Susceptibility of Static Energy Meters Electromagnetic Compatible Energy Measurements
Tom Hartman is a PhD student in the department Power Electronics. (Co)supervisors are prof.dr.ir. F.B.J. Leferink and dr.ir. D.J.G. Moonen from the faculty of Electrical Engineering, Mathematics and Computer Science.
Considering that static energy meters (SMs) are being deployed across Europe, it is important that these new SMs are accurate and electromagnetically compatible with the electronic environment they are placed in. However, the electronic environment of a household has changed over the years due to, for instance, the increase in non-linear equipment. Standards regarding the electromagnetic compatibility (EMC) of equipment generally lag behind such changes and electromagnetic interference (EMI) problems with SMs were reported. The goal of this thesis is to find and address a root-cause for the EMI problems with SMs and improve the EMC of their energy measurements with non-linear currents, to recover the consumer confidence in this transition towards smart meters.
This thesis proves that SMs used for the billing purposes in households are susceptible to EMI generated by non-linear household equipment drawing pulsed currents. This susceptibility results in over estimations of the measured energy, creating increased energy bills, as well as extreme under estimations, resulting in perceived energy generation, essentially earning money for the consumer. These problems show the large impact EMC issues can have on the everyday consumer.
Because these non-linear impulsive currents, caused by commercial off-the-shelf equipment, are still interfering with the energy measurements of SMs, while both adhere to their respective standards, the need for time-domain testing is emphasized. A multichannel time-domain measurement technique is developed to analyze the waveforms in order to investigate the relation between critical waveform parameters and the SM errors. Important time-domain parameters found to correlate with the erroneous energy measurements are the peak amplitude, rise- and fall time, slew rates (SRs) and firing angle (FA).
For assessing the underlying problem behind the SM misreadings the time-domain EMI measurement approach is extended to on-site scenarios. These measurements are used to identify, analyze and characterize the fundamental parameters of the interfering waveforms in the field. However, to address the root-cause behind the errors it is necessary to control the current waveforms that are causing interference. Adjusting parameters in a controllable practice will quantitatively determine the effect a parameter, or a combination of parameters, has on the energy measurements.
For this reason an ac controlled-current load has been designed, and ultimately built, used to directly relate certain waveform parameters of the impulsive currents to SM errors in a controllable setup. From this a root-cause for the erroneous measurements of SMs utilizing a Rogowski coil as their current transducer was found. Namely, amplifier clipping caused by the Rogowski coil as a result of the high frequency components of the current waveform results in distorted current waveforms after integration. With the ac controlled-current load a direct relation between parameters such as, the rise time, fall time, SRs and FA, and the corresponding SM errors is validated with measurements. Furthermore, a change in the paradigm of accurate energy measurements is proposed based on the orthogonality of power flow. Focusing on fundamental active power and lower harmonics for energy metering instead of continuously increasing the measurement bandwidth simplifies the electronics and makes them more robust against EMI. Combining these two findings resulted in the implementation of a low-pass filter between the current transducer and the amplifier, which improves the robustness against conducted EMI without lowering the accuracy of the energy measurements. This shows a complete reduction of significant errors caused by the equipment from the initial reporting that resulted in SM misreadings.
Showing a root-cause for SM energy measurement errors can be considered the main contribution to this thesis. Especially combining this root-cause with the orthogonality of power flow creating a low-cost solution which allows for energy measurements that are more robust against EMI. This should improve the consumer confidence during the transition towards SMs and more specifically, smart meters.
