As illustrated in 1ec600441, the class x current transformer is needed. According to ansiieee the intersection of the excitation curve with a 45 0 tangent line is the knee point. The knee point is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. What is the use of knee point voltage electrical4u. Testing for magnetization characteristics knee point voltage of ct cores. 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. The secondary star point is grounded through a 9 ohm ngr. The knee point voltage kpv of the current transformer tells the information about the magnetization characteristics of the current transformer. However, detail information concerning ct and voltage transformer vt. This is well above the kneepoint voltage of the ct, and is at best a marginal application. This concept is the one used for testing the knee point in current transformers with the.
The impact of currenttransformer saturation on transformerdifferential protection article pdf available in ieee transactions on power delivery 303. As there are different classes of current transformer cores, how it is decided whathow much will be the knee point voltage of that core. These ct shall be installed in all ends of the protected object. 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. 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. For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. Knee point voltage of current transformer this is the significance of saturation level of a ct core mainly used for protection purposes. 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. What is knee point voltage, use of knee point voltage in. According to the manufacturer, the internal burden of the auxiliary ct is 1. Hi, the ct requirements for the electrical system are to be specified in the initial detail engineering stage itself for any project. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the. Instrument security factor is the ratio of instrument limit primary electric current to the rated primary current.
If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. What is the secondary limiting voltage of a current. Consider now the same application with the addition of a 5. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping.
To measure this first demagnetise the ct and apply. Current transformer testing swp check this is the latest process zone version before use. Function description for high impedance busbar protection. Knee point voltage of current transformer and how to. 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.
We are offering ct knee point tester to our clients c. According to the manufacturer, the internal burden of the auxiliary ct. Please help me how to calculate the same its very argent. Ct kneepoint determination iec ratings electric power. Sometimes, it specifies the maximum value of the magnetising current io at knee point voltage. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. Ct burden, knee point voltage, core saturation details of. 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 sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200. Record the reading of voltage and current at several points. The obtained vk should be greater than specified one. The knee point voltage of ct is very important parameter for the ct used for differential protection. 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. This is well above the knee point voltage of the ct, and is at best a marginal application.
Knee point voltage of current transformer electrical concepts. 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. It then explainhow s this core activity corresponds to the ct equivalent circuit, ansi voltage ratings, and the familiar ct excitation graph. Knee point voltage of current transformer electrical.
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. A practical guide to ct saturation ariana hargrave, michael j. The ct manufacturer asks for the following information before designing the ps class ct. At the knee point the voltage across the secondary tends to remain constant whereas the current continues to rise. 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. Ct knee point voltage calculation transformer volt. Knee point voltage, internal secondary resistance remanence no limit of knee point voltage. Polarity class, saturation voltage, knee point voltage excitation characteristics. Knee point voltage for ps class ct cr4 discussion thread. 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. 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. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core.
Function description for high impedance busbar protection resp. According to iec, t he knee point voltage is defined as a point where 10% increase in the secondary voltage cause an. So for choosing ct for differential protection of transformer, one should consider current transformer ps class rather its convectional protection class. From the ct junction points a measuring branch is connected. The knee point voltage can be said as the rated voltage at which a transformer works. Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. Insulation tester ct knee point tester manufacturer from. Current transformer saturation voltage disturbance. How to calculate the knee point voltage of a current.
We know that ct core is made of crgo cold rolled grain oriented silicon steel. Magnetization and primary injection test of current. For this ct the knee point voltage for psclass core is v k 120v for 3ss3 winding and v k 60v for 3ss2 winding. 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%. 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. 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. T are air core hence bh curve of air, as core material, could be used, to find knee point flux density and the amp.
The following illustrates the method to size a class x current transformer. Knee point voltage of current transformer ps class. When the primary of ct is energized a working mmf is produced in the core. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. The system fault current and the burden of the ct should be known for calculating the knee point voltage. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly understood by protective relay engineers. Every protection class ct name plate contains details of knee point voltage as shown in below figure. Ct kneepoint determination iec ratings redtrumpet electrical 29 dec 03 11. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Oct 04, 2017 the knee point voltage can be said as the rated voltage at which a transformer works. Abstractcurrent transformer ct saturation, while a fairly. 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. In order to make a scheme all cts belonging to one phase shall be connected in parallel. Polarity class, saturation voltage, knee point voltage.
Iec defines knee point as the intersection of straight lines extended from nonsaturated and saturated portions of the excitation curve. Ct saturation tolerance for 87l applications mpcomments. You simply read the knee point from the excitation curve published by the manufacturer. C400 protection current transformer assuming i nom 5a vvolt 0,01 0,1 1 10 excitation currentamp 400. This means that an increase in 50 % current will lead to just an increase in 10% voltage. Presentation on all products with detailed ccv and ct. The point is known as the knee point where the voltage at this point. I read knee point is a point at which 10% increase in voltage leads to 50% increase in current. This value was used in reference 4 for all the ct performance analysis, and will be used here also.
Current transformer errors and transformer inrush as measured. What is knee point voltage, use of knee point voltage in current transformer duration. Ct burden can be expressed in two terms as example. Ct knee point voltage check by cpc100 omicron duration. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. This is the significance of saturation level of a ct core mainly used for protection purposes. Ansi knee is more conservative compared to iec 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. Ct knee point voltage calculation free download as pdf file. Now increase the voltage and monitor the excitation current up to the ct reaching near to saturation point. Current transformer errors and transformer inrush as. Knee point voltage of a current transformer is the magnitude of secondary of current transformer. Pdf the impact of currenttransformer saturation on. Why do current transformer cores have different knee point. 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. 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. Knee point voltage is nothing but a magnitude of the saturation limit of the current transformer. 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. 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. Instrument security factor or isf of current transformer. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. Power transmission and distribution author page 247 200708. It also imposes a maximum value of rct ct secondary winding resistance. The accuracy of a current transformer depends among others of.
For differential protection for equipment like generator,transformer,bus etc. British method here 5a is rated secondary current and 2volt across secondary. Jul 24, 2018 ct knee point voltage check by cpc100 omicron duration. What is meant by knee point voltage of a current transformer. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. But this to be remembered that the cable length may increase as per the cable route layout.
Restricted earth fault protectionct selectionpart 2. The saturation voltage is graphically located by the cross point of the straight slopes of the excitation curve 1. Testing and commissioning of hv power transformers. For this ct the knee point voltage for psclass core is v k 120v for 3ss3 winding and v. In the flux current graph after a point the flux changes with the current insignifficantly. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core, as shown in 1. This class specifies the minimum value of the knee point voltage vk of the ct. This concept is the one used for testing the knee point in current transformers with the pte100c. What is knee point voltage of current transformer answers. Applying high impedance differential protection with ied 670. 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.
Hence it is always advisable to keep adequate margin on knee point voltage. 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. This power factor is not the power factor of secondary load. With respect to ansi cts, some of the major details that one would look for in the ct are. 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 knee point is determined by observing a small voltage increase causing a large increase in current. Knee point voltage, internal secondary resistance remanence no limit 10 % increase in voltage of ct secondary results in 50 % increase in secondary current.