♣Air-Gap Adjustment. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase. The relay setting is adjusted based on its location in the network. a tap value of “5” means that approximately 5 amps of CT secondary current is required for induction disk pickup). The criteria used for setting the High-set Instantaneous Overcurrent Relay are: The relay must be set to operate for faults up to, but not beyond, the first tap from the feeder. Types of Overcurrent Relay. Electromechanical 51 (time overcurrent) relays are more complicated in design, using a rotating metal “induction disk” to physically time the overcurrent event, and trip the circuit breaker only if the overcurrent condition persists long enough. This CT should have a ratio smaller than the phase CT’s, and the relay pickup range in conjunction with the neutral CT should allow a pickup as … Instantaneous overcurrent protection is where a protective relay initiates a breaker trip based on current exceeding a pre-programmed “pickup” value for any length of time. Spring tension holds the trip contacts open, but if the magnetic field developed by the CT secondary current becomes strong enough to overcome the spring’s tension, the contacts close, commanding the circuit breaker to trip: The protective relay circuit in the above diagram is for one phase of the three-phase power system only. MIXED IDMT AND HIGH SET INSTANTANEOUS OVERCURRENT PROTECTION RELAYS A high-set instantaneous device can be utilized where the source impedance is small in comparison with the protected circuit impedance. This is a more sophisticated form of overcurrent … When electromechanical relays were more popular, the overcurrent protection was made up from separate single-phase units. Basically overcurrent relay is a type of protective relay which operates when the load current exceeds a preset value. The time dial setting adjusts the time delay before the relay operates whenever the fault current reaches a value equal to, or greater than, the relay current setting. Its value may be varied by adjusting a movable magnetic pole inside the core of the relay. This is the simplest form of overcurrent protection, both in concept and in implementation (relay design). A fine adjustment is provided in the form of a variable resistor in series with the stator coils. Modern relays may contain more than one stage of protection each stage includes each own current and time setting. If the 50 relay is connected with motor feeder for motor protection then pickup will be higher than the starting current of the motor. ♣Operation Time Less Than 3 Cycles is is tutorial provides a theoretical foundation on instantaneous and definite time overcurrent relays. Earth fault protection for an electric motor is provided by means of . Figure 2 – Overcurrent relay arrangement with CT’s, including 50/51N. Time overcurrent protection is where a protective relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration, the relay tripping sooner with greater current magnitude. As the aluminum disk rotates through the permanent magnet’s field, eddy currents induced in the disk set up their own magnetic poles to oppose the disk’s motion (Lenz’s Law). The settings of this kind of relay at different locations in the network can be adjusted in such a way that the breaker closest to the fault is tripped in the shortest time and then the other breakers in the direction toward the upstream network are tripped successively with longer time delay. Next, we tested a 60Hz SEL-321 relay 50Q instantaneous negative sequence overcurrent element. This allows a decrease in the … The peg may only lightly touch the stationary contact when it reaches its final position, failing to provide a secure and lasting electrical contact when needed. The 51Q element is set to protect the transformer in the event of a low-side single-phase-to-ground fault. The most important feature of the relay is their speed of operation. The ANSI device number is 50 for an instantaneous overcurrent (IOC) or a Definite Time overcurrent (DTOC) and 51 for the Inverse Definite Minimum Time. In switchgear application, an overcurrent relay usually is used on each phase of each circuit breaker and often one additional overcurrent relay is used for ground-fault protection. The operating time of both overcurrent definite-time relays and overcurrent inverse-time relays must be adjusted in such a way that the relay closer to the fault trips before any other protection. The monitoring of all three line currents is necessary because power line faults are usually unbalanced: one line will see a much greater share of the fault current than the other lines. If the CT current exceeds the pickup value for a long enough time, the disk rotates until it closes a normally-open contact to send 125 VDC power to the circuit breaker’s trip coil. For a low-resistance-grounded system, the use of an overcurrent relay connected to a CT in the service transformer or generator neutral is usually the best option. The amount of CT secondary current necessary to activate the 50 relay is called the pickup current. A reed relay, one per pole, is used as the current detecting component, each is inserted 10) sets the resting position farther away from contact, so that the disk must rotate farther to trip. Calibration of an instantaneous overcurrent (50) relay consists simply of verifying that the unit “picks up” within a reasonably short amount of time if ever the current magnitude exceeds the prescribed pickup value. ♣Spring Adjustment. In such a relay, the relay contacts close immediately after the current in … A photograph of a General Electric time-overcurrent induction-disk relay appears here: The round disk you see in the photograph receives a torque from an electromagnet coil assembly acting like the stator coils of an induction motor: alternating current passing through these coils cause alternating magnetic fields to develop through the rear section of the disk, inducing currents in the aluminum disk, generating a “motor” torque on the disk to rotate it clockwise (as seen from the vantage point of the camera in the above photo). After the proper pickup value has been set, the time value is established by rotating a small wheel called the time dial located above the induction disk. There is always an inherent time delay of the order of a few milliseconds. It will help you. The relay protects the system from earth fault and also used for protecting the system from circulating current. Instantaneous Overc1r11e1nt Relay; Inverse Time Overcurrent Relay (IDMT Relay) Directional overcurrent relay; 1Mixed IDMT and high set instantaneous; Instantaneously overcurrent relay operates when current exceeds its Pick-up value. 21 : Distance relay . All 51 relays are inverse in that the amount of time to trip varies inversely with overcurrent magnitude: the greater the sensed current, the less time to trip. This relay operates only when the impedance between the source and the relay is less than that provided in the section. Thus, is has a time setting and pick up adjustment. overcurrent relay having one opto-isolated input, two output contacts, and a set of three-phase current inputs for each of the independent relays X and Y. More specifically ideally there is no time required to operate the relay. Each tap is labeled with the number of whole amperes (AC) delivered by the secondary winding of the CT required for relay pick-up (e.g. in [1], “instantaneous overcurrent relays may be used, but sensitive settings will probably result in incorrect operations from dissimilar CT saturation and magnetizing inrush. Protective Relaying System .  Pickup Current Setting ♣Taps in the Relay Current Coil. Ideally, only the device closest to the fault will trip, allowing power to be maintained at all “upstream” locations. The content is copyrighted to EEP and may not be reproduced on other websites. The relay setting is adjusted based on its location in the network. The effect is akin to having the disk rotate through a viscous liquid, and it is this dynamic retarding force that provides a repeatable, inverse time delay. Optional output configuration allows the relay to be used as a phase-selective fault detector. These relays are non-directional and instantaneous when in oper­ ation. An instantaneous relay is one in which there is no time delay provided intentionally. It is of two types: instantaneous over current (IOC) relay and definite time overcurrent (DTOC) relay. An instantaneous over-current relay is an overcurrent relay which has no intentional time delay for operation. The contacts of the relay are closed instantly when the current inside the relay rises beyond the operational value. Relay setting, Over current relay, IDMT relay. The pickup value for the induction disk (i.e. In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous for at least a time equal to the time setting of the relay. 1) means the disk need only rotate a small amount to close the contact; a high number on the time dial (e.g. It is an amusing anachronism that even in digital 51 relays containing no electromagnets or induction disks, you will find parameters labeled “pickup” and “time dial” in honor of legacy electromechanical relay behavior. Time overcurrent protection is where a protective relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration, the relay tripping sooner with greater current magnitude. the minimum amount of CT current necessary to overcome the spring’s torque and begin to rotate the disk) is established by the spring tension and the stator coil field strength. 87 : Differential relay . 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