Here we will discuss the formation of sequence networks in the power system. Computation of unsymmetrical faults in power systems requires knowledge of sequence networks. Load studies require knowledge of positive sequence networks whereas for studies including unsymmetrical faults, negative and zero sequence networks knowledge is also essential.
For the calculation of symmetrical fault, a 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.
A negative sequence network is similar to that of a 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 the same.
For zero sequence networks; zero-sequence sub-networks of different power system elements can be combined 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 a sequence network.
Figure: Power System Network
Secondly consider a base MVA and base kV and bring all the reactance to per unit of this base value.
Then let base MVA be 50 MVA and base kV for generator side be 11kV and transmission side be 220kV.
Above all, it is suitable to take the base MVA equal to the highest generator MVA present in the system. Therefore in our case, it the generator G1 with 50MVA.
At first, per unit reactance of generator G1
Secondly, per unit reactance of generator G2
Thirdly, per unit reactance of transformer T1
Fourthly, per unit reactance of transformer T2
Finally, per unit reactance of transmission line 1
There is no data available for the zero sequence reactance of the transmission line so we will assume that the zero sequence reactance of the transmission line is 3.5 times its positive or negative sequence reactance.
Furthermore per unit reactance of transmission line 2
Then the neutral reactance of j0.024 up of G2 and j0.03 pu of T2 will be,
In a zero-sequence network, this reactance will appear as 3 times to the neutral reactance i.e.
Finally, the sequence network diagrams are drawn below.
Positive Sequence Network
Negative Sequence Network
Zero Sequence Network
Another Example
Similarly, consider a power system network as shown, and let’s draw a sequence network for a given power system network.
Figure: Power System Network
| Equipment | MVA rating | Voltage rating | X1 pu | X2 pu | X0 pu |
| Generator (G1) | 100 | 11kV | 0.25 | 0.25 | 0.05 |
| G2 | 100 | 11kV | 0.2 | 0.2 | 0.05 |
| T1 | 100 | 11/220kV | 0.06 | 0.06 | 0.06 |
| T2 | 100 | 11/220kV | 0.07 | 0.07 | 0.07 |
| Transmission line-1 | 100 | 220kV | 0.1 | 0.1 | 0.3 |
| Transmission line-2 | 100 | 220kV | 0.1 | 0.1 | 0.3 |
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 piece of 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 a zero-sequence network of transformer which is a bit complex.
And, the neutral reactance has to be multiplied by 3.
Finally, the sequence networks are drawn below.
Positive Sequence Network
Negative Sequence Network
Zero Sequence Network
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