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. INSTANTANEOUS OVERCURRENT AND VOLTAGE RELAYS TYPES PJC AND PJV INTRODUCTION APPLICATION * The Type PJC and Type PJV plunger relays are designed for general service. Definite (Instantaneous)-Current Protection. It is of two types: instantaneous over current (IOC) relay and definite time overcurrent (DTOC) relay. is is tutorial provides a theoretical foundation on instantaneous and definite time overcurrent relays. 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. Overcurrent relay characteristics for both types of relays are discussed. In small, self-tripping circuit breakers, this type of protection is best modeled by “magnetic” breakers where the tripping mechanism is actuated by the magnetic field strength of the line conductors: any amount of current greater than the tripping threshold will cause the mechanism to unlatch and open the breaker. If the 50 relay is connected with motor feeder for motor protection then pickup will be higher than the starting current of the motor. Legacy electromechanical time-overcurrent (51) relays implemented these different inverse curve functions by using induction disks with different “cam” shapes. Relay setting, Over current relay, IDMT relay. There are three types of operating characteristics of overcurrent relays: This relay is referred as definite (instantaneous) overcurrent relay. 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. A single 50 relay sensing current on a single line would not provide adequate instantaneous overcurrent protection for all three lines. The instantaneous overcurrent relay … 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. The purpose of overcurrent protection is to guard against power distribution equipment damage, due to the fact that excessive current in a power system dissipates excessive heat in the metal conductors comprising that system. This is known as time grading. 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. 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. Types of Overcurrent Relay. An instantaneous overcurrent relay is one in which no intentional time delay is provided for operation. In small, self-tripping circuit breakers, this type of protection is best modeled by “thermal” breakers where the tripping mechanism is actuated by the force of a bimetallic strip heated by line current: excessive current heats the metal strip, which then forces the mechanism to unlatch and open the breaker. Features of Electromechanical 50 Elements ♣ Suitable for AC and DC Systems. Published under the terms and conditions of the, Electric Power Measurement and Control Systems, Instantaneous and Time-overcurrent (50/51) Protection, Electrical Sensors: Potential Transformers (PTs) and Current Transformers (CTs), Creative Commons Attribution 4.0 International Public License. a tap value of “5” means that approximately 5 amps of CT secondary current is required for induction disk pickup). Overcurrent relays are normally supplied with an instantaneous element and a time-delay element within the same unit. if \(I_{pickup}\) = 4.5 amps, a 9.0 amp signal would be \(M = 2\)). In the feeder with small impedance, distinguis… 20% off code for Pro Plan: EEP2020. The relay located furthest from the source, operates for a low current value. This means we want overcurrent protection devices at the remote end(s) of a power system to be more sensitive and to trip faster than devices closer to the source, where a trip would mean an interruption of power to a greater number of loads. ♣Spring Adjustment. 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. A silver-colored permanent magnet assembly at the front of the disk provides a consistent “drag” force opposing disk rotation. 5.3 Setting Overcurrent Relays. Overcurrent protection is also applied to machines such as motors and generators for the exact same reason: electric current dissipates heat in the windings’ resistance (\(P = I^2 R\)), and excessive heat will damage those winding conductors. uses an Relay 2 instantaneous phase overcurrent element, a phase inverse-time overcurrent (51P) element and a51Q element. The relay’s seal-in function will subsequently maintain the trip command until some external contact opens to break the trip circuit, usually an auxiliary contact within the circuit breaker itself. Instantaneous Overcurrent Relay Features High speed operation Not slowed by dc transients Settings readily adjustable on relay front plate Two phase and earth fault relay Description A connection diagram for this relay is shown in Figure 2. It starts operating instantly when the value of current is more than the relay setting. There is no intentional time delay set. Figure 2 – Overcurrent relay arrangement with CT’s, including 50/51N. 67 : Directional-overcurrent relay . Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. There is always an inherent time delay of the order of a few milliseconds. More specifically ideally there is no time required to operate the relay. There is no intentional time delay set. In this type of relays, operating time is inversely changed with the current. Its value may be varied by adjusting a movable magnetic pole inside the core of the relay. Ideally, only the device closest to the fault will trip, allowing power to be maintained at all “upstream” locations. • Relay 1 ground time-overcurrent settings − 51G pickup = 240 A The relay operates as soon as the current gets higher than a preset value. In practice, three different protective relay circuits (three CTs, and three 51 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 51 relays detect a timed overcurrent condition: The seal-in unit is shown as an electromechanical relay connected with its contact in parallel with the induction disk contact, but with its actuating coil connected in series to sense the current in the 125 VDC trip circuit. 52 : Circuit breaker . Although there is some time delay which can not be avoided. in [1], “instantaneous overcurrent relays may be used, but sensitive settings will probably result in incorrect operations from dissimilar CT saturation and magnetizing inrush. 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 amount of CT secondary current necessary to activate the 50 relay is called the pickup current. The relay setting is adjusted based on its location in the network. As the current coil is an inductor, there would be a certain delay to reach the current in the coil to its maximum value. Modern relays may contain more than one stage of protection each stage includes each own current and time setting. 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. 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. Earth fault protection for an electric motor is provided by means of . Instantaneous overcurrent relay takes no time in giving tripping command once overcurrent is sensed. 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. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase. 1.67A negative sequence current) and we applied 1A prefault current a balanced 3-phase 15A fault at the MTA. Overcurrent relays. This is the simplest form of overcurrent protection, both in concept and in implementation (relay design). This can be avoided by using a short-time overcurrent relay with a sensitive setting.” Care must be exercised in understanding an element’s fun-damental operation. Modern microprocessor-based 51 relays contain multiple curve functions as mathematical formulae stored within read-only memory (ROM), and as such may be programmed to implement any curve desired. Learn about power engineering and HV/MV/LV substations. Here also consider the relay catalogue as mentioned earlier. Thus, is has a time setting and pick up adjustment. Following are the important features of an Instantaneous Over-current Relay: 1) Operates in a definite time when current exceeds its Pick-up value. If desired, the ground­overcurrent curve and the instantaneous­ pickup and definite­time­delay attributes can be disabled. Next, we tested a 60Hz SEL-321 relay 50Q instantaneous negative sequence overcurrent element. Protective Relaying System . Create one now. Calibration of the time-overcurrent protective function must be performed at multiple values of current exceeding the pickup value, in order to ensure the relay trips within the right amount of time for those current values. This “seal-in” contact ensures a reliable circuit breaker trip even if the peg momentarily brushes or bounces against the stationary contact. It will help you. The adjustment of definite-time and inverse-time relays can be carried out by determining two settings: time dial setting and pickup setting. 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. In the feeder with small impedance, distinguishing between the fault currents at both ends is difficult and leads to poor discrimination and little selectivity at high levels of short-circuit currents. Overcurrent protective devices at 480V are set to open at or below the downline cable or busway ampacity per NEC Section 240.3; except when the ampacity does not correspond to a standard rating, the next higher standard rating may be used as long as this rating does not exceed 800 amps. When E­speed or K­speed curves are selected, the ground­overcurrent pickup setting is disabled as are the 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. 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. The tap in this first photograph happens to be set at the 4 amp position: Proper setting of the pickup tap value is determined by the maximum continuous current rating of the system being protected and the ratio of the current transformer (CT) used to sense that current. 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. 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.  Pickup Current Setting ♣Taps in the Relay Current Coil. 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. Electrical Engineering Books and Technical Guides. 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). TARGET AND SEAL-IN UNIT TABLE I ,.---2 Amp Tap Carry-Tripping Duty 30 Amps Carry Continuously 3 Amps This relay is referred as definite (instantaneous) overcurrent relay. From left to right we see the disk in the resting position, partially rotated, and fully rotated: The mechanical force actuating the time-overcurrent contact is not nearly as strong as the force actuating the instantaneous overcurrent contact. 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. 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. An instantaneous over-current relay is an overcurrent relay which has no intentional time delay for operation. These relays are non-directional and instantaneous when in oper­ ation. Arcing at the peg momentarily brushes or bounces against the stationary trip contact short-time overcurrent relay with. Setting is adjusted based on its location in the relay operates as soon as the current inside core... 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