Font Size: a A A

Research On Several Issues On Operation And Control In Modular Multilevel Converter

Posted on:2017-01-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:W LiaoFull Text:PDF
GTID:1222330488477067Subject:Electrical engineering
Abstract/Summary:PDF Full Text Request
Among all kinds of high voltage converter topology, modular multilevel converters(MMC) do not need high-voltage dc-link capacitors and IGBT(or series connected). It has several attractive features such as high voltage capability and scalability, the excellent quality of the output voltage waveforms, modular structure,the common dc bus configurations, low switching stress, capability of four-quadrant operation, transformerless and filterless configurations, which make the MMC as a very attractive candidate for high-voltage direct current transmission, high voltage motor driver, power quality management. Because of its numerous sub-modules(SM), the internal energy changing mechanism of capacitance is complex, the switching transitions and switching interval are restricted under the different voltage balancing algorithm, and power grid faults will also change the working state of the inner sub modules. How to ensure the stability and optimize the operation of MMC under various conditions is the goal of scholars study. This paper studies on the MMC through the theoretical analysis, software simulation and experiment through the financial support from the international cooperation in science and technology projects(China)(2011DFA62240) and the National Natural Science Fund(51377050). The research is focus on the sub-modules capacitor voltage control, switching frequency and switching losses analysis and calculation under the modified voltage balancing algorithms conditions, operation and control in the asymmetric fault of AC grid. The detail contents are as follows:MMC uses distributed energy storage capacitor structure, and the sub-module capacitor voltage is easily affected by the arm parameters and control method. The stability of the sub-module capacitor voltage would have a direct impact on the performance and stability of MMC. The relationship between DC component, fundamental component, second hormonic component of circulating current and the capacitor voltage are analyzed. The sub-module capacitor voltage control methods including total capacitor voltage control, unbalanced voltage control and capacitor voltage fluctuation suppression are proposed. Based on the operation principle, the dynamic equations between the capacitor voltage and circulating current can be obtained. The total capacitor voltage in upper and lower arm can be controlled based on the dc component of the circulating current, and the unbalance voltage between upper and lower arm can be controlled based on fundamental circulating current, which enable the MMC to operate under inconsistent parameters between upper and lower arms. At last, capacitor voltage fluctuation suppression method is proposed based on instantaneous power balance of input and output power. This method can reduce the second frequency fluctuation by injecting the second harmonic circulating current. The amplitude and phase of the circulating current do not need in real time. Furthermore, the injection current and natural second harmonic circulating current is compared.The disadvantage of traditional voltage balance control with NLC(Nearest Level Modulation) is that it has no fixed switching transitions, and small sub-module capacitor voltage difference in the same arm would result in unnecessary switching operations for capacitor voltage balancing control, which increases the switching frequency and switching losses. Many optimization algorithms for switching frequency Reducing have been proposed, there is little literature focus on the switching process analyzing and calculating switching frequency in MMC, it resulting in the lack of theoretical guide for thermal design. This paper proposes a new method to evaluate the switching frequency and switching losses of MMC under the optimization voltage balancing control methods. The analytic equations of the switching frequency and losses based on definite integration approximate substitution are derived. The factors affecting the switching frequency and losses are summarized. It reveals the influence of unbalanced voltage, modulation ratio and power factor with switching frequency and losses. According to the analysis, the advantages and disadvantages of the two algorithms are compared, which provides theoretical support for engineering applications.The circulating current and sub-module capacitor voltage of MMC would change under asymmetric grid faults conditions, and this will threaten the safe and stable operation of MMC. The traditional analysis and control method of the MMC is no longer applicable under the asymmetric grid faults.Two common control strategies on MMC-HVDC system under asymmetric grid faults are presented, one of which is to suppress negative sequence current and the other is to suppress the second harmonic fluctuation of the active power. The circulating current of these two methods are compared and analyzed, the mathematical expressions of circulating current are derived. A new suppression strategy of circulating current based on the sub-module capacitor voltage control is proposed, which does not require a separate zero sequence current controller and calculate the refernece of circulating current. The fundamental and second frequency fluctuation of sub-module capacitor voltage are compared and analyzed under the two common control strategies.Finally, a 9-level single-phase MMC laboratory prototype is also built to validate this paper analyses and proposed method. The parameter design of MMC laboratory prototype is presented, including sub-module capacitor, the arm inductance, and the control mainboard with DSP+FPGA circuit. The total sub-module capacitor voltage control method, the unbalanced sub-module capacitor voltage control method, the fluctuation suppression method of sub-module capacitor, the switching frequency and loss under the two optimization voltage balancing control methods, are fully experiment on the laboratory prototype. The experimental results demonstrate that the accuracy of analytical results and the validity of proposed method.
Keywords/Search Tags:Moudular multilevel converter, Capacitor voltage control, Switching frequency, Optimization voltage balancing control method, Switching losses, Asymmetric grid faults, VSC-HVDC
PDF Full Text Request
Related items