First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented. Iec standards and nonconventional instrument transformers this article, completing a series of three. What is knee point voltage, use of knee point voltage in. Current transformers, along with voltage or potential transformers, are instrument transformers. The mct provides a microprocessor controlled variable voltage output and. The process of saturation can be shown by expressing the. Ct saturation calculations are they applicable in the.
It produces a current in its secondary which is proportional to the current in its primary. Ct analyzer current transformer testing, calibration and. If the result is that the saturation influence leads to unacceptable relay performance then diverse remedies can be considered, such as using a greater lead crosssection to reduce the burden of the connection between ct and relay the leads usually contributing much more to the external ct burden than the connected relay input. Current transformer ct saturation leads to inaccurate current measurement and. Calculator that provides voltage drop caused by the starting of motors at the substation transformer primary and secondary leads. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1. Xr ration also have effect on timetosaturate if you have some residual flux permanently left in a ct, say a c100 class ct, do you think you can drive the ct to the saturation point by build up to same level of voltage or in other words pass same level of the. Calculation of the current transformer accuracy limit factor. A practical guide to ct saturation ariana hargrave, michael j. The calculator below can be used to determine the desired burden resistor value.
Support ct saturation calculator ideally, we select cts that normally operate in their linear region. This page is dedicated to showing examples of ct and pt calculations. Mar 16, 2018 the device evaluates the current transformer as per the iec or ieee standard within seconds with the push of a button. International standard iec 600441 has been prepared by iec technical committee 38. Pdf saturation of current transformers and its impact on. Abaout ttt eocneccbivbycosnabcyaecntecagcnl in some balanced protection systems where sensitivity and stability levels required are high, classes 5p10 or 10p10 may. Ct saturation in industrial applications analysis and. Sep 14, 2005 ct saturation calculations are they applicable in the modern world. A routine series of diagnostics and testing for your current transformer should include ct saturation testing to identify the knee point against ieee or iec standards, ct ratio tests to identify that the voltage ratio is within expected parameters and current transformer resistance testing for both the transformer windings and ct insulation. Abstractalthough there is an abiding interest in the application of current transformers ct for relaying, few written rules exist for selecting ratings. Abstractalthough there is an abiding interest in the application of current transformers ct for relaying, few. This consolidated version of iec 600441 consists of the first edition 1996 documents. Pdf the impact of currenttransformer saturation on.
Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. An excel spreadsheet has been developed by the ieee psrc for the. Current transformers ct are used in high voltage hv and medium voltage mv 1 installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. There are four typical types of current transformers. Medium voltage products technical guide protection. Depending on the level of saturation the measured current on the secondary is much smaller than the value present in the prim. Iec, ieee or local create a ct analyzer fingerprint measurement for further onsite comparison verify the ct design. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Ct analyzer the number one in current transformer testing. Ct saturation in industrial applications analysis and application guidelines mv current transformers photo credit. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead.
Required for the safer side of ct sizing according to iec 600446. An excel spreadsheet has been developed by the psrc for the purpose of easy application. Analyze your current transformer ct with the push of a button. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Current transformer saturation voltage disturbance. So i have been getting a lot of requests about how to do ct and pt calculations. A current transformer ct is used to measure current in a shunt inductor connected on a 120 kv network. Index terms line current differential relay, ct saturation tolerance.
The standard iec protection class ct s are of class p that only takes the ac behaviour into account in iec 600441. Understanding ct magnetization curve electric power. Jan 28, 2015 asymmetrical faults like phasephase may generate dc components contributing to current transformer saturation that. Analyze the effect of ct saturation you can export the measurement results to network. What happens if current transformer gets saturated. There are a few examples with ct calculations only and a few examples with both ct and pt calculations.
Furthermore, if the current is increased beyond this point, saturation occurs and the sine wave is cutoff at an angle less than 1800. Ct saturation in industrial applications analysis and application guidelines 3 of 34 get8501 figures 1 and 2 below present plots of the proportional secondary ct current, and the simulated secondary current for a 50. The voltage rating of the ct is the rms value of the sine wave where the fluxlimited volttime area just fits under the half cycle of the sine wave. What happens in current transformer core during ct saturation. Ct saturation tolerance for 87l free download as powerpoint presentation. Selecting cts to optimize relay performance gabriel benmouyal, ireq jeff roberts and stanley e. Although also resulting in nonlinear output, the saturation of a current transformer magnetic core has a different characteristic from other saturation. Ct is a very dependable and accurate device for the most part except when in goes in to saturation. With a better modeling of ct saturation in an existing system, protection engineers can optimize their relay settings and understand the behavior of the relay under such conditions. Iec standards and nonconventional instrument transformers article in ieee industry applications magazine 181.
Actually if we want to check ct saturation that means ouput current must be. Iec602551871 and real current recordings from relays. Class px ct s are defined by the position of the kneepoint saturation point or kneepoint voltage and magnetising current and the secondary wire resistance r ct. The iec shall not be held responsible for identifying any or all such patent rights. Finally, we explain how ct saturation can affect relay operation and show how to detect ct. A ct secondary excitation characteristics obtained from the test. Current transformer performance during transient conditions and. Scribd is the worlds largest social reading and publishing site. A more precise calculation of the current transformers transient response in the overall system can contribute to significant. Comtrade export function allows further analysis and even hardware testing.
The impact of currenttransformer saturation on transformerdifferential protection article pdf available in ieee transactions on power delivery 303. If a ct is rated as a turns ratio, then it likely does not contain a burden resistor. Older relays, especially electromechanicals, handled this poorly, reacting in barely comprehensible fashion, if at all. Current transformer saturation, ratio and polarity test set description the megger mct1600 test set is a lightweight, robust, portable unit used to automatically or manually perform saturation, ratio, polarity, demagnetizing tests and insulation tests on current transformers.
Industry standards do not require the curves to be an accurate representation of true measurable values. The primary winding which consists of a single turn passing through the ct toroidal core is connected in series with the shunt inductor rated 69. Short circuit and full load current calculator transformer and short circuit calculator that provides base load current and short circuit currents for identified ge transformers. Instrument transformers application guide abb group. Ct saturation theory to understand ct saturation, it is very important to understand the basic concept of how cts work and what is actually happening when they saturate.
Scope this document describes the calculation of the actual accuracy limit factor f a for protectiontype p current transformers ct. In iec 618692 standard, the knee point voltage is defined as the voltage. With a better modeling of ct saturation in an existing system, protection. Current transformer saturation, ratio and polarity test set. Ieee cts and pts 610 free download as powerpoint presentation. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac.
Current transformer errors and transformer inrush as measured. This section describes what happens in the core of a ct during symmetrical saturation, asymmetrical. Saturation of current transformers and its impact on digital overcurrent relays. Ct saturation is to know at which point of current the ct gets saturated and so the ct is no more. Ct saturation cannot be calculated, it is tested with some instruments. Calculator, an excel spreadsheet created by the ieee power. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the knee point is reached. Current transformer with a given core dimension can only support a given amount of maximum flux density in the core. Burden and ct saturation analysis at asymmetrical current condition. If something is wrong and the terminal voltage at your ct exceeds the saturation voltage of the ct, then that identical and proportional business goes out the window. When selecting a ct, the calculated saturation voltage should be isz.
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