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. 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. Figure 2 â Overcurrent relay arrangement with CTâs, including 50/51N. Overcurrent Relay Setting 50 Elements: Pickup Setting 51 Elements Pickup setting Time delay setting definite time: time setting inverse time: curve selection Selecting an Overcurrent Relay Time Curve According to American Standrd Time-overcurrent relays (ANSI 51 relays) have two basic settings: uses an Relay 2 instantaneous phase overcurrent element, a phase inverse-time overcurrent (51P) element and a51Q element. 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 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. 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. 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. The basic element in overcurrent protection is an overcurrent relay. Types of Overcurrent Relay. The 3I2 pickup setting was 5A secondary, (i.e. Here also consider the relay catalogue as mentioned earlier. ï§ Pickup Current Setting â£Taps in the Relay Current Coil. Next, we tested a 60Hz SEL-321 relay 50Q instantaneous negative sequence overcurrent element. â£Air-Gap Adjustment. 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. 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. 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 relay operates only when the impedance between the source and the relay is less than that provided in the section. Test Results on a Digital Relay Negative Sequence Overcurrent Element. The disadvantage of this type of protection is that it’s difficult to coordinate and requires changes with the addition of load and that the short-circuit fault close to the source may be cleared in a relatively long time in spite of its highest current value. It starts operating instantly when the value of current is more than the relay setting. The relay located furthest from the source, operates for a low current value. This is the simplest form of overcurrent protection, both in concept and in implementation (relay design). There is always an inherent time delay of the order of a few milliseconds. A photograph of the tap wire setting (coarse pickup adjustment) and resistor (fine pickup adjustment) are shown here. 1.67A negative sequence current) and we applied 1A prefault current a balanced 3-phase 15A fault at the MTA. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase. 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. TARGET AND SEAL-IN UNIT TABLE I ,.---2 Amp Tap Carry-Tripping Duty 30 Amps Carry Continuously 3 Amps The relay located furthest from the source, operates for a low current value. 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. 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. 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). In electromagnetic relays such as the General Electric model showcased here, this setting may be coarsely adjusted by connecting a movable wire to one of several taps on a transformer coil inside the relay, varying the ratio of CT current sent to the induction disk stator coils. A set of three photographs show the motion of a peg mounted on the induction disk as it approaches the stationary trip contact. This is a more sophisticated form of overcurrent â¦ IAC Time-overcurrent Relay Type IAC relays are used in the protection of industrial and utility power systems against either phase or ground overcurrent. Instantaneous over-current 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. 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. 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. The parallel seal-in contact also helps reduce arcing at the peg’s contact by carrying most of the trip coil current. 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. In such a relay, the relay contacts close immediately after the current in â¦ Don't have an Control account? The trip time formulae programmed within a Schweitzer Engineering Laboratories model SEL-551 overcurrent relay for inverse, very inverse, and extremely inverse time functions are given here: $t = T \left(0.18 + {5.95 \over {M^2 - 1}} \right) \hskip 30pt \hbox{Inverse curve}$, $t = T \left(0.0963 + {3.88 \over {M^2 - 1}} \right) \hskip 30pt \hbox{Very inverse curve}$, $t = T \left(0.0352 + {5.67 \over {M^2 - 1}} \right) \hskip 30pt \hbox{Extremely inverse curve}$, $$T$$ = Time Dial setting (typically 0.5 to 15), $$M$$ = Multiples of pickup current (e.g. The adjustment of definite-time and inverse-time relays can be carried out by determining two settings: time dial setting and pickup setting. 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. 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. 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. 21 : Distance relay . In addition to the differential function, a backup overcurrent protection for 1 winding/ star point is integrated in the relay. The relay setting is adjusted based on its location in the network. Instantaneous overcurrent relay takes no time in giving tripping command once overcurrent is sensed. 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. A reed relay, one per pole, is used as the current detecting component, each is inserted 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. 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. Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. 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. Time overcurrent protection allows for significant overcurrent magnitudes, so long as these overcurrent events are brief enough that the power equipment avoids heat damage. Basically overcurrent relay is a type of protective relay which operates when the load current exceeds a preset value. ï§Features of Electromechanical 50 Elements â£ Suitable for AC and DC Systems. 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. 20% off code for Pro Plan: EEP2020. â£Operation Time Less Than 3 Cycles This allows a decrease in the â¦ The pickup value for the induction disk (i.e. Instantaneous-overcurrent relay . Overcurrent relays are normally supplied with an instantaneous element and a time-delay element within the same unit. The relay setting is adjusted based on its location in the network. It is of two types: instantaneous over current (IOC) relay and definite time overcurrent (DTOC) relay. This “seal-in” contact ensures a reliable circuit breaker trip even if the peg momentarily brushes or bounces against the stationary contact. 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. Study specialized technical articles and papers. 5.3 Setting Overcurrent Relays. 52 : Circuit breaker . The contacts of the relay are closed instantly when the current inside the relay rises beyond the operational value. This is known as time grading. Definite (Instantaneous)-Current Protection. An instantaneous relay is one in which there is no time delay provided intentionally. 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. While, when the impedance of feeder is high, the instantaneous protection has advantages of reducing the relay’s operating time for severe faults and avoiding the loss of selectivity. The most important feature of the relay is their speed of operation. In this type of relays, operating time is inversely changed with the current. The relay operates as soon as the current gets higher than a preset value. This means the operating time delay is zero and hence relay operation is instantaneous. This is a more sophisticated form of overcurrent protection than instantaneous, expressed as a “time curve” relating overcurrent magnitude to trip time. If desired, the ground­overcurrent curve and the instantaneous­ pickup and definite­time­delay attributes can be disabled. Although there is some time delay which can not be avoided. Optional output configuration allows the relay to be used as a phase-selective fault detector. 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. 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. elay settings are The r summarized as follows. Protective Relaying System . These relays are non-directional and instantaneous when in oper­ ation. Overcurrent relay characteristics for both types of relays are discussed. A step-by-step guide on designing and testing Instantaneous and definite time overcurrent relays (DTOC) in Matlab/Simulink is also provided. We refer the pre-set value of current in the relay coil as pickup setting current. A silver-colored permanent magnet assembly at the front of the disk provides a consistent “drag” force opposing disk rotation. In protective relay-based systems, the time overcurrent protection function is designated by the ANSI/IEEE number code 51. 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. They are available with standard inverse, very inverse and extremely inverse characteristics. There is no intentional time delay applied. 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 protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. Instantaneous Overcurrent Relays operate without time delay, (0.1s) ï§The pickup current or threshold is adjustable **. Electromechanical 50 (instantaneous overcurrent) relays are models of simplicity, consisting of nothing more than a coil817, armature, and contact assembly (a “relay” in the general electrical/electronic sense of the word). is is tutorial provides a theoretical foundation on instantaneous and definite time overcurrent relays. Learn about power engineering and HV/MV/LV substations. 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. This wheel functions as an adjustable stop for the induction disk’s motion, positioning the disk closer to or farther away from the trip contact in its resting condition: The amount of disk rotation necessary to close the trip contact may be set by adjusting the position of this time dial: a low number on the time dial (e.g. The communication system allows the user to read and modify the relay settings, and to access data gathered by the relayâs metering and recording functions. 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. In protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. Legacy electromechanical time-overcurrent (51) relays implemented these different inverse curve functions by using induction disks with different “cam” shapes. 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. There is always an inherent time delay of the order of a few milliseconds. 67 : Directional-overcurrent relay . There are three types of operating characteristics of overcurrent relays: This relay is referred as definite (instantaneous) overcurrent relay. Time overcurrent relays exhibit different “curves” relating trip time to multiples of pickup current. In the feeder with small impedance, distinguisâ¦ 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. 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. Tell us what you're thinking... we care about your opinion! 1. More specifically ideally there is no time required to operate the relay. The relay protects the system from earth fault and also used for protecting the system from circulating current. Following are the important features of an Instantaneous Over-current Relay: 1) Operates in a definite time when current exceeds its Pick-up value. â¢ Relay 1 ground time-overcurrent settings â 51G pickup = 240 A These ratings include voltage, ampere and interrupting rating. They generally have current setting multipliers ranging from 50 to 200% in steps of 25% which is refered to as plug setting [PS] for each relay is determined by the fault current. The difference in operating time of these two relays for the same fault is defined as discrimination margin. â£Spring Adjustment. 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. When E­speed or K­speed curves are selected, the ground­overcurrent pickup setting is disabled as are the The 51Q element is set to protect the transformer in the event of a low-side single-phase-to-ground fault. 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. The amount of CT secondary current necessary to activate the 50 relay is called the pickup current. 1) means the disk need only rotate a small amount to close the contact; a high number on the time dial (e.g. Ideally, only the device closest to the fault will trip, allowing power to be maintained at all “upstream” locations. In Definite Time Overcurrent Relay, there is a â¦ in [1], âinstantaneous overcurrent relays may be used, but sensitive settings will probably result in incorrect operations from dissimilar CT saturation and magnetizing inrush. Calibration of a time overcurrent (51) relay consists first of verifying that the unit “picks up” (begins to time) if ever the current magnitude exceeds the prescribed pickup value. 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. The content is copyrighted to EEP and may not be reproduced on other websites. 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. BE1-50, Instantaneous Overcurrent Relay Provides high-speed detection of phase and/or ground faults. If the 50 relay is connected with motor feeder for motor protection then pickup will be higher than the starting current of the motor. INSTANTANEOUS OVERCURRENT AND VOLTAGE RELAYS TYPES PJC AND PJV INTRODUCTION APPLICATION * The Type PJC and Type PJV plunger relays are designed for general service. 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