A current transformer (CT) is a type of transformer that is used to measure AC current. Bar-type Current Transformer- This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. Bar Type CT: One that has a fixed and straight single primary winding turn passing through the magnetic circuit. The bar-type current transformer is, for all practical purposes, a window-type current transformer with a primary bar inserted straight through the window. Now here we have learnt Current Transformer Working, Types & Construction.You can download this article as pdf, ppt. Save my name, email, and website in this browser for the next time I comment. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. In short, it should be never operated at no-load condition when the current is flowing through the primary. In practice, they are intended to be used over insulated conductors when the conductor voltage exceeds 600 V. It is common practice to utilize a 600 V class window type in conjunction with air space between the window and the conductor on higher-voltage systems. The accuracy class of a CT is the percent allowable error, and is associated to burden power, the apparent power, expressed in VA, that is taken up from the secondary core (secondary load), and for which the accuracy is assured. Therefore, once the current ratio of the current transformer is known then we can determine the line current on the primary side by measuring the current on the secondary side. These transformers with low range ampere meters are used to measure the current in the high voltage circuits. ANSI C51.13.1978 CTs and VTs CANADIAN CSA CAN3-C13-M83 CTs and VTs AUSTRALIAN AS 1675-1986 CTs s 5 Current Transformer Construction BAR PRIMARY . Your email address will not be published. It is used to activate the trip coil of a circuit breaker. The ammeter is connected in the secondary. Accuracy class for relaying CTs: Relaying CTs requires lower accuracy. CDG current setting = 10% = 0.5 A. Volts required to operate relay = 2 / 0.5 = 4 V. Therefore voltage required at 10 times setting = 10 x 4 = 40 V. Hence for a flux density of 1.6T the C.T. In special cases when taps are employed, they are distributed such that any connection made would utilize the entire core-periphery. A window current transformer has an open area in the centre for a power line to be passed through as the primary . Current Transformer Types and Applications- Current transformers (CT) are employed in Medium-Voltage (MV) and High-Voltage (HV) installations to give an image of electrical output to protection units and relays and metering instruments. Current transformers decrease the high-voltage values to a much lower current and supply a convenient method of safely measuring the actual electrical current moving in an AC transmission line applying a standard ammeter. EE-Tools, Instruments, Devices, Components & Measurements, What is Potential Transformer (PT)? The current rate will determine the turn ratio, and as the primary winding commonly includes one or two turns, whereas the secondary winding can include several hundred turns, the rate between the primary and secondary can be relatively large. A bar type current transformer of toroidal construction requires 400A to magnetize it, and 300 A to supply the iron loss for each volt per turn induced in the secondary winding and rated frequency. SUPERBUTE Product Overview: Current Transformers. The core of the current transformer is built up with lamination of silicon steel. The bar may be of rectangular or circular cross-section. Large alternating currents that cannot be measured directly by normal ammeters or by wattmeters current coil to measure power can be done by using a current transformer. In terms of voltage, it is a step-up transformer which steps-down the level of current. Thus, accurate and small instruments and monitoring devices can be applied with CTs since they are insulated away from any high-voltage energy lines. In the case of a large number of secondary winding turns, requiring more than one winding layer, the frequently adopted technique is to sectionalize the secondary winding so as to considerably reduce the peak voltage between layers. A current transformer in which the primary winding consists of a bar of suitable size and the material forming an integral part of the transformer.The figure below shows bar type transformer. The primary of a CT typically has only one turn, which is nothing but the current-carrying conductor passed through its window. Your email address will not be published. The rated burden of the system is of the voltage and current on the secondary winding when the CT supports the relay or instrument with its maximum rated amount of the current. A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.. CT Accuracy Class or Current Transformer Class. Another type of current transformer which falls into the C class is a bar-type current transformer, where the primary conductor passes through the current transformer window but there is only . These are present for sensing currents from 100 A up to 5000 A, with square window forms from 25 mm to 300mm. HV CT are commonly installed outdoors, in AIS substations (Air Insulated Substation) Figure 2 or indoors, in GIS substations (Gas Insulated Substation) Figure 3. There are several core types used, from low-reactance toroid to high-reactance cut . Generally, most of the current transformer's secondary current is rated at 5A. Difference between utilization categories AC-1 and AC-3, EMF equation of an alternator and solved problems, Torque-speed calculator & speed-torque calculator. Based on the construction, there are two types of current transformer. The primary winding consists of very few turns and, therefore, there is no appreciable voltage drop across it. The primary winding and secondary winding(s) are insulated from each other and from the core(s) and are . Their structure can be one single primary turn as in Doughnut, Toroidal, or Bar forms, or a few wound primary turns, commonly for low current applications. The reassembly of the core halves is critical. Protection current transformer: used to protect the circuit through protective relays.generally having class 5P10,10P10,5P20etc. They are also known as bar-primary CTs. Construction. The alternating current produces an alternating magnetic field in the ferromagnetic core. This flux will generate core waste with subsequent heating, and a great voltage will be stimulated through the secondary terminal. Either way, the primary conductor is a single turn through the window, fully insulated from the secondary winding. In a ring-type core structure, the secondary winding is wound on the inner core of the bakelite former. They are also used to step down the current at a specific ratio to insulate the instrument from the high voltage lines. . Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating Ammeter (i.e. Toroidal Current Transformers: These do not include a primary winding. Rated primary currents from 400 A to 3200 A. Bus- bar type split core current transformer Class 1 accuracy Currents from 400 A to 3200 A Up to 10 Bus- bar isolated. The insulation of Bar Type Rectangular Current Transformer provides an enhanced safety. Answer.1. 50,60. The working principle of a current transformer is especially different from the power transformer. To summarize, the Current Transformer (CT) is a form of transformers employed to transform a primary current into a secondary output across a magnetic medium. According to the above referred IEC Standard, CT most common accuracies and burdens are: Class PX is the most accurate and is used usually for main protections. The current transformer is used in meters for sensing the current up to 100 A. Ring type CT are commonly used in cables, busbars and transformers bushings. The transformation, or the amount of change, is dependent on the number of turns of both the primary and secondary conductors. So, for instance, a current transformer with a ratio of, say, 300/5 can be modified to another of 150/5 or even 100/5 by passing the basic primary conductor within its interior window two or three times. This is a common construction for HV and EHV current transformers. B. false. Rather, a toroidal transformer has a window or hole through which the current flowing in the network passes. Current transformers are available in various sizes and shapes and are used as an interface between high currents and measuring/sensing devices. A current transformer is generally used in metering and protective relays in the electrical power industry. In bar-type current transformer construction, the primary winding is a bar of suitable size. A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). The secondary section will provide a current into a resistive load or into either a short circuit, in the form of an ammeter until the voltage generated in the secondary is heavy enough to saturate the core or result in a failure from additional voltage breakdown. [2] IEC: International Electrotechnical Commission. Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the . A variant of the . Current and voltage transformers are available in voltage classes ranging from 5 kV to 34.5 kV. Clamp meters are specially designed current measurement devices, that allows measurement of the current in a conductor by clamping it around a current carrying conductor. It never has more than a very few numbers of turns in its primary. A current transformer in which the primary winding consists of a bar of suitable size and the material forming an integral part of the transformer.The figure below shows bar type transformer. Toroidal Current Transformer [wp_ad_camp_1] Bar-type Current Transformer: The primary of the current transformers is considered as the line or cable or busbar of the load circuit. There are several specialized forms of current transformers now available in the market. Measuring current transformers are constructed to feed on other . Due to which an extremely high voltage appears at the secondary of the transformer which is dangerous to personnel and the CT itself. See Terms of Use and Privacy Policy. They are also known as CTs. Bar type current transformers consist of a single bar as a primary winding. Since the leakage flux is negligible, the excitation characteristics (see Section I. At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. Shell Type Transformer Construction. Such use may be seen in isolated-phase bus compartments. Buy Equipment or Ask for a Service. Bar type; Wound type; Wound-type Current Transformer: The transformer's primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit.The magnitude of the secondary current is dependent on the turn ratio of the transformer. In the shape of a doughnut, these transformers are designed without having a primary winding as part of their construction. Generally, the windings do not utilize the magnetic path efficiently. The primary section of the current transformers carries the current which should be measured, and it is attached to the main circuit. #2 - Wound Current Transformer There is an ongoing development and limited use of optical-type current transducers (OCT), which rely on the principles of light deflection. Eaton's 5A solid-core current transformer is ideal for new construction and retrofit applications where no current transformer exists. The current transformer (CT) is a type of instrument transformer designed to measure high-value current in the power system. A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. Transformer Basics and working principle,Construction,Types of . Also, the voltage in the secondary winding will be 200 times the primary voltage. These barriers can be overcome using a CT. There are several metering cases and usages for current transformers, such as with power factor meters, Wattmeters, protective relays, watt-hour meters, or as trip coils in MCBs or magnetic circuit breakers. While aluminium wire is lighter and generally less expensive than copper wire, a larger cross sectional area of conductor must be used to carry the same amount of current as with copper so it is used mainly in larger power transformer applications. In other words, the secondary of the current transformer should be never left open under any circumstances. Current transformer are used for measuring current in a bus bar without disconnecting it by means of ammeter.There are three types 1.bar type current transformer 2.toroidal current transformer 3 . A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. least 40 V. With bar primary, number of turns = 20. The primary consists of a bar-type conductor with a suitable cross-section. d) secondary winding having more than one full turn wound on core. Table 1 Common accuracies and burden powers of CT and error limits. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. With wound primary current transformers this particular problem is rarely met but it is of importance to try to obtain good relative positioning of primary and secondary winding coils, thus minimizing the axial forces on both coils caused by primary winding short circuit currents. Some CT has special primary coils that allow a double ratio, when an increase of the installation is foreseen (example: 200-400/1 A) see Figure 8. If there is current flow in the primary of the current transformer and its secondary winding is kept closed, the current flowing through the secondary builds up a back EMF and counters the primary magnetizing force. The current transformer connected to a small line voltage system uses tape or varnish as insulation. Below are the types of current transformers in the manner of construction and applications. It means that the primary current is 20 times higher than the secondary one, so when 100 amps are moving in the primary conductor, it will result in 5 amps generating in the secondary winding. Then the primary current or load current I1 is given as. Current Ratio Error expressed in percent is given by the formula:Credit: https://talema.com/introduction-current-transformers/, Kn = Rated transformation ratioIp = Actual primary currentIs = Actual secondary current when Ip is flowing, under conditions of measurement. CT secondary circuit must be grounded, and grounded at one point only. Bar Current Transformers. These current transformers have an opening where the conductor carrying the primary load is passed. Conversely, the minimum primary current a CT can accurately measure is light load, or 10% of the rated current. Therefore, the secondary of the current transformer may never be open when the primary part is carrying the current. In the manufacture of current transformers assembly of lamination stacks demands somewhat greater core cross-section than ordinary transformers in order to keep down the reluctance of the interleaved corners as low as possible so as to minimize the magnetizing current.Sometimes cut cores are used. Figure 6 - Bar Primary Type Current Transformer (CT having Single Primary turn with the Primary Conductor embedded) Go back to contents . The secondary winding is fully distributed around the periphery of the core. Either way, the primary conductor is a single turn through the window, fully insulated from the secondary winding. Why is a Transformer Rated in kVA, but Not in kW? Why are Air-Conditioners (AC) Rated in Tons, Not in kW or kVA? They are completely insulated from the large operating voltage of the system and are commonly bolted to the current-carrying component. RITZ offers a large portfolio with a variety of different products. If the turns ratio between primary and secondary is 100:05; then it will step up the level of voltage in terms of 20:01 or it will step down the level of current in terms of 01:20. Current transformers are used extensively in metering and circuit protection. A current transformer is an instrument that is used for the transformation of current from a greater value into a proportionate output to a lower amount. It transforms the large-voltage current into the low-voltage current, due to which the heavy current moving across the transmission lines are safely controlled by the ammeter. Current transformers are used with ammeters, kWh meters, power factor meters and energy meters for current measurements. The ammeter, or wattmeter current coil, are connected directly across the secondary winding terminals. 3. The maximum current up to which the transformer must maintain its current ratio is 40,000 amperes. In most case, the secondary current of a current transformer shall be 5A. 2. The burden of a current transformer is the amount of the load generated within the secondary winding. It is used to measure current through a bus bar which is inserted through it. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Linquipis a Professional Network for Equipment manufacturers, industrial customers, and service providers, Copyright 2022 Linquip Company. Itproduces an alternating current (AC) in its secondary which is proportional to the AC currentin its primary. View Answer. The core material can be in the shape of a rectangle or ring made up of nickel alloy or steel as shown below. Accuracy classes are defined by IEC and IEEE and are different for metering CTs and relaying CTs. They have separate primary and secondary winding. One of the terminals of the secondary winding is earthed so as to protect equipment and personnel in the vicinity in the event of an insulation breakdown in the current transformer. CT consists of three main components: a primary winding . But in a practical current transformer, there is a phase difference between the secondary and the primary current since the primary output has also supplied the portion of the exciting current. We are specialized in offering Summation Current Transformer to our clients. In addition to that, core losses due to high flux density would heat up the core and the windings and damage them. Exciting Current) can . Types of current transformer based on construction, https://talema.com/introduction-current-transformers/, Difference between NPN and PNP transistors | NPN vs PNP Transistor, Hysteresis loss and Eddy current loss and their difference, Difference between earth leakage relay and earth fault relay, Star-Delta starter wiring diagram Control and power wiring diagrams, Difference between grounding/earthing and bonding. The, is the ring type or Window type, examples of which are given in the below figure which shows three commonly used shapes i.e., stadium, circular and rectangular orifices. Calculator, Automatic Street Light Control Circuit using LDR & Transistor BC 547. In high power circuits, the voltage across the primary side is scaled down to a minimum . 3. They are directly connected to the current-carrying conductor. The bus bar type MV CTs construction usually have resin cast CT with bur bar built into the construction. If for some reasons, the ampere meter is disconnected from the secondary windings, it is necessary to short circuit the secondary winding. Required fields are marked *. Current Transformers (600 Volt, Indoor Bar Type) Specifications File Reference: Title: Publish Date: Format: JCM-3/4/5: Models JCM-3/JCM-4/JCM-5: 2017-11-08 [417k] document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); (adsbygoogle = window.adsbygoogle || []).push({}); All Rights Reserved by Electrical Engineering Info @2023, The primary winding consists of very few turns and, therefore, there is no appreciable voltage drop across it. The current transformer is used in the AC device, meters, or monitoring apparatus where the current to be sensed is of such magnitude that the instrument coil or meter cannot conveniently be created from adequate current carrying capacity. The core can be of solid or split type construction. The ratio of a CT is the relation between the values of the primary and the secondary currents; the usual secondary values are 1 A and 5 A. If the ammeter on the secondary winding is read 1.5 amps i.e, I2 = 1.5A. 5A ammeter). There are two type of current transformer. Based on the function performed by the current transformer, it can be classified is follows: . CT are connected in series and protection devices and metering equipment are connected to the secondary of the CT in series association, as shown in Figure 1. These CT's include the laminated core and a secondary winding. This bar assembly can be permanently attached or held in place with brackets. The secondary output is commonly rated at a standard 1 A or 5 A for greater primary current ratings. 2.7.4 Current transformer construction. Instrument transformers are of two types - Current Transformer (C.T.) The primary side . Our articles are written by the electrical engineers in a simple and easy way. Required fields are marked *. Save my name, email, and website in this browser for the next time I comment. The current transformer (CT), also known as an instrumental transformer, is used to either reduce or propel the primary current across the secondary winding. 10. Secondary Rating. Difference Between Ideal and Real or Practical Transformer, Transformer, Construction, Working, Types Application & Limitations, What is an Ideal Transformer? . Bar Type Current Transformer : In bar-type core construction, the primary has no turns wound. Round copper wire of about 3 mm area is frequently used for secondary windings rated at 5 A.Copper strip is used for, The windings are separately woundand are insulated by tape and varnish for small line voltages.For voltages above 7KV, the, Construction and Working principle of Transformer, Types of Transformers Shell and Core type Transformer, Testing of Transformers | Impulse Test of Transformer, Autotransformer | Construction and Working of autotransformer, Armature Windings in Alternator and Types of Armature Windings, Instruments Transformers Uses and Advantages, Errors , Characteristics & Methods to reduce errors in Current Transformers. How to Check Whether a Multimeter is Working or Not! RF, which is a characteristic of metering and energy metering cores, represents the amount by which the primary load current may be increased over its nameplate rating without exceeding the allowable temperature rise, that is to say, the transformers overload capability. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated current. According to IEC[2] Standard 61869-2, Clause 5.201, rated primary currents of CT are: 10 12,5 15 20 25 30 40 50 60 75 A and their decimal multiples or fractions. . Accuracy class describes how accurate the current transformation of a current transformer would be within when the burden within the allowable limits. Answer: A. Clarification: The split core transformer consists of a split core. It is best suited for general-purpose monitoring and temporary installations. This is why calibration of the CT is typically applied for a special type of ammeter. Types of Current Transformers CTs are manufactured in various sizes, shapes, and ratings for a broad range of applications. Insulation Voltage: 3000 volts. It is important to monitor current and voltages at the substations and generating stations which are at high values. There is also some uncertainty in the repeatability of performance from installation to installation. The magnitude of the primary and secondary voltages depends on the number of rolls on the windings. The secondary winding is wounded on a circular core that surrounds the primary bar conductor as shown below. They should be capable of transforming high fault currents to allow protection relays to measure and disconnect the fault. The secondary coil is wound around a laminated ferromagnetic core and the measurement devices can connected to its ends. . The instruments (ammeter etc.) We develop your standard equipment, but also . The manner in which the windings are arranged on the core affects the reactance since it is a geometric function. The working or operation of a current transformer is similar to that of a normal two-winding transformer. Based on the type of construction, current transformers are classified into three types as follows: This type consists of a hollow core through which the current-carrying conductor or cable is passed. In dc circuits it is easy to measure large currents by adding suitable shunts to the low range ammeters. As a result, if the secondary component is opened and the current moves across the primary windings, then there will be no demagnetizing environment because of the secondary current. The body of the CT itself provides insulation between the primary circuit and ground. Figure 1 Schematic connection of a current transformer. The primary of a CT is connected in series to the current-carrying conductor and measurement devices are connected to its secondary. The following are different types of current transformers: A current transformer having a primary winding of more than one full turn wound on core. In a low voltage wound type current transformer the secondary winding is wound on a bakelite former or bobbin and the heavy primary conductor is either wound directly on top of secondary winding, suitable insulation being first applied over the secondary winding or the primary is wound entirely separately, taped with suitable insulating material and then assembled with the secondary winding on the core. Where Kt is the turn ratio, Ip is the primary current, and Is is the secondary current. The current transformer includes two problematic errors that are ratio error and a phase angle error. Figure 3. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to currents of 20 or 30 times rated current during faults, and is most critical on differential protection that will be discussed later. C. 4. Most CT have rating factors for 35 C and 55 C. Common value for RF is 1.5. Across the secondary terminal is connected an impedance of 2 ohm with a phase angle . This turns ratio is equal to: T.R=n=\frac{{N}_{P}}{{N}_{S}}=\frac{{I}_{S}}{{I}_{P}}, Secondary Current, {I}_{S}={I}_{P}(\frac{{N}_{P}}{{N}_{S}}). Clamp meters close and open around a current transporting conductor and control its current by identifying the magnetic field around it, supplying a quick measurement reading commonly on a digital display without opening or disconnecting the circuit. The level of the primary and the secondary output is introduced as a current transformer rate of the system. Why Cant a Transformer Be Operated on DC Supply? The ammeter is required to give a full scale deflection when the primary current is 800 Amps. By using Linquip RFQ Service, you can expectto receive quotations from various suppliers across multiple industries and regions. The primary current and secondary output of the current transformers are related to each other. They are typically 600 V class since they are used in the low-voltage circuit. Various primary bar arrangements can be supplied to suite most applications. This type of current transformer is available to measure AC currents from 100A to 600A, at 50 to 400HZ. This arrangement is ideal to use in cases where the primary conductor cannot be opened or broken. The secondary current replaces the primary side by reversing the secondary output and multiply by the turn rate. 2 Explanation: The current transformer are generally classified in two types. A current transformer of say 5/500 will create 5 amps in the secondary winding for 500 amps in the primary part, 100 times higher. 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