Paralleling generator systems—part three: installation considerations

When designing generator systems, electrical engineers must ensure that the generators and the building electrical systems that they support are appropriate for the specific application. Whether providing standby power for health care facilities or prime power for processing plants, engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria.


This article is peer-reviewed.

Learning objectives

  • Learn best practices for paralleling generators, touching on dependability, cost savings, efficiency, synchronization, and other aspects.
  • Know the requirements for emergency, standby, and backup power loads.
  • Explain the benefits of parallel power-generation systems.

Editor’s note: Because of the extent of this topic, this article is divided into three parts:

Part 1: The need for backup power, code requirements, and generator ratings; why diesel is preferred; and the benefit of paralleling generator systems.
Part 2: Paralleling switchgear, their components, and common paralleling modes.
Part 3: Installation considerations, interconnection with the utility, and generator sizing. Also, two existing parallel generator systems will be presented and their paralleling elements highlighted.

Figure 1: Paralleling diesel generators with wall-mount radiators, control panels, mufflers, power and control conduits, step-up transformers, and isolation pads are located indoors. All graphics courtesy: JBA Consulting EngineersInstalling and commissioning paralleled generator sets is a complex process. A qualified manufacturer or integrator will have experience with protective relaying, system grounding, and other paralleling issues beyond the generator set functionality.

Installation considerations

Working with a manufacturer who has substantial paralleling experience over a wide range of applications and will assume responsibility for a correct installation is key to a successful project even in the most basic paralleling application. There are several considerations that an experienced installer will address.

Selective coordination. NFPA 70-2017: National Electrical Code (NEC) requires selective coordination for emergency and legally required standby loads. Any downstream protective devices must coordinate with upstream overcurrent protection, such as paralleling breakers or a generator-mounted circuit breaker. Coordinating with a genset-mounted molded-case circuit breaker (MCCB) with an instantaneous trip will be very difficult and will require, in most cases, that the downstream breakers be supplied from the same breaker manufacturer as the genset-mounted MCCB. It is much easier to coordinate with a power breaker, which is most often used in paralleling switchgear because it is 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. The NEC requires that emergency, legally required standby, and optional standby equipment and distribution be separated from each other. With paralleled generator sets, that means that the emergency, legally required standby, and optional standby loads must be fed from the paralleling switchgear (PSG) load bus by separate breakers in separate physical compartments or sections in the switchgear lineup.

Isolation. To enable maximum reliability and safety, there must be means to individually disconnect each generator from the paralleling bus located at the PSG. Without this disconnecting means (typically an incoming breaker), a fault on one generator may cause all generators to become inoperable. All generators will then need to be locked out for maintenance work on any one generator in compliance with NFPA 70E-2015: Standard for Electrical Safety in the Workplace. Without disconnecting means, much of the value of having a redundant generator will be lost.

Connecting standby generation to the utility

Each utility has specific requirements and regulations for paralleling generators to utility distribution systems. The utilities are responsible for protecting their distribution system from faults, power surges, poor power quality, or an interruption of power to their customers. For some utilities, it may be the first time that they are allowing the connection of a closed-transition paralleling generator system. Therefore, it is crucial that the engineer starts the utility connection process early in the design phase and continues the coordination process to a successful signoff of the generator paralleling system. Not meeting any of the utility paralleling requirements or concerns can delay start-up and possibly increase project costs as well as delay occupancy or operation of the facility.

Most utilities will consider closed-transition standby generation systems greater than 1 MW to be similar to distributed generation systems. In many cases, the utility interconnection requirements also will affect the facility’s service switchgear. Below are some of the interconnecting requirements that may apply to the installations of paralleled standby generator systems.

Timeline. The utility will identify timelines and milestones from utility receipt of application to initial review by the utility, the feasibility study agreement and findings, the impact study agreement and findings, the facilities study agreement and findings, and the interconnection agreement. For standby systems, it has been the experience of this firm that many of these milestones may not always apply because the utility companies have implemented interconnect standards for standby generation applications.

Utility fees. These fees cover application fees, utility charges for time spent on an interconnection and study (those mentioned previously in “Timeline”), and charges for utility facility upgrades, if necessary.

Equipment. The utility may require interconnecting equipment including:

  • Protective relays
  • Overfrequency/underfrequency monitoring
  • Overvoltage/undervoltage monitoring
  • Reverse power
  • Ground-fault protection
  • Overcurrent protection
  • Metering
  • Redundant relays
  • 110 or 48 Vdc power supply with alarm remote monitoring
  • Nickel cadmium batteries
  • dc trip units
  • Seismic ratings for switchgear and battery systems
  • Housing and connections of utility telemetry equipment.

Utility inspections. Usually, the utility will perform several inspections to ensure compliance with their requirements and include physically inspecting the generating facility and testing of the protective-relay operations. Upon completion of these inspection requirements, the utility notifies the applicant that the interconnection application is approved and the standby generation system can go online.

Figure 2: The Encore/Wynn Las Vegas 15- kV paralleling switchgear lineup.Sizing gensets

It is generally good practice to size the paralleling system using generator-sizing software and request the help of a manufacturer’s representative. Many factors influence genset sizing, making it time-consuming to manually calculate required generator capacity. Sizing software available from the major power-generation manufacturers will greatly simplify this process and allow for analysis of alternatives to optimize the generator size. Parameters that determine the required generator set size include minimum generator set load, maximum allowable step voltage dip and step frequency dip, altitude and temperature, duty cycle, fuel, phase, frequency, and voltage. It is important to identify every type and size of load the generator set will power. Additionally, when nonlinear loads are present, it may be necessary to oversize the alternator.

The more you know about the parameters that affect sizing, the better off you will be. The following is a general discussion of how various loads and electrical factors affect the sizing of generator sets.

Single-phase loads and load imbalance. Single-phase loads should be distributed as evenly as possible between the three phases of a 3-phase generator set to fully use generator set capacity and limit voltage imbalance.

Peak loads. Peak loads are caused by loads that cycle on and off, such as welding equipment, medical imaging equipment, or motors. Depending on how cyclic loads are taken into account, the generator size could significantly increase or decrease the size of the recommended generator set despite painstaking efforts to place loads in a step starting sequence.

Motor loads. Calculating specific motor loads is best handled by sizing software that will convert types of motors into load-starting and -running requirements. All motor loads greater than 50 hp should be modeled. A large motor with high inrush current may cause a generator voltage dip, which would affect other sensitive systems. The manner in which generator voltage recovers from this dip is a function of the relative size of the generator. Depending on the severity of the load, the generator should be sized to recover to rated voltage within a few seconds, if not cycles. Various types of reduced-voltage motor starters are available to reduce the starting kVA of a motor in applications where reduced motor torque is acceptable. Reducing motor-starting kVA can reduce the voltage dip, the size of the generator set, and provide a softer mechanical start.

Battery-charger loads. A battery charger is a nonlinear load requiring an oversized alternator based on the number of rectifiers (pulses)—up to 2.5 times the steady-state running load for three-pulse loads and up to 1.15 times for 12-pulse. These loads are typically found in telecommunications systems.

Uninterruptible power supply (UPS) loads. Another type of nonlinear load, the UPS system uses rectifiers or other static devices to convert ac voltage to dc voltage for charging storage batteries. Larger alternators are required to prevent overheating due to the harmonic currents induced by the rectifiers and to limit system voltage distortion by lowering alternator reactance.

Medical imaging loads. These include CT scan, MRI, and X-ray equipment. The generator set should be sized to limit the voltage dip to 10% when the medical-imaging equipment is operated with all other loads running to protect image quality.

Lighting loads. In addition to lamp wattages, all ballast wattages and starting/running power factors should be considered.

Regenerative loads. For loads such as elevators, cranes, and hoists, the power source is often relied on for absorbing power back fed during braking. That is usually not a problem when the utility is supplying power, because it can be considered as an infinite power source with many loads. A generator set, in comparison, is able to absorb far less power, especially with no other loads connected. Generally, the regeneration problem can be solved by making sure there are other connected loads that can absorb the regenerative power. Excessive regenerative load can cause a generator set to overspeed and shut down.

Future needs

A last step in the sizing equation has to do with future needs. Power use is not always fixed and tends to grow over time. Therefore, any generator set sizing exercise needs to take system expansion into consideration. Even with sophisticated software solutions, the final decision on generator set size needs to be tempered with judgment.

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