Current transformers,voltage transformers and instrument. Sensors which output directly to a voltage can be connected to most standard test equipment, so long as the expected voltage does not exceed the ratings of the equipment. You simply read the knee point from the excitation curve. 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. Ct saturation tolerance for 87l applications zhihan xu ge digital energy, matt proctor ge digital energy. Vk is the minimum kneepoint 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. For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.
Knee point voltage of current transformer is the significance of saturation level of a ct core mainly used for protection purposes. The diagram below can be used to select wire size if the lengths of the leads are known. If you carefully read the definition, you will notice that it is something related to. A corresponding correcting voltage is applied in series with the cts output terminals during a current pulse. Ct application guide ge grid solutions online store. 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. Medium and high voltage ct and pt vendors continental. With respect to ansi cts, some of the major details that one would look for in the ct are. The meter will give connection diagrams for the various ct and voltage probe wiring configurations. Knee point voltage of a current transformer is the magnitude of secondary of current transformer. The main body of cts outdoor type has to be grounded at the points provide by the manufacturer. Matlab was used in plotting the magnetization curves. Ct knee point voltage calculation transformer volt scribd. What is the secondary limiting voltage of a current transformer.
A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer. Knee point voltage for ps class ct cr4 discussion thread. This power factor is not the power factor of secondary load. Effects of the difference in tube voltage of the ct. The metering class ct operate around around knee point and protection class ct operates around ankle point so as the limiting value of secondary voltage is. This is a plot of the ct secondary current against secondary voltage. The basics of current transformers nk technologies. Ct burden can be expressed in two terms as example. Upto knee point the change nature is linear in charateristics and beyond knee point the curve looses its linearity property. 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. 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 output voltage of a current transformer ct can be tested in the field using an digital multimeter with a millivolt ac mvac range. How to calculate the knee point voltage of a current.
Definition of the ct kneepoint voltage iec 600441 class px formerly british standard bs3938. This test is useful to confirm that the ct is working properly and that current is flowing in the conductor that the ct is installed on. Ensure that ct is earthed at one point in the whole circuit. 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 current transformer also provides insulation against the high voltage of the power circuit and hence protect the apparatus and personnel from high voltage. In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. It means that meter connected to ct via control cable will read 100 a as 5 a. This means the secondary voltage the ct will support across a standard burden with 20 times rated current without exceeding 10 percent ratio correction. For this ct the knee point voltage for psclass core is v k 120v for 3ss3 winding and v k 60v for 3ss2 winding. It then explainhow s this core activity corresponds to the ct equivalent circuit, ansi voltage ratings, and the familiar ct excitation graph.
This concept is the one used for testing the knee point in current transformers with the. The following illustrates the method to size a class x current transformer. Ct, right section ii, subsection e will saturated cts cause my relay to misoperate. An open circuit secondary will allow the ct to saturate at a fairly low primary current. For example, a 600v window ct cannot be installed on or around a bare 2400v conductor, however a 600v window ct can be installed around a 2400v cable, if the ct is installed around the insulated portion of the cable and the insulation. The magnitude of this voltage is based on the magnitude of the resetting voltage sensed during previous switching cycles. Dear all, please guide me for the calculation of knee point voltage for ps class ct for ref protection of trafo transformer rating. Knee point voltage of current transformer ps class electrical4u. Plot the curve and evaluate the vk and img from the graph. Current transformers testing and commissioning guidelines. The obtained vk should be greater than specified one. What is the purpose of vt and ct in an electrical circuit. Ct knee point voltage calculation free download as pdf file.
Ct saturation tolerance for 87l applications mpcomments. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping primary winding open circuited. Wattnode meters and current transformers cts manufactured by continental control systems ccs are rated for use up to 600 vac. Current transformer circuits for power electronics. How to calculate knee point voltage of current transformer. The paper relates the concept of kneepoint and the 10 ampere excitation voltage of a conventional excitation curve to the actual secondary waveforms produced at and above the ansi voltage rating. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. On secondary side, star point of the current transformers are to be earthed at one point. The ct voltage class determines the maxim voltage that a ct can be in direct contact with. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. Current transformer ct and potential transformer pt sensors how to measure electrical power usage.
These curves illustrate how high the secondary voltage will in rise in order to force the rated secondary current. Altuve, normann fischer, gabriel benmouyal, and dale finney, schweitzer engineering laboratories, inc. Power transmission and distribution author page 247 200708. Ct is current transformer which steps down high ampere value to low ampere value. Please help me how to calculate the same its very argent. Now increase the voltage and monitor the excitation current up to the ct reaching near to saturation point. If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve.
Abstractthis paper discusses the factors to consider for sizing current transformers cts for line protection applications. Power transmission and distribution author page 244 200708. The knee point determines the saturation voltage of a. According iec, knee point voltage of a current transformer is defined as the voltage at which 10 % increase in voltage of ct secondary. Knee point voltage of current transformer this is the significance of saturation level of a ct core mainly used for protection purposes. Ct application guide for the 489 generator management relay 2 ge power management lead resistance if the relay is a great distance from the ct that supplies the currents, heavy gauge ct cables must be used to ensure good performance. Also the excitation current should less than or equal to 100ma at v k 4 for anyone of core3 windings.
Ct output voltage test continental control systems, llc. 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. When the standby service is at the transmission voltage level 60 kv and above or at the same service point as the generators interconnection, the generator shall provide the required combination metering units ptct. When the ct saturates, the secondary voltage of many cts will be at a lethal level. Ct knee point voltage calculation sheet primary secondary volt resistance resistance length in mva in amp in kv in ohm in ohmkm in meter in %. The whole point of a current transformer is that it transforms current, not voltage. The knee point voltage given in the transformer specs is an indication of the secondary voltage at or near the saturation point. The original magnitude can be determined by just multiplying the result with the transformation ratio. You simply read the knee point from the excitation curve published by the manufacturer. Sizing current transformers for line protection applications. A method of minimizing droop by indirect sensing of the cts peak core flux excursion is then presented.
In general, medium voltage is about 1 kvac to 69 kvac and high voltage from 100 kvac through 345 kvac. 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%. You can transform the voltage or current down with a transformer whose turns ratio is accurately known, then measuring the stepped down magnitude with a normal range instrument. The family of curves below describe the excitation characteristics for the 6005 multi ratio bushing current transformer shown on the previous sheet. Ct burden is an impedance of secondary circuit expressed in ohms and power factor. The paper uses the volttime area concept to introduce ct saturation and the voltage rating using an idealized bh curve. 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. Current transformer danger when secondary is left open. Knee point voltage of current transformer electrical. The current transformer is mainly used for relay protective scheme because it reduces the high magnitude of primary current to the suitable value for relay operation. With this argument, can i short the the secondary sides of current transformer and voltage transformer together on a busbar and then ground this busbar to create a single grounding point.
For example, a 600v donut ct cannot be installed on or around any bare 2400v conductor. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Knee point voltage of current transformer electrical concepts. British method here 5a is rated secondary current and 2volt across secondary. Vkp required ct knee point voltage ift max transformer through fault in ampere rct ct secondary winding resistance in ohms rw loop impedance of pilot wire between ct and the k ct transformation ratio. Current transformer ct and potential transformer pt are used to sense current and. How to calculate the knee point voltage of a current transformer and. Current transformer white paper the basics of current transformers ratio the ct ratio is the ratio of primary current input to secondary current output at full load.
The ct voltage class determines the maximum voltage the ct may come in direct contact with. A practical guide to ct saturation ariana hargrave, michael j. Revenue metering applications involving installations served from 60 kv through 230 kv. 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. However, a 600v donut ct can be installed around a 2400v cable if the ct is installed around the. What is the secondary limiting voltage of a current. After or beyond this voltage the linearity between primary and secondary circuit that is the desired property of current transformer does not work any more. Record the reading of voltage and current at several points. Ct knee point formula scottf electrical 10 jul 12 10. The kneepoint voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input. Instrument security factor of ct is the significant factor for choosing the metering instruments which to be connected to the secondary of the ct. If you are a design engineer and are designing an iron cor. Current transformer, simplified equivalent circuit. A point worth considering is that the meter has to be installed near the measuring point due to.
Ansi voltage ratings, and the familiar ct excitation graph. What is the significance of knee voltage for protection ct. Kneepoint voltage determination for current transformer. Instrument security factor is the ratio of instrument limit primary current to the rated primary current. A highvoltage current transformer may contain several cores, each with a secondary winding, for different purposes such as metering circuits, control, or protection.