Lawan, Abdurrahman Umar
(2019)
Enhanced decoupled current control with compensation algorithm for modular multilevel converter based static synchronous compensator.
PhD thesis, University of Nottingham.
Abstract
The major tasks in the control of the MMC based STATCOM are associated with control of external (i.e. output ac voltage and current controls), and internal (i.e. circulating current and the capacitor voltages) variables. Reactive power (VAR) compensation using static compensators (STATCOMs), plays a role in improving system voltage, power factor and/or harmonics by the external control. Control schemes have been employed in providing the reference VAR needed. Among the schemes the vector control, direct control and power synchronization are commonly employed, the vector control is mostly employed for its faster current control. This current control is achieved using mostly proportional (P) controllers and proportional-integral (PI) controllers among others. PI controllers have the advantage of low harmonics and small variations. However due to PI controllers’ dependency on the system parameters and complex transfer functions of the MMC, which are often obtained based on estimation, and the STATCOM non-ideal operations, variations even in the steady-state still occur that affect the MMC based STATCOM performance. In this Thesis, an improved performance of MMC based STATCOM is proposed using a compensation algorithm based on the vector control using the PI-controllers. The proposed control is derived to tackle the effects of the dynamics of the MMC, and the STATCOM ideal variations. This control is achieved by providing compensation that minimizes the variations of the system and enhances the PI controller’s response. To evaluate the superiority of this work, simulation and experimental studies are carried out and presented. The results illustrate that the performance of the STATCOM reactive current has improved where the conventional load current has more fluctuations with a higher settling time of not less than 15%. The proposed control based output voltage has FFT of 1.50 % while in the conventional method is 2.0%. Even though both have complied with IEEE standard (i.e. less than 5% THD). The circulating current harmonics under conventional PI-control method and the proposed method are 19.04% and 18.44% respectively.
Another challenge in the MMC is the performance variations that arise due to the flow of load and circulating currents through the arms of the MMC internally which usually result in voltage disturbance on the submodule capacitor voltages. This leads to the circulating current harmonics. The typical control methods based on PI and resonant controllers depends on the bandwidth of the MMC current controller. Furthermore, the PI-controller has zero static error in tracking the circulating current reference, its impact in suppressing the voltage disturbance needs to be improved, even at steady-state, the PI-controlled circulating current is reported to exhibit presence of large harmonics. This paper proposes a circulating current control method in order to reduce circulating current harmonics. The proposed method improves the performance of the system by providing current and voltage compensations in the internal control loops of the MMC through virtual-impedance loops without the use of any additional filtering. The enhancements using the virtual-impedance are implemented in the internal control and the circulating current harmonics are found to be 15.32% and 19.04% for the proposed and conventional schemes respectively under the STATCOM operations. This means, there is a reduction of not less than 3.72% THD in the harmonics level. The voltage distortion of the MMC has improved from a value of 2.0% to 1.50% with external compensation, and then to 1.35% with combined external and internal compensations. Further control enhancement is considered in this Thesis to improve the MMC performance at internal control stage. The harmonic components of the circulating current have reduced such as the case when the reference average capacitor voltage is modified. The THD levels obtained are 1897%, 1485%, and730% for the typical PI-control method, PI-control with a second-order resonant controller, and the proposed method respectively. Similar improvement in harmonics elimination are obtained under the internal control enhancement using state-feedback controller where the THDs are 2572%, 1722%, and 1200% for the typical PI-control method, PI-control with a second-order resonant controller, and the proposed method respectively.
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