# Knee point voltage of ct pdf

Ct knee point voltage calculation transformer volt. Instrument security factor is the ratio of instrument limit primary electric current to the rated primary current. According to ansiieee the intersection of the excitation curve with a 45 0 tangent line is the knee point. Before understanding knee point voltage of current transformer and current transformer ps class we should recall the terms instrument security factor of ct and accuracy limit factor instrument security factor or isf of current transformer. The impact of currenttransformer saturation on transformerdifferential protection article pdf available in ieee transactions on power delivery 303. Consider now the same application with the addition of a 5. For this ct the knee point voltage for psclass core is v k 120v for 3ss3 winding and v. To measure this first demagnetise the ct and apply. Abstractcurrent transformer ct saturation, while a fairly. This is the reason, in definition it is said that 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. Apr 25, 2018 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. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly understood by protective relay engineers. I read knee point is a point at which 10% increase in voltage leads to 50% increase in current.

As illustrated in 1ec600441, the class x current transformer is needed. Why do current transformer cores have different knee point. Jul 24, 2018 ct knee point voltage check by cpc100 omicron duration. Knee point voltage is nothing but a magnitude of the saturation limit of the current transformer. This concept is the one used for testing the knee point in current transformers with the. Function description for high impedance busbar protection. This class specifies the minimum value of the knee point voltage vk of the ct. You simply read the knee point from the excitation curve published by the manufacturer. The kneepoint voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. Plot the curve and evaluate the vk and img from the graph. Their performance is defined in terms of a knee point voltage kpv, the magnetizing current imag at the knee point voltage or 12 or 14 the knee point voltage, and the resistance of the ct secondary winding corrected to 75c. We are offering ct knee point tester to our clients c.

According to the manufacturer, the internal burden of the auxiliary ct is 1. Steady state ac saturation the next step is to apply anticipated faults to the system and determine if the voltage that will be impressed upon the ct will be greater than the ct rating. Now increase the voltage and monitor the excitation current up to the ct reaching near to saturation point. Restricted earth fault protectionct selectionpart 2.

Record the reading of voltage and current at several points. What is the secondary limiting voltage of a current. Knee point voltage of current transformer electrical concepts. Current transformer errors and transformer inrush as measured.

Function description for high impedance busbar protection resp. In order to make a scheme all cts belonging to one phase shall be connected in parallel. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. Jul 25, 2018 current transformer ps classbefore understanding knee point voltage of current transformer and current transformer ps class we should recall the terms instrument security factor of ct and accuracy limit factor. Magnetization and primary injection test of current. Current transformer errors and transformer inrush as. From the ct junction points a measuring branch is connected.

For this ct the knee point voltage for psclass core is v k 120v for 3ss3 winding and v k 60v for 3ss2 winding. As there are different classes of current transformer cores, how it is decided whathow much will be the knee point voltage of that core. This is well above the kneepoint voltage of the ct, and is at best a marginal application. Their performance is defined in terms of a knee point voltage kpv, the magnetizing current imag at the knee point voltage or 12 or 14 the knee point voltage, and the resistance of the ct secondary winding corrected to. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core, as shown in 1. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core. 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. We know that ct core is made of crgo cold rolled grain oriented silicon steel. The saturation voltage is graphically located by the cross point of the straight slopes of the excitation curve 1. This value was used in reference 4 for all the ct performance analysis, and will be used here also. Ct burden is an impedance of secondary circuit expressed in ohms and power factor. Hence it is always advisable to keep adequate margin on knee point voltage. Ct saturation tolerance for 87l applications mpcomments. Instrument transformers itr rmk site december 2005 59 59 typical maximum lead lengths based on voltage level of substations voltage level in kv lead length in meters 400 220 800 2 500 instrument transformers itr rmk site december 2005 60 60 typical burdens analog and digital meters current transformer circuit.

It must also be defined in the future standard iec 600441 under the name of class px. Current transformer testing swp check this is the latest process zone version before use. Testing and commissioning of hv power transformers. Current transformers,voltage transformers and instrument.

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. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. Ct burden can be expressed in two terms as example. Applying high impedance differential protection with ied 670. Instrument security factor or isf of current transformer. C400 protection current transformer assuming i nom 5a vvolt 0,01 0,1 1 10 excitation currentamp 400. In the flux current graph after a point the flux changes with the current insignifficantly. The saturation point is at a voltage 10 to 20% greater than the true knee point of the curve. Oct 04, 2017 the knee point voltage can be said as the rated voltage at which a transformer works. What is knee point voltage of current transformer answers.

The following illustrates the method to size a class x current transformer. What is knee point voltage, use of knee point voltage in. When the primary of ct is energized a working mmf is produced in the core. Hi, the ct requirements for the electrical system are to be specified in the initial detail engineering stage itself for any project. 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. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. Knee point voltage of current transformer electrical. The obtained vk should be greater than specified one. The knee point voltage can be said as the rated voltage at which a transformer works. Dear all, please guide me for the calculation of knee point voltage for ps class ct for ref protection of trafo transformer rating. The accuracy of a current transformer depends among others of.

Power transmission and distribution author page 247 200708. According to iec, t he knee point voltage is defined as a point where 10% increase in the secondary voltage cause an. For differential protection for equipment like generator,transformer,bus etc. What is knee point voltage, use of knee point voltage in current transformer duration. This is the significance of saturation level of a ct core mainly used for protection purposes. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. Knee point voltage of current transformer and how to. Ct knee point voltage calculation free download as pdf file. Ct burden, knee point voltage, core saturation details of. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Insulation tester ct knee point tester manufacturer from. This concept is the one used for testing the knee point in current transformers with the pte100c.

After or beyond this voltage the linearity between primary and secondary circuit that is the desired property of current transformer does not work any more. According to the manufacturer, the internal burden of the auxiliary ct. Iec defines knee point as the intersection of straight lines extended from nonsaturated and saturated portions of the excitation curve. But this to be remembered that the cable length may increase as per the cable route layout. So for choosing ct for differential protection of transformer, one should consider current transformer ps class rather its convectional protection class. Ps stands for protection special which is defined by knee point voltage of electric current transformer v k and excitation electric current i e at v k2. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping. If you carefully read the definition, you will notice that it is something related to saturation of current transformer ct. The secondary voltage rating is the voltage the ct will deliver to a standard burden at 20 times rated secondary current without exceeding 10% ratio correction. Knee point voltage, internal secondary resistance remanence no limit 10 % increase in voltage of ct secondary results in 50 % increase in secondary current.

British method here 5a is rated secondary current and 2volt across secondary. The sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200. Ct kneepoint determination iec ratings redtrumpet electrical 29 dec 03 11. The knee point is determined by observing a small voltage increase causing a large increase in current. The ct manufacturer asks for the following information before designing the ps class ct. The system fault current and the burden of the ct should be known for calculating the knee point voltage. 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.

Knee point voltage of current transformer ps class. The knee point voltage kpv of the current transformer tells the information about the magnetization characteristics of the current transformer. This power factor is not the power factor of secondary load. Pdf the impact of currenttransformer saturation on.

Vk is the minimum knee point voltage in volts, k is a parameterto be specified by the purchaser which depends on the system fault level and the characteristics of the relay intended to be used, is is the rated secondary current of the current transformer or the secondary current as derived from a specified turns ratio and primary current. A practical guide to ct saturation ariana hargrave, michael j. The knee point voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. The secondary star point is grounded through a 9 ohm ngr. These ct shall be installed in all ends of the protected object. Knee point voltage of a current transformer is the magnitude of secondary of current transformer. At the knee point the voltage across the secondary tends to remain constant whereas the current continues to rise. Knee point voltage of current transformer this is the significance of saturation level of a ct core mainly used for protection purposes. It then explainhow s this core activity corresponds to the ct equivalent circuit, ansi voltage ratings, and the familiar ct excitation graph. It also imposes a maximum value of rct ct secondary winding resistance. What is meant by knee point voltage of a current transformer.

Sometimes, it specifies the maximum value of the magnetising current io at knee point voltage. Magnetization and primary injection test of current transformer for protective relaying jacob tsado 2 general lack of even the most elementary knowledge concerning their performance characteristics and limitations among engineers who are continually using them 1, 2. Knee point voltage, internal secondary resistance remanence no limit of knee point voltage. This is well above the knee point voltage of the ct, and is at best a marginal application. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the. T are air core hence bh curve of air, as core material, could be used, to find knee point flux density and the amp. What is the use of knee point voltage electrical4u. Presentation on all products with detailed ccv and ct. Knee point is that point, in the voltage vs magnetizing current characteristics of a ct,beyond which the ct current changes drastically with respect to change of voltage. Dear all,waht is knee point voltage of ps class ct,when i am order for 40001a ps class ct,vendor ask me for the knee point voltage of the same. If we consider the magnetising curve vio of the ct, the knee point voltage vk is defined as the point on this curve from which a 10% increase in voltage causes a 50% increase in the magnetising current io. Testing for magnetization characteristics knee point voltage of ct cores.

Please help me how to calculate the same its very argent. Knee point voltage for ps class ct cr4 discussion thread. With respect to ansi cts, some of the major details that one would look for in the ct are. Ct knee point voltage check by cpc100 omicron duration. Polarity class, saturation voltage, knee point voltage. Current transformer saturation voltage disturbance. However, detail information concerning ct and voltage transformer vt. The knee point voltage of ct is very important parameter for the ct used for differential protection. This means that an increase in 50 % current will lead to just an increase in 10% voltage. Ct kneepoint determination iec ratings electric power. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. The knee point is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%.

Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. Page 3 of 9 standard work practice sp0507 ver 5 ergon energy corporation limited abn 50 087 646 062 ergon energy queensland pty ltd abn 11 121 177 802 rated knee point voltage 950 volts, maximum secondary winding resistance 3 ohms. The point is known as the knee point where the voltage at this point. For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. Ratio test for approved ct ratios in all the cores including complete wiring in the secondary side to ensure correct ratio and continuity of the wiring.

Every protection class ct name plate contains details of knee point voltage as shown in below figure. How to calculate the knee point voltage of a current. Polarity class, saturation voltage, knee point voltage excitation characteristics. Ansi knee is more conservative compared to iec curve.

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