1. INTRODUCTION TO SWITCHGEARS
PRESENTED BY
EWETUMO, A . A .

2. SWITCHGEARS : Definition
Switchgear is a general term covering all equipment used for :
- switching,
- protection,
- control and
- isolation in a power system.
All equipment used for fault clearing is covered by the term switchgear.
Switchgears are used in Generation, Transmission and Distribution Systems, whereas, Control Gears are used in Consumer Circuits.

3. Necessity of Switchgears
Switchgears are necessary at every switching point in the power system because there are several voltage levels and fault levels which has to be controlled and protected by accessible switching devices and for isolation, if the need arises.

4. Principal Switchgears
Principal Switchgears are the main equipment concerned with the process of switching and isolating circuits in a power system.

5. Auxiliary Switchgears
Auxiliary Switchgears are secondary or subsidiary equipment which assist the main switchgear equipment in the control, measurement, protection and fault-clearing process.

6. Examples of Principal Switchgears:
SWITCHING DEVICES
(a) Circuit Breakers
(b) Isolators (Disconnector or
Disconnecting Switch)
(c) Earthing Switches
(d) Load Switches (Ring Main Units)
(e) Contactors

7. Examples of Auxiliary Switchgears
PROTECTION DEVICES
(i) Protection Relays
(j) Lightning Arresters
(k) Feeder Pillars
(l) Fuses.

8. Examples of Auxiliary Switchgears
SENSING DEVICES
Voltage (Potential) Transformers
Current Transformers

9. Examples of Auxiliary Switchgears :
CONTROL (COMPENSATION) DEVICES
(m) Series Inductive Reactors
(n) Shunt Inductive Reactors
(o) Series Capacitive Reactors
(p) Shunt Capacitive Reactors

10. Examples of Auxiliary Switchgears
AUXILIARY POWER SUPPLY DEVICES
Tripping Units (Battery Bank & Charger)

11. Current Transformers; Voltage Transformers; Reactors; Tripping Units.
Switch gear - It's function
SWITCHGEAR : Gear for Switching
Switching Devices
Control & Sensing Devices
Decision Making Devices
Protective Relays
Lightning Arresters
Fuses
Circuit breakers
Current Transformers; Voltage Transformers; Reactors; Tripping Units.
Isolators
Switches

12. CIRCUIT BREAKERS
(a) Definition: A CB is a device capable of making and breaking an electric circuit under normal and abnormal conditions such as short circuit.
(b) Functions: A CB is a switching and current interrupting device

13. The CB serves two basic purposes:
(i) Switching during normal operating
conditions for the purpose of operation and maintenance.
(ii) Switching during abnormal conditions such as short circuits and interrupting the fault current.

14. 6. CLASSIFICATION OF CBs
CBs are classified according to the following categories:
(a) RATED VOLTAGE:
(b) MEDIUM OF ARC EXTINCTION;
(c) TYPE OF OPERATING MECHANISM;
(d) TYPE OF CONSTRUCTION;
(e) STRUCTURAL FORM.

15. 7. ARC-EXTINCTION MEDIA EMPLOYED IN CBs
(a) Atmospheric Air
(b) Compressed Air
(b) Dielectric Oil (Transformer Oil).
(c) Sulphur Hexafluoride (SF6) Gas.
(d) Vacuum.

16. 8. CURRENT INTERRUPTION MODES
(a) High Resistance Current Interruption Mode is employed in all HVDC CBs and in ABCBs by the automatic insertion of high resistance in the CB
circuit, milli-seconds, prior to current- interruption, to divert and extinquished arcing.

17. (b). Zero-Point Current Interruption mode is
(b) Zero-Point Current Interruption mode is employed in all ac cbs by extinguishing the arcing by the timing of the zero- value position of the current wave form with the rapid operation of the operating mechanism to achieve rapid build-up of dielectric strength of the contact space.

18. FAULT CLEARING PROCEDURE OF A CB
PRESENTED BY
EWETUMO, A. A.

19. FAULT-CLEARING PROCESS OF A CIRCUIT BREAKERS
If we consider a fault occurrence and clearing process of a circuit breaker, the following order or sequence of operation is observed.

20. FAULT OCCURS: As the fault occurs, the fault impedance being low, the currents increases and the relay gets actuated. The moving parts of the relay move because of the increase in the operating torque. The relay takes some time to close its contacts.

21. RELAY CONTACT CLOSES: The relay contacts closes the trip – circuit of the circuit breaker and the trip-coil is energised from the control voltage supply from a battery bank of tripping unit.

22. OPERATING MECHANISM OPERATES:
As the trip – coil is energised, appropriate plunger, toggles, latches or valves are operated, which enables the actuation of the operating mechanism for the opening operation to separate the circuit breaker contacts.

23. ARCING: Arc is drawn between the separating contacts, because the charged electrons surrounding the separating contacts tries to re-establish potential between the contacts.

24. CLEARANCE: ISOLATION: The arc is extinguished in the circuit breaker by appropriate suitable techniques at some natural currents zero of a.c. wave. The process of current interruption is completed when the arc is extinguished, current reaches final zero value and the arc does not restrike again. The fault is said to be cleared.