Knee point voltage of current transformer is of importance in protection class ct i. Ct burden, knee point voltage, core saturation details of. Ct knee point voltage calculation transformer volt scribd. Power transmission and distribution author page 247 200708. Oct 04, 2017 the knee point voltage can be said as the rated voltage at which a transformer works. 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. A current transformer ct is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree ct accuracy class or current transformer class. If you are a design engineer and are designing an iron cor. 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. Why do current transformer cores have different knee point. The whole and sole requirement of a protection class ct is to maintain its measurement accuracy during fault as well as abnormal condition.
You simply read the knee point from the excitation curve. Protection class ct is normally specified as ps protection special. 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. 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. The system fault current and the burden of the ct should be known for calculating the knee point voltage. The saturation point is at a voltage 10 to 20% greater than the true kneepoint 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. The saturation point is at a voltage 10 to 20% greater than the true kneepoint of. Current transformer errors and transformer inrush as.
A practical guide to ct saturation ariana hargrave, michael j. How to calculate the knee point voltage of a current. Current transformer saturation, ratio and polarity test set specifications input 100 to 265 v, 1o, 50 or 60 hz, 15 a max. Current transformer saturation voltage disturbance. As show in fig 1 the ct secondary voltage vs not should be saturate and exciting. 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. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly.
Knee point voltage is nothing but a point at which the ct saturates. 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. Current transformer errors and transformer inrush as measured. 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 75c. The knee point voltage of ct is very important parameter for the ct used for differential protection. The formula i use for knee point voltage vk if for example we have a protection class ct rated as 30va 5p50. Knee point voltage of current transformer electrical concepts. Thi s ratio must be chosen hig her than 2 in order to ensure. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac.
The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Current transformers, along with voltage or potential transformers, are instrument transformers. How to calculate knee point voltage of current transformer. Ct application guide ge grid solutions online store. With respect to ansi cts, some of the major details that one would look for in the ct are. British method here 5a is rated secondary current and 2volt across secondary. According to the manufacturer, the internal burden of the auxiliary ct is 1.
To measure this first demagnetise the ct and apply. 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. Based on the above calculation, check the rated knee point voltage of ct. If you carefully read the definition, you will notice that it is something related to. The kneepoint is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. For voltages greater than theknee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. Current transformers,voltage transformers and instrument. Instrument security factor is the ratio of instrument limit primary current to the rated primary current. The knee point voltage can be said as the rated voltage at which a transformer works.
Current transformer saturation, ratio and polarity test set. Pls find below the following formula for calculating the knee point voltage. The knee point is determined by observing a small voltage increase causing a large increase in current. The ct core is entering the magnetically saturated region.
Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. This is well above the kneepoint voltage of the ct, and is at best a marginal application. Iec defines knee point as the intersection of straight lines extended from nonsaturated and saturated portions of the excitation curve. 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. The developed voltage across the relay terminals should be lower than the knee point voltage of the cts. Abstractcurrent transformer ct saturation, while a fairly. Since the requirement at 20001 a tap is 2v per turn,the selected knee point voltage for 30001 a tap is 6000v. A current transformer chosen for protection purpose are defined by protection class p. Therefore slope at knee point voltage will be, slope increase in voltage increase in current 0. Consider now the same application with the addition of a 5. Ct burden is an impedance of secondary circuit expressed in ohms and power factor. Knee point voltage of the ct is of valid importance. C400 protection current transformer assuming i nom 5a vvolt 0,01 0,1 1 10 excitation currentamp 400. 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.
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 this because the curve beyond point k becomes nonlinear. The point is known as the knee point where the voltage at this point. Thompson, and brad heilman, schweitzer engineering laboratories, inc. T he ratio of the knee point voltage to stability voltage determines the tripping time of the restricted earth f ault protectio n. According to ansiieee the intersection of the excitation curve with a 45 0 tangent line is the knee point. 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. Ct burden can be expressed in two terms as example. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. Please help me how to calculate the same its very argent. The knee point is a important parameter for ct selection for a particular electrical installation. Pdf the impact of currenttransformer saturation on. As there are different classes of current transformer cores, how it is decided whathow much will be the knee point voltage of that core.
The core of ps class is such that very high current is needed for saturation of the core. How to calculate the knee point voltage of a current transformer and. Accuracy limit factor of current transformer is a very important factor while selecting a ct for protection purpose. 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. Thatsy its always better to design the cts which can operate without saturation even at a voltage appears in the ct during fault condition. 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 knee point is defined as the voltage at which a 10% increase in appliedvoltage increases the magnetizing current by 50%.
Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly understood by protective relay engineers. 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 transformer volt. This power factor is not the power factor of secondary load.
As illustrated in 1ec600441, the class x current transformer is needed. Knee point voltage calculation electrical impedance. Ansi knee is more conservative compared to iec curve. Knee point voltage of current transformer ps class. Pdf high impedance restricted earth fault protection. Method of establishing relay setting current relay setting current is given by. This is in line with the knee point voltage requirement being specified for 400 kv cts. 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. Knee point voltage of current transformer electrical. Sometimes, it specifies the maximum value of the magnetising current io at knee point voltage. If maximum fault current at the location, in amperes. Calculation of the current transformer accuracy limit factor. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current.
According to iec, knee point voltage of a current transformer is defined as the voltage at which a 10 % increase in voltage of ct secondary results in a 50 % increase in secondary current. The accuracy of a current transformer depends among others of. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. It produces a current in its secondary which is proportional to the current in its primary. 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. This is in line with the knee point voltage requirement being specified for 400 kv ct s.
Sep 14, 2018 based on the above calculation, check the rated knee point voltage of ct. Sizing current transformers for line protection applications. If the knee point voltage calculated on the basis of the power network is higher than the specified knee point voltage of ct, then there is a chance of ct saturation during a fault in the power network, and probably protection relay will not operate. What is knee point voltage, use of knee point voltage in. 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 5p10p or 5p20 class cts cannot match the characteristic as that of ps class. This concept is the one used for testing the knee point in current transformers with the pte100c. Instrument security factor or isf of current transformerinstrument security factor is the ratio of instrument limit primary current to the. The sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. References 1 ieee standard requirements for instrument transformers, ieee std c57. In the flux current graph after a point the flux changes with the current insignifficantly. Ct knee point voltage calculation free download as pdf file. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. The knee point determines the saturation voltage of a. You simply read the knee point from the excitation curve published by the manufacturer. The following illustrates the method to size a class x current transformer. Instrument transformers scale the large values of voltage or current to small.