The burden of a current transformer is defined as the volt-ampere loading that is permissible without exceeding the accuracy limits for a given CT accuracy class. Simply put, it is the load connected to the CT secondary. CT burdens are usually specified as 5 VA, 10 VA, 15 VA, 20 VA, and 30 VA. The burden of a CT is determined based on the rough calculation of the burden of leads and meters connected to its secondary. To ensure safety, the burden value is often slightly inflated. However, choosing an incorrect burden can lead to problems, which we will discuss next.

Consequences of Overestimating Burden

So, what happens if we overestimate the burden? A higher burden can lead to damage to connected meters and measurement errors.

Let’s go deeper into these two issues. A higher burden extends the saturation characteristics of the CT core. Now, during a fault condition, the current increases significantly. If the instrument safety factor, or ISF, of the CT is assumed to be five, the metering core should saturate if the fault current exceeds five times the rated current. This helps in limiting secondary current, thus protecting the connected meters.

However, when we overestimate the burden, the CT’s saturation characteristics change, meaning the CT may not saturate even at higher fault currents. This results in an increased ISF, allowing excessive secondary current to flow, which can potentially damage the connected meter.

Calculating Saturation Voltage and ISF

Let’s consider an example. Suppose the actual CT burden requirement is 5 VA, but due to miscalculation we select a burden of 15 VA. The required ISF is 5, and the CT ratio is 500 to 1 ampere. For an ISF of five, the saturation current should be 5 amperes.

Now, let’s calculate the saturation voltage.

To find the saturation voltage, we take the burden in VA, divide it by the saturation current for the given ISF, and then multiply by ISF. So, in this case, we take 5 VA, divide it by 5 amperes, and then multiply by 5. This gives us 5 volts as the saturation voltage.

But since we selected a burden of 15 VA instead of 5 VA, we need to calculate the new saturation voltage.

Again, we take 15 VA, divide it by 5 amperes, and multiply by 5. This results in 15 volts as the new saturation voltage.

Calculating New ISF

Now, let’s calculate the new ISF. The ISF is determined by multiplying the saturation current by the new saturation voltage and then dividing by the actual burden in VA.

So, in this case, we take 15 volts, multiply by 5 amperes, and divide by 5 VA. This results in a new ISF of 15. So, instead of an ISF of 5, we now have an ISF of 15. This means the CT won’t saturate when it should, and the connected meter could be damaged.

Impact on Measurement Accuracy

Apart from meter damage, an incorrect burden selection also affects measurement accuracy. A CT is most accurate when the actual burden matches the rated burden. For example, if a CT designed for 15 VA burden is operated at just 5 VA, there will be higher measurement errors even when the CT is working at its rated current. This inaccuracy can affect power system monitoring and reporting.

So, to sum it up: always choose a CT with the appropriate burden. Overestimating burden extends the saturation voltage, increasing ISF and risking meter damage. CT accuracy is best when the actual burden matches the rated burden. Avoid overestimation while calculating the burden to ensure precise and safe operation.