What is the Purpose of Energy Meters in an Electrical System?

Energy metering in electrical systems utilise devices that record the electrical energy load in units of Kilo-Watt hour (kWh). Power utilities use this information to bill electricity users.

[Smart power meter from Schneider Electric]

The principals behind Energy Meters

Energy meters generally measure load over a set period of time. An energy meter thus generally records the instantaneous power consumption of a load and then integrates the result over a predetermined time frame. The energy is then displayed in kWh units. In Mathematical format, the power of the load is depicted as follows:

For single phase systems:

For three phase systems:

Where θ is the phase angle of the load

The energy is then:

The instantaneous voltage and current measurements are used to determine the power usage of a load at any given time.

Types of Energy Meters

There are various different forms of energy meters, from Electro-mechanical Induction type meters to Electronic energy meters. The inner working of these meters is generally the same as most systems require some sort of power measurement, power recording and power integration over time.

The main difference between meter types is the method used to measure current. The current can either be measured directly (Direct Reading) or by the use of a current transformers (CT’s). In the direct measurement methodthe current supplied to a load is directly measured by either mechanical or electronic methods. In the case of a current transformer the current carrying conductor is placed in the center of a current transformer. A proportionally smaller current is then produced in the secondary winding of the transformer which is proportionally smaller to the primary current. The size of the secondary current is dependent of the turns-ratio of the transformer. The secondary current is then recorded. This allows for the measurement of very large currents while not subjecting the meter to them.

[A schematic representation of a current transformer. Image created by Author.]

Why use a Current Transformer?

The current transformer might seem unnecessary at first, due to the transformer adding complexity to the system. The CT is however vital in some energy metering systems.  High currents could damage direct measurement instruments of a similar size and therefore CT’s are  used in energy meters rated at 150A and higher.

The difference between Building Management Systems (BMS) and Energy Management Systems (EMS) in energy metering.

Let’s first define these two concepts. A BMS is usually a hierarchic control scheme where a central server or computer gathers data about various processes within a building. This ranges from energy management to HVAC processes and uses proprietary software to do so. An EMS will usually  monitor the energy usage of a building or set of buildings using energy metering or sub-metering. The EMS is also controlled by a server or computer.

There is usually a misconception that a BMS will provide the same capabilities as an EMS when managing the energy consumption of a building. This is however not true as the two systems serve different goals and should be used to complement each other. The BMS is normally used as a real time data collection service which helps to manage the instantaneous requirements of a building. The BMS is also usually used as a means to balance the real time state of a system.

The EMS however, helps one to identify the long term energy requirements of a building as well as predict future load profiles. The EMS is thus a better solution when one wants to optimise the energy use and cost of a building over a long period of time. The EMS thus gives a better overview of the energy requirements of a system.

The emergence of smart energy meters which save data either to non-volatile memory or cloud based computer servers have greatly enhanced the use of modern day energy and building management systems. These energy monitors can be readily installed in most buildings, allowing the relatively fast setup of an EMS or BMS.

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Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.

jannes9000@gmail.com