bar type current transformer construction

See Terms of Use and Privacy Policy. The burden of a current transformer is the amount of the load generated within the secondary winding. The figure below shows thewound type transformer. The center of the current transformer is accumulated with lamination of silicon steel. The primary windings of the potential transformer are directly connected to the power circuit whose voltage is to be measured. Good to Know: According to the above referred IEC Standard, CT most common accuracies and burdens are: Class PX is the most accurate and is used usually for main protections. 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. 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. A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). This is why calibration of the CT is typically applied for a special type of ammeter. Why Cant a Transformer Be Operated on DC Supply? In a ring-type core structure, the secondary winding is wound on the inner core of the bakelite former. It is presented as the output in V.A (volt-amperes). 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. The bar-type current transformer is, for all practical purposes, a window-type current transformer with a primary bar inserted straight through the window. Buy Equipment or Ask for a Service. Usually HV CT use oil or gas (SF6) as insulation medium and MV CT use synthetic resins. The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. Due to the absence of primary windings and their simplicity, they are much preferred in low voltage circuits and switchboards. If the secondary of CT is left unloaded a risk of explosion exists. Then the primary current or load current I1 is given as. The working or operation of a current transformer is similar to that of a normal two-winding transformer. A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. 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. The construction of the current transformer can be wound or bar type. It is a very good information about CT for electrical knowledge. C. 4. 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. Cable is passing through the centre of CT and in that way forms the primary winding. [2] IEC: International Electrotechnical Commission. 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. The windings must be designed to withstand, without damage, the large short circuit forces that are caused when a short circuit takes place on the system in which the current transformer is connected. To this end, 'bar' type primaries are used, wherever practicable. Tamper-Resistant Outlet: What is it? Both windings are permanently assembled on the core. When large currents pass through the primary winding, it induces small currents in the secondary winding (depending upon the turns ratio). 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. There are three main kinds of current transformers: toroidal, wound, and bar. View Answer. The primary consists of a bar-type conductor with a suitable cross-section. Unlike the structure of a power or voltage transformer, the current transformer includes only one or very few turns as its primary section. The level of the primary and the secondary output is introduced as a current transformer rate of the system. The primary winding of a current transformer is wound with a few turns (one or more) of a thick cross-sectioned conductor. Your email address will not be published. The auxiliary CT is a wound type used in secondary circuits for totalizing, summation, phase shifting, isolation, or changing ratio. Plz, give the potential transformer theory, construction and working. 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. The secondary of a current transformer must not be open-circuited at any time when the primary is energized. The current transformer is used as a controlling device to operate protective devices such as relays and pilot lights. It means that the primary current is 20 times higher than the secondary one, so when 100 amps are moving in the primary conductor, it will result in 5 amps generating in the secondary winding. Most CT have rating factors for 35 C and 55 C. Common value for RF is 1.5. 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. Therefore, if we know the current ratio and the rating of AC ampere meter, we may determine the line current. If there is current flow in the primary of the current transformer and its secondary winding is kept closed, the current flowing through the secondary builds up a back EMF and counters the primary magnetizing force. Notify me of follow-up comments by email. 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. There are some window types that can be rated for higher voltages as stand-alone units, for use with an integral high-voltage stress shield, or for use with an insulating sleeve or tube made of porcelain or some polymeric material. Under normal performance states, the secondary winding of a current transformer is attached to its burden, and it is normally closed. There is an ongoing development and limited use of optical-type current transducers (OCT), which rely on the principles of light deflection. Depending upon the construction there are two types of current transformers used for measuring large currents. Therefore, the current transformer performs based on the secondary circuit states. placed on panel boards can be used (Fig. At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. Window-type current transformers generally have a round window opening but are also available with rectangular and oval openings. In dc circuits it is easy to measure large currents by adding suitable shunts to the low range ammeters. Insulation Voltage: 3000 volts. Based on the construction, there are two types of current transformer. The primary winding consists of more than one full turn or multiple turns which is wounded on the core. The instruments (ammeter etc.) They have very similar construction to that of normal transformer. A typical connection . The ammeters scale is adjusted according to the turns ratio. 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. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. 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 distribution of the cut(s) and gap(s) helps control the magnetizing losses. Thus if the secondary is opened, there is no counter MMF in the secondary winding to oppose the primary MMF. Exciting Current) can . The secondary part, however, may have a great number of coil rolls wound on a laminated component of low-loss magnetic material. The manner in which the windings are arranged on the core affects the reactance since it is a geometric function. One is Live Tank CT and another one is Dead Tank CT. This process is shown in fig below. An approximate inverse turns level exists between the two currents in most current transformers in the secondary and primary windings. They may contain multiple cores and secondary coils as well as added insulation. Types of current transformer based on construction. Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the . They are classified into four basic types: Hairpin tank type, Cascade/eye bolt type, Top core type and Combined current-voltage type. This large secondary voltage could harm the insulation or result in electric shock if the terminals of the current transformer are accidentally touched. 3). The accuracy classed defined by IEC are 0.2 or 0,2S, 0.5 or 0.5S and those defined by IEEE are 0.15 or 0.15S, 0.3 or 0.6. They are . This accuracy class was retained by IEC in 1966 in the Amendment nr. 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. 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 design features are described below: . The working principle of a current transformer is especially different from the power transformer. Types & Working of Voltage Transformers, Difference Between Current Transformer & Potential Transformer. oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. 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 magnitude of the primary and secondary voltages depends on the number of rolls on the windings. A true bar type current transformer is a toroidal type transformer with a bus bar as an integral part of the current transformer which . [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 . Therefore, the difference between the two phases is introduced as a phase angle error. A busbar or feeder whose current measurement is required are inserted in the opened core and the trigger released to close the core. The primary of a CT is connected in series to the current-carrying conductor and measurement devices are connected to its secondary. As the name suggests in wound type current transformers both primary and secondary windings are of wound type with a suitable number of turns. 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. Round copper wire of about 3 mm area is frequently used for secondary windings rated at 5 A.Copper strip is used for primary winding, the dimensions of which depend upon the primary winding current. A magnetic core made up of silicon steel laminations links both the windings for providing a low reluctance path as shown below. The type of wire used as the main current carrying conductor in a transformer winding is either copper or aluminium. The primary winding in most cases is a single conductor centrally located in the window. Toroidal Current Transformers: These do not include a primary winding. The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuously wound type. 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. Why are current transformers (CT) necessary? Construction can be one single primary turn as in Toroidal . 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. The current transformer has high electrical isolation. 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. Current Transformers Resolution: A bar type CT is an in-line CT, much more costly and is mounted in line with the bus bars (bolted on both sides). When applying auxiliary CTs, the user must be aware of their reflected impedance on the mainline CT. The 100 to 5 and 20 to 1 types are the most common forms of current transform existence in the market. It is used to activate the trip coil of a circuit breaker. This transformer as a low leakage reactance for which the knowledge of the transformers secondary excitation characteristics, secondary winding resistance, secondary burden resistance and turns ratio is sufficient to assess its performance in relation to the protective relay system with which it is to be used. Below are the types of current transformers in the manner of construction and applications. They are constructed to supply a current in its secondary winding related . The secondary of the transformer has many turns depending on the magnitude of the current to be stepped down. A current transformers primary winding is often configured in series form with the main conductor. The point on the magnetizing curve at which the CT operates is dependent upon the resistance of the CT secondary circuit. . For a low voltage wound type CT, the secondary turns are wound . Figure 2 Current transformer in an AIS substation, Figure 3 Current transformer in a GIS substation, Figure 4 Current transformer in MV switchgear. 2.5, 3.75. Ek = 4.44 B A f N. MV CT are usually installed indoors, in MV Switchgears Figure 4. 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. The bar type CT is fully insulated to the switchgear voltage. The conductor itself acts as a single turn primary winding. For low and medium voltage applications, there are three main classifications based on construction; window, bar, and wound. Types of current transformer based on construction, https://talema.com/introduction-current-transformers/, Difference between NPN and PNP transistors | NPN vs PNP Transistor, Hysteresis loss and Eddy current loss and their difference, Difference between earth leakage relay and earth fault relay, Star-Delta starter wiring diagram Control and power wiring diagrams, Difference between grounding/earthing and bonding. The bar-type current transformer is, for all practical purposes, a window-type current transformer with a primary bar inserted straight through the window. Instrument transformers are of two types - Current Transformer (C.T.) The secondary output lags behinds the secondary stimulated voltage by a particular angle (). TRue Engineer on October 24, 2019: The core of the current transformer is built up with lamination of silicon steel. Potential Transformer (P.T.) 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. For example, a current transformer has a ratio of 200:1. 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. The current carrying conductor or feeder acts as a single turn primary while the secondary winding is connected across the standard ampere-meter. 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