Generator Selection

Listed below are some of the main considerations involved in generator selection.

• Synchronous versus induction. Synchronous generators are slightly more efficient than induction units and may be used for any capacity application and any type of load. They can operate independently of the grid to provide power during standby or isolated operation. Induction generators are used for smaller applications (usually under 1,000 kW), where isolated operation is not required. They are less expensive than synchronous generators in smaller capacity ranges, have a more simple construction, and are easier to install and connect to the grid because they do not require synchronizing controls. Induction generators tend to place a high parasitic kVAR load on the utility-derived system and, in most cases, will require PF correction.

• Voltage selection. Generator voltage is determined by the voltage at the point of interconnection with the system. If the alternator operates in an isolated mode only (no parallel operation with the utility), then the economical choice would be to generate at the load utilization voltage. If parallel operation with the utility-derived system is contemplated, it may be desirable to operate at the delivery voltage. Other considerations could include cable and switchgear costs or line losses.

• Power ratings. Prime mover/generator systems are rated in terms of standby power and prime power (or continuous duty). Standby ratings are designated for a brief and infrequent operating regime and the standby power rating will usually be greater than the prime power rating.

• Insulating material temperature rating. This will largely depend on the operating duty. Cost is considered with respect to the type of duty and the expected operating life and reliability of the unit.

• Power factor (PF). Generators are rated by their total current-carrying capacity in kVA and most three-phase units are rated at a minimum allowable PF of 0.8. With increased load PF, more output power will be delivered per kVA. Required kVA for a given load can be calculated as follows:

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