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. More complex specifications are sometimes mentioned, which include the xr ratio of the network or the magnetising current of the power transformer. A 20 cm 2 i max primary 40 ka find maximum secondary burden permissible if no saturation is to occur. Why to use class ps ct s for differential protection. Pdf abb instrument transformers current transformer. What is the knee voltage of ct, and how is it calculated. 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.
Pdf the impact of currenttransformer saturation on. Sizing current transformers for line protection applications. A ct is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Therefore, the calculation of burden, vknee point voltage and alf show that requirement for ct is adequate. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. Ct, the knee point voltage vk is defined as the point on this curve from. Polarity class, saturation voltage, knee point voltage. Please help me how to calculate the same its very argent. 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. You simply read the knee point from the excitation curve published by the manufacturer. Combining this equation with the definition of ks in section a. The first approach to determine if a ct is rated for its application is to calculate whether the ac voltage that will be impressed on its secondary during a fault will exceed the ac volt. The sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200.
Instrument transformers application guide abb group. In order to properly test the kneepoint the connections must be as follows. Current transformer ct class ratio error phase angle error. Knee point voltage of ct provided by cgl for ref ct 115v. 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. A current transformer is, in many respects, different from other transformers. The knee point determines the saturation voltage of a.
To measure this first demagnetise the ct and apply voltage gradually from secondary keeping primary winding open circuited. If you are a design engineer and are designing an iron cor. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. With respect to ansi cts, some of the major details that one would look for in the ct are. Why to use class ps cts for differential protection.
This characteristics are current transformer burden, knee point voltage, saturation of current transformer. Current transformer saturation, ratio and polarity test set one button automated test. As the current transformer has two windings, in some cases it is confusing about the load of it. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. This is to keep the voltage less than a specified value usually less than ct knee point voltage. The purpose of a current transformer ct is to produce an exact replica of the primary current waveform with its magnitude reduced in proportion to the turns ratio. The impact of currenttransformer saturation on transformerdifferential protection article pdf available in ieee transactions on power delivery 303. This is a software package for high performance and visualization, combining. Ct saturation tolerance for 87l applications mpcomments. 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. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. Calculation of the current transformer accuracy limit factor. 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. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core, as shown in 1.
Knee point voltage, internal secondary resistance remanence no limit voltage is, therefore, 3. This concept is the one used for testing the knee point in current transformers with the. 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%. Understanding ct magnetization curve electric power. 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. This value was used in reference 4 for all the ct performance analysis, and will be used here also. 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.
Consider now the same application with the addition of a 5. Abb instrument transformers current transformer theory technical level training course 201203. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. The definition here is applicable for nongapped ct core. How do we select the proper current transformer for a. Power transmission and distribution author page 247 200708. 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. The knee point is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. The minimum knee point voltage is imposed and depends on the resistance of the secondary winding rct of the ct and of its real load r r. It is possible to combine the use of abb cvts as voltage transformers with using. Current transformer saturation, ratio and polarity test. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current.
The saturation point is at a voltage 10 to 20% greater than the true knee point of the curve. As can be seen from the above formula, here the customer is specifying the level of saturation, duly taking into account the maximum possible fault current in his network, the actual burden connected to the ct, etc. A ct according to ansiieee is also specified by the kneepoint voltage. A 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. Vk required ct knee point voltage volts if max maximum value of fault current a rct ct secondary winding resistance ohms rl single lead resistance from ct to relay ohms rt effective impedance of. 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. Current transformer errors and transformer inrush as. Ct burden, knee point voltage, core saturation details of. The equation must be evaluated for all likely ct secondary current distributions for. Class x corresponds to a better metering accuracy than classes 5p and even more so 10p see figure 3. The minimum knee point voltage vk required is given by the formula. 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. Definition, ratings, general functional requirements and classifications.
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. Knee point voltage of current transformer ps class. Ct beyond knee point voltage transformer electrical impedance. Knee point voltage of current transformer this is the significance of saturation level of a ct core mainly used for protection purposes. If you carefully read the definition, you will notice that it is something related to saturation of current transformer ct. 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. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion. This relay does not need an interposing ct see fig. Ct is a very dependable and accurate device for the most part except when in goes in to saturation. How to calculate the knee point voltage of a current.
Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Pdf high impedance restricted earth fault protection. According to the manufacturer, the internal burden of the auxiliary ct. Kneepoint voltage determination for current transformer. Also note that a voltage limiting mov connected across the highimpedance relay is shown in the figure. Prior to joining schweitzer engineering laboratories, inc. Four special differential protections and their respective. This is well above the knee point voltage of the ct, and is at best a marginal application. Knee point, saturation voltage, or c rating, depending on the users decision. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. But the fact should be clear that the current of primary circuit is the load of a current transformer. It then explainhow s this core activity corresponds to the ct equivalent circuit, ansi voltage ratings, and the familiar ct excitation graph. 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. Current transformers,voltage transformers and instrument.
A practical guide to ct saturation ariana hargrave, michael j. Demagnetization, ct ratio, knee point, saturation, polarity and phase deviation test displays multiple instantaneous saturation curves with knee point integrated 1 kv dc insulation test system color daylight viewable graphical display. What is knee point voltage, use of knee point voltage in. Ct,rated knee point, saturation voltage, or c rating, depending on the users decision. 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. By combining the equations 1 and 2, we find the following formula. Current transformer, kneepoint voltage, functional identification. Ct saturation tolerance for 87l applications zhihan xu ge digital energy, matt proctor ge digital energy. Current transformer saturation voltage disturbance.
1005 874 73 546 284 387 1004 373 916 701 1375 957 1469 968 470 560 1286 580 1203 1447 1356 69 1002 550 1163 218 62 438 301 554 220 1006 737 51 1109 1174 998