Because of the varied methods of generation at high voltage, there are a greater number of potential schemes to be considered. These are:
Single HV generators and low voltage generators with transformers:
a) Operation in isolation from external supplies;
b) Operation as an alternative to an external supply;
and
c) Operation in parallel with an external supply.
Multiple HV generators and low voltage generators with transformers:
d) Operation in isolation from any external supply;
e) Operation as an alternative to an external supply;
and
f) Operation in parallel with an external supply.
g) Mixed generation schemes where low voltage generators with transformers and high voltage generators operate in parallel are a special case and are considered separately.
In addition, generators that are installed only to operate in parallel with an external supply, such as in combined heat and power systems are considered.
a) Single generators operating in isolation from external supplies.
This is similar to the low voltage case, where the star point of the generator is normally connected to earth, via a test link and earth electrode. All extraneous metalwork is connected to the earth terminal to avoid the possibility of high touch potentials existing under fault conditions.
For most high voltage generator systems, it is normal for the star-point earth connection to include a resistance or reactance for the reasons discussed above. The impedance will be provided with a high voltage connection terminal, which must be connected to the generator using a suitable rated cable for up to the phase-to-neutral voltage. The low voltage side of the earthing impedance will be connected to the earth terminal, via a bolted test link, using conventional earthing conductors.
For low voltage generators provided with a transformer, a delta connected transformer LV winding is normally used, the delta winding providing a path to attenuate third harmonic currents. The star point of the generator is solidly earthed. A star connected HV winding is usually provided and is earthed as for the high voltage generator case. If a delta connected HV winding is used, the electrical system may be referenced via separate earthing transformer.
b) Single generators operating as an alternative to an external supply
For high voltage generators and low voltage generators equipped with transformers, the application is similar to the isolated case a) above, since HV system neutrals are not usually distributed, as would be the case for LV systems. Care must be taken to interlock the switchgear to avoid the risk of accidental parallel operation of the two systems, since this would effectively apply an additional reference to the external system. For manual changeover schemes a system of key interlocks may be used, while automatic schemes will require the use of electrical interlocking.
c) Single generators operating in parallel with an external supply.
These systems require considerable care in design and neutral-earth switching will be required in most cases; in order to avoid placing an additional earth reference on the external system, which may compromise the protection scheme of this system. The neutral-earth switching is provided either for the generator star point earth, for HV generators; for the generator transformer star-point earth, for LV generators operating with transformers; or for the neutral earthing transformer star-point earth connection.
The neutral-earth switchgear must be rated to pass the current that will flow under all fault conditions and must remain closed while fault clearance is in progress. The control system logic must allow the switch to be closed at all times except when the generator is operating in parallel with the external supply system. In the event of loss of the external supply while running in parallel, the neutral-earth switch must be immediately closed in order to maintain generator system reference.
Apart from the introduction of the neutral-earth switch, which will be sited on the HV side of any impedance, generator system earthing will be as for the isolated case above.
d) Multiple generators operating in isolation from any external supply.
When designing a scheme involving multiple HV generators, it is important to ascertain whether the generators can be operated in parallel with each other with their star points coupled, as this can result in significant savings in switchgear costs.
If the generators can be operated with their star points coupled, then star point earthing may be applied similar to case a) above, with the multiple generators being treated as one. Impedance earthing of the star points is normal in this case.
D42
Generation
Earthing
Section D
Typical Earthing Arrangements
Generation
Earthing
Typical Earthing Arrangements
D44
Generation
Earthing
Section D
High voltage multiple generators operating in isolation from any external supply.
Particular care must be taken if applying unrestricted earth fault protection to generators operating in parallel.
Generation
Earthing
Consideration must be given to isolation of the generators for maintenance, since the HV side of the neutral earth impedance, to which all generator star points will be connected, is capable of rising to phase voltage, with respect to earth, under fault conditions. In this case, access to any generator’s terminals and high voltage conductors can only be granted if all generators connected to the impedance are shut down, locked off and their high voltage conductors earthed. This may be an important consideration for prime power schemes.
Where generators cannot be operated in parallel with their star points coupled, neutral-earth switchgear will be necessary and should be designed to provide the necessary isolation for generator maintenance purposes.
Where double-wound generator transformers are used in multiple LV generator installations, each generator LV installation can be treated as a separate electrical system and the combination is then treated as a high voltage generator. In this case, solid earthing of each generator star point, as if it were a single LV generator, provides an economic solution. Unrestricted earth fault protection, applied to the LV neutral-earth connection, will protect the generator, cables and transformer LV winding and will not be affected by the HV system configuration.
Where the transformers have similar ratings and impedances, it is usual to couple the HV star points and to earth these via a suitable impedance, sizes as described earlier. If the multiple generator transformers are not matched in terms of size and impedance, parallel operation may result in circulating currents and neutral-earth switching or the application of an neutral-earthing transformer to the high voltage busbar may be necessary.
e) Multiple generators operating as an alternative to an external supply
For high voltage generators, the HV system can be treated in the same manner as for the isolated case d) above, if care is taken to ensure that the generator system cannot become accidentally paralleled with the external system.
In the case of LV generator / transformer installations, the combination of each generator and transformer can again be likened to a high voltage generator. The LV system of each combination may be solidly earthed as before, since this is isolated from the external supply and earth fault protection applied as required.
f) Multiple generators operating in parallel with an external supply.
Multiple high voltage generators are treated in the same manner as single HV generators when paralleled with an external supply. The same considerations regarding operating generators with their star points coupled will apply.
In the case of generators that can be operated in parallel with their star points coupled, the common star point connection is taken to a neutral-earth switch and via the earthing impedance to the earth electrode.
Where the generators cannot be operated in parallel with their star points coupled, each generator must be provided with a neutral-earth switch that will be used to select the generator that is to provide the system reference when not in parallel with the external system.
When the generators are in parallel with each other and with the external system, all neutral earth switches are open.
Generator/transformer combinations are treated in an identical manner with the star-point earth switch connected to all transformers in the case of operation with star points coupled and a separate earth switch for each transformer if this is not possible.
It is important to include automatic detection for incorrect operation of the earth reference switches as accidental operation with an unearthed neutral is potentially hazardous.
g) Mixed generation schemes where low voltage generators with transformers and high voltage generators operate in parallel.
These schemes will require special consideration, as the effective impedances of the generator combinations will be significantly different.
Generally, the most convenient treatment of these schemes is to avoid parallel operation with star-points coupled and to provide a neutral earthing transformer on the common busbar. This should be zigzag wound to attenuate third harmonic currents.
When a neutral earthing transformer is used on systems that parallel with an external supply, the star point earth is switched out when in parallel, in the same manner as for generators. Earthing impedances can be provided in the same manner as for generators and should be sized to take into account the impedance of the earthing transformer.
The circuit supplying the earthing transformer should be protected in order to prevent hazard in the event of a winding fault. However, this protection must intertrip with the generators to avoid the possibility of operating the system without a reference. Providing the transformer with a separate fuse-switch supply from the HV busbar will satisfy this requirement. Rupture of any of the fuses will cause an alarm contact to close, which can be used to trip the system.
Generation
D46