Ct s that are used to monitor motor loads can be sized from the nameplate full load motor amps. 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. Abstractalthough there is an abiding interest in the application of current transformers ct for relaying, few written rules exist for selecting ratings. First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented. A current transformer ct is necessary if you need to measure the current, energy, frequency. This situation is exacerbated by at least one wellknown pacs vendor using percentages for their inbuilt ratio measurement. Calculation of the current transformer accuracy limit factor. A ratio is a proportional factor between two similar objects of different sizes. Applying cts in protection schemes for transformers. Go back to contents important design considerations 1. Cts are required because standard current measuring devices are not configured to measure highcurrent levels. First apply the ct ratio test and find out the information and match with the testing data sheet of that particular ct. In fact, depending on the types of machines and industrial process, the protection functions to be selected can. Current transformer selection guide efine customer requirements based on the primary circuit and the metering and d protection chains.
Ct characteristics are normally specified at a single frequency such as 50 or 60 hz. 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. If the primary current changes, the secondary current output will change. 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. The current transformer will operate into either a short circuit or a resistive load until the. Example the distance between the cts and the protection relay is 15 metres, 4 mm 2 cu conductors in 4wire connection are used. If the installer did not provide the meters instruction manual, then try googling for a pdf copy. Remember, on multiratio current transformers there are several secondary outputs. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. Whilst measurement current transformers are intended to reach saturation point as quickly as possible once they exceed their operational current range expressed by the overcurrent factor fs in order to avoid an increase in the secondary current with a fault e. It is possible to calculate the time to saturate for any ct given the set of. T35t60 reference winding selection and ct ratio mismatch. Sizing current transformers for line protection applications hector j. Relay setting calculationrestricted earth fault protection.
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. C ratio error can be determined by calculation from the. The selection of a current transformer is a simple task. The ct ratio is the ratio of primary current input to secondary current output at full load. Using 5, calculate if, which is the fault current in per unit of ct nominal rating. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. The basics of current transformers nk technologies. When isn 5a when isn 1a o selection of rated burden. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or monitor the current levels. Chapter 16 current transformer design university of north. Selecting cts to optimize relay performance gabriel benmouyal, ireq jeff roberts and stanley e.
Selecting a current transformer ct if the current ratio, burden and class are selected, the current transformer ct has been specified basically. The magnitude of the secondary current is dependent on the turns ratio of the transformer. As per ansi, the ct ratio shall be chosen so that the maximum symmetrical fault current on the ct secondary is limited to 100a. Ct based monitoring is the most simple and costeffective solution for retrofitting systems. Scope this document describes the calculation of the actual accuracy limit factor f a for protectiontype p current transformers ct. It is usual to express the ratio to two decimal places, e. Calculation of the current transformer 1mrs 755481 accuracy limit factor 3. Wound current transformer the transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The values of burden as voltamperes of some equipment are shown in table. 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. Of course additional requirements like type, frequency, isolation level, maximal short circuit current and environmental conditions should also be specified. 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. A ct defined for 50 hz can be installed on a 60 hz network with the same level of accuracy. After that you can calculate, observe and perform practically.
Current transformers selection guide electrical engineering portal. In a belt drive system, a ratio is used to determine the speed relation between two vbelt pulleys. 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. Instrument transformers application guide abb group. How do i figure out the correct ct ratio to use for an energy.
Selecting current transformers janitza electronics. T35t60 reference winding selection and ct ratio mismatch application note 3 we use the information in the following panels to input values into the following equation set. Total ratio, from high voltage side of cvd to secondary side of emu. As your circuit indicates, the ratio is simply 6005120. So perhaps you are really asking how to measure the ratio. How do i figure out the correct ct ratio to use for an. Selecting ct s a selection guide is provided in this catalog to aid in selecting the type of ct for. Current transformers, or cts, are transformers used to step down the current level of highpower transmission systems to measure or. What happens when cts are used with variable frequency drives vfds.
The standard current transformer secondary winding is rated at 5a as per ansi standards. Ct selection guide 600v rated1 cat 5 ethernet cable can be used to extend up to 4 cts. That property of the circuit connected to the secondary winding that determines the active and reactive power at the secondary terminals. For a given distance between the burden relay or instrument and the ct, the adoption of 1 a as the rated secondary current reduces the lead burden to 1 25th of its value at 5 a secondary current. Application guide to selecting current transformers. For a nonreferenced unit, it is vital to indicate the rated frequency on the order from. The factor k ssc is relatively easy to understand and relates to the liner portion of a ct characteristic. Ct application guide ge grid solutions online store. Abstractthis paper discusses the factors to consider for sizing current transformers cts for line protection applications. Current transformer basics and the current transformer.
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. This page is dedicated to showing examples of ct and pt calculations. How do i figure out the correct ct ratio to use for an energy meter. Toroidal current transformer these do not contain a primary winding.
The current goes to a high value if a fault occurs. Ct and pt calculations a how to guide learn metering. 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. In practice, the designer must consider various factors in selecting a current transformer. Unique to a current transformer the current transformer function is different than that of a voltage transformer. As per ansi, the ct ratio shall be chosen so that the maximum symmetrical. Because imargin3 is less than imargin2 and imargin3 is less than imargin1, the winding 3 is the reference winding by auto selection. How to determine the ratio of a current transformer quora. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the ct output is 40 amps. Current transformers selection guide schneider electric.
For cts that are linear to approximately 10x rated primary current at 60 hz, the volts hertz ratio is approximately constant. The current transformer is expected to deliver about 5a or lower under healthy load conditions. A ct steps down current and a ct specified as stepping down in the ratio 50. The primary circuits of the current transformer must withstand the constraints. Threephase ac current measurement using current transformer. A current transformer is, in many respects, different from other transformers. Divide the maximum primary fault current if by the per unit current to determine the ct nominal current rating. Then determine the total burden in per unit of the ct standard burden. 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.
Important main factors when selecting current transformers are. Altuve, normann fischer, gabriel benmouyal, and dale finney, schweitzer engineering laboratories, inc. Sizing current transformers for line protection applications. Instrument transformer basic technical information and. So i have been getting a lot of requests about how to do ct and pt calculations. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1.
The current transformer can provide a constant current or voltage output. There are a few examples with ct calculations only and a few examples with both ct and pt calculations. In reality, however, it is not possible to neglect the excitation. Joule losses by wire resistance are 25 times higher with 5a than with 1a. Medium voltage products technical guide protection criteria. The diagram below can be used to select wire size if the lengths of the leads are known.
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