IJSRD - International Journal for Scientific Research & Development| Vol. 6, Issue 02, 2018 | ISSN (online): 2321-0613

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Relay Coordination of over Current Relay in Presence of DG System

using FCL

Pushpa Bhatiya1 Shruti Khatri2

1,2Department of Electrical Engineering 1,2Vadodara Institute of Engineering and Research, Vadodara, India

Abstract— In the Power System presence of Distributed

generation (DG) is one of attractive phenomena. With the

presence of DG units in distribution system, its function

would generally be changed and it would variously be

affected by DGs. When DGs are introduced into the power

system, the value and direction of both system’s power flow

in normal operation and short circuit current under fault

condition are changed results the disturbance to relay

coordination. In this, GA-NLP approach is presented to solve

the directional over current relay coordination problem,

which arises from installing DG in distribution system. This

approach involves the implementation of Fault Current

Limiter (FCL) to limit the DG fault current. In this thesis,

simulation of 3-bus and 9-bus system is implemented in

ETAP software. The optimum value of time multiplier setting

(TMS) and plug setting (PS) are found by using MATLAB

Optimization toolbox.

Key words: Genetic Algorithm (GA); Nonlinear

Programming (NLP); Overcurrent Relay Coordination; Time

Multiplier Setting (TMS); Plug Setting (PS), Fault Current

Limiter (FCL)

I. INTRODUCTION

OVERCURRENT relays have been widely used as an

economic alternative option for the protection of transmission

and distribution system [1]. To diminish the power outages

care should be taken that backup relays should work

efficiently and chances of their mal-operation should be

avoided. Hence coordination in case of overcurrent relays is

a matter of major. Overcurrent relay must be efficient enough

to provide protection to the affected primary zones. In case if

the primary protection fails to clear the fault, the back-up

protection should instigate tripping. Each protection relay

needs to be coordinated with the relays protecting the

neighboring equipment thereby making the overall protection

coordination complicated [2].

Distributed generation (DG) presence in power

systems is one of the attractive phenomena in power industry.

With the presence of DG units in distribution systems, its

function would generally be changed and it would variously

be affected by these units. One of the most important effects

of these units is on distribution systems protection. Since the

number of DG units not only can be varied but also has a great

deal of wide spread, Distribution systems protective devices

manner is completely changed with the presence of

distributed generation[3]. DG units are electrical energy

sources which are connected to distribution systems and in

comparison with the large scale power stations, have the

lower generation capacity and also have a lower starting cost.

II. PROBLEM FORMULATION

The coordination problem of overcurrent relays is an

optimization problem, where the sum of the operating times

of the relays of the system is to be reduced,

min Z= ∑ Wi . ti,k (i=1 to m) (1)

Where,

m- number of relays;

Wi- operating time of the relay;

ti,k - weight assigned for operating time of the relay.

As the lines are short and are of approximately equal

length, weight is equal for operating times of all the relays.

The purpose of reducing the total operating times of relays is

to be achieved under five sets of constraints, as discussed

below.[4.]

A. Constraint Set I—Coordination Criteria

The backup relay should operate only in case if the primary

relay fails to provide protection and causes maloperation.

The operating time of primary relay decides

minimum operating time of backup relay. The sum of

operating time of CB associated with primary relay, and the

overshoot time is called the coordination time interval (CTI).

If Rj is the primary relay for fault at k, and Ri is backup relay

for the same fault, then the coordination constraint can be

stated as –

ti,k – tj,k ≥ Δt (2)

Where,

ti,k - operating time of the primary relay, Rj

tj,k - operating time of the backup relay, Ri

B. Constraint Set II—Bounds on Relay Operating Time

Relay needs minimum time for operation. The constraint

imposed due to time related restriction on the operating time

of relays can be stated as –

ti,min ≤ ti.k ≤ ti,max (3)

Where,

ti,min - minimum operating time of relay at i

ti,max - maximum operating time of the relay at i

C. Constraint Set III—Bounds on the TMS ofRelays

Relays directly affect the operating time of relays, which puts

bounds on of relays. It can be stated as

TMSi,min ≤ TMSi.k ≤ TMSi,max (4)

Where,

TMSi,min - minimum value of relay;

TMSi,max - maximum value of relay.

D. Constraint Set IV—Bounds on the PS of Relays

The bounds on PS of relays can be stated as

PSi,min ≤ PSi.k ≤ PSi,max (5)

PSi,min - minimum value of PS on relay;

PSi,max - maximum value of PS on relay.

Relay Coordination of over Current Relay in Presence of DG System using FCL

(IJSRD/Vol. 6/Issue 02/2018/910)

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E. Constraint Set V—Relay Characteristics

A nonlinear and popular overcurrent relay characteristic

function is as below-

top =(λ.TMS) / ((Irelay / PS)γ - 1) (6)

Where,

top- relay operating time;

Irelay – current through coil of relay.

III. APPLICATION OF HYBRID GA-NLP APPROACH TO RELAY

COORDINATION OPTIMIZATION

The problem of optimum coordination of OCRs is formulated

as NLPP. After formulation of the problem, few generations

of GA are used to determine the TMS and PS of OCRs and

Xfcl of fault current limiter. These values are then used as

initial choice in NLP method, which gives the global

optimum solution[2].

A. Finding Initial Solution Using GA

For applying GA technique to find the initial solution, the

problem is first converted into unconstrained optimization

problem. As the objective function, coordination constraints

and operating time constraints are written using the relay

characteristics, the relay characteristic constraint gets

automatically incorporated in the objective function,

coordination constraints and operating time constraints. The

constraints due to bounds on TMS and PS and Xfcl are taken

care of by defining the lower and upper limit of the variables

representing TMS and PS of relays and Xfcl, respectively, in

the GA program[2]. The constraints due to operating time of

relays, and the constraint due to coordination criteria, and the

constraint due to FCL are included in the objective function

using penalty method and thus the problem gets converted

into an unconstrained optimization problem[2]. Number of

bits to represent each parameter is decided and then a

population of suitable number of chromosomes is generated

randomly. The value of variables in each chromosome is

bounded by lower and upper limits, as described above. After

this, the iterations (generations) of GA are started. The

population is passed through the fitness function (objective

function). Then the population is sorted according to fitness.

As the objective function is of minimization type the

chromosome giving minimum value is most fit chromosome.

This chromosome has been treated as elite chromosome. The

chromosomes with higher _tness value survive (reproduced

in the next generation) and are called parent chromosomes.

These are used for mating. Pairs of parent chromosomes are

made for mating. Using the pairs of parent chromosome,

crossover is performed. For each pair the crossover site is

selected randomly. One pair (two parent chromosomes)

generates two offsprings after crossover. All the parent

chromosomes and all offsprings are placed together to form

the population for the next generation. The same population

size is maintained in all generations. The "mutation" is

applied after crossover. The number of mutations to be

performed is decided by mutation rate, which is one of the

GA parameter to be supplied at the beginning of the program.

For each mutation the chromosome is selected randomly (if

selected chromosome is elite then mutation is not performed

on this chromosome and another chromosome is selected).

The bit to be mutated is selected randomly from the

chromosome and is replaced by its complement. At this point,

an iteration of GA is complete. As, in this paper, the stopping

criteria has been considered as number of iteration of GA, the

process is stopped after performing pre-specified number of

iterations. The elite chromosome, at this stage, gives the

result[2].

B. Finding Final (Optimum) Solution Using NLP

The function available in MATLAB optimization toolbox, to

solve a constrained nonlinear optimization problem, can be

used to find the global optimum solution of the relay

coordination problem. The results obtained, after performing

prespecified number of iterations of GA, are taken as the

initial values of variables while applying NLP method. The

lower and upper bounds of variables (TMS and PS of OCRs),

are decided as per the mentioned constraints. The objective

function and the constraints (relay operating time constraints,

and coordination constraints), are defined in two different

function files. These functions are called in the main program

where the MATLAB optimization toolbox function is used to

give the optimum solution[2].

IV. SYSTEM INFORMATION

A. Three bus system without DG

The Hybrid GA -NLP method was tested for three bus system

as shown in fig 1. In this, eight relays have been considered

as numerical relay with standard IDMT characteristics. A

multi loop distribution system with 3-buses, and 8 relays, as

shown in fig.1 was considered.

Bus 1 is receiving the power, which has been

represented by a source of 25 MVA, 11 kV with a source

impedance of (0+j0.25)p.u. Base MVA is 25 MVA and base

kV is 11 kV. Lines between bus-1 and bus-2, and line

between bus-2 and bus-3 have an impedance of ( 0+j0.3) p.u.

Line between bus-1 and bus-3 has an impedance of (0+j0.6)

p.u. The overcurrent relays (OCRs) are numbered as 1 to 8.

B. System information with DG

The Hybrid GA -NLP method was tested for three bus with

DG system as shown in fig 2.In this eight relays have been

considered as numerical relay with standard IDMT

characteristics. A multi loop distribution system with 3-buses,

and 8 relays, as shown in fig 2 was considered.

Fig. 1: Single line diagram of three bus system

Relay Coordination of over Current Relay in Presence of DG System using FCL

(IJSRD/Vol. 6/Issue 02/2018/910)

All rights reserved by www.ijsrd.com 3360

Fig. 2: Single line diagram of three bus system with DG

Bus 1 is receiving the power, which has been

represented by a source of 25 MVA, 11 kV with a source

impedance of (0+j0.25)p.u. Base MVA is 25 MVA and base

kV is 11 kV. Lines between bus-1 and bus-2, and line

between bus-2 and bus-3 have an impedance of ( 0+j0.3) p.u.

Line between bus-1 and bus-3 has an impedance of (0+j0.6)

p.u. The overcurrent relays (OCRs) are numbered as 1 to 8.

Five wind generators are connected on bus 2 and rating of

each is 2.3 MW.

V. SIMULATION RESULTS OF THREE BUS SYSTEM

The proposed method was tested for the three bus system with

the help of different optimization method like genetic

algorithm, NLP and Hybrid GA-NLP. Short circuit Analysis

of three bus (with DG and without DG) system have been

done using E-TAP software. MATLAB optimization toolbox

function is used to get the optimum solution.

Table 1: Maximum load current and maximum fault current

(without DG)

Table 2: Values of TMS and PS for relays using GA

(without DG)

Table 3: Values of TMS and PS for relays using NLP

(without DG)

Table 4: Values of TMS and PS for relays using hybrid GA-

NLP (without DG)

Relay Coordination of over Current Relay in Presence of DG System using FCL

(IJSRD/Vol. 6/Issue 02/2018/910)

All rights reserved by www.ijsrd.com 3361

Table 5: Maximum load current and maximum fault current

(with DG)

Table 6: Values of TMS and PS for relays using GA (with

DG)

Table 7: Values of TMS and PS for relays using NLP (with

DG)

Table 8: Values of TMS and PS for relays using hybrid GA-

NLP (with DG)

VI. CONCLUSION

The values of TMS and PS for each relay of the three bus and

nine bus distribution system without DG using only genetic

algorithm technique were not optimal. Optimization

terminated and no feasible solution found using NLP method

for the same system. By using hybrid GA-NLP method a

feasible solution is achieved with minimum objective

function value. CTI was maintained between primary and

backup relay pairs. The operating time of all the relays were

minimized using Hybrid GANLP approach and at the same

time coordination was maintained. The values of TMS and

PS for each relay and XFCL of the three bus distribution

system with DG using only genetic algorithm technique were

not optimal. Optimization terminated and no feasible solution

found using NLP method for the same system. The optimal

values of TMS and PS and XFCL for 3-bus distribution

system with DG were found using hybrid GA-NLP approach

and CTI was maintained between primary and backup relay

pairs. The operating time of all the relays were minimized

using Hybrid GA-NLP approach and at the same time

coordination was maintained.

VII. FUTURE SCOPE OF WORK

There are some recommendations for further study in this

topic:

Used different types of relays such as directional

overcurrent relay, earth fault protection and others. Some

distribution system used different types of relay like

earth fault relay and directional overcurrent relay. The

different types of relay can be used to compare the

operating time at different setting of relays.

Combined all the protection system for distribution

system and transmission system. For transmission

system, the protection that applied was different with

distribution system. So, if distribution and transmission

system was combined, we should get the different setting

and coordination according to the system.

The further research suggested is the development of the

total intelligent protection system that is adaptive to all

Relay Coordination of over Current Relay in Presence of DG System using FCL

(IJSRD/Vol. 6/Issue 02/2018/910)

All rights reserved by www.ijsrd.com 3362

system conditions. Two major areas are improvement on

optimization method and real-time coordination and

adaptive grading margin.

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