The electricity demand is increasing,
and the need for electrical energy is growing in India as well as in other
countries. Energy demand is increasing 4.9 % per year on an average from 900
Terawatt-Hours (TWh) in 2013 to almost 3300 TWh by 2040 1. India holds for almost 17 % of the increase in global electricity
demand from 2013 to 2040 2. Renewable sources
like wind power, solar power, etc. are the reliable and the cost-effective
solution for the sustaining energy shortages 3.
Wind power is emerging as the fastest developing technology and provides the
largest share of the distributed generation in electrical power generation. The
installed capacity of the wind power generation has been increasing in the
world at an annual growth rate of about 20 % 4-5.

The major disadvantage of the power
generation from wind power is that its output is intermittent in nature. The
dynamic nature of wind power causes mal-operation of relays which affects the
stability and security of the system 6. The
wind generators of the wind farm functions only when the wind speed is between
the cut in and cut out speed of the wind turbine and the wind farm must be
always connected to the grid. Normally the settings of the relay are fixed by
assuming that the wind farm is connected to the power grid at its rated capacity
7. The penetration of distributed generated
power changes the conventional distribution system’s short circuit power, fault
level, amplitude of fault current and direction due to the variation in source
impedance 8. The addition of intermittent wind
power causes mal-operation in the differential relay, over reach or under reach
problem in distance relays and over current relay coordination problems 9-10.    

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The over current relays are the most commonly used
protection scheme for the feeders as they are economic and effective. The over
current relays have two settings; pick up current settings and Time Multiplier Settings
(TMS). To determine the above settings, two different methods are commonly
adopted, i.e., conventional approach, and optimization techniques 11. The conventional method is based on the
predetermination of all fault currents, abnormal conditions, system contingencies,
etc. The conventional methods are based on the network topology which includes
graphical selection procedure 12,
identification of Minimum Break Point Set (MBPS) using expert set 13 and linear graph theory 14.
The proper relay coordination is ensured only if the operating time of the
secondary relays is greater than the corresponding primary relays. The
conventional method is not applicable in large systems with contingencies
including generator outage, line outage etc., as it is time consuming. Relay
coordination in power network having multi generators including DG sources
becomes unfeasible through conventional optimization technique. Optimization
approaches solve the protection coordination problem by minimizing the
objective function subject to the constraints by maintaining the Coordination
Time Interval (CTI). The coordination of over current relays is formulated as
the optimization problem using linear programming approach in 15 using simplex and two phase simplex methods. In these
methods, the pickup current settings are predetermined and the operating time
of the relay is considered as the linear function of TMS. These optimization
techniques are not capable of handling the complicated problems and the
obtained results may be trapped in local minimum values 16. More over the accurate predetermination of the dynamic nature
of pick up current setting is not possible due to changes in network
topologies. When both the pickup current setting and TMS are to be determined
simultaneously, the coordination problem becomes Non Linear Programming (NLP)
problem. In 17, NLP problem is solved using
Gauss-Seidel iterative procedure. Subsequently the application of various
heuristic optimization techniques have also been proposed including genetic
algorithm approach 18, particle swarm
optimization technique 19, differential
evolution algorithm 20, ant colony
optimization 21, etc. These evolutionary
algorithms get trapped into local minima and as the result these algorithms
need to be run repeatedly by varying the parameters. In medium and large
interconnected networks, it is very difficult to satisfy all the coordination
constraints 22. Thus the relay coordination
problem is reformulated as unconstrained programming problems by using penalty
functions for avoiding constraint violations 23.