How does a Idmt relay work?
IDMT relay is inverse definite minimum time relay. It is one in which Time of operation is inversely proportional to magnitude of fault current near pickup value and becomes substantially constant slightly above the pickup value of the Relay.
What is IDMT over current relay?
Inverse definite minimum time (IDMT) over-current Relay is one in which the operating time is approximately inversely proportional to the fault current near pick-up value and then becomes constant above the pick-up value of the relay.
What is the use of IDMT relay?
What is idmt relay: The IDMT relays are protection relays. They are used in the transmission lines to see that the line current does not exceed the safe values and, if necessary, operates the circuit breaker. IDMT means the minimum minimum time spent.
How is Idmt relay calculated?
Calculation of Over Current Relay Setting:
- Over Load Current (In) = Feeder Load Current X Relay setting = 384 X 125% =480 Amp.
- Required Over Load Relay Plug Setting= Over Load Current (In) / CT Primary Current.
- Required Over Load Relay Plug Setting = 480 / 600 = 0.8.
What are the drawbacks of IDMT relay?
The use of IDMT overcurrent relay has some disadvantages. Most of the problems relate to the coordination between the main and the backup relay. One of the problems faced by the overcurrent relay is fault severity variation.
Why IDMT relays are better?
The IDMT relay uses the electromagnetic core because it can easily saturate for the current having larger magnitude than pick up current. The relay is used for the protection of the distribution line. The inverse characteristic of the relay is more than the IDMT.
What is pick up current?
The pickup current is the current that passes through the coil in creation of the deflecting force. The contact will move once the deflecting force is greater than the controlling force. Therefore, the minimum pickup current will be 1.75 of steady state current.
Why IDMT relays are widely used for over current protection?
Suitable for the protection of machines against overheating. The use of extremely inverse overcurrent relays makes it possible to use a short time delay in spite of high switching-in currents. Used when Fault current independent of normal changes in generating capacity.
How do you calculate pickup current?
Pick-up current = Plug Position x Rated CT Secondary Current. The plug or tapping is provided on the Relay Coil so that changing the position of Plug changes the number of turns of the relay coil as shown in figure below.
What is the limitation of the IDMT type overcurrent relay?
How to calculate IDMT over current relay setting?
IDMT Relay High Current setting :Plug setting of Relay is 2.5 Amp and Time Delay (TMS) is 0.100 Sec, Relay Curve is selected as Normal Inverse Type Calculation of Over Current Relay Setting: (1) Low over Current Setting: (I>) Over Load Current (In) = Feeder Load Current X Relay setting = 384 X 125% =480 Amp
How does the operating time of an IDMT curve decrease?
This means that the operating time decreases with increasing current magnitude. However, like instantaneous overcurrent relays, IDMT relay curves have a definite minimum operating time. Hence the name Inverse Definite Minimum Time.
What is the relay setting for inrush current?
Inrush current is generally 10-15 times of transformer FLC. So setting is=910×5/120=37.9 A or (910/120) X In=7.58 X In as per the available Relay functions.
How to check the power of a relay?
Procedure: 1. Make the connection as shown in the circuit diagram. 2. Select current setting (set phase trip) less than 100%, keeping phase TMS at maximum position. 3. Select any time setting. 4. Switch on variac and check power ON indication provided on relay front panel.