Table 1 of IEEE C37.99-2000 arranges capacitor bank protections in two groups: bank protection (failed element, fault to case, arcing, overvoltage from lost units, rack-to-rack fault) and system protection (lightning, overvoltage, harmonics, bus fault, inrush current, loss of voltage, line trip and breaker failure). Each row links a threat with its device and its notes.
Short answer
Table 1 of IEEE C37.99-2000 is the summary of the whole guide: each row links a threat with the device that covers it and a note. It is divided into bank protection, for faults inside the bank, and system protection, for what the network imposes on the bank and what the bank causes to the network (cl. 7). The main threat inside the bank is covered by unbalance. The other rows back it up.
Bank protection
| Threat | Device | Notes from the guide |
|---|---|---|
| Failed element | External or internal fuse in fused banks; the weld that forms at the fault in banks without fuses | The fuse must be fast to coordinate with fast unbalance, and not operate on switching or external faults |
| Element-to-case fault, terminal fault or defective connection in the unit | Fuse in externally fused banks; unbalance in internally fused or fuseless banks | In banks without external fuses, unbalance must be fast to prevent case rupture, without operating on switching or external faults |
| Arcing inside the bank, outside a unit | Unbalance, with a bandpass filter relay for the fundamental | Fast, to limit damage to other units (cl. 7.1.4) |
| Continuous overvoltage on elements or units from faults or operated fuses | Unbalance, with bandpass filter | Trip when healthy units exceed 110% of their rated voltage, or what the manufacturer recommends. Alarm at 5% unbalance or one unit out |
| Rack-to-rack flashover in wye banks with two series groups and overlapping phases | Phase or negative-sequence overcurrent; unbalance current in double wye | Fast operation (cl. 7.1.4 and 7.1.5) |
Two points of clause 7.1 clarify the previous rows:
- In an externally fused bank, several element breakdowns can occur before the fuse removes the unit. Unbalance takes the bank out when the overvoltage on the healthy units is excessive.
- In an internally fused bank, the link isolates the element in a few cycles, which reduces the risk of successive failures. In fuseless or unfused banks, the failed element is left shorted by the weld.
Each threat has its own guide: externally fused bank, internally fused bank, fuseless bank, unfused bank, capacitor fuses: expulsion or current-limiting, external arcing and rack faults and ambiguous indications and faults that unbalance cannot see.
System protection
| Threat | Device | Notes from the guide |
|---|---|---|
| Surge overvoltage | Surge arresters | Selection may require considering the bank energy, especially in large banks |
| Power-frequency overvoltage | Phase voltage relays | The capacitor dielectric is sensitive to peak voltage if the waveform is distorted |
| Overload from harmonic current | Relay sensitive to harmonic current | Where excessive harmonics are expected |
| Fault on the installation bus or major bank fault | a) Circuit breaker or circuit switcher with conventional relays, or b) power fuses | As fast as possible, without false trips from the discharge current into nearby faults |
| Fault in or near the substation, outside the installation | Inrush and outrush current-limiting reactors | They protect breakers, current transformer circuits and other components from excessive induced currents and voltages |
| Excessive inrush current | a) Preinsertion resistor or reactor in the breaker, b) reactors between banks, c) synchronous (zero-voltage) closing | Energizing near another energized bank can damage breakers, blow fuses, and cause high voltages in current transformers and relays |
| Loss of system voltage | Undervoltage relays | A bank energized through an unloaded transformer may require disconnection before the system is re-energized |
| Trip of the line feeding the bank | a) Transfer trip, or b) undervoltage relays | A bank connected directly to an unloaded line may require disconnection before the line is reconnected |
| Breaker failure | Conventional breaker failure relays | The local or remote breakers must have capacitive switching capability if they trip the bank with no load in parallel |
The bank voltage protection (59, 59B and 27B), overcurrent and breaker failure in a bank, the capacitor unit limits: 110% voltage and 135% power and bank switching expand on that part; the harmonic filter protection and capacitors in SVC and TSC cover filter banks and SVCs; large banks are in extra-high-voltage bank protection.
How to read the table
Unbalance carries the bank’s workload. The three rows on arcing, overvoltage on units and defective connection refer to it. But it does not generally detect a defective connection, which can deteriorate until it produces a failure. See capacitor bank unbalance protection.
Overcurrent is not enough for an arc. The guide warns that overcurrent protection is not normally sensitive to an arc inside the bank and that, if it detects it, it is usually too slow (cl. 7.2.1). Instantaneous relays are not effective against rack faults because of the high setting they require.
Two trips that do not lock out. System overvoltage and loss of voltage take the bank out, but as they are not a fault inside the bank, they do not leave it locked out (cl. 7.2.2 and 7.2.4).
The inrush and breaker failure rows depend on the switching equipment: special-purpose switch for banks and motors and automatic control of capacitor banks. For the undervoltage relay, see undervoltage protection (27); for voltage, overvoltage protection (59); for breaker failure, 50BF protection.
Points of attention from the guide
- Unbalance scheme with false sensitivity to the system. Some schemes respond to system voltage unbalance. If they are not compensated, they require a long delay to coordinate with line protection (cl. 7.1.5).
- Voltage transformer fuses. Many bank schemes depend on voltage. The guide says not to install primary fuses on the voltage transformers of these protections and to run the circuit directly to the panel, with no other loads (cl. 7.2.5).
- Breakers without capacitive capability in the breaker failure backup.
What we do
This table is the map: the settings depend on the bank type, the connection and your system. At Tiza Group we configure your protective relays remotely: unbalance, overvoltage, undervoltage, overcurrent and breaker failure of the bank, with its trip and lockout logic, and we quote the work according to the scope.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Bank voltage protection: 59, 59B and 27B
- Overcurrent and breaker failure in a bank
- External arcing and rack faults in a capacitor bank
- Capacitor bank protection
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: Table 1 (bank and system protection), cl. 7.1 (bank protection) and 7.2 (system protection).
Frequently asked questions
Why does the guide separate bank protection from system protection?
Because they cover different things. Bank protection handles faults inside the bank: elements, units, arcs. System protection protects the bank from what the network imposes (overvoltage, harmonics, energization) and protects the network from what the bank can cause. One does not replace the other.
What is the main protection of a capacitor bank?
Unbalance protection. It detects the change in capacitance from lost units or elements and arcing inside the bank. Overcurrent and voltage protections back it up.
Must an unbalance relay have a filter?
For the rows on arcing inside the bank and overvoltage on units, Table 1 asks for the unbalance relay to have a bandpass filter for the fundamental current or voltage, to provide security against harmonics and transients.