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Home / Technical Articles / Voltage Imbalance Protection of Synchronous Generator (ANSI Code 60)

Synchronous Generator Protection

In an apparatus protection perspective, generators constitute a special class of power network equipment because faults are very rare, but can be highly destructive and therefore very costly when they occur. If for most utilities, generation integrity must be preserved by avoiding erroneous tripping, removing a generator in case of a serious fault is also a primary if not an absolute requirement.

Voltage Imbalance Protection of Synchronous Generator (ANSI Code 60)
Voltage Imbalance Protection of Synchronous Generator (ANSI Code 60)

Furthermore, protection has to be provided for out-of-range operation normally not found in other types of equipment such as overvoltage, overexcitation, limited frequency or speed range, etc.

Typical generator-transformer protection scheme
Figure 0 – Typical generator-transformer protection scheme

Table below provides a list of protective relays and their functions most commonly found in generator protection schemes. These relays are implemented as shown on the single-line diagram of Figure 0 above. .

As shown in the Relay Type column, most protective relays found in generator protection schemes are not specific to this type of equipment but are more generic types.

 ANSI Function DescriptionRelay Type 
87GGenerator phase phase windings protectionDifferential protection
87TStep-up transformer differential protectionDifferential protection
87UCombined differential transformer and generator protectionDifferential protection
40Protection against the loss of field voltage or current supplyOffset mho relay
46Protection against current imbalance. Measurement of phase negative sequence currentTime-overcurrent relay
32Anti-motoring protectionReverse-power relay
24Overexcitation protectionVolt/Hertz relay
59Phase overvoltage protectionOvervoltage relay
60Detection of blown voltage transformer fusesVoltage balance relay
81Under- and overfrequency protectionFrequency relays
51VBackup protection against system faultsVoltage controlled or voltage-restrained time overcurrent relay
21Backup protection against system faultsDistance relay
78Protection against loss of synchronizationCombination of offset mho and blinders

Now, let’s have an overview of specific protection ANSI 60 – Detection of blown voltage transformer fuses.

Loss of a voltage phase signal

The loss of a voltage phase signal can be due to a number of causes. The primary cause for this nuisance is a blown-out fuse in the voltage transformer circuit. Other causes can be a wiring error, a voltage transformer failure, a contact opening, a misoperation during maintenance, etc.

Since the purpose of these VTs is to provide voltage signals to the protective relays and the voltage regulator, the immediate effect of a loss of VT signal will be the possible misoperation of some protective relays and the cause for generator overexcitation by the voltage regulator.

Among the protective relays to be impacted by the loss of VT signal are:

  • Function 21 – Distance relay. Backup for system and generator zone phase faults.
  • Function 32 – Reverse power relay. Anti-motoring function, sequential tripping and inadvertent energization functions.
  • Function 40 – Loss-of-field protection.
  • Function 51V –  Voltage-restrained time overcurrent relay.

Normally these functions should be blocked if a condition of fuse failure is detected.

It is common practice for large generators to use two sets of voltage transformers for protection, voltage regulation, and measurement.

Therefore, the most common practice for loss of VT signals detection is to use a voltage balance relay as shown in Figure 1 on each pair of secondary phase voltage. When a fuse blows, the voltage relationship becomes imbalanced and the relay operates. Typically, the voltage imbalance will be set at around 15%.

Example of voltage balance relay
Figure 1 – Example of voltage balance relay

The advent of digital relays has allowed the use of sophisticated algorithms based on symmetrical components to detect the loss of VT signal.

When a situation of loss of one or more of the VT signals occurs, the following conditions develop:

  • There will be a drop in the positive sequence voltage accompanied by an increase in the negative sequence voltage magnitude. The magnitude of this drop will depend upon the number of phases impacted by a fuse failure.

  • In case of a loss of VT signal and contrary to a fault condition, there should not be any change in the current’s magnitudes and phases. Therefore, the negative and zero sequence currents should remain below a small tolerance value. A fault condition can be distinguished from a loss of VT signal by monitoring the changes in the positive and negative current levels. In case of a loss of VT signals, these changes should remain below a small tolerance level.

All the above conditions can be incorporated into a complex logic scheme to determine if indeed a there has been a condition of loss of VT signal or a fault.

Symmetrical component implementation of fuse failure detection
Figure 2 – Symmetrical component implementation of fuse failure detection

Figure 2 represents the logic implementation of a voltage transformer single and double fuse failure based on symmetrical components.

If the following conditions are met in the same time (and condition) during a time delay longer than T1:

  • The positive sequence voltage is below a voltage set-value SET_1,
  • The negative sequence voltage is above a voltage set-value SET_2,
  • There exists a small value of current such that the positive sequence current I1 is above a small set-value SET_4 and the negative and zero sequence currents I2 and I2 do not exceed a small set- value SET_3,

then a fuse failure condition will pick up to one and remain in that state thanks to the latch effect. Fuse failure of a specific phase can be detected by monitoring the level voltage of each phase and comparing it to a set-value SET_5. As soon as the positive sequence voltage returns to a value greater than the set- value SET_1 and the negative sequence voltage disappears, the fuse failure condition returns to a zero state.

Reference: The electric power engineering handbook – L.L. Grigsby (Purchase hardcover book from Amazon)

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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. Cody
    Mar 27, 2018

    Have you every ran into a GE 489 Generator protection throwing a VT fuse failure alarm when the generator is not running. after arriving onsite and clearing the alarm the alarm does not return, Did it 3 times last year and not returned a year later doing the same thing.

  2. Minju
    Mar 28, 2016

    Thanks for the tech info. What about 110v dc failure protection? I understand there s no relay like that but incase the dc system which powers the relays n spring charging motors fails, what happens to the synch generators? Gen will be running blind, since the relays r dead. What do u suggest?

    • Gautham
      Dec 26, 2016

      I cannot say with certainty, but for any reason where the end result is that of a breaker not opening (like the relay dc source failure), then there will be redundant schemes that use a different dc source and set of relays or remote breaker failure schemes, which is made remote because it is powered by a different source in an upstream location since it is far away. I guess however in this case you can’t get any more upstream than the generator so redundant schemes might be applied. Additionally, there will be monitoring and communication schemes that always keep a check on the health of these critical systems, though I can’t get to the technical details yet without reading again. But you could google redundant breaker schemes and the like to get more info.

  3. hamza
    Feb 06, 2015

    Its top application

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