4 Switchgear
4.1 Switchgear Classifications
There are many different types of enclosure designs for medium voltage switchgear use. However, the most commonly accepted and used style is metal-enclosed, with segregated and insulated apparatus compartments.
AuCom's MVX soft starter range and medium voltage distribution range are available in this style of enclosure.
Panel compartments
1 Busbar compartment 2 Cable compartment 3 Switching compartment 4 Low voltage compartment
Busbar Compartment
The busbar compartment houses the main busbar system, which is connected to the fixed upper isolating contacts of the main switchgear apparatus by means of branch connections. The main busbars are made of high conductivity copper. The busbar compartment of each panel is isolated from the busbar compartments of the neighbouring compartments.
Single or double busbar configuration is used depending on the current rating.
Cable Compartment
The cable compartment houses some of the following components:
Branch connections
Earthing busbar
Earth switch
Power cables
Surge arrestors
Instrument transformers (current transformers, voltage transformers) Switching Compartment
The switching compartment houses the bushing insulators containing fixed contacts for the connection of the switching apparatus to the busbar and cable compartment. The bushings are single-pole type and are made of cast resin. They are covered by metallic shutters.
The metallic shutters operate automatically during movement of the switching apparatus from the test position (racked-out) to the service position (racked-in) and vice versa. Shutters may be locked if required.
The position of the switching apparatus can be seen from the front of the panel through an inspection window.
1
2 3
4
11158.A
SWITCHGEAR
IEC Switchgear Classification
IEC 62271-200 classifies metal-enclosed switchgear based on:
compartment types
method of access to compartments
safety levels provided during access
effect on continuation of service during access
type of insulation barriers between compartments
internal arc endurance (refer to section on internal arc classification)
The manufacturer must state which areas of the switchgear are accessible and provide a clearly defined switchgear classification.
Classification related to personnel safety in case of internal arc
Types of compartments with regard to accessibility Features
Operator-accessible
compartment Interlocked-based accessible
compartment.
Intended to be opened for normal operation and maintenance.
No tools for opening – Interlocking allowing access only when HV parts are dead and earthed.
Procedure-based accessible compartment.
Intended to be opened for normal operation and maintenance.
No tools for opening – Provision for locking to be combined with operator procedures, to allow access only when HV parts are dead and earthed.
Special accessible compartment Tool-based accessible compartment.
Possible for user to open, but not intended to be for normal operation and maintenance.
Tools necessary for opening. No specific provision to address access procedure.
Special procedures may be required to maintain performances.
Non-accessible compartment Not possible for user to open (not intended to be opened).
Opening destroys compartment or clear indication to the user. Accessibility not relevant.
Switchgear classification with regard to the loss of service continuity when opening accessible compartments
Features
LSC1 Other functional units or some of them
shall be disconnected.
LSC2 LSC2A Other functional units can be energized.
LSC2B Other functional units and all cable
compartments can be energized.
Switchgear classification with regard to the nature of the barrier between live parts and opened accessible compartment
Features
PM Metallic shutters and partition between
live parts and open compartment – (metal-enclosed condition maintained).
PI Insulation-covered discontinuity in the
metallic partitions/shutters between live parts and open compartment.
Switchgear classification with regard to mechanical, electrical and fire hazards in case of internal arc during normal operation
Features
IAC No ejection of parts, no ignition of cloths,
enclosure remains earthed.
Source: IEC 62271-200
ANSI-defined switchgear
ANSI defined switchgear is equivalent to IEC classification LSC2B-PM, with the following characteristics:
the main switching device is withdrawable, with disconnecting auxiliary control circuits
separate compartments are provided for voltage transformers and control power transformers
busbar compartments are divided between adjacent enclosures
metal barriers isolate the withdrawable compartment, when the main switching device is drawn-out into test position
main circuit busbars and connections are covered with fire resistant insulating material
mechanical interlocking prevents stored energy discharge of withdrawable parts
a locking method prevents the withdrawable switching device from being moved into service position
low voltage control parts are segregated from medium voltage apparatus
all voltage transformers must have primary circuit current limiting fuses
Switchgear Ratings
Switchgear is rated according to IEC 62271-1. When choosing switchgear, its rating must be sufficient for the electrical characteristics at the point of installation, the environmental conditions it needs to operate under, and the safety requirements. Future expansion of the switchgear distribution system needs to be considered, as this may affect initial rating requirements.
Switchgear selection is determined by considerations including:
Electrical conditions
System operating voltage (U)
System operating frequency (f)
Nominal operating current (I)
Short circuit current levels at point of installation (ISC, Idyn, etc)
Horizontal busbar arrangement Environmental conditions
Ambient temperature
Altitude
Pollution degree
Indoor or outdoor installation Personnel safety considerations
Internal Arc Classification (IAC)
Interlocking of access areas and switchgear apparatus
Access method (eg tools, keys, process, etc)
Withdrawable switchgear apparatus
Switchgear information for enquiries or ordering
When enquiring about, or ordering switchgear, the supplier should at minimum provide the following information.
When enquiring, advise the supplier of any unusual operating condition requirements (eg altitude 1800 metres).
System characteristics
nominal system voltage and frequency
expected highest voltage
type of neutral earthing system Service conditions
any non-standard service requirements which differ from normal routine Installation specifics
indoor or outdoor installation
SWITCHGEAR
rated insulation level (Ud, Up)
rated nominal current of main busbars and feeders (Ir)
rated short-time withstand current (Ik)
duration of short time withstand (tk)
rated peak withstand current – typically 2.5 Ik at 50 Hz (Ip)
protection degree for enclosure and apparatus Operating device specifics
types of operating devices
rated auxiliary supply voltage (if any)
rated auxiliary supply frequency (if any)
rated gas pressure (if any)
special interlocking requirements
Switchgear derating
Switchgear must be derated for altitudes exceeding 1000 metres and ambient temperatures exceeding 40 °C.
Insulation derating according to altitude
The relevant standards specify the derating required for equipment installed at an altitude greater than 1000 metres.
Guideline: derate by 1.25% U peak, per 100 metres above 1000 metres.
This applies for lightning impulse withstand voltage and for power frequency withstand voltage 50 Hz - 1 minute.
Derating for altitude only applies to air-insulated switchgear, not vacuum or SF6-insulated equipment.
Current derating
IEC 62271-1 defines the maximum permissible temperature rise for each device, material and dielectric medium, using a reference ambient temperature of 40 °C.
The actual temperature rise is affected by:
the rated current
the ambient temperature
the cubicle type and its protection index (IP rating)
Guideline: derate by 1% Ir per degree above 40 °C.
Current derating coefficient
erating coefficient k 10.9
0.8 0.7 0.6
0.5 13325.A