Genny Neutral Earthing

[ccdubs]

How do you determine whether a generator for grid generation requires a neutral earthing resistor when star connected? I am also having difficulty with the concept of switched star point earthing.

All help appreciated!

 

[jbartos]

Suggestions:
1. Low voltage power distribution system having the supply with star connection usually has its neutral solidly grounded. This is because of advantages of solidly grounded neutral in terms of simplicity, applicability to dual voltage systems e.g. 208/120, and relatively small damage be the short circuit arc (relatively small short circuit arc energy is released).
2. Medium voltage (1kV-100kV), especially on its lower end, has frequently applied a resistor, which is often called "medium resistance system neutral grounding." However, there is some applicability advantages and disadvantages to be considered. Therefore, there is another type frequently used, which is called the "high-resistance system neutral grounding." This type of grounding allows much smaller current to produce arcing at the fault and it can be traced where the arc is. The fault can be cleared within a short time or stay the longer time depending on the insulation level (100%, 133%, etc.). D. Beeman "Industrial Power Systems Handbook", McGraw-Hill, 1955 describes this methods fairly popularly.
3. If there are more power supplies with the star connection, then one high or medium resistance grounding is implemented at one power supply since it is sufficient and convenient. In case that maintenance and testing are to be done on the power supply that has the neutral grounding in effect, then the grounding system is switched to another power supply so that the high or medium resistance grounding has the continuous functionality of the power distribution system.

 

[jwerthman]

If the generator is connected directly to a 4-wire distribution grid (no step-up transformer), then the generator neutral must either be solidly grounded or grounded through an inductor rated to carry the continuous line-to-neutral unbalance current. Resistance grounding in this application cannot be used because the continuous neutral unbalance current would result in poor line-to-neutral voltage regulation, and even low levels of neutral load current would result in unacceptable power loss and overheating of the resistor. Solid grounding is seldom used above 600 volts. Generator standards require that the ground fault current at the generator terminals be limited to not more than the 3-phase fault current unless the generator is designed specifically for solid grounding. This requires some neutal impedance since the generator zero sequence reactance is almost always less than the positive and negative sequence reactances. The neutral reactor must be rated to carry the expected continuous neutral unbalance current.

 

[peterb]

Note also that if your connection to the grid is through a dedicated unit transformer, the generator would normally (depending on size) be equipped with high resistance neutral earthing, usually through a distribution transformer and secondary resistor.