Commercial Generators
Written by Mike Morin, NEI Electric-MN
When considering a commercial generator system, there are questions a customer needs to consider: Why is a generator system needed? Is the generator for emergency power only? Will it be used for peak-shaving? Will the customer want to export power to the utility? What type of transfer switch will they need? Will multiple generators be operated in parallel? What type of fuel will be available to operate the generator?
NEI Electric recently worked with a customer who was in need of emergency backup power. The customer performed various mechanical tests at their facility and was experiencing power outages from the utility company which ruined test results and became an unexpected obstacle.
The customers’ electrical service for their facility was 1600 amp, 480 volt three phase. After analyzing their peak demand, we determined that they could reduce their service to 400 amps and install a 130 Kw generator. As a result of these findings, we separated the bus of the switchgear between the main fused switch and the distribution section and wired the automatic transfer switch (ATS) between these sections. The final connection for this work was performed after normal business hours because we had to de-energize the switchboard.
The location of the generator was unique for this particular site. The customer is located near residential housing and adjacent to a commercial building so noise from the generator would be an issue. The decision was made to install the generator inside the building on the second floor and duct the radiator and exhaust to the exterior. NEI hired a rigging contractor to accept the generator at their facility, transport to the customer site and raise it to an opening cut through the exterior wall. A rigging company is hired because they have the right equipment and knowledge to move heavy equipment, reducing the chance of injuries or damage to the equipment.
Upon completion of the installation, commissioning of the generator was scheduled. Commissioning included having a factory technician on-site to check out and start the generator. Part of the testing included simulating a power outage to verify the generator started properly, the transfer switch operated as designed and the generator carried the load of the building correctly.
A second function of a generator system is peak shaving. Peak shaving is a term used to describe a generator installation with the purpose of reducing the load on the electrical utility company.
When peak shaving, the generator is sized to carry the full peak load of the customers’ facility which is predetermined by the utility metering departments. Generally the minimum load the customer must have on their system to consider peak shaving is 50 Kva. In an agreement with the utility company, the customer agrees to operate their generator when requested by the utility company. The benefit to the customer is a reduced electric rate year round. Check with your local utility company regarding the peak shaving programs and rates they offer.
Another decision to consider is what type of transfer switch is needed. If the generator is installed strictly for emergency power then an open transition transfer switch is adequate. This type of transfer switch opens the normal contacts before it closes the emergency contacts and vice versa when normal power is restored. When this process is used, the facility lights will blink and motors will stop when the power is transferred.
If any power interruption is an issue, then either a closed transition or a soft load transfer switch would be required. A closed transition switch has the emergency and normal contacts parallel for .1 second assuring the power transfer goes unnoticed. If exporting to the utility company or the demand is high, a soft load transfer switch is considered necessary. It provides a seamless load transfer with no interruption of power and meets all protective requirements laid out by the utility company.
Additional considerations to make in a generator system design: At facilities with multiple generators operating in parallel, more switchgear will be required to provide paralleling of the generators. The type of fuel available to operate the generator; generally the smaller units operate on natural gas and the larger units operate on diesel fuel.
A generator system design is a critical decision for a customer. NEI Electric has professionals who can assist you in assessing your current electrical system and in making the decisions that will best fit your company and facility.