31-07-2013, 02:49 PM
Genetic PID Controlled Hydrothermal AGC with CES
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Abstract
This paper discusses automatic generation control (AGC) of an interconnected hydrothermal power system fitted with Capacitive Energy Storage in both the areas. In addition, a PID controller is also considered in both the areas instead of a PI controller as in conventional AGC .The gain settings are optimized using Genetic Algorithm approach. Integral Time Squared Error (ITSE) Technique is used for tuning optimum gain settings. Appropriate Generation Rate Constraints (GRCs) are also taken into account. Simulation studies are carried out and a comparison of dynamic performances brings out the superior performances of CES unit and PID controller in suppressing the frequency and tie-line power oscillations followed by a step load perturbation.
INTRODUCTION
P ower system is mainly consisting of different control areas. The control areas are interconnected through tie-lines which are used for energy transfer between areas and to support inter-areas during abnormal operations. Many studies have been reported in the past about the Load frequency control (LFC) (which is also termed automatic generation control (AGC))[1-7].Different control strategies, such as classical control, optimal control, sub optimal control etc have been employed in the past to obtain an optimum controller for LFC.
An energy storage device with abrupt response time can be inserted into the system can reduce the oscillation in frequency and tie line power oscillation due to the inherent characteristics of changing the loads. A variety of storage technologies are in the market but the most viable are battery energy storage systems (BESS), pumped storage hydroelectric system, superconducting magnetic energy storage (SMES) and capacitive energy storage(CES). Some of the disadvantages of BESS include limited life cycle, voltage and current limitation and environmental hazards. The disadvantages of pumped hydro electric units are larger in size, environmental and topographic limitation. For SMES, the limitations are SMES coil cannot be so upgraded also it requires continuous operating liquid helium system. In addition to that SMES requires continuous flow of current [8-10].
Hydrothermal Power Systems Model
A two area multiunit hydrothermal power system is considered for investigation as shown in figure 1. Area 1 consisting of two non-reheat thermal units and area 2 with two units of hydro. Each having a generation constraint of 10% per minute for thermal units and for hydro units 270% per minute for raising and 360% per minute for lowering generation are considered as in IEEE Committee Report on power plant response to load charges [14]. The transfer function models used for this work are developed as in IEEE Committee report on dynamic modes for steam and hydro turbines in the power system studies [15].
Fig.1 show the linear time invariant (LTI) transfer function model of two area multiunit hydrothermal power system with PID controller and CES block. Area participation factors (apf) are also considered as the system under investigation is a multi unit two area system. apfs are the ratios in which generating units adjust their power output. It is noted that in area 1, apf11+ apf22 = 1 and in area 2, apf21+apf22=1.Here apf11 = apf12 = apf21 = apf22 = 0.5 has been considered, which means all the units share the load change equally. The values of the constants in the block diagram for the areas, PID controllers and CES unit are given in the appendices.
Capacitive Energy Storage
The main aim of incorporating CES is to supply or absorb some energy due to sudden increase or decrease of load. So obviously for implementing CES a power handling system is needed. This is also called power conditioning system (PCS). Capacitor is an energy storage device which stores energy in its electrostatic field created between its plates in accordance with the applied potential.PCS consists of a power conversion system, bypass resistor, DC breaker with dump resistor, Reversing switch arrangement and the capacitor bank. Power conditioning system consists of 3-phase dual converters. Capacitor hank is having many capacitors connected in parallel, having an equivalent capacitive C as shown in Fig.2. Resistance Ri which is connected in parallel to the capacitor C which represents its leakage and dielectric loss [12].
Overview of Genetic Algorithm (GA)
GA has been used for optimizing the parameters of the system that are complex and difficult to solve by using conventional optimization methods. GA maintains a set of candidates called population and repeatedly modifies them in each steps called generation. At each generation, the GA selects individuals from the current population as parents and uses them to produce the offspring’s for the next generation. Candidate solutions are usually represented as strings of fixed length, called chromosomes. A fitness or objective function is used to obtain the goodness of each member of the population. For a given random initial population, GA operates in steps called generations, as follows [17]:
• Goodness of each member in the population is evaluated using a fitness function.
• The population undergoes reproduction in a number of iterations. One or more parents are chosen according to their fitness values having higher probability of contributing an offspring.
• Genetic operators, such as crossover and mutation, are applied to parents to produce offspring.
• The old population is replaced by offspring’s to form new population until convergence criteria is meet.
The flow chart of the GA optimization approach followed in the present paper is shown in Fig. 4
Conclusion
In this work a LTI mathematical model of two area multi-unit hydro thermal power system with and without CES and PID controllers has been studied. As per IEEE Committee report on power plant response to load change have also been considered in the simulation.
The Analysis seems to have revealed that the area of CES and PID controller in AGC is capable of damping oscillations and also the amplitudes of the deviations in frequency and tie-line power are reduced significantly. Because of the inherent characteristics of changing loads, the operating point of a power system may change very much during a daily cycle.