5.1 ONSHORE SUPPLY SYSTEMS (GENERAL)
Every onshore system
draws its supplies from the National Grid.
In England and Wales they will be taken through an Area Supply Board,
but in Scotland they are taken direct from the North of Scotland Hydro-Electric
Board (NOSHEB) or the South of Scotland Electricity Board (SSEB).
The Supply Authority
concerned provides power through its own grid transformers, offering it to the
customer at 33kV or 11kV and carrying out its own metering. The transformers themselves and their supply
switchgear on the high-voltage side remain the property and responsibility of
the supply authority. All switchgear and equipment downstream of this is the
property of the consumer.
To illustrate the
supply and distribution of power within an onshore installation, the Mossmorran
complex is taken as a typical example.
5.2 MOSSMORRAN INSTALLATION
5.2.1 Supply
Authority Feeders and High Voltage Distribution
Figure 5.1 shows in simplified one-line
form the incoming feeders from the SSEB grid transformers and the high-voltage
distribution from those transformers.
Two underground cables carry power at 132kV to the SSEB grid
transformers, each rated 30MVA. These
supply the Mossmorran plant at 11kV, and each has sufficient capacity to feed
the whole plant load on its own.
The two incoming supplies are taken to a 11kV switchboard in Substation
A, from which they supply 11kV power to two large motors. They are further distributed through
step-down transformer feeders to lower voltage switchboards in Substation A and
B, to the plant modules and to the Administration and Workshop building.
The principal circuits of Substation A are depicted schematically in
Figure 5.1, which also shows the SSEB incomers supplying the 11kV
switchboard. Also shown are the feeder
connections from that board through step-down transformers to the 3.3kV
switchboard and to the 415V Utilities switchboard. Other transformer feeders from the 11kV board
go to Substation B, to Module 1 and Module 2 and to the administration/workshop
building. There are also feeders from
the 3.3kV switchboard to the Control Building for heating and ventilation. The various voltage levels are distinctively
coloured as follows:
132kV yellow
11kV
busbars blue
3.3kV
busbars green
4l5V
busbars red
Equipment above the shaded line is the property of the supply authority
(SSEB).
FIGURE 5.1
MOSSMORRAN SUBSTATION A
Switchgear on the 11kV switchboard consists
of 750MVA oil circuit-breakers for all purposes, including motor feeders. The 3.3kV switchgear comprises oil
circuit-breakers for incomers, section breakers and transformer feeders, and
vacuum contactors backed by HRC fuses for the motor feeders. The 415V switchgear consists of air-break
circuit-breakers together with the normal motor control centre distributing
equipment.
Normally both the SSEB incoming circuits are used together, sharing the
total load, and the 11kV bus-section circuit-breaker is kept open. Thus the entire installation is powered in
two separate halves which operate independently. Care is taken (see para. 5.2.5) that the two
halves cannot be paralleled inadvertently at the 11kV switchboard or at any
lower voltage switchboard.
If either main SSEB incoming supply fails,
its main circuit-breaker opens and the 11kV bus-section breaker closes
automatically, thus putting the whole installation load onto one transformer,
which is designed to carry it. The
failed incomer circuit-breaker is locked open so long as the section breaker
remains closed, so preventing paralleling of the two main supplies, which would
otherwise raise the fault level on the 11kV system beyond the breaking capacity
of the switchgear.
FIGURE 5.2
MOSSMORRAN SUBSTATION A
UTILITIES, EMERGENCY AND SECURE SUPPLIES
SWITCHBOARDS
5.2.2 Low Voltage
Distribution, Substation A
Figure 5.2 shows the
arrangement of the 415V Utilities switchboard in Substation A and the principal
circuits which it feeds. In particular
this switchboard distributes to two ‘Secure Power’ switchboards and to the Fire
Pump House. Because of the distance of
the pump house from Substation A the two feeders to it are stepped up in
voltage from 415V to 3.3kV and then stepped down again at the pump house to
415V.
The 415V Utilities
switchboard has two extensions, emergency busbar ‘A’ and emergency busbar ‘B’. These two extensions are connected to the
main busbar through circuit-breakers which act as section breakers. The two secure power switchboards and certain
other important services are connected to these two extensions, which are
supplied normally from the incoming transformers TR 10 and TR 11 through the
main busbar. In the event of loss of
supply at the main busbar the two extensions are supplied direct from the
emergency generator, and the interconnectors with the main busbar are locked
out.
Figure 5.2 also shows
the emergency generator which is rated at 577kW. It is diesel-driven and starts automatically
on loss of main power at the 415V Utilities switchboard.
The generator feeds
its own emergency switchboard from which feeders are taken direct to the
emergency extensions ‘A’ and ‘B’ of the main utilities switchboard and are
automatically switched.
FIGURE 5.3
MOSSMORRAN
SUBSTATION B
5.2.3 Low Voltage
Distribution, Substation B
The arrangement of
Substation B is shown in Figure 5.3.
Substation B receives
power from Substation A at 11kV and transforms it to 3.3kV. A number of motors are supplied direct at
this voltage through vacuum contactors.
There are also further transformer feeders to a 415V Utilities
switchboard in Substation B and to the LPG Tanks Heating switchboard.
5.2.4 Security and
Duplication
The safe and
satisfactory operation of the whole plant is dependent on a secure supply and a
secure distribution system. Any
possibility of loss of supplies to the many points of consumption within the
plant is virtually eliminated by duplication of feeders.
The SSEB supply
originates from two different sources, chosen because of the unlikelihood of
both being unavailable at the same time.
Either of the SSEB supply transformers is capable on its own of
supplying the projected maximum demand of the plant, with a margin in
hand. The operation of the changeover
arrangement is explained in para. 5.2.1.
In a similar manner
the 3.3kV and 415V switchboards of Substation A are each supplied from either
of two transformer feeders as shown in Figure 5.1. The feeders from the 415V Utilities
switchboard of Substation A to the two ‘secure power’ switchboards and to the
fire pumps are duplicated in a like manner (see Figure 5.2).
Substation B is also
supplied by duplicated feeders from Substation A, and in turn feeds its 415V
Utilities switchboard and the LPG tanks heating switchboard by duplicated
feeders in each case (see Figure 5.3).
In the unlikely event
of complete failure of both SSEB feeders the essential minimum of power is
available from the emergency generator.
The 415V switchboard of Substation A feeds the most vital consumers -
the fire pump house and the two ‘secure power’ switchboards - and therefore the
emergency generator is arranged to feed this switchboard. Once again there are duplicate feeders: the
emergency generator is able to supply the three most vital users through either
of two circuits connected to the wings of the 415V switchboard as shown in
Figure 5.2.
Apart from the matter
of security of supply the duplication of feeders affords great flexibility in
operation. It also gives freedom to
perform regular routine maintenance on the distribution system without
interruption of supply.
5.2.5 Automatic
Transfer
Of those switchboards
which receive duplicated feeder supplies, five are arranged for automatic
transfer. The switchboards so equipped
are the 11kV switchboard in Substation A, the two 3.3kV switchboards
(Substations A and B), and the two 415V Utilities switchboards (Substations A
and B).
Normal operation
requires all feeder transformers to be energised and connected, and all
low-voltage bus-section breakers to be open.
Each switchboard is thus normally operating as two separate sections,
each section with its own supply feeder.
If supply is lost in
either of these feeders, the auto-transfer control immediately disconnects that
feeder and closes the bus-section switch of the switchboard affected, thus
restoring service to the consumers connected to the affected half of that
switchboard.
In other switchboards
with duplicate feeders, for example the two ‘secure power’ switchboards in
Substation A (Figure 5.2) and the LPG tanks heating switchboard in Substation B
(Figure 5.3), such transfer is done manually under the control of key-operated
interlocks.
5.2.6 Tariff
Metering
Tariff metering of the
supply and monitoring of the loading is under the control of the SSEB and
centralised in Substation A. Information
is taken from both incoming feeders and all outgoing circuits on the 11kV
switchboard and is supplied through a microprocessor to the plant Control Room
and to a computer.
5.2.7 Alarms
Alarm annunciators are
located in Substations A and B, with common alarms annunciated in the Control
Room.
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