Cable Insulation Rating question

[rand]

Hello,

I have a question regarding cable insulation rating.

To provide you with a background, we are tapping the iso-phase bus duct to power up an Auxiliary Bank for a steam turbine unit. The nominal voltage rating of this bus is 13.8kV; generator output is 13.8kV, and GSU xfmr is rated 13.8kV-230kV. The BOP design and construction company who built this unit installed medium voltage cable rated at 25kV, 133% insulation when tapping the iso-phase to the aux bank high side. Unfortunately, they installed a conduit that just the minimum size of what was needed, and we have come to a point where we need to up-size the cable.

My proposal is the following: instead of going with the same insulated-rated cable and having to install new conduit, downsize to 15kV, 133% insulated cable. This way, the overall insulation thickness of the cable is reduced and we can up the conductor size while still remaining within NEC fill requirements. In addition, this cable is readily stocked by Okonite (a cable mfr). Compared to this, 25kV rated cable (100% or 133% insulated cable) requires minimum order of 3000 feet and a 12 week lead time.

A plant electrician says we cannot do this due to the swings from the generator and grid-side. But I have dug up metering data from the past 2+ years that shows the max voltage occurrence that has ever happened on the iso-phase bus to be 14.65kV. In addition, I believe that the 133% insulation level will also provide plenty of added protection in case of potential faults, future voltage swings, etc.

Am I missing anything here? Is there a reason why the BOP contractor decided to go with 25kV @ 133% insulated cable? Is there any counter argument anyone can provide on why I cannot use 15kV @ 133% insulated cable for this 13.8kV system?

My suspicion of why this cable (i.e. 25kV @ 133%) was installed in the first place is because another unit built by the same contractor at the same has its' iso-phase bus rating of 18kV. The BOP contractor had to buy 25kV rated cable in bulk (cable manufacturers require minimum orders of 3000' as stated above) for this unit so they decided to use it for this other 13.8kV system instead of wasting the 25kV bulk-ordered cable. I haven't been able to confirm this with the BOP contractor since they built these units before my time at the plant.

I appreciate any help you guys can provide. Please let me know if you have any questions.

 

[ron]

Seems like a reasonable approach

 

[rand]

Seems like a reasonable approach

Thank you for your reply! Yeah, I am guessing I have my bearings right on this, but wanted to see if I can get feed back from anyone else who could argue why I shouldn't install 15kV rated cable in this situation.

 

[Jraef]

As long as you don't put it on the 230kV side...

There are probably hundreds of thousands of miles of 15kV cable strung around the world carrying 15kV and under... If it needed to be rated for 25kV, why make it? Sometimes people do that with the idea that "someday" they might change out the transformers and boost the voltage to get more power out of the circuit, so that way they don't have to pull new cables.

 

[ron]

We are installing 13.8kV feeders currently with 15kV rated conductors. I never would have considered anything higher. The client would have kicked me in the pants for wasting their $$$.

 

[kingpb]

You may be correct on the assumption about the other 18kV unit, but in power generation, assumptions are hazardous.

The measurements that you mention from metering are good to have but from a design standpoint not so useful. I believe what the plant operator may be referring to is the original design requirements that are required for the unit to be capable of; regardless of whether it has actually ever happened. These are regulatory requirements imposed by the system operator.

You need to confirm the maximum voltage swing capability of the system and the maximum voltage capability of the generating unit and also what is the guarantee operating capability.

The next question you need to ask yourself is, are you giving the client what they want? Plant guys have belts and suspenders mentalities, and don't like to to take risks and usually aren't looking for the cheapest solution. A unit going down can cost them a lot of headaches not to mention a unscheduled shutdown can cost the operator millions in lost generation. So if there is any question on the right approach, better to error on the conservative side. They won't question replacing the conduit (at a higher cost), but believe me you don't want to get the call if there is a problem.

 

[Julius Right]

In ungrounded systems where a ground fault will be cleared in 1 h then 133% is required [UL 1072].If the fault will be cleared in more than 1h 173% insulated cable is required. See for instance EPRI-EL-5036-V4/1987 pg.4-9.

 

[Julius Right]

See NEC [2014] Art.310.104(E)

 

[rand]

You may be correct on the assumption about the other 18kV unit, but in power generation, assumptions are hazardous.

The measurements that you mention from metering are good to have but from a design standpoint not so useful. I believe what the plant operator may be referring to is the original design requirements that are required for the unit to be capable of; regardless of whether it has actually ever happened. These are regulatory requirements imposed by the system operator.

You need to confirm the maximum voltage swing capability of the system and the maximum voltage capability of the generating unit and also what is the guarantee operating capability.

The next question you need to ask yourself is, are you giving the client what they want? Plant guys have belts and suspenders mentalities, and don't like to to take risks and usually aren't looking for the cheapest solution. A unit going down can cost them a lot of headaches not to mention a unscheduled shutdown can cost the operator millions in lost generation. So if there is any question on the right approach, better to error on the conservative side. They won't question replacing the conduit (at a higher cost), but believe me you don't want to get the call if there is a problem.

Thank you for the reply. You are absolutely right so I did further research on our side. The auxiliary transformer is fed from the iso-phase bus duct. It primarily will be used during the start-up of our steam turbine unit to power the auxiliary boiler and seal the turbine. This means that the generator circuit breaker (GCB) will be open and the aux transformer's primary feed will be getting power from the grid (i.e. the station's 230kV switchyard) via the main bank of the unit. The main bank ratio is 13.8kV-230kV. The absolute max swings per the customer on the 230kV side is +/- 5% meaning the highest voltage the iso-phase bus duct, and thus the aux transformer, can see is 14.48kV. The generator itself has over voltage protection set at 120% based on its nominal output voltage which is 13.8kV translating to 16.56kV. Anything higher and the generator circuit breaker will trip. Again, the AVR will for the most part keep the generator output close to its' 13.8kV rated output as shown by the metering data I found. Based on the 133% insulation rating of the 15kV rated cable, it can handle this swing nominally.

In addition, the relay protection scheme which controls the GCB, the 230kV rack circuit breakers, etc. acts on fault conditions on a matter of cycles to isolate equipment to prevent damage.

 

[rand]

See NEC [2014] Art.310.104(E)

Thank you for your reply. There is a relay protection scheme in place to clear any faults within cycles. So due to this, 133% shouldn't be needed as the fault will not be lasting anywhere near a minute or higher. But I would like to install the 133% insulated cable to handle the potential over voltage swings of >15kV.

 

[kingpb]

Thank you for the reply. You are absolutely right so I did further research on our side. The auxiliary transformer is fed from the iso-phase bus duct. It primarily will be used during the start-up of our steam turbine unit to power the auxiliary boiler and seal the turbine. This means that the generator circuit breaker (GCB) will be open and the aux transformer's primary feed will be getting power from the grid (i.e. the station's 230kV switchyard) via the main bank of the unit. The main bank ratio is 13.8kV-230kV. The absolute max swings per the customer on the 230kV side is +/- 5% meaning the highest voltage the iso-phase bus duct, and thus the aux transformer, can see is 14.48kV. The generator itself has over voltage protection set at 120% based on its nominal output voltage which is 13.8kV translating to 16.56kV. Anything higher and the generator circuit breaker will trip. Again, the AVR will for the most part keep the generator output close to its' 13.8kV rated output as shown by the metering data I found. Based on the 133% insulation rating of the 15kV rated cable, it can handle this swing nominally.

In addition, the relay protection scheme which controls the GCB, the 230kV rack circuit breakers, etc. acts on fault conditions on a matter of cycles to isolate equipment to prevent damage.

Other than some short circuit condition, sounds like you are on right track. Have you plotted the TCC with the existing protection settings along with cable damage curve you propose to use. This may be a good idea.

 

[kingpb]

I did think of something else; you did not mention what the voltage configuration is on the 13.8KV side, i.e. delta or wye. In the back-feed situation you can (I don't know your set-up) end up being in an ungrounded condition. If a phase were to go to fault, the voltage can rise. This should be looked at just to make sure you have covered all the bases.

Another possibility is that during unit operation, if there were to be a sudden removal of load, because of load rejection the voltage can momentarily shoot to 1.4 per unit. I think the 15kV 133% may be marginal at the value.

Just a couple points to look at.

 

[rcwilson]

Check capcitance

Check capcitance

Things to check:
1. Is there a grounding transformer on that section of isophase? Does that section operate ungrounded when the generator breaker is open? GSU and Auxiliary transformers are probably delta on the 13.8kV. If there are suitably sized wye connected VT's on the isophase bus or line side of the generator circuit breaker with ground detection a ground fault will elevate the other two legs to 13.8 kV to ground. VT windings should have a broken delta secondary with a loading resistor to supply capacitance charging current and create a high impedance ground. If the resistor is undersized, there may be excessive voltages. Note that the generator neutral is probably high impedance grounded and will provide the neutral ground when the breaker is closed.

2. Total capacitance connected to the 13.8kV? Cable capacitance determines the sizing of the damping resistor in the broken delta winding. I may be recalling this incorrectly but I think the 13.8 kV smaller diameter cable has higher capacitance than the 25 kV thicker insulation.

We have had to use 25 kV cable on 13.8kV isophase taps for the above reason, but that was usually on a gas turbine 5 kV, LCI, VFD static starting system that uses the generator as a 4 kV starting motor to spin up the gas turbine by energizing the generator side isophase at 4KV. LCI required a certain minimum capacitance to avoid operating issues.

Probably not the case with your STG, but something to check.