost-effective solution is to combine the SVC and the STATCOM. However, uncoordinated operation would reduce the performance.

[0084] Another exemplary embodiment for implementing the method according to the invention and the compensation device according to the invention is shown in FIG. 9. With respect to the description of FIG. 9, reference is again made to the description of all preceding figures where the same elements are mentioned. The following description concentrates only on the differences. In the further method according to FIG. 9, the VSC 10 compensates for that which the TCR does not manage to compensate for.

[0085] The compensation requirement is al above the performance of the TCR. Therefore, the VSC must always assume the compensation requirement which is not managed by the TCR. For this purpose, like in the preceding examples, a current measurement signal or current measured value is obtained from the ammeter 16, which measurement signal or measured value represents the current between the load 1 and the supply network 2 and is used here as the first current measurement signal (load current measurement signal). The TCR control unit 17 receives the first current measurement signal in unchanged form here as the current measurement signal. It therefore receives here, as input signals, the first current measurement signal directly from the ammeter 16 and the voltage measurement signal directly from the voltmeter 19.

[0086] In contrast, the VSC control unit 18 receives, as the current measurement signal, a sum of the first current measurement signal (load current measurement signal) from the first ammeter 16 and a second current measurement signal (TCR current measurement signal) from a second ammeter 29 which measures a current between the supply network 2 and the TCR 8. For this purpose, an adder 30 adds the first current measurement signal and the second current measurement signal and delivers the sum signal to the VSC control unit 18. The latter also obtains the voltage measurement signal from the voltmeter 19.

[0087] The TCR control system therefore compensates for the reactive load as well as it can. The TCR 8 opposes the capacitively acting passive filter circuit 15 with a corresponding inductive power. The VSC compensates for the remaining reactive power which the TCR does not manage to compensate for. For this purpose, the sum of the first current measurement signal (load current) and the second current measurement signal (TCR current) is supplied to the VSC control system. The compensation current from the VSC 10 then corresponds to the difference between the total (capacitive) filter current, which flows between the passive filter 15 and the supply network 2, and the (inductive) load current together with the (inductive) TCR current. The VSC must therefore only correct the difference which was not managed by the TCR 8. The performance of the method again falls into the band 24 from FIG. 6.