Summary:Three phase meter current transformers are used to convert the primary current in line conductors in...
Three phase meter current transformers are used to convert the primary current in line conductors into a secondary current that is suitable for measuring by an electricity meter.
They are connected to the electricity meter using wires. They are most often used in residential, commercial and industrial electrical systems to measure energy use, but can be found in other applications as well. In this article, we’ll explore the basics of three phase meter CTs and how they work with electricity meters to report power usage.
In most cases, the CTs are installed as part of a “stack” of devices including revenue metering and protection circuits. This allows different sets of CTs to be used for each purpose – for example, protection devices needing higher overload performance may run their secondary coils through a set of high-performance split core CTs while revenue metering uses lower-cost conventional CTs. This also allows the same set of CTs to be used across multiple phases for measurement purposes and provides isolation between the metering and protection circuits - a requirement in most electrical codes.The most common CT configurations include donut and bar-type devices.
Donut-type CTs have a primary conductor passing through their center and a secondary winding that is formed by a number of turns of magnet wire wrapped around an iron core. Bar-type CTs are similar, but include a fixed bar that forms the primary winding.
Both types of CTs are available in many different ratios that are specified on their nameplate. The ratio describes how much of the primary current that passes through the CT is converted into a secondary current by the secondary winding. The higher the ratio, the more current a given CT is capable of handling.
The other important specification on a CT is its accuracy, which is described as Class on the nameplate. This describes how accurately the primary current and the secondary current are measured in comparison to one another - for example, a Class 1 CT will produce an error of no more than 1% at its full rated current.
It is possible to use a single-phase CT in a three-phase delta circuit, but only if the grounded leg of the delta (called a GND or zero sequence leg) is not monitored by any current sensors. In most cases, this will not be practical, and it is better to monitor both of the non-grounded legs to avoid a potential issue in the event of an earth fault.
If you are installing a new three-phase meter or replacing an existing one, it’s good to know that most meters can be either CT operated or Direct Connected. CT operated meters require the use of CTs to operate, while Direct Connected meters don’t need them. This is because the three-phase meters need to be able to detect all of the phase currents in order to calculate total watt hour energy consumption.Most electricity meters are self-contained, meaning they have their own internal current transformers to measure the total electric energy consumption.
They can be either transformer rated or non-transformer rated. Non-transformer rated meters are typically used on services under 200 amperes, while transformer rated meters are used on larger commercial and industrial supplies.