Designing a GA-Based Robust Controller For Load Frequency Control ( LFC )

Power systems include multiple units linked together to produce constantly moving electric power flux. Stability is very important in power systems, so controller systems should be implemented in power plants to ensure power system stability either in normal conditions or after the events of unwanted inputs and disorder. Frequency and active power control are more important regarding stability. Our effort focused on designing and implementing robust PID and PI controllers based on genetic algorithm by changing the reference of generating units for faster damping of frequency oscillations. Implementation results are examined on two-area power system in the ideally state and in the case of parameter deviation. According to the results, the proposed controllers are resistant to deviation of power system parameters and governor uncertainties. Keywords-load frequency control; LFC; drop characteristics


INTRODUCTION
A power system includes multiple power plant units constantly connected to each other while power flux is moving among them.Power systems should be operated in such a way that they remain stable or return to stable condition as soon as possible.If the power system frequency -which is equal to the frequency of production units electrical power output-is deviated, this leads to a difference between the speed of stator and rotor rotating magnetic fields in the air gap of synchronous generators.If this difference is low or quickly eliminated, synchronous generator remains in stable condition but if this difference becomes greater, the rotating magnetic fields will not be stable in the coupled state.Any synchronous generator output power in the production unit will be injected into the power system.Following this, the load of generator will be eliminated.So, rotational speed of rotor shaft will suddenly increase, which is probable to cause damage like cracks in the rotor or even total destruction of the power system stability.So, frequency of different areas and the power flux between them can be considered as fundamental stability factors in a power system.When load increases, the turbine speed will be reduced.As long as governor coordinates input vapor with new load, speed reduction may lead the power system to instability.Load Frequency Control (LFC) problem is a way to restore nominal values of frequency by adding PI or PID controllers as supplementary controller system.

II. LOAD FREQUENCY CONTROL
Since real power (P) with frequency (f) and reactive power (Q) with voltage amplitude (|V|) are associated, by considering the importance of keeping frequency and voltage amplitude at desirable level, control of real and reactive power is very important.As noted, for a power system to perform in favorable state it is necessary for the frequency to remain constant.In order to maintain frequency stability it is necessary to prevent deviation in the outflow of production units.However, since power in a power system is supplied by a large number of generators, it is necessary the extra power requested to be divided appropriately among production units.Main speed control in each production unit is performed by governor.In other words, governor is the primary frequency controller.Higher control process is performed by other controllers added to the system as supplementary controllers.This process is called load frequency control (LFC).

PROBLEM
Several attempts to solve LFC problem have been carried out.The installation of photovoltaic (PV) in [1] and possible uncertainties caused by time delays in [2] can be mentioned.In [3] complex and nonlinear parameters of a decentralized LFC problem have been inspected.In [4] authors designed a differential controller based on fuzzy logic to solve the LFC problem in a restructured environment.In [5] the problem is solved using a method called SOA.In order to solve LFC problem PID controllers based on genetic algorithm are used.In [6] a two-stage controller which prevents disorders from entering the power system is proposed.LFC solving methods are divided into following categories.

A. Classic Controller
In this method, error signal is based on the integrated control error.In the classical approach, to reach desirable gain margin (K m ) and phase margin (φ m ) in Bode plot, Nyquist curve is used as reference locus.This controller is simple to implement but according to the results obtained in [7]

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