Power System Stabilizers: Comparison
Please note this is a comparison between Version 1 by Adrian Nocoń and Version 2 by Camila Xu.

A power system (PS) is a complex system consisting of devices for the generation, transmission, distribution, processing, and consumption of electric energy. The expansion of a PS causes a number of new stability problems that scientists try to solve in various ways, including the use of appropriate devices and control systems, the most popular of which are power system stabilizers (PSSs). It should be emphasized that the use of classic power system stabilizers in the modern digital voltage regulators of synchronous generators does not require large financial outlays (a PSS is implemented as an additional fragment of the voltage regulator program code) compared to other solutions improving PS stability (e.g., FACTS systems).

• power systems
• power system stabilizers
• optimization

## 1. Introduction

A power system (PS) is a complex system consisting of devices for the generation, transmission, distribution, processing, and consumption of electric energy. The purpose of PS operation is to ensure an uninterrupted supply of electricity of appropriate voltage and frequency while minimizing the costs of its generation and transmission.
A power system is subject to constant changes resulting from many reasons. There is a permanent transient state in PSs. The classification of PS transients can be performed based on various criteria. One of them is the criterion resulting from the type of changing physical quantities. Electromagnetic, electromechanical, and thermal transient states are usually distinguished (Figure 1). This division is also related to the rates of change (i.e., the time over which these changes take place) of given quantities.
Figure 1.
Classification of transient states.
Electromagnetic transient states are associated with fast changes in magnetic and electrical quantities, including, among others, magnetic fluxes, currents, and voltages. They occur in the shortest time, usually not longer than a few seconds, after the phenomenon that cause them (e.g., short circuits).
Electromechanical transient states refer to situations in which electrical quantities (currents and voltages) and mechanical quantities (mechanical powers and rotational speeds of electrical machines) change simultaneously. The concept of electromechanical swings is inextricably linked with electromechanical transient states in systems with synchronous machines. This concept is defined as oscillating changes in the position of a rotor with respect to the stator. In electromechanical transient states, changes also take place after stabilization of the electromagnetic quantities. The duration of electromechanical transients is from several to several dozen seconds.
Thermal transient states associated with changes in the temperature of the elements included in a PS are the last group of transients. Thermal phenomena are usually the slowest. Their duration ranges from tens of seconds to minutes or hours. Visible changes in values in thermal transient states usually begin after the disappearance of electromechanical changes.
It should be emphasized that all the above-mentioned transient states are interrelated—one occurring transient state entails the occurrence of another state being slower.
Regardless of the type of transient, a state threatening the security of energy supply, i.e., a failure, may arise in a PS. Therefore, properly designed systems are used in PSs to eliminate possible failures or to minimize their effects. Among the devices eliminating threats in electromagnetic transient states, one should mention electric power protection automatics. Devices operating during electromechanical transient states include devices that stabilize PS operation. The elimination of unfavorable temperature changes in PSs takes place thanks to appropriate heating or cooling devices and thermal protections.
This research concerns devices whose operation is aimed at stabilizing PS operation in electromechanical transient states. It should be emphasized that the literature on the analysis of electromechanical transient states and the related stability of PSs [1][2][3][4][1,2,3,4] is exceptionally wide. Research is conducted all the time. Recently, the interest in this topic has been increasing as new elements, including renewable sources and energy storage that influence these transients, have been installed in PSs. The expansion of a PS causes a number of new stability problems that scientists try to solve in various ways, including the use of appropriate devices and control systems, the most popular of which are power system stabilizers (PSSs).

## 2. Classic Solutions for PSSs

Ensuring PS stability is one of the basic technical problems of power engineering. It should take place at the design and construction (expansion) stage of a system. Control systems such as power system stabilizers are only additional elements, i.e., means of improving stability and mitigating transient states. Synchronous generators, as the primary power sources in PSs, are equipped with damping circuits generating relatively high electromagnetic damping torques. However, the operation of excitation systems, especially fast static ones, can reduce the values of these torques, adversely affecting the waveforms of electromechanical transient states. This unfavorable influence of voltage regulation systems can be reduced, among others, by the use of additional, regulating elements called PSSs— in this section, understood as classic systems [2]. It should be emphasized that the use of classic power system stabilizers in the modern digital voltage regulators of synchronous generators does not require large financial outlays (a PSS is implemented as an additional fragment of the voltage regulator program code) compared to other solutions improving PS stability (e.g., FACTS systems). It is worth noting that the improvement in the damping of electromechanical transient states of PSs thanks to the use of PSSs installed in excitation systems deteriorates the quality of voltage regulation [2][4][5][6][7][2,4,5,6,7]. Nevertheless, the effects of classic PSSs can be comparable to the effects of other systems, provided that they are properly selected (in terms of structure and parameters). Therefore, the use and scientific research of classic PSSs are still justified. Currently, various types of stabilizers are used in practice, from the simplest single-input to complex, broadband, multi-input ones. Single-input stabilizers (e.g., PSS1A-type) are simple in construction and tuning but have their drawbacks (e.g., a PSS with one input from rotational speed when the speed is measured only in one place of the generator shaft can amplify the torsional vibrations of the generating unit). These disadvantages can be eliminated by using multi-input PSSs. Then, various causes of electromechanical swings can be eliminated. However, tuning PSSs then becomes more difficult.

## 3. Artificial-Intelligence-Based PSSs

Techniques and methods based on artificial intelligence have been present in scientific research for many years. They are also constantly being developed, both on the theoretical and application level. In case of complex, nonlinear objects that behave unpredictably, artificial intelligence methods are willingly used due to their high potential. A power system is such an object. Therefore, it is not surprising that there are many scientific papers describing the use of artificial intelligence methods in PS control.  In the analyzed papers related to electromechanical transients, three techniques of artificial intelligence are discussed, i.e., artificial neural networks, fuzzy logic, and their combination, i.e., neural-fuzzy systems. Recently, however, there has been no greater interest in expert systems used as an element of systems stabilizing PS operation.

## 4. Modern Control Systems in PSs

The constantly developing theory of automatic control causes new control systems to be used to control the processes taking place in complex systems, including PSs. Attempts to adapt new theories have been reflected in numerous scientific publications. A similar effect is achieved by the constant development of power electronic converter systems, thanks to which it becomes possible to control voltage, power flow, etc. in PSs. Some researchers of publications have tried to use new control systems as elements to improve waveforms in the electromechanical transients in PSs. It should be emphasized that some of these systems have been a permanent element of PSs for some time (e.g., reactive power compensation systems). Nevertheless, the development works on them are still carried out on a large scale, and new possibilities of these systems are still considered.

## 5. PSSs in Networks with Renewable Sources

Recently, there has been a significant increase in the use of renewable electricity sources. Among them, the most popular in the context of research on power system stability are wind and photovoltaic sources, especially because wind and photovoltaic sources do not work continuously. They only produce electricity when wind and solar radiation energy is available. Consequently, PSs have to continuously compensate for the changes in active power to keep frequency constant. Additional problems may be caused by the presence of converter systems installed in renewable sources. Power electronic systems allow for the quick control of power (active and reactive) and voltage, which may adversely affect the voltage and frequency values in a PS. Moreover, the presence of wind and photovoltaic sources in PSs reduces the resultant PS inertia, which results in increased susceptibility of PSs to electromechanical swings.