Reliability considerations in simple paralleling applications


Control system

A robust control system is critical for a reliable paralleling system. A control system must minimize single points of failure and have built-in fault tolerance measures. Key factors in a paralleling control include the following. 

Eliminate single points of failure. The most effective way to eliminate single points of failure in a control system is to use distributed logic and control rather than centralized control. Critical control functions such as generator starting, bus voltage sensing, synchronizing, and closing to the bus should be executed by individual generator set controls rather than a master control. This way, the system will have redundancy in critical control functions in addition to redundant generators, eliminating the single point of failure. 

  • Generator starting: In a simple standby isolated bus paralleling application, the start signal is sent directly from the transfer switches that sense the utility failure to the generator sets. Sending the signal through a master control adds no value and introduces an unnecessary failure mode. Sending the start signal directly to the generator sets for the transfer switch contacts is the simplest, most reliable means of starting the generators.
  • Paralleling bus voltage sensing: For reliable paralleling, each generator must sense the bus voltage independently rather than rely on a signal from a separate control
  • Closing to a dead bus: When closing to a dead bus, the system must include an arbitration scheme to prevent multiple generators from closing to the bus at the same time. To provide the fastest and most reliable service to a dead bus, the arbitration and breaker control logic must be resident in the generator controls rather than in a master control. Waiting for a permissive signal from a master slows the system down and adds an unnecessary failure mode.
  • Synchronizing and closing to a live bus: Generator sets synchronize reliably and quickly when the bus sensing and synchronizing logic is part of the generator set control. External controls adjusting the generator set frequency and voltage in an attempt to synchronize with the bus introduce unnecessary complexity into the system.  

Figure 1: Implementing a load add function: Connecting the normally closed aux contact of each genset’s paralleling breaker in parallel to the ATS transfer inhibit input will prevent the ATS from transferring to the emergency source until all of the gener

Load add and load shed. Load add and load shed schemes ensure that there is always sufficient capacity to serve the most critical loads; less critical loads are served as capacity becomes available. Two levels of load add (one level for emergency loads and one level for all other loads) and one level for load shed (emergency loads are never shed) are sufficient for most simple, isolated bus paralleling applications. This can be implemented without the use of a master control. However, a master control may be required for additional levels of load add/shed. Although a master control can present a single point of failure, the system can be designed so that failure of the master will not impact the most critical loads. 

A load add scheme is required in a paralleling system when a single generator is not large enough to carry all of the loads in the system. A simple load add scheme with two levels can be implemented using the inhibit function of the non-emergency load transfer switches and the aux contacts of the genset paralleling breakers (see Figure 1). Emergency load transfer switches should not be inhibited and should close to the bus as soon as it is live. Non-emergency transfer switches can be inhibited until all of the generator sets come on line.

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