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Home / Technical Articles / Earth Fault Protection On Insulated Networks and Petersen Coil Earthed Networks

Insulated Networks

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.

Earth Fault Protection On Insulated Networks and Petersen Coil Earthed Networks
Earth Fault Protection On Insulated Networks and Petersen Coil Earthed Networks

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.

It is vital that detection of a single phase-earth fault is achieved, so that the fault can be traced and rectified. While system operation is unaffected for this condition, the occurrence of a second earth fault allows substantial currents to flow.

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.

  1. Using residual voltage element
  2. Sensitive earth fault

1. Residual Voltage

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.

Current distribution in an insulated system with a C phase–earth fault
Figure 1 – Current distribution in an insulated system with a C phase–earth fault

2. Sensitive Earth Fault

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.

Phasor diagram for insulated system with C phase-earth fault
Figure 2 – Phasor diagram for insulated system with C phase-earth fault

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.

The polarising quantity used is the residual voltage. By shifting this by 90°, the residual current seen by the relay on the faulted feeder lies within the ‘operate’ region of the directional characteristic, while the residual currents on the healthy feeders lie within the ‘restrain’ region.

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.

The effect is therefore similar to having an insulated system. The effectiveness of the method is dependent on the accuracy of tuning of the reactance value – changes in system capacitance (due to system configuration changes for instance) require changes to the coil reactance.

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.

Earth fault in Petersen Coil earthed system
Figure 3 – Earth fault in Petersen Coil earthed system

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.

Distribution of currents during a C phase-earth fault – radial distribution system
Figure 4 – Distribution of currents during a C phase-earth fault – radial distribution system

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°.

C phase-earth fault in Petersen Coil earthed network: theoretical case – no resistance present in XL or XC
Figure 5 – C phase-earth fault in Petersen Coil earthed network: theoretical case – no resistance present in XL or XC

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.

Zero sequence network showing residual currents
Figure 6 – Zero sequence network showing residual currents

However, in practical cases, resistance is present and Figure 7 shows the resulting phasor diagrams.

If the residual voltage Vres is used as the polarising voltage, the residual current is phase shifted by an angle less than 90° on the faulted feeder and greater than 90° on the healthy feeders.

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.

C phase-earth fault in Petersen Coil earthed network: Practical case with resistance present in XL or XC
Figure 7 – C phase-earth fault in Petersen Coil earthed network: Practical case with resistance present in XL or XC

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:

  1. Sensitive earth fault and
  2. Zero sequence wattmetric

1. Sensitive Earth Fault Protection

To apply this form of protection, the relay must meet two requirements:

  1. Current measurement setting capable of being set to very low values
  2. 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.

An often used setting value is the per phase charging current of the circuit being protected. Fine tuning of the relay characteristic angle (RCA) is also required about the 0° setting, to compensate for coil and feeder resistances and the performance of the CT used.

In practice, these adjustments are best carried out on site through deliberate application of faults and recording of the resulting currents.


2. Sensitive Wattmetric Protection

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.

Resistive components of spill current
Figure 8 – Resistive components of spill current

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.

This method of protection is more popular than the sensitive earth fault method, and can provide greater security against false operation due to spurious CBCT output under non-earth fault conditions.

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)

where:

  • 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

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

4 Comments


  1. mohsen jahankhah
    Mar 02, 2018

    hello Edvard

    i follow articles from 2 years ago.
    very useful site.

    mohsen jahankhah
    electrical engineer


  2. Manuel Cruz
    Feb 13, 2018

    What happen with the free PDF download?


  3. Klaus Achtelik
    Feb 12, 2018

    Top article about application of Petersen spule.


  4. Bruno Almeida
    Feb 12, 2018

    Excellent article!

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