A Medium Frequency Transformer-Based Wind Energy Conversion System Used for Current Source Converter Based Offshore Wind Farm
Abstract
Offshore wind farms with series-interconnected structures are promising configurations because bulky and costly offshore substations can be eliminated. In this work, a medium-frequency transformer (MFT)-based wind energy conversion system is proposed for such wind farms based on current source converters. The presented configuration consists of a medium-voltage permanent magnet synchronous generator that is connected to a low-cost passive rectifier, an MFT-based cascaded converter, and an onshore current source inverter. Apart from fulfilling traditional control objectives (maximum power point tracking, dc-link current control, and reactive power regulation), this work endeavors to ensure evenly distributed power and voltage sharing a the constituent modules given the cascaded structure of the MFT-based converter. In addition, this paper thoroughly discusses the characteristic of decoupling between the voltage/power balancing of the modular converter and the other control objectives. Finally, both simulation and experimental results are provided to reflect the performance of the proposed system.
EXISTING SYSTEM:
On the basis of the connection methods of wind turbines in offshore wind farms and the characteristics of the power to be delivered, the wind energy conversion system (WECS) proposed in literature and implemented practically can be classified into four types parallel ac connection and high voltage alternating current (HVAC) transmission systems, parallel ac connection and high voltage direct current (HVDC) transmission systems, parallel dc connection and HVDC transmission systems, and series dc connection and HVDC transmission systems. The HVAC system is suitable for application where the transmission distance is lower than 50 km, while HVDC system dominates the market when the transmission distance is longer than 50 km. All these configurations except the fourth one (series dc connection and HVDC transmission system) need offshore substation which is very bulky and costly. Aside from considering reliability and efficiency as main requisites for all onshore conversion systems, the footprints and weights of the components are particularly important for offshore infrastructure. The total weight of the system that is dominated by the offshore substation significantly affects the cost and complexity of the offshore wind farm. Therefore, the fourth one (series dc connection and HVDC transmission system) is increasingly emphasized in research because it can save significantly cost given that the bulky and costly offshore substation can be eliminated. The electric generators used to convert mechanical energy into electrical energy have been well developed. They are divided in two main groups: induction generators (squirrel-cage, doubly-fed induction generator) and synchronous generators (permanent-magnet, wound-rotor synchronous generator). A these generators, PMSG is gaining increased attention in the research given its low maintenance cost and negligible rotor loss. Moreover, medium-voltage (MV) PMSG-based WECS with voltage levels that range between 3–4 kV is considered the most suitable and economical approach when a power rating exceeds 3 MW.
PROPOSED SYSTEM:
A two-level VSC in which switching devices are connected in series has been proposed for MV-based WECS , and the back-to-back neutral point clamped (NPC)-based WECS is widely studied in previous works. An active NPC (ANPC)-based WECS was proposed to solve the uneven power loss problem associated with NPC. A four-level diode clamped converter can be used to achieve MV operation levels, as well as the multi-level ANPC and matrix converter-based WECS. The power flow in WECS is unidirectional; therefore, low-cost passive converters (diode rectifiers) can be employed at the generator side instead of the aforementioned pulse width modulated (PWM) active converters. On the other hand, CSCs features natural advantages with simple structure, grid-friendly waveforms, controllable power factor, and reliable grid short-circuit protection. In addition, these converters have successfully been used in high-power MV-based industrial drives and are considered to be highly promising converters for MV-based WECS applications . Therefore, the key words of the present work can be highlighted with: (a) CSC-based offshore wind farm; (b) series dc connection and HVDC transmission system; and (c) MV PMSG-based WECS. Unfortunately, the CSC-based WECS has not been studied extensively in literature. The thyristor-based current source line commutated converter (LCC), which is a proven and well-established technology in HVDC transmission system, has been investigated for WECS application focus on techniques for reactive power control and harmonics compensation, while discusses an LCC-based wind farm with series-interconnected structures. However, an LCC-based WECS has the following disadvantages [21]: the need for large passive filters to reduce low-order harmonics, a large footprint, a low dynamic performance, dependent active and reactive power control, and susceptibility to ac network disturbance. A parallel-connected wind farm structure was established in wherein all wind turbine modules are connected in parallel; only one onshore current source inverter (CSI) delivers the total power of the offshore wind farm to the grid. The most significant challenges faced by this configuration are the losses of the transmission cables and the power rating of the onshore CSI that needs to withstand the full power of the wind farm
CONCLUSION
In this work, an MFT-based WECS is proposed for CSC-based offshore wind farms. The proposed configuration is composed of an MV PMSG, a passive rectifier, a modular MFT-based converter, and a CSI. It is characterized by (a) no offshore substation; (b) high power density due to the adaption of a modular MFTs instead of a low-frequency transformer; (c) high reliability and flexibility due to the use of a modular converter; and (e) all the advantages of a CSC. Apart from traditional control objectives (MPPT, dc-link current and reactive power control) of a WECS, additional effort is made to ensure an evenly distributed power and voltage sharing a the constituent modules. The characteristic of decoupling between voltage/power balance control and the other control objectives is analyzed as well. Finally simulation and experimental verification are provided to demonstrate the converter’s performance of the proposed WECS.
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