UDE-Based Current Control Strategy for LCCL-Type Grid-Tied Inverters

ABSTRACT:

LCL filter is usually used as an interface between inverters and the grid. However, due to the characteristics of LCL filter and system uncertainties, it is complex to design a controller with proper parameters. In this paper, with LCCL filter, the order of the inverter control system can be reduced from third order to first order, and an uncertainty and disturbance estimator based control strategy for grid-tied inverters with LCCL filter is proposed. Specifically, the proposed control strategy consists of differential feed forward, proportional–integral controller, and grid voltage feed forward. Moreover, with one-sampling computation plus half-sampling pulse width modulation delays considered, a simple and clear tuning algorithm for the proposed control strategy is presented. Finally, the inverter system with the proposed control strategy is investigated, and the effectiveness is supported by the tuning and comparative experiments with a 2-kW inverter.

KEYWORDS:

  1. Current control
  2. Inverter
  3. LCCL filter
  4. Tuning algorithm
  5. Uncertainty and disturbance estimator (UDE)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. System topology of the grid-tied inverter with LCCL filter.

EXPECTED SIMULATION RESULTS

 

 Fig. 2. Result of UDE-based control without grid voltage feed forward. (a) Injected grid current i2 . (b) Spectrum of the injected grid current.

Fig. 3. Tuning results of UDE-based control with the same α = 10 000 rad/s, β = 5000 rad/s, and different k. (a) k = 10 000 rad/s. (b) k = 9000 rad/s. (c) k = 7000 rad/s.

Fig. 4. Result of UDE-based control under i*12 (s) = 10 A with α = 10 000 rad/s, β = 5000 rad/s, and k = 8000 rad/s. (a) Injected grid current i2 . (b) Spectrum of the injected grid current.

Fig. 5. Result of PI control under i*12 (s) = 10 A with kp = 17 and ki  = 14400. (a) Injected grid current i2 . (b) Spectrum of the injected grid current.

CONCLUSION:

For grid-tied inverter, LCL filter is widely used to attenuate the high switching frequency harmonics caused by PWM. However, due to the characteristic of LCL filter and uncertainty, it is complex to design a controller with proper parameters. In this paper, with LCCL filter, the inverter control system can be degraded from third order to first order. And a UDE-based injected grid current control strategy was built. The proposed strategy unified the system uncertainty and disturbance into the lumped disturbances, and the closed-loop system adjusted by PI regulator approached to the reference model. Meanwhile, the PI controller can be expressed in the error feedback gain, the desired closed-loop bandwidth, and the approximate lumped disturbance bandwidth. Moreover, with one-sampling computation plus half-sampling PWM delays considered, a simple and clear tuning algorithm for the proposed control strategy was provided. Finally, the proposed strategy was verified by the tuning and comparative experiments on a 2-kW inverter.

REFERENCES:

[1] M. Lindgren and J. Svensson, “Control of a voltage-source converter connected to the grid through an LCL-filter-application to active filtering,” in Proc. IEEE Power Electron. Spec. Conf., May 1998, pp. 229–235.

[2] E. Twining and D. G. Holmes, “Grid current regulation of a three-phase voltage source inverter with an LCL input filter,” IEEE Trans. Power Electron., vol. 18, no. 3, pp. 888–895, May 2003.

[3] G. Shen, D. Xu, L. Cao, and X. Zhu, “An improved control strategy for grid-connected voltage source inverters with an LCL filter,” IEEE Trans. Power Electron., vol. 23, no. 4, pp. 1899–1906, Jul. 2008.

[4] G. Shen, X. Zhu, J. Zhang, and D. Xu, “A new feedback method for PR current control ofLCL-filter-based grid-connected inverter,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2033–2041, Jun. 2010.

[5] R. P. Alzola, M. Liserre, F. Blaabjerg, R. Sebasti´an, J. Dannehl, and F. W. Fuchs, “Analysis of the passive damping losses in LCL-filter-based grid converters,” IEEE Trans. Power Electron., vol. 28, no. 6, pp. 2642–2646, Jun. 2013.

Design of External Inductor for Improving Performance of Voltage Controlled DSTATCOM

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, 2015

ABSTRACT: A distribution static compensator (DSTATCOM) is used for load voltage regulation and its performance mainly depends upon the feeder impedance and its nature (resistive, inductive, stiff, non-stiff). However, a study for analyzing voltage regulation performance of DSTATCOM depending upon network parameters is not well defined. This paper aims to provide a comprehensive study of design, operation, and flexible control of a DSTATCOM operating in voltage control mode. A detailed analysis of the voltage regulation capability of DSTATCOM under various feeder impedances is presented. Then, a benchmark design procedure to compute the value of external inductor is presented. A dynamic reference load voltage generation scheme is also developed which allows DSTATCOM to compensate load reactive power during normal operation, in addition to providing voltage support during disturbances. Simulation and experimental results validate the effectiveness of the proposed scheme.

KEYWORDS:

  1. Distribution static compensator (DSTATCOM)
  2. Current control
  3. Voltage control
  4. Power factor
  5. Power quality

 SOFTWARE: MATLAB/SIMULINK

EQUIVALENT CIRCUIT DIAGRAM:

 

 Fig. 1. Three phase equivalent circuit of DSTATCOM topology in distribution system.

EXPECTED SIMULATION RESULTS:

Fig. 2. Voltage regulation performance of conventional DSTATCOM with resistive feeder. (a) PCC voltages. (b) Load Voltages. (c) Source currents. (d) Filter currents. (e) Load currents.

Fig. 3. Simulation results. (a) During normal operation (i)-(v). (b) During voltage sag (vi)-(x). (c) During voltage swell (xi)-(xv).

CONCLUSION:

This paper has presented design, operation, and control of a DSTATCOM operating in voltage control mode (VCM). After providing a detailed exploration of voltage regulation capability of DSTATCOM under various feeder scenarios, a benchmark design procedure for selecting suitable value of external inductor is proposed. An algorithm is formulated for dynamic reference load voltage magnitude generation. The DSTATCOM has improved voltage regulation capability with a reduced current rating VSI, reduced losses in the VSI and feeder. Also, dynamic reference load voltage generation scheme allows DSTATCOM to set different constant reference voltage during voltage disturbances. Simulation and experimental results validate the effectiveness of the proposed solution. The external inductor is a very simple and cheap solution for improving the voltage regulation, however it remains connected throughout the operation and continuous voltage drop across it occurs. The future work includes operation of this fixed inductor as a controlled reactor so that its effect can be minimized by varying its inductance.

REFERENCES:

[1] M. H. Bollen, Understanding power quality problems. vol. 3, IEEE press New York, 2000.

[2] S. Ostroznik, P. Bajec, and P. Zajec, “A study of a hybrid filter,” IEEE Trans. Ind. Electron., vol. 57, no. 3, pp. 935–942, Mar. 2010.

[3] C. Kumar and M. Mishra, “A voltage-controlled DSTATCOM for power quality improvement,” IEEE Trans. Power Del., vol. 29, no. 3, pp. 1499– 1507, June 2014.

[4] Q. Liu, L. Peng, Y. Kang, S. Tang, D. Wu, and Y. Qi, “A novel design and optimization method of an LCL filter for a shunt active power filter,” IEEE Trans. Ind. Electron., vol. 61, no. 8, pp. 4000–4010, Aug. 2014.

[5] T. Aziz, M. Hossain, T. Saha, and N. Mithulananthan, “VAR planning with tuning of STATCOM in a DG integrated industrial system,” IEEE Trans. Power Del., vol. 28, no. 2, pp. 875–885, Apr. 2013.

Design of External Inductor for Improving Performance of Voltage Controlled DSTATCOM

 

ABSTRACT:

A distribution static compensator (DSTATCOM) is used for load voltage regulation and its performance mainly depends upon the feeder impedance and its nature (resistive, inductive, stiff, non-stiff). However, a study for analyzing voltage regulation performance of DSTATCOM depending upon network parameters is not well defined. This paper aims to provide a comprehensive study of design, operation, and flexible control of a DSTATCOM operating in voltage control mode. A detailed analysis of the voltage regulation capability of DSTATCOM under various feeder impedances is presented. Then, a benchmark design procedure to compute the value of external inductor is presented. A dynamic reference load voltage generation scheme is also developed which allows DSTATCOM to compensate load reactive power during normal operation, in addition to providing voltage support during disturbances. Simulation and experimental results validate the effectiveness of the proposed scheme.

KEYWORDS:

  1. Distribution static compensator (DSTATCOM)
  2. Current control
  3. Voltage control
  4. Power factor
  5. Power quality

 SOFTWARE: MATLAB/SIMULINK

EQUIVALENT CIRCUIT DIAGRAM:

 Fig. 1. Three phase equivalent circuit of DSTATCOM topology in distribution system.

EXPECTED SIMULATION RESULTS:

 Fig. 2. Voltage regulation performance of conventional DSTATCOM with resistive feeder. (a) PCC voltages. (b) Load Voltages. (c) Source currents. (d) Filter currents. (e) Load currents.

Fig. 3. Simulation results. (a) During normal operation (i)-(v). (b) During voltage sag (vi)-(x). (c) During voltage swell (xi)-(xv).

CONCLUSION:

This paper has presented design, operation, and control of a DSTATCOM operating in voltage control mode (VCM). After providing a detailed exploration of voltage regulation capability of DSTATCOM under various feeder scenarios, a benchmark design procedure for selecting suitable value of external inductor is proposed. An algorithm is formulated for dynamic reference load voltage magnitude generation. The DSTATCOM has improved voltage regulation capability with a reduced current rating VSI, reduced losses in the VSI and feeder. Also, dynamic reference load voltage generation scheme allows DSTATCOM to set different constant reference voltage during voltage disturbances. Simulation and experimental results validate the effectiveness of the proposed solution. The external inductor is a very simple and cheap solution for improving the voltage regulation, however it remains connected throughout the operation and continuous voltage drop across it occurs. The future work includes operation of this fixed inductor as a controlled reactor so that its effect can be minimized by varying its inductance.

REFERENCES:

[1] M. H. Bollen, Understanding power quality problems. vol. 3, IEEE press New York, 2000.

[2] S. Ostroznik, P. Bajec, and P. Zajec, “A study of a hybrid filter,” IEEE Trans. Ind. Electron., vol. 57, no. 3, pp. 935–942, Mar. 2010.

[3] C. Kumar and M. Mishra, “A voltage-controlled DSTATCOM for power quality improvement,” IEEE Trans. Power Del., vol. 29, no. 3, pp. 1499– 1507, June 2014.

[4] Q. Liu, L. Peng, Y. Kang, S. Tang, D. Wu, and Y. Qi, “A novel design and optimization method of an LCL filter for a shunt active power filter,” IEEE Trans. Ind. Electron., vol. 61, no. 8, pp. 4000–4010, Aug. 2014.

[5] T. Aziz, M. Hossain, T. Saha, and N. Mithulananthan, “VAR planning with tuning of STATCOM in a DG integrated industrial system,” IEEE Trans. Power Del., vol. 28, no. 2, pp. 875–885, Apr. 2013.

Improved Active Power Filter Performance For Renewable Power Generation Systems

 

ABSTRACT:

 An active power filter implemented with a four-leg voltage-source inverter using a predictive control scheme is presented. The use of a four-leg voltage-source inverter allows the compensation of current harmonic components, as well as unbalanced current generated by single-phase nonlinear loads. A detailed yet simple mathematical model of the active power filter, including the effect of the equivalent power system impedance, is derived and used to design the predictive control algorithm. The compensation performance of the proposed active power filter and the associated control scheme under steady state and transient operating conditions is demonstrated through simulations and experimental results.

 KEYWORDS:

  1. Active power filter
  2. Current control
  3. Four-leg converters
  4. Predictive control.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Three-phase equivalent circuit of the proposed shunt active power filter.

Fig. 2. dq-based current reference generator block diagram.

 EXPECTED SIMULATION RESULTS:

Fig. 3. Simulated waveforms of the proposed control scheme. (a) Phase to neutral source voltage. (b) Load Current. (c) Active power filter output current. (d) Load neutral current. (e) System neutral current. (f) System currents. (g) DC voltage converter.

Fig. 4. Experimental transient response after APF connection. (a) Load Current iLu , active power filter current iou , dc-voltage converter vdc , and system current isu . Associated frequency spectrum. (c) Voltage and system waveforms, vsu and isu , isv , isw . (d) Current reference signals i*ou , and active power filter current iou (tracking characteristic).

Fig. 5. Experimental results for step load change (0.6 to 1.0 p.u.). Load Current iLu , active power filter current iou , system current isu , and dc-voltage converter vdc .

Fig. 6. Experimental results for step unbalanced phase u load change (1.0 to 1.3 p.u.). (a) Load Current iLu , load neutral current iLn , active power filter neutral current ion , and system neutral current isn . (b) System currents isu , isv , isw , and isn .

CONCLUSION:

Improved dynamic current harmonics and a reactive power compensation scheme for power distribution systems with generation from renewable sources has been proposed to improve the current quality of the distribution system. Advantages of the proposed scheme are related to its simplicity, modeling, and implementation. The use of a predictive control algorithm for the converter current loop proved to be an effective solution for active power filter applications, improving current tracking capability, and transient response. Simulated and experimental results have proved that the proposed predictive control algorithm is a good alternative to classical linear control methods. The predictive current control algorithm is a stable and robust solution. Simulated and experimental results have shown the compensation effectiveness of the proposed active power filter.

REFERENCES:

 

[1] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodriguez, “Control of power converters in AC microgrids,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4734–4749, Nov. 2012.

[2] M. Aredes, J. Hafner, and K. Heumann, “Three-phase four-wire shunt active filter control strategies,” IEEE Trans. Power Electron., vol. 12, no. 2, pp. 311–318, Mar. 1997.

[3] S. Naidu and D. Fernandes, “Dynamic voltage restorer based on a fourleg voltage source converter,” Gener. Transm. Distrib., IET, vol. 3, no. 5, pp. 437–447, May 2009.

[4] N. Prabhakar and M. Mishra, “Dynamic hysteresis current control to minimize switching for three-phase four-leg VSI topology to compensate nonlinear load,” IEEE Trans. Power Electron., vol. 25, no. 8, pp. 1935– 1942, Aug. 2010.

[5] V. Khadkikar, A. Chandra, and B. Singh, “Digital signal processor implementation and performance evaluation of split capacitor, four-leg and three h-bridge-based three-phase four-wire shunt active filters,” Power Electron., IET, vol. 4, no. 4, pp. 463–470, Apr. 2011.

A PLL Based Controller for Three Phase Grid Connected Power Converters

A PLL Based Controller for Three Phase Grid Connected Power Converters

 ABSTRACT

The current control of three-phase grid-connected converters is typically carried out by using a proportional resonant controller or synchronous reference frame proportional integral regulator. The implementation of these controllers often requires knowledge of the grid voltage frequency/phase angle, which is typically provided by a synchronization unit. It implies that dynamics and possible inaccuracies of the synchronization unit have a considerable impact on the current controller performance. The aim of this letter is to design an adaptive current controller by using a conventional synchronous reference frame phase-locked loop (SRF-PLL). In this way, the current controller and synchronization part are merged into a single unit, which results in a simpler and more compact structure. The effectiveness of the proposed controller is verified using experimental results.

KEYWORDS:

  1. Current control
  2. Distributed generation (DG) systems
  3. Phase-locked loop (PLL)
  4. Power converters
  5. Synchronization
  6. Three phase grid

 SOFTWARE: MATLAB/SIMULINK

CONTROL SYSTEM CIRCUIT DIAGRAM:

Three-Phase Grid

Fig. 1. Power stage of a three-phase VSC with the proposed PLL-based controller and a harmonic/imbalance compensator.

EXPECTED EXPERIMENTAL RESULTS:

PLL Based Controller

Fig. 2. Experimental results for the test 1.

Three Phase Grid

Fig. 3. Experimental results for the test 2.

Three Phase Grid

Fig. 4. Experimental results for the test 3.

 CONCLUSION

In this letter, a PLL-based controller for grid-connected converters was proposed. This controller, which is realized by adding a positive feedback loop to the conventional SRFPLL, eliminates the need for a dedicated synchronization unit and, therefore, results in a more compact structure. To enhance the harmonic/imbalance rejection capability of the suggested controller, multiple complex integrators tuned at low-order disturbance frequencies is employed. To simplify the tuning procedure, a simple yet accurate linear model describing the frequency estimation dynamics of the proposed controller was was verified using some experimental results. The main contribution of this letter is not the proposed controller. It is actually demonstrating the possibility of making a frequency-adaptive controller from a standard PLL. The importance of this contribution will be more evident when we notice that there are a large number of advanced PLLs which can be explored for the controller design.

REFERENCES

  • M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C. P. Guisado, M. A. M. Prats, J. I. Leon, and N. Moreno-Alfonso, “Powerelectronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1016, Jun. 2006.
  • Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.
  • K. Bose, “Power electronics and motor drives recent-progress and perspective,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 581–588, Feb. 2009.
  • Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.
  • Zeng and L. Chang, “An advanced SVPWM-based predictive current controller for three-phase inverters in distributed generation systems,” IEEE Trans. Ind. Electron., vol. 55, no. 3, pp. 1235–1246, Mar. 2008.