Reliability considerations in simple paralleling applications
A load shed scheme is required so that when generators are overloaded, the noncritical loads can be taken off line so that there will be sufficient capacity to serve the critical loads. Most paralleling generator set controls have a load shed or load dump output that can be connected to the load shed input of the transfer switches that serve the non-emergency loads (see Figure 2). This will take the non-emergency loads off line in the event that the generator sets are overloaded. Note that to properly shed load, transfer switches must be three position switches with center-off positions. Using two position switches to shed load is not recommended as this often results in connecting the load to a failed utility.
Isochronous vs. droop load sharing. Most load sharing systems today are isochronous, meaning that the voltage and frequency are held constant. However, there are still controls being produced that use droop load sharing, which allows voltage and frequency to vary with load. Droop load sharing controls were once popular because they allowed generator sets to parallel with each other without communicating with each other. Due to the variation of frequency and voltage with a droop paralleling system, the quality of power provided to the load typically is not very good and may not be suitable for some electrical equipment. Isochronous load sharing is the appropriate technology to use.
Random access paralleling vs. exciter paralleling. Random access paralleling refers to a system in which the first generator at rated speed and voltage closes to the dead bus and then all the other generators actively synchronize and close to the bus. This type of paralleling is the most reliable method and is most commonly used in critical applications. A less expensive paralleling method known as exciter paralleling is used in some paralleling applications. In an exciter paralleling system, all of the generators start with their paralleling breakers closed and their excitation circuits de-energized. When the generators start, they are connected to the bus but produce no voltage. When all generators reach starter disconnect speed, their excitation circuits are energized and the generator bus voltage ramps up with the generators forcing each other into sync. Because exciter paralleling systems will not work until all generators either come up to speed or are locked out, they are not used in critical applications. Another disadvantage of an exciter paralleling system is that if one generator set shuts down it cannot be brought back on line unless the entire system is shut down and restarted. Random access paralleling with active synchronization should always be used when paralleling gensets in critical applications.
Fault tolerance—manual mode. A key consideration in assessing the reliability of a system is its ability to operate when certain components have failed. Having a predefined sequence of operations for a user to follow in a worst-case scenario to manually provide power to loads is often a requirement in critical applications. A user should be able to manually start generators, initiate synchronization, and close paralleling breakers. Manual operation does not mean that the generator control is not operating; it means that functions are user initiated rather than system initiated. All system protection functions will still be active. The control will not allow a paralleling breaker to close if the generator and the paralleling bus are not in phase with each other.