The basics of current transformers nk technologies. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. They are defined by their ratio, power and accuracy class. The current that intially passes through the transformer when it is switched on is known as the transformer inrush current. Current transformer, standard for steadystate performance. Transformer basics electric machines single phase transformer three phase transformer transformer type instrument transformer transformer test transformer accessories transformer oil maintenance of transformers installation of transformers transformer protection. The potential transformer is connected in parallel with the circuit. They are a simple devices designed to produce an alternating current in the secondary winding which is directly proportional to the current in.
Principles of electrical grounding john pfeiffer, p. For an examplelets use load voltage 480 volts, load current, 80 amps and line voltage 208 volts. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. Basics of current transformer engineering tutorial. The current transformer is designed to connect in series with the line to transform the line current to the. These are used in protective relays and for measurement purposes. 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. This protects connected metering devices from overloading in the presence of fault level current flows. A transformer being an ac machine however cannot step up or down a dc voltage or dc current. Sf6 110 kv current transformer tgfm series, russia. The magnetic field around the winding is constantly in motion. Basic electronics transformers according to the principle of electromagnetic induction, we have already learnt that, a varying flux can induce an emf in a coil. They are very useful in highpower circuits, where the current.
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. Tech electrical, iii year, v semester k72 submitted to mr. Correction factor limits for cts accuracy 100% rated current 10% rated current class min max min max. At 5% rated current through 100%, the current transformer must maintain 0. Determine the burden of the device connected to the ct in va or ohms impedance. A german mathematician, george simon ohm, formulated this relationship in the 19th century.
The ct ratio is the ratio of primary current input to secondary current output at full. Chapter 16 current transformer design university of north. Basics, maintenance, and diagnostics vii contents continued page 4. Measure the length of the wire between the current transformer and the burden. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8 diameter. Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. There are four typical types of current transformers. 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 essentials of current transformers in power circuits. When an input voltage is applied to the primary winding, alternating current starts to flow in the primary winding.
Similarly, current transformers take a high current and reduce it to typically 5 amps on the secondary winding so that it can be used with a watthour meter, ammeter. Voltage transformer or potential transformer theory. Figure 144 shows the magnetic field during one cycle. Cable is passing through the centre of ct and in that way forms the primary winding. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to.
Abb group 1 jan 5, 2012 current transformers introduction main tasks of current transformer are i1. In high frequency transformers eddy current losses are reduced by. A transformer with a primary winding of turns and a secondary winding of 100 turns has a turns ratio of. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. Learn how to specify current transformers photo credit. 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. Oct 08, 2016 calculation of cts if the current transformer has primary to secondary current ratio of 100. The primary windings of the potential transformer are directly connected to the power circuit whose voltage is to be measured. Learn the basics of differential protection of transformers. This is the first part of video series called as current transformer in this video i have discussed about definition of current transformer and working principle of it. The current transformer function is different than that of a voltage transformer. As the current flows, a changing magnetic field is set up in the transformer core. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. In a delta transformer, the line current doesnt equal the phase current as it does in a wye transformer.
Transformer design and test are sometimes viewed as an art rather than a science. The dc current transformer dcct is a type of dccapable current transformer, which produces an output current proportional to the input current all the way down to dc. His law ohms law stated that current is directly proportional to voltage and inversely proportional to. Read pdf basic electrical engineering equations transformer of chapter single phase transformer of subject basic electrical. Instrument transformer basic technical information and application. A single turn primary winding is passed through a wellinsulated toroidal core transformer which is wounded with many turns, which makes a current transformer. The following are some points regarding selection, operation, and maintenance of cts. Just like the transformer used for stepping down purpose, potential transformer i. It is most commonly used to increase step up or decrease step down voltage levels between circuits. To analyze how an electrical shock occurs and how grounding is applied, you need to look at the circuit involved. A current transformer ct is a type of transformer that is used to reduce or multiply an. It can be designed to step up or step down voltages and works on the magnetic induction principle. The basics of power transformer theory, operation, testing and protection. Open circuit test and short circuit tests are performed on a transformer to determine equivalent circuit, voltage regulation and efficiency of transformer.
Types of instrument transformer construction simple basic forms current transformer voltage transformer h1 x1 h2 x2 primary secondary current example 5. A test set operator connects the leads, pushes the buttons, and records the. Ring type is the most usual type of corebalance current transformer cbct. Back to the basics current transformer ct testing as test equipment becomes more sophisticated with better features and accuracy, we risk turning our field personnel into test set operators instead of skilled field service technicians. Figure 2 illustrates the basic circuit that consists of a source, a transformer or. Complete basics and theory of electrical transformer.
Presentation on current transformers submitted by roopal pancholi b. The primary winding of the transformer is connected in series with the load and carries the actual current flowing to the load, while the secondary winding is connected to a measuring device or a relay. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. A 1005 ratio ct means that the primary current is 20 times greater than the secondary current. Pdf instrument transformer basic technical information.
Correction factor limits for cts accuracy 100% rated current 10% rated current class min max min max 1. The basic principle of the current transformer is the same as that of the power transformer. For an ideal transformer, the power delivered in the secondary v sec isec equals the power in the primary v pri ipri therefore. Transformers multiple choice questions and answers mcqs electrical engineering interview transformers viva lab manual pdf free download skip to content engineering interview questions,mcqs,objective questions,class notes,seminor topics,lab viva pdf free download. Current transformer in general, large currents hundreds of amperes cannot be directly input to powermeters for measuring power. Electrical tutorial about current transformer basics and current transformer theory on how the current transformer works with just one secondary winding. Figure 6 bar primary type current transformer ct having single primary turn with the primary conductor embedded go back to contents 2. Jun 24, 2018 the transformer core is submerged in the transformer oil.
Nov 17, 2019 transformer basics and working principle. Basic transformer operation can be described by two formulae relating the transformation ratio to the turns ratio of the transformer windings. A transformer is a static device which transfers electrical energy from one circuit to another through the process of electromagnetic induction. The fundamentals of current sense transformer design patrick a. The basics of transformers by raising and lowering voltage levels, transformers make power systems versatile when alternating current ac prevailed over direct current dc in the late 1800s, one of the deciding factors was the need for voltage levels to be raised and lowered throughout the power system to make the system efficient and safe. Tape wound measuring and protection current transformers. Instrument transformer basic technical information and. Transformers are imperfect devices, and there will be differences between a transformer s design values, its test measurements, and its performance in circuit. An alternating voltage v p applied to the primary creates an alternating current i p through the primary. By increasing the number of secondary windings, the secondary current can be made much smaller than the current in the primary circuit. Their class accuracy as a function of ct load and of overcurrent is chosen according to the application. A transformer consists of two electrically isolated coils and operates on faradays principal of mutual induction, in which an emf is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents.
A transformer is an ac machine that steps up or steps down an alternating voltage or current. The current transformers provide current in the secondary circuit proportional to the current in the primary circuit. 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. This technology is commerically available, and offers amazing accuracy. A transformer changes the voltage level or current level on its input winding to another value on its output winding using a magnetic field. For pricing and availability of a specific ct model, give us a call at 614 8896152 or use the quote request form. When 100 amps is flowing in the primary conductor, it will result in 5 amps flowing in the secondary winding. Oct 02, 2016 learn the basics of differential protection of transformers. Transformer theory the action of a transformer is such that a timevarying ac voltage or current is transformed to a higher or lower value, as set by the transformer turns ratio.
The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings. In this article we will be learning about transformer basics and working principle. Primary of this transformer is connected across the phase and ground. Class px as defined in iec600441bs en600441 allows the manufacturer of a protection device to detail more specific current transformer.
If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were. The basics of power transformer theory, operation, testing. Dec 20, 2015 current transformer in general, large currents hundreds of amperes cannot be directly input to powermeters for measuring power. 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. Therefore, the current is passed through a ct current transformer prior to measurement. Burden, current transformer, 114 potential transformer, 3 bus protection, automatic reclosing, 292 by backup relays, 275, 291 circuitbreaker bypsasing, 292 combined power transformer and bus, 287 current differential with overcurrent relays, 278 current differential with overvoltage relays, 288 current. Recommendations are provided by manufacturers see application guides. A highvoltage current transformer may contain several cores, each with a secondary winding, for different purposes such as metering circuits, control, or protection. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors.
It buffers the meter from experiencing excessive torques that might be. Power transmission and distribution author page 242 200708. Knee point voltage of current transformer ps class. A transformer consists of two electrically isolated coils and operates on faradays principal of mutual induction. A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer. A distinction has to be made between a metering class and a protection class current transformer. The current flows first in one direc tion, stops, then reverses and flows in the other direc274 unit 74 transformers tion. Dec 30, 2018 a voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer.
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. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor. 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. Fundamentals of electricity there is a definite relationship between the three primary electrical characteristics. A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. The current transformer basically consists of an iron core upon which primary and secondary windings are wound. The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. Current transformers are used to measure, or monitor, the current in the lead of an ac power circuit. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. This types of transformer is of low cost with a disadvantage of environmental impact transformer oil is not environment friendly and possibility of fire hazard. Current transformers used in metering equipment for threephase. The electrical current flowing in a transformer is al ternating current.
On subtractive polarity transformers the h1 primary lead and the x1 secondary lead are on the same side of the transformer. Transformer basicsworking principle,construction,types. The power required for these tests on transformer is equal to the power loss occurring in the transformer. Dry type transformer aluminum and resin is used as the structural material in dry type transformer. In practical transformers the off load current in the primary is actually very low. 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. Current transformer basics and the current 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. Current transformers cts are used extensively throughout electrical distribution systems for measuring, metering, and protection applications.
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