An Experimental Study of the Effect of Current THD to kWh Meter’s Energy Measurement

Abstract

An experimental study of the effect of constant rms current with different THD to the single phase digital kWh meter is presented in this paper. The study focused on how harmonics from non-linear load with constant voltage supply and constant rms current affect the energy measurement of single phase digital watt-hour meter. Traditionally, analog electromechanical meter is using the rms current as its main measurement parameter. In digital meter, different techniques and sampling are used to measure the power consumed by the consumers. Although different techniques and sampling are used, the rms current parameter is still the dominant parameter used to measure and calculate the energy measurement. Both electromechanical and digital meter show good measurements under linear load condition. On the other hand, under non-linear load, harmonics are often neglected in energy measurement. In addition, harmonics in heavily distorted current originated from non-linear load and contributed to lot of losses and it will not be recorded in the billing. To make the situation worse, harmonics can propagate through the transformer and they will be affected the power network either if the harmonics are injected from the source or load side. Analysis from the measurement data clearly showed the effect of the harmonics to the energy measurement. In consequence, either utility company or consumer shall be affected if the harmonics parameter is neglected in the energy measurement purpose. The results showed that no significant difference when the actual digital watt-hour meter energy reading is compared to the calculated energy consumption based on rms current value. Therefore, either harmonics are taken or not taken into account during the energy measurement, the consumer is not affected and still paying the same prices. However, the utility company will be burdened by some losses if harmonics are not taken into consideration.

Harmonics influence on power and energy measurements

“…The effects described making the energy meters based on the induction mechanism correctly measure the energy at the fundamental frequency (50 or 60hz), have an appreciable error for second and third harmonics and practically do not respond for harmonics of greater order than 5 (Masri et al, 2017).…”

“…These standards are general recommendations and do not always force consumers to connect harmonic filters or compensators. In some load analyzers, the angle between the voltage and current harmonics is taken into account but cannot be defined if the energy is generated or consumed by the load (Masri, 2017). Petkovi & Stevanovi (2014), proposes to use the measurement of the distortion power as an indicator of the presence of harmonics and to use that parameter as a penalty element.…”

Active power measurement uncertainty modelling and propagation analysis in case of harmonically distorted signals

“…where the single components will be addressed in the discussion that follows, while the index Y denotes the quantity to which the concrete uncertainty component is attributed. The first two uncertainty components presented in (10), 𝑢 A,Y and 𝑢 R,Y , are mathematically evaluated in a unique manner, no matter the signals' parameter regarded. The uncertainty component 𝑢 A,Y is calculated according to Type A evaluation principle [17] and it is a result of the statistical scattering of measurement data, if multiple, 𝑁, recordings for any signal parameter are conducted:…”

“…The single components 𝑢 C,𝑥ℎ% are evaluated according to (10) and the discussion presented in 4.1.1, while the sensitivity coefficients 𝜕𝑇𝐻𝐷 𝜕𝑥 ℎ,% ⁄ are calculated according to (4). If the substitutions are made, (25) becomes:…”

“…where 𝑢 C,𝑋 is evaluated according to (10) and the conclusions presented in 4.1.2, 𝑢 THD is calculated according to (25)-( 26) and the partial derivatives 𝜕𝑋 1 𝜕𝑋 ⁄ and 𝜕𝑋 1 𝜕𝑇𝐻𝐷 ⁄ are determined from (5).…”