A ct is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. Current transformer errors and transformer inrush as. By combining the equations 1 and 2, we find the following formula. 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. Why to use class ps cts for differential protection. 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. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. Four special differential protections and their respective. Abb instrument transformers current transformer theory technical level training course 201203. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion.
A 20 cm 2 i max primary 40 ka find maximum secondary burden permissible if no saturation is to occur. Combining this equation with the definition of ks in section a. What is the knee voltage of ct, and how is it calculated. Ct saturation tolerance for 87l applications mpcomments. Saturated is defined as the secondary current no longer behaving linearly to the primary current within some. How do we select the proper current transformer for a. A practical guide to ct saturation ariana hargrave, michael j.
Pdf abb instrument transformers current transformer. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. Ct, the knee point voltage vk is defined as the point on this curve from. What is knee point voltage, use of knee point voltage in. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. 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. Class x corresponds to a better metering accuracy than classes 5p and even more so 10p see figure 3. The definition here is applicable for nongapped ct core.
Current transformer saturation voltage disturbance. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current. A current transformer is, in many respects, different from other transformers. But the fact should be clear that the current of primary circuit is the load of a current transformer.
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. Current transformer saturation, ratio and polarity test. Also note that a voltage limiting mov connected across the highimpedance relay is shown in the figure. Current transformer saturation, ratio and polarity test set one button automated test. Understanding ct magnetization curve electric power. The knee point determines the saturation voltage of a. The saturation voltage vsat is graphically found by locating the intersection of the straight portions of the excitation curve on loglog axes 3. Sizing current transformers for line protection applications. Knee point voltage of current transformer ps class. Pdf high impedance restricted earth fault protection. Current transformers,voltage transformers and instrument. It then explainhow s this core activity corresponds to the ct equivalent circuit, ansi voltage ratings, and the familiar ct excitation graph.
Knee point voltage of ct provided by cgl for ref ct 115v. This relay does not need an interposing ct see fig. Polarity class, saturation voltage, knee point voltage. If you are a design engineer and are designing an iron cor. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. 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. It is possible to combine the use of abb cvts as voltage transformers with using.
Knee point, saturation voltage, or c rating, depending on the users decision. 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. Thompson, and brad heilman, schweitzer engineering laboratories, inc. According to the manufacturer, the internal burden of the auxiliary ct.
More complex specifications are sometimes mentioned, which include the xr ratio of the network or the magnetising current of the power transformer. How cts work in its simplest form, a ct consists of two sets of wire windings around an iron core, as shown in 1. Consider now the same application with the addition of a 5. 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. Current transformer, kneepoint voltage, functional identification. How to calculate the knee point voltage of a current. With respect to ansi cts, some of the major details that one would look for in the ct are. 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.
In order to properly test the kneepoint the connections must be as follows. This is a software package for high performance and visualization, combining. 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 you carefully read the definition, you will notice that it is something related to saturation of current transformer ct. Knee point voltage, internal secondary resistance remanence no limit voltage is, therefore, 3. 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.
The minimum knee point voltage vk required is given by the formula. Ct,rated knee point, saturation voltage, or c rating, depending on the users decision. As the current transformer has two windings, in some cases it is confusing about the load of it. The impact of currenttransformer saturation on transformerdifferential protection article pdf available in ieee transactions on power delivery 303.
The sample ct in figure 2 has an ansi knee point of about 200v, a saturation voltage of about 275v, and is class c200. Please help me how to calculate the same its very argent. The saturation point is at a voltage 10 to 20% greater than the true knee point of the curve. Calculation of the current transformer accuracy limit factor.
This concept is the one used for testing the knee point in current transformers with the. The equation must be evaluated for all likely ct secondary current distributions for. 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. Definition, ratings, general functional requirements and classifications.
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. Hi i m doing a knee point using a cpc100 but manufacturer results say knee point from 1s1 to 1s5 is 4. 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%. 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. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping primary winding open circuited. Knee point voltage of current transformer electrical. Current transformer ct class ratio error phase angle error. Ct beyond knee point voltage transformer electrical impedance.
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. Therefore, the calculation of burden, vknee point voltage and alf show that requirement for ct is adequate. 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. 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 transformers application guide abb group. The knee point is defined differently in ansi standards and the iec standards, and neither is as easily definable as the saturation voltage. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. A ct according to ansiieee is also specified by the kneepoint voltage. The following illustrates the method to size a class x current transformer. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required.
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. Why to use class ps ct s for differential protection. Power transmission and distribution author page 247 200708. 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. Knee point voltage of current transformer this is the significance of saturation level of a ct core mainly used for protection purposes. Ct burden, knee point voltage, core saturation details of. This is to keep the voltage less than a specified value usually less than ct knee point voltage. You simply read the knee point from the excitation curve published by the manufacturer. Ct saturation tolerance for 87l applications zhihan xu ge digital energy, matt proctor ge digital energy. 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 well above the knee point voltage of the ct, and is at best a marginal application. Prior to joining schweitzer engineering laboratories, inc.