Differential Protection Method And Differential Protection Device For Performing A Differential Protection Method

Abstract

A differential protection method for generating a fault signal includes measuring current measurements at least at two different measuring points of a multiphase transformer for each phase. The current measurements for each phase are used to form differential current values and stabilization values. The fault signal is generated if it is determined during a trigger region check that a measurement pair of at least one of the phases, being formed by using one of the differential current values and the associated stabilization value in each case, is in a predefined trigger region. In order to be able to selectively and reliably distinguish an external fault from an internal fault, the transformer has a grounded star point and a zero system current flowing through the star point is used to form the stabilization values. A corresponding differential protection device is provided for performing the differential protection method.

Claims

1-8. (canceled)

9. A differential protection method for generating a fault signal, the method comprising the following steps: measuring respective current measurements at least at two different measuring points of a multiphase transformer for each phase; forming differential current values and stabilization values with the current measurements for each phase; using a zero system current flowing through a grounded star point of the transformer to form the stabilization values; and generating the fault signal if it is determined during a trigger region check that a pair of measurements, created by using one of the differential current values and a respectively associated stabilization value, of at least one of the phases lies in a predefined trigger region.

10. The differential protection method according to claim 9, which further comprises using a maximum value from the current measurements formed at the respective measuring points and the zero system currents flowing on respective sides of the transformer.

11. The differential protection method according to claim 9, which further comprises determining the zero system current by measuring a current flowing through the star point.

12. The differential protection method according to claim 9, which further comprises determining the zero system current computationally from the current measurements acquired for the individual phases.

13. An electrical differential protection device for forming a fault signal, the device comprising: terminals for direct or indirect connection to at least two different measuring points of a multiphase transformer having a grounded star point through which a zero system current flows; and an evaluation device configured to form differential current values and stabilization values using current measurements acquired at the measuring points; said evaluation device configured to generate a fault signal if a pair of measurements, formed by using one of the differential current values and a respectively associated stabilization value, lies in a predefined trigger region; and said evaluation device configured to use the zero system current flowing through the star point to form the stabilization values.

14. The electrical differential protection device according to claim 13, wherein said evaluation device is configured to determine a respective stabilization value by determining a maximum value from current measurements formed at the respective measuring points and zero system currents flowing on respective sides of the transformer.

15. The electrical differential protection device according to claim 13, which further comprises: a terminal of the differential protection device for a direct or indirect connection to a current measuring point of the star point; said evaluation device configured to determine the zero system current by measurement of a current flowing through the star point.

16. The electrical differential protection device according to claim 13, wherein said evaluation device is configured to determine the zero system current computationally from the current measurements acquired for individual phases.

Description

[0028] Here

[0029] FIG. 1 shows a schematic view of a differential protection device monitoring a transformer; and

[0030] FIGS. 2-4 show trigger diagrams with pairs of measurements of differential current values and stabilization values entered by way of example.

[0031] For reasons of simplified illustration, a transformer with two sides is assumed in the context of the exemplary embodiment. To apply the invention to transformers with more than two sides, the method described has to be carried out for all the other sides in a corresponding manner.

[0032] FIG. 1 shows a schematic view of a section of a three-phase (phase conductors A, B, C) electrical energy supply system with a two-sided transformer 10 in star-delta connection with a grounded star point 14 on the high-voltage side 10a. The transformer 10 is monitored by means of a differential protection device 11 in respect of the occurrence of internal faults (e.g. short-circuits, ground shorts, winding faults). Current measurements I.sub.A, I.sub.B, I.sub.C are acquired for this purpose at a first measuring point M1 on the high-voltage side 10a of the transformer 10 by means of current measuring devices (e.g. inductive transducers or so-called non-conventional transducers), and are supplied to corresponding terminals of a measurement acquisition device 12 of the differential protection device 11. In a corresponding manner, current measurements I.sub.a, I.sub.b, I.sub.c are acquired at a second measuring point M2 on a low-voltage side 10b of the transformer 10 by means of current measuring devices, and are correspondingly supplied to further terminals of the measurement acquisition device 12 of the differential protection device 11. The current measurements I.sub.A, I.sub.B, I.sub.C, I.sub.a, I.sub.b, I.sub.c can here be transferred in analog or digital form to the measurement acquisition device 12. If the current measurements I.sub.A, I.sub.B, I.sub.C, I.sub.a, I.sub.b, I.sub.c are present as analog measurements at the measurement acquisition device 12, they are filtered and subjected to an A/D conversion there. Otherwise filtering and A/D conversion already takes place outside the measurement acquisition device 12, for example by means of a so-called remote terminal unit or a merging unit. The digitized measurements are transferred in this case to the differential protection device 11 over a process bus.

[0033] The measurement acquisition device 12 is connected on its output side to an evaluation device 13 of the differential protection device 11, which can, for example, consist of an appropriately configured hardware computing component (ASIC, FPGA), a central microprocessor assembly, a digital signal processor (DSP) or a combination of the said devices. The evaluation device 13 is configured through software-determined and/or hardware-determined programming, to use the current measurements from both sides of the transformer 10 to carry out a differential protection method in order to be able to detect any internal faults and to switch off.

[0034] Because changes of amplitude and phase angle of the current output on the low-voltage side in comparison with the sizes present on the high-voltage side occur in the transformation of current and voltage by the transformer 10, it is first necessary in order to carry out the differential protection method, for the amplitude and the phase angle of the current measurements to be adjusted. An adjustment of this sort in respect of the currents I.sub.a, I.sub.b, I.sub.c on the low-voltage side 10b of the transformer 10 is described below, although it would, alternatively or in addition, equally be possible to adjust the current measurements of the high-voltage side 10a.

[0035] For the amplitude-related adjustment, the current measurements I.sub.a, I.sub.b, I.sub.c are adjusted using the transformer ratio n. This gives the ratio of the number of windings of the high-voltage winding to those of the low-voltage winding, and determines the amplitude-related change of the current during the transformation process. An adjustment of the phase angle between the high-voltage side and the low-voltage side is also carried out. The change in the phase angle results primarily from the constructively predetermined vector group and from the position of any tap switch. These adjustments are sufficiently well known, and are therefore not explained in further detail at this point. After the adjustment of the amplitude and of the phase angle, adjusted current measurements I′.sub.a, I′.sub.b, I′.sub.c are present at the output side.

[0036] On the high-voltage side 10a of the transformer 10, a zero system current component I.sub.0 can occur as a result of the grounding of the star point 14. This is compensated for through appropriate correction prior to performing the differential protection method. The following equation represents the zero system current correction for the current measurements I.sub.A, I.sub.B, I.sub.C acquired at the high-voltage side 10a:

[00001]$\begin{array}{cc}\left(\begin{array}{c}{\hat{I}}_A\\ {\hat{I}}_B\\ {\hat{I}}_C\end{array}\right)=\left(\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right).Math.\left(\begin{array}{c}{I}_A\\ I_B\\ I_C\end{array}\right)+\left(\begin{array}{c}{I}_0\\ I_0\\ I_0\end{array}\right).& \left(1\right)\end{array}$

[0037] Î.sub.A, Î.sub.B, Î.sub.C here represent the current measurements, corrected for the zero system current, of the high-voltage side 10a; I.sub.0 represents zero system current.

[0038] Zero system current I.sub.0 can here be determined, for example computationally, from the current measurements I.sub.A, I.sub.B, I.sub.C:

3I.sub.0=I.sub.A+I.sub.B+I.sub.C.

[0039] The zero system current can alternatively also be determined through measurement of the star point current I.sub.St if an appropriate measuring device is present in the current path between the star point 14 and ground, and transmitted to the differential protection device 11 (not shown in FIG. 1).

[0040] The amplitude-adjusted and phase-angle-adjusted current measurements I′.sub.a, I′.sub.b, I′.sub.c of the low-voltage side 10b can now be employed, together with the zero current-corrected current measurements Î.sub.A, Î.sub.B, Î.sub.C acquired on the high-voltage side 10a, for the differential protection comparison. With the formation of a respective differential current value I.sub.Dif, the difference between the sizes of the current measurements belonging in each case to a phase is formed here:

I.sub.Dif,A=|Î.sub.A−I′.sub.a|,

I.sub.Dif,B=|Î.sub.B−I′.sub.b|,

I.sub.Dif,C=|Î.sub.C−I′.sub.c|.

[0041] In order to adjust the differential protection method dynamically to the size of the current flowing at any one time, and to compensate for any transducer errors in the current measuring devices used, a stabilization value I.sub.Stab is additionally formed for each phase from the current measurements corrected for the zero system current, or from the amplitude-adjusted and phase-angle adjusted current measurements, Î.sub.A, Î.sub.B, Î.sub.C and I′.sub.a, I′.sub.b, I′.sub.c respectively. The incorrectly determined differential current value I.sub.Dif resulting from transducer errors namely increases in proportion to the current flowing through the transformer, and in the event of external short-circuit currents can rise so strongly with transducer saturation that without stabilization it would lead to triggering, although the fault is not located within the protection region, that is within the transformer 10.

[0042] On the basis of the calculated differential current value I.sub.Dif and of the associated stabilization value I.sub.Stab, the position of a pair of measurements comprising differential current value I.sub.Dif and stabilization value I.sub.Stab is checked for each phase in a trigger diagram. If the pair of measurements of at least one phase of the transformer 10 is located within a trigger region, then a fault signal indicating the fault is generated, and can be used by the differential protection device 11 to form a trigger signal TRIP for a power switch (not illustrated in FIG. 1), in order to prevent further damage to the transformer 10. The trigger signal TRIP causes the power switch to open appropriate switch contacts in order to disconnect the transformer 10 from the rest of the energy supply network.

[0043] In conventional approaches, the stabilization value I.sub.Stab of transformers is either determined as the sum of the sizes of the corresponding zero system current-corrected or adjusted current measurements

I.sub.Stab,A=|Î.sub.A|+|I′.sub.a|,

[0044] as the size of the difference of the corresponding zero system current-corrected or adjusted current measurements

I.sub.Stab,A=|Î.sub.A|−|I′.sub.a|,

[0045] or as the maximum value from the corresponding zero system current-corrected or adjusted current measurements

I.sub.Stab,A=Max(|Î.sub.A|;|I′.sub.a|)

[0046] The above equations are formulated by way of example in each case for the phase A of the transformer 10; the equations for calculating the stabilization values I.sub.Stab,B and I.sub.Stab,C for the two other phases B, C are to be set up correspondingly.

[0047] With this sort of conventional formation of the stabilization value, however, weaknesses in terms of the treatment of the zero current at a grounded star point in the presence of heavy-current external faults and transducer saturation that may occur, have emerged. Such an external fault, i.e. one located outside the transformer, between phase A and ground at the fault location F on the cable section between a generator 15 and the transformer 10 is illustrated in FIG. 1. This external fault must be recognized as such by the differential protection device 11, and should not lead to unwanted triggering.

[0048] In the case of the external, single-pole fault illustrated, with contact to ground and feed through the grounded transformer, the short-circuit current −I.sub.KA flows through the transformer star point 14 as the zero system current I.sub.0 (the short-circuit current flowing is suggested in FIG. 1 by arrows; the size of the respective short-circuit current that is flowing is suggested by the number of arrows, where more arrows stand for a higher short-circuit current). The short-circuit current is represented on the high-voltage side 10a of the transformer 10 equally in all three phases A, B, C. The zero-system current I.sub.0 on the other hand is not represented on the low-voltage side 10b of the transformer 10, since this is implemented as a delta-winding. For this reason, in the transformer differential protection, prior to the formation of the differential current value I.sub.Dif and of the stabilization value I.sub.Stab, the zero-system current correction explained above is performed in respect of those sides of the transformer 10 that have a grounding (in the present example, this is only the high-voltage side 10a).

[0049] In terms of the single-pole fault in FIG. 1, the fault current on the high-voltage side 10a is equal in size in terms of amplitude and phase angle in all three phases. The following thus applies to the high-voltage side 10b when the transformer 10 is unloaded:

I.sub.A=I.sub.B=I.sub.C=1/3I.sub.kA

[0050] and

3I.sub.0=−I.sub.kA

[0051] The zero-current-treated current measurements of the high-voltage side 10a thus yield, according to equation (1):

[00002]$\left(\begin{array}{c}{\hat{I}}_A\\ {\hat{I}}_B\\ {\hat{I}}_C\end{array}\right)=\left(\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right).Math.\left(\begin{array}{c}\frac{1}{3}.Math.I_{\mathrm{kA}}\\ \frac{1}{3}.Math.I_{\mathrm{kA}}\\ \frac{1}{3}.Math.I_{\mathrm{kA}}\end{array}\right)-\frac{1}{3}.Math.\left(\begin{array}{c}{I}_{\mathrm{kA}}\\ I_{\mathrm{kA}}\\ I_{\mathrm{kA}}\end{array}\right)=\left(\begin{array}{c}0\\ 0\\ 0\end{array}\right).$

[0052] As can be seen from the above equation, if only the short-circuit current I.sub.kA is considered, i.e. without taking the load current into account, the zero-current-treated current measurements on the high-voltage side 10a become zero. The above-described conventional calculation methods for the stabilization values thus lead to stabilization values that only take the load current on the low-voltage side 10b of the transformer 10 into account. When the transformer is unloaded, such a load current can also be zero, so that altogether a stabilization value of zero (or close to zero) is determined.

[0053] If a cable current transducer does not accurately transmit the current (e.g. when the transducer is saturated, but also as a result of inaccurate measurements), the incorrect component in the former method is represented with equal proportions in the differential current value and in the stabilization value. This ratio of approximately 1 appears in the trigger diagram as a characteristic fault line in the case of the internal fault with single-sided feed, and unwanted triggering results. This case is illustrated by way of example in the trigger diagram of FIG. 2, which, for the sake of simplicity (as is also true of the subsequent trigger diagrams in FIGS. 3 and 4) is only drawn for one phase, for example phase A. In the trigger diagram, pairs of measurements consisting of determined differential current values and associated stabilization values are checked to examine their position. The characteristic fault line 20 can be seen as a diagonal in the trigger diagram. The characteristic trigger line 21 separates the trigger region 23 from the normal region 24. The pair of measurements 25 of the differential current value I.sub.Dif1 and the stabilization value I.sub.Stab1 calculated in the conventional manner is located in the trigger region 24, and therefore leads—in spite of the fault being external—to triggering.

[0054] It is proposed that to solve this problem, the calculation of the stabilization value is changed, in that the stabilization value is determined by also taking a zero system current that is present into account. The stabilization value is preferably formed through a selection of the maximum value from the respective current measurements, adjusted and/or zero-current-corrected if relevant, for each phase, as well as from the measured or calculated zero system current (considered below for phase A by way of example):

I.sub.Stab,A=Max(|Î.sub.A|;|I.sub.0,s1|;|I′.sub.a|;|I.sub.0,s2|)

[0055] Here I.sub.0,s1 and I.sub.0,s2 represent the calculated or measured zero system currents on the respective sides of the transformer (S1: side 1, the high-voltage side 10a in the present case; S2: side 2, the low-voltage side 10b in the present case). If a zero system current is not present—as is the case here for the low-voltage side 10b—this term is correspondingly omitted from the determination of the stabilization value; in the present case it follows that only the zero system current I.sub.0,s1 on the high-voltage side is included in the calculation of the stabilization value. The stabilization value is determined separately for each phase. The number of sides of the transformer in use in which zero currents occur and which therefore require zero current correction, also determines the number of zero currents to be considered for the formation of the stabilization value.

[0056] As a result of the changed determination of the stabilization value, an incorrectly formed zero system current is now included directly in the stabilization of the differential protection method, and is appropriately considered in the check of the trigger region. The solution illustrated therefore solves the problem of an incorrect triggering in the case of the external fault illustrated in FIG. 1. This is illustrated by way of example in FIG. 3. If the value of the differential current value I.sub.Dif1 remains the same in comparison with the trigger diagram in FIG. 2, the significantly higher zero system current is now included in the calculation of the stabilization value I.sub.Stab. As a result, instead of the stabilization value I.sub.Stab1 used in the case of FIG. 2, the higher value I.sub.Stab2 is now used; the pair of measurements 31 comprising I.sub.Dif1 and I.sub.Stab2 is now located in the normal region 24. Accordingly, triggering is not initiated by the differential protection device 11 for the external fault.

[0057] FIG. 4 shows, by way of example, the case of an internal fault (not illustrated in FIG. 1). As a result of the differential current value I.sub.Dif2, which is now larger, the pair of measurements 41 comprising I.sub.Dif2 and I.sub.Stab2 now falls in the trigger region 23. The differential protection device 11 correspondingly initiates a triggering of a power switch to switch off the internal fault.

[0058] The solution described advantageously allows the formerly known, proven basic principles of the standard differential current protection to be retained, with the same settings for the characteristic trigger curve 21, so that in this respect no changed settings have to be made. The formation of the differential current value I.sub.Dif is also unchanged. A change only occurs in the way in which the stabilization value I.sub.Stab is formed. This is also important, in order to avoid over-stabilization in the case of internal faults, with the associated under-function. At the same time, however, an improved stabilization is achieved with external faults.