It is important to know that the parameters of the above formula are dependent on the actual current transformer characteristics and the protection application requirements. Explanation of accuracy specifications for current transformers. Current transformer definition of current transformer by. Definition of current transformerct a ct is an instrument transformer in which the secondary electric current is substantially proportional to primary electric current and differs in phase from it by ideally zero degree. The accuracy class is only applicable if the total load including cable losses is approximately equal to the current transformer burden. Instrument transformers application guide abb group. The externalload applied to the secondary of a current transformer is called the oburden. Instrument transformers scale the large values of voltage or current to small, standardized. Class px current transformers are generally being defined in terms of the internal characteristics of current transformer as follows. Knee point voltage of current transformer electrical.
Both of these ct accuracy standards describe the typical secondary output of a current transformer as 5 amps or 1 amp with an external burden. But in the case of protection, the ct may not have the accuracy level as good as metering ct although it is desired not to be saturated during high fault current passes through primary. The new current transformer and metering speci cations like the iec618692 table above now require accuracies from 0. Definition of current transformer ct a ct is an instrument transformer in which the secondary electric current is substantially proportional to primary electric current and differs in phase from it by ideally zero degree. The accuct act0750 and act1250 families have builtin burden resistors and provide voltage outputs nominally 0. Full testing and calibrating of all products meters and current sensors. The current transformer works on the principle of variable flux. 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. Accuracy classes of current transformers are guarantee at between 100% and 120% of rated. Accuracy class of current transformer is the highest permissible. 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.
By using a pt, voltage is reduced that can be safely handled by meter or relay circuits. Current transformer saturation voltage disturbance. Their class accuracy as a function of ct load and of overcurrent. Calculation of the current transformer accuracy limit factor. Accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. Nov 17, 2019 the degree of accuracy of a metering ct is expressed by ct accuracy class or simply current transformer class or ct class. 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. A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer.
For protective current transformers, the accuracy class is determined by the highest. A question is often asked about what the difference is between the tolerances specified by magnelab and other companies. Ct accuracy standards continental control systems, llc. Understanding current transformer accuracy classes victor. Currenttransformer burden all ct accuracy considerations require knowledge of the ct burden. This means the ct can supply the rated current to a burden of 1. Feb 21, 2017 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. A general rule is a c200 current transformer can supply. 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. Selecting a current transformer ct svri electrical engineering. According iec, knee point voltage of a current transformer is defined as the voltage at which 10 % increase in voltage of ct secondary results in 50 % increase in secondary current.
The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuity from the high voltages or currents. So, with the help of ct, high value of current can be measured easily by using low value meters. A ct is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Current transformer ct class ratio error phase angle error. A relaying accuracy class is designated by two symbols which effectively describe the capability of the transformer as follows. The class designation is the measure of the cts accuracy. 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 for metering application is provided as an example 0. 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. When looking at the specifications for a ct, the accuracy rating or class.
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. If the current ratio, burden and class are selected, the current transformer ct has been specified basically. Difference between metering and protection current. The precision classes of current transformers are related to the measured value. Class x cts are current transformers which are used for special applications. Although literally, load and burden carry nearly same meaning in english. Accuracy classes for various types of measurement and at standard loads in the secondary circuit burdens are defined in iec 61869. Current transformers high accuracy measurement sensors from 100 to 2000 a function advantages socomec current transformers deliver a standard current to the secondary that is proportional to the primary current and adapted to the rating of the associated energy meter. Three metering accuracy classes are defined for instrument transformers. Ps is defined by knee point voltage of current transformer vk and excitation current ie at vk2. There are two different ways to represent this, depending if the current transformer is to be use for protection or metering. The burden of ct when used for protection purpose is quite high when compared with metering class ct, which means that voltage drop across the burden will be high.
Because of the way tcf is defined, the resulting gain accuracy limits and phase. Technical parameters of instrument transformer following are some of the normally used terms associated with. If you carefully read the definition, you will notice that it is something related to saturation of current transformer ct. 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. The transformer correction factor tcf, accuracy, and phase angle limits of these ct accuracy standards can be applied to the voltage output accuct series of current transformers. Usually for energy meter circuits are used transformer of class 0. It produces a current in its secondary which is proportional to the current in its primary. This is a class of current transformers, and is a fairly low class. For example, a c100 ct class means that the ct is capable of passing 20. Electricity metering accuracy explained ecd online. 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. Measurement type cts are required to transform the primary current, at various classes of accuracy, as specified by the class designation, over a current range from 1 to 120 per cent of its rated primary ratio. Explanation of accuracy specifications for current.
C means the transformer ratio can be calculated, i. This has to do with what kind of burden can be placed on the secondary. When looking at the specifications for a ct, the accuracy rating or class can be. Meter, ct, and system accuracy accuct current transformer. Current transformer accuracy is specified by the standards ieee std c57. Knee point voltage of current transformer electrical concepts. Current transformers convert an alternating current usually high into a proportional lower one, depending on their use. Current transformers,voltage transformers and instrument. The new and old standards are essentially identical, but iec 618692 consolidated two parts of the older standard.
Potential transformer accuracy class voltage disturbance. The number 20 called accuracy limit factor indicates that the ct will sense the current with the specified accuracy even with 20 times of its secondary current flows in the secondary. Accuracy limit factor and instrument security factor of current. A current transformer chosen for protection purpose are defined by protection class p. As such the instrument transformers must be sized correctly the right class. Accuracy limit factor of current transformer electrical. The basics of current transformers nk technologies. Current transformers, along with voltage or potential transformers, are instrument transformers.
A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system. 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. Ct class definitions ge grid solutions online store. They are specified by current ratio, window size, and ct class. Current transformer ct class ratio error phase angle error in. 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. C100 class for example, a c100 ct class means that the ct is capable of passing 20.
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. A 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. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. 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.
Class a for highend products class a is a higher accuracy class for real time measurement 2. Oct 02, 2016 protection class ct is normally specified as ps protection special. Class ps transformers are the special purpose protective current transformers. Accuracy limit factor and instrument security factor of. Current transformers accuracy is defined as per iec 600441, classes 0. The ct accuracy class is listed on the label or the nameplate of the ct and. This fa is concerned with relay accuracy class resolution. According to the definition, the voltage error is positive if the secondary voltage. The accuracy of a current transformer depends among others of. 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 read more. In more practical terms, it guarantees a 10% accuracy provided the voltage across. Current transformers accuracy are defined as per iec 600441, classes 0. Cts are categorized as protection ct, special protection ct and measuring ct. Defining ct characteristics instrument transformers.
Selecting current transformers janitza electronics. The voltage transformer must be suited to requirements relating to protection and measuring devices. Instrument transformer basic technical information and. The foreseen application of the voltage transformer is used to determine the rated secondary voltage, the accuracy power, the accuracy class and the thermal power limit. A highvoltage current transformer may contain several cores, each with a secondary winding, for different purposes such as metering circuits, control, or protection.
Their class accuracy as a function of ct load and of. O the burden is expressed pr eferably in ter ms of the impedance of the load and its r esistance and r eactance components. Technical parameters of instrument transformer following are some of the normally used terms associated with instrument transformer, that. Accuracy classes for various types of measurement and at standard loads in the secondary circuit burdens are defined in iec 618691. Iec and ansiieee define kneepoint voltage different. Dont be deceived by the numbers or catch phases, know what you are getting when ordering current transformers cts. Understanding current transformer accuracy classes.
The x indicates that they do not belong to any standardized class. The burden of a current transformer is indicated in its nameplate. Jan 06, 2018 accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. 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. 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.