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. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly understood by protective relay engineers. Dimensioning of current transformers for protection applications 4 ge multilin tends to drastically reduce the effect of the remanent flux left in the core as a result of its lower magnetizing impedance and much lower secondary time constant. Ct saturation tolerance for 87l applications mpcomments.
Dimensioning of current transformers for protection applications. You simply read the knee point from the excitation curve published by the manufacturer. The knee point determines the saturation voltage of a ct, right. This concept is the one used for testing the knee point in current transformers with the. The fault level in mva at the point where the current transformer is to be installed. Ct knee point voltage can be calculated using formula. Ct kneepoint voltage mismatch can thus be tolerated and the risks in finding the best equipotential point for connection of ct secondaries in parallel become a thing of the past. 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. Dimensioning of current transformers for protection applications ge multilin 3 voltage on ct secondaries during faults description testing and experience have shown that the performance of many relays can be adversely affected by moderate degrees of ct saturation.
Insulation class as per is a,e,b,f,h system voltage. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. Plot the curve and evaluate the vk and img from the graph. Dec 10, 2014 knee point calculation the minimum knee point voltage for a given ps class ct is calculated by. Aug 08, 2011 the formula for knee point voltage is. Understanding ct magnetization curve electric power.
Knee point voltage, internal secondary resistance remanence no limit download as powerpoint presentation. 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. 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. Ct knee point voltage calculation transformer volt. Definition of the ct kneepoint voltage iec 600441 class px formerly british standard bs3938. This concept is the one used for testing the knee point in current transformers with the pte100c. However, after 3 cycles, the flux asymmetry produced by the primary current causes ct saturation, thus producing large distortion of ct secondary voltage. Saturated is defined as the secondary current no longer behaving linearly to the primary current within some tolerance, like 10% or 5%. Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. The following illustrates the method to size a class x current transformer. In order to select the proper indoor type current transformer for an application, there are four phases need to be addressed.
Observe that for the first 3 cycles, the flux stays lower than the saturation knee point 10 pu. Ct knee point voltage calculation free download as pdf file. Thompson, and brad heilman schweitzer engineering laboratories, inc. R is the total resistance of the secondary of the ct. Sizing current transformers for line protection applications. Earth fault protection an overview sciencedirect topics. How to calculate the knee point voltage of a current. Now increase the voltage and monitor the excitation current up to the ct reaching near to saturation point. In this case, the excitation voltage is somewhat higher than the ct knee voltage, so a different tap should be considered. Ifs stands for maximum through fault current as reflected at the ct secondary terminalsifsct ratio. A practical guide to ct saturation ariana hargrave, michael j.
The saturation voltage is graphically located by the cross point of the straight slopes of the excitation curve 1. As illustrated in 1ec600441, the class x current transformer is needed. Their performance is defined in terms of a kneepoint voltage kpv, the magnetizing current imag at the knee point voltage or 12 or 14 the kneepoint voltage, and the resistance of the ct secondary winding corrected to. The effect of the remanent flux is also to reduce the time to saturation. The obtained vk should be greater than specified one. Importantly, in the event of breaker failure, the relays are able to identify the individual failed breaker. 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.
Saturated is defined as the secondary current no longer behaving linearly to the primary current within some. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. Oct 04, 2017 the knee point voltage can be said as the rated voltage at which a transformer works. Current transformer saturation, ratio and polarity test. What is the secondary limiting voltage of a current. If maximum fault current at the location, in amperes. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. Current transformer saturation, ratio and polarity test set. Current transformers,voltage transformers and instrument. Knee point voltage of current transformer ps class.
Calculation of the current transformer accuracy limit factor. The saturation point is at a voltage 10 to 20% greater than the true knee point of the curve. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. Neglecting saturation, the worstcase voltage on the ct would be v e z burden 2. This value was used in reference 4 for all the ct performance analysis, and will be used here also. C400 protection current transformer assuming i nom 5a vvolt 0,01 0,1 1 10 excitation currentamp 400. The ct voltage output v2 then follows the primary current. The kneepoint is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Instrument transformers application guide abb group.
How do we select the proper current transformer for a given. Pdf the impact of currenttransformer saturation on. May 09, 20 the knee point voltage of a current transformer is the magnitude of the secondary voltage after which the output current ceases to linearly follow the input current within declared accuracy. At the knee point the voltage across the secondary tends to remain constant whereas the current continues to rise. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, up until the. This because the curve beyond point k becomes nonlinear. 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 formula i use for knee point voltage vk if for example we have a protection class ct rated as 30va 5p50.
Because the relay has a high impedance the voltage developed across it during an internal fault may be sufficiently high so as to force the cts into saturation. Application guidelines for microprocessorbased, high. The knee point is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. Standard ct for laboratory purposes earth fault protectiondifferential protectionbusbar protection systems bushing type, oilimmersed ct in power transformer. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Application guidelines for microprocessorbased, highimpedance bus differential relays. How do we select the proper current transformer for a. Ifs stands for maximum through fault current as reflected at the ct secondary terminalsifs ct ratio.
Jul 24, 2018 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. Instrument security factor is the ratio of instrument limit primary current to the rated primary current. This concept is the one used for testing the kneepoint in current transformers with the. How to calculate knee point voltage of current transformer.
The point is known as the knee point where the voltage at this point. Mar 23, 2018 for the love of physics walter lewin may 16, 2011 duration. Note in the ct excitation curve in figure 2 that, at the indicated ansi knee point, the magnetizing impedance is about 5000. Ct knee point voltage calculation transformer volt scribd. In the flux current graph after a point the flux changes with the current insignifficantly. Pdf current transformers ct are the key device in power system protection. Ct knee point voltage check by cpc100 omicron youtube. The kneepoint voltage of a current transformer is the magnitude of the secondary voltage after which the output current ceases to linearly follow the input current within declared accuracy. It is therefore necessary when applying high impedance relay protection to calculate the relay circuit resistance and ct knee point voltage such that the ct is still able to produce sufficient output under saturated conditions. Withstand test of power frequency alternating current. C ratio error can be determined by calculation from the.
Record the reading of voltage and current at several points. Polarity class, saturation voltage, knee point voltage excitation characteristics. K same as c class, except the kneepoint must be greater. 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. The knee point determines the saturation voltage of a. The knee point voltage can be said as the rated voltage at which a transformer works. Pdf saturation analysis of current transformer researchgate. To measure this first demagnetise the ct and apply. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the. With respect to ansi cts, some of the major details that one would look for in the ct are. The saturation point is at a voltage 10 to 20% greater than the true kneepoint of the curve. What is knee point voltage, use of knee point voltage in. Kalpa electrikal lv current transformers kalpa electrikal.
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. Alternative method there is also an alternative method for calculating f a, that is, by first calculating the voltage on the ct secondary side, using the rated values. Portable ct pt analyzertransformer testgfuve electronics. 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. Dimensioning of current transformers for protection. Power transmission and distribution author page 247 200708.
Knee point voltage of current transformer electrical. Automatic calculation of excitation knee point value. If you are a design engineer and are designing an iron cor. A knee point voltage of 2700 v, tells you that with a 1 ohm burden connected to the ct, you can drive the primary up to 2700 times 10ka before the unit saturates. Please help me how to calculate the same its very argent.
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