DIFFERENTIAL PROTECTION METHOD AND DIFFERENTIAL PROTECTION DEVICE

Abstract

A differential protection method generates a fault signal which indicates that a fault has occurred at an electrical component of an energy supply network. Differential current values and stabilization values are formed from current measurement values at different measurement points of the component and the fault signal is generated if a measured value pair, formed from one of the differential current values and the respectively associated stabilization value, lies in a predetermined tripping range. The stabilization current values in a predetermined observation period are compared with the respective associated differential current values and, based on the comparison, a fault indicator is formed, which indicates whether the proportion of the differential current values or the proportion of the stabilization current values is greater. The generation of the fault signal is blocked if the fault indicator indicates a greater proportion of the stabilization current values.

Claims

1-12. (canceled)

13. A differential protection method for generating a fault signal which indicates a fault occurring at an electrical component of an electrical power supply system, the method comprising: measuring current measurement values at each of at least two different measurement locations of the component, and forming differential current values and stabilization current values from the current measurement values; comparing the stabilization current values within a predefined observation time period with associated differential current values in each case and, based on a comparison result, forming a fault indicator that indicates whether a proportion of the differential current values or a proportion of the stabilization current values dominates; and performing a triggering range test and, if the triggering range test indicates that a measurement value pair formed from one of the differential current values and the associated stabilization value in each case lies in a predefined triggering range, generating the fault signal; and blocking the generation of the fault signal if the fault indicator indicates a predominant proportion of the stabilization current values.

14. The differential protection method according to claim 13, which comprises, for forming the fault indicator, determining an integral mean value from differences formed through the use of the differential current values and the stabilization current values.

15. The differential protection method according to claim 13, wherein the electrical component is a transformer or a generator.

16. The differential protection method according to claim 15, which comprises determining the fault indicator differently in dependence on a type of the electrical component.

17. The differential protection method according to claim 13, which comprises: comparing the fault indicator with a predefined threshold value; and blocking the generation of the fault signal if the fault indicator falls below the threshold value.

18. The differential protection method according to claim 17, which comprises determining the threshold value differently in dependence on a type of the electrical component.

19. The differential protection method according to claim 13, which comprises determining the fault indicator through a direct use of the differential current values or through a use of values derived from the differential current values.

20. The differential protection method according to claim 13, wherein the step of forming the fault indicator comprises carrying out the following calculation: $J\left(t\right)=\frac{1}{T_1}.Math.\underset{t-T_{1.Math.}}{\overset{t}{}}.Math.\left(i_{\mathrm{op}}-K.Math.i_r\right).Math.t$ where J(t): fault indicator; t: time; T.sub.1: predefined observation time period; i.sub.op: triggering current formed from the differential current values; K: adjustment factor for adjusting to the type of electrical component; i.sub.r: stabilization current.

21. The differential protection method according to claim 13, which comprises: determining the fault indicator for each phase of a multiphase electrical component; and blocking the generation of the fault signal only if the fault indicators for all phases fall below the predefined threshold value.

22. A differential protection device, comprising: a measurement value detection device for detecting current measurement values determined at different locations at an electrical component of an electrical power supply system; and an analysis device connected to said measurement value detection device, said analysis device being configured: for forming differential current values and stabilization values from the current measurement values; for generating a fault signal if a triggering range test indicates that a measurement value pair formed from one of the differential current values and the associated stabilization value in each case lies in a predefined triggering range; for comparing the stabilization current values in a predefined observation time period with the associated differential current values in each case and, based on a result of the comparison, to form a fault indicator which indicates whether the proportion of the differential current values or the proportion of the stabilization current values predominates; and for blocking a generation of the fault signal if the fault indicator indicates a predominant proportion of the stabilization current values.

23. The differential protection device according to claim 22, wherein said analysis device is configured to determine an integral mean value from differences formed by using the differential current values and the stabilization current values for the formation of the fault indicator.

24. The differential protection device according to claim 22, wherein: said analysis device is configured to compare the fault indicator with a predefined threshold value; and to block the generation of the fault signal if the fault indicator falls below the threshold value.

Description

[0035] FIG. 1 shows a schematic view of a component of an electrical power supply system monitored via a differential protection device;

[0036] FIG. 2 shows a flow diagram for explaining an exemplary embodiment of a differential protection method;

[0037] FIGS. 3 to 4 show diagrams for explaining a first exemplary embodiment of the analysis of current measurement values via a differential protection method;

[0038] FIGS. 5 to 6 show diagrams for explaining a second exemplary embodiment of the analysis of current measurement values via a differential protection method;

[0039] FIGS. 7 to 8 show diagrams for explaining a third exemplary embodiment of the analysis of current measurement values via a differential protection method; and

[0040] FIGS. 9 to 10 show diagrams for explaining a fourth exemplary embodiment of the analysis of current measurement values via a differential protection method.

[0041] In FIG. 1, a section 10 of an electrical power supply system, which is not depicted further, is apparent. The section 10 has an electrical component 11 which, for example, may be a transformer or a generator. The component 11 is monitored for faults, for example, short circuits, by means of a differential protection method described in greater detail below.

[0042] For this purpose, for the individual phase conductors R, S, T, similar instantaneous values of the phase currents i.sub.R1.sub._.sub.a, i.sub.S1.sub._.sub.a, i.sub.T1.sub._.sub.a are detected at a first measurement location 12a, and phase currents i.sub.R2.sub._.sub.a, i.sub.S2.sub._.sub.a, i.sub.T2.sub._.sub.a are detected at a second measurement location 12b, and are supplied to a differential protection device 13. The differential protection device has a measurement value detection device (not shown in FIG. 1), via which the phase currents are detected and converted via sampling into corresponding current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2. In deviation from the depiction according to FIG. 1, the sampling may in this case also be carried out by means of the current converters used for detecting the current or a sampling device downstream from it; in this case, the sampled current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 would already be transmitted directly to the differential protection device 13.

[0043] The differential protection device 13 is equipped with an analysis device (also not shown in FIG. 1), to which the individual current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 are supplied. The analysis device analyzes the current sampled values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 through the use of a differential protection method and generates a fault signal F if an internal fault, i.e., a fault in the area of the component 11, has been detected. The fault signal F may, for example, be used to generate a triggering signal for a switching device, for example, a power switch, via which the faulty component is disconnected from the rest of the electrical power supply system.

[0044] In order to increase the reliability of the fault detection, the analysis device of the differential protection device 13 also checks whether certain operating states of the electrical power supply system, for example, an activation operation, exist, which may influence the carrying out of the differential protection method in such a way that the fault signal F could also be generated if no fault actually exists in the area of the component 11. If such an operating state of the electrical power supply system is detected, the generation of the fault signal F is blocked in order to avoid an erroneous triggering of the differential protection device 13.

[0045] In FIG. 2, a flow diagram is shown by way of example which shows an exemplary embodiment of the functioning of the differential protection device 13 according to FIG. 1. In this case, the flow chart is depicted in the form of a block diagram; however, the implementation of the described differential protection method may be carried out both in the form of software and hardware, as well as a combination of the two.

[0046] Initially, the current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 are supplied to a first analysis step 21. If the electrical component 11 is a transformer, in a preceding step, the current measurement values of both voltage sides of the transformer must be adjusted to each other. For this purpose, a voltage level adjustment, a switching group adjustment, and possibly a zero-voltage correction or elimination are carried out in a manner known to those skilled in the art. In the following equations, all values are calculated in relation to the nominal current of the component (unit: p.u.) and at consecutive sampling points n.

[0047] In the first analysis step 21, differential current values i.sub.d and stabilization current values i.sub.r are formed from the current measurement values as described below. The formation is in turn carried out by phase; however, the following exemplary embodiment is depicted for only one phase for the sake of simplicity.

[0048] The differential current values i.sub.d of each phase R, S, T are calculated according to equation (1):

i.sub.d=i.sub.1i.sub.2.(1)

[0049] In this case, as already mentioned, the respective phase-related current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 are to be used for the phase-wise calculation, instead of i.sub.1 and i.sub.2.

[0050] The stabilization current values i.sub.r are determined according to equation (2) as follows:

i.sub.r=max[|i.sub.r1|;|i.sub.r2|],(2)

[0051] In this case, i.sub.r1 and i.sub.r2 represent the stabilization currents at the respective measurement locations 12a or 12b. They are, for example, determined according to the following equations (3a) and (3b):

[00002]$\begin{array}{cc}{i}_{r.Math..Math.1}=\max \left[.Math.i_1.Math.;\frac{1}{}.Math..Math.i_1^{}.Math.\right],& \left(3.Math.a\right)\\ i_{r.Math..Math.2}=\max \left[.Math.i_2.Math.;\frac{1}{}.Math..Math.i_2^{}.Math.\right],& \left(3.Math.b\right)\end{array}$

[0052] In this case, the respective phase-related current measurement values i.sub.R1, i.sub.S1, i.sub.T1 or i.sub.R2, i.sub.S2, i.sub.T2 are again to be used for the phase-wise calculation, instead of i.sub.1 and i.sub.2. In addition, i.sub.1 and i.sub.2 represent the respective first derivative of the current measurement values, and represents the angular frequency of the current. The first derivatives may be determined, for example, through the use of a difference quotient (shown here by way of example for i.sub.1):

[00003]$\begin{array}{cc}{i}_1^{}=\frac{1}{T_s}.Math.\left(i_1\left(n\right)-i_1\left(n-1\right)\right).& \left(4\right)\end{array}$

[0053] In this case, T.sub.s represents the sampling period. In a next analysis step 22, value pairs are formed in each case from a differential current value and an associated stabilization current value, i.e., one formed from simultaneously ascertained current measurement values, and checked with respect to their position in a triggering diagram. If the value pair lies within a triggering range, this is rated as an indication of an internal fault, and a corresponding indication signal S.sub.F is passed to a following analysis step 23. The following analysis step 23 is used for the generation of the fault signal if the indication signal S.sub.F exists.

[0054] The differential current values and stabilization current values formed in the analysis step 21 are also supplied to an analysis step 24, which uses them to form a fault indicator via which it may be checked whether a proportion of the differential current values or a proportion of the stabilization current values predominated in a previous observation time period T.sub.1. The fault indicator may, for example, be ascertained as depicted in equation (5):

[00004]$J\left(t\right)=\frac{1}{T_1}.Math.\underset{t-T_{1.Math.}}{\overset{t}{}}.Math.\left(i_{\mathrm{op}}-K.Math.i_r\right).Math.t$

where [0055] J(t): fault indicator; [0056] t: time; [0057] T.sub.1: length of the predefined observation time period; [0058] i.sub.op: triggering current formed from the differential current values; [0059] K: adjustment factor for adjusting to the type of electrical component; [0060] i.sub.r: stabilization current.

[0061] In this equation, the triggering current i.sub.op is ascertained directly or indirectly from the differential current values and may, for example, be formed as specified in equation (6):

[00005]$i_{\mathrm{op}}=\max \left[.Math.i_d.Math.;\frac{1}{}.Math..Math.i_d^{}.Math.\right].$

[0062] In this case, a difference quotient may again be used for the formation of the first derivative i.sub.d, similarly to the approach according to equation (4).

[0063] The adjustment factor K is used for adjusting the formation of the fault indicator J to the type of electrical component and may, for example, assume the value 0.3 in the case of a transformer and the value 0.7 in the case of a generator. It is used for weighting the stabilization current values for the comparison with the differential current values or the triggering current formed from them. The exact value of the adjustment factor may, for example, also be ascertained via experiments or simulations.

[0064] In a following analysis step 25, the fault indicator J is compared with a threshold value H. The threshold value H may also be determined as a function of the type of electrical component; for example, it may assume the value 2.0 in the case of a transformer and the value 0.0 in the case of a generator. The exact value of the threshold value may, for example, also be ascertained via experiments or simulations.

[0065] The fault indicator J is formed individually for each phase and compared with the threshold value H. In the analysis step 25, exactly one blocking signal B is then generated if the fault indicators of all three phases fall below the threshold value H. The blocking signal B is supplied to the analysis step 23. In the case of a pending blocking signal B, the generation of the fault signal F is prevented in the analysis step 23.

[0066] In the following figures, the behavior of the described differential protection method is depicted for different scenarios based on diagrams with exemplary profiles of differential current values and the corresponding fault indicator in each case.

[0067] FIGS. 3 and 4 relate to a first scenario, in which an activation operation exists in the electrical power supply system. The monitored component is a transformer. In FIG. 3, the profile of the differential current values for the individual phases is depicted, while FIG. 4 shows the profile of the fault indicator for each individual phase. It is clearly apparent in the diagram in FIG. 3 that after approximately 700 ms, the current converters go into saturation due to a direct current component. Without further stabilization, the comparatively high differential current due to the activation operation and the transformer saturation would result in the generation of a fault signal F. However, as a result of the additional analysis of the fault indicator J, it may be achieved that the generation of the fault signal F is blocked, since the fault indicator lies permanently below the threshold value H having the value 2.0 (for transformers).

[0068] FIGS. 5 and 6 relate to a second scenario in which an internal fault exists in the component 11. The monitored component is a transformer. In FIG. 5, the profile of the differential current values for the individual phases is depicted, while FIG. 6 shows the profile of the fault indicator for each individual phase. The comparatively high differential current, in combination with the fault indicator J lying above the threshold value H having the value 2.0, results in a reliable generation of the fault signal F.

[0069] FIGS. 7 and 8 relate to a third scenario, in which an external fault (a fault outside the component) exists. The monitored component is a transformer. In FIG. 7, the profile of the differential current values for the individual phases is depicted, while FIG. 8 shows the profile of the fault indicator for each individual phase. It is clearly apparent in the diagram in FIG. 7 that current transformer saturation occurs approximately 60 ms after the occurrence of a fault. Without further stabilization, the comparatively high differential current due to the transformer saturation would result in the generation of a fault signal F. However, as a result of the additional analysis of the fault indicator J, it may be achieved that the generation of the fault signal F is blocked, since the fault indicator lies permanently below the threshold value H having the value 2.0 (for transformers).

[0070] Finally, FIGS. 9 and 10 relate to a fourth scenario, in which a near-generator external fault exists having a low conductor current. The monitored component is a generator. In FIG. 9, the profile of the differential current values for the individual phases is depicted, while FIG. 10 shows the profile of the fault indicator for each individual phase. Transformer saturation again occurs due to existing direct-current components. Without further stabilization, the comparatively high differential current would result in the generation of a fault signal F. However, as a result of the additional analysis of the fault indicator J, it may be achieved that the generation of the fault signal F is blocked, since at no point in time do the fault indicators of all three phases lie above the threshold value H having the value 0.0 (for generators).