Review and evaluation of L index for voltage stability in power systems

Dissertation for receiving the master's degree " M.sc "

With the new structure change that has emerged in power systems in recent years and has caused production units to pass more and more electric power through transmission lines, it is expected that we will see a wider voltage collapse and more power systems. For example, passing too much power of a transmission line will cause excessive voltage drop and reduce the capacity of electric power transmission to a certain part of the power system. In recent years, there have been major disturbances in the power systems of various countries around the world, which have involved these countries economically, politically, and judicially in many cases that are still ongoing in some places.

Since the industrial and economic efficiency of the world depends on the safe supply of electricity to industries, any disruption in this matter causes heavy economic losses for those countries. Therefore, the macro management and planning of the electricity industry is moving towards the reduction of these phenomena.. Voltage instability is one of the most important issues and the study of it has been felt for years. Many engineers and researchers have taken steps in this direction, and today, with the change in the structure of power systems, this phenomenon will be more noticeable.

In the late last century as well as the beginning of the new century, voltage instability has caused several huge collapses in various networks, examples of which are given below:

l The disturbance of the Florida system in December 1982

l Swedish system disturbance in 1983

l French system disturbance in 1987

l Japanese system disturbance in 1987

As a result, today the subject of some articles and researches in scientific societies and conferences is devoted to the issue of voltage stability.

Although it is possible that the low voltages caused by the mismatch of the rotor angles between two groups of machines are close to 180° or more than that, the gradual departure of the machines from the coordination leads to very low voltages in the middle points of the network. But in such cases, low voltage is the result due to rotors being out of coordination. The system enters instability when a disturbance, an increase in the required load, or a change in the system's position causes an increasing and uncontrollable decrease in voltage [2]. 1-1 Voltage collapse The main factor of instability of the power system is the inability to face the demand for reactive power. In practice, power system operators need a quick and accurate indicator for voltage stability to help them monitor and make the right decisions to prevent voltage collapse and loss of all or part of the power system. Problems related to voltage control and stability have existed in the power industry for a long time, mainly related to weak systems and long lines. In recent years, more attention has been paid to the issue of voltage stability of developed power systems under heavy load conditions. On the one hand, due to the increasing growth of electric energy consumption and on the other hand, due to economic and environmental considerations, macro-management policies in the world's electricity industry are aimed at operating power systems closer to their permitted limits.

Under such conditions, voltage instability in the system seems likely. This voltage instability will cause the unstable operation of the power system. In recent years, voltage instability has caused several huge collapses in the power grids of different countries that have been mentioned.

Voltage instability in power systems may be caused by a disturbance, lack of system reactive power support, or both.

Voltage instability usually occurs in systems under heavy load. There may be other reasons for voltage collapse, but the main problem is the inherent weakness of the power system. The factors inherent in the power system. The main factors of wattage collapse are: limitations of reactive power control or generator voltage, load characteristics, characteristics of reactive compensation devices, and the operation of voltage control devices such as transformers with tap changers under load.

Whenever we encounter an increase in reactive power demand in a power system, if this additional demand is met with the help of reactive power reserves of generators and compensators, the system will be established at an auxiliary voltage level. But if, due to the combination of a number of events at the same time and the state of the system, the demand for reactive power is not met by storage devices and compensators, eventually the voltage drop in the system will turn into a voltage collapse, which will cause a major part of the system or all of it to fail [3].

     Several voltage collapse events have occurred worldwide, based on these events, the voltage collapse can be specified as follows.

1 – The triggering event may occur for various reasons: small gradual system changes such as a natural increase in system load or large sudden disturbances such as the loss of a generating unit or a line under heavy load. Sometimes a seemingly insignificant initial disturbance may lead to a series of events. which will eventually cause the collapse of the system.