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Monday, March 25, 2013

CHAPTER 5 TYPICAL ONSHORE INSTALLATION SUPPLY SYSTEM




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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