By Ni Shisheng, Jiangsu Linyang Electronics Co., Ltd.
The working principle of the traditional electronic energy meter is to get the calculus value on the basis of time of the product of the instantaneous value of voltage and current. The accuracy of this energy calculation is guaranteed under the condition of quality energy — that is, the voltage and current curves are the periodic sine waves. However, as the technology development and the increasing use of non-linear load make grid power quality a problem, which is reflected by the distortion of the voltage and current waveform, more errors occur if we continue to adopt the traditional way of energy calculation. According to the experiments and assessments carried by the Electric Power Research Institute under the China Research Institute, the errors caused by harmonics in the medium and low voltage power grid are up to 0.5% – 1%. Therefore, how to improve the accuracy of energy measurement under the conditions of harmonics is a topic worth studying.
This article suggests that the instantaneous reactive power theory can be used for the calculation of energy measurement to improve the real-time performance and accuracy of the energy measurement under the condition of harmonics.
MAIN PROBLEMS OF THE TRADITIONAL WAY OF ENERGY MEASUREMENT UNDER THE CONDITIONS OF HARMONICS
Due to the application of a large quantity of electrical and electronic equipments and non-linear loads, there are waveform distortions of supply voltage, or voltage fluctuation and flickering. What we get from the traditional way of energy measurement is the sum of the fundamental energy and harmonic energy (this sum is not so accurate due to the poor real-time measurement). This sum may be larger or less than the fundamental energy due to the uncertainty of the direction and phase of the harmonics (randomness); that is to say, the traditional way of energy measurement cannot reflect accurately the actual energy consumption of users.
According to the information from the manufacturers of energy metering IC, the response of the electronic energy meters to each harmonic is close to the actual power consumption, but there are still errors:
- Users of linear loads: their energy is equal to the sum of the fundamental energy and energy of each harmonic, but greater than the fundamental energy;
- Users of non-linear loads, their energy is equal to the difference between the fundamental energy and energy of each harmonics, but less than the fundamental energy;
MAIN WAYS TO IMPROVE MEASUREMENT ACCURACY UNDER THE CONDITION OF HARMONICS
- Measure the fundamental energy in an effective and real-time way: Digital Signal Processors are used to make FFT analysis for the sampling signals and extract the fundamental components for energy measurement; or Digital Signal Processors are used to filter and remove harmonic components for ADC sampling data for energy measurement. The energy measurement of the fundamental electricity meters are currently based on this method.
- Measure the periodic harmonic energy and non-periodic harmonic energy in an effective and real-time way: the common way to be used is to get the difference between the aggregate energy and the fundamental energy. But this method includes many errors because the traditional way of energy measurement only applies to periodic harmonics, but cannot get the right result of energy measurement of non-periodic harmonic energy and transient mutant harmonic energy.
Both fundamental energy and harmonic energy can be measured and calculated correctly if the instantaneous reactive power theory is to be used.
The three-phase circuits’ instantaneous reactive power theory, which was proposed by Mr Hirofumi Akagi, a Japanese scholar, in 1983, is mainly to be used for the real-time detection of the APF harmonics and reactive current, but I think it can also be extended to real-time detection of active power and reactive power.
Because the active power and reactive power in the traditional theory are defined on the basis of the average volume, they only hold under the condition that the waveform of voltage and current are sine waves. However, the concepts in the instantaneous reactive power theory are defined on the basis of the instantaneous value, so they not only apply to the sine wave, but also apply to any non-sine wave and any other operation status during transition. The concepts in the instantaneous reactive power theory are quite similar to those in the traditional theory, so the instantaneous reactive power theory, which has larger application, can be seen as the extension of the traditional theory.
In the instantaneous reactive power theory, matrix transformation and low-pass filter can be used in a more convenient way to get the fundamental current components. Time delay of the test is not more than a power cycle, furthermore, the accuracy of the fundamental components are not affected by the harmonic components. Therefore, the instantaneous reactive power theory is applicable to the calculation of electrical energy under the condition of harmonics, and can be used to improve the accuracy of energy measurement.
This article is my rough opinion about the improvement of accuracy of energy measurement under the condition of harmonics. Linyang is making a plan to cooperate with the School of Electrical Engineering of Xi’an Jiaotong University to verify the feasibility of the above opinion through simulation and experiments. If feasible, Linyang will produce and install some multi-functional electricity meters, which will include the fundamental energy, harmonic energy and aggregate energy, and then study the feedback from users.