Current transformer definition is a transformer whose primary carries the whole of an alternating current to be measured or controlled and whose secondary is connected to a measuring or control device. Practically, it offers constructive assistance for technicians who have reached a dead end. One of the most typical representations of the current sense transformer model is the linear model, comprising an ideal transformer, magnetizing inductance and distributed winding capacitance. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. This is connected in series with the line carrying high current. They are fully insulated from the high operating voltage of the system and are usually bolted to the currentcarrying device. Turns ratio tests optional test this test is to ensure the turns ratio of ct at all taps.
As the current transformer has two windings, in some cases it is confusing about the load of it. Definitions current transformers are available from ge multilin for use as phase current inputs with motor protection relays. Pdf method of current transformer metrological properties. 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. It is most commonly used to increase step up or decrease step down voltage levels between circuits. Cts current transformers pts potential transformers. Accuracy limit factor of current transformer electrical. Definition of the ct kneepoint voltage iec 600441 class px formerly british standard bs3938. Cts that transform current isolate the high voltage.
A buckboost transformer is classified as an autotransformer. Current transformer article about current transformer by. In some, the bar carrying high current may act as a primary. 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.
But the fact should be clear that the current of primary circuit is the load of a current transformer. The primary current of minimum of 25% rated primary current to be injected on primary side of ct with secondaries shorted and the secondary current can be measured and recorded for all cores. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. A transformer constructed of three coils, each having its own core, magnetically independent from each other, and contained in one enclosure to function as a three phase transformer. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. A current transformer, abbreviated ct, is a type of transformer used to measure the current of a given circuit. 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.
The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. Exciting current, im, may be defined as the portion of the primary current that satisfies the hysteresis and eddy current losses of the core. A general overview of abbs current and voltage transformers. A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits. Jan 25, 2012 the burden on the current transformer is imposed by the connected device and the impedance of the connecting wires which connect it. Scribd is the worlds largest social reading and publishing site. 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. The original magnitude can be determined by just multiplying the result with the transformation ratio. As a current passes through the primary winding it induces a magnetic flux in the steel core. Instrument transformer basic technical information and. Current transformer ct construction and working principle. 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. Current transformer saturation voltage disturbance. The 66kv current transformer is outdoor,single phase and oil immersed dead tank type current transformer,which is used for metering and relay protection of 66kv,50or 60hz system.
The primary and secondary current of the current transformers are proportional to each other. 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. Apr 16, 2011 current transformers principle of operation of ct. Cts are required because standard current measuring devices are not configured to measu. 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.
Current transformer basics and the current transformer. Effects of a current transformers magnetizing current on. Oct 08, 2016 calculation of cts if the current transformer has primary to secondary current ratio of 100. First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented.
C electrical power from one voltage to another voltage keeping the frequency same by. 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. Exploring current transformer applications power electronics. 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. 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. For example, the potential transformer mentioned above having a rating of 480. Ieee standard terminology for power and distribution. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1. 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. 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. 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 importance of instrument transformer tests is often underestimated. 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 flux flows through the core and induces a current on the secondary winding proportional to the ratio of turns on the primary to the secondary.
C100 class for example, a c100 ct class means that the ct is capable of passing 20. Pdf transformer is a static device which transforms a. Pdf current transformer ct is one of the important element of power system. Calculation of the current transformer accuracy limit factor. China current transformer, voltage transformer, potential. The knowledge of amps in per amps out is the basis for use of the current transformer. Transformers do what their name impliesthey transform voltages from one value to another the term voltage is used rather than emf, because transformers have internal resistance. Such specially constructed transformers with accurate turns ratio are called as instrument transformers. Scope this document describes the calculation of the actual accuracy limit factor f a for protectiontype p current transformers ct. Using a reactance to represent the magnetizing leg of a ct, as shown in figure 1, is a useful visual concept.
The terms rated and base current in this paper and referenced standards, however, refer to the winding. Current transformer a current transformer produces a reduced current accurately proportional to the current in the circuitis which is too high to apply directly to metering and protecting instruments. A current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is. May 17, 2020 a transformer is defined as a passive electrical device that transfers electrical energy from one circuit to another through the process of electromagnetic induction. Instrument transformers are not a perfect device and incur losses. You can transform the voltage or current down with a transformer whose turns ratio is accurately known, then measuring the stepped down magnitude with a normal range instrument. Current transformers schneider electric documentsdownloads. For example, many cell phones, laptops, video games, and power tools and small appliances have a transformer built into their plugin unit like that in figure 1 that changes 120 v or 240 v ac into whatever. Measuring instruments are connected to a secondary winding. A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value. Transformer an electrical device, without continuously moving parts, which, by electromagnetic induction, transforms energy from one or more circuits to other circuits at the same frequency, usually with changed values of voltage and current. Quite often, the end users provide the primary with a wire through the center of the transformer.
Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments. This is a plot of the ct secondary current against secondary voltage. With current transformers, the user must know the current ratio to use the transformer. This permits computation of current on all sides of a transformer for a through fault e. There are four typical types of current transformers. The burden of a current transformer is the value of the load connected across. Pdf during transformation of power frequency currents inductive current transformers cts are. Ct burden, knee point voltage, core saturation details of. The main purpose of a current transformer is to translate the primary current in a high voltage power system to single level that can be handled by delicate electromechanical or electronic device. Transformer definition, a person or thing that transforms. A current transformer is a device which is used for the transformation of current at a higher value to a lower value with respect to the earth potential. The family of curves below describe the excitation characteristics for the 6005 multi ratio bushing current transformer shown on the previous sheet. A typical connection diagram of a current transformer is shown in figure below.
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. The primary current of minimum of 25% rated primary current to be injected on primary side of ct with secondaries shorted and the secondary current can be measured and recorded for all cores limits. Composite error is defined in the iec 600441 standard for. Bartype current transformer this type of current transformer uses the actual cable or busbar of the main circuit as the primary winding, which is equivalent to a single turn. 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. Current transformers have a primary winding to which the current to be measured is fed. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. Power transmission and distribution author page 244 200708. Current transformer, standard for steadystate performance. They are specified by current ratio, window size, and ct class. The circuit connection shall be made as shown figure 4.
Abb instrument transformers current transformer theory technical level training course 201203. A varying current in any one coil of the transformer produces a varying magnetic flux in the transformers core, which induces a varying electromotive force across any other coils wound around the same core. Current transformers are the very important aspect of. Pdf abb instrument transformers current transformer. The classification of protection current transformers follows the following. It produces a current in its secondary which is proportional to the current in its primary. Power transmission and distribution author page 247 200708. Current transformers play an important role in the monitoring and protection of electrical power systems.
A current transformer is a device which produces an alternating current in its secondary which is proportional to the alternating current in its primary. Definition of the ct kneepoint voltage iec 600441 class px. A transformer changes the voltage level or current level on its input winding to another value on its output winding using a magnetic field. A current transformer ct is a type of transformer that is used to reduce or multiply an. The current transformer is used for measuring the high voltage. Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating ammeter i. This characteristics are current transformer burden, knee point voltage, saturation of current transformer. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. The ct ratio is the ratio of primary current input to secondary current output at full load. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Knee point voltage of current transformer electrical. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit.
Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or monitor the current levels. A current transformer ct basically has a primary coil of one or more turns of heavy crosssectional area. As protection engineers we consider the rated or base current as the transformer rated line current. The va load of the device can be obtained from the datasheet. The ip5 a ratio current transformer delivers at the secondary a current is of 0.
A buckboost transformer provides a means of raising boosting or lowering bucking a supply line voltage by a small amount usually no more than 20 percent. Current transformers cts can be used for monitoring current or for transforming primary current into reduced secondary current used for meters, relays, control equipment and other instruments. Transformer tests per nema st20, routine transformer production tests are performed on each. The basics of current transformers nk technologies. Current transformer definition of current transformer by. Current transformers, along with voltage or potential transformers, are instrument transformers. Its secondary winding then provides a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions.
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