Stability and Interaction Analysis of Inverter-Based Resources in Power Grids

The increasing penetration of inverter-based resources (IBRs) introduces some unexpected dynamic issues, including low-frequency oscillations. To investigate these phenomenon, a laboratory-scale grid-following voltage-source converter (VSC) system is implemented to demonstrate weak grid oscillations. Grid-following control is applied in VSC to provide active power, reactive power or ac voltage control. The test bed is also replicated in electromagnetic transient (EMT) simulation environment (MATLAB/SimPowerSystems) for benchmark purpose. Case studies are carried out to demonstrate low-frequency oscillations under real power/ac voltage or real power/reactive power control. An analytical model is carried out to examine the stability condition and compared with EMT or hardware test bed results by using eigenvalue analysis. To enhance the stability of an IBRs-integrated power system, reactive power (VAR) compensation is an effective method. Synchronous condenser (SynCon) and static synchronous compensator (STATCOM) are widely used among VAR devices. They have the capability of increasing the transmission system stability and efficiency by absorbing or generating reactive power. A comparison of SynCon and STATCOM under the condition of zero reactive power injection will be presented. The two devices are integrated into a grid-connected type-4 wind farm to examine their effects on system stability. It is found that SynCon is capable of stability enhancement while STATCOM does not have such capability. To explain the difference, we measure the $dq$-frame admittance frequency-domain responses of the two devices using frequency scans. Vector fitting method is then utilized to convert the admittance frequency-domain measurements to an $s$-domain model. $s$-domain admittance-based eigenvalue analysis further confirms that SynCon is advantageous in stability enhancement. The difference of SynCon and STATCOM can be summarized as SynCon providing a steady-state reactance while STATCOM acting as a current source at steady state. Moreover, the controller interaction of STATCOM in type-4 wind farm system is also investigated. Reactive power and voltage control are implemented on STATCOM controller. The control loops are disconnected and a series of harmonic signals are injected into the system. With the frequency response data, a linear model can be derived using vector fitting method. Then, the stability analysis for different control parameters are carried out by root locus diagram. The proposed method is tested in a type-4 wind farm system, EMT simulation results verify the accuracy of the stability analysis