STATCOM Impedance Measurement and Sensitivity Analysis of Design and Control Parameters

Sammanfattning

In the past decades, there has been an increase of renewable energy in the energy sector. This has led to new challenges to ensure power grid stability. One of these challenges is the increase of harmonic distortions caused by power electronic converters. A Static Synchronous Compensator (STATCOM) is a device utilized both in transmission and utility applications. Its main purpose is to provide reactive power compensation that enhances the power quality in the grid. However, for an inadequate designed, harmonic distortions can make it unstable if located within its weak spots in the frequency domain, potentially resulting in instabilities in the grid. In a passive system, all active power dissipates, which means that manufacturers can guarantee that their STATCOM will remain stable within a designated frequency range by proving passivity at those frequencies. The system is passive if the admittance is positive for all frequencies. The impedance is the inverse of admittance and facilitate a more straightforward analysis. Further research is needed to understand how various design and control parameters influence the impedance characteristics. This thesis aims to analyse the characteristics of the STATCOM impedance and investigate how seven design parameters and two control parameters influence its impedance and passivity. An arbitrarily selected STATCOM model for transmission applications was analysed. Its impedance and admittance were measured using a voltage perturbation injection method. The results showed that the design parameters primarily influence the impedance in the high frequency range, while the control parameters influence the impedance at low frequencies. When examining the design parameters, it was found that the STATCOM size (number of cells in the converter), pulse number and reactor’s reactance all influenced the impedance characteristics. However, neither the rated reactive power of the STATCOM or the transformer’s reactance showed any significant influence. Exchanging the RC-circuit for a high-pass filter increased the overall impedance at high frequencies. The impedance characteristics depend on both the size of the filter and the tuning frequency. The results showed that when both parameters are high, the overall impedance at frequencies above 2 kHz gives a lower impedance than a smaller highpass filters at the same tuning frequency. For the control parameters, the impedance characteristics near the operating frequency changed significantly when the STATCOM was in reactive power control mode compared to voltage control mode, resulting in an extension of its passive region at lower frequencies. The passive region also increases when the regulator gain was lowered.