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Power Ratings of Diesel Generator as per ISO 8528
A specific model of a Diesel Generator set can deliver more power for fewer hours per year, or less power continuously. The runtime, type of load, and application are the factors affecting the power rating of the diesel generator set.
The Diesel Generating set is an alternate, reliable source of power often used for meeting the electrical power requirement of establishments such as Industries, commercial, residential buildings, Hotels, Railway Stations, Telephone exchanges, Hospitals, Shops, etc. in the absence of main grid power.
Diesel generating sets are extremely useful appliances meant to supply electrical power during situations like a main grid power failure and are a prime source of electric power supply to critical process industries, commercial establishments, and places where Grid Power is not available.
Diesel Generator sets must be capable of delivering the necessary power required for an anticipated, number of run hours per year, to ensure reliable power generation.
A specific model of a Diesel Generator set can deliver more power for fewer hours per year, or less power continuously. The runtime, type of load, and application are the factors affecting the power rating of the diesel generator set.
To meet power and run-time criteria, manufacturers have developed standard rating guidelines designed to size the generator set to meet the load requirements of an application.
A manufacturer’s rating definitions are designed to allow correct machine selection and guide consultants and end users. But many generator manufacturers are using differing specifications.
This makes equipment comparisons across manufacturers more difficult, as some published rating definitions may appear more aggressive. This can give a potentially false appearance of higher capabilities in machines from various suppliers, causing some confusion.
The ISO has developed a standard set of power rating definitions to avoid confusion. Specifically, ISO 8528 was created for gas and diesel generator sets. This standard was developed to provide consistency across manufacturers and industries and to improve communication and understanding between the manufacturer and the customer.
It is important to note that generator set manufacturers may establish product ratings that go above and beyond the requirements prescribed in the standard.
Diesel Generators must provide the anticipated power required reliably and without damage and this is achieved by the manufacturers giving one or more ratings to a specific generator set model.
This blog post will provide you with an overview of a Diesel Generating set power ratings, as per ISO 8528.
This will surely help you to get an insight into, the selection of the proper Diesel Generator set for the specific application.
The ISO 8528 standard defines various classifications for diesel generating sets’ application, rating, and performance. The classifications put forth in ISO 8528 are intended to help align the generator set manufacturer and customer by providing customers with a common basis on which to compare generator set ratings from different manufacturers. It is important to note that generator set manufacturers may establish product ratings that go above and beyond the requirements prescribed in the standard.
These standard ratings allow valid comparisons among manufacturers. It prevents manufacturers from rating their machines differently and guides designers.
Diesel Generator Set Power Rating Guidelines
- These ratings describe maximum allowable loading conditions on a generator set.
- The generator set will provide acceptable performance and life when applied according to the published ratings.
- It is also important to operate generator sets at a sufficient minimum load to achieve normal temperatures and properly burn fuel.
- It is recommended that a diesel generator set should be operated at a minimum of 30% of its nameplate rating.
The ISO 8528 standard describes four power ratings for Diesel Generator Set:
- Emergency Standby Power (ESP)
- Prime Power (PRP)
- Limited-Time Running Prime (LTP)
- Continuous Power (COP)
Definition of Diesel Generator Set Power Ratings
Emergency Standby Power (ESP)
The ESP rating commonly referred to as the “Standby” rating, is applicable for supplying emergency power for the duration of normal power interruption. No sustained overload capability is available for this rating.
The ISO 8528-1 standard defines standby power as the maximum power available from the diesel generator set to energize a variable electric load profile, where it has been determined that the total annual runtime doesn’t exceed 200 hours of operation. In addition, the average load factor over 24 hours shall not exceed 70 percent of ESP unless otherwise approved by the generator set manufacturer. The Figure below shows how to calculate the average power.
Note that when calculating the average power of a variable load profile, powers that are less than 30 percent of the ESP rating should be considered to be equal to 30 percent, and the time at which the generator set is at standstill should not be counted.
Prime Power (PRP)
The PRP rating, commonly referred to as “prime” is applicable when supplying electric power using generator sets instead of or in addition to Grid power.
In Prime Power rating, maximum power is available for a varying electrical load for unlimited hours. A 10% overload is available for 1 hour in 12 hours of operation.
ISO 8528-1 defines prime power as the maximum power which a generator set can provide a variable electrical load sequence for an unlimited number of hours per year. In addition, the maximum average load factor over 24 hours shall not exceed 70 percent of Prime Power unless otherwise approved by the generator set manufacturer. The Figure below shows how to calculate the average power.
- Unlimited number of hours per year (8760 hours/year less service and maintenance hours)
- Variable load
- Not to exceed 70% average of the prime rating during any operating period of 24 hours
- 10% overload capability is available in accordance with ISO 3046, AS 2789, DIN 6271, and BS 5514
Typical application – Applicable when supplying electric power instead of commercially purchased power
Limited-Time Running Prime (LTP)
The ISO 8528-1 standard defines LTP as the maximum power that a generator set can provide for up to 500 operating hours per year under operating conditions and maintenance intervals approved by the generator set manufacturer.
The LTP rating is typically applied in constant load applications such as interruptible, load curtailment, peak shaving, and other applications that normally operate parallel to utility operation and where the yearly generator set run time is to not exceed 500 hours.
- Limited number of hours per year (500 hours/year)
- Non-Variable Load
- Applications not to exceed 100% of the prime power rating
Continuous Power (COP)
The continuous power rating is used for applications for which there is no grid power, and the generator set is relied upon to supply a constant load for an unlimited number of hours annually.
These applications such as remote power stations typically use multiple generator sets to achieve this constant load, which is also known as a “base load power station.” The base load is the minimum amount of power that a utility must make available to meet its customers’ demands for power.
ISO-8528-1 states that a Continuous Power rated generator set must provide power “for an unlimited number of hours per year under the agreed operating conditions with the maintenance intervals and procedures being carried out as prescribed by the manufacturers”.
- COP 8760 hours per year less maintenance time
- No Variation of Load allowed.
Typical application – Applicable for supplying power continuously to a load up to 100 percent of the base rating for unlimited hours.
Continuous power rating is used in applications where supplying power is at a constant 100% load for an unlimited number of hours each year. Continuous power, rated units are most widely used in applications where the power grid is unreachable. Such applications include mining, agriculture, construction, or military operations.
Standby power rating is used in applications where the main provider of power – usually the Grid fails.
Standby Generator is not run regularly to provide the main power it must be run up periodically by the maintenance team to ensure that it starts and supplies the building load when required. A Standby Generator should be fitted with a trickle charge battery charger to keep the starting batteries ready for action and it is also advisable to fit a jacket water heater which keeps the engine at a preset temperature and significantly aids in cold starting, reduced engine wear and reduced smoking from the engine when cold.
Limited time prime rating is used in applications where the Prime power is accessible for a limited number of hours in non-variable load situations. Limited prime power is intended for an expected power outage, such as a planned utility power reduction.
Here, it is important to never exceed the prime power rating. The end user should be aware that constant high load use will reduce the life of any engine. It is recommended that any application requiring over 500 hours per year that the engine be continuously power rated.
As an example, if in a particular set the Standby Rating is 1000 kW, then a Prime Power rating might be 850 kW and the Continuous Rating 800 kW. However, these ratings vary according to the manufacturer and should be taken from the manufacturer’s datasheet.
A generator rated at 500 kVA Prime Power and 550 kVA Standby Power.
When used for Prime Power the generator can run at 500 kVA, but the average load should be 70% of this rating i.e., 350 kVA. The number of annual run hours is not limited. For every 1 hour in 12 hours of operation, the generator can run at a 10% overload giving 550 kVA.
When used for Standby Power the generator can run at 550 kVA, but the average load should be 70% of this rating i.e., 350 kVA. There is no overload available for Standby Power and the manufacturer’s restrictions on run hours should be considered.
It’s important to remember that most rating considerations are engine-based, but there are also generator-end characteristics that must be considered, such as the allowable generator temperature rise for the operating hours and load of the generator set.
Diesel Generator Set Power Ratings
Conclusion
In this blog post, we have walked you through the Power ratings of a Diesel Generator Set as per ISO 8528.
Diesel generating sets are extremely useful appliances and hence they must be capable of delivering the necessary power required for an anticipated, number of run hours per year, to ensure reliable power generation.
You have understood that a specific model of a Diesel Generator set can deliver more power for fewer hours per year, or less power continuously. The runtime, type of load, and application are the factors affecting the power rating of the diesel generator set.
You have also understood that the ISO has developed a standard set of power rating definitions. Specifically, ISO 8528 was created for diesel generator sets, to make equipment comparisons across manufacturers and industries simpler and to improve communication and understanding between the manufacturer and the customer.
This will surely help you to get an insight into, the selection of the proper Diesel Generator set for the specific application.
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Diesel fuel is a resource that comes with a cost. We should value it as a resource that has to be used carefully and wisely.
In this post, we have tried to compile all freely available information on various internet sites, so ultimately you do not need to spend your time searching for it.
Now next in the series on Power Generation we will post about the Alternators.
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