FTEEE-1697 A Medium Frequency Transformer-Based Wind Energy Conversion System Used for Current Source Converter Based Offshore Wind Farm – IEEE EEE Project 2016-2017
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 among 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.
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). Among 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.
The overall structure of the CSC-based offshore wind farm. N numbers of the proposed MV PMSG-based WECSs are connected in series with one common dc-link inductor Ldc. The onshore CSIs are connected to the grid through multi-winding transformers. The topology of the proposed MFT-based WECS; the configuration consists of an MV PMSG, a three-phase diode rectifier, a modular MFT-based converter, and a CSI that is connected to the grid through a transformer. Cf is the output capacitor and Lg represents the grid-side inductance. The detailed topology for the modular MFT-based converter, in which N numbers of voltage-fed, current-output converters (without an output capacitor filter) are connected in series, then directly connected to the dc-link inductor Ldc. CN (N = 1, 2 …, N) is input capacitor of each module in the cascaded converter.
sIt is beneficial for achieving both MPPT and grid-side control. Second, MFT is employed because of the generator insulation issue which has been discussed in the previous section, thus not repeated here. Instead of using bulky low-frequency transformer, MFTs are employed given their advantages of high power density and easy offshore construction. Furthermore, a modular design is implemented based on a number of cells that are connected in series at the input and output. In contrast to a single MFT, such design helps reduce the burden of implementation as one transformer accounts for only one part of a megawatt-level power.
- Determination of the number of cells
- Input capacitor voltages sharing in the modular converter
- Control scheme of the proposed WECS 5. Simulation Setup
- Report Generation
Determination of the number of cells:
The minimum required number of cells for the proposed configuration depends on: input dc voltage, the voltage rating of the selected insulated-gate bipolar transistor (IGBT), and the chosen cell voltage. The rated input dc voltage is approximately 5000 V for a 4000 V PMSG-based system. 1700 V IGBT is selected because it is the most suitable switching device in terms of cost, voltage utilization, and failure in time rate for MV applications. Given a cell voltage of 1000 V, five cells (without redundancy) must be connected in series at both the input and the output. A converter with six cells can also be chosen for redundancy (N+1); nonetheless, this work considers a modular converter with five cells.
The topology of each module that is employed in the MFT-based converter is a voltage-fed, current-output, full-bridge converter with a common inductor (Ldc) filter. In this study, the MFT plays two roles: First, this transformer can help realize zero-voltage switching for the primary switches through leakage inductance LpN (N = 1, 2 …, 5) without requiring additional components. Second, MFT performs an isolation function. As mentioned in the previous section, the MFT must withstand a full transmission voltage at most; hence the issue of MFT insulation must be considered in practical design and manufacture.
Input capacitor voltages sharing in the modular converter:
The constituent modules of the modular converter are designed to be identical. Given existing manufacturing techniques, however, the components used may not display exactly same characteristics As a result, the operation of the cascaded converter is destabilized if a common duty ratio alone is employed. The module with the lowest turn ratio has highest input capacitor voltage and constitutes the largest proportion of total power. Therefore, input capacitor voltages must be shared among the constituent modules
Control scheme of the proposed WECS:
Two objectives are controlled at the generator side: MPPT and capacitor voltage sharing. In a variable-speed WECS, generator speed is adjusted to achieve MPPT. MPPT methods are widely discussed in literature, including MPPT with turbine power profiles, optimal tip speed ratios, and optimal torque control. The current study mainly focuses on converter control; therefore, the simple optimal tip speed ratio is applied to achieve MPPT.
The major control objectives for the grid-side CSI are dc-link current and reactive power control. Unlike in VSC-based WECS where dc-link voltage is normally controlled at a constant value, the dc-link current in CSC-based WECS is variable according to different levels of captured power to minimize loss. It is determined by both the generator and the grid side to achieve all the control objectives and a minimum WECS loss.
The wind turbine and the PMSG used in this simulation are provided by Mat lab/Simulink. The turbine model receives the wind speed and provides an optimized reference speed to the control system. The inertia constant of a megawatts-level WECS is normally around a few seconds; in this simulation, the constant is reduced to achieve a faster speed response compared with that in a real system.
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 no offshore substation; high power density due to the adaption of a modular MFTs instead of a low-frequency transformer; high reliability and flexibility due to the use of a modular converter; and 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 among 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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