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Home / Technical Articles / Mastering switchgear control circuits – AC/DC circuits and circuit breaker closing circuit

MV Switchgear Control Circuits

Control circuits are vital to the operation of medium voltage switchgear. The integrity of these control circuits is essential to the switchgear’s proper operation, so commissioning and maintenance activities records are extremely important for troubleshooting practices. Furthermore, one has to comprehend the reading and interpretations of the control circuits in order to develop the skills of troubleshooting and sequence of operation follow.

Mastering switchgear control circuits – AC/DC circuits and circuit breaker closing circuit
Mastering switchgear control circuits – AC/DC circuits and circuit breaker closing circuit

Although the control circuits are many, they boil down to the circuit breaker close/trip coils irrespective of the many details involved.

The good news is that the principles addressed in this technical article apply to almost all types of switchgear with some exceptions. The control circuits for Gas Insulated Substation (GIS), for example, are much more detailed and complicated, but they have many common principles with 13.8 kV switchgear.

Hence, it is extremely important to master the skills of control circuit interpretation for better and faster troubleshooting activities.

This article tackles the closing coil control circuits of a 13.8 kV capacitor bank feeder. These circuits are AC/DC circuits and circuit breaker closing circuit, which in turn contains two sub-circuits: motor charging circuit and anti-slam/pumping protective circuit.

In addition, a brief view of busbar arrangements utilized in different applications is introduced, as it has a direct effect on the interlocking contacts that might be used in the control circuits. The next article will talk about substation single-line diagram, circuit breaker tripping circuit, bay-control-protection-unit (BCPU) and alarm circuits, indication circuits, and mechanical/electrical interlocking circuits.

Table of Contents:

  1. Busbar Arrangement Overview
  2. Switchgear Components
  3. Substation Single-Line Diagram
  4. Substation Control Systems
    1. AC and DC Control Circuits:
      1. AC Circuit
      2. DC Circuit
    2. Circuit Breaker Closing Circuit
      1. Motor Charging Sub-Circuit
        1. Troubleshooting (Motor Not Charging)
      2. Closing Circuit
      3. Circuit Breaker Closing Coil Interlocking Sub-Circuit
  5. About My Next Article (Trip Coils, Control/Alarm Circuits, Interlocking Schemes)

1. Busbar Arrangement

A variety of busbar configurations exist in which there is a trade-off in complexity, cost, and reliability. The main criteria for selecting the busbar arrangement are summarized below.

  • Simplicity.
  • Ease of equipment maintenance.
  • Outage duration during maintenance.
  • Future provision of extension with demand growth.

The most common practices for the three major substation types are as below:

1. Generation stations:

  1. Large generators:
    1. Double bus, single breaker
    2. Double bus, double breaker
    3. Combination double/single, with double breakers for the generator circuits and a single breaker for outgoing lines or transformers
    4. Double bus, breaker and a half
  2. Small generators:
    1. Single bus
    2. Various forms of line tap

2. Network switching stations:

  1. Breaker and a half
  2. Double bus, usually single breaker
  3. Single bus, single breaker
  4. Ring bus

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Salem Alshahrani

Electrical engineer (BEE & Meng). Specialized in substation design, especially in LV/MV switchgears and transformers. Passionate in power system planning, analysis, and stability studies.

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