There are situations when power system is run completely insulated from earth. The advantage of this is that a single phase-earth fault on the system does not cause any earth fault current to flow, and so the whole system remains operational.
The system must be designed to withstand high transient and steady-state overvoltages however, so its use is generally restricted to low and medium voltage systems.
The absence of earth fault current for a single phase-earth fault clearly presents some difficulties in fault detection. Two methods are available using modern relays.
When a single phase-earth fault occurs, the healthy phase voltages rise by a factor of 3 and the three phase voltages
no longer have a phasor sum of zero. Hence, a residual voltage element can be used to detect the fault.
However, the method does not provide any discrimination, as the unbalanced voltage occurs on the whole of the affected section of the system. One advantage of this method is that no CTs are required, as voltage is being measured.
Grading is a problem with this method, since all relays in the affected section will see the fault. It may be possible to use definite-time grading, but in general, it is not possible to provide fully discriminative protection using this technique.
This method is principally applied to MV systems, as it relies on detection of the imbalance in the per-phase charging currents that occurs.
Figure 1 above shows the situation that occurs when a single phase-earth fault is present. The relays on the healthy feeders see the unbalance in charging currents for their own feeders. The relay in the faulted feeder sees the charging currents in the rest of the system, with the current of its’ own feeders cancelled out.
Figure 2 shows the phasor diagram.
Use of Core Balance CTs is essential. With reference to Figure 2, the unbalance current on the healthy feeders lags the residual voltage by 90°.
The charging currents on these feeders will be √3 times the normal value, as the phase-earth voltages have risen by this amount. The magnitude of the residual current is therefore three times the steady-state charging current per phase. As the residual currents on the healthy and faulted feeders are in antiphase, use of a directional earth fault relay can provide the discrimination required.
Thus, the relay characteristic angle (RCA) required is 90°. The relay setting has to lie between one and three times the per-phase charging current.
This may be calculated at the design stage, but confirmation by means of tests on-site is usual. A single phase-earth fault is deliberately applied and the resulting currents noted, a process made easier in a modern digital or numeric relay by the measurement facilities provided.
Application of such a fault for a short period does not involve any disruption to the network, or fault currents, but the duration should be as short as possible to guard against a second such fault occurring.
It is also possible to dispense with the directional element if the relay can be set at a current value that lies between the charging current on the feeder to be protected and the charging current of the rest of the system.
Introduction to Petersen Coil earthing
Petersen Coil earthing is a special case of high impedance earthing. The network is earthed via a reactor, whose reactance is made nominally equal to the total system capacitance to earth. Under this condition, a single phase- earth fault does not result in any earth fault current in steady-state conditions.
In practice, perfect matching of the coil reactance to the system capacitance is difficult to achieve, so that a small earth fault current will flow.
Petersen Coil earthed systems are commonly found in areas where the system consists mainly of rural overhead lines, and are particularly beneficial in locations subject to a high incidence of transient faults.
To understand how to correctly apply earth fault protection to such systems, system behaviour under earth fault conditions must first be understood. Figure 3 shows a simple network earthed through a Petersen Coil. The equations clearly show that, if the reactor is correctly tuned, no earth fault current will flow.
Figure 4 shows a radial distribution system earthed using a Petersen Coil. One feeder has a phase-earth fault on phase C.
Figure 5 shows the resulting phasor diagrams, assuming that no resistance is present.
In Figure 5(a), it can be seen that the fault causes the healthy phase voltages to rise by a factor of √3 and the charging currents lead the voltages by 90°.
Using a CBCT, the unbalance currents seen on the healthy feeders can be seen to be a simple vector addition of Ia1 and Ib1 and this lies at exactly 90° lagging to the residual voltage (Figure 5(b)). The magnitude of the residual current IR1 is equal to three times the steady-state charging current per phase.
On the faulted feeder, the residual current is equal to IL – IH1 – IH2, as shown in Figure 5(c) and more clearly by the zero sequence network of Figure 6.
However, in practical cases, resistance is present and Figure 7 shows the resulting phasor diagrams.
Hence a directional relay can be used, and with the relay characteristic angle (RCA) of 0°, the healthy feeder residual current will fall in the ‘restrain’ area of the relay characteristic while the faulted feeder residual current falls in the ‘operate’ area.
Often, a resistance is deliberately inserted in parallel with the Petersen Coil to ensure a measurable earth fault current and increase the angular difference between the residual signals to aid relay application.
Having established that a directional relay can be used, two possibilities exist for the type of protection element that can be applied:
To apply this form of protection, the relay must meet two requirements:
- Current measurement setting capable of being set to very low values
- Relay characteristic angle (RCA) of 0°, and capable of fine adjustment around this value
The sensitive current element is required because of the very low current that may flow – so settings of less than 0.5% of rated current may be required. However, as compensation by the Petersen Coil may not be perfect, low levels of steady-state earth fault current will flow and increase the residual current seen by the relay.
In practice, these adjustments are best carried out on site through deliberate application of faults and recording of the resulting currents.
It can be seen in Figure 7 above that a small angular difference exists between the spill current on the healthy and faulted feeders. Figure 8 shows how this angular difference gives rise to active components of current which are in antiphase to each other.
Consequently, the active components of zero sequence power will also lie in similar planes and a relay capable of detecting active power can make a discriminatory decision. If the wattmetric component of zero sequence power is detected in the forward direction, it indicates a fault on that feeder, while a power in the reverse direction indicates a fault elsewhere on the system.
Wattmetric power is calculated in practice using residual quantities instead of zero sequence ones. The resulting values are therefore nine times the zero sequence quantities as the residual values of current and voltage are each three times the corresponding zero sequence values.
The equation used is:
Vres × Ires × cos (φ − φc)
= 9 × V0 × I0 × cos (φ − φc)
- Vres = residual voltage
- Ires = residual current
- V0 = zero sequence voltage
- I0 = zero sequence current
- φ = angle between Vres and Ires
- C = relay characteristic angle setting
The current and relay characteristic angle (RCA) settings are as for a sensitive earth fault relay.
Reference // Network Protection & Automation Guide by Alstom Grid