Burden in Instrument Transformers/Introduction to Burden

Instrument transformers are instruments for measuring current, voltage, impedance, power factor, energy dissipated, etc. They are also called measurement instruments or measurement circuits designed as transformer-operated meters. 

Burden in instrument transformers is of utmost importance. If the impedance/burden in the instrument transformers is greater than what was promised or claimed for that particular transformer, it will end up delivering a value lower than the actual value. Similarly, if the impedance/burden in the instrument transformers is smaller than what was claimed, it will start showing a value greater than the actual one.

This blog post will discuss how you can measure the burden in instrument transformers and what are standard burden ratings for CTs and VTs.

Standard Burden Ratings for CTs

In a Current Transformer (CT), the burden is defined as the impedance connected to the instrument transformer’s secondary, including the transformer’s own secondary impedance as well the impedance of the device and connected cables.

The IEEE C57.13 standard provides comprehensive guidelines for the implementation of current transformers (CTs) rated at 5A, 60Hz. These ratings are expressed in terms of Burden to ensure reliable and uniform performance from different CT models.

Standard Burdens for Current Transformers with 5 A Secondaries

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Detailed Breakdown of Burden Components

Current transformers (CTs) are essential for measuring high currents in electrical systems. However, a critical factor impacting their accuracy is the CT burden. This section breaks down the various components contributing to CT burden, empowering you to understand its influence on CT performance.

  • Internal CT Burden: Every CT has an inherent burden due to its design. This burden is a combination of the CT’s internal resistance (R) and reactance (X), typically represented as a complex impedance (Z) value in ohms. The manufacturer specifies this impedance in the CT’s datasheet.
  • Secondary Cable Resistance: The resistance of the cable connecting the CT’s secondary winding to the measuring device (relay, meter, etc.) also contributes to the burden. This resistance value (Rcable) depends on the cable length, cross-sectional area, and material properties. It can be calculated using the following formula:

Rcable = (ρ * L) / A

where:

  • ρ (rho) is the resistivity of the cable conductor material (e.g., copper) in ohms per meter (Ω/m)
  • L is the cable length in meters (m)
  • A is the cable’s cross-sectional area in square meters (m²)
  • Burden of Measuring Instrument: The final piece of the puzzle is the impedance (Zmeter) of the measuring instrument itself. This value, also specified in ohms, represents the instrument’s internal resistance and reactance.

Understanding these three components empowers you to calculate the total CT burden, a crucial step in ensuring accurate, current measurement within your electrical system. Selecting a CT with a sufficient burden rating safeguards against inaccuracies that could arise from exceeding the CT’s limitations. This ensures reliable current readings, critical for proper functioning and protection within your electrical system.

Standard Burden Ratings for VTs

The IEEE C57.13 standard outlines the varied burden ratings for Voltage Transformers (VT) to ensure safety and optimal performance – ranging from 120V all the way down to 69.3V, creating a reliable foundation of electricity distribution infrastructure in homes and businesses worldwide.

Standard Burdens for Voltage Transformers

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Burden Calculations

The burden is calculated by the following formula:

Burden ()=CT Secondary Resistance ()+Wire Resistance () +Device Impedence ()

For instance, you are asked to calculate the burden of a 200A: 5A CT, given that:

  • CT secondary winding resistance = 0.08
  • Total cable resistance (between CT & Relay) = 0.3
  • Relay coil resistance = 0.02

The burden will be calculated by the above formula as:

  • Burden ()=0.08+0.3 +0.02
  • Burden ()= 0.4

Since the burden is another name for Resistance or Load, another way to calculate Burden is using Power & Ohm’s formulas. For instance, you are asked to calculate the Burden of 0.5VA @ 5A in Ohms.

  • Since,
    Power, P or VA= 0.5VA or 0.5Watt
    Current, I= 5A

The burden will be calculated as:

  • P= VI
    • (Since V= IR)
  • P= IR(I)
  • P= I²R
  • 0.5= (5)²R
  • 0.5/(5)²= R
  • R= 0.02

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Conclusion

A burden is a resistance that a device or system offers to the flow of electric current. This is measured in ohms, and it is important to consider when designing or selecting an instrument transformer. Transformers are selected for their ability to handle a certain amount of burden without exceeding the maximum allowable temperature rise. The selection process can be simplified by using a chart that takes into account the type of insulation, winding material, and ambient temperature. 

By using the calculations discussed above, you can easily find the burden in instrument transformers. This manual approach of calculating burden in an instrument transformer can help you find if it has any errors in terms of deviation in actual burden from the claimed one.

To learn more about circuits and their important calculations, including burden in instrument transformers (VT/CT), and to learn about how to calculate power factor, energy, current, etc., using instrument transformers, feel free to check out our FE Electrical and PE Power Exam Prep Books.

wasim-smal

Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.