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. When electromechanical relays were more popular, the overcurrent protection was made up from separate single-phase units. In practice, the relay is set to operate for faults up to 80% of the distance to the first tap. 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. This relay is referred as definite (instantaneous) overcurrent relay. â£Air-Gap Adjustment. TARGET AND SEAL-IN UNIT TABLE I ,.---2 Amp Tap Carry-Tripping Duty 30 Amps Carry Continuously 3 Amps is is tutorial provides a theoretical foundation on instantaneous and definite time overcurrent relays. Study specialized technical articles and papers. 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). In the relay setting procedure, the user is prompted for relay X or Y and then for the application. A simplified diagram of an induction disk time-overcurrent relay is shown in the following diagram, for one phase of the three-phase power system only. Instantaneous-overcurrent relay . For this reason, a seal-in relay actuated by current in the 125 VDC trip circuit is provided to maintain firm electrical contact closure in parallel with the rotating peg contact. Instantaneous Over current Protection (50): This is typically applied on the final supply load or on any protection relay with sufficient circuit impedance between itself and the next downstream protection relay. â£Spring Adjustment. So setting is=910x5/120=37.9 A or (910/120) X In=7.58 X In as per the available Relay functions. Definite time overcurrent relay is used as a backup protection of distance relay of transmission line with time delay, backup protection to differential relay of power transformer with time delay and main protection to outgoing feeders and bus couplers with adjustable time delay setting. A set of three photographs show the motion of a peg mounted on the induction disk as it approaches the stationary trip contact. Don't have an Control account? Figure 2 â Overcurrent relay arrangement with CTâs, including 50/51N. The difference in operating time of these two relays for the same fault is defined as discrimination margin. However, the function of trip time versus overcurrent magnitude is a curve, and several different curve shapes are available for United States applications: Time curves standardized by the Swiss standards agency IEC (International Electrotechnical Commission) include: The purpose for having different curves in time-overcurrent relays is related to a concept called coordination, where the 51 relay is just one of multiple overcurrent protection devices in a power system. This relay is referred as instantaneous over current relay, as ideally, the relay operates as soon as the current in the coil gets higher than pick upsetting current. The amount of CT secondary current necessary to activate the 50 relay is called the pickup current. The instantaneous overcurrent relay â¦ Over current relay protection is usually provided by either instantaneous or time delay over current relays. 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. In practice, three different protective relay circuits (three CTs, and three 50 relays with their trip contacts wired in parallel) would be connected together to the circuit breaker’s trip coil, so that the breaker will trip if any of the 50 relays detect an instantaneous overcurrent condition. 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. In such a relay, the relay contacts close immediately after the current in â¦ Instantaneous over-current relay. 5.3 Setting Overcurrent Relays. The more modern microprocessor protection has a three-phase overcurrent unit and an earth-fault unit within the same â¦ Other overcurrent protection devices include fuses and additional 51 relays at different locations along the same line. uses an Relay 2 instantaneous phase overcurrent element, a phase inverse-time overcurrent (51P) element and a51Q element. ï§ Pickup Current Setting â£Taps in the Relay Current Coil. In this type of relays, operating time is inversely changed with the current. 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). 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. There is no intentional time delay applied. Instantaneous Overcurrent Relays operate without time delay, (0.1s) ï§The pickup current or threshold is adjustable **. Legacy electromechanical time-overcurrent (51) relays implemented these different inverse curve functions by using induction disks with different “cam” shapes. These ratings include voltage, ampere and interrupting rating. Like process instruments which are often calibrated at five points along their measurement range, time-overcurrent relays must also be checked at multiple points along their prescribed “curve” in order to ensure the relay is performing the way it should. BE1-50, Instantaneous Overcurrent Relay Provides high-speed detection of phase and/or ground faults. 1. Although there is some time delay which can not be avoided. 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 protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. 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