The key protection of a capacitor bank is unbalance protection, which detects failed elements or units before the healthy ones are overstressed. If the bank trips on unbalance, first assume there is real internal damage. If there is none, the typical causes are poorly compensated natural unbalance, a problem with the neutral CT or VT, or unbalance in the system. Overcurrent or overvoltage trips are usually due to harmonic resonance, energization or high system voltage.
Symptom
A capacitor bank in a substation or industrial plant goes out of service on a protection trip. It may be on unbalance, overcurrent, overvoltage or undervoltage. Sometimes the bank returns to service without problem and trips again days later; other times the trip coincides with energization, with an hour of low demand or with the start of loads with drives.
A bank is made up of many units in series and parallel, and each unit of many elements. When an element fails, its internal fuse operates, a unit loses its external fuse or, in fuseless banks, the element is left shorted. In every case the capacitance of one branch changes, the voltage is redistributed and the healthy units are subjected to more voltage. Unbalance protection compares currents or voltages that, with a healthy bank, are nearly equal, and trips before the overvoltage on the healthy units exceeds the permissible limits, per IEEE C37.99 and IEC 60871-1. How that unbalance is measured and how alarm and trip are staged is explained in capacitor bank unbalance protection.
Possible causes, from most to least likely
| Cause | Why it happens | What gives it away |
|---|---|---|
| Failed elements or units | Aging, overvoltages, harmonics, temperature | Unbalance trip; earlier unbalance alarm that was growing |
| Poorly compensated natural unbalance | Manufacturing tolerances; compensation not updated after units were replaced | Unbalance present since energization with a healthy bank |
| Faulty neutral CT or VT | Wrong ratio, loose connection, saturation | Unbalance measurement inconsistent with the phase currents |
| System unbalance | In banks with a grounded neutral measured by current, system voltage unbalance and zero-sequence harmonics show up in the neutral | Unbalance that follows the system load, not the bank |
| Harmonic resonance | The bank capacitance and the system inductance resonate near a harmonic that is present | Bank current with high harmonic content, overload or peak overvoltage |
| Inrush current | Discharged bank or energization with another bank already connected (back-to-back) | Instantaneous overcurrent trip on closing |
| System overvoltage | Low demand, transformer taps, the bank itself raising the voltage | 59 trip during low-load hours |
| Undervoltage | Voltage sag or loss of supply; the 27 disconnects the bank on purpose | 27 trip coinciding with a system event |
| Restrike on opening | A breaker that restrikes on opening and generates overvoltages | Overvoltages in the oscillography of the opening |
What to check first
- Which function tripped: unbalance, overcurrent, overvoltage or undervoltage. Each points to a different group of causes.
- History of the unbalance measurement: whether it had been rising in steps (elements failing one by one) or appeared suddenly.
- Inspection of the bank de-energized and discharged: operated external fuses, bulged or leaking units, signs of heating.
- Harmonic content of the bank voltage and current in the oscillography or with an analyzer. See IEEE 519 harmonic study.
- Time of the trip: energization, low-load hours, start of other loads or system switching.
- Which function tripped?
- Unbalance: Had the unbalance been rising, or are fuses operated?
- Yes: Failed elements or units: do not re-energize without inspecting the bank
- No: Poorly compensated natural unbalance, neutral CT or VT, or system unbalance
- Overcurrent: Did it happen on energization?
- Yes: Inrush current or back-to-back switching
- No: Harmonic resonance: measure the harmonic content
- Overvoltage or undervoltage: System voltage out of range or system event
- Unbalance: Had the unbalance been rising, or are fuses operated?
How to confirm
- Capacitance measurement of each unit or branch, compared with the nameplate and the manufacturer’s tolerances. It locates units with failed elements.
- Natural unbalance measurement with a healthy bank and recalibration of the relay compensation, per the manufacturer.
- Harmonic measurement with the bank connected and disconnected, and calculation of the resonance frequency using the short-circuit power of the bus. See active versus passive harmonic filter.
- Tests on the neutral CTs and VTs and on the protection with secondary injection. See secondary injection.
- Energization analysis: peak and frequency of the inrush current against the instantaneous setting.
How to keep it from coming back
- Unbalance alarm ahead of the trip, to schedule unit replacement.
- Re-compensate the natural unbalance every time a unit is replaced.
- Harmonic study before installing or expanding the bank; if there is resonance risk, a detuned bank or a filter. See how to size a capacitor bank.
- Current-limiting reactors or synchronized switching where back-to-back energization is frequent.
- Bank control by voltage or by reactive power that takes it out at low demand.
Bank configurations and what each unbalance protection measures
How unbalance is measured depends on how the bank is connected, and it determines which fault is seen and which false alarm is likely:
| Connection | What is measured | Note |
|---|---|---|
| Ungrounded wye | Voltage between the bank neutral and ground | Neutral voltage grows when a phase loses capacitance; sensitive to residual system voltage |
| Grounded wye | Current through the ground connection | Neutral current includes system unbalance and zero-sequence harmonics |
| Double wye (split wye) | Current between the two neutrals | Compensates a good part of the system unbalance, because both halves share it |
| H-bridge per phase | Current in the bridge | Each phase has its own measurement and the affected phase can be identified |
IEEE C37.99 is the reference guide for choosing the connection and protection of shunt banks, and the selection depends on the voltage, the size of the bank and whether it has internal, external or no fuses. Each configuration brings its own alarm and trip thresholds, calculated from the number of units, the number of elements per unit and the permissible overvoltage on the remaining units.
Operating limits of the units
Capacitor standards set a limit for permanent overvoltage. IEEE 18 and, for medium and high voltage, IEC 60871-1 allow continuous voltage of up to 110 % of rated. They also set limits for peak voltage and for the resulting reactive power. Those limits are for contingencies and not a normal design criterion. The exact values are in the edition of the standard that applies to the contract. After the bank is opened, each unit must discharge through internal resistors to a low residual voltage, within a time defined by the standard. The bank must not be re-energized before that time, because the residual charge adds to the system voltage and produces a closing overvoltage. The controls of many banks include a discharge timer with that function; verify the manufacturer’s value, not a generic one.
Resonance: when the bank amplifies harmonics
A bank and the source inductance form a resonant circuit whose approximate harmonic order is the square root of the ratio between the bus short-circuit power and the bank rating. If that order falls near a harmonic produced by the loads, for example the 5th or 7th from drives and rectifiers, the bank current is amplified and can overload the units. That is why, when adding a bank or changing the system short-circuit power, the resonance order must be calculated again. Detuned banks with a series reactor move the resonance below the harmonics present; the detuning factor and the rated voltage of the units are chosen together, because the reactor raises the voltage across the capacitors.
What not to do
- Do not re-energize a bank that tripped on unbalance without inspecting it: the overvoltage on the healthy units speeds up new failures and can end in a major fault.
- Do not raise the unbalance setting to stop it from tripping.
- Do not touch a de-energized bank without discharging, shorting and grounding each unit.
- Do not add capacitance to a bus with harmonics without studying the resonance.
When to call us
If your capacitor bank trips and it is not clear why, send us the events and the oscillography: we quote the trip analysis, a remote service available worldwide. We test the bank and its protection on site with protection testing, in Colombia or elsewhere on request, run the power quality study to evaluate resonance, and our team sets the unbalance relay through remote relay configuration. Write to us.
Related guides
- Capacitor bank
- How to size a capacitor bank
- Power factor in industry
- IEEE 519 harmonic study
- Active versus passive harmonic filter
- Overvoltage protection 59
- Capacitor bank unbalance 60: protection
References
- IEEE C37.99, Guide for the Protection of Shunt Capacitor Banks.
- IEC 60871-1, shunt capacitors for a.c. power systems having a rated voltage above 1 000 V.
- IEEE 18, shunt power capacitors.
- IEEE 519, recommended practice for harmonic control in electric power systems.
Frequently asked questions
Why does a capacitor bank trip on unbalance with no visible fault?
Because unbalance protection detects internal element failures or internal fuse operations that cannot be seen from outside. It can also operate from poorly compensated natural unbalance, a faulty neutral CT or VT, or a setting that does not match the real bank configuration.
Why does the bank trip on overcurrent without a fault?
The most common causes are inrush current, especially when a bank is energized with another already connected to the same bus, and harmonic currents. If the bank capacitance and the system inductance form a resonance near a harmonic that is present, the bank current and voltage rise sharply.
Can a bank be re-energized right after the trip?
Not until the residual voltage has dropped to the value the standard and the manufacturer allow. The discharge devices set that time, and many controls block closing meanwhile. In addition, if the trip was on unbalance, the bank must be inspected before re-energizing.