Voltage transformer or potential transformer theory. What is the working principle of a current transformer. A centre tapped transformer works in more or less the same way as a usual transformer. It transforms the high voltage current into the low voltage current due to which the heavy current flows through the transmission lines is safely monitored by the ammeter. Current transformers theory of operation butler winding. Basic transformer theory vp primary voltage ip primary current np primary turns pp primary power.
The output of a constantvoltage transformer is essentially a square wave, which is desirable for rectifier output applications while, also. Electrical tutorial about current transformer basics and current transformer theory on how the current transformer works with just one secondary winding. Apr 25, 2017 the current transformer the current transformer c. Ring type is the most usual type of corebalance current transformer cbct. An alternating voltage v p applied to the primary creates an alternating current i p through the primary. The current flows first in one direc tion, stops, then reverses and flows in the other direc274 unit 74 transformers tion. They are very useful in highpower circuits, where the current. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Figure 1 gives a circuit schematic of a current transformer application. C indicates accuracy can be calculated based on design.
Figure 1 represents the essential elements for a transformer a magnetic core, with a primary and secondary coil wound on the limbs of the magnetic core. Primary of this transformer is connected across the phase and ground. Cattermole typically, the transformer is designed so that the impedance of the primary winding is much greather than the shunt resistance reflected into the primary side and thus the primary impedance is mostly resistive. Current transformers are used to measure, or monitor, the current in the lead of an ac power circuit. Aresistanceshunted byacondenserisplacedinthemovingcoilcircuitoftheseries. Figure 144 shows the magnetic field during one cycle. The difference lies in just the fact that its secondary winding is divided into two parts, so two individual voltages can be acquired across the two line ends. Generally current transformers and ammeters are used together as a matched pair in which the design of the current transformer is such as to provide a maximum secondary current corresponding to a fullscale deflection on the ammeter.
The voltage transformer is designed to connect in parallel. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value. In the typical current transformer application, the primary winding consists of one to a few turns of wire. The constantvoltage transformer cvt has a wide application, particularly where reliability and inherent regulating ability against line voltage changes are of prime importance. Testing current transformers burden test only if we cannot get to the primary side of the ct we can. Some most recent works are also discussed to give readers a feel for the latest trends in transformer technology. In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. The first author worked in the transformer industry for 11 years before joining academia.
Im going to connect a current transformer to a mcu ade7755. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. Its electrical purpose is to transfer power from the primary winding to the other windings with no energy storage or loss. Magnetics design 4 power transformer design texas instruments. Optimiza tion techniques, programming and algorithm theory. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. The ideal transformer equations we use are sufficient to design a transformer that works under certain constraints. I want to make a current transformer using toroidal ferrites to measure 2205060hz ac current40a max. A study of the current transformer with particular reference. Optimization techniques, programming and algorithm theory. The designer should consider the maximum gate trigger current as the minimum trigger current value that must be applied to the device in order to assure its proper triggering. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the. Basics, maintenance, and diagnostics vii contents continued page 4. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another.
Instrument transformer theory and testing steve hudson, pe. Nov 15, 2019 a current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is proportional to the primary current under normal conditions of operation and differs from it by an angle which is approximately zero. A transformer basically is very simple static or stationary electromagnetic passive electrical device that works on the principle of faradays law of induction by converting electrical energy from one value to another. Igt gate trigger current the maximum value of gate current required to switch the device from the off. In high frequency transformers eddy current losses are reduced by using a core made of a ceramic. They are accurate, easy to implement, and reliable under harsh. Power transformers operating theory butler winding. The current transformer function is different than that of a voltage transformer. Thus, in an ideal transformer the power ratio is equal to one unity as the voltage, v multiplied by. Feb 12, 2017 in this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer.
The core of the current transformer is built up with lamination of silicon steel. The arrangement of primary and secondary windings on the transformer core is shown in figure 9 below. In an ideal transformer ignoring any losses, the power available in the secondary winding will be the same as the power in the primary winding, they are constant wattage devices and do not change the power only the voltage to current ratio. For an ideal transformer, the power delivered in the secondary v sec isec equals the power in the primary v pri ipri therefore. If the voltage is stepped up in the secondary, the current is stepped down by the same amount if the voltage is stepped down in the secondary, the current is stepped up by the same amount. Transformer tlt some baluns are narrowband because they depend on a section of transmission line that is a particular fraction of a wavelength. Your load resistance should be small relative to the impedances of the inductors in the transformer, at least at the frequency you want it to work at. As long as the current to be measured are small and at low voltage, the direct method of connecting the instruments and relays in the circuits is convenient and adequate. Pdf optimal design of protective current transformer using pso. Basic transformer theory vp primary voltage ip primary current np primary turns. C indicates accuracy can be calculated based on design of this ct. The primary wire size is much larger than the secondary wire size. Generally current transformers are designed such a way that, its.
This current is known as the magnetizing or exciting. The number of secondary winding turns is a selected multiple of the primary turns. Instrument transformer theory and testing steve hudson, pe engineering manager 10737 lexington drive knoxville, tn 37932. In nonideal transformers this current is greater than zero. Current sense transformer application design guidelines by ariel general applications engineer, datatronic distribution inc. The electrical current flowing in a transformer is al ternating current. A voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer.
The standard for precise current measurement in instrumentation and other high reliability equipment applications has been the current sense transformer. The voltage transformer can be thought of as an electrical component rather than an electronic component. A transformer uses the principles of electromagnetism to change one a. Cable is passing through the centre of ct and in that way forms the primary winding. The regulation characteristics at various lines and loads are shown in figure 116. In the ideal transformer the current value is zero. Based on the fundamentals of electromagnetics, this clear and concise text explains basic and applied principles of transformer and inductor design for power electronic applications. If there is a zero sequence impedance of transformer, the power and leakage impedance rating of the transformer may be calculated. Leakage inductance delays the transfer of current between switches.
The current transformer is used for measuring the high voltage current because of the difficulty of inadequate insulation in the meter itself. Chapter 16 current transformer design university of north. Pdf current transformers ct are the key device in power system protection. Current transformer design example the following information is the design specification for a current transformer, as shown in figure 169. Shortcircuit design basic theory radial force due to axial flux. Force is proportional to square of current transformer consulting services inc. Pdf saturation analysis of current transformer researchgate. It becomes very difficult to optimize a power transformer design.
Design procedure for compact pulse transformers with. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. If there is a zero sequence impedance of transformer, the power and leakage impedance rating. Power transformer design involves many interdependent parameters. The fundamentals of current sense transformer design patrick a. The internal process is the same, which is when an. However, most baluns are made from short sections of transmission lines compared to l surrounded by magnetic media with their ends connected to show transformerlike properties. He has vast experience in the design and development of transformers. A current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is proportional to the primary current under normal conditions of operation and differs from it by an angle that is approximately zero. When an ac voltage is applied to the primary winding, a timevarying current flows in the primary winding and causes an ac magnetic flux to appear in the transformer core.
The above theory is sufficient to give a general view. A transformer has no moving parts and is a completely. It can be designed to step up or step down voltages and works on the magnetic induction principle. The basic theory covers all types from small high frequency transformers using ferrite core, current transformers typically a round wound core and a.
Generally, the metering its require high accuracy in the range of normal oper. In case of a transformer of normal design, the no load current will generally be less than about 2% of the full load current. The basics of current transformers nk technologies. When the circuit burden gets too large, the ct secondary voltage becomes distorted. H i g h p o w e r e d r e s e a r c h f o r t h e r e a l w o r l d december 2011 issue 1 modeling current transformer ct saturation for detailed protection studies 6 the wound rotor wr and squirrel cage sq induction machine models. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were. The choice of circuit topology obviously has great impact on the transformer design. Detailed descriptions of many types of transformer are also given together with typical applications. Figure 6 bar primary type current transformer ct having single primary turn with the primary conductor embedded go back to contents 2. Instrument transformer basic technical information and. Transformer book is still in demand is a tribute to the publishers and to the authors who have carried the torch to light our way for 75 years.
The shortcircuit current is limited and set by the series inductance, l. Current transformer white paper the basics of current transformers ratio the ct ratio is the ratio of primary current input to secondary current output at full load. Current transformer basics and the current transformer. H i g h p o w e r e d r e s e a r c h f o r t h e r e a l w o r l d december 2011 issue 1 modeling current transformer ct saturation for detailed protection studies 6 the wound rotor wr and squirrel. Oct 08, 2016 calculation of cts if the current transformer has primary to secondary current ratio of 100. Current transformer design guide an effective design of a ring type c.
The essentials of current transformers in power circuits. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. The current transformer is used in meters for measuring the current up to 100 amperes. A study of the current transformer with particular. The minimum primary inductance required l p is a function of how accurate and linear the volts per ampere versus current transfer function of the transformer needs to be.
Modeling current transformer ct saturation for detailed. Design requirements for current transformers for general protection purposes are frequently laid out in terms of kneepoint e. The voltage, current and flux due to the current in. It details both the theory and practice of inductors and transformers employed to filter currents, store electromagnetic energy, provide physical isolation between circuits, and perform stepping up and down of dc. Isolation transformers can allow separate circuits to be physically isolated from each other whilst still allowing current and. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the meter or relay. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8 diameter. A distinction has to be made between a metering class and a protection class current transformer.
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