多段柔性直流输电

sum to a large ac grid-side VSC. Because of Schwarz’s Inequality
where Si (i=l,2, ...,n) is the complex power, the MVA ofthis large converter is smaller than the sum of the MVAs of converters serving individual DFIGs. Aside from the introduction, this paper is organized into six parts. Part I1 is a short description about MTDC-DFIG system. Part I11 demonstrates how the wind energy can be optimally captured and converted by a wind turbine with a doubly fed induction generator. Part IV shows how the stator side Ps and Qs can be controlled in a decoupled way by the rotor side converter voltage. Part V introduces the control of the grid-side voltage regulator. Part VI lists some simulation results to show that acquiring optimal wind power at varying wind velocities can be carried out in a DFIG and the MTDC-DFlG system. Part VI1 gives some conclusions.
I.
INTRODUCTION
11.
DESCRIPTION MTDC-DFIG SYSTEM OF
Wind energy is presently one of the most widely used renewable energy sources. Doubly-Fed Induction Generators (DFIGs) are ideal for wind turbines because both their real and reactive powers can be controlled on the rotor side using back-to-back converter pairs at slip (reduced) MVA ratings [ 1-31. This paper shows that there can be a further reduction in component count and in MVA by organizing the converter pairs in a multi-terminal DC (MTDC) system. In the MTDC system, the many individual grid-side converters and transformers (needed for voltage matching) are merged into a single large converter and transformer. The MTDC arrangement exploits the fact that wind power is never evenly distributed across the wind farm [I]. As a matter of fact, compensating high wind velocity at one turbine location with low velocity wind at a neighboring location is already seen as a method of averaging the temporal stochastic variations. Another factor which needs emphasizing is that the real power outputted on the rotor side of a DFIG is sPairgap, where s is the slip and, , . P is the air gap power. As the wind velocity drives the rotor speed U& above and below the synchronous speed w.y., the rotor power reverses direction with the positive or negative sign of the slip. The dc bus of the MTDC system collects the rotor powers of varying magnitudes and polarities before channeling the algebraic
In conventional operation, when all the DFIGs rotate at super-synchronous speeds (negative slip), the rotor ac power of each DFIG is rectified by its VSC. The dc outputs of the VSCs are connected to the dc bus ofthe MTDC system which aggregates the slip powers of the rotors before inverting them to the ac bus through the VSC (higher rating) which operates as a DC Voltage Regulator. The role of the DC Voltage Regulator is to maintain the dc voltage at the reference setting. To achieve this it invertslrectifies all the power that is rectifiedlinverted so as to prevent the dc capacitors from
In general, a wind farm consists of tens or hundreds of wind turbine-generators but for the purpose of this study it is sufficient to represent them as three DFIGs as shown in the single line diagram of Fig.1. The stators of the three DFIGs are connected to an ac bus directly. The power from these stators are collected at the ac bus and transmitted to the utility grid. Every rotor of the DFIGs is connected to a VSC (Voltage-Source Converter). The kVA rating of each of these VSCs is only s I S I where s is the slip and I S 1 is the complex powerrating ofthe DFIG. The dc terminals ofthese VSCs are connected to a common dc bus. The VSC, laheled as DC Voltage Regulator, maintains the dc power balance in the dc bus and at the same time can provide reactive power on the ac side. A transformer matches the ac voltage levels.
Lianwei Jiao* Geza Joos Chad Abbey Fengquan Zhou Boon-Teck Ooi Electrical and Computer Engineering Department, McGill University, Montreal, Canada *Department of Electrical Engineering, Tsinghua University, Beijing, China, Iwjiao@power.ece.mcgill.ca
Abstract- Of the environmentally friendly, renewable energy sources of the future, wind turbines are, cost-wise, amongst the most competitive. Doubly-Fed Induction Generators (DFIGs) are ideal for wind turbines because both their real and reactive powers can he controlled on the rotor side using back-to-hack converters at slip (reduced) MVA ratings. This paper shows that there can be a further reduction in component count and total MVA by organizing the back-to-back converters in a multi-terminal DC (MTDC) system. In the MTDC system, the many ac grid-side VSCs (and transformers needed for voltage matching) are merged into a single or a few VSC(s). The MVA of the large VSC is smaller than the sum of the MVAs of VSCs serving individual DFlGs because the wind power is never evenly distributed across the wind farm. Simulations show that optimal capture of wind power and ac voltage regulation are achievable under MTDC control.