Grid codes require wind farm to remain on-grid and inject specific reactive current when grid fault occurs. To satisfy the requirements, reactive power devices such as the static synchronous compensator (STATCOM) are usually used in modern wind farms. In order to produce reactive currents, the wind energy generation system (WECS) and the STATCOM are normally controlled with the phase locked loop (PLL)-oriented vector control methods. Due to the active power imbalance between the generation and consumption, the wind farm has the risk of losing synchronization with the grid under severe fault conditions. This paper analyzes the dynamic synchronization mechanism and stability criteria of the wind farm and proposes a coordinated current control scheme for the WECS and the STATCOM during severe grid fault period. The synchronization stability of both the WECS and the STATCOM is remained by the active power balancing control of the wind farm. The control objectives of the generator- and grid-side converters for the WECS are swapped to avoid the interaction between the dc-link voltage control loop and the synchronization loop. The synchronized STATCOM produces additional reactive currents to help the wind farm meet the requirements of the grid code. Effectiveness of the theoretical analyses and the proposed control method are verified by simulations.
- Low voltage ride through (LVRT)
- Permanent magnet synchronous generator (PMSG)
- Wind farm
- Coordinated current control
Fig. 1. Configuration of the PMSG-based wind farm
EXPECTED SIMULATION RESULTS:
Fig. 2. System response of the PMSG-based wind farm with conventional control strategy during severe fault
Fig. 3. System response of the PMSG-based wind farm with proposed strategy during severe fault
This paper studied the LOS mechanism and the coordinating LVRT scheme of the PMSG based wind farm when severe grid voltage dip occurs. The following conclusions can be derived from the theoretical analyses and simulation verification:
(1) Variable-speed wind turbines and STATCOM both have the LOS risk when the grid voltage dip is severe.
(2) The proposed active power balancing control scheme which relies on the frequency dynamic of the PLL can achieve the synchronization stability of the WECS. However, reactive current capability of the WECS would be sacrificed to implement such scheme.
(3) The coordinated current control between the PMSG based WECS and the STATCOM can achieve both the synchronization stability and the reactive current support according to the grid code under severe grid fault. The analysis results and proposed scheme are also available for the LVRT of other renewable energy conversion systems.
(4) It should be pointed out that this paper focuses on the symmetrical fault conditions. In practical applications, unsymmetrical faults occur more often than symmetrical ones. Some Europe grid codes, such as “VDE-AR-N 4120” code in Germany, are requiring the WECS to provide negative sequence current compensation during unsymmetrical fault period. In such cases, the advanced PLL, such as the second order generalized integrator (SOGI) PLL , should be employed to separate the positive and negative sequence components from the grid voltage. The advanced PLLs have much more complicated structures and models compared with the conventional one as indicated in this paper. Also the synchronization stability should be discussed in both positive and negative sequences. By further considering the coupling of the PLL and control loops during grid faults similarly with the case discussed in this paper, the synchronization issue would be more complicated. More studies are expected in this issue and would be our future work.
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