Thesis-1: Automatic Generation Control (AGC) – Part_2

Posted: June 27, 2012 in Electrical Engineering, Electrical Thesis, Thesis Papers
Tags: , , , , , , , , ,


2.1. Introduction

This chapter represents the effect of loads on the voltage and frequency. The system frequency deviation affects the changes of real power and reactive power is affected by the voltage magnitude and frequency. We discuss frequency effect.

2.2. Control loop

Changes in real power affect mainly the system frequency, while reactive power is less sensitive to changes in frequency and is mainly depended on changes in voltage magnitude. Thus the real and reactive powers are controlled separately. The load frequency control (LFC) loop controls the real power frequency, and automatic voltage regulator (AVR) loop regulator the reactive power and voltage magnitude [5].

2.2.1. Basic Generator Control loops

An isolated and interconnected power system, Load frequency Control (LFC) and Automatic Voltage regulator (AVR) equipments are installed for each generator. Figure1.1 saw the schematic diagram of the Load Frequency Control loops and the Automatic Voltage regulator (AVR) control loops of a synchronous generator. The controllers are set frequency operating condition and take care of small changes in load demand to the frequency and voltage magnitude within the specified limits

Fig 2.1: Load Frequency Control loops and the Automatic Voltage regulator (AVR) control loops of a synchronous generator.

Small changes in real power are mainly dependent on changes in the rotor angle δ and thus the frequency. Reactive power is mainly depends on the voltage magnitude (i.e. on the generator excitation).the excitation system time constant is much smaller than the prime over constant and its transient decay is much faster and does not affect the LFC dynamic. Thus, the cross-coupling between the LFC loop and the AVR loop is negligible, and the load frequency and excitation voltage control are analyzed independently [4, 5].

2.2.2. Megawatt-Frequency, or P-f control loops

The objective of this loop this is to exert control of frequency and simultaneously of the real power exchange via outgoing lines. Frequency sensor senses the frequency ‘error’ ∆f and the increments in the real tie line powers, which will indirectly provide information about the incremental static error, ∆δ. These sensor signals are amplified, mixed and transformed into a real power command signal in ∆PV which is sent to the prime mover to the call for an increment in the torque. As a result, a change in ∆PG, in the real generator, which will then change the state increments sensed.

2.2.3. Megavar-Voltage, or Q-V Control Loops

The objective of this loop this is to exert control of the voltage state │Vi│. The voltage error ∆│Vi│ is sensed and this signal is transformed into a reactive power command signal ∆QV, which is fed to excitation source. The result is a change in the rotor current, and thus in the generator EMF, which finally adds up to an increment change in the reactive generator ∆QG.

2.2.4. Dynamic Interaction Between P-f and Q-V Control Loops

In static sense, and for small deviations, there is little interaction between the p-f and Q-V control loops [6].

During dynamic perturbations we encounter considerable coupling between the two control channels, for two different responses.

  1. As the voltage magnitude fluctuates at a bus, the real load of that bus will likewise change as a result of the voltage load characteristics (∂PD/∂│V│).
  2. As the voltage magnitude fluctuates at a bus, the synchronizing coefficient (or “electric spring constants”) of all outgoing transmission lines will change.

A dynamic perturbation in the QV loop thus will affect the real-power balance in the system. In general, the QV loop is much faster than the Pf loop, due to the mechanical inertia constants (see next section) in the latter. If it can be assumed that the transients in the QV loop are essentially over before the Pf loop reacts, then the coupling between loops can be neglected.

This is a reasonable assumption, and we shall adopt it in the following analysis, due to the resulting simplicity.

2.3. Conclusion

A load change affected the system frequency and the voltage magnitude as well as the real power and reactive power. Interaction between two control loops is negligible [6].in this research, only the Load Frequency Control (LFC) have been discussed for single area power system.

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