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Question 7.9: Consider a Δ/Y-connected, 20-MVA, 33/11-kV transformer with ...

Consider a Δ/Y-connected, 20-MVA, 33/11-kV transformer with differential protection applied, for the current transformer ratios shown in Figure 7.37. Calculate the relay currents on full load. Find the minimum relay current setting to allow 125 percent overload.

7.37
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The primary line current is given by

I_{P} =\frac{20\times 10^{6} }{\left(\sqrt{3} \right)\left(33\times 10^{3} \right) }= 349.91 A

The secondary line current is

I_{S} =\frac{20\times 10^{6} }{\left(\sqrt{3} \right)\left(11\times 10^{3} \right) }=1049.73A

The C.T. current on the primary side is thus

i_{P} =349.91\left\lgroup\frac{5}{300} \right\rgroup =5.832 A

The C.T. current in the secondary side is

i_{S} =1049.73\left\lgroup\frac{5}{2000} \right\rgroup \sqrt{3} =4.545A

Note that we multiply by  \sqrt{3} to obtain the values on the line side of the Δ- connected C.T.’s. The relay current on normal load is therefore

i_{r} =i_{p}- i_{s}=5.832- 4.545=1.287 A

With 1.25 overload ratio, the relay setting should be

I_{r}=\left(1.25\right) \left(1.287\right) =1.61 A

 

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