Single line diagrams of substations 66/11 kV and 11/0.4 kV (reading & analysis)

Substation single line diagrams

This technical article describes single line diagrams of two typical power substations 66/11 kV and 11/0.4 kV and their power flow, principles of incoming lines (incomers) and outgoing lines (feeders), busbar arrangement functionality and so on.

Regarding elements in single-line diagrams, they were already explained in previous article, so if you didn’t read it, it’s advisable to do it first.

66/11 kV outdoor substation

Single line diagram

Figure 1 shows the single-line diagram of a typical 66/11 kV substation. Let’s explain the main parts of it and how it actually works.

There are two 66 kV incoming lines marked ‘incoming 1’ and ‘incoming 2’ connected to the bus-bars. Such an arrangement of two incoming lines is called a double circuit. Each incoming line is capable of supplying the rated substation load.

Both these lines can be loaded simultaneously to share the substation load or any one line can be called upon to meet the entire load.

The double circuit arrangement increases the reliability of the system. In case there is a breakdown of one incoming line, the continuity of supply can be maintained by the other line.

The substation has duplicate busbar system: one ‘main busbar’ and the other spare busbar. The incoming lines can be connected to either busbar with the help of a bus-coupler which consists of a circuit breaker and isolators.

The advantage of double busbar system is that if repair is to be carried on one busbar, the supply need not be interrupted as the entire load can be transferred to the other bus.

Generally, one transformer supplies the entire substation load while the other transformer acts as a standby unit. If need arises, both the transformers can be called upon to share the substation load.

The 11 kV outgoing lines feed to the distribution substations located near consumers localities. Both incoming and outgoing lines are connected through circuit breakers having isolators on their either end.

NOTE! Whenever repair is to be carried over the line towers, the line is first switched off and then earthed.

The potential (or voltage) transformers (PTs or VTs) and current transformers (CTs) and suitably located for supply to metering and indicating instruments and relay circuits (not shown in the figure). The PT is connected right on the point where the line is terminated. The CTs are connected at the terminals of each circuit breaker.

The lightning arresters are connected near the transformer terminals (on high voltage side) to protect them from lightning strokes.

There are other auxiliary components in the substation such as capacitor bank for power factor improvement, earth connections, local supply connections, d.c. supply connections etc. However, these have been omitted in the single-line diagram for the sake of simplicity.

11 kV/400 V indoor substation

Single line diagram

Figure 2 shows the single-line diagram of a typical 11 kV/400 V indoor substation. Let’s explained this scheme a little bit.

The 3-phase, 3-wire 11 kV line is tapped and brought to the gang operating switch installed near the substation. The gang operated switch (G.O. switch) consists of isolators connected in each phase of the 3-phase line.

From the G.O. switch, the 11 kV line is brought to the indoor substation as underground cable. It is fed to the HV side of the transformer (11 kV/400 V) via the 11 kV C.B. The transformer steps down the voltage to 400 V, 3-phase, 4-wire.

The single phase residential load is connected between any one phase and neutral whereas 3-phase, 400 V motor load is connected across 3-phase lines directly.

The CTs are located at suitable places in the substation circuit and supply for the metering and indicating instruments and relay circuits.

Source: Elements of Power Systems by Pradip Kumar Sadhu and Soumya Das (Purchase hardcover from Amazon)