Protection Systems

Protection Systems

DOI: 10.4018/978-1-4666-9429-3.ch008
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In the previous chapter, we assumed that there is no fault in the converter. To achieve a converter without failure, we presented the methods for stress reduction. However, a fault may occur in a system even operating with low stress. In the current chapter, we take one further step and assume that a fault occurs in the converter but there is a short time interval between fault occurrence and catastrophic damage to the converter. Therefore, the topic of this chapter is the methods for saving the converter in this condition. In this chapter, protection methods for saving the system against damaging faults are presented. Based on background of chapter two, protection systems should be able to bypass the effect of failure factors on electric power converter. Methods for current limiting and voltage clamping as the usual factors of failure in converters are described. Circuit diagram of a snubber is presented and its operation is described based on safe operating area of solid state power switches. Operating diagrams of fuses as emergency circuit breakers are presented. Measurement methods and devices used in protection systems are explained. Experimental samples and standard diagrams are presented to clarify the theoretical notes in all cases.
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Introduction: Protection For Rapid Isolation

In previous chapter, we tried to postpone the failure time with reducing the stress on converter. However, the converter must operate and it is logically affected by stress. It is true that we reduce the stress but the stress still exists and cause to failure in long term. Now, we are faced with a fault in the converter. What should we do? This chapter deals with one of the methods for preventing catastrophic damage in faulty converters: protection systems. Protection systems acts when a fault occurs in converter. Their performance is very important; isolation of the converter is not always the best choice because this strategy has a bad effect on availability of the converter. We talk about this manner in the next chapter. In the current chapter, we are not sensitive to this concern. However, we consider this concern from another view; any failure factor needs to a time interval for damaging the converter. If the protection system is very fast, it protects the converter but conflict with noise is possible. Therefore, protection systems should be fast enough to protect the converter but not so fast to incorrect operation. Figure 1 shows the state of this chapter in the flowchart of the book. In an electric power system, a fault is any abnormal electric current. For example, a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a “ground fault” or “earth fault”, charge flows into the earth. The prospective short circuit current of a fault can be calculated for power systems. In power systems, protective devices detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.

Figure 1.

State of chapter 8 in the flowchart of the book


In a polyphase system, a fault may affect all phases equally which is a “symmetrical fault”. If only some phases are affected, the resulting “asymmetrical fault” becomes more complicated to analyze due to the simplifying assumption of equal current magnitude in all phases being no longer applicable. The analysis of this type of fault is often simplified by using methods such as symmetrical components.

Design of systems to detect and interrupt power system faults is the main objective of power system protection.

Transient Fault

A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored. Many faults in overhead power lines are transient in nature. When a fault occurs, equipment used for power system protection operate to isolate the area of the fault. A transient fault will then clear and the power-line can be returned to service. Typical examples of transient faults include:

  • Momentary tree contact

  • Bird or other animal contact

  • Lightning strike

  • Conductor clashing

Transmission and distribution systems use an automatic re-close function which is commonly used on overhead lines to attempt to restore power in the event of a transient fault. This functionality is not as common on underground systems as faults there are typically of a persistent nature. Transient faults may still cause damage both at the site of the original fault or elsewhere in the network as fault current is generated.

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