4 typical designs for connecting generator sets to low voltage system
Connection of generating sets to the LV system
Many different system designs are possible, but for maximum reliability, systems are usually configured so that the generator set (s) are connected at low voltage. With the minimum number of transformers and circuit breakers between the generator set and the load to be served.
4 typical designs for connecting the generator sets to the low voltage system (photo credit: Cummins)
Local laws often require that emergency loads be electrically separated from non-urgent and privileged loads in service, so that overloads result in the elimination of non-emergency loads, as they provide the greatest reliability of service to the most critical loads in the world system.
In most cases, a neutral conductor will be required. Since many loads and their controls at low voltage will be single phase, requiring a return conductor. Particular attention should be paid to the need for grounding and neutral switching of the system. This design could also be used for a small main power application.
Let’s see the four most common designs for connecting generator sets to low voltage systems:
1. Generator set serving common loads
2. Several generators serving common loads
3. Single generator set backup applications
4. Several generators, several ATS applications
1. Generator set serving common loads
Generator sets are generally equipped with a main circuit breaker mounted on the generator set and service to the loads is provided by a separate distribution panel, as shown in Figure 1.
Generators should have overcurrent protection which can come in various forms, including a circuit breaker mounted in the distribution panel as shown in Figure 1.
Over-current protection is usually required for generator sets, but short circuit protection is not. For example, it is not necessary to protect the short circuit between the generator set and the main circuit breaker.
The importance of this is that the protection can be located on the generator set or in a remote panel. If the generator set breaker is omitted, a disconnect switch may still be required by code on the generator set, in order to provide a point of isolation. Refer to local codes and standards for generator disconnection or isolation requirements.
2. Multiple generators set serving common loads
Figure 2 shows a similar application with groups in parallel replacing the single generator. In this situation, the generator sets can be specifically selected to be of multiple sizes to help minimize fuel consumption at a site by closely matching the capacity of the operating equipment to the loads in the system.
The use of generators of different sizes may require specific system grounding arrangements.
3. Emergency applications for single generator set
Shows a typical example diagram of single assembly power transfer for low voltage utility power, as can be applied to many households, commercial and industrial applications.
An Automatic Transfer Switch (ATS), which may use contactors, circuit breakers, or a dedicated transfer module, is used to transfer electrical power at the utility load to the generator.
Three-pole generator and circuit breakers or fused switches are often used to limit the level of fault present on the ATS. The ATS can be a 3-pole (solid, unswitched neutral) or 4-pole (switched neutral) device. Generally, two-pole ATS equipment is used in applications where it is necessary to isolate the supply neutral from the generator neutral.
The choice of neutral switching equipment can be related either to safety considerations or if the system is required to incorporate earth fault detection devices. The utility provider should be consulted to confirm the type of earthing system (earthing) used in the utility distribution system supplying a site and to verify that the grounding arrangements offered at the site of the client are appropriate.
Power transfer switches and generator sets should not be connected to a utility before this review (and utility approval, if required by local law).
Note that some local codes and standards require the use of multiple transfer switches due to the need to isolate emergency loads from standby loads.
In these cases, the transfer switches may be located on the load side of the utility distribution panel, and the generator set may also require a distribution panel when the feeder circuit breakers for ATS equipment cannot be mounted on the generator set.
4. Several generators, several ATS applications
Larger systems can use multiple ATS units and protection located near loads. These are often considered more reliable than those using a single large ATS, as distribution system failures are more likely to occur on the load side of a distribution system and using multiple switches would result in disruption. Lesser of the system when a fault occurs.
Figure 4 illustrates a design suitable for large installations, in particular when several buildings are served by the same generator installation.
In this system, three ATS units are used, powered by a common utility and generator system. This scheme can be further adapted to operate from separate utility systems.
Four-pole switching devices are commonly used with three-pole generators and utility or fused switches. Each ATS has automatic utility failure detection and will send a start signal to the generator system and switch to generator power when the generator is within an acceptable tolerance.
This scheme allows a versatile generation system to be built and can be easily adapted to multiple sets.
Cummins 250kVA Generator Set — Introduction and Testing
Load test of a 500 kVA generator
A follow-up to my August 2011 generator test video. Every six months (unless there is a power failure in the meantime), we perform a load test on our main standby generator in the office. Rather than hiring a dummy load, we just use the electrical equipment in the office itself, which also allows us to identify any “easy trigger” breakers in the building.
We don’t do a full load test, but a combination of all the lighting, air conditioning, and the computer server room gives us around 150kW, or almost 40% load.
500KVA Cold Start Diesel Generator Set
500KVA Cummins Diesel Generator cold start / Backfire, stayed up for a while and the temperature was -20 ° C. The controller stopped him after he straightened up at the end.
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