Capacitor Bank Case Study

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Chapter 5: Protection of Capacitor Bank and Power Transformer 5.1. Introduction The function of protection system is to cause the prompt removal from service of any element of a power system when it suffers a fault; short circuit or when it starts to operate in any abnormal condition that might cause damage or otherwise disturb the operation of the rest of the system. The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment [79]. This chapter presents all types of protection devices on capacitor bank to protect it from high inrush current and transient overvoltage during switching. Furthermore,…show more content…
Internal faults are caused by failures of capacitor elements composing the capacitor units, and units composing the capacitor bank. Also other faults are inside the bank such as a flashover within the rack. External power system conditions cause stress to SCBs such as bank overload due to the combined effect of sustained overvoltage and increased level of harmonics. On the other hand, the SCB may stress the power system if remained connected during abnormal system conditions possibly causing overvoltages that may damage both the SCB and other components of the system. The purpose of the protection is also to limit the effect of overload to a safe and acceptable level, and to prevent the abnormal system conditions from damaging the SCB by disconnecting it in case of loss-of supply condition. Additionally, reconnection of the SCB must be prevented if not fully discharged. Also short circuits and earth faults between the circuit breaker and SCB terminals must be cleared by the SCB protection scheme in due time in order to minimize damage and stress to the system. Protection on capacitor divided into: [80]…show more content…
Unbalance Protection Unbalance protection is provided against internal faults related to capacitor element/unit failures and against arcing faults within the bank. Ideally, the neutral unbalance voltage or current equals zero for a healthy SCB. However, manufacturing tolerances of individual capacitor units and the natural unbalance of the power system causes neutral unbalance to be measured even in a healthy SCB. Capacitor unit and element failures cause this neutral unbalance to change. The unbalance protection of SCBs is based on the magnitude or the change of the neutral unbalance current or voltage. [83] 5.2.2. Overload Protection The SCB may be subjected to overvoltage resulting from combined fundamental and harmonic content. This overvoltage increases the load current drawn by the SCB and stresses the layers of film that compose the individual capacitor elements. This kind of capacitor element is sensitive to a peak voltage across it, so the peak voltage measurement principle is preferred when implementing the protection. If the stress exceeds the SCB capability, the bank should be removed from service in due

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