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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.

Practical handbook for electrical engineers (beginners)
Practical handbook for electrical engineers (beginners)

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.

Power can flow in both directions: from the high-voltage side to the low-voltage side or from the low-voltage side to the high-voltage side.

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.

Step-down transformer
Figure 1 – Step-down transformer

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.

Instrument transformers are used to connect high-power equipment to low-power electronic instruments for monitoring system voltages and currents at convenient levels. Instrument transformers include CTs and VTs (i.e., current transformers and voltage transformers). These instrument transformers connect to metering equipment, protective relaying equipment, and telecommunications equipment.

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.

Distribution power transformer
Figure 2 – Distribution power transformer

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.

Oil circuit breaker
Oil circuit breaker

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.

Title:A practical handbook for electrical engineers (beginners) – Steven W. Blume
Size:22.0 MB
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A practical handbook for electrical engineers (beginners)
Practical handbook for electrical engineers (beginners)

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  1. Norberto Ardila
    Mar 26, 2023

    Thanks You. May i had a copy?…

    • Abraham Abeiku Bassaw
      Mar 28, 2023

      Please I want a copy

  2. James
    Mar 24, 2023

    HI everyone , Can you please send me a copy

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