Energy Metering Using Potential and Current Transformers

Energy metering is one of the critical functionalities of today's metering devices. Providing accurate measurement of the energy consumption of commercial buildings, industrial plants and residential housing is the basis for billing by utility companies.

For commercial buildings, industrial plants and factories, the utilities normally provide electrical connections at a medium voltage level, typically at 15kv system voltage and 34.5kv system voltage. In these cases, there are a couple of connections to measure the power consumption including outdoor installations where outdoor medium voltage potential transformers and current transformers are mounted on overhead metering assemblies or for indoor installations where the indoor medium voltage potential transformers and current transformers are mounted in indoor MV switchgear panels.

In both cases, the potential transformers transform the medium voltage system to lower utilization voltages, typically at 120Vac, which is suitable for connections to the metering devices. The current transformers provide isolation from the systems voltages and the secondary terminals provide 0-5A outputs that are connected to the metering devices.

To assure correct billing by the utility companies, the combined functionality of the potential transformers and current transformers guarantee the proper reading of the metering devices monitoring the energy consumption at the customer’s premises.

For outdoor installations in a typical 15kv system, the model JCK-5C outdoor current transformer and JVW-5C outdoor potential transformer can be used as these instrument transformers are highly accurate with the JCK-5C having 0.3B0.5 metering class accuracy and the JVW-5C having 0.3WXMYZ accuracy class. This accuracy meets the utility metering accuracy requirements.

The model PTG5 indoor potential transformers and JKM-5C indoor current transformers installed in MV switchgears also have metering accuracy of 0.3WXMYZ and 0.3B1.8 respectively. There are instances in MV switchgear installations where window type current transformers such as the model 780, are used in the circuit breaker bushing. While window type current transformers are only rated for 600v, they can be used on higher voltages if the cables are fully insulated.

For other commercial and residential applications where the system voltages are at the utilization level like 480Vac, low voltage potential transformers and window type current transformers are used for metering applications. The model 456 potential transformer provides utility grade accuracy at 0.3WXM and can be combined with the model JAK-0C current transformer which has 0.3B0.5 metering class for revenue grade metering at the low voltage level.