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Neutral system - Single earthed or multi earthed? (Photo by WM Dyer ElectricalContractors)

http://electrical-engineering-portal.com/neutral-single-earthed-or-multi-earthed April 14, 2013

Neutral system Single earthed or Multi earthed?

jiguparmar

In distributionsystem threephase load isunbalance andnon linear so theneutral plays anvery importantrole indistributionsystem.

Generally,distributionnetworks areoperated in anunbalancedconfigurationand also serviceto consumers.This causescurrent flowing

through neutralconductorandvoltage droppingon neutral wire.The unbalanceload andexcessive current in neutral wire is one of the issues in three phase four-wire distributionsystems that causes voltage drop through neutral wire and makes tribulations for costumers.

The existence of neutral earth voltage makes unbalance in three phase voltages for threephase customers and reduct ion of phase to neutral voltage for single phase customers.

Mult i-grounded three-phase four-wire service is widely adopt ed in modern power distributionsystems due to having lower installat ion costs and higher sensit ivity of fault protection thanthree-phase three-wire service.

The neutrals play an important role in power quality and safety problems. The mult i groundedneutral system is the predominant electrical distribution system used in the United States.

It allow an uncont rolled amount o f electric current to f low over the earth unrestrained, posingthe pot ential of harm to the public and to animals causing electric shocks and is presumedresponsible for undetected electrocutions.

The protect ive grounding used in low voltage,600-volt and below, applications will bedescribed and used to explain the hazards involved with the present day mult i grounded neutraldistribution System, used in the United States. This will allow the reader to see the parallelsbetween the safe low voltage distribution system and the dangerous medium voltage multigrounded neutral distribution system.

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Three-phase four wire multigrounded neutral

Three-phase four wire singleneutral grounded

The reasons for the development of the three phase, four-wire, multi-grounded systemsinvolve a combination of safet y and economic considerations. The three-phase, four-wiremulti-grounded design has been successfully used for many years and is well documented inthe standards including the National Electrical Code (NEC). It is crucial decisions to adopt Mult iGrounded Neutral System save money by the adopt ion of the mult i grounded neutralelectrical distribution system in the cost o f the publics safety.

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Multi Grounded Neutral System (MEN)

Figure on left shows the mult i-grounded neutral systemscommonly used by the elect ric utilit ies in North America.The neutral grounding reactoris used by some ut ilities toreduce the available ground f ault current while at the sametime st ill maintaining an effectively grounded system.

The mult iple earthed neutral (MEN) system of earthing is one

in which the low voltage neutral conductor is used as the lowresistance return path for fault currents and where itspotential rise is kept low by having it connected to earth at anumber of locations along its length. The neutral conductor isconnected to earth at the distribution t ransformer, at eachconsumers installat ion and at specif ied poles or undergroundpillars. The resistance between the neutral conductor of thedistribution system and the earth must not exceed 10 ohmsat any locat ion.

NEC Article 250 Part X Grounding of Systems and Circuits 1 kV and Over (High Voltage)

1. Mult iple Grounding: The neutral of a solidly grounded neutral system shall be permitt ed tobe grounded at more than one point.

2. Mult i-grounded Neutral Conducto r: Ground each transformer, Ground at 400 m intervalsor less, Ground shielded cables where exposed to personnel contact.

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Single Grounded Neutral

Figure on left shows single grounded neut ral which isdifferent f rom mult i grounded system. Figure shows theneutral also connected to earth, but the neutral conductor isextended along with the phase conductors. Theconf iguration shown in figure allows electrical loads,transformers to be placed between any of the three phaseconductors, phase-to-phase and/or phase-to-neutral.

This connect ion, phase to neutral will force electric currentto f low over the neutral back to the transformer. So far, thiselectrical connection is acceptable, as long as the neutral is

insulated or treated as being pot entially energized, butmodifications will be made in the future that will negatesafety for the public and animals.

The ground connection would typically be located in the distribution substat ion. This may

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appear insignif icant, but the diff erences are significant

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Advantages of Multiple grounded neutral Systems

(1) Optimize the Size of Surge Arrestor:

Surge arresters are applied to a power system based on t he line-to-ground voltage undernormal condition and abnormal conditions. Under ground-fault conditions, the line-to-ground voltage can increase up to 1.73 per unit on the two, unfaulted.

Application of surge arresters on a power system is dependent on the effectiveness ofthe system grounding. The over voltage condit ion that can occur during a ground faultcan be minimized by keeping the zero sequence impedance low. Therefore, opt imizat ionin sizing the surge arresters on the system is dependent on the system grounding.

An effectively grounded power system allows the use of a lower rated surge arrester. Thelower rated surge arrester provides better surge protection at a lower cost. An effectivelygrounded system can only be accomplished using a properly sized, multi-groundedsystem neutral.

With Single Grounded Neutral System require the use of full line-to-line voltage ratedarresters. This increases the cost of the surge arresters while at t he same time reducesthe protection provided by the surge arrester. In addit ion, if the fourth wire neutral is notmult i grounded, it would be good pract ice to place surge arresters at appropriatelocations on that conductor.

(2) The zero sequence impedance is lower for a multi grounded system than the single pointgrounded neutral system.

(3) Freezing and arctic conditions have an adverse impact on the zero sequence impedance. Amulti-grounded system neutral will still lower the zero sequence impedance over a single pointground. In fact, without the multi-grounded system, it is more probable that insuff icient faultcurrent will f low to properly operate the ground fault protection.

(4) Cost o f Equipment f or the multi-grounded system is lower.

(5) Safety Concerns on Cable Shields.

Medium voltage and high voltage cables typically have cable shields (NEC requirementabove 5 kV) that need to be grounded. There are several reasons for t his shield:

To conf ine electric f ields within the cable

To obtain uniform radial distribution of the electric field

To prot ect against induced voltages

To reduce the hazard of shock

If the shield is not grounded, the shock hazard can be increased. With the shield groundedat one point, induced voltage on the shield can be signif icant and create a shock hazard.Therefore, it is common practice to apply multiple grounds on the shield to keep the

voltage limited to 25 volts.

This practice of mult i grounding cable shields includes the grounding of concentricneutrals on power cables thereby extending the need for mult i grounding of neutrals onthe power system.

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Ground fault relay

at neutral

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Disadvantages of Multiple neutral grounding

(1) Less Electrical Safety in Public and Private Property.

With a mult i grounded neutral distribution system it is necessary to have an electricalconnect ion to earth at least 4 t imes per mile to keep the voltage on t he multi grounded

neutral f rom exceeding approximately 25 volts making it safe for t he linemen should theycome into contact with the neutral and the earth.

As per NESC Rule 096 C in the section with the mult i grounded neutral conductorconnected to earth at least 4 times per mile and at each transformer and lightningarrester there are now multiple paths over and through the earth that the hazardouselectric current can f low over continuously, uncont rolled.

The path that this current f low takes through the earth cannot be determined. We cannotput an isotope on each electron and trace its path as it f lows uncont rolled through theearth. It is irresponsible to permit stray uncont rolled electric current t o f low into and over

private property.The National Electrical Code (NEC) requires the neutral in the service disconnect and overcurrent panel board to be connected to t he earth also. Now the secondary neutral isconnected to earth a second time. A parallel connection of the neutral to earth nowexists permit t ing hazardous electric current to f low cont inuously uncont rolled over theearth.

(2) Earth Fault Protection Relay setting is complicated.

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Advantages of Single Grounded Neutral System

(1) More Reliable and Safe System.

(2) Protection Relay Setting is more easy in Single Grounded Neutral:

Prot ective relays need to sense abnormal conditions, especially those involving a groundfault . The single point grounded system, with or without a neutral conductor, currentflowing into the ground should be considered abnormal (excluding normal chargingcurrent). For sensing of ground faults are:

A current t ransformer in the locat ion where the neutral is groundedcan be used to sense the ground fault (zero sequence) current.

A zero sequence CT enclosing the three phase and neutralconductors.

Four CT residue circuit (Three CT residual with neutral CTcancellation).

Prot ecting against ground faults on a multi-grounded neutralsystem is more dif f icult than the single point grounded system

since both neutral and ground fault currents must be considered.

Neutral current and likewise ground fault current can f low in both

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CT res idualcircuit

. ,as the amount of neut ral current which may flow in the circuit , and theground fault sett ing must be above this neutral current. This is selfexplanatory f rom Fig.

(3) Sensing of Ground Fault current:

While the sensing of the ground fault current in the single pointgrounded system is less complex than the mult i grounded system, the

amount of ground fault current on t he single-point grounded systemmay be greatly limited due to the fact t hat all ground fault current mustreturn through the earth. This is especially t rue where the earthresistivity is high, the soil is frozen or the soil is extremely dry.

REFERENCES:

John P. Nelson Fellow, IEEE ANSI/IEEE Std 142-1991

Westinghouse Electric Corporation, Electrical Transmission and Distribution ReferenceBook NFPA 70

Jeffery Leib, Train-Car Crashes on the Rise,Denver Post Newspaper, November 7, 2002

R.T. Beck and Luke Yu, Design Considerations for Grounding Systems