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.

SOFTWARE: MATLAB/SIMULINK

CONTROL SYSTEM CIRCUIT DIAGRAM:

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

EXPECTED EXPERIMENTAL RESULTS:

Fig. 2. Experimental results for the test 1.

Fig. 3. Experimental results for the test 2.

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

[1]   J. 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.

[2]   F. 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.

[3]   B. K. Bose, “Power electronics and motor drives recent-progress and perspective,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 581–588, Feb. 2009.

[4]   F. 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.

[5]    Q. 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.

Experimental Design of a Nonlinear Control Technique for Three-Phase Shunt Active Power Filter

ABSTRACT

This paper presents a nonlinear control technique for a three-phase shunt active power filter (SAPF). The method provides compensation for reactive, unbalanced, and harmonic load current components. A proportional–integral (PI) control law is derived through linearization of the inherently nonlinear SAPF system model, so that the tasks of current control dynamics and dc capacitor voltage dynamics become decoupled. This decoupling allows us to control the SAPF output currents and the dc bus voltage independently of each other, thereby providing either one of these decoupled subsystems a dynamic response that significantly slower than that of the other. To overcome the drawbacks of the conventional method, a computational control delay compensation method, which delaylessly and accurately generates the SAPF reference currents, is proposed. The first step is to extract the SAPF reference currents from the sensed nonlinear load currents by applying the synchronous reference frame method, where a three-phase diode bridge rectifier with R–L load is taken as the nonlinear load, and then, the reference currents are modified, so that the delay will be compensated. The converter, which is controlled by the described control strategy, guarantees balanced overall supply currents, unity displacement power factor, and reduced harmonic load currents in the common coupling point. Various simulation and experimental results demonstrate the high performance of the nonlinear controller.

KEYWORDS:

1.      Active power filter

2.      Control delay compensation,

3.      Modeling

4.      Nonactive load current compensation

5.      Nonlinearc control

6.      Power quality.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Basic circuit of SAPF.

EXPECTED SIMULATION RESULTS:

Fig. 2. Steady-state response of the SAPF.

Fig. 3. Spectrum of phase 1. (a) Load current. (b) Source current after

compensation.

Fig. 4. Dynamic response of SAPF under varying distorted nonlinear load

conditions.

Fig. 5. Steady-state response of SAPF with nonlinear load unbalances.

Fig. 6. Spectrum of load currents and source currents after compensation for

asymmetrical load conditions.

CONCLUSION

The nonlinear control algorithm of an SAPF has been implemented to enhance its response for compensation of nonactive load currents. The nonlinear control technique of the SAPF has been designed, which is based on two inner current loops and an outer dc bus voltage regulator loop. It addition to good performance in both steady-state and transient operations. Simulation and experimental results have validated the nonlinear control approach of the SAPF. It has been shown that the system has 1.5 cycles for the outer voltage loop and 0.5 cycles for the inner current loop and is able to keep the THD of the supply current below the limits specified by the IEEE- 519 standard. The obtained results have demonstrated the high performance of the SAPF.

REFERENCES

[1]   S. Senini and P. J. Wolfs, “Hybrid active filter for harmonically unbalanced three phase three wire railway traction loads,” IEEE Trans. Power Electron., vol. 15, no. 4, pp. 702–710, Jul. 2000.

[2]   S. Rahmani, K. Al-Haddad, H. Y. Kanaan, and B. Singh, “Implementation and simulation of a modified PWM with two current control techniques applied to a single-phase shunt hybrid power filter,” Proc. Inst. Elect. Eng.—Electr. Power Appl., vol. 153, no. 3, pp. 317–326, May 2006.

[3]   B. Singh, V. Verma, and J. Solanki, “Neural network-based selective compensation of current quality problems in distribution system,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 53–60, Feb. 2007.

[4]   B. R. Lin and C. H. Huang, “Implementation of a three-phase capacitor clamped active power filter under unbalanced condition,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1621–1630, Oct. 2006.

[5]   R. Grino, R. Cardoner, R. Costa-Castello, and E. Fossas, “Digital repetitive control of a three-phase four-wire shunt active filter,” IEEE Trans. Ind. Electron., vol. 54, no. 3, pp. 1495–1503, Jun. 2007.

.

An Improved Control Scheme for Grid Connected Voltage Source Inverter

ABSTRACT

In grid connected Distribution Generation systems, Voltage Source Inverters are used for interfacing the renewable energy source to the utility grid. DG has variety of problems during grid integration. The power quality problems may cause problems to the industries ranging from malfunctioning of equipment to complete plant shut down. Disturbances from the utility grid including voltage sags, harmonics and the grid impedance will affect the grid connected voltage source inverters connected to the grid. Hence the control of the grid connected inverter plays an important role in feeding a grid with high quality power. This report presents an analysis of the stability problem of a grid connected with Voltage Source Inverter and with a LC filter. The possible grid-impedance variations have a significant influence on the system stability. Whenever the grid inductive impedance increases, the low frequency gain and the bandwidth of the Proportional Integral (PI) controller have to be decreased to maintain the system stable, thereby degrading the tracking performance and disturbance rejection capability. To overcome this problem an H∞ controller is proposed with an explicit robustness in terms of grid impedance variations to incorporate the desired tracking performance and stability margin. The proposed method is simulated by using MATLAB/SIMULINK. The results of the proposed H∞ controller and the conventional PI controller are compared, which validates the performance of the proposed control scheme.

KEYWORDS:

1.      Distributed Generation (DG)

2.      Voltage Source Inverter (VSC)

3.      LC Filter

4.      H∞ Controller

5.      Total Harmonic Distortion (THD).

SOFTWARE: MATLAB/SIMULINK

SIMULINK BLOCK DIAGRAM:

Figure.1.Overall Simulink Model

EXPECTED SIMULATION RESULTS:

Figure.2.Waveform for output voltage of PV module

Figure.3.Output current waveform of overall system

Figure.4.THD analysis with Lg=0.3 mH and rg= 0.2Ω

Figure.5.THD analysis with Lg=0.15 mH and rg=0.2Ω

CONCLUSION

In the grid connected VSI with LC filters, the possible wide range of grid impedance variations can challenge the design of the controller, particularly when the grid impedance is highly inductive. In this project, the suitability of an H∞ controller to get the desired tracking performance and stability margin is investigated. From the software results it is seen that the grid current THD of the H∞ controller are always lower than that of the PI controller, which satisfy the THD requirement of IEEE Std.1547-2003 (i.e.,5%). Further simulation work is based on demonstrating the operation of a grid in Grid connected mode and intentional islanded mode. Through this, the system is able to determine whether or not it is safe to remain connected to the grid. An islanding detection algorithm is used to act as a switch between the two controllers and this minimizes the effect of losses in the time of transition, and also to prevent the undesirable feeding of loads during fault conditions.

REFERENCES

[1]   .Asiminoaei, L., Teodorescu,R.,Blaabjerg,F., and Borup, U., “A New Method Of On-Line Grid Impedance Estimation for PV Inverter,” in Proc. IEEE APEC, San Diego, CA, Feb. 2004, pp. 1527–1533.

[2]    Bierhoff, M. H., and Fuchs,F. W., “Active Damping for Three-Phase PWM Rectifiers with High-Order Line-Side Filters,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 371–379, Feb. 2009.

[3]    Bin Yu and Liuchen Chang, “Improved Predictive Current Controlled PWM for Single-Phase Grid-Connected Voltage Source Inverters,” in Proc. IEEE PESC, 2005, pp. 231 - 236.

[4]    Blaabjerg, F., Teodorescu, R., and Liserre, M., “Overview of Control and Grid Synchronization for Distributed Power Generation Systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006. 

Variable and Fixed Switching Frequency Based HCC Methods for Grid-Connected VSI with Active Damping and Zero Steady State Error

ABSTRACT

This paper presents variable and fixed switching frequency based hysteresis current control (HCC) methods for single-phase grid-connected voltage source inverters (VSI) with LCL filter. The main feature of the proposed HCC methods is that the reference inverter current is generated through a proportional-resonant (PR) controller for achieving zero steady-state error in the grid current. The consequence of using PR controller is eliminating the need for using derivative operations in generating the reference inverter current. Furthermore, active damping method is employed to damp the LCL resonance. An equation is derived for variable switching frequency. Fixed switching frequency operation is achieved by modulating the hysteresis band. The performance of both HCC methods has been validated by simulation and experimentally. It is reported that the proposed HCC methods not only preserve the inherent features of the conventional HCC methods, but also damp the LCL resonance using an active damping method and guarantee zero steady-state error in the grid current.

KEYWORDS:

1.      Fixed switching frequency

2.      Grid-connected inverter

3.      Hysteresis current control

4.      Variable switching frequency.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Single-phase grid-connected VSI with the proposed HCC methods. (a) Variable switching frequency based HCC, (b) Fixed switching frequency based HCC.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation and experimental results of , , , and obtained by: (a) the variable switching frequency based HCC, (b) the fixed switching frequency based HCC. 1 i * ,1damp i2 i2 *2 i i

Fig. 3. Simulation and experimental results of , , and obtained by: (a) the variable switching frequency based HCC, (b) the fixed switching frequency based HCC. 2 ig vsw f

CONCLUSION

In this study, variable and fixed switching frequency based HCC methods are presented for single-phase grid-connected VSI with LCL filter. Unlike the conventional HCC methods, the proposed HCC methods employ the reference inverter current generated with the PR current controller by processing the grid current error. The use of PR controller ensures zero steady-state error in the grid current independently from the hysteresis bandwidth. In addition, the need for using derivative operations in generating the reference inverter current is eliminated by using PR controller. Also, active damping method is employed to damp the LCL resonance. A formula is derived for the variable switching frequency. The fixed switching frequency operation is achieved by modulating the hysteresis band obtained from the derived variable switching frequency equation. The performances of both HCC methods are validated by experimental investigation. These results show excellent performance in terms of dynamic response, robustness, zero steady-state error, and low THD in the grid current. Hence, the proposed HCC methods not only preserve the inherent features of the HCC methods, but also damp the LCL resonance using an active damping method and guarantee zero steady-state error in the grid current.

REFERENCES

[1]   F. 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.

[2]   H. Abu-Rub, M. Malinowski, and K. Al-Haddad, Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications. Hoboken, NJ, USA: Wiley, 2014.

[3]   R. Pena-Alzola, M. Liserre, F. Blaabjerg, R. Sebastion, 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.

[4]   J. Dannehl, F. W. Fuchs, S. Hansen, and P. B. Thogersen,”Investigation of active damping approaches for PI-based current control of grid-connected pulse width modulation converters with LCL filters,” IEEE Trans. Ind. Appl., vol. 46, no. 4, pp. 1509-1517, Jul./Aug. 2010.

[5]   D. Pan, X. Ruan, C. Bao, W. Li, and X. Wang, “Capacitor-current feedback active damping with reduced computation delay for improving robustness of LCL-type grid-connected inverter,” IEEE Trans. Power Electron., vol. 29, no. 7, pp. 3414–3427, Jul. 2014.

Fixed-Frequency Generalized Peak Current Control (GPCC) for Inverters

ABSTRACT

A fast and robust fixed switching frequency peak current controller for dc-ac converters is presented. The method is specifically elaborated for single-phase grid-connected distributed generation (DG) applications. This method is called generalized peak current control (GPCC) as it can mimic any known pulse width modulation (PWM) strategy. It is shown that additional control objectives can be achieved by adaptive bands of the GPCC, which are proposed to provide active damping for inverters with LCL output filters. The proposed approach features all the advantages of peak current controllers such as simplicity, fast transient, and optimum dynamic response; with the superiority of fixed switching frequency and harmonic free output. Feasibility and performance of the controller is shown by simulations and experimental results.

KEYWORDS:

1.      Current Control

2.      DC-AC Converters

3.      Generalized Peak Current Control (GPCC)

4.      Switching Scheme

5.      Single-Phase Grid-Connected Inverter

SOFTWARE: MATLAB/SIMULINK

  

BLOCK DIAGRAM:

Fig. 1. Block diagram of the proposed controller along with the cost effective

active resonant damping technique.

EXPECTED SIMULATION RESULTS:

Fig. 2. (a) Grid current for the LCL filter case when the active damping

branch is disabled, (b) Current of inverter and grid with the proposed active damping.

Fig. 3. Dynamic performance evaluation of proposed GPCC. Peak current

reference jumps from 1A to 3A at t = 0:1s.

CONCLUSION

A fixed switching frequency Generalized Peak Current Control (GPCC) method for inverters is proposed . While controlling the peak value of the inverters’ current, the proposed approach can mimic any known PWM strategy. As a result, the GPCC features all the advantages of peak current controllers, along with a fixed switching frequency and the clean output harmonic spectrum inheriting from the original PWM scheme. It is shown that the proposed technique is able to obtain additional control objectives by its adaptive bands. As an example, the GPCC is applied to a unipolar PWM scheme and the controller is elaborated for both Land LCL-type output filters. Demonstrating the advantages of resulting controller, a simple active damping strategy based on adaptive bands of the controller is proposed. Simulations and experimental results are presented to validate the method.

REFERENCES

[1]   R. Gupta, “Generalized frequency domain formulation of the switching frequency for hysteresis current controlled vsi used for load compensation,” Power Electronics, IEEE Transactions on, vol. 27, no. 5, pp. 2526–2535, May 2012.

[2]   F. Blaabjerg, R. Teodorescu, M. Liserre, and A. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” Industrial Electronics, IEEE Transactions on, vol. 53, no. 5, pp. 1398–1409, Oct 2006.

[3]   L. Malesani and P. Tenti, “A novel hysteresis control method for currentcontrolled voltage-source pwm inverters with constant modulation frequency,” Industry Applications, IEEE Transactions on, vol. 26, no. 1, pp. 88–92, 1990.

[4]   L. Malesani, L. Rossetto, and A. Zuccato, “Digital adaptive hysteresis current control with clocked commutations and wide operating range,” Industry Applications, IEEE Transactions on, vol. 32, no. 2, pp. 316– 325, 1996.

[5]   B. Bose, “An adaptive hysteresis-band current control technique of a voltage-fed pwm inverter for machine drive system,” Industrial Electronics, IEEE Transactions on, vol. 37, no. 5, pp. 402–408, Oct 1990.

Adaptive Hysteresis Band Current Control for Transformer-less Single-Phase PV Inverters

ABSTRACT

Current control based on hysteresis algorithms are widely used in different applications, such as motion control, active filtering or active/reactive power delivery control in distributed generation systems. The hysteresis current control provides to the system a fast and robust dynamic response, and requires a simple implementation in standard digital signal platforms. On the other hand, the main drawback of classical hysteresis current control lies in the fact that the switching frequency is variable, as the hysteresis band is fixed. In this paper a variable band hysteresis control algorithm will be presented. As it will be shown, this variable band permits overcoming the aforementioned problem giving rise to an almost constant switching frequency. The performance of this algorithm, together with classical hysteresis controls and proportional resonant (PR) controllers, has been evaluated in three different single-phase PV inverter topologies, by means of simulations performed with PSIM. In addition, the behavior of the thermal losses when using each control structure in such converters has been studied as well.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Basic Current Control Scheme in a single phase inverter.

EXPECTED SIMULATION RESULTS:

Fig. 2. Behavior of the current and the voltage at the output of the converter

when using the H5 topology.

Fig. 3. Behavior of the current and the voltage at the output of the converter

when using the HERIC topology.

Fig. 4. Behavior of the current and the voltage at the output of the converter

when using the or HB-ZVR topology.

CONCLUSION

A hysteresis current control algorithm based on an adaptive hysteresis band for single phase PV converter topologies has been presented in this paper. As it has been shown analytically and by means of simulations this algorithm permits obtaining a fixed switching frequency in all the tested topologies. The main drawback of the conventional fixed hysteresis band current control is that generates excessive current ripple because modulation frequency varies within a band. This modulation frequency variation makes complicated the output filter design. Adaptive hysteresis band current control keeps the good performance of the fixed band hysteresis current control and additionally permits an easier output filter design due that the switching frequency is almost constant. On the other hand, switching losses can be reduced by using this adaptive hysteresis band current control. The analyzed topologies are the more widely used in transformerless single-phase PV systems (H5 and HERIC). Based in the previously comparative simulations results it can be concluded that in the case of H5 topology losses are concentrated in S5. In case of HERIC topology losses are located among S1, S2, S3 and S4. Finally in HB-ZVR single phase topology, losses are located in S5. These results mean that in each case, the losses distribution is not the same and a different thermal design should be done.

REFERENCES

[1]   L. Malesani, P. Mattavelli, P. Tomasin, “High Performance Hysteresis Modulation Technique for Active Filters”, IEEE Transactions on Power Electronics, Volume 12, September 1997.

[2]   J. Holtz and S. Stadtfeld, “A Predictive Controller for the Stator Current Vector of AC Machine-fed from a Switched Voltage Source”, in Proc. Int. Power Electronics Conference Rec. (Tokyo), 1983, pp. 1665-1675.

[3]   M. Ciobotaru, R. Teodorescu, and F. Blaabjerg, “Control of Single-Stage Single-Phase PV Inverter”, European Conference on Power Electonics and Applications, 2005.

[4]   Y. Hayashi, N. Sato, K. Takahashi, “A Novel Control of a Current- Source Active Filter for ac Power System Harmonic Compensation”, IEEE Transactions on Industrial Applications, Vol. 27, No. 2, March/April 1991.

[5]   T. Kato, K. Miyao, “Modified Hysteresis Control with Minor Loops for Single-Phase Full-Bridge Inverters”, Doshisha University, Kyoto Japan, 88CH2565-0/88/0000-0689$01.00, 1988 IEEE.