Switch Gear & protection
Chapter-01
(Concepts of switch gear and protection)
    -Asiqur Rahman Milon,EETE-DIU,EEE-IEB(Con)
#What is switch Gear?
-In an  electric power system switch gear is the combination of electrical disconnected switches,fuses or circuit breakers used to control,protect,and isolate electric equipements.Switch gear are used both to de-energize equipement to allow work to be done or to clear faults downstream.

#What are the main component of switch gear?
- The main component of switch gear are-
1)ACB(Air circuit breaker)
2)MCCB(Moulded case circuit breaker)
3)Contactor
4)Relay
5)SPP
6)Capacitor
7)Starter
8)Isolator
9)MCB(Miniature circuit breaker)
10)ELCB(Earth liakage circuit breaker)
11)Changeover circuit

1)    ACB(Air circuit breaker)- Air circuit breaker is the primary protecting device of any switch gear.Any high power consuming device such as panel board,power is distributed through ACB.This device cuts the main circuit during earth fault,under voltage,and short circuits due to which circuits connect through it are prevented from getting fail or damage.
   
         Air circuit breaker is 3-pole or 4-pole.It ranges from 630amp to 10,000Amp.
 

2)    MCCB(Moulded case circuit breaker)-Moulded case circuit breaker is used to protect high load current devices from fault overloading.Tripping mechanism in MCCB is thermal based,microprocessor based or electronic based.
                 MCCB is 3-pole or 4-pole.
3)    Contactor-   Contactor is the most demanding component of switch gear.This is an electrically operated switch.Inside contactor there is an electomachanical system connected to the coil.It works on the predefined supply being provided to the coil.
4)    Relay-Relay is a switching device which is electromechanically or electronically controlled to open or close the relay.Relay are different types such as thermal relay,CT operated relay,solid state relay,hybrid relay,electromechanical relay and electronic relay.
5)    SPP (single phase preventer)-Motor wiring or circuit may get damage due to failure of any of the phase in 3-phase motor.SPP detects fault in 3 phase and blocks current in case of failure of any phase.
6)    Capacitor-To maintain power factor capacitor is use.Capacitor is generally air cooled or water cooled which is dry type,MPP9metalized polypropylene)type and APP(all polypropylene)type.
7)    Stater- DOL(Direct online starter) means DOL.It consists of contactor and relay.Star Delta starter comprises of 3 contactor.1 timer and 1 relay.In any circuit if the current increases from a predefined limit,the relay in starter gets trip hence blocks the flow of current through the contactor thus prevents the motor from damaging.
8)    Isolator-To operate a circuit manually (ON/OFF),isolate is use.Isolator does not contain any protection or tripping device. It’s operating system is manually  independent,quick make and quick break.
9)    MCB (Miniature circuit breaker)-Miniature circuit breaker is an electromechanical device which protects low electrical circuits from overcurrent.MCB has tripping mechanism which can be reset easily.MCB is 1 pole,2-pole,3-pole and 4-pole.  
10) ELCB-Earth liakage circuit breaker is a safety device thst protects circuits from fault and earthing and human from electric shock.
11) MPCB(Motor protection circuit breaker)



#What are the essential feature/characteristics of switch gear ?
- The essential feature/characteristics of switch gear are-
1.     Complete reliability: With the continued trend of interconnection and the increasing capacity of generating stations, the need for a reliable switchgear has become of paramount importance. This is not surprising because switchgear is added to the power system to improve the reliability. When fault occurs on any part of the power system, the switchgear must operate to isolate the faulty section from the remainder circuit.
2.     Absolutely certain discrimination: When fault occurs on any section of the power system, the switchgear must be able to discriminate between the faulty section and the healthy section. It should isolate the faulty section from the system without affecting the healthy section. This will ensure continuity of supply.
3.     Quick operation: When fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, transformers and other equipment by the short-circuit currents. If fault is not cleared by switchgear quickly, it is likely to spread into healthy parts, thus endangering complete shut down of the system 
4.     Provision for manual control: A switchgear must have provision for manual control. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control. 
5.     Slecctivity-Swich gear must have discrimination power between used protective gear and device.It find out in system which part has fault that fault part need to isolate.
6.     Sensitivity-If over current flow in a system then need to isolate the system then is happen when switch gear has sensitivity power.
7.     High speed-Switch gear need to take swift action as early as possible cause if it works early then device don’t damage. Device longibility is high.
8.     Stability-After protection that fault then need to again start work with previous capacity.
9.     Simplicity-Protective gear and device structure and architecture must be simple.


#What is the classification of switch gear?
- The classification of switch gear is-
Switchgear can be classified on the basis of voltage level in to the following

1.     Low voltage (LV) Switchgear
2.     Medium voltage (MV) Switchgear 
3.     High voltage (HV) Switchgear
1. Low Voltage Switchgear
Generally electrical switchgear rated upto 1 kV is termed as low voltage switchgear. 


The term LV Switchgear includes low voltage circuit breakers, switches, off load electrical isolators, HRC fuses, earth leakage circuit breaker, miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) etc i.e. all the accessories required to protect the LV system.

The most common use of LV switchgear is in LV distribution board.

2. Medium Voltage Switchgear
From 3 kV to 36 kV switchgear system is categorized as medium voltage switchgear or MV switchgear. 


These switchgears are of many types. They may metal enclosed indoor type, metal enclosed outdoor type, outdoor type without metal enclosure, etc. The interruption medium of this switchgear may be oil, SF and vacuum. 


The main requirement of MV power network is to interrupt current during faulty condition irrespective of what type of CB is used in the MV switchgear system. Although it may be capable of functioning in other conditions also.


A medium voltage switchgear, should be capable of,
1.     Normal ON/OFF switching operation.
2.     Short circuit current interruption.
3.     Switching of capacitive currents.
4.     Switching of inductive currents.
5.     Some special application.
3. High Voltage Switchgear
The power system deals with voltage above 36kV, is referred as high voltage. 


As the voltage level is high the arcing produced during switching operation is also very high. So, special care to be taken during designing of high voltage switchgear. 


High voltage circuit breaker, is the main component of HV switchgear, hence high voltage circuit breaker should have special features for safe and reliable operation. 


Faulty tripping and switching operation of high voltage circuit are very rear. Most of the time these circuit breakers remain, at ON condition, and may be operated after a long period of time. So CBs must be reliable enough to ensure safe operation, as when required.

 By using switch gear is two types-
1) Indoor
2) Outdoor