Shunt capacitor Banks
Shunt capacitor Banks or SCBs are typically used to deliver capacitive reactive power compensation or power factor correction. The use of shunt capacitor units has been quite advantageous because it being quite affordable, simple to install and commission, and can be placed anywhere in the electrical distribution system.
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Its usage has additional advantages for the electrical distribution system such as:
Enhancement of the voltage at the load side,
Increased voltage regulation,
Decrease of power losses and
Decrease or postponement of investments in electrical transmission networks.
The primary weakness of the shunt capacitor bank is that its reactive power generation is related to the square of the voltage, and accordingly, when the voltage is low, the electrical system needs it the most, it then delivers the least amount of the reactive power.
The capacitor unit
The capacitor unit is the essential element of a shunt capacitor bank. The capacitor unit consists of individual capacitor segments, connected in parallel/series arrangements, within a metal case. The internal discharge element is a resistor that decreases the unit residual voltage to 50V or less in 5 min. Capacitor units come in a range of voltage ratings (240 V to 24,940 V) and ratings (2.5 kvar to about 1,000 kvar).
Protection of shunt capacitor bank calls for knowledge of the advantages and restrictions of the capacitor unit and related electrical devices that include: individual capacitor elements, bank switching equipment, fuses, voltage, and current sensing elements. Capacitors are meant to be run at or below their rated voltage and frequency since they are highly sensitive to these parameters. The reactive power produced by a capacitor element is related to both voltage and frequency as Kvar = V2/Z ; Z = XC and XC = 1/2𝜋fC
Standard sizes of the capacitor elements made for shunt interconnection to AC electrical systems are given in IEEE Std 18-1992 and Std 1036-1992.
As per the IEEE standards,
Shunt capacitor units need to be designed for continuous service up to 110% of rated terminal RMS voltage and a crest voltage not exceeding 1.2×√2 of rated RMS voltage, considering harmonics but omitting transients.
The shunt capacitor units should also be able to withstand 135% of nominal current.
Shunt capacitor units should not provide less than 100% or more than 115% of rated reactive power at rated sinusoidal voltage and frequency.
Shunt capacitor units are not supposed to be suited for continuous service at up to 135% of the rated reactive power made by the mixed impacts of:
Voltage in excess of the nameplate rating at the fundamental frequency, but not over 110% of the rated RMS voltage, Harmonic voltages laid over on the fundamental frequency, and
Reactive power fabrication margin of up to 115% of the rated reactive power.
Shunt capacitor bank arrangements
The function of the fuse for protection of the shunt capacitor elements and their location (inside the capacitor unit or outside the unit) is a significant topic in the design of shunt capacitor banks. They impact the failure modality of the capacitor element and also impact the setting of the capacitor bank protection. Depending on the usage, any of the described arrangements are appropriate for shunt capacitor banks:
External fuse
A fuse, externally installed between the capacitor element and the capacitor bank bus bar, generally protects each shunt capacitor element. The shunt capacitor elements can be made for a comparatively high voltage since the external fuse can clear a high-voltage fault. The application of capacitor elements with the greatest possible voltage rating will lead to less number of shunt capacitive units of the series groups.
The fault of the capacitor element welds the foils together and causes short circuit currents to flow between capacitor elements arranged in parallel in the same group. The remaining capacitor elements in the bank thus stay in operation with an increased voltage across them than before the fault. If a second element breaks down the procedure repeats, causing an even greater voltage for the remaining elements. Sequential faults within the same bank will make the fuse trip, unplugging the capacitor element.
Externally fused shunt capacitor units are assembled using one or more series groups of parallel-connected capacitor elements per phase. The unbalanced signaling level reduces as the number of series groups of capacitors is raised or as the number of capacitor elements in parallel per series group is increased. However, the kilovar rating of the individual capacitor unit may be smaller because a minimum number of parallel units is required to allow the bank to remain in service with one unit out.
Shunt capacitor bank with internal fuses
Each capacitor element has a fuse inside the capacitor element. The fuse is a basic part of the wire sufficient to limit the current and is capsulized in a wrapper that can resist the heat generated by the arc. Upon a capacitor element fault, the fuse takes out the struck element only. The remaining elements, linked in parallel in the same arrangement, stay in operation but with a somewhat increased voltage across them.
A common capacitor bank that uses capacitor elements with an internal fuse is shown below. Generally, shunt capacitors using capacitor elements with internal fuses are assembled with less capacitor elements in parallel and more series groups of elements than that utilized in banks employing elements with internal fuses. The capacitor elements are typically large because the whole unit is not anticipated to break down.
Fuseless shunt capacitor banks
Fuse-less shunt capacitor banks are normally used for applications where the failure (short-circuiting) of one capacitor element will not cause excessive voltage on the remaining elements in that string. This is usually at or above 34.5 kV. The capacitor units are normally designed with two bushings with the elements insulated from the case. The capacitor units are connected in series strings between phase and neutral (or between line terminals for delta-connected or single-phase installations). The protection is based on the capacitor element’s failure in a shorted mode. The discharge energy is small because no capacitor units are connected directly in parallel.
Unfused Shunt Capacitor Units
Opposite to the fuseless arrangement, where the units are linked in series, the unfused shunt capacitor bank applies a series/parallel arrangement of the capacitor units. The unfused arrangement would typically be utilized on units below 34.5 kV, where a series chain of capacitor units is not practical, or on higher voltage units with small parallel energy. This arrangement does not need as many capacitor units connected in parallel as a bank with external fuses.
The design aspect and the connection to the utility bus will be discussed in the following article.
