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

Dissertation for receiving the master degree "M.sc"

Field: Electricity, Trend: Power Electronics

Abstract Voltage stability is related to the power system's ability to maintain acceptable voltages in all system buses under normal conditions and after disturbance. The system enters an unstable state 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. The main cause of voltage instability is the inability of the power system to meet the demand for reactive power. Voltage instability generally occurs in heavily loaded systems. The voltage stability status of the power system can be checked using voltage stability indicators. These indicators, which can be based on static analysis or dynamic models of the power system, have the ability to determine critical buses, evaluate the stability of any line connected between two buses, or evaluate the stability margin of the system. Voltage instability is basically a local phenomenon that first occurs in the weak voltage area and then spreads to the rest of the system. Therefore, if the occurrence of voltage instability can be predicted, it can be prevented from spreading with appropriate corrective measures. In this thesis, a method is presented that has the ability to predict the occurrence of voltage instability. The presented method uses three factors of voltage size, voltage change and voltage change rate to predict the occurrence of voltage instability. The simulations carried out in two IEEE 9-bus and 39-bus standard systems show the proper performance of this index in predicting the occurrence of voltage instability. One of the key features presented is its low computational load, which makes it suitable for online applications. To deal with voltage instability, there are different tools, some of which are: quick change of the reference voltage of generators through the generator voltage control loop, switch of capacitor banks, controlled zoning of the network with the aim of balancing the reactive power of production and consumption, introducing units with low start-up time into the circuit, controlling the tap of transformers either by blocking the tap or reducing the set point, redistribution Fast power transfer between generators and load shedding. In most references, load removal is introduced as a last resort but a very effective way to deal with instability. It should be noted that with the passage of time, the amount of load that must be removed in order for the system to return to normal conditions will increase. Therefore, determining the right time to remove the load is one of the important points in removing the voltage reduction load. Also, in this research, an algorithm has been presented that determines the appropriate time to remove the load by using the proposed index to predict the occurrence of voltage instability. Numerous simulations show the proper performance of this algorithm in determining the appropriate time to remove the load [1]. Introduction The increase in scattered productions and the increasing use of electronic power equipment as well as renewable energies in the coming years doubles the need for further study on voltage stability and their effect on power systems.  

With the new structure 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 and more voltage collapse in 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 way, and today with the change in the structure of power systems, this phenomenon will be more noticeable.

At the end of the last century as well as the beginning of the new century, voltage instability has caused several massive breakdowns in different networks, examples of which are below:

l Florida system disturbance in December 1982

l Sweden 1983 system disturbance

l France 1987 system disturbance

l Japan system disturbance 1987

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

Although it is possible that the low voltages due to the lack of coordination between the rotor angles between two groups of machines are close to or more than 180°, the machines slowly getting out of 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 is the power system's 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 also 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 long existed in the power industry, which are 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 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, a 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 face 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 additional 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].

1-2 General specification based on real events

There have been several voltage collapse events in the world.

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

2- The main problem is the inability of the system to meet its reactive demands. Usually, but not always, voltage collapse involves a situation with heavily loaded lines. When it is difficult to transfer reactive power from nearby areas, any change that requires an increase in reactive power can lead to voltage collapse. 3- Voltage collapse usually appears as a slow voltage damping, which is the result of a cumulative process of the operation and interference of many devices, controllers, and protection systems. The time range of the collapse in such cases may be about a few minutes.

Of course, the duration of the voltage collapse in some cases may be much shorter and about a few seconds.