AC Current Transformer Burden

By tchenwa | Published: May 21, 2019 – 4:15 pm

The AC current transformers used for the accurate measurement of AC electric currents are designed to transform the AC primary current to a proportional AC voltage signal or a lower proportional AC current signal level that is appropriate for micro-processor based systems. The VA burden that is imposed on the AC current transformer secondary signal can have a significant impact on the accuracy performance of the AC current transformer. To achieve the Accuracy Class desired for specific application, the AC current transformer model and the AC primary to AC secondary ratio must be carefully matched with the VA burden of the monitoring device to provide the optimum accuracy performance.

AC Current Transformer Secondary

The secondary burden or impedance on the AC current transformer has a direct impact on the accuracy performance of the AC current transformer. Overloading the AC current transformer secondary coil can have a significant adverse impact on the AC current transformer accuracy performance. Careful consideration should be given to the interface between the monitoring device and the current transformer to assure optimum accuracy performance.

AC Voltage Secondary

For a typical AC current transformer with a AC voltage secondary signal, maintaining a secondary burden of ≥ 10,000 ohms is typically adequate to assure optimum accuracy performance. Minimal power is drawn from the AC current transformer secondary coil.

AC Current Secondary

For a current transformer with an AC current secondary signal, the secondary burden is critical to the accuracy performance of the current transformer.

For the purposes of this BLOG post, the CTSB0816 Split-core current transformer with a 5A secondary output signal at Rated Primary Current will be discussed. This example will compare two of the same model CTSB0816 split-core current transformer, each configured for different Rated Primary Currents:

  • Model CTSB0816-500A/ 5A with a Rated Primary Current of 500A and secondary signal at Rated Primary Current (500A) of 5A and
  • Model CTSB0816-5000A/ 5A with a Rated Primary Current of 5,000A and secondary signal at Rated Primary Current (5,000A) of 5A.



Accuracy Class
Model0.51.03.0
CTSB0816-500A/ 5A2.5VA5.0VA10.0VA
CTSB0816-5000A/ 5A20.0VA30.0VA40.0VA

The comparison of the two models shows the significant difference in burden allowed to meet a specific Accuracy Class performance. In this example, to obtain an Accuracy Class 0.5 for the CTSB0816-500A/ 5A the secondary burden must be less than 2.5VA, whereas the CTSB0816-5000A/ 5A the secondary burden must be less than 20.0VA.

Consideration in the calculation of the Current Transformer Burden (VA) should not only consider the measuring device burden, but especially for the lower VA burdens, the length of lead wire from the current transformer to the measuring device. For the CTSB0816-500A/ 5A configuration the current transformer coil impedance (inductance and resistance) and the length of the wire from the current transformer to the measuring device could significantly impact accuracy performance.

Download CTBS0816 product information: CTSB Brochure (pdf 349kb)



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Accuracy Class for Current Transformers

By tchenwa | Published: April 16, 2019 – 5:18 pm

The accuracy performance of an AC current transformer’s primary current to secondary signal transformation (where the secondary is either a current or voltage) is stated as Accuracy Class. The Accuracy Class for current transformers is measured in accordance with the IEC61869 standard. The IEC61869-2 standard specifies transformation accuracy for current transformers at different percentage levels of Rated Primary Currents. Rated Primary Current is the AC primary current which will result in a secondary signal output equal to the current transformer’s design full scale (for example a model CTSB0816-500A/ 5A Rated Primary Current is 500A with a secondary output at Rated Primary Current of 5A).

IEC61869 Current Transformer Accuracy Class 0.2:

  • ±0.75% ratio error @ 5% of rated primary current,
  • ±0.35% ratio error @ 20% of rated primary current,
  • ±0.20% ratio error @ 100% of rated primary current
  • ±0.20% ratio error @ 120% of rated primary current

IEC61869 Current Transformer Accuracy Class 0.5:

  • ±1.50% ratio error @ 5% of rated primary current,
  • ±0.75% ratio error @ 20% of rated primary current,
  • ±0.50% ratio error @ 100% of rated primary current
  • ±0.50% ratio error @ 120% of rated primary current

The current transformers offered by T.I. Chen Associates are designed for the accurate measurement of AC currents up to 120% of the Rated Primary Current.

The Accuracy Class requirements as applied to a specific model of a current transformer may limit the Rated Primary Current ranges that can meet those accuracy requirements.

Example:CTSB0816 Split-core Current Transformer Accuracy Class 0.5:

The CTSB0816 is a split-core current transformer with an opening of 80mm(3.15″) x 160mm(6.30″. Typical applications would be large primary conductors and BUS bars. Download the CTSB Brochure (pdf 349kb)

Accurate measuring range is 5%-120% of Rated Primary Current, for rated primary currents from 500A-2,000A (e.g. models CTSB0816-500A/ 5A to CTSB0816-2000A/ 5A).

  • CTBS0816-500A/ 5A: Class 0.5 measurement range would be 25A to 600A.
  • CTSB0816-2000A/ 5A: Class 0.5 measurement range 100A to 2,400A
Accurate measuring range is 1%-120% of Rated Primary Current, for rated primary currents from 2,000A-5,000A;
  • CTSB0816-2000A/ 5A: Class 0.5 measurement range would be 20A to 2,400A.
  • CTSB0816-5000A/ 5A: Class 0.5 measurement range 50A to 6,000A
These examples demonstrate that to achieve the best accuracy over the anticipated primary current operating range, the Rated Primary Current of the current transformer must be carefully consider. So if the application typically measures less than 500A, the CTSB0816-500A/ 5A would be the appropriate selection where 500A is the Rated Primary Current for the current transformer.



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DC Current Measurement

By tchenwa | Published: April 16, 2019 – 5:02 pm

The non-intrusive measurement of a DC current is accomplished through the use of electronic sensors that use the Hall Effect to monitor and measure electrical currents. Hall Effect sensors used for DC current measurement are configured with an opening for the primary conductor. Matching the opening size to the outside diameter of the primary conductor assures optimal sensor accuracy. Two enclosure options are standard, either a solid body where the primary conductor must be taken offline for installation or split-core where the sensor may be installed without interfering with the primary conductor.

What is the Hall Effect?

The Hall Effect is the principle that a magnetic field applied perpendicular to a current will create a proportional Hall voltage perpendicular to the two fields. In a typical application, the DC current in the primary conductor creates the magnetic field which is proportional to the amount of DC current flowing through the conductor.

This magnetic field acts on a current flowing through the Hall Effect sensor resulting in a Hall voltage proportional to the primary conductor DC current. This technology allows non-intrusive DC current and DC pulse measurements.

Open Loop Sensor

The basic Hall Effect electronic sensor is configured as an “open-loop” sensor. It measures the Hall voltage to determine the primary conductor DC current. For example, see our open-loop Hall Effect sensor: HOS-Q11 Open-loop Hall Effect Sensor

Closed Loop Sensor

A “closed-loop” sensor configuration is a more accurate Hall Effect electronic sensor. The “closed-loop” design incorporates a second magnetic field, which is used to offset the primary conductor magnetic field. The amount of power necessary to zero out the primary conductor field is then the representation of the DC primary conductor current. The zeroing of the magnetic flux provides a highly accurate representation of the primary conductor current. For example, see our closed-loop Hall Effect sensor: HCS-C5 Closed-loop Hall Effect Sensor


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AC Voltage Measurement, Intelligent Devices

By tchenwa | Published: February 7, 2019 – 12:24 pm

The accurate measurement of AC Voltage is complicated by the necessity to minimize the burden the measuring instrument places on the primary circuit. The incorporation of a 1:1 current style voltage transformer in the AC voltage measuring circuit offers several advantages.


Features

  • A minimal burden on the primary voltage circuit, with essentially zero primary circuit load,
  • Isolation of the primary AC circuit and the secondary output signal, and
  • Exceptional accuracy with a minimal phase shift.

The AC current style voltage transformer is designed with either a 1mA to 1mA or 2mA to 2mA ratio. An example of the implementation using an operational amplifier I/V (current to voltage) circuit or a resistor sampling circuit;

The input resistor R limits the current to the 1mA or 2mA input. An application note document provide additional information – Application Note 1:1 Voltage Transformer (pdf 510kb).
The TV31 with UL Recognition Certification is an example of a current style AC Voltage Transformer.


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AC Current Measurement, Intelligent Devices

By tchenwa | Published: January 14, 2019 – 8:15 pm

Toroidal, Solid-core Current Transformer - AC Current Measurement - Intelligent DevicesThe Supervisory Control and Data Acquisition (SCADA) systems used by electric utilities to manage the distribution of electric power are highly dependent upon the intelligent, micro-processor based devices installed throughout the electric power distribution grid.

These intelligent devices acquire in real time the critical performance measurements (e.g. current, voltage), transmitting that information back to the central SCADA control center.

The SCADA control center can issue operate commands (CONTROL actions) to the intelligent devices.

These CONTROL actions can operate a switch, operate equipment that adjusts voltage or current, operate equipment that adjusts phase shift or any number of actions necessary to manage the electric power distribution grid.

Intelligent Devices Measurements

The intelligent devices typically measure AC current based upon the secondary output of a primary current transformer, typically 0 to 5 ampere AC.

Transforming the 0 to 5 ampere AC signal to level appropriate to a micro-processor based circuit is handled by solid-core, toroidal current transformers.

The 0 to 5 ampere conductor is looped through the center opening of the current transformer. A solid core current transformer offers superior transformation accuracy at a very competitive component price. Solid-core current transformer can be designed to perform beyond the rated primary before magnetic core saturation offering the ability to measure AC current surges.

The advent of digital signal processors (DSP) offers high signal sampling rates, enabling the measurement of the AC base frequency and the harmonics of the base frequency. Surface Mount Technology (SMT) components offers compact designs capable of operating in harsh operating environments.

3 Phase SCADA Remote Terminal Unit

3 Phase SCADA Remote Terminal Unit A three(3) phase SCADA Remote Terminal Unit (RTU) incorporates analog input measurement of AC current and AC voltage, digital signal processing necessary for the calculation of power, power factor, harmonics amplitudes, etc. and data transmission to the SCADA control center.




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