A current transformer can be modeled as a constant current source where ratio current is injected into a magnetizing impedance in parallel with the burdens shown in figure 1. If the primary current of the current transformer becomes a the secondary current will still be 1. If the applied voltage is 250 v, calculate the output voltage of the transformer. Voltage is produced in any conductor in a changing magnetic field current flowing in the coil on one side of the transformer induces a. The primary winding can consist merely of the primary current conductor passing once through an aperture in the current transformer core window or bartype, or it may consist of two or more turns wound on the core together with the secondary winding wound type. I am je of nagbhir sub station of 225 kv newly electrified section and testing started of current transformer when test ct ratio by injecting current in primary bushing of old ct which was 20 years old ct installed in circuit,after testing values not found according to ratio,100 amps inject and found in secondary terminal 3. Cts are required because standard current measuring devices are not configured to measure high current levels. Continue reading current transformers theory of operation. A potential transformer is a conventional transformer having primary and secondary windings. Basic theory the turns ratio of a transformer is defined as the number of turns on its secondary divided by the number of turns on its primary. Presume that the load linked to the secondary has an impedance of 10 ohms. Instrument transformer basic technical information and. Also, please note that the word ratio in these formula refer to a fraction, not an actual ratio. The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift.
The primary winding is the power input winding which is connected to the ac power source. Cts used with watt transducers enable the owner to control demand as well as monitor. The primary wire size is much larger than the secondary wire size. Cts are required because standard current measuring devices are not configured to measu. Because the action of a transformer is nearly perfect, the power in both primary and secondary. However, if the ct saturates, more current is required to magnetize the core and as a result the secondary current i s available as inputs to the relay may not be an identical scaled down replica of the actual primary current i p. Voltage transformers are designed so the through impedance rs,r,xp, and x is as low as possible, while current transform. The auto greek for self prefix refers to the single coil acting alone, not to any kind of automatic mechanism. 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 transformer turns ratio tells you how much the voltage changes through the transformer as current passes from the primary windings to the secondary windings. Since the load current passes through primary windings in the ct, screw terminals are provided for the load and secondary conductors. If the secondary circuit is closed, a current will flow through the secondary circuit in proportion to the ct ratio. Ct, screw terminals are provided for the load and secondary conductors.
Current transformers cts basic theory basic formula. For transformation ratio to be equal to turn ratio. This means that there should not be any core loss in the current transformer and burden should be purely resistive. The magnetized iron core induces a voltage in the secondary coils. On window type units with low primary current ratings, where the primary conductor size is small, the ratio of the transformer can be changed by taking multiple wraps of the primary conductor through the window. While the latter configuration would not be used today, one may be required to make this calculation on an existing system. In reality, however, it is not possible to neglect the excitation. The ratio of the primary to the secondary, the ratio of the input to the output, and the turns ratio of any given transformer will be the same as its voltage ratio. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer.
Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. Exact burden calculations are usually not necessary unless the ct ratio is below 200. Vs vp ns np the current ratio of an ideal transformer is inversely related to the turns ratio. Complete basics and theory of electrical transformer. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. How to calculate transformer turns ratio sciencing.
Modeling current transformer ct saturation for detailed. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. The transformation from primary to secondary current requires a small amount of energy to. 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. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8 diameter. The essentials of current transformers in power circuits. Using a reactance to represent the magnetizing leg of a ct, as shown in figure 1, is a useful visual concept. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. Kod ktf remanence only considered in extra high voltage systems ehv k tfvalues acc. Turns ratio test results should not deviate more than onehalf percent from either the adjacent coils or the calculated ratio. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or monitor the current levels. Introduces no phase shift apparent power balance p s p p s s s s e i e i no power losses in idea transformer current ratio current ratio is the inverse of the voltage ratio a 1 i i s p p ais 1 i. Basic transformer operation can be described by two formulae relating the transformation ratio to the turns ratio of the transformer windings.
Now we will proceed one further step toward a more practical aspect of a transformer. Current transformers, standard for transient performance iec 600446 specifies four classes. The secondary current ratings are of the order of 5a, 1a and 0. Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output. Voltage is produced in any conductor in a changing magnetic field current flowing in the coil on one side of the transformer induces a voltage in the coil on the other side. Transformation ratio of transformer turns, current. Testing current transformers ratio vs applied burden cts installations can be fully verified in the field we can verify that the ratio is correct and constant as burden changes. The open circuit situation resolves to high voltages and lethal. Theory of transformer on load, with resistive winding, but no leakage reactance. Primary exciting current is divided by the ct ratio to determine equivalent. As your circuit indicates, the ratio is simply 6005120.
Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. In a real ct ct analyzer model the 3 parts of the cta model. 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. In the typical current transformer application, the primary winding consists of one to a few turns of wire. Figure 1 gives a circuit schematic of a current transformer application. The iron core inside the transformer is magnetized. 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. Current flowing in a wire produces a magnetic field. The basics of current transformers nk technologies. An alternating voltage v p applied to the primary creates an alternating current i p through the primary.
The transformer consists of two separate windings placed over the laminated silicon steel core. That is, the ct secondary current is an identical but scaled down replica of the primary current. The current in the windings of a transformer is inversely proportional to the voltage in. Another very common type of construction is the socalled window. A study of the current transformer with particular. Current transformers theory of operation butler winding. Oct 08, 2016 calculation of cts if the current transformer has primary to secondary current ratio of 100. This makes the replacement of instrument transformer. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. This makes the replacement of instrument transformer very easy. Transformers transformer theory 6 current ratio the current in the windings of a transformer is inversely proportional to the voltage in the windings. 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 typical excitation current test data pattern for a threelegged core transformer is two similar current readings and one lower current reading. It also improves the performance of current transformers and subjects their performance to simple calculation. Download fulltext pdf calculation of current transformer transient errors article pdf available in ieee transactions on power apparatus and systems 976. The current primary ratings vary from 10a to 3000a or more. The current ratio of the transformer is usually high. Referring all quantities to the secondary winding, the simplified equivalent circuit of figure 3. The internal process is the same, which is when an. Abb instrument transformers current transformer theory technical level training course 201203.
O the ratio errors of current transformers used for relaying are such that, for a given magnitude of primary current, the secondary current is less than the marked ratio would indicate. Potential transformers pts or voltage transformers vts pts or vts are the most common device used. In most current transformers an approximate inverse turns ratio exists between the two currents in the primary and secondary windings. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. The amount of current flow in the secondary circuit can be computed making use of the formula the primary current can be computed by using the same formula that was utilized to calculate primary current for an isolation sort of transformer the quantity of power input and output for the. For example, the potential transformer mentioned above having a rating of 480. So perhaps you are really asking how to measure the ratio.
A centre tapped transformer works in more or less the same way as a usual transformer. The number of secondary winding turns is a selected multiple of the primary turns. Theory of transformer on load and no load operation. Pdf calculation of current transformer transient errors. In contrast, an ordinary transformer has separate primary and secondary. This is required to minimize the transmission loss and making the transmission line. The ideal transformer has infinite noload input impedance, 100% magnetic coupling between transformer windings hence no leakage inductance, zero winding resistance, zero core losses, and no capacitance. Calculation of cts if the current transformer has primary to secondary current ratio of 100. A transformer has turns on the primary winding and 200 on the secondary.
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. When the transformer is connected to a load, the secondary current i 2 produces a demagnetizing flux proportional to the secondary ampereturns i 2 n 2. Phase angle of current transformer is the angle between the primary current i p and secondary current i s when reversed. Modern ac transmission technology requires the voltage to be higher such as 11kv, 33kv, 3kv, 232kv. A read is counted each time someone views a publication summary such as the title, abstract, and list of authors, clicks on a figure, or views or downloads the fulltext. Scribd is the worlds largest social reading and publishing site. Current and power when a load resistor is connected to the secondary winding, there is a current through the resulting secondary circuit because of the voltage induced in the secondary coil this results in current in both the primary and secondary coils which is also effected by the turns ratio. The current transformer shown represents an ideal transformer. Current transformer basics and the current transformer. An autotransformer is an electrical transformer with only one winding. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were.
The windings of a standard singlephase transformer are called the primary winding and the secondary winding as shown in fig. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance. There are four typical types of current transformers. Primary current by the factor of the turns ratio in an ideal ct. Zb burden impedance ip primary current ns number of secondary turns ratio to 1 v 3. The simplified equivalent circuit of a current transformer, as shown in figure 162, represents the important. The voltage ratio of an ideal transformer is directly related to the turns ratio.
The ct ratio is measured by shorting the secondary the 5 a winding and then injec. 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 ct ratio is the ratio of primary current input to secondary current output at full. The measuring circuit is not isolated electrically from the power circuit. Pdf power transformers principles and applications. There are three basic styles used to transformer system voltages to control voltages. 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. 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. How to determine the ratio of a current transformer quora.
Pdf abb instrument transformers current transformer theory. Tests have shown values ranging from 500 to 11,000 volts. These devices use standard transformer theory with 23 windings one primary with one or two secondary windings with a magnetic connection via an iron core. Pdf abb instrument transformers current transformer. Unlike a voltage transformer, the primary current of a current transformer is not dependent of the secondary load current but instead is controlled by an external load. The ratio of the primary current and the secondary current is known as a current transformer ratio of the circuit. 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.
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