A study on load shedding including islanded power system

Considering daily increase and changes in the electrical industry, major developments can be observed that are called as revamping the structure of electricity industry. In general, power system is exposed to fault and because of fault, black out may occur in whole area or country, entailing irreparable and dangerous impacts. To prevent this, this thesis proposes a plan ensuring improvement and continuity in giving electricity to urgent customers when the power system equipment exits the network. In power systems, frequency and voltage are two important factors for the stability; the frequency control is related to active power and voltage control which depend on the reactive power of the network. Any factor or fault driving out the frequency and voltage from their allowable range can cause instability in the power system. We know that under normal conditions, generated active power is equal to loads᾽ active power (Pg=PL), it means that system is operating in a stable state. In this thesis, two possible modes that can occur for frequency in the power system are considered. Mode 1) If generated active power is less than loads᾽ active power (PgPL), then considering the swing equation, the frequency starts to increase and if this increase is more than the allowable frequency, power system will go into unstable state. To prevent this, when governors cannot supply the frequency to allowable range, over frequency relays by proposed algorithm are embedded to drive out the generators with priority from the power system till frequency reaches to its allowable limit. It should be noted that, when generators are driven out from the power system, the generators᾽ economic aspects like reconnection costs must be taken into account. Also, in this thesis the islanding mode is considered. In this mode, the power system is divided into two or more parts, meaning that we might have frequency increase in a part or may have frequency decrease in other part of the power system. Accordingly to control frequency in this mode, a combination of the above states could be used. With performance of frequency control relays, voltage magnitude of bus bars may be improved somehow; but bus bars voltage control in power systems is tasked with the AVRs. Since they operate slowly, SVC systems are used to control voltage .With the concepts that, if voltage is higher than the allowable range in bus bars, the inductive mode of SVC systems is used and if voltage in bus bars is lower than the allowable range, the capacitive mode of SVC systems is used. In the present thesis, a frequency control algorithm in presence of SVC systems has been used to control the frequency of the system while maintaining the bus bars voltage in standard ranges to ensure incurring the least damage possible to power system equipment and customers. The main purpose is to continuous supply of power for urgent customers when a fault in power system equipment occurs. The software used in the proposed design and method is DIgSILENT Power Factory 15.1. This method is able to improve the reliability of power system in cases where the equipment exits the network. As mentioned above and considering the importance of voltage and frequency control in the power system, the contents of this thesis is organized as follows: In the first chapter, load shedding is introduced and the importance of using it in the network to improve the control of the power system is explained. The second chapter contains literature review. This chapter addresses articles pertaining to design and improve the frequency and voltage control algorithm in the power system. The third chapter describes the methodology used in this thesis, referring to the relationships and algorithms used in the UFLS, as well as a brief explanation of the SVC systems to control and improve the voltage. The specifications and network parameters used to perform the simulation are presented in Chapter 4. The results of frequency and voltage control in the under-frequency / under voltage mode are discussed in the fifth chapter. In this chapter, the frequency and voltage statuses of the system are described before and after the load shedding and SVC systems. The sixth chapter refers to the results of frequency and voltage control in over-frequency /over voltage mode. In this chapter, the frequency and voltage of the system are discussed before and after the application of over-frequency relays and SVC systems. The seventh chapter addresses the islanding mode. In this chapter, a brief explanation of the islanding mode is presented, and the results of frequency and voltage control are discussed before and after applying frequency control relays and SVC systems.