Relay-Assisted Commissioning

Transcription

1 Relayssisted ommissioning asper Labuschagne and Normann Fischer Schweitzer Engineering Laboratories, nc. Presented at the 59th nnual onference for Protective Relay Engineers ollege Station, Texas pril 4 6, 6 Originally presented at the nd nnual Western Protective Relay onference, October 5

2 Relayssisted ommissioning asper Labuschagne and Normann Fischer, Schweitzer Engineering Laboratories bstract Power transformer differential relays were among the first protection relays with digital technology. These new power transformer relays offered such improvements over electromechanical relays as automatic calculation of TP values, and the use of calculations instead of current transformer (T connections to eliminate zerosequence currents. While these improvements reduced setting complexity, Elmore [] identifies 9 different ways in which errors can occur during the commissioning of a power transformer. There is a clear need for relays to offer commissioning personnel assistance during commissioning of a power transformer. This paper describes an algorithm that checks for correct T polarities, consistent T ratios, and the existence of any crossedphase wiring errors, provided minimum, balanced load current flows. Other algorithms [] [] describe systems that test for similar wiring errors, but these systems fail to calculate an alternate vectorgroup compensation setting if the present relay vectorgroup compensation setting is incorrect. The algorithm this paper describes includes a vectorgroup compensation calculation. fter verifying T connections, the algorithm calculates the correct T compensation connections for the particular vector group within the transformer. The algorithm also produces a commissioning report that includes information such as phase rotation, measured load current during the test, and the recommended vectorgroup setting resulting from these load current and operating current measurements. ommissioning personnel can use the commissioning report to confirm immediately and conclusively the correctness of wiring and the integrity of differential element configuration settings.. NTRODUTON n general, commissioning protection equipment involves verification of physical connections (cabling, wiring, etc., relay settings, and proper operation of the complete system. Usually, a commissioning engineer first verifies physical connections, and then tests the appropriate analog signals and applies appropriate digital signals. To verify the complete system, the commissioning engineer takes a number of measurements during injection testing and compares these measured values against expected values. When differences between measured values and expected values exceed predetermined margins, the commissioning engineer suspects the existence of system errors and performs further tests to determine the cause of these errors. ommissioning engineers most often use the operating (differential current of the current differential element to determine whether commissioning errors exist. However, use of the operating current as a catchall method for detecting all commissioning errors can result in ambiguous conclusions. For example, both incorrect T polarity and a T connected to the incorrect T tap result in the presence of operating current. n fact, operating current can exist in an errorfree installation []. lthough numerical relays compensate for most unbalances, the socalled TP compensation can result in operating current. ecause standard T ratios seldom match the full load current of the transformer, transformer relays adjust each phase current to compensate for the ratio mismatch between installed Ts and the transformer full load current. To determine the adjustment for each phase current, the relay uses either Equation or Equation to calculate a scaling factor called TP. ( wye connected Ts MV TP ( kv TR TP MV kv TR ( delta connected Ts ( where: MV transformer rating in MV kv nominal system linetoline rated voltage in kv TR T ratio (normalized f the commissioning engineer measures operating current while the transformer tap position does not correspond with the rated voltage of the network (nominal tap position, operating current can be present. n this case, there is operating current present although there may be no setting or commissioning errors at the installation. Therefore, although the presence of excessive operating current indicates commissioning error(s, the commissioning engineer cannot identify the specific cause of the unbalance by the mere presence of operating current. learly, we need to take measurements other than just the differential current to identify the specific cause of the operating current. Table shows the measurement methods we use in the algorithm this paper describes. TLE MESUREMENT METHODS TO DENTFY VROUS USES OF OPERTNG URRENT Error Measuring Method nsufficient load current Two crossed phases T connected to the incorrect tap ncorrect T polarity Vectorgroup compensation selection urrent magnitude measurement Negativesequence current measurement Expected current to measured current magnitude comparison; negativesequence current measurement ngular comparison between a reference phase and all other phases Operating current and phase angle measurement

3 . MESUREMENT QUNTTES s we see from Table, the relay uses operating current to select the correct vectorgroup compensation. ecause we can apply the vectorgroup selection resulting from this algorithm as a relay setting (overwrite officially approved settings, we must eliminate all possible ambiguities and error sources. To avoid errors resulting from primary or secondary current injection, we require at least 5 m (for a 5 secondary relay of balancedload current (approximately five percent of full load instead of injected current. We specify 5 m load current because T characteristics can vary significantly among phases at low current values (ankle point. For greater current magnitudes, Ts operate on the linear portions of their respective magnetization (/H curves. On the linear portion of the /H curve, the T characteristics are substantially similar to each other, and the secondary currents from the Ts are balanced. t is important to have balanced load current, because we use symmetrical components in some of the tests (see Table, and unbalanced load current can distort test results. The value of 5 m also ensures that relay errors do not obscure proper compensation selection. Table shows the various differential current values for each phase shift. From Table we see that, with the correct compensation selection on both windings, the differential current is (ideally zero. With 5 m secondary current flowing, differential current resulting from a phase error (5 instead of 8, for example is ± m. ecause a current of m is substantially larger than any relay error, the relay can conclusively make correct compensation selections. ngular Error TLE DFFERENTL URRENT FOR PHSE SHFTS Phase HV Phase LV Differential urrent No error 5 m 5 8 m. m error 5 m 5 5 m m 6 error 5 m 5 m 5. 6 m 9 error 5 m 5 9 m m error 5 m 5 6 m 4. m 5 error 5 m 5 m m 8 error 5 m 5 m 5 m. URRENT ENSTON We define current compensation in three parts: vectorgroup compensation (phaseangle correction, zerosequence removal, and scaling (TP. We can achieve vectorgroup compensation and zerosequence removal either by appropriate T connections or by mathematical calculations. n numerical relays, actual compensation transformer taps (used for TP compensation in electromechanical relays do not exist, and all determinations of TP are by way of mathematical calculations (Equation or Equation. Electromechanical relays require deltaconnected Ts to compensate for wyeconnected power transformer windings; wyeconnected Ts generally provide more information. ecause numerical relays compensate for input currents mathematically, deltaconnected Ts are no longer necessary. The algorithm this paper describes assumes that all Ts are wyeconnected, regardless of the transformer vector group. V. PHSENGLE ENSTON Phase angle differences come about when the vector group of one set of power transformer windings differs from the group for another set of power transformer windings (such as wyeconnected and deltaconnected windings. For example, consider the YD (YNd connection shown in Fig.. Taking the phase of the HV winding as reference, the delta connection causes the phase of the LV winding to differ by with respect to the phase HV winding. HV Winding LV Winding Phase Difference 6 bc ca Fig. Phase Shift etween HV and LV Sides of a YD (YNd Transformer When electromechanical relays are in use, Ts from wyeconnected power transformer windings are connected in delta, and Ts from deltaconnected power transformer windings are connected in wye to compensate for the phase shift. When both HV and LV Ts are wyeconnected, T connections cannot compensate for this phase difference, and the secondary current from the HV winding and the secondary current from the LV winding are phaseshifted by. For correct differential operation, we need to correct for the phase shift of wyedelta transformers in the relay software. To achieve this phaseshift correction, the relay software calculates the appropriate delta connection. Equations through 5 show the three line current equations for the YD transformer connection. ab bc ca ( c ( a b b ( a (4 c c ( b (5 a f we were to write Equations through 5 in matrix format, the placeholders for the current vectors would be as follows:

4 a a a b c b c so that ab a b becomes b c a [ ] b ( c c We can then rename ab to and complete the current relationships of the YD transformer in matrix form as follows (divide by to scale the magnitude, see ppendix : n the same manner, we can form other delta matrices for transformer vector groups that require integer multiples of phaseshift correction (see ppendix. V. ZEROSEQUENE ELMNTON Why eliminate zerosequence current? Fig. shows a wyedelta transformer with the wye winding grounded. Ground faults on the HV side of the transformer result in current flowing in the lines of the wyeconnected windings and therefore the HV Ts. This current distribution is different in the LV windings of the transformer. Fault current for ground faults on the HV side of the transformer circulate in the deltaconnected windings, but no zerosequence current flows in the LV lines or in the LV Ts. ecause fault current flows in the HV Ts only, the differential protection is unbalanced and can misoperate. Secondary Fault urrent HV Side LV Side F wye winding delta winding No Secondary Fault urrent Fig. System Fault on the Wyeonnected Winding of a YD transformer learly, we need to eliminate zerosequence currents from Ts connected to all grounded, wyeconnected transformer windings or where a grounding transformer is installed on the delta winding within the differential zone. ecause all Ts are wyeconnected, we must remove the zerosequence currents mathematically in the relay. One way to remove the zerosequence currents is by means of the delta matrices we use for phaseangle correction. For a D delta, connecting a b, b c, and c a phases forms the delta connection. Of these three groups, consider the a b connection. Equation 6 and Equation 7 express a and b in terms of symmetrical components, phase being the customary reference. b (6 a α α (7 a b α α (8 ( α ( α a b (9 where: α is the alpha operator, i.e., Equation 8 shows the a b connection in terms of symmetrical components. From Equation 9, we see that the zero sequence currents cancel, and only positivesequence and negativesequence currents flow. lthough delta connections effectively eliminate zero sequence currents, delta connections also create phase shifts. We may need this phase shift in wyedelta transformers, but we do not need a phase shift in autotransformers or wyewye connected transformers. With autotransformers or wyewye connected transformers, the HV and LV currents are in phase with each other (or 8 out of phase. Use of a delta connection to remove zerosequence current introduces an unnecessary phase shift between the HV and LV currents. Fortunately, numerical relays make it possible to remove zerosequence current mathematically without creating a phase shift. Perform the following calculation to remove zero sequence current from the phase current: (, where ( ( ( ( Similarly for the and phases: ( ( rranging the results in matrix form yields the following: Matrix is the identity matrix; it does not alter the currents:

5 4 dding the even matrices (M, M4,.. M, see ppendix brings the total number of available matrices to (including the identity matrix. Even with all Ts connected in wye configuration, there are 4, possible matrix combinations for a fourwinding installation. learly, with so many combinations, it is easy to make an error in selecting the correct matrix combination when setting the differential element. The following discussion describes an algorithm that automatically selects the correct matrices. HV usbar T T Yconnected Ts Power Transformer OP OP OP Differential Relay V. UTOMT VETORGROUP SELETON. Overview Fig. shows a typical twowinding transformer installation. oth HV Ts (T and LV Ts (T are wye connected. n the figure, the differential protection obtains threephase current inputs (only one phase shown from current transformers T and T. There are three restraintdifferential elements inside the differential relay, one element per phase. Each of the three differential elements calculates operate current (OP through OP and restraint current (RT through RT. The vectorgroup selection algorithm reverses the process that engineers usually follow when setting and commissioning protection relays. Usually, the setting engineer selects relay settings and the commissioning engineer takes suitable measurements to verify the correctness of these relay settings. For example, if the commissioning engineer measures high restraint current ( per unit, for example and low operating current (.5 per unit, then the commissioning engineer concludes that the differential settings are correct. With the vectorgroup selection algorithm, the inverse applies: we first do the measurement, then we select the relay settings. LV usbar T Yconnected Ts T4 Fig. Typical TwoWinding Transformer nstallation The vectorgroup selection algorithm is essentially in two parts: a part that checks for correct T wiring (single contingency, balanced conditions and a part that calculates vectorgroup compensation, two windings at a time (i.e., a reference and a test winding, for a power transformer. oth parts require that 5 m balanced, load current flows through the transformer. Part : the algorithm checks for correct T polarities, consistent T ratios, and whether any crossedphase wiring errors exist. Part : after verifying the T connections in Part, the algorithm calculates the correct T compensation for the particular vector group within the transformer. fter calculating the correct compensation settings, the relay accepts the new setting only if the tester confirms the setting change.. Part. T hecks Through the use of balanced load current, we can identify the occurrence of one of the following T errors (wyeconnected Ts: T secondary wire connected to the incorrect tap on the T rossed phases ncorrect T polarity Fig. 4 shows a T secondary wire connected to the incorrect tap on the T, as well as a connection that results in an

6 5 incorrect T polarity. Fig. 5 shows the crossing of two phases. ncorrect T Tap Relay Fig. 4 ncorrect T Ratio or T Polarity Relay rossed Phases ncorrect T Polarity Fig. 5 rossed Phases ncorrect T polarity To check for correct T polarities, the relay uses the phase current as a reference for all tests. Table shows the angular relationship between phases when the polarities of all three Ts are correct in an phasesequence power system. TLE NGULR RELTONSHP FOR ORRET T POLRTES Phase Phase Phase Table 4 shows the threephase angular relationships for incorrect polarity of each of the three phases. ecause phase is the reference, the phase angle remains at. TLE V NGULR RELTONSHP FOR NORRET T POLRTES Phase Phase Phase * 6 6 * 6 * 6 *ncorrect polarity rossed Phases The relay considers the relay phase rotation setting ( or and uses Equation to calculate the positivesequence current of the reference winding. The relay uses Equation to calculate the negativesequence current of the reference winding for an phase rotation. REF (REF αref α REF ( REF ( REF α REF αref ( where: REF Positivesequence current of the reference winding REF Negativesequence current of the reference winding phase current of the reference winding phase current of the reference winding phase current of the reference winding α alphaoperator ( n a separate calculation, the relay calculates the positivesequence current and the negativesequence current of a test winding. The relay declares a crossedphase condition when the positivesequence current is less than percent of the negativesequence current. ncorrect T Tap Position We use the fact that the HV/LV turns ratio describes the relationship between the HV current and the LV current of a power transformer, i.e., the LV current is a scaled version of the HV current. Furthermore, when the transformer is on the nominal tap, we can use the linetoline voltage ratio instead of the transformer turns ratio. Use Equation to calculate the scaling factor N. VHV N ( VLV Fig. 6 shows the logic to detect an incorrect T tap connection. The relay uses the voltage ratio (Equation, to scale the measured HV current (W N for each HV phase and compares this result (expected LV current to the measured LV current (W of the corresponding LV phase. f the difference between the expected LV current and measured LV current exceeds.4 per unit, the relay declares an incorrect T tap connection. To determine the location (HV side or the LV side of the offending T, the relay calculates the negativesequence currents from the HV side and the negativesequence currents from the LV side and identifies the side with the greater negativesequence current as the side with the T on the incorrect tap.

7 6 W N W W N W W N W.4 pu TLRM phase HV T phase HV T phase HV T connected Ts. The direction of the rotation ( clockwise or counter clockwise is a function of the transformer vector group and the choice of reference winding. For example, consider the YD vector group shown in Fig. 8. When we take the HV winding as reference, the LV current resulting from the connection leads the phase HV current by. To correct for this angular difference, we rotate the LV current clockwise by. fter rotating the current clockwise by, the LV current is in phase with the HV current. W a (W a(w W a (W a(w Note: a e j N HVT LVT phase LV T phase LV T phase LV T Fig. 6 Logic to Detect the Phase With ncorrect T Tap onnection To avoid possible misleading results from unbalanced loading, the relay provides a separate alarm that is independent of the negativesequence calculation. ecause the previously discussed three cases are wiring errors and not setting errors, the algorithm does not attempt to rectify such errors. When detecting one of the wiring errors, the algorithm reports that such an error exists and suspends the selection process. This suspension gives the commissioning engineer the opportunity to correct the wiring error before continuing with the selection process.. Part. alculation of VectorGroup ompensation alculating the correct vector group consists of two separate calculations: vectorgroup selection through use of operate current, and vectorgroup selection through use of relative phase angles. Fig. 7 shows vectorgroup selection through use of operate current for a twowinding transformer. Selection Using The Operate urrent HV usbars T T LV usbars Reference Winding Test Winding Power Transformer Matrix M Matrix M Differential Relay (Reference M M Fig. 7 VectorGroup Selection for a TwoWinding Transformer HV M LV Mn n,... through M n general, to correct an angular difference of between two windings, we need to rotate the T secondary current phasors of one winding by with respect to the T secondary current phasors of the other winding (assuming wyewye HV Winding Phase LV Winding (Reference Difference 6 bc ca after clockwise rotation Fig. 8 YD Transformer With the HV Winding as Reference Taking the HV winding as reference is arbitrary; we could just as easily take the LV winding as reference. Fig. 8 shows the same YD vector group, but with the LV winding as reference. LV Winding (Reference HV Winding Phase Difference bc 6 ca Fig. 9 YD Transformer With the LV Winding as Reference after counterclockwise rotation With the LV winding as reference, we must rotate the HV currents counterclockwise to correct for angular difference. fter we rotate the phasecurrent counterclockwise by, the HV current is in phase with the LV current. Therefore, to compensate for the angular difference of the YD transformer, an HV/LV matrix combination of M/M is equally correct as an M/M combination or as an M/M combination. learly, it is not necessary that we know the actual transformer vector group; to correct for an angular difference, we simply declare one set of secondary current phasors as reference and rotate the other set of secondary current phasors either clockwise or counterclockwise by the appropriate amount. fter we select the reference winding (WDG in Fig., the vectorgroup compensation algorithm assigns Matrix to the reference winding. ssume for this example that we select WDG as the test winding. With Matrix M assigned to the reference winding, we now sequentially assign, starting from Matrix M, all matrices to the test winding. The objective is to find the combination of matrices that produces an operate current that is (ideally zero (see Table. ecause the existing relay settings can be the correct matrix combination, the relay calculates the operate current and restraint current with the existing settings before assigning matrices to any of the windings. f the operate current is less than.5 per unit, the existing settings are correct and the

8 7 relay records the numbers of the two matrices. lthough the relay found the correct combination with the first calculation, the relay still assigns all matrices to the test winding, and records all calculated values. f the operate current is greater than.5 per unit with the present settings, the relay does not record the matrix combination, but it assigns Matrix M to the test winding. y keeping the reference winding at Matrix M and assigning all matrices in succession to the test winding, the algorithm finds, by process of elimination, the correct matrix combination. Fig. shows a flow diagram of the selection process. t the conclusion of the test, the relay displays the recorded, calculated values from the calculations in a commissioning report. Table 5 summarizes the operatecurrent vectorgroup selection process. combination to compensate for the angular difference between HV and LV windings. To provide visual confirmation to the commissioning engineer, the relay displays a commissioning report of the operate currents and the restraint currents for each of the matrix combinations. Fig. shows an example commissioning report that includes information such as the phase rotation setting and the operate currents and the restraint currents with original matrix settings. Set W DG Set Value Set W DG Set Value Record OP,RT OP,RT OP,RT TLE V. VETORGROUP SELETON PROESS Steps ctivity omment Step alculate OP, the operating current, and RT, the restraint current. Evaluate the existing relay settings OP<.5 OP<.5 OP<.5 No Yes Record Matrix number for W DG and W DG Step ssign Matrix to the reference winding. Declare Matrix as the reference. Step Step 4 ssign Matrix as the initial matrix for the test winding. alculate OP, the operating current, and RT, the restraint current. ssign the M M matrix combination. Get the data to evaluate in Step 5 and data for the commissioning report. Set WDG M (matrix Set WDG M (matrix Record OP,RT OP,RT OP,RT Step 5 f OP is less than.5 per unit, record the matrix number, and assign the next matrix to the test winding. f OP is greater than.5 per unit of RT, do not record the matrix number and assign the next matrix to the test winding. f OP is less than.5 per unit, the present matrix combination is the correct combination. However, we continue to evaluate the remaining combinations. OP<.5 OP<.5 OP<.5 No Yes Record Matrix number for W DG and W DG Selection Using Relative Phase ngles ecause we can permanently assign the selected matrix as the relay setting, a further test is necessary. Relative angle selection provides a second, independent method for determining the correct matrix combination. We can confirm that the calculation using the operate current determined the correct matrix combination by checking (after reversing the polarity of the test winding Ts that the reference winding and test winding current phasors are in phase (± 5 with each other. Fig. shows the logic to compare the HV winding phase phasor with the LV winding phase phasor. ecause of the T polarity connections, the HV and LV phasors are 8 out of phase. rbitrarily selecting the HV phasors as reference, we add 8 to the LV phasors and test whether the HV phasors and LV phasors are in phase with each other (± 5. arg(hv arg(lv ±5 Output Fig. Phase VectorGroup Selection ngular Verification When both selection processes agree, the relay considers the present calculated matrix combination to be the correct Set WDG M (matrix Set WDG M (matrix Record OP,RT OP,RT OP,RT OP<.5 OP<.5 OP<.5 END No Yes Fig. lgorithm for OPMatrix Selection W DG for the entire test duration only W DG is increm ented Record M atrix num ber for WDG and WDG

9 8 utomatic Matrix Selection Successful. Phase Rotation: Reference Winding: Winding Z Matrix assigned to Winding Z: Matrix Test Winding: Winding X Matrix autoselected for Winding X: Matrix Y V. OMMSSONNG REPORT With the present matrices, the differential measurements are: OP(pu OP(pu OP(pu... RT(pu RT(pu RT(pu... With the autoselected matrices, the differential measurements are: OP(pu OP(pu OP(pu... RT(pu RT(pu RT(pu... phase Values From ll Matrices (Winding Z Matrix: Matrix Matrix Matrix OP(pu RT(pu OP(pu RT(pu.... Matrix Matrix 4 OP(pu RT(pu OP(pu RT(pu.... Matrix 5 Matrix 6 OP(pu RT(pu OP(pu RT(pu.... Matrix 7 Matrix 8 OP(pu RT(pu OP(pu RT(pu.... Matrix 9 Matrix OP(pu RT(pu OP(pu RT(pu.... Matrix Matrix OP(pu RT(pu OP(pu RT(pu.... Fig. ommissioning Report

10 9 V. ONLUSON The relay uses balanced, minimum load current to detect such singlecontingency T errors as crossed phase, incorrect T polarity, and incorrect T ratio. lthough the algorithm can be used for deltaconnected Ts, deltaconnected Ts increase the risk for undetected doublecontingency errors. The vectorgroup compensation algorithm this paper describes uses two independent compensation selection methods to calculate the correct transformer differential protection matrix combination. With this assistance, commissioning transformer differential protection is much easier for both experienced and inexperienced protection personnel. With the commissioning report the algorithm produces, commissioning personnel can immediately and conclusively confirm the correctness of relay wiring (balanced test and the integrity of differential element configuration settings. When relays assist commissioning personnel during commissioning, increased relay complexity need not mean increased complexity to protection personnel. X. PPENDX. VETORGROUP ENSTON USNG MTRX LGER vector (or phasor is a quantity with both magnitude and direction, as opposed to a scalar quantity that has magnitude only. n the rectangular form, we represent a vector (Z as follows: Z x where Z vector x real component y imaginary component j n the polar form, we represent a vector as follows: where jy Z Z θ Z x y y θ tan x We require only basic vector algebra to manipulate the vector quantities. For example, calculate the difference between and in Fig.. Fig. ddition of Vector and Vector Reference Line if y dividing by, we have a vector with magnitude, that is advanced by. For example, to calculate the compensated values of three system currents (taking as reference, multiply the three system currents (,, and by matrix M: M

11 X. PPENDX. MTRES Fig.. shows the matrices (including M, the unit matrix available to the relay to determine test winding compensation. M M M M M4 M5 M6 M7 M8 M9 M M M Fig. Thirteen vailable Matrices X. REFERENES [] W.. Elmore, Ways to ssure mproper Operation of Transformer Differential Relays, in 99 44th nnual onference for Protective Relay Engineers Proceedings. [] M. Young, J. Horak, ommissioning Numerical Relays, in th nnual Western Protective Relay onference Proceedings. [] M. Thompson, J. R. losson, Using OP haracteristics to Troubleshoot Transformer Differential Relay Misoperation, in nternational Electric Testing ssociation Technical onference Proceedings. X. OGRPHES asper Labuschagne has years of experience with the South frican utility Eskom, where he served as senior advisor in the protection design department. He began work at SEL in December 999 as a product engineer in the Substation Equipment Engineering group. He earned his Diploma (98 and Masters Diploma (99 in Electrical Engineering from Vaal Triangle Technicon, South frica. He is registered as a Professional Technologist with ES, the Engineering ouncil of South frica. Normann Fischer joined Eskom as a Protection Technician in 984. He received a Higher Diploma in Technology, with honors, from the Witwatersrand Technikon, Johannesburg, in 988, a.sc. in Electrical Engineering, with honors, from the University of ape Town in 99, and an M.S.E.E. from the University of daho in 5. He was a Senior Design Engineer in Eskom s Protection Design Department for three years, then joined ST Energy as a Senior Design Engineer in 996. n 999, he joined Schweitzer Engineering Laboratories as a Power Engineer in the Research and Development Division. He was a registered professional engineer in South frica and a member of the South frica nstitute of Electrical Engineers. Previously presented at the 6 Texas M onference for Protective Relay Engineers. 6 EEE ll rights reserved. 59 TP68