Auto Tr tap changing ( Tap raise output volt reduces !!) 1

[bvc]

Auto transformer tap changing

Dear Friends,
would like to share an interesting experience during my work as a commissioning Er. We were working in a 380 kV/132/13.8 kV transmission SS. The transformers were Areva make Autotransformers 502 MVA each ( 2 nos). The first charging of the transformer was done from 132 kV side. After 4 days, it was decided to switch on a 150 km 380 kV line on No- load on the 380 kV side bus of the above Tr. As soon as the line was energised, the voltage at the remote end was reported as around 405 kV ! (due to Ferranti effect). Now, the maintenance Ers. gave instructions to reduce the voltage by lowering the Tr. Tap ! Immediately, the voltage on 380 kV side increased further. There was some heated arguements between the maintenance and commissioning personnels as to why the voltage was increasing when the tap is lowered ! What followed was a beautiful explanation of the transformer output equation and concept of emf/turn from one of our seniors. The variation of the transformer output due to tap changing gets reversed based on which is the source side for the transformer. Thus, the general idea that "if the output voltage is low, raise the tap and if the output voltage is high , lower the tap" doesn't work when the voltage source side is on the secondary side. We had to raise the tap to lower the primary side voltage.

Thanks

bvc

 

[cranky108]

Typically the load changing tap changer is on the low side of the transformer, and it is assumed the high side is the source. However when back feeding a transformer, if you want to lower the high side voltage, you must rase the tap on the low side. It's all in the transformer turns ratio as to why it is backwards.

Most people don't have a chance to see this happen because the network is typically energized on both ends of the line.

Another issue is why you don't have switchable reactors on this line?

 

[ScottyUK]

Most GSUs have the OLTC on the HV side because changing taps on a winding carrying 10 - 20kA is a bit harder than switching a few hundred amps at transmission voltage. Where currents are more reasonable then it makes sense to switch taps on the LV side.

 

[nagasivasankar]

In IEEE Transformers Committtee I have presented a tutorial on "Taps in Autotransformers". In the tutorial the above topic is covered. The paper can be accessed in Google and also in

http://www.transformerscommittee.org

 

[cranky108]

Scotty, I guess I might agree, however I have never come across an OLTC on a larger GSU here. It's normally not done because it is usually determined that the costs out weigh the benifits.
Is it common there?

I have seen NLTC on GSU's, and they are on the high side, but that's also the way on load serving transformers.

I guess it comes down to knowing your equipment.

He did say an autotransformer, so I assume this might not be a GSU. At least we don't normally use autotransformers for GSU's here. A wye-delta is normally used for a GSU, because between that and the high impedance of the neutral resistor/reactor, the ground fault is very low, and the ground relaying can be very sensitive on the generator.
I haven't looked into generator protection outside the US, so I may learn something.

 

[bvc]

Thanks

 

[ScottyUK]

Hi Cranky,

Missed that this was an autotransformer. Oops.

OLTCs are pretty much standard for generator transformers over here. Our Grid Code is fairly prescriptive in terms of generating plant capability in order for it to be connected to the system: I can't remember if provision of a tapchanger is explicitly required by the Grid Code (I don't think so) but for the machine to achieve its declared output through the defined range of operating conditions an OLTC is required.

Protection-wise I don't think we do anything radically different to protect our generator / GSU blocks compared with what you do over that side of the water.

 

[cranky108]

Scotty, have you had instability problems with your set up?
We do use capacitor banks for adding transmission vars, or reactors, at times, to reduce transmission vars. But we also require generators to produce some amount of spinning vars.

The problem is that as the voltage drops, the reactive output of capacitors drops very quickly, and we can see voltage stability problems. This can get worse with large inputs of wind generation, as some of the wind machines use capacitors for var support.

The solution so far has been to use rotating var support, or DVAR units or large sub-transmission capacitor banks to help break the link between voltage and var output of sources.

 

[prc]

Please have a look at the rating plate.There the voltages corresponding to different voltages will be indicated.Generally tap no 1 will correspond to the highest voltage and the voltage will come down as the tap number goes up.This is the convention followed.

From what you have mentioned, for this unit the tappings may be for LV voltage variation.You can confirm it from rating plate.In such auto transformers, tappings can be provided for HV variation also.In that case, if you had raised the tap number the LV voltage would have gone up.This is because for the same in coming voltage if you increase the tap number, the turns on HV side comes down,increasing the per turn voltage,thereby increasing the LV voltage.

 

[ScottyUK]

Transmission isn't really my field, but we don't have a lot of stability problems as far as I'm aware. The UK has a geographically small and heavily interconnected grid compared with the US, so some of the stability problems you might see in the US with very long interconnectors don't affect the UK to the same extent.

Most of our wind generation is connected into the distribution system rather than on to the grid, typically at 11kV or 66kV. One of the other UK members - Marmite - works for one of the DNOs and can probably tell you a lot more about how embedded generation such as wind affects the distribution network.