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+ Nile Valley UniversityFaculty of Engineering & TechnologyDepartment of Electrical & Electronic Engineering

Power System Stability

/ Othmanabdeen_nvu@hotmail.com

Power System Stability

1. INTRODUCTION :- Power system security in general may be defined as the system robustness to operate in an equilibrium state under normal and perturbed conditions. Power system security covers a wide range of aspects, usually subdivided into static and dynamic phenomena. Power system stability currently refers to the dynamic part of security.Power system stability may be defined broadly as that property of a power system that enables it to remain in a stable equilibrium state under normal operating conditions and to regain an acceptable equilibrium state after being subjected to a disturbance.

Figure 1.1 Types of power system stability phenomena.

1.1 Power system transients :-Power system transients are power-quality disturbances that can be harmful to electronic equipment.THE TERM transient originates from electric circuit theory where it denotes the voltage and current component that occurs during the transition from one (typically sinusoidal) steady-state to another steady-state.Electric circuits are described by means of differential equations, whose solutions are the sum of a homogenous solution and a particular solution. The particular solution corresponds with the steady-state; the homogeneous solution corresponds with the transient.

In electric circuit theory a transient is always associated with a change in steady state due to a switching action.In power systems the term transient is used in a slightly different way: it denotes those phenomena in voltage and current with a short duration. There is no clear limit, but phenomena with aduration of less than one cycle (of the power-system frequency, 50 or 60 Hz) are generally referred to as transients.The interest in power system transients has traditionally been related to the correct operation of circuit breakers and to over voltages due to switching of high-voltage lines. But more recently transients are viewed as a potential power-quality Problem.CATEGORIZATION OF POWER SYSTEM TRANSIENTSA. Impulsive TransientsAn impulsive transient is a sudden change in the steady state condition of voltage, current or both, that is unidirectional in polarity (primarily either positive or negative).Impulsive transients are normally characterized by their rise and decay times.The most common cause of impulsive transients is lightning .Fig 1. 2 shows an impulsive transient measured in a 132 kV network.

Fig 1.2 Voltage waveform of an impulsive transient (measurement in a 132-kV network).B. Oscillatory Transients

A typical example of an oscillatory transient is caused by the energizing of a capacitor bank.The oscillation frequency is mainly determined by the capacitance of the capacitor bank and the short-circuit inductance of the circuit feeding the capacitor bank.Fig.1. 3 Distribution system for the simulation of voltage amplification due to capacitor energizing.

Fig.1. 3 Distribution system for the simulation of voltage amplification due to capacitor energizing.C. Multiple Transients With a Single CauseDue to one single switching action. However in many cases the transient waveform is due to more than one switching action leading to overlapping transients.Other examples of multiple transients with a single cause are current chopping and restrike. Current chopping occurs when the current during opening of a circuit breaker.Restrike may occur when a capacitor is de-energized by a slowly moving switch. An example of multiple restrikes is shown in Fig 1.4 As shown in the figure multiple restrikes can lead to an escalating voltage over the capacitor leading to an internal flashover and serious damage to the equipment.

Fig 1.4 Voltage waveform for multiple restrikes during capacitor de-energizing. From top to bottom: (a) line side; (b) capacitor side. TABLE 1.1 CATEGORIZATION OF TRANSIENTS BASED ON WAVEFORM SHAPES AND THEIR UNDERLYING CAUSES (OR EVENTS)

1.2 Electromagnetic and Electromechanical transients :-Electromagnetic transients

Electromagnetic transient phenomena are usually triggered by changes in the network configuration, which may be caused by the closing or opening action of circuit breakers or power electronic equipment, or by equipment failure or faults, such as a lightning stroke on a transmission line.

The study of electromagnetic transient phenomena includes switching surges, transient recovery voltage, and etc... Electromagnetic transients programs (EMTP) are common digital computer tools for the analysis of this class of transients.Electromechanical transientsElectric drive systems consist of an electric motor, a transfer mechanism, an electrical energy converter and a control system .The control system consists of a microcontroller with data connection interfaces, data channels (data network), sensors and actuators (motors).

To couple electrical motor with mechanical load, the mechanical drives are used. The basic types of mechanical drives are: a) Geared transmission, which provides specific fixed type ratios. b) Belt drives, providing flexibility in the positioning of the motor. c) Chain drives, providing infinitely variable speeds. d) Traction drives, which provides adjustable speed with relatively high speed. Thus, electromechanical system consists of two parts: electrical and mechanical. Converter of electric energy and control system compose an electrical part, moving masses form a mechanical part (see Fig1.5).

Figure 1.5 Structure of electromechanical system

Electromechanical transients are slower transients that are caused by a mismatch between power production and consumption, and therefore involve the oscillation of machine rotors because of an unbalance between turbine and generator torques.

Table 1.2. Various dynamic phenomena