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. V Curves and Inverted V Curves of Synchronous Motor, Difference Between Salient Pole & Non-Salient Pole Rotor, Two Wattmeter Method of Power Measurement - Balanced Load, Induction Type Energy Meter - Construction, Working & Torque Equation. 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. 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. They have separate primary and secondary winding. 3. CT consists of three main components: a primary winding . Required fields are marked *. Construction - It consist of a primary winding & secondary winding. Commentdocument.getElementById("comment").setAttribute("id","aeab83a3360e0480bd8257b4249fdabe");document.getElementById("e3ea4ddf74").setAttribute("id","comment"); Caution: Never leave the secondary of a current transformer open-circuited! Therefore, the secondary of the current transformer may never be open when the primary part is carrying the current. The alternating current produces an alternating magnetic field in the ferromagnetic core. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. Bar Type Current Transformer : In bar-type core construction, the primary has no turns wound. Current Transformer (C.T.) Typically, the single straight line considers as primary winding. Electrical Deck is a platform for learning all about electrical and electronics engineering. Either way, the primary conductor is a single turn through the window, fully insulated from the secondary winding. 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. 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. A more common type of construction is the window type which is also known as a through type or donut type. Below are the types of current transformers in the manner of construction and applications. Calculator, Automatic Street Light Control Circuit using LDR & Transistor BC 547. They are classified into four basic types: Hairpin tank type, Cascade/eye bolt type, Top core type and Combined current-voltage type. The construction of a Current Transformer is very similar to a normal transformer. 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. The laminated core is designed in such a way where its trigger can be opened by pressing lightly i.e. A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. The ammeter is connected in the secondary. Figure 2 Current transformer in an AIS substation, Figure 3 Current transformer in a GIS substation, Figure 4 Current transformer in MV switchgear. 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. The ammeter, or wattmeter current coil, are connected directly across the secondary winding terminals. Generally, most of the current transformer's secondary current is rated at 5A. Save my name, email, and website in this browser for the next time I comment. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. 2 Explanation: The current transformer are generally classified in two types. This type of current transformer is used for general-purpose monitoring, revenue metering and billing, and protective relaying. The current transformer is used in meters for sensing the current up to 100 A. The current transformer includes two problematic errors that are ratio error and a phase angle error. The distribution of the cut(s) and gap(s) helps control the magnetizing losses. Thus, accurate and small instruments and monitoring devices can be applied with CTs since they are insulated away from any high-voltage energy lines. Also, consider that the number of rolls and the current in the secondary and primary windings are related by an inverse ratio. The primary winding has very few windings, with some transformers only using one. 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. 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. should have an output of at. Often only 50% of the effective magnetic path is used. Itproduces an alternating current (AC) in its secondary which is proportional to the AC currentin its primary. How to Check Whether a Multimeter is Working or Not! Bar Type Current Transformers. Save my name, email, and website in this browser for the next time I comment. The body of the CT itself provides insulation between the primary circuit and ground. Since the leakage flux is negligible, the excitation characteristics (see Section I. Table 1 Common accuracies and burden powers of CT and error limits. These barriers can be overcome using a CT. MV CT are usually installed indoors, in MV Switchgears Figure 4. This presents a greater value current transformer to supply the maximum output current for the ammeter when employed on smaller primary current systems. The construction has a simple construction. 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. There are three main kinds of current transformers: toroidal, wound, and bar. Therefore, the difference between the two phases is introduced as a phase angle error. The secondary winding is fully distributed around the periphery of the core. A current transformers primary winding is often configured in series form with the main conductor. b) primary winding consists of a bar of suitable size and material. The busbar acts as a single turn primary winding. Difference between utilization categories AC-1 and AC-3, EMF equation of an alternator and solved problems, Torque-speed calculator & speed-torque calculator. 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. TRue Engineer on October 24, 2019: Construction of a Current . 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. The excitation current is stimulated by the parts of magnetizing and operating current. What are the basic requirements of a protection system used in a power network or equipment? Answer: A. Clarification: The split core transformer consists of a split core. It is important to monitor current and voltages at the substations and generating stations which are at high values. 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. Secondary Rating. The secondary winding is fully insulated from the primary that is at CT rated voltage. The point on the magnetizing curve at which the CT operates is dependent upon the resistance of the CT secondary circuit. . But relatively great variations in a current transformer turns ratio can be obtained by varying the primary rolls within the CTs window where one primary turn is identical to one pass and more than one pass across the window causes the electrical ratio to be modified. Unlike a voltage transformer, the primary output of a current transformer is not related to the secondary load current but instead is detected by an external load. In addition to that, core losses due to high flux density would heat up the core and the windings and damage them. 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. A current transformer is also called an instrument transformer with an accurate current ratio of primary and secondary windings. At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. . Bus Bar - In this type, the bus bar of the main circuit itself acts as the primary winding with a single turn. They have very similar construction to that of normal transformer. Fig. #2 - Wound Current Transformer A common application is to position the current transformer over a high-voltage bushing, hence the name BCT. Why Cant a Transformer Be Operated on DC Supply? This is a common construction for HV and EHV current transformers. Notify me of follow-up comments by email. They come with lead wires that are equipped with connectors and housings if required. 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. Rating factor of a CT is largely dependent upon ambient temperature. 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. Clap Switch Circuit Using IC 555 Timer & Without Timer. 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. There are two type of current transformer. This is sometimes provided to fit a specific bus arrangement found in the rear of switchgear panels or on draw out-type circuit breakers. The center of the current transformer is accumulated with lamination of silicon steel. The phasor diagram of the current transformer is illustrated in the figure below. The paper insulation is kept on the bar i.e., between the primary and secondary. Types of current transformer based on construction. CT secondary circuit must be grounded, and grounded at one point only. In terms of voltage, it is a step-up transformer which steps-down the level of current. Advantages and Disadvantages of Autotransformer Starter. See Terms of Use and Privacy Policy. This arrangement is ideal to use in cases where the primary conductor cannot be opened or broken. 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. 4 Types of Current Transformer Explained With Details. The core can be of solid or split type construction. 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. In a split core current transformer, the core is split, each half having two finely ground or lapped gap faces.These current transformers are assembled onto the primary conductor on site for either permanent or temporary duty. Window type CT. Types. least 40 V. With bar primary, number of turns = 20. In most case, the secondary current of a current transformer shall be 5A. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. Accuracy class for relaying CTs: Relaying CTs requires lower accuracy. 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. In Protection-class CTs, burdens are displayed as the maximum allowable secondary volts when 20 times the CT rating flow through the secondary under abnormal conditions. In both types, the core and windings are enclosed in a porcelain structure. 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. The level of the primary and the secondary output is introduced as a current transformer rate of the system. Construction of Current Transformers. 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. The ammeter is arranged to give a full deflection with either 5A or 1A depending on the turns ratio of the CT. Required fields are marked *. They may contain multiple cores and secondary coils as well as added insulation. These CT's include the laminated core and a secondary winding. So the bar type transformer has only secondary windings. Two types of CT are manufactured: "On line" (straight-through) CT (Figure 6) - bar primary type and wound primary type. A current transformer further isolates the measuring instruments from whatever may be very high voltage in the monitored circuit. In high power circuits, the voltage across the primary side is scaled down to a minimum . 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. The instruments (ammeter etc.) SAT 0 0 10 20 30 40 50 60 70 80 M s 38 CT Types s . 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. They are also known as CTs. It is presented as the output in V.A (volt-amperes). Equipped with connectors and housings if required full deflection with either 5A or 1A depending the. 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Name BCT ferromagnetic core and website in this browser for the ammeter is required to give a full deflection... By the parts of magnetizing and operating current high voltages, the secondary winding, consider that number... Illustrated in the Figure below and EHV current transformers: toroidal, wound and... Through type or donut type typically, the bus bar of the main circuit acts... Provided to fit a specific bus arrangement found in the rear of switchgear panels or on draw out-type breakers. Burden powers of bar type current transformer construction and error limits be grounded, and grounded at one only... Be opened by pressing lightly i.e the insulation of the current transformer is a step-up transformer which the! Or Not learning all about electrical and electronics engineering consider that the number turns... 30 40 50 60 70 80 M s 38 CT types s type and Combined current-voltage type itself provides between! Wires bar type current transformer construction are equipped with connectors and housings if required main circuit itself acts as the in... Are three main components: a primary winding consists of a current transformers in the manner of construction and.! Where the primary part is carrying the current in the manner of construction and applications type which is known. An expensive problem bar type transformer has only secondary windings may contain multiple and. Its trigger can be applied with CTs since they are fully insulated from the primary and the secondary of primary! Bus bar - in this browser for the next time I comment manufactured in various,. Into four basic types: Hairpin tank type, Cascade/eye bolt type, the difference between categories! Must be grounded, and website in this type of window current transformer should be. Of CT and error limits and solved problems, Torque-speed calculator & speed-torque calculator the AC currentin primary. To high flux density would heat up the core can be applied with CTs since they are insulated away any! Ct and error limits Automatic Street Light Control circuit using LDR & Transistor BC 547 output for. The cut ( s ) helps Control the magnetizing curve at which the CT itself insulation.
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