Selecting a generator set is not only about choosing the kVA/kW power that suits you from the catalogue. The key is the operating mode and what it will be used for.

Most often you will encounter three generator operating scenarios:
  1. As an emergency power source.
  2. As a prime power source.
  3. As a continuous power source.

You also need to take into account how many hours per year the generator will operate, with what load variability and whether you need permissible overload. That is exactly why the ISO 8528-1 standard was created. Below you will find information in brief, what this standard is about and what to consider in order to correctly select a generator set.

ISO 8528-1 – what kind of standard is it and why should you know it?

ISO 8528-1 is part of the international ISO 8528 series concerning AC generating sets driven by a reciprocating internal combustion engine (engine + alternator + equipment/control and auxiliary systems). The standard defines application classifications, ratings (power classes) and performance requirements for such sets – i.e. generator sets.

Why is the rated power (kVA – kW) from the nameplate not enough? Why do I need a “Genset Rating”?

In terms of ISO 8528-1, a rating is not “one, always true power”. It is power + permissible operating conditions, including:

  • load variability (variable or almost constant),
  • annual operating hours limit (or no limit),
  • permissible average generator load level (e.g. as load factor),
  • information whether short-term overload is permissible (overload).

That is why the same generator can have several power values in the catalogue – and each will be correct, but for a different operating scenario. In practice, when you see in a datasheet “ESP/PRP/COP/LTP”, it is a reference to the ratings described in ISO 8528-1.

The most common ISO 8528-1 ratings are:

 ESP, LTP, PRP, COP

Below you have the practical meaning of the most common classes.
Generator set ratings according to ISO 8528-1 (ESP, LTP, PRP, COP) – comparison of operating conditions

Generator Rating
( ISO 8528 - 1 ) 		Emergency Standby Power Emergency (Standby) power for supply in emergency situations ( ESP ) Limited Time Prime For supply, for operation for a limited time ( LTP ) Prime Rated Power For operation as the main power source with variable load (24/7). ( PRP ) Continuous Operating Power For continuous operation at constant load (24/7), without overloading. ( COP )
Permitted Load Type Constant + Variable Load Constant Load Only Constant + Variable Load Constant Load Only
Permitted Operating Hours Per Year 200 500 No Limit No Limit
Permitted Average Generator Load Level 70% 100% 70% 100%
Permitted Short-Term Overload NO NO 10%
1 Hour In Each 12-Hour Operating Period 		NO
Rated parameters of the alternator according to NEMA MG 1 – Part 32 Emergency Operation Emergency Operation Continuous Operation Continuous Operation
Duty Cycle (according to IEC 60034-1) S10 - Operation Under Discrete Loads
( multi-level ) 		S10 - Operation Under Discrete Loads
( multi-level ) 		S1 - Continuous Operation S1 - Continuous Operation
Alternators Should Meet the Following OVERLOAD CAPABILITY REQUIREMENTS in a given operating mode.

!!! WARNING - The fact that the alternator meets the class requirements does not mean that a generator set with such an alternator will have such parameters and that it can be overloaded by that amount momentarily !!!
Overload capability in this case concerns CURRENT [A] and not POWER [kVA]

  • Overload capability 150% for 40 seconds
  • Overload capability 163% for 27 seconds
  • Overload capability 150% for 40 seconds
  • Overload capability 163% for 27 seconds
  • Insulation Class H - high-temperature up to 180°C
    Overload capability 125% for 40 seconds
  • Insulation Class F - temperature up to 155°C
    Overload capability 125% for 40 seconds
  • Insulation Class B - temperature up to 130°C
    Overload capability 80% for 40 seconds
  • Insulation Class H - high-temperature up to 180°C
    Overload capability 125% for 40 seconds
  • Insulation Class F - temperature up to 155°C
    Overload capability 125% for 40 seconds
  • Insulation Class B - temperature up to 130°C
    Overload capability 80% for 40 seconds

The same rating and generator set application table in the original as a graphic file.



End of theory, time for practice. 

  1. Define what you need the generator for. Application and rating are key. Whether for power supply ESP / LTP  / PRP / or COP.
  2. Calculate the real load profile. You need information such as:
    • Determine whether the load is constant or variable (this decides COP vs PRP) and the data I write about below.
      IN 99% OF INSTALLATIONS WE DEAL WITH VARIABLE LOAD - SO MOST OFTEN WE SELECT POWER ACCORDING TO THE RATING (PRP)
    • Maximum power you reach. Define the maximum power demand.
    • Continuous power of the loads. Define what average power you will operate with.
    • Power peaks. Define the largest power fluctuations that may occur in a short time.
    • Current peaks. Define the largest current increases that may occur in a short time.
  3. Leave a margin for operating conditions (temperature, altitude, ventilation) and an expansion plan.

TO OBTAIN THE DATA WE TALK ABOUT ABOVE, WE RECOMMEND CONNECTING A POWER NETWORK ANALYZER FOR 2-3 DAYS OF OPERATION IN YOUR INSTALLATION.
For Customers who plan to purchase a generator from us, we do this service free of charge. Thanks to this we are always sure what power machine to select.

EXAMPLE CALCULATION AND SELECTION FOR POWER 100 [kW] 

Assumption for example A - 100 [kW] - 100 [kW] AS MAXIMUM (PEAK) POWER
Assumption for example B - 100 [kW] - 100 [kW] AS AVERAGE POWER IN 24h. (CONTINUOUS AVERAGE 100 kW - 24h)

Common assumptions:
Power factor of the installation: cos φ = 0.8 (often assumed for selection)
FORMULA - S [kVA] = P [kW] / cos φ
Margin for expansion/uncertainty:  +10% Power  (Sometimes you need to assume more, for example when the generator operates at a high altitude above sea level, sudden current increases [A], or when other conditions occur)

CASE - A - 100 [kW] AS MAXIMUM (PEAK) POWER

P_max = 100 [kW]
S_max = 100 / 0.8 = 125 [kVA]

Step 2:  we assume a margin - margin 10%

P_selection = 100 × 1.10 = 110 [kW]
S_selection = 125 × 1.10 = 137.5 [kVA]

In this case, a generator with rated power P.R.P. - 140 [kVA] = 112 [kW] will be appropriate

CASE - B - 100 [kW] AS AVERAGE POWER in 24h. (CONTINUOUS AVERAGE 100 kW - 24h)

The 70% condition for P.R.P. applies here.

Since: P_avg - (average active power)  ≤ 0.7 × P.R.P.
then 
P.R.P. ≥ P_avg (average active power) / 0.7 = 100 / 0.7 = 142.9 [kW]
Add a 10% power margin
P.R.P. selection = 142.9 × 1.10 = 157.2 [kW]
Conversion to [kVA] (cosφ 0.8)
157.2 [kW] / 0.8 = 196.5 [kVA]
In this case, a generator with rated power P.R.P. - 200 [kVA] = 160 [kW] will be appropriate

For such selected power, so that the selection is not only “from P (from power)  and cosφ”, usually you also check and take into account what we wrote about above, i.e.:
- the largest load step (step load) – e.g. simultaneous switching on of a large part of the loads
- motor starts (DOL/softstart/VFD) – often they force higher [kVA]
DOL - (Direct-On-Line) – direct start - the motor gets full voltage immediately from the grid/generator.
Softstart (soft starter) – softstarter / soft start - a system (usually thyristor-based) gradually increases voltage during start.
VFD (Variable Frequency Drive) – inverter / frequency converter. It controls the frequency and voltage of the motor (smooth acceleration, speed control). Usually the least “painful” start for the generator (starting current close to In), but note: it is a non-linear load and can generate higher harmonics and require a larger alternator and better voltage quality (stability).
- THDi / non-linear loads (UPS, inverters) - as above, often affects oversizing of the alternator.
- Derating (temperature/altitude) – calculated according to the generator manufacturer’s data and requirements.

THE END :)

As you can see, selecting a generator is not complicated.
However, sometimes it is worth asking someone who deals with it, because many, many times we have already replaced generators after an unsuccessful selection at various Customers.
Choosing a generator with a lower power is not always a good choice and in fact most often it is a bad choice.
The most common factor in choosing a too small (too weak) generator is:
- lower price, desire to save
- pushing by companies selling the machine, quoting the highest apparent powers of the generators and misleading Customers
- thinking that a smaller (weaker generator) will burn less fuel, which is not true at all

Are you looking for a proven partner who will deliver a generator and HELP YOU WITH SELECTION ;)
We invite you to cooperate.
APEX POWER GENERATORS