How to determine the ratio of a current transformer quora. The speed ratio would be stable if slippage did not occur. Current transformer ct class ratio error phase angle error in. Therefore, the ac saturation can be avoided by properly increasing the ct saturation voltage, ct ratio, or decreasing ct burden. Current transformer selection guide efine customer requirements based on the primary circuit and the metering and d protection chains. Correct selection of the primary nominal current is important for the accuracy of measurement. Selecting current transformers janitza electronics.
Selecting cts to optimize relay performance schweitzer. Cts are required because standard current measuring devices are not configured to measure high. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. The ct ratio is measured by shorting the secondary the 5 a winding and then injec. The current goes to a high value if a fault occurs. As your circuit indicates, the ratio is simply 6005120. After that you can calculate, observe and perform practically.
Ct and pt calculations a how to guide learn metering. Current transformers principle of operation of ct a current transformer is defined as as an instrument transformer in which the secondary current is substantially proportional to the primary current under normal conditions of operation and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Enter the information below to calculate the burden resistor required for current transformers that do not contain a builtin burden resistor. Current, power and energy registers are dynamically calculated using the ct ratio set in the meter. The magnitude of the secondary current is dependent on the turns ratio of the transformer. A practical guide to ct saturation ariana hargrave, michael j. Determining the transformer ratio ipis for the transformer primary current, select the standard value immediately higher than the current to be measured. It is possible to calculate the time to saturate for any ct given the set of. The current transformer is expected to deliver about 5a or lower under healthy load conditions. R1 must serve as backup for r2 and therefore selection of r2 ct ratio and tap must be based on ll fault at bus 3. In a belt drive system, a ratio is used to determine the speed relation between two vbelt pulleys. How do i figure out the correct ct ratio to use for an energy. Apr 09, 2020 selecting a current transformer ct if the current ratio, burden and class are selected, the current transformer ct has been specified basically. Ct application guide for the 489 generator management relay 6 ge power management it is important to remember that manufacturer ct excitation curves are based on calculation and are intended to specify minimum guaranteed performance.
In real applications, a commonly used rule of thumb is to select a ct with a c voltage rating at. Sizing current transformers for line protection applications hector j. Using 5, calculate if, which is the fault current in. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or. The standard current transformer secondary winding is rated at 5a as per ansi standards.
Part1 ct ratio selection for ref relay setting in this video we have explained calculation of ct ratio for restricted earth fault relay setting. Example the distance between the cts and the protection relay is 15 metres, 4 mm 2 cu conductors in 4wire connection are used. Where possible, use multitap ct s so that the ratio can be reduced if the maximum load is much less than 80% of the breaker size. As per ansi, the ct ratio shall be chosen so that the maximum symmetrical fault current on the ct secondary is limited to 100a. Pass the x1 secondary lead is through the transformer window 15 times from h1 to h2. Recommended is a ratio slightly beyond the measured defined maximum load current in. The auxiliary ct is installed as close as possible to the current transformers. Jan 28, 2019 part1 ct ratio selection for ref relay setting in this video we have explained calculation of ct ratio for restricted earth fault relay setting calculation. Then determine the total burden in per unit of the ct standard burden. Using cts of burden values higher than required, is unscientific since it leads to inaccurate reading meter or inaccurate sensing of fault reporting conditions. Practicing ct disinfection is the safe thing to do. Wound current transformer the transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. To calculate the percentage ratio error,divide the exciting current by the secondary current times 100.
This reduces the ct burden by reducing the length of the ct secondary current conductors. Altuve, normann fischer, gabriel benmouyal, and dale finney, schweitzer engineering laboratories, inc. Thompson, and brad heilman, schweitzer engineering laboratories, inc. Ct based monitoring is the most simple and costeffective solution for retrofitting systems. Ct disinfection is the disinfection standard in alberta. Abstractthis paper discusses the factors to consider for sizing current transformers cts for line protection applications. Divide the maximum primary fault current if by the per unit current to determine the ct nominal current rating. Joule losses by wire resistance are 25 times higher with 5a than with 1a. The nominal current can also be defined on the basis of the following considerations. Apr 16, 2011 current transformers principle of operation of ct a current transformer is defined as as an instrument transformer in which the secondary current is substantially proportional to the primary current under normal conditions of operation and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.
To optimize the test average diagnostic performance, the ct cutoff should be selected when diagnostic odds ratio is maximal. There are a few examples with ct calculations only and a few examples with both ct and pt calculations. Ct factor calculation guidelines what is a ct factor. Nov 17, 2019 suppose, you have one ct with current ratio 4001 a and its protection core is situated at 500 a. The basics of current transformers nk technologies.
Ct sizing calculation analog to digital converter relay scribd. A ct steps down current and a ct specified as stepping down in the ratio 50. Some of this information is repeated from the cts and pts page. Where possible, use multitap cts so that the ratio can be reduced if the maximum load is much less than 80% of the breaker size. Selecting a current transformer permissible current in a busbar accuracy of current transformers important warning for cts. Current transformer ct class ratio error phase angle error. A correctly selected ct, on the other hand, enables fast and reliable protection. A ct defined for 50 hz can be installed on a 60 hz network with the same level of accuracy. Application guide to selecting current transformers.
Relay setting calculationrestricted earth fault protection. Ctbased monitoring is the most simple and costeffective solution for retrofitting systems. As of today, ct disinfection is also the water treatment industry standard for disinfection. Home download center electrical engineering books and technical guides electrical engineering guides current transformers selection guide electrical characteristics primary circuits the primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected. It is important to remember that the resolution of the wattson readings depends on the ct and pt ratio settings also. Since this is what we used for setting r2, the ct and tap will be the same as r1. Recently we had a question on ct current transformer sizing often called dimensioning. 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. How to calculate ct ratio and pt ratio cr4 discussion thread. While the latter configuration would not be used today, one may be required to make this calculation on an existing system. Even though we have the wiki page, i thought it may be good. 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. For a nonreferenced unit, it is vital to indicate the rated frequency on the order from.
Current transformer basics and the current transformer. Calculation of the current transformer accuracy limit factor. Abstractalthough there is an abiding interest in the application of current transformers ct for relaying, few written rules exist for selecting ratings. Which current transformer ct is more sensitive and why. Abstractcurrent transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. When the current transformer ratio is selected primarily to meet the full. This situation is exacerbated by at least one wellknown pacs vendor using percentages for their inbuilt ratio measurement. Although the selection of a cutoff based on misclassification cost depends on infection prevalence, the selection based on predictive values does not. Threephase ac current measurement using current transformer.
Calculation of the current transformer 1mrs 755481 accuracy limit factor 3. It is intended to outline an easy to follow procedure that will. Energy is accumulated without the ct ratio, therefore changing the ct ratio will affect the accumulated energy readings. It is usual to express the ratio to two decimal places, e. If the primary current changes, the secondary current output will change. The selection of a current transformer is a simple task.
Selection ratio number of hires number of applicants because it is a ratio, the selection ratios value must range between 0 and 1 theoretically it really ranges from 1 to 1, but in the real world you will never find a negative selection ratio in a hiring situation, with 0 indicating a low selection ratio and 1 indicating a very high. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or monitor the current levels. Selection of a cutoff value for realtime polymerase chain. How do i figure out the correct ct ratio to use for an. Some of this information is repeated from the ct s and pts page. Some report cardiothoracic ratio as a percentage, however this is incorrect, as it is a ratio.
Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. Note for rated secondary current as shown previously, the smaller the ct burden in voltamperes, the better higher the actual accuracy limit factor. Ct s that are used to monitor motor loads can be sized from the nameplate full load motor amps. Selecting ct s a selection guide is provided in this catalog to aid in selecting the type of ct for. So perhaps you are really asking how to measure the ratio.
Selecting cts a selection guide is provided in this catalog to aid in selecting the type of ct for. A ratio is a proportional factor between two similar objects of different sizes. Cts that are used to monitor motor loads can be sized from the nameplate full load motor amps. Selecting higherratio cts to prevent saturation and match the breaker load. A ct is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. The reason a ct saturates is related to what physically occurs inside a ct during the electromagnetic induction process. It was prepared as a presentation for the alberta water and wastewater operators association annual conference in banff, alberta on march, 2002. Ct saturation in industrial applications analysis and application guidelines 3 of 34 get8501 figures 1 and 2 below present plots of the proportional secondary ct current, and the simulated secondary current for a 50. Of course additional requirements like type, frequency, isolation level, maximal short circuit current and environmental conditions should also be specified.
As per ansi, the ct ratio shall be chosen so that the maximum symmetrical. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the ct output is 40 amps. The iron core of a ct is made up of a fixed number of. As part of the answer i referred the person asking the question to our note on ct sizing ct dimensioning. How do i figure out the correct ct ratio to use for an energy meter. The standard has changed because we know we can do better than just 20 minutes of chlorine contact time. Threephase ac current measurement using current transformer reference design the current flowing through a conductor can be detected using a resistive shunt, current transformer ct, hall effect sensor, and so forth. Ct shorting terminal strips the illustration below shows the termination of a multi ratio ct on a special shorting terminal strip. Basically, such high value of design burden extends saturation characteristics of ct core. Using 5, calculate if, which is the fault current in per unit of ct nominal rating. Industry standards do not require the curves to be an accurate representation of true measurable values.
Selecting cts to optimize relay performance gabriel benmouyal, ireq jeff roberts and stanley e. If the primary current of the ct becomes a the secondary current will still be 1. Cts are required because standard current measuring devices are not configured to measure highcurrent levels. This page is dedicated to showing examples of ct and pt calculations. Ct saturation tolerance for 87l applications mpcomments. So i have been getting a lot of requests about how to do ct and pt calculations. Toroidal current transformer these do not contain a primary winding. Sizing current transformers for line protection applications. That is, for all other conditions held the same at 6 hz, the ct will be linear to only 1x rated current and at 30 hz the ct will be linear to 5x rated current. So, if you have not already checked it out head over there for an overview of what ct s and pts are and their role in metering. For cts that are linear to approximately 10x rated primary current at 60 hz, the volts hertz ratio is approximately constant. I am je of nagbhir sub station of 225 kv newly electrified section and testing started of current transformer when test ct ratio by injecting current in primary bushing of old ct which was 20 years old ct installed in circuit,after testing values not found according to ratio,100 amps inject and found in secondary terminal 3. Abstractalthough there is an abiding interest in the application of current transformers ct for relaying, few. However, when a ct saturates, i s does not accurately replicate i p.
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