Formation of Sequence Networks
Here we will discuss about the formation of sequence networks in power system. Computation of unsymmetrical faults in power system requires knowledge of sequence network. Load studies requires knowledge of positive sequence network whereas for studies including unsymmetrical faults, negative and zero sequence networks knowledge is also essential.
For calculation of symmetrical fault, positive sequence network is drawn by examining the one line diagram or single line diagram. For unsymmetrical fault analysis negative and zero sequence networks are also required.
Negative sequence network is similar to that of positive sequence network. But the only difference is that it does not contain any voltage or EMF source. For power system elements like transformer and transmission lines, the positive and negative sequence networks are same.
For zero sequence networks; zero sequence sub networks of different power system elements can be combined together to form a complete zero sequence network. It doesn’t contain any voltage sources and it will consist of any impedance present in the generator or transformer neutral.
Procedure for Formation of Sequence Networks
At first we will consider the following power system network for which we will form sequence network.
We will consider a base MVA and base kV and bring all the reactance to per unit of this base value.
Let base MVA be 50 MVA and base kV for generator side be 11kV and transmission side be 220kV.
[NOTE: It is always suitable to take the base MVA equal to the highest generator MVA present in the system. In our case, it the generator G1 with 50MVA.]
Per unit reactance of generator G1
Per unit reactance of generator G2
Per unit reactance of transformer T1
Per unit reactance of transformer T2
Per unit reactance of transmission line 1
There is no data available for the zero sequence reactance of transmission line so we will assume that the zero sequence reactance of the transmission line is 3.5 times of its positive or negative sequence reactance.
Per unit reactance of transmission line 2
The neutral reactance of j0.024 pu of G2 and j0.03 pu of T2 will be,
In zero sequence network, these reactance will appear as 3 times to the neutral reactance i.e.
Now the sequence network diagrams are drawn below.
Now let’s draw sequence network for given power system network.
|Equipment||MVA rating||Voltage rating||X1 pu||X2 pu||X0 pu|
Now considering base MVA of 100MVA and base kV of 11kV for LT (low tension) side and 220kV for HT (high tension) side. As, every equipment has the specified ratings of MVA and kV equal to our considered base values so we don’t need to convert them to our base value.
Now we will form the sequence network as before.
We have to be careful while drawing zero sequence network of transformer which is a bit complex.
And, the neutral reactance have to be multiplied by 3.
The sequence networks are drawn below.
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