Active and Reactive Power Adjustment of a Synchronous Generator. Connected to Infinite Bus Systems by Fuzzy Logic Controller

The fuzzy logic controller based on the fuzzy set theory provides a useful tool for converting the linguistic control from the expert knowledge into automatic control rules [I]. By using fuzzy automatic rules from the heuristic or mathematical strategies, complex processes can be controlled effectively in many situations. But the most important and difficult point is how to obtain the proper control rules for a given system. In this study, active and reactive power control of a generator connected to infinite bus system was fulfilled effectively by the fuzzy logic controller. And the software of the system's simulation results were given. Introduction: Power system operation considered so far was under conditions of steady load. However, both reactive and active power demands are never steady and they continually change with rising or falling trend. Steam input to turbo generators (or water input to hydro-generators) must, therefore, be continuously regulated to match the reactive power demand with reactive power generation, otherwise the voltages at various system busses may go beyond the prescribed limits [2]. In modem large interconnected systems, manual regulation is not feasible and therefore automatic generation and voltage regulation equipment is installed on each generator. 1. Active and Reactive Power Control of a Synchronous Generator Connected to Networks Where the word network's means is that it has a very small internal impedance, which can be ignored, with a constant voltage and is infinite power compare with considered machine [3]. Because of its connection of such a network, both terminal voltage (Vt) and parameters of synchronous machine (Ra,Xs) are constant. Thus for the reactive and active power adjustment of a synchronous generator, we can change both the excitation current and the torque is given to machine shaft , respectively. Excitation current control of synchronous generator is operated paralleled to net\Vork (infinite bus systems) cause to change in its internal emk (Ef) and just only reactive power of the machine is effected. Active power of the machine stay at constant value [4]. Machine active power can be calculated in two ways. From the tap conditions of the machine, active power can be written for the first one as below: P = Vt. la. cose Where, Vt = terminal voltage la = annature current cose = power factor For the second one, by ignoring the stator resistance equations (1) can be rev,111tten relative to load angle, synchronous reactance and il1tenralemk by the excitation current. p = _E/_· r_'f . sin 8 Xs Both tenninal voltages (Vt) and active powers (P) stay at constant value. From the equations (') and (2), the constant values k 1 and k2 can be written as below: fa 'cosO = kl E/ ·sin8 = k2 By the increasing excitation current, the vector Ef and load current will change wit~in the range of k2 and k I, respectively As explained above, active power is not based on the excitation current. When the field current is constant in the machine, Efvoltage is also constant. Thus for the Adjustment of the active power, the torque angle should be changed. To make bigger the angle 0 as big as ,10, the given torque to generator's shaft should be increased and accelerated in a short time by opening the valve of the water turbine or steam turbine (figure-2). And depending on the torque angle, terminal voltage, synchronous reactance and internal emk, equation (3) can be rewritten as equation (4) [5] Vt Q = -.. (Ef ·cosO Vt) Xs 2. Design of Fuzzy Logic Controller For Active And Reactive Power Control at Synchronous Generators Connected to Infinite Bus System As known, active and reactive power demand changes in a various time of a day. Thus active and reactive power demand should be kept up with the power generations by adjusting field cunent (If) and torque angle (0). For our system, fuzzy controller is designed as below : M.,thtm.,· lie." If Opo' alor Q Figure 3. Design of fuzzy logic controller for active and reactive power control at synchronous generators connected to infinite bus system As seen in the block diagram of the controller, information obtained from SCADA system measure some values at energy systems determine which generators should be loaded at which level of active and reactive power. [6] From the equations (2) and (4) torque angle 0 and internal emk Ef are determined as shown equations (5) and (6). As well as this, by using saturation curve of the synchronous generator (shown figure 4), If can be found. Ef= (. (.AS ] V S'"tarct~ (;~ ~r J( V Q)-l] 8 =arct, P'Xs p 300