Metering accuracy cts or relaying accuracy cts protection. 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. Similarly to power meters, standard iec 600441 defines the accuracy classes of cts as shown below. Current transformers can be manufactured according to any accuracy class defined by international standards. Depending on their accuracy class, cts are divided into. Current transformers technical datasheet schneider electric is the global specialist in energy management with the most complete power. Current transformer selection the selection of a current transformer includes factors such as range of current being measured, turns ratio and the class. Selecting current transformers janitza electronics. The community should debate the benefits of tightening the constraints of each test to come to a. The accuracy class for current transformers is measured in accordance with the iec61869 standard. Current transformer ct selection guide january 18, 2017 contents. Hence, it is essential to inspect and calibrate those instrument transformers at regular intervals. Hence, it is imperative that the choice of ct is made with full knowledge of the application and the number of relays and meters which are to be connected to it.
In reality, however, it is not possible to neglect the excitation. Validation of measurement solution according to accuracy class it consists in controlling the right adaptation of the ct on the accuracy class aspect. 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. Cts are expected to provide the secondary output current based upon their accuracy class. The essentials of current transformers in power circuits. Accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. Current transformers mak7430 for currents from 100 a to 800 a.
Accuracy class accuracy class describes the performance characteristics of a ct and the maximum burden allowable on the cts secondary. Threephase ac current measurement using current transformer. This means that the ct must be accurate to within 0. Instrument transformers scale the large values of voltage or current to small. Pdf accuracy of current transformer with current errors at.
They are defined by their ratio, power and accuracy class. The resultant accuracy depends on these parameters. Rated ratio accuracy rating is a number which is the rated ratio expressed as a percent. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. For multi ratio current transformers, if only one accuracy rating is assigned, it shall apply to all ratios. Voltage transformers voltage transformers are connected across the points at which the voltage. Supreme accuracy current transformer super accuracy for 0. For protective current transformers, the accuracy class. Before understanding accuracy limit factor and instrument security factor of ct or current transformer we must try to understand different technical parameters of instrument transformer i. Unlike a current transformer, the rcti and rcti3ph require a load impedance of 10k.
Instrument transformer theory and testing steve hudson, pe. Cts are categorized as protection ct, special protection ct and measuring ct. A class 5p15 protection transformer indicates that it has an accuracy rating of 1% that it does not become saturated until the primary current reaches. Understanding current transformer accuracy classes victor. Socomec current transformers deliver a standard current to the secondary that is proportional. Rs isolsecs englishspeaking engineers put their expertise in transformers at your disposal to help you determine the most appropriate transformer. All cts are built to ansi standards and have an accuracy class to outline what effect the ct losses have on the ct secondary current under normal conditions. Nov 09, 2017 a ct is primarily used for three purposes in power system.
The new current transformer and metering speci cations like the iec618692 table above now require accuracies from 0. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. Three metering accuracy classes are defined for instrument transformers. Accuracy limit factor of current transformer electrical. A protective current transformer type 5p10 has, for example, the accuracy class 5p and the rated accuracy limit factor 10. Technical parameters of instrument transformer following are some of the normally used terms associated with instrument transformer, that. It produces a current in its secondary which is proportional to the current in its primary. As per vde0410, the measuring class of measuring instruments has to be class 1. If a current transformer is not properly sized based upon secondary loop burden, it may result in a decrease in ct secondary current. On the name plate of current transformer, rated accuracy limit factor is indicated following the corresponding output and accuracy class. Like a current transformer, the rct1a is a current output device. Current transformers 1505 a measurement sensors of 0. These are used where the required characteristics of the current transformers cannot be conveniently expressed in terms of the ordinary protection current transformers like class 5p, 10p and 15p current transformers.
Accuracy of a current transformer the percentage of error, produced in a transformer, is established by iec600441. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. This transformer ensures stable revenue accuracy over a broad current range for solidstate metering, and is best suited for power generation facilities. Class current transformers type cxm current transformers provide best accuracy of 0. 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 meter or relay. Indooroutdoor current transformer elk ct0 instrument. Accuracy limit factor and instrument security factor of. Instrument transformers for metering purposes must have a high degree of accuracy up to class 0. In the past, current transformers were designed to accurately measure down to 10% of the current rating on the name plate of the transformer. Depending on their accuracy class, cts are divided into metering accuracy cts or. There are two different ways to represent this, depending if the current transformer is to be use for protection or metering. For example, on name plate of ct, you may find 30 va, 5p10 or 305p10. Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output.
Accuracy class of current transformer electrical concepts. We always recommend current transformers with the same precision class for the umg measuring devices. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and. Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays.
Current transformer design guide an effective design of a ring type c. It is designed to be compatible with equipment traditionally connected to instrument current transformers. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Selecting ansi class metering current transformers 142 h b0. The new standard iec618692 and old standard iec600441 states that current transformers used at energy monitoring equipment and in energy usage monitoring systems in order to obtain higher accuracies that will soon be considered the new industry standard. The range benefits include ratio ratings from 10a to 5000a, accuracy up to class 0. Current transformers, along with voltage or potential transformers, are instrument transformers. High accuracy extended range current transformers can increase utility revenue through improved metering. For metering and indication purpose, accuracy of ratio, between primary and secondary currents are essential within normal working range. 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. Current transformers elkct0 english pdf manual current transformers elkct0. May 12, 2017 each current transformer has a secondary burden when connected in a relay or metering circuit.
Understanding current transformer accuracy classes. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. Lt6ct1001 current transformer tesys t lt6ct 1001 a accuracy. 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. Current transformer accuracy is specified by the standards ieee std c57. The basics of current transformers nk technologies. This occurs on systems with variable currents such as wind or solar power generation. Current transformer white paper the basics of current transformers ratio.
Find current transformer accuracy class related suppliers, manufacturers, products and specifications on globalspec a trusted source of current transformer accuracy class information. 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. Jul 11, 2019 before understanding accuracy limit factor and instrument security factor of ct or current transformer we must try to understand different technical parameters of instrument transformer i. Core crosssection calculation and accuracy class curves of instrument current transformer yusuf kursat ekiz1, nursel akcam2 1aselsan inc. Apr 18, 2020 ct accuracy class or current transformer class. Current transformers,voltage transformers and instrument. Instrument transformer basic technical information and. Threephase ac current measurement using current transformer reference design 4 system design theory the tida00753 ti design has been designed to meet high accuracy demands when measuring wide ac current ranges for motors. Current transformers occupy a vital part in the measurement and protection scheme of any electric installation. Ctsb0816 splitcore current transformer accuracy class 0.
The new and old standards are essentially identical, but iec 618692 consolidated two parts of the older standard. Current transformer testing swp check this is the latest process zone version before use. As such the instrument transformers must be sized correctly the right class. A current transformer chosen for protection purpose are defined by protection class p. Technical parameters of instrument transformer following are some of the normally used terms associated with instrument transformer. 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. In the case of metering ct s, accuracy class is typically, 0. Definitions current transformers are available from ge multilin for use as phase current inputs with motor protection relays. Specify whether it is an instrument or relay transformerfrom 10 amps to 0 amps this is often evident from the given class specification a minimum specification for a relay transformer with a ratio of a to 5a, burden 30va, accuracy class 5%, overcurrent factor 20 20 times the rated current and central hole of min. Calculation of the current transformer accuracy limit factor. National grid current transformers for protection and general use technical specification ts 3. Class ps transformers are the special purpose protective current transformers. The ctsb0816 is a splitcore current transformer with an opening of 80mm3. Actual accuracy limit factor in operation is higher as the ct is normally underburdened.
Percent transformer correction factor with a wattmeter or watthour meter, the. The metering ct has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated. As seen from the tables, under full load conditions both classes have similar accuracy. A current transformer with a wrong burden rating read more. They are specified by current ratio, window size, and ct class. A high accuracy standard for electricity meters a new standard accuracy class conditions and influences load, pf, temperature, etc. The accuracy class requirements as applied to a specific model of a current transformer may limit the rated primary current ranges that can meet those accuracy requirements. When we talk about metering we mean energy tariff calculation. Furthermore, most relaying applications can tolerate what for metering. Introduction the rated accuracy limit factor f n is the ratio of the rated accuracy limit primary current to the rated primary current. The range is available in both 5a and 1a versions, with an integral terminal. Subjecting a current transformer to load currents above the manufacturers rated current specification will saturate the core.
Iec 618691, iec 618692, iec 600446 protective types for transient performance, ansiieee c57. Iec 618692 defines the new current transformer accuracy classes intended to replace the old standard, iec600441 note that iec 618691 is designed for instrument transformers. The design uses current transformers ct, which have a very high turns ratio and are used when. Previously i have put a few posts in the site, going into more detail on current transformer classes. The accuracy class of a current transformer depends on the transformers apparent power va and the consumption of the entire measurement line. Page 1 of 9 standard work practice sp0507 ver 5 ergon energy corporation limited abn 50 087 646 062. Lt6ct1001 current transformer tesys t lt6ct 1001 a. Difference between metering and protection current. 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. Current transformers features and benefits the as series of moulded case current transformers offers a wide range of current ratings, apertures and case sizes to suit every application. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance. Jan 24, 2017 below 60 amps the accuracy is not guaranteed. Need advice on selecting an instrument transformer. The accuracy performance of an ac current transformer s primary current to secondary signal transformation where the secondary is either a current or voltage is stated as accuracy class.
601 941 1433 1362 480 154 1398 471 1198 1230 1402 306 414 663 643 871 1055 127 598 1179 232 1143 20 1570 1216 427 841 1150 22 336 1287 1218 969