The burden connected to a current transformer's secondary circuit has a profound effect on the CT's performance, influencing its accuracy, saturation characteristics, and overall reliability. The burden-which consists of the connected instruments, relays, and connecting wires-determines the voltage that the CT must generate in its secondary winding to drive the secondary current. This voltage, in turn, determines the core flux and the excitation current, which are the primary sources of CT error. Understanding the relationship between burden and CT performance is essential for proper system design and for troubleshooting accuracy problems.
The most direct effect of burden on CT performance is on ratio and phase errors. As the burden increases, the CT must generate a higher secondary voltage to drive the same secondary current through the increased impedance. This higher voltage requires a larger core flux, which increases the excitation current. Since the excitation current is diverted from the secondary output, an increase in excitation current causes an increase in ratio error and phase error. The relationship between burden and error is approximately linear for burdens within the CT's rated range, but errors increase rapidly when the burden approaches or exceeds the rated VA.
The effect of burden on accuracy is specified through the CT's rated burden. The CT's accuracy class is only guaranteed when the actual burden is equal to or less than the rated burden. If the actual burden exceeds the rated burden-a condition known as overburdening-the CT's errors may exceed the limits specified by its accuracy class. This can lead to inaccurate energy measurements, billing discrepancies, and improper operation of protection relays. For this reason, the burden calculation must be performed carefully during system design to ensure that the total burden does not exceed the CT's rated VA.
Conversely, a very low burden-a condition known as underburdening-also affects CT performance. A lower actual load rating increases the overcurrent factor, and measuring devices may be damaged in case of a short-circuit. With insufficient burden, the secondary voltage during faults may rise excessively, potentially exceeding the insulation level of the secondary circuit or damaging connected instruments. In some cases, a loading resistor may be added to bring the burden within the recommended range. The sum of the consumer power should be close to the transformer rated power, otherwise the measured value may be too positive.
The burden also affects the CT's saturation characteristics. Higher burden increases the secondary voltage required for a given secondary current, which increases the core flux and brings the CT closer to saturation. During fault conditions, when the secondary current is many times the rated value, the secondary voltage and core flux can become very large, causing saturation. The effect of burden on saturation is particularly important for protection applications, where the CT must maintain accuracy during faults to ensure proper relay operation. The CT saturation characteristics have a huge influence on the safety of both personnel and equipment and on the reliability of the protection and metering system.
The burden affects the CT's performance differently at different frequencies. The influence of burden on the accuracy of current transformers at wide frequency ranges has been studied, with results showing that the burden value and type (resistive, inductive, or capacitive) can affect the CT's response to harmonics and transients. Capacitive burdens, in particular, can cause the core flux to contain a component that changes linearly with time, leading to saturation within a few cycles. This is particularly problematic for protection applications, where the CT must reproduce the fault current accurately over the duration of the fault.
Temperature effects on burden should also be considered. The resistance of copper conductors-both in the CT secondary winding and in the connecting cables-increases with temperature at approximately 0.4% per degree Celsius. As the burden increases due to temperature rise, the CT errors may also increase. In installations with high ambient temperatures or limited ventilation, the actual burden at operating temperature may be significantly higher than the burden calculated at reference temperature.
Practical burden management involves careful calculation and selection of all components in the secondary circuit. The first step is to determine the burden of each connected device from its specifications or by measurement. The second step is to calculate the burden contribution of the connecting cables, using the formula: Lead burden (VA) = I² × R, where I is the rated secondary current and R is the total loop resistance of the cables. The third step is to sum all burden components and compare the total to the CT's rated VA. If the total burden exceeds the rated VA, measures such as increasing the cable cross-section, reducing cable length, or selecting a CT with a higher VA rating must be taken.
In summary, the burden of a current transformer is not merely a passive load but an active factor that directly determines the CT's accuracy, saturation behavior, and overall performance. Proper burden calculation and management-considering all connected devices, cable characteristics, temperature effects, and application requirements-are essential for ensuring that the CT operates within its specified accuracy limits and provides reliable service throughout its life.