Tuomas Messo

Modern electric distribution systems typically contain several feedback-controlled parallel inverters that together form a complex power distribution system. Consequently, a number of issues related to stability arise due to interactions among multiple inverter subsystems. Recent studies have presented methods where the stability and other dynamic characteristics of a paralleled inverter system can be effectively analyzed using impedance measurements. This paper presents implementation techniques for a comprehensive online stability analysis of grid-connected paralleled inverters using power hardware-in-the-loop measurements based on an OPAL-RT real-time simulator. The analysis is based on simultaneous online measurements of current control loop gains of the inverters and the grid impedance, and aggregated terminal admittance measurements of the inverters. The analysis includes the measurement of the inverters’ aggregated output impedance, inverters’ loop gains, global minor loop gain, and grid impedance. The presented methods make it possible to rapidly evaluate the system on both global and local level in real time, thereby providing means for online stability monitoring or adaptive control of such systems. Experimental measurements are shown from a high-power energy distribution system recently developed at DNV GL, Arnhem, Netherlands.

Grid characteristics have a drastic impact on the stability and control performance of grid-connected systems. Because the grid conditions typically vary over time, online measurements are most desirable for the stability assessment, protection design, and control-system optimization of the systems. Previous studies have presented methods based on Fourier techniques and broadband sequences with which the frequency responses of the grid-connected systems, such as the grid impedance or inverter control loops, can be measured. However, online measurements under unbalanced grids with harmonic voltages have not been comprehensively considered. The present paper demonstrates how the previously applied online-measurement methods fail in the presence of unbalanced grid and voltage harmonics due to the spectral leakage caused by Fourier transform. This work also proposes a simple signal-design method to avoid the leakage. Experimental results based on a high-power grid-connected system are shown to demonstrate the effectiveness of the proposed method.

Grid impedance is an important parameter which affects the control performance of grid-connected power converters. Several methods already exist for optimizing the converter control system based on knowledge of grid impedance value. Grid impedance may change rapidly due to fault or disconnection of a transmission line. Therefore, online grid identification methods have been recently proposed to have up-to-date information about the grid impedance value. This is usually done by perturbing the converter output current and measuring the response in output voltage. However, any parallel converters connected to the same interface point will cause errors, since the measured current differs from the current that is flowing through the grid interface point. This paper points out challenges and errors in grid impedance identification, caused by parallel converters and their internal control functions, such as grid-voltage support. Experimental grid-impedance measurements are shown from the power hardware-in-the-loop setup developed at DNV-GL Flexible Power Grid Lab.

Online measurements of the battery impedance provides valuable information on the battery state-of-charge and state-of-health which can be utilized for improving the safety and the performance of the associated system. The electrochemical-impedance- spectroscopy (EIS) is widely used for battery impedance measurements but it is not the most applicable solution for online measurements due to its slowness and complexity. These drawbacks can be improved using broadband signals, such as, pseudo-random-sequences (PRS), which are fast and easily implementable. However, the non- linear behavior of batteries have significant effect on the impedance measurements and the selection of the PRS signal. Majority of the PRS signals are applicable for measurements of linear systems but also signals for non-linear system identification do exist. Moreover, the reduced accuracy and signal-to-noise ratio of the PRS signals compared to the EIS make the filtering of the results as well as the amplitude design important aspects. This paper demonstrates the use of two PRS signals, the pseudo-random-binary-sequence (PRBS) and a ternary- sequence with better toleration to battery non-linear effects, with comprehensive amplitude and filtering design for battery impedance measurements. The results are referenced and validated to practical EIS measurements in various operating conditions for lithium-iron-phosphate cell.

Renewable energy-based generation of electrical energy is currently experiencing rapid growth in electrical grids

Impedance-ratio-based interaction analyses in terms of stability and performance of DC-DC converters is well established. Similar methods are applied to grid-connected three-phase converters as well, but the multivariable nature of the converters and the grid makes these analyses very complex. This paper surveys the state of the interaction analyses in the grid-connected three-phase converters, which are used in renewable-energy applications. The surveys show clearly that the impedance-ratio-based stability assessment are usually performed neglecting the cross-couplings between the impedance elements for reducing the complexity of the analyses. In addition, the interactions, which affect the transient performance, are not treated usually at all due to the missing of the corresponding analytic formulations. This paper introduces the missing formulations as well as explicitly showing that the cross-couplings of the impedance elements have to be taken into account for the stability assessment to be valid. In addition, this paper shows that the most accurate stability information can be obtained by means of the determinant related to the associated multivariable impedance ratio. The theoretical findings are also validated by extensive experimental measurements.

AC microgrid is an attractive way to energize local loads due to remotely located renewable generation. The AC microgrid can conceptually comprise several grid-forming and grid-following power converters, renewable energy sources, energy storage and local loads. To study the microgrid dynamics, power-hardware-in-the-loop (PHIL)-based test setups are commonly used since they provide high flexibility and enable testing the performance of real converters. In a standard PHIL setup, different components of the AC microgrid exist as real commercial devices or electrical emulators or, alternatively, can be simulated using real-time simulators. For accurate, reliable and repeatable results, the PHIL-setup should be able to capture the dynamics of the microgrid loads and sources as accurately as possible. Several studies have shown how electrical machines, dynamic RLC loads, battery storages and photovoltaic and wind generators can be emulated in a PHIL setup. However, there are no studies discussing how a three-phase grid-following power converter with its internal control functions should be emulated, regardless of the fact that grid-following converters (e.g., photovoltaic and battery storage inverters) are the basic building blocks of AC microgrids. One could naturally use a real converter to represent such dynamic load. However, practical implementation of a real three-phase converter is much more challenging and requires special knowledge. To simplify the practical implementation of microgrid PHIL-studies, this paper demonstrates the use of a commercial high-bandwidth voltage amplifier as a dynamic three-phase power converter emulator. The dynamic performance of the PHIL setup is evaluated by identifying the small-signal impedance of the emulator with various control parameters and by time-domain step tests. The emulator is shown to yield the same impedance behavior as real three-phase converters. Thus, dynamic phenomena such as harmonic resonance in the AC microgrid can be studied in the presence of grid-following converters.

Here is information on how to publish, spread your research, and search for your publications. Most of LiU's research publications are directly imported into DiVA from the Web of Science. Before registering a publication, always check if the publication is already in DiVA!

Here is information on how to publish, spread your research, and search for your publications. Most of LiU's research publications are directly imported into DiVA from the Web of Science. Before registering a publication, always check if the publication is already in DiVA!