Does a High Accuracy CT Really Get More Revenue?
Does a high accuracy current transformer really get more revenue?
By Steve Lindsay
In short, Yes! A current transformer (CT) with higher accuracy will typically generate more revenue for a utility than will a standard accuracy CT.
First, let’s take a look at the accuracy classes of a CT. The IEEE C57.13.6-2005 standard has various levels of accuracy but let’s focus on just three. There is the standard accuracy class defined as 0.3 which is the minimum a CT can be rated for revenue metering purposes. There is the 0.15 class which is more accurate than the 0.3 class although this class is not often referenced anymore. And lastly, the IEEE standard defines the 0.15S class that is the highest recognized accuracy class. An accuracy class means that a CT will meter current to within plus-or-minus that amount. Therefore, a standard accuracy CT (0.3 class) will meter within 0.3% of the nominal current. Each CT is required to be tested for accuracy before being delivered to a customer.
Second, let’s take a slightly deeper look at what the accuracy class means. A CT is only required to measure at the rated accuracy at nominal current or above. A good example of this is a standard accuracy CT with a 600:5 ratio. This CT is only accurate at 0.3% from 600 amperes to the rating factor. From 600 amperes to 10% of the nominal current, or 60 amperes, this CT only has to measure at 0.6% accuracy or better. Below 10%, or 60 amperes, there is no guaranteed accuracy. In summary, the lower the current, or load, the less accurate a standard CT will measure.
The question you should be asking yourself is, “What happens to our revenue when we have services that fall well below that during low load periods?”
The answer to that question is that a utility is losing potential revenue during periods of low loads. This is why it is beneficial for a utility to use high accuracy, extended range current transformers that meet or exceed the 0.15S standard.
Many CTs on the market today exceed the 0.15S accuracy standard. The standard stipulates that a 0.15S CT must meter at 0.15% or better from 5% of nominal current to the rating factor. In our above example of a 600:5 ratio CT, that would mean the CT would be accurate down to 30 amperes. With the advent of lower loss core metals, many CTs now can meter accurately down to 1% of nominal current which in the above case would be 6 amperes. This gives the CT an “extended range”. If a utility uses a typical high accuracy ratio of 600:5, that would give the utility an extended range of metering from 6 amperes to 1200 amperes (considering a rating factor of 2).
A high accuracy CT is most effective for a utility during periods of low load in what is commonly referred to as a “variable load” installation. One example of this could be a factory with 2 or 3 shifts with lower capacity during 2nd or 3rd shift. Another example could be a church that has a small staff during the day, but has a higher load on Sunday mornings and Wednesday nights when larger crowds are present.
In an independent study done by a large municipal utility, it was determined that a high accuracy CT could generate anywhere from 0.2% to 0.8% more revenue depending on the load characteristics at the installation.