Jun 02, 2015 as an example, a typical 100 to 5a current transformer has a nominal ratio of single primary turn to 20 secondary turns or 1. 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 transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. The various types of transformer shown in the figure above are explained in detail below. This behavior is used to calculate the value of the burden resistor for an optimum currenttovoltage relationship. Protection relaysrelay panels earth fault protection bussing type, oilimmersed ct in power transformer.
Though the magnitude of inrush current may be less compared to short circuit current, the frequency and duration of inrush current is more frequent, leading. Then i created another transformer with more turns added turns proportionally to get desired secondary current at 1a on primary. 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. Circulating current in thick, medium and thin laminations figure 2. Transformer only stepup or stepdown the level of ac voltage or ac current. As an example, a typical 100 to 5a current transformer has a nominal ratio of single primary turn to 20 secondary turns or 1. Current transformers perform the following functions. A current transformer, abbreviated ct, is a type of transformer used to measure the current of a given circuit. Inrush current in transformer often gets less importance compared to other faultseffects. Magnetization and primary injection test of current transformer for protective relaying jacob tsado 8 current transformers. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings. 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. Equivalent current transformer circuit x 1 primary leakage reactance r 1 primary winding resistance x 2 secondary leakage reactance z 0 magnetizing impedance r 2 secondary winding resistance z b secondary load note. 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.
Though the magnitude of inrush current may be less compared to short circuit current, the frequency and duration of inrush current is more frequent, leading to more adverse effects compared to other. 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. A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. If the primary current of the current transformer becomes a the secondary current will still be 1. Current transformer basics and the current transformer. 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. Current transformer accuracy is specified by the standards ieee std c57.
What is the working principle of a current transformer. Effects of a current transformers magnetizing current on. This transformer has a 500 a1 a current ratio and a rated output load of 1 va, which corresponds to a nominal burden of 1. It may occur whenever the transformer is energised. Different types of transformers and their applications. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. Indoor current transformer instrument transformers product features. This type of transformer is categorized on the basis of a number of turns in the primary and secondary windings and the induced emf. The classification of protection current transformers follows the following. Current transformers,voltage transformers and instrument. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to.
Pdf effects of a current transformers magnetizing current. 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. Coupled with an ammeter, cts are used to protect the main lines of the power grid, by measuring highvoltage electrical current and preventing damage to sensitive equipment. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. 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. Magnetization and primary injection test of current. The constant current transformer, usually called a regulator, has a movable secondary winding that automatically changes position to provide constant current for any load within its fullload rating. The basics of current transformers nk technologies. These are the most commonly used transformer types for all the applications. Transformer operates when both circuits take effect of mutual induction. Apr 16, 2011 current transformers principle of operation of ct.
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. After all this i had to make sure that core is not saturating i was increasing current until secondary current was not rising proportionally to primary current. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the. The constantcurrent transformer, usually called a regulator, has a movable secondary winding that automatically changes position to provide constant current for any load within its fullload rating. Primary current range from 5 amp to 5000 amp with secondary current 5a, 1a, or 01. Current transformer terminal block with short circuit devices.
Types of transformer different types of transformer. Energy is stored in the magnetic field of the transformer when the pulse is applied. Cts current transformers pts potential transformers. Eddy current and current induced by the external magnetic field figure 2. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Apr 25, 2017 the current transformer the current transformer c. You will also get clear idea about importance of burden in current transformer factors which affect the errors and what is difference between wound type current transformer, window or. These comprise a crossdisconnect terminal with measuring and test equipment, insulated bridges for grounding and short circuiting of the current transformer. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. Basics, maintenance, and diagnostics vii contents continued page 4. Current transformer design guide an effective design of a ring type c. Transformer basicsworking principle,construction,types. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. Figure 5 shows the result of the curve fitting for core 1, considering the y axis to be the current and x axis to be the voltage, the following expression give the standard.
The fundamentals of current sense transformer design patrick a. Selecting current transformers janitza electronics. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. When secondary current vector lags the primary current vector. 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.
A current transformer modeling article pdf available in compel international journal of computations and mathematics in electrical 214. Siemens industry catalog energy lowvoltage power distribution lowvoltage components sentron protection, switching, measuring and monitoring devices measuring devices and power monitoring measuring devices accessories 4nc current transformers for measuring purposes. Transformer cant stepup or stepdown the level of dc voltage or dc current. Whilst measurement current transformers are intended to reach saturation point as quickly as possible once they exceed their operational current range expressed by the overcurrent factor fs in order to avoid an increase in the secondary current with a fault e. Here we discuss different types of transformers are the step up and step down transformer, distribution transformer, potential transformer, power transformer, 1. The balance point between coil weight and magnetic force may be adjusted to provide the desired output current. Due to this, it can easily recover from dc magnetization, even with a low ac measured current.
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