Prior to joining schweitzer engineering laboratories, inc. We have discussed about the theory of ideal transformer for better understanding of actual elementary theory of transformer. Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Instrument transformers scale the large values of voltage or current to small. Abstractthis paper addresses the security of transformer differential protection with low levels of second harmonic during. The alternate current transformer in theory and practice. Current transformer polarity can be defined by permanent markings typically h 1 x 1 or polarity dots. A transformer is one of the most common devices found in electrical system that links the circuits which are operating at different voltages. Current transformers,voltage transformers and instrument. Tape wound measuring and protection current transformers.
The joining point where the two halves of the housing meet inside the primary. Chapter 16 current transformer design university of north. A distinction has to be made between a metering class and a protection class current transformer. Instrument transformers application guide abb group. A current transformer ct is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree ct accuracy class or current transformer class.
Current transformers are designed to provide a scaled down replica of the current in the hv line and isolate the measuring instruments, meters, relays, etc. A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. It is based on the twostage principle, and it includes electronic compensation for. Calculation of the current transformer 1mrs 755481 accuracy limit factor 3. Kod ktf remanence only considered in extra high voltage systems ehv k tfvalues acc. Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case. Testing current transformers burden test only if we cannot get to the primary side of the ct we can. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Energy is stored in the magnetic field of the transformer when the pulse is applied. Their role in electrical systems is of primary importance as they are a means of stepping down the current or voltage of a system to measurable values, such as 5a or 1a in the case of a current transformers or 110v or 100v in the case of a voltage transformer. A current transformer with electronic assistance was developed, intended for current to voltage transducers. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. Cts current transformers pts potential transformers.
Courtesy siemens current transformers cts are used to convert highlevel currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment. Basics, maintenance, and diagnostics vii contents continued page 4. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. Instrument transformer theory and testing steve hudson, pe. The following explanation arose from a question from physics. The transformer core is used to provide a continuous magnetic path with small air gap. Assume you have a working transformer, current flows in the primary and the secondary. Pdf transient behaviour of conventional current transformers. Potential transformer pt definition the potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. If the load is disconnected from the secondary winding of the transformer and a high voltage is applied to the primary winding of the transformer, a magnetizing current will flow in the primary. Example the distance between the cts and the protection relay is 15 metres, 4 mm 2 cu conductors in 4wire connection are used.
Current transformers principle of operation of ct a current transformer is defined as as an instrument transformer in which the secondary. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watthour meters, etc. Calculation of the current transformer accuracy limit factor. Using a special grade of heattreated grainoriented silicon steel laminations can minimize the hysteresis loss. The generator transformer is the largest transformer on a power station and connects the generator output to the grid. Wire effective resistivity cm total rms winding current, ref to pri itot a desired turns ratios n2n1, n3n1, etc.
A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. The sheets are laminated and coated with an oxide layer to reduce eddy current loss. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were. In principle, the only difference between a current transformer and a voltage transformer normal type is that the former is fed with a constant currentwhile the latter is fed with a constant voltage, where constant has the strict circuit theory meaning. Eddy current losses in transformer windings and circuit wiring, included in this manual, is a useful supplement. 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.
Current transformers have a primary winding to which the current to be measured is. Current, voltage and combined transformers, oilpaper or gas insulated up to 800 kv. It can be designed to step up or step down voltages and works on the magnetic induction principle. Pdf twostage current transformer with electronic compensation. The path taken at dc and low frequencies can be quite different from. Class px as defined in iec600441bs en600441 allows the manufacturer of a protection device to detail more specific current transformer. Pdf the transient behaviour of a conventional current transformer ct is. 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 invention provides a kind of current transformer simulation examination system and method for work, students machine that described system comprises teachers machine, communicates to connect with teachers machine, described students machine comprises slave computer and current transformer detection simulation platform, the simulator that described current transformer detection. High voltage sf6 current transformers image source.
Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an ac. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. This type of transformer is categorized on the basis of a number of turns in the primary and secondary windings and the induced emf. Thus, in an ideal transformer the power ratio is equal to one unity as the voltage, v multiplied by. What is the working principle of a current transformer. Transformer cant stepup or stepdown the level of dc voltage or dc current.
The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift between the input and output voltage is also minimum. Basic understanding on current transformer transformer. 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. The following explanation arose from a question from. B2 current and voltage transformers schneider electric. 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. If the secondary circuit is opened with primary current flowing, all the primary ampereturns become magnetizing ampereturns and usually will produce an excessively high secondary. A range of 3in1 current transformers combine three traditional current. Generally, the primary winding of a transformer is connected to the input voltage supply and converts or transforms the electrical power into a magnetic field. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing operational data and. Protection relaysrelay panels earth fault protection bussing type, oilimmersed ct in power transformer. Transformer is a nonrotational device and motor is a rotational machine. The basics of current transformers ratio the ct ratio is the ratio of primary current input to secondary current output at full load.
Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. It is possible to combine the use of abb cvts as voltage transformers with using. Introduction to transformers construction, working. Introduction the rated accuracy limit factor f n is the ratio of the rated accuracy limit primary current to the rated primary current.
If the primary current of the current transformer becomes a the secondary current will still be 1. In other words, for a transformer there is no direct electrical connection between the two coil windings, thereby giving it the name also of an isolation transformer. Conservation of energy like water running downhill, electrical current always takes the easiest path available. Basic transformer operation can be described by two formulae relating the transformation ratio to the turns ratio of the transformer windings. Pdf a transformer model based on the jilesatherton. Typically current transformers consist of a silicon steel ring core wound with many. Voltage and current transformers for low primary voltage or current ratings. A protective current transformer type 5p10 has, for example, the accuracy class 5p and the rated accuracy limit factor 10. However, it is possible to understand the electric field distribution in detail with such threedimensional analyses and to find a reasonable insulation structure. Basic transformer theory vp primary voltage ip primary current np primary turns. Transformer basic objective introduce basic transformer theory as it relates to diagnostics provide a better understanding of the diagnostic test environment identify important information that should be collected in the diagnostic test process. Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors.
Basic transformer theory is simple ratio of turns, iron core, power in power out for ideal transformers etc but now the sticky bit. Equivalent current transformer circuit x 1 primary leakage reactance r 1 primary winding resistance x 2 secondary leakage reactance z 0 magnetizing impedance r. 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. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. Theory of transformer on load and no load operation. Primary current range from 5 amp to 5000 amp with secondary current 5a, 1a, or 01. The various types of transformer shown in the figure above are explained in detail below. The current transformer function is different than that of a voltage transformer.
The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings. Bogdan kasztenny, normann fischer, and yu xia, schweitzer engineering laboratories, inc. Transformer only stepup or stepdown the level of ac voltage or ac current. Generator transformer an overview sciencedirect topics. For example, the potential transformer mentioned above having a rating of 480. Instrument transformer basic technical information and.
There is a generator transformer for each generating unit and it is rated according to the size of that unit. Current transformers, along with voltage or potential transformers, are instrument transformers. Transformer basicsworking principle,construction,types. The fundamentals of current sense transformer design patrick a. Chapter 31 alternating current phasors and alternating currents resistance and reactance magneticfield energy the lrc series circuit power in alternating current circuits resonance in alternating current circuits transformers. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. Basic understanding on current transformer free download as powerpoint presentation. Now suppose the resistance of the secondary circuit is reduced. Oct 08, 2016 in principle, the only difference between a current transformer and a voltage transformer normal type is that the former is fed with a constant currentwhile the latter is fed with a constant voltage, where constant has the strict circuit theory meaning. These are commonly used in applications where there is a need of ac voltage conversion from one voltage level to another.
Types of transformer different types of transformer. By combining the equations 1 and 2, we find the following. Ideal for installation at metering points due to its very high. 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. 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. The classification of protection current transformers follows the following. In these circumstances, the traditional electromagnetic current transformer model based on the tcircuit 24, 25 and operating in the rated load mode was applied. It produces a current in its secondary which is proportional to the current in its primary.
Finally, we explain how ct saturation can affect relay operation and show how to detect. Current transformer basics and the current transformer. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. The singlephase distribution transformer consists of a primary winding and a secondary winding wound on a laminated steel core. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8 diameter. Transformer operates when both circuits take effect of mutual induction. Mathematical model of a current transformer the general equivalent circuit of a current transformer should contain the primary section and the secondary section combined with the burden. If the currents exceeding several hundreds of amperes have to circulate continuously through the. The basics of current transformers nk technologies.
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