UNIQUE CHARACTERISTICS OF GROUND FAULTS
It is assumed here that the transmission has multiple grounding points at wye-connected transformer neutrals, located throughout the system. When this condition is satisfied, any arcing fault between a phase conductor and the ground will be supplied by zero-sequence currents originating in the neutral connection of the high-voltage transformer banks.
We often refer to these neutral connections as the “sources” of ground current, since very little current would flow to the ground fault if there were no grounded neutrals to provide a complete circuit for the fault current. When there are multiple ground sources, the current flowing to the ground may be very large.
Any current flowing to the ground contains zero-sequence components and, under undergrounded conditions, a zero-sequence voltage will be measured at any nearby relay installation. Negative-sequence currents and voltages will also be observed, and these are sometimes used by the protective system. However, most ground relay systems depend on detecting zero-sequence currents, for this is a sure sign of an abnormal system condition.
No significant zero-sequence currents flow during normal operation of the power system, with those that do appear to be the result of the unbalance in the operating condition of the three phases. These unbalanced currents are very small compared to fault currents, so it is a good approximation to think of the normal power system as being free of zero-sequence voltages or currents.
This is the first principle of ground fault relaying, namely, that a unique type of current exists during a ground fault and the relay needs only to be designed to detect the zero-sequence current in order to make positive identification of a ground fault.
Zero-sequence currents are confronted by zero-sequence impedances that depend on the structure of the power system. This structure does not change based on the loading of the power system and changes only when switching occurs. Therefore, except for occasional switching, the zero-sequence impedances are almost constant.
The zero-sequence impedance is affected by the generation and will change slightly as generators are added or removed. However, the line impedances are more important than the generator impedances for most fault currents.
This situation is quite different from positive-sequence currents, which fluctuate with the loadings of the lines as they respond to system load and generation changes. This is the second principle of ground relaying, viz., that the impedance seen by the zero-sequence fault currents is nearly constant from maximum load to minimum load conditions.
Another characteristic of the zero-sequence network is the magnitude of the impedance of the transmission lines. Zero-sequence line impedance is two to six times greater than positive-sequence line impedance. This means that, over the length of a transmission line, there will be a large difference in impedance seen by the fault current as the fault is moved from one end of the line to the other.
It should be noted that this may not be true if the line is mutually coupled with another nearby transmission line. There are two important points to observe here. First, there is a large difference in the fault current as the fault is moved from the relay location to the far end of the line. Second, the source impedances are usually small compared to the line impedance, hence the far-end fault currents are about the same at both ends.
Another requirement of ground faults is the need to determine the direction of the fault current. For a radial line, there is no problem in determining the direction of current flow, but this is not true in other parts of a power system. For this reason, many ground relays are directional relays. In order to get a sense of zero-sequence current direction, it is necessary to have a reference current or voltage against which the actual fault current can be compared.
This type of comparison is called polarization. By means of polarization, it is possible for the ground relay to determine if the fault is ahead or behind the relay location, giving the relay a measurement of the current direction as well as its magnitude.
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