Are you sure you know
Medium Voltage terms
perfectly ?
Introduction to prefabricated
metal-enclosed equipment
We are reminding you
of the main definitions
in usual terms.
Keep in mind
the following
points
We are not giving you hereafter,
the exact standard terms.
The definitions are issued by the
International Electrical Commission.
oltage, current, frequency
short circuit power are
are the four main bases
which define the electrical
equipment.
V
e need to consider
5 criteria to specify
the design of the
compartments of a
metal-enclosed equipment
he insulation function,
the control function
and the protection
function of electrical
networks, are carried out
through the switchgears.
ll the documents,
standards, regulations,
recommendations
and local customs are in
reference to the electrical
equipment and will not have
the same legal impact.
T
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■ Merlin Gerin ■ Square D ■ Telemecanique
W
A
Presentation
factory-built equipment
Introduction
In order to design a medium-voltage cubicle, you need to
know the following basic magnitudes:
#
#
#
#
Voltage
Current
Frequency
Short-circuit power.
The voltage, the rated current and the rated frequency are often known or
can easily be defined, but how can we calculate the short-circuit power or
current at a given point in an installation?
Knowing the short-circuit power of the network allows us to choose the
various parts of a switchboard which must withstand significant
temperature rises and electrodynamic constraints. Knowing the voltage
(kV) will allow us to define the dielectric withstand of the components.
E.g.: circuit breakers, insulators, CT.
Voltage
Operating voltage U (kV)
This is applied across the equipment terminals.
Rated voltage Ur (kV)
The rated voltage is the upper limit of the highest voltage of system for
which the equipment is intended
Example:
# Operating voltage: 20 kV
# Rated voltage: 24 kV
# Power frequency voltage
50 Hz 1 mn: 50 kV rms.
# Lighnting impulse voltage
125 kV peak.
The rated voltage is always greater than the operating voltage and,
is associated with an rated insulation level.
Rated insulation level Ud (kV rms. 1 mn) and Up (kV peak)
The rated insulation level is specified by the rated lighning impulse withstand
voltage phase to earth (Up). It is linked to rated short duration power-frequency
withstand voltage (Ud)
Ud: rated short duration power-frequency withstand voltage
It is simulated in a laboratory by the rated power-frequency withstand
for one minute.
Us: switching impulse voltage
Up: rated lightning impulse withstand voltage
The voltage wave that results is simulated in a laboratory
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N.B.: IEC 60694, article 4 sets the various voltage values together with, in article 6,
the dielectric testing conditions.
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Presentation
Metal-enclosed,
factory-built equipment
0
Apart from special cases, MERLIN GERIN equipment is in conformity with
list 2 hereunder
Rated
voltage
Rated
power-frequency
withstand voltage
Up
1 minute kV rms.
Normal
operating
voltage
Ur
kV rms.
Rated lightning
impulse withstand
voltage
Up
kV peak
7.2
12
17.5
24
36
list 1
40
60
75
95
145
20
28
38
50
70
3.3 to 6.6
10 to 11
13.8 to 15
20 to 22
25.8 to 36
list 2
60
75
95
125
170
kV rms.
Insulation levels apply to switchgear under operating conditions in
accordance with IEC 60694 §2.1If not, derating should be considered.
Each insulation level corresponds to a distance in air which guarantees
equipment withstand without a test certificate.(IEC60071-2)
Rated
voltage kV rms.
7.2
12
17.5
24
36
Rated impulse
withstand voltage
1.2/50 µs kV peak
Distance/earth
in air cm
60
75
95
125
170
10
12
16
22
32
STANDARD IEC VOLTAGE
U
Um
0,5 Um
t
rated voltage
0
rated short duration power-frequency
withstand voltage
rated withstand voltage values
for lightning impulse voltage
20 7,2
12
28
38
50
70
Ud
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17,5
24
36
Ur
60
75
95
125
170
Up
NOTE:different "across the isolating distance" values are mentioned for the
switchning device to meet the safety requirements specified for disconnectors
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Metal-enclosed,
factory-built equipment
0
Current
Rated normal current: Ir (A)
This is the rms. value of current that equipment can withstand when
closed, without exceeding the temperature rise allowed in standards.
The table below gives the temperature rises authorised by the IEC
according to the type of contacts.
Rated normal current:
Type of mechanism
of material
Max. values
Max. temperature
Max. temp. rise
of conductor (°C)
= tmax°- 40 °C
contacts in air
bare copper or copper alloy
75
silver or nickel plated
105
tin-plated
90
bolted connections or equivalent devices
bare copper, bare copper alloy
or aluminium alloy
90
silver or nickel plated
115
tin-plated
105
35
65
50
50
75
65
N.B.: rated currents usually used by Merlin Gerin are:
400, 630, 1 250, 2 500 and 3 150 A.
Examples:
Operating current: I (A)
# For a switchboard with a 630 kW motor
feeder and a 1 250 kVA transformer feeder
at 5.5 kV operating voltage.
This is calculated from the consumption of the devices connected to the
circuit in question. It is the current that really passes through the
equipment.
If we do not have the information to calculate it, the customer has to
provide us with its value. The operating current can be calculated when
we know the power of the current consumers.
5 calculating the operating current
of the transformer feeder:
Apparent power:
S = UI 3
S
1 250
- = 130A
I = ----------- = ------------------------5 ,5 / 1 ,732
U 3
5 calculating the operating current
of the motor feeder:
cosϕ = power factor = 0.9
η = motor efficiency = 0.9
630
P
I = ----------------------------- = ----------------------------------------------------------- =82A
5.5 • 1.732 • 0.9 • 0.9
U 3 cos ϕη
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Presentation
Metal-enclosed,
factory-built equipment
0
Minimal short-circuit current: Isc (kA rms.)
(see explanation in "Short-circuit currents" chapter.)
Rated short-time withstand current (Ik)
(kA rms. 1 s or 3 s)
the r.m.s value of the current which the switchgear can carry in the closed
position during a specified rated duration (generally 1s or 3s)
Rated peak withstand current (Ip)
(kA peak) value of the initial peak in the transient period
for a rated frequency of 50Hz and below it is equal to 2.5 Ik (2.6 for 60Hz)
Note: some installations,close generator for example, can have higher
Rated frequency fr (Hz)
peak coefficient.
# Two frequencies are usually used throughout the world:
5 50 Hz in Europe
5 60 Hz in America.
For other value of frequency, contact the technical department
(the IEC standard values of the rated frequency are 16 2/3; 25; 50; 60 Hz)
Switchgear functions
Designation
and symbol
function
operating current fault current
switching
breaking
Disconnecter
isolates
Earthing disconnecter
earthes
Switch
✔
switches,
does not isolate
Disconnecter switch
switches
isolates
✔
Fixed circuit breaker
✔
switches
protects
does not isolate
✔
Withdrawable circuit breaker
switches / protects
isolates in some case
if withdrawn
✔
✔
Fixed contactor
switches
does not isolate
✔
Withdrawable contactor
switches
isolates in some case
if withdrawn
✔
Fuse
protects
does not isolate
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Presentation
is
al vocabulary
Electrotechnic
IEC 60694
defined by the
Metal-enclosed,
factory-built equipment
The insulation function, the control function and
the protection function of electrical networks,
are carried out through the switchgear.
T
he term “Switchgear” is a general one. It is applied to apparatus with
mechanical contacts and their arrangement with devices, of control,
measurement and protection.
It equally concerns the grouping of such apparatus with connections,
accessories and enclosures…
THE INSULATION
FUNCTION
The insulation function establishes or isolates
an unloaded circuit.
THE DISCONNECTOR
The disconnector is a mechanical switching device which provides in the
open position, an isolating distance which ensures the 2 sub-installations
to run separatly.
The disconnector has two stable positions : open and closed.
As the disconnector does not have breaking and closing capacity, the
opening and closing must be unloaded.
In the “open” position, the isolating distance allows the dielectric withstand
between input and output.
To ensure the dielectrical independence of the 2 sub-installations,
the input/output distance is superior to the phase/earth distance.
In the “closed” position, it has to withstand the service current and the short
circuit current during the specified duration.
The two positions must have a locking device forbidding the manœuvre
when the circuit is on load.
THE EARTHING SWITCH
The earthing switch is a disconnector used to earth the circuit.
It is capable of supporting the short circuit currents within one second and
to feed the discharged current of the cables.
The earthing switch can have, in certain cases, closing capacity.
The earthing switch must have a locking device in the “closed” position.
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Presentation
THE CONTROL FUNCTION
Metal-enclosed,
factory-built equipment
The control function of the switchgear is used to
close and to open the circuits on load.
here are three types of control devices :
the switch, the switch disconnector and the contactor.
All this equipment has to withstand the short-circuit currents within a
specified time.
T
THE SWITCH
The switch is a control device with two stable positions, open and closed,
which allows the feeding of a circuit (transformer, distribution cables…) making.
and breaking the service current.
THE SWITCH DISCONNECTOR
The switch disconnector is a switch which, in the “open” position, complies
with the insulation conditions of the disconnector.
THE CONTACTOR
The contactor is an apparatus, electrically controlled, able to establish or to
break a circuit will a strong rhythm of manœuvres.
In general, it is used in motor feeder cubicles.
It is often associated with fuses to limit the peak and to break the shortcircuit current.
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Presentation
THE PROTECTION
FUNCTION
Metal-enclosed,
factory-built equipment
The protection function has to quickly eliminate all
overcurrent, overload or short-circuit, in a network
to assure the safety of the personnel and to limit all
damages.
W
e use fuses or circuit breakers in association with protective
systems.
FUSES
The fuses are protective devices which have to open the circuit by blowing
up one or several of its elements, when the current passes within a
specified time, to a determined value.
A fuse is designed to eliminate the short-circuits.
If the overcurrent is too low, the fuse may fail and not break properly.
A fuse is specified by its melting curve (see technical leaflets).
☞ remarks : the choice of fuses depends on the voltage, the service
current, the type of feeder, the starting currents and its manufacturer.
In general, the service current = nominal current
2
THE CIRCUIT BREAKER
The circuit breaker is an apparatus which assures the control and the
protection of a network.
The circuit breaket is establish, to withstand and break the service currents
as well as the short-circuit currents.
The tripping is automatically released by means of a protection chain
(TC, TP, relay, tripping coil).
When the circuit breaker is in a withdrawn position, it assures the insulation
function and is similar to a disconnector when tested in accordance.
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Metal-enclosed,
factory-built equipment
Presentation
5 criteria have to be considered to
specify compartments in accordance
with the customer's need
-architecture and accessibility s
-service continuity of the switchgear
-partition class
-degree of protection (IP) of the enclosure
-internal arc classification
METAL-ENCLOSED
COMPARTMENT
CHARACTERISTICS
.
ARCHITECTURE AND ACCESSIBILITY: four types of
compartment
Three accessible to the user, one non-accessible
non-accessible compartment: the opening may
destroy the integrity of the compartment.
For example: welded tank of a RMU. ; completely welded
GIS tank....
Accessible compartments
- "interlock-controlled accessible compartment" :
use of interlocks to ensure that all live parts inside are dead
and earthed before opening
- "procedure-based accessible compartment":
relies on user procedure and locking , use of padlocks or
equivalent
- "tool-based accessible compartment":
no built-in feature to ensure electrical safety before opening.
They need tools to be opened
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Presentation
Metal-enclosed,
factory-built equipment
SERVICE CONTINUITY OF THE SWITCHGEAR
(LSC1,2A,2B) ONLY FOR ACCESSIBLE COMPARTMENT
For switchgear and controlgear, the Loss of Service
Continuity (LSC) describes the extent to which the other
compartments and/or functional units may remain
energized when the main circuit is opened
Category LSC1: this form does not provide service
continuity during maintenance and require complete
disconnection of the swithgear from the system and making
dead before accessing the interior of the enclosure.
Ex: the complete installation (HV cables; busbars..) must be
shut down before opening the CB enclosure
Category LSC2A: when accessing components of one
functional unit ,the other functional units of the switchgear
may be kept in service.
Ex: the busbar may be kept alive (also the other cubicles of the
switchgear) during the maintenance of the circuit breaker.
Category LSC2B:this form is intended to allow the
maximum continuity of service.
Ex: during the maintenance of the circuit breaker , MV cables and
busbar may stay alive.
PARTITION CLASSES
There are 2 types of partitioning class
Class PM:when an accessible compartment is open the
equipment is always in accordance with the metal-enclose
criteria. The opened compartment is surrounded by earthed
metallic partition and shutters
No electric field is present in the opening compartment
and no electrical field change may occur in the
surrounding compartments.
Class PI:is not a PM class, there are one or more
non-metallic partitions or shutters between opened
accessible compartments and live parts of the main circuit.
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Presentation
Metal-enclosed,
factory-built equipment
DEGREE OF PROTECTION OF THE ENCLOSURE
see 5.13 of IEC 60694
The degree of protection of persons provided by an
enclosure against access to hazardous parts of the main
circuit is indicated in the here under table
Metal-enclosed swithgear satisfies as a minimum IP2X
INTERNAL ARC CLASSIFICATION
Metal-enclosed swithgear that satisfies the requirements
of the IECstandard is designed to prevent the occurrence
of internal fault. Experience has shown that faults are
.
more
likely to occur in some locations inside an enclosure .
Designs which have been successfully tested according
to Annex A of IEC 62271-200 are classified as IAC
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Where the risk is considered negligible, IAC class is not necessary
Where the risk is considered to be relevant, only metal-enclosed
switchgear IAC classified should be used.
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Presentation
Metal-enclosed,
factory-built equipment
All the documents, standards, regulations,
recommendations, and local customs do not have
the same legal value.
THE REGULATIONS
In order to describe the equipment, each contract
refers to a gathering of regular texts,
standards, recommendations and local customs.
The objective of these regulations is to guarantee the safety of people and
to limit the damage.
THE REGULATIONS
The regulations, either fixed a circular, must be respected imperatively by
the constructors.
THE STANDARDS
The standard has a contractual character and acts as a reference. Only
when a fixed or a decree imposes it, must it be respected imperatively by
the supplier.
The standards define the switchgear and controlgear and their field of
application.
THE RECOMMENDATIONS
The standards are nationwide and are often inspired by international
recommendations (eg. the IEC).
THE LOCAL CUSTOMS
The local customs, though theoretically negociable, are difficult to pass.
We have to work with these.
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Metal-enclosed,
factory-built equipment
Presentation
THE REGULATIONS
MAIN NATIONAL STANDARDS
(cont’d)
France
Germany
UTE
DIN-VDE-TGL
Great Britain Belgium
BS
NBN
Spain
USA
Italy
UNE
ANSI
CEI*
* Comitato Electrotecnico Italiano.
INTERNATIONAL REFERENCES
Three recommendations often act as international references :
■
I.E.C. : International Electrical Commission.
■
A.N.S.I. : American National Standard Institute (US).
■
N.E.M.A. : National Electrical Manufacturer Association (US).
MAIN IEC PUBLICATIONS ON MEDIUM VOLTAGE
THE
298
publications
IEC
Publication 60050: International Electrotechnical Vocabulary
Publication 60265: high voltage switches.
■ Publication 60470: high-voltage alternating current contactors
and contactors-based motor-starters
■ Publication 60529: degrees of protection provided by enclosures (IP Code)
■ Publication 60694 :common clauses for high voltage switchgear and
controlgear standards.
■ Publication 62271: high-voltage switchgear and controlgear
part 100: high-voltage alternating current circuit breakers
part 102: alternating current disconnectors and earthing switches
part 105: alternating current switch-fuse combinations
part 200: AC metal-enclosed switchgear and controlgear for
rated voltage above 1kV and up and including 52kV
part 203: gas-insulation metal-enclosed swithgear for rated voltages > 52kV
. duty T100a
part 308: guide for asymmetrical short circuit breaking test
■
■
WHERE CAN YOU ORDER THE IEC PUBLICATIONS ?
Central Office at the International Electrotechnical Commission,
3 rue de Varembé PO Box 131 CH-1211Geneva 20- Switzerland
you may find the IEC standard on Intranet or Email: inmail@iec.ch
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