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Let's analyse phase-to-earth faults in a single-incomer power system
Let’s analyse phase-to-earth faults in a single-incomer power system

Detect and isolate…

Power-system protection should detect faults and isolate the faulty parts of the power system while keeping the fault-free parts in operation. Protection units are chosen according to the power-system configuration (parallel operation of generators or transformers, loop or radial power system, neutral earthing arrangement…).

Consideration must be given to:

  1. Earth fault protection, linked to the neutral earthing arrangement:
    Resistance earthing on the transformer
    Resistance-earthed neutral at busbars
    Reactance-earthed neutral
    Isolated neutral
    Solidly earthed neutral
    Compensated neutral
  2. Phase-to-phase fault protection

Ok, this time I’ll analyse earth fault protection in a single-incomer power system…


Resistance earthing on the transformer

Earth fault protection units (ANSI 51N) are installed on the feeders, incomer and neutral earthing connection.

Phase-to-earth fault protection (resistance-earthed neutral at transformer)
Figure 1 – Phase-to-earth fault protection (resistance-earthed neutral at transformer)

Time-based discrimination is used between the different protection units. These units are necessarily different from phase fault protection units since the fault currents are in a different range.

The feeder protection units are set selectively in relation to the incomer protection unit, which is itself set selectively in relation to the neutral earthing protection unit (in accordance with discrimination intervals). The fault current flows through the capacitances of the fault-free feeders and the earthing resistance.

All the fault-free feeder sensors detect capacitive current.

To prevent inadvertent tripping, the protection unit on each feeder is set higher than the feeder’s capacitive current.

  • fault at 3: the D1 circuit breaker is tripped by the protection unit linked to it,
  • fault at 4: the A circuit breaker is tripped by the incomer protection unit,
  • fault at 5: the protection unit on the neutral earthing connection trips circuit breaker H on the transformer primary circuit. (fig. 1).

The protection unit at D must be selective in relation to the downstream protection units: if the delay required for protection A is too long, logic discrimination should be used. The neutral earthing protection unit at H acts as back-up should the incomer protection unit at A fail to trip. The incomer protection unit at A acts as back-up should a feeder protection unit at D fail to trip.

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Resistance-earthed neutral at busbars

A zero sequence generator is used for resistance-earthing. Earth fault protection units (ANSI 51G) are installed on the feeders, incomer and zero sequence generator.

Phase-to-earth fault protection (resistance-earthed neutral at busbars)
Figure 2 – Phase-to-earth fault protection (resistance-earthed neutral at busbars)

Time-based discrimination is used between the different protection units. The feeder protection units and incomer protection unit are set selectively in relation to the earthing impedance protection unit. As in the previous case, the protection unit on each feeder is set higher than the feeder’s capacitive current.

In the event of a fault on feeder 1, only the D1 feeder circuit breaker trips. In the event of fault on the busbars 2, only the protection unit on the earthing connection detects the fault. It trips circuit breaker A.

In the event of fault on the transformer secondary circuit 3, the incomer protection units detects the fault. It trips circuit breaker H.

NOTE // when circuit breaker A is open, the transformer secondary circuit neutral is isolated. It may be necessary to protect it by a neutral voltage displacement measurement (ANSI 59N).

The zero sequence generator protection unit acts as back-up should the incomer protection unit at A or a feeder protection unit at D fail to trip. If the condition IsD > 1.3 Ic cannot be satisfied for a feeder, a directional earth fault protection unit may be used to discriminate between fault current and capacitive current.

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Neutral earthing arrangement

Reactance-earthed neutral

The same procedure is used as for resistance-earthing at the transformer or busbars.

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Isolated neutral

A fault, regardless of its location, produces current which flows through the capacitance of the fault-free feeders. In industrial power systems, this current is generally weak (a few amperes), allowing operations to carry on while the fault is being tracked.

Phase-to-earth fault protection (isolated neutral)
Figure 3 – Phase-to-earth fault protection (isolated neutral)

Time-based discrimination is used between the different protection units. The fault is detected by an insulation monitoring device or a neutral voltage displacement protection unit (ANSI 59N).

When the total capacitive current of a power system is high (in the range of ten amperes), additional measures must be taken to quickly clear the fault. Directional earth fault protection can be used to selectively trip the fault feeder.

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Solidly earthed neutral

This is similar to resistance-earthing at the transformer, but the capacitive currents are negligible compared to the fault current, so the protection function is simpler to implement.

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Compensated neutral

The power system is earthed at the transformer. Faults are detected by a specific directional earth fault protection unit (ANSI 67NC), which monitors the active residual current and recognizes faults during their initial transient phase.

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Reference: Electrical Network Protection Guide – Merlin Gerin (Schneider Electric)

About Author //

author-pic

Edvard Csanyi

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

7 Comments


  1. Gamal
    Aug 11, 2015

    Appreciated…


  2. karam
    Mar 15, 2015

    thanks a lot


  3. Eddie Liu
    Nov 26, 2014

    I am a technical engineer.


  4. simon f
    Oct 20, 2014

    Edvard – you refer to the returning earth fault currents through adjacent fault free feeders as capacitive currents. I have not heard them described as capacitive before. Why do you label them so?

    Cheers

    Simon


  5. Ahmed
    Oct 19, 2014

    Very nice


  6. Ahmed
    Oct 19, 2014

    Good


  7. Vasil
    Oct 17, 2014

    Хорошая статья!

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