Transcript Switchgear08a_Earthing_System

E54530
Earthing systems,
Harmonics Currents and GFP
Contents
 Basic Principle
3
 Earthing System Technique and Suitable Devices
o IEC 60364
o NEC and GFP
6
 GFP Technique
19
 Harmonics and Neutral conductor
o network studies
o Multi-source and Multi-grounding
o conclusion NEC
27
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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 PE
Insulation Fault current
E56885
Basic Principle
L1
L2
L3
N
PE
Measuring this fault current is the principle of Earthing System
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PE conductor
 The A point (star point of
LV transformer) can be:
o solidly grounded
o isolated
o linked with an impedance
to the PE
 Fault current
in A, the vectorial sum of
the current is equal to 0
SIph + IN+ I PE = 0
o 2 ways to measure If ( =IPE)
o don’t forget the Neutral
current
E56886
Basic Principles
A
L1
L2
L3
N
PE
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Basic Principles
Main Ideas
 Measuring the current in the PE conductor is the principle
of the Earthing System Techniques
 Any other Normal current mustn’t flow in the PE (especially the
Neutral current) in order to avoid the tripping nuisances
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Earthing System Techniques
E56887
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 60364
TT system
E56888
Definition
 The Neutral point of
LV transformer is directly
connected to an earth
electrode
L1
L2
L3
N
 The exposed conductive
parts of the installation are
connected to an electrically
separate earth electrode
PE
Rn
Ru
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Earthing system IEC 60364
Earth fault study
 The fault current generates
a dangerous touch voltage
Uc > UL
 The SCPD is usually not
suitable for eliminating this type
of 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)
Uo = 230 V
E56889
TT system
L1
L2
L3
N
400V / 230V
If =
11,5A
Rn
10 W
Exposed
conductive
metallic part
Ru
10 W
Uf =
115 V
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Earthing system IEC 60364
Definition
 The star point of the LV
transformer is directly
connected to an earth
electrode (source earth)
 The exposed conductive
parts of the installation are
connected by the PE to the
same earth electrode
E56890
TN system
L1
L2
L3
N
PE
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Earthing system IEC 60364
Definition
 The PE and Neutral
conductor are separate
E56890
TN-S system
L1
L2
L3
N
PE
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Earthing system IEC 60364
Definition
 The PE and Neutral
conductor are common
= the PEN
E56892
TN-C system
L1
L2
L3
PEN
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Earthing system IEC 60364
Earth fault study
 Fault current is equal to a
Phase/Neutral short-circuit
 The fault current generates
dangerous touch voltage
E56893
TN system
L1
L2
L3
N
PE
 Tripping of the circuit breaker
by Short Time Protection or
Magnetic
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Earthing system IEC 60364
Definition
 The star point of the LV
transformer is not connected
to an earth electrode
 The exposed conductive parts
of the loads are connected by
the PE conductor to a common
earth electrode
E56894
IT system
L1
L2
L3
N
PE
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Earthing system IEC 60364
Earth Fault Study
 There isn’t a fault current
 The fault is not dangerous
but it must be signaled
by PIM
E56895
IT system
L1
L2
L3
N
PE
Principle of PIM
 Injection of current,
 Measurement of IR (*).
(*) Insulation Resistance
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Earthing system IEC 60364
Principle of the FTD (*)
 Detection of fault current
Type of FTD (*):
 Movable tracking
 Fixed tracking
E56896
IT system
L1
L2
L3
N
PE
(*) Fault Tracking Device
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NEC / US Installation Standard
 Defines IT - or IG (Insulated Grounding) - System
and TN-S - or SG (Solidly Grounded) - System
 TN-S details
o Neutral conductor is not broken
o the protective conductor PE is achieved by bonding the metallic parts
of equipment
o protection against fire is achieved by GFP
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NEC / US Installation Standard
E56897
Example of an installation complying with the NEC
 Protection against fire
o the NEC imposes a maximum protection sensivity of: 1200 A
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Earthing System conclusions
IEC 60364
NEC
Earthing
System
TN-C
TN-S
TT
IT
idem
TN-S
I fault
High
High
Low
Very low
High
Protection
of people
SCPD
SCPD
RCD
No problem
SCPD
Fire
Protection
Forbidden
Not
recommended
Recommended
with RCD
Recommended
with RCD
GFP
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
o measurement of all Currents
(don’t forget the Neutral)
 GFP co-ordination
o with the CB Magnetic or ST tripping
o with the other GFP
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GFP Types
E56898
 Residual Sensing: vectorial
sum of the currents in the
phases and the Neutral
L1
L2
L3
 Source Ground Return: single
sensor placed on the Neutral
earth link
L1
L2
L3
N
E56900
 Zero Sequence: - for low
current detection - single
sensor placed all live
conductors
E56899
N
L1
L2
L3
N
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External CT installation
E56901
 3 poles CB on 3 phases + N:
on T option, the External CT
must be compulsory both
o installed on primary and
o connected on secondary
according to this diagram
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Upstream GFP must be
coordinated with any downstream
Short Circuit Protection
Device - SCPD - or GFP
 Discrimination types
o current and Time discrimination
with the respect of
- asymptote at 1200A
- 3000 A / 1s point
o logic (ZSI)
E56902
GFP coordination
GFP
upstream
GFP
downstream
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Discrimination rules
 Fuses
o GFP sensibility setting up > 10
to 15 times down. fuse rating
o GFP I²t function
 Circuit breakers
o GFP sensibility setting up > 1,5
times down. CB short
time setting up
o GFP short delay setting up
> down. CB breaking time
setting
E56903
Upstream GFP / downstream SCPD
T
Downstream Downshort stream Upstream
delay fuse 2 GFP 1
step 2
DI
30 %
I downstream
I upstream
step 1
I
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Discrimination rules
 Ampermetric
o up. GFP sensibility setting up
> 30% down. GFP senstivity
setting up
 Chronometric
o up. GFP short delay setting
up > down. SCPD breaking
time setting
o Dt1 (GFP1) > D t2 (GFP2)
+ dt2 (SCPD2)
E56904
Upstream GFP / downstream GFP
T
Downstream Upstream
GFP 2
GFP 1
30 %
3000 A
1s
step 2
step 1
I downstream
I up1200 A 3000 A
stream
I
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Fluorescent lighting
 Electrical diagram
E56915
E56914
 Is current
Ph
Is
FL
T
N
o capacitive impedance
 Harmonics breakdown
E56916
for harmonics current
o THDI = 53%
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Computer power supply
 Is current
E56891
E56913
 Electrical diagram
Is
C
T
o THDI = 115%
 Harmonics breakdown
E56917
o single phase loads
high level of harmonics
o I3=85%, I5=65%, I7=40%, I9=20%
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Harmonics current
Their effects
E56905
 On the equipment:
o capacitor, transformer, motor, alternator, …
o 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
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Harmonics current conclusion
IEC 60364
Earthing
System
TN-C
EM
Forbidden
disturbances PE and
Neutral are
together
(PEN)
NEC
TN-S
TT
IT
idem
TN-S
Good
But be careful
avoiding PE
and Neutral to
be in contacts
Very good
No problems
even if Neutral
and PE are in
contacts
Very good
No problems
even if Neutral
and PE are in
contacts
Good
But be
careful
avoiding
PE and
Neutral
to be in
contacts
There isn't any fault current in TT and IT System even if the PE
and Neutral are in contact
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Application
E56912
 Study of network
o one source / two sources
o with harmonics
o TN- S with Neutral conductor not broken (according to NEC standard)
o with GFP
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Harmonics current / Earth fault
(one source)
E56907
E56906
 Harmonics current without earth fault
no fault current measured by GFP
 Harmonics current and earth fault
the right fault current is measured by GFP
PE
N
GFP
Service ground
PE
Ground bus
N
GFP
Service ground
Ground bus
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Harmonics current / Earth fault
(two sources or more)
 Both the main circuit breakers are closed, the bus coupler
E56907
is open
GFP
GFP
Neutral
PE
Ground bus
Service ground
Loads
Service ground
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Harmonics current / Earth fault
(two sources or more)
E56909
 Harmonic current without earth fault
a "false" earth fault current is measured,
harmonic current are present throughout the metallic frames
GFP
GFP
Neutral
PE
Ground bus
Service ground
Loads
Service ground
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Harmonics current / Earth fault
(two sources or more)
E56910
 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 this
current may be present throughout the metallic frames
GFP
GFP
Neutral
PE
Ground bus
Service ground
Loads
Service ground
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How to improve Network dependability ?
E56911
 Dealing with the problem
by using particular GFP diagrams (Modified GFP, for instance)
 Avoiding to have the problem
by using a 4 poles switch on the bus coupler (to break the Neutral)
GFP
GFP
Neutral
PE
Ground bus
Service ground
Loads
Service ground
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How to improve network dependability ?
E56911
Breaking the Neutral conductor
 Is the more efficient solution
 Is the sole solution to avoid EM effects
GFP
GFP
Neutral
PE
Ground bus
Service ground
Loads
Service ground
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