Generator Protection: Which Function Covers Which Fault

Short answer

Synchronous generator protections fall into three groups: those that cover internal faults (87G, stator and rotor ground), those that cover abnormal conditions that are not short circuits (loss of excitation, negative sequence, reverse power, overexcitation, frequency, out-of-step) and those that back up the system (21 or 51V). The size and connection of the machine decide which are installed.

Three groups of protections

A synchronous generator can suffer internal electrical faults, but also abnormal conditions that are not faults and that damage it if they persist. It must also back up system faults that are not cleared in time. The functions are ordered accordingly:

Internal faults of the stator and the rotor.
Abnormal operating conditions.
Backup for system faults.

The guide on the generator protection scheme explains how they combine and what each one trips. This guide goes the other way: it starts from the fault or condition and leads to the function.

Internal faults

Fault	Function	What it does
Phase-to-phase short circuit in the stator	87G differential	Compares the currents at the neutral and at the terminals; see 87G
Ground fault in most of the stator	Neutral overvoltage 59N	Measures the voltage that appears at the high-impedance grounded neutral; see 59N
Ground fault near the stator neutral	100 % stator ground	Completes the 59N with third harmonic or low-frequency injection; see 64S
Ground fault in the rotor	64R or 64F	A first fault does not damage the machine; a second produces magnetic imbalance and vibration; see 64F

Abnormal conditions

Condition	Function	What it prevents
Loss of field current	Loss of excitation 40	The generator absorbing reactive power from the grid and behaving like an induction machine; see 40
Unbalanced currents	Negative sequence 46	Rotor heating from double-frequency currents; see 46
Generator starting to run as a motor	Reverse power 32	Damage to the prime mover; see 32
Excessive core flux	Overexcitation 24 (V/Hz)	Core saturation and heating of structural parts; see 24
Swing that loses synchronism with the grid	Out-of-step 78	Pole slipping with high currents and pulsating torques; see 78
Frequency or voltage out of range	81, 27 and 59	Operation outside the limits of the machine; see 81
Stopped machine that receives voltage	Inadvertent energization 50/27	Damage to the rotor, stator and bearings; see 50/27

None of these conditions is a short circuit. That is why conventional overcurrent does not detect them and each has its own function.

System backup

If a grid fault is not cleared by its own protection, the generator keeps feeding current into it. Two functions back it up:

Distance 21, which looks toward the grid; see distance 21.
Voltage-restrained overcurrent 51V, which becomes more sensitive when the voltage falls because of the fault; see 51V.

Breaker failure 50BF of the unit breaker closes the scheme; see 50BF.

What each function trips

Not all functions trip the same things. A serious internal fault must open the unit breaker and the field breaker and shut the turbine. A system fault only separates the unit and leaves the machine ready to resynchronize. That assignment is the trip matrix, and it is where the most errors are found in the field. The four modes are described in the generator protection scheme.

The analysis of a generator trip is in Fault analysis in generators and the case of a generator set, in Generator set protection. The meaning of each number is in ANSI numbers of protection functions; you can also look it up in the free ANSI table tool. To go further: Basler BE1-11 and BE1-951 relays.

What the grid requires of a connected generator

When a synchronous generator connects to a utility grid, the utility sets minimum protection functions, and the design and settings are adapted to the local code and the utility’s requirements. A typical list includes synchronism check (25), phase and ground overcurrent, voltage-controlled overcurrent (51V) and negative sequence (46). It also includes overvoltage (59), frequency (81) and anti-islanding protection. On small machines, loss of field (40) and out-of-step (78) may be treated as optional. A protection setting and coordination study is normally also required. The guide on generator in parallel with the grid details this. This is complemented by Woodward HighPROTEC MCDG relay.

What data is needed to set them

The settings come from the data of the generator manufacturer, the turbine or engine and the excitation system: rating, capability curve and thermal limits. Typical values are not good enough. Two coordinations deserve attention from the design stage:

Loss of excitation and overexcitation with the voltage regulator limiters. The regulator must act before the protection.
Frequency and voltage with load shedding and with the system operator’s requirements. IEEE C37.106 covers abnormal frequency protection of generating plants.

How many functions each generator carries

Generator	Usual functions
Low-voltage or emergency set	Those of the set controller: overcurrent, voltage, frequency and reverse power
Medium-voltage generator in industry or cogeneration	Complete generator relay; if it runs in parallel with the grid, also interconnection functions
Power plant unit at high or extra-high voltage	Two generator relays, overall unit differential and step-up transformer protection

For small sets see the emergency diesel plant and the DEIF and SELCO generator relays. For an industrial generator that operates with the grid, see generator in parallel with the grid. This is complemented by generator reverse power (32) trip.

How they are tested

Each function is tested separately with secondary injection and then together, with the complete trip matrix. A function that trips the wrong breaker is a common error at commissioning. It is also checked that the 86 lockout relays hold the command and that the breaker failure 50BF trip acts if the unit breaker does not open. This is complemented by substation trip matrix.

How we help

At Tiza Group we configure generator relays with our remote relay configuration: trip matrix, loss of excitation, reverse power, negative sequence and backup functions. We verify them with secondary injection and quote the trip analysis if your generator has already gone out of service without a clear cause. Write to us.

References

IEEE C37.102: guide for AC generator protection.
IEEE C37.2: device function numbers.
IEEE C37.106: guide for abnormal frequency protection for power generating plants.

Frequently asked questions

Are all protection functions mandatory on a generator?

No. The scheme scales with size, importance and whether the generator runs in parallel with the grid. A small island-mode set may need few functions; a large one in parallel needs a complete scheme. The protection study defines which ones and with what settings.

Why does a generator need protection for conditions that are not short circuits?

Because some conditions damage it if they last, even when the current is not high: loss of excitation, unbalanced currents, motoring and overexcitation. None of them is a short circuit, and overcurrent does not detect them.

Does the generator set controller protection replace the generator relay?

On small sets the controller functions are usually enough. On a medium-voltage generator, or one that runs in parallel with the grid, the study may call for a generator relay with additional functions. Generator set controllers have their own guide.