Principles of Shunt Capacitor Bank Application and Protection
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Principles of Shunt Capacitor Bank Application and Protection Satish Samineni, Casper Labuschagne, and Jeff Pope Schweitzer Engineering Laboratories, Inc. Presented at the 64th Annual Georgia Tech Protective Relaying Conference Atlanta, Georgia May 5–7, 2010 Previously presented at the 63rd Annual Conference for Protective Relay Engineers, March 2010, and 9th Annual Clemson University Power Systems Conference, March 2010 Originally presented at the 36th Annual Western Protective Relay Conference, October 2009 1 Principles of Shunt Capacitor Bank Application and Protection Satish Samineni, Casper Labuschagne, and Jeff Pope, Schweitzer Engineering Laboratories, Inc. Abstract—Shunt capacitor banks (SCBs) are used in the electrical industry for power factor correction and voltage support. Over the years, the purpose of SCBs has not changed, but as new dielectric materials came to market, the fusing practices for these banks changed from externally fused to internally fused, fuseless, and finally to unfused [1]. This paper gives a brief overview of the four most common types of SCBs. What are the differences between them? Which is the best one to use? What type of protection is best suited for each bank configuration? The paper provides a quick and simple way to calculate the out-of-balance voltages (voltage protection) or current (current protection) resulting from failed capacitor units or elements. While the identification of faulty capacitor units is easy with an externally fused bank, it is more complex with the other types of fusing, making maintenance and fault investigation difficult. This paper presents a novel method to identify the faulted phase and section in capacitor banks. Fig. 1. Four most common capacitor bank configurations I. INTRODUCTION A. Grounded/Ungrounded Wye SCBs mean different things to different people. From the Most distribution and transmission-level capacitor banks system operator’s viewpoint, an SCB is a system tool that are wye connected, either grounded or ungrounded. provides voltage support, power factor correction, and/or Characteristics of a grounded bank are as follows: harmonic filtering. To use this tool, the protection and control • Provides a low impedance to ground for lightning scheme must provide the information and the means to control surge currents the SCBs. From a designer’s viewpoint, an SCB provides a • Provides a degree of protection from surge voltages challenge to find the optimum balance of system • Reduces recovery voltages for switching equipment requirements, cost, maintenance, and spares. From a (approximately twice normal peak voltage) protection engineer’s viewpoint, the protection must cover all • Provides a low impedance to ground for triplen and faults internal and external to the SCB, and it must be immune other harmonic currents to transients, fast, sensitive, and dependable. Characteristics of an ungrounded bank are as follows: This paper provides information for both the design • Does not provide a path for zero-sequence currents, engineer and the protection engineer by giving an overview of triplen, and other harmonic currents bank fusing and grounding, and the more common protection • Does not provide a path for capacitor discharge used for these applications. It also shows a simple way to currents during system faults calculate current and voltage out of balance for use during • Requires the neutral to be insulated to full line voltage commissioning or setting calculations. The final section of the paper shows a novel method that identifies the phase and III. GENERAL UNIT CAPABILITIES AND CONSTRUCTION section with the faulty unit/element in a shunt capacitor bank. IEEE Std C37.99-2000 [1] defines a number of operating II. SHUNT CAPACITOR BANKS criteria for capacitor units. From a fusing viewpoint, the following two requirements are important: Fusing and protection are the two aspects that determine • Abnormal operating conditions must be limited to 110 the optimum bank configuration for a given capacitor voltage percent of rated root-mean-square (RMS) terminal rating. voltage Fig. 1 shows the four most common wye-connected capacitor bank configurations [1]: • The capacitor should be able to carry 135 percent of nominal RMS current Capacitor banks are constructed by the series/parallel combination of capacitor units. Units are connected in parallel (parallel groups) to meet the VAR specification of the 2 capacitor bank. These parallel groups are then connected in series to meet the nameplate voltage rating of the capacitor units. Capacitor units are available over a wide voltage range (216 V to 24,940 V), and VAR ratings (2.5 kVAR to around 800 kVAR [1]). With this wide range of VAR and voltage ratings, the bank designer must find a good compromise between cost (number of units in the bank, the complexity of the bank construction, maintenance and spares) and the impact of an element/unit failure. For example, when a fuse blows in an externally fused bank, one whole unit is disconnected. If the bank used only a few units of large kVAR rating, a significant amount of kVAR could be lost. Depending on the function of the bank, a large kVAR (capacitance) loss impacts the amount of available voltage support, the degree of power factor correction, or the effectiveness of the filtering (filter banks). IV. TO FUSE OR NOT TO FUSE? Although many factors influence the design of a capacitor bank, developments in the dielectric play a major role in determining the character of element failures within a unit. Earlier capacitor units used kraft paper with a PCB impregnant as dielectric. Although the kraft paper was highly refined, there were still many non-uniformities in the paper [2]. To avoid weak spots in the dielectric, capacitor units had several layers of paper inserted between the foil layers. When dielectric material of this type failed, the foil layers did not weld together to form a solid connection. Instead, the cellulose continued to arc, resulting in charring of the paper that generated gas inside the sealed capacitor unit. In many cases, this gas buildup caused the unit to rupture, resulting in damage beyond the failure of a single element. Present-day dielectrics are manufactured with as few as two to three layers of impregnated polypropolene film (as Fig. 2. Three stages of a fuse blowing opposed to many layers of kraft paper). Because the film The labels in Fig. 2 are as follows. layers are thin, failures now cause the foils to weld together, XC = The reactance of each element/unit (10 Ω) thus forming a solid connection between the foils without XP = Reactance of a parallel group of elements/units arcing or charring. XT = Total reactance of the circuit To visualize the three stages of a fuse blowing, consider the arrangement in Fig. 2. This arrangement shows four series VF = Voltage across the faulted parallel group of groups of 10 capacitors in parallel, with an applied voltage of elements/units 12 V. A capacitor symbol represents either one row of an VH = Voltage across the healthy parallel group of internally fused unit or a complete unit in an externally fused elements/units bank. In Fig. 2(a), the system is healthy and the voltage across each of the four series groups is 3 V. Fig. 2(b) depicts the circuit just after a short circuit occurs, but before the fuse blows (fused application). In a fuseless bank, Fig. 2(b) shows the final state. In this state, the following circuit conditions prevail: • All the elements/units in parallel with the faulted element/unit are shorted out • The total reactance decreases • The total current increases • The voltage across the healthy series elements/units increases • The increased voltage is evenly distributed among the healthy series elements/units 3 Fig. 2(c) depicts the circuit after the fuse blew. At this unit, many elements can fail before unbalance tripping is time, the following circuit conditions prevail: necessary. • The reactance of the faulted parallel group increases (1) Impact on Bank Design • The voltage across all the elements/units in the faulted To construct a bank with the fewest number of units, select parallel group increases units with the highest available kVAR rating. The minimum • The total reactance increases number of units in parallel should be two units; the maximum • The total current decreases number of units in parallel depends on the value of discharge • The voltage across the healthy elements/units current from the parallel units. Select this value according to decreases the capacitor manufacturer’s recommendation. • The decreased voltage is evenly distributed among the c) Fuseless Banks Based on modern-day high-quality dielectrics, fuseless healthy elements/units units are similar in construction to externally fused units (few When a capacitor element fails, there is an increase in elements in parallel, but many elements in series). When an current through the fuse of the failed element. This current element fails (welds together), the entire row of elements consists of two components: shorts out (Fig. 2[b]). However, unlike the fused installations, • Increase in fundamental frequency current resulting there are now no fuses to blow, and the effect of a failed from the decrease in reactance element on the bank is permanent. Because there are no fuses • Increase in transient current, resulting from the in this bank, we can visualize the bank in terms of elements discharge from the healthy parallel elements rather than units. Both components must be considered when selecting a fuse (1) Impact on Bank Design size: the fuse on the healthy elements must not blow when Fuseless banks are constructed with at least 10 elements in discharging into an adjacent faulted capacitor, but must series to ensure that when an element fails, the remaining quickly and effectively remove a failed element/unit. series elements do not exceed the 110 percent overvoltage a) Externally Fused Units requirement. For example, if there are 10 capacitor units in Units in an externally fused bank have a few elements in series, and each unit has 10 elements (total 100 elements in parallel but many elements in series [2].