What is the common power rating of a diesel generator set? How can the power rating of a diesel generator be identified?

I. How to Identify the Power Rating of a Diesel Generator
In China, diesel generator sets are typically rated by their continuous power—i.e., the maximum power they can deliver continuously over a 24-hour period. This is what we refer to as “continuous power.” However, under certain operating conditions, the standard allows for a one-hour overload of up to 10% above the continuous power rating within any 12-hour window. The power output during such an overload period is what is commonly called “prime power” or “standby power.” In other words, if you purchase a prime-rated 400 kW unit, you may operate it at 440 kW for one hour within any 12-hour period. On the other hand, if you buy a standby-rated 400 kW unit and operate it at its rated 400 kW without ever exceeding that limit, the unit will in fact be running in an overloaded condition almost all the time (since its actual rated power is only 360 kW). This practice is highly detrimental to the generator set, as it significantly shortens its service life and increases the likelihood of failures.
Typically, the power rating of a diesel generator set is clearly marked on the nameplate; however, the output power ratings specified by different manufacturers vary and are categorized as standby power, prime power, and continuous power.
1. Standby Power: Defined as the maximum power that a generator set can continuously deliver for 300 hours between scheduled maintenance intervals under specified ambient conditions, with a maximum of 500 operating hours per year. This is equivalent to the Limited-Time Operation Power (LTP) specified in national and ISO standards. It is generally suitable for occasional emergency applications with highly variable loads, such as telecommunications and building systems.
2. Common Power: Defined as the maximum power level within a variable power sequence that can be continuously operated for an unlimited number of hours per year under specified maintenance intervals and environmental conditions; this is equivalent to the Prime Rated Power (PRP) as defined in national standards and ISO standards. It is generally suitable for applications with relatively stable load profiles, such as industrial plants, mines, and military facilities, where operating conditions are characterized by minimal load fluctuations.
3. Continuous Power: Defined as the maximum power that can be sustained within a constant power profile over an unlimited annual operating period, under specified maintenance intervals and prescribed environmental conditions. This is equivalent to the Continuous Operating Power (COP) as defined in national standards and ISO standards. It is generally applicable to continuous-operation conditions with minimal load variations, such as power plant applications or grid-connected operation with the utility grid.
The power output of a diesel generator set cannot be compared with that of the utility grid, because a diesel generator converts mechanical energy from the diesel engine into electrical energy, and the diesel engine experiences power loss during operation. The relevant power ratings are listed below:

1. Emergency Standby Power (ESP) of Diesel Generators: Under agreed operating conditions and in accordance with the manufacturer’s maintenance requirements, ESP refers to the maximum power output at which the generator set can operate under variable load for up to 200 hours per year in the event of a mains power failure or during test conditions. Within a 24-hour operating cycle, the allowable average power output shall not exceed 70% of the ESP, unless otherwise agreed with the manufacturer.

2. Limited-Time Running Power (LTP) of Diesel Generators: Under agreed operating conditions and in accordance with the manufacturer’s maintenance requirements, the generator set may operate at its maximum power for up to 500 hours per year. At 100% LTP, the annual operating time shall not exceed 500 hours.

3. Prime Power (PRP) of a diesel generator: the maximum power at which the generating set can operate continuously under variable load and with no limit on annual operating hours, provided that it is operated under the agreed operating conditions and maintained in accordance with the manufacturer’s specifications. The average power output (Ppp) over a 24-hour operating cycle shall not exceed 70% of the PRP, unless otherwise agreed with the engine manufacturer. In applications where the permitted average power output Ppp exceeds the specified value, Continuous Power (COP) shall be used.

4. Continuous Operating Power (COP) of Diesel Generators: Under the agreed operating conditions and in accordance with the manufacturer’s maintenance requirements, the maximum power at which the generator set can operate continuously under a constant load with no limit on annual operating hours. This standard also specifies the site conditions for generator operation: such conditions are determined by the user; when the site conditions are unknown and no specific provisions have been made, the following rated site conditions shall apply.

1. Absolute atmospheric pressure: 89.9 kPa (or equivalent to an altitude of 1,000 m). 2. Ambient temperature: 40°C. 3. Relative humidity: 60%.

II. Regarding Unit Power Correction

The engine is rated at its power output under standard atmospheric conditions in accordance with ISO 3046 at the factory. If the site conditions differ from the standard conditions, the engine’s power output must be corrected in accordance with the applicable engine power correction procedure.

Correction of power involves accounting for the adverse effects of real-world conditions—such as altitude, relative humidity, and ambient temperature—to reduce the engine’s maximum output power relative to its rating under standard atmospheric conditions. Failure to adjust the unit’s output power in accordance with actual environmental conditions may result in the following:

1. Increased exhaust temperature (resulting in greater thermal stress on the valves and turbocharger).

2. Increased smoke exhaust volume.

3. The turbocharger speed increases.

III. Matching the Diesel Generator Set with Its Load Power

To properly select a standby diesel generator set, it is essential to understand the characteristics of the load. Motors started by direct-on-line typically have a starting current 4 to 7 times their rated current; in contrast, when using variable-frequency soft starters, the starting current is usually 2 to 3 times the rated current.