Lead Numbering
The Wye (Star) configurations and the Delta configurations are the most common generator lead connections. The following three-phase connection diagrams illustrate the proper connection and lead identification.
The leads are numbered clockwise from the top and from the outside inward. The diagrams show lead numbering for the six and twelve lead generators.
Wye Configuration Diagrams
6 Lead
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| Illustration 1 | g00611486 |
6 Lead Wye Configuration Terminals T4, T5, and T6 become neutral connection when the terminals are tied together. | |
12 Lead
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| Illustration 2 | g00661863 |
12 Lead Wye Configuration - High Voltage Terminals T10, T11, and T12 become neutral connection when the terminals are tied together. | |
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| Illustration 3 | g00611608 |
12 Lead Wye Configuration - Low Voltage Terminals T10, T11, and T12 become neutral connection when the terminals are tied together. | |
Delta Configuration Diagrams
6 Lead
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| Illustration 4 | g00669319 |
6 Lead Delta Configuration | |
12 Lead
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| Illustration 5 | g00669312 |
12 Lead Delta Configuration Terminals T6 and T9 become the neutral connection when the terminals are tied together and grounded. This position reflects the terminal T2 and T10 connection as the high phase. | |
Grounding the Frame
In any generator set installation, the frame of the generator is positively connected to an earth ground. This connection is the first one that is made at the installation. This connection is the last one that should be removed. The ground connection must be flexible in order to avoid possible breakage in later operation.
Ground connection cable or straps should have at least the current carrying capacity of the largest line lead to the connected load. Joints in cables or straps must be clean, free of electrical resistance, and protected from possible oxidation. Bolted ground connection joints eventually oxidize. The joints are frequent sources of radio frequency interference (RFI). Silver soldered and bolted joints are electrically and mechanically sound.
Neutral Connections
The generators with grounded configuration usually have the neutral grounded when the generator is installed. However, there are some cases when definite measures can be taken in order to prevent ground faults on the load side. The purpose of the grounding of the neutral is to prevent load side equipment damage. The purpose of the grounding of the neutral is also to prevent harm to personnel.
If the neutral wire and one of the phase leads becomes grounded, the excessive current will open a load circuit breaker to isolate the fault. The excessive current will collapse the generator voltage, if the circuit breaker does not trip first. The result depends on the particular generator electrical characteristics, type of fault and trip rating of the circuit breaker. An undervoltage device may be required in order to provide an adequate short circuit protection.
There are some instances in which grounding the neutral wire is undesirable. An ungrounded generator neutral lead is acceptable in applications in which definite measures have been taken to prevent grounds to the phase leads. Examples of such measures are ground fault protective circuits. Ground fault protection requires that the entire group of distribution circuits should be studied and treated as a system. The owner should engage a certified and registered consultant if a new distribution system is being developed. The owner should also engage a certified and registered consultant if an existing system should be modified for the ground fault protection.
Neutral resistors and reactors may be added to the system for two reasons: to provide protection during faults and to limit neutral currents.
Single Units
In a three-phase, four-wire system, the neutral wire should be grounded according to local wiring codes.
In applications, in which definite measures are taken in order to prevent grounds to the load leads, an ungrounded neutral can be used. Be sure to check your local wiring codes.
Multiple Units
Operation of multiple generators in parallel, having all neutrals grounded, may result in the circulating current through the neutral connections. In order to eliminate the possibility of circulating currents, ground the neutral of only one generator. If multiple generators are alternated on line, a switch should be installed in the neutral ground circuit of each generator. In this case all neutral ground circuits except one can be opened. Be sure that one of the neutral ground circuits is closed.
Parallel to Utility
When a Wye (Star) connected generator operates in parallel with a utility system (infinite bus) with the secondary of the step-down transformer also a Wye connection, the following may happen. The grounding of both Wye neutrals may result in circulating currents through the neutrals. Also, the coordination of ground fault protection requires an entire system study. A study should be done by a certified and registered consultant who is familiar with generator systems. The study will determine which grounding method should be used.