Handbook of Electrical Installation Practice - BeKnowledge

Standby Power Supplies 125
quickly corrected by the governor/fuel pump system. The momentary drop in speed
is load dependent so that where prescribed frequency limits are important (e.g.
standby for computers), it is necessary to ensure that only a partial electrical load
is applied as a single step. Turbocharged engines tend to have lower load acceptance
capability than naturally aspirated engines, owing to the response time of the
exhaust turbocharger, and the maximum load step may therefore be restricted to
60% or 80% of rated load.
Standard governors usually give a speed drop of a few per cent between no load
and rated load and this is advantageous in equalisation of load between parallel
running sets. Special governors are available which adjust the no load and full load
speeds to be equal, and such isochronous governing may be specified for certain
critical installations. Parallel running with isochronous governing requires load
sensing input to the governors to ensure proper load sharing between individual
sets.
The characteristic of speed reduction with increasing load is known as speed
droop. If two diesel generators operating in parallel have the same no-load speed
and droop then they will share load in proportion to their rating even if they differ
in rating. If either of these conditions are not achieved or drift with time, then the
full system capacity cannot be achieved.This restriction could be particularly serious
for a standby system since full availability is vital. The use of engine management
systems with load control provides close tolerance load share between parallel
running generators and compensates for drift in governor and combustion
characteristics.
Governing performance is defined by BS 5514-4 and different classes of governing
accuracy and response can be specified to suit the application.
A typical generating set will consume about 0.3 litres of fuel per kWh generated.
Fire regulations limit the amount of fuel that can be stored in the generator room
and the usual arrangement of fuel supply comprises a small service tank (also
referred to as the day-tank) on or near the set which is topped up from a bulk tank
situated outside.
Cooling and exhaust system
All internal combustion engines are relatively inefficient and Table 6.1 shows how
the energy in the fuel is transformed into useful work and heat loss. Column A shows
the figures relative to 100% fuel energy, while column B bases the figures on 100%
rated set output, after allowing for generator inefficiency.
Table 6.1
A B
Total fuel energy 100 333
Heat loss to exhaust 30 100
Heat loss to cooling system 27 90
Mechanical and radiation losses 10 33
Generator losses 3 10
Energy to load 30 100