Medium voltage switchgear application guide

Electrical installations
contain receivers, the
power supply to which
must be controlled by
switchgear.
Medium voltage switchgear
application
T
guide
his behaviour guide is of based electrical on the equipment analysis of (transformers, phenomena arising motors, from etc.), the
type and during characteristics normal or of faulty the device running.Its best adapted aim is to to help your you needs. choose the
Switchgear provides
circuit electrical
protection,
disconnection and
control.
date
11/94
- B•3•2 -
revised
05/2004
■ Merlin Gerin
■ Square D ■ Telemecanique

Medium voltage switchgear application guide
CONTENTS
1
- Transformer 3
1 - 1 Switchgear to be used 3
1 - 2 Device characteristics 3
2 - Motor 7
2 - 1 Switchgear to be used 7
2 - 2 Device characteristics 7
3 - Capacitor 10
3 - 1 Switchgear to be used 10
3 - 2 Device characteristics 11
12 Switchgear Device characteristics to be used 4 - 3 Determination of a generator circuit breaker 13 15
4 - Generator 13
5 - Lines and cables 16
5 - 1 Switchgear to be used 16
5 - 2 Device characteristics 16
6 - Arc furnace 18
6 - 1 Switchgear to be used 18
6 - 2 Device characteristics 18
date
11/94
- B•3•2 -
revised
12/95
1: 2: motor transformer
3: 4: capacitor
5: lines generator
6: arc furnace and cables
Appendix 7: 8: diode fused switch-disconnector and thyristor applications and fused contactor association
7 - Appendices 20
page 2

Medium voltage switchgear application guide
1 - TRANSFORMER 1 - 1 Switchgear to be used
A fused switch-disconnector, fused contactor and circuit breaker are
usually used for operating and protecting high power transformers.
type power transformer controlled by
MV/LV P ≤ 2 MVA dry or immersed type fused switch-disconnector
transformer
or circuit breaker
or fused contactor
2 MVA < P ≤ 4 MVA dry or immersed type fused switch-disconnector
or circuit breaker
or fused contactor
MV/MV P ≤ 4 MVA dry type circuit breaker
transformer immersed type circuit breaker
P > 4 MVA immersed type circuit breaker
The switch and contactor make sure the transformer is switched on and off
during normal and overload running.
network The fuse short-circuit limits and breaks power. short-circuit currents generated by the upstream
The as short-circuit breaker currents. can make, withstand and break operating currents as well
The protection system (CT, VT, relay, release…) automatically causes tripping
when a fault is detected.
1 - 2 Device characteristics
The device must be able to:
■ withstand and operate the continuous operating current and
eventual overloads,
■ break the fault current at the connection point; on the transformer’s
secondary terminals,
■ withstand short-circuit switching and no-load transformer
switching peaks,
■ break the no-load currents without excessive overvoltage
(downstream open).
The control and/or protection device must be located upstream of the
transformer.
Transformer faults cannot be picked up by downstream protection alone.
A downstream control mechanism cannot isolate the fault transformer.
date
11/94
- B•3•2 -
revised
12/95
page 3

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Medium voltage switchgear application guide
1 - TRANSFORMER
(cont’d)
fig. 1: transformers feeder
MV/LV transformers
MV upstream busbars
date
11/94
MV busbars
fig. 2: transformers incomer
MV downstream busbars
- B•3•2 -
revised
05/2004
MV/MV transformer
THE DEVICE MUST BE ABLE TO:
■
EVENTUAL
WITHSTAND
OVERLOADS
AND OPERATE THE CONTINUOUS OPERATING CURRENT AND
rated The device current must and be eventual dimensioned overloads. to be able to withstand the transformer’s
The rated current Inis given
U:
in the
operating
following
voltage
equation:
Ir =
In specific 3
S (in kVA)
• U (in
transformer cases kV)
manufacturer where the must transformer supply overloads must operate likely in to overload, be applied the
device depending on the ambient temperature. to the
This The current overload value is expressed that should in be time taken and into as a account percentage is the of following: the rated power.
Ir overload= Ir •Xoverload
Examples:
630 ■MV/LV kVA MV/LV transformer transformer; tee-off (fig.
Unprimary:
1) 20 kV
Ir au primaire : control device 3 S
•
rating U = must
3630
• 20 = 18.18 A
The be higher than or equal to 18.18 A.
400 standardised values used by Merlin Gerin are:
If - 630 - 1250 - 2500 - 3150 A.
■a the control device is:
200 fused A and switch-disconnector: the real rating of the assembly the fuse association becomes the limits same this as current the fuse. to
■
device a fused contactor: it is impossible to use a fused contactor as an control
■a circuit since breaker: the contactor the rating has which a maximum we approve rated is voltage 400 A. of 12 kV.
3150 ■MV/MV transformer incomer (fig. 2)
temporary kVA MV/MV load of transformer; 20% one hour.
Un secondary= 5.5 kV operating with a
Ir secondary: 3150
Ir overload= 331
3 •
x
5.5 = 331 A
We shall choose a
1.2
circuit
= 397
breaker
A.
with a rated current of 630 A.
■
The
BREAK
breaking
THE
capacity
INSTALLATION’S
must be higher
FAULT
than
CURRENT
circuit current at the point of installation. or equal to the maximum short-
When must be a fused higher contactor than the current is used limited as a control by the device, fuse. the breaking capacity
The short-circuit current limited
Uk: transformer
by the transformer
short-circuit
is equal
voltage
to:
Ik = Ir transfo. x 100
Uk transfo.
You will find in appendix 1 some values of Uk
page 4

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Medium voltage switchgear application guide
1 - TRANSFORMER
(cont’d)
date
11/94
0
- B•3•2 -
revised
I 1 = current giving maximum
breaking capacity
I 2 = current giving the matching
arc energy conditions
I 3 = minimum breaking
current
05/2004
I r = rated current
definition of fuse operating areas
(see fuse technical leaflet)
Example:
Transformer: Secondary voltage: 10 MVA10 kV; primary voltage: 20 kV
Short-circuit
Insecondary: 577 voltage: A 9%
a) the device is upstream of the transformer
The standardised breaking capacity values must are: be 8 - equal 12.5 - to 16 or - higher 20 - 25 than - 31.5 the - network 40 - 50 kA.
Ik
b) the device is downstream of the transformer
The current breaking limited capacity by the transformer must be equal as in to the or following higher than equation: the short-circuit
Ik = In x 100
We shall
Uk
= 577 •
choose a device
9
100 = 6411 A
with a breaking capacity equal to 8 kA.
The Fuse fuse protection
network as makes a result sure of that a downstream short-circuit fault currents are broken. generated by the upstream
protection, In order to determine you must know: the required fuse rating to ensure transformer
■the ■power transformer’s (S in kVA), characteristics
■short-circuit ■rated current voltage with eventual (Ukin%),
■the characteristics of the fuse overload family (A).
■time/current characteristics at 0.1 s), used
■minimum ■the installation breaking and current operating (I3in conditions A).
open a cubicle, air,
■in
In order
fuse
to
chambers.
■choose the determine rated current the fuse of the rating, you must:
■in ■if the general: installation transfo. and Ir1.3 operating ≤fuseIr≤transfo. conditions are Ir1.5,
a fuse current rating immediately higher than transfo. not fully Ir1.5. known, choose
■check secondary that short-circuit transformer current switching does does melt not the melt fuse the with fuse the and equation: that the
IkxI3

Medium voltage switchgear application guide
1 - TRANSFORMER
(cont’d)
Transformer Circuit breaker protection protection
thresholds: is provided mainly by amperimetric relays with two
■an low immediate threshold high acts threshold on downstream acts on upstream faults. or internal transformer faults.
■must Different take threshold transformer settings:
account. switching, load current and fault current peaks into
■for and downstream the downstream protection protection is ensured. device so that selectivity between upstream
The ■thermal following image devices relays: are protection also used:
■overvoltage relays: protection against against overvoltage, overloads and overheating,
■earth ■tank protection overcurrent relays, relays,
■restricted ■differential earth relays. relays,
■
TRANSFORMER
WITHSTAND THE
SWITCHING
SHORT-CIRCUIT
PEAKS
SWITCHING CURRENT AND THE NO-LOAD
The of the device short-circuit making current capacity (Ik2.5 must according be higher to than IEC or standard). equal to the peak value
■
(DOWNSTREAM
BREAK NO-LOAD
OPEN)
CURRENTS WITHOUT EXCESSIVE OVERVOLTAGE
The No-load transformer transformer insulation breaking may is be like damaged small inductive by overvoltage. current breaking.
peak value must be limited. The linking circuit between the device The overvoltage
transformer plays an important role. The presence of cables (thus of and the
capacitances) be checked. greatly reduces overvoltage. Insulation co-ordination must
Such value overvoltage accepted by must standards. not be higher than 3.5 pu (1 pu =Ur3 2 ) , general
SF6 breaking devices are particularly well adapted to this utilization.
date
11/94
- B•3•2 -
revised
05/2004
page 6

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Medium voltage switchgear application guide
2 - MOTOR 2 - 1 Switchgear to be used
A fused contactor, contactor and circuit breaker are usually used
for operating and protecting motors.
M M M
type power or current controlled by
asynchronous I < 200 A fused contactor
I < 315 A
contactor
P < 2 MW
circuit breaker
synchronous P ≥ 2 MW circuit breaker
The fused operating contactor rate is is high, used to operate the motor when:
■the power operating is low voltage (P < 1,500 is lower kW), than or equal to 11 kV.
The short-circuit fuse breaks power. short-circuit currents generated by the upstream network
The circuit operate breaker rate is is low, used to operate the motor when:
■the power operating is high voltage (I > 315 is higher A), than 12 kV.
2 - 2 Device characteristics
date
11/94
- B•3•2 -
revised
05/2004
The control and/or protection device must be able to:
■ withstand and operate continuous operating current,
■ break the fault current,
■ withstand the short-circuit fault current (I k ),
■ break currents during starting without excessive overvoltage.
THE DEVICE MUST BE ABLE TO:
■
The
WITHSTAND
device must
AND
be dimensioned
OPERATE THE
to
OPERATING
be able to withstand
CURRENT
current.
the motor load
The rated current Ir is given in the following equation:
Ir (A) = U cos = ϕ= phase motor phase power voltage factor in kV
3
P (in kW)
• U • cos ϕ•η
η= motor output
Example:1160 kW motor under 6.6 kV; cos ϕ = 0.92; η = 0.94
I (A) = 3
P (in kW)
• U • cos
■fused We shall contactor: choose:
= 3 1160
ϕ•η • 6.6 • 0.92 • 0.94 = 117.35 A
■circuit breaker: 400 200 A. A or 250 A
page 7

Medium voltage switchgear application guide
2 - MOTOR(cont’d) ■ BREAK FAULT CURRENTS
fuse time-current curve
t
125 150 200 250
t d
I
I d I dm
date
11/94
- B•3•2 -
revised
05/2004
When capacity a fused must contactor be compatible is used with as the a control need arising device, from the co-ordination contactor breaking
the fuse and the different protective mechanisms. with
The short-circuit fuse breaking current capacity possible. must be higher than or equal to the maximum
When breaking a circuit capacity breaker must or be a higher contactor than alone the maximum is used as short-circuit a control device, current its
possible.
The
Fuse
specific
protection
motor which
type
they must
of stress
protect
that
and
the fuses
the network
must withstand
on which
comes
it is located.
from the
■stress ■rated current due to (Ir) the motor
■starting current time Id(Id= 6 to 7 Ir for direct starting)
tddepends on the
td
■the number of consecutive motor (values startings. between 1 and 30 seconds)
■stress ■rated voltage due to the (< 11 network
■prospective short-circuit kV). Only current. fuses with Ur ≤12 kV are used
■choice ■ideal starting of fuse current characteristics
The ideal starting current calculation Idmis equal
(Idm)
2.4 Idif the ratio Id/ to:
- 2 Idif the ratio Id/
Ir given by the manufacturer is a design value
This Idmvalue guarantees
Ir given two by consecutive the manufacturer start-ups is or a measured six start-ups value.
intervalsper hour.
at regular
■to record determine the Idmand the fuse starting rating
shown in figure 1 time value on the fuse time-current curve as
intersection - choose the value immediately higher than the Idmand tdright angle
On When figure the 1, fused the fuse contactor rating is to installed be taken in is the 250 cubicle, A.
by 20%.
Idmmust be increased
The Circuit motors breaker are protected protection
■thermal image against overloads, by the following protection devices:
■independent ■earth overcurrent time for overcurrent insulation for faults, short-circuits,
inverse ■against component slow start-ups maximum and rotor-locking. for unbalance,
The undervoltage motors must relay. be Ingeneral, protected against a single voltage relay is drops installed by on a time-delay
carries out load shedding to all the associated devices. the busbar and
page 8

Medium voltage switchgear application guide
2 - MOTOR(cont’d) ■ WITHSTAND THE FAULT SWITCHING CURRENT (Ik)
The equal contactor to the peak or circuit short-circuit breaker current making value. capacity It is equal must be to 2.5 higher Ikaccording than to the IEC.
■
The
BREAK
motors are
CURRENTS
sensitive
DURING
to overvoltage
STARTING
since,
WITHOUT
for technological
EXCESSIVE
reasons,
OVERVOLTAGE
are not as well insulated. The switchgear operation creates an overvoltage they
insulation for each start-up. which cause Repeated it to wear overvoltage down prematurely, creates weak if not points destroy in the it. turn
Devices and do not using need the motor SF6 technique protection do devices. not generate dangerous overvoltage
Rotating devices are arc likely techniques to provoke should higher be kept currents if they than work the well. rotating Puffer arc technique
therefore more dangerous overvoltage (see“selling points” folder, and
overvoltage file).
date
11/94
- B•3•2 -
revised
05/2004
page 9

Medium voltage switchgear application guide
3 - CAPACITOR 3 - 1 Switchgear to be used
A fuse-switch combination, fused contactor, circuit-breaker-switch
and circuit breaker are usually used for operating and protecting
capacitor banks.
type power I capacitance current
(1) controlled by
delta P ≤ 1050 kvar I ≤ 240 A fused contactor
connection U ≤ 11 kV I ≤ 160 A ISF1circuit-breaker-switch
bank
I ≤ 60 A
SM6 fixed fused
switch-disconnector
star 600 ≤ P ≤ 1050 kvar I ≤ 160 A ISF1 circuit-breaker-switch
connection 11.7 ≤ U ≤ 21.8 kV
bank 1050 ≤ P ≤ 4200 kvar I ≤ 240 A fused contactor
U ≤ 11 kV
1050 ≤ P ≤ 4800 kvar I ≤ 160 A ISF1 circuit-breaker-switch
U ≤ 21.8 kV
1050 ≤ P ≤ 21000 kvar 440 A 630 A circuit breaker
U ≤ 21.8 kV 875 A 1250 A circuit breaker
1750 A 2500 A circuit breaker
2200 A 3150 A circuit breaker
(1) capacitive current that the breaking device is capable of breaking
The on and switch off during and the normal contactor running. make sure that the capacitor bank is switched
The currents fuse-switch generated or fuse-contactor by the upstream combination network short-circuit make sure power that short-circuit
General protection and breaking device with the required breaking are capacity broken.
are The necessary circuit breaker even is if able capacitors make, are withstand designed and with break an operating internal fuse.
well as short-circuit currents. Fault tripping is carried out automatically currents by theas
protection The capacitor system banks (CT, are VT, either relay, mounted release, in etc.).
bank (fig. 2). a single bank (fig. 1),or in multi steps
The head
multi
circuit
steps capacitor
breaker ensuring
bank with
the
the
general
following
protection
arrangement is often used:
■a switch (ISF1)
control each tap. or circuit breaker (if the number of operations is low)
fig. 1: single bank
fig. 2: automatic banks
circuit breaker or
fused contactor
switch
or
circuit breaker
date
11/94
- B•3•2 -
revised
05/2004
banks with delta
or double star connections
banks with delta
or double star connections
page 10

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Medium voltage switchgear application guide
3 - CAPACITOR(cont’d) 3 - 2 Device characteristics
date
11/94
- B•3•2 -
revised
05/2004
The control and/or protection device must be able to:
■ withstand the continuous operating current,
■ break the fault current,
■ withstand the network switching current and switching peaks
generated when power to the capacitors is switched on,
■ make and break without excessive overvoltage.
THE DEVICE MUST BE ABLE TO:
■
The
WITHSTAND
device must
THE
be dimensioned
CONTINOUS OPERATING
to be able to
CURRENT
capacitor bank rated current. withstand 1.43 times the
The Irdevice rated = current Ircapacitor of the x device 1.1 x 1.3 is given = I capacitor by the following x 1.43 equation:
1.3 currents. to take into account overheating due to the presence of harmonic
1.1 The take rated into capacitive account current the tolerance must be over lower the than capacitance the capacitive value.
the device is able to break (value given by the manufacturer). current that
Irdevice Example:Ircapacitor = 100 A
The = I capacitor x 1.43 = 100 •1.43 = 143 A
than breaking 143 A. device must have a rated current value equal to or higher
The following control devices are the ones we would choose:
■for switch: ISF1 (Ibrokencapacitor =160 A)
■a circuit a contactor: breaker Rollarc with a 630 (Ibrokencapacitor A rating (Ibrokencapacitor = 240 A) = 440 A).
■
The
BREAK
breaking
THE
capacity
FAULT CURRENT
maximum short-circuit current of the device possible. must be higher than or equal to the
When be higher the device than the is current a fused limited contactor, by the the fuse. contactor breaking capacity must
The
Fuse
fuse
protection
bank. This
rating
coefficient
must be
takes
between
into account
1.7 and
a
1.9
downgrading
times the rated
of 1.3
current
due to
of
the
the
Ircapacitor
presence of
x
harmonic
1.7 ≤Irfuse
currents
≤Ircapacitor
and the derating
x 1.9
due to the inrush currents
Protection against overloads must also be provided on each phase.
Protection Circuit breaker against protection
phase. short-circuits and overloads must be provided on each
page 11

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Medium voltage switchgear application guide
3 - CAPACITOR(cont’d) ■
The
WITHSTAND
making capacity
THE SWITCHING
of the device
CURRENT
greatest of the following values: must be equal to or higher than the
■Ipnetwork ■bank switching (2.5 Ik
current according Ie. to IEC)
The bank switching current I e must always be less than I capacitor x 100.
Otherwise arcing contact it must wear. be limited by installing dumpingreactors in order to reduce
This current plays an important role in the mechanical endurance of the device.
maximum
Example:Rollarc
number contactor, of operations
Ir = : 400 300 A; 000 Ibrokencapacitor in mechanical latching. = 240 A;
Imax.
number
switching
of operations
current: 8 kA at peak;
The determination of switching
Imax. switching
current
current=
values 10,000. is given in appendix 3.
■ MAKE AND BREAK WITHOUT EXCESSIVE OVERVOLTAGE
If 1 pu =Ur
the overvoltage 3
2
single bank: must 2 not pu be higher than:
■for a automaticbank (n identical taps):
2 n 2 + n1 SF6 devices = pu
are especially well adapted to this application.
date
11/94
- B•3•2 -
revised
05/2004
page 12

Medium voltage switchgear application guide
4 - GENERATOR 4 - 1 Switchgear to be used
The control device usually used for operating and protecting
generators is a circuit breaker.
The ■low two power most station current generator uses for the protection generator circuit breaker are:
In (generator/transformer practical cases, power set). station A VHV generators circuit breaker are built is used as self-contained to operate and units
■protection them.
circuits in the of event auxiliary of a generators distribution providing network failure. a power supply to priority
In Its this characteristics case, the circuit are those breaker described used in as chapter a transformer 1 (transformer). protection incomer.
power station generator
G
generator
distributor power supply
power
generator
G
essential
power supply
MV circuit breaker
LV
transformer
MV
MV capacitors
MV
transformer
VHV
VHV circuit breaker
VHV network
4 - 2 Device characteristics
date
11/94
- B•3•2 -
revised
05/2004
When in use, the generator circuit breaker comes up against very
different operating conditions. It must:
■ withstand high load currents (overheating),
■ break strong and highly asymmetrical fault currents which may
be higher than 100% (generator terminal fault),
■ close on very high currents generated by the asymmetry,
■ withstand transient recovery voltage with highly increasing speeds,
■ withstand phase displacement.
DIFFERENT IN OPERATION, CONDITIONS THE GENERATOR OF RUNNING. CIRCUIT IT MUST BREAKER BE ABLE MEETS TO: VERY
■
The
WITHSTAND
rated current
HIGH
is given
LOAD
by
CURRENTS
the following equation: I ≥
3 S
• U
page 13

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Medium voltage switchgear application guide
4 - GENERATOR(cont’d) ■
fig. 1: evolution of a generator
short-circuit current
I (kA)
I dc
t o
time
to = circuit breaker switching time
+ relay time
Iac aperiodic component
Idc = component
date
11/94
- B•3•2 -
revised
05/2004
I ac
BREAK HIGHLY ASYMMETRICAL FAULT CURRENTS
When current a speed short-circuit is the one occurs represented on a network in figure close 1. to a generator the fault
It values is essential at the moment to know the the evolution circuit breaker of the arcing short-circuit contacts current separate: and the
aperiodic Current asymmetry
(Idc) and can periodic reach
(Iac)
very component high values, value.
which delays the natural passages of the current sometimes through zero higher that than are100%,
In necessary highly asymmetrical for breaking.
asymmetry for a total asymmetrical breaking, the current SF6 technique equal to is 2 times limited the to peak 100%
symmetrical short-circuit current.
•
Iasym. =Isym. 1 + 2A2 with A = % asymmetry =Idc 100
The or equal breaking to the capacity maximum of short-circuit the generator current circuit possible. breaker
Iac peak
must be higher than
The value short-circuit to be taken current into supplied account is by the the greatest network, of:
■the asymmetrical current of the generator at the moment the contacts open.
is
Example:when
29 kA with 80% the asymmetry contacts separate, at the periodic short-circuit current
to
Asymmetrical current = Isym.•
29 •1.35
1
=
+ 2A2
39.15
=
kA
29 • 1 + 2 80% 2
We The short-circuit current supplied by the network is 25 kA.
If shall choose a circuit breaker with a breaking capacity equal to 40 kA.
the the breaking capacity required is higher than the breaking capacity of
order device, one solution consists in delaying circuit breaker switching in
open due to avoid to a an fault. overly high asymmetry when the circuit breaker has to
■
The
SWITCHING
making capacity
WITH
must
VERY
be
HIGH
higher
CURRENTS
short-circuit current made. than or equal to the peak value of the
The ■2.5 value times to the be breaking taken into capacity account of is the the circuit greatest breaker, of the following:
asymmetry ■the peak value of the of generator the short-circuit current (figure current, 1). taking into account the
■
The
INVERSION
short-circuit
OPERATION
breaking capacity current given value by the in manufacturer phase inversion of the must circuit be lower breaker. than the
■
The
TRANSIENT
TRV values
RECOVERY
as well as the
VOLTAGE
TRV increase
VALUES
time
(TRV)
the circuit breaker in all types of situations (see circuit must breaker be compatible technical with
guide § 5.11).
page 14

Medium voltage switchgear application guide
4 - GENERATOR(cont’d) 4 - 3 Characteristics required for the determination
of a generator circuit breaker
must To be be able satisfied: to select a generator circuit breaker, the following characteristics
■the ■generator rated current power in in MVA continous or kVA, operation
■operating ■generator voltage rated current. in kV,
short-circuit asymmetrical symmetrical current and current the continuous rms kA
closing current in peak kA component value
rated peak value withstand of the current TRV (in (rms peak kA/1 kA) s)
■the rated operating cycle (O- 3 mn - CO as - well 3 mn as - the CO increase advised). time in µs
The following generator values circuit are breaker known: can operate in phase inversion which implies
■the short-circuit peak value TRV current, and increase time.
date
11/94
- B•3•2 -
revised
05/2004
page 15

Medium voltage switchgear application guide
5 - CABLES OR
CABLES/LINES
5 -1 Switchgear to be used
A switch, fused switch-disconnector and circuit breaker are usually
used to operate and protect lines and cables.
lines feeder
MV busbars
lines incomer
MV upstream busbars
MV downstream busbars
type current controlled by
cables I ≤ 630 A switch
I ≤ 200 A
I ≤ 3150 A
5 - 2 Device characteristics
fused switch-disconnector
circuit breaker
The operating and/or protection device must be able to:
■ carry the load current,
■ withstand the short-circuit current,
■ break the fault current if the device is an incomer,
■ operate weak currents (no-load cables or lines) without
excessive overvoltage.
THE DEVICE MUST BE ABLE TO:
■
The
CARRY
rated current
THE LOAD
depends
CURRENT
receivers installed on the outlet directly feeder. on the supply power and voltage of the
Installed power: S = U I3 ⇒ I = US3
date
11/94
- B•3•2 -
revised
05/2004
page 16

Medium voltage switchgear application guide
5 - CABLES OR
CABLES/LINES(cont’d)
■
The
BREAK
breaking
THE
capacity
INSTALLATION’S
must be higher
FAULT
than
CURRENTS
circuit current possible at the installation point. or equal to the maximum short-
If the device is used as an incomer
The circuit breaking current capacity possible must at the be installation higher than point. or equal to the maximum short-
The on the short-circuit type and current section limited of the conductors. by the overhead or underground link depends
Ik = 3 U•Zk ■for X 0.4 an ohm/km overhead = U
3 • R2 +X2
link
R = ρL
Three-phase ■for an S with ρ= 3.310-6 ohm cm2
underground cables: link
for Almelec
Single-phase R = ρL cables: X = 0.1 0.1 to to 0.15 0.20 ohm/km ohm/km
S with ρ=2.810-6 ρ=1.810-6 ohm ohm cm2
cm2
for copper
for aluminium
If the device is used as an outlet
The protection switch device does not located have upstream a short-circuit which current provides breaking protection capacity. against It is shortcircuits.
the
the It must time however needed be to eliminate able to withstand it. the short-circuit current induced for
If or protection the fuse-switch against combination short-circuits must is required be used. the fused switch-disconnector
the The short-circuit breaker current must at have the connection a breaking capacity point. higher than or equal to
■
Applying
OPERATE
voltage
WEAK
to a
CURRENTS
no-load line
WITHOUT
can be
EXCESSIVE
compared
OVERVOLTAGE
to capacitor switching.
date
11/94
- B•3•2 -
revised
05/2004
page 17

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Medium voltage switchgear application guide
6 - ARC FURNACES 6 - 1 Switchgear to be used
The ISF2 switch-circuit breaker was designed for arc furnace application.
The maximum characteristics of the ISF2 are given in the table below.
max. voltage max. load breaking no. of daily
power current capacity operations
180 MVA 40.5 kV
156 MVA 36 kV 2500 A 20 kA 200
104 MVA 24 kV
6 - 2 Device characteristics
date
11/94
- B•3•2 -
revised
05/2004
The switch must be able to:
■ operate the load current,
■ break the installation’s short-circuit currents,
■ operate weak currents (no-load transformer) without
excessive overvoltage,
■ carry out a large number of operations.
THE DEVICE MUST BE ABLE TO:
■
The
OPERATE
rated current
THE
depends
LOAD CURRENT
Furnace power: S = U I 3
directly on the furnace supply voltage and power.
⇒ I =
S: U: apparent primary voltage power of of furnace US3
transformer
Example:
Furnace Supply voltage: power: 30 100 kVMVA
I = US3 = 30100
• 3 •10 3 = 1924 A
■ BREAK THE SHORT-CIRCUIT CURRENTS
furnace switch current must may be able be calculated to break the in electrode a perfectly short-circuit general way. current.
The short-circuit impedance is essentially due to the LV links.
Zk LV = 2.510-3 Ω whatever the furnace size is
The primary short-circuit power is:
Sk MV=UMV •Ik MV• 3 andUMV=Zk •Ik
The primary short-circuit current is:
Ik MV = UMV withZk MV = UMV
3 •Zk MV
Ik MV =UMV (ULV)2
3 1
•
(UMV)2 • Zk LV
ULV 2 • Zk LV
page 18

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Medium voltage switchgear application guide
6 - ARC FURNACES
(cont’d)
■
OVERVOLTAGE
OPERATE TRANSFORMER NO-LOAD CURRENTS WITHOUT EXCESSIVE
In transformer: the melting 50% process of the (see case appendix upon opening 6) a and switch 90% operates of the case with upon the no-load closing.
Operating This overvoltage overvoltage arises must during be furnace limited when transformer the arc operation. furnace is It installed.
to the progressive destruction of the transformer’s internal insulation contributes
reduces its lifespan.
and
The (installation network of must overvoltage be adapted protection to the breaking capacitors device’s for example). characteristics
With The means SF6 technology of limiting operating overvoltage overvoltage are simple. is low.
A simple RC circuit placed upstream of the furnace transformer is sufficient.
■
In the
MECHANICAL
melting process
AND ELECTRICAL
a switch operates:
ENDURANCE
with currents between 0.8 Iloadand 2.6 Iloadand
50% of 90% the case of the upon case opening
closing with load currents of 2.5 upon
Iload
The mechanical endurance of the ISF2 is 50,000 operations.
The supplying electrical the endurance furnace transformer depends during on the the average process value and of the the number current
operations carried out under this current. of
The load expected current is lifespan given by of the the curve poles below. (number of CO cycles) depending on the
the expected lifespan of the poles
(number of FO cycles)
Example:for
supplied by a 52 an MVA arc furnace
in accordance with the load current
under 30 kV with a load transformer
cycles
1000 A, the expected lifespan current
50,000
(or of an 50,000 ISF2 is CO 25,000 cycles CO if the cycles
45,000
set is replaced). pole
40,000
35,000
30,000
25,000
0
1
2
20,000
15,000
10,000
5,000
I (A)
date
11/94
- B•3•2 -
revised
05/2004
500 1,000 1,500 2,000 2,500
0 curve without replacement of the poles
1 curve with 1 replacement of the poles
2 curve with 2 replacements of the poles
page 19

Medium voltage switchgear application guide
7 - APPENDICES Appendix 1: transformer
Appendix 2: motor
Appendix 3: capacitor
Appendix 4: generator
Appendix 5: cables
Appendix 6: alternating current melting arc furnaces
Appendix 7: diode and thyristor applications
Appendix 8: fused switch-disconnector or fused contactor
date
11/94
- B•3•2 -
revised
05/2004
page 20

Medium voltage switchgear application guide
Appendix 1:
transformer
A transformer is defined by its rated power, its rated primary and
secondary currents, its rated primary and secondary voltages, its
insulation level, its short-circuit voltage and its coupling.
The peak-switching current is also a parameter that should not be
forgotten in the determination of protective switchgear.
1 - RATED POWER
The
S = U
rated power (S in kVA or MVA) of a transformer is equal to:
• 3 • I r
2 - RATED CURRENT OF ONE WINDING
The rated current Inis equal to:
I r = 3 S •U
3 - RATED VOLTAGE AND INSULATION LEVEL OF ONE WINDING
This in “no-load” is the specified operating voltage between to be the applied terminals (primary) of one or transformer developed (secondary)
The rated primary voltage must be higher than or equal to that of the winding. network.
rated
voltage
U r (kV)
insulation level
rms kV (50 Hz - 1 mn) kV impulse (1.2/50 µs)
7.2 20 60
12 28 75
17.5 38 95
24 50 125
36 70 170
4 - SHORT-CIRCUIT VOLTAGE
The short-circuit short-circuit current voltage to be (expressed calculated. in %) allows the transformer’s secondary
The short-circuit current is given in the following equation:
•
I k =In
Uk: Ir: rated
U
100
short-circuit current
k
andP k =U r •I k • 3
in voltage Ampsin %
given The usual in the Ukvalues tables on depending the following on transformer page. voltage and power are
The real values must be requested from the manufacturer.
date
11/94
- B•3•2 -
revised
05/2004
page 1

Medium voltage switchgear application guide
Appendix 1:
transformer(cont’d)
For a MV/LV transformer
transformer
no-load secondary voltage
power U k (% U k (%)
(kVA) 410 V 237 V
50 to 630 4 4
800 4.5 5
1000 5 5.5
1250 5.5 6
1600 6 6.5
2000 6.5 7
2500 7 7.5
3150 7 7.5
For a MV/MV transformer
transformer U k primary secondary
power voltage voltage
(MVA) (%) (kV) (kV)
5 8 24 or 36 7.2 or 12 or 17.5 or 24
10 9
20 11
25 12
5 9 72.5 7.2 or 12 or 17.5 or 24
10 9
20 9.5
25 9.5
5 - COUPLING
The ■the coupling respective is a connection conventional modes symbol of the indicating:
■The angular gap between the two vectors high representing and low voltage the voltage windings,
between the neutral point (real or ideal) and the corresponding high and values
voltage terminals. This phase displacement is expressed by a time index. low
connection HV LV
mode symbol symbol
delta D d
star Y y
zig-zag Z z
neutral out N n
To give low you powers an example, (25 to 160 the kVA): most Yzn11 widely or used Dyn11; couplings are the following:
■for medium high powers: powers Yyn11, (250 Ydn11 kVA to or 3150 Dyn11. kVA): Dyn11;
date
11/94
- B•3•2 -
revised
05/2004
page 2

Medium voltage switchgear application guide
Appendix 1:
transformer(cont’d)
6 - SWITCHING CURRENT PEAK
When peaks the are power produced. is switched on, very high signals or switching
Ie
I r
date
11/94
- B•3•2 -
revised
05/2004
transient state of the current during no-load transformer switching
is They very may quickly be 14 absorbed. times greater The time than constant the rated of current. the damp This down switching depends current
the winding resistance and the secondary load. The real values must beon
requested from the manufacturer.
Time constant and peak-switching current values depending on the immersed transformer power
(high voltage switching).
transformer
power
(kVA)
n e = I e peak
I r rms
100 14 0.15
160 12 0.20
250 12 0.22
400 12 0.25
630 11 0.30
800 10 0.30
1000 10 0.35
1250 9 0.35
1600 9 0.40
2000 8 0.45
5000 8 0.50
10000 8 0.50
15000 8 0.50
time
constant
(s)
20000 8 0.50
and In the the event time of constant low voltage divided switching, by 1.5 in the relation ratio neshould to the values be multiplied given in the by 2
table These above.
protection switching is defined. peaks must be taken into account when the type of
The transformer may have its own optional devices.
following: Depending on the type, the transformer’s own protective are the
gas ■the leak, waterproof pressure protection and temperature unit for complete detector. filling transformer:
■Buchholz gaz leak and protection pressure relay detector. for transformer fitted with a conservator:
monitoring. ■thermostat, thermometer for immersed-type transformers: temperature
■thermostatic probes for dry-type transformers: temperature control.
t
page 3

Medium voltage switchgear application guide
Appendix 2: motor
A motor is defined by its mechanical power, its voltage, its
insulation level, its starting type (torque, current, time) as well as
1 - RATED POWER
calculate A motor’s the power rated is the power mechanical by taking power the output which into it develops. account. We may
The
P = U
rated power (P in kW) of a motor is equal to:
• 3 •In •cos ϕ• η
U: Ir: rated phase current to phase in Amps; voltage in V;
cos ϕ: power factor (generally cos ϕ
η: efficiency (generally η: 0.94 is used) = 0.92)
2 - RATED CURRENT
The rated current Iris equal to:
I r = P
3 •U •cos ϕ • η
3 - RATED VOLTAGE AND INSULATION LEVEL
The insulation rated level. voltage must be higher than that of the network as well as its
rated insulation
voltage level
U r (kV) rms kV (50 Hz - 1 mn) impulse kV (1.2/50 µs)
3 2 U + 1 kV * 4 U + 5 kV *
3.3 2 U + 1 kV * 4 U + 5 kV *
5 2 U + 1 kV * 4 U + 5 kV *
6.6 2 U + 1 kV * 4 U + 5 kV *
10 2 U + 1 kV * 4 U + 5 kV *
11 2 U + 1 kV * 4 U + 5 kV *
* for modern motors
☞ note:the real values should be requested from the manufacturer.
date
11/94
- B•3•2 -
revised
05/2004
page4

Medium voltage switchgear application guide
Appendix 2: motor
(cont’d)
4 - DIFFERENT MEDIUM VOLTAGE MOTORS
Alternating and 100 kW current to 40 MW medium ranges. voltage motors are situated in the2 kV to 14 kV
The applications motors can are be given classed in the in table 4 families below. whose main characteristics and
motor characteristics, advantages applications
type
drawbacks
asynchronous ■ highly robust due to their ■ intensive use
motors with simple construction ■ aggressive or
single cage, ■ hardly any on-load speed dangerous atmosphere
double cage,
or deep slots
variation
■ high quantity of reactive power
absorbed with weak load
asynchronous ■ easy optimisation of starting ■ machines with high starting
motors with torque or signal torque
widing rotor ■ rotor resistances and rings needing ■ machine needing strong
adjustment and maintenance
counter-current breaking
■ power factor usually
■ tending to give way to
lower than 0.9
double cage motors
synchronous ■ same technology as ■ P > to 2000 kW
motors
alternators
■ high transient state
■ good output and good power
factor
synchronised ■ mixture of two previous ones: ■ tending to disappear
asynchronous good starting torque and good
motors
power factor
■ complex starting co-ordination
5 - DIFFERENT MV ASYNCHRONOUS MOTOR STARTING TYPES
J L
J C J L
J C
J N
J L J L
J R3
M
R 3
J R2
R 2
J R1
M
M
M
R 1
date
11/94
- B•3•2 -
revised
05/2004
direct auto-transformer reactance rotors
6 - ASYNCHRONOUS MOTOR CHARACTERISTICS
Direct characteristics
asynchronous starting
voltage
are motors
power
The the most widely used.
starting current
influencing main characteristics motor operation no-load current
are opposite. given in the table
asynchronous motor
2 kV to 14 kV
100 kW to 40 MW
6 to 7 times motor I n
≈ 0.3 times motor I n
starting current cos ϕ 0.1 to 0.2
no-load current cos ϕ ≈ 0.1
load current cos ϕ 0.7 to 0.9
page 5

Medium voltage switchgear application guide
Appendix 3: capacitor
A capacitor bank is defined by the power, the voltage, the insulation
level and the rated current. Depending on the connection mode
(single or multi-steps bank), the switching current is an important
parameter which must be taken into account.
1 - REACTIVE POWER
The reactive power (Q in kvar or Mvar) of a capacitor bank is equal to:
Q =Un • 3 •Ic orU r
2 •C•ω
Ur: Ic: rated rated current voltage or of current the bank absorbed network by the voltage capacitor of the network (kV)
C: capacitance
ω = 2 •π •ƒnetwork= network pulsation
2 - RATED CURRENT
The rated current Icis equal to:
Ic = Qc
3 •Un
3 - RATED VOLTAGE
The insulation rated level. voltage must be the same as that of the network as well as its
rated voltage
U r (kV)
insulation level
rms kV (50 Hz - 1 mn) impulse kV (1.2/50 µs)
7.2 20 60
12 28 75
17.5 38 95
24 50 125
36 70 170
date
11/94
- B•3•2 -
revised
05/2004
page 6

Medium voltage switchgear application guide
Appendix 3: capacitor
(cont’d)
date
11/94
- B•3•2 -
revised
05/2004
4 - SWITCHING CURRENT
The current switching of the capacitance. current must always be lower than 100 times the rated
It is generally limited to 10 kA peak by impulse reactors.
The switching current value depends on the type of bank used. It is equal to:
1) for a single bank:
Ie = 1
L0 •C
Ie =Ic • 2 •
S •I L
A B
c • • 2 1ω
I c
k
G
U
Ie: C: peak-switching capacitance of one current
L0: network short-circuit tap
Sk: supply short-circuit power reactor in MVA
Qc: S capacitor = 3 •U •Isc bank with power U k
3 =L0 in Mvar
•Isc =U2
• ω
ω = 2 •π •ƒnetwork= network pulsation
2) for a multi-steps bank with n identical steps:
1 2 n + 1
l l l
C C C
U: Ie: peak-switching network phase to current
n: number of energized phase tapsvoltage
C: l: link capacitance reactor of one tap
ω = 2 •π •ƒnetwork= network pulsation
ƒinherent = 2
1
l • π • •C
C
L0 • ω
Qc
Ie =Ic • 2 •
Ie = 2
n + n1 ƒinherent
3 •U •
n + n
• ƒnetwork
1 c • l
page 7

Medium voltage switchgear application guide
Appendix 4: generator
A generator is defined by its power, its voltage and its internal
impedance.
1 - RATED POWER
The
S
rated power (S in kVA or MVA) of a generator is equal to:
In
r = U • 3 •Ig
large power stations, the power ranges between 500 and 1500 MVA
2 - RATED CURRENT
The rated current is equal toIg = 3
S
•U
r
3 - RATED VOLTAGE
Generator standardised. voltage — contrary to distribution network voltage — is not
There a variety of rated voltage values for generators.
generator power
(MVA)
S n < 200 10 and 16
200 - 400 14 and 21
400 - 600 16 and 23
600 - 1000 18 and 26
S n > 1000 20 and 27
generator rated voltage
(kV)
4 - INTERNAL IMPEDANCE
Manufacturers The resistance generally values are specify always generator negligible impedance before the reactance expressed values. in %.
subtransient reactance reactance (X''d ≈20%),
■the earth reactance (X'd ≈30%), (X'o ≈6%).
These making values capacities will allow as well us to as determine protection the setting. circuit breaker breaking and
Ik: (three-phase) short-circuit kA current
Ur: Sr: generator generator rated rated power voltage (MVA)
X'cc: transient reactance expressed (kV) in % by the manufacturer.
date
11/94
- B•3•2 -
revised
05/2004
I k = 3
S
•U
r •
r X' 1
cc = 345
•10 100 30 •
= 8.66 kA
page 8

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Medium voltage switchgear application guide
Appendix 5: cables
MV cables
single-pole cable
individually screened
three-pole cable
screen
belted three-pole cable
conductor
The purpose of all cable systems is to allow the transit of electrical
energy which implies that in order to determine the system layout,
it is necessary to know the following basic data:
■ rated voltage,
■ laying procedure,
■ operating and short-circuit currents,
■ conductor type.
1 - RATED VOLTAGE
The Uo: rms rated rated voltage power is defined frequency by voltage three values.
the earth or the metal screen. between the phase conductor and
U: Um: rms rms rated maximum power power frequency frequency voltage voltage between between two phase two phase conductors.
Belted cables have the same phase to phase and phase to earth conductors.
cables are only used for voltages of 10 kV or less. insulation.
These three values are written: Uo/U (Um) kV
Example:
individually belted cable: screened 6/6 (7.2) cable: kV 12/20 (24) kV
2 - OPERATING AND SHORT-CIRCUIT CURRENT
When temperature calculating at the the surface allowable of the current conductor for a and cable, screens the maximum must be allowable
account:
taken into
cable, ■allowable its section permanent and how operating it is laid. temperature: this depends on the type of
screens ■allowable (phase-earth short-circuit fault) current: must the be dimensioned conductor sections so that (phase the maximum fault) and
temperatures circuit temperatures. reached during the fault remain below the allowable short-
3 - CABLE-LAYING PROCEDURE
The manufacturer characteristics correspond of allowable to specific steady ambient operating temperatures currents indicated and cable-laying by the
conditions The ambient which temperatures are taken are as a sometimes reference.
so that they heat each other or cool down with higher greater and the difficulty. links are often laid
the So that currents allowable must be adjusted temperatures in accordance of the cable with constituents the installation are respected, conditions.
date
11/94
- B•3•2 -
revised
05/2004
page 9

Medium voltage switchgear application guide
Appendix 5: cables
(cont’d)
MV cables capacitance
single-pole cable
individually screened three-pole cable
K
C
C
C
belted three-pole cable
4 - TYPE OF CONDUCTOR
A and cable mechanically is comprised united of either conductors. one conductor, or several electrically distinct
The ■single-pole different types individually of cables screened insulated cables, by extruded solid dielectrics are:
■individually ■belted three-pole screened cables. three-pole cables,
■the The metal type part of metal: of the annealed conductor electrolytic is defined copper by:
annealed aluminium, or 3/4 hard or sometimes
■electric ■self-induction resistance coefficient per unit (L of in length H/km) (R which in ohm/km),
geometrical size and the relative disposition of depends the conductors. essentially on the
■the rated cables section capacitance. (mm2),
5 - MV CABLES CAPACITANCE
The capacitance of a cable depends on the construction type:
■ single-pole
cable capacitance
cables:the
C is that conductor measured is between surrounded the conductor by a screen and and the the
which is earthed.
screen
■ individually screened three-pole
by a screen and the cable capacitance
cables:each
is that measured conductor between is surrounded
conductor and its individual screen which is earthed. each
■ belted three-pole
there is a capacitance
cables:a
K between screen conductors surrounds and a capacitance three conductors C between and
a conductor and the screen which is earthed.
Ic= The 3 capacitive C ωV which current causes Icof capacitance a network in C the only event to intervene of an earth whatever fault is:
cable type.
the
In practice, C for for a star-connected individually screened capacitance, cables. the cable makers indicate:
Upon ■the value request, 3 K the + C cable for belted maker cables.
measured between the three inter-connected will tell you the conductors capacitance and the C2(C2= metal sleeve. 3 C)
date
11/94
- B•3•2 -
revised
05/2004
page 10

Medium voltage switchgear application guide
Appendix 6: alternating
current melting arc
furnaces
1 - MELTING PROCESS
Arc furnace operating principle
The operations standard including processing scrap cycle metal of loading, an arc furnace melting, comprises liquid steel a refining series ofand
casting.
alternating current melting arc furnace
scrap metal
loading
melting
liquid bath
refining
casting of liquid
steel in ladles
and inter-casting
intermediary
reloading
furnace ■scrap is metal stopped. loading For and a single intermediary processing loading cycle, is carried two or three out when basket the
loads ■scrap are metal required.
part of the energy melting is consumed. during which the solid load is liquefied and the main
■liquid bath steel refining casting which in ladles only involves requires the thermal furnace loss tipping compensation.
metal is poured from the furnace into the ladles and the time so needed that the for liquid
furnace to return to its original position. the
The ■frequent scrap metal arcing melting and arc period suppression is characterised at the beginning by:
■strong current variations caused by arc displacement of during each the basket,
period,
melting
■short-circuits, generated by the metal as it collapses.
2 - OPERATING FREQUENCY
The number of casts per day furnace: depends 13 on casts the type per of day. furnace.
■furnace with refining in the ladle furnace: 22 casts per day.
date
11/94
- B•3•2 -
revised
05/2004
page 11

Medium voltage switchgear application guide
Appendix 6: alternating
current melting arc
furnaces(cont’d)
3 - ELECTRICAL POWER SUPPLY
The transformer) power supply or on must the factory be channelled distribution from network. the HV (with step-down
The from power 15 to 40 available kV. varies from 10 to 100 MVA with voltage values ranging
arc furnace power supply
on the internal factory network
MV (15 to 40 kV)
4
MV/LV
1 2 3 1
1 = disconnector
2 = circuit breaker
3 = switch
4 = arrester
5 = overvoltage limiting capacitors
6 = earth switch
5 6
furnace
10 to 100 MVA
The furnace switch controls all devices downstream of the transformer.
For ≈8 operations an average per of 13 cast. casts per day the switchgear carries out:
≈100 ≈30,000 operations operations per per day. year.
voltage The furnace (15 to transformer, 40 kV) to pass located to the next low to voltage the furnace, used by allows the furnace intermediate
1000 V).
≈800 to
date
11/94
- B•3•2 -
revised
05/2004
page 12

Medium voltage switchgear application guide
Appendix 7: diode and
thyristor applications
MV circuit breaker
MV
transformer
LV
rectifier bridge
The transformer’s type coupling determines the choice of the breaking
device.
through In a diode the or contactor thyristor application, the circuit the breaker shape located of the load upstream current of which the MV/LV goes
rectifier transformer bridge is greatly and coupling. deformed. It depends on the transformer’s type of
1 - SWITCHGEAR TO BE USED
We have to consider two types of feeding:
■ the feeding transformer has the same primary and secondary coupling.
The primary and secondary currents have the same shape.
M
direct
current motor
d i /d t low
All our devices can interrupt this type of current because the current
passes through zero during long periods owing to the switching cycle.
no specific recommandations
■ the feeding transformer has a different coupling on the primary and the
secondary.The primary current is shaped as a stair with a high value of the
di/dtwhen the current reaches the zero
d i /d t high value
■ if the rectifier bridge is with diodes and if the number of operations is
acceptable, the vaccum technology or the SF6 auto-expansion (LF) are OK.
■ if the rectifier bridge is with thyristors:
■ the puffer technology (SF) can be used without any particular precautions.
■ the rotating arc technology (LF or Rollarc) can be used if any tripping
commands are accompanied by a thyristor locking command.
date
11/94
- B•3•2 -
revised
05/2004
page 13

Medium voltage switchgear application guide
Appendix 7: diode and
thyristor applications
(cont’d)
The information corresponding should be relay taken must directly be positively from its safe auxiliary and the contacts device’s (opening opening
contact) operating and device. relayed as directly as possible to the thyristor trigger
than In this the case, given the values. arc is maintained in the device as long as the di/dtis higher
MV
+ Vdc
+ Vdc
S4
stop PB
S4
1 NC
KM
tripping
- Vdc
trip.
KM
1 NO
MV side fault
1 NC
MV
+ Vdc
LV
K1
1 NO
K1
LV contactor
- Vdc
LV =
M
=
The vaccum technology allows the breaking in every cases
date
11/94
- B•3•2 -
revised
05/2004
page 14

Medium voltage switchgear application guide
Appendix 8: fused
switch-disconnector or
fused contactor
35
21
20
I limited current
(peak kA)
250
160
This is a good association if the switchgear breaking capacity is
higher than the rated breaking current of the fuse (I 3 ) and if the
fuse is able to withstand the load current
(generally I load ≤ I r fuse )
2
1 - SWITCHGEAR CHARACTERISTICS
which The fused are: contactor co-ordination imposes two parameters on the contactor
presence ■the maximum of the fault fuses. current that it is likely to break, taking into account the
■the fuses electrodynamic which depend on withstands the prospective of the contactor short-circuit without current fuses value and and with
rating.
the
The figure below illustrates the fused contactor association.
example of characteristics required of contactor when protection is provided by an
interior Fusarc type fuse (250 A /7.2 kV) installed on a 6.6 kV network with a prospective
short-circuit current of 50 rms kA.
I 1 : current giving
maximum breaking
capacity (50 rms kA)
I sc : maximum network
short-circuit current
8.75
I sc
(rms kA)
I limited
(35 peak kA)
I 3 : minimum breaking
current (2,200 rms A)
I r : rated current
(250 rms A)
electrodynamic withstand (1)
(25 peak kA)
breaking capacity
(10 rms kA)
I r (315 rms A)
I load
1 3.5 8 50
limited broken current amplitude
characteristics
fuse characteristics
contactor characteristics
(1) A device that withstands 25 peak kA without a fuse could, if it had a fuse, withstands a fuse
peak current of over 35 peak kA without being destroyed. In fact, contact repulsion takes place
above 25 peak kA but the destructive effect is linked to the thermal constraint which is relatively
weak in the case of fuses intervening in less than 10 ms.
short-circuit The fuse must current not be is required the most to intervene equal to the in Irto I1of I3range. the fuse The used. network
Rated voltage fuses matching the network voltage must be used.
0
date
11/94
- B•3•2 -
revised
05/2004
page 15

Medium voltage switchgear application guide
Appendix 8: fused
switch-disconnector or
fused contactor (cont’d)
2 - DEVICE FUNCTIONS
Each associated element ensures different and complementary functions.
The role of the switch or
■make or break the load
contactoris
current, to:
■break ■cause fault three-phase currents tripping which are as not a result high of enough fuse breaking. to melt the fuse,
The role of the fuse is to:
■break ■limit switching short-circuit currents currents,
■steadily withstand the load (in the current event without of a fault excessive circuit being overheating produced),
depending on the installation and/or operating conditions.
date
11/94
- B•3•2 -
revised
05/2004
page 16