Assistant Professor, Port Said University
operating MMC HVDC networks to transfer power under DC faults
Modular multi-level converters (MMC) are considered a promising topology for HV applications like HVDC transmission systems. Such systems must be designed to provide high reliability against different types of faults. MMCs using Half-Bridge (HF) cells are well known for lacking the capability to block short circuits in the DC link. On the contrary, Full-Bridge (FB) can block the short circuit current in the DC link, they also enable the implementation of fault ride-through (FRT) functionalities, as the cells can generate bipolar voltages in this case. This paper presents a DC short circuit fault ride-through strategy for HVDC transmission system using an MMC topology with FB cells (FBMMC).
Abstract: To understand the operation principle of the modular multilevel converter (MMC) deeply, it is necessary to study the harmonic characteristics of the MMC theoretically. Besides, the analytical harmonic formulas of the MMC are useful in designing the main circuit, reducing the losses and improving the waveform quality. Based on the average switching function and the Fourier series harmonic analysis, this paper deduces the analytical expressions for such electrical quantities as the arm voltage, the arm current, the capacitor voltage, the capacitor current and the circulating current of the MMC. Finally, a digital model of a 21-level MMC-HVDC system is realized in PSCAD/EMTDC. The results of the analytical expressions coincide with the simulation results, which verify the effectiveness and feasibility of the proposed analytical expressions.
Pub.: 09 Sep '13, Pinned: 17 Aug '17
Abstract: HVDC link interconnections under construction or planned in France are part of a highly meshed ac network. This is a relatively new operating condition. The impact of their operation and the risk of abnormal interaction may have an influence on the ac network. The use of voltage source converters (VSCs) with the modular multilevel converter (MMC) topology is becoming more attractive mainly due to their higher performances and lower cost. This paper analyses the operation and interaction of MMC–HVDC links embedded in an ac grid. First a MMC–HVDC link model suitable for small-signal analysis is presented. This small-signal HVDC model is then validated against an EMT-type model for ac systems having different SCR (Short-Circuit Ratio) values. Modal analysis and parametric studies are performed in order to study the impact of an ac line connected in parallel with an HVDC link.
Pub.: 11 Mar '16, Pinned: 17 Aug '17
Abstract: A vector current controller based on quasi-proportional resonant (PR) in α β stationary reference frame is used to control the positive and negative sequence currents uniformly. Through analyzing the power characteristics under unbalanced grid conditions, control strategies to suppress negative sequence current and active power ripples are realized without the additional transformation between α β and d q reference frame. A more general method is proposed to analyze the circulating current features and sequence components; then a circulating current controller is designed based on quasi-PR controller. The traditional voltage-balancing algorithm is also improved to reduce switching times by adjusting the voltage sorting frequency according to the capacitor voltage standard deviation (CVSD).
Pub.: 04 Jun '16, Pinned: 17 Aug '17
Abstract: The Modular Multi-level Converter (MMC) topology is one of the most important switch mode converter topology and provides indispensable potential applications in the power systems industry, specifically, for High Voltage Direct Current (HVDC) transmission systems. Recent applications of MMC in HVDC transmission systems has posed problems such as circulating currents in the converter legs, which threatens the safe operation and also deteriorates the system’s performance. The circulating currents occur mainly due to the fluctuations in the capacitor voltages in the Sub Modules (SMs) of the MMC. Hence, in order to nullify the SM’s unbalanced voltage, this article aims to use a new counting sort based technique in conjunction with Carrier Phase Disposition Pulse Width Modulation Technique (CPD PWM). The increased harmonic content in the output voltage and current waveforms may be attributed to the unpredictable dynamics of capacitors. To address this, the article also proposes a cascaded voltage and current H∞ repetitive control scheme under normal and fault conditions. This entire scheme has been applied for back to back connected HVDC systems for mitigating the Total Harmonic Distortion (THD) in current and voltage respectively. Further, stability analysis for the proposed controllers has been performed to prove its robust operation and finally all the results have been explored.
Pub.: 05 Sep '16, Pinned: 17 Aug '17
Abstract: Modular multilevel converter based multi-terminal HVDC (MMC-MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to isolate dc fault currents and ensure the continuous operation of the healthy parts of the MMC-MTDC system under dc faults presents a huge challenge. Based on a comprehensive analysis on the dynamic behavior of the dc fault current of a MMC, along with the operation characteristics of hybrid dc circuit breakers (CBs), a protection strategy will be developed. It coordinates the system parameters to ensure the hybrid dc CBs cut off the dc fault current before triggering MMC blocking in bipolar dc faults. The mathematical approximation of the dc fault current without converter blocking is achieved through theoretical analysis and numerical calculation, and the smoothing inductance at the dc side of MMC station has shown a major influence on the dynamic behavior of the dc fault currents. The smoothing inductance can be determined based on the mathematical expression of the dc fault current, as well as the breaking capacity of hybrid dc CBs. Data analysis and simulation are performed based on a three-terminal dc test system to validate the effectiveness of the coordination protection strategy.
Pub.: 17 Feb '17, Pinned: 17 Aug '17