CHAPTER 6 DIESEL GENERATOR SETS

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.