This test is performed automatically during a ct saturation test or manually with the output control knob and metering display. Measuring the knee point is looking for the point where the ct is beginning to saturate, and there is no longer a linear relationship between the current in the primary and secondary windings. Every protection class ct name plate contains details of knee point voltage as shown in below figure. Oct 28, 2020 the degree of accuracy of a metering current transformer is expressed by current transformer accuracy class or simply current transformer class or ct class. Thompson, and brad heilman, schweitzer engineering laboratories, inc. 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. Voltage output to 250 v is connected to the secondary terminals of the current transformer between the common of the main current output and the active tap of out1 so as to allow the measurement.
Current transformer saturation, ratio and polarity test. 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. Ct, the knee point voltage vk is defined as the point on this curve from. Single and multiratio current transformers can be tested. Instrument transformers scale the large values of voltage or current to small. 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. The required the knee point voltage used equation 4. The mct1600 will also provide the operator with a direct reading ratio, thus removing the need to calculate ct ratio manually. The ct secondary resistance should be calculated according to tnb technical. A protection core is designed to transform a distortion free signal. Please help me how to calculate the same its very argent. Ct kneepoint determination iec ratings electric power. Since the requirement at 20001 a tap is 2v per turn,the selected knee point voltage for 30001 a tap is 6000v. This allows testing even the larger generation class ct with a portable instrument.
Further voltage increases will make the ct draw larger currents. Through this video you will get overall knowledge on ct characteristics, knee point voltage. Another possible backup protection scheme is low voltage 27 or unbalanced voltage detection 47. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. A current transformer is, in many respects, different from other transformers. Oct 27, 2020 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. It produces a current in its secondary which is proportional to the current in its primary. Pdf current transformers ct are the key device in power system protection. Test of cts with very high kneepoint voltages up to 15 kv. Sizing current transformers for line protection applications. But in the case of protection, the current transformer may not have the accuracy level as good as metering current transformer although it is desired not to be saturated during high fault. Ct analyzer current transformer testing, calibration and assessment omicron.
The knee point determines the saturation voltage of a. Instrument security factor or isf of current transformer. Note that the polarities for this simulation are such that a positive remanence is the worstcase condition for premature saturation. Test of cts with very high knee point voltages up to 15 kv. 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. Knee point voltage of current transformer electrical. The following illustrates the method to size a class x current transformer. 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. Feb 05, 2015 purpose this test is intended to obtain the knee point voltage of current transformer ct. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. Oct 08, 2020 if the knee point voltage 45 degree slope point is 80% of vs, then the maximum remanence value is 0. Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Published on sep 14,2018ct knee point voltage calculation. But the fact should be clear that the current of primary circuit is the load of a current transformer. Ct saturation tolerance for 87l applications mpcomments.
If there is a possibility of over voltage on the units due to local generation or a transformer being placed at the end of a long line the feranti effect, voltage relays 24 and 59, section 4. Current transformer saturation, ratio and polarity test set. 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 voltage of current transformer electrical concepts. The knee point voltage represents the point beyond which the ct becomes nonlinear. The standards adopted the concept of knee voltage and it is still common to find knee voltage voltage being used as a ct sizing parameter. 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 the current transformer has two windings, in some cases it is confusing about the load of it. The knee point determines the saturation voltage of a ct, right. Ct knee point voltage calculation transformer volt. Beyond the knee of the curve, the ct is no longer working properly. Beyond a certain level, the primary and secondary windings physically exist next to each other, but the magnetic coupling becomes insignificant. Hi, the ct requirements for the electrical system are to be specified in the initial detail engineering stage itself for any project.
The ct analyzer is a test system for protection and metering cts according to ieee and iec standards. It is usually defined as the point where a 10% increase in secondary voltage results in a 50% increase in secondary injected current. This because the curve beyond point k becomes nonlinear. So that we have to calculate the maximum allowable voltage limit, that can be applied to the secondary winding of the current transformer. The ieee 45% and 30% definitions apply to class c cts and refer to the point on the excitation curve where the tangent is at 45 or 30 degrees to. Ct burden, knee point voltage, core saturation details of. The point on the excitation curve where a 10% increase in secondary exciting voltage results in a 50% increase in secondary exciting current is the british standard definition of knee point voltage. Calculation of the current transformer accuracy limit factor. Also note that a voltage limiting mov connected across the highimpedance relay is shown in the figure.
A protection core is designed to transform a distortionfree signal. 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. Thompson, and brad heilman schweitzer engineering laboratories, inc. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly understood by protective relay engineers. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Pdf saturation analysis of current transformer researchgate. The opening paragraph of moretons classic paper 1 st. For differential protection for equipment like generator,transformer,bus etc. Ct knee point voltage calculation transformer volt scribd.
Ct analyzer current transformer testing, calibration and. Iscc max maximum short circuit currentthree phase 63000 amps vk ct knee point voltage specified 3700 volts xr system ratio 17 if maximum value of through fault current. This is to keep the voltage less than a specified value usually less than ct knee point voltage. As illustrated in 1ec600441, the class x current transformer is needed. Shortcircuit the free winding on the measured transformer leg to reduce the. A practical guide to ct saturation ariana hargrave, michael j. The paper relates the concept of kneepoint and the 10 ampere excitation.
As per iec, the 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. Knee point voltage, vk vk 3700 v at 2000a tap ct excitation current at 2000a tap in ma ie ct calculation where for 2000 amps vk knee point voltage of ct 3700 volts rb resistance of relay p543 0. The latest publications can be downloaded from the schneider electric internet web. Knee point voltage of a current transformer is the magnitude of secondary of current transformer. Aug 19, 2020 sometimes, it specifies the maximum value of the magnetising current io at knee point voltage. K same as c class, except the knee point must be greater. Apr 16, 2011 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. Second formula provides the voltage transformation in relation to. This result should be compared with current transformer factory test and be sure that result within acceptable tolerance.
The knee point voltage has importance when the ct is used for zone or differential protection. Current transformer concepts schweitzer engineering laboratories. Ct test and current transformer test procedure with testing. C ratio error can be determined by calculation from the. The knee point voltage of the current transformers should be at least twice the relay setting voltage.
Mov prevents the high voltage to be built up across the ct during internal fault and so preventing the relay and the cts from damaging. The point at which when the voltage is increased 10% and correspondingly the current increase 50%. The saturation test can test kneepoints up to 30 kv using an new innovative dc technique. Knee point voltage calculation electrical impedance. The latest publications can be downloaded from the schneider electric internet web site. Selected knee point voltage for 20001 a tap 4000vsame as that for 400 kv cts note. The saturation voltage is graphically located by the cross point of the straight slopes of the excitation curve 1. What is the significance of knee voltage for protection ct. 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% in differential and restricted earth fault ref. In order to properly test the knee point the connections must be as follows. The knee point is determined by observing a small voltage increase causing a large increase in current. This characteristics are current transformer burden, knee point voltage, saturation of current transformer. Jun 02, 2018 knee point voltage of a current transformer. Why do current transformer cores have different knee point.
Instrument transformers application guide abb group. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current. May 12, 2017 excitation tests are performed by applying an ac voltage to the secondary winding of the ct and increasing the voltage in steps until the ct is in saturation. Current transformers, along with voltage or potential transformers, are instrument transformers. May, 2014 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. Ct knee point voltage calculation free download as pdf file. Current transformers,voltage transformers and instrument. The mvct will calculate the rated knee point in compliance with either ieee c57. Knee point voltage of current transformer ps class. The knee pointis defined as the voltage at which a 10% increase in appl. The knee point voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of the current transformer which when increased in magnitude by 10% causes the magnetizing current to increase by 50%. The empiric formula for a ct free from saturation is.
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