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
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)


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.


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


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.


  • 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.


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.


AC Electric Power Monitoring and Measurement Sensors

By tchenwa | Published: December 3, 2018 – 2:45 pm

Retrofitting Existing Distribution Networks

AC Current Transformer Sensors - Retrofitting Existing Distribution Networks The current generation of electric power AC current sensors that are suitable for retrofitting existing distribution networks have obtained a level of accuracy capable of near revenue grade metering. These split-core sensor designs permit the installation of the AC current sensor without taking the primary conductor off-line. The standard solid-core AC current sensor used by electric power meters requires that the primary conductor be disconnected and threaded through the solid core current sensor for installation. The split-core design is simply clamped around the primary conductor and physically secured to the primary conductor.

AC Current Transformer Sensors Most Widely Used

Split-Core Rogowski Coil Current Sensor - Retrofitting Existing Distribution NetworksThe technologies most widely used are the split-core AC current transformer sensors which incorporate advanced materials magnetic cores (e,g, silicon steel cold rolled grain oriented, Mn-Zn ferrite, …) and the Rogowski Coil current sensor “rope style split-core design. AC current transformer sensors offer excellent price/ performance. However, the AC primary current range and frequency response for each sensor design is relatively limited. The rigid physical design is not conducive to irregular primary conductor profiles (something other than a circular or rectangular profile).

Benefits of Rogowski Coil AC Current Sensors

Rope style Rogowski Coil AC current sensors offer a very wide AC primary current range and frequency response. The flexibility of the “rope” style split-core design can be used for irregularly shaped primary conductor profiles. The Rogowski Coil current sensor does require additional electronics to perform the integration of the dv/ dt secondary output of the sensor.

See Specs on the Following:

AC Current Transformer Sensors Rogowski Coil Current Sensors


Micro-Grid, Option for Restoring Electric Power to Puerto Rico

By tchenwa | Published: September 17, 2018 – 3:54 pm

I have read several interesting stories of organizations proposing and installing Micro-Grids as a means of restoring electric power to areas in Puerto Rico. The Micro-Grid systems incorporate electric power generation, electric power storage and/ or smart-grid connection to the electric utility distribution network. The installations goals are;

  • Restoring electric power.
  • Providing backup to critical infrastructure (e.g. hospitals) should the electric utility supply be inconsistent.
  • Incorporating alternative electric energy generation sources.

Hurricane Maria severely damaged the electric utility distribution grid. That damage coupled with the failing electric utility infrastructure left many areas without service for extended periods of time. These organizations are proposing to use solar arrays as the electric power generation source with battery backup for electric power storage. Technically, this solution is very doable. I would also assume that for remote locations that may not regain electric service any time soon, this may be the only solution. However, these systems are relatively complex to maintain and have the same miss-match between when power generation is at its mid-day peak and when usage is desired during evening and at night.

I do believe that solar power will be part of the future electric generation mix. Maybe what has to change is our thinking and go back to the Thomas Edison idea of DC power. I have been exposed to the mathematics which justifies AC for transmitting electric power over distance. By taking the output of the solar array, converting the power to AC to power some usage, converting the electric power back to DC to charge the battery there are enormous loses. Maybe Micro-Grids should be DC, only using inverters to power the huge power loads (e.g. oven, washer, dryer).

Tim C

California Zero Net Energy Program: Part 2

By tchenwa | Published: September 23, 2016 – 4:38 pm

The California Zero Net Energy program discussed in the previous post will have a significant impact on new construction. The average residential electric power usage in the State of California is 6,741 kWh per year (U.S. Energy Information Administration) or 18.5 kWh per day on average. Presumably a residential home will generate electric power using a PV (solar) system. The average PV system is rated at 5 kWh (approximately 400 SFT of solar panels). This would appear to be sufficient to offset any usage but depending upon PV module efficiency, which will vary with time of day and weather, the electric power produced will be significantly less than rated. The difference will have to be made up through energy efficiency and the shifting of usage from the time of day with peak loads (e.g. evening) to time of day with peak generation (e.g. mid-day).

I would think that from an electric utility perspective, electricity available at 7pm to 10pm during peak usage will be worth a lot more than electricity available at 12:00 noon. Time of day rates may become the norm.


California Zero Net Energy Program

By tchenwa | Published: September 16, 2016 – 1:56 pm

I recently read an article about the California Zero Net Energy program whose stated goal is that all new residential construction in California will be zero net energy by 2020. All new commercial construction will be zero net energy by 2030. Zero Net Energy (ZNE) is defined to be that the building will produce as much energy as the building uses over a one year period. This goal will be achieved through a high level of energy efficiency and through the addition of clean, on-site renewable power generation, presumably solar PV.

Wow … The implications are dramatic for both the home owner and the electric utilities that service the building. As we discussed in previous posts, the electric utility distribution networks have been designed over the last 100 years or so to be a one-way street. The regulated electric utilities built huge electric power generating plants that transmitted the electric power through a network of power transmission lines at ever decreasing voltage levels until the power arrives at the home at typically 120VAC or 240VAC. Balancing the power requirements for the service territory is the support base provided by the large electric power generation capability augmented with “standby” generators for peak loads. A fairly predictable power distribution model.

What happens when a significant share of the end electric power user base becomes at times a power generator? Is the sun shinning in Los Angeles but Santa Barbara is overcast? (FYI Santa Barbara is northwest of LA along the Pacific coast). The source of the electric power generation becomes highly volatile and variable.

More information can be found on the ZNE web site .


Free Electric Power

By tchenwa | Published: November 13, 2015 – 1:39 pm

The New York Times published an article recently on how TXU Energy (Dallas/ Fort Worth, Texas) is offering their customers free electricity during certain times of the day.

A little background may be helpful. There are three electrical grids that cover the mainland United States, essentially East coast, West coast and Texas. Several years ago, the Texas electrical power market was split into generation, transmission and distribution. The intent was to encourage competition at the generation level and distribution level where the electric power is used.

According to the article, the abundance of wind electric power generation has resulted in a surplus of power in the evening. The TXU strategy is to train their customers to shift their variable loads to the evening hours, hopefully reducing the peak energy loads during the day. The article interviews several residential customers who shift their washing machine and dish washer use to the evenings. All of this is made possible by the implementation of the industry term Smart Grid. Intelligent electric meters that can transmit and receive user level data make this time-of-day rate application possible. Is this the killer app that will help to justify the internet of everything?

I wonder if this program is available to large industrial users? A night shift from 9:00pm to 5:00am might be very cost efficient.

New York Times article: “A Texas Utility Offers a Nighttime Special: Free Electricity”, Clifford Krauss, Diana Cardwell.