Current at Star Point on WYE generator 4

[Dakotahawk]

The star point on a Wye generator connects to Phases A, B, C, and ground through a nuetral transformer. In a perfectly balanced system, is there current at the star point?

 

[stevenal]

Just for fun, lets unbalance the load. The current at the star point is still zero. We know that the sum of currents entering any point is always zero.

 

[peterb]

Yes, but now the zero sequence (unbalanced) current will flow in the neutral - see above!

 

[SteveKW]

I have a simple question...

If I make a light bulb out of three filaments and "star" connect them in the bulb and then bring out the three leads so they can make the "star" of my generator, will the bulb burn when the generator load is "balanced" or will it only burn when the generator is "unbalanced"?

 

[peterb]

A simple answer - it is abundantly clear from everything that has been said above that load current flows at all times in the phase leads connected to the star point. Current will only flow in the neutral lead connected to the star point when there is an unbalance condition involving ground or when triple-n harmonic currents are present.

 

[dipstick]

The argument that current flow can only be measured between two points does not negate the fact that current is defined as the flow of electrical charge carriers. They are measured in amperes which are defined as 6.24 X 10 24th power electrons moving past a specific point in one second. In a balanced three phase generating system, the current flowing in, out, around or through a star point has to occur or the generator would not work. The argument has been made that when added by vectors, the phase currents sum to zero and they do, however they still flow. There are electrons flowing in, out, around and through the star point. They are equal to the rms value of single phase peak current and they are constant. They have four possible routes of travel and as long as everything is in balance, they will use three.The answer as I understand your question is yes. The voltages when added in vectors equal zero and that is why no current flows through the neutral lead to ground. The star point and ground are at equal potential constantly.

 

[Dakotahawk]

Thanks Dipstick! I think you summed up quite clearly the "current flows" side of the argument. PeterB gets the atta-boy on the "no current flow" side of the argument. It's interesting that you can argue either side of this question and make complete sense. This has turned into quite a discussion on-the-job also.

 

[jbartos]

Suggestions:
1. Differential-mode currents cancel in a balanced voltage supply and load system, i.e. Iad+Ibd+Igd=0
2. Common-mode currents can flow through the neutral and ground even for the balanced supply and load.
(Iad+Igcm)+(Ibd+Igcm)+(Icd+Igcm)=3Igcm
where 3Igcm=common-mode currents, which may be caused by various causes, e.g. by a dc component. Hypothetically, one can connect a battery between the star neutral point and ground to generate the common-mode currents.

 

I have recently come across a similar issue in dealing with medium voltage drives. Does anyone have any comments on the common-mode voltage problems (voltages that cause shift from the neutral point in medium voltage motors) associated with retrofitting medium voltage motor with new medium voltage VFDs?

The waveform created by the switching algorithm is Additive PWM and 9 step, in cases where the gated switches simultaneously connect to the + or the - busway, a more severe neutral point shift is created with respect to voltage across teh motor windings. This is not a big deal with LVM because of the inverter wire that is so widely used (spike resistant), as well as the fact that the voltages are not so high. However, with 4160V this can be a real problem, especially with retrofits (old insulation sytems).
It has been determined that this problem can be resolved by employing the combination pf an input isolation xfmr and a complicated control scheme that involves a Neutral Point Clamp.
the switches are 3300V IGBTs.
Can anyone explain how the two work together to solve this problem? I can sort of visualize it, but its complex as hell!

Input xfmr has 12 taps, 22.5, 7.5, -7.5, and -22.5, (so it provides 24 pulses to the rectifiers)

 

[TheDOG]

tuffone,
Differential mode currents are caused by VSD's because of the switching of the IGBT's - voltages at the motor terminals are not a sine wave and therefore the vector sum of the three phases does not equal 0. See jbartos' post above.
Is the motor connected in star or delta? And what is "complicated control scheme that involves a Neutral point Clamp."?