Earthing systems,Harmonics Currents and GFP

E54530

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Contents

Basic Principle 3

Earthing System Technique and Suitable Devices 6 IEC 60364 NEC and GFP

GFP Technique 19

Harmonics and Neutral conductor 27 network studies Multi-source and Multi-grounding conclusion NEC

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Basic Principle

Definition The 3 phases The Neutral conductor N The Protection conductor PE

Role Active conductors: 3 phases + Neutral

Normal current in these conductors

Protection conductor PEInsulation Fault current

Measuring this fault current is the principle of Earthing System

E56885

L1L2L3N

PE

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Basic Principles

PE conductor The A point (star point of

LV transformer) can be: solidly grounded isolated linked with an impedance

to the PE Fault current

in A, the vectorial sum of the current is equal to 0Iph + IN+ I PE = 0

2 ways to measure If ( =IPE) don’t forget the Neutral

current

E56886

L1L2L3N

PE

A

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Basic Principles

Main Ideas

Measuring the current in the PE conductor is the principleof the Earthing System Techniques

Any other Normal current mustn’t flow in the PE (especially theNeutral current) in order to avoid the tripping nuisances

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Earthing System Techniques

The target Protection of people Protection against fire risks

The economical constraints Continuity of the power supply Avoiding tripping nuisance due to the EM Effects

IEC 60364 and NEC standards have got the same targets and the same constraints

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Earthing system IEC 60364TT system

Definition

The Neutral point of LV transformer is directly connected to an earth electrode

The exposed conductive parts of the installation are connected to an electrically separate earth electrode

Rn RuPE

E56888

L1L2L3N

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Earthing system IEC 60364TT system

Earth fault study The fault current generates

a dangerous touch voltageUc > UL

The SCPD is usually not suitable for eliminating this typeof fault(If = a few A eg from 1 to 10 A)

Earth Leakage Relays (ELR)must be used(Setting at 300 mA to 10 A)

400V / 230V

L1L2L3N

If = 11,5A

Ru10

Rn10

Uo = 230 V

Uf =115 V

Exposedconductivemetallic part

E56889

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Earthing system IEC 60364 TN systemDefinition The star point of the LV

transformer is directlyconnected to an earthelectrode (source earth)

The exposed conductiveparts of the installation areconnected by the PE to thesame earth electrode

E56890

L1L2L3NPE

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Earthing system IEC 60364 TN-S systemDefinition The PE and Neutral

conductor are separateL1L2L3NPE

E56890

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Earthing system IEC 60364 TN-C systemDefinition The PE and Neutral

conductor are common = the PEN

E56892

L1L2L3PEN

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Earthing system IEC 60364 TN systemEarth fault study Fault current is equal to a

Phase/Neutral short-circuit

The fault current generatesdangerous touch voltage

Tripping of the circuit breakerby Short Time Protection orMagnetic

E56893

L1L2L3NPE

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Earthing system IEC 60364 IT systemDefinition The star point of the LV

transformer is not connectedto an earth electrode

The exposed conductive partsof the loads are connected bythe PE conductor to a commonearth electrode

E56894

L1L2L3NPE

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Earthing system IEC 60364 IT systemEarth Fault Study There isn’t a fault current The fault is not dangerous

but it must be signaled by PIM

Principle of PIM Injection of current, Measurement of IR (*).

(*) Insulation Resistance

E56895

L1L2L3NPE

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Earthing system IEC 60364 IT systemPrinciple of the FTD (*) Detection of fault current

Type of FTD (*): Movable tracking Fixed tracking

(*) Fault Tracking Device

E56896

L1L2L3NPE

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NEC / US Installation Standard

Defines IT - or IG (Insulated Grounding) - Systemand TN-S - or SG (Solidly Grounded) - System

TN-S details Neutral conductor is not broken the protective conductor PE is achieved by bonding the metallic parts

of equipment protection against fire is achieved by GFP

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NEC / US Installation Standard

Example of an installation complying with the NEC

Protection against fire the NEC imposes a maximum protection sensivity of: 1200 A

E56897

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Earthing System conclusions

IEC 60364 NEC

Earthing TN-C TN-S TT IT idemSystem TN-S

I fault High High Low Very low High

Protection SCPD SCPD RCD No problem SCPDof people

Fire Forbidden Not Recommended Recommended GFPProtection recommended with RCD with RCD

TT and IT Systems naturally manage the Fire Risk by limiting the fault current

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Ground Fault Protection GFP

GFP types

External CT installation measurement of all Currents

(don’t forget the Neutral)

GFP co-ordination with the CB Magnetic or ST tripping with the other GFP

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Residual Sensing: vectorialsum of the currents in thephases and the Neutral

Source Ground Return: singlesensor placed on the Neutralearth link

Zero Sequence: - for lowcurrent detection - single sensorplaced all live conductors

GFP Types

E56898

E56900

E56899

L1L2L3N

L1

L2

L3

N

L1L2L3N

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External CT installation

3 poles CB on 3 phases + N:on T option, the External CTmust be compulsory both

installed on primary and connected on secondary

according to this diagram

E56901

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GFP coordination

Upstream GFP must becoordinated with any downstreamShort Circuit ProtectionDevice - SCPD - or GFP Discrimination types current and Time discrimination with the respect of

- asymptote at 1200A - 3000 A / 1s point

logic (ZSI)

E56902

GFPupstream

GFPdowns-tream

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Upstream GFP / downstream SCPD

Discrimination rules Fuses GFP sensibility setting up > 10

to 15 times down. fuse rating GFP I²t function Circuit breakers GFP sensibility setting up > 1,5

times down. CB shorttime setting up

GFP short delay setting up> down. CB breaking timesetting

E56903

Down-streamshortdelay

Down-streamfuse 2

UpstreamGFP 1

I30 %

I down-stream

I up-stream

T

I

step 2

step 1

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Upstream GFP / downstream GFP

Discrimination rules Ampermetric up. GFP sensibility setting up

> 30% down. GFP senstivitysetting up

Chronometric up. GFP short delay setting

up > down. SCPD breakingtime setting

t1 (GFP1) > t2 (GFP2)+ t2 (SCPD2)

E56904

DownstreamGFP 2

UpstreamGFP 1

I down-stream

I up-stream

T

I1200 A 3000 A

30 %3000 A1s

step 2

step 1

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Is current

THDI = 53% Harmonics breakdown

Electrical diagram

capacitive impedancefor harmonics current

Fluorescent lighting

E56915

E56914

E56916

T

FLIsPh

N

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Computer power supply

Electrical diagram

single phase loadshigh level of harmonics

E56913

E56891

E56917

Is

CT

Is current

THDI = 115% Harmonics breakdown

I3=85%, I5=65%, I7=40%, I9=20%

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On the equipment: capacitor, transformer, motor, alternator, … overload and overheating, loss of the available power,

added vibration On the installation

E M disturbance On the Neutral conductor

cumulating of harmonic 3 and their multiples

Harmonics currentTheir effects

E56905

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Harmonics current conclusion

IEC 60364 NEC

Earthing TN-C TN-S TT IT idemSystem TN-S

EM Forbidden Good Very good Very good Gooddisturbances PE and But be careful No problems No problems But be

Neutral are avoiding PE even if Neutral even if Neutral carefultogether and Neutral to and PE are in and PE are in avoiding(PEN) be in contacts contacts contacts PE and

Neutralto be incontacts

There isn't any fault current in TT and IT System even if the PEand Neutral are in contact

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Application

Study of network one source / two sources with harmonics TN- S with Neutral conductor not broken (according to NEC standard) with GFP

E56912

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Service ground

PE GFP

Ground bus

N

Harmonics current / Earth fault (one source) Harmonics current without earth fault

no fault current measured by GFP Harmonics current and earth fault

the right fault current is measured by GFP

Service ground

PE GFP

Ground bus

N

E56907

E56906

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Harmonics current / Earth fault (two sources or more)

Both the main circuit breakers are closed, the bus coupleris open

E56907

Service groundService ground

Ground busPE

GFP GFP

Neutral

Loads

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Harmonics current / Earth fault (two sources or more) Harmonic current without earth fault a "false" earth fault current is measured, harmonic current are present throughout the metallic frames

E56909

GFP GFP

PE

Service groundService ground

Ground bus

Neutral

Loads

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Earth fault with / without harmonic current the "right" earth fault current is not measured by the "right" GFP,

a "false" earth fault current is measured by the other one thiscurrent may be present throughout the metallic frames

Harmonics current / Earth fault (two sources or more)

E56910

GFP GFP

PE

Service groundService ground

Ground bus

Neutral

Loads

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Dealing with the problem by using particular GFP diagrams (Modified GFP, for instance) Avoiding the problem by using a 4 poles switch on the bus coupler (to break the Neutral)

How to improve Network dependability ?

E56911

GFP GFP

PE

Service groundService ground

Ground bus

Neutral

Loads

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Breaking the Neutral conductor Is the more efficient solution Is the sole solution to avoid EM effects

How to improve network dependability ?

E56911

GFP GFP

PE

Service groundService ground

Ground bus

Neutral

Loads