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.

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.

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.

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.

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%.

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.

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

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