A practical handbook for electrical engineers (beginners)

Transmission and distribution substations

The major types of equipment found in most transmission and distribution substations are discussed in this handbook. The purpose, function, design characteristics, and key properties are all explained. After the equipment is discussed, planned and essential predictive maintenance techniques are discussed.

The reader should get a good fundamental understanding of all the important aspects of the major equipment found in substations and how they are used and operated.

The substation equipment discussed in this handbook includes:

• Transformers
• Regulators
• Circuit breakers and reclosers
• Air disconnect switches
• Lightning arresters
• Electrical buses
• Capacitor banks
• Reactors
• Static VAR compensators
• Control building
• Preventative maintenance

Power transformers

Transformers are essential components in electric power systems. They come in all shapes and sizes. Power transformers are used to convert highvoltage power to low-voltage power and vice versa.

Generation plants use large step-up transformers to raise the voltage of the generated power for efficient transport of power over long distances. Then step-down transformers convert the power to subtransmission, as in Figure 1, or distribution voltages, as in Figure 2, for further transport or consumption.

Distribution transformers are used on distribution lines to further convert distribution voltages down to voltages suitable for residential, commercial, and industrial consumption. There are many types of transformers used in electric power systems.

Regulating transformers are used to maintain proper distribution voltages so that consumers have stable wall outlet voltage. Phase shifting transformers are used to control power flow between tie lines.

Transformers can be single phase, three phase, or banked together to operate as a single unit.

Circuit breakers

The purpose of a circuit breaker is to interrupt current flowing in the line, transformer, bus, or other equipment when a problem occurs and the power has to be turned off. Current interruption can be for normal load current, high-fault current (due to a short-circuit current or problem in the system) or simply tripped by protective relaying equipment in anticipation of an undesirable event or disturbance.

A breaker accomplishes this by mechanically moving electrical contacts apart inside an interrupter, causing an arc to occur that is immediately suppressed by the high-dielectric medium inside the interrupter.

Circuit breakers are triggered to open or close by the protective relaying equipment using the substation battery system.

The most common types of dielectric media used to extinguish the arc inside the breaker interrupter are listed below:

• Oil (clean mineral)
• Gas (SF6 or sulfur hexafluoride)
• Vacuum
• Air

These dielectric media also classify the breaker, such as oil circuit breaker (OCB), gas circuit breaker (GCB), and power circuit breaker (PCB).

Compared to fuses, circuit breakers have the ability to open and close repeatedly, whereas a fuse opens the circuit one time and must be replaced. Fuses are single-phase devices, whereas breakers are normally gang operated three-phase devices.

Breakers can interrupt very high magnitudes of current. They can close into a fault and trip open again.  They can be controlled remotely. They need periodic maintenance.