Modeling and Analysis of Resistive Type Superconducting Fault Current Limiters for Coordinated Microgrid Protection

Transcription

1 Modeling and Analysis of Resistive Type Superonduting Fault Current Limiters for Coordinated Mirogrid Protetion R. Haider, M. S. Zaman, S. B. A. Bukhari, Y. S. Oh, G. J. Cho, M. S. Kim, J. S. Kim, and C. H. Kim Abstrat-- The penetration of distributed generation (DG) through mirogrid systems is inreasing ontinually to meet growing energy demand. The energy delivered through mirogrids has inreased the overall reliability of power systems, but at the same time protetion issues are observed. These problems need to be addressed in order to failitate smooth operation of power system. Superonduting fault urrent limiters (SFCLs) provide unique protetion features to eletrial power system during various fault onditions. The paper desribes modeling, haraterization, and implementation of SFCL to enhane the transient stability of power system. The system is modeled and developed in MATLAB/ Simulink environment and the performane of proposed model is validated through simulation results. It is also shown that proposed SFCL provides means of better relay oordination during various fault onditions. This is ahieved by inorporating well-alulated design parameters of SFCL and its plaement at optimal loation. Keywords: superonduting fault urrent limiter (SFCL), distributed generation (DG), mirogrid (MG), fault urrent, protetion. A I. INTRODUCTION S the ountries develop and living values improve, energy demand grows signifiantly resulting in inreased number of power plants and influx of eletrial interonnetions. With these inreasing energy needs and reduing dependeny on onventional power generation tehnologies, the mirogrid systems integrated with distributed energy resoures (DER) are being developed. The integration of DERs in eletrial power distribution system (EPDS) give rise to diverse levels of fault urrent and may interrupt oordination of protetion system [1]. The inreased interonnetions of modern eletrial power system among distributed and entralized power resoures may ause ompliated short iruit senarios. A short iruit fault in eletrial power system affets the ontinuity of supply and hazardous for equipment. This work was supported by the National Researh Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A ). The authors are with the College of Information and Communiation Engineering, Sungkyunkwan University, Suwon City, , South Korea ( razahaider@skku.edu; saeed568@skku.edu; s.basit41@skku.edu; fivebal2@naver.om; thug1220@naver.om; lovelykiku25@naver.om; kjs7107@naver.om; hkim@skku.edu ). Paper submitted to the International Conferene on Power Systems Transients (IPST2017) in Seoul, Republi of Korea June 26-29, 2017 The eletrial system s strength against a fault an be enhaned by applying superonduting fault urrent limiter (SFCL) whih has the prospetive apabilities to suppress ritial fault urrent and protet the relevant equipment against any worst ondition. Due to their unique features (e.g. very quik response, high reliability beause of independent operation from external signals, minimal selfreovery time, no wearable parts, energy effiieny, et.), SFCL are promising andidate for their inreased role in power systems [2]. The well-known fast protetion feature of SFLCs is due to HTS (high temperature superonduting) materials. In general, an SFCL offers superior tehnial performane to limit fault urrents in omparison to onventional protetive approahes. The SFCL shows high impedane during fault period and negligible impedane during normal operating onditions in the iruit [3]. The resistive and indutive are two basi types of SFCLs. In resistive type SFCLs, the superondutor is diretly onneted in series to the line to be proteted. On the other hand, in the indutive type SFCLs, the superondutor is magnetially oupled with the line. Under normal operation, SFCLs have negligible influene on a power system. In ase of fault, they limit short-iruit urrent to a value lose to nominal urrent. With disovery and advanement in HTS materials, SFCLs have strong potential for power system appliations due to their numerous advantages [4]. There is signifiant literature whih has looked into the use of SFCL for power system stability. In [5] the results show that the power quality is signifiantly enhaned by use of SFCLs. The SFCLs limit the fault urrents and ontrols voltage sags on other feeders onneted to power transformer. In addition, the stresses on other equipment (transformers, iruit breakers, et.) are greatly redued, and this prolongs the useful life of the equipment. The use of SFCLs to handle asymmetrial fault urrent has been addressed in [6]. Fault urrents in power systems always reate troubles and adversely affet system s performane and effiieny. The performane of advaned power systems is more vulnerable to these onditions beause they inorporate intermittent and omparatively weak distributed generation (DG). The onventional protetion shemes annot operate effiiently in EPDS integrated with DG beause of bidiretional power flows and unertain urrent and voltage levels under fault onditions. Therefore, it is required to develop new protetion shemes for proteting MG and

2 EPDS integrated with DG. Amongst various protetion shemes, inlusion of SFCL is a promising solution for MG protetion. An SFCL offers several advantages for power system protetion espeially for EPDS integrated with DG beause it has negligible power loss and high initial fault urrent limiting apability usually at first half yle [2], [7]. The relevant literature shows that the protetion of MG is not ahieved with the same methodologies that have been used to protet onventional distribution systems [8], [9]. In general, the MG an operate in both the grid-onneted mode and the islanded mode, whereas the overurrent relays (OCRs) are set for higher fault urrents under grid-onneted mode, but under islanding operation the response is slow. These relays annot operate reliably beause of low fault urrent ontribution from DG [10]. When an EPDS is integrated with multiple DG units as MG, and a short iruit fault ours in the upstream or downstream network, the MG may be tripped off due to protetion equipment setting and poor fault-ride-through (FRT) apabilities of some DG units [11]. Therefore, the protetion equipment design parameters are modified for DER integrated with EPDS. In this study, we have onsidered some pratial senarios whih an be observed in atual mirogrid systems. The fault urrent level varies signifiantly during gridonneted and islanded modes of mirogrid. This results in inreased omplexity for reliable operation of protetion system. There are several protetion methodologies proposed in literature to address this problem [12]. This paper proposes modeling and use of SFCL to inrease transient stability of integrated power system and to ahieve relay oordination of mirogrid in both grid-onneted and islanded modes of operation. The organization of rest of the paper is as follows: in setion-ii SFCL is modeled and analyzed in MATLAB/Simulink environment and its various harateristis are explained. This setion also shows the development of oordinated protetion system apable to work in both modes of mirogrid operation. In setion III, test system is presented and simulation results are disussed. The onlusion of paper is presented in setion-iv. II. MODELING AND ANALYSIS This setion has been divided into three sub-setions whih desribe fundamental priniple of resistive SFCL, modeling and harateristis of resistive SFCL using MATLAB, and development of protetion relaying system to best work with developed SFCL. A. Fundamental Priniple of Resistive Type SFCL A resistive type SFCL is merely a length of superondutor whih is onneted in series with a line to limit high fault urrents. This lass of urrent limiters is based on the physial properties of an oxide erami superondutor onneted in series with the system. In the normal operating ondition, this behaves nearly perfet eletrial ondutor without ohmi resistane. If the passing urrent exeeds the urrent arrying apaity of the oxide erami material (whih happens during short iruit onditions), it suddenly drops its superonduting property and reverts rapidly to its normal state (i.e. its impedane inrease to a defined value). In this way, by shaping up as a high ohmi resistane in the network, resistive type SFCLs limit the short-iruit urrent in the first half-wave. The oxide erami elements of the SFCL, whose temperature rises briefly during the limiting proess, automatially revert to the superonduting state after ooling bak to normal operating temperature. Generally resistive SFCL works on a zero resistane priniple. i.e. during normal ondition when value of urrent and temperature are below their ritial values I and T respetively, the value of resistane is zero. During fault ondition this value of I and T inreases above their ritial values and hene, there is transition from superonduting to normal state, whih ause the inrease in resistane value with inreased urrent. In this way urrent limitation takes plae in resistive SFCL. The relation among temperature, magneti field, and urrent density, known as T-B-J harateristis of superondutor, desribe the behavior of superonduting materials. These show signifiant physial property of SFCL performane and give idea of three states of atual superondutors with different power laws. These sates are superonduting state (E 1), flux flow state (E 2), and normal onduting state (E 3), as shown in Fig. 1. Suh parameterization in general holds for all types of high temperature superonduting ( HTS) materials [13]. Eletri field (V/m) Fig. 1. E 0 E Resistive region E J J J 0 Flux-flow region E J Superonduting region E J (flux-reep) Current density (A/m 2 ) Charateristi urve and states of SFCL B. Modeling and Design Charateristi As mentioned earlier, the working of resistive type SFCL is divided into three regions. Eah region has its own physial harateristis whih an be expressed by mathematial equations [14]. The superonduting state E 1 is expressed in (1), where J

3 is the ritial urrent density, E is the ritial eletrial field intensity defined at E = 1 V/m of superonduting material and J (T) is temperature dependent urrent density by an exponential power law. J J ( T) T E1 E ( ) (1) The exponent α (T) is derived as Whereas, ( T) max[, '( T)] (2) high magnitude of passing urrent, and developed eletri field show validity of modeled SFCL. The value of resistane is almost zero in superonduting state (state 1) of SFCL. In this region, the temperature is nearly 77F and eletri field intensity is also negligible. As the value of urrent exeed about 600A, the resistane of SFCL inreases and eletri field builds up. In the 2 nd state of SFCL, the value of resistane exeeds enough to signifiantly redue the fault urrent. The normal onduting state (state 3) has the maximum resistane and hinders fault urrent the most. E0 log( ) E '( T ) (4) 1 1 J (77 ) (1 ) k E0 (77 k ) log[( ) ( ) ] J ( T ) E The flux flow state (E 2) is expressed as, E / (77 ) 2 ( k J(77 k) J E Eo ) ( ) (4) Eo J ( T) J (77 k) And the normal onduting state (E 3) is given by (5) as, E3 ( T ) J (5) Whereas, is the normal resistivity and T is the ritial temperature of the HTS material. The alulated values of E 1, E 2, and E 3 gives the variable impedane of resistive type SFCL and haraterizes the nonlinearity of HTS material. We modeled resistive type SFCL using parameters shown in Table I. TABLE I SFCL MODEL PARAMETERS Parameter Value alpha_77k 6 beta 3 Critial urrent density at 77K, A/m 2 1.5e 7 Critial temperature, K 95 ρ(t), Ωm 1e -6 E0, V/m 0.1 E, V/m 1e -4 Length of super ondutor, m 50 Diameter of super ondutor, m Area of super ondutor, m e -5 Before implementing the modeled SFCL in test system, the funtionality of developed model was first tested by applying a urrent from 0 amps to 3000 amps and values of various SFCL parameters were evaluated as shown in Fig. 2. The values of SFCL resistane, temperature of SFCL due to Fig. 2. Effet of urrent on SFCL parameters. (a) The resistane of SFCL inreases with applied urrent (b) The temperature of SFCL () Eletri field intensity (d) States of SFCL C. Development of Protetion System As mentioned in Setion-I, the protetion of MG integrated with DERs is a hallenging area. Sine there is signifiant differene between fault urrents during gridonneted and islanded modes, therefore, speial attention is required when protetion shemes for suh mirogrids are developed. In this work, we exploit the use of external devie (i.e. SFCL) to obtain the protetion objetive. It is proposed that fault urrent during grid-onneted mode should be limited by proper design and plaement of SFCL suh that it mathes the fault urrent level during islanded mode. By ahieving this, a uniform relay setting an work in both grid onneted and islanded modes. For protetion of test system (presented in setion-iii), we alulated the SFCL design parameters and plaed it at the loation where protetion objetive an be ahieved. In this subsetion, details of modeled relay and trip harateristis of important breakers are provided. The overurrent relay (OCR) is installed in eletrial power system for primary and/or bakup protetion. It operates instantly or with a preset time delay when the fault urrent exeeds a preset value and disonnets the fault

4 setion from the system by tripping the assoiated iruit breaker. In order to validate protetion oordination with SFCL, we have modeled a relay in MATLAB with three relaying elements (instantaneous, inverse time, and definite time) whih are built-in to ommonly available digital relays. The interfae of modeled relay is shown in Fig. 3. For inverse time harateristis, IEC type C urve has been adopted whose trip time an be alulated by (6), where I is the urrent flowing through iruit, I p is pik-up value (or plug multiplier setting, PMS) of urrent, and T p is time dial setting of the relay. 80 T. T 2 ( I / I ) 1 p (6) p Fig. 4. Trip time harateristis of three important breakers of test system (a) trip harateristis of 4 MVA PV array outgoing breaker (b) trip harateristis of MV breaker () Trip harateristis of utility breaker Fig. 3. The user interfae of modeled relay For test system (shown in Fig. 5), appropriate relay oordination has been developed for proper operation of protetion system. The tripping harateristis of three important breakers (outgoing breaker of 4 MVA PV array, main breaker of MV named as MV Br, and outgoing breaker of utility) of system are shown in Fig. 4. The lower urrent setting of utility breaker is due to the fat that the utility side voltage is high. The utility breaker is not shown in test system of Fig. 5. III. TEST SYSTEM AND SIMULATIONS RESULTS The single-line diagram of the test system onsidered in this study is illustrated in Fig. 5. The system has been adopted from [15] and only medium voltage (MV) side has been onsidered whih is relevant to our study. The parameters of test system are summarized in Table II. The main utility grid is onneted to a distribution network onsisting of eletrial loads and two DGs through a 10 MVA 35 kv/10 kv transformer. The rated apaities of PV system and diesel generator are 4-MVA and 7-MVA respetively. The plaement of SFCL near M1 yields optimal results in this study. The impat of SFCL plaement at various loations of mirogrid has been disussed in [7]. To verify the performane of developed model of SFCL and proposed protetion strategy, the fault F1 has been onsidered with and without SFCL in grid-onneted and islanded modes. The details of these simulations are desribed below. TABLE II TEST SYSTEM PARAMETERS Parameter Utility Voltage and VA Transformer Ratio PV Capaity Diesel Generator Capaity Line Positive Sequene Resistane Line Positive Sequene Reatane Line Zero Sequene Resistane Line Zero Sequene Reatane Load 6 Load 7 Load 8 Value 35kV, 100MVA 35 /10 kv 4 MVA 7 MVA 0.38Ω/km 0.45Ω/km 0.76Ω/km 0.32Ω/km 5MVA 1 MVA 5 MVA

5 Utility Grid 35/10kV 10MVA PV-4MVA L-7 L-8 Fig. 5. Test mirogrid MV Breaker M1 Line 6A Line 6B Line 7 SFCL 1KM 2KM 3KM F1 F2 M2 L-6 F3 M3 Diesel Generator 7MVA A. Case 1: F1 in grid-onneted mode without SFCL In this situation, the ontribution of fault urrent is from utility as well as distributed resoures as shown in Fig. 6. In this ase, the fault urrent is very large whih is dangerous for eletrial equipment and needs earliest possible tripping. C. Case 3: F1 in islanded mode with and without SFCL The level of fault urrent in ase of islanded mode is shown in Fig. 8. The level of fault urrent is onsiderably less due to no ontribution from utility and limited fault urrent apaity of distributed energy resoures. The fault urrent in islanded mode during F1 fault is small enough that it is not limited by SFCL. Hene, fault urrent shown in Fig. 8 is same in ase of SFCL installed or otherwise. Fig. 6. Fault urrent in ase of F1 in grid-onneted mode without SFCL B. Case 2: F1 in grid-onneted mode with SFCL As shown in Fig. 7, the fault urrent is redued to lower level very quikly due to inreased resistane of SFCL. The advantage of SFCL is well reognizable over here. This is the reason why SFCLs lessen the stress on iruit breakers [16] and transformers [17] by limiting fault urrents. Fig. 8. Fault urrent in ase of F1 in islanded mode From Fig. 7 and Fig. 8, it is notable that fault urrent level in grid-onneted mode with SFCL is omparable with fault urrent level in islanded mode. This helps in ahieving uniform relay setting in both modes. On the other hand, there is signifiant differene in fault urrent levels when fault ours in grid-onneted mode without SFCL as observed in Fig. 6 and Fig. 8. Fig. 7. Fault urrent in ase of F1 in grid-onneted mode with SFCL IV. CONCLUSION Superonduting fault urrent limiter is a promising devie that an limit the fault urrent levels. This feature of SFCL an be utilized for oordinated protetion of mirogrid during various modes. This paper presents a omprehensive study on the basi priniple, modeling, and performane analysis of resistive type SFCL. In this work, a protetion strategy that exploits the use of SFCL is proposed and analyzed for mirogrid protetion. The proposed strategy is apable of working in both modes of mirogrid operation and does not require any advaned or ompliated system

6 onfiguration. As shown in the paper, the proposed model of SFCL an help in improving system s transient stability. It does not only limit the initial short iruit fault urrent but also lowers the stress on other protetion and operational equipment as well. The developed model of SFCL and proposed oordinated mirogrid protetion sheme is evaluated on a test system. The results show that overall performane of SFCL and proposed sheme is very promising. Protetion Coordination, IEEE Trans. Appl. Superond., vol. 26, no. 7, pp. 1 8, Ot [17] H.-C. Seo, C.-H. Kim, S.-B. Rhee, J.-C. Kim, and O.-B. Hyun, Superonduting Fault Current Limiter Appliation for Redution of the Transformer Inrush Current: A Deision Sheme of the Optimal Insertion Resistane, IEEE Trans. Appl. Superond., vol. 20, no. 4, pp , Aug V. REFERENCES [1] T. Ghanbari and E. Farjah, Unidiretional Fault Current Limiter: An Effiient Interfae Between the Mirogrid and Main Network, IEEE Trans. Power Syst., vol. 28, no. 2, pp , May [2] M. Noe and B. R. Oswald, Tehnial and eonomial benefits of superonduting fault urrent limiters in power systems, IEEE Trans. Appl. Superond., vol. 9, no. 2, pp , [3] L. Ye, L. Lin, and K.-P. Juengst, Appliation studies of superonduting fault urrent limiters in eletri power systems, IEEE Trans. Appl. Superond., vol. 12, no. 1, pp , [4] A. Abramovitz and K. Ma Smedley, Survey of Solid-State Fault Current Limiters, IEEE Trans. Power Eletron., vol. 27, no. 6, pp , Jun [5] J. R. Cave et al., Testing and modelling of indutive superonduting fault urrent limiters, IEEE Trans. Appl. Superond., vol. 7, no. 2, pp , [6] C.-H. Kim, H.-C. Seo, S.-B. Rhee, and C.-H. Kim, Deision of optimal insertion resistane of superonduting fault urrent limiter for reduing asymmetrial fault urrent, in Developments in Power System Protetion (DPSP 2014), 12th IET International Conferene on, 2014, pp [7] Y. Kim, H.-C. Jo, and S.-K. Joo, Analysis of Impats of Superonduting Fault Current Limiter (SFCL) Plaement on Distributed Generation (DG) Expansion, IEEE Trans. Appl. Superond., vol. 26, no. 4, pp. 1 5, Jun [8] S. Parhizi, H. Lotfi, A. Khodaei, and S. Bahramirad, State of the Art in Researh on Mirogrids: A Review, IEEE Aess, vol. 3, pp , [9] S. A. Gopalan, V. Sreeram, and H. H. C. Iu, A review of oordination strategies and protetion shemes for mirogrids, Renew. Sustain. Energy Rev., vol. 32, pp , Apr [10] Lei Chen, Changhong Deng, Fang Guo, Yuejin Tang, Jing Shi, and Li Ren, Reduing the Fault Current and Overvoltage in a Distribution System With Distributed Generation Units Through an Ative Type SFCL, IEEE Trans. Appl. Superond., vol. 24, no. 3, pp. 1 5, Jun [11] T. Ghanbari and E. Farjah, A Multiagent-Based Fault-Current Limiting Sheme for the Mirogrids, IEEE Trans. Power Deliv., vol. 29, no. 2, pp , Apr [12] Y. Zhao, O. Krause, T. K. Saha, and Y. Li, Stability enhanement in distribution systems with DFIG-based wind turbine by use of SFCL, in Power Engineering Conferene (AUPEC), 2013 Australasian Universities, 2013, pp [13] L. Ye and K.-P. Juengst, Modeling and Simulation of High Temperature Resistive Superonduting Fault Current Limiters, IEEE Trans. Appiled Superond., vol. 14, no. 2, pp , Jun [14] S. Nemdili and S. Belkhiat, Modeling and Simulation of Resistive Superonduting Fault-Current Limiters, J. Superond. Nov. Magn., vol. 25, no. 7, pp , Ot [15] W. Huang, T. Nengling, X. Zheng, C. Fan, X. Yang, and B. J. Kirby, An Impedane Protetion Sheme for Feeders of Ative Distribution Networks, IEEE Trans. Power Deliv., vol. 29, no. 4, pp , Aug [16] H. He et al., Appliation of a SFCL for Fault Ride-Through Capability Enhanement of DG in a Mirogrid System and Relay