Reactive Power Control of Hybrid Multi-terminal HVDC Systems Considering the Interaction Between the AC Network and Multiple LCCs
Abstract : A strategy was developed for reactive power control of the line-commutated converters (LCCs) of hybrid multi-terminal high-voltage direct current (MT-HVDC) systems. In the conventional method, the reactive power outputs of hybrid MT-HVDC systems are regulated discontinuously by only using switched shunt devices (SSDs). If continuous reactive power compensation is required, additional devices, including a static compensator (STATCOM), are essential, but these are costly solution. However, in the proposed method, the reactive power outputs of hybrid MT-HVDC systems can be continuously regulated without any additional compensating device by adjusting DC voltage and using SSDs. Therefore, with the proposed method, an HVDC operator can control reactive power outputs and reduce costs, simultaneously. To achieve our objectives, we first describe how to determine the DC voltage set points that regulate the reactive power outputs of multiple grid-connected LCCs, and then realize coordinated control using SSDs. The utility of the method is demonstrated by case studies simulated using PSCAD and MATLAB.
? The voltage margin method for interchange power control was applied to the DC voltage control block and the active power control block in the terminal controller.
? In this paper the simulation of Hybrid multiinfeed HVDC link on RTDS is studied for its steady state and transient performance.
? In developing countries to meet the energy demand long HVDC transmission lines are planned to the load centers. This resulted in two or more HVDC links terminating to close electrical proximity forming multiinfeed HVDC systems.
? The application of Line Commutated Converter (LCC) HVDC systems has become the proven technology.
? To increase the DC voltage level, the number of sub-modules of each arm should be increased drastically, leading to many technical problems in communication design, optical fibre instalment, cooler system design and so on.
? To overcome the problems above, a hybrid HVDC system consisted of LCC and cascaded hybrid MMC valves is proposed to realize ultra HVDC transmission.
? Thus, the application of the hybrid HVDC transmission system based on single hybrid MMC in ultrahigh voltage power transmission is faced with many technical problems.
? The system is able to ride through the DC fault. There won’t be problems for MMC to be applied to the ultra-high voltage direct current(UHVDC) system.
• Power Control Method on VSC-HVDC in a hybrid multiinfeed HVDC system is proposed in ref that can realize more effective support to system voltage by fully using the VSC-HVDC in comparison with the traditional control method and significantly improve the system voltage stability.
• To retain the capacitor voltages at a desired value, the approaches proposed in literature are (1) to use separate DC sources for each capacitor, (2) to use an auxiliary converter to inject current in the neutral point of the NPC to balance the dc side voltage, (3) to provide current path between neutral point of NPC to balance the DC side voltages, (4) another approach is to modify the converter switching pattern according to a control strategy to balance and maintain the required value of DC-side voltages.
? The design of the topology and controllers, the DC fault ride-through strategy and the online switching strategy for MMCs will be presented in this paper. Finally, the performance during DC faults and online switching are verified by extensive simulations.
? Based on the analysis above, MMC1 is used to control the DC voltage. MMC2 is used to control the active power.
? The AC control circuit is used to control the average capacitor voltage of all of the sub-modules to be a constant. Thus, at any instant, the power transmission between the AC and DC side can be achieved.
? For MMC1, S is set to ?, the control circuit is used to control the DC voltage of the converter. The other control type is adopted in MMC2, the control circuit will control the active power of the converter.
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