Mis-operation Cases on Transformer Differential Protection Yiyan Xue, Zachary Campbell, Sudhakar Chidurala, Charles Jones American Electric Power Company Abstract- Modern digital relays not only help to increase the speed, sensitivity and dependability for transformer protection, but also help to simplify the differential (87T) protection in circuit design and setting calculations. However, mis-operations of digital 87T relays still happen from time to time. This paper presents a few relay mis-operation cases that occurred in real life. One of the common lessons learned is: even though digital relays are superior and appears simpler for the user, the thinking process on protection can never be waived. In order to ensure the quality of relay settings, the relay engineer needs to know not only the protection fundamentals, but also needs to gain some insight of how digital relay works. I. INTRODUCTION The theory and practice of transformer protection has been mature and comprehensive for many years. But in reality, transformer protection mis-operations still happen from time to time. The common reasons of mis-operations are: ⋅ Incorrect settings of 87T function ⋅ Incorrect settings of transformer overcurrent protection – the setting is either too sensitive or lack of coordination with adjacent lines or feeders ⋅ Inrush current during energization or voltage recovery ⋅ CT polarity error in design or construction ⋅ False operation of non-electric protection, such as sudden pressure relays, Buchholz relay, etc. ⋅ Relay failure This paper will focus on the first type of mis-operations. As the primary protection for large or mid-size transformers, the 87T function is considered to be reliable, sensitive, fast and selective. Before modern microprocessor relays, the 87T schemes built upon electromechanical (EM) relays were prone to human error mainly because of the auxiliary CTs used for current compensation. Modern digital relays have greatly simplified the 87T scheme with regards to secondary circuit design and settings, but setting-related mis-operations still happen from time to time. This paper presents a few cases that were caused by incorrect settings of digital 87T relays. In the first case, the mis-operation was caused by a setting mistake on differential current compensation. The second case is about a refurbish project in which the old 87T relay was replaced but the old circuit was maintained. In that project, the 87T settings of the digital relay were following the relay manual, but the mis-matching between the old circuit and the digital relay have created a mistake that was not easy to identify. The third mis-operation case may or may not be labeled as “user error”, since it was the “automatic” setting of the digital relay that caused the mis-operation under heavy through fault condition. The fourth case has power electronics involved and it reminds us of another possible source of error in differential current. The fifth mis-operation case appears to be caused by inrush current during energization. But it turns out to be the issue of CT ratio and 87T pickup setting. II. 87T FUNCTION OF A DIGITAL RELAY Like other digital relays, the 87T relay will perform filtering and phasor estimation as the first step of signal processing. After the current phasors are derived, they will be compensated before computing the two key quantities of the 87T function – the differential current (Id) and the restraint current (Ir). Most 87T relays would use the percentage characteristic to compare Id and Ir to determine if the fault is within the protection zone. Winding 1 Ph-A Current Winding 2 Ph-A Current Winding ‘n’ Ph-A Current Filtering and Phasor Calc. Filtering and Phasor Calc. Filtering and Phasor Calc. Magnitude, Phase angle and zero sequence compensation Magnitude, Phase angle and zero sequence compensation Magnitude, Phase angle and zero sequence compensation Maximum magnitude, or Sum of magnitude, etc. Vector Sum of Phasors Differential Current Iad Restraint Current Iar Id Ir Figure II.1. The 87T Implementation in a Digital Relay Fig.II.1 uses phase-A currents as an example to show the main signal flow of the 87T relay. The key step is the current compensation that includes magnitude compensation, phase angle compensation and the optional zero sequence removal. Different type of relays may have different implementations, but the purpose is the same - to achieve zero differential current during normal operation. In the EM relay era, the auxiliary CTs were used for the compensation, which may incur human error during design or construction. A digital 87T relay would use settings to simplify the compensation and is more secure by using the advanced 87T characteristic to override the spurious differential current caused by CT error, Page 1 of 9 relay error, magnetizing current, tap changer, etc. Some digital relay also incorporates an external fault detector that is based on the detection of current change pattern. However, all the advantages of digital relays are subject to a premise: the relay settings have to be correct. In reality, setting errors related to mis-operations happen from time to time. How to reduce human error on settings is a challenge that both relay users and manufacturers need to think about. For an 87T relay, the setting error is usually associated with current compensation. The following simple example explains how current compensation works. In Fig. II.2, the transformer 138kV windings are connected in delta, 12kV windings are wye-grounded. The CT’s at both sides are wye-grounded. caused by D/Y conversion. The relay manuals [2], [3] have detailed descriptions on how to set the relay for phase angle compensation. In this example, the 12kV side currents need to be converted as follows, IL − ILB IL − ILC ILconvert _ ph _ A = A , ILconvert _ ph _ B = B , 3 3 IL − ILA ILconvert _ ph _ C = C 3 Such conversion is equivalent to connecting the CT in delta, like that in EM relay scheme. Combining the two compensations (assuming TAP is used for magnitude compensation), the following currents can be used to calculate differential and restraint currents, 138kV Side IH A IH convert _ A = TAPH IH convert _ B = IH convert _ C IH B TAPH IH C = TAPH 12kV Side IL − ILB 1 ⋅ ILconvert _ A = A TAPL 3 ILconvert _ B = ILconvert _ C = ILB − ILC ⋅ 1 TAPL ⋅ 1 TAPL 3 ILC − IL A 3 Figure II.2. The 87T Implementation in a Digital Relay Most relays’ 87T function is on a per-phase basis. Using phase-A as example, the relay will see the following currents under nominal load condition, 50MVA 5 IH no min al = ⋅ = 2.615 A 400 3 ⋅ 138kV ILno min al = 50MVA 5 = 4.01A 3 ⋅ 12kV 3000 ⋅ In some cases, the zero-sequence current (I0) removal is needed because the zero sequence current for an external ground fault may flow at only one side of transformer and be treated as differential current by 87T relay. In the above example, the 12kV side phase compensation would remove zero sequence current, so there is no extra step for zero sequence current removal. The following mis-operation case 1 will give an example on zero sequence current removal. III. MIS-OPERATION CASE 1 Where ‘H’ and ‘L’ represents high and low voltage side respectively. In order to get zero differential current under normal condition, one method of magnitude compensation is to use the nominal current as the conversion base, which is also called TAP. In this case, TAPH = 2.615, TAPL = 4.01. So, for any normal operation currents, the converted currents are IHconvert_magnitude = IH/TAPH, ILconvert_magnitude=IL/TAPL. In this case, the substation has a 138/69/46kV autotransformer, as shown in Fig. III.1. On 2012 March 15th, an external B-G fault occurred on the 46kV line due to lightning. The transformer 87T relay mis-operated for this fault. 138kV Bus Station Service Another method for magnitude compensation is to use multiplication. In this example, if the multiplier for low side current IL is defined as 1.0, the multiplier for high side current IH should be 4.01 /2.615=1.533. I.e., MH=1.533, ML=1.0. So, for any normal operation currents, the converted currents are IHconvert_magnitude=IH*MH, ILconvert_magnitude=IL*ML. 138/69/46kV 87T Regardless which method of magnitude compensation is used, under load condition or external fault condition, there should be |IHconvert_magnitude| = |ILconvert_magnitude|, such that differential current can be zero. 46kV Bus 69kV Bus Because the transformer winding is DY1 in this case, the relay also needs to account for the phase shift and the 3 factor Figure III.1. The Simplified Oneline Diagram For Case 1 Page 2 of 9 The reason was found to be related to the 87T relay transformer settings that are listed in the following table, Winding/CT inputs CT ratio Rated MVA Nominal voltage(kV) Transformer Winding Connection Grounding Angle with regards to Wnd1 W1 100 62.5 138 Wye Within zone 0 W2 100 62.5 69 Wye Within zone 0 W3 100 35.7 46 Delta Not Within zone -30 ∆ Figure III.2. The 87T relay settings in Case 1 ∆ The “Grounding” setting of the transformer delta side was “Not Within Zone”, which sounds fine for transformer delta side. However, in this case there is a grounding transformer at the 46kV side and it is within the 87T zone. Used also as grounding and station service transformer, it has a zigzag/delta configuration. For an external ground fault at the 46kV side, the zig-zag winding will facilitate a zero sequence current path such that the 87T scheme 46kV side will see zero sequence current that the other two sides of transformer cannot see. This explains the erroneous differential current in the record and the mis-operation. The correct setting of “Grounding” for the 46kV winding should be “Within zone”, even per literal meaning of this setting. However, what does this setting do inside the relay? The relay manual has the answer. The MathCAD sheet appended to this paper also includes the calculations. Explicitly, the zero sequence removal for the winding 3 current inputs in this case is performed as follows, 2 ⋅ I A − I B − IC I convert _ ph _ A = 3 2 ⋅ I B − I A − IC I convert _ ph _ B = 3 2 ⋅ IC − I A − I B I convert _ ph _ C = 3 Such conversion is just like connection of the CTs in delta, but without producing a phase shift. This case is not hard to figure out. The human error reminds the relay engineers that each setting needs to be checked carefully. IV. MIS-OPERATION CASE 2 The mis-operation in Case 2 happened in a power plant. The 87T relay provides generator-transformer unit protection. Per the simplified Oneline Diagram in Fig. II.1, four groups of CT inputs connected to the 87T relay make a protection zone that covers the generator and the 765kV/26kV step-up transformer. Before the refurbish project in 2012, the 87T scheme was built upon an EM relay that had been in service for over 30 years. In 2012, the old EM relay was replaced by a digital relay. Figure IV.1. The Simplified Oneline Diagram for Case 2 A special thing in this refurbish project was that CT’s used by the 87T scheme for the generator branch are installed on each winding and the generator windings are in Delta configuration. In the old circuit, 8.66:5 auxiliary CT’s were added for this 87T branch. When the new relay was installed, the auxiliary CT’s were not removed. All the other CT’s were connected to the 87T relay directly with Wye connection. Figure IV.2. CT Circuitry for 87T Scheme in Case 2 On 04/30/2012, not long after the power plant was put back in service after the overhaul, the 87T relay tripped. From the event record shown in Fig. II.3, there was no fault. It was the restraint 87T function that tripped under load condition. Figure IV.3. Case 2 Event Record Page 3 of 9 From the event record, differential current did exist and was above the 87T pickup setting. Since all the currents look normal, the suspicion was on 87T settings that are listed as follows, In this case, the primary currents for the relay are IAB, IBC, ICA. Since an 87T relay will typically compare each phase individually, the first step is to get the phase currents IA, IB, IC. To make the calculation intuitive, a number can be assumed for the primary currents. E.g., • Current Transformer Data CTRS = 600 CTRY = 600 CTRU = 6980 CTRW = 7000 CTCONU = D CTCONW = Y CTCONS = Y CTCONT = Y • Transformer Data TSCTC = 12 TTCTC = 12 TUCTC = 1 TWCTC = 1 VTERMS = 765.0 VTERMT = 765.0 VTERMU = 26.0 VTERM = 26.0 MVA = 1500 • 87T Settings O87P = 0.30 pu SLP1 = 25.00 SLP2 = 60.00 U87P = 16.00 pu I AB = 10000∠0o , I BC = 10000∠ − 120o , I CA = 10000∠120o Other settings and calculations for this case can be found in the Appendix II. For this relay, 4 groups of CT inputs are connected to the 87T relay separately. And they are identified by letters S,T,U,W in setting names of this relay. These 87T related settings (MVA, CT ratio CRTx, CT connection CTCONx, transformer winding connection TxCTC, nominal voltage VTERMx, x=S, T, U, W) provide information on the transformer and CT’s, which are critical for the 87T scheme because correct compensation on currents are relying on these settings. In 87T scheme, the generator branch is labeled as “U” winding by the relay. The auxiliary CTs were included for this branch and the auxiliary CT’s were connected in Delta before connecting to the 87T relay. Does this mean that the CT connection setting CTCONU should be set as “D” (Delta)? No matter how many steps of current conversion are involved, the digital 87T relay just needs to know the relationship between the primary current and the secondary current that goes directly into the relay for each phase. The easiest way for the digital 87T relay is to connect all the CT’s in Wye, such that at least one step of Wye-Delta conversion can be waived. If the relationship between primary and secondary is not straightforward, like the generator branch in this case, it is recommended to go through some calculations to know the correct relationship (ratio and phase shift). Two steps are given below to clarify such relationship in this case: Per Fig. IV.4 and Kirchhoff Law I A = I CA − I AB = 10000 ⋅ 3∠150o I B = I AB − I BC = 10000 ⋅ 3∠30o (1) I C = I BC − I CA = 10000 ⋅ 3∠ − 90o Step2. Calculate the secondary currents that flow into the relay Figure IV.5. Simplified Secondary Circuit for 87T Scheme Generator Branch Current inputs Per Fig. IV.4, the CT ratio is 20000/5, so the secondary currents out of the CT are I ab = 2.5∠0o , I bc = 2.5∠ − 120o , I ca = 2.5∠120o The auxiliary CT’s are connected in Delta, and the ratio is 8.66/5. Per Fig. IV.5 and Kirchhoff Current Law, the currents flowing into the 87T relay are, I aR = I cax − I abx = (5 / 8.66) ⋅ 2.5 ⋅ 3∠150o I bR = I abx − I bcx = (5 / 8.66) ⋅ 2.5 ⋅ 3∠30o (2) I cR = I bcx − I cax = (5 / 8.66) ⋅ 2.5 ⋅ 3∠ − 90o From (1) and (2), the relationship between the primary phase currents and the secondary phase currents that are flowing into the relays are I A / I aR = 6928∠0o , I B / I bR = 6928∠0o , I C / I cR = 6928∠0o After this exercise, it turns out that primary and secondary currents are actually in phase, equivalent to that from Wye connection of CT’s, and the ratio is 6928. Therefore, for the relay, the correct setting for CT inputs should be CTCONU=Y. Figure IV.4. Generator Windings and CT’s for 87T Step1. Convert the primary currents from phase-to-phase currents to phase currents. Inside the digital relay, the settings of CT ratio, CT connection and nominal voltage are all used to calculate the compensation Page 4 of 9 factor. The root cause of this mis-operation is that the generator branch currents for 87T scheme are amplified by 3 times after compensation, due to the incorrect setting “CTCONU=D”. The alternative solution could be using a different CT ratio setting or a different nominal voltage setting for generator branch, if the CTCONU is retained as D. 138kV The lesson of this mis-operation case is that one cannot assume the digital relays are smart enough to make the 87T calculation by itself, and it is not rigorous just to apply relay settings per literal meaning. In this example, the auxiliary CT was in Delta connection and this was why the setting CTCONU=D was made. But some questions should be asked: why did the ancestor engineer used 8.66/5 auxiliary CT’s connected in Delta for the EM relay scheme? What was the difference between the EM 87T relay and the digital 87T relay? What would be the difference if the generator windings were in Wye connection or the CT’s were installed outside of the generator terminal? If these questions were thought about and calculations were performed, a correct setting could have been made. W5 W2 W6 W3 87T Relay W4 W1 WEST AUTO 345/138/13.8kV (SPARE) MIDDLE AUTO 345/138/34.5kV 345kV Figure V.1. Simplified Oneline Diagram for Case 3 Assuming the auxiliary CT’s were removed in this case (which is recommended because auxiliary CT’s are not needed for the digital relay scheme), should CTCONU be set as Y or D? Through similar exercise above, the correct answer is that CTCONU should still be “Y”. However, there will be a 30 degree phase shift between the primary and secondary currents, so the TUCTC should be set as 12 instead of 1. The event record of the 87T relay is shown in Fig. V.2. From the record, it was the unrestraint 87T function that operated and the recorded phasor of differential current Ibd magnitude was 9.66pu or 48.3 secondary amps. Since the setting of the unrestraint 87T function was 8.0 pu, the relay operated per setting. The question is: why did the relay see such high differential current for an external fault? As a side note, most digital relays have a differential current metering function. So in this case, if the differential current was monitored when the load current started to increase, the mis-operation might be avoided. To automate the monitoring, the relay may be programmed to give alarm when differential current is over a threshold but less than the 87T pickup settings. In the appendix II, the MathCAD sheet is used to describe the 87T calculations of the digital relay in this project. It can also be used as a tool for fault analysis. V. MIS-OPERATION CASE 3 On August 24th, 2011, an 87T relay mis-operated during an external fault condition. The simplified oneline diagram is shown in Fig. V.1. The fault was caused by a thunderstorm, during which a tree had fallen on the 138kV line outside of the station. The 138kV line protection tripped correctly. When the line breaker reclosed after 5 seconds, not only did the 138kV line tripped again due to the persistent fault, the transformer 87T protection also operated. Figure V.2. Case 3 Event Record In Fig. V.1, the 87T relay has 6 groups of CT inputs. And the relay had the following settings that were related to 87T function, Page 5 of 9 CT ratio Rated MVA Nominal voltage(kV) Transformer Winding Conn. Grounding W2 400 675 138 W3 800 62 34.5 W4 800 675 345 W5 600 675 138 W6 800 143.7 13.8 Wye Wye Delta Wye Wye Delta Idiff C = I Cconv [1] + I Cconv [2] + ... + I Cconv [6] Within zone Within zone Within zone Within zone Withi n zone Not Within zone 0 0 0 0 0 -30 From the above equations, the magnitude compensation factor M would amplify 138kV side current input 10 times before it was used for differential current calculation. In this case, the through fault current (phase A, B) at 138kV side was about 20kA RMS in primary or 50A in secondary. So after compensation, it would become 200kA in primary or 500A in secondary, which looks suspicious. Angle with regards to Wnd1 Reference Winding Selection Automatic Selection 8.0 pu ( 40A secondary) Unrestraint Differential Protection The CT’s and connections were verified after the event. From the event record, there was no sign of CT saturation even though some current were relatively high. There was not apparent mistake with the relay settings either. To analyze the event, the relay internal 87T calculations were reproduced on the MathCAD sheet. Then an issue emerged during this exercise. This type of relay has a setting called “Reference Winding Selection”. Any CT inputs can be selected as the setting. But by default, the setting is “Automatic Selection”. Since “Automatic” is such a magic word, no one would ever want to change this default setting prior to this event. According to the relay manual, the reference winding is used to determine the compensation factor. When “Automatic Selection” is used, the relay will select the reference winding that gives the minimum Imargin, which is defined by CT _ Pr imary[ w] I m arg in [ w] = , where w=1, 2, 3, 4, 5, 6 I rated [ w] In this case, the rated current and the margin current for each current input are listed in the following table, Winding 1 2 3 4 5 6 1129.6 2824.0 1037.6 1129.6 2824.0 6012 Irated 1.771 1.416 1.157 1.771 1.416 0.665 Imargin Since the Imargin[6] is the smallest, the winding 6 input for the relay is used as reference per “Automatic Selection” setting. Once the reference winding is defined, the magnitude compensation factor M for each winding is calculated per following equation, I primary [ w] ⋅ Vno min al [ w] , where w=1, 2, 3, 4, 5, 6 M [ w] = I primary [ wref ] ⋅ Vno min al [ wref ] Winding M The differential currents are calculated by W1 400 675 345 1 12.5 2 10.0 3 0.75 4 12.5 5 10.0 6 1.0 After magnitude and phase compensation, the converted currents for 87T scheme are I Aconv [ w] = I A [ w] ⋅ M [ w] ⋅ AngCompA [ w] I Bconv [ w] = I B [ w] ⋅ M [ w] ⋅ AngCompB [ w] I Cconv [ w] = I C [ w] ⋅ M [ w] ⋅ AngCompC [ w] where the AngComp X [ w] represents the phase compensation. Idiff A = I Aconv [1] + I Aconv [2] + ... + I Aconv [6] Idiff B = I Bconv [1] + I Bconv [2] + ... + I Bconv [6] The above M factor calculation is similar to that mentioned in Section II. The initial suspicion by the AEP engineer was that when compensated current was so high, the error could be magnified and consequently more differential current could be produced. However, it was still hard to imagine such a differential current as high as 9.66 pu or 48.3A for an external fault. In the end, the relay vendor indicated that the relay has an internal threshold for secondary current and the threshold is 64pu or 320A in peak value, regardless the current is the direct measurement or the converted value. If the secondary current is over the threshold, numerical error would occur inside the relay, which explains the false differential current and the misoperation. To correct the setting error, the reference setting can be changed to either Winding 1 or 2. By using Winding 2 as reference, the M factors become, Winding 1 2 3 4 5 6 M 1.25 1.0 0.075 1.25 1.0 0.1 As can be seen, the new M factors would not change the original currents too much for the main windings. But they did change the tertiary winding currents significantly. And this change means reduced sensitivity. However, since the tertiary winding is not as critical as the main winding, and there are other transformer protection elements, these M factors are acceptable. Another possibility is to add auxiliary CT’s to the tertiary winding currents, but it is not desirable for a digital relaying scheme. In this case, the “Automatic setting” selected the 13.8kV tertiary winding as the reference winding. This is questionable if one is curious - the 13.8kV winding is auxiliary for a large autotransformer and it’s actually the winding of the spare transformer, why would it be used as the reference winding? By using the minimum Imargin as the criteria to select the reference winding, it can end up with a higher compensation factor M for a relatively smaller secondary current. In general, this may improve the numerical accuracy of differential calculation during normal operation. For example, a high CT ratio is used for a 2-winding transformer high side so that the secondary current of high side under normal operation is low. Page 6 of 9 The “Automatic Selection” will then select the transformer low side as the reference and end up with a higher M factor for the high side. Reasonable though it sounds, a condition should be added: the M factor shall not be too high! In this case, because of the large difference in voltage level, using the tertiary winding as reference resulted in overly high M factors for other windings. This case tells us, even if the relay setting has the magic word such as “Automatic”, the relay engineer needs to be cautious and try to understand what is behind this setting. Otherwise, the automatic setting may bring trouble. Another lesson learned is: for a transmission autotransformer, the tertiary winding should not be used as reference for this type of relay. W1 400 100 138.0 kV Wye Within zone 0° Slope 1 Break 1 30% 1.25 pu CT ratio Rated MVA Nominal voltage Transformer Winding Connection Grounding Angle with regards to Wnd1 Percent Differential Pickup 0.099 pu W2 1200 100 19.5 kV Delta Not Within zone -330° Slope 2 Break 2 80% 3.0 pu From the event record, it was a percent differential Phase B operation. At the moment of tripping, the Idiff_B = 0.11 pu, Irestraint_B = 0.36 pu. Since the point (0.36pu, 0.11pu) was just above the 87T characteristic shown in Fig. VI.3, the 87T operation was per setting. VI. MIS-OPERATION CASE 4 On May 7th 2015, an 87T relay for a SVC transformer misoperated for an external ground fault. The simplified oneline diagram and event record are shown in Fig. VI.1 & 2. The SVC-2 transformer high side breaker tripped for a ground fault on a radial 138kV line. Since there was no high-speed scheme on the 138kV radial line, the fault lasted about 27 cycles. The SVC-2 transformer was tripped by the 87T relay in about 5 cycles after fault inception. 138kV Bus#2 138kV Bus#1 138kV Bus#3 Figure VI.3. Operating Point on 87T Characteristic The CT’s were checked and no problems were found. The settings about the transformer and CT’s were also correct. The 87T settings look normal except the pickup setting 0.099pu was relatively low, but still acceptable in a general sense. Neither the relay vendor nor the SVC vendor could provide a solid explanation to the spurious differential current and the mis-operation. The following analysis gives the possible reason, but it is still supposition. 138kV Bus#4 400/5 YnD11, 100MVA YnD11, 100MVA (Spare) YnD11, 100MVA 87T 1200/5 19.5kV 19.5kV SVC#1 SVC#2 Figure VI.1. Simplified Oneline Diagram in Case 4 Figure VI.4. The Event Records from SVC Controller The SVC controller recorded the external fault event as well. From Figure VI.4., the waveforms of the SVC’s TCR (Thyristor Controlled Reactor) branch and the TSC (Thyristor Switched Capacitor) branch can be seen with designations as I_TCR and I_TSC, respectively. As the fault occurs, the TCR branch halts conduction. As this occurs, the TSC branch begins conduction. One cycle after this shift in output from the SVC changes, the 87T relay operates. Figure VI.2. Case 4 Event Record The relevant 87T settings are: There is significant distortion on current of each phase in all of the current waveforms. In another word, there was significant harmonic content in the current. Using Fourier analysis, each Page 7 of 9 phase current of the transformer contains high percentage of 2nd harmonics. This alludes to CT saturation or transformer saturation. Since the magnitude of the primary current was not high and CT’s are of C800 class, it was very unlikely to be CT saturation issue. The transformer saturation was the most possible reason because if the transformer core saturates, the magnetizing current would increase such that the spurious differential current would increase. Checking the record of adjacent line relays, the voltage prior to the fault was about 1.03~1.04pu. Additionally, the event record from the SVC controller illustrates that low side bus voltage during the fault was approximately 1.5pu while high side voltage was about 1.15pu. Since the flux level is proportional to the voltage level, the transformer core may be saturated already before the fault or close to be saturated, and was exacerbated during the fault. This can drive the transformer into the saturation mode and cause more magnetizing current. many literatures discussing this topic. The security and sensitivity of the traditional 2nd harmonic block method seems a dilemma forever unless another blocking method is used to prevent 87T operation under energization. However, it turns out the inrush current was not the key problem in this case. After two attempts of energization, the event record and relay settings (shown below) were reviewed. Another possible source of error might be the SVC switching operation. During the fault, because of the voltage sag on the faulty phase, the TSC switched in the capacitor bank. Such switching could produce significant harmonics in the currents. Even though the digital relays have filters and the 87T is supposed to cancel the harmonics. The harmonics can still have certain impact to the phasor estimation, especially the phase angle error. In this case, the 87T pickup was set at 0.099pu, which is biased towards sensitivity. Such setting is generally acceptable to account for CT error, relay error and magnetizing current. However, this setting may not be able to override the increased magnetizing current plus other errors. If the pickup is set at 0.2pu, the mis-operation would not happen. So the decision for the mis-operation mitigation was to increase the 87T pickup setting to 0.2pu and increase the 87T Slope 1 from 30% to 35%. Since the transformer is also protected by other elements such as restricted earth fault protection, negative sequence differential protection, Buchholz relay, etc. the slight setting changes would not compromise the protection for the transformer. The lesson learned in this case is: The pickup setting of the 87T function should not be set too sensitive. If the transformer is part of a FACTS device (Flexible AC Transmission System, such as SVC, HVDC, etc.), extra margin may be added to the 87T pickup setting. VII. MIS-OPERATION CASE 5 On Aug. 15th 2015, a transformer (70MVA, 345KV/13.8KV, Delta/Wye) was tripped by 87T relay during the energization. The energization was attempted twice, and the 87T relay tripped twice. CT ratio Rated MVA Nominal voltage 87T Pickup Slope 1 Break 1 Break 2 Slope 2 Inrush Inhibit Function Inrush Inhibit Mode Inrush Inhibit Level W1 W2 400 600 70 70 345.0 19.5 kV kV 0.05 pu 30% 1.53 pu 7.66 pu 75% Adaptive 2nd 2-out-of-3 15% Figure VII.1. Case 5 Event Record and 87T settings The 2nd harmonics is used by the 87T relay to prevent false operation due to magnetizing inrush current. Different from the traditional method, this type of 87T relay not only checks the percentage of 2nd harmonics within the differential current, but also the phase angles of the 2nd harmonics with regards to the fundamental frequency phasors [4]. The setting also selects the 2-out-of-3 inhibit mode, which means that 2nd harmonics need to be significant in more than one phase before blocking the 87T operation. The three phase current waveforms in the event record shows typical inrush currents during energization. Using Fourier analysis, each phase current actually contains significant 2nd harmonics, about 40-50%. So why did the relay still operate? The mis-operation due to inrush current was not uncommon. Similar events had occurred from time to time and there are Page 8 of 9 Another two issues were noticed when checking the setting file. First, the 87T pickup setting of 0.05pu (0.25A secondary) is very low. Second, the 345kV side CT ratio was 2000/5, which might be the reason of the low 87T pickup setting. For the 70MVA transformer in this case, the nominal current of 345kV side is only 117A. According to the relay vendor, there is a cut-off level of 0.04pu for differential current calculation. This is understandable: if the current input is too small, there could be more error in the phasor and differential current calculation. In this case, Phase C differential current (Icd) was below 0.04pu all the time. So in the event record the phasor Icd was shown 0.0. The phasors of Iad and Ibd were above 0.05pu. At the beginning, the 87T operation was blocked due to significant 2nd harmonics in both A and B phases. After a few cycles, when the current on Phase B (Ibd) dropped to be less than 0.04pu, both the differential current and 2nd harmonics on B phase became zero inside the relay. So the 2-out-of-3 logic on harmonics blocking would not block 87T operation anymore. The relay tripped because Iad was still above 0.05pu. Therefore, the root cause of this case is actually not due to the inrush blocking settings. The high CT ratio and low 87T pickup setting are the culprits. The lessons learned are: • Do not use an overly high CT ratio for 87T protection. An 87T relay with 5A nominal inputs should be able to see 1~5A current under normal operation. • The rule of thumb of 87T pickup setting is: never set the pickup lower than 0.1pu. If certain calculation indicates the need of lower-than-0.1pu setting, change the CT ratio. VIII. SUMMARY AND RECOMMENDATIONS In this computer era, intelligent electronic devices (IED) have helped a lot to simplify relaying schemes. However, it is risky to put blind faith on the intelligence of the digital relays. The knowledge and the judgement of a protection engineer are still the most important tools to ensure the quality of setting work. Ideally, a relay engineer not only needs to have knowledge of the protection principle but also needs to know how the digital relay works internally to a certain degree. And calculation is always necessary. Using the 87T relay as example, a set of standard settings may be just fine to provide good protection for most transformers. However, when there are slight variations with the transformer, the CT or the system, if the settings are not adjusted accordingly, mis-operation may occur. This paper presented five mis-operation cases. These misoperations could have been avoided, if each of the 87T relay setting was contemplated carefully. For example, in the first case, the “Not Within Zone” was set for “Grounding” of Delta winding because the setter did not think of the grounding transformer at Delta side. In the second case, the CT connection setting of the relay was set as Delta because the setter only looked at the auxiliary CT’s connection, without thinking of the main CT’s and the generator configuration. This type of over-simplified thinking process should be avoided. In addition, a relay engineer should always carefully balance the sensitivity and security. In the Case 4 and Case 5, the 87T pickups were set too sensitive and caused misoperation. The only mis-operation case that seemed hard to avoid is the Case 3, but if one has a questioning attitude with the default or “automatic” settings, the issue could emerge before the setting was issued. A methodology to help with the understanding of the digital relays is to mimic the relay internal calculations for a specific application. Most relay vendors have 87T calculations and the compensation equations included in the manuals, so it is not too hard to go through the calculations step by step. Modern CAD tool can also be utilized to automate the calculation and to gain insight of the relay internal process at the same time. At the end of this paper, the MathCAD calculation sheets for two types of 87T relays are provided as reference. REFERENCE [1] J.L. Blackburn and T.J. Domin, “Protective Relaying Principles and Applications, Third Edition”, CRC Press, 2006 [2] GE-T35 Instruction Manual, Available: http://www.gedigitalenergy.com [3] SEL- 487E Instruction Manual, Available : http:// www.selinc.com [4] What is an operating principle for magnetizing inrush inhibit on the T60 and SR745 relays? Available: http://www.gedigitalenergy.com/products/support/t60/get8429.pdf Yiyan Xue received his B.Eng. from Zhejiang University in 1993 and M.Sc. from the University of Guelph in 2007. He is currently an Engineer Principal in American Electric Power (AEP), working on protection and control standards, relay settings, fault analysis, simulation studies, etc. Before joining AEP in 2008, he had worked in GE for 3 years, in ABB for 10 years and in GEC-ALSTHOM for 1 year. Yiyan Xue is a senior member of IEEE and a Professional Engineer registered in the state of Ohio. Zachary P. Campbell received his B.S.E.E. degree from the University of Akron, in Akron, Ohio, in 2008, and his M.Sc. degree from The Ohio State University, in Columbus, Ohio, in 2012. He has been an engineer at American Electric Power (AEP) since 2008, working in various capacities within protective relaying departments including field services and engineering. Zak is a member of IEEE, CIGRE and is a registered professional engineer in the state of Ohio. Sudhakar Chidurala received his B.Eng. from Osmania University in 1989 and Master of Technology from REC, Kakatiya University in 1999. He is currently a Protection and Control Engineer in AEP. Before joining AEP in 2007, he had worked 3 years in Hydro One Inc., Canada and 13 years in AP Transmission Corp., India at various capacities in Protection and Control Engineering. Sudhakar is active Senior Member of IEEE, MIE of India and is registered professional engineer in the state of Oklahoma and Texas. Charles Jones received his BSEE from West Virginia University in 1982 and MEEE from the University of Idaho in 2011. He is currently a Staff Engineer at American Electric Power, working on protection and control standards, relay setting templates, fault analysis, etc. He has been with American Electric Power for 30 years. Charles Jones is a member of IEEE and a Professional Engineer registered in the state of West Virginia and Ohio. Page 9 of 9 Appendix I: Relay Type-I 87T Function Calculator Notes This example is only for 3-winding transformer and 3 groups of CT inputs to 87T relay Color Codes: Equation to be updated by user MVA ≡ 1000000⋅ V⋅ A pu ≡ 1 ∠ ( mag , ang) ≡ mag⋅ ( cos ( ang⋅ deg) + i⋅ sin ( ang⋅ deg) ) 1. Relay Settings nWnd := 3 Number of windings: Enter the MVA, nominal kV and CT primary for each winding 62.5MVA Snom := 62.5MVA 35.7MVA 138kV Vnorm := 69kV 46kV ReferenceWinding := 0 500A CT prim := 500A 500A (0="Auto", otherwise, enter the winding# as reference) "Y" Connection := "Y" "D" "Within Zone" Grounding := "Within Zone" "Not Within Zone" Phase angle with regards to Winding #1: 0 AngWrtW1 := 0 −30 • CTsec := 5A 87T Curve settings Pickup := 0.2 Slope1 := 40% Break1 := 2.0 Slope2 := 100% 2. CT Ratio Check • CT ratio • Nominal primary and secondary current for each winding 100 CTR := = 100 CTsec 100 CT prim → Snom Inorm_prim := Vnorm⋅ 261 = 523 A 3 448 1 Break2 := 15.0 → 2.615 Inorm_prim Inorm_sec := = 5.23 A CTR 4.481 Tthe selection of CT ratio should make the secondary nominal current 1~5A for each winding. 3. Magnitude Compensation Factors → 1.912 CTprim Imargin := = 0.956 Inorm_prim 1.116 • The automatic reference winding is ( ( ) ) RefW_Auto := match min Imargin , Imargin + 1 = ( 2 ) • The reference winding is RefW := RefW_Auto0 if ReferenceWinding = 0 = 2 ReferenceWinding otherwise • The magnitude compensation factors (M) are used to convert each winding current before the differential and restraint current are calculated → 2 CT prim⋅ Vnorm = 1 M := ⋅ Vnorm CTprim RefW− 1 RefW− 1 0.667 If the setting of Reference Winding is "Automatic Selection", the winding cooresponding to the smallest Margin Factor will be selected by the relay as the reference winding. If you see a overly high M factor, please set the reference winding manually instead of using "Automatic Selection". 3. Phase Compensation Reference i := n← 0 while Connectionn = "Y" n← n + 1 n i=2 RefAngle := AngWrtW1i = −30 −30 AngWrtRef := RefAngle − AngWrtW1 = −30 0 4. Fault Analysis 2 Enter fault current phasors in primary value. The angle is in Degree. 175.85 ∠ −283.2 IAFprim := 83.6 ∠ −103.37 ⋅ A 80.52 ∠ −84.01 → IAFprim IAF := CTR • 112.84 ∠ −64.45 IBFprim := 102.95 ∠ −204.83 ⋅ A 592.83 ∠ −286.92 112.9 ∠ −145.43 ICFprim := 119.15 ∠ −341.61 ⋅ A 236.57 ∠ −318.7 → → IBFprim IBF := CTR ICFprim ICF := CTR Phase and zero sequence compensation IAp0gnd := 2⋅ IAF − IBF − ICF 3 IAp30lag := IAF − ICF IBp0gnd := IBp30lag := 2⋅ IBF − IAF − ICF 3 IBF − IAF ICp30lag := 2⋅ ICF − IAF − IBF 3 ICF − IBF 3 3 IAp30lead := ICp0gnd := IAF − IBF IBp30lead := 3 IBF − ICF ICp30lead := ICF − IAF 3 3 3 IAW ( n) := s ← IAFn if AngWrtRef n = 0 ∧ Groundingn = "Not Within Zone" IBW ( n) := s ← IBFn if AngWrtRef n = 0 ∧ Groundingn = "Not Within Zone" ICW ( n) := s ← ICFn if AngWrtRef n = 0 ∧ Groundingn = "Not Within Zone" IAp0gndn if AngWrtRef n = 0 ∧ Groundingn = "Within Zone" IAp30lead n if AngWrtRef n = 30 IAp30lagn if AngWrtRef n = −30 s⋅ Mn⋅ CTsec− 1 IBp0gndn if AngWrtRef n = 0 ∧ Groundingn = "Within Zone" IBp30leadn if AngWrtRef n = 30 IBp30lagn if AngWrtRef n = −30 s⋅ Mn⋅ CTsec− 1 ICp0gndn if AngWrtRef n = 0 ∧ Groundingn = "Within Zone" ICp30leadn if AngWrtRef n = 30 ICp30lagn if AngWrtRef n = −30 s⋅ Mn⋅ CTsec− 1 • Calculate Differential and Restraint Currents ii := 0 .. nWnd − 1 IAXii := IAW ( ii) IBXii := IBW ( ii) ICXii := ICW ( ii) 3 nWnd− 1 IdA := ∑ IAXk ( ) IdB := k=0 → IrA := max IAX • ∑ IBXk ( ) IdC := → IrB := max IBX ∑ ICXk ( ) k=0 k=0 → IrC := max ICX Differential current, magnitude in pu IdA = 0.342 • nWnd− 1 nWnd− 1 IdB = 0.316 IdC = 0.325 Restraint current, magnitude in pu IrA = 0.62 IrB = 0.79 0 Pickup vx1 := Slope1 Break1 Break2 vx := IrC = 0.34 Pickup Pickup vy1 := Break1⋅ Slope1 Break2⋅ Slope2 vx1 if xlimit ≤ Break2 vy := stack ( vx1 , xlimit) otherwise vy1 if xlimit ≤ Break2 stack ( vy1 , xlimit⋅ Slope2) otherwise Differential Current (Unit in pu) 87T Characteristic vy 2 IdA IdB Change the limit of X-axis and Y-axis: IdC xlimit ≡ 3 1 ylimit ≡ 3 0 0 1 2 3 vx , IrA , IrB , IrC Restraint Current (Unit in pu) 4 Appendix II: Relay-2 87T Function Calculator Notes Color Codes: Y ≡1 D ≡ Equation to be updated by user pu ≡ 1 3 ∠ ( mag , ang) ≡ mag ⋅ ( cos ( ang ⋅ deg) + i ⋅ sin( ang ⋅ deg) ) The TxCTC setting represents one of the following matrixes. The CTC(12) produces no phase shift, but it removes the zero-sequence components. 2 −1 −1 1 CTC12 := ⋅ −1 2 −1 3 −1 −1 2 1 −1 0 1 CTC1 := ⋅ 0 1 −1 3 −1 0 1 D lags Y by 30 1 0 −1 1 CTC11 := ⋅ −1 1 0 3 0 −1 1 D leads Y by 30 1. Relay Settings MVA_ := 1500 ICOM := "Y" Windings:S,T,U,W CTRS := 600 CTCONS := Y TSCTC := CTC12 VTERMS := 765 CTRT := 600 CTCONT := Y TTCTC := CTC12 VTERMT := 765 CTRU := 6928 CTCONU := D TUCTC := CTC1 VTERMU := 26 CTRW := 7000 CTCONW := Y TWCTC := CTC1 VTERMW := 26 CTRX := 1 CTCONX := Y TXCTC := CTC1 VTERMX := 1 O87P := 0.3 U87P := 16 SLP1 := 25% SLP2 := 60% DIOPR := 1.2 DIRTR := 1.2 E87HB := "N" E87HR := "Y" PCT2 := 20% PCT4 := 20% PCT5 := 35% S87QP := 1.0 SLPQ1 := 100 S87QD := 100 2. Check the TAP In order to compensate for the differential current due to CT ratios and transformer ratio, the relay will calculate the scaling factor, namely TAPn, for each winding current. The calculation method is, ⋅ 1 TAPS := TAPT := TAPU := TAPW := TAPX := MVA_ ⋅ 1000 3 ⋅ VTERMS ⋅ CTRS ⋅ CTCONS = 1.89 MVA_ ⋅ 1000 3 ⋅ VTERMT ⋅ CTRT ⋅ CTCONT = 1.89 MVA_ ⋅ 1000 3 ⋅ VTERMU ⋅ CTRU ⋅ CTCONU = 8.33 MVA_ ⋅ 1000 3 ⋅ VTERMW ⋅ CTRW MVA_ ⋅ 1000 3 ⋅ VTERMX ⋅ CTRX ⋅ CTCONW = 4.76 ⋅ CTCONX = 866025.4 2. 87T Calculation and Plots • Fault Current Phasors For each winding, enter the A, B, C phase current from top to bottom. 237 ∠ 44 IFS := 274.8 ∠ 280.7 311.3 ∠ 163.2 527.8 ∠ 36.9 IFT := 504.8 ∠ 278.7 480.9 ∠ 155 13641.4 ∠ 186.6 IFU := 13590.2 ∠ 68 13915.6 ∠ 307.5 951.5 ∠ 328.9 IFW := 997.3 ∠ 207 963 ∠ 87.4 • 0 ∠ 0 IFX := 0 ∠ 0 0 ∠ 0 87T Calculation ISC := TSCTC ⋅ IFS ⋅ CTCONS CTRS ⋅ TAPS IWC := TWCTC ⋅ IFW ⋅ CTCONW CTRW ⋅ TAPW ITC := TTCTC ⋅ IFT⋅ CTCONT CTRT ⋅ TAPT IXC := TXCTC ⋅ IFX ⋅ CTCONX CTRX ⋅ TAPX 0.234 + 0.177i DiffSEL := ISC + ITC + IUC + IWC + IXC = 0.042 − 0.296i −0.276 + 0.118i 2 IUC := TUCTC ⋅ IFU ⋅ CTCONU CTRU ⋅ TAPU ResA := ISC0 + ITC0 + IUC0 + IWC0 + IXC0 = 1.11 ⋅ pu DiffA := DiffSEL0 = 0.29 ⋅ pu ResB := ISC1 + ITC1 + IUC1 + IWC1 + IXC1 = 1.14 ⋅ pu DiffB := DiffSEL1 = 0.3 ⋅ pu ResC := ISC2 + ITC2 + IUC2 + IWC2 + IXC2 = 1.14 ⋅ pu DiffC := DiffSEL2 = 0.3 ⋅ pu The following variables are just for plotting. 0 O87P vx1 := SLP1 xlimit O87P vy1 := O87P xlimit ⋅ SLP1 O87P vx2 := SLP2 xlimit O87P xlimit ⋅ SLP2 vy2 := 87T Characteristic 87T Settings: O87P = 0.3 ⋅ pu SLP1 = 25 ⋅ % Differential Current (pu) 4 SLP2 = 60 ⋅ % Fault Quantity Calc: DiffA DiffA = 0.29 ⋅ pu DiffB 3 DiffB = 0.3 ⋅ pu DiffC DiffC = 0.3 ⋅ pu vy1 vy2 2 ResA = 1.11 ⋅ pu ResB = 1.14 ⋅ pu ResC = 1.14 ⋅ pu 1 Change the scale: 0 xlimit ≡ 5 0 1 2 3 4 ResA , ResB , ResC , vx1 , vx2 Restraint Current (pu) 3 5 ylimit ≡ 5
