Electrical protection system is to identify the changes in the current and voltage parameter as a consequence of a fault which may result in damage to the equipment and beings. The detection then result simultaneous isolation of the faulty equipment.
There are four aspects involved in designing protection system for any installation namely
- Characteristics of the relay- Definite time Relays; Inverse time Relays with definite minimum time(IDMT); Instantaneous Relays IDMT with inst. ; Stepped Characteristic; Programmed Switches; Voltage restraint over current relay
- Logic for comparing different parameters – Differential; Unbalance; Neutral Displacement; Directional; Restricted Earth Fault; Over Fluxing; Distance Schemes; Bus bar Protection; Reverse Power Relays ; .Loss of excitation; negative Phase Sequence Relays etc.;
- Actuating Parameters – Current, voltage, frequency and power relay
- Application- Primary and Secondary relay
Most primitive and old system had been protection by rewire-able fuse wire. As the name implies, wire fuses when more than rated current flows through it thus isolation the defective equipment. Fuse rating is generally given in terms of gauge of the wire. The current rating is in directly proportion to 1.5 power of diameter. Indian Standard defines a fuse as
“ A device that by fusion of one or more of the specially designed and proportional components, opens the circuit in which it is inserted when the current through it exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete device.”
It is cheap, simple in construction but had the disadvantage of giving a sign of sound, flash and smoke while fusing, environmental effect and unreliability of using any size of the wire during replacing. Use of rewire-able fuse has been discontinued for all application in Indian Railways.
This problem is overcome by enclosing the fuse wire and called cartridge fuse. There are different designs for cartridge fuse and very commonly used in electrical general services, locomotive, EMU, Train Lighting and Air conditioning.
Miniature Circuit Breaker (MCB):
Fuse has to be replaced and till such time system remains out of circuit even if it is a transient fault. MCB provides the facility of resetting of the tripping mechanism. Trip mechanism works on thermal and elecro-magnetic sensing. Rated current is of the order of 100A
Moulded Case Circuit Breakers (MCCBs):
MCCBs are used for low voltage circuit protection of distribution system. They are suitable for application as main breakers and for protection of branch and feeder circuit & connected
equipments / load. It provides overload and short circuit protection for all circuit elements.
They are designed for use in Switchboards, Control Panels, Combination Starters, in separate enclosures and meet the requirements of lighting, distribution and power circuits. MCCBs are very commonly used in almost all applications for protection in Indian Railways.
Rated current is of the order of 1000A with provision of trip mechanics resetting and adjustment of trip characteristics.
Physics of Arc formation during breaking
Current interruption leads to induction of high voltage at terminals depending on Ldi/dt. This high voltage causes breakdown in the air gap resulting in spark. This is an undesirable part of braking and need is to extinguish the spark without any delay.
Air Blast Circuit Breaker
Air blast circuit breaker was very commonly used by Indian Railways for rolling stock application called ABDJ(Air blast Dis-Jointer). The arc formed at the time of opening is extinguished by a blast of air. Tripping coil of the relay is triggered by a set of relays which monitors the functioning of different equipments of locomotive. ABDJ are now being replaced with VCBs for many advantages associated with it.
Vacuum Circuit Breaker
Technological advancement in vacuum arising due to the high vacuum in developing semi-conductor technology has opened opportunities for development of vacuum circuit breaker at the beginning of 1960s. Air provides conducting path for an arc and absence of the same prevents its formation. Vacuum also provides high level of insulation as it is the ionisation of air which results in breakdown.
Advantages of VCB over ABDJ
- Excellent interruption capability
- Compact, less moving parts and less maintenance, long life
Desired properties of VCB
- Good conductor during closed condition
- Good Insulator in open condition
- Open to close in short period of time
- Quenching of arc during opening operation and containing transients
- Current chopping ability, Recovery of die-electric strength and high frequency current quenching
Process of arc quenching
- Vacuum arc is different than the ionise arc produced in air blast circuit breaker. The arc that appears between the contacts of VCB is a result of metal vapour, ion and electron emission.
- Vacuum arc is extinguished with small movement of piston at first natural current zero.
- While interrupting small current, arc extinguishes even before natural zero, which produces transient recovery voltage. This is called current chopping. During current chopping, the current declines with very high di/dt, producing very high over voltages due to inductance of the circuit. This overvoltage re-ignites the arc causing high frequency current flow which again extinguishes on its natural zero. Design of contacts, provision of shield around the contact reduces chances of TRV.
- When VCBs were initially introduced on electric locomotive, ET2 flashing was observed.
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