Isolated Phase Busduct grounding 1

[radug]

Hello,

I have calculated the grounding grid of a power plant for one phase to earth faults and now I am concerned with the double phase to earth faults on isolated phase buses between generators and transformers.

I have come across two pages of ABB about it but I have many concerns.
-Should the IPB enclosures be grounded? In how many points?
-If they are not to be grounded, can the isolation from ground be assured? What happends with the isolation between IPB and transformer or generator bushings?
-Should I increase the size or add more grounding grid conductors below those areas?

I have been reading previous similar threads but have not been able to find specific information about this topic. Neither have I been successful finding information in the net, or the IEEE.


Thanks.

 

[7anoter4]

I think you have to connect the IPHB Enclosure to the Grounding Grid in a single point in order to avoid an eddy current loop. The Generator box should be insulated , the same all the supports, insulator heaters, blowers and all the instruments- thermometers, moisture testers and so on. The heaters, blowers and other shall be grounded at their power source. The connection shall be suitable for three phases or two phases short-circuit [the maximum of them].
As the return grounding current is very large the Grounding Grid shall be designed accordingly, from generator along the IPHB up to Step-up transformer.

 

[ScottyUK]

I think you are contradicting yourself a little when you propose a single earth bond between the IPB shield (casing) and the earth grid, and then say there may be very high fault currents in the earthing grid. There may well be very large currents flowing in the IPB shield and its bonding bars in the event of a LLG fault, but these will not flow through the earthing grid. The return path to the generator neutral is limited by the earthing resistor / earthing transformer, so any earth current is necessarily small.


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If we learn from our mistakes I'm getting a great education!

 

[RRaghunath]

Earthing the IPB at both ends or only at one end (insulating the other end) is a matter of design. Please look at the design basis for the project.
Earthing at both ends of the IPB is termed as 'zero flux' design and is practiced by many utilities (not withstanding the higher costs and copper losses).

 

[ScottyUK]

By 'zero flux design' do you mean the situation where there is near-perfect cancellation of the external magnetic field? That is achieved by interconnection of the three IPB shields, and is not a function of whether it is earthed or not. The bonding bars between the IPB shields carry the full image current and are essential to correct operation of the IPB. Additional earth connections may well interfere with the flow of image currents and result in imperfect flux cancellation, depending on where the earths are attached. The regions outside of the bonding bars does not have flux cancellation, so there are normally very high magnetic fields near transformer bushing boxes and machine main terminals where the IPB connects to other equipment.



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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

Thank you Scotty for your remark. I am not involved in utility activity but only in design power station. But I am still not convinced in the double earth fault case–one at IPB and one in generator or step-up transformer the short-circuit current will not flow through the grounding grid. I heard about a short-circuit flowing through the rebar of generator foundation producing rebar heating and even cracks in the foundation concrete. Do you think it is not possible?

 

[ScottyUK]

That is a good point and I had overlooked that case - you're correct, that would result in a significant current in the earthing grid.


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If we learn from our mistakes I'm getting a great education!

 

[prc]

Grounding of IPB depends on the type of construction.Generally these are of continuous enclosure construction ( so called zero flux) esp when high currents are involved.Here the enclosure will be carrying almost the same current as the inside conductor but in opposite direction.At both ends the three phases of enclosures will be shorted cancelling the currents.These IPBs are not isolated from the supporting metallic structure or the ground. Earthing can be done at both ends after the interphase shorting.

In case of IPB with non-continuous enclosure construction,each section shall be isolated from ground and earthed at one point to avoid circulating currents.

Please see section 5.3.4 IPB Grounding in colour book IEEE 665-1995 Generating station grounding.

 

[7anoter4]

You are right prc this is mentioned in IEEE std 665 ch.5.3.4 Isolated phase bus grounding. I forgot it!
Usually the IPHB is interrupted at the power station wall continuing outside up to step-up transformer. Thanks.

 

[RRaghunath]

Thanks ScottyUK for correcting me. I agree the zero flux design IPB need not be earthed at both ends (shorting the three enclosures at both ends is ofcourse is required).

 

[radug]

Hello,

I have reviewed bids by different supplyers (ALSTOM, SIEMENS, ABB, Delta-Unibus and DAQ0). They offer almost no details about the LLG faults and grounding. I also read IEEE 665 5.3.4. It says that in case of continuos enclosures, they should be bonded at both sides and also says that the busduct does not require isotation from the ground or structures. So, in case of continous enclosures, could I connect the IPB enclosures, generator, generator circuit breaker and transformer to the ground (sized for LLG faults) without care for ground loop losses?
Does any of you know about any good reference about this topic?

Thanks.

 

[electricpete]

I have calculated the grounding grid of a power plant for one phase to earth faults and now I am concerned with the double phase to earth faults on isolated phase buses between generators and transformers.

I haven’t followed your thought process closely, but I thought the whole idea of an ISOLATED phase bus was that L-L faults and L-L-G faults would not be credible.

So, in case of continous enclosures, could I connect the IPB enclosures, generator, generator circuit breaker and transformer to the ground (sized for LLG faults) without care for ground loop losses?

Yes, the enclosures are connected to ground periodically along the length. It should not cause dramatic increase in losses. In general, the sum of currents entering ground at a given location along the bus is close to zero since the three phase current sum to zero and three enclosure currents almost sum to zero. Since they may not exactly sum to zero, periodic grounding (rather than just periodic crossover bars) helps ensure there is a path for full cancellation of flux of each phase and also helps avoid voltage buildup.

Special attention is required at the ends. For example transformer low voltage bushing enclosures (doghouses) are insulated from the bus so that the phase crossover currents do not flow in the transformer.


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(2B)+(2B)' ?

 

[radug]

electricpete,

"I thought the whole idea of an ISOLATED phase bus was that L-L faults and L-L-G faults would not be credible"

A document from ABB says:
"In discussions on IPB earthing with ABB customers, it must be repeatedly explained that 2-pole faults are possible even for 1-phase IPB's. More than enough actual examples are a matter of recorded... It is possible that a broken support, or one of the wooden transport safety blocks which has not been removed, could lead to an earth-fault of one phase after commissioning the IPB. In this event the voltage level in the other two phases increases. In the example, an earth-fault occurs in the transformer phase R and the double earth-fault current starts to flow to the second fault location as shown by the green path. The second fault can naturally also occur in any of the other components. As already mentioned, the earthing connections between generator, transformer, protection cubicle etc. and the enclosure of one phase must be by an earthing conductor with AEmin thermic short-circuit proof connection so that the double earth-fault current can be safely carried until disconnection. "

What concerns me is that reading the above paragraph, for me the supplier considers "that a broken support, or one of the wooden transport safety blocks which has not been removed, could lead to an earth-fault of one phase after commissioning the IPB." I have no experience in commisioning, but, couldn't this topic be checked and repaired during commisioning? And if it was totally repaired, the LLG fault risk could be totally eliminated?

Last, what are crossover currents? What do you mean with "so that the phase crossover currents do not flow in the transformer"?

 

[ScottyUK]

ePete,

It's one of those 'perfect storm' scenarios where an uncleared earth fault exists somewhere on the IPB at the same time as another earth fault occurs in (say) the GSU transformer or in the machine. Considering the time taken to clear an earth fault I'm not sure how likely it is for a second fault to occur at a separate location during that period but it is 'possible', if only on paper.

I disagree about the use of additonal earth bonds within the flux-cancelling region, at least from a theoretical perspective. The amount of image current diverting into the earthing system may be small enough for it not to make a significant difference but it does still reduce flux cancellation to a lesser level than without the additional earths. It also knowingly and intentionally introduce normal operational current into an earthing system, which generally isn't allowed.


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If we learn from our mistakes I'm getting a great education!