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curious
| Joined: | Tue Sep 5th, 2006 |
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Posted: Wed Feb 7th, 2007 02:33 pm |
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| Is there a general rule to follow when calculating the value of generator neutral earthing resistance?
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George McDowell
| Joined: | Mon Aug 15th, 2005 |
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Posted: Wed Feb 7th, 2007 03:53 pm |
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The primary purpose of inserting an impedance between the star point of a generator and earth is to limit earth fault current. This is usually only necessary on high voltage generators (ie generators above 1000 volts) so as to limit the fault energy that is released at the fault location. By comparison low voltage generators tend to be directly earthed. With this in mind the the impedance that is inserted can be characterised as being of a high or low type. Low impedance earthing is conventionally defined as an impedance that limits the prospective fault current to the full load current of the generator. By comparison high impedance earthing limits the prospective earth fault current to much less than full load current and may be as low as tens of amps.
Most industrial applications in the UK (typically at voltages of 3.3 to 11kV) will use low impedance earthing. The value of impedance required is easily calculated to a reasonable approximation by dividing the rated phase voltage by the rated phase current of the generator. Obviously be extension the value for high impedance earthing is then calculated by dividing the rated phase voltage by the prospective low level of earth fault current required.
High impedance earthing is often applied in power stations to earth turbo generators for which the circuit is limited to the generator winding and the generator step-up transformer to which it is connected. The upper value of the high impedance must be limited so as not to create earth fault levels that are comparable to the current components that will also flow to earth through distributed system capacitiance. At these high values of impedance there is a risk that fault currents could become oscillatory and insufficiently damped.
Neutral earthing impedance is conventionally achieved using resistors rather than inductors (as in the case of reactance earthing) so as to limit the tendency for the fault arc to persist due to inductive energy storage. These resistors will dissipate considerable heat when earth fault current flows and are usually only short term rated (typically 30secs) so as to achieve an economic design.
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