DG set rating

dear sir,

what you have heared is the general practice going on. Infact, due to variable load cycle of the general industries, it is obious that you may not be able to load the DG set to its 100% capacity, even though, you wish for.

So, what is the best most efficient way of selecting the dg size, is, on its efficiency - i.e. kwh generated /ltr. of fuel consumed. You will just see that, below 60% of engine capacity, things get most inefficient.

So, considering peak load / higher starting torques / efficiency .... in general, most of the time, 70% loading formula works better.

You have used undefined terms "prime power" , "rating", "design rating" and "restricted" [>= or <= 70%?]. A manufacturer's data might give "continuous rating" and "maximum rating", even a standard which defined these.

If a single generator, as sole supply, of size from a few kW to hundreds, the size may be decided by the capability to start the largest load motor, it draws, say, twice its rated continuous kW [it is possible to oversize the alternator to meet the 6 - 8 times overcurrent during start] .

Design questions are "how long without stopping?" or "how long between overhauls?".

I was once involved with power for a microwave system having 60 repeaters [two sets/repeater running alternate weeks without mains power] & 30 standby to mains.

The system ran for over a year, needing only gaskets, oil & fuel filters, lube oil & fuel - except for 1 broken rocker. One "standby" set ran 10,000 hours, because mains was so bad it was less trouble to switch off mains. This set and the longest hours pair from no-mains stations were stripped at the end of the period and parts found to be within new tolerances. Some sets were nominally at "continuous rating", but many were 70% [because e.g. 2 cylinder set was two small so 3 cylinder used] - in any case the load is usually over-estimated to allow for tolerances. If necessary, resistance load was added to keep engine load > 30% continuous, any less and the engine "soots up" with unburnt fuel and oil & maintenance cost exceeded the extra fuel cost.

As another indication, in the early 1970s, Trinity House - the UK marine lighthouse/lightship operator - did trials for unmanned operation and concluded that a small DG rated at 70% continuous could run for a year without stopping. The oil supply was a massive sump & filters oversized to ensure vital oil and fuel quality.

If you are talking national power grids [nets], nearly all the sets run steady at continuous rating for contracted period, maybe 1 in 10 is at 90% rating ready to pick-up load of a failed major generator or station. These get paid more/kWh because at 100% load they could earn more & fuel/KWh is higher at part load. The engines have much in common with ship engines, which are efficient and reliable [even many super-tankers had only one engine]. System load may vary 2:1 over a day, so "Base Load" sets meet about half the peak.