The primary purpose of this guide is to give the reader a basic understanding of how to apply instrument transformers in a practical way while observing good engineering practice. Validation of measurement solution according to accuracy class it consists in controlling the right adaptation of the ct on the accuracy class aspect. This means the ct can supply the rated current to a burden of 1. In the past, current transformers were designed to accurately measure down to 10% of the current rating on the name plate of the transformer. Currenttransformer burden all ct accuracy considerations require knowledge of the ct burden. There are two different ways to represent this, depending if the current transformer is to be use for protection or metering. If the burden of a current transformer for metering is much higher than the load, instruments and devices could be damaged if short circuit occurs somewhere.
The accuracy class is only applicable if the total load including cable losses is approximately equal to the current transformer burden. 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. Accuracy class describes the performance characteristics of a ct and the maximum burden allowable on the cts secondary. Accuracy class for metering application is provided as an example 0. Instrument transformers application guide abb group. After saturation occurs, the current transformer output will be distorted and the performance of the relay protection system will be affected. It explains how the system accuracy meter with cts can be much worse than just the meter accuracy or even the naive addition of the meter and ct accuracies a 0.
Supreme accuracy current transformer super accuracy for 0. In addition the classification of protection current transformers indicates accuracy class. Abb instrument transformers current transformer theory technical level training course 201203. Meter, ct, and system accuracy accuct current transformer. Apr 27, 2014 the second part of the ct accuracy class is a letter that designates the application for which the ct is rated.
Class ps transformers are the special purpose protective current transformers. The accuracy class of a current transformer depends on the transformers apparent power va and the consumption of the entire measurement line. Like all transformers, a current transformer can only transform a finite amount of energy. First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented.
A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Current transformer burden all ct accuracy considerations require knowledge of the ct burden. Similarly to power meters, standard iec 600441 defines the accuracy classes of cts as shown below. Class current transformers type cxm current transformers provide best accuracy of 0. 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. It is not always satisfactory for revenue and precision metering. In more practical terms, it guarantees a 10% accuracy provided the voltage across. A current transformer chosen for protection purpose are defined by protection class p. 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. The range is available in both 5a and 1a versions, with an integral terminal.
As seen from the tables, under full load conditions both classes have similar accuracy. Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output. Abaout ttt eocneccbivbycosnabcyaecntecagcnl mr transformers are used to accurately measure high alternating primary currents, converting the primary current into a. In reality, however, it is not possible to neglect the excitation. Calculation of the current transformer accuracy limit factor. The range benefits include ratio ratings from 10a to 5000a, accuracy up to class 0. Instrument transformers will operate successfully under many atmospheric conditions and transients, and switching overvoltages.
The basics of current transformers nk technologies. 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. C100 class for example, a c100 ct class means that the ct is capable of passing 20. Lt6ct2001 current transformer tesys t lt6ct 2001 a. 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. Metering current transformers are designed to be within quite accurate values up to say two x rated current, and down to current transformer will get a heavily saturated waveform during fault conditions and it wont work properly. This protects connected metering devices from overloading in the presence of fault level current flows. The burden of a current transformer is indicated in its nameplate. A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. Ctsb0816 splitcore current transformer accuracy class 0.
Their class accuracy as a function of ct load and of overcurrent is chosen according to the application. Typical burden of modern protection and metering equipment is very low in range 510 va and considering that accuracy class is fulfilled down to 25% only a low rated burden should be applied. 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. Learn how to specify current transformers photo credit. Full testing and calibrating of all products meters and current sensors. A protective current transformer type 5p10 has, for example, the accuracy class 5p and the rated accuracy limit factor 10. Denominator is the secondary rated current typically.
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. Current transformer accuracy is specified by the standards ieee std c57. The accuracy performance of an ac current transformers primary current to secondary signal transformation where the secondary is either a current or voltage is stated as accuracy class. Apr 17, 2012 the transformer class relates to its accuracy. Class a for highend products class a is a higher accuracy class for real time measurement 2. Scope this document describes the calculation of the actual accuracy limit factor f a for protectiontype p current transformers ct. The whole and sole requirement of a protection class ct is to maintain its measurement accuracy during fault as well as abnormal condition. Pdf accuracy of current transformer with current errors at. Understanding current transformer accuracy classes february 21, 2017 fehlberg victor leave a comment go to comments 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. It is not intended to make the reader an instrument transformer designer. This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage.
Accuracy class accuracy class describes the performance characteristics of a ct and the maximum burden allowable on the cts secondary. Understanding current transformer accuracy classes. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1. The externalload applied to the secondary of a current transformer is called the oburden. A current transformer may have dual ratings and be used in metering or protection applications, if both ratings are listed on the nameplate.
At 5% rated current through 100%, the current transformer must maintain 0. This occurs on systems with variable currents such as wind or solar power generation. Jan 06, 2018 accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. Percent transformer correction factor with a wattmeter or watthour meter, the. Iec class p cts for standard idmt oc or even distance may only be 10% accuracy it is. A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system. O the burden is expressed pr eferably in ter ms of the impedance of the load and its r esistance and r eactance components. Lt6ct2001 current transformer tesys t lt6ct 2001 a accuracy. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. The accuracy class is the minimum accuracy guaranteed by the manufacturer and the ct may be built to higher standards. An easy way to select the amperage needed is to match the breaker or disconnect rating that you will be. Accuracy limit factor of current transformer electrical. The current transformer works on the principle of variable flux. National grid current transformers for protection and general use technical specification ts 3.
Instrument transformer basic technical information and. Depending on their accuracy class, cts are divided into metering accuracy cts or. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance. Current transformer saturation voltage disturbance. The new current transformer and metering speci cations like the iec618692 table above now require accuracies from 0. 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. Pdf abb instrument transformers current transformer. High accuracy extended range current transformers can increase utility revenue through improved metering. Iec 618692 transformer type sirax ct100 sirax ct110 accuracy class 0. Accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. They are specified by current ratio, window size, and ct class. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device.
It buffers the meter from experiencing excessive torques that might be. On the other hand, if you are going to drive a switchboard type instrument with a 1% accuracy, you will want a current transformer with a 0. The accuracy class is defined for 25100% of rated burden. 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. You must keep in mind that the accuracy ratings are based on rated primary current flowing and per ansi standards may be doubled 0. Instrument transformers will transmit the signal to the metering and protective relaying instrumentation, with the purpose of protection of important equipment. Previously i have put a few posts in the site, going into more detail on current transformer classes. Cts are categorized as protection ct, special protection ct and measuring ct. Accuracy classes for transient cores according to iec 618692. The accuracy class for current transformers is measured in accordance with the iec61869 standard. Current transformers,voltage transformers and instrument. 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. For protection relays intended to operate during a fault the core output under transient conditions is of.
If the primary current is 10 times to the rated primary current of the ct, the ct will function perfectly, within the rated composite accuracy class 5 %. While the physical principles are the same, the details of a current transformer compared with a voltage transformer will differ owing to different requirements of the application. Control of the conformity of the measurement chain. The third part of the ct accuracy class is the maximum burden allowed for the ct. The ctsb0816 is a splitcore current transformer with an opening of 80mm3.
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. Difference between metering and protection current. This transformer ensures stable revenue accuracy over a broad current range for solidstate metering, and is best suited for power generation facilities. The energy limitation of a ct is referred to as the maximum burden. For protective current transformers, the accuracy class is determined by the highest. Accuracy classes for current transformers for measuring are 0. If this limit is exceeded, ct accuracy is not guaranteed. Current transformer ratio ctr the current transformer ratio, ctr, expressed as a fraction, is the ratio between the magnitudes of the primary and the secondary current for ideal working conditions of the current transformer.