Synchronization and Reactive Current Support of PMSG based Wind Farm during Severe Grid Fault

ABSTRACT:

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.

 

KEYWORDS:

  1. Low voltage ride through (LVRT)
  2. Permanent magnet synchronous generator (PMSG)
  3. Wind farm
  4. Coordinated current control

 

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

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

 

CONCLUSION:

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 [31], 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.

 

REFERENCES:

  • BDEW Technical Guideline, Generating Plants Connected to the Medium- Voltage Network [EB/OL], June 2008 issue.
  • Grid code-high and extra high voltage, E. ON Netz GmbH, 2006. Tech. Rep., [EB/OL].
  • Geng, C. Liu and G. Yang. LVRT Capability of DFIG-Based WECS Under Asymmetrical Grid Fault Condition [J]. IEEE Transactions on Industrial Electronics, vol. 60, no. 6, pp. 2495-2509, June 2013.
  • Chinchilla M., Arnaltes S., Burgos J. C. Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the grid [J]. IEEE Transactions on Energy Conversion, 2006, 21(1): 130-5.
  • Conroy J. F., Watson R. Frequency Response Capability of Full Converter Wind Turbine Generators in Comparison to Conventional Generation [J]. IEEE Transactions on Power Systems, 2008, 23(2): 649-56.

Fault Ride-Through of a DFIG Wind Turbine Using a Dynamic Voltage Restorer During Symmetrical and Asymmetrical Grid Faults

ABSTRACT:

 The application of a dynamic voltage restorer (DVR) connected to awind-turbine-driven doubly fed induction generator (DFIG) is investigated. The setup allows the wind turbine system an uninterruptible fault ride-through of voltage dips. The DVR can compensate the faulty line voltage, while the DFIG wind turbine can continue its nominal operation as demanded in actual grid codes. Simulation results for a 2 MW wind turbine and measurement results on a 22 kW laboratory setup are presented, especially for asymmetrical grid faults. They show the effectiveness of the DVR in comparison to the low-voltage ride-through of the DFIG using a crowbar that does not allow continuous reactive power production.

 KEYWORDS:

  1. Doubly fed induction generator (DFIG)
  2. Dynamic voltage restorer (DVR)
  3. Fault ride-through and wind energy

 SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fault Ride-Through of a DFIG

Fig. 1. Schematic diagram of DFIG wind turbine system with DVR.

 EXPECTED SIMULATION RESULTS:

 

Fig. 2. Simulatin of DFIG performance with crowbar protection during 37 % two-phase voltage dip. (a) Line voltage. (b) DVR voltage. (c) Stator voltage. (d) Stator current. (e) RSC current. (f) Crowbar current. (g) Mechanical speed. (h) Active and reactive stator power. (i) Active and reactive DVR power.

Fig. 3. Simulation of DFIG performance with DVR protection during 37 % two-phase voltage dip. (a) Line voltage. (b) DVR voltage. (c) Stator voltage. (d) Stator current. (e) RSC current. (f) Crowbar current. (g) Mechanical speed. (h) Active and reactive stator power. (i) Active and reactive DVR power.

Fig. 4. Measurement results for DFIG with crowbar protection: (a) stator

voltages, (b) stator currents, and (c) rotor currents.

Fig. 5. Measurement results for DFIG with DVR protection: (a) line voltages, (b) DVR voltages, (c) stator voltages, (d) stator currents, and (e) rotor currents.

CONCLUSION:

The application of a DVR connected to a wind-turbine-driven DFIG to allow uninterruptible fault ride-through of grid voltage faults is investigated. The DVR can compensate the faulty line voltage, while the DFIG wind turbine can continue its nominal operation and fulfill any grid code requirement without the need for additional protection methods. The DVR can be used to protect already installed wind turbines that do not provide sufficient fault ride-through behavior or to protect any distributed load in a microgrid. Simulation results for a 2 MW wind turbine under an asymmetrical two-phase grid fault show the effectiveness of the proposed technique in comparison to the low-voltage ridethrough of the DFIG using a crowbar where continuous reactive power production is problematic. Measurement results under transient grid voltage dips on a 22 kW laboratory setup are presented to verify the results.

REFERENCES:

[1] M. Tsili and S. Papathanassiou, “A review of grid code technical requirements for wind farms,” Renewable Power Generat., IET, vol. 3, no. 3, pp. 308–332, Sep. 2009.

[2] R. Pena, J. Clare, and G. Asher, “Doubly fed induction generator using back-to-back pwm converters and its application to variable-speed windenergy generation,” Electr. Power Appl., IEE Proc., vol. 143, no. 3, pp. 231–241, May 1996.

[3] S.Muller,M.Deicke, andR.DeDoncker, “Doubly fed induction generator systems for wind turbines,” IEEE Ind. Appl.Mag., vol. 8, no. 3, pp. 26–33, May/Jun. 2002.

[4] J. Lopez, E. Gubia, P. Sanchis, X. Roboam, and L. Marroyo, “Wind turbines based on doubly fed induction generator under asymmetrical voltage dips,” IEEE Trans. Energy Convers., vol. 23, no. 1, pp. 321–330, Mar. 2008.

[5] M. Mohseni, S. Islam, and M. Masoum, “Impacts of symmetrical and asymmetrical voltage sags on dfig-based wind turbines considering phaseangle jump, voltage recovery, and sag parameters,” IEEE Trans. Power Electron., to be published.

Single- and Two-Stage Inverter-Based Grid Connected Photovoltaic Power Plants With Ride-Through Capability Under Grid Faults

IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 6, NO. 3, JULY 2015

 ABSTRACT Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1) excessive dc-link voltage; 2) excessive ac currents; and 3) loss of grid-voltage synchronization. In this paper, the control of single and two stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes’ (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed. The results presented illustrate the capability of the system to ride-through different types of grid faults.

 

KEYWORDS:

  1. DC–DC converter
  2. Fault-ride-through
  3. Photovoltaic (PV) systems
  4. Power system faults
  5. Reactive power support
  6. single and two stage inverter

 

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

single and two stage inverter

Fig. 1. Diagram of a single-stage GCPPP

 single and two stage inverter

Fig. 2. Diagram of the two-stage conversion-based GCPPP

 

EXPECTED SIMULATION RESULTS:

Fig. 3. Short-circuiting the PV panels: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at MV side of the transformer.

Fig. 4. Short-circuiting the PV panels: (a) overall generated power; (b) injected active power; and (c) reactive power to the grid.

Fig. 5. Turning the dc–dc converter switch ON: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at the MV side.

Fig. 6. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages; (b) grid currents; (c) dc-link voltage; (d) input voltage of the dc–dc converter; (e) estimated duty cycle; and (f) actual duty cycle under a 3LG with 45% voltage sag at MV side.

Fig. 7. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages under a 3LG with 45% voltage sag at MV side; (b) related grid currents for G = 300 W/m2; and (c) related dc-link voltage; (d) grid voltages under an SLG with 65% voltage sag at theMV side; (e) related grid currents for G = 1000 W/m2; (f) related dc-link voltage; (g) related grid currents under G = 300 W/m2; and (h) related dc-link voltage.”

single and two stage inverter

CONCLUSION

Performance requirements of GCPPPs under fault conditions for single and two stage grid-connected inverters have been addressed in this paper. Some modifications have been proposed for controllers to make the GCPPP ride-through compatible to any type of faults according to the GCs. These modifications include applying current limiters and controlling the dc-link voltage by different methods. It is concluded that for the single-stage configuration, the dc-link voltage is naturally limited and therefore, the GCPPP is self-protected, whereas in the two-stage configuration it is not. Three methods have been proposed for the two-stage configuration to make the GCPPP able to withstand any type of faults according to the GCs without being disconnected. The first two methods are based on not generating any power from the PV arrays during the voltage sags, whereas the third method changes the power point of the PV arrays to inject less power into the grid compared with the prefault condition. The validity of all the proposed methods to ride-through voltage sags has been demonstrated by multiple case studies performed by simulations.

 

REFERENCES

  1. Trilla et al., “Modeling and validation of DFIG 3-MW wind turbine using field test data of balanced and unbalanced voltage sags,” IEEE Trans. Sustain. Energy, vol. 2, no. 4, pp. 509–519, Oct. 2011.
  2. Popat, B. Wu, and N. Zargari, “Fault ride-through capability of cascaded current-source converter-based offshore wind farm,” IEEE Trans. Sustain. Energy, vol. 4, no. 2, pp. 314–323, Apr. 2013.
  3. Marinopoulos et al., “Grid integration aspects of large solar PV installations: LVRT capability and reactive power/voltage support requirements,” in Proc. IEEE Trondheim Power Tech, Jun. 2011, pp. 1–8.
  4. Islam, A. Al-Durra, S. M. Muyeen, and J. Tamura, “Low voltage ride through capability enhancement of grid connected large scale photovoltaic system,” in Proc. 37th Annu. Conf. IEEE Ind. Electron. Soc. (IECON), Nov. 2011, pp. 884–889.