Generator Neutral

[Wilbur]

I am installing a new 480Y/277 500kw generator. I have received conflicting data on what to do with the neutral of the generator. The manufacturer does not wish to make a
recommendation. The manuf's rep says to float the neutral.
My idea was to run a neutral from the generator to the
transfer switch and bond the neutral to the system ground.
The 480v distribution is 3W, so there is no neutral to connect to. With a generator of this size, it is impractical to install a transformer to develop a neutral.
Any suggestions?

 

[genRman]

My reccomendation would be to press the manufacturer to find out why he does not wish to make a reccomendation.

You need to know if the neutral is fully rated or not.

Some manufacturers do not bring a fully rated neutral outside of the stator winding and hence their machnies fault carrying capacity phase to neutral is severly limited.

Have you any other generators on site, which you will be connecting in parallel with?
If the answer is yes, you need to know what pitch winding they all are.

 

[nitro]

Grounding
1. In any generator set installation, the frame of the generator must be positively connected to an earth ground or to the hull of a vessel. This connection is the first one made at installation, and the last one to be removed. If the generator set is on flexible or resilient pads, the ground connection must be flexible 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, and are frequent sources of radio frequent y interference. Silver soldered, bolted joints are electrically and mechanically sound. Where local codes permit, an effective ground connection can be made to an underground all metallic water piping system. Do not ground to plastic and cement-type water pipes, One or more approved corrosion resistant ground rods, driven at least ten feet (three meters) into damp soil, are effective. In dry, sandy soil, an extensive wire ground mat may be required.
2. “Y’’-connected generators should have the neutral (and frame) grounded when the generator is
installed unless definite measures are taken to prevent grounds on the load side.
a. If the neutral wire is grounded and one of the phase leads becomes grounded, the excessive current will open the load circuit breaker or collapse the generator voltage. The result depends on the particular generator electrical characteristics, type of fault, and circuit breaker trip rating. An undervoltage device may be required to provide adequate short circuit protection.
b. There are some instances in which it is undesirable to ground the neutral wire. In these
applications where definite measures (ground fault protective circuits) have been taken to
prevent grounds to the phase leads, an ungrounded generator neutral lead is
acceptable. Ground fault protection requires that the entire group of distribution circuits be studied and treated as a system. The owner should engage a competent consultant if a new distribution system is being developed, or if an existing system is to be modified for ground fault protection.
There have been reports of excessive circulating neutral current occurring when two or more
generators are operated in parallel. Generators should be installed with the assumption that no
excessive circulating neutral current will occur with the generators paralleled at full load. The
phase current should not exceed 100% of the generator nameplate ampere rating. In those cases where excessive neutral currents are encountered, either a neutral lead reactor or selective grounding of the neutrals may be used. If the selective grounding of one generator neutral is
used, a disconnect switch should be installed in each generator ground lead. The disconnect switch should have about 25% more current-carrying capacity than the generator line ampere rating.
Note: Three phase, “Y’’-connected generators can have a third harmonic voltage that does not cancel at the neutral. It is this voltage that causes the excessive circulating neutral current that can occur when operating in parallel.
Instructions concerning excessive neutral current and grounding the generator neutral also apply to former model generators. Most radio and audio frequency interference problems are corrected by proper grounding of the generator frame. In other instances, the cause is not the generator set, but equipment that may be


related or unrelated to it. A small, hand-held battery- operated radio is a useful tool in locating the source of interference. Tune the radio receiver to the frequency of the interference and use the receiver as a probe around pipes, conduits, or other metal parts. Emitted noise will increase as the source approached. Pipes rubbing together can cause radio frequency interference. There have been instances where steel and galvanized pipe connections create interference even though the joint is mechanically sound and leak proof. In areas adjacent to high power radio frequency generators (transmitters, heating equipment, etc.), a corroded joint or connecting acts like a very poor rectifier, and can emit interference at frequencies different from the exciting frequency.

The solution in all such instances has been physical bonding of the joint or conduit. A clean copper strap bridging the joint normally clears up the interference.

 

[dpc]

How is your generator going to operate - as a stand-alone system, or in parallel with other generators and/or utility?

As you probably know, if you have any existing 277V loads, you must solidly-ground the neutral.

If not, there are a lot of options and possible Code implications. Traditionally, at least in the U.S., we would normally solidly-ground the neutral, but you need to make sure generator is braced for this, since line-ground short circuit current will generally be higher than three-phase fault currents.

dpc

 

[Etrix]

Hello,

I'm going off on a tangent here, but this is w.r.t. what nitro brought up in his reply. Regarding the third harmonic voltage and circulating currents arising from that, what are the typical solutions to alleviate this problem in parallel transformers and generators? Can anyone point me in the right direction? Thank you.

Etrix