Grid Impedance Identification Research Articles

Design Scheme of Signal-to-Noise Ratio for Broadband Generalized Grid Impedance Identification

Design Scheme of Signal-to-Noise Ratio for Broadband Generalized Grid Impedance Identification

Distributed Coordinated Control for Stabilization of Multi-Inverter Power Plant

For a power plant integrated with multiple parallel inverters, the impedance variation of the power grid and the natural plug-and-play property of the inverters bring stability challenges. One of the issues is nonlinear wideband oscillations of the grid current and voltage. This paper proposes a distributed coordinated control for the stabilization of the multi-inverter power plant. The passivity-based control (PBC) scheme designs the controller of a single grid-connected inverter from the perspective of energy reshaping, which ensures the stability of the entire system when it is extended to multiple inverters in parallel. A nonlinear observer is proposed to discover the changeable frequency drifting of wideband oscillations so that the PBC scheme can adapt to nonlinear disturbances, such as inverter plug-and-play and fluctuated grid impedance. Moreover, to achieve mutual coordination and distributed control of damping characteristics among multi-inverters to ensure the good dynamic performance of the system, an adaptive dynamic coordination loop based on grid impedance identification is incorporated in the PBC scheme. The Lyapunov stability of the system with the proposed PBC controller is proven in the paper. The effectiveness of the proposed scheme is verified by simulation and experimental results.

Impedance Adaptive Dual-Mode Control of Grid-Connected Inverters With Large Fluctuation of SCR and Its Stability Analysis Based on D-Partition Method

The stable operation of grid-connected inverters (GCIs) with traditional current source mode (CSM) control is affected by the large fluctuations of short-circuit ratio (SCR) under weak grids. Improved CSM control enhances the stability of GCIs under weak grids but remains unstable for very weak grids, while the stability of GCIs with voltage source mode (VSM) is just the opposite. Therefore, previous research works have proposed the impedance adaptive dual-mode control strategy: when the SCR is large, the GCI is controlled by CSM, and when the SCR is small, it is controlled by VSM. However, the existing literature still lacks the analysis of the parameter stability region and mode switching boundary of GCI in different modes. Therefore, based on the D-partition method, this article derives the parameter stability region of GCI in different modes under the constraints of multiple performance indexes such as phase and gain margin, current-loop bandwidth and phase-locked loop bandwidth, and the switching boundaries of CSM and VSM are also proposed. Finally, based on the grid impedance identification algorithm, the impedance adaptive dual-mode control is realized, which effectively improves the stability of GCI when SCR fluctuates greatly, and the experimental results verify the correctness of the above analysis.

Identification of Grid Impedance by Broadband Signals in Power Systems with High Harmonics

Grid impedance is an important parameter and is used to perform impedance-based stability analysis for the operation of grid-connected systems, such as power electronics-interfaced solar, wind and other distributed power generation systems. The identification of grid impedance with the help of broadband signals is a popular method, but its robustness depends strongly on the harmonic disturbances caused by non-linear loads or power electronics. This paper provides an in-depth analysis of how harmonics affect the identification of grid impedance while using broadband measurements. Furthermore, a compensation method is proposed to remove the disturbing influences of harmonics on broadband impedance identification. This method is based on exploiting the properties of the used maximum-length binary sequence (MLBS). To explain the methodology of the proposed method, the design basis for the excitation signal is discussed in detail. The analysis from simulations and a real measurement in an industrial power grid shows the effectiveness of the proposed method in compensating the disturbing influences of harmonics on broadband impedance measurements.

Analysis, Design, and Implementation of Impulse-Injection-Based Online Grid Impedance Identification With Grid-Tied Converters

Grid impedance information occupies a fundamental and crucial position for the operation of grid-tied converters. Consequently, accurate and fast extraction of grid impedance is vital. The impulse injection method applied under the control loop of a grid-tied converter, which is commonly used to extract linear grid impedance online, is relatively reasonable in terms of device cost, measurement speed, simplicity, and design flexibility. However, the selection of impulse and design of impulse parameters mostly depend on rules of thumb, without theoretical guidance, leading to insufficient measurement accuracy in target frequency section and greatly burdening the system. To address these issues, a systematic guidance is proposed for analysis, design, and implementation of impulse-injection-based grid impedance identification. By comparing the spectra of several basic impulses with unipolar and bipolar forms and analyzing the impact when impulse parameters are altered, a guidance for impulse selection for different measurement scenarios is presented. Based on this guidance, an asymmetric bipolar sawtooth wave with a defined asymmetry ratio of $\boldsymbol{\rho } = 0.5$ is chosen in this article for broadband measurement of grid impedance. Furthermore, a related parameter design procedure with optimal injection position and instant is also proposed to reduce system burden and eliminate overmodulation when injecting the impulse. Finally, theoretical results are validated by experiments.

Identification of Three-Phase Grid Impedance in the Presence of Parallel Converters

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.

Spectral Grid Impedance Identification on the Low-, Medium- and High-Voltage Level – System Design, Realization and Measurement Results of Grid Impedance Measurement

Spectral Grid Impedance Identification on the Low-, Medium- and High-Voltage Level – System Design, Realization and Measurement Results of Grid Impedance Measurement

Identification of Grid Impedance for Purposes of Voltage Feedback Active Filtering

A voltage feedback active filter is vulnerable to unknown grid impedance. To overcome this problem we propose an identification method, which uses the control system of a frequency selective active filter to measure the grid impedance at selected frequencies. The usefulness of the method is experimentally demonstrated with a 19 kVA active rectifier with a voltage feedback active filtering function. The voltage feedback active filtering is performed in a case in which the active filter control is not stable before the impedance is identified with the method proposed. It is shown that the use of the measured grid impedance in the control system greatly enhances the dynamic stability of the system. Also, the grid impedance measurements are provided in two cases.