Electrical Engineering Substation Automation Lab Experiments

Control of Bus Voltages Through Onload Tap Changes

Aim of the experiment : Control Of Bus Voltages Through On-load Tap Changer.

The transformer in substation are equipped with on-load tap changer (OLTC) to control the bus voltages. The taps can be lowered of raised depending on the request. The standard adapted is +5% to -15% in step of 1.25%. This process is simulated as below.

  1. The tap lower indicates raise in secondary bus voltage, Tap raise indicates decrease in secondary Bus voltage.
  2. One click on lower or raise button changes tap by 1.25% and alters voltages correspondingly.
  3. The taps are changed by observing the variation in 33KV incomer voltage. The lowest voltage on 11 kV side in set as 10.7 kV.
  4. The following table shown below gives the number of times tap to be lowered to increase secondary voltage, when input voltage at 33 kV incomer is less than 33kV.
  5. V1 is 33 kV incomer.
  6. V2 is 11 kV with normal tap settings.
  7. V2T is 11 kV after tap changing.

The incomer voltage can be selected from the table and in order to achieve constant 11kV secondary voltage, tap can be changed by clicking tap lower or tap raise button.

To increase secondary Bus voltage by lowering taps when there is decrease in primary voltage:

V1(kV) V2(kV) No of times tap lower
to be clicked		 Percentage of
tap lowered 		V2 T (kV)
33 11 --- --- 11
32.5 10.83 1 -1.25% 10.97
32 10.66 3 -3.75% 11.08
31.5 10.5 4 -5.00% 11.05
31 10.33 5 -6.25% 11.02
30.5 10.16 7 -8.75% 11.14
30 10 8 -10.00% 11.11
29.5 9.83 9 -11.25% 11.07

Illustration

  • if the incoming primary voltage(V1) = 32.5 kV.

Then , V1=32.5 kV
V2 = (32.5 / 3) = 10.83kV
With one time clicking on tap lower button, tap lowered by 1.25%
Hence V2T= (V1/3)/ (1-0.0125) = (32.5/3)/0.9875=10.97kV.

To decrease secondary Bus voltage by increasing taps when there is increase in primary voltage:

V1(kV) V2(kV) No of times tap lower
to be clicked		 Percentage of
tap lowered 		V2 T (kV)
33 11 --- --- 11
33.5 11.16 1 1.25% 11.02
34 11.33 2 2.50% 11.05

Illustration

  • if the incoming primary voltage(V1) = 33.5 kV.

Then , V1=33.5 kV
V2 = (33.5 / 3) = 11.16 kV
With one time clicking on tap raise button, tap raised by 1.25%
Hence V2T= (V1/3)/ (1+0.0125) = (33.5/3)/(1.0125)=11.02 kV.