A Common Capacitor Based Three Level STATCOM and Design of DFIG Converter for a Zero-Voltage Fault Ride-Through Capability

ABSTRACT:

To meet the augmented load power demand, the doubly-fed induction generator (DFIG) based wind electrical power conversion system (WECS) is a better alternative. Further, to enhance the power flow capability and raise security margin in the power system, the STATCOM type FACTS devices can be adopted as an external reactive power source. In this paper, a three-level STATCOM coordinates the system with its dc terminal voltage is connected to the common back-to-back converters. Hence, a lookup table-based control scheme in the outer control loops is adopted in the Rotor Side Converter (RSC) and the grid side converter (GSC) of DFIG to improve power flow transfer and better dynamic as well as transient stability. Moreover, the DC capacitor bank of the STATCOM and DFIG converters connected to a common dc point. The main objectives of the work are to improve voltage mitigation, operation of DFIG during symmetrical and asymmetrical faults, and limit surge currents. The DFIG parameters like winding currents, torque, rotor speed are examined at 50%, 80% and 100% comparing with earlier works. Further, we studied the DFIG system performance at 30%, 60%, and 80% symmetrical voltage dip. Zero-voltage fault ride through is investigated with proposed technique under symmetrical and asymmetrical LG fault for super-synchronous (1.2 p.u.) speed and sub-synchronous (0.8 p.u.) rotor speed. Finally, the DFIG system performance is studied with different phases to ground faults with and without a three-level STATCOM.

KEYWORDS:

  1. Doubly-fed induction generator (DFIG)
  2. Field oriented control (FOC)
  3. Common-capacitor based STATCOM
  4. Voltage compensation
  5. Balanced and unbalanced faults
  6. Zero-voltage fault ride through

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Figure 1. Grid-Connected DFIG With Three Levels Statcom Converter.

EXPECTED SIMULATION RESULTS:

Figure 2. DFIG Operation With 50% Voltage Dip (I) Using Method In [27]. And (Ii) Using Proposed Method.

Figure 3. DFIG Operation With 50% Voltage Dip (I) Using Method In [28] And (Ii) Using Proposed Method.

Figure 4. DFIG Operation With (I) 30% Dip, (Ii) 60% Dip And (Iii) 80% Dip In Grid Voltage.

Figure 5. Rotor, Stator Gsc And Grid Terminal Current Waveforms With The Proposed Technique With Slg Fault.

CONCLUSION:

A generalized DFIG wind energy conversion system based test-bed system connected to the grid is considered in the paper. The work tested in the starting cases with two different research papers works with proposed method under an 80% dip. Later, proposed methodology compared under 30%, 60%, and 80% dip, and the DFIG behavior is examined. Further, under three different cases, LG, LLG and LLG faults without and STATCOM are compared to show STATCOM controller’s effectiveness. An improved field-oriented control scheme for the DFIG with real and reactive power lookup-based control in the outer control loops. It is observed that, there is a rapid development in the back emf and decoupled current regime in the paper’s inner control loops is proposed. A three-level SATCOM is used in this paper, with the rectifier end dc-link connected to the common capacitor between the DFIG back-to-back converters. A better damping factor is observed for torque, powers, current, voltage, and speed at 60%, 80%, and 100% dip with the proposed scheme.

The proposed method employs the adjustment of external real and reactive powers using the optimal lookup table method as shown in Table 3, rotor speed, and terminal voltage in the outer control loops of both RSC and GSC. The inner control loop is fast-acting current control and back emf- based voltage injection near the decoupling voltage loop. The strategy works on decoupled real and reactive power flow controls in synchronous rotating frames leads to individual power control. This technique improves performance under normal conditions and during grid faults, with better rotor voltage control, rotor speed, and damping. The post-fault behavior of an overall system improved using the proposed technique.

Further improvement in the system behavior is observed with the common- dc link STATCOM. The rectifier end dc link is connected to the capacitor between the DFIG converters, which will reduce the cost for capacitor and measurement sensors. This paper demonstrates the DFIG-based WECS with better active and reactive power and EMT damping, surge current reduction, speed control, and effective LVRT capability. There are distortions in the rotor current waveform during the zero-voltage fault ride during the fault and considerably more when the rotor speed is at super synchronous speed. When the rotor speed is beyond the synchronous speed, the rotor current is injected into the stator terminal from the rotor side windings with RSC control scheme. Under this condition, the fault inrush current from the dc-link capacitor will pass through this rotor terminal and reach the stator windings. Under sub-synchronous speed, the rotor winding will receive the current for the stator windings, so the fault effect is less influenced at lower speeds than with higher speeds. The rotor voltage is maintained at both speeds during the LG fault. However, waveform is less distorted with lower rotor speed.

The dc-link voltage distortions during the fault are more with super- synchronous speed than sub-synchronous speed operation for zero voltage ride through. The dc-link voltage is more stubborn and stable when the rotor speed is lesser than the synchronous speed. The STATCOM current is observed to be more in faulty phase than with other two healthy phases. The reason and analysis are the same as that with the symmetrical fault study. The deviation of the fault current at the STATCOM terminal is re-injected to the grid via the closed path with the dc-link capacitor terminal. The post-fault performance is superior with a serious 100% voltage dip case and also found better dynamic response because of the RSC and GSC proposed technique and the STATCOM controller. Further, an effective operation is experienced with a common link dc capacitor STATCOM than with a conventional topology. Hence, simulated results show better performance and profitable operation during and after the faults than the earlier famous methods.

With the proposed method, rotor and stator current during fault are maintained, not getting zero value and limiting surge currents to a dangerous value. However, stator and rotor current is not supported to their pre-fault value during the fault period. The torque reduction to a smaller value observed increases the grid fault dip value, but there are no surges and oscillations with the proposed method. The rotor speed is also maintained almost constant even for significant voltage dip. As a result, the post fault recovery in the DFIG is smooth and instantaneous, observed for winding voltages, currents, EMT, active and reactive powers, dc-link capacitor voltage, and rotor speed.

All the objectives specified in the Introduction section are met 1) rotor and stator current surges are limited, current surges ate within 1.5 times the operating value, mitigation in the rotor voltage observed. Furthermore, the reactive power support by STATCOM, RSC and GSC improved the DFIG WECS during and after the fault. Thereby 1) enhancement in overall dynamic and transient stability is observed. 2) The rotor speed is almost constant even for a significant grid voltage dip which is better than many research papers. 3) The electromagnetic torque (EMT) and active and reactive power flow oscillations are damped completely, and sustainable control observed with the technique. 4) The proposed method is suitable for grid faults like symmetrical, asymmetrical, and recurring faults. Better DFIG performance is expected with LVRT capability for symmetrical and asymmetrical faults with future research activities.

REFERENCES:

[1] H. A. Mohammadpour, A. Ghaderi, H. Mohammadpour, and E. Santi, “SSR damping in wind farms using observed-state feedback control of DFIG converters,” Electr. Power Syst. Res., vol. 123, pp. 57_66, Jun. 2015.

[2] F. Blaabjerg and K. Ma, “Future on power electronics for wind turbine systems,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 1, no. 3, pp. 139_152, Sep. 2013.

[3] T. D. Vrionis, X. I. Koutiva, and N. A. Vovos, “A genetic algorithm based low voltage ride-through control strategy for grid connected doubly fed induction wind generators,” IEEE Trans. Power Syst., vol. 29, no. 3, pp. 1325_1334, May 2014.

[4] A. M. Eltamaly and H. M. Farh, “Maximum power extraction from wind energy system based on fuzzy logic control,” Electr. Power Syst. Res., vol. 97, pp. 144_150, Apr. 2013.

[5] Y. Weng and Y. Hsu, “Sliding mode regulator for maximum power tracking and copper loss minimisation of a doubly fed induction generator,” IET Renew. Power Gener., vol. 9, no. 4, pp. 297_305, May 2015.

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