If you are a design engineer and are designing an iron cor. Oct 04, 2017 the knee point voltage can be said as the rated voltage at which a transformer works. How do we select the proper current transformer for a given. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. However, after 3 cycles, the flux asymmetry produced by the primary current causes ct saturation, thus producing large distortion of ct secondary voltage. Polarity class, saturation voltage, knee point voltage excitation characteristics. 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. 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. Portable ct pt analyzertransformer testgfuve electronics. The point is known as the knee point where the voltage at this point. You simply read the knee point from the excitation curve published by the manufacturer.
C ratio error can be determined by calculation from the. Saturated is defined as the secondary current no longer behaving linearly to the primary current within some. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. Calculation of the current transformer accuracy limit factor. At the knee point the voltage across the secondary tends to remain constant whereas the current continues to rise.
With respect to ansi cts, some of the major details that one would look for in the ct are. The saturation voltage is graphically located by the cross point of the straight slopes of the excitation curve 1. K same as c class, except the kneepoint must be greater. This concept is the one used for testing the kneepoint in current transformers with the. Understanding ct magnetization curve electric power.
Automatic calculation of excitation knee point value. 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. 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. Ifs stands for maximum through fault current as reflected at the ct secondary terminalsifs ct ratio.
Kalpa electrikal lv current transformers kalpa electrikal. 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. 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. To measure this first demagnetise the ct and apply. 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. How to calculate knee point voltage of current transformer. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. Ct knee point voltage calculation transformer volt scribd. The formula i use for knee point voltage vk if for example we have a protection class ct rated as 30va 5p50. 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. What is the secondary limiting voltage of a current. 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.
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. Application guidelines for microprocessorbased, high. Thompson, and brad heilman schweitzer engineering laboratories, inc. 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. R is the total resistance of the secondary of the ct. 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. Record the reading of voltage and current at several points. Dimensioning of current transformers for protection. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. 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 kneepoint voltage iec 600441 class px formerly british standard bs3938. Standard ct for laboratory purposes earth fault protectiondifferential protectionbusbar protection systems bushing type, oilimmersed ct in power transformer. C400 protection current transformer assuming i nom 5a vvolt 0,01 0,1 1 10 excitation currentamp 400.
In this case, the excitation voltage is somewhat higher than the ct knee voltage, so a different tap should be considered. Power transmission and distribution author page 247 200708. How do we select the proper current transformer for a. Current transformer saturation, ratio and polarity test.
Ct knee point voltage calculation free download as pdf file. Ifs stands for maximum through fault current as reflected at the ct secondary terminalsifsct ratio. The saturation point is at a voltage 10 to 20% greater than the true knee point of the curve. The effect of the remanent flux is also to reduce the time to saturation. Pdf saturation analysis of current transformer researchgate.
A practical guide to ct saturation ariana hargrave, michael j. The knee point voltage can be said as the rated voltage at which a transformer works. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Ct knee point voltage calculation transformer volt. Instrument transformers application guide abb group. Please help me how to calculate the same its very argent. The fault level in mva at the point where the current transformer is to be installed. The saturation point is at a voltage 10 to 20% greater than the true kneepoint of the curve. The kneepoint is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. In the flux current graph after a point the flux changes with the current insignifficantly. The ct voltage output v2 then follows the primary current.
In order to select the proper indoor type current transformer for an application, there are four phases need to be addressed. Application guidelines for microprocessorbased, highimpedance bus differential relays. Current transformers,voltage transformers and instrument. Plot the curve and evaluate the vk and img from the graph. Knee point voltage of current transformer ps class. 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. Sizing current transformers for line protection applications. What is knee point voltage, use of knee point voltage in. Knee point calculation the minimum knee point voltage for a given ps class ct is calculated by. The knee point determines the saturation voltage of a. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. Instrument security factor is the ratio of instrument limit primary current to the rated primary current.
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. If maximum fault current at the location, in amperes. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. This concept is the one used for testing the knee point in current transformers with the. Ct knee point voltage check by cpc100 omicron youtube. 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. The obtained vk should be greater than specified one. Knee point voltage, internal secondary resistance remanence no limit download as powerpoint presentation. 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. This because the curve beyond point k becomes nonlinear. 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. Observe that for the first 3 cycles, the flux stays lower than the saturation knee point 10 pu.
Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. This concept is the one used for testing the knee point in current transformers with the pte100c. 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. 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. The following illustrates the method to size a class x current transformer. The knee point determines the saturation voltage of a ct, right. This value was used in reference 4 for all the ct performance analysis, and will be used here also. Insulation class as per is a,e,b,f,h system voltage. Knee point voltage of current transformer electrical. Importantly, in the event of breaker failure, the relays are able to identify the individual failed breaker. Ct saturation tolerance for 87l applications mpcomments. Pdf the impact of currenttransformer saturation on. Ct knee point voltage can be calculated using formula. T are air core hence bh curve of air, as core material, could be used, to find knee point flux density and the amp.
How to calculate the knee point voltage of a current. Aug 08, 2011 the formula for knee point voltage is. Saturated is defined as the secondary current no longer behaving linearly to the primary current within some tolerance, like 10% or 5%. Dec 10, 2014 knee point calculation the minimum knee point voltage for a given ps class ct is calculated by. Pdf current transformers ct are the key device in power system protection. Dimensioning of current transformers for protection applications. Withstand test of power frequency alternating current. 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.
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