Once powered, the winding generates a magnetic field and stores. The minimum aid ratio guarantees that wealthier towns receive at least a minimum amount of ecs aid. If the primary current changes the secondary current output will change proportionally. A ct which has an opening in the center to accommodate a primary conductor which may be busbar or cable through it. For example, the potential transformer mentioned above having a rating of 480. The log inactivation calculation adjusts the wtps ct value to account for the disinfection chemical reaction process variables that influence the disinfection process efficiency. The voltage and current across a ct are linear only up till a certain value normally specified as a multiple of the nominal rating, after which the ct will saturate and the curve will level off. To use ratiocorrectionfactor curves, one must calculate the ct burden for each value of secondary current for which he wants to know the ct accuracy. To use ct to demonstrate that you have sufficient water treatment, you need to do the following three things. The required excitation voltage from the auxiliary ct is 4. The current transformer is expected to deliver about 5a or lower under healthy load conditions.
Calculation of current transformer transient errors article pdf available in ieee transactions on power apparatus and systems 976. The ct burden impedance decreases as the secondary current increases, because of saturation in the magnetic circuits of relays and other devices. Ratio testing with applied burden is the most accurate and complete approach for testing at ct in service. Current transformer basics and the current transformer. Ct saturation occurs when the flux required to produce the secondary current exceeds the saturation flux density of the core as dictated by the physical dimensions of the ct. It has a round iron core with a coil of insulated wire wrapped around it. How to calculate ct ratio and pt ratio cr4 discussion thread. The reason a ct saturates is related to what physically occurs inside a ct during the electromagnetic induction process.
Page 1 ct calc 2009 chlorine contact time calculations steps to solving a math problem. The ct ratio is the ratio of primary current input to secondary current output at full load. Current transformers are the very important aspect of. The basics of current transformers nk technologies. Some types of equipment employ this method to calibrate the equipment or to permit a single ratio ct to be utilized for several different ampacities of equipment. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to saturate the core or cause voltage breakdown. As your circuit indicates, the ratio is simply 6005120. Too high ct ratio may be problematic for relays settings. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1. It is possible to calculate the time to saturate for any ct given the set of. The standard current transformer secondary winding is rated at 5a as per ansi standards. The primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected. The calculator below can be used to determine the desired burden resistor value. Example the distance between the cts and the protection relay is 15 metres, 4 mm 2 cu conductors in 4wire connection are used.
The secondary current is n times less than the current being measured for a ct with a turns ratio of n for example if a. This situation is exacerbated by at least one wellknown pacs vendor using percentages for their inbuilt ratio measurement. Sizing current transformers for line protection applications. First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an ac.
As per ansi, the ct ratio shall be chosen so that the maximum symmetrical. A ct rated at say 5p20 will stay linear to approximately 20 times its nominal current. Insulation and basic impulse insulation level bil kv. Scope this document describes the calculation of the actual accuracy limit factor f a for protectiontype p current transformers ct. However, it is the negligible impedance during ct saturation that will most affect relay settings. Reflected to the primary the auxiliary ct secondary burden is 4. It is convenient to define the excitation curves in units of ampereturns. The value of ir is determined by dividing the value of ct actually achieved by the treatment plant by that of ct required to achieve the desired level of pathogen inactivation see section 5. Ct knee point, connected burden, and system xr ratios. Current transformer saturation voltage disturbance. With respect to ansi cts, some of the major details that one would look for in the ct are. To use ratiocorrectionfactor curves, one must calculate the ct burden for each. The iron core with the coil of wire is also called the winding. This ct can be interpreted as a protection class ct having an accuracy of 5% over a current range of 10 times of normal primary current rating.
The log inactivation calculations equations 1 and 2 in the lefthand bottom box use the wtps ct ctcalc and the epadeveloped ct log inactivation tables ct99. So perhaps you are really asking how to measure the ratio. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. A toroidal transformer is a transformer shaped like a doughnut. Now we can divide 100 by 40 and get our answer of 2. How to calculate kwh of ct pt energy meter multiplying factor ashok electrical guru. This ct will maintain its measurement accuracy of 5% over 10xrated primary current i. A critical concern is the performance of fast protective schemes instantaneous or differential elements during severe saturation of low ratio cts.
Pdf calculation of current transformer transient errors. Solving the differential equation of the fundamentals of the inductive circuit. Accuracy limit factor of current transformer electrical. Typical secondary excitation characteristic for a multiratio ct. How to calculate kwh of ct pt energy meter multiplying. Instrument transformer basic technical information and.
Ct ratio with burden testing secondary connection is made through the test switch same connection that is used for the rest of the site testing. Ct ratio core crosssectional area connected burden. As per ansi, the ct ratio shall be chosen so that the maximum symmetrical fault. If phase distance relays at bus a are to reach 80% of line ab, what should the relay reach be set to in secondary. At this point, even though i p to flow, there is no more change in flux, v. The polarity of a ct is determined by the direction the coils are wound around the core of the ct clockwise or. Cross out anything given that has nothing to do with solving the problem. The main purpose of a current transformer is to translate the primary current in a high voltage power system to single level that can be handled by delicate electromechanical or electronic device. The circuit connection shall be made as shown figure 4. This is the first calculation that we need to do as this will give us the multiplier.
Turns ratio tests optional test this test is to ensure the turns ratio of ct at all taps. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the ct output is 40 amps. As a reminder, the 5 in the 5p20 would be the ct accuracy class and the p signifies a. How to determine the ratio of a current transformer quora. Determine what is being requested with the correct units. Base aid ratio adjustment the act creates the base aid ratio adjustment factor, which is a bonus added to a towns base aid ratio if the town is ranked in the top 19 connecticut towns based on points awarded through the. It is usual to express the ratio to two decimal places, e. The primary current of minimum of 25% rated primary current to be injected on primary side of ct with secondaries shorted and the secondary current can be measured and recorded for all cores limits. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. If a ct is rated as a turns ratio, then it likely does not contain a burden resistor.
Calculation of the current transformer 1mrs 755481 accuracy limit factor 3. Ct and pt calculations a how to guide learn metering. Highest ct ratio that can be used a given application be used to get best performance. A current transformer where primary winding is provided by user.
The ct ratio is measured by shorting the secondary the 5 a winding and then injec. Singlephase fault the a phase ct will have an excitation voltage of v exa i asecz ct 2z lead z phase z. C ratio error can be determined by calculation from the. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or. Perunit and primarysecondary calculations perunit basics a perunit value is the actual value of some quantity divided by the base value of that quantity. The method used to determine if a surface water treatment plant has met the daily minimum disinfection requirements. Ct is the concentration of chlorine in your water times the time the water is in contact with that chlorine. The impedance of the excitation branch varies tremendously from one ct design to the next, the tap ratio used, and the v exc seen by the ct. However, when a ct saturates, i s does not accurately replicate i p. Equation 2 can be expressed in perunit pu values as. Ipu i pusp 3 2 real ct behavior real cts have copper losses, core losses, and leakage flux and require a certain current to magnetize the core. Current transformer errors and transformer inrush as. The iron core of a ct is made up of a fixed number of. Before the positive halfcycle is over, at p oint b, all of the magnets available in the core are lined, and the core has reached maximum flux density saturation.
Some report cardiothoracic ratio as a percentage, however this is incorrect, as it is a ratio. First apply the ct ratio test and find out the information and match with the testing data sheet of that particular ct. For a given core, the excitation curve can be scaled to any number of turns, so only one excitation curve is needed in a multi ratio ct or ct supplied on the same core with different turn ratios. The current goes to a high value if a fault occurs. How do i figure out the correct ct ratio to use for an. For multi ratio ct, using a lower tap reduces the effective accuracy and increases the chance of saturation. So, if there are 100 amps on the primary side of the ct. As per ansi, the ct ratio shall be chosen so that the maximum symmetrical fault current on the ct secondary is limited to 100a. Calculation of the current transformer accuracy limit factor.
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