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 alternating magnetic flux passes through the secondary winding. When using low amperage 200a cts with a summing transformer, be sure to check the burden on the cts as the summing transformer adds burden. 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. This thermal voltampere rating corresponds to the fullload rating of a power transformer. Current transformers,voltage transformers and instrument. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. Like every other transformer, cts can only produce a finite amount of energy. The burden va can be calculated as burden va voltage drop x rated ct sec. Raising the voltage produced on the output of a ct reduces its accuracy. Exploring current transformer applications power electronics. Modeling current transformer ct saturation for detailed. Nov 15, 2019 the value of the rated secondary current is 5a.
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. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. Safety of current transformers regarding safety issues,it is considered important as the secondary of a current transformer should not be disconnected from its burden while current is in the primary, as the secondary will attempt to continue driving current into an effective infinite impedance up to its insulation breakdown voltage and thus. The externalload applied to the secondary of a current transformer is called the oburden. Any constant value of alternating current would, in theory, produce an infinite primary voltage drop.
A potential transformer is a conventional transformer having primary and secondary windings. Current transformers, along with voltage or potential transformers, are instrument transformers. When using summing transformers, add the primary ct currents to calculate the kw and kwh outputs. Saturation makes the point of equality between the excitation current and the input occur at a flux level lower than would be expected from linear theory. 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. Burden of interconnecting leads can be calculated by using the above formula. Instrument transformer theory and testing steve hudson, pe engineering manager 10737 lexington drive. 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. Similarly, the burden of an electromechanical over current relay is about 5va, whereas the burden of a digital microprocessor based over current relay. 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. Accuracy decreases with increased burden load or low line.
The current transformer function is different than that of a voltage transformer. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. Burden test is to ensure the connected burden to ct is with in the rated burden, identified on the nameplate. Current transformer in 3phase, 4wire system 4th wire is the. Used in ac system for the measurement of current, voltage, power and energy. Rated instrument limit primary current is the value of the minimum primary current transformer is equal to or greater than 10 percent, the secondary is equal to or greater than 10 percent, the secondary burden being equal to the rated burden.
The current transformer is connected to a measuring instrument or a protective relay by means of wires. Kod ktf remanence only considered in extra high voltage systems ehv k tf. There are four typical types of current transformers. Turns compensation may well be needed to achieve the measurement performance. Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Instrument transformer basic technical information and. The burden is calculated to provide a defined va load, and it is the current and not the output voltage that is the most accurate reflection of the input current value. If the secondary is connected to a burden, the alternating. O the burden is expressed pr eferably in ter ms of the impedance of the load and its r esistance and r eactance components. Without the load or burden the current transformer acts like an inductor and would completely block the primary current flow. Transformers used for voltage potential measurement steps down the voltage to the level of voltmeter.
It buffers the meter from experiencing excessive torques that might be. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. Current transformer, standard for steadystate performance. The essentials of current transformers in power circuits. A secondary current rating of 2a and 1a may also be used in some cases if the number of secondary turns is low and the ratio cannot be adjusted within the required limits by the addition or removal of one turn, if the length of secondary connecting lead is such that the burden due to them at higher secondary current.
The voltage transformer is designed to connect in parallel. Currenttransformer burden all ct accuracy considerations require knowledge of the ct burden. With respect to ansi cts, some of the major details that one would look for in the ct are. Current transformer circuits for power electronics applications. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. Current transformers cts basic theory basic formula. Download fulltext pdf calculation of current transformer transient errors article pdf available in ieee transactions on power apparatus and systems 976.
Current transformer circuits for power electronics. Current transformer ct working principle, how an electrical current transformer works, uses, types, load resistor concept is explained in the video lecture by. Tests have shown values ranging from 500 to 11,000 volts. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. Pdf calculation of current transformer transient errors. The secondary of a current transformer must always have a burden load connected. When the rated primary amps is exceeded, which is usually the case when a fault occurs on the system, the amount of secondary current will increase. Testing current transformers burden test only if we cannot get to the primary side of the ct we can check that the output is constant as burden changes. Instrument transformers scale the large values of voltage or current to small. The burden resistor completes or closes the ct secondary circuit. Cts to be oversized so the total secondary current does not exceed the 5 amp rating, which reduces accuracy. Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. As an example, a typical 100 to 5a current transformer has a nominal ratio of single primary turn to 20 secondary turns or 1. Demagnetization of current transformers using pwm burden article pdf available in ieee transactions on magnetics 511.
The burden value is chosen to provide a voltage proportional to the secondary current. Testing current transformers ratio vs applied burden cts installations can be fully verified in the field we can verify that the ratio is correct and constant as burden changes. Oct 08, 2016 calculation of cts if the current transformer has primary to secondary current ratio of 100. Current transformer basics and the current transformer. Abb instrument transformers current transformer theory technical level training course 201203. Pdf demagnetization of current transformers using pwm burden.
Compare this with the burden of the current coil of modern day digital trivector meter, which is less than 0. Point a, the magnets in the core are aligned in random directions no remanence, which is discussed later in this paper. 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. In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. The va load of the device can be obtained from the datasheet provided by the manufacturer. Protection ratings are expressed in terms of rated burden, class, and accuracy limit factor, for example 10va class 10p10. Zb burden impedance ip primary current ns number of secondary turns ratio to 1 v 3. Connecting too low a burden compared to the design burden of the current transformer will allow higher currents through the metering. Rated primary current selection of the current transformer low voltage current transformer low voltage current transformer general reference standards iec600441, vde0414441, din57414, bs3938, bs7626, en600441, gb12082009 measuring current transformers are constructed to feed on other low. If the primary current of the current transformer becomes a the secondary current will still be 1. When secondary current vector lags the primary current vector. Ct sensors an introduction learn openenergymonitor. In reality the current transformera s input inductance hence also impedance cannot be infinity. How to convert current obtained from ct to precise voltage.
Back to the basics current testing valence electrical. Since this method requires ideal conditions, it is generally better to use a summing transformer. Supreme accuracy current transformer super accuracy for 0. Morgan lacey in 1925, based on a series of pamphlets entitled transformer abstracts that were. Ideally, before the current is applied at point a, the magnets in. 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. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and. Pdf abb instrument transformers current transformer. Typically e rated current limiting fuses are used to protect pts. Calculation of current transformer transient errors article pdf available in ieee transactions on power apparatus and systems 976.
The burden is the termination impedance of the measuring instrument. Refer to the table at the right to determine the resistance, in ohms or va, of the wires that connect the secondary of the current transformer to the device. A secondary current rating of 2a and 1a may also be used in some cases if the number of secondary turns is low and the ratio cannot be adjusted within the required limits by the addition or removal of one turn, if the length of secondary connecting lead is such that the burden due to them at higher secondary current would be excessive. How we can calculate burden of current transformer. It produces a current in its secondary which is proportional to the current in its primary. It is higher than the voltampere rating used to classify potential transformers as to. Ct accuracy a ct is most accurate at rated current with a low burden load. Secondary current of ct as a function of primary current with zero ct burden. H i g h p o w e r e d r e s e a r c h f o r t h e r e a l w o r l d december 2011 issue 1 modeling current transformer ct saturation for detailed protection studies 6 the wound rotor wr and squirrel cage sq induction machine models.
With 150 amps through the 300 amp rated primary, the secondary current will be 2. All instruments that use a current transformer to measure line current must terminate the ct with a resistance impedance in same case which means there is some inductance involved. The ct energy limitation is also included in the accuracy class but is shown as a maximum burden. Feb 12, 2017 in this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an ac. Transformer book is still in demand is a tribute to the publishers and to the authors who have carried the torch to light our way for 75 years. Pdf abb instrument transformers current transformer theory.
The burden value needs to be low enough to prevent ct core saturation. Injected the rated secondary current of the ct, from ct terminals towards load side by isolating the ct secondary with all connected load and observe the voltage drop across the injection points. Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. The measuring instrument can be an analog or digital energy meter, a data logger or a recorder. While the latter configuration would not be used today, one may be required to make this calculation on an existing system. The basics of current transformers nk technologies. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. That is, the ct secondary current is an identical but scaled down replica of the primary current. Measurement ratings are expressed in terms of rated burden and class, for example 15va class 0. However, if the ct saturates, more current is required to magnetize the core and as a result the secondary current i s available as inputs to the relay may not be an identical scaled down replica of the actual primary current i p. This is usually the voltamp va rating for normal transformers. The applied burden is the total of instrument and relay burdens. What happens next depends on the load connected to the secondary winding.
Energized but unused cts must be kept shortcircuited. Current transformers theory of operation butler winding. Secondary current of ct as a function of primary current with finite ct burden. For example, the potential transformer mentioned above having a rating of 480. These devices use standard transformer theory with 23 windings one primary with one or two secondary windings with a magnetic. A burden resistor connected across the secondary produces an output voltage proportional to the resistor value, based on the amount of current flowing through it. The burden on a current transformer is critical to it providing accurate output. Pdf current transformer ct is one of the important element of power system protection.
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