How is the burden rating of an instrument transformer calculated and why is it important to consider during design?

The burden rating of an instrument transformer (IT), which includes current transformers (CTs) and voltage transformers (VTs), represents the maximum apparent power (VA or volt-amperes) that the transformer can supply to its secondary circuit while maintaining its specified accuracy. It indicates the load that can be connected to the secondary winding without exceeding the transformer's design limits and compromising its performance. The burden is typically expressed as VA at a specific power factor (usually 1.0 or 0.8). The burden rating is calculated based on the secondary current or voltage rating and the maximum allowable secondary impedance. For a CT, the burden rating (VA) is calculated as: VA = (Secondary Current)^2 (Secondary Impedance). For a VT, the burden rating (VA) is calculated as: VA = (Secondary Voltage)^2 / (Secondary Impedance). The secondary impedance includes the impedance of the connected devices (relays, meters, instruments) and the impedance of the connecting wires. It is important to consider the burden rating during the design phase for several reasons. First, if the actual burden connected to the secondary of an IT exceeds its burden rating, the transformer's accuracy will be compromised. This can lead to incorrect measurements, improper relay operation, and potential damage to equipment. CT saturation can occur if the burden is too high. This will cause the CT to produce a distorted secondary current waveform, which can affect the performance of protective relays. For VTs, exceeding the burden rating can cause voltage drops and inaccuracies in voltage measurements. Secondly, selecting ITs with an appropriate burden rating ensures that the connected devices operate within their specified voltage and current ranges. This improves the reliability and accuracy of the entire measurement and protection system. Finally, considering the burden rating during design helps to minimize the cost of the ITs. Over-specifying the burden rating can result in the selection of larger and more expensive ITs than are necessary. Therefore, accurately calculating the required burden and selecting ITs with an appropriate burden rating is crucial for ensuring the reliable, accurate, and cost-effective operation of a power system.