Transformer Tap Settings

[cjcoronel21]

Hey everyone,

I am looking into adjusting a tap on your 110MVA Main step up transformer. I know the voltages are a direct correlation to the turn ratios. The issue we are having is the voltage on the low-side is getting to high, forcing inverters to trip since they are out of their voltage operating range. The Tap is currently set to neutral. It is a automatic load tap transformer however we are having issues with the automatic adjustments, therefore will set to a single tap setting.

What else should i look at inorder to determine the optimal tap setting. I have low side and high side voltages for the past week but are there any other equations that will fine tune my selection besides Vp/Vs = turns ratio = corresponding tap setting?

 

[jghrist]

What are the primary & secondary voltages? On which side are the inverters? You call it a step up transformer. Do you have generation on the low side and a utility source on the high side? Is the tap changer on the low side or the high side?

 

[cjcoronel21]

The primary and secondary are 138kv and 34.5kv respectively. The inverters are on the secondary side of a medium voltage transformer, stepping the voltage from 370V to 34.5kV to the collection system. Yes generation is on the low side, and connected to the utility on the high side. The tap is on the secondary side( 34.5kv)

 

[rockman7892]

In industrial applications I have typically seen load flow studies performed in order to determine the optimal transformer tap setting based on the secondary voltages at low and peak load conditions.

Along with this there is other system conditions to consider like weather or not the utility voltage is high/low, etc...

 

[pwrengrds]

Fix the automatic tap changer would be the best solution, do you know what's wrong with it? (If it's operating backwards that's an easy fix).

To determine an optimal setting for a fixed tap, determine the high side average voltage and put the tap in the middle of the operating band, that should get the 34.5kV to mid band. If the 34.5kV voltage goes consistently too high raise the HV one tap at a time until it gets to a band the plant can live with.

 

[rcw retired EE]

If your inverters are capable of putting out reactive power (KVARS) there will be a significant voltage rise through the transformers. If these were normal generators with a voltage regulator controlling generator voltage the auto tap changer and the voltage regulator would fight each other, depending on which side of the transformer your auto tap changer control is monitoring.

If the tap changer is trying to control voltage on the 138kV side, it will not be able to move the voltage much since it is pushing against the whole utility system. The controller is probably monitoring the 34.5 kV bus and trying to control the 34.5 kV voltage. But that voltage is also controlled by the inverter output voltage controller and affected by the amount of kVAR's flowing through the system. Usually the solution is to turn off the tap changer control during generation.

You best bet is to model the system in ETAP or Easy-Power, run a few simulations over a range of full power out to minimum power at various utility voltages and pick the best compromise.

 

[prc]

I think your step up transformer is 0.37/34.5 KV unit and 110 MVA is the collector unit. Look at the rating plates of both units and find out on which side the taps are shown in the diagrams. From the tables on rating plates, find out on which side the voltage is varied with tap changing. There can be taps on HV for LV variation. Once these details are available, you can select appropriate taps on both units so as to mitigate your problem.

 

[prc]

One of the trends coming with smart grid( ie with renewable energy generators feeding to grid) is distribution transformers with automatic on-load tap changers(OLTC).MR has developed a small compact OLTC for fitting on distribution transformers(MV side)

 

[ScottyUK]

They look useful for certain locations prc. Have you any idea of cost?

 

[prc]

Scotty, I have no idea of price. But from the look, it must be very much inexpensive. Of course it is used in special cases, but I thought this particular case under discussion may demand such units.
Many years ago, in India, when under voltage was a perennial problem at distribution end, we used to make DTs (1MVA 33kV) with 20%tap range, locally made OLTCs.