Knee point voltage calculation free download as pdf file. The kneepoint voltage is less applicable for metering current transformers as their accuracy is generally much higher, but constrained within a very small range of the current transformer rating, typically 1. Ct application guide ge grid solutions online store. Normally flux density is chosen near knee point of the magnetization curve with some margin to overcome over fluxing, voltage variation and. Current transformer errors and transformer inrush as. A practical guide to ct saturation ariana hargrave, michael j. Knee point voltage calculation electrical impedance. Ct knee point formula gilly1 electrical 11 jul 12 04. In magnetic circuit calculations, it is more common to use the mag. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve.
Since the requirement at 20001 a tap is 2v per turn,the selected knee point voltage for 30001 a tap is 6000v. This is well above the kneepoint voltage of the ct, and is at best a marginal application. This characteristics are current transformer burden, knee point voltage, saturation of current transformer. Reverse power flow relay are used in generating stations protection. Also note that a voltagelimiting mov connected across the highimpedance relay is shown in the figure. 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. If the fault current and knee point voltage are such that the intersection of these two points plots below the curve, then the application will be safe with respect to the voltage limits. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Class, saturation voltage, knee point voltage excitation characteristics voltage rating, current rating, thermal ratings etc. The loglog graph paper shall be with square decades. How to calculate the knee point voltage of a current. T are air core hence bh curve of air, as core material, could be used, to find knee point flux density and the amp. C400 protection current transformer assuming i nom 5a v volt 0,01 0,1 1 10 excitation currentamp 400.
The kneepoint is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. You simply read the knee point from the excitation curve. Again from the equation above, it is found that, secondary voltage of. Instrument security factor or isf of current transformer. Ct burden, knee point voltage, core saturation details of. 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. Knee point voltage of current transformer ps class electrical4u. Instrument security factor is the ratio of instrument limit primary current to the rated primary current.
Ct knee point voltage calculation free download as pdf file. The knee point is determined by observing a small voltage increase causing a large increase in current. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as. Why do current transformer cores have different knee point. However, the concept of knee point voltage is very pertinent to protection current transformers, since they. What is knee point voltage, use of knee point voltage in. 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. Thompson, and brad heilman, schweitzer engineering laboratories, inc. 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.
As there are different classes of current transformer cores, how it is decided whathow much will be the knee point voltage of that core. Voltage transformer which is also called as a potential transformer. Instrument security factor is the ratio of instrument limit primary electric current to the rated primary current. Knee point voltage is calculated for electrical current transformers and is very important factor to choose a ct. Please help me how to calculate the same its very argent. But the fact should be clear that the current of primary circuit is the load of a current transformer. The basic calculation method of a magnetic circuit is the same as.
What is the secondary limiting voltage of a current. Upt primary voltage ust secondary voltage 1 fault inception 2 aperiodic damping of ust 3 periodic damping of ust time ts in ms ratio classes 3pt1 6pt1 3pt2 6pt2 3pt3 6pt3 10. The knee point voltage can be said as the rated voltage at which a transformer works. As illustrated in 1ec600441, the class x current transformer is needed. How to calculate knee point voltage of current transformer. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. The point is known as the knee point where the voltage at this point. The following illustrates the method to size a class x current transformer. I read knee point is a point at which 10% increase in voltage leads to 50% increase in current. This concept is the one used for testing the kneepoint in current transformers with the pte100c. When the operating point bd is above the knee point of the bh curve. If voltage ur1is smaller than the set voltage, then the relay will not operate for external fault and will remain stable. Class x current transformer class x current transformer is use in conjunction with high impedance circulating current differential protection relay, eg restricted earlh fault relay.
The saturation point is at a voltage 10 to 20% greater than the true kneepoint of the curve. This condition defines the stability limit for the scheme. As the current transformer has two windings, in some cases it is confusing about the load of it. 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. This is in line with the knee point voltage requirement being specified for 400 kv cts. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. This is to keep the voltage less than a specified value usually less than ct knee point voltage.
Knee point voltage of current transformer ps class. Medium voltage products technical guide protection. How to calculate the knee point voltage of a current transformer and. The knee point determines the saturation voltage of a. Voltage drop calculation methods with examples explained.
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%. In this graph, the knee point voltage is defined as the point on the excitation curve where the tangent is at 45 degree to the abscissa. You simply read the knee point from the excitation curve published by the manufacturer. Consider now the same application with the addition of a 5. Value of flux density in the transformer determines the core area and yoke area, hence computation of flux density is very important and crucial part in design. Function description for high impedance busbar protection. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current. 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. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping primary winding open circuited. Current transformer ps classbefore understanding knee point voltage. The sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200. 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. The knee point voltage of the current transformers should be at least twice the relay setting voltage.
The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. According to the manufacturer, the internal burden of the auxiliary ct. Scribd is the worlds largest social reading and publishing site. If you are a design engineer and are designing an iron cor. At the kneepoint the voltage across the secondary tends to remain constant whereas the current continues to rise. Rudolf class x ct with knee point voltage of 1b5v is suitable to be use in thls application. 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.
The saturation point is at a voltage 10 to 20% greater than the true kneepoint of. Calculation and analysis of transformer inrush current based on parameters of transformer and operating conditions. If you carefully read the definition, you will notice that it is something related to saturation of current transformer ct. In the flux current graph after a point the flux changes with the current insignifficantly. 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 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. Sizing current transformers for line protection applications. If maximum fault current at the location, in amperes. Ct knee point voltage calculation transformer volt scribd. It employed in electrical energy power system for stepping down the voltage of the system to some protected value which is often. One final point, if the reverse bias voltage vr applied to the diode is increased to a sufficiently high enough value, it will cause the diodes pn junction to overheat and fail due to the avalanche effect around the junction.
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