Why is Relay Coordination Study Important?
Widely used feeder over current protection devices are
- Fuses
- Relays with circuit breaker having shunt trip features
- LV circuit breaker series trip releases
In addition, in overhead distributions lines, automatic reclosers and sectionalisers are used
FUSE
The simplest form of overcurrent protection is the fuse.
The fuse is capable of operating in less than 10ms for very large values of current, thus considerably limiting fault energy.
Fuse require no maintenance and are cheaper when compared to other circuit interrupting devices of equal breaking capacity.
Fuse, however, it does have a number of disadvantages as listed below.
- Can be difficult to co-ordinate.
- Its characteristic is fixed.
- Needs replacing following fault clearance.
- Has limited sensitivity to earth faults.
- Causes single phasing.
Fuse I-t Curve
LV CIRCUIT BREAKER
LV Circuit Breaker with Releases
In addition to providing a means to open and close its contacts manually, the LV circuit breakers (LVCB) are equipped with features to automatically activate opening of its main contacts when an overcurrent condition is sensed
The trip unit is the part of the circuit breaker that determines when the contacts will open automatically
LV Circuit Breaker with Releases
LV circuit breaker, in case of fault, opens the circuit in a manner dependent on the magnitude of the overcurrent and on the tripping characteristics of the LV CB’s tripping unit.
Each circuit breaker is equipped with a trip unit which actuates the circuit breaker release mechanism and allows opening on the basis of the current flowing through it.
Two types of trip units are used in low voltage circuit breakers:
- Thermomagnetic (thermomagnetic trip unit)
- Electronic (electronic trip unit)
The thermomagnetic trip unit consists of two parts:
- the thermal trip unit, used for over load protection, is made up by a bimetal thermal device with a delay depending on the overcurrent value.
- the magnetic trip unit, intended for short circuit protection, is made up by an electromagnetic device, with fixed or adjustable instantaneous trip thresh
Characteristics of thermo-magnetic trip unit
The electronic trip units using microprocessor to process the current signal initiates LVCB opening in case of fault.
These releases provide the following protection functions:
- the long time-delay trip function (ac inverse time overcurrent relay equivalent)
- the short time-delay trip function (ac inverse or definite time overcurrent relay)
- the instantaneous trip function (instantaneous overcurrent relay equivalent)
- the ground-fault trip function (ac inverse or definite time earth fault overcurrent relay).
LV Circuit Breaker with Releases-Characteristics of electronic LSIG trip unit
