Wye Winding Neutral Cables? 2

[cabletray]

When a generator with a wye winding is connected to a wye-wye xfmr, is a cable connection required between the two winding neutral points in generator and xfmr primary? (Such an existing installed system here has four full-sized neutral cables, but the consultant design drawings show none.)

 

[RRaghunath]

Your current arrangement allows the generator to feed the earth faults on the secondary side of generator transformer most effectively (I don't know whether that was the intention of the designer).

I think this is helpful if the system on secondary side of generator transformer has no other zero sequence source and depends exclusively on the generator.

 

[rbulsara]

Agreeing with raghunath, there is no need for the neutral between the generator and the transformer primary.

 

[PWR]

If you don't connect the neutrals, you will not have a stabilized L-N voltage and cannot connect single phase loads on the secondary winding except line to line.

 

[cabletray]

Regarding PWR's post, the xfmr secondary voltage in this case is 12.47 kV and feeds the plant main standby bus--so no single-phase loads--but thanks anyway for discussing the broader principle.

The consultant design did include equipment grounding conductors to carry the earth faults.

 

[rbulsara]

Pwr:

I think OP is referring to the primary side. The norm for Y-Y xfmr is that the primary N and secondary N are internally bonded together and secodnary N is grounded for gorunding the system. The system shoud be grounded whether or not single phase load exist. But there is no need for running neutral with the primary side of a xmr. It is a 3 phase load by itself.

 

[alehman]

In order for zero sequence (ground) current to flow, both transformer neutrals must be grounded and connected to the generator neutral.

Also as mentioned by PWR, the generator would be operating essentially ungrounded. This is typically not desirable due to the possibility of high transient overvoltages.

 

[mc5w]

Under US rules the primary of a wye-wye transformer must have the primary neutral connected to the supply neutral. This is to prevent parasitic overvoltage due to 3rd harmonics and slight differences in the magnetizing current of each phase of the transformer. If the secondary is used as a grounded wye system then normally both neutrals are solidly bonded together.

An abnormal situation would be a dairy farm where the primary and secondary neutrals would be connected together using a 5 KV class lightning arrestor or inductance. The 2 neutrals then use 2 separate sets of grounding electrodes that are say 20 feet apart. At the far end of a long distribution circuit the voltage drop in a perfectly good primary neutral is enough to cause a step potential that would bother cows. When cows are being milked the situation is similar to how I got a nasty shock form picking up a 1.5 volt dry cell by the ends - I had let too much perspiration salts build up on my hands. Cows do not like the Cow Voltmeter Method.

However, sometimes a wye transformer secondary is used instead of a delta secondary, particularly at 480 volts or 600 volts. In that case the secondary neutral needs to be taped up. Primary neutral still must be connected to the supply neutral to prevent parasitic overvoltages.

In some cases the secondary system is resistance grounded. Primary neutral still must be connected to the supply neutral. In some cases the secondary grounding electrode system is isolated from the primary neutral.

If you are connecting a wye generator to a wye system using a wye wye transformer then the transformer needs to be a 5 legged or 6 legged core 3-phase unit or three single phase transformers. This is so that any zero sequence voltage or current will magnetize the core(s) so that there will be a zero sequence impedance that will prevent circulating current. A wye generator has almost no zero sequence impedance of its own since zero sequence current produces almost no stator magnetization. The reason why I say almost zero is that a straight phase to neutral single phase load encounters negative and positive sequence impedance which makes things a little more complicated.

If your generator step up transformer is wye-delta-wye then the delta tertiary provides the secondary zero sequence current. Therefore, the generator neutral and tranformer wye winding that is connected to the generator can both be resistor grounded. This gets rid of circulating current in the generator circuit and provides a path for fault current that will operate protective relays for most ground faults in the generator circuit. At the same time, ground fault current is limited to a nondestructive value. However, to detect ground faults close to the neutral point of the generator or transformer primary you need to inject a test current such as 115 volts 400 Hertz from and aircraft ground power unit using a current limiting resistor.

 

[rbulsara]

mc5w:

What US rules are these?

 

[PWR]

Extending alehmans remarks: You cannot simply ground either or both sides of the transformer and expect that you will have a stabilized neutral. You will not unless you connect the source neutral to the transformer. I have witnessed what happens when a wye wye transformer is solidly grounded but supplied by a source that is not and the neutral is not carried to the transformer. In that case, the line to ground voltage will float and if you have the misfortune to have lightning arresters rated for solid grounded service some of them will likely fail rather quickly.

 

[rbulsara]

I do not agree with advise of running the neutral to xfmr primary. Grounding the generator (at the source) is essential, regardless of the what type of transformer it feeds.

What do you do if the primary were delta? The primary of the transformer does not stabilize the voltage as PWR correctly mentioned, but that has nothing to do with running or not running the neutral. The tx may be wired for Y primary, it is still a 3 wire load, no neutral is necessary, not different than say a star connected motor.