In short. Unbalanced three-phase current creates a negative-sequence component that produces double-frequency currents in a generator’s rotor, heating it. Negative-sequence protection trips when this exceeds the generator’s withstand. For motors, unbalance protection prevents overheating.
Why it matters
Balanced current creates a field rotating with the rotor. An unbalanced load adds a counter-rotating field that induces currents at twice the system frequency in the rotor surface and damper windings, heating them. The tolerance is expressed as a continuous negative-sequence current limit and a short-time limit, usually as a constant K in the form I2² × t = K.
I2² × t = K
Causes of unbalance
- Single-phase loads unevenly spread.
- Open phase (blown fuse, broken conductor, failed contact).
- Unbalanced faults, such as phase-to-phase or phase-to-earth, until cleared.
- Poor connections.
Protection design
| Stage | Purpose |
|---|---|
| Alarm (continuous level) | Warn of sustained unbalance below the continuous limit |
| Inverse-time trip | Follows I2² × t = K for larger, shorter unbalances |
| Definite-time trip | Backup for severe unbalance |
Setting steps
- Get the generator’s continuous I2 limit and K constant from the manufacturer.
- Set the alarm just below the continuous limit.
- Set the inverse-time characteristic with the K value, and a minimum and maximum time.
- Coordinate with line protection so that the system clears external unbalanced faults first.
Motors and feeders
Motors lose efficiency and overheat with voltage unbalance, because small voltage unbalance causes larger current unbalance. Phase-failure and unbalance relays protect them. Reference engineering notes on unbalance are in motor and feeder protection.
Illustrative example
A generator feeds a site with many single-phase loads on one phase. Measurement shows a continuous negative-sequence current close to the alarm level. The alarm sounds, the loads are rebalanced and the level falls. Without the alarm, the rotor would have run hot for months.
Why negative sequence matters
Unbalanced load or an open phase creates a negative-sequence current component that produces a magnetic field rotating opposite to the rotor. This induces double-frequency currents in the rotor surface and wedges and causes rapid heating. Machines have a limit expressed through the thermal capability constant, often written as I2² t = K.
I2² × t = K, so allowable time t = K ÷ (I2 per unit)²
For example, if K = 20 and the negative-sequence current is 0.2 per unit, the allowed time is 20 ÷ 0.04 = 500 seconds. At 0.4 per unit, it is 125 seconds. The constant is a machine property; take it from the generator maker.
Typical causes
- Loss of a phase on a feeder.
- Heavily unbalanced single-phase loads.
- Unsymmetrical faults not yet cleared.
- Open conductors or failed breaker poles.
Relay functions
| Function | Use |
|---|---|
| Negative-sequence overcurrent with inverse-time curve | Protect machine thermal limit |
| Voltage unbalance or negative-sequence voltage | Protect motors and alarm on supply issues |
| Phase failure / sequence relay | Protect motor feeders |
Setting approach
- Obtain the machine’s continuous negative-sequence capability and K.
- Set an alarm at a lower level and trip on the inverse-time curve.
- Coordinate with downstream protections so a feeder fault clears first.
Practical point for generator sets
The most common cause on sites is an unbalanced building load. Balance circuits. See independent-phase regulation for the voltage side of the problem.
Common mistakes
- No K value used, so generic settings.
- Ignoring single-phase load distribution.
- Not alarming sustained unbalance.
What to send us
Send the load distribution and generator data. Send your application details. See also generator relay architecture.
Questions engineers ask
What is negative-sequence current?
A component of unbalanced current that rotates against the rotor and heats it.
How much unbalance is acceptable?
The generator manufacturer gives a continuous limit, often a small percentage of rated current.
Can single-phase loads be run on a generator?
Yes, within limits. Spread them across phases.
Will this trip on external faults?
It can, which is why it is coordinated with system protection.
This guide gives general principles. Limits come from the manufacturer.