Model verification and common mode current analysis of grid connected three phase two level converters

Abstract

In this Master’s thesis an in-depth model of a shunt active filter for high frequency analysis is constructed. The purpose is to enable analysis of higher frequency behavior reaching 1MHz. Using the high frequency model, common mode currents can be simulated.

The model is built using high frequency impedance measurements of the internal components in one of Comsys ABs active filters. The measurement is done up to 10MHz and equivalent circuits are constructed to mimic the parasitic components. The impedance is measured over the components and from the component terminal to ground. The model containing the equivalent circuits is made and calibrated to match the device under test.

Using the measured component values and the measured parasitic elements, a simulation model could be designed. Equivalent circuits were used to model the non-idealities in the components and thus their high frequency behavior. An initial model was run and the simulated signals were compared to the lab setup measurements. The component values were tuned to better match the measured frequencies and damping resulting in a final simulation model. The outcome of the thesis is a model that simulates the overall frequency behavior up to 2 MHz. The major oscillations caused by switching IGBTs and how they are extended through the machine can be observed in the simulation. Fast Fourier transform analysis supports the similarities between measurements and simulations where peaks at relevant frequencies are observed. Common mode currents flowing through parasitic couplings can be approximated using the model. Discussions concerning the frequency and damping adjustments are made where areas such as eddy currents and measuring uncertainties are touched upon. A comparison between reality and simulation is done where differences are analysed and discussed.

The thesis provides a model that can be used up to 2MHz where common mode currents can be approximately simulated, therefore the purpose was achieved. It lays a foundation for continued work concerning EMC and high frequency behavior in active filters. Further work to improve upon the model could be simulating non-ideal IGBTs and locating the large damping resistance present in the lab setup. To use the model in deeper analysis the model should be run in different operation states to ensure the performance of the model is satisfactory.

Popular Abstract

Ideally, power would be transferred by the interaction of balanced and perfectly sinusoidal currents and voltages, in-phase with one another. However, power is not perfect. Disturbing currents in the grid affect connected electrical equipment. Shunt active filters are electronic devices designed to clean the currents. By using transistors in a smart way, shunt active filters inject currents in the grid that counteract the disturbances, resulting in almost perfect power transfer. However, this process creates its own minute disturbing currents, that should not enter the grid. To understand these minute currents created by the shunt active filter a simulation model that mimics the high frequency behavior has been developed.

In this project, a model was made to simulate the high-frequency behavior of a shunt active filter from Comsys AB. The model can simulate the active filter (AF) adequately up to 2MHz. This was an increase in frequency from the previous model, that reached less than 200kHz.

All conducting materials have inherently present internal resistances and inductances. All conducting materials close to each other will have a capacitive coupling. These properties are called parasitic elements, and can be measured using an impedance analyser. Using the high resolution model, minute disturbing currents flowing through the parasites can be approximated. Common mode currents are disturbing currents that flow through the parasites to electrical ground. These were of certain interest in this study.

To construct the model, the individual components of the AF were measured with an impedance analyser. These measurements enabled modelling of the components' high frequency characteristics and include eventual parasites. An AF supplied by Comsys AB was set up in a lab environment and run to verify the simulation results. As with any other electronic equipment, AFs need to pass the requirements of electromagnetic compatibility (EMC) for it to be used and sold commercially. EMC-directives limit the radiated electromagnetic emissions and the conducted current emissions from the device. Common mode currents are one such sort of conducted current emissions.

The resulting model was able to simulate the AF up to 2MHz. Compared to the experimental measurements, the model was able to predict high frequency oscillations and common mode currents with reasonable accuracy. However, it was not enough with just measuring the individual components on their own to accurately represent the system. A fine tuning of the parasitic components was required, resulting in overall reasonable values, except for an unexpectedly high parasitic resistance of 70$\Omega$. This will require further work to identify. This is an important step in further understanding AFs at higher frequencies.

In an increasingly electrified society EMC grows more and more important. To be able to lower the conducted current emissions from an AF, a simulation model is an incredibly helpful tool. Solutions on how to limit common mode currents impact can be investigated in a matter of hours instead of days or weeks.

• author: Lindvall, Simon and Haraldsson, Richard
• supervisor: Mats Alaküla; Nils Lundström; Francisco Marquez
• organization: Industrial Electrical Engineering and Automation
• alternative title: Model verification and common mode current analysis of a grid connected active filter
• course: EIEM01 20211
• year: 2021
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: active filter, EMC, EMI, converter, common mode, differential mode, shunt active filter, parasitic elements
• publication/series: CODEN:LUTEDX/TEIE
• report number: 5471
• language: English
• id: 9062305
• date added to LUP: 2022-01-27 11:02:23
• date last changed: 2022-01-27 11:02:23