Power Engineering Consultants plc

Capacitive Currents in Unearthed HV Systems.

Although it is generally the practice to provide an earth path in HV systems, there are examples where this is not the case and the HV power system operates in isolation from earth.  We call these unearthed systems.  The term ungrounded is also used.

During an earth fault, a suitable protection relay has to be able to detect the fault.  In the case of an earthed HV system, the earth path allows the earth fault current to flow, so that current operated protection relays can detect and clear the fault.
In the case of an unearthed HV system, during an earth fault, some current does flow in the capacitance of the generators, cables and transformers.  A suitable protection relay should still be provided to detect such earth faults to either operate the circuit breaker, close down the prime mover and switch off the generator excitation, or else initiate an alarm.

In this article, we investigate the capacitive currents that flow, during an earth fault, in an unearthed HV system comprising a generator, with isolated neutral at the star point, a cable connection and a dYn step-up transformer.  Suitable protection relays can then be applied to detect the presence of an earth fault.  A core balance current transformer is located at the generator terminals. 

1. Modelling of an unearthed system during an earth fault

The isolated system under investigation is shown in Figure 1 below. The capacitance of each of the system components is represented separately, i.e. the generator winding, cable and transformer winding capacitance.

The assumption made is that the capacitance of each component is balanced i.e. it is the same for each phase.  In this example, the cable connecting the generator to the transformer terminals is short and so its contribution to capacitance current is not great.  If the cables are of considerable length, exceeding say 100 metres, the cable capacitance becomes significant, even dominant, the earth fault current increases and the distribution of capacitance currents is modified.

2. Analysis of the capacitive currents and applications in E/F protection design

In this case an earth fault was applied on phase A and the corresponding system voltages and capacitive currents were drawn. A core balance CT was then added, in order to determine the capacitive currents it would be able to detect, depending on the location of the earth fault.

The location of the fault is of great importance, as far as the CBCT operation is concerned. Investigation of the system below can reveal that capacitive currents due to an earth fault within the generator winding will not be detected by the CBCT. This occurs because the main source of capacitive currents and the fault location reside on the same side of the CBCT, while the cable and transformer capacitance, as a source of capacitive currents, is negligible. Therefore, only earth faults in the cable or the transformer delta winding can produce capacitive currents that an CBCT can detect.

Nevertheless, if long cables are installed, which comprise a sizable source of capacitive currents, it is possible to detect earth faults occurring throughout the isolated HV system.

In Figure 2, the capacitive currents and voltages are represented in vector form, in order to demonstrate their phase relation. These can then be used to set up a sensitive directional E/F relay, which would be able to determine the fault location by the direction of the capacitive fault currents. The angle between the currents and the voltages is the only way to identify the location of the fault in an isolated system and is an accurate method of discriminating between internal generator winding and external earth faults.

Figure 2. Vector form representation of the capacitive currents and system voltages during an earth fault