The things to know when sizing a generator Genset
Terminology can be tough to understand!
The terms that are often used incorrectly can cause confusion and cost you money.
The biggest culprit is service duty, particularly the “prime” and “continuous” (or baseload) duty ratings. We are often asked to supply a continuous rated generator set simply because the application calls for 24/7 power from the generator.
However, just because your Genset will be running 24/7 doesn’t automatically mean you need a continuously rated machine. While both systems can provide power for an unlimited number of hours per year, there are some significant differences between prime and continuous ratings: overload capability, average load factor, and cost. The ISO-8528-1* standard limits prime-rated Gensets to a 10% overload capacity of one hour in every 12 and an average load factor of 70% of the nameplate rating in a 24-hour period. However,
the standard does not afford overload capability to continuously rated machines but it does allow for a 100% average load factor. Ok, what does this all mean, Take two 500kW diesel gen-sets, one rated for continuous duty and the other for the prime duty: – The 500kW continuous rated unit can supply up to 500kW all day and night. – The 500kW prime rated genset is designed to provide a maximum of 550kW for one hour in 12 and is limited to an average of 350kW per hour over a 24-hour period. 70% load factor, remember? Let’s break down a 24-hour period down to further illustrate the load profile:
Midnight – 4am: 500kW (4 hours @ 100% load = 2000kW-Hours)
4am – 5am: 550kW (1 hour @ 110% load = 550kW-Hours)
5am – 8am: 400kW (3 hours @ 80% load = 1200kW-Hours)
8am – Noon: 250kW (4 hours @ 50% load = 1000kW-Hours)
Noon – 4pm: 300kW (4 hours @ 60% load = 1200kW-Hours)
4pm – 8pm: 250kW (4 hours @ 50% load = 1000kW-Hours)
8pm – Midnight: 300kW (4 hours @ 60% load = 1200kW-Hours)
Total KW-Hours: 2000 + 550 + 1200 + 1000 + 1200 + 1000 + 1200 = 8150kW-Hours
Nameplate Rating x 24 Hours: 500kW x 24 Hours = 12000kW-Hours Load Factor: 8150kW-Hours Actual Load / 12000kW-Hours Nameplate Rating = 68% A 500kW prime rated generator would be an excellent match for the above load profile
because the machine would be utilized to within a percentage or two of its maximum design capacity. Ok, but what about using a 500kW continuous rated unit for the same load profile? Well, there are some problems with that: 1) 4am – 5am calls for 550kW, or 50kW over the design capacity. This might not be mechanically possible. However, you should read the fine print on your warranty card regardless if the machine is able to provide the additional kW or not. 2) The above load profile has varying loads. However, continuously rated Gensets are purpose-built for non-varying loads. Such loads are commonly seen at mines or remote power stations where there are multiple generators running in parallel to achieve a constant load, often referred to as a ‘baseload power station’. 3) KW for KW, continuous rated machines cost much more than prime-rated machines simply because both a larger engine and generator-end are required to meet the continuous rating spec. No sense paying all that extra cash if you’re not using the generator at full capacity.