Metering accuracy cts or relaying accuracy cts protection. The transformation from primary to secondary current requires a small amount of energy to. A protection current transformer ct must saturate sufficiently high to allow a relatively accurate measurement of the fault current by. The following illustrates the method to size a class x current transformer. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. Depending on their accuracy class, cts are divided into. Burden the ieee standard burdens for relaying are 1, 2, 4, and 8 ohms, all with an impedance angle of.
The accuracy class for current transformers is measured in accordance with the iec61869 standard. Current transformer accuracy is specified by the standards ieee std c57. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. A ct can have dual ratings and be used in metering or protection applications if both ratings are listed on the nameplate. C100 class for example, a c100 ct class means that the ct is capable of passing 20.
Current transformer basics and the current transformer. High accuracy extended range current transformers can increase utility revenue through improved metering. A typical egauge installation will measure the amperage of multiple conductors via multiple ct sensors. The secondary winding is connected to the measuring instrument s or relay. Like the power transformer, the current transformer also contains a primary and a secondary winding. The x indicates that they do not belong to any standardized class. The burden of a current transformer is indicated in its nameplate. There are two different ways to represent this, depending if the current transformer is to be use for protection or metering.
Lower or specialized ratios may be obtained by adding primary and secondary turns as shown on page 52. Primary current is calculated from the following equation. The magnetized iron core induces a voltage in the secondary coils. Courtesy siemens current transformers cts are used to convert highlevel currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment. Select the proper window size or slightly larger to fit your primary conductor. It is not always satisfactory for revenue and precision metering. Selecting a current transformer ct if the current ratio, burden and class are selected, the current transformer ct has been specified basically. This allows them to be used in combination with measurement equipment. 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 basics of current transformers nk technologies. Aug 14, 2018 current transformer application considerations. 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.
Pdf abb instrument transformers current transformer. This occurs on systems with variable currents such as wind or solar power generation. Rating factor rf is a term, which applies to a current transformer. Determine that the primary range of the current transformer encompasses the application requirement. With respect to ansi cts, some of the major details that one would look for in the ct are. High voltage sf6 current transformers image source. Pdf abb instrument transformers current transformer theory. They are defined by their ratio, power and accuracy class. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating.
Their class accuracy as a function of ct load and of overcurrent is chosen according to the application. Apr 17, 2012 the transformer class relates to its accuracy. Class tps low leakage current transformer performance is defined by the secondary excitation characteristic and turns ratio limits. 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 iron core inside the transformer is magnetized. How to select a current transformer the steps to select a transformer from our catalogue are as follows. It buffers the meter from experiencing excessive torques that might be. They are customized transformers therefore it is necessary to specify the individual characteristics of the current transformers such as the turns ratio, rated primary and secondary current, voltage knee point, the maximum magnetizing current at. Magnetic flux through a surface of area a placed in a uniform magnetic field b is defined as cos. Maximum secondary current should not exceed 20 times the rated current 100a for 5a rated ct for ansi class c ct. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or monitor the current levels. Cts are required because standard current measuring devices are not configured to measu.
Current transformer saturation voltage disturbance. Nov 15, 2019 the basic principle of the current transformer is the same as that of the power transformer. Back to the basics current testing valence electrical. This protects connected metering devices from overloading in the presence of fault level current flows. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. They are specified by current ratio, window size, and ct class. In the past, current transformers were designed to accurately measure down to 10% of the current rating on the name plate of the transformer. The new current transformer and metering speci cations like the iec618692 table above now require accuracies from 0. This guide is intended to help you select the right quantity and type of current transformers cts needed for an egauge installation. If the secondary circuit is closed, a current will flow through the secondary circuit in proportion to the ct ratio.
Dimensions dimensions of casemm nsq30 nsq35 30a3000a 5a,1a bs7626,iec441 0. Transformer with the primary current be turned twice which help to keep the samerated turns ratio1. Class px as defined in iec600441bsen600441 allows the manufacturer of a protection device to detail more specific requirements of the current transformer so. There are four typical types of current transformers. Definitions current transformers are available from ge multilin for use as phase current inputs with motor protection relays. For the encapsulated type current transformer, the windings are enclosed in a. Of course additional requirements like type, frequency, isolation level, maximal short circuit current and environmental conditions should also be specified. Selecting current transformers janitza electronics. Class x cts are current transformers which are used for special applications. Previously i have put a few posts in the site, going into more detail on current transformer classes. We always recommend current transformers with the same precision class for the umg measuring devices.
Current transformer split core current transformer. If the application requirements are not fully defined, refer to. In addition the classification of protection current transformers indicates accuracy class. The ip5 a ratio current transformer delivers at the secondary a current is. The ip5 a ratio current transformer delivers at the secondary a current is of 0 to 5 a that is proportional to the current measured at the primary ip. Burden the ieee standard burdens for relaying are 1, 2, 4, and 8 ohms, all with an impedance angle of 60 degrees. The transformation from primary to secondary current requires a. Class x current transformer is use in conjunction with high impedance circulating current differential protection relay, eg restricted earth fault relay.
The letter distinguishes what application the ct is rated for. The name instrument transformer is a general classification applied to current and voltage devices used to change currents and voltages from one magnitude to. These cts output is then further fed to protection relay which in turn gives tripping comma. It is possible to use a 600v class toroidal type current transformer in higher voltage class circuits if the primary conductor is fully insulated and shielded or the current transformer is placed on a bushing that is designed and rated for the circuit voltage class. The primary winding is connected in series with the load and carries the load current to be measured. Instrument transformers scale the large values of voltage or current to small. Operating voltage 720v maximuminsulation test voltage 3kv ac for 1 minutecompliance standard ve404. Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing operational data and. Their class accuracy as a function of ct load and of overcurrent is chosen according to the application a protection current transformer ct must saturate sufficiently high to allow a relatively accurate measurement of the fault current by the protection whose operating threshold can be very high. Electromagnetic induction and alternating current class 12. Current transformers, along with voltage or potential transformers, are instrument transformers.
Protection class cts are used to sense the fault current under fault conditions. The basic principle of the current transformer is the same as that of the power transformer. Rated highest equipment voltage shall be given v or kv. Determine the usual application of current transformer that best suits the application for which it is intended. A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system. The following table describes the characteristics of the ansi class transformer. It consists in controlling the right adaptation of the ct on. Validation of measurement solution according to accuracy class. A current transformer is an instrument transformer in which the current ratio is within the specified limit. The air gap between the spout and the case of the current transformer the 600 volt class insulation of the current transformer this series combination of various insulating mediums provides an insulating system that goes through the dielectric testing along with the entire switchgear system. Whenever an alternating current flows through the primary winding, alternating magnetic flux is produced, which then induces alternating current in the.
Supreme accuracy current transformer super accuracy for 0. The application of 600 volt class current transformers in. A transformer is an electrical device for converting an alternating current at low voltage into that at high voltage or vice versa. It produces a current in its secondary which is proportional to the current in its primary.
A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. In its application to a current transformer, it is the number representing the amount by which the primary load current may be increased over its nameplate rating without exceeding the allowable temperature rise. For example, the potential transformer mentioned above having a rating of 480. Abb instrument transformers current transformer theory technical level training course 201203. When extremely accurate measurement is required, or when revenue is generated from a power meter, ansi class current transformers are generally selected. The ct sensors are installed around the current carrying conductors of the load or generation source you. Ct primary current rating should be equal to or greater than the maximum expected current for the associated breakers or transformers.
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