Coordination of protective relays in MV transformer stations using EasyPower Protector software S. Nikolovski, Member, IEEE, I. Provci and D. Sljivac In this paper, the analysis of digital protection relays setting in a MV transformer station grounded via a low-ohmic resistor is presented. Theoretical background on low-ohmic resistor grounding in a transformer station is provided. EasyPower PowerProtector software is used for simulation and relay settings. Protective relay coordination after the simulation of a 3- phase fault and a line-to-ground fault on different selected buses in transformer station 35/10 kv is performed. Digital ABB relays series REF 541 were used for low-ohmic resistor thermal, shortcircuit and overcurrent protection and overload protection of the low voltage transformer side. The results of analysis indicate correct selectivity of overcurrent protection devices in all cases. Index Terms -- digital relay, fault, low-ohmic resistor, protection, transformer station T I. INTRODUCTION HE purpose of the power system neutral grounding is to limit the fault current, as well as to limit the values of overvoltages induced by line-to-ground faults. In the middle voltage (10 kv, 20 kv and 35 kv) networks neutral grounding is performed by voltaic connection between the energy transformers secondary wyes winding neutral and the ground. In the case when transformers connection type is not the same or transformers do not have the neutral point (delta winding), it is necessary to perform the artificial wye neutral grounding of the secondary winding. Artificial neutral is achieved by earthing the transformer with specific technical characteristics. There are basically two different types of grounding: direct and indirect. Indirect grounding can be performed using resistance or reactance. To the present, low-ohmic resistor grounding is the only grounding type used in the middle voltage power system of the Croatian National Grid. If the secondary winding neutral of all transformers in a transformer station is accessible, two grounding schemes or approaches are possible: 1. one low-ohmic resistor in station 2. one low-ohmic resistor at transformer The usual grounding scheme in the distribution area "Elektroistra" Pula is the one only one low-ohmic resistor in both 110/35 kv and 35/10(20) kv transformer stations. Current magnitudes and transient duration of power transformers switching into parallel operation, as well as the single-line-to-ground (SLG) fault resistance value are the important parameters which condition the correct operation of resistor relay protection. Inconsistent use of a typical solution for resistor protection settings or of back up lines protection can result in the false response of relay protection. False protection response indicates a different cause of failure, which makes the establishment of normal system operation more difficult. In order to avoid false protection response, current magnitude and response time are increased intuitively using engineering experience. However, care must be taken since an increased tap setting, may decrease the sensitivity during SLG faults and lead to resistor overheating and possible damage. Increase of the response time due to resistor bridging can result in high induced touch and step voltage voltages in a transformer station, as well as in damaging the grounding system, especially in the case of currents exceeding several ka. Information on resistor characteristics, network SLG conditions, and energy transformers operational transients leads to a different approach to selection of resistor protection in order to decrease consumer supply interruptions. II. MATHEMATICAL BACKGROUND A. Technical Characteristics of Low Ohmic Resistor Middle voltage resistor grounding characteristics are chosen for short overloads. The resistor is defined by these values: 1. nominal current with 5 seconds permitted duration 2. permanent permitted current 3. 10 minutes permitted current These are the parameters, which determine the choice and the protection settings of a resistor. Knowing the material of the resistive element, together with the nominal current with 5 seconds permitted duration, it is possible to calculate the maximum time of permitted overload for various current values above the nominal. SLG fault current in 35 kv network is typically limited to a nominal current of 300 A. SLG fault current in a 10 (20) kv network is limited to 150 A current in meshed or 300 A in a cable network. Calculation of the permitted overload duration will be conducted for those nominal resistor currents
according to the following expression: I a ln I n 2 I a 1 I n 2 t = T B max ϑ (1) where: T ϑ is the resistive element heating time constant; I a is the resistor current; I n is the nominal current of the resistive element and t B max is the maximum overloading duration. For the 300 A current, the resistive element is made with the permitted permanent current equal to or higher than 65 A, while for the 150 A current with permitted permanent current is equal to or higher than 32 A. Calculated current values are obtained according to manufacturer's characteristics, but in reality they are higher than those declared by manufacturers. The diagram in figure 1 presents calculated results. TABLE I TIME-CURRENT CHARACTERSITICS: α AND β CONSTANTS Time-current characteristic α β Normal inverse Very inverse Extremely inverse Long-time inverse 0.02 1.0 2.0 1.0 0.14 13.5 80.0 120.0 Fig. 1. Calculated values of resistor currents There are four international standard time-current characteristics. Time and current ratio is defined according to the BS 142 and IEC 255-4 standards as: t( s) k β = α I I > 1 where: t is the response time; k is the time multiplier; I is the fault current and I> is the overcurrent setting. α and β constants are dependent on the type of the characteristics according to table I. Fig. 2 (using table I) represents an example of choosing the time-current characteristics for 300 A nominal current low-ohmic resistor protection. (2) Fig. 2. Time-current characteristics for low-ohmic resistor For example, low-ohmic resistor protection in a 300 A current area, i.e. resistor thermal protection area, is performed using the long-time inverse characteristic defined by parameters k=1 and I=15 A. Low-ohmic resistor protection in a 150 A current area, i.e. resistor bridging protection area, is performed using extremely inverse characteristic defined by parameters k=0.05 and I=200 A. III. DIGITAL RELAYS SIMULATION IN EASYPOWER PROTECTOR SOFTWARE For the simulating the protection relays and proper coordination the EasyPower PowerProtector software module was used. It has the short circuit (SC) and the Time Current Curves (TCC) programs completely integrated with the one-
line diagram. The user does not have to create separate onelines for the TCC plots. Also all SC currents are automatically placed on the one-line diagram of the TCC Plots, and the TCC plot itself so there is a huge time savings in the analysis. EasyPower PowerProtector can model all the different types of Multi-Function relays on the one-line as a single relay, so the user actually sees the real protective system. The MF relays can perform many functions (plotted on the TCC) like a real unit. EasyPower Protector has the graphics capabilities to model all the different protective device relays, CT's, etc. so the real protection system is modeled. The problems associated with transformers working in parallel operation, and transients in the case of SLG faults, as well as SLG high-ohm faults, could be solved using several electromechanical or static relays with various current characteristics and various time-current settings. However, this kind of solution is very expensive. By using digital multifunctional relays which can perform several functions in a single device, it is possible to simplify the solution of these multiple problems and therefore to make the solution less expensive, together with decreasing the number of false protection operations. It is possible to configure the digital relays to simultaneously use the inverse time-current characteristics and definite time-currents characteristics together with supervision of the switching state of power station components important for operational logic of relay protection. A. Sample case: Transformer station 35/10 kv Pula Zapad The sample case of using the digital relays installed in a low voltage side of power transformer field terminals in a 35/10 kv transformer station Pula Zapad is described further in a text. The analysis of the real transformer station in cases of 3- phase and single line-to-ground faults was performed. Fig. 3. presents the protection scheme of TS 35/10 kv Pula Zapad. Using protection blocks in digital relays the following protections are achieved: 1. High-ohmic fault protection using definite time-current characteristic with settings: I=5 A and t=5 min (300 s) - used for switching off power transformers. 2. Thermal protection of low-ohmic resistor using long time inverse characteristic with settings: k=1, I=15 A (fig. 2.). 3. Low-ohmic resistor bridging using extremely inverse characteristic with settings: k=0.05, I=200 A (fig 2.). 4. SLG fault protection of the lines using definite time characteristic with settings: I=100 A and t=5 s. 5. 10(20) kv bus fault protection using definite time characteristic with settings: I=900 A and t=0.2 s. 6. Overload protection of LV side of power transformers using definite time characteristic with settings: I=492 A and t=1.8 s. According to the data from [6] the topology of the distribution network and the transformer station 35/10 kv Pula Zapad were simulated in the Protector module of EasyPower software. The REF 541 digital relay from ABB was used for lowohmic resistor thermal protection, short-circuit and overcurrent protection and overload protection of the low voltage transformer side. Due to the situation in TS Pula Zapad, it was neccessary to obtain the multi-function combination of 51DT/50, 51DT/50, 51/50N protection functions. Therefore, the tap time, time dial time, the instantaneous pickup time and the instantaneous delay time needed to be adjusted according to the calculated data from Dispatching Center of the Croatian Grid Company. The field terminal of REF 541 has: three protection blocks of 3-phase non-directional overcurrent protection (NOC3), three protection blocks of non-directional earth fault protection (NEF1) and three protection blocks of directional earth fault protection (DEF2). All relays setting are present in Fig. 4. Fig. 3. Protection scheme of TS 35/10 kv Pula Zapad Fig. 4. Database of ABB REF 541 digital relay settings
Since this relay is in the multi-function connection, these settings needed to be performed for 51/50N, as well as for 51DT/50N function. Protective settings of 10 kv feeders and transformers were adjusted and the 10/0.4 kv transformer was also introduced in the analysis in order to obtain the information on protective response in TS 35/10 kv Pula in cases of faults in low voltage networks. Low-resistor impedance on 10 kv side of transformers in TS Pula Zapad was 20 Ω. B. The results of the analysis The six sample cases were studied and assessment of timecurrent relays setting was performed: 1. 3-phase fault on 10 kv bus of TS Pula Zapad 2. 3-phase fault in 0.4 kv network 3. 3-phase fault in 10kV network with fault impedance10 Ω 4. Single line-to-ground fault on 10 kv bus of TS Pula Zapad 5. Single line-to-ground fault on 10 kv bus of TS Pula Zapad with zero low-resistor impedance 6. Single line-to-ground fault on 10 kv bus of TS Pula Zapad with variable fault impedance of: 20 Ω, 50 Ω, 100 Ω, 200 Ω, 500 Ω, 1000 Ω and 1200 Ω Fig. 6. Time-current curve for 3-phase fault on 10 kv bus of TS Pula Zapad Time-current curves indicate the response of the protective scheme in each case. Fig. 7. Relay s tripping and setting in case of a 3-phase fault on 10 kv bus Fig. 5. Single-line diagram for 3-phase fault on 10 kv bus of TS Pula Zapad For example, Figures 6 and 9 present time-current curves for 3-phase and a single line-to-ground fault on 10 kv bus of TS Pula Zapad respectively, with zero low-resistor impedance. The most important results of the analysis are described further in a text. Tables II, III and IV represents the simulated protection operation results in case of a 3-phase fault on 10 kv bus of TS Pula Zapad, a 3-phase fault in 0.4 kv network and a single line-to-ground fault on 10 kv bus of TS Pula Zapad, respectively.
Relay TABLE II 3-PHASE FAULT ON 10 KV BUS OF TS PULA ZAPAD Voltage level ( kv ) Device Function Fault Current I> ( A ) Responce Time t ( s ) Fault on low voltage on 0.4 kv bus of TS Pula Zapad was performed to point out the need to increase the response time of relay R-5 so that a high voltage fuse in transformer branch TX-3 has time to burn out and therefore to switch off the power transformer, but not the rest of the network. R-1 35 51DT/50 1169 2.012 R-3-1 10 51DT/50 4094 0.206 R-5-1 10 51DT/50 8189 0.051 From Table II. it can be seen that coordination of protective relays is done well, since the selectivity for a 3-phase fault is achieved. First, the relay in cable terminal R-5-1 will trip with an instantaneous delay of t=51ms, after that relay R-3-1 on the secondary side of the transformer station with an instantaneous delay of t=206 ms, and the last relay will be the relay on the primary side of the transformer station R-1, with definite time delay t=2.012 sec. From Fig. 5 it can be noticed that fault currents that flow from the supply network to 10 kv bus of TS where the fault is simulated crosses time-current characteristics of relays and those values are presented in Table II Fig. 9. Time-current curve for 3-phase fault on 0.4 kv bus of TS Pula Zapad It should be mentioned that in a sample network there is no HV fuse of 10/0.4 kv transformer when simulating the 0.4 kv fault, but in reality selectivity of 10 kv lines fault protection operation to operation of HV fuse of 10/0.4 kv transformers in case of close faults in 0.4 kv network is desirable. TABLE IV SINGLE LINE-TO-GROUND FAULT ON 10 KV BUS OF TS PULA ZAPAD Relay Device Function Fault Current I 0 > ( A ) Response Time t ( s ) Fig. 8. Single-line diagram for SLGfault on 10 kv bus of TS Pula Zapad Relay Voltage level ( kv ) TABLE III 3-PHASE FAULT IN 0.4 KV NETWORK Device Function Fault Current I> ( A ) Response Time t ( s ) R-3-1 10 51DT/50 515 0.206 R-5-1 10 51DT/50 1030 0.049 R-3-3 51/50N 150 13.874 R-3-4 51DT/50N 150 5.024 R-5-1 51DT/50 293 1.003 R-5-2 51DT/50N 293 0.614 The analysis of relay protection coordination in case of the SLG fault at 10 kv bus in TS Pula Zapad indicates that the selectivity of the relays is also satisfactory. Relay R-5-1 is also presented in Fig. 6. although it is used for 2 or 3-phase faults. The reason is that overcurrent protection functions always "see" SLG faults due to their 3-phase performance, so there is a real possibility that a 10 kv feeder overcurrent element will be excited in the fault affected phase.
1, R-3-2, R-3-3 and R-3-4 will react on the same current value of 6.411 ka, but the faster response has relay R-3-2 operating first which is set according to the very-inverse characteristic, resulting in a very good protection of the low-ohmic resistor, meaning that the response time of relay R-3-2 in case of resistor bridging is only 0.022 s. IV. CONCLUSIONS Fig. 10. Time current curve for single line-to-ground fault on 10 kv bus of TS Pula Zapad Furthermore, the simulation of relay protection response on various fault impedances in the case of SLG fault on 10 kv bus of TS Pula Zapad is simulated. Fault resistance is changed from 20 ohm to 1000 ohm, for high-ohmic SLG faults. The results are presented in table V. It can be noticed that by increasing the fault impedance (i.e. decreasing the fault currents) the response current is decreases, while the response time is increases according to the TCC curves. TABLE V SINGLE LINE-TO-GROUND FAULT ON 10 KV BUS OF TS PULA ZAPAD Feeder field Transformer field Z (Ω) R-5-1 R-5-2 R-3-3 R-3-4 20 (293A; 0.9s) 50-100 - 200-500 - 100 0 - (293A; 0.5s) (118A; 0.5s) (58.8A; 0.6s) (29.3A; 0.5s) (11.8A; 176.7s) (5.89A; 174.1s) (151; 13.9s) (59.6A; 42.1s) (30.2A; 14.9s) - - (150.1; 5.1s) (59.6A; 126.3s) (31.2A; 298.7s) (15A; 329.5s) (6A; 312.5s) - - Additionally, simulation of the grounding resistor which is short circuited is performed. Results indicate that relays R-3- The analysis of the digital protection relays setting in MV a transformer station grounded via low-ohmic resistor is presented in this paper. EasyPower PowerProtector software is used for simulation and coordination of the relay protection in MV TS 35/10 kv. Several functions of digital relays are simulated: High-ohmic fault protection using definite timecurrent characteristic. Thermal protection of low-ohmic resistor using long time inverse characteristic Low-ohmic resistor bridging using extremely inverse characteristic. SLG fault protection of the lines using definite time characteristic The 10(20) kv bus fault protection using definite time characteristic. Overload protection of LV side of power transformers using definite time characteristic. The analysis of relay protection co-ordination in case of the three-phase fault and SLG fault at 10 kv bus in TS Pula Zapad indicates that relay selectivity is satisfactory. V. ACKNOWLEDGMENT The authors gratefully acknowledge the contributions of Igor Vickovic, BSc from HEP - Croatian National Grid Company for his help and also to the Electrical System Analysis Inc. for their financial support to this project. VI. REFERENCES [1] S. H. Horowitz, Power System Relaying, 2nd ed, Research Studies Press LTD. Somerset, England, 1995. [2] A. T. Johns and S. K. Salman, Digital Protection for Power Systems IEEE Power Series 15, London, 1995. [3] S.Nikolovski, Basis of Relay Protection in Power System, Osijek, 2001. [4] ABB: Protective Relaying Theory and Aplication, 1994. [5] ABB: User s Manual and Tehnical Description REF 54_1999. [6] S. Drandić and N. Rudan, "Choice and setting of relay protection of lowohmic grounding resistor in MV electrical networks using digital relays," in Proc. 2001 CIGRE Croatian Comitee Conference, November 4-8, Cavtat, Croatia VII. BIOGRAPHIES Srete Nikolovski was born in Belgrade on 01. October, 1954. He obtained his BSc degree in 1978 and Master of Science degree (MSc) in 1989, both in the field of Electrical Engineering, from the Faculty of Electrical Engineering, University of Belgrade. He received his PhD degree from the Faculty of Electrical Engineering and Computing, University of Zagreb, Croatia in 1993. Currently he is Associate Professor with the Power System Engineering Department at the Faculty of Electrical Engineering, Osijek, Croatia. His main interest is in power system modeling, simulation and analysis, especially in reliability assessment of power system and power system protection.
Igor Provci was born in Travnik, Bosnia and Hercegovina, on October 19, 1976. He graduated from the Faculty of Electrical Engineering in Osijek in 2002. In 1999. He received a scholar ship from HEP - Croatian National Grid Company, Direction section - Opatija. He was an active member of IAESTE (The International Association for the Exchange of Students for Technical Experience) during his unergraduate studies and carried out two practical trainings, one in Tunisia and the other one in Ireland. Damir Sljivac was born in Osijek, Croatia on February 4, 1974. He obtained his BSc degree in Electrical Engineering in 1997, from the Faculty of Electrical Engineering, University of Osijek, Croatia and his MSc degree in 2000, from the Faculty of Electrical Engineering and Computing, University of Zagreb, Croatia. At present he is a research assistant with Power System Department at the Faculty of Electrical Engineering, University of Osijek and a PhD student at Faculty of Electrical Engineering and Computing, University of Zagreb. In 2002, he was an academic visitor at a University of Manchester, Institute for Science and Technology (UMIST), UK. His main field of interest is in power system analysis, particularly power system reliability in deregulated electricity markets.