Optimail Strategy for Over Current Relay Coordination Using Genetic Algorithm

Rania A. Swief, Almoataz Y. Abdelaziz" Member IEEE, A. Nagy

Department of Electrical Power & Machines, Faculty of Engineering, Ain Shams University, Cairo, Egypt

Abstract - In conventional methods coordination of Over

Current relays (OCR) is obtained through careful time

grading. Due to the expansion of electric systems, the need

for an efficient coordinated protective system is crucial.

The optimal time for protection given the coordination

problem turned to be little complex to be calculated. To

solve the problem of the coordination of OC relays,

Genetic Algorithm (GA) technique is applied. The purpose

of the OC relay coordination is to find an optimum relay

setting to minimize the time dial setting (TDS) and

calculate the pickup current (Ip). Setting of the relay is the

core of the coordination study to satisfy the primary and

secondary operation. This paper presents an application of

GA technique for optimal coordination ofOC relays to a 6-bus ring system.

Keywords - Over current relay coordination, Genetic algorithms , power system protection

I. INTRODUCTION

Directional over-current relay is an important protective device in power system. It is used to protect electric power equipment in power system when a fault occurs. Power system protection is mainly divided into protective zones. Each zone is responsible for prevention and protections operate in separate zone of responsibility as quickly as possible from the system when fault occur in the system. Over current (OC) relays are commonly used for the protection of interconnected sub transmission systems, and distribution systems [1]. This level of protection is called primary protection system and if primary protection fails or does not operate a secondary protection must be operated, which is responsible for backup the operations of the primary protection. Therefore the relay position has served as primary relay or backup relay in case of a fault occurred. To provide more effective protection, relays must be coordinated in power systems [2].

OC time setting characteristics have three criterions: constant time, instantaneous and inverse time characteristics. OC with inverse time characteristics is having fast fault clearing times, as the magnitude of the current increases. These relays are provided in electrical power system to isolate only the fault lines of the faulted section from the system.

Relay is a logical element and issues a trip signal to circuit breaker if a fault occurs within the relay protective zone and is placed at both ends of each line. Relay co

.ordination proble� is

to determine the sequence of relay operatIOns for each possIble fault location so that faulted section is isolated [3]. Good OC relay coordination is very important for industrial plant, poor

978-1-4799-5807-8/14/$31.00 @2014 IEEE

coordination may spread fault zones wider caused unnecessary power blackout or damage equipment which are avoidable , or even more affect backup utility substations [4].

The setting of OC relays has to satisfy all possible network configurations subject to type and location of all faults. It is not easy to find a proper OC relay setting to meet this requirement by traditional methods [5]. The setting of the OC relay is consisted of defining two terms, the pick-up current which the relay starts to operate and the time delay setting TDS. For the difficulty of defming the protection problems given many constraints at the same time, an essential need arise for using intelligent techniques like GA to select the suitable pick up current (Ip) and operating time (TDS). The structure of the fundamental protective functions is met under the requirements of sensitivity, selectivity, reliability and speed [6].

Many search technique have been implemented to solve the coordination problem starting from linear programming [5] till the artificial intelligent techniques like particle swarm [1, 7], differential evolution [3, 6, 8], genetic algorithm [4] and multi­agent system [9]. GA is different from other search techniques in several aspects. First, the algorithm is a multipath that searches many peaks parallel, hence reducing the possibility of local minimum trapping. Second, the GA works with a coding of parameters instead of parameters themselves. The coding of parameter will help the genetic operator to evolve the current state into the next state with minimum computations. Hence GA gives the global optimum solution. GA optimization method has been employed to many power system problems [9-11] and it is applied in this paper to solve the optimum coordination of OC relays.

This paper consists of five sections. Section I presents a review for the coordination problem and the need for artificial intelligence techniques to set the optimization problem. Section II discusses the construction of fitness function and constraints of the coordination problem. Section III describes the GA outlines for OC coordination problem Section IV describes the application of GA to solve the OCR optimal coordination problem on IEEE 6-bus ring system. Section V discusses the results and conclusion.

II. COORDINA nON OF OC RELAYS

A. Objective function

In the coordination problem, the purpose is to minimize the TDS and calculates Ip of each relay, so that the sum of the operating time of the primary relay, for near end fault, is to �e minimized. The objective function can be defined as follows m equation 1:

(1)

Where; n is the number of relays and ti is the operating time of the i relay for near-end fault. The weight wi depends upon the probability of a given fault occurring in protection zone and usually set to one.

B. Relay characteristics

In this study all relays are assumed to be identical. The characteristic equation can be defined as follows in equation 2:

_ O.14xTDSi lik -1

- ( )0.02

Ipi

(2)

Where Iik is the short circuit current passing through the relay and Ipi is the pickup current settings of relay Ri

C. Coordination constraints 1. Selectivity constraints for all relay pairs

Selectivity means that the faulted line is the only part to be disconnected which means that the primary time must be greater than the secondary time with certain delay as shown in equation 3.

Tbackup -Tprimary 2: eTI (3)

Where Tbackup is operating time of backup relay.

Tprimary is the operating time of primary relay. cn is

coordination time interval, is equal to 0.3 seconds. This number based on using electromechanical relays and can be reduced using the electronic relays.

2. Bounds on TDS

There setting boundaries must be fulfilled as described in equation 4.

TDS; . ::;; TDS; ::;; TDS; <mln < <max (4)

Where TDSimin is the lower limit and TDSimax is upper

limit ofTDSi. These limits are 0.05 and l.l respectively.

3. Limits on primary operation times

This constraint imposes constraint on each term TDS of the objective function to lie between 0.05 and l.2.

III. OUTLINES OF GENETIC ALGORITHM PROGRAM FOR

OCR COORDINATION

Figure 1 shows a flow chart which describes the outlines of the genetic algorithm program as applied to the OCR coordination problem. The setting parameters for the genetic algorithm are as in Table I.

Table I - GA Parameters Number of generation 100 Number of population 100 Crossover scattered

Selection uniform

Formulate the objective function (operating time) and the constraints

GA initialization and parameters' setting

Output pick up currents and the time setting

chromosome

Figure 1 - Genetic Algorithm Program Flow Chart for Coordination of Over Current Relay

IV. THE IEEE 6-BUS MODEL

The system under study is the IEEE 6-bus ring system which is shown in Figure 2 [3]. The proposed numbering of the over current relays is shown in Figure 2. Values of relay setting parameters depend upon the number of turns in the equipment current transformer (CT). CT is used to reduce the level of the current so that relay can withstand it. The parameters of primary relays, their back up relays and their coordination are presented in Tables II & III with the relay currents and their fault currents.

Two strategies are adopted in this study. The fIrst one is to take into consideration both near and far end times (Part I) or the only the far end time delay setting (Part II).

(2) -.2 1+

7

e 12

4

The value of constants, a and c, are maximum fault currents while band d are load currents. These parameters are of primary relay conditions. The constraints are shown in equations 8, 9, and 10. They are summarized in Table II.

T�ackuP - T�rimary - CT I :2: 0 (8)

(9)

(10)

The ps is the plug setting value and this value is one of the unknown in the objective equation. The values of constants, e and g, are the maximum fault currents while f and h are the load currents. These parameters are of backup relay conditions which are summarized in Table II & Table III.

t TABLE II - SOME PARAMETERS OF PRIMARY RELAY

11

3 5

0 -.

0 14 13

Figure 2 - A typical IEEE 6-bus OC relays coordination problem model

In Figure 2, 14 over-current relays are equipped in a 6-bus system [3].

• Part I

The fIrst strategy is to calculate the sum of the near end and the far end time as follows in equations 5,6, and 7.

OBI = If�l TJr_cun + II!l T�rJar_bus

Where

ri . =

O.14*TDSi pr JUn (�)O.02

ps!*b!

rj = O.14*TDSj prJar_bus (�)O.02

psJ *dJ

(5)

(6)

(7)

T�TJUn TjT-!aT_bus

TDS a' b' TDS c

' d

TDSI 2.5311 0.2585 TDS2 5.9495 0.2585

TDS2 2.7376 0.2585 TDSI 5.3752 0.2585

TDS3 2.9723 0.4863 TDS4 6.6641 0.4863

TDS4 4.1477 0.4863 TDS3 4.5897 0.4863

TDS5 1.9545 0.7138 TDS6 6.2345 0.7138

TDS6 2.7678 0.7138 TDS5 4.2573 0.7138

TDS7 3.8423 1.746 TDS8 6.3694 1.746

TDS8 5.618 l.746 TDS7 4.1783 l.746

TDS9 4.6538 1.0424 TDSI0 3.87 1.0424

TDSIO 3.5261 1.0424 TDS9 5.2696 1.0424

TDSII 2.584 0.7729 TDS12 6.1144 0.7729

TDS12 3.8006 0.7729 TDSII 3.9005 0.7729

TDS13 2.4143 0.5879 TDS14 2.9011 0.5879

TDS14 5.3541 0.5879 TDS13 4.335 0.5879

TABLE III - SOME PARAMETERS OF BACKUP RELAY

T�aCkUP T�TimaTY

P e' f Q g' hi

8 4.0909 l.746 1 5.3752 0.2585

11 1.2886 0.7729 1 5.3752 0.2585

8 2.9323 l.746 1 2.5311 0.2585

3 1.6658 0.4863 2 2.7376 0.2585

3 1.6658 0.4863 2 5.9495 0.2585

lO 0.0923 1.0424 3 4.5897 0.4863

10 2.561 1.0424 3 2.9723 0.4863

13 1.4995 0.5879 3 4.5897 0.4863

1 0.8869 0.2585 4 4.l477 0.4863

1 1.5243 0.2585 4 6.6641 0.4863

12 2.5444 0.7729 5 4.2573 0.7138

12 1.4549 0.7729 5 1.9545 0.7138

14 1.7142 0.5879 5 4.2573 0.7138

3 1.4658 0.4863 6 6.2345 0.7138

3 1.1231 0.2585 6 6.2345 0.7138

11 2.l436 0.7729 7 4.1783 1.746

2 2.0355 0.2585 7 4.1783 1.746

11 1.9712 0.7729 7 3.8423 1.746

2 1.8718 0.2585 7 3.8423 1.746

13 1.8321 0.5879 9 5.2696 1.0424

4 3.4386 0,4863 9 5.2696 1.0424

13 1.618 0.5879 9 4.6538 1.0424

4 3.0368 0.4863 9 4.6538 1.0424

14 2.0871 0.5879 11 3.9005 0.7729

6 1.8138 0.7138 11 3.9005 0.7729

14 1.4744 0.5879 11 2.584 0.7729

6 1.1099 0.7138 11 2.584 0.7729

8 3.3286 1.746 12 3.8006 0.7729

2 0.4734 0.2585 12 3.8006 0.7729

8 4.5736 1.746 12 6.1144 0.7729

2 1.5432 0.2585 12 6.1144 0.7729

12 2.7269 0.7729 13 4.335 0.5879

6 1.6085 0.7138 13 4.335 0.5879

12 1.836 0.7729 13 2.4143 0.5879

lO 2.026 1.0424 14 2.9011 0.5879

4 0.8757 0.4863 14 2.9011 0.5879

lO 2.7784 1.0424 14 5.3541 0.5879

4 2.5823 0.4863 14 5.3541 0.5879

Tables IV shows the results of time delay settings and pick up currents relative to the fIrst proposed criterion (part 1) using the genetic algorithm and satisfying each and every constraint.

TABLE IV - THE OPTIMUM VALUES OF (TDS) AND (lp) OBTAINED USING GA TECHNIQUE - FIRST

CRITERION

Relay Number TDS Ip

Rl 0.12064 1.26713

R2 0.20392 1.30078

R3 0.l0198 l.25512

R4 0.11339 1.25

R5 0.05 1.25452

R6 0.05 1.26615

R7 0.05 1.25002

R8 0.05123 1.2539

R9 0.05 1.25055

RIO 0.06547 l.25182

R11 0.08488 1.25

R12 0.06172 1.29608

R13 0.05624 l.3133

R14 0.09684 1.25926

The optimum total time setting as it is described in details in Table IV is equal to:

FITNESS FUNCTION VALUE: lO.734481982442581

• Part II

Part II is dealing with the optimum function based on the far end values as revealed on equations 11 and 12.

OBi = Lf!l T�rJar_bus Where

(11)

(12)

Tables IV shows the results of time delay settings and pick up currents relative to the second proposed criterion (Part II) using the genetic algorithm and satisfying each and every constraint.

TABLE V - THE OPTIMUM VALUES OF (TDS) AND (Ip ) OBTAINED USING GA TECHNIQUE

Relay Number TDS Ip

Rl 0.11929 1.25

R2 0.18862 1.47913

R3 0.09688 1.26389

R4 0.11446 1.25078

R5 0.05028 1.25091

R6 0.05079 1.25

R7 0.05 1.25

R8 0.05099 1.25

R9 0.05 1.25

RIO 0.06073 1.25

Rll 0.08467 1.25

R12 0.06392 1.25

R13 0.05781 1.27076

R14 0.09673 1.25731

The optimum total time setting as it is described in detail in Table V is equal to:

FITNESS FUNCTION VALUE: 5.9225973157780185

From Tables IV & V, the results almost show the same pick up currents even if the time is calculated from far end terminal or both near and far end terminals.

CONCLUSION

Coordination of relays is a vital issue in power system studies. Good results are obtained in solving the coordination problem with the help of Genetic Algorithms (GA) on the IEEE 6 bus ring system. Two different time settings calculation strategies were applied, one technique is based on the sum of both far and near end time calculation and the other technique is based on the swn of the far end time setting only. For both strategies, the pickup currents were the same, so the only criterion to differentiate between both strategies is the time setting values. The result shows promising results of the second strategy. The results show that for vital loads which currents can never be allowed to be attained for long time, the designing of the time dial setting (TDS) based on far end time calculations provides better performance.

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