FIELD OF THE INVENTION

The invention relates to a method of and an apparatus for supervising the operation of a plurality of current transformers and prevention of any malfunction, more specifically, false tripping of power in an electrical system.

BACKGROUND OF THE INVENTION

Power transformers are considered as prime power equipment in an electrical power system and faults in power transformers may lead to major consequences, both in form of power failure for a large group of customers and in the form of cost/time associated with the repair or replacement of the transformer. Major power equipments are protected against faults with use of protection devices. These devices detect various fault conditions in the power system and provide trip signals to circuit breakers associated with power equipment to isolate the faulty equipment or the protected power equipment from the system.

In the invention, a protection device may also be referred as an intelligent electronic device (IED), protection relay or simply as a relay. One of the protection devices for a power is a differential relay. Traditional power transformer differential relays utilize individual phase currents from different windings of the transformer in order to form the phase-vise differential currents to monitor faults in the power transformer or the power system and protect the power transformer. The individual phase currents from different windings are measured with help of current transformer (CT) connected in the path of each windings of the power transformer.

Some of the techniques of detecting abnormal operating conditions, particularly unbalanced conditions in a power transformer (in general, applicable for any three phase equipment and transmission system) and resulting faults due to unbalanced operation is to use phase sequence information (for example negative sequence currents). It is important that the protection relay provides a trip signal to its respective circuit breaker before any damage or degradation can occur in the power equipment. Usually, performance within one AC cycle time (power cycle) is desired.

Failure in CT, example CT secondary winding failure by breakage or winding disconnections, can cause malfunction of the protective relays resulting into false tripping (spurious operation). Thus, the detection of abnormal operating condition in the system may not be authentic.

Further, in modern numerical multifunction protection relays, CT secondary failure can also cause malfunctioning of other protection functions (like broken primary conductor protection) or can pose safety hazard because of high voltage build up at the terminal as a result of breakage in CT secondary winding/connection.

Detection of authentic abnormal operating condition (faults) for transformers are carried out by additionally monitoring other parameters or derivable values from these additional measurements (current/voltage) made in the system. Some common examples are use of voltage values as a reference together with the monitored electrical current values, use of current values from other CT as a reference etc. These arrangements requires additional channel in the IED to supply reference voltage or current to the IED.

An exemplary illustration is provided to illustrate the difficulty in supplying reference voltage or current signals to the protection relay. Here, the CT secondary supervision is performed with use of a separate reference current input but the drawback is that reference CT input would be needed for each CT set separately, thus making it rather expensive solution and more difficult to integrate into the protective relay. In the three winding differential protection relay, totally 12 CT inputs would be needed to cover both the protection requirements (9CT) and the CT secondary supervision requirements (3CT) at the same time. Some known solutions use voltage as reference for the current in detection of CT secondary failure, but also this solution requires some extra measurements on top of the differential protection itself. Therefore, there is a need for a solution that makes use of signals or measurements commonly provided for the differential protection relay to detect authentic abnormal conditions in the protected power equipment.

The invention provides a method and a device for detection of authentic abnormal operating condition for differential protection of three phase electrical equipment, specifically, the method and the device for supervision of abnormal condition in current transformers used in the electrical system to provide measurements required for differential protection and thus to enable the differential protection device to initiate trip procedures for authentic faults affecting the three phase electrical equipment.

SUMMARY

In one aspect of the invention, a method for supervising the operation of a plurality of current transformers (CTs) used along with a protection relay for protection of a multiphase power equipment is provided. The method used by the protection relay comprises the steps of:

a) Measuring electrical parameters (current magnitude and phase) from the plurality of current transformers for the multi-phase power equipment; b) Computing a fault indicative parameter for the multi-phase power equipment in the electrical power system from the measurements made with-the plurality of current transformers;

c) Detecting a current zero state (fault condition) in at least one electrical measurement made from the plurality of current transformers, the current transformer providing current or voltage zero state is said to be suspected faulty current transformer and the other current transformers from the plurality of the current transformers are said to be healthy current transformers; d) Detecting a change in the computed fault indicative parameter from the at least one electrical parameter measurement made from the said at least one healthy current transformer on detection of the current zero state in the suspected faulty current transformer; and

e) Comparing the detected change in the computed fault indicative parameter with a preset threshold value for the fault indicative parameter to determine the condition of at least one electrical equipment in the electrical power equipment;

The fault indicative parameter being one of the condition parameter selected from a group consisting of a negative sequence current parameter computed from measurements made with at least one healthy current transfrmer, a phase angle difference parameter computed from measurements between current signals from any two healthy current transformers from the plurality of current transformers and a combination parameter set based on the negative sequence current parameter and the phase angle difference parameter. The combination parameter set having multiple preset threshold values corresponding to the parameter set or is in form of a derived parameter based on an expression consisting of negative sequence current parameter and the phase angle difference parameter. The selection of condition parameter being as a fault indicative parameter being made based on the loading condition in the power equipment.

In an embodiment, the change in the computed negative sequence current is detected by comparing the current value of the negative sequence current magnitude with a previous value stored in the memory of the protection relay. The health state of the current transformer/multi-phase power equipment is determined based on comparison of the change in the computed negative sequence current with a pre-defined threshold value. For the purpose of computation, the healthy current transformers in the system are used.

In addition to the negative sequence current values, the change in phase angles of current signals from the healthy current transformer may also be consider, specially, in the condition where the change in the current magnitude may be small due to poor loading of the transformer. The health state of the current transformer is said to be faulty if the value of change in the computed negative sequence current or the change in the phase angle is smaller than the pre-defined threshold value else the fault is attributed to the power equipment i.e. it is not a CT fault.

In another aspect, a relay for protection of a multi-phase power equipment in a electrical power system that is adapted for supervision of operation of one or more current transformers used for measurement of current in the multi-phase power equipment is provided. The adaptation may be carried out by additional algorithmic or hardware/firmware solutions provided completely in the relay or with other supporting/collaborating automation devices (other IEDs or servers used in the distribution or substation network). In an embodiment, the relay comprises of:

a) A current sensing module for measuring current parameters from the one or more current transformers and for detecting a current zero state in any of the measurements made from the one or more current transformers;

b) A computation module for computing change in negative sequence current value for the multi-phase power equipment in the electrical power system based on the data provided by the current sensing module on the detection of current zero state;

c) A comparator module for comparing the change in at least one negative sequence current value and/or the change in phase angles from measurement from at least two current transformers, wherein the two current transformers are different from the current transformer associated with the current zero state, with corresponding pre-set threshold values, to determine the health state of the current transformer associated with the current zero state;

d) A blocking module for internal blocking of trip signal generated for protection of the multi-phase power equipment based on the result from the comparator module.

In another embodiment, a relay computes respectively preset threshold values for comparison with the change in negative sequence current value from a health CT and for comparison with the change in phase angles adaptively based on statistical techniques applied on the data gathered during normal operation of the electrical power system. This feature is useful to have automatic settings and for reliable measurements in the power equipment that is unbalanced and poorly loaded.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: Figure. 1 - 4 are exemplary electrical power systems having a fault in electrical power equipments;

Figure 5 illustrates the method for determining CT failure as per an embodiment of the invention;

Figure 6 illustrates the apparatus for determining CT failure as per an embodiment of the invention.

DETAILED DESCRIPTION

The application of the invention is not limited to any specific system or power equipment, but it can be used in connection with various electric three-phase AC power equipment where numerical differential protection is used.

The invention introduces a numerical method for detecting CT secondary failure that operates with the conventional CT inputs that are required for protection functions. It is fast, independent of the transformer vector group and can distinguish CT secondary failure from other primary system faults (like broken primary conductor). The method uses fast, sub cycle, measuring principle to detect possible CT secondary failure using "current zero" criteria (in the faulty CT set), thus making it fast enough to block the necessary protection functions before false tripping.

The method also monitors the change in negative sequence current (ΔΙ2) in healthy CT sets, thus making it sensitive to primary system faults (like broken primary conductor) and also effective under low load conditions.

Another advantage of using the change of negative sequence current (ΔΙ2) principle is to make the threshold also independent of the varying negative sequence current levels introduced by the varying transformer loading conditions.

The invention is described with reference to the accompanying drawings, in which Figure 1- 4 show a typical measurement system for protection having a three-winding power transformer 110 where three CT sets 120 are used for connection into the apparatus (not shown) that is performing CT secondary failure supervision with the three CT sets. The apparatus in addition may also be providing protection (eg transformer differential protection) to the power transformer. Figure 1 indicates an example of broken phase 'A' 130 in CT secondary 1. Power transformer loading is unsymmetrical and thus the change of loading also introduces some varying negative sequence current levels in different windings in normal operation when no primary or secondary fault exists.

Figure 5 illustrates the method, 500, used according to an embodiment in the invention. Current parameters (magnitude and phase angle) are measured (step 510) for each phase current separately from each winding with the CT sets (triplets). Negative sequence current (12), step 520, a condition parameter, is further calculated for each CT set from measured phase currents by the differential protection relay (not shown in Figure 1). There usually is sufficient amount of current passing in at least two CT sets, one in the CT set where CT secondary failure is detected and another one operating as a reference (healthy CT set) to make reliable measurements. Depending on the load conditions, there may be several healthy CT sets available as reference, but at least one is required for the method described in the invention and the reference CT set may be selected dynamically with a computer program or defined as part of pre-configuration.

The differential protection relay in step 530 detects current zero state, arising from example, fault condition due to CT secondary failure, and computes change in the negative sequence current (step 540) from measurements made with the reference healthy CT in the electrical system. The extent of the change in the negative sequence current is used for determining the health of the CT depicting current zero state and that of the three phase power transformer (step 550).

In another embodiment, illustrated with Figure 6, the electrical system 600 for performing

CT supervision is provided. The apparatus 610 is electrically connected to measure current parameters provided by the CTs 120. The apparatus (relay) has the current sensing module

620 to measure current and also detect current zero state (fault condition). Negative sequence levels are calculated by the computation module 630 in the relay and the value is updated continuously in the memory of the relay during normal load conditions, i.e. when there is no primary system fault detected or CT secondary failure detected. Current zero condition (also referred as current zero state) is continuously monitored for each phase measured with raw sample based measuring algorithm. This algorithm provides dedicated sub cycle raw samples to detect current zero within one fundamental power system period, thus providing fast enough blocking information to prevent false tripping of the protective relay. Here, continuous or large numbers of zero values (current zero) are detected within a sub power cycle.

Processing of the measured signals includes use of numerical peak measuring filter where the window length of samples in the filter is set more or equal to the number of samples in AC power system half cycle. However, the window length/size is always set less than the number of samples for full cycle. The numerical peak measuring filter is designed to capture any instantaneous fast change or interruption in the signals.

After current zero, in any phase, has been determined and confirmed, CT secondary failure is determined by comparing the change of negative sequence current (ΔΙ2) in a healthy CT set as a reference against a pre-defined threshold (carried out by comparator module 640). In case threshold is not exceeded, the CT secondary failure is confirmed. Threshold value can be selected to suit the desired sensitive level and the value needs to consider the normal measuring errors in ΔΙ2, involved in measurement and computations. The threshold value may be set by the operator as an independent setting 670 or may be adaptively based (for example using statistical methods) from the history of the measurements performed in normal (no faults) conditions, carried out by statistical threshold module 680.

The relay has a protection module 660 configured to provide trip signals under various fault conditions for which the relay is configured. For CT failures, by using the method described in the invention, the trip signal generated by the protection module 660 maybe internally blocked with the blocking module 650.

For a genuine CT secondary failure the magnitude of negative sequence current I ₂ will change only on the side where current zero has been detected. The change in magnitude of I ₂ (ΔΙ ₂ ) on other sets of the current transformer (other than where current zero is detected), if found to be unchanged for the healthy sets of CT, it is treated as an indication of CT secondary failure.

Thus, the method detects the CT secondary failure without additional reference inputs but uses the healthy CT set also required by the differential protection, as reference. The method for CT secondary failure detection comprises the steps of

(a) determining current zero in at least one phase within a sub power cycle;

(b) determining change in negative sequence current change (ΔΙ2) in at least one of the healthy reference CT sets;

(c) comparing (ΔΙ2) against predefined threshold; and

(d) indicating of the CT secondary failure when current zero is detected and the negative sequence change doesn't exceed the defined threshold limit.

Further, the working of the method is demonstrated along with simulation results/case illustrations.

For simulation purpose, the three winding transformer with rating 132kV/l lkV/1 lkV and type YN/yn/dl is considered. The MVA rating is 30MVA, with 21MVA on secondary 1 (SEC l) and 9MVA on secondary 2 (SEC 2). The power transformer has secondary 1 with 10% loading, the load being 0.9% unsymmetrical.

The following cases are illustrated as examples:

Case A)

Fault condition:

CT secondary failure 130 occurs on Phase 'A' of CTS SEC l as shown in Figure 1.

The transformer is having load of 10% at the time when CT secondary failure happens on Phase Ά' of CTS SECJ . These will result into current zero in phase Ά' of CTS SEC l. However this being a CT secondary problem, false tripping should not result and the relay blocks tripping based on the method of the invention. As per the method described in the invention, on detection of current zero in phase 'A' of CTS SEC l, the change in negative sequence current is evaluated on healthy sets of CTs i.e. on CTS PRI and CTS SEC 2. As current zero is due to CT secondary phase 'A' broken, no significant change in negative sequence current (less than the pre-set threshold value) on CTS PRI and CTS_SEC_2 are observed. The relay blocks trip function.

Case B)

Fault condition:

Broken phase 'Α' 210 on secondary 1 as shown in Figure 2.

If the current zero is due to broken line conductor, this result into change in negative sequence current on CTS PRI as well as on CTS SEC 2 side at the instance of current zero. The instantaneous change is an indication of system failure and not the case of CT secondary open. The relay on detection of current zero detects significant change in negative sequence current and executes the trip function i.e. no blocking of the trip signal.

Case C)

Fault condition:

CT secondary failure 310 occurs on Phase 'A' of CTS PRI as shown in Figure 3.

The transformer is having load of 10% at the time when CT secondary failure happens on Phase 'A' of CTS PRI. This results into current zero in phase 'A' of CTS PRI. However this being a CT secondary problem, false tripping should not result.

As per the method described in the invention, the change in negative sequence current is further evaluated on healthy sets of CTs i.e. on CTS SEC l and CTS SEC 2 on detection of current zero in phase 'A' of CTS PRI. As current zero is due to CT secondary phase 'A' broken, no change in negative sequence current on CTS SEC l and CTS SEC 2 are observed i.e. negative sequence current remains constant (less than the preset threshold) even after current zero occurrence in CTS PRI. Based on this, the relay block the trip function. Case D)

Fault condition:

Broken phase 'A' 410 on primary as shown in Figure 4.

As the current zero is due to broken line conductor, this result into change in negative sequence current on CTS SEC l as well as on CTS SEC 2 side at the instance of current zero. The instantaneous change is an indication of system failure and not due to CT secondary open. The relay on detection of current zero and change in negative sequence current, executes the trip function.

Thus, the method is established through simple illustrations that the system failure and CT failures are distinguishable based on the observations of current zero in the CT secondary and changes in the condition parameter, negative sequence current. The CT failures show no change (change less than preset threshold values) when current zero in CT secondary is observed.

Though the illustration was described based only on the magnitude of current, it is indicated that the relay also computes change in phase angles (condition parameter) for all the three set of CT. This is specifically useful to confirm fault when the change in negative sequence current is small, which is a case when the loading of the transformer is small. Thus, more than one condition parameter may be computed by the relay.

Exemplary cases are illustrated for various loading of transformer (10% and 100% loading) by simulation.

Case 1 : 100% loading of transformer

For this case, the phase 'Α' of the power transformer secondary gets open (phase discontinuity) as shown in Figure 2, and the transformer is 100% loaded.

The CT SEC l would measure current as zero. The fault would cause unbalance condition resulting in change in negative sequence current. On primary side, this asymmetry of secondary 1 gets reflected i.e. for phase 'A' current magnitude changes compared to what it was before fault. Similarly depending upon type of secondary 2 (Star, Y or Delta, D), the asymmetry of secondary 1 also get reflected on secondary 2 side (though for simplicity, this can be neglected). Eventually negative sequence current is detected on both CT SEC2 and CT PRI. The transformer being 100% loaded, the change in magnitude getting detected is of sufficient quantity for the IED to easily sense. Simulation results depict this change in CT SEC2 to be approximately 13.2A (=0.028pu) and in CT PRI to be approximately 46Amp (=0.33pu) which are detectable by the relay. In worst case, the change in CT PRI side is detected and that is sufficient to indicate that the fault is system fault and not the CT secondary circuit problem.

Case 2: 10% loading of power transformer

For the case fault condition as in Case 1 (Figure 2) but with 10% loaded condition for power transformer, the detection in the change in magnitude of the negative sequence current becomes difficult. From the simulation results, the negative sequence current on CT SEC2 is approximately 1.32 A (=0.0028pu) and that in the CT PRI is approximately 4.6A (=0.033pu), which are difficult to detect i.e. the change in negative sequence current both in the healthy sets CT SEC2 and CT PRI is so small that we cannot measure and IED is likely to consider this as a CT secondary circuit problem and will mall operate if the method is based only on detected change in the magnitude.

As one can see from the presented simulated results, the method to detect change in negative sequence current needs to consider both the magnitude and the phase i.e., the change in vector for all the three sets of CT signals.

During normal condition, phase angle difference between healthy phases is 120 degrees. A system fault condition on Phase 'A' will result into change in phase angle difference between phases 'Β' and 'C. This change in phase angle difference between healthy phases is evaluated in all three sets of current transformer, and if the change is detected in any set of CT, it is an indication of system failure.

For the fault condition illustrated in Figure 2, the change in phase angle difference on

CT PRI (Phase B - Phase C) was observed in simulation as approximately 17 degrees, which is measureable even when the transformer has small load (10% loading condition).

For the CT secondary problem, no such shift in phase angle would result. The method is algorithmically implemented in a differential protection relay that would block the differential protection relay to trigger power trip signal using the method described in the invention in the case of CT secondary failure. Further the relay includes reset timer to not to allow CT failure indication to be reset in case of intermittent type of CT secondary failures. The method may also be used in a dedicated apparatus/device deployed only for supervision of faults in the CT sets.

In an embodiment, the protection relay measures current parameters (magnitude and phase) with current transformers electrically coupled at various terminals of multiphase power equipment. The relay computes negative sequence current from the measurements made at different terminals of the power equipment. On detection of current zero condition in any of the terminals, the protection relay looks for an abrupt change in the computed negative sequence current value to determine the health of the current transformer used for measurement of current at the terminal measuring current zero condition and that of the power equipment.

In another embodiment, the protection relay measures current parameters (magnitude and phase) with current transformers electrically coupled at various terminals of multiphase power equipment. The relay computes negative sequence current from the measurements made at different terminals of the power equipment. On detection of current zero condition in any of the terminals, the protection relay looks for an abrupt change in the computed negative sequence current value to determine the health of the current transformer used for measurement of current at the terminal measuring current zero condition and that of the power equipment. In addition to the supervision of any abrupt change in the computed negative sequence current value, the relay also supervises the computed change in current phase angle with respect to a healthy current transformer to detect any abrupt change corresponding to the detection of current zero. Thus the relay monitors multiple condition parameters to determine health of the current transformer reporting current zero condition.

In yet another embodiment, the protection relay measures current parameters (magnitude and phase) with current transformers electrically coupled at various terminals of multiphase power equipment. The relay computes one or more condition parameter based on the negative sequence current value, current phase angle and the power equipment loading factor (% loading of the power equipment). A suitable condition parameter may be selected as a fault indicative parameter based on the load condition in the power equipment. The fault indicative parameter may directly be the negative sequence current or the phase angle or a combination parameter derived as a mathematical function of negative sequence current and phase angle based on the loading factor.

In yet another embodiment, one or more condition parameters including combination parameter derived as a mathematical function is provided as a combination parameter set. Thus, a combination parameter set is provided with one or more condition parameter including derived condition parameters as a fault indicative parameter. On detection of current zero condition in any of the terminals, the protection relay looks for an abrupt change in the computed fault indicative parameter to determine the health of the current transformer used for measurement of current at the terminal measuring current zero condition and that of the power equipment. For determining the health state of power equipments in the electrical system, the condition parameters in the condition parameter set are compared with the preset threshold value corresponding to the condition parameter in the condition parameter set. One or more condition parameters in the condition parameter set may be selected based on the load condition of the multi-phase power equipment.

In another embodiment, the method covers a short circuit fault in any of the CT windings resulting in a fault condition. Here, the relay may make use of voltage signals for detection of faults. The relay computes negative sequence current/voltage as found suitable to distinguish the fault. The method described using the current signals in the embodiment is now based on voltage signals i.e. on detection of current/voltage zero, the change in negative sequence voltage/current or the values of fault indicative parameter based on electrical parameters (current/voltage) is checked. If the change is significant, the fault is indicated as system problem. It is to be noted that the method is capable to detect short circuit or open circuit faults in the CT winding with measurement of current/voltage (electrical) parameters.

It is to be noted that the protection principle of the present invention can easily be extended and applied for supervising current transformers in the protection of multi-winding power transformers or any other multi-phase power equipment. The method described in the invention may be partly or completely embedded in the relay, the relay functioning along with another computer device external to the relay. Also, the method described in the invention can easily be extended to any current sensor for measurement of current.

While the present invention has been described in terms of the preferred embodiments, the invention is not limited thereto, but can be embodied in various ways without departing from the principle of invention as defined in the appended claims.