6.1 GENERAL
In large onshore installations power is derived from
the National Grid. On platforms the main generating sets are
always driven by gas turbine, using the platform’s own gas as fuel when
available, with liquid fuel as an alternative in some cases.
Onshore
the grid supply can sometimes fail, and on platforms main generators may also
fail, or under certain conditions they may be deliberately shut down. In either case there is loss of main power
supply, and it is important that there should be immediately available a
quick-starting alternative supply - and this means diesel generation.
All
platforms, and most large onshore installations, have one or more
diesel-generator sets. In many cases
they are arranged to start automatically on loss of mains voltage and to switch
themselves onto an emergency switchboard.
It is never the intention that such generators should replace the lost
main ones, but they should provide limited power for only really essential
services such as some degree of lighting, safety, instrumentation,
communications, fire and gas detection and so on.
Diesel-driven
generators are also required for ‘black-start’ conditions when no main
generators are running but whose auxiliaries must be run in order to start
them. Such diesel sets must of course be
entirely self-contained, requiring no external assistance to start them.
The
construction of a diesel engine is well known and will not be described
here. It is usually multi-cylinder,
turbo-charged and jacket-cooled through a water/air radiator, some times
assisted by a cooling fan. It is usually
battery-started, and some sets have an alternative hydraulic starter, hand
pumped, for use if the battery becomes discharged, for example after a
prolonged shutdown. It is vitally
important for diesels which drive emergency generators which are automatically
started that the batteries are maintained fully charged ready for an instant
start; also that practice starts should be exercised regularly.
6.2 BASIC SERVICES
In all
installations the really essential services, which it is vital to keep running
even when the normal main power has been lost, are offshore termed Basic
Services. The diesel-driven generator is
called the ‘Basic Services Generator’ and its switchboard the ‘Basic Services
Switchboard’, both shown in red in Figure 6.1.
The system is usually at low voltage (440V), and positive steps are
taken to see that the basic services generator does not feed back into any
non-basic low-voltage services or into the high-voltage system. (There are however some exceptions to this
practice.)
Under
normal conditions on an offshore platform the basic services switchboard is
part of the complete 440V distribution system.
It is in continuous use and is normally fed through an interconnector
from a main 440V board, as shown in Figure 6.1.
If power on the main board fails, the basic services board is isolated
from it and can be fed direct by its basic services generator, which normally
has sufficient capacity for that board and no more. The generator may start automatically on
failure of the main 440V power, but quite commonly it must be manually started. The incomer circuit-breaker from the
generator is interlocked with the incomer from the main 440V board so that both
cannot be closed at the same time; therefore the generator can never feed back
into the remainder of the 440V system or, through the transformer, into the HV
system (other than with the exceptions mentioned above).
FIGURE
6.1
TYPICAL BASIC
SERVICES AND BLACK START GENERATOR ARRANGEMENT
Auto-starting
is achieved by providing the basic services busbar with an undervoltage relay
which causes the interconnector to open on loss of main supply and the basic services
generator to start. When the generator
has started and run up, it closes its incomer breaker automatically and in so
doing locks out the interconnector. Even
when main power is restored, the interconnector breaker cannot be reclosed onto
the basic services board until the operator has first opened the generator
incomer breaker, so lifting the interlock.
The normal interconnector incomer breaker can then be closed, and the
system reverts to normal. The basic
services generator is afterwards stopped manually and left in a condition to
restart whenever needed.
Where
the start is manual no undervoltage trip is provided, but instead the act of
manually closing the generator incomer breaker also trips and locks out the
interconnector incomer breaker. When
power is restored, the process is reversed manually.
With
regard to the exceptions referred to above, on some of the newer platforms
larger diesel-generator sets are fitted which have a capacity appreciably
greater than that needed only for the basic services switchboard and its
essential loads. In those cases some
limited feedback into the system is allowed to power other less essential but
still important loads, such as utilities.
In that case the interlock between the generator and interconnector
breaker is not fitted.
6.3 AVAILABILITY OF BASIC SERVICES GENERATOR
A basic
services generator is nearly always needed in a hurry, whether automatically or
manually started. It is therefore always
left in a ‘ready-to-run’ state. If
automatic, the selector switch is left on ‘Auto’, even if it had been turned to
‘Local’ for the previous manual stopping.
Ready-use fuel tanks are kept full, oil and water levels correct,
battery fully charged and heaters on.
These things are checked daily, and always after the machine has
been run.
Where
basic services generating sets are automatically started on loss of main
supply, this feature is regularly tested to ensure that it functions
correctly. Manual starts on all
auxiliary sets are also regularly exercised.
6.4 BASIC SERVICES GENERATOR UTILITIES
Most
diesel engines are electrically started from a local battery, usually 24V. When the engine is not in use this battery is
kept fully charged by a charger fed from the main a.c. system. An engine-driven d.c. generator charges the battery when the
engine is running.
Basic
services diesel engines are provided with ready-use fuel tanks with a capacity
sufficient for at least 24 hours’ full-load running. As main supplies are assumed to have been
lost, fuel pumping facilities may not be available, and it may be necessary to
refill the tank by hand-pumping from barrels.
Each
diesel generator unit is provided with a local control panel on or near the
engine mounting, from which the output can be controlled and monitored for
speed and voltage. No remote control is
exercised from the Electrical Control Room on the generator and interconnector
circuit-breakers. All control is local,
but there is usually some remote instrumentation in the Electrical Control
Room.
2 comments:
The basic services, how to handle diesel generators and availability details are useful for me..thanks for sharing this detailed information with us.
Cummins diesel generator
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