IS YOUR ELECTRICAL SYSTEM’S NEUTRAL EARTHED?

D.G.DUNCAN Consulting Engineer (B.Sc.Eng., B.Comm, MSAIEE)
Test-A-Relay / Ron Slatem and Associates

SYNOPSIS: Electrical systems are generally designed to have their neutral points resistively or solidly earthed. This earthing is installed when the system is initially built, but the system may become “unearthed” for various reasons. An arcing earth fault on such an unearthed system can lead to dangerous overvoltages which can cause multiple failures in other equipment.

1.0 GENERAL

Electrical systems are never completely “unearthed” as system earth capacitance always allows some current to flow when an earth fault occurs on a phase. When the fault is an arcing type instead of solid or “bolted”, dangerous overvoltages up to five or more times normal phase to ground peak voltage may be developed.
Such overvoltages could cause insulation failure anywhere on the system.

For this reason, systems at medium voltages (3,3kV to 33kV)are usually intentionally earthed, either solidly or through some impedance. The policy recommended by SABS is neutral earthing resistance (NER) with typically 300 amp limits per transformer .

This intentional earth connection is installed on the neutral of a transformer or the neutral created by a Neutral Earthing Compensator or Neutral Electromagnetic Coupler (NEC).

2.0 PRACTICAL PROBLEMS

This intentional neutral earthing reduces the risk of dangerous overvoltages causing additional insulation damage when an arcing earth fault occurs. However, there have been many instances where this intentional neutral earth is no longer the same as the original design. The various reasons found are:-

a) The NER has failed or become an open circuit.

b) The NER has had part or all of it’s resistance shorted out.

c) The copper strap from the NER to earth has been stolen.

d) Water has evaporated from Liquid Earthing resistors

The results of a), c) and maybe d) above leave the system vulnerable to overvoltages or a higher probability of “Cross Country” faults. A “Cross Country” fault is where a second fault occurs on a healthy phase at some other point on the system, when an initial failure causes a fault to earth and thus offsets the three phase voltages and increases the voltage on the healthy phases. This occurs for all systems with some or infinite impedance between the neutral and earth. Where the fault is not cleared by normal earth fault current protection, the increased time that the high voltages may remain on the healthy phases (1.732x Vpn for a failed NER) increases the risk of a “Cross Country” fault occuring on one of the healthy phases.

“Cross Country” fault clearance results in confusion for operators as the tripped circuits can be closed individually even with permanent faults, but trip when both are closed. Also these fault currents are much higher than the limited earth fault current which should have occurred with a normal NER.

For case b), the subsequent earth faults result in higher fault currents and increased heating of the NER with the possibility of further elements shorting or the NER burning off completely.

Liquid Earthing resistors use a solution of potassium hydroxide in water contained in a tank with a centre electrode to create a high power resistor for use in neutral earthing. When water evaporates, the specific gravity of the liquid and the length of immersed electrode reduces and changes the resistance. These type of resistors require maintenance! Too large a variation of resistance will create protection problems and with excessive loss of liquid, an “unearthed” condition.

All these conditions are a danger to both personnel and equipment, and should be detected as early as possible and rectified.

3.0 SOLUTIONS TO FAILURES OF EARTHING

The ideal solution is a system robust enough to ensure failures cannot occur and secure enough to prevent theft of copper. Nither conditions are likely to be designed into practical systems, especially the necessary security. Failures have already occurred and will continue to occur in future.

Other solutions are:-

i) Changing to Solid Earthing.

ii) Regular checking by physically measuring the NER’s resistance to ground.

iii) A high set Instantaneous E/F relay on a CT in the neutral of the transfomer or NEC (not the NER’s earthed end).

iv) Tripping of the system when “Neutral Displacement” occurs during an earth fault.

v) The automatic detection of the “unearthed” condition and an alarm.

Solution i) removes the possibility of NER failure, and is a preferred short term solution when an NER fails. However, the advantage of the use of NERs is the substantially lower damage at the point of fault which results when an earth fault occurs. All the other advantages put forward in the various articles on NER earthing must also be considered before changing to solid earthing.

Solution ii) can leave the system vulnerable to overvoltages caused by an arcing fault, should one occur between checks. Checking requires the NER (and associated transformer) to be isolated for the tests which limits the number of checks to be carried out in practice.

Solution iii) only covers the condition of shorted resistor elements in the NER. The setting is made higher than the earth fault current possible from the normal NER earthed system. With shorted NER elements, rapid clearance of a fault occurs and further degredatiopn of the NER is reduced, but the complete system is isolated for a fault on any feeder or item of equipment. Operation of this protection must be followed up with checks on the NER resistance value. An open circuited NER is not covered.

Solution iv) can also leave the system vulnerable to overvoltages during the time delay of the neutral displacement protection. Such time delay is necessary to allow for the normal condition of the NER being healthy and correctly tied to earth, otherwise with instantaneous operation the normal earth fault protection using time grading will not operate and the total system isolated for a fault on any feeder or item of equipment. Neutral displacement protection will at least isolate an earth fault when the first fault occurs and reduces the probability of the higher phase to phase current of a “cross country” fault to occur.

Solution v) allows the faulty earthing to be detected and repaired before an arcing fault is likely to occur, drastically reducing the chance of damage to equipment and danger to personnel. However, as operators may not react to an alarm, a first fault could leave the system vulnerable to a second fault resulting in a “cross country” condition.

The recommended solution is a combination of both iii) and iv). A substantial improvement is the continuous measurement of the value of the NER’s actual resistance to detect changes as soon as possible, both higher or lower in value.

4.0 PROBLEM AREAS

The continuous measurement of the NER’s resistance creates a problem when earth faults occur on a live system. The voltage across the NER will be up to Phase to Neutral value during an earth fault, i.e. 6.35kV for an 11kV system. As this dangerously high voltage must not be brought back into the relay protection panel, some form of isolation must be provided. This neutral displacement voltage must not cause spurious alarms during an earth fault, thus the alarm must be suitably time delayed.

As unbalanced capacitance and third harmonic voltages result in small AC earth currents, these must not affect any measurement of the condition of earthing resistors. Also, multiple earthing for two or more transformers in parallel could have an effect on any measurement, therefore the lower alarm may need to be set less than the paralleled NERs’ resistance value.

As A.C. supplies could fail when a trip would be initiated during an earth fault, reliability of trip operation can be substantially improved by using D.C. tripping supplies for auxiliary power. A universal input for D.C. (and A.C.) with a wide voltage range to cover the various supply voltages possibly in use is an added advantage for such protection.

5.0 CONCLUSIONS

Although equipment should be reliable, NERs have been found to fail from corrosion, overloading, underspecification, underdesign or poor maintenance.

The need for addition protection and monitoring has arisen because of the many failures experienced and the results of copper theft. Consideration now needs to be given to reducing danger to personnel and equipment. Protection and monitoring equipment is on the market and needs to be assessed with regard to the requirements of the particular supply system.

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