Power Factor Measurement Methods in Revenue Energy Meters

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Power Factor Measurement Methods in Revenue Energy Meters

Today’s industrial life extensively uses all types of electronic devices such as monitors and servers that require a lot of power. Our customers at Powerpoint Engineering regularly pose questions relating to power usage and the costs involved. I have put together the following information that I hope will give you a better understanding of how efficiently we utilise power, keeping the cost-effectiveness in mind. One way to analyse this through consideration of the power factor.

Power Factor

There are two aspects to power – active and reactive. Active or real power is the power used in all electrical appliances. This is expressed in kilowatts (kW). Reactive power is the power needed to generate and sustain a magnetic field in order to operate. The reactive and active powers together comprise the apparent power.

Power factor is the ratio of working power to the apparent power. It does not have a dimension and its value ranges from 0 to 1. An appliance with a high PF utilises power more efficiently than a one with a low PF.

Revenue Meters

To identify the energy consumption of devices, energy meters are used. There are three general types of energy meters. They are:

  • Induction Type Energy Meters
  • Electronic Energy Meters
  • Smart Meters

The first two types mentioned are conventional energy meters. They measure the power used by both domestic and industrial sectors. However, a smart meter, measures not just the energy consumed but also indicates when it was consumed (Time of Use – TOU). In addition, it is also capable of measuring consumption of natural gas or water.

Smart meters enable bidirectional communication. They can transmit data such as parameter values, alarms, etc. to utilities for retrieving information such as reconnect/disconnect instructions and meter software upgrades. Modems are fitted onto these meters to facilitate communication through wireless media. Revenue metering helps prevent power theft.

Measurement of Power Factor

The Commissioner for Energy Regulation – Ireland (CER) clearly define the accuracy requirements of revenue electrical energy metering systems and their components for kWh & kVArh over a range of supply voltages and over a range of supply kVA thresholds. Section 5.7.1 of the CER metering code –  (http://www.cer.ie/docs/000820/CER%20Metering%20Code.pdf ) states:

“The accuracy of the various items of measuring equipment shall conform to the relevant national IEC Standards (or equivalent European Standards).”

There are different Methods of Measurement for Power Factor:

  • Displacement Power Factor (DPF) is defined as the Cosine of the Phase Angle between Supply Voltage and Load Current.
  • Effective Power Factor (PF) is the ratio of Active Power / Apparent Power.

Generally DPF is almost equal to PF. However, in a situation where the connected load contains Harmonic Distortion, these power factor values can be quite different.

There is a lack of clarity from IEC and the CER for which method of power factor measurement should be used when the connected load current contains harmonics. It is unclear if the mentioned Wattless Energy [kVArh] in section 5.7 of the CER Metering Code is the reactive energy component at 50Hz only, or over the EN50160 Voltage compliance spectrum of 50 harmonics. The kVAr will vary significantly and consequentially, so will the power factor.

This affects billing costs and how Power Factor Correction (PFC) equipment is configured and may operate optimally.

The relevant IEC standard for Electricity Metering Equipment is IEC 62053-24, in particular Annex E, E1. A summary is highlighted below. The IEC do not yet have a standard for measurement of power factor / reactive power in harmonic environments. They have stated that it a known issue, but a standard has not been developed yet.

“Excerpt from IEC Standard 62053-24:

Annex E

(Informative)

Treatment of harmonics and tests for harmonics

E.1 Non-sinusoidal conditions and reactive power definition

Many meter types designed prior to the establishment of this standard exhibit large differences in metered reactive energy under non-sinusoidal conditions (in the presence of harmonics). Earlier standards for electricity meters for reactive energy were based on a definition of reactive energy for sinusoidal currents and voltages. Consequently, they could not specify any requirements for the performance under non-sinusoidal conditions. In order to ensure that the differences in measurement results between different meter types remain within reasonable limits, it is necessary to base the standard on a definition of reactive energy that allows including performance requirements in the presence of harmonics. There are many definitions available for reactive, or non-active, power under non-sinusoidal conditions. Some of them are more theoretical, some are very suitable for some particular applications, but none of them have the same wide usefulness as the definition of reactive energy under sinusoidal conditions. A wide consensus on a single definition of reactive power under non-sinusoidal conditions, suitable for a wide spectrum of applications is not expected in the foreseeable future. ……. Harmonics have no particular phase angle and do rather average out than add in the network. This makes billing for harmonic current controversial. Additional standards for meters for non-active energy based on other definitions may be developed in the future.”

 

(IEC Standard 62053-24)

For more information please contact Garrett Kelly BSc MIET, Senior Project Engineer at Powerpoint Engineering or call us on 057 866 2162. 

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