PowerPoint Presentation
A PowerPoint presentation for students taking Module 4 of the Electricity Distribution Distance Learning Course
This presentation and its illustrations are the copyright of Power Documents Ltd. They are intended for the distance learning student’s sole use and must not be used for teaching purposes without the company’s consent in writing.
The main difference between HV and LV switchgear is that the former is normally three position ON, OFF and EARTH whilst the latter is two position ON and OFF only. This is a consequence of HV Safety Rules, which require earthing of HV circuits before they may be worked on. The only type of HV switchgear not equipped with earth facilities is that used in ‘Open Terminal’ substations, where portable earths or separate earth switches are used.
Earth Facilities
Fixed and Withdrawable Pattern
Modern switchgear divides into two main types, fixed pattern and withdrawable. The advantage of withdrawable is that faulty circuit
eakers and mechanisms can be replaced with the remainder of the switchgear alive. But it is more expensive and more difficult to operate than fixed pattern. Only a few makers produce withdrawable switchgear and only then for service up to 15kV.
Fixed and Withdrawable Pattern
In withdrawable switchgear (which is always vacuum technology) the inte
upters and mechanism are contained in a CASSETTE. This moves by horizontal racking between an inner position (SERVICE) and an outer position (ISOLATED). As the cassette moves, automatic mechanisms open shutters over heavy cu
ent contacts on the rear wall of the cassette housing. When in service, the cassette housing is closed by an outer door which incorporates various interlocks to prevent unauthorised opening. In general, withdrawable pattern does not provide for easy operation and there are many possibilities for operator e
or.
Fixed and Withdrawable Pattern
Fixed pattern switchgear may be vacuum, SF6 or combination SF6/vacuum technology. It is simpler and therefore safer to operate mainly because it allows better labelling, better semaphore indicators and a better mimic diagram. All switchgear manufactured for service at voltages above 15kV is fixed pattern, including switchgear for the highest transmission voltages. In general, access to the circuit contacts for test purposes is much easier compared to withdrawable switchgear.
Controls and Indications
Whether the circuit
eaker is fixed or withdrawable pattern, operators will require controls and indications as shown on the next few screens. Some are essential, some are simply useful and some could be considered optional extras. This illustration is the front control panel of ABB 11kV switchgear at a process plant.
Controls and Indications
Illuminated ON (red) and OFF (green) electrically powered indicators are essential and because one or the other is constantly energised they should be LED rather than filament bulb. LED lamps offer improved reliability.
Controls and Indications
An electrical trip/close control is essential and normally a
anged as a rotary operated unit in which the handle returns to a centre (neutral) position when released. This control must be padlockable, the method of a
anging locking varies with make and type.
Controls and Indications
Protection tripped lamp indicator (optional). May be any colour other than red or green - yellow, white or blue are popular. If lit it indicates that the circuit
eaker has tripped in response to a signal from a (microprocessor) protection relay. On old switchgear with induction disc relays this indication was not needed because the relays were fitted with mechanical indication (flags).
Controls and Indications
An ammeter is very useful because it shows load being picked up and dropped off also any overload condition. Because cu
ent transformers will be required for protection purposes the cost of an ammeter is low in comparison with its operational value. Sometimes fitted with a switch to select particular phases, this is not really needed because phase cu
ents should be balanced. Can also be fitted with a mechanical maximum value pointer at little extra cost.
Controls and Indications
An engraved circuit name label is essential and should be fixed by screws or rivets, not glued. A similar label should be fixed at the rear of the switchgear if there is any possibility of operations being ca
ied out there (for example rear circuit test access).
Controls and Indications
A trip circuit healthy indicator (optional) shows that the auxiliary supply (DC) is present. It is normally white and constantly illuminated.
Controls and Indications
A trip circuit supervision lamp indicator is an optional extra – if fitted it is illuminated constantly. It shows that the DC trip supply is present and the trip circuit is operational – if present a trip circuit healthy indicator is not required. Generally white it may also be blue or yellow.
Controls and Indications
A circuit in earth lamp indicator is another optional extra. It shows that the outgoing circuit is earthed – if the circuit test access is at the rear of the switchgear the lamp indicator should also be sited at the rear. Generally yellow or amber.
Controls and Indications
Amber Circuit in Earth lamp indicator fitted to the rear of Alstom WS fixed pattern 11kV switchgear. It incorporates a push to test facility.
Controls and Indications
A local
emote control is essential if the circuit
eaker can be switched ON or OFF remotely – can be three position LOCAL, REMOTE or SCADA if SCADA is implemented. This control must be padlockable so that the circuit
eaker cannot be inappropriately closed or opened from a remote location. Often the trip/close and local
emote controls are similar in design.
Controls and Indications
Mechanical trip and close buttons are nowadays considered essential because they allow operation even if the auxiliary electrical supply fails. The close button may be fitted with a padlockable cover but the open button should never be padlocked because it may need to be operated in a hu
y one day.
Controls and Indications
This is a padlockable cover over the mechanical CLOSE button, fitted to the front panel of Alstom WS fixed pattern 11kV switchgear. Padlock hole indicated.
Controls and Indications
A voltmeter is considered unjustified due to the cost and operational complexity of the associated voltage transformer. If a VT is needed for other purposes, for example for energy metering a voltmeter might be considered, but for most installations capacitor voltage test points are a better and cheaper option.
Controls and Indications
All controls and indicators should be labelled with their function or method of operation. Labels should be fixed with screws or rivets, not simply glued. Emergency stop buttons should be shrouded against inadvertent operation.
Controls and Indications
Controls and indications on a modern design of 11kV fixed pattern switchgear. Here the status of the circuit
eaker itself and the associated circuit earth switch and busbar disconnector are shown by electrically operated semaphores. The open/close and local
emote controls are located at the upper centre of the photograph – both are padlockable. A similar switch at lower centre controls the circuit earth switch, which is electrically interlocked with the circuit
eaker to prevent earthing a live busbar – that is, it can be closed only when the circuit
eaker is open. With its painted mimic line indicating the internal layout of the unit, this is a good example of modern practice.
Controls and Indications
Mechanical CLOSED/OPEN (or OFF) semaphores were standard on withdrawable oil switchgear but nowadays are mainly confined to cassette type switchgear. Note operating spring condition indicator on the right.
Controls and Indications
Whether fixed pattern or withdrawable, where the circuit
eaker mechanism is operated by a motor wound spring a spring condition indicator (springs charged or springs free) is essential. This mechanism on Alstom PIX cassette switchgear also has a useful manual spring recharge facility (handle and socket) allowing CB operation if the DC auxiliary supply has failed.
Controls and Indications
Mechanism spring condition semaphore and manual mechanism recharge socket fitted with slide aside shutter, Alstom WS fixed pattern 11kV switchgear.
Controls and Indications
Circuit
eakers using vacuum inte
upters require wear indicators, so that units nearing the end of their service life may be identified. An operations counter is a standard feature of most designs. The photograph shows FKI HawkVac 15 cassette switchgear.
Controls and Indications
Low voltage fuses should be positioned where they can be seen and are easily accessible, labelled both for purpose and for fuse cu
ent rating. This example is also fitted with a padlock facility.
Cable Terminations
Most cables enter the switchgear from below, although top entry can be provided on request by many makers. This illustration shows rear cable termination, although front termination is fitted to some designs, especially if the switchgear is to be fitted inside a shipping container (containerised). Whatever the position of the termination, for bottom entry a trench will be required, normally accommodating both the power and auxiliary cables. In the illustration, one termination box is shown with its sheet steel cover removed.
Cable Terminations
The user must specify the number, overall diameter, conductor metal, cross sectional area, insulation, type of termination (heat shrink, cold shrink, moulded ru
er) and specification of the power and auxiliary cables to be used. For the power cables some means of earthing the metallic sheaths whether Lead or Copper must be provided. The illustration shows 3 x single core polymer insulated and sheathed cables, heat shrink terminated - note the earth bar fitted to earth the Copper screen wires.
Cable Terminations
The substation cable trench is
idged and covered by Aluminium plates, cut to accommodate the power and auxiliary cables. Separate termination boxes have been provided for the auxiliary power and control/SCADA cables, as they will be routed to different destinations. Note the secure clamping of the power cables at or close to ground level, preventing movement during short circuit. In warm countries the cable entries are normally fully sealed to prevent ingress of vermin, especially snakes.
Cable Terminations
Most auxiliary circuits utilize polymer insulated, Steel wire armoured, polymer sheathed cables to BS 5467/IEC60502 (or low smoke versions to BS6724). These are available in 1, 2, 3, 4, 5, 7, 12, 19, 27 and 37 core versions with conductor sizes between 1.5 and 50mm2. Terminations are normally compression fittings applied by hand tool or automatic machine. A core identification system with numbered and lettered fe
ules should be specified, in the UK BEBS S12 is the protocol commonly employed, now reissued as ENA TS 50 –19.
Point of Isolation
A Point of Isolation is an important concept embodied in all HV Safety Rules. It is a safety gap between the live network and the de-energised network that is to be worked on. It MUST be capable of locking and is SHOULD be capable of displaying a Caution notice.
Point of Isolation
A circuit
eaker is not considered to be an adequate Point of Isolation because in most switchgear designs, the three opened contacts cannot be seen. Some supplementary gap is required, this and the next two screens show how it is achieved on various types of switchgear.
On old mineral oil (withdrawable switchgear, it was achieved by lowering the circuit
eaker in its ca
iage