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