Abb instrument transformers current transformer theory technical level training course 201203. The composition of a transformer core depends on voltage, current, and frequency. Power transformer used in between the power plant power generation up to the distribution network. Potential transformer definition potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. What is a voltage transformer and a current transformer. The line voltage, eline, will be the same as the actual winding voltage of the transformer. The saturation flux density, bs, is dependent on the annealing process of the magnetic material. The current transformer is used for measuring the high voltage current because of the difficulty of inadequate insulation in the meter itself. Module 14 test instruments and measuring devices this module describes electrical measuring and test equipment and includes the. Voltage transformer secondary windings are rated at 120 v for phasetophase voltage connections, or, equivalently, at 69. But, since the voltage at the secondary of a step down transformer is lesser than that at the primary, the current at the secondary will be increased in order to balance the total power in the transformer. An alternating voltage v p applied to the primary creates an alternating current i p through the primary.
Phase current, line current, and voltage in a delta system in a threephase delta circuit, such as the one shown in figure 127, the line voltage and line current are commonly called phase voltage and phase current. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased va rating for a given size and mass. Then to summarise this transformer basics tutorial. A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. 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. Current transformer basics and the current transformer. Module 14 test instruments and measuring devices this module describes electrical measuring and test equipment and includes the parameters measured and the principles of operation of common instruments. A transformer is a device used to change the voltage levels of electricity to facilitate the transfer of electricity from generating stations to customers.
When the circuit burden gets too large, the ct secondary voltage becomes distorted. The intermediate transformer must be of very small ratio error. This is because at the high voltage side lower will be the current and at low voltage side the current is higher. The accuracy in transforming the primary currents by high voltage and intervening ct determines the. Typically current transformers consist of a silicon steel ring core wound with many. Secondary terminal voltage which the ct must maintain within the c rating which is 200v in this example. Figure 1 gives a circuit schematic of a current transformer application. Some other differences between the current and the potential transformer are explained below in the the comparison chart. Current transformers theory of operation butler winding. From the above equation, by knowing secondary ammeter current and current ratios, we can easily determine the current flowing through the primary which is connected to the main line. 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.
Recommendations are provided by manufacturers see application guides. Transformer theory quiz questions free online quiz test transformer theory and practice. A potential transformer is a conventional transformer having primary and secondary windings. Ideal transformer theory equations example problems. Transformers used for voltage potential measurement steps down the voltage to the level of voltmeter. Commonly used core materials are air, soft iron, and steel. 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. Current and voltage relation in step down transformer. Performance with nonrectangular current pulses theory experimental readings with triangular current waveform comparison with theory discussion and summary of chapter application of switchedmode circuitry for supplying correcting voltage to the unidirectional current pulse transformer with reset voltage feedback introduction proposed technique. Presume that the load linked to the secondary has an impedance of 10 ohms. 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. Voltage transformer or potential transformer theory. Continue reading current transformers theory of operation.
Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output. Both the potential divider and the intermediate transformer have the ratio and insulation requirement which are suitable for economical construction. Grounding system theory and practice ced engineering. A primary voltage will be induced in the primary coil i1 and i3 and due to magnetic induction the voltage will be transferred to the secondary coil. Nevertheless, the primary winding is still connected to electricity and forms a closed circuit. Feb 24, 2020 the difference in a power transformer and a current transformer being the voltage on the primary, there is no transformed voltage in a ct. The transformer has a range of output voltage taps that can be used to set the start voltage.
Current transformers, standard for transient performance. Motor start theory auto transformers thermal overload 3 phase auto transformer b start contactor a start contactor run contactor 3 m the auto transformer starter employs an auto transformer to reduce the voltage during the start period. Step up increases the secondary voltage and thus the secondary current is decreased. The current transformer is used in meters for measuring the current up to 100 amperes. It will help you to understand what assumptions were needed while deriving the ideal transformer equations we use. A transformer consists of two electrically isolated coils and operates on faradays principal of mutual induction, in which an emf is induced.
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. Transformer theory quiz questions electrician practice tests. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. 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. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. The output of a constant voltage transformer is essentially a square wave, which is desirable for rectifier output applications while, also. Current and voltage transformers may be freestanding devices, or they may be built inside the bushing of some power apparatus such as a circuit breaker or a power transformer with a grounded tank. The basics of current transformers nk technologies. The voltage may be stepped up or down depending on. It buffers the meter from experiencing excessive torques that might be. The voltage transformer can be thought of as an electrical component rather than an electronic component. Current and voltage instrument transformers technical. Each of these materials is suitable for certain applications.
To verify that the ct is working at the site we have to test the entire circuit. 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. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. 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. A voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer. A transformer has no moving parts and is a completely. The primary wire size is much larger than the secondary wire size. The output voltage regulation of a constantvoltage transformer, for a change in line voltage, is a function of the squareness of the bh loop, as shown in figure 119. The voltage applied to the primary of the intermediate transformer is usually of the order 10kv. A stepup transformer increases the voltage while a stepdown transformer decreases it. Medium voltage instrument transformers ritz instrument. Theory of operation of singlephase transformers custom. Transformer equation notes this file contains a more detailed derivation of the transformer equations than the notes or the experiment 3 writeup. Hence, the output voltage in a step down transformer is less than that of the primary voltage and the output current is more than the input current.
This is required to minimize the transmission loss and making the transmission line. To insulate the metering circuit from the primary high voltage system. Step down decreases the secondary voltage and thus secondary current is increased. In the typical current transformer application, the primary winding consists of one to a few turns of wire. 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. Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. This current produces an alternating magnetic flux in the magnetic core.
The open circuit situation resolves to high voltages and lethal currents. It employed in electrical energy power system for stepping down the voltage of the system to some protected value which is often provided to low rating meter. The constantvoltage transformer cvt has a wide application, particularly where reliability and inherent regulating ability against line voltage changes are of prime importance. Correct operation of current current, voltage voltage, or current voltage devices usually depends on the relative instantaneous directions. The core of the current transformer is built up with lamination of silicon steel. The flux flows in the part of the core represented by the lefthand exciting branch of the equivalent circuit shown in figure 1. Knee point voltage of current transformer ps class electrical4u. A transformer uses the principles of electromagnetism to change one a.
As an example, a typical 100 to 5a current transformer has a nominal ratio of single primary turn to 20 secondary turns or 1. The output of a constantvoltage transformer is essentially a square wave, which is desirable for rectifier output applications while, also. Dec 30, 2018 voltage transformer or potential transformer theory. 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 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. 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. Used in ac system for the measurement of current, voltage, power and energy. Flux produced by one coil must link to other coil to induce voltage. K tf values required in practice depend on relay type and design. Theory of operation of singlephase transformers definition of transformer an electrical power transformer is a static device, which transforms electrical energy from one circuit to another without any direct electrical connection. Voltage measurement on a current transformer ct physics.
Sep 16, 2017 this is even applicable to a step down transformer. It can be designed to step up or step down voltages and works on the magnetic induction principle. Saturation makes the point of equality between the excitation current and the input occur at a flux level lower than would be expected from linear theory. A transformer is a passive electrical device that transfers electrical energy between two or more circuits. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance.
Pdf current transformers ct are the key device in power system protection. Current and voltage transformer symbols and simplified concepts 5 i 1 i 2 v 1 v 2 symbol of a current transformer symbol of a voltage transformer conceptual picture of a current transformer. Figure 5 wound primary current transformer low ratio, wound primary current transformer, suitable for primary currents from 2. Pdf current transformer ct is one of the important element of power system protection. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. It is used for auxiliary current transformers and for many low or moderate ratio current transformers used in switchgear of up to 11kv rating. Generally, aircore transformers are used when the voltage source has a high frequency above 20 khz. This module introduces transformer theory and includes the types of transformers, voltage current relationships, and application. Instrument transformer theory and testing steve hudson, pe engineering manager 10737 lexington drive. The constant voltage transformer cvt has a wide application, particularly where reliability and inherent regulating ability against line voltage changes are of prime importance. To do the derivation, we will use the figure pictured below.
The operation and theory behind a center tapped transformer is similar to a normal secondary transformer. Voltage transformers have only one iron core with attached secondary winding s. Finds a wide application in protection circuits of power system ex. Pdf abb instrument transformers current transformer theory. A transformer changes the voltage level or current level on its input winding to another value on its output winding using a magnetic field. However, since the current leads by 90 in the capacitive circuit, current zero occurs at the instant of a voltage maximum. When alternating voltage is applied to the primary winding, an alternating current will flow that will magnetize the magnetic. Kod ktf remanence only considered in extra high voltage systems ehv k tfvalues acc. 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. In this case, the primary winding behaves as a coil with a core. Current flowing in a wire produces a magnetic field. Stationary coils, time varying flux due to ac current flow. So in the primary, the ct will pick up the magnetic effect of the current but has no known voltage. Transformer theory the action of a transformer is such that a timevarying ac voltage or current is transformed to a higher.
One of the major difference between them is that the current transformer convert the high value of current into low value whereas the potential or voltage transformer convert the high value of voltages into low voltage. When a circuit is completed between the secondary winding and a load, this voltage causes current to flow. Maximum allowed torques for screw connections of current transformers maximum allowed torque for screw connection of voltage transformer is 20 nm. Cts current transformers pts potential transformers. Modern ac transmission technology requires the voltage to be higher such as 11kv, 33kv, 3kv, 232kv. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Module transformers this module introduces transformer theory and includes the types of transformers, voltage current relationships, and application. Faradays work in the 19th century showed that a changing current in a conductor e. But a voltage must still be present across the secondary for a current to flow. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device.
Cts are basically stepup voltage transformers, on the other hand, these are stepdown transformers in view of current. The essentials of current transformers in power circuits. Design procedure for compact pulse transformers with. 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. In case of bus ct, there must be always connected ct shielding to the primary bar.
Bartype current transformer this type of current transformer uses the actual cable or busbar of the main circuit as the primary winding, which is equivalent to a single turn. Jun 02, 2015 the operation of current transformer is similar to the conventional power transformer. The meter measures only the voltage and current reaching the meter terminals. Now we will proceed one further step toward a more practical aspect of a transformer. Another way to consider transformer voltages is by volts turn. An example of an application of an autotransformer is one style of travelers voltage converter, that allows 230volt devices to be used on 120 volt supply circuits, or the reverse. Current and voltage instrument transformers applicaton guide for current transformers, the value of the reactance x is determined in a special way so that it represents the leakage flux. While the physical principles are the same, the details of a current transformer compared with a voltage transformer will differ owing to different requirements of the application. The transformer winding voltage ratio is directly proportional to the winding turns ratio. Voltage transformer which is also called as a potential transformer. Primary of this transformer is connected across the phase and ground.
Transformers used for current measurement steps down the current to the level of ammeter. Jul 25, 2018 current transformer ps classbefore understanding knee point voltage of current transformer and current transformer ps class we should recall the terms instrument security factor of ct and accuracy limit factor. Pdf abb instrument transformers current transformer. All merlin gerin instrument transformers are in conformity with iec standards. Theory of transformer on load and no load operation. Theory of transformer on load, with resistive winding, but no leakage reactance. The number of secondary winding turns is a selected multiple of the primary turns. Current and voltage transformer symbols and simplified concepts 5 i 1 i 2 v 1.
Transformer core the composition of a transformer core depends on voltage, current, and frequency. Additionally, the turn ratio of the pulse transformer offers a degree of freedom that allows adapting the modulator design to the current and voltage ratings of available switch technology. 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. Pdf saturation analysis of current transformer researchgate. From the above analysis, we can define a step down transformer as a device which converts a high voltage and low current alternating source to a low voltage and high current alternating supply. Instrument transformers application guide abb group. In other words, you have to use a type of constantly reversing electricity called alternating current ac with a transformer. Current transformer circuits for power electronics. Complete basics and theory of electrical transformer. One important thing to note is that this trick works only if the current is fluctuating in some way.
356 1545 356 56 114 1049 222 888 1176 1083 198 1349 1065 563 1090 1588 103 1 1540 481 285 1099 1337 1525 1269 281 1428 536 382 115 781 915 1125 704 1299 1166 54 502 1400 1474 472 465 960 459 52 840 1063 1282 1050