Active Harmonic Filtering with a Static Synchronous Compensator in High Voltage Applications
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Harmonisten aktiivisuodatus STATCOMilla suurjännitesovelluksissa
Harmonics compensation has become increasingly important as the presence of nonlinear loads and the use of power electronic devices, both generating harmonics, have increased. Harmonics increase losses and have unwanted impacts on different equipment and on the entire power system. Standards and transmission operators’ specifications set limits for harmonics in the grid. Passive power filters, the traditional solution to compensate harmonics, have a number of shortcomings especially under changing grid conditions. Thus, the harmonic limits are not always met. Active harmonic filters, representing newer technology, can automatically correct their tuning to varying grid and component characteristics thus providing an advanced solution to this harmonic issue gathering increasing attention. In this thesis, two harmonics detection methods were studied: a method based on the conventional dq-theory in the synchronous reference frame (SRF) and another based on a Multiple Second Order Generalized Integrator (MSOGI) in the stationary reference frame. The developed active filter features were designed as an add-on feature on a reactive power compensation system Static Synchronous Compensator (STATCOM). The operation principles and theory behind these harmonics detection methods were studied comprehensively. Methods for positive and negative sequence extraction as well as grid synchronization were also considered. Moreover, the suitability of the studied methods to be used in a full-scale Modular Multilevel Converter (MMC) based STATCOM system was considered. Simulations for the studied methods were performed in PSCAD environment in order to demonstrate and compare their feasibilities in a steady-state operation. According to the simulation results, both methods were able to compensate selected harmonics completely in STATCOM’s maximum capacitive, maximum inductive and zero-operation points. Compensation of positive and negative sequence harmonics worked similarly and neither of the methods significantly strengthened individual uncontrolled grid harmonics. The structure of the MSOGI based method appeared to be slightly more complex and the control implementation in MSOGI’s stationary reference frame was considered much more challenging than corresponding control in the conventional synchronous reference frame (SRF based method). On the other hand, the suitability of the MSOGI based active filter method for both three-phase and single-phase applications was found superior compared to the studied SRF based method, which in turn is only suitable for three-phase applications.