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First, identify ground faults and status of the neutral and then protection...
Define protection but first identify ground faults and status of the neutral (photo credit: ABB)

Ground faults and status of the neutral

To identify ground faults in a network and therefore carry out effective protection, it is necessary to know in detail how the neutral is run. Identification of ground faults is made by means of voltage and/or homopolar current measurements and therefore knowing the existence and order of these parameters is fundamental in being able to select and set the protection system.

Unlike the protections against overload or polyphase short circuit, no signal (voltage or current) normally comes to the protections which have to identify ground faults, but, on the other hand, only comes when there is a ground fault in the network.

This condition makes the protection system to be provided very simple, generally only requiring one threshold (voltage and/or current) with relatively short trip times.

By analysing the various types of status of the neutral, the types of protections which can be associated can be defined.

  1. Isolated neutral
  2. Solidly grounded neutral
  3. Neutral grounded by means of resistance
  4. Neutral grounded by means of impedance (Petersen coil)

1. Isolated neutral

In networks with isolated neutral, no circulation of homopolar current is generated deliberately (by means of grounding systems) in the case of a fault between a phase and ground. However, there is a circulation of homopolar current in the plant linked to the phase ground capacities of the machines and cables (for what regards the transformers, the phase to ground capacities are very small and they can be overlooked).

The difficulty (in any set-up the network may be found to run in) of being able to identify ground faults using selective protections which measure the fault current can be deduced from this.

The only way to be able to ensure identification of the fault is measurement of the homopolar voltage. Homopolar voltage is the voltage normally equal to zero in the absence of a fault and different from zero only in the presence of a phase to ground fault.

Unfortunately the voltage homopolar protection (like all voltage protections for that matter) is not of the selective type. This means that it (voltage homopolar protection) is not able to identify the position of the fault, but is only able to indicate that there is a fault in the network without specifying its position.

Networks with isolated neutral
Networks with isolated neutral

Homopolar current, homopolar voltage and angle between voltage and homopolar current in a network are:

  • Homopolar current only from capacitive contribution (operation of the metallically interconnected network) of variable value in any case and, in general, not guaranteed for all the conditions the network can be run in. Identification of the faults is not always certain by means of homopolar current measurements.
  • Homopolar voltage always present in the case of a ground fault. It is therefore definite identification but with uncertainty linked to the position of the fault since the voltmetric signal is practically the same for the whole network and does not allow selective identification.
  • Angle between voltage and homopolar current – the current is in advance by 90° compared with the voltage (capacitive type of network).

Go back to the neutral statuses ↑


2. Solidly grounded neutral

With solidly grounded neutral, the single-phase to ground fault current is in the same order of size as the short circuit current for polyphase faults.

Consequently simple and selective identification of the faults by means of protections which measure the homopolar current is possible (or the homopolar protection could even be omitted and only the phase protection used).

Networks with solidly grounded neutral
Networks with solidly grounded neutral

Homopolar current, homopolar voltage and angle between voltage and homopolar current in the network are:

Homopolar current of high value – Therefore identification of the faults by means of measuring the current is always certain and of selective type (the part of the network seat of the fault can be identified correctly).

Homopolar voltage – if this voltage is measured between star point and ground, the voltage is nil, whereas, if the vectorial sum of the three phase voltages is measured, this is different from zero and gives indication of a fault in the network (but not of selective type).

Angle between voltage and homopolar current – the current is late (typical values 75-85°) compared to the voltage (inductive type of network source).

Go back to the neutral statuses ↑

3. Neutral grounded by means of resistance

Grounding the neutral by means of resistance allows a definite current to be obtained in the case of a fault and consequently to be able to carry out selective protection of the network.

Depending on the value of the resistance installed, fault current values which are higher or less high are obtained, but:

  • The lower the fault current is, the small the damage to the machines is.
  • The higher the fault current is, the more easily the fault is identified (and the protection with lower sensitivity is required).
Networks with neutral grounded by means of resistance
Networks with neutral grounded by means of resistance

Homopolar current, homopolar voltage and angle between voltage and homopolar current in the network are:

Homopolar current of known value. Identification of the faults is possible by measuring the homopolar current. The protection is therefore of the selective type.

Homopolar voltage – if this voltage is measured between the star point and ground, the voltage varies according to the value of the grounding resistance. For grounding resistances of high value one falls back into the situation of isolated neutral, for grounding resistances of very small value, one falls back into the situation of solidly grounded neutral.

If the vectorial sum of the three phase voltages is measured, it is different from zero and gives indication of a fault in the network (but not of the selective type).

Angle between voltage and homopolar current – theoretically equal to zero (in phase). In reality, the angle is in any case capacitive for the contribution of the to ground capacity of the network.

There are various methods to create network grounding according to the availability or lack thereof of the star point as shown in the figure.

Methods to create network grounding according to the availability or lack thereof of the star point
Methods to create network grounding according to the availability or lack thereof of the star point (click to expand)

Go back to the neutral statuses ↑


4. Neutral grounded by means of impedance

Petersen coil

Grounding the neutral by means of impedance allows the network capacitive currents to be compensated and therefore to reduce the current to relatively small values in the case of a fault (in Italy, utilities limit the fault current to 40-50 A) and with a fault angle about equal to zero (compensated network).

Networks with neutral grounded by means of impedance (Petersen coil)
Networks with neutral grounded by means of impedance (Petersen coil)

Homopolar current, homopolar voltage and angle between voltage and homopolar current in network are:

Homopolar current of known value – Identification of the faults by means of homopolar current measurement is possible. The protection is therefore of the selective type.

Homopolar voltage – The measurement of the vectorial sum of the three phase voltages is different from zero and gives indication of a fault in the network (but not of the selective type).

Angle between homopolar voltage and current – Theoretically equal to zero (network tuned). In actual fact, the angle can in any case diverge slightly both in advance and delayed according to the setting of the compensation reactance and to changes in the network set-up.

Go back to the neutral statuses ↑


Measurement of the ground fault current and identification of the faulted phase

Since the advent, first of electronic and then of digital protections which have low absorption on the current circuit, the use of ring type CTs has been possible (able to generally give very small performances). This allows the vectorial sum of flux to be measured instead of the vectorial sum of the three currents (residual connection).

Holmgreen connection - connection principle when the earth-fault current is measured through the residual connection of the three phase CTs
Holmgreen connection – connection principle when the earth-fault current is measured through the residual connection of the three phase CTs. The stabilising resistor is connected in series with the earth-fault current input of the protection relay. The purpose of the voltage-dependent resistor is to limit the voltage of the secondary circuit to a safe level. The need for a VDR is case-specific and can be checked by calculation.

When a homopolar overcurrent protection is connected to the residual connection of the phase CTs (Holmgreen connection) it performs a vectorial sum of the currents. Resultant is therefore affected by the aperiodic components linked to magnetisation of the transformers or to motor starting.

In this case, very conservative settings of the protections are required and the stability of these is not normally guaranteed (risk of unwanted trips). It is therefore suggested to systematically use (obviously where possible) CTs of ring type associated with the homopolar overcurrent protection.

In the case where it is necessary to identify which of the phases is the seat of the ground fault, identification is possible using undervoltage protections with measurement for each independent phase connected between the phase to ground (obviously to the VT secondary).

Reference // Complete guide to the good protection of medium voltage networks by ABB

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

4 Comments


  1. Michael R,. Cole
    Aug 07, 2016

    A ground fault that is close to or at the neutral point of a generator, transformer, motor, inverter, or other source or load is very difficult to detect. I have seen this happen when one of the other electricians moved the liquidtight conduit from 1 motor junction box opening to another. He screwed the box connector right into the uninsulated crimp barrel of a neutral ring terminal on the nut block.

    This was causing the variable frequency drive to trip about 5 seconds after starting because the carrier
    frequency produces a very strong zero sequence tracing tone. If it had been a solidly grounded system we would never have noticed that an ungrounded system had been accidentally converted to a 277Y480-volt system.

    For ungrounded and resistance grounded systems one way to do this is to is to inject a 2.5 Hertz subsonic tracing tone into the system that is compatible with the Merlin Gerin Vigilohm system. This has both permanently installed and portable ground fault relays. It also has a portable transmitter that can be used on a deenergized or energized circuit.

    You can also temporarily resistance ground one of then corners of the system instead of the neutral but that needs to be done periodically and automatically.

    Mike Cole. Ohio Lic. No. EL45,008


  2. alok
    Aug 06, 2016

    Sir explain me about economy associated with thickness of neutral and phase wire in transmission and distrubution


  3. adolfo f. ponce de leon PE
    Aug 05, 2016

    I am dismay by the lack of knowlege by the members.
    The article fails to disclose that is about systems grounding.

    God bless

    • Edvard
      Edvard
      Aug 05, 2016

      The main topic here is the neutral grounding status in order to properly set protection. It’s not about systems grounding.

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