Generator differential protection (ANSI 87G, logical node PDIF in IEC 61850) compares, phase by phase, the current on the neutral side with the current at the stator terminals. A difference means a phase-to-phase fault inside the winding or in its connections, and the relay trips the breaker, de-excites the machine and shuts down the prime mover. It does not see turn-to-turn faults within one phase, nor, in general, ground faults on generators grounded through high impedance.
What 87G generator differential protection is
The stator winding of a synchronous generator has two accessible ends per phase: the neutral side and the terminal side. In normal operation, and for a fault outside the machine, the current entering one end leaves through the other. Generator differential protection measures both with current transformers (CTs) and compares them phase by phase. If a difference appears, current is flowing into a fault inside the winding or in the connections between the two CT sets.
It is identified as follows:
- ANSI/IEEE C37.2: device 87, with the suffix G by custom (87G).
- IEC 61850: logical node PDIF.
It is the main protection against phase faults in the stator, which can damage the magnetic core in a very short time. Such a fault is not cleared by opening the breaker: the machine keeps feeding it as long as it is excited. That is why the 87G trip opens the generator breaker, de-excites the field and stops the prime mover, normally through the lockout relay 86.
Where it is applied
- Generators in hydro, thermal and gas power plants, from medium to large ratings.
- Cogeneration and standby generators in industry, when their size and importance justify it.
- Generator-transformer units, where an overall differential covering both machines is also used.
On small generators without access to the neutral ends, other protections are used. The complete application is summarized in synchronous generator protection.
How it works
The numerical relay calculates per phase:
- Differential current: the phasor sum of the neutral-side and terminal-side currents, using the relay’s polarity convention.
- Restraint current: a measure of the through current; each manufacturer defines it in its own way.
It trips when the differential current exceeds a percentage characteristic with a minimum pickup and one or two slopes. Unlike the transformer differential 87T, there is no transformation in between: no vector group to compensate and no inrush current, so the 87G can be very sensitive and needs no harmonic blocking.
The challenge lies elsewhere. On an external fault close to the machine, the current has a DC component that decays slowly, because the generator X/R ratio is high. That component can saturate one CT before the other and create a false differential current. That is why relays include high slopes for large currents, saturation detection or directional logic that confirms the fault is internal.
What the 87G does not see:
- Turn-to-turn faults within the same phase: the current entering and leaving the phase is still the same. These are covered with split-phase differential (windings with parallel circuits) or other functions.
- Ground faults on generators grounded through high impedance: the current is only a few amperes. These are covered with neutral overvoltage 59N and third-harmonic or injection schemes.
Setting criteria
- Generator rated current and the ratio of the neutral-side and terminal-side CTs. Ideally both sets have the same ratio, accuracy class and burden, so they behave alike on external faults.
- Maximum external fault, with its DC component and the machine time constant, to check stability and CT performance (IEEE C37.110).
- Minimum internal fault to be detected, to set the pickup.
- Slopes according to the CT errors and the manufacturer’s recommendation.
- Trip logic coordinated with the other generator functions.
The actual values come from the short-circuit study and the manufacturer’s recommendation.
How each manufacturer implements it
- Siemens SIPROTEC (DIGSI) treats the generator as a protected object with its measuring sides.
- SEL (AcSELerator QuickSet) offers the differential element in its generator relays, with restraint options and programmable logic.
- Relion (Hitachi Energy 670/650; ABB 615/620) (PCM600) includes it as a block in the generator application configuration.
- GE Vernova (Multilin, EnerVista) defines current sources first and then the stator differential element.
- Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) sets it in the generator protection menus; some applications also use a high-impedance scheme.
They differ in the definition of the restraint quantity, the shape of the characteristic, the methods for CT saturation and whether the generator-transformer unit differential runs in the same device or a separate one.
Why it trips or blocks
- Trip on an external fault or at synchronizing: uneven CT saturation from the DC component, CTs of different class or burden, or too low a slope.
- Trip with load: reversed polarity in one CT set, wrong ratio entered or phases crossed between the neutral and terminal sides. With the machine on line, the measured differential current should be almost zero.
- No trip on a ground fault: this is the expected behavior with high-impedance grounding.
- Open CT circuit or test block not properly closed after maintenance.
- Under which condition did the 87G operate or fail to operate?
- On an external fault or at synchronizingUneven CT saturation from the DC component, CTs of different class or burden, or too low a slope
- With the machine on lineReversed polarity in one CT set, wrong ratio entered or phases crossed between the neutral and terminal sides
- No operation on a ground faultExpected with high-impedance grounding; that fault is covered by the 59N
Start by downloading the oscillography and comparing the neutral-side and terminal-side currents phase by phase, looking for one that distorts before the other.
How it is tested
With secondary injection, using a test set such as the Omicron CMC 356, the usual checks are the pickup, points on the percentage characteristic in all phases, stability with through current, operating times and the actual trip to the breaker, the excitation and the prime mover. During commissioning, the machine’s three-phase short-circuit test is also carried out when the plant procedure allows it: with primary current, polarity is confirmed and the differential current is checked to be practically zero.
Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms and Contact Designations.
- IEEE C37.102, Guide for AC Generator Protection.
- IEC 60255-187-1, Functional requirements for differential protection (motors, generators and transformers).
- IEC 61850-7-4, logical node models (PDIF).
- IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why did the 87G trip for a fault on the system and not in the generator?
The typical cause is uneven saturation of the neutral-side and terminal-side CTs driven by the DC component of the fault current, which decays slowly in generators. CTs of different characteristics, unbalanced secondary burdens or too low a restraint slope also play a part.
Does the 87G protect against stator ground faults?
Only if the ground fault current is large enough. On generators grounded through high impedance that current is very small and the 87G does not see it; 59N, 64G or third-harmonic and injection schemes are used for that.
What is the difference between the 87G and the generator-transformer unit differential?
The 87G covers the stator only. The unit differential covers the generator, the step-up transformer and sometimes the station service transformer, so it must compensate the vector group and block on inrush like an 87T.