Planning and Coordination of Relays in Distribution System

Background/Objectives: A protective relay should proficiently distinguish between fault, normal and abnormal environments. The prime aim is to plan and coordinate the relays effectively during power system faults. Methods/ Statistical Analysis: ETAP simulation tool is adopted for appropriately operating the backup relays, after giving acceptable time discernment for operating the primary region relays. The optimal identification of relay settings for both Main and Standby Protection is achieved using Fault Current Limiters (FCL) and Coordination Time Interval (CTI). The distribution set-up of Surana industries is considered for the analysis. Findings: The early detection of fault and automatic cutting off of defected portion ensures high degree of protection and instigates proper planning in any power system network. Normally the distribution system protection involves both principal relays and standby relays. The whole distribution system is divided into numerous intersecting zones to guarantee complete protection. Protective relays must function only for the precise protection for which they are intended, without functioning for any normal and short term tolerable abnormal happenings. Hence, during the failure of primary protection, the backup relays provide protection after some indicated time interval. The modern power systems use distributed network wherein which insertion and removal of DG units are common. This disturbs the relay coordination and initiates frequent operation of standby relays. Deciding the proper time interruption for the working of backup relays decides the protection quality in distribution link. Application/ Improvements: The protection coordination of distribution set-up of Surana industries are formulated in ETAP after getting the power flow results using Newton-Raphson algorithm. Results show improvement over the existing methods. Planning and Coordination of Relays in Distribution System


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 chief goal of protecting the power system is to separate a defective section of electrical power system from remaining section of the system enabling the healthy section to perform reasonably without much harm because of the excessive current during defect.
Fundamentally circuit breaker detaches the imperfect portion from the perfect portion and these breakers inevitably open during any mistake by receiving a signal from over-current relay for isolation. The protection of power system does not mean to avert the movement of fault current nevertheless it prevents the persistence of flow of over-current. This is realized by rapidly disconnecting the defective part from the system. It is decided by the magnitude of fault current, sensitivity of the relay and pick up value. 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, the magnitude of operating time is decided from the location of fault i.e. failure within the border or on the border 1 .

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 adequate time discernment for the operation of the main zone. The defective portion must be quarantined when a short circuit is identified to evade destruction and to ensure security of the personnel. The programmed relays observe and check the machine. The machine is disengaged by the relay at the event of fault 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.
The following points are ensured in relay coordination 2 : Protection equipment are coordinated with each other • so that 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. • The extent and duration of service interruption is • limited. Fault clearing time is reduced. •

Test System
The test system considered for analysis is Surana steel plant. The plant consists of 46 buses with following six different voltage levels: 1. 220 kV (Raichur sub-station) 2. 110 kV (Chikkasagur sub-station) 3. 33 kV (VCB panel) 4. 11 kV (VCB panel) 5. 415 V (Service) 6. 220 kV grid voltage The MW and MVAr demands are 23.336MW and 23.082MVAr respectively. In all types of relay coordination schemes some basic steps are exercised. They are: 1. Investigation of Load Flow 2. Investigation of Short Circuit 3. CT as well as 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 described in the subsequent segments. A sample line data of Surana steel plant is given in Table 1.

Investigation of Load Flow
This study is the basis for relay coordination and design. Load flow study is a technique that provides elementary computation formula in order to govern the characteristics of a power system during steady state and then to assess the several functional conditions of a prevailing system 3,4 . Power flow learning generally uses shortened notation such as a single-line drawing and per-unit system, and prominences on many forms of electric parameters and AC power (i.e. voltages, voltage angles, active power and reactive power). This investigation is exercised for the test scheme to obtain the particulars of nodal voltages, phase angle, power injection at all the buses and the power flow through interconnecting power channels. Newton-Raphson algorithm is used for the power flow. The sample load flow analysis through ETAP is displayed in Table 2. Figure 1 demonstrates the result of load flow output in ETAP.

Investigation of Short Circuit
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 . The sample short circuit analysis with ETAP is provided in Table 3.
This analysis is very much significant while applying relay coordination in distribution system since it helps in relay coordination problem involves different parameters and hence different limitations are taken into account for solving the objective function. Here the optimization function is the summation of the working seconds of the over-current relays linked to the arrangement. In any power system, main protection has a private standby for assuring reliability. A method has to be evolved to optimally find the over-current relay locations and the same can be employed for the entire probable DGs.
Coordination Time Interval (CTI) is the factor to be accounted for properly synchronizing the main and standby relays. It is a predetermined sequencing time delay which differs based on the kind of over-current relays used. CTI ranges from 0.3 to 0.4s for electromagnetic relays, and it may even take up a value between 0.1and 0.2s for programmable relays 7 .
The processor based relays and solid state relays use less time for coordination due to the absence of moving parts

Optimal Coordination Problem
The various coefficients of objective function are set by optimization algorithm in literatures 6 . Generally, the and faster resetting time. To guarantee the consistency of the protecting scheme, the standby relays should not be activated unless the main relay becomes unsuccessful in taking suitable measure. The standby relay should work one and only when the current transformer interval is exceeded the set value. This is mathematically expressed as given in equation (1). (1) Where, T s is the operating time of the standby relay. T m is the operating time of the main relay. CTI is Current Transformer Interval. Accounting the above factors, the objective is framed mathematically as shown in equation (2). (2) Here, N represents the total count of over-current relays.

T-I Characteristics
The T-I characteristics are very much important in setting the interval of working of time over current relays and they show the time of operation of over-current relay against the short circuit current. This helps in providing the discernment in the operation of main and standby relays. The variation of operating time with respect to highest probable load currents will be plotted on T-I characteristics. A standard T-I characteristics is shown in Figure 3. Relay synchronization is to be assessed for highest and lowest defective conditions and for several probable complex arrangements. If a circuit has numerous stages of main and standby relay levels, the operation of last relay from the point of short circuit becomes quite late because of consecutive time discrimination 8 .
Hence, it becomes essential to guarantee the segregation of defective portion immediately by feasibly synchronizing the relays father from the point of short circuit. The relay setting should be instantaneous and it must be capable of completely distinguishing between the internal faults and external faults to enhance the degree of protection. Figure 4 and 5 show the time current characteristics simulation and output respectively.

Relay Characteristics
An Inverse Time Directional Over current relay essentially comprises of two components, a directional instantaneous

Selection of CT
The foremost purpose of a 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.

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 customary to use FCL to safeguard the equipment during fault [11][12][13] . 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 Figure 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 equations (8, 9 and 10). Before fault the current is limited by Z L and Z S . Hence: (8) Figure 6. An equivalent circuit during fault with FCL of main and standby relays are the setting of time knob (TDS) and the magnitude of operating current of overcurrent relay (Ip) 9 . TDS is also referred as Time Multiplier setting and a plug setting multiplier is used to fix the minimum current value for operating over-current relay.
A coefficient M is decides the operating time and is given as the ratio of short-circuit operating current to pick up current as given by equation (3). (3) The characteristic function governing the operating time of over-current relay is given by equation (4): (4) It is estimated as shown in equation (5): (5) , and are coefficients that are influenced by the type of device used.
The subsequent two cases are tested in this paper to reach the objective. Case I: A variable Time Dial Setting TDS, assuming constant pick up current as formulated by equation (6) is considered.
Case II: A constant Time Dial setting TDS, assuming variable pick up current as formulated by equation (7) is considered.
Once the plug and time setting are decided, the proper Current Transformers (CTs) are to be selected for proper coordination. During fault the load impedance become less predominant and omitted since the line is shorted through the fault. Also, without any FCL, the current becomes: (9) Current, I of equation (9) is maximum since Z F is too small. Hence to limit the fault current, FCL is added in the circuit. The impedance of Z FCL is added in series with the other impedances. The current during fault with FCL is lesser than that of without FCL and is expressed mathematically as shown in equation (10).
The fault current is limited by Z FCL since this impedance is added in series with the existing impedances and increases the effective impedance of the circuit. During normal operating condition FCL is deactivated. So, there is no voltage drop and no energy loss. A Hybrid Shuffled Frog Leaping Algorithm has been used for designing the FCL 14. Literature shows that the magnitude and phase angle of fault current changes based on the nature and type of faults 15 .The use of DG's in modern power system may also change the fault current level 16 . The sudden insertion and removal of DG units in a distributed system disturbs the coordination of relays [17][18][19] . The stability analysis has been carried out for a standard test system using Swarm optimization 20. The over current and destructive current in power system is limited by FCL 18 . This paper focuses the coordination with FCL but not exposes the removal (or) addition of DG units. 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 root mean square value of balanced breaking current into peak value whereas the factor 1.8 is considered to account the direct current component. The short time rating or capacity is usually calculated by dividing the breaking current using 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. The capable breaker with reference to the type of fault and fault current magnitude receives signal from the relay at the event of potential fault and changes from normally closed position to normally open position and segregates the unhealthy section from the entire power system.

Conclusion
The challenges involved in relay coordination of a protective system during abnormal condition are 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 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 Annexure. 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.

Acknowledgement
I acknowledge the help the help rendered by Surana Industries Limited (SIL), Mumbai for having provided the data of their distribution network for analysis.