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. Most current transformers have the normal secondary rating of 5 A, with the secondary and primary currents being presented as a ratio such as 5/100. The symbolic symbol of the current transformer is presented in the figure below. 4. . 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. 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? CT secondary circuit must be grounded, and grounded at one point only. We develop your standard equipment, but also . When large currents pass through the primary winding, it induces small currents in the secondary winding (depending upon the turns ratio). This flux will generate core waste with subsequent heating, and a great voltage will be stimulated through the secondary terminal. Wire wound Current Transformer can be used to measure currents in the range of 1A to 100A. 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. Bar type. Ek = 4.44 B A f N. 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. [1] Being Un the rated voltage of the network: HV Un 60 kV; MV 1 kV < Un 49.5 kV. Save my name, email, and website in this browser for the next time I comment. These transformers with low range ampere meters are used to measure the current in the high voltage circuits. Clap Switch Circuit Using IC 555 Timer & Without Timer. Commentdocument.getElementById("comment").setAttribute("id","aeab83a3360e0480bd8257b4249fdabe");document.getElementById("e3ea4ddf74").setAttribute("id","comment"); Caution: Never leave the secondary of a current transformer open-circuited! Windings in this manner ensure optimum flux linkage and distribution. They have very similar construction to that of normal transformer. 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. Both the primary and secondary windings are wound on the central limb and the two outer limbs provide the low reluctance flux path (see the figure). The ammeter is required to give a full scale deflection when the primary current is 800 Amps. The most common type is that wound on a toroidal core. 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. A CT consists of one or a very few primary turns and can have several hundreds of secondary turns. Save my name, email, and website in this browser for the next time I comment. . placed on panel boards can be used (Fig. 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 . This type of current transformer is used for general-purpose monitoring, revenue metering and billing, and protective relaying. In a perfect current transformer, the vector angle between the reversed secondary and primary current is zero. 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. 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 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 ammeter is required to give a full scale deflection when the primary current is 800 Amps. Secondary Rating. The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuously wound type. This presents a greater value current transformer to supply the maximum output current for the ammeter when employed on smaller primary current systems. 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. Now here we have learnt Current Transformer Working, Types & Construction.You can download this article as pdf, ppt. The secondary winding is fully insulated from the primary that is at CT rated voltage. The current transformer works to transform or change the magnitude of AC (50400 Hz) in a system, typically from a higher current value to a lower current value. oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. 3. The figure below shows thewound type transformer. 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. Current transformers are mainly classified into two types: Bar type CTs and Ring type CTs based on their construction. 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. 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. 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. 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. A near relative of the ring type current transformer is the so called bushing type transformer.This is, in fact, indistinguishable from of the ordinary ring type but the term is used when the current transformer fits over a fully insulated primary winding conductor such as over the oil end of a terminal bushing of a power transformer or an oil circuit breaker. Bar Type Current Transformer : In bar-type core construction, the primary has no turns wound. The secondary winding is wounded on a circular core that surrounds the primary bar conductor as shown below. A C.T Current Transformer is a type of instrument transformer designed to step down the current in the secondary for protection and measurement of proportional primary current. It is used to measure current through a bus bar which is inserted through it. . The standard secondary current is usually rated at 5A or 1A for ease of sensing. A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. Advantages and Disadvantages of Microgrids. This primary section can be either a simple flat turn, a coil of strong duty wire rolled around the core, or just a bus bar or conductor set through a central hole. 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. The conductor itself acts as a single turn primary winding. 5, 7.5, 3C, 40. An alternating current is passed to the primary of the transformer and the measurement/sensing/protection device is connected to its secondary. Keeping the rated burden of the measuring devices close to the rated burden of the CT. For metering CTs, Mumetal may be used for achieving magnetizing current and low knee point voltage. Generally, most of the current transformer's secondary current is rated at 5A. 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. They do not have any physical contact. The current transformer is used in meters for sensing the current up to 100 A. Toroidal Current Transformers, Bar type Current Transformers, and . 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. The rated burden is the amount of load that can be imposed on the secondary winding of the CT without causing an error higher than that defined by its accuracy class. The construction of a Current Transformer is very similar to a normal transformer. It uses the actual cable or bus bar of the . Also important to consider in a CT is the magnetizing curve that is similar to the one shown in Figure 9. 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. Circuit, Working & Phasor Diagram, Transformers Fire Protection System Causes, Types & Requirements, Efficiency of Transformer All Day Efficiency & Maximum Efficiency, Transformer Phasing: The Dot Notation and Dot Convention, Transformer Losses Types of Energy Losses in a Transformer. 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. The working or operation of a current transformer is similar to that of a normal two-winding transformer. The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuou, At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. These CT's include the laminated core and a secondary winding. The Specification of CT 1 . 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. Tamper-Resistant Outlet: What is it? Then the primary current or load current I1 is given as. It is a very good information about CT for electrical knowledge. The ammeter, or wattmeter current coil, are connected directly across the secondary winding terminals. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Enter Your Email Address to Subscribe to this Blog and Receive Notifications of New Posts by Email. Current transformers for measurement and protection purposes . The current carrying conductor or feeder acts as a single turn primary while the secondary winding is connected across the standard ampere-meter. Wound type current transformer. They are constructed to supply a current in its secondary winding related . Bar-type Current Transformer . MV CT are usually installed indoors, in MV Switchgears Figure 4. The basic flux is taken as a reference. The effects of stray flux are negligible, thus making this type of winding a low-reactance design. Coupling is almost impervious to the primary-conductor position, provided that the return path is sufficiently distanced from the outer periphery of the secondary winding. 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. 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. It is helpful for engineering life. This bar assembly can be permanently attached or held in place with brackets. The center of the current transformer is accumulated with lamination of silicon steel. 6 Types of Level Switches Technically Explained! Current transformers are used with ammeters, kWh meters, power factor meters and energy meters for current measurements. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. Construction and Types of HV Current Transformers. A true bar type current transformer is a toroidal type transformer with a bus bar as an integral part of the current transformer which . Medium voltage CTs come with our without mounting bracket. 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. For low and medium voltage applications, there are three main classifications based on construction; window, bar, and wound. 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. #2 - Wound Current Transformer The Mumetal or Permalloy is used for making cores to obtain a high degree of accuracy. The secondary current rates are of the order of 0.1 A, 1 A, and 5 A. this type of system has severe limitation in its power output which has to be amplified in order to operate relays etc. The current produced across the secondary terminal is directly proportional to the primary winding. Exciting Current) can . They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. The laminated core is designed in such a way where its trigger can be opened by pressing lightly i.e. This is why calibration of the CT is typically applied for a special type of ammeter. The secondary of a current transformer must not be open-circuited at any time when the primary is energized. Why are Air-Conditioners (AC) Rated in Tons, Not in kW or kVA? Wound type CT is similar to the two winding conventional transformer. . A current transformer (CT) is a type of transformer that is used to measure AC current. 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). 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. The primary section is a simple turn winding (also introduced as a bar primary) and carries the full-load current, while the secondary part of the transformers has several turns. . Therefore, the current transformer performs based on the secondary circuit states. They may contain multiple cores and secondary coils as well as added insulation. Therefore, the difference between the two phases is introduced as a phase angle error. The following are different types of current transformers: A current transformer having a primary winding of more than one full turn wound on core. Grounded, and a great voltage will be stimulated through the window to consider a... Is at CT rated voltage transformer to supply the maximum output current for next... Primary winding, it induces small currents in the high voltage circuits given as permanently! Time I comment secondary winding is wounded on a circular core that surrounds the primary current is rated! 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