Transcript document

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.
Earth Facilities
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.
Fixed and Withdrawable Pattern
Modern switchgear divides into two main types, fixed pattern and withdrawable. The
advantage of withdrawable is that faulty circuit breakers 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
interrupters 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 current 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 error.
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 breaker 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 arranged 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 arranging 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 breaker
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 current 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 currents 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 carried 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/remote control is essential if the circuit breaker 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 breaker cannot
be inappropriately closed or opened from a remote location. Often the trip/close
and local/remote 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 hurry 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 breaker itself
and the associated circuit earth switch and
busbar disconnector are shown by
electrically operated semaphores. The
open/close and local/remote 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 breaker to
prevent earthing a live busbar – that is, it
can be closed only when the circuit
breaker 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 breaker 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 breakers using vacuum interrupters 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 current 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 rubber) 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
bridged
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 ferrules
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 breaker 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 breaker in its
carriage – an interlock prevents
lowering (or raising) a CLOSED CB
which
could
have
catastrophic
consequences.
Point of Isolation
On modern cassette (withdrawable)
switchgear a Point of Isolation is
achieved by moving the cassette with its
vacuum interrupters horizontally in the
housing from an inner SERVICE position
to an outer ISOLATED position – again an
interlock prevents moving a CLOSED CB
from service to isolated (or vice-versa)
with catastrophic consequences. The
photograph shows an Alstom PIX
cassette circuit breaker in its ISOLATED
or outer position – returning it to the
SERVICE or inner position involves using
a racking handle to rotate a drive screw –
this may or may not require opening the
front panel door.
Point of Isolation
Inside the housing, movable shutters on the rear wall cover the busbar and feeder
contacts. The mechanism on the left hand side wall and another on the right hand
side wall (not visible) open or close these shutters as the cassette moves in or out.
These mechanisms may be padlocked to secure the shutters closed. In the
photograph, the cassette has been removed entirely from the housing.
Point of Isolation
On fixed pattern switchgear the
circuit breaker is supplemented by a
separate disconnector, interlocked
with the circuit breaker, preventing its
operation either opening or closing
unless the circuit breaker is open. On
this example the disconnector status
is shown by an electrical semaphore.
The switchgear is ABB 33kV ZS1. On
many types of switchgear all three
phases of this disconnector are
visible through a viewing window, to
provide
extra
confidence
to
operational personnel. Disconnectors
may be electrically powered or
manually operated.
Circuit Earthing
In addition to a Point of Isolation, any HV circuit or equipment that is to be worked
on must be EARTHED. Further, this earth must be safety LOCKED ON whilst work
is in progress. The means of earthing a circuit varies with the type of circuit
breaker; but it is the provision of an EARTH position that distinguishes HV
switchgear from LV switchgear, which is simply ON or OFF.
Circuit Earthing
In withdrawable mineral oil switchgear,
circuit earth is obtained by moving the
circuit breaker carriage to a position
further into the housing. When closed, the
circuit breaker earths the outgoing circuit.
Circuit earthing through the circuit
breaker is good practice, because it is
possible that the circuit is live from a
distant source. If it is, the rated circuit
breaker will be able to safely withstand
the high short circuit current. The circuit
breaker can be LOCKED ON in this
position.
Circuit Earthing
In withdrawable cassette switchgear, earthing is not through the vacuum interrupter
but through a separate disconnector, fully rated to withstand short circuit current.
This disconnector is interlocked with the circuit breaker so that it cannot be closed
unless the circuit breaker is open and the cassette is in the ISOLATED position. The
example is on Alstom WBD 11kV switchgear – note the facility to lock the socket for
the operating handle closed and the status semaphore to the left.
Circuit Earthing
Circuit earthing on fixed pattern
switchgear may be through the circuit
breaker (a less common arrangement)
or through a separate disconnector
rated to withstand short circuit current
(a more common arrangement). In both
cases, operation of the circuit earth
disconnector is interlocked with the
circuit breaker, so that it cannot be
closed unless the circuit breaker is
open. This example (shown with circuit
earth applied) is on ABB ‘Safeplus’ SF6
11kV switchgear, note that the earth
disconnector is operated manually by a
handle that fits into a socket, which is
shuttered and may be locked closed.
Note
the
several
mechanical
semaphores on this equipment and the
shutter protected open and close
mechanical buttons at the top.
Circuit Earthing
Another example of the use of a
disconnector in association with a
circuit breaker, this time on FKI
‘Eclipse’
fixed
pattern
11kV
switchgear. The manually operated
disconnector (at the top) is three
position, (1) through to busbar (as
shown), (2) open and (3) through to
earth. Interlocked with the circuit
breaker it can be moved only when
the circuit breaker is open. When
the disconnector is in position 3, the
circuit breaker can be closed,
earthing the outgoing circuit. Note
the symbols indicating that this unit
is fitted with voltage test points
(marked VPIS) and with a VT. Also
that the manual trip lever (red) may
be padlocked.
Circuit Earthing
Where circuit earthing is obtained
through a circuit breaker, a
method
of
preventing
inappropriate operation of the
manual trip button is essential.
This photograph shows a lockable,
swing aside cover on South Wales
Switchgear
C4X
withdrawable
mineral oil 11kV switchgear. Some
utility/company Safety Rules also
require removal of the trip circuit
fuses.
Voltage Transformers
Voltage transformers are sometimes associated with complex protection schemes
but more often with energy metering. Where fitted, they require a padlockable
means of isolation to prevent any possibility of energising the HV circuit from the
transformer LV side – that is, the Point of Isolation concept applies. For 11 and
33kV switchgear, both HV side and LV side isolation is customary. For higher
voltages, only LV side isolation is normally possible. The photograph shows a VT
on mineral oil 11kV switchgear – isolation is obtained by moving the de-energised
VT backwards on a track, isolating both HV and LV connections simultaneously.
Voltage Transformers
On withdrawable cassette and
fixed pattern switchgear, the VT
is fixed and fitted with its own
manually operated HV side
isolator. This example is on
Alstom WBD 11kV switchgear –
note the warning against live
operation
and
the
twin
padlocking points. VT LV side
isolation is obtained by removing
fuses or less often by opening a
miniature circuit breaker (MCB).
Voltage Transformers
For circuit breakers fitted with energy metering, Neon indicators showing that the
VT secondary side circuits are live are very useful. They help prevent disputes
where a metering VT fuse has blown and the meter therefore under – registers.
Voltage Indication
Fitting a voltage transformer simply to know whether a circuit is live or dead is not
justified because of the high cost and added operational complexity. It is better to fit
capacitor voltage indicators, which can also be used with electronic phase
comparators. This example is on Alstom WBD 11kV fixed pattern switchgear.
Circuit Test Access
Electrical contact with the circuit conductors is required for two reasons:
1.
To apply test voltages so that circuit faults can be located.
2.
To prove that a circuit has been properly repaired and is fit to restore to
service.
Clearly, safety is an issue and so two interlocks are normally specified:
1.
The circuit cannot be accessed unless it is isolated and earthed and
2.
Once test access is obtained, circuit earth may be removed.
Circuit Test Access
On mineral oil switchgear,
accessing the circuit meant
crawling inside the housing and
manually moving the circuit
shutter aside – the yellow
shutters in this example. Test
prods (sometimes called probes)
are then inserted and fixed in
position. This was a thoroughly
unpleasant task.
Circuit Test Access
On modern fixed pattern switchgear, a separate test access is provided, under a
padlockable and interlocked cover – this example is on ABB ZV2 33kV switchgear.
Note the circuit in earth lamp indicator and electrical interlock.
Circuit Test Access
Where a proper circuit test access is provided, many examples of modern fixed
pattern switchgear still require the use of test prods. This is a prod for the ABB ZV2
switchgear whose test access was shown on the previous screen. In use they must
be secured with the hexagon head set screws contained in the plastic bag.
Circuit Test Access
On withdrawable cassette type switchgear, special test cassettes are needed. Using
them requires complete removal (and eventual replacement) of the standard
cassette, which is a laborious procedure.
Busbar Earthing
On multi-panel switchboards,
some means of safely earthing
the busbars is required, so that
maintenance can be carried out
or a new panel added. There are
many methods of providing a
busbar earth, few are fitted with
interlocks because busbars may
be energised from many sources.
Safety procedures alone must be
relied upon. Note the GAS
GAUGE on this switchgear – a
gauge is always required where
pressurised SF6 gas is used for
arc quenching or for insulation.
Remote Operation
Remote ON/OFF operation of HV switchgear from an adjacent room is the most cost
effective method of ensuring operator safety and likely to be much cheaper than arc
proofing. Some company Safety Rules insist on it. At its most basic it requires only a
box, a control switch, two indicator lamps and a connecting cable. This shows a
remote control box at a steelworks substation, controlling a five panel 11kV
switchboard. This type of facility can be easily incorporated into SCADA.
LV Fuses
LV fuses should be accessible and clearly labelled as to purpose and size. Some
Safety Rules require padlocking facilities on fuse holders, this twin swivelling bar
arrangement is one method by which it can be achieved.