Insulation coordination study for lightning overvoltages in 420 kV power substation

Substation Model

Busbar type of the substation is designed according to the importance of the substation. This means that effects of energy interruption versus capital expenditure of the substation. A substation where energy interruption is less important and capital expenditure cost should be low is designed as simple type.

However, in some substations, energy availability is so critical that more expensive but more reliable busbar type is designed. The main busbar types of substations are:

• Single Main Busbar,
• Single Main Busbar with Transfer Busbar,
• Double Main Busbar,
• Double Main Busbar with Transfer Busbar,
• Ring Busbar.

Since this study is focused on THVES and double main busbar with transfer busbar is the most common type, the busbar type of the substation used in this thesis study is accepted as double main busbar with transfer busbar.

The single line diagram of the feeders modeled in this study is shown in Figure 1. This figure demonstrates the typical 420 kV single line diagram consisting of line feeder, transformer feeder and coupling-transfer feeder.

It is obvious that although substations are designed according to double main busbar with transfer busbar type, the arrangement can be changed upon the substation area and direction of the lines and transformers in order to fit the site and connect to OHL with lower investment.

Although it has more feeders, it is assumed that the other feeders are out of service in order to analyze the worst possible condition. Therefore, the lightning waveform is not divided and proceeds from the line feeder to the transformer feeder.

Figure 2 shows the typical general layout drawing of the line feeder of 420 kV substations with double busbar system with transfer busbar. This is the top view of the substation and equipment.

Pantograph type disconnectors are used in this substation configuration. As seen from the figure, the line entrance is from right hand side. At the end of the bay, there are voltage transformer and lightning arrester. After this equipment, line is connected directly to the upper line, and then there is a connection between the upper line and current transformer.

After that, the line is connected to the busbars via pantograph type disconnectors.

The typical general layout of the bus coupler-transfer feeder and transformer feeder of 420 kV substations are shown in Figure 3.

As seen from the figures, since the transformer is located at the left end side of the transformer bay, there is no need for the upper line. Transformer is connected directly to the transformer bay through the lightning arrester and voltage transformer at the transfer busbar side.

Complete Simulation Models

At the previous parts of this chapter, the sections introduce the lightning strike model, overhead line model, lightning arrester model, substation equipment model and substation model. These models are connected to compose the complete simulation models. The complete models where simulations performed on are presented in this section.

As explained in previous parts, two different models are developed in the scope of this thesis study: the shielding failure model and the back flashover failure model.

The model used on the shielding failure analyses is given in Figure 4. The parameters of shielding failure which are calculated at the previous parts are used at this model. The signed lightning arrester model is not fixed for the defined different cases.

The model implemented for the back flashover failure analyses is in Figure 5. The parameters of back flashover failure used at this model are calculated at the previous sections. The signed lightning arrester model with red circular is not fixed for all cases.

These models, presented in Figure 4 and Figure 5, are base models whose parameters and configurations will be changed for different simulation cases.

In both shielding failure and back flashover failure models, the corona effect, which is an important factor reducing the steepness of the incoming surge, is neglected in this thesis study in order to analyze the worst case scenarios.

In addition, since the lightning strike locations are close to the substation and the path, that surge propagates through, is short in simulations of this thesis, the effect of corona is very limited.