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. The working principle of a standard current transformer is moderately different from that of an ordinary voltage transformer. 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. Either way, the primary conductor is a single turn through the window, fully insulated from the secondary winding. It is presented as the output in V.A (volt-amperes). They isolate the measurement and sensing devices from the main circuit. A current transformer is connected in series to the current-carrying conductor and an ammeter is connected to its secondary. Why are Air-Conditioners (AC) Rated in Tons, Not in kW or kVA? should have an output of at. This bar is connected in series with the circuit conductor. A CT consists of one or a very few primary turns and can have several hundreds of secondary turns. Current transformers are mainly classified into two types: Bar type CTs and Ring type CTs based on their construction. 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). Save my name, email, and website in this browser for the next time I comment. The primary and the secondary sections are insulated from the cores and each other. 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. Now, the whole primary current acts as a magnetizing current and induces large voltages in the secondary. Looks Like You're Using an Ad Blocker. Answer: A. Clarification: The split core transformer consists of a split core. 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. 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. Most CT have rating factors for 35 C and 55 C. Common value for RF is 1.5. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. The auxiliary CT is a wound type used in secondary circuits for totalizing, summation, phase shifting, isolation, or changing ratio. This is the same concept used in the clamp meters. 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. Nearly all window-type current transformers manufactured today are rated 600 V class. The wound-type current transformer is a special type of CT that has a primary winding that is fully insulated from the secondary winding. Product scope Primary current 100 A - 2500 A Highest voltage for equipment: 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 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. We develop your standard equipment, but also . Figure 8 Schematic connection of primary CT windings with double ratio. Bar type current transformer. What are the basic requirements of a protection system used in a power network or equipment? Common value for RF is 1.5. 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. Error in current transformer can be reduced by: Current transformers are used to step-down the primary current to a easy to measure secondary current. Current transformers, along with voltage or potential transformers areInstrument transformer.Current. They have separate primary and secondary winding. Secondary Rating. MV CT are usually installed indoors, in MV Switchgears Figure 4. 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. Burden in VA. 1. This is due to the fact the ratio of 100/5 presents the input/output current rating and not the practical ratio of the primary to the secondary outputs. CT secondary circuit must be grounded, and grounded at one point only. The paper insulation is kept on the bar i.e., between the primary and secondary. The working principle of a current transformer is especially different from the power transformer. 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. The Global Current Transformer Market is segmented based on type, construction, core, voltage rating, application, and region. The windings should be close together to reduce the secondary winding leakage reactance as the leakage reactance increases the ratio error. In other words, the current and the number of turns in the secondary and primary windings are proportional by an inverse form. CTs used in substations are usually mounted outdoors. The current transformer can be used in commercial-grade applications, such as control panels and panelboards, and for most industrial metering and relay applications in switchboards, switchgear and motor control centres. For example, a current transformer has a ratio of 200:1. If for some reasons, the ampere meter is disconnected from the secondary windings, it is necessary to short circuit the secondary winding. 4 Types of Current Transformer Explained With Details. Required fields are marked *. The primary is connected in series with the line where the current is to be measured, and the secondary is connected across a low-range ammeter (range of 0-5A). Save my name, email, and website in this browser for the next time I comment. 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. Based on the function performed by the current transformer, it can be classified is follows: . In other words, the secondary of the current transformer should be never left open under any circumstances. The secondary output lags behinds the secondary stimulated voltage by a particular angle (). and the resistance of secondary winding is 0.5 ohm. Current transformers are available in various sizes and shapes and are used as an interface between high currents and measuring/sensing devices. Accuracy class for relaying CTs: Relaying CTs requires lower accuracy. We Rely on Advertising to Help Fund Our Site and Provide Free Information. Types of Current Transformers CTs are manufactured in various sizes, shapes, and ratings for a broad range of applications. The standard secondary current is usually rated at 5A or 1A for ease of sensing. To this end, 'bar' type primaries are used, wherever practicable. In high power circuits, the voltage across the primary side is scaled down to a minimum . This causes excessive core losses and produces excessive heat. Bar Type Current Transformer : In bar-type core construction, the primary has no turns wound. 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. RITZ is one of the world's leading specialists in the field of Instrument Transformers, cast resin insulated Bus Bar Systems, cast resin insulated Power Transformers as well as cast resin parts according to customer specifications. [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. 1 A, 5A. 50,60. Where Kt is the turn ratio, Ip is the primary current, and Is is the secondary current. Electrical Deck is a platform for learning all about electrical and electronics engineering. 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. 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. The secondary windings of the transformer carry the current proportional to the current to be measured, and it is connected to the current windings of the meters or the instruments. In a current transformer, there exists an inverse ratio relation between the current and the number of turns in the primary and secondary windings. The 100 to 5 and 20 to 1 types are the most common forms of current transform existence in the market. They are also used to step down the current at a specific ratio to insulate the instrument from the high voltage lines. Shell Type Transformer Construction. 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. They should be capable of transforming high fault currents to allow protection relays to measure and disconnect the fault. The insulation medium used whether polymeric, oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. The primary winding consists of more than one full turn or multiple turns which is wounded on the core. A busbar or feeder whose current measurement is required are inserted in the opened core and the trigger released to close the core. 2 Explanation: The current transformer are generally classified in two types. The current transformer has high electrical isolation. Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the . c) secondary winding having one full turn wound on core. The transformers are designed and manufactured in conformity with following standards: IEC, VDE, ANSI, BS,GOST and CSN. 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 . 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. oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. The primary winding may be a conductor, strip etc whose current is to be reduced. The point on the magnetizing curve at which the CT operates is dependent upon the resistance of the CT secondary circuit. This accuracy class was retained by IEC in 1966 in the Amendment nr. What is the bar type current transformer? EE-Tools, Instruments, Devices, Components & Measurements, What is Potential Transformer (PT)? These contain a copper or aluminium busbar surrounded by the secondary winding wound over the ferromagnetic core. Burden: 2 to 30 VA. A typical connection . Our articles are written by the electrical engineers in a simple and easy way. Based on the construction, there are two types of current transformer. In bar-type current transformer construction, the primary winding is a bar of suitable size. 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}}). 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. They are directly connected to the current-carrying conductor. Notify me of follow-up comments by email. Construction can be one single primary turn as in Toroidal . The primary has few numbers of the turn of heavy wire, and the secondary of the current transformer has many turns of fine wire. 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. If the secondary of CT is left unloaded a risk of explosion exists. ANSI outdoor medium voltage transformers are encapsulated in hydrophobic cycloaliphatic epoxy (HCEP) using the automatic pressure gelation (APG) process. Clap Switch Circuit Using IC 555 Timer & Without Timer. Types of Wind Turbines: Type 1, 2, 3, and 4 Explained! The turns ratio of a current transformer is made such that the full load current in its primary give a secondary current of either 5A or 1A. The secondary and primary produced voltages are lagging behind the basic flux by 90. These current transformers have an opening where the conductor carrying the primary load is passed. How Does it Work? 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}}. The center of the current transformer is accumulated with lamination of silicon steel. 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. Visit here to explore more about the construction of a current transformer. The transformer is fundamentally a step-down transformer (with respect to current), consisting of primary and secondary windings without any electrical connection between them. 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. These transformers with low range ampere meters are used to measure the current in the high voltage circuits. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. 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. A current transformer (CT) is a type of transformer that is used to measure AC current. B. 1 to former Standard 60044 to include the accuracy class X, defined on the withdrawn BS 3938:1973. 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. The primary winding of a current transformer is wound with a few turns (one or more) of a thick cross-sectioned conductor. 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. Fig. 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. It uses the actual cable or bus bar of the . 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. 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.