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3 phase Power transmission question

11/27/2021 4:09 AM

I'll use my simplified model of 3-phase generation. Assume the alternator has 3 coils at 120⁰ apart; each is connected to the primaries of 3 step-up transformers. One terminal each of the secondaries of the transformer is connected to the HT transmission cable forming a 3-wire long distance cable. Let the other end of the secondary coils be joint together and let this node be Z; assume Z is earthed.
My questions are:
1) Is my simplified model acceptable.
2) If 1) is acceptable, then can the model work if Z is not connected to earth, say it is left hanging in air.

3) If Z can be left hanging in the air, is there a consensus if Z should be earthed? Or what is the actual practice?

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#1

Re: 3 phase Power transmission question

11/27/2021 7:57 AM

You are describing a delta-wye transformer where point Z is the neutral. If the neutral is not grounded, if there is a fault to ground of one of the phases, the other two phases will have a higher than normal voltage with respect to ground, which is an unsatisfactory situation.

https://en.wikipedia.org/wiki/Delta-wye_transformer

https://www.eeeguide.com/ungrounded-neutral-system/

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#2
In reply to #1

Re: 3 phase Power transmission question

11/27/2021 8:46 AM

Thanks. Exactly cleared my doubts.

I have near zero knowledge of 3-phase ac transmission. Just speculating from my rudiments of electromagnetic physics.

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#3

Re: 3 phase Power transmission question

11/27/2021 11:54 AM

"Delta is dangerous" is a known phrase, because Delta has one leg at 240 volts while the other two legs are 120 volts. I don't know why. Wye three phase has all three legs at 120 volts. You can get bulbs fried if you connect their breaker to the 'hot' leg in a Delta breaker box. Words of wisdom and experience.

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#4
In reply to #3

Re: 3 phase Power transmission question

11/27/2021 1:39 PM

Are you referring to the typical residential service where there are two "phases" 120 vac to neutral and 240vac phase to phase?

This is not 3 phase delta power but a single phase 240 vac from the transformer secondary on the pole.

The center tap (neutral) is grounded giving 120 vac lines which are 180 degrees out of phase resulting in 240 vac line to line.

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#5
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Re: 3 phase Power transmission question

11/27/2021 6:30 PM

Your picture looks like wye to me, three phase. I was talking about three phase Delta, where motors will get all three hot wires. A breaker box with Delta in it can power three phase motors from a three leg breaker, single phase items from a two leg breaker, and lights and outlets from a single leg breaker. Just don't put that single breaker on the 'hot' leg, test with a voltmeter before installing that breaker. That breaker box will have a 'white' terminal for single phase wires, and a 'green' terminal for earth/ground.

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#9
In reply to #3

Re: 3 phase Power transmission question

12/02/2021 5:05 AM

In a 4 wire,delta connected system, where only the mid point of one phase is connected to ground,the High leg conductor(or busbar) is required to be Orange in color at every point where there is a connection, if the grounded conductor is present.

Other permanent effective means can be used,but orange is the most frequently used color.

Utilities may use any color they please,as they are not subject to the NEC.

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#6

Re: 3 phase Power transmission question

11/27/2021 11:29 PM

The winding of the generator side of the transformer can have an influence on the ground fault characteristics of the secondary.

A long transmission line will have reactance, resistance and distributed capacitance. These usually cannot be ignored in any power transmission analysis. The voltage class of the line has a big influence on the values of these parameters. Also, the construction methods have a big impact on those values, unbalanced reactance on open lines due to phase transposition, for example.

If it is a short line, then those factors can be safely ignored, but there is not much need for any analysis.

You may have the line ungrounded. The BIL of the line can be adjusted for this case, so your equipment needs to be rated with a higher withstand voltage, at a higher price. A ground fault can be left on the system, but the risk of equipment damage due to an arcing ground fault generating very high transient voltages is possible. A second ground fault now becomes essentially a phase to phase fault. So generally a transmission line above 15kV is solidly or intentionally grounded.

While the voltage to ground on a faulted ungrounded system is offset, the voltages between the phases does not change, so the load equipment will operate normally, providing the insulation can take the higher voltage, i.e. 33kV to ground instead of 19kV on a normal 33kV unfaulted system. Your system may be grounded, but could be considered effectively ungrounded. The BIL of such a system might be adjusted up from 150kV BIL to 200kV BIL, considering an effectively ungrounded system.

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#7

Re: 3 phase Power transmission question

11/29/2021 2:42 AM

"HT transmission cable forming a 3-wire long distance cable" - this is not normal. I suppose there will be a HT circuit breaker at the transformer HT terminals.

There is a risk of resonance taking place between inductive reactance of the transformer (especially when the transformer is unloaded or lightly loaded or when the grid voltage is higher than normal) and the capacitive reactance of long cable

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#8

Re: 3 phase Power transmission question

11/30/2021 5:09 AM

<...Is...simplified model acceptable...>

To whom?

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