METHOD AND DEVICE FOR CONTROLLING AN ELECTRIC VOLTAGE

Abstract

A method and a device control a voltage of a power grid by use of a reactive power device connected to the power grid. The method includes: determining a change over time of a previous voltage of the power grid, determining a change over time of a reactive power previously output into the power grid by the reactive power device, and subsequently outputting reactive power into the power grid by the reactive power device, which reactive power is determined in dependence on the changes over time.

Claims

1-10. (canceled)

11. A method for controlling an electric voltage of a grid by a reactive power device which is connected to the grid, which method comprises the steps of: determining a first change over time of a previous voltage on the grid; determining a second change over time of a previous reactive power previously output into the grid by the reactive power device; and subsequently outputting reactive power into the grid by means of the reactive power device, the reactive power being determined in accordance with the first and second changes over time.

12. The method according to claim 11, wherein the outputting of the reactive power which is output subsequently by the reactive power device comprises the further substeps of: setting at least one controller parameter of a controller of the reactive power device in accordance with the first and second changes over time, wherein the controller receives a deviation of the previous voltage from a target value as an input; outputting by the controller a control value for the reactive power to a converter of the reactive power device; generating the reactive power by the converter and injecting the reactive power into the grid.

13. The method according to claim 12, wherein the setting of the at least one controller parameter further comprises the following substeps: determining a previous short-circuit power on the grid, in accordance with the first and second changes over time; and determining the at least one controller parameter on a basis of the previous short-circuit power thus determined.

14. The method according to claim 13, wherein the previous short-circuit power of the grid is determined in accordance with a respective magnitude and/or symbol of first changes over time of the previous voltage on the grid and the second change over time in the previous reactive power output by the reactive power device.

15. The method according to claim 13, wherein the previous short-circuit power of the grid is determined in accordance with a previous relationship of first changes over time in a system voltage with the second change over time in the previous reactive power output of the reactive power device, by an application of a determination logic which is determined or verified on the basis of a simulation.

16. The method according to claim 15, wherein the determination logic further comprises the following substeps of: weighting of the previous relationship relative to a further previous short-circuit power which is higher, where a greater of at least one of the magnitudes of the first changes over time in a previous system voltage and/or of the second change over time in the previous reactive power previously output by the reactive power device; and/or employment of the previous relationship or a weighted previous relationship for the determination of the previous short-circuit power, where a magnitude of the first changes over time in the previous system voltage and/or of the second change over time in the previous reactive power previously output by the reactive power device exceeds a respective threshold value; and/or where a symbol of the first change over time in the previous system voltage and a symbol of the second change over time in the previous reactive power previously output by the reactive power device are the same, wherein the reactive power output by the reactive power device is subject to a rising valuation, in an event of an increasing injection of capacitive reactive power.

17. The method according to claim 16, wherein the previous short-circuit power KSL_previous, where Q/V>0, is determined as follows:
KSL_previous=((1w)*KSL_further-previous+w*Q_previous/V_previous)/2 where: V_previous is the first change over time in the previous voltage on the grid; Q_previous is the second change over time in the previous reactive power previously output by the reactive power device; KSL_previous is the previous short-circuit power of the grid; KSL_further-previous is the further previous short-circuit power of the grid; and w is a weighting factor between zero and one.

18. The method according to claim 17, wherein the Q0, V0 are threshold values, and: w=0, where V<V0 and/or Q<Q0, w=1, where V>=V0 and/or Q>=Q0, or where w is proportional to (QQ0).

19. The method according to claim 11, wherein: the first and second changes over time are determined in a time interval which lies between 100 s and 10 ms; the method is repeated cyclically; and the converter comprises a switch mode voltage converter.

20. The method according to claim 12, wherein: the at least one controller parameter is controller amplification; and the converter is a reactive power compensation installation of the reactive power device.

21. The method according to claim 13, wherein the at least one controller parameter determined on the basis of the previous short-circuit power is further determined in consideration of an influence of other voltage controllers which are active in the grid.

22. The method according to claim 13, wherein the previous short-circuit power of the grid is determined in accordance with a respective magnitude and/or symbol of first changes over time of the previous voltage on the grid and the second change over time in the previous reactive power output by the reactive power device, and on a basis of a further previously determined short-circuit power of the grid.

23. The method according to claim 15, wherein parameters of the determination logic are maintained constant over a short-circuit power range from 50 MVA to 20 000 MVA.

24. The method according to claim 11, wherein: the first and second changes over time are determined in a time interval which lies between 500 s and 5 ms; the method is repeated cyclically; and the converter comprises a switch mode voltage converter.

25. The method according to claim 11, which further comprises setting a speed of control in consideration of other voltage controllers which are present in the grid.

26. A device for controlling an electric voltage of a grid, the device comprising: a reactive power device being connectable to the grid; a controller configured to determine a first change over time in a previous voltage on the grid, and to determine a second change over time in a previous reactive power which has previously been output into the grid by said reactive power device; and said reactive power device configured to output a reactive power into the grid which is determined in accordance with the first and second changes over time.

Description

[0051] Further advantages and features of the present invention proceed from the following exemplary description of currently preferred forms of embodiment. The individual figures in the drawing attached to the present application are to be considered as schematic only, and are not true to scale.

[0052] FIG. 1 shows a schematic illustration of a device for controlling an electric voltage, according to one form of embodiment of the present invention;

[0053] FIGS. 2 to 8 illustrate graphs of electrical variables which are determined, considered or adjusted, according to the form of embodiment of the present invention;

[0054] FIGS. 9 to 15 illustrate graphs of electrical variables which are determined, considered or adjusted, according to the form of embodiment of the present invention;

[0055] FIGS. 16 to 22 illustrate graphs of electrical variables which are determined, considered or adjusted, according to the form of embodiment of the present invention;

[0056] FIGS. 23 to 27 illustrate graphs of electrical variables which are determined, considered or adjusted, according to the form of embodiment of the present invention;

[0057] FIGS. 28 and 29 illustrate graphs of the output of reactive power or the system voltage, which are considered or adjusted according to forms of embodiment.

[0058] The device 1, which is schematically illustrated in FIG. 1, for controlling an electric voltage of a grid 3 according to one form of embodiment of the present invention comprises a controller 5, which is configured to determine a change over time in a previous voltage 7 on the grid 3, and to determine a change over time in a reactive power which has previously been output into the grid 3 by a reactive power device 9, as a measured value 11. The device 1 further comprises the reactive power device 9, which is connectable to the grid 3 and is particularly connected by means of a line 13, and is configured to output a reactive power 15 into the grid 3 via the connecting line 13 which is determined in accordance with changes over time in the previous voltage 7 and in the previous reactive power 11 which has been output into the grid 3.

[0059] The device 1 is particularly configured to execute a method for controlling an electric voltage of a grid by means of a reactive power device according to one form of embodiment of the present invention. By means of a measuring device or a measuring sensor 17, the voltage 7 on the grid is measured at a plurality of previous time points, particularly with a scanning frequency between 10 s and 10 ms. From the plurality of previously measured voltages 7, the controller 5, particularly a determination module 19, determines the change over time in the previous voltage 7 on the grid 3. By means of a further measuring probe or a measuring device 21, reactive power or the previous reactive power output into the grid 3 by the reactive power device 9 at a plurality of previous time points is also measured, and is fed to the module 19 of the controller 5 as a measured value 11. On the basis of changes over time, the module 19 determines at least one controller parameter of the controller 23, and feeds this at least one controller parameter to the controller 23, e.g. a PID controller, by means of a signal 24.

[0060] As an input, the controller 23 receives a deviation 25 between a target voltage 27 (which e.g. is fed in from the exterior or can be saved in a memory of the controller 5) and the previous voltage 7 on the grid 3, wherein the deviation 25 is determined by means of a differential element 26. On the basis of the deviation 25, the controller 23 calculates a reference value 29 for reactive power, and feeds said reference value to a converter 31 of the reactive power device 9. The converter 31 can further comprise a gate driver circuit which generates gate driver signals for power transistors on the basis of the reference value 29 for reactive power.

[0061] In other forms of embodiment, a thyristor-controlled installation can be employed, which can comprise a thyristor stack with an inductance (a variable) arranged down-circuit, or a capacitor bank (SVC Classic engineering).

[0062] By the appropriate actuation of power transistors and/or thyristors, the converter 31 can generate and output a reactive power 15 into the grid which is defined according to the reference value 29 for reactive power. The requisite reactive power can be delivered within milliseconds.

[0063] The grid 3 can comprise e.g. a transmission line 4, for example a high-voltage transmission line. The device 1 can be arranged at various points within the grid 3, in order to permit the execution of voltage control in various regions of the grid 3.

[0064] The control structure illustrated in FIG. 1 and proposed according to this form of embodiment of the present invention can operate passively wherein, during the determination of the short-circuit power of the grid, an active variation of the output power (e.g. the reactive power output 15) is no longer necessary, but is still possible. The measured system voltage 7 and the measured power 11 of the operational equipment (current, power or reactive power) of the installation can be differentiated (in order to determine a change per unit of time). The relationship between these two changes over time can be employed as a measure of the short-circuit power of the electricity transmission grid (e.g. the grid 3). The short-circuit power thus calculated can be employed subject to specific criteria, but can also be discarded under specific conditions. A decision-making logic or calculation logic, depending upon the form of embodiment of the present invention, can be employed for this purpose. Other options can also be implemented.

[0065] According to forms of embodiment of the present invention, the decision-making or determination algorithm can incorporate two criteria:

[0066] The first criterion considers the magnitude of two dispersions, i.e. the dispersion of the voltage 7 on the grid and of the power 11 which is output or has been outputted by the reactive power device 9 into the grid. According to the calculation algorithm or logic (particularly fuzzy logic), it is decided whether the change in output power is caused by the change in voltage. If the output power of a reactive power compensation installation is predominantly capacitive, the system voltage must rise. If the output power is predominantly inductive, the system voltage must fall. Thus, according to one form of embodiment of the present invention, only the direction of change can or need be critical, but not the absolute value of the output power.

[0067] According to one form of embodiment of the present invention, fuzzy logic is employed for the determination of these two criteria. The input values are the change in power (the change over time in the voltage or the power output 15, or in the resulting measured value 11) and the change in voltage on the grid (i.e. the change over time in the system voltage 7). These values are then applied to formulae which state whether the input value rises or falls, and whether or not the magnitude thereof is significantly greater than zero. These values can then be mutually combined thereafter. A number between zero and one can thus be obtained, which can be considered as a weighting factor. A weighting value of one can indicate that the short-circuit value of the grid thus calculated is to be adopted. A weighting factor of zero can indicate that the calculated short-circuit value is not to be adopted, i.e. is to be discarded. For interim values, particularly where the weighting factor lies between zero and one, the short-circuit value can be proportionally calculated in combination with the further previously determined short-circuit value, and a weighted combined value can be saved and further employed for voltage control. For this selection logic, further options or forms of implementation can also be employed.

[0068] The logic function can or must monitor the controller output at all times. Only in this case is it rational for the previously calculated value to be considered. By means of the constant monitoring and triggers associated with fuzzy logic, in comparison with a conventional method, in which it is necessary for the controller to be disabled, a far greater number of measured values can be generated, and thus the quality of measurement can be successively improved. The consideration of previous measured values permits the combination of a plurality of measurements. In conventional methods, conversely, a measurement is only executed approximately once every 12 hours, as it is not desirable for the controller to be disabled frequently, on the grounds of the resulting reduction in the availability of the installation.

[0069] Any other interconnection of input parameters according to the above-mentioned two criteria is possible in order to arrive at this resolution of the issue. The selection logic can also be expanded to incorporate other parameters. Thus, the short-circuit power value can also be discarded, in the event of a substantial change in the system voltage associated with flashover in the electricity transmission grid. In particular, the voltage rise associated with the clearance of a grid fault is not caused by the installation, and is consequently not to be employed, according to this form of embodiment of the present invention. In particular, the method can provide particular advantages and can operate effectively in this case.

[0070] In order to permit the setting of the presently optimum controller amplification at any time point, in the absence of the occurrence of a change in the grid, a change in the grid can be simulated, independently of other components in the grid. To this end, a temporary step in the reactive power of the intrinsic converter can be generated. This step can function in the grid in the manner of a change associated with a switched element. The intrinsic measuring system can thus be triggered, and the controller amplification recalculated and reset.

[0071] By means of this solution, the present grid short-circuit power equivalent, incorporating the influence of other fast-response voltage controllers, can be determined at any time point.

[0072] An active grid test of this type can be initiated at any time point.

[0073] A combination of a conventional method and the method illustrated in the forms of embodiment of the present invention can also be executed. The short-circuit power (particularly the grid short-circuit equivalent) of the electricity transmission grid can thus be determined, and a correct response to other dynamic voltage control elements can be provided.

[0074] According to a form of embodiment of the present invention, changes over time in the system voltage and the change over time in output power are employed, and selection criteria and a logic function are applied for the execution of control.

[0075] A conventionally executed variation of the output power in order to determine a short-circuit power of the grid is not necessary according to the presently described method, but can be executed additionally in an optional manner. However, the calculation described is no longer restricted to a few cyclical measurements, but essentially can also be executed continuously, i.e. particularly in each pulse cycle. This can be advantageous in the event of a very substantial change in short-circuit power within a short time (e.g. in the event of a partial loss of the power grid), as the controller immediately calculates a new short-circuit power, and is thus not required to wait until a certain time has elapsed before a cyclical test is executed, or the system becomes unstable. Moreover, the influence of other dynamic voltage control elements can be considered at the same time. The proposed method can determine a lower short-circuit power value of the grid, and thus shows a reduced response to a voltage variation where another dynamic voltage control element is in electrical proximity and contributes to control. Thus, a stable control performance can also be achieved in this case.

[0076] FIGS. 2 to 8, 9 to 15 and 16 to 22 respectively illustrate graphs which represent the system voltage 7, the dispersion over time of the system voltage 7, the output power 15 (or measured value 11), the dispersion over time of the output power 15, the control current, the individual phase currents or a system factor as considered, defined or set according to forms of embodiment of the present invention. In all the graphs, time is plotted on the x-axis, and the magnitude of the electrical parameter is plotted on the y-axis.

[0077] The curves 37a, 37b, 37c in FIGS. 2, 9 and 16 illustrate the characteristic of the system voltage 7. In FIGS. 2, 9 and 16, a target value 38a, 38b, 38c of the system voltage is additionally plotted. The curves 39a, 39b and 39c in FIGS. 3, 10 and 17 respectively illustrate the dispersion of the system voltage over time. The curves 41a, 41b and 41c in FIGS. 4, 11 and 18 respectively illustrate the output power 15, particularly reactive power. The curves 43a, 43b and 43c in FIGS. 5, 12 and 19 illustrate the dispersion over time of the reactive power output by the reactive power device. The curves 45a, 45b and 45c illustrate the control current in FIGS. 6, 13 and 20. The curves 47a, 49a, 51a, 47b, 49b, 51b and 47c, 49c and 51c illustrate the converter currents of the three phases A, B and C in FIGS. 7, 14 and 21. Finally, the curves 53a, 53b and 53c illustrate a system factor in FIGS. 8, 15 and 22.

[0078] In response to a rise in the system voltage in FIGS. 2 and 3, the system responds with a reduction in the output of reactive power, according to FIGS. 4 and 5. At the point of strongest decline in the variation of the system voltage, at a time point 55, the system factor of the curve 53a rises substantially, achieves a maximum and declines again, immediately the voltage variation falls back below a threshold value.

[0079] FIGS. 23, 24, 25, 26 and 27 illustrate further curves 57, 59, 61, 63 and 65, which represent the system voltage, the control current, reactive power, SCL or an amplification, as considered, measured or set according to forms of embodiment of the present invention.

[0080] FIGS. 28 and 29 illustrate curves 67 and 69 respectively, which represent the temporal characteristic of output power or the temporal characteristic of the system voltage. The abscissa in FIG. 28 represents the value of the variation in reactive power and in FIG. 29 represents the variation in voltage. If a concordant variation in both the power output and the system voltage is observed, it can be concluded that the control installation itself is responsible for the variation in voltage in the grid. This concordant behavior is present if, e.g. the voltage reduces and the power reduces simultaneously, or if the voltage increases and the power also increases simultaneously.

[0081] Only if a concordant behavior of this type is observed can a short-circuit power, which is calculated from a relationship of changes over time in both the system voltage and in the reactive power output, be employed for the calculation of a control parameter. If this is not the case, the measured value is discarded, and the grid short-circuit equivalent determined at the most recent time point is employed.