Solar Storms Impact on High-Voltage Saturable-Core Transformers and Mitigation Methods

Transcription

1 Solar Storms Impat on High-Voltage Saturable-Core Transformers and Mitigation Methods Rui Fan 1, Student Member, IEEE, Yu Liu 1, Student Member, IEEE, Zhenyu Tan 1, Student Member, IEEE, Liangyi Sun 1, Student Member, IEEE, and Yiran An 2 1 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 and they 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 very harmful beause they an saturate transformer ores and generate harmonis in the windings, ausing many detrimental effets or even failures of transformer. High-voltage transformers are extremely vulnerable to solar storms beause the low resistivity to ground makes it easier for GICs to find the eletri path. In this paper, the model of high-voltage transformers with saturable ores is introdued for the purpose of resiliene studies. Transformer ores are modeled by high-fidelity nonlinear equations to represent the nonlinear magnetization harateristis. Numerial simulations are onduted to study the impat of solar storms on 5kV three-phase transformers. The study shows the impat of solar storm inludes reative power loss, voltage flutuation, relay misoperation, and transformer heating. Several mitigation methods are also presented against the geomagneti disturbanes aused by solar storms. Index Terms- Solar Storms, geomagnetially indued urrents (GICs), high-voltage transformer saturation, harmonis, mitigation methods 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 [1]-[2]. Solar storms release large amount of harged partiles, whih travel about 1 to 3 days until they arrive at 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 1 volts/km or higher [4]. The ESPs in turn produe geomagnetially indued urrents (GICs) through the neutral onnetions of transformers and transmission lines at the affeted regions. The frequeny of GICs are typially below 1 Hz, thus GICs are onsidered as quasi-diret or diret urrents for the purposes of eletri grid analysis. The quasidiret GICs an saturate transformer ores and generate harmonis in the windings, ausing detrimental effets suh reative power losses, voltage flutuations, relay misoperation, and transformer heating [5]-[6]. GICs reorded in the neutrals of transformers are onsiderably larger in magnitude than antiipated, and the frequeny of ourrene is higher than expeted [7]. Researhers had pointed out that solar storms may be the major ause of transformer problems during 198 to 1994 [8]. There were also reords indiating transformer failures at the Eskom network of South Afria were aused by solar storms in late Otober and early November of 23 [9]. High-voltage transformers are espeially vulnerable to the geomagneti disturbanes aused by solar storms. The low resistivity of high-voltage transformers makes it easier for GICs to find the eletri path. Moreover, high-voltage transformers are typially designed to operate near the knee region of the B-H urve for the purposes of saving material ost and reduing the volume. The quasi-diret GICs draw inreased magnetizing urrent, whih an easily saturate the transformer ores and generate harmonis in the windings. Saturation and harmonis aused by severe GICs onsume large amount of reative power, ausing voltage flutuations and threatening the stability of nearby eletri systems. It is important to understand that solar storms annot be prevented, while engineers an plan for, ride through and reover from the geomagneti disturbanes with proper transformer resiliene studies. The resiliene studies range from understanding how to model the high-voltage transformers, assessing the vulnerability to designing mitigation strategies against geomagneti disturbanes. To analyze the impat of solar storms, the fundamental thing is to build the appropriate model of transformers that an mimi the ores saturation aused by GICs. This paper introdues the model of high-voltage saturable-ore transformers. Transformer ores are modeled by equations with high fidelity to represent the nonlinear magnetization harateristis. If GICs enter the transformers, their ores will saturate in realisti manners aording to introdued models. Resiliene studies are onduted based on the presented transformer model. Speifially, numerial simulations are performed to study the impat of solar storm

2 on 5kV three-phase transformers. The simulation analyzes the influene on ores saturation and harmonis, relay misoperation and transformer heating. To mitigate these influene, several strategies are also presented at the end of the paper. This paper is organized as follows. In Setion II, the models of saturable-ore transformers are introdued. In Setion III, the impat of solar storms is disussed. In Setion IV, numerial simulations on three-phase high-voltage transformers are presented. The mitigation strategies are disussed in Setion V. Setion VI presents onlusions. II. MODELING OF SATURABLE-CORE TRANSFORMERS The model of high-voltage saturable-ore transformers is presented in this setion. For simpliity, the models are introdued by a single-phase transformer with two-windings, as is shown in Figure. 1. However, the models an be easily generalized to three-phase multi-winding transformers. v () t 1 i () t 1 v () t 2 i () t 2 r 1 N 1 N 2 L 1 g LM i () t m (a) i t i () t 2 1 () + e () t (b) jl 12 Figure 1. Single-phase transformer model: (a) two-winding transformer (b) equivalent iruit + L 2 r 2 v () t 3 i () t 3 v () t 4 i () t 4 The model of the single-phase transformer is represented with following differential algebrai equations: i ( t) i ( t) i ( t) g e ( t) (1) 1 1 m i ( t) i ( t) i ( t) g e ( t) (2) 2 1 m i ( t) i ( t) (3) 3 2 i ( t) i ( t) (4) 4 2 di () t di () t dt dt (5) 1 3 v1 ( t) v2 ( t) ri 1 1( t) L1 L12 e ( t) di ( t) di ( t) N v ( t) v ( t) r i ( t) L L e ( t) dt dt N1 (6) d() t e ( t) dt (7) N i ( t) N i ( t) (8) where v () t and i () t are the terminal voltages and 1~4 1~4 urrents, r1, r2, L1, L2, L are the orresponding resistanes, 12 indutanes and mutual indutane, N1 and N2 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. The transformer ore is modeled by a high-fidelity equation to represent the nonlinear magnetization harateristis [1]: () t im ( t) i sign ( t) (9) where i and are the equation onstants, n is the exponent, and sign is the sign of funtion. For typial materials used for transformer ores, the exponent n an be 11 to 13 resulting in a prefund nonlinearity. n III. SOLAR STORMS IMPACT ON TRANSFORMERS With the geomagneti disturbanes aused by solar storm, quasi-diret GICs flow though the transformers and ause many detrimental effets. The main effets inlude ores saturation and harmonis, reative power losses, voltage flutuations, relay misoperation, and transformer heating. A. Core Saturation and Harmonis As is mentioned, transformers are typially designed to operate near the knee region of the B-H urve, as shown in Figure 2. The AC magnetizing urrent is non-sinusoidal in the region. The quasi-diret GICs aused by solar storms draw inreased magnetizing urrent, whih an easily saturate the transformer ores. Currents in the saturated transformers ontains large amounts of harmonis. These harmonis not only inrease transformer losses and derease its life expetany, but they also affet the operation of relays and ause reative power and voltage problems. Flux Density (B) Knee Point Magneti Field Strength (H) Figure 2. Transformer ore B-H urve

3 B. Reative Power Loss and Voltage Flutuations When transformers are saturated by GICs, they will onsume more reative power. Inreased reative power losses lead to under-voltage or voltage flutuations. In some extremely severe situations, it may result in an area grid blak-out if voltage limits are exeeded. C. Relay Misoperation The impat on transformer relays is identified in two ways. First, severe GICs may saturate the CTs, ausing the misoperation of perentage-differential relays due to the saturation on one side of the transformer only [11]. Seond, harmonis aused by GICs may ause the misoperation of negative-sequene differential relays, or mislead harmonirestraint differential relays to blok the orret trip signals. For example, harmoni-restraint differential relays will inhibit any trip signals if the seond-harmoni level in the operating urrent is higher than a pre-set threshold (15%~25%). Therefore, if faults happen inside a transformer and at the same time GICs ause saturations and harmonis, the transformer may not be proteted by the relays. D. Transformer Heating Transformer heating is another problem aused by the GICs. The inreased magnetizing urrents result in inreased opper losses ( ir 2 ) at transformer windings. In addition, exessive eddy-urrent losses ramp up in the transformer ores due to the harmonis. The relationship between eddyurrent losses and harmonis is as follows [12]-[13]: hmax 2 2 EC EC1 h h1 P P I h (1) where P EC is the total eddy-urrent losses, PEC 1 is the eddyurrent losses at full load, h is the harmoni order and I h is th the urrent RMS value at h order harmoni. The opper losses and eddy-urrent losses together inrease transformer heating both in the transformer ore and in other metalli omponents through the flux leakage [7]. Without proper ooling or proteting methods, inreased heating an break the windings insulation, redue the lifespan and even take down the transformer if the severe overheating ours. IV. NUMERICAL SIMULATIONS AND RESULTS Numerial simulations are performed to study the impat of solar storms on high-voltage saturable-ore transformers, as shown in Figure 3. The 5 kv three-phase transformers onneted with 3 km transmission lines are presented with aforementioned models. 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 level of indued ESPs. The duration of GMDs is 1 minutes. 5kV, 3km Transmission Line Transformer + - ESP, 2volts/km Transformer 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 two 5 kv transformers, and the resistanes of transmission lines, transformers and their groundings. The parameters of transformers and transmission lines are listed in Table I and Table II. Table I. Transformer parameters Power Rating (MVA) 4 Voltage (kv) 115/5 Leakage Reatane (p.u.).1 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) 1.67 The simulation results are used to analyze the impat of solar storms in three aspets: ores saturation and harmonis, relay misoperation and transformer heating. A. Impat One: Cores Saturation and Harmonis The impat of solar storms on transformer ores saturation and harmonis is shown in Figure 4. Voltage Phase AN (kilovolts) Current Phase A (Amps) 1 5 with GICs without GICs Time (s) Figure 4. Impat on ores saturation and harmonis

4 The blue dash lines represent the transformer terminal voltage and urrent measurements at normal operations; while the red solid lines represent those measurements under the influene of GICs. The voltage and urrent waveforms are sinusoids when the transformer is working normally. When the quasi-diret GICs flow through the transformer, obvious distortions our to the waveforms beause of the ores saturation. Moreover, transformer voltage waveforms flutuate during the solar storms period. Although it is not learly shown in the figure, a 3.9% voltage drop ours to the transformer due to the inreased reative power losses. The saturated transformer ontains large amount of even and odd harmonis in the winding urrents. The fundamental value and 2 nd to 7 th order harmonis are shown in Figure 5. These harmonis are extremely harmful, leading to relay misoperation and transformer heating. Harmoni Perentage (%) Harmoni Order Figure 5. Harmoni in transformer urrents B. Impat Two: Relay Misoperation The impat of solar storms on transformer harmonirestraint differential relays is shown in Figure 6. The priniple of harmoni-restraint differential funtions is that relays will blok any possible trip signal if the seond-harmoni level in the operating urrent is higher than a pre-setting (typially a pratial setting is 2%) in order to prevent false tripping from inrush urrents. As is shown in Figure 6, the operating urrent (refer to the primary side) is obviously non-sinusoidal and it ontains large amount of harmonis. The seond-harmoni level in the operating urrent is about 49%, whih is muh higher than the pratial threshold settings. As a onsequene, if faults happen inside the transformer during the period of geomagneti disturbanes, harmoni-restraint differential relays may blok the trip signals sent by other relay funtions. Thus the transformer may not be proteted during the dangerous period. Operating Current (Amps) Seond Harmoni (%) Setting Threshold Time (s) Figure 6. Impat on harmoni-restraint differential relay C. Impat Three: Transformer Heating With a simplified transformer thermal model [14], the impat of solar storms on transformer heating is shown in Figure 7. At the beginning transformer operates normally, the temperatures of hotspots (ore, oils, oil and tank) are around 5. When GICs flow through the transformer opper losses and eddy-urrent losses are inreased, leading to the rise of hotspot temperatures. At the end of the GMDs period, the highest hotspot temperature reahes about 85 (typially the temperature limit for transformer is 15 ). It is notied that the onduted simulation has only used a moderate-level 2 volts/km ESP that lasts for 1 minutes to study the impat on transformer heating, yet some extreme levels of indued ESPs an reah more than 1 volts/km and have a longer duration. Stronger solar storms result in higher rise of the hotspot temperatures. It is highly possible that transformer will be taken down by overheating if it is attaked by more severe solar storms. Temperature in Celsius GMDs Period Core Primary Coil Seondary Coil Oil Tank Time (mins) Figure 7. Impat on transformer urrents

5 V. MITIGATION METHODS In this setion, several mitigation methods are presented against the geomagneti disturbanes aused by solar storms. A. Foreasting and Monitoring With modern tehnologies and robust spae monitoring arhitetures, sientists an provide short-term warnings (6 to 12 hours) in advane [8]. Therefore, a lot of preparations an be set up before the solar storms truly affet the Earth. For example, operators an ativate GICs bloking devies at transformer neutrals or the series apaitors onneted in the series ompensated transmission lines before GMDs really our. Given the 6 to 12 hour preparation time, loal utilities an also gather more labor and equipment to ride through and restore from the geomagneti disturbanes. B. Transformer Neutral Resistane Grounding and DC Bloking High-voltage transformers are more vulnerable to solar storms beause of their low-resistivity groundings. Therefore, the quasi-diret GICs an be effetively redued or bloked by installing resistane groundings or DC bloking devies at the ground onnetion of transformers [7]. The first grounding method inreases the neutral resistane of transformers, reduing the GICs flow through the transformers to tolerable levels. The DC bloking devies disonnet the transformers from the ground for the quasidiret GICs, with no influene on the system AC flows. These two grounding methods are reognized as the most effetive methods to mitigate the impat of solar storms [15]. C. Series Compensated Lines Series apaitors in the transmissions lines are typially used to ompensate the reative power, while they an also be used to mitigate the impat of solar storms [6]. The priniple is the same as transformers neutral DC bloking devies: the series apaitors blok the quasi-diret GICs and allow the system AC flows. D. Protet Critial Transformers The key point of transformer resiliene is that GMDs aused by solar storms is unpreventable. Therefore, if it is unrealisti to over eah transformer when the disturbanes our, it will be better just to protet the most ritial transformers and redue the losses as muh as possible. VI. CONCLUSIONS In this paper, the model of high-voltage transformers with saturable-ores is introdued to study the impat of solar storms. Transformer ores are modeled by high-fidelity nonlinear equations to represent the nonlinear magnetization harateristis. Simulation results indiate that GICs generated by solar storms an saturate the high-voltage transformer and reate large amount of harmonis, ausing many detrimental effets suh as reative power loss, voltage drop, misoperation of harmoni-restraint differential relay, and transformer heating. At the end of the paper, some mitigation methods have been presented to prevent or alleviate the impat of solar storms. REFERENCES [1] L. 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