Scribd is the worlds largest social reading and publishing site. Flux of each limb uses the other two limbs for its return path with the three magnetic fluxs in the core generated by the line voltages differing in timephase by 120 degrees. Theory of transformer on load, with resistive winding, but no leakage reactance. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to.
The core material for wound type is nickeliron alloy or oriented electrical steel. B2 current and voltage transformers schneider electric. 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. Its secondary winding then provides a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions. A selection guide is provided in this catalog to aid in selecting the type of ct for your. Indoor current transformer cable kola current transformers type kola indoor dry type, cast resin insulated are designed for rated voltages up to 0.
Cable is passing through the centre of ct and in that way forms the primary winding. Current transformers contents omega ring type omega 20 tw cts omega 20 tw cts sifam tinsleys omega tw series round type cts are housed in robust case with classical. A current transformer is a device which produces an alternating current in its secondary which is proportional to the alternating current in its primary. A comprehensive range of measuring ring current transformers for installation where reliability, accuracy and quality are required. Lowvoltage single ratio metering current transformers are either a ring type or plastic molded case. The working procedure on how transformers work types.
A complete switchgear means, assembly of required switching devices, protection device as well as metering device. Now we will proceed one further step toward a more practical aspect of a transformer. When the circuit burden gets too large, the ct secondary voltage becomes distorted. Ring type current transformer protection class sensor 56 current transformers which are well matched and operating below saturation, will deliver no current to the earth fault relay, since 3phase currents sum to zero. Typically, the wound type is insulated to only 600 volts. The shell type transformer has a double magnetic circuit.
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. Therefore, the current transformer does the current conversion from high currents to a measurable range of currents. 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. Do not use current transformers opened between secondary output terminals. The core has three limbs and both the winding are placed on the central limbs. Current transformer ct construction and working principle. Core balance current transformer or cbct is a ring type current transformer through center of which a three core cable or three single core cables of three phase system passes. Current transformers, along with voltage or potential transformers, are instrument transformers. A current transformer ct is a type of transformer that is used to measure alternating current ac.
Before using zerophase current transformers do not drop or apply any other mechanical stress, as such stresses may change performance characteristics. It is a static device, totally enclosed and usually oil immersed, and therefore the fault occurs on them are usually rare. Consider the case of charging up a capacitor c which is connected to very long wires. Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. A current clamp uses a current transformer with a split core that can be easily wrapped around a conductor in a circuit. The core is having two rectangular limbs and the coils are placed on both the limbs in the core type. A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. But the effect of even a rare fault may be very serious for a power transformer. Medium voltage mv indoor current transformer ct cable. Then to summarise, the current transformer, ct is a type of instrument transformer used to convert a primary current into a secondary current through a magnetic medium. A comprehensive range of measuring ring current transformers for installation where reliability. Here is an example in which current does not gives rise to the magnetic field.
Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. The wound type provides excellent performance under a wide operating range. Current sensors zct, zerophase current transformers. Ring core cts are available for measuring currents from 50 to 5000. Theory of transformer on load and no load operation. Current transformers cts can be used for monitoring current or for. The current transformer presents a negligible load to the primary circuit. Rmu of different voltage mostly 12 kv and 24 kv and current ratings are.
The impedance of the secondary circuit in ohms and power factor. 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 equivalent circuit is the same as for figure a. A current transformer ct is a type of transformer that is used to reduce or multiply an. Since the through type current transformer must be capable of operating under over saturated conditions short circuits, and their accuracy at lower ratings is poor, most of the bushing transformer are used at high voltages and low currents. For the encapsulated type current transformer, the windings are enclosed in a. Construction high grade silicon iron cores are carefully selected, then insulated and protected by. Ring main unit,rmu is a compact, enclosed and sealed type of switchgear used for medium voltage power distribution. Some commonly used shapes for the stampings of window type current transformers are shown in the figure below. The current transformer function is different than that of a voltage transformer. 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 threelimb core type threephase transformer is the most common method of threephase transformer construction allowing the phases to be magnetically linked. The specific application of current transformers involves in various considerations such as type of mechanical construction, ratio of primary to secondary currents, type of insulation oil or dry type, thermal conditions.
Faradays work in the 19th century showed that a changing current in a conductor e. From the symmetry it is easy to see that an application of amperes law will produce. Ring main unit rmu introduction electrical concepts. Application suitable for measuring or protective applications, available in standard class 1, class 0. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. A range of ring type tape wrapped current transformers suitable for primary currents from 1a 0a with 1a or 5a secondary output. Since the load current passes through primary windings in the ct, screw terminals are provided for the load and secondary conductors. Instrument transformers are also used with protective relays for protection of power system basic function of instrument transformers is to step down the ac system voltage and current. Before installing the secondary winding on the core it is insulated with the help of end.
Core balance current transformer electrical concepts. The essentials of current transformers in power circuits. A transformer uses the principles of electromagnetism to change one a. This makes the replacement of instrument transformer. Medium voltage current transformers cable type kola are suitable for the measurement of the sum of threephase currents in a 3phase cable. The current transformers can be ring type or window type. Basics, maintenance, and diagnostics vii contents continued page 4. Ct sensors an introduction learn openenergymonitor. There are four typical types of current transformers. Many current transformers have a ringshaped core, sometimes built up from. This is a common method used in portable current measuring instruments but permanent installations use more economical types of current transformer. Exact burden calculations are usually not necessary unless the ct ratio is below. Current transformers ct are used in high voltage hv and medium voltage mv 1 installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. Ring type is the most usual type of corebalance current transformer cbct.
The current transformer is designed to connect in series with the line to transform the line current to the. Shell type transformers are the most popular and efficient type of transformers. Whenever the values of voltage or current in a power circuit are too high to permit. The core of a ct is normally ringtype, with the secondary evenly distributed to minimize the secondary flux losses. Current transformer basics and the current transformer. The primary winding can consist merely of the primary current conductor passing once through an aperture in the current transformer core window or bar type, or it may consist of two or more turns wound on the core together with the secondary winding wound type. An alternating voltage v p applied to the primary creates an alternating current i p through the primary. In reality, however, it is not possible to neglect the excitation. Ring type current transformer protection class sensor. It produces a current in its secondary which is proportional to the current in its primary. Differential protection of a transformer the transformer is one of the major equipment in power system. A range of ring type current transformers suitable for primary currents from 1a 5000a with 1a or 5a secondaries. 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.
The secondary of the current transformer is portable. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Window type bar type wound x 1 x2 x3x4x5 20 10 50 40 600. Typically current transformers consist of a silicon steel ring core wound with many. 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 standard range of measuring and protective cts is available in tape wrapped style. Application suitable for measuring or protective applications with accuracies of class 0.
Instrument transformers are used in ac system for measurement of electrical quantities i. The primary of the current transformers is considered as the line or cable or busbar of the load circuit. Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Transformer with the primary current be turned twice which help to keep the samerated turns ratio1.
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