. 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. Medium voltage CTs come with our without mounting bracket. 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. At 750 kV, cascaded current transformers are used or alternatively a coaxial shunt is used to modulate a radio frequency signal that is transmitted from the shunt placed in the high voltage line to receiving equipment on the ground, thereby overcoming the insulation problem.However. 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. 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. 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. A current transformer should never be left in open-circuited condition or performed with no-load connected when the basic primary current is moving across it, just as a voltage transformer type should never perform on a short-circuit condition. One is Live Tank CT and another one is Dead Tank CT. It must be considered, however, that a current transformer introduced as 100/5 is not identical to one expressed as 20/1 or subdivisions of 100/5. The bar type CT is fully insulated to the switchgear voltage. In dc circuits it is easy to measure large currents by adding suitable shunts to the low range ammeters. Rather, a toroidal transformer has a window or hole through which the current flowing in the network passes. In high line voltage systems oil-immersed current transformers are used. In most case, the secondary current of a current transformer shall be 5A. The figure below shows a circuit for measurement of current and power with a current transformer. Shell Type Transformer Construction. It is helpful for engineering life. 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. Measuring Current Transformer: Used to measure high current in a safe manner.generally class 0.1,0.2,1.0. 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. The current transformer includes two problematic errors that are ratio error and a phase angle error. d) secondary winding having more than one full turn wound on core. 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. Ring type CT are commonly used in cables, busbars and transformers bushings. There are several core types used, from low-reactance toroid to high-reactance cut . 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. How rating of a circuit breaker for a system is determined? As the name suggests in wound type current transformers both primary and secondary windings are of wound type with a suitable number of turns. The secondary winding consists of many turns of thin wires which are connected in parallel to the terminals of the ampere meter. Notify me of follow-up comments by email. Also, higher voltage levels make these devices dangerous to connect. Insulation Voltage: 3000 volts. Keep in mind that the internal resistance of ampere-meter is very low, thus, a current transformer works as a short circuit. SAT 0 0 10 20 30 40 50 60 70 80 M s 38 CT Types s . Bar Type Current Transformer: In bar-type core construction, the primary has no turns wound. A current transformer is generally used in metering and protective relays in the electrical power industry. For this CT to operate satisfactorily at maximum fault currents, it must operate on the linear part of the magnetizing curve, i.e., below the point at which saturation occurs, which is known as the knee point. The ammeter is arranged to give a full deflection with either 5A or 1A depending on the turns ratio of the CT. 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. This structure is filled with oil which provides necessary cooling to core as well as winding. 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 winding needs to be physically connected in series to current carrying conductor. Burden in VA. 1. HV CT are commonly installed outdoors, in AIS substations (Air Insulated Substation) Figure 2 or indoors, in GIS substations (Gas Insulated Substation) Figure 3. The current transformer is used in meters for sensing the current up to 100 A. View Answer. The bar must be sized to handle the continuous current to be passed through it, and it must be mechanically secured to handle high-level short-circuit currents without incurring damage. Whenever possible secondary windings should utilize the whole available winding length on the core, the secondary winding turns being suitably spaced to accomplish this and the insulation between secondary winding and core bed earth must be capable of withstanding the high peak voltages caused if the secondary winding is open-circuited when primary winding current is flowing. There are several specialized forms of current transformers now available in the market. Difference Between Single Phase and Three Phase Transformer. Measuring current transformers are constructed to feed on other . Figure 1 Schematic connection of a current transformer. It uses the actual cable or bus bar of the . Bar Type Current Transformer : In bar-type core construction, the primary has no turns wound. There is an ongoing development and limited use of optical-type current transducers (OCT), which rely on the principles of light deflection. Also, the voltage in the secondary winding will be 200 times the primary voltage. Figure 3. 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. The simplest form any current transformer can take is the ring type or Window type, examples of which are given in the below figure which shows three . The potential transformer is mainly classified into . For Metering Class CTs burden is in terms of ohms impedance and for Protection-class CTs it is expressed in terms of volt-amperes (VA). 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. The most common type is that wound on a toroidal core. It is considered to be an incomplete transformer assembly since it consists only of a secondary winding wound on its core. Types of Instrument Transformers. In addition to that, core losses due to high flux density would heat up the core and the windings and damage them. The secondary output can be created much lower than the current in the primary winding by increasing the number of secondary windings, Ns, being sensed because as Ns rises, Is (Current of secondary part) reduces by a related amount. A more common type of construction is the window type which is also known as a through type or donut type. The secondary winding is wound on this core. Based on this form of configuration, the current transformer is always introduced as a series transformer as the primary section, which never has more than a very few turns, is in series form with the current-carrying conductor providing a load. 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 . Construction. For instance, assume that the current rate of the primary part is 100A and the secondary winding has the normal rate of 5A. Bar type. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuously wound type. . 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. In some, the bar carrying high current may act as a primary. Bar type current transformers consist of a single bar as a primary winding. They isolate the measurement and sensing devices from the main circuit. The induced alternating current in the secondary will be proportional to the primary current and its magnitude depends on the turns ratio. The secondary and primary produced voltages are lagging behind the basic flux by 90. Accuracy classes for metering CTs: Metering devices such as ammeters, wattmeters, energy meters, kVar and kVarh meters requires high accuracy and has a low burden. 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. Bar Current Transformers. In a bar type current transformer, the core and secondary windings are the same as in a ring type transformer but the fully insulated bar conductor constituting the single turn primary winding is now an integral part of the current transformer.The insulation on the primary winding conductor may be bakelized paper tube or a resin directly moulded on the bar. 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. In bar-type current transformer construction, the primary winding is a bar of suitable size. The magnitude of the secondary output is based on the turns ratio of the instrument. 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. There are different types of design and construction of current transformers which will be explained later, whose performance can be calculated or must be tested to determine. Two principal types of transformer construction embody the requirements of economics, ease of manufacture, insulation, mechanical strength, and ventilation: core-type and shell-type. [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. As regards the voltage rating, a current transformer induces a large voltage on the secondary compared to the primary. In special cases when taps are employed, they are distributed such that any connection made would utilize the entire core-periphery. D. 5. Tamper-Resistant Outlet: What is it? The core material can be in the shape of a rectangle or ring made up of nickel alloy or steel as shown below. Kraft paper provides insulation between core and . 50,60. 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. The primary winding consists of very few turns and, therefore, there is no appreciable voltage drop across it. 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 also known as bar-primary CTs. The bar-type current transformer is, for all practical purposes, a window-type current transformer with a primary bar inserted straight through the window. Usually HV CT use oil or gas (SF6) as insulation medium and MV CT use synthetic resins. A current transformer further isolates the measuring instruments from whatever may be very high voltage in the monitored circuit. A bar-type current transformer has no primary winding. An alternating current is passed to the primary of the transformer and the measurement/sensing/protection device is connected to its secondary. The current transformer is used with its primary winding connected in series with line carrying the current to be measured and, therefore, the primary current is dependant upon the load connected to the system and is not determined by the load (burden)connected on the secondary winding of the current transformer. The distance between the two windings is kept very small to reduce flux leakage. The current rate of the transformer is normally high. There are four major types of current transformers: Window-type, which includes bushing-type (BCT); bar-type; split-core-type; and wound-type, the latter having both a primary and secondary winding. Most of the construction part of a current transformer is similar to a normal transformer. It passes through the center of the hollow metalcore. A common application is to position the current transformer over a high-voltage bushing, hence the name BCT. . The construction of a current transformer is similar to that of a conventional transformer. Required fields are marked *. 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. Over the secondary winding, the primary winding is wound on the outer core with suitable insulation between the two windings. A current transformer is a device that is used for the transformation of current from a higher value to a lower value. The laminated core is designed in such a way where its trigger can be opened by pressing lightly i.e. The current transformer is used as a controlling device to operate protective devices such as relays and pilot lights. This bar assembly can be permanently attached or held in place with brackets. . The knee point is defined as the point at which a 10% increase in voltage produces a 50% increase in magnetizing current. The primary of a CT is connected in series to the current-carrying conductor and measurement devices are connected to its secondary. Answer.1. Electrical Deck is a platform for learning all about electrical and electronics engineering. The burden of a current transformer is the amount of the load generated within the secondary winding. This voltage resulted in the breakdown of the insulation, and also the waste of accuracy in the future can occur since the additional MMF leaves the residual magnetism in the system. #2 - Wound Current Transformer 10. When large currents pass through the primary winding, it induces small currents in the secondary winding (depending upon the turns ratio). The primary winding consists of more than one full turn or multiple turns which is wounded on the core. 1 to former Standard 60044 to include the accuracy class X, defined on the withdrawn BS 3938:1973. Wire wound Current Transformer can be used to measure currents in the range of 1A to 100A. Bus Bar - In this type, the bus bar of the main circuit itself acts as the primary winding with a single turn. The Current Transformer (C.T) is a form of instrument transformer that is constructed to generate an alternating current in its secondary winding, which is related to the current being sensed in its primary. The secondary part, however, may have a great number of coil rolls wound on a laminated component of low-loss magnetic material. The transformation, or the amount of change, is dependent on the number of turns of both the primary and secondary conductors. Its winding and core construction is such that it can hinge open, or totally separate into two parts. An approximate inverse turns level exists between the two currents in most current transformers in the secondary and primary windings. These transformers with low range ampere meters are used to measure the current in the high voltage circuits. Equivalent Circuit of Electrical Transformer, Parallel Operation of Single-Phase & Three-Phase Transformers, Polarity Test of a Transformer Circuit Diagram and Working, Sumpners Test or Back-to-Back Test on a Transformer, Short Circuit Test and Open Circuit Test of Transformer, How to Size a Single Phase and Three Phase Transformer in kVA? Save my name, email, and website in this browser for the next time I comment. A bar current transformer can withstand the stresses of heavy . This is sometimes provided to fit a specific bus arrangement found in the rear of switchgear panels or on draw out-type circuit breakers. 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By pressing lightly i.e most current transformers are used to measure the current transformer is used in for. Bar inserted straight through the center of the hollow metalcore turn or turns. Carrying conductor will be proportional to the primary has no turns wound provided to fit a specific bus arrangement in.
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