Stability Analysis of Energy System with Gridconnected Converter
The structure of the energy system with a high share of renewable energy has changed. There will be more gridconnected converters in power generation and transmission stage. On the other hand, DC network, AC / DC hybrids and power electronic loads are increasing. This shows that the future energy system has a clear trend towards gridconnected converters. In contrast to conventional power system devices with wellknown inherent characteristics, the operational behaviour of modern gridconnected converters strongly depends on implemented control and protection algorithms. This leads to fundamental changes in the theory of stability analysis and optimal operation of the power system.
Problem definition and approach
Due to the nonstandardization and nonlinearization of control strategy also the wide timescale of dynamic characteristics (from microsecond class to second class), it is very difficult to analyze the energy system with gridconnected converter in a comprehensive and integrated stability. In addition, the interaction between the control strategies of multiple gridconnected converters exacerbates the difficulty of analysis.
Combining the modeling of gridconnected converters and the stability analysis of steadystate and transientstate, this project proposes a systematic, comprehensive and universal method for stability analysis of energy system with gridconnected converter. Based on this set of stability analysis methods, this project provides a theoretical basis and method for optimization and testing of converter.
Steadystate Stability Analysis
The steadystate stability analysis method linearizes the system near the operating point of the device, idealizing the gridconnected converter as a controlled current source or voltage source, and a series or parallel virtual impedance, as shown in Figure 1. In this project, the linearization of the grid synchronization control unit is carried out, such as phaselocked loops and Pf control loops. In the modeling and analysis process, the control unit coupled with the grid synchronization control unit is also considered. Using this analysis method can clearly explain the mechanism of instability phenomenon including subsynchronous oscillation and harmonic oscillation, as shown in Figure 2. It can easily analyze the interaction between largescale gridconnected converter.
Figure 1. The equivalent circuit diagram of the invertergrid system
Figure 2. The measurement data for the Photovoltaic Charging Station
Transientstate Stability Analysis
The analysis object of the transient stability analysis method is the stability of gridconnected converter when the grid situation changes suddenly. In this process, the operation point of gridconnected converter is far away from its steadystate operation point, so a nonlinear analysis method must be used.
Qualitative analysis of the system can use a method similar to the phase portrait method, that is, the powerangle stability analysis method in a classic power system. The qualitative analysis method can intuitively judge the transient stability, as shown in Figure 3.
Figure 3. Analysis of phaselocked loop of converter based on phase portrait like method
The estimation of domain of attraction method is used for quantitative analysis of the system. According to the Lyapunov stability criterion, the system's domain of attraction is estimated, as shown in Figure 4, and the robustness of transient stability of the system is obtained. This analysis method can be used to explain and analyze the mechanism including LVRT failure (Figure 5) and voltage phase jump instability phenomenon.
Abbildung 4 Anziehungsbereich eines netzgekoppelten Konverters` PLL
Abbildung 5 Instabilität des Konverters nach LVRT
Publications
2019
Zhang, Z., Schuerhuber, R., Chen, G., & Zhang, Y.
Stability and stability tests of inverter based power generation equipment.
In Energy Transition in Power SupplySystem Stability and System Security; 13th ETG/GMASymposium (pp. 16). VDE.
Zhang, Z., Schürhuber, R., Fickert, L., Liu, X., Chen, Q., & Zhang, Y.
Hardwareintheloop Based Grid Compatibility Test for Power Electronics Interface.
In 2019 20th International Scientific Conference on Electric Power Engineering (EPE) (pp. 16). IEEE.
Zhang, Z., Gercek, C., Renner, H., Reinders, A., & Fickert, L.
Resonance instability of photovoltaic Ebike charging stations: Control parameters analysis, modeling and experiment.
Applied Sciences, 9(2), 252.
2018
Zhang, Z., Schürhuber, R., Fickert, L., & Zhang, Y.
Stability of Gridconnected Photovoltaic Inverters During and After Low Voltage Ride Through.
In 8th Solar Integration Workshop: International Workshop on Integration of Solar into Power Systems.
Zhang, Z., & Fickert, L.
Power hardwareintheloop test of the energy feedback device in urban rail transportation system.
In 2018 19th International Scientific Conference on Electric Power Engineering (EPE) (pp. 14). IEEE.


