If you are a design engineer and are designing an iron cor. Jan 21, 2020 hi we generally do knee point voltage test for special class protection of current transformers,like transformers and generators differential protections etc. Instrument transformers application guide abb group. May 27, 2015 there was a query in all this about how to measure knee point on a transformer. The essentials of current transformers in power circuits. 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. The saturation point is at a voltage 10 to 20% greater than the true kneepoint. Knee point voltage of a current transformer is the magnitude of secondary of current transformer. You should then hook the transformer up to a circuit breaker of an equal or higher current rating so that the breaker will not trip under normal operation of the transformer. Knee point voltage and current transformers cr4 discussion. It employed in electrical energy power system for stepping down the voltage of the system to some protected value which is often.
Aug 08, 2011 hi i m doing a knee point using a cpc100 but manufacturer results say knee point from 1s1 to 1s5 is 4. Transient response for capacitor voltage transformers. In this case we need observe the name plate details of current transformer,there the man. The knee point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating. Current transformers,voltage transformers and instrument. When connecting a transformer to an electrical power source, you need to calculate the current, it will draw through the primary. Apr 16, 2011 sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. If you continue to use this site we will assume that you are happy with it. Reverse power flow relay are used in generating stations protection. Knee point calculation the minimum knee point voltage for a given ps class ct is calculated by. Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. A ct steps down current and a ct specified as stepping down in the ratio 50. Current transformer, kneepoint voltage, functional identification. Knee point voltage is nothing but a point at which the ct saturates.
Voltage transformer which is also called as a potential transformer. Instrument security factor or isf of current transformer. Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Knee point voltage of current transformer electrical. What is the knee point test for a current transformer. The following illustrates the method to size a class x current transformer. 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. Current transformer, simplified equivalent circuit.
If you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. 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. That point on the magnetizing curve where an increase of 10% in the flux density voltage causes an increase of 50% in the magnetizing force current. Knee point voltage is nothing but a magnitude of the saturation limit of the current transformer. Hi, the ct requirements for the electrical system are to be specified in the initial detail engineering stage itself for any project. When the currents in a number of feeders need not be individually metered but summated to a single meter or instrument, a summation current transformer can be. For differential protection for equipment like generator, transformer,bus etc.
We use cookies to ensure that we give you the best experience on our website. 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. Altuve, normann fischer, gabriel benmouyal, and dale finney, schweitzer engineering laboratories, inc. Observe that for the first 3 cycles, the flux stays lower than the saturation knee point 10 pu. Enter the information below to calculate the burden resistor required for current transformers that do not contain a builtin burden resistor. Abstractthis paper discusses the factors to consider for sizing current transformers cts for line protection applications. The ct voltage output v2 then follows the primary current. The ratio then is not accurate enough and due to this reason it may cause misoperation of the protective relays. The knee point voltage is defined as that minimum sinusoidal e. The k rating is a proposed rating where the kneepoint is at least 70% of the secondary voltage rating. Knee point voltage of current transformer is the significance of saturation level of a ct core mainly used for protection purposes. Ct burden, knee point voltage, core saturation details of.
The secondary voltage rating is the voltage the ct will deliver to a standard burden at 20 times rated. Abstract current transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Current transformer ps classbefore understanding knee point voltage of. The behavior of a 3phase transformer bank is analyzed in the same way as for a singlephase unit assume that both the primary and secondary windings are connected in wye configuration consider only one transformer the primary voltage is the linetoneutral voltage, vh1, n the secondary voltage is the linetoneutral voltage. The k rating is a proposed rating where the knee point is at least 70% of the secondary voltage rating. 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.
What is the use of knee point voltage electrical4u. If you carefully read the definition, you will notice that it is something related to. Transformer current calculator calculate transformer. Note in the ct excitation curve in figure 2 that at the indicated ansi knee point the magnetizing impedance is 5000. Current transformer errors and transformer inrush as measured. It transforms the high voltage current into the low voltage current due to which the heavy current flows through the transmission lines is safely monitored by the ammeter. 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 as we know ct core is made of crgo cold rolled grain oriented silicon steel. Ct application guide ge grid solutions online store. Specifications of cts 6 knee point voltage it is defined as that sinusoidal voltage of rated freq. Knee point voltage of current transformer electrical concepts. 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%. At the kneepoint the voltage across the secondary tends to remain constant whereas the current continues to rise. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance. May 09, 20 from the knee point upwards, the magnetizing current increases abruptly even with small increments in voltage across the secondary terminals.
Thatsy its always better to design the cts which can operate without saturation even at a voltage appears in the ct during fault condition. Each protection class ct has to be operated in its non saturation region. What is the secondary limiting voltage of a current transformer. The basics of current transformers nk technologies. Current transformer saturation, ratio and polarity test. What is the secondary limiting voltage of a current. A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value. Mar 15, 2008 the knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. Knee point voltage is calculated for electrical current transformers and is very important factor to choose a ct. Require low saturation level to protect instruments, thus use nickel iron alloy core with low exciting current and knee point at low flux density.
In this lecture we will solve some tutorial problems on ct and analyse. The excitation current can then be read from the curve. The secondary voltage rating is the voltage the ct will deliver to a standard burden at 20 times rated secondary current without exceeding 10% ratio correction. We know that ct core is made of crgo cold rolled grain oriented silicon steel.
You simply read the knee point from the excitation curve published by the manufacturer. Mar 14, 2018 if you are the user of an iron core ct, then you dont try to calculate the knee point of the curve. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times. Knee point voltage of current transformer ps class.
May 12, 2017 current transformers play an important role in the monitoring and protection of electrical power systems. So for choosing ct for differential protection of transformer, one should consider current transformer ps class rather its convectional protection class. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current. Again from the equation above, it is found that, secondary voltage of. Definition of the ct knee point voltage iec 600441 class px formerly british standard bs3938. Full descriptions of the relay characteristics are found in the 489 manual. Knee point voltage of current transformer april 25, 2018 october 2, 2016 by admin 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 concept is the one used for testing the kneepoint in current transformers with the pte100c. What is meant by knee point voltage of a current transformer. Current transformer design guide an effective design of a ring type c. As illustrated in 1ec600441, the class x current transformer is needed. Trivector meter is an energy meter which accurately measures all the parameters of supply such as voltage, current, power factor, active load, reactive load, apparent load etc.
The knee point determines the saturation voltage of a. 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. Knee point voltage of ct provided by cgl for ref ct 115v. The point of contact between the rod and the metal surfaces gets heated to the melting point, fusing a part of the rod and both surfaces into a single piece. Sizing current transformers for line protection applications.
Pls find below the following formula for calculating the knee point voltage. Sizing current transformers for line protection applications hector j. So that we have to calculate the maximum allowable voltage limit, that can be applied to the secondary winding of the current transformer. What is the difference between ct secondary limiting voltage and knee point voltage. Introduction the knee point of any transformer is the point at which the core starts to saturate presenting thus a low impedance path for the current to flow. Apr 25, 2018 this because the curve beyond point k becomes nonlinear. How to calculate knee point voltage of current transformer. Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output. Transformer short circuit current calculation and solutions. Current transformer saturation, ratio and polarity test set. To provide fast operation on an in zone fault, the current transformer should have a knee point voltage at least twice the setting voltage of the relay. To measure this first demagnetise the ct and apply voltage gradually from secondary keeping primary winding open circuited. How to calculate the knee point voltage of a current. A practical guide to ct saturation ariana hargrave, michael j.
Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments. A 20 cm 2 i max primary 40 ka find maximum secondary burden permissible if no saturation is to occur. However, after 3 cycles, the flux asymmetry produced by the primary current causes ct saturation, thus producing large distortion of ct secondary voltage. Kneepoint voltage determination for current transformer. Thompson, and brad heilman, schweitzer engineering laboratories, inc. 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 relates to the ability of the secondary winding of a current transformer to produce enough voltage to drive the required current around the connected burden and what excitation current is lost inside the transformer to do that. Jul 25, 2018 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. Bartype current transformer this type of current transformer uses the actual cable or busbar of the main circuit as the primary winding, which is equivalent to a single turn.
Utilising the british standards, cts were defined by the knee point voltage u kn and the internal secondary resistance r i. When the primary of ct is energized a working mmf is produced in the core. Current transformer calculation extreme electronics. Knee point voltage of current transformer ps class electrical4u.
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