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.

Power Quality Improvement of PMSG Based DG Set Feeding Three-Phase Loads

IEEE Transactions on Industry Applications, 2015

ABSTRACT: This paper presents power quality improvement of PMSG (Permanent Magnet Synchronous Generator) based DG (Diesel Generator) set feeding three-phase loads using STATCOM (Static Compensator). A 3-leg VSC (Voltage Source Converter) with a capacitor on the DC link is used as STATCOM. The reference source currents for the system are estimated using an Adaline based control algorithm. A PWM (Pulse Width Modulation) current controller is using for generation of gating pulses of IGBTs (Insulated Gate Bipolar Transistors) of three leg VSC of the STATCOM. The STATCOM is able to provide voltage control, harmonics elimination, power factor improvement, load balancing and load compensation. The performance of the system is experimentally tested on various types of loads under steady state and dynamic conditions. A 3-phase induction motor with variable frequency drive is used as a prototype of diesel engine with the speed regulation. Therefore, the DG set is run at constant speed so that the frequency of supply remains constant irrespective of loading condition.

KEYWORDS:

  1. STATCOM
  2. VSC
  3. IGBTs
  4. PMSG
  5. PWM
  6. DG Set
  7. Power Quality

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1 Configuration of PMSG based DG set feeding three phase loads.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Dynamic performance at linear loads (a) vsab, isa,isb and isc ,(b) vsab, iLa,iLb and iLc (c) Vdc, isa,iLa and iCa

CONCLUSION:

The STATCOM has improved the power quality of the PMSG based DG set in terms voltage control, harmonics elimination and load balancing. Under linear loads, there has been negligible voltage variation (From 219.1 V to 220.9 V) and in case of nonlinear load, the voltage increases to 221 V. Thus, the STATCOM has been found capable to maintain the terminal voltage of DG set within ± 0.5% (220 ±1 V) under different linear and nonlinear loads.

Under nonlinear loads, the load current of DG set is a quasi square with a THD of 24.4 %. The STATCOM has been found capable to eliminate these harmonics and thus the THD of source currents has been limited to 3.9 % and the THD of terminal voltage has been observed of the order of 1.8%. Therefore, the THDs of source voltage and currents have been maintained well within limits of IEEE-519 standard under nonlinear load.

It has also been found that the STATCOM maintains balanced source currents when the load is highly unbalanced due to removal of load from phase ‘c’. The load balancing has  also been achieved by proposed system with reduced stress on the winding of the generator.

The proposed system is a constant speed DG set so there is no provision of frequency control in the control algorithm.

However, the speed control mechanism of prototype of the diesel engine is able to maintain the frequency of the supply almost at 50 Hz with small variation of ±0.2 %.

Therefore, the proposed PMSG based DG set along with STATCOM can be used for feeding linear and nonlinear balanced and unbalanced loads. The proposed PMSG based DG set has also inherent advantages of low maintenance, high efficiency and rugged construction over a conventional wound field synchronous generator based DG set.

 REFERENCES:

[1] Xibo Yuan; Fei Wang; Boroyevich, D.; Yongdong Li; Burgos, R., “DC-link Voltage Control of a Full Power Converter for Wind Generator Operating in Weak-Grid Systems,” IEEE Transactions on Power Electronics, vol.24, no.9, pp.2178-2192, Sept. 2009.

[2] Li Shuhui, T.A. Haskew, R. P. Swatloski and W. Gathings, “Optimal and Direct-Current Vector Control of Direct-Driven PMSG Wind Turbines,” IEEE Trans. Power Electronics, vol.27, no.5, pp.2325-2337, May 2012.

[3] M. Singh and A. Chandra, “Application of Adaptive Network-Based Fuzzy Inference System for Sensorless Control of PMSG-Based Wind Turbine With Nonlinear-Load-Compensation Capabilities,” IEEE Trans. Power Electronics, vol.26, no.1, pp.165-175, Jan. 2011.

[4] A. Rajaei, M. Mohamadian and A. Yazdian Varjani, “Vienna-Rectifier-Based Direct Torque Control of PMSG for Wind Energy Application,” IEEE Trans. Industrial Elect., vol.60, no.7, pp.2919-2929, July 2013.

[5] Mihai Comanescu, A. Keyhani and Dai Min, “Design and analysis of 42-V permanent-magnet generator for automotive applications,” IEEE Trans. Energy Conversion, vol.18, no.1, pp.107-112, Mar 2003.