Z-NET Pre-normalisation of grid impedance measurement in the power line communication frequency

• Forschungsstelle: METAS
• Projektnummer: F-5213.30107

Schlüsselwörter (Englisch)

Grid impedance, power line communication, smart meters, standardisation

Kurzbeschreibung (Englisch)

The large scale deployment of smart meters that is about to take place in Switzerland relies heavily on the usage of power line carrier technology for its communication. On top of various interferences, the frequency dependant impedance of the low voltage network greatly influences the propagation the power line signals and can thus impact the reliability of the communication channel. The precise measure of the line impedance is presently ill defined and only possible with some experimental instruments. Furthermore, the various proposed methods tend to yield non-comparable results. The aim of this project, is to establish a metrological traceable grid impedance standard that enables the objective comparison of the proposed measurement techniques and open the road towards standardisation. The normalisation of an agreed upon measurement method should lead to the apparition of instruments and grid connected components permitting the resolution of power line communication problems for difficult locations.

Projektziele (Englisch)

Power Line Communication signals are affected by the injection of electrical noise by the various power electronic equipment used in renewable sources or electronic loads. Additionally low grid impedances also contributes to signal attenuation. Grid impedance is affected by resonances in EMC filter build in electronic devices and converters. The so-called ‘notching effect’ by resonances has been recognised, but is not yet considered in the standardisation process.

Today, no agreed upon methods for the measurement of the noise nor impedance in the frequency range used by PLC for smart metering exist. As a result the normalisation of these measuring methods is not possible. In term of grid spectral impedance several methods have been proposed by various research instances. However the comparison of these methods is made difficult by the absence of a metrological traceable reference. To be useful this metrological impedance standard must be defined in both frequency and time domains. The frequency dependence of the impedance reflects the various field conditions that can affect PLC at different frequencies. The Impedance standard must be able to emulate these conditions in deterministic fashion. In the same way, the fluctuations of the grid conditions in time must be reproduced in laboratory in a deterministic fashion. Such a standard can also be used for the assessment of the susceptibility of a PLC system to the network impedance.

A comparison of the various grid impedance measurement techniques developed by key research organisations in Switzerland and Europe will be made possible with such an impedance standard and will constitute the first significant step towards standardisation.

Abstract (Englisch)

WP0: Specifications for a reference impedance for grid impedance meters comparison:

The purpose of the WP0 was to establish preliminary specifications for the Static Impedance Standard (WP1), the Programmable Impedance Standard (WP2) and the Time Variant Impedance Network (WP3) prototypes. the standard reference impedance was carefully designed to also contain some important dynamic range of impedance with some strong variations of magnitude and phase over the considered frequency range, in order to emulate some damped resonances. It was decided to couple the SIS with a commercially available LISN. This approach presents the advantage to be affordable and easily applicable in all the labs involved in the project.

WP1: Design of a fully characterized static impedance network

The goal of WP1 was to design and characterize a Static Impedance Standard (SIS) according to WP0's specifications. After simulating the LISN, the project manager designed, simulated and built the RLC network and the printed circuit board and the enclosure for the SIS. The impedance of the device was then measured over the 1-500 kHz frequency range (Figure 3) with a calibrated precision impedance measurement bridge. All the impedance measurements were performed at low voltage and current, disconnected from the grid. Once the SIS was built and characterized, the intercomparison was started in January 2020 and successfully finished its course around Europe in June 2020 after being measured by all the partners.

WP2: Design of a fully characterized programmable impedance network:

The RLC circuit is based on the same principle as the SIS, but in the Programmable Impedance Standard (PIS), several values of the RLC components were placed on the PCB and could be switched on or off by using MOSFET-based AC switches. The uncertainty budget calculation was automated, thus allowing a meaningful comparison between METAS measurements and partners.

WP3: Design of a fully characterized programmable and time variant impedance network:

A synchronization circuit based on an isolation transformer, a band-pass filter centered on 50 Hz and a zero-crossing detector was added to the PIS hardware and implemented on the power circuit. A real-time micro-controller was also implemented in the control circuit. The main processor programs the micro controller according to the user inputs and then let the real-time side handle the switching of the impedance. To add flexibility, some BNC interfaces were added for an external trigger signal as well as a synchronization signal for the impedance switching.

Both static and dynamic modes of the device can be selected and configured in the web based control interface. The user can also synchronize the impedance switching with an external TTL signal to allow further flexibility.

Umsetzung und Anwendungen (Englisch)

The devices developed by METAS in the scope of the Z-Net project were used to measure a reference impedance with several grid impedance measurements devices. This allowed comparing the results and the different measurement techniques used in the labs, providing precious information for the amelioration and design improvements of the grid impedance meters.

The intercomparison of devices also raised the question of a measurement protocol, which was not clearly defined before the project started.

Finally, the devices developed and built by METAS are now finding a further usage in the scope of the supraEMI project, where the Static Impedance Standard is used as a reference standard for intercomparison and the Programmable Impedance Standard is used to generate harmonics on sinusoidal waveforms to test sampling algorithms.

Publikationen / Ergebnisse (Englisch)

Dominique Roggo, Jonathan Braun, Jan Meyer, David de la Vega, Blaise Evequoz, Cédric Blaser, Robert Stiegler, Igor Fernandez: "Pre-normalisation of grid impedance measurement in the power line communication frequency band" CIRED 2021 Conference