The classification of protection current transformers follows the following. The various types of transformer shown in the figure above are explained in detail below. In the typical current transformer application, the primary winding consists of one to a few turns of wire. 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. 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. The alternate current transformer in theory and practice. Current transformer design guide an effective design of a ring type c. If the primary current of the current transformer becomes a the secondary current will still be 1. 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. 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. Chapter 6 introduction to transform theory with applications 6.
A transformer uses the principles of electromagnetism to change one a. Transformer only stepup or stepdown the level of ac voltage or ac current. The current transformers reduce the level of current to the instrument or relay operating range, whereas potential transformers transforms the high voltage to a circuit operating low voltage. 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 latest version of the software can be downloaded for pcs running windows xpvista. Figure 1 gives a circuit schematic of a current transformer application. Voltage and current relationship of a threephase delta circuit. Now we will go through the practical aspects one by one of an electrical power transformer and try to draw vector diagram of transformer in every step.
Phase voltage, line voltage, and current in a wye system the relationship between the line voltage, and line current, and the winding, or phase voltage and phase current, in a threephase wye circuit can be seen in figure 128. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. We have discussed about the theory of ideal transformer for better understanding of actual elementary theory of transformer. Transformer cant stepup or stepdown the level of dc voltage or dc current. The current transformer function is different than that of a voltage transformer. Current transformers perform the following functions. The energy is drawn out from the second coil and thus can be called as the secondary winding. Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing. Commercially available relays and meters used for protection and metering, are designed for low voltage. 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.
If the currents exceeding several hundreds of amperes have to circulate continuously through the. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. Apr 16, 2011 current transformers principle of operation of ct. Typically current transformers consist of a silicon steel ring core wound with many. Jun 01, 2015 the current transformers reduce the level of current to the instrument or relay operating range, whereas potential transformers transforms the high voltage to a circuit operating low voltage.
Figure 81 shows a oneline diagram of the primary and secondary automobile ignition system. Assume you have a working transformer, current flows in the primary and the secondary. Basic transformer operation can be described by two formulae relating the transformation ratio to the turns ratio of the transformer windings. 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. The fundamentals of current sense transformer design patrick a. Otto taylor, jim overmyer, ron michaelis in partnership with njatc transformer principles and applications provides a comprehensive overview of transformer operation, maintenance, installation, and troubleshooting this fullcolor textbook is designed to develop basic competencies of electrical apprentices and beginning.
An autotransformer is generally used either for providing a second leg in a single phase system giving you 120240 vac from a single 120 vac line or to provide a 120 vac tap from a 240 vac. This is the current required to produce the flux in ferromagnetic core. Calculation and analysis of transformer inrush current based on parameters of transformer and operating conditions electronics and elect rical engineering. A current transformer ct is a special type of transformer that is generally used for stepping down from high current levels to measurable lo w levels. Current transformer terminal block with short circuit devices. There are four typical types of current transformers. Current transformers,voltage transformers and instrument.
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. 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. When the circuit burden gets too large, the ct secondary voltage becomes distorted. 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.
Calculation and analysis of transformer inrush current. I intend to use the current transformer to power a device which requires only 3vdc and about 2040ma when active, and 0ma when inactive. Current transformers theory of operation butler winding. A current transformer is defined as as an instrument transformer in which the secondary current is substantially proportional to the primary current under normal conditions of operation and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Now suppose the resistance of the secondary circuit is reduced. Assuming an ideal transformer and the phase angles. The basics of current transformers nk technologies. Instrument transformers application guide abb group.
Our antivirus analysis shows that this download is malware free. What is the working principle of a current transformer. 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. 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. In this article we are going to discuss in detail about the potential transformers. Transfer of electric power from one circuit to another. Voltage transformer or potential transformer theory. 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.
The primary wire size is much larger than the secondary wire size. The magnetizing current in a real transformer when the ac power source is connected to a transformer, a current flows in the primary winding, even when the secondary winding is opencircuited. High voltage sf6 current transformers image source. 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 transformer basics and the current transformer. This type of transformer is categorized on the basis of a number of turns in the primary and secondary windings and the induced emf.
A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and. 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. The open circuit situation resolves to high voltages and lethal currents. Basics, maintenance, and diagnostics vii contents continued page 4. Theory of transformer on load and no load operation. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. Within electrical systems, current transformers are essential to ensure the correct functioning. Using a reactance to represent the magnetizing leg of a ct, as shown in figure 1, is a useful visual concept.
Transformer operates when both circuits take effect of mutual induction. Pdf calculation and analysis of transformer inrush current. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. One is a continuous operation level and one is for emergency conditions. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were. Transformer principles and applications njatc, textbook. Transformer theory electrical values and their definition in a power transformer voltage, current, number of turns, impedance and their interrelation operation of power transformers in a power system thermal performance, loading, paralleling, tapchanging, connections and vector groups power transformer protection surge protection. 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.
The alternating current supply is given to the first coil and hence it can be called as the primary winding. 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 transformer theory is simple ratio of turns, iron core, power in power out for ideal transformers etc but now the sticky bit. In short, a transformer carries the operations shown below. A treatise on the theory, operation, and application of transformers l. 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. The number of secondary winding turns is a selected multiple of the primary turns. Transformer basics and transformer principles of operation. Effects of a current transformers magnetizing current on. These comprise a crossdisconnect terminal with measuring and test equipment, insulated bridges for grounding and short circuiting of the current transformer.
The following explanation arose from a question from physics. In order to short circuit current transformers and for the purpose of recurrent comparative measurements it is recommended that special terminal block for din rails be used. The current transformer isolates measurement and control. The current installer available for download occupies 288. An alternating voltage v p applied to the primary creates an alternating current i p through the primary. 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. 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.
They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. Current transformers cts are used to convert high level currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment. Current transformers are commonly used in metering and protective relaying, where they facilitate the safe measurement of large currents. Current transformer basics and current transformer theory. 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 transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. Practical power transformers operation and maintenance. Faradays work in the 19th century showed that a changing current in a conductor e. This direct current transformer will demonstrate the actions of a stepup transformer and provide stopaction analysis of the moving magnetic field. A current transformer ct is a type of transformer that is used to reduce or multiply an.
Transformer basicsworking principle,construction,types. 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. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Chapter 16 current transformer design university of north. Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Types of transformer different types of transformer. A current transformer is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary.
S note that the order of the numbers when expressing a transformers turns ratio value is very important as the turns ratio 3. Nov 17, 2015 the magnetizing current in a real transformer when the ac power source is connected to a transformer, a current flows in the primary winding, even when the secondary winding is opencircuited. 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. 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. The following explanation arose from a question from. An autotransformer is a kind of transformer with only one winding, but one or more taps. The primary circuit, or power source side, includes the battery positive. Operation of current transformers janitza electronics. The most popular versions among the software users are 3. Continue reading current transformers theory of operation. The singlephase distribution transformer consists of a primary winding and a secondary winding wound on a laminated steel core.
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