Earth fault detection methods

In an isolated neutral system the earth fault current is smaller than load current and thus the detection of earth fault cannot be utilized by use of over current relay. The possible indicators of earth fault are fundamental frequency neutral point displacement voltage, change in fundamental frequency phase voltage, fundamental frequency asymmetric cur-rent, harmonics in current or voltage and high frequency current changes. Detection with fault current harmonics is based on the fact that earth fault current contains 5 th harmonic.

The high frequency current changes occurs during the first moments of earth fault. Basi-cally the earth fault detection in substations is taken care of by directional relay which operates based on the measured asymmetric current Ir and measured asymmetric voltage between system neutral and earth. The asymmetrical current can be measured by the sum connection of current transformers or with cable type current transformer.

The current that relay sees during an earth fault is smaller than the total earth fault in fault point. This is because the current generated in faulted feeder is transmitted in both direc-tion through the current measurement device. In Figure 7 an equivalent circuit of an iso-lated system is represented whose capacitances of two lines are separated. 3Cj represents the earth capacitance of faulted feeder.

Figure 7. The equivalent circuit of an earth fault in isolated system. The capacitances of faulted feeder are separated. [1]

Since the same zero sequence voltage V0 effects on both capacitances 3Cj and 3(C-Cj), it follows equation (12) for the residual current Ir that is seen by a relay during an earth fault. [5]

𝐼_𝑟= (𝐶 − 𝐶_𝑗)

𝐶 𝐼_𝑓 (12)

For directional earth fault protection to detect the faulted feeder current Ir and voltage V0

needs to exceed the defined limits and the angle between negative V0 and Ir have to be close to 90˚. Therefore the third operating condition of directional earth fault relay is an angle sector represented in equation (13). The suitable angle tolerance ∆𝜑 depends in neutral isolated system on the system conductance and resistances of the lines. [5]

90˚ − ∆𝜑 < 𝜑 < 90˚ + ∆𝜑 (13) In which 𝜑 is the angle between Ir and V0, and ∆𝜑 is the angle tolerance.

The single phase equivalent circuit of a system consisting of both distributed and central-ized coils is presented in Figure 8. L^BG is the coil inductance located in background net-work (centralized and distributed coils) and L^Fd is the inductance of the coils located in the inspected feeder.

Figure 8. The single phase equivalent of an earth fault in centralized and distributed compensated system [7], modified from [5]

The absolute value of the residual current for the faulted feeder in system represented in Figure 8 can be calculated with equation (14) [7].

𝐼_𝑟 = √1 + (𝑅_𝐿(3𝜔(𝐶 − 𝐶_𝐹𝑑) − 1

Where L is the total inductance of all coils in the network.

In compensated neutral system the Ir is mainly generated in the parallel resistance of the centralized coil and is thus mainly resistive. The parallel resistance of the coil can be configured to work differently in different systems depending on the desired purposes.

Parallel resistance can be connected all the time or it can be connected only during an earth fault if the earth fault have not disappeared after small time delay. The parallel re-sistance increases the resistive earth fault current and it is thus easier for relays to detect earth fault selectively. In case of resonant earthed neutral system the directional over cur-rent relay is set to trip if the angle between curcur-rent and voltage is maximum ±∆𝜑. Since the system is often operated close to resonance, the angle 𝜑 might alternate significantly during earth fault and angle tolerance is thus usually set remarkably higher than in neutral isolated systems. [5]

In system where only distributed compensation coils is used the earth fault current is mainly reactive. Thus the earth fault detection can be utilized as in neutral isolated system.

In this case the changes in switching state causes large changes in earth fault current gen-eration of the feeder consisting of local coil, which causes problems in sensitive earth fault detection. Locations of the coils should be carefully selected to avoid the situation where local coils overcompensate the feeder and therefore the earth fault current produced by the feeder would become inductive. Protection settings might not been adjusted to take such an operation into account. [8]

The advantage of the traditional earth fault protection methods is that they are commonly known and the setting principles are familiar to protection engineers. The performance of the traditional protection methods is considered adequate, but there are some limitations especially systems with distributed coils, multiple types of fault resistance and intermit-tent earth faults. These kind of systems will be in use since the cabling increases but part of the lines remain overhead lines. Also one limitation of the traditional earth fault detec-tion method is that relay settings must be changed if the compensadetec-tion coil is discon-nected, which complicates the daily operation of distribution network. [9] Recent studies indicates that admittance based protection would solve these problems, since its inherent immunity to fault resistance, good sensitivity and easy setting principles. [8]