The Impact of Solar Storms on Protective Relays for Saturable-Core Transformers

Transcription

1 The Impat of Solar Storms on Protetive Relays for Saturable-Core Transformers Rui Fan, Student Member, IEEE, Yu Liu, Student Member, IEEE, Aniemi Umana, Member, IEEE, Zhenyu Tan, Student Member, IEEE, Liangyi Sun, Student Member, IEEE, and Yiran An 2 Shool of Eletrial and Computer Engineering, Georgia Institute of Tehnology, USA 2 State Grid Shanghai Pudong Eletri Power Supply Company, China rfan7@gateh.edu Abstrat- Solar storms often ause transient variations in the Earth s magneti field. These storms are alled geomagneti disturbanes (GMDs). GMDs an generate quasi-diret geomagnetially indued urrents (GICs) through the neutral onnetions of transformers and transmission lines at the affeted regions. GICs are quite detrimental to transformers beause they an saturate transformer ores and generate harmonis in the windings. Power Transformers are typially proteted by the ommerial relays in whih several protetive funtions are implemented. When GICs flow through a transformer, the protetive relays for the transformer may be affeted. Studies have shown the impat of solar storm inludes reative power loss, voltage flutuation, and transformer heating. In this paper, numerial simulations are onduted to study the impat of solar storms on the protetive relays for saturable-ore transformer. Also, the ommonly applied transformer protetion funtions in ommerial relays are investigated in relation to GICs. The results indiate that the protetive relays may not provide adequate protetion for the transformers in the presene of solar storms. Index Terms- Solar Storms, geomagnetially indued urrents (GICs), transformer saturation, protetive relays G I. INTRODUCTION eomagneti disturbanes (GMDs) are the transient variations in the Earth s magneti field that are aused by the solar storms or oronal mass ejetions []-[2]. Solar storms release large amount of harged partiles, whih travel about to 3 days until they get to the Earth [3]. The harged partiles ause short-term variations in the Earth s magneti field and indue earth surfae potentials (ESPs) with values up to volts/km or higher [4]. The ESPs in turn produe geomagnetially indued urrents (GICs) through the neutral onnetions of transformers and transmission lines in the viinity of the affeted regions. The frequeny of GICs are typially below Hz, therefore GICs are onsidered quasidiret or diret urrents for the purposes of eletri grid analysis. The quasi-diret GICs an saturate transformer ores and generate harmonis in the windings, ausing detrimental effets suh as reative power losses, voltage flutuations, and transformer heating [5]-[6]. The magnitude of GICs reorded on the neutrals of transformers, and the frequeny of ourrene are onsiderably larger than antiipated [7]. Researhers had pointed out that solar storms may be the major ause of transformer problems from 98 to 994. There were also reords indiating that the transformer failures at the Eskom network of South Afria were aused by solar storms in late Otober and early November of 23 [8]. High-voltage transformers are espeially vulnerable to the geomagneti disturbanes aused by solar storms. The low resistivity of high-voltage transformer makes it easier for GICs to find an eletri path. Moreover, the quasi-diret GICs draw inreases magnetizing urrent, whih an lead to ore saturation and the generation of harmonis in the windings. The ore saturation and harmonis in the windings make proteting the transformer with traditional relays hallenging. Transformer heating is another problem aused by the GICs. The inreased magnetizing urrents result in inreased per losses ( ir) 2 in the transformer windings. Exessive eddyurrent losses are also experiened in the transformer ores due to the harmonis. The inreased temperature in the transformer ould ause thermal protetion relays to trip. The detrimental impat of solar storms on transformer protetive relays is twofold. First, severe GICs may saturate the transformers, generating an unbalaned differential urrent. If the differential urrent is higher than the setting, the perentage-differential relays would tend to mis-erate (whih is not neessary) during normal erations [9]-[]. Seond, many transformer protetive relays are implemented with harmoni-restrained differential protetive funtions. The purpose of these funtions is to prevent unneessary, false-tripping during transformer energization. However, sine the harmonis aused by GICs depend on the severity of the GMDs, strong GMDs may signifiantly inrease the seond or fourth harmoni levels in the harmoni-restrained differential relays. If the levels are higher than the relay settings, orret trip signals ould be bloked during transformer internal faults. To analyze the impat of solar storms on saturable-ore transformer relays, apprriate model of transformers are

2 neessary to study the ores saturation aused by GICs []. In this paper, the transformer ores are modelled with highly non-linear equations to represent the non-linear magnetization harateristis. If GICs flow into a transformer, its ore will saturate in a manner onsistent with its non-linear models. Studies of the solar storm impat are onduted based on the presented non-linear transformer model. Speifially, numerial simulations are performed to study the impat of a solar storm on 5kV three-phase transformers. Several ommonly used protetive strategies have been summarized in this paper to show the effets of solar storms on transformers relaying system. This paper is organized as follows. In Setion II, the nonlinear models of saturable-ore transformers are introdued. In Setion III, the transformer relay funtions are disussed. The numerial simulations on three-phase high-voltage transformers are presented in Setion IV and in Setion V the onlusion is disussed. II. NON-LINEAR MODEL OF SATURABLE-CORE TRANSFORMERS Transformers are ommonly treated as linear models. However, to study the impat of GICs, the non-linear model is presented in this setion to mimi atual ore saturation onditions. For simpliity, the non-linear model is introdued using a single-phase, two winding transformer, as is shown in Figure. Obviously, this single-phase, non-linear model an be easily generalized to three-phase multi-winding transformers. v () t i () t v () t 2 i () t 2 r N N 2 L g LM i () t m (a) i t i () t 2 () + e () t (b) jl 2 + L 2 r 2 v () t 3 i () t 3 v () t 4 i () t 4 Figure. Single-phase transformer model: (a) two-winding transformer, (b) equivalent iruit. The model of the single-phase saturable-ore transformer is represented with following differential algebrai equations: i ( t) i ( t) i ( t) g e ( t) () m i ( t) i ( t) i ( t) g e ( t) (2) 2 m i ( t) i ( t) (3) 3 2 i ( t) i ( t) (4) 4 2 di () t di () t dt dt (5) 3 v ( t) v2 ( t) ri ( t) L L2 e ( t) di ( t) di ( t) N v ( t) v ( t) r i ( t) L L e ( t) dt dt N d() t e ( t) dt 2 2 (6) (7) N i ( t) N i ( t) (8) () t im ( t) i sign( t) (9) where v () t and ~4 i () ~4 t are the terminal voltages and n urrents respetively. The terms: r, r2, L, L2, L 2 are the orresponding resistanes, indutanes and mutual indutane. N and N 2 are the number of turns at the primary and seondary sides, g is the exitation ondutane, im () t is the magnetizing urrent and () t is the flux linkage through the iron ore. Lastly, i and are the equation onstants, n is the exponent, and sign is the sign of funtion. The transformer ore model is represented by equation (9), whih is highly non-linear. For typial materials used for transformer ores, the exponent n an be in the order of to 3, resulting in a profound non-linearity [2]. By adting this non-linear model of the ore, non-linear magnetization harateristis when the GICs go through the transformer is aurately represented. III. PROTECTIVE FUNCTIONS OF TRANSFORMERS RELAYS Transformer are ritial omponents of the power system network. Therefore, these transformers, depending on the size, are often proteted with advaned protetive relays. These transformer relays, espeially the mirroessor-based relays, are equipped with multiple protetion funtions [3]. The most ommonly used protetive funtions inlude perentage-differential funtion, harmoni-restrained differential funtion, negative-sequene differential funtion, overurrent funtion, thermal protetive funtion and Gasand-pressure protetive funtion. This protetive funtions of the transformer relays are summarized next. The perentage-differential protetion sheme is one of the most pular legay transformer protetion shemes, as shown in Figure 2. It monitors the urrents oming to the transformer and alulates the erating urrent I I I and restraining urrent Ires Is I, where s2 s s2 2 I s and I s2 are the seondary urrents of CTs on the terminals of the single phase transformer, respetively. Ideally, the erating urrent I remains zero unless an internal fault

3 ours. However, the existene of variable-tap transformers and instrumentation errors make this simple riterion inapprriate for pratial appliations. To overome this problem, a minimum pikup urrent I min and differential ratio K I / I are introdued. The transformer is only tripped if () I res a ertain threshold. I ; and (2) the ratio K I / I min res exeeds transformers protetive relays, as shown in Figure 3. The 5/5 kv three-phase transformers, onneted with a 3 km transmission line are presented with the aforementioned non-linear model. Solar storms generate the indued earth surfae potential (ESP) with a value of 2 volts/km. It should also be noted that 2 volts/km is far from the extreme levels of indued ESPs, whih ould have values as high as volts/km. 5kV, 3km Transmission Line Transformer Transformer + - ESP, 2volts/km Figure 2. Perentage-differential protetion method Harmoni-restrained differential protetion monitors the 2 nd or 4 th harmoni level in the erating urrent, in addition to all the quantities that perentage differential protetion funtion monitored. It bloks any trip signal if the harmoni levels are higher than the settings to avoid relay mis-eration during transformer normal energization. Negative-sequene differential protetion is based on the fat that internal faults reate disturbanes of the symmetry of transformer terminal urrents [4]. Similar to the perentagedifferential protetion, this method uses the negativesequene erating urrent I (Q) and negative-sequene restraining urrent I res(q) of transformer to make trip deisions. The negative-sequene differential protetion only trips the transformer if () I( Q) Imin( Q) ; and (2) the ratio K I / I exeeds a preset threshold. ( Q) ( Q) res( Q) The overurrent protetion is also widely used for transformer protetion. It is set to trip the transformer if the values of the terminal urrents exeed the pikup urrent for a duration determined by the inverse/definite time urve. Thermal protetion is used to protet the transformer from exessive heat, whih would be detrimental to the transformer windings. This heat ould be due to overload onditions, over-exitation or a malfuntioning transformer ooling system. If the temperature heat exeeds the setting, this funtion will trip the transformer. Gas-and-pressure relays monitor the aumulation of gas and sudden hange in pressure inside the transformer tank to detet the internal faults. A ombined gas-aumulator and pressure relay, alled the Buhholz relay, has been in suessful servie for over 7 years. IV. NUMERICAL SIMULATIONS AND RESULTS Numerial simulations have been performed to study the impat of solar storms on high-voltage saturable-ore Figure 3. Test system for numerial simulations The urrents that flow in the network depends on both the indued voltage between the neutral grounding points of the two 5 kv transformers, and the resistanes of transmission lines, transformers and their groundings. The parameters of transformers and transmission lines are listed on Table I and Table II. Table I. Transformer parameters Power Rating (MVA) 4 Voltage (kv) 5/5 Leakage Reatane (p.u.). Resistane (p.u.).2 Table II. Transmission lines parameters Power Voltage (kv) 5 Length (km) 3 Resistane per Phase (Ohms/km).59 Indutane per Phase (mh/km).67 Several ommonly used protetive strategies have been seleted to protet the transformer. The simple perentage differential funtion is not applied for transformer protetion beause of the danger of undesirable tripping on inrush. While the gas-and-pressure model is too ompliated that it is not inluded as well. The orresponding transformer relays have the following settings: (a) harmoni-restrained differential protetion: the perent differential threshold setting is 2%, the minimum pikup erating urrent is A (referred to the primary side) and the 2nd harmoni bloking level is 2%; (b) negative-sequene differential protetion: the perent differential threshold setting is 2%, the minimum pikup erating urrent is.a (referred to the primary side); () time-overurrent protetion: the pikup urrent referred to the primary side is 2 A and the time dial is. and very inverse; (d) thermal protetion: the temperature limit is 5. The simulation results are used to analyze the impat of solar storms on the transformer and the assoiated protetive relays as follows.

4 A. Core saturation and harmonis in the windings Core saturation: The transformer terminal voltage and urrent (with and without GICs) are shown in Figure 4. The blue dash lines represent the transformer terminal voltage and urrent measurements at normal erations; while the red solid lines represent those measurements under the influene of GICs. The voltage and urrent waveforms are pure sinusoids when the transformer is erating under normal onditions. When the quasi-diret GICs flow through the transformer, obvious distortions our to the waveforms beause of the ores saturation. Voltage Phase AN (kilovolts) 5 with GICs without GICs 3.9% dr period. The performane of the transformer protetive funtions are presented next. Harmoni-restrained differential protetion: The results of harmoni-restrained differential protetion are also shown in Figure 6. The erating urrent is about 26 A, whih is larger than the A setting. The restraining urrent is about 72A. The differential perent is 73%, whih is also more than the 2% setting. However, the 2 nd harmoni levels in the erating urrent is higher than the 2% setting, whih means this funtion will blok the trip signal. Therefore, the harmoni-restraint differential protetion will not falsely trip the transformer before the winding fault ours. However, when the % winding fault atually takes plae, the transformer trip ommand is still be inhibited by this harmoni relay, thereby preventing the transformer from being tripped. 68. A 26.4 A 25.6 A Operating_I (A) Restraining_Ires (A) Current Phase A (Amps) Time (s) Figure 4. Impat of solar storms on transformer ore saturation Harmonis in the windings: The saturated transformer ontains large amounts of even and odd harmonis as a result of the solar storm. The fundamental value and 2 nd through 7 th order harmonis are shown in Figure 5. These harmoni levels in transformer terminal urrents are very high. The 2 nd harmoni is about 3% and the 3 rd harmoni is about 2%. These harmonis are extremely harmful to the transformer A 8.7 % 73.2 % 56.4 % 37.3 % 88.6 A 54. A 3. A A % 7.73 % Diff_Perent (%) I_2nd_Harmoni_Level (%) Neg_I (A) Neg_Ires (A) Neg_Diff_Perent (%) 9.87 s.9 s Harmoni Perentage (%) Harmoni Order Figure 5. Harmoni in transformer urrents B. Impat on transformer protetive relaying funtions In this part, a % winding fault near the neutral terminal ours on the transformer at.3s, during the solar storm Figure 6. Perentage differential, harmoni-restraint differential and negative-sequene differential protetion results Negative-sequene differential protetion: The results of negative-sequene differential protetion are also shown in Figure 6. The situation for negative-sequene differential protetion in this event is very similar to the perentage differential protetion funtion. As shown in Figure 6, the negative-sequene erating urrent and differential perent are always higher than the settings. Therefore, this funtion will falsely trip the transformer before the fault ours. Time-overurrent protetion: The result of overurrent protetion is shown in Figure 7. Before the winding fault ours, the RMS value of transformer primary-side urrent is about 88 A, whih is less than the setting (2A). When the % winding fault ours, the RMS value goes up to 257A, whih is still below the setting. Therefore, this protetion funtion does not offer adequate, sensitive protetion for this type of internal fault.

5 257.6 A 88.8 A Pri_I_RMS (A) s.9 s harmonis. The transformer saturation and generated harmonis ould ause assoiated protetive relays fail to erate during solar storms. If the protetive relay is implemented with harmoni restrained differential protetive funtions, it will help prevent mis-erations when the transformers are in normal onditions. However, if an internal fault ours during solar storms, the affeted transformer might not be prerly proteted by the relays. Figure 7. Result of overurrent protetion Thermal protetion: The result of thermal protetion is shown in Figure 8. During the entire solar storm period, the temperature of transformer is ontinually rising. There is only minimal hange in temperature when the fault ours. The temperature is below the 5 C setting, thus thermal protetion funtion annot trip the transformer when the fault is initiated C C C C 59.3 C 59. C 53.7 A 53.7 A C C Core (C) Pri_Coil (C) Se_Coil (C) Oil (A) Tank (C) s.9 s Figure 8. Result of thermal protetion The tripping deision of the transformer protetive relay is dependent on the summary of all the above protetive funtions. Before the fault ours, some protetive funtions tend to falsely trip the transformer beause of the energization aused by GICs. However, the harmoni-restraint differential funtion will blok any trip signal at this time, thus the transformer will not be falsely tripped by the transformer protetive relay. When the fault ourred, some protetive funtions failed to detet the fault while other funtions issued trip signals. However, beause of the harmoni-restraint differential funtion again, the trip signals are bloked. As a onsequene, the protetive relay will not send a trip signal to protet the transformer beause of the influene of solar storm. V. CONCLUSIONS This paper studies the impat of solar storms on saturableore transformer and its protetive relays. To simulate the transformer saturation, the transformer ore is modeled using high-fidelity, non-linear equations. Simulation results indiate that GICs generated by solar storms an saturate the high-voltage transformer and reate large amount of REFERENCES [] L. Hogan, "Solar Storms," New York: Simon and Shuster, 997. [2] J. Kappenman, "Geomagneti storms and their impats on the U.S. power grid," Oak Ridge National Laboratory, Oak Ridge, Tennessee, 2. [3] W.A. Radasky, "Overview of the impat of intense geomagneti storms on the U.S. high voltage power grid," in 2 IEEE International Symposium on Eletromagneti Compatibility (EMC), pp.3-35, Aug. 2. [4] A.P.S. Melioulos, E.N. Glytsis, G.J. Cokkinides and M. Rabinowitz, "Comparison of SS-GIC and MHD-EMP-GIC effets on power systems," IEEE Trans. on Power Delivery, vol.9, no., pp.94-27, Jan [5] T.R. Huthins and T.J. Overbye, "The effet of geomagneti disturbanes on the eletri grid and apprriate mitigation strategies," in 2 North Amerian Power Symposium (NAPS), pp.-5, 4-6 Aug. 2 [6] J.E. Berge, "Impat of geomagnetially indued urrents on power transformers," Ph.D. dissertation, Dept. Elet. Eng., the University of Western Ontario, London, Ontario, Canada, 2. [7] V.D. Albertson, J.M. Thorson, R.E. Clayton and S.C. Tripathy, "Solar-Indued-Currents in power systems: Cause and Effets," IEEE Trans. on Power Apparatus and Systems, vol.pas-92, no.2, pp , Mar [8] C.T. Gaunt and G. Coetzee, "Transformer failures in regions inorretly onsidered to have low GIC-risk," in 27 IEEE Lausanne Power Teh, pp.87-82, July 27. [9] G.K. Stefoulos, G.J. Cokkinides and A.P.S. Melioulos, "Quadratized model of nonlinear saturable-ore indutor for time-domain simulation," in 29 Power & Energy Soiety General Meeting (PESGM), pp.-8, July 29. [] B. Bozoki, S. Chano, L. Dvorak, W. Feero, G. Fenner, E. Guro, C. Henville, J. Ingleson, S. zumdar, P. MLaren, and K. Mustaphi, The effets of GIC on protetive relaying, IEEE Trans. on Power Delivery, vol., no.2, pp , 996. [] D.H. Boteler and R.J. Pirjola, "Modelling geomagnetially indued urrents produed by realisti and uniform eletri fields," IEEE Trans. on Power Delivery, vol.3, no.4, pp.33-38, Ot [2] R. Fan, A.P.S. Melioulos, G.J. Cokkinides, L. Sun and Y. Liu, "Dynami state estimation-based protetion of power transformers," in 25 IEEE Power & Energy Soiety General Meeting (PESGM), July 25. [3] M.A. Rahman, and B. Jeyasurya, A state-of-the-art review of transformer protetion algorithms, IEEE Transations on Power Delivery, vol. 3, no. 2, pp , 988 [4] R. Fan, A.P.S. Melioulos, L. Sun, Z. Tan and Y. Liu, Transformer Inter-Turn Faults Detetion by Dynami State Estimation Method, 26 North Amerian Power Symposium, Denver, CO, 26.