The protection of shunt capacitor banks against internal faults involves several protective devices/ elements in a coordinated scheme. Typically, the protective elements found in a SCB for internal faults are: individual fuses, unbalance protection to provide alarm/ trip and overcurrent elements for bank fault protection.
Removal of a failed capacitor element or unit by its fuse results in an increase in voltage across the remaining elements/ units causing an unbalance within the bank. A continuous overvoltage (above 1.1pu) on any unit shall be prevented by means of protective relays that trip the bank. Unbalance protection normally senses changes associated with the failure of a capacitor element or unit and removes the bank from service when the resulting overvoltage becomes excessive on the remaining healthy capacitor units.
Unbalance protection normally provides the primary protection for arcing faults within a capacitor bank and other abnormalities that may damage capacitor elements/ units. Arcing faults may cause substantial damage in a small fraction of a second. The unbalance protection should have minimum intentional delay in order to minimize the amount of damage to the bank in the event of external arcing.
In most capacitor banks an external arc within the capacitor bank does not result in enough change in the phase current to operate the primary fault protection (usually an overcurrent relay) The sensitivity requirements for adequate capacitor bank protection for this condition may be very demanding, particularly for SBC with many series groups. The need for sensitive resulted in the development of unbalance protection where certain voltages or currents parameters of the capacitor bank are monitored and compared to the bank balance conditions.
Capacitor unbalance protection is provided in many different ways, depending on the capacitor bank arrangement and grounding. A variety of unbalance protection schemes are used for internally fused, externally fused, fuseless, or unfused shunt capacitor.
Capacitor Element Failure Mode
For an efficient unbalance protection it is important to understand the failure mode of the capacitor element. In externally fused, fuseless or unfused capacitor banks, the failed element within the can is short-circuited by the weld that naturally occurs at the point of failure (the element fails short-circuited).
This short circuit puts out of service the whole group of elements, increasing the voltage on the remaining groups. Several capacitor elements breakdowns may occur before the external fuse (if exists) removes the entire unit. The external fuse will operate when a capacitor unit becomes essentially short circuited, isolating the faulted unit.
Internally fused capacitors have individual fused capacitor elements that are disconnected when an element breakdown occurs (the element fails opened). The risk of successive faults is minimized because the fuse will isolate the faulty element within a few cycles. The degree of unbalance introduced by an element failure is less than that which occurs with externally fused units (since the amount of capacitance removed by blown fuse is less) and hence a more sensitive unbalance protection scheme is required when internally fused units are used.
Schemes with Ambiguous Indication
A combination of capacitor elements/ units failures may provide ambiguous indications on the conditions of the bank. For instance, during steady state operation, negligible current flows through the current transformer between the neutrals of an ungrounded wye-wye capacitor bank for a balanced bank, and this condition is correct.
However, the same negligible current may flow through this current transformer if an equal number of units or elements are removed from the same phase on both sides of the bank (Fig-A). This condition is undesirable, and the indication is obviously ambiguous.
Where ambiguous indication is a possibility, it is desirable to have a sensitive alarm (preferably one fuse operation for fused banks or one faulted element for fuseless or unfused banks) to minimize the probability of continuing operation with canceling failures that result in continuing, undetected overvoltages on the remaining units.
It may also be desirable to set the trip level based on an estimated number of canceling failures in order to reduce the risk of subjecting capacitor units to damaging voltages and requiring fuses to operate above their voltage capability when canceling failures occur.
Undetectable Faults
For certain capacitor bank configurations some faults within the bank will not cause an unbalance signal and will go undetected. For example: a) rack-to-rack faults for banks with two series groups connected phase-over-phase and using neutral voltage or current for unbalance protection; and, b) rack-to-rack faults for certain H-bridge connections.
Inherent Unbalance and System Unbalance
In practice, the unbalance seen by the unbalance relay is the result of the loss of individual capacitor units or elements and the inherent system and bank unbalances. The primary unbalance, which exists on all capacitor bank installations (with or without fuses), is due to system voltage unbalance and capacitor manufacturing tolerance.
Secondary unbalance errors are introduced by sensing device tolerances and variation and by relative changes in capacitance due to difference in capacitor unit temperatures in the bank. The inherent unbalance error may be in the direction to prevent unbalance relay operation, or to cause a false operation. The amount of inherent unbalance for various configurations may be estimated using the equations provided in reference (1).
If the inherent unbalance error approaches 50% of the alarm setting, compensation should be provided in order to correctly alarm for the failure of one unit or element as specified. In some cases, a different bank connection can improve the sensitivity without adding compensation. For example, a wye bank can be split into a wye-wye bank, thereby doubling the sensitivity of the protection and eliminating the system voltage unbalance effect.
A neutral unbalance protection method with compensation for inherent unbalance is normally required for very large banks. The neutral unbalance signal produced by the loss of one or two individual capacitor units is small compared to the inherent unbalance and the latter can no longer be considered negligible. Unbalance compensation should be used if the inherent unbalance exceeds one half of the desired setting.
Harmonic voltages and currents can influence the operation of the unbalance relay unless power frequency band-pass or other appropriate filtering is provided.
Unbalance Trip Relay Considerations
The time delay of the unbalance relay trip should be minimized to reduce damage from an arcing fault within the bank structure and prevent exposure of the remaining capacitor units to overvoltage conditions beyond their permissible limits.
The unbalance trip relay should have enough time delay to avoid false operations due to inrush, system ground faults, switching of nearby equipment, and non-simultaneous pole operation of the energizing switch. For most applications, 0.1s should be adequate. For unbalance relaying systems that would operate on a system voltage unbalance, a delay slightly longer than the upstream protection fault clearing time is required to avoid tripping due to a system fault. Longer delays increase the probability of catastrophic bank failures.
With grounded capacitor banks, the failure of one pole of the SCB switching device or a single phasing from a blown bank fuse will allow zero sequence currents to flow in system ground relays. Capacitor bank relaying, including the operating time of the switching device, should be coordinated with the operation of the system ground relays to avoid tripping system load. The unbalance trip relay scheme should have a lockout feature to prevent inadvertent closing of the capacitor bank switching device if an unbalance trip has occurred.
Unbalance Alarm Relay Considerations
To allow for the effects of inherent unbalance within the bank, the unbalance relay alarm should be set to operate at about one-half the level of the unbalance signal determined by the calculated alarm condition based on an idealized bank. The alarm should have sufficient time delay to override external disturbances.