Bipolar Operation Investigation of Current Source Converter-Based Wind Energy Conversion Systems
Abstract
A series-connected current source converter (CSC)-based configuration has recently been proposed for offshore wind energy conversion systems. A big challenge exists for such a system that its maximum insulation level is the full transmission voltage due to its monopolar operation. This introduces significant burden to the system in terms of cost, reliability, and flexibility. To solve this issue, a bipolar operation giving a half insulation requirement is proposed and investigated in the present work. However, a unique challenge exists for the CSC-based system when operating under bipolar mode, that is the dc-link current control. There are two equivalent paths for the dc-link current which introduces a concern for proper operation of the bipolar system. Accordingly, an optimized dc-link current control is developed in this study. In summary, the bipolar system with the help of the optimized dc-link current control features lower insulation requirement, higher reliability, higher efficiency, and higher flexibility. Finally, both simulation and experimental results are provided.
EXISTING SYSTEM:
One common thing for existing CSC-based series-connected configurations is they are all operating under monopolar mode leading significant challenge for system insulation. The wind generator that is farthest from the grounding point must be capable of withstanding a full transmission level which is impractical. To tackle this issue, a three-phase low-frequency high-power transformer is normally connected between the generator and the front-end converter. This transformer, however, is heavy and bulky increasing burden on offshore construction because of the limited space either in the nacelle or in the tower of the wind turbine. On the other hand, a modular medium-frequency transformer (MFT)-based solution is proposed in. Compared with the low-frequency transformer in, the modular MFT gives smaller size and weight that is particularly important for offshore construction. However, the maximum insulation requirement of the system under monopolar operation is still the full transmission level. This introduces significant challenges to the system in terms of cost, reliability, and flexibility. With a special focus on reducing the maximum insulation level, the present work proposes and investigates a bipolar system. Bipolar mode that typically used in VSCs gives a half insulation requirement compared with monopolar mode. However, a big concern exists for the CSC-based system operating under bipolar mode, that is the dc-link current control. Unlike monopolar mode where there is only one equivalent current path, the bipolar operation mode has two equivalent current paths. Thus, an optimal dc-link current control is required to ensure all the control objectives and give higher efficiency, reliability, and flexibility to the system. Accordingly, an optimized dc-link current control is developed in this study.
PROPOSED SYSTEM:
Fig. 1 shows the recently proposed PWM CSC-based configuration for offshore wind farms. In the offshore part, a number of medium-voltage (MV) permanent magnet synchronous generator (PMSG)-based wind generation units are connected in series to reach a high-voltage direct current (HVDC) level. A modular MFT-based converter with a series-input and series-output structure is used interfacing each generator. In the onshore part, a number of MV CSCs are connected in series to form a dc/ac converter. The step-up multi-winding transformers are employed to connect the converters to the grid, providing isolation and grid integration. A dc-link inductor, that is L dc , is shared by offshore and onshore converters. The overall control scheme of the PWM CSC-based wind farm consists of two parts. One is the control scheme for offshore converters. The other is the control for onshore converters. The onshore control objectives include the maximum power point tracking (MPPT) and voltage balancing control, while the dc-link current control and reactive power control are implemented on the onshore converters. It is worth noting that the dc-link reference current determination shown in Fig. 1 plays a crucial role in the control system which will be thoroughly presented in the following section.
CONCLUSION
In this work, the performance of the CSC-based series-connected offshore wind farm under bipolar operation mode is investigated. Compared with monopolar mode, bipolar mode gives lower insulation level, thus contributing to significant cost saving and higher reliability. In addition, an optimized dc-link current control is developed, based on which the bipolar system is equivalent to two independent monopolar systems that can operate with their own dc-link reference independently with the same earth return. Compared with conventional dc-link current control, the proposed one features higher efficiency and flexibility. Furthermore, the bipolar system can be extended to a multi-terminal system with larger power capacity. In summary, the bipolar system with the help of the optimized dc-link current control features lower insulation level, higher reliability, higher efficiency, and higher flexibility. Both simulation and experimental results are provided in the end. dc2 : 2 A/div, I g1_abc o
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