A practical approach for integrated power system vulnerability analysis with protection failures

Protection system failure is one of the main causes of cascading outages. This paper proposes an integrated scheme to study power system vulnerability considering protection system failures. In this scheme, both adequacy and security based reliability analysis are conducted. A new protection system reliability model including two major failure modes is established to demonstrate their effects on power system reliability. The mechanism and scheme of protection systems have been analyzed for their contribution to cascading outages as well as system stability after a fault occurs. All contingencies and the responses in the power system are depicted in their inherent stochastic manner. The power system vulnerability is assessed by both adequacy indices, such as Bus Isolation Probability (BIP), Loss of Load Probability (LOLP) and Expected Power Loss (EPL), and the security index Probability of Stability (POS). In addition, a new vulnerability index, Integrated System Vulnerability (ISV), is introduced to give a more comprehensive description of the system vulnerability. A nonsequential Monte Carlo simulation approach is used to implement the stochastic properties of contingencies, protective response and protection system failures. The IEEE Reliability Test System is used to illustrate the methodology and present the results.

I. Introduction

Protection system plays an important role in power system operation in terms of safety and security. During the development of modern power systems, protection system dependability (the ability to function correctly when required) has taken priority over consideration of global system security (the ability to refrain from unnecessary operations). While reinforcing the protection systems to guarantee system dependability, the probability of their incorrect operation may increase as a result of higher complexity. It has been observed that protection system hidden failures commonly lead to multiple or cascading outages, which consequently can cause large-scale power system blackouts. A study by the North American Electric Reliability Council (NERC) shows that protective relays are involved in about 75 percent of major disturbances [1]. There have been several large-scale cascading failures in recent times affecting large populations of customers in the Western United States. All of these blackouts are related to protection system hidden failures, which remain dormant when everything is normal and manifest as a result of other system disturbances [2]. There are two major failure modes in protection system: “failure to operate” and “undesired tripping” [5]. The former means that when a fault occurs in a power system, the protection system fails to clear the fault. The latter refers to either spontaneous operation in the absence of a fault or trip for faults outside the protection zone. Large-scale power system blackout is a rare event. However, when it occurs, the impact on the system is catastrophic [3]. Therefore, study of the origin and propagation of cascading outages, their impact and preventive actions are becoming more and more imperative. Fig. 1. State transition diagram of a component and its protection system.