if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'electricalclassroom_com-box-3','ezslot_2',190,'0','0'])};__ez_fad_position('div-gpt-ad-electricalclassroom_com-box-3-0');A current transformer is a type of instrument, used to measure or sense large alternating currents by scaling it to a smaller, safer and measurable value. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. Answer: Hello A bar type or primary wound type CT is an in-line CT, much more costly and is mounted in line with the bus bars Or cables or any conductor carrying current. It is considered to be an incomplete transformer assembly since it consists only of a secondary winding wound on its core. The secondary of the transformer has many turns depending on the magnitude of the current to be stepped down. Therefore, the secondary of the current transformer may never be open when the primary part is carrying the current. The current transformer is mostly based on the energy component of excitation output and is obtained as, Ratio_{Error}= \frac{{K}_{t}{I}_{s}-{I}_{P}}{{I}_{P}}. Bar type current transformer. It transforms the primary current to a proportional secondary value in terms of magnitude and phase. The windings are separately woundand are insulated by tape and varnish for small line voltages.For voltages above 7KV, the transformers are oil immersed or compound filled. Why Cant a Transformer Be Operated on DC Supply? The distance between the two windings is kept very small to reduce flux leakage. Based on the function performed by the current transformer, it can be classified is follows: . 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. Itproduces an alternating current (AC) in its secondary which is proportional to the AC currentin its primary. Various primary bar arrangements can be supplied to suite most applications. It produces a current in its secondary which is proportional to the current in its primary. Some current transformers contain a split core which permits them to be opened, installed, and closed without disconnecting the system to which they are joined. Thus a current transformer operates its secondary winding nearly under short circuit conditions. Types of Instrument Transformers. Accuracy classes are defined by IEC and IEEE and are different for metering CTs and relaying CTs. The loads impedance or burden on the secondary component has moderately differed from the power types in a current transformer. In short, it should be never operated at no-load condition when the current is flowing through the primary. How Does it Work? Bar type. The uses are the same as the window type. 4. In general, current transformers and ammeters are employed together as a particular pair in which the model of the instrument is such as to supply a maximum secondary current controlled by a full-scale deflection on the ammeter. Current Transformer (C.T.) In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work , use of current tran. Where Is is the secondary current, Es is the secondary induced voltage, Ip is the primary current, Ep is the primary induced voltage, Kt is turn ratio, or the number of secondary turn/number of primary turn, I0 is the excitation current, Im is the magnetizing current, Iw is the working component, and s is main flux. 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. Commentdocument.getElementById("comment").setAttribute("id","aeab83a3360e0480bd8257b4249fdabe");document.getElementById("e3ea4ddf74").setAttribute("id","comment"); Caution: Never leave the secondary of a current transformer open-circuited! 3. Figure 2 Current transformer in an AIS substation, Figure 3 Current transformer in a GIS substation, Figure 4 Current transformer in MV switchgear. The secondary current replaces the primary side by reversing the secondary output and multiply by the turn rate. Home > Transformer > What is the Current Transformer? 50,60. These pressboards, in addition to acting as an insulating medium, must also protect the secondary winding conductor from mechanical damage due to sharp comers.The secondary winding conductor is put on the core by a toroidal winding machine although hand winding is still frequently adopted if the number of secondary winding turns is small. In the shape of a doughnut, these transformers are designed without having a primary winding as part of their construction. The current rate of the transformer is normally high. 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. It is difficult to make measurement devices such as an ammeter or a wattmeter(kWh) meter and protection relays that can carry hundreds or thousands of amperes. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. Answer: A. Clarification: The split core transformer consists of a split core. Special precautions must be taken when connecting CT primary (connection points are usually identified by P1 and P2), and secondary (connection points are usually identified by S1 and S2) in order to assure the correct flow of electric current and the proper functioning of the devices, what is explained in Figure 5. Can we operate a 60HZ Transformer on 50Hz Supply Source and Vice Versa? 3. The doughnut fits over the primary conductor, which constitutes one primary turn. So the bar type transformer has only secondary windings. As stated above the bar-type CT carries full current through its primary whereas the ring-type CTs are installed over the current-carrying conductors. The symbolic symbol of the current transformer is presented in the figure below. Why are current transformers (CT) necessary? A current transformer, like any other type of transformer, should satisfy the amp-turn formula. The current transformer is employed for sensing the high-voltage current due to the difficulty of insufficient insulation in the meter itself. Typically, the single straight line considers as primary winding. The split half consists of 2 finely grounded or lapped gap faces. When large currents pass through the primary winding, it induces small currents in the secondary winding (depending upon the turns ratio). . Rating factor of a CT is largely dependent upon ambient temperature. They do not have any physical contact. 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 . 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. The working principle of a current transformer is especially different from the power transformer. Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the . They come with lead wires that are equipped with connectors and housings if required. View Answer. "Ring type" (doughnut) CT (Figure 7) "Ring type" CT is constructed of an iron toroid, which forms the core of the transformer, and is wound with secondary turns. There is also some uncertainty in the repeatability of performance from installation to installation. Save my name, email, and website in this browser for the next time I comment. The split-core type current transformer is a special case of a window-type current transformer. The insulation medium used whether polymeric, oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. At 750 kV, cascaded current, Whenever possible secondary windings should utilize the whole available, 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, The windings should be close together to reduce the secondary winding leakage reactance as the leakage reactance increases the ratio error. This process is shown in fig below. For low and medium voltage applications, there are three main classifications based on construction; window, bar, and wound. 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. Types of Current Transformers CTs are manufactured in various sizes, shapes, and ratings for a broad range of applications. 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. 3: Bar Type CT. Precautions in the use of C.T. Accuracy class for relaying CTs: Relaying CTs requires lower accuracy. The windings should be close together to reduce the secondary winding leakage reactance as the leakage reactance increases the ratio error. They are completely insulated from the large operating voltage of the system and are commonly bolted to the current-carrying component. least 40 V. With bar primary, number of turns = 20. 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. This causes excessive core losses and produces excessive heat. A. true. 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. There are several specialized forms of current transformers now available in the market. The primary conductor constitutes the primary winding of the CT (one pass through the window is one turn on the primary). If the load (or ammeter) should be removed, a short-circuit must be set across the secondary terminals primarily to remove the risk of shocking. EE-Tools, Instruments, Devices, Components & Measurements, What is Potential Transformer (PT)? The paper insulation is kept on the bar i.e., between the primary and secondary. Over the secondary winding, the primary winding is wound on the outer core with suitable insulation between the two windings. Bar Current Transformers. 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. Let us see the construction of the current transformer. Exciting Current) can . When the current moves across the primary part, it often flows within secondary windings, and amperes turns of each section are subsequently identical and opposite. These current transformers have an opening where the conductor carrying the primary load is passed. This structure is filled with oil which provides necessary cooling to core as well as winding. Where required for test purposes, terminals in the form of test blocks are provided on the switchgear for the connection of external instruments and metering equipment. A bar-type current transformer which has 1 turn on its primary and 160 turns on its secondary is to be used with a standard range of ammeters that have an internal resistance of 0.2. They are classified into four basic types: Hairpin tank type, Cascade/eye bolt type, Top core type and Combined current-voltage type. By using Linquip RFQ Service, you can expectto receive quotations from various suppliers across multiple industries and regions. The reassembly of the core halves is critical. Current transformers (CT) are used in High Voltage (HV) and Medium Voltage (MV)[1] installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. The window-type current transformer is the simplest form of instrument transformer. Bus Bar - In this type, the bus bar of the main circuit itself acts as the primary winding with a single turn. With this connection the directions of primary and secondary currents are: When testing a CT using an Omicron test equipment it is possible to verify if the CT is correctly connected: Ring type CT is constructed of an iron toroid, which forms the core of the transformer, and is wound with secondary turns. Construction - It consist of a primary winding & secondary winding. The design features are described below: . B. false. Most CT have rating factors for 35 C and 55 C. Common value for RF is 1.5. Wound-type current transformers, T-type classification, are . The current transformer (CT), also known as an instrumental transformer, is used to either reduce or propel the primary current across the secondary winding. The Specification of CT 1 . 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. Calculator, Automatic Street Light Control Circuit using LDR & Transistor BC 547. The current transformer (CT) is a type of instrument transformer designed to measure high-value current in the power system. In dc circuits it is easy to measure large currents by adding suitable shunts to the low range ammeters. A common and portable kind that can be employed to detect the circuit loading is introduced as clamp meters or Handheld type, as shown. In some, the bar carrying high current may act as a primary. Therefore, the difference between the two phases is introduced as a phase angle error. An approximate inverse turns level exists between the two currents in most current transformers in the secondary and primary windings. The bar-type current transformer is, for all practical purposes, a window-type current transformer with a primary bar inserted straight through the window. Construction. 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. Usually HV CT use oil or gas (SF6) as insulation medium and MV CT use synthetic resins. 1 A, 5A. CT may have one or more cores; typical applications of these cores are: The use of more than one core for protection is justified when the installation have two protections sets main and backup. 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 key distinction between the two types lies in the core and winding placement. Documentary information about C.T. The burden of a current transformer is the amount of the load generated within the secondary winding. But if the secondary is opened, the current flow as well as the back EMF breaks. 2.5.3 Core-Balance Current Transformers. Bar-type Current Transformers: This form of current transformers employs the actual wire or bus-bar of the main circuit as the primary section, which is identical to a single turn. Otherwise, it will be like operating a voltage transformer in short-circuit condition which leads to damage to the insulation as well as the chance of danger of electric shock to the operator. . Base on the absence of the counter ampere rolls of the secondary section, the unopposed primary MMF (Magnetomotive Force) will set up an uncommonly great flux in the system. Current and voltage transformers are available in voltage classes ranging from 5 kV to 34.5 kV. When applying auxiliary CTs, the user must be aware of their reflected impedance on the mainline CT. The bar type CT is fully insulated to the switchgear voltage. [3]A magnetic material is said to be saturated when the increase of applied external magnetic field does not increases the magnetization of the material. The bar may be of rectangular or circular cross-section. Assume that a bar primary type 2000/5A (CROSS core) current transformer having a core csa area of 20 square cm's is available with a secondary resistance of 0 31 ohm.
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