Generator Set Sizing and Ratings
Generator set is maybe the most expensive part you can find in a substation. Therefore, many factors must be considered when determining the proper size or electrical rating of an electrical power generator set. If wrong size or rate is selected and delivered on site, the only thing you can do is to sit and cry.
The engine or prime mover is sized to provide the actual or real power in kW, as well as speed (frequency) control through the use of an engine governor.
The generator is sized to supply the kVA needed at startup and during normal running operation. It also provides voltage control through the use of a brushless exciter and voltage regulator. Together the engine and generator provide the energy necessary to supply electrical loads in many different applications encountered in today’s society.
The generator set must be able to supply the starting and running electrical load. It must be able to pick up and start all motor loads and low power factor loads, and recover without excessive voltage dip or extended recovery time.
The type of fuel (diesel, natural gas, propane, etc.) used is important as it is a factor in determining generator set response to transient overloads.
It is also necessary to determine the load factor or average power consumption of the generator set. This is typically defined as the load (kW) x time (hrs. while under that particular load) / total running time. When this load factor or average power is taken into consideration with peak demand requirements and the other operating parameters mentioned above, the overall electrical rating of the genset can be determined.
Other items to consider include the unique installation, ambient, and site requirements of the project. These will help to determine the physical
configuration of the overall system.
Typical rating definitions for diesel gensets are: standby, prime plus 10, continuous and load management (paralleled with or isolated from utility).
Any diesel genset can have several electrical ratings depending on the number of hours of operation per year and the ratio of electrical load/genset rating when in operation.
Different generator set manufacturers use basically the same diesel genset electrical rating definitions. These are based on International Diesel Fuel Stop Power standards from organizations like ISO, DIN and others.
Standby diesel genset rating
Typically defined as supplying varying electrical loads for the duration of a power outage with the load normally connected to utility, genset operating <100 hours per year and no overload capability.
Prime plus 10 rating
Typically defined as supplying varying electrical loads for the duration of a power outage with the load normally connected to utility, genset operating <500 hours per year and overload capability of 10% above its rating for 1 hour out of 12.
Typically defined as supplying unvarying electrical loads (i.e., base loaded) for an unlimited time.
Load management ratings
Apply to gensets in parallel operation with the utility or isolated/islanded from utility and these ratings vary in usability from <200 hours per year to unlimited usage.
It’s highly advisable to refer to generator set manufacturers for further definitions on load management ratings, load factor or average power consumption, peak demand and how these ratings are typically applied.
Even though there is some standardization of these ratings across the manufacturers, there also exists some uniqueness with regard to how each manufacturer applies their generator sets.
Electrical rating definitions for natural gas powered gensets are typically defined as standby or continuous with definitions similar to those mentioned above for diesels.
A natural gas engine is challenged with air-fuel flow dynamics and a much more indirect path from the engine governor (throttle actuator) and fuel delivery system (natural gas pressure regulator, fuel valve and actuator, carburetor mixer, aftercooler, intake manifold) to the combustion chamber. This results in a less responsive engine-generator.
Diesel gensets recover about twice as fast as natural gas gensets!
Considering factors such as site conditions, load characteristics and required performance, SpecSizer (Caterpillar’s genset software) provides accurate and timely technical data, robust load models and optimized algorithms to assist in specifying a properly sized generator set to best meet your power needs.
Several features set SpecSizer apart as a revolutionary tool in generator set sizing, including its ability to evaluate load types such as air conditioners, elevators, ultraviolet lights, single-phase NEMA and single-phase IEC motors.
For the actual calculations involved for sizing a genset, there are readily accessible computer software programs that are available on the genset manufacturer’s websites or from the manufacturer’s dealers or distributors. These programs are used to quickly and accurately size generator sets for their application.
The programs take into consideration the many different parameters discussed above, including:
- The size and type of the electrical loads (resistive, inductive, SCR, etc.),
- Reduced voltage soft starting devices (RVSS),
- Motor types,
- Fuel type,
- Site conditions,
- Ambient conditions and
- Other variables.
to reduce the inrush current, improving system power factor and other methods.
The computer software programs are quite flexible in that they allow changes to the many different variables and parameters to achieve an optimum design. The software calculates how to minimize voltage dips and can recommend using paralleled gensets vs. a single genset.
Genset Sizing Guidelines
Some conservative rules of thumb for genset sizing include:
- Oversize genset 20–25% for reserve capacity and for across the line motor starting.
- Oversize gensets for unbalanced loading or low power factor running loads.
- Use 1/2 hp per kW for motor loads.
- For variable frequency drives, oversize the genset by at least 40% for sixpulse technology drives.
- For UPS systems, oversize the genset by 40% for 6 pulse and 15% for 6 pulse with input filters or 12 pulse.
- Always start the largest motor first when stepping loads.
For basic sizing of a generator system, the following example could be used:
Step 1 – Calculate Running Amperes
- Motor loads:
- 200 hp motor: 156 A
- 100 hp motor: 78 A
- 60 hp motor: 48 A
- Lighting load: 68 A
- Miscellaneous loads: 95 A
- Running amperes: 445 A
Step 2 – Calculating Starting Amperes Using 1 25 Multiplier
- Motor loads:
- 200 hp motor: 195 A
- 100 hp motor: 98 A
- 60 hp motor: 60 A
- Lighting load: 68 A
- Miscellaneous loads: 95 A
- Starting amperes: 516 A
Step 3 – Selecting kVA of Generator
- Running kVA = (445 A × 480 V × 1.732) / 1000 = 370 kVA
- Starting kVA = (516 A × 480 V × 1.732) / 1000 = 428 kVA
Generator must have a minimum starting capability of 428 kVA and minimum running capability of 370 kVA.
Generator Set Installation and Site Considerations
There are many different installation parameters and site conditions that must be considered to have a successful generator set installation. The following is a partial list of areas to consider when conducting this design.
Some of these installation parameters include:
- Foundation type (crushed rock, concrete, dirt, wood, separate concrete inertia pad, etc.)
- Foundation to genset vibration dampening (spring type, cork and rubber, etc.)
- Noise attenuation (radiator fan mechanical noise, exhaust noise, air intake noise)
- Combustion and cooling air requirements
- Exhaust backpressure requirements
- Emissions permitting
- Delivery and rigging requirements
- Genset derating due to high altitudes or excessive ambient temperatures
- Hazardous waste considerations for fuel, antifreeze, engine oil
- Meeting local building and electrical codes
- Genset exposure (coastal conditions, dust, chemicals, etc.)
- Properly sized starting systems (compressed air, batteries and charger)
- Allowing adequate space for installation of the genset and for maintenance (i.e., air filter removal, oil changing, general genset inspection, etc…)
- Flex connections on all systems that are attached to the genset and a rigid structure (fuel piping, foundation vibration isolators, exhaust, air intake, control wiring, power cables, radiator flanges/duct work, etc.)
- Diesel fuel day tank systems (pumps, return piping)
- Fuel storage tank (double walled, fire codes) and other parameters
Please see the generator set manufacturer’s application and installation guidelines for proper application and operation of their equipment.
4x1225KW, 13.8KV Diesel Generator sets in Parallel
Source: Power Distribution Systems by Eaton