0
Over Current (Inverse Time) Over current relay function monitors the general balanced overloading and has current/time settings. This is determined by the overall protective discrimination scheme. There advantage over definite time relays is that they can have much shorter tripping times can be obtained without any risk to the protection selection process. These are classified in accordance with there characteristic curves, this indicates the speed of the operation. Based on this they are defined as being inverse, very inverse or extremely inverse. The typical settings for these relays are 0.7-2In (normal or rated generator current) in 1-10 second. Inducing a calibrated test current through the normal load current tests this relay.
Wiki User
∙ 17y ago
Add your answer:
Earn +20 pts
What is an Inverse Time Device?
an inverse time protective device is a protective relay with definite minimum operating current and definite minimum operating time for a given current. the device will ensure that increasing current in a fault, that the operating time will be decreased. Preventing an overload.
What is Inverse Definite Minimum Time?
(I.D.M.T.) RELAYThe over load inverse time relay is shown in fig 26. It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc have been displaced in phase from the flux entering the disc from the lower magnet. The deflecting torque is produced on the disc in the fashion as already explained. The spindle of the disc carries a moving contact which bridges two fixed contacts after the disc has rotated through a certain angle which has been set before. Any setting for this angle is possible varying from 0 to 360°. The variation of this angle imparts to the relay, various time settings.The speed of rotation of the disc is dependent upon the torque which in turn is dependent on the current setting, when the load current increases from this setting it will increase the speed of rotation of the disc resulting into decrease of operation time. Thus the time current characteristics of the relay observe inverse-Square law. The definite minimum time characteristics of the relay are obtained by the use of a saturated upper magnet. This ensures that there is no further increase in f1ux when the current has reached a certain value and any further increase of current will not affect the relay operation. This results in a flattened current time characteristic and the relay obtains its name as Inverse definiteminimum time lag (I.D.M.T.) relayThe current time characteristics of the relay have been illustrated in Fig. 27. It represents the time required to close the trip contacts for different values of over current. Its horizontal scale is marked in terms of current-setting multipliers i.e. number of times the relay current is in excess of current setting
What is the inverse time of the thermal overload?
what is the inverse time of the theraml overload?
How can you replace a diode with a relay in a circuit?
In certain cases, yes. A diode is a valve for current. A relay is a switch that is actuated by current. If you have a flow of current going from A to B and you only want it to go from A to B you can use a diode in series between A and B. You can also use a switch in series between A and B that is controlled by a relay that is in parallel with the switch. Add a large resistor in series with the relay so that it does not permit too much current to back flow through it. Configure the relay to open the switch when the current is trying to go from B to A and to close when the current is trying to go from A to B. This is not a leak free valve. Current will be going back through the relay while it is holding the switch open. As mentioned put a large resistor in series with it to reduce this. You cannot have this resistance too large though or the relay will not have enough current to maintain the switch. Another caveate is that this will not be a fast acting valve. There will be some time during current direction changes when the switch will be spending time openning and closing. It is a mechanical switch after all.
What is relay coil?
An electrical device, typically incorporating an electromagnet, which is activated by a current or signal in one circuit to open or close another circuit.Based on Characteristic the protection relay can be categorized as-Definite time relaysInverse time relays with definite minimum time(IDMT)Instantaneous relays.IDMT with inst.Stepped characteristic.Programmed switches.Voltage restraint over current relay.Based on of logic the protection relay can be categorized as-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.Based on actuating parameter the protection relay can be categorized as-Current relays.Voltage relays.Frequency relays.Power relays etc.Based on application the protection relay can be categorized as-Primary relay.Backup relay.Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when primary relay fails to be operated during fault. Hence backup relay is slower in action than primary relay. Any relay may fail to be operated due to any of the following reasons,The protective relay itself is defective.DC Trip voltage supply to the relay is unavailable.Trip lead from relay panel to circuit breaker is disconnected.Trip coil in the circuit breaker is disconnected or defective.Current or voltage signals from CT or PT respectively is unavailable.As because backup relay operates only when primary relay fails, backup protection relay should not have anything common with primary protection relay.Some examples of Mechanical Relay are-Thermal(a) OT trip (Oil Temperature Trip) (b) WT trip (Winding Temperature Trip) (C) Bearing temp trip etc.(a) Buchholz (b) OSR (c) PRV (d) Water level Controls etc.Float typePressure switches.Mechanical interlocks.Pole discrepancy relay.
What is an Inverse Time Device?
an inverse time protective device is a protective relay with definite minimum operating current and definite minimum operating time for a given current. the device will ensure that increasing current in a fault, that the operating time will be decreased. Preventing an overload.
What are time delay relay characteristics?
Time delay protective relays used in the power industry can be time over current or voltage, or definite time over / under voltage / over current.Definite time type relays use an over / under voltage / current element that picks up when voltage / current reach a specified level. When this picks up, a timer is started. If current / voltage stays above / below the pickup value for long enough to allow the timer to expire, the relay trip contacts are closed.Time over current elements use an inverse time - current curve to define speed of operation, so the more current is present, the faster the relay will operate.
What is the difference between idmt and dmt?
idmt means inverse definite minimum time. so in this type of relay as the value of operand that is current increase time taken by the relay to operate decrease. higher the current lower is the time taken by the relay to operate. AND dmt definite minimum time (DMT) relay take its minimum time to operate regardless of the value of the current. The definite mean time lag in disc type EM relays is inverse to current as the disc is faster at higher current and hence the less time.Definite mean time lag is possible in static relays by setting the time lag.
What is idmt relay?
Inverse Definite Minimum Time Lag = IDMT relay It's a electromagnetic type rotating disk relay. Tripping time of relay decreases with increasing fault current. see http://myweb.tiscali.co.uk/robert.booth/uni/docs/Power%20Supply%20Assignment%203.pdf
What is meant by IDMTL type relay?
Inverse definite minimum time lag relay
What is DTOC relay?
A definite time over-current relay operates like an instantaneous over-current relay coupled with a timer. Once current reaches the pick-up value, it initiates the timing circuit. As long as current stays above this pick-up value, the timer will continue to time. Once the definite time setting is reached, the relay gives trip signal to the circuit breaker.
What is instantaneous over current relay?
Instantaneous over current relay, which operates very fast with no intentional time delay and the operating time of these relay can be as low as 0.01sec . These relay operates only when the impedance between the relay and the source zsis very small compared to the impedance to the impedance of the protected section zl .
What is Inverse Definite Minimum Time?
(I.D.M.T.) RELAYThe over load inverse time relay is shown in fig 26. It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc have been displaced in phase from the flux entering the disc from the lower magnet. The deflecting torque is produced on the disc in the fashion as already explained. The spindle of the disc carries a moving contact which bridges two fixed contacts after the disc has rotated through a certain angle which has been set before. Any setting for this angle is possible varying from 0 to 360°. The variation of this angle imparts to the relay, various time settings.The speed of rotation of the disc is dependent upon the torque which in turn is dependent on the current setting, when the load current increases from this setting it will increase the speed of rotation of the disc resulting into decrease of operation time. Thus the time current characteristics of the relay observe inverse-Square law. The definite minimum time characteristics of the relay are obtained by the use of a saturated upper magnet. This ensures that there is no further increase in f1ux when the current has reached a certain value and any further increase of current will not affect the relay operation. This results in a flattened current time characteristic and the relay obtains its name as Inverse definiteminimum time lag (I.D.M.T.) relayThe current time characteristics of the relay have been illustrated in Fig. 27. It represents the time required to close the trip contacts for different values of over current. Its horizontal scale is marked in terms of current-setting multipliers i.e. number of times the relay current is in excess of current setting
What is the difference between high set and low set over current protection?
in on project that i saw for over current & earth fault protection high set setting had standard inverse curve and low set had l.t.i ( long time inverse) curve
What is dmt and idmt relay?
DMT relay stands for definitive mean time, and IDMT stands for inverse definitive mean time. In IDMT relay, there is a minimum time delay with magnitude.
What is high set and low set current setting?
The low-set stage is a current with a definite time or inverse-time operation. The high-set stage has a definite time characteristic only without the inverse-time operation.
What does the term inverse time mean pertaining to overcurrent protection?
Inverse time is a setting which determines how much quicker the breaker trips relating to the level of over current. Similar to a logarithmic curve. It usually relates to Electronically controlled breakers.
