PLANNING AND COORDINATION OF RELAY IN DISTRIBUTION SYSTEM

Transcription

1 PLANNING AND COORDINATION OF RELAY IN DISTRIBUTION SYSTEM 1 J.Jaishree And 2 Dr.S.Thangalakshmi 1 PG scholar, M.E. Power Systems Engineering., Department of Electrical and Electronics Engineering, G.K.M College of Engineering and Technology, Chennai. 2 Associate Professor, Department of Electrical and Electronics Engineering, G.K.M College of Engineering and Technology, Chennai. Abstract- In any power system network, protection should be planned such that protective relays segregate the faulted portion of the network at the earliest. Normally the distribution system protection comprises of both primary relays and backup relays. The entire distribution network is divided into various overlapping zones to ensure complete protection. The essential requirement of protective relays is that they must be capable of differentiating between faulted conditions, normal operating conditions and abnormal operating conditions. At the same time, they must function only for the specific protection for which they are designed, without operating for any normal and short term acceptable abnormal happenings. Hence, during the failure of primary protection, the backup relays provide protection after some specified time interval. Deciding the proper time discrimination for the operation of backup relays decides the quality protection in distribution network. In this paper, a simulation tool is used for the proper operation of backup relays. It is ensured that the relays located in the backup zone, work to isolate the fault, after giving adequate time discrimination for the operation of the primary zone relays if primary relays do not function and clear the fault. It is capable of optimally identifying set of relay settings for both Primary and Back-up Protection. The fault conditions and the protection coordination problem are formulated and simulated in ETAP. The use of Fault Current Limiters (FCL) and the design of Coordination Time Interval (CTI) are discussed in this paper. The distribution network of Surana industries is considered for the analysis. Index Terms Distribution network, Coordination Time Interval (CTI), Fault Current Limiters (FCLs), protection coordination, relay protection, primary relay, backup relay, ETAP simulation software. I. INTRODUCTION A power system encompasses generation, transmission, distribution, and consumption by consumers. To ensure continuity of supply to consumers and to avoid damage to the power system components as well as equipment, a faultless operation is required. However, operating the power system without any fault is not practicable. The power system operation is said to be effective when there is minimum fault in the system or when the faults are cleared then and there. During fault conditions, the fault should be slickly cleared and faulty portion must be isolated. In fact, the prime objective of power system protection is to separate a faulty section of electrical power system from rest of the live system so that the healthy section can perform the role satisfactorily without any 154 severe harm due to fault current. Essentially circuit breaker isolates the faulty system from rest of the healthy system and these breakers automatically open during fault condition because they receive trip signal from protective relay.the protection of power system does not mean to prevent the flow of fault current through the system but it only prevents the continuation of flowing of fault current. This is realized by quickly disconnecting the short circuit path from the system. This process depends on the magnitude of fault current, sensitivity of the relay, pick up value etc., the decision making is effective in unit system of protection while it is difficult in case of non-unit system of protection where the discrimination is only relative and not absolute. In distance protection, which is a non-unit system of protection, when the fault is well within the protection zone, the lower rise time especially in magnitude can accelerate the decision making while for the faults close to the boundary of operation, lower settling time is more effective to accelerate decision making [1]. II. PROBLEM STATEMENT The equipment is damaged when the primary protection fails while fault occurs. There is an ambiguity in proper isolation of fault when there are many faults. This ultimately leads to power outage. When the primary protection fails backup must operate to isolate the fault, after sufficient time discrimination for the operation of the primary zone. The faulty portion should be shut down when a fault is experienced to avoid damage and for the safety of the personnel and computer based relays monitor the machine and disconnect it during the faults [2]. The relays are designed to discriminate between faulted and normal conditions and provide protection. The fault Current limiters decrease the intensity of the fault condition and outage of power is minimized. It is concluded by many researchers that relay coordination plays a vital role in providing efficient protection.

2 The following points are ensured in relay coordination [2]. Protection equipment are coordinated with each other so that channels. Newton-Raphson algorithm is used for the power flow. Fig.1 shows the result of load flow output in ETAP. when fault occurs only the corresponding relay sends the trip signal and other relay remains idle. Selective fault isolation is provided. Outage possibility of the system is minimized. Limits the extend and duration of service interruption. Fault clearing time is maximized. 2.1 Test System The test system considered for analysis is Surana steel plant. The plant consists of 46 buses with following six different voltage levels: (i) 220kV (Raichur sub-station), (ii) 110kV (Chikkasagur sub-station), (iii) 33kV (VCB panel) (iv) 11kV (VCB panel) (v) 415V (Service) (vi) 220kV grid voltage. The MW and MVAr demands are MW and MVAr respectively. The basic steps involved in relay coordination are: 1. Load Flow analysis. 2. Short Circuit Analysis. 3. CT and CB Selection using time-current (T-I) characteristics. 4. Co-ordination of relays and finding the Critical Clearing Time. The step by step analysis is explained in the following sections. Fig.1 Load Flow Output in ETAP IV. SHORT CIRCUIT ANALYSIS The power system operations are balanced during normal operating conditions. Under irregular condition (fault) the system becomes unbalanced [3]. The short circuit analysis reveals a clear idea about the system under short circuited conditions and it is helpful in power system estimation [5]. III. LOAD FLOW ANALYSIS The load flow analysis is the basis of relay coordination and design. Load flow study is technique that provides basic calculation procedure in order to determine the characteristics of a power system under steady state and to evaluate the various operating states of an existing system [3, 4]. A power flow study usually uses simplified notation such as a one-line diagram and per-unit system, and focuses on various forms of AC power (i.e. voltages, voltage angles, real power and reactive power). The load flow is run for the test system to obtain the details of nodal voltages, phase angle, power injection at all the buses and the power flow through interconnecting power Fig.2. Short Circuit Output in ETAP 155

3 This analysis is very much significant while applying relay coordination in distribution system since it helps in determining the maximum current flowing during the fault. The short circuit analysis is made by simulating the fault/abnormal conditions using ETAP. The output of short circuit analysis in ETAP is shown Fig.2. V. OPTIMAL COORDINATION PROBLEM The various coefficients of objective function are set by optimization algorithm in literatures [6]. Generally, the relay coordination problem involves different parameters and hence different limitations have to be considered in solving the objective function. Here the objective function to be optimized is the sum of the operating times of the relays connected to the system. In any power system, a primary protection has its own backup for assuring a reliable power system. The method capable of optimally identifying one set of relay settings is valid for all possible future DG planning of the system considered. Coordination time interval (CTI) is the criteria to be considered for coordinating the primary and back-up protective schemes. It is a predefined coordination time interval and it varies based on the type of relays used. CTI is of the order of 0.3 to 0.4s for electromagnetic relays, while for a microprocessor based relay, it is of the order of 0.1 to 0.2s [7]. The processor based relays and solid state relays use less time for coordination due to the absence of moving parts and faster resetting time. To ensure reliability of the protective system, the back-up scheme should not be activated unless the main relay becomes unsuccessful in taking appropriate action. Only when CTI is exceeded than the set value, backup relay should work. This is mathematically expressed as: Tbackup Tprimary CTI (5.1) Where, Once the relays are coordinated, the discrimination in the operation of primary and backup relays and their coordination with the maximum possible load currents will be plotted on the Time Current characteristics (T-I). The standard time-current characteristics is shown in Fig.3. Relay coordination needs to be assessed for maximum and minimum fault conditions and for various probable network arrangements. If a network has numerous stages of primary and backup relay levels, the source end relay operation becomes quite late due to consecutive time discrimination [8]. Fig.3. Time Current Characteristics In such situations, it may be essential to guarantee the isolation of fault at the earliest by feasibly coordinating the source end relays with much quicker faithful relays in the downstream. The relay setting should be instantaneous and it must be capable of completely distinguishing between the faults in the primary and subsequent zones. This is indispensable to confirm that the instantaneous settings will not entertain the operation of relays for the faults external to its primary protective zone. Fig.4 and Fig.5 show the time current characteristics simulation and output respectively. T backup is the operating time of the backup relay. T primary is the operating time of the primary relay. Fig.4. T-I Characteristics simulation in ETAP CT is Current Transformer. Accounting all these criteria, the problem can be framed mathematically as: (5.2) Where, n represents the number of relays. VI. T-I CHARACTERISTICS 156

4 Fig.5. T-I Characteristics Output in ETAP 6.1 Relay Chacteristics A typical Inverse Time Directional (6.5) Once the plug and time setting are decided, the proper CTs are to be selected for proper coordination. Over current relay consists of two elements, an instantaneous unit and a time overcurrent unit. The calculation of the Time Dial Setting (TDS) and the pickup current (Ip) setting of the relays is the core of the optimal coordination of primary and secondary relays [9]. The overcurrent unit has two values to be set: Pick up current value (Ip) and the Time Dial setting (TDS)/ Time Multiplier setting (TMS). The pickup current value (Ip) which is also referred as plug setting is the minimum current value for which the relay operates. The Time Dial Setting which is also referred as time setting defines the operation time (T) of the device for each current value, and is normally given as a curve T vs M, where M is the ratio of relay fault current I, to the pickup current value: (6.1) In general, overcurrent relays respond to a characteristic function of the type: This Function can be approximated as, (6.2) ( ) (6.3) Where, and are constants that depend upon the exact device that is to be simulated. The following two cases have been considered in this paper for obtaining the objective function and minimizing it. Case I: The Linear equation is formulated in terms of TDS by taking Ip to be constant at a particular value in equation (6.3) Where, (6.4) Case II: The Non-Linear equation is formulated in terms of Ip by taking TDS to be constant at a particular value in equation (6.3) Where, 157

5 VII. SELECTION OF CT The foremost purpose of a Current Transformer (CT) is to convey the three phase primary current in a high voltage power system to single level that is bearable the relays connected to its secondary side [10]. The following procedure is applied while selecting the current transformer: Customer requirement based on the primary circuit, the metering and protection chain is defined. The most proper CT unit is selected based on the customer necessity from the catalogue of referenced CT. If none is selected from the general catalogue the standardised CT would be the fit unit for the customer. If even standardised CT is not suitable then a feasibility study has to be carried out. Or a special unit can be manufactured. VIII. FAULT CURRENT LIMITERS The next step of relay coordination is selection of circuit breakers. This is decided by type of fault and magnitude of FCL. It is customery to use FCL to safeguard the equipment during fault [13, 14, and 15]. The selection of circuit breaker is hence decided by the fault impedance as well as by the impedance of FCL. An equivalent circuit can be modelled as shown in Fig.6. This shows the series combination of source voltage V S, source impedance Z S, load impedance Z L and fault current limiter Z FCL. The fault impedance Z F shunts the load impedance during fault (or) during the opening of circuit breaker. The current before and after installing FCL can be calculated using the Eq. (8.1, 8.2, 8.3). Before fault the current is limited by Z L and Z S. Hence, During fault without FCL, (8.1) (8.2) I of Eq.(8.2) is maximum since Z F is too small. Hence FCL is added in the circuit. The impedance of Z FCL is added in series with the other impedances. During fault with FCL, (8.3) Fig.6. A Simple Electric Circuit FCL It is limited by ZFCL. During normal operating condition FCL is deactivated. So, there is no voltage drop and no energy loss. Literature shows that the magnitude and phase angle of fault current changes based on the nature and type of faults [16].The use of DG s in modern power system may also change the fault current level [17]. The sudden insertion and removal of DG units in a distributed system disturbs the coordination of relays [18, 20]. The over current and destructive current in power system is limited by FCL [19]. This paper focusses the coordination with FCL but not exposes the removal (or) addition of DG units. IX. SELECTION OF CB The Circuit Breaker are chosen based on the following duties to be performed Breaking capacity. Making capacity. Short time capacity The symmetrical and asymmetrical breaking capacity of a CB depends on base MVA, pre-fault voltage and the short circuit current. The making capacity can be found by multiplying the symmetrical breaking current with 1.8 and. The factor is accounted for converting the r.m.s value of symmetrical breaking current into peak value whereas the factor 1.8 is considered to account the d.c component. The short time rating (or) short time current is usually the ratio of breaking current to rated normal current. It is evident that the rating of the CB (or) the selection of CB is decided by the short circuit current. The current calculation was discussed in the previous section with the use of FCL. X. CONCULSION The challenges involved in relay coordination of a protective system during abnormal condition is discussed in this paper. The step involved in relay coordination is elaborated. The data of distribution system of surana steel industries is taken as a test case. Newton-Raphson algorithm is applied to conduct the power flow. A short-circuit 158

6 analysis is carried out to get the magnitude of fault current. The selection of CT and CB has been detailed with the use of FCL. The entire test system is simulated using ETAP and the screen shots are presented. Sample data are provided in Annexures. It is inferred that the relay coordination in modern power system is disturbed by the insertion and removal of DG units. The fault condition is simulated in the test system and the relays used in the system had been coordinated. insertion and removal of DG units. The fault condition is simulated in the test system and the relays used in the system had been coordinated 159

7 Coordination of Directional Overcurrent Relays for Distribution Systems Considering DG IEEE TRANSACTIONS ON SMART GRID.s REFERENCES [8] Alvin Yiek and Keith Harker., [1] Mohammad R. Dadash Zadeh, and NATIONAL GRID Zhiying Zhang., A New DFT- EXPERIENCE WITH Based Current Phasor Estimation MANAGEMENT OF MULTIfor Numerical Protective FUNCTIONAL NUMERICAL Relaying, IEEE RELAY SETTINGS. TRANSACTIONS ON POWER [9] Dharmendra Kumar Singh and DELIVERY,VOL.28,NO.4,OCTO Dr.S.Gupta, Optimal BER Coordination of Directional [2] Rama Hamma. Faults Overcurrent Relays: A Genetic IdentificationinThree-Phase Algorithm Approach 2012 IEEE Induction Motors Using Support Students Conference on Vector Machines Bowling Green Electrical, Electronics and State University Scholar Works Computer [10] [3] PUSHP RAJ., Load flow and Sachin Tiwari1 and Aditya short circuit analysis of 400/220 Pande., Current Transformer kv substation Internal Journal of Sizing & Saturation Calculation creative research thoughts volume with Transient Performance 1, Issue.4.April Analysis of CT Using ATP [4] Rajesh Krishnasamy, Software International Journal of A.Bhuvanesh and Advanced Research in Electrical, Karuppasamypandiyan.M. Electronics and Instrumentation, Power flow analysis of 230/110 Engineering (An ISO 3297: 2007 kv substation using Etap. Certified Organization) Vol. 4, [5] Daljeet Kaur, Dr. S.K.Bath, Issue 5, May Darshan Singh Sidhu, Short [11] Circuit Fault Analysis of Electrical Niraj Kumar Choudhary, Soumya Power System using MATLAB Ranjan Mohanty and Ravindra IOSR Journal of Electrical and Kumar Singh., Coordination of Electronics Engineering (IOSR- Overcurrent Relay in Distributed JEEE) e-issn: , p- System for Different Network ISSN: , Volume 9, Issue Configuration Journal of Power 2 Ver. III (Mar Apr. 2014), PP and Energy Engineering, 2015, 3, [6] Seyed Hadi Mousavi Muztagh., [12] and Kazem Mazlum., Optimal Working Group on Distribution Overcurrent Relay Coordination Protection: P.T. Carroll, Using Optimized Objective Chairman, C. Fink, Vice Chairman Function Hindawi Publishing Contributing Members: J. Corporation ISRN Power Appleyard, J. R. Boyle, B. Engineering Volume 2014, Article Jackson, J. Johnson, L. Kojovic, E. ID Krizauskas, L. P. Lawhead, P. J. [7] Lukasz Huchel and Hatem H. Lerley, D. Miller, A. Napikoski, Zeineldin., Planning the R.D. Pettigrew, W. M. Strang, C. oso 160

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9 ANNEXURE 1. Line Data ID SIZE ADJ. %TOL #/PHAS T R1 X1 Y1 ID E Line Line1 1 Line Line2 2 Line Line3 3 Line Line Sample Load Flow Output ID MW Mvar Amp %Pf Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Sample Short Circuit Output From Bus To Bus Mag. Bus 16 Total Bus 15 Bus Bus 37 Bus Bus 14 Bus Bus 17 Bus Bus 18 Bus Bus 19 Bus Bus 20 Bus Bus 26 Bus Bus 28 Bus Bus 30 Bus Lump 5 Bus