Reliability considerations in simple paralleling applications
Installing and commissioning paralleled generator sets is not a simple process. A qualified manufacturer will have experience with protective relaying, system grounding, and other paralleling issues beyond the generator set functionality. Working with a manufacturer that has substantial paralleling experience over a wide range of applications and will assume responsibility for a correct installation is a key to a successful project even in the most basic paralleling application. There are several considerations that an experienced installer will address.
Selective coordination. The National Electrical Code (NFPA 70) requires selective coordination for emergency and legally required loads (Sections 700.27 and 701.27). Any downstream breakers must coordinate with upstream overcurrent protection such as paralleling breakers or a genset mounted breaker. Coordinating with a generator set mounted molded case circuit breaker (MCCB) with an instantaneous trip will be very difficult and will require in most cases that the downstream breakers are supplied from the same breaker manufacturer as the genset mounted MCCB. It is much easier to coordinate with power breakers as are most often used in paralleling switchgear as they are typically equipped with a programmable trip unit specifically for the purpose of coordination. When the generator control includes integral, UL listed overcurrent protection, coordination between the genset and the paralleling breaker is simplified because the overcurrent trip curve is optimized to allow the maximum permissible time delay while still protecting the alternator.
Separation of circuits. The National Electrical Code (700.10(B)(5)) requires that emergency loads are separated from legally required and optional loads. NEC 700.10(B)(5)(d) allows the use of “single or multiple feeders to supply distribution equipment between the emergency source and the point where the combination of emergency, legally required or optional loads are separated.” In this context the “emergency source” can refer to either a single generator set or the bus in the case of paralleled generator sets. Emergency circuits must be separated from legally required and optional standby circuits with separate circuit breakers installed in separate switchboard sections or enclosures. Emergency loads must be switched using a dedicated transfer switch, separate from legally required and optional standby circuits.
There is some ambiguity in the code regarding whether legally required and optional standby circuits need to be separated from each other. The language in the code doesn’t explicitly require separation of legally required loads from optional standby loads, but all of the exhibits in the NEC handbook show them as separate. Some authorities having jurisdiction (AHJs) have enforced separation of legally required loads from emergency standby loads.
Isolation of generators from the paralleling bus. To enable maximum reliability and safety, there must be means to individually disconnect each generator from the paralleling bus located at the paralleling switchboard. Without this disconnecting means, typically an incoming breaker, a fault on one generator can make all generators inoperable and all generators will have to be locked out to do maintenance work on any generator in compliance with NFPA 70E lockout-tagout requirements (see Figure 3). Without this disconnecting means, much of the value of having a redundant generator will be lost.
10-second start requirement. The National Electrical Code requires that emergency loads are served within 10 seconds of a utility failure. As 10 seconds typically isn’t enough time to start, synchronize, and close multiple generator sets, this requirement means that each of the generator sets in the system must be large enough to carry all of the emergency loads on its own. For example, a system of three paralleled 600 kW generator sets will not meet the NEC requirement if the emergency load exceeds 600 kW. To meet this requirement, the sequence of operations should not require any interaction with a master controller. All control functions should be carried out independently by the generator and the transfer switch controls with no communication required other than a genset start command.
Supplier of distribution switchboard. Sourcing the generator and paralleling controls separately from the switchboard complicates a project. The contractor and consulting engineer must have a clear definition of what is included in the scope of supply for each part of the system. Dividing scope of supply won’t be limited to equipment but will also include assignment of responsibility for meeting code requirements like selective coordination and separation of circuits, system testing and start-up, and ongoing maintenance and service. Having the same entity responsible for supply, installation, and maintenance helps promote an efficient maintenance program.