This “seal-in” contact ensures a reliable circuit breaker trip even if the peg momentarily brushes or bounces against the stationary contact. 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: Here also consider the relay catalogue as mentioned earlier. 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. if $$I_{pickup}$$ = 4.5 amps, a 9.0 amp signal would be $$M = 2$$). The content is copyrighted to EEP and may not be reproduced on other websites. Once the induction disk contact closes to initiate current in the DC trip circuit, even momentarily, the seal-in coil will energize which closes the seal-in contact and ensures the continuation of DC trip current to the circuit breaker’s trip coil. Other overcurrent protection devices include fuses and additional 51 relays at different locations along the same line. 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. 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. Ideally, only the device closest to the fault will trip, allowing power to be maintained at all “upstream” locations. This is a more sophisticated form of overcurrent â¦ The basic element in overcurrent protection is an overcurrent relay. Protective Relaying System . 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. The difference in operating time of these two relays for the same fault is defined as discrimination margin. The relay located furthest from the source, operates for a low current value. 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. 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. 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. 5.3 Setting Overcurrent Relays. Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. In protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. Types of Overcurrent Relay. a tap value of “5” means that approximately 5 amps of CT secondary current is required for induction disk pickup). In protective relay-based systems, the time overcurrent protection function is designated by the ANSI/IEEE number code 51. â£Operation Time Less Than 3 Cycles 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 pickup value for the induction disk (i.e. The adjustment of definite-time and inverse-time relays can be carried out by determining two settings: time dial setting and pickup setting. So setting is=910x5/120=37.9 A or (910/120) X In=7.58 X In as per the available Relay functions. Create one now. 1. This relay is referred as definite (instantaneous) overcurrent relay. Definite (Instantaneous)-Current Protection. 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. More specifically ideally there is no time required to operate the relay. 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. INSTANTANEOUS OVERCURRENT AND VOLTAGE RELAYS TYPES PJC AND PJV INTRODUCTION APPLICATION * The Type PJC and Type PJV plunger relays are designed for general service. 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. These ratings include voltage, ampere and interrupting rating. 1.67A negative sequence current) and we applied 1A prefault current a balanced 3-phase 15A fault at the MTA. Overcurrent relays. BE1-50, Instantaneous Overcurrent Relay Provides high-speed detection of phase and/or ground faults. 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. In this type of relays, operating time is inversely changed with the current. elay settings are The r summarized as follows. 20% off code for Pro Plan: EEP2020. A fine adjustment is provided in the form of a variable resistor in series with the stator coils. The 3I2 pickup setting was 5A secondary, (i.e. 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. 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. Learn about power engineering and HV/MV/LV substations. 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. 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. There is always an inherent time delay of the order of a few milliseconds. 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. It will help you. 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. 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. Over current relay protection is usually provided by either instantaneous or time delay over current relays. 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. Instantaneous-overcurrent relay . 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. The most important feature of the relay is their speed of operation. 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. 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. Don't have an Control account? 1) means the disk need only rotate a small amount to close the contact; a high number on the time dial (e.g. Legacy electromechanical time-overcurrent (51) relays implemented these different inverse curve functions by using induction disks with different “cam” shapes. 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. is is tutorial provides a theoretical foundation on instantaneous and definite time overcurrent relays. uses an Relay 2 instantaneous phase overcurrent element, a phase inverse-time overcurrent (51P) element and a51Q element. A spiral spring applies a counter-clockwise restraining torque to the disk’s shaft. 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. Test Results on a Digital Relay Negative Sequence Overcurrent Element. In the relay setting procedure, the user is prompted for relay X or Y and then for the application. A silver-colored permanent magnet assembly at the front of the disk provides a consistent “drag” force opposing disk rotation. This CT should have a ratio smaller than the phase CTâs, and the relay pickup range in conjunction with the neutral CT should allow a pickup as â¦ Next, we tested a 60Hz SEL-321 relay 50Q instantaneous negative sequence overcurrent element. The amount of CT secondary current necessary to activate the 50 relay is called the pickup current. Electrical Engineering Books and Technical Guides. 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. â£Spring Adjustment. The more modern microprocessor protection has a three-phase overcurrent unit and an earth-fault unit within the same â¦ A reed relay, one per pole, is used as the current detecting component, each is inserted 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. Instantaneous over-current relay. The instantaneous overcurrent relay â¦ Optional output configuration allows the relay to be used as a phase-selective fault detector. In practice, the relay is set to operate for faults up to 80% of the distance to the first tap. The relay setting is adjusted based on its location in the network. 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. This allows a decrease in the â¦ 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 contacts of the relay are closed instantly when the current inside the relay rises beyond the operational value. Instantaneous Over-current relay is employed where the impedance between the source and the Relay is small as compared with the impedance of the section to be provided. An instantaneous overcurrent relay is one in which no intentional time delay is provided for operation. In protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. A photograph of the tap wire setting (coarse pickup adjustment) and resistor (fine pickup adjustment) are shown here. 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. â¢ Relay 1 ground time-overcurrent settings â 51G pickup = 240 A 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. 67 : Directional-overcurrent relay . 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. A. Instantaneous over-current relay B. Instantaneous relay having a setting of approximately 30% of motor rated current in the residual circuits of two CTs C. Ground wire D. Both B and C The relay then presents a group of elements to be set. 29. This is a more sophisticated form of overcurrent protection than instantaneous, expressed as a “time curve” relating overcurrent magnitude to trip time. This relay operates only when the impedance between the source and the relay is less than that provided in the section. A step-by-step guide on designing and testing Instantaneous and definite time overcurrent relays (DTOC) in Matlab/Simulink is also provided. TARGET AND SEAL-IN UNIT TABLE I ,.---2 Amp Tap Carry-Tripping Duty 30 Amps Carry Continuously 3 Amps The relay setting is adjusted based on its location in the network. Earth fault protection for an electric motor is provided by means of . There is no intentional time delay set. 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. 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. There is no intentional time delay set. 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. Following are the important features of an Instantaneous Over-current Relay: 1) Operates in a definite time when current exceeds its Pick-up value. Hence, this relay has current setting range as well as time setting range. ï§Features of Electromechanical 50 Elements â£ Suitable for AC and DC Systems. This can be avoided by using a short-time overcurrent relay with a sensitive setting.â Care must be exercised in understanding an elementâs fun-damental operation. In addition to the differential function, a backup overcurrent protection for 1 winding/ star point is integrated in the relay. The relay located furthest from the source, operates for a low current value. If desired, the ground­overcurrent curve and the instantaneous­ pickup and definite­time­delay attributes can be disabled. Basically overcurrent relay is a type of protective relay which operates when the load current exceeds a preset value. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase. There are three types of operating characteristics of overcurrent relays: This relay is referred as definite (instantaneous) overcurrent relay. 21 : Distance relay . 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 relay operates as soon as the current gets higher than a preset value. 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 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 set of three photographs show the motion of a peg mounted on the induction disk as it approaches the stationary trip contact. This is known as time grading. We refer the pre-set value of current in the relay coil as pickup setting current. The parallel seal-in contact also helps reduce arcing at the peg’s contact by carrying most of the trip coil current. 52 : Circuit breaker . 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. Its value may be varied by adjusting a movable magnetic pole inside the core of the relay. There is no intentional time delay applied. Time overcurrent relays exhibit different “curves” relating trip time to multiples of pickup current. Relay setting, Over current relay, IDMT relay. An instantaneous relay is one in which there is no time delay provided intentionally. 87 : Differential 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. A single 50 relay sensing current on a single line would not provide adequate instantaneous overcurrent protection for all three lines. 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. Thus, is has a time setting and pick up adjustment. Overcurrent relay characteristics for both types of relays are discussed. 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). Instantaneous overcurrent relay takes no time in giving tripping command once overcurrent is sensed. 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. 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 such a relay, the relay contacts close immediately after the current in â¦ Phase­overcurrent pickup settings and ground ­overcurrent pickup settings are made independently. The relay operates as soon as the current gets higher than a preset value. Tell us what you're thinking... we care about your opinion! These time dial values are linear multipliers: a time dial setting of 10, for example, exhibits twice the time to trip than a setting of 5, for any given overload condition. 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. 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. It starts operating instantly when the value of current is more than the relay setting. 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. They are available with standard inverse, very inverse and extremely inverse characteristics. In the feeder with small impedance, distinguisâ¦ This means the operating time delay is zero and hence relay operation is instantaneous. Although there is some time delay which can not be avoided. 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. 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. Time overcurrent protection allows for significant overcurrent magnitudes, so long as these overcurrent events are brief enough that the power equipment avoids heat damage. ï§ Pickup Current Setting â£Taps in the Relay Current Coil. The relay protects the system from earth fault and also used for protecting the system from circulating current. The 51Q element is set to protect the transformer in the event of a low-side single-phase-to-ground fault. Figure 2 â Overcurrent relay arrangement with CTâs, including 50/51N. 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. 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. If the 50 relay is connected with motor feeder for motor protection then pickup will be higher than the starting current of the motor. When E­speed or K­speed curves are selected, the ground­overcurrent pickup setting is disabled as are the â£Air-Gap Adjustment. Modern relays may contain more than one stage of protection each stage includes each own current and time setting. In Definite Time Overcurrent Relay, there is a â¦ Study specialized technical articles and papers. 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. 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. As the current coil is an inductor, there would be a certain delay to reach the current in the coil to its maximum value. 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. 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. So, high current will operate overcurrent relay faster than lower ones. Instantaneous Overcurrent Relays operate without time delay, (0.1s) ï§The pickup current or threshold is adjustable **. 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. 10) sets the resting position farther away from contact, so that the disk must rotate farther to trip. 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. When electromechanical relays were more popular, the overcurrent protection was made up from separate single-phase units. These relays are non-directional and instantaneous when in oper­ ation. 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. 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. 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. 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. 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. They are single phase (although some models contain more than one unit), non-directional, current sensitive, AC devices. Example, when the overcurrent relay is connected to the end of distribution feeder, Inverse Time relays are also referred to as, Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! IAC Time-overcurrent Relay Type IAC relays are used in the protection of industrial and utility power systems against either phase or ground overcurrent. 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). Overcurrent relays are normally supplied with an instantaneous element and a time-delay element within the same unit. As discrimination margin are available with standard inverse, very inverse and extremely inverse characteristics includes own... Or ground overcurrent in Matlab/Simulink is also provided ) element and a element. Content is copyrighted to EEP and may not be reproduced on other websites ï§ pickup current or threshold adjustable! 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