Premium Membership ♕

CYBER MONDAY OFFER 💥 Save 20% on PRO Membership Plan and Video Courses with the coupon CYM23 and learn from experienced engineers.

Home / Technical Articles / Backup fault protection for generators in case of a failure at the generation station

The purpose of generator backup protection

It is a common practice to use the differential relay as primary fault protection for the generator. Backup fault protection is also highly recommended to protect the generator from the effects of faults that are not cleared because of failures within the normal protection scheme. The backup relaying is automatically applied to provide protection in the event of a failure at the generation station, on the transmission system, or both.

Backup fault protection for generators in case of a failure at the generation station
Backup fault protection for generators in case of a failure at the generation station

Specific generating station failures would include the failure of the generator or Generator Step Up (GSU) transformer differential scheme. On the transmission system, failures would include the line protection relay scheme or the failure of a line breaker to interrupt.

Table of contents:

  1. Implementation of backup fault protection
  2. Standard overcurrent relays
  3. Voltage-dependent relays
  4. Voltage supervised overcurrent relays
    1. Voltage-controlled and voltage-restrained relays
    2. Application options and fault sensitivity
  5. Other distance relay applications

1. Implementation of backup fault protection

Figure 1 shows the sample system generator. Backup protection is provided by distance relays (Device 21) or voltage supervised overcurrent relays (Device 51V). These relays can be connected to CTs at the neutral end of the generator or they can be connected to CTs at the generator terminals.

The neutral end configuration is preferred because this connection will allow the relaying to provide protection when the unit is off line. Terminal connected relays will not see internal generator faults for this condition, because there is no relay current.

If the scheme is intended to provide backup protection for both generating station and system faults, the backup relays should initiate a unit shutdown. This entails tripping the breaker on the high-voltage side of the GSU, the generator field breaker, the auxiliary transformer breakers and initiating a prime mover shutdown.

If the station configuration included a generator breaker it would be tripped instead of the high-voltage breaker.

When relays are applied solely to backup transmission line relaying, only the GSU transformer or generator breaker need be tripped. This would allow a faster resynchronizing after the failure has been isolated. This assumes the unit can withstand the effects of the full load rejection that will occur when the outlet breaker opens.

If the unit cannot withstand this transient, a unit shutdown must be initiated.

Generator online protection scheme
Figure 1 – Generator online protection scheme

Go back to Contents Table ↑

2. Standard Overcurrent Relays

Standard overcurrent relays are not recommended for backup protection of a generator. The backup relay must be capable of detecting the minimum generator fault current. This minimum current is the sustained current following a three-phase fault assuming no initial load on the generator and assuming the manual voltage regulator in service.

If the automatic voltage regulator where service, it would respond to the fault-induced low terminal voltage and boost the field current, thus increasing the fault current. The assumption of no initial load on the generator defines the minimum field current to drive the fault.

Typically, a generator’s synchronous reactance, which controls the value of the sustained fault current, is greater than unity. If the generator is unloaded and at rated terminal voltage (Et = 1.0) prior to the fault, the sustained short-circuit current will be 1/Xd which will be less than full load current. In the case of the sample system generator Xd = 1.48 and the resulting sustained three-phase fault current would be 0.67 pu or 67% of full load current.

A standard overcurrent relay must be set above load and could not detect the minimum sustained fault current. Tripping would be dependent on rapid relay operation before the fault current decays below the relay’s pickup setting.

Figure 2 plots the decaying current for the minimum fault condition on the sample system generator vs. an overcurrent relay set to carry full load. The figure shows that the relay must be set with a very short time delay (Time Dial = 1/4) to intersect the current plot to assure tripping.

This fast tripping is undesirable, because it would preclude coordination with system relays and could cause misoperation during system disturbances that do not require protective action.

Fault clearing with overcurrent relay
Figure 2 – Fault clearing with overcurrent relay

Go back to Contents Table ↑

3. Voltage-Dependent relays

The problems associated with standard overcurrent protection can be overcome if fault detection is based on current and voltage. At full load, the generator terminal voltage will be near rated voltage. Under sustained three-phase fault conditions, the internal generator impedance will increase to the synchronous value and the terminal voltage will decrease sharply.

Both distance relays and voltage supervised overcurrent relays use the voltage degradation to differentiate between load current and a sustained fault current condition. Because of this design, these backup relays are supervised by a potential failure detection element, device 60. This element blocks tripping in the event of an open phase or blown fuse in the potential circuit.

Without this blocking feature, these instrument circuit malfunctions would trip the fully loaded unit.

The decision to use a 21 or a 51 V function as backup protection is normally dependent on the type of phase protection applied on the transmission or distribution system to which the generator is connected.

Distance backup protection is chosen if phase distance relaying is applied on the transmission system. A 51 V function is chosen if overcurrent relays are used for phase protection on the connected system. These choices are made to facilitate relay coordination.

Go back to Contents Table ↑

4. Voltage Supervised Overcurrent Relays

4.1 Voltage-controlled And Voltage-restrained Relays

There are two kinds of voltage-supervised overcurrent relays used in generator backup applications. The voltage-restrained overcurrent relay is normally set 125–175% of full load current. The relay uses voltage input from the generator terminals to bias the overcurrent setpoint.

At rated voltage, a current equal to the setpoint is required to actuate the relay. As input voltage decreases, presumably
due to a short circuit, the overcurrent setpoint also decreases. Typically a current equal to 25% of the setpoint is require to operate the relay at zero volts input.

Figure 3 is a typical pickup characteristic for a voltage-restrained relay.

The voltage-controlled relay is set below full load with sufficient margin to detect the minimum fault current. The relay includes an undervoltage element that senses generator terminal voltage. If the voltage is above the undervoltage element setting, the overcurrent unit is not functional.

When voltage is depressed by a fault, the undervoltage element drops out, allowing the relay to operate as a standard overcurrent relay in accordance with its pickup and time delay settings.

Voltage-restrained overcurrent relay characteristic
Figure 3 – Voltage-restrained overcurrent relay characteristic

The voltage-restrained relay is more difficult to apply because operating time is a function of both current and voltage.

The voltage-restrained relay has two adjustable setpoints, a voltage-dependent minimum pickup current, and a time delay setting. The voltage-controlled relay has a voltage-independent current pickup setting, a time delay setting, and an undervoltage drop out setting.

Go back to Contents Table ↑

4.2 Application Options and Fault Sensitivity

Voltage-supervised overcurrent relays allow many input options. The 51 V function comprises three single-phase units. The current and voltage connections are not standardized. Phase-to-neutral or phase-to-phase voltages can be applied in conjunction with line or delta currents.

There is also the option of voltage-controlled or voltage-restrained relays.

Go back to Contents Table ↑

5. Distance Relays

The term distance relays refers to a general class of relays that measure circuit impedance. The relay distinguishes between fault current and load current in a manner similar to the 51 V functions. The  voltage applied to the distance relay tends to restrain operation, while current promotes operation.

Both phase and ground distance relays are applied on the transmission system. Unique relay designs are required for phase and ground fault protection.

There are many different algorithms  used in these relays, but in all cases the common goal is to measure the positive sequence impedance from the relay to the fault. When full fault protection is provided by distance relaying, six elements are required, phase elements A–B, B–C, C–A and ground elements A–G, B–G, and C–G.

Phase distance relays are applied at generators for system backup protection. Ground distance relays are not applied. Most generators are grounded through impedance to limit the ground fault current. Specialized ground fault protection schemes are required.

When a generator is solidly grounded and connected to a distribution system directly or through a wye-wye transformer, overcurrent ground relays provide superior fault sensitivity and economy when compared to ground distance relays. Overcurrent ground relaying is applicable because generator ground faults do not decay to values less than full load current and ground overcurrent relays are not subject to setting limitations due to load current.

Likewise, when a generator is connected to a system through a delta-wye grounded transformer, backup ground protection is usually provided by a time overcurrent ground relay connected in the transformer neutral.

For example, SEL-700G protection relay offers three choices for system backup protection. You can select one or more of the available elements:

  • Distance (DC),
  • Voltage Restraint (V), or
  • Voltage Controlled (C) Overcurrent elements.

Modern protective relays provide four zones of phase step distance protection. Functions are positive sequence voltage polarized mho characteristics. The reach of the three forward looking zones can be compensated for a delta-wye transformer.

Zone 4 is reversed and disregards any transformer between the relay and the fault in the forward direction. Zones 1, 2, 3, and 4 each include independent timers for phase step distance protection.

Out-of-step blocking monitors swing condition and blocks tripping. Out-of-step tripping logic is provided with a choice of two or three mho type characteristics with adjustable shapes.

Forward and reverse share a common maximum reach angle. Loss of synchronism or a power swing between two areas of the power system is detected by measuring the positive sequence impedance seen by the relay over a period of time as the power swing develops.

Generator protection relay SEL-700 functionalscheme
Figure 4 – Generator protection relay SEL-700 functionalscheme

Go back to Contents Table ↑

5.1 Other Distance Relay Applications

Other applications of the 21 function are also possible. Phase distance relaying can be connected to CTs at the generator terminals with the 21 function connected to look into the generator instead of the system. This relay can be applied without a time delay to provide fast backup clearing for generator faults when connected to the system.

Many generator protection microprocessor packages include two phase distance relay functions. One zone can be implemented with a short reach and a short time delay sufficient to coordinate with high-speed bus and line relaying plus breaker failure time if applicable. The second zone is then set to see into the transmission system with a delay sufficient to coordinate with zone 2 line relaying and applicable breaker failure time.

This scheme can provides 0.3 sec clearing for high current faults in the vicinity of the generator as opposed to the single zone scheme that would require a delay of about a second to coordinate with zone 2 and breaker failure relaying.

Go back to Contents Table ↑


  1. Protective relaying for power generation systems by Donald Reimert
  2. SEL-700G Generator Protection Relay by SEL
  3. LPS-O System backup for generators and transmission lines by GE

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Giorgi
    Jan 28, 2021


    Nov 21, 2020

    Can it be restrained or delayed Instantaneous relay protection between feeder to LV and GC,Like Grading Of Instantaneous Over Current Protections.?

  3. vcks
    Sep 23, 2020

    can you elaborate the generator online protection scheme single diagram in detail? and why standard overcurrent relays are discussed here?

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

8  +  two  =  

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge