Distribution, dedicated vs tertiary feed? 1

[Mbrooke]

What are the disadvantages of using the tertiary windings of a 400kv to 132kv 500MVA Grounded wye auto transformer to feed a 22kv distribution system over having a separate 132-22kv transformer?

 

[bacon4life]

Putting overhead distribution exposes the tertiary to a high number of faults. Tertiaries windings have much higher fault currents than either the primary or secondary windings, leading to a higher likelihood of through fault damage. Is your distribution system a delta system?

 

[Marmite]

At first glance it might look like you are getting something for nothing, but the reality is you exposing your large,important, strategic and very expensive transmission transformer to the risk of damage for faults on the distribution system. An uncleared 22kV feeder fault could also bring out the transmission transformer and that could have significant effects on your transmission system with potentially 500MVA worth of lost capacity. Utilities tend to limit tertiary loading to reactive plant and not general distribution. I have seen some obscure tertairy voltages used in some substations which may have been deliberate to discourage someone supplying general distribution load.

Regards
Marmite

 

[Mbrooke]

Thanks! I had considered the risk of tripping a large power transformer, but not the fault currents. Can a tertiary winding be ordered with a standard distribution impedance such as 10%Z? Or would a large air choke reactor be able to knock down fault currents below 12 Ka?

Could a large power transformer be ordered as to have a similar cumulative fault withstand to a 132-22kv distribution transformer?



The distribution system is low impedance grounded. All loads out on the line are delta so the line itself is delta. To obtain a zero sequence source a fault duty zig-zag grounding transformer will be placed on the distribution bus bonded to the ground mesh via a 5 ohm resistor.



The reason it is being considered is because the cost is substantial reduced from not having separate distribution transformers.

 

[bacon4life]

Also, the voltage regulation on a tertiary is quite bad.

 

[Mbrooke]

How bad is the voltage regulation? Im assuming that any non symmetrical components recirculate in the delta that causes this?

 

[prc]

In one major country , when they were introducing their first 1500 MVA 765/420/33 kV auto transformer bank, utility planned to load tertiary by 5 MVA to feed station load. To meet this they wanted special impedances to tertiary so that regulation can be at manageable levels. All these made transformers complicated, costly and put reliabilty down by several points for the 765 kV system. I could finally persuade them to abandon this risky plan and get station supply from a separate 132 kV line.

In the current case,also it is bettter to avoid loading distribution system from tertiary. However if it is essential there is no problem. The tertiary impedances will be of the order of 60/45 % ( from HV and MV sides on 500 MVA base) But since the tertairy loading will be at much lower levels, regulation may not be an issue at tertiary terminals.The main point is whether you want disturbance in your 400 kv from faults on DT level.

 

[Mbrooke]

Regulation is making me double think this. What determines the level of regulation or lack there of?

 

[bacon4life]

I used the term regulation a bit differently than prc. He is right that the voltage drop due to the tertiary winding impedance will be fairly small.

The regulation I was referring to was attempting to keep the 22kV feeder at exactly 22 kV. In my neck of the woods, the 500 kV buses swing as can swing all the up 550 kV (+10%). The system operator uses the 500/230 kV and 230/115 kV transformer LTCs to regulate voltage at the lower levels to only swing a couple of percent. Depending on the exact configuration of the windings and LTC, the voltage variation on tertiary will be driven by the voltage variation on the common winding.

 

[Mbrooke]

But as long as the 400kv buss remains steady, the terirtary feed will always be around the same Pu voltage as 132kv secondary?

I did look into it, and you are correct, the available fault current coming from the tertiary windings will be significant to the point air choke reactors will be needed before feeding the distribution buss.



The other option was adding separate 132-22kv transformers right in front of the 132kv buss fed by the power transformers.

 

[prc]

In this case one more point is to be checked. With 400/132 kv autos, it is normal practice to provide tappings at neutral end(eg UK) of common winding. Then the transformer will be variable flux regulation type ie the flux density in core will vary with tap change, thereby the tertiary terminal voltage also.Please check the rating plate of the transformer.

 

[Mbrooke]

I will look it now that you mention it. That's actually a really good point If no regulation exists, then the pu ratio would remain about the same between EHV-HV-MV?


Just to ask, I could be wrong but in the UK, do any real world situations exist where an auto transformer tertiary feeds 33kv sub transmission? Ive heard if it being done but never found out how it was done, if it all.