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Licensed copy: Tarhafra Con

GENERATORS G G13-1Earthing of

Electrical Safety CouncilGENERATORS - Earthing of

Contents

1. Introduction

2. Generator sets operating in parallel with each other

3. Generator sets operating as a switched alternative to a supply obtained from the system for distributing electricity to the public

4. Generator sets operating in parallel with a supply obtained from the system for distributing electricity to the public

1. Introduction

This topic provides guidance on the earthing arrangements for generator sets. Inparticular it covers arrangements to prevent circulating currents where generator setsoperate:

in parallel with each other;

as a switched alternative to, or in parallel with, a supply forming part of a TN system,obtained from a public electricity distribution network.

In general, the neutral point of a generator sets windings will need to be connected withEarth and, in such cases, a source earth electrode will be required for the generatorset. Where there is more than one generating set, the neutral point of each generatorsets windings may be connected with Earth, either individually, in groups or collectively;see Fig 1.sulting Ens. Co.,Tue Jun 8 01:22:57 2010

ELECTRICAL SAFETY COUNCIL ESSENTIAL GUIDE April 2010 1 of 8



Electrical Safety CouncilAlternative arrangements for connecting the neutral points of a number of generating sets with Earth

Fig 1

2. Generator sets operating in parallel with each other

Where generator sets operate in parallel with each other and more than one generatorsneutral point is connected with Earth at the same time, harmonic currents (predominatelythird harmonic current), produced by the generators, can circulate between these sourcesof supply; see Fig 2.

a) Generator neutral points connected to individual earth electrodes

G1 G2 G3 G4

b) Generator neutral points connected to a group earth electrode

G2 G4

c) Generator neutral points connected collectively to a common earth electrode

G2 G3 G4

G1 G3

G1sulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety CouncilCirculating harmonic currents where two generator sets operate in parallel with each other

Fig 2

Where the magnitude of the circulating current is problematic because it will causeoverheating of the conductors or unwanted operation of a generator sets earth faultprotection, the earthing arrangements can be adapted to overcome the problem by:

(i) providing a switching arrangement to switch the connections between the neutralpoint of each generator set and Earth, such that the neutral point of only one of theon-line generating sets is connected with Earth at any given time (see Fig 3), or

(ii) installing a suitable reactor in each of these connections to attenuate the harmoniccirculating current (see Fig 4).

Where a switching arrangement such as that described in indent (i) and shown in Fig 3 isused, it must be interlocked such that fault protection and fault current protection are notimpaired at any time (Regulation 551.5.2 refers).

Where a reactor is installed in the connection between the neutral point of each generatorset and Earth, such as described in indent (ii) and shown in Fig 4, the electrical systembecomes an IT system.

Load

Generator 1

Generator 2

Line current

Neutral currentsulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety CouncilUse of switching arrangement

Fig 3

The earthing arrangement shown in Fig 3 is that generally used for a TN system, where aswitching arrangement to prevent circulating currents is provided. This, however, is notthe only option. For example, the neutral point of each generator sets windings could beprovided with its own source earth electrode but, in such cases, the switching arrangementbecomes more complex as the connection(s) from the exposed-conductive-parts andextraneous-conductive-parts of the installation need to be switched such that these arealways connected to the source earth electrode being used at the time.

Generator No 1

To exposed-conductive-parts (For TN systems ONLY)

To extraneous-conductive-parts (For TN systems ONLY)

Generator No 2

Neutral

To Loadssulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety CouncilUse of neutral point earthing reactor

Fig 4

3. Generator sets operating as a switched alternative to a supply obtained from the system for distributing electricity to the public

Where a generating set is to operate as a switched alternative to a supply obtained fromthe system for distributing electricity to the public forming part of a TN system, theconnection to the means of earthing of that system (which is provided by the distributor) isnot to be relied upon for fault protection when the generating set or sets are selected asthe source of supply (Regulation 551.4.3.2.1 refers). This is because the connection tothat means of earthing may be lost during a failure of that supply. Consequently, one ofthe earthing arrangements as described in item 1 will be required.

Generator No 1

Generator No 2

To exposed-conductive-parts

To extraneous-conductive-parts

Neutral

To Loads

Neutral point

earthingreactor

Neutral point

earthingreactor

Installation earth electrodesulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety CouncilFurthermore, when the generating set or sets are selected as the source of supply, it maybe necessary to disconnect the earth connection between the Main EarthingTerminal (MET) and the means of earthing provided by the distributor, since currents asdescribed in item 2 could otherwise circulate between the generator set or sets and thesource of the distributors TN supply, see Fig 5.

Circulating currents where a generator operates as a switched alternative to a supply from the system of distributing electricity to the public

Fig 5

Generally, these circulating currents are interrupted by switching the neutral, as well as theline conductors, when selecting the source of supply. However, where the neutral is notswitched and circulating currents are a problem, they will need to be interrupted bydisconnecting the earth connection as previously described.

Similarly, when the supply from the system for distributing electricity to the public isselected as the source, and the neutral is not switched, the circulating currents can beinterrupted by disconnecting the connection between the neutral point of the windings ofthe generator set or sets and Earth as shown in Fig 3.

It is important that any arrangement for switching the earth connections is interlocked suchthat fault protection and fault current protection are not impaired at any time.

Non-essential loads Essential loads

MET

PUBLIC SUPPLYGENERATOR

NEU

TRA

L

NEU

TRA

L

PRO

TEC

TIV

EC

ON

DU

CTO

R

PRO

TEC

TIV

EC

ON

DU

CTO

Rsulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety Council

4. Generator sets operating in parallel with a supply obtained from the system for distributing electricity to the public

Where a generator set operates in parallel with a supply obtained from the system fordistributing electricity to the public, currents as described in item 2 can circulate betweenthe generator set and the source of the public supply. In such cases it may be necessaryto disconnect the connection between the neutral point of the generator set and its sourceearth while the generator set is operating in parallel with the TN supply, in order to preventthe currents circulating.The Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 containsrequirements for the installation and operation of generating equipment that is to operatein parallel with a supply obtained from a system for distributing electricity to thepublic. Where the output of the on-site generation exceeds 16 A per phase, thedistributor must be consulted and the technical and safety arrangements for paralleloperation agreed. Such consultations may have an impact on the earthing arrangementsfor both the generating set(s) and the installation.

Where the output of the on-site generation does not exceed 16 A per phase, therequirement to consult with the distributor and agree the safety arrangements for paralleloperation is waived, subject to the following conditions being met (see ESQCR):

appropriate and necessary equipment is provided to prevent danger or interferencewith the network or with the supply to consumers

the consumers installation complies with British Standard Requirements, such asBS 7671

the source of energy is configured to disconnect itself electrically from the parallelconnection when the distributors equipment disconnects the supply of electricity tothe persons installation

the person installing the source of energy ensures that the distributor is advised ofthe intention to use the source of energy in parallel with the network before, or at thesame time of, commissioning the source.sulting Ens. Co.,Tue Jun 8 01:22:57 2010




Electrical Safety Councilsulting Ens. Co.,Tue Jun 8 01:22:57 2010


Topics referred to in this text:

None

Topics not referred to in this text, which are related and may be of interest:

G13-9 GENERATORS: Parallel operationG13-13 GENERATORS: Switching between supplies

BS 7671 (Requirements for electrical installations)Some of the most important requirements are found in:

Low voltage generating sets, Fault protection 551.4Low voltage generating sets, Protection against overcurrent 551.5Low voltage generating sets, Additional requirements for installationswhere the generating set provides a supply as a switched alternative tothe system for distribution of electricity to the public (standby systems) 551.6

GENERATORSContents1. Introduction2. Generator sets operating in parallel with each other3. Generator sets operating as a switched alternative to a supply obtained from the system for distributing electricity to the public4. Generator sets operating in parallel with a supply obtained from the system for distributing electricity to the public1. IntroductionThis topic provides guidance on the earthing arrangements for generator sets. In particular it covers arrangements to prevent circulating currents where generator sets operate:In general, the neutral point of a generator sets windings will need to be connected with Earth and, in such cases, a source earth electrode will be required for the generator set. Where there is more than one generating set, the neutral point of ...Alternative arrangements for connecting the neutral points of a number of generating sets with EarthFig 1

2. Generator sets operating in parallel with each otherWhere generator sets operate in parallel with each other and more than one generators neutral point is connected with Earth at the same time, harmonic currents (predominately third harmonic current), produced by the generators, can circulate betwe...Circulating harmonic currents where two generator sets operate in parallel with each otherFig 2Where the magnitude of the circulating current is problematic because it will cause overheating of the conductors or unwanted operation of a generator sets earth fault protection, the earthing arrangements can be adapted to overcome the problem by:(i) providing a switching arrangement to switch the connections between the neutral point of each generator set and Earth, such that the neutral point of only one of the on-line generating sets is connected with Earth at any given time (see Fig 3), or(ii) installing a suitable reactor in each of these connections to attenuate the harmonic circulating current (see Fig 4).

Where a switching arrangement such as that described in indent (i) and shown in Fig 3 is used, it must be interlocked such that fault protection and fault current protection are not impaired at any time (Regulation 551.5.2 refers).Where a reactor is installed in the connection between the neutral point of each generator set and Earth, such as described in indent (ii) and shown in Fig 4, the electrical system becomes an IT system.

Use of switching arrangementFig 3The earthing arrangement shown in Fig 3 is that generally used for a TN system, where a switching arrangement to prevent circulating currents is provided. This, however, is not the only option. For example, the neutral point of each generator sets...

Use of neutral point earthing reactorFig 4

3. Generator sets operating as a switched alternative to a supply obtained from the system for distributing electricity to the publicWhere a generating set is to operate as a switched alternative to a supply obtained from the system for distributing electricity to the public forming part of a TN system, the connection to the means of earthing of that system (which is provided by t...Furthermore, when the generating set or sets are selected as the source of supply, it may be necessary to disconnect the earth connection between the Main Earthing Terminal (MET) and the means of earthing provided by the distributor, since currents a...Circulating currents where a generator operates as a switched alternative to a supply from the system of distributing electricity to the publicFig 5Generally, these circulating currents are interrupted by switching the neutral, as well as the line conductors, when selecting the source of supply. However, where the neutral is not switched and circulating currents are a problem, they will need to ...Similarly, when the supply from the system for distributing electricity to the public is selected as the source, and the neutral is not switched, the circulating currents can be interrupted by disconnecting the connection between the neutral point of...It is important that any arrangement for switching the earth connections is interlocked such that fault protection and fault current protection are not impaired at any time.

4. Generator sets operating in parallel with a supply obtained from the system for distributing electricity to the publicWhere a generator set operates in parallel with a supply obtained from the system for distributing electricity to the public, currents as described in item 2 can circulate between the generator set and the source of the public supply. In such cases i...The Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 contains requirements for the installation and operation of generating equipment that is to operate in parallel with a supply obtained from a system for distributing electricity ...Where the output of the on-site generation does not exceed 16 A per phase, the requirement to consult with the distributor and agree the safety arrangements for parallel operation is waived, subject to the following conditions being met (see ESQCR):Topics referred to in this text:Topics not referred to in this text, which are related and may be of interest:BS 7671 (Requirements for electrical installations) Some of the most important requirements are found in:

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