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. Ek = 4.44 B A f N. The laminated core is designed in such a way where its trigger can be opened by pressing lightly i.e. Below are the types of current transformers in the manner of construction and applications. Often only 50% of the effective magnetic path is used. Wound Current Transformers: The transformers primary section is physically configured in series form with the conductor that carries the detected current flowing in the system. The primary current and the full scale deflection current of the measurement devices determines the turns ratio of the a CT. Types of current transformer based on construction. The core of the current transformer is built up with lamination of silicon steel. Window-type current transformers generally have a round window opening but are also available with rectangular and oval openings. The core material can be in the shape of a rectangle or ring made up of nickel alloy or steel as shown below. Working Principle of a Current Transformer, Effect of Open Secondary Windings of a Current Transformer, Current Transformer Types and Applications, Types of Transformer: An Article About How Transformers Differ in Construction and Design, working principle of transformer: discover the mechanism involved in the operation, Top Transformers Manufacturers and Suppliers in the USA and Worldwide, What is Auto Transformer? SUPERBUTE Product Overview: Current Transformers. Save my name, email, and website in this browser for the next time I comment. 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 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). 3. Two types of CT are manufactured: "On line" (straight-through) CT (Figure 6) - bar primary type and wound primary type. CT are connected in series and protection devices and metering equipment are connected to the secondary of the CT in series association, as shown in Figure 1. 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. it opens the core of the instrument transformer. See Terms of Use and Privacy Policy. c) secondary winding having one full turn wound on core. 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 . Whereas in alternating supply it is difficult to measure large currents and it is dangerous to the operator while working with a system carrying large currents (more than 100A). One of the major differences between them is . Current transformers, along with voltage or potential transformers areInstrument transformer.Current. "Ring type" (doughnut) CT (Figure 7) "Ring type" CT is constructed of an iron toroid, which forms the core of the transformer, and is wound with secondary turns. . 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. Thus, accurate and small instruments and monitoring devices can be applied with CTs since they are insulated away from any high-voltage energy lines. Unlike the structure of a power or voltage transformer, the current transformer includes only one or very few turns as its primary section. In bar-type core construction, the primary has no turns wound. Construction can be one single primary turn as in Toroidal . Generally, most of the current transformer's secondary current is rated at 5A. 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. The level of the primary and the secondary output is introduced as a current transformer rate of the system. This causes excessive core losses and produces excessive heat. 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. 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. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. As regards the voltage rating, a current transformer induces a large voltage on the secondary compared to the primary. Useful information on Current Transformers, Current Transformers are widely used in electrical industries. While there are several different types of current transformers, the basic construction is much the same. They may contain multiple cores and secondary coils as well as added insulation. It is important to monitor current and voltages at the substations and generating stations which are at high values. Window type CT. Types. Home > Transformer > What is the Current Transformer? How rating of a circuit breaker for a system is determined? Frequency Range: 0.0480 to 0.0620 kHz. Rating factor of a CT is largely dependent upon ambient temperature. The ammeters scale is adjusted according to the turns ratio. 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. Depending upon the construction there are two types of current transformers used for measuring large currents. Bar Type C. T. It has a circular ring type core, on which secondary is Wound. This arrangement is ideal to use in cases where the primary conductor cannot be opened or broken. . 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. . With wound primary current transformers this particular problem is rarely met but it is of importance to try to obtain good relative positioning of primary and secondary winding coils, thus minimizing the axial forces on both coils caused by primary winding short circuit currents. Current Ratio Error expressed in percent is given by the formula:Credit: https://talema.com/introduction-current-transformers/, Kn = Rated transformation ratioIp = Actual primary currentIs = Actual secondary current when Ip is flowing, under conditions of measurement. A current transformer is also called an instrument transformer with an accurate current ratio of primary and secondary windings. Accuracy class describes how accurate the current transformation of a current transformer would be within when the burden within the allowable limits. 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. Current transformers (CT) are used in High Voltage (HV) and Medium Voltage (MV)[1] installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. Switched Reluctance Motor Drive: Definition and Function. Our articles are written by the electrical engineers in a simple and easy way. Then the primary current or load current I1 is given as. The alternating current produces an alternating magnetic field in the ferromagnetic core. 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. The primary winding needs to be physically connected in series to current carrying conductor. Difference Between Single Phase and Three Phase Transformer. Why is a Transformer Rated in kVA, but Not in kW? 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. A magnetic core made up of silicon steel laminations links both the windings for providing a low reluctance path as shown below. Why Cant a Transformer Be Operated on DC Supply? The potential transformer is mainly classified into . 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. Current transformers are considered to be applied as proportional current instruments. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'electricalclassroom_com-box-3','ezslot_2',190,'0','0'])};__ez_fad_position('div-gpt-ad-electricalclassroom_com-box-3-0');A current transformer is a type of instrument, used to measure or sense large alternating currents by scaling it to a smaller, safer and measurable value. An approximate inverse turns level exists between the two currents in most current transformers in the secondary and primary windings. This is a common construction for HV and EHV current transformers. Toroidal Current Transformers, Bar type Current Transformers, and . . The primary windings of the potential transformer are directly connected to the power circuit whose voltage is to be measured. 4. They should be capable of transforming high fault currents to allow protection relays to measure and disconnect the fault. A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. The use of this type should be with discretion since this construction results in higher-than-normal errors. Looks Like You're Using an Ad Blocker. Therefore, if we know the current ratio and the rating of AC ampere meter, we may determine the line current. 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. Basically, it is a step-down transformer (with respect to current) consisting of two windings, primary and secondary without any electrical connection between them. Indoor and Outdoor Current Transformers Overview - Specifications. MV CT are usually installed indoors, in MV Switchgears Figure 4. this type of system has severe limitation in its power output which has to be amplified in order to operate relays etc. 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. The loads impedance or burden on the secondary component has moderately differed from the power types in a current transformer. 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. Instead, the line that runs the current flowing in the system is threaded across a hole or window in the toroidal transformer. Based on the type of construction, current transformers are classified into three types as follows: Window CT or toroidal CT; Bar CT; Wound CT Over the secondary winding, the primary winding is wound on the outer core with suitable insulation between the two windings. d) secondary winding having more than one full turn wound on core. One is Live Tank CT and another one is Dead Tank CT. 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 of a current transformer is wound with a few turns (one or more) of a thick cross-sectioned conductor. Current transformers are used extensively in metering and circuit protection. 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. [2] IEC: International Electrotechnical Commission. If the ammeter on the secondary winding is read 1.5 amps i.e, I2 = 1.5A. 6 Types of Electrical Switches In The Home Explained! 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. A. true. A Clampon (also known as Clipon) is a type of amperemeter based on current transformer. These current transformers have an opening where the conductor carrying the primary load is passed. The primary has few numbers of the turn of heavy wire, and the secondary of the current transformer has many turns of fine wire. 3. 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. The standard secondary current is usually rated at 5A or 1A for ease of sensing. For this reason, the secondary of a current transformer should always be short-circuited before removing the device connected to its secondary. SAT 0 0 10 20 30 40 50 60 70 80 M s 38 CT Types s . 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. Essentially, C relaying accuracy class applies to a current transformer of the toroidal, bushing or window type, commonly called donut-type transformers. View Answer. 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. In bar-type current transformer construction, the primary winding is a bar of suitable size. The window-type current transformer is the simplest form of instrument transformer. 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. (1 to over 12). The doughnut fits over the primary conductor, which constitutes one primary turn. [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. Answer.1. Bar Type CT: One that has a fixed and straight single primary winding turn passing through the magnetic circuit. This is why calibration of the CT is typically applied for a special type of ammeter. 3). 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}}). The secondary winding is fully distributed around the periphery of the core. The secondary rolls will be 1% and 2% less than the primary ones, and the difference being employed in the magnetizing core. For core-type transformers, the windings encircle the core, while in shell-type transformers, the core encircles the windings. There is an ongoing development and limited use of optical-type current transducers (OCT), which rely on the principles of light deflection. Bar type current transformers consist of a single bar as a primary winding. Some current transformers contain a split core which permits them to be opened, installed, and closed without disconnecting the system to which they are joined. oil or even air-in conjunction with its rated voltage class dictates the core and coil construction. 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 center of the current transformer is accumulated with lamination of silicon steel. In most case, the secondary current of a current transformer shall be 5A. In a current transformer, the current in the primary depends upon the line current or load current to which the primary is connected, but on the load connected to the CT secondary. Thus a CT must not be opened in working condition, it must always be earthed. Two principal types of transformer construction embody the requirements of economics, ease of manufacture, insulation, mechanical strength, and ventilation: core-type and shell-type. Your email address will not be published. The current transformer is employed for sensing the high-voltage current due to the difficulty of insufficient insulation in the meter itself. . Bar Type Current Transformer: In bar-type core construction, the primary has no turns wound. It produces a current in its secondary which is proportional to the current in its primary. least 40 V. With bar primary, number of turns = 20. Accuracy classes are defined by IEC and IEEE and are different for metering CTs and relaying CTs. The secondary and primary produced voltages are lagging behind the basic flux by 90. These units are typically used by utilities in substations. They are also known as CTs. The reassembly of the core halves is critical. The windings should be close together to reduce the secondary winding leakage reactance as the leakage reactance increases the ratio error. 5A ammeter). Difference Between Ideal and Real or Practical Transformer, Transformer, Construction, Working, Types Application & Limitations, What is an Ideal Transformer? [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. Most CT have rating factors for 35 C and 55 C. Common value for RF is 1.5. Required fields are marked *. The phasor diagram of the current transformer is illustrated in the figure below. 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. Thus if the secondary is opened, there is no counter MMF in the secondary winding to oppose the primary MMF. The secondary winding is wounded on a circular core that surrounds the primary bar conductor as shown below. Cable is passing through the centre of CT and in that way forms the primary winding. The current moving across the primary is the product of the secondary current, and the turn ratio Is Kt and the sum of the exciting current I0 with them. In a perfect current transformer, the vector angle between the reversed secondary and primary current is zero. HV CT are commonly installed outdoors, in AIS substations (Air Insulated Substation) Figure 2 or indoors, in GIS substations (Gas Insulated Substation) Figure 3. Some CT has special primary coils that allow a double ratio, when an increase of the installation is foreseen (example: 200-400/1 A) see Figure 8. 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. 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