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Accuracy Classification and Burden

The IEEE has established standardized strategies of classifying instrument transformers on accuracy and burden. Associate in Nursing accuracy classification for Associate in Nursing instrument electrical device includes the quality burden also because of the most % error limits for line power factors between 100 percent and hr insulating material.  A typical CT classification may be zero.3 B0.5 wherever the zero.3 is that the % allowable error and also the B0.5 is that the secondary burden in ohms resistance.  The accuracy relies on the burden.

It is very necessary at now to possess an awfully clear understanding of the term “burden” because it is employed in reference to instrument electrical device accuracy classifications.  The term “burden” has been adopted to tell apart it from “load” that is usually related to the first, particularly with current transformers. as an example, the load rating of a current electrical device indicates the load (in current) which can be applied to its primary, whereas the burden rating indicates the number of resistance (in ohms) and inductance (in milli-henries) which can be connected to its secondary while not inflicting a metering error larger than such as by its accuracy classification.

The types of meters and relays and also the size and length of wire connected to the second aspect of the instrument electrical device form up its burden.

These values will be calculated by changing every device into terms of volt-amperes and power issue and doing a vector analysis to work out what the whole effective burden on the electrical device is.  An additional sensible method is to get from the manufacturer the burden of every device in terms of watts and vars and calculate the whole the whole effective burden on the instrument electrical device.  A typical example may be as follow:

A utility is attempting to work out what accuracy and burden classification CT to buy for a specific metering application. the varied meters and instruments to be used square measure celebrated and also the distance and size of wire to be used between the CT and also the meters square measure celebrated

Standard ANSI values of accuracy burdens for CT’s and PT’s square measure listed below, also as within the IEEE standards manual, C57.13.

CONCLUSION: Since half dozen.94 VA at .95 P.F. exceed B0.2 burden (which is five.0 VA at .9 P.F.) the utility should use an electrical device that encompasses a zero.3B0.5 classification (or twelve.5 VA at .9 P.F. capability

Metering Accuracy

There area unit 2 sources of error in instrument transformers, particularly magnitude relation error and phase error. during a given electrical device, the metering error is that the combination of the 2 separate errors. this mixture is named electrical device Correction issue (TCF), IEEE has established accuracy categories for each current and potential transformers.  The limit of permissible error during a potential electrical device for a given accuracy category remains constant over a variety of voltage from 100 percent below to 100 percent higher than rated voltage. within the figure to the correct could be a commonplace check card provided by the manufacturer showing the performance of the CT at 100 percent and 100 percent of rated current.

The limit of permissible error during a current electrical device accuracy category has one worth at 100 percent rated current and permits double that quantity of error at 100 percent rated current. generally zero.3% the error is suitable for watthour metering, 0.6% to 1.2% error for indicating instruments.  The figure to the correct shows the performance limits of a customary metering zero.3% accuracy CT with a rating issue of four.0.

High Accuracy Instrument Transformers

Two new accuracy categories are developed by IEEE C57.13.6 to accommodate the shift towards electronic relays and meters from the standard induction devices.  Consequently, makers have begun to boost the accuracy of instrument transformers to require advantage of the lower resistance of the devices. enclosed within the new high accuracy commonplace area unit new testing points and burdens to verify performance.  New burdens of E.04, (1.0 var at 5Amp, unity power factor), E0.2, (5.0 var at 5Amp, unity power factor), and low current check purpose of fifty versus the standard 100 percent rated current, area unit currently needed.  

0.15 Accuracy Instrument Transformers Current transformers should maintain zero.15% accuracy from the rated current through rating issue at rated burden.  At five-hitter rated current through 100 percent, this electrical device should maintain zero.3% accuracy.  No accuracy is secure at levels below five-hitter.  Voltage transformers area unit zero.15% accuracy from 90%-110% of rated voltage. 

0.15S Accuracy Instrument Transformers Current transformers should maintain zero.15% accuracy from five-hitter rated current through rating issue at rated burden.  No accuracy is secure at levels below five-hitter.  Voltage transformers area unit zero.15% accuracy from 90%-110% of rated voltage

Relay Accuracy of a Current electrical device

Current transformers that square measure accustomed to operating relays for management and system protection should have sure accuracy throughout over-current conditions.  The electrical device should be able to not solely stand up to the high currents concerned, however, should additionally remodel current to a lower worth appropriate for application to the relay terminals, and try this with an inexpensive accuracy.  A typical relay accuracy classification can be C200 or T200. 

The “C” stands for calculated and implies that the window and bar kind units that have a totally distributed secondary on a coffee run flux core so leading itself to calculated values.  The “T” stands for tested as a result of wound kind units don’t have totally distributed windings, they need to be tested as a result of the run electrical phenomenon isn’t predictable. The last variety is that the secondary voltage which will be developed at the secondary terminals while not saturation.

Thus the which means of the relay classification!

C200 would be (10% accuracy inferred at twenty X traditional current X secondary ohmic resistance


V = IR two hundred potential unit = (20 X five amps) X B2.0 ohms

Thus, this CT would have a mistake of no larger than 100 percent at twenty times traditional secondary current with a secondary burden of two.0 ohms.

Manufacturers can usually supply a graph of the excitation performance of a specific CT.  The graph permits the top user to work out the performance of the CT over the whole vary of secondary current and make sure that the CT can operate pro re nata.  The figure to the correct shows a typical excitation curve of a relay category CT.

Connections – Potential Transformers

Potential transformers area unit ordinarily connected across 2 lines of the circuit during which the voltage is to be measured. ordinarily they’ll be connected L-L (line-to-line) or L-G (line-to-ground). A typical association is as follows:

When a section relationship of “direction of flow” is of no consequence, like in a very meter that operates solely consistent with the magnitude of the voltage, there’s no ought to observe the polarity of the electrical device.  However, in watthour meter applications, polarity should always be ascertained.

Most potential transformers have one winding secondary as antecedently shown, however, they’ll have broached secondary windings or twin secondary windings.

Connections – Current Transformers

CT’s with wound primaries continuously have their primary windings connected serial with the road and also the load and their secondary windings connected to the burden (the watthour meter current coil) as show below:

Current transformers having a middle broached secondary area unit stated as a twin magnitude relation CT. they’re employed in applications wherever it’s necessary to own on the market 2 ratios of primary to secondary current from an equivalent secondary of the CT. this might be accomplished by adding a faucet within the secondary to induce a second magnitude relation.  The magnitude relation obtained by the faucet is typically simple fraction the magnitude relation obtained by the total secondary.  A schematic example is shown below. With two hundred amperes flowing within the primary

a association X2 – X3 can turn out five amperes out of the secondary.  Then because the load grows to four hundred amperes, the secondary circuit are reconnected to X1 – X3 to provide five amperes within the secondary. it’s not counseled to reconnect whereas the unit is energized, the secondary terminals should be short circuited therefore as to not induce high voltage within the secondary circuit once the circuit is opened to create the association.  Voltage from many hundred volts to many thousand volts, captivated with the planning, is developed within the secondary circuit once it’s open circuited with current flowing within the primary coil.  On a twin magnitude relation broached secondary CT, each the total winding and also the broached winding can’t be operated at the same time.  The unused terminal should be left receptive avoid short circuiting some of the secondary.

Another style of CT quite ordinarily used is that the double secondary CT. during this configuration the CT has 2 cores, 2 secondary windings and one common primary coil.  Its application would be for victimization one CT to each meter and relay a standard circuit wherever the metering circuit should be isolated from the relaying circuit.

A schematic of this is able to like this:

In this style, if each the circuits don’t seem to be attending to be used at the same time, then the unused circuit should be short-circuited whereas the opposite is energized otherwise you can develop associate degree iatrogenic high voltage on the open-circuited unused CT.

Connections – commonplace Metering

Typical current electrical device connections on 3 common circuits can illustrate the principles concerned in creating CT installations

You can watch the following video or read the written tutorial below.