Class Notes on Power System Control and Operation

Modern Power System

Modern power system is a complex system, spread over a large geographically area. The power system has been expanded manifold in the recent past due to increased demand and industrial growth. The power system has also seen new emerging trend in its technology, operation and planning.

The issues such as, reactive power and active power control, angle stability and voltage stability, inter-area power transfer, power quality, automatic generation and frequency control for multi-machine system, reliability evaluation operation in competitive environment, are important factors in operation and control of the power system.

Generation, transmission, distribution and loads make up the fundamental structure of a power system. Generators, substations, transmission circuits, transformers, circuit breakers, metering, demand, load, are the main physical devices of an electric power system.

Moreover, if the system is subjected to faults, it may become unstable. Therefore, it is necessary to study and analyze the system state and to determine the system stability and security.

For this purpose, we need to use many numerical techniques and modem stochastic or metaheuristics computational techniques. After studying this subject, one may be able to address and understand the issues related the power system.

Fault Analasys

Symmetrical Components

The power systems are subjected to faults. Most of the faults in power system are unsymmetrical in nature. However symmetrical faults such as short-circuits or 3-phase faults may also occur.

The unsymmetrical faults are:

1. Lme to Ground faults,
2. Lme to Line faults or Double Line faults,
3. Double Line to Ground faults

Hence a three phase unbalanced systems can be resolved into three balanced systems as shown in Figure 1 above.

V_a1, V_b1, V_c1 – Positive sequence voltages of three phases a, b, c equal in magnitudes and displaced from each other by 120° in phase. Subscript ‘1’ represents positive sequence i.e. having the same phase sequence as the original system.

V_a2, V_b2, V_c2 – Negatrve sequence voltages of three phases a,b,c equal in magnitudes and displaced from each other by 120° in phase. Subscript ‘2’ represents negative sequence i.e. having the opposite phase sequence as the original system.