EPE Association: EPE 2007 - Subtopic 06-1 - LS: Converter control, current/voltage control

EPE 2007 - Subtopic 06-1 - LS: Converter control, current/voltage control
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2007 - Conference > EPE 2007 - Topic 06: 'Modulation strategies and specific control methods for static converter' > EPE 2007 - Subtopic 06-1 - LS: Converter control, current/voltage control

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   Control of Grid Connected AC-DC Converters with Minimized DC Link Capacitance under Unbalanced Grid Voltage Condition
By J. George Hwang; Peter W. Lehn; Manfred Winkelnkemper [View]
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Abstract: Due to the limited lifetime of electrolytic capacitors, use of film capacitors is gaining popularity in AC-DC converter applications. In order to constrain both cost and converter volume while using film capacitors, small DC link capacitance values must be employed. Two challenges are associated with converters that have small DC link capacitors. First, such converters are highly susceptible to transient over/under voltages on the DC link. Secondly, even minor grid voltage imbalance can cause large 2nd harmonic DC link voltage ripple to appear, possibly interfering with the load. This paper details a control scheme that overcomes both these problems. A feedforward matrix is employed to generate a set of reference current commands that are processed by high bandwidth stationary frame current controllers. A novel DC link voltage controller combines load feedforward with a resonant term to reduce transient over/under voltages and improve immunity to 2nd harmonic voltage ripple under unbalanced grid conditions. Experimental results demonstrate the efficacy of the proposed converter controller.

   Four leg parallel Z-source inverter based DG systems to enhance the grid performance under unbalanced conditions
By C. J. Gajanayake; R. Teodorescu; F. Blaabjerg; D. M. Vilathgamuwa; P. C. Loh [View]
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Abstract: This paper presents a DG system based on four-leg parallel Z-source inverters in integrating a renewable generation system into the grid. Particularly, four-leg distribution schemes give flexibility into the DG system by supporting other functions of power distribution like control of zero sequence components and unbalance mitigation. To increase the capacity and to have redundancy, a parallel structure for the Z-source inverter is proposed. The emphasis is given to component count and the modular structure, thereby reducing the cost while achieving the system reliability. A modulation method is proposed based on interleaved carriers to reduce the output current ripple. Separate controllers are designed in stationary reference frame for the AC side of each inverter. The AC side controller is designed using a combination of p+resonance and negative feedback time delay. The combined controller would deliver good reference tracking and harmonic rejection properties. Another controller is designed for the DC side Z-source impedance network to mitigate the fluctuations in the renewable source. The whole system is driven from a higher level controller that would generate current references to operate the total system in two operating modes, to deliver specified power and to control the unbalances and zero sequence. Proposed control method is tested with simulation results obtained using Matlab/Simulink and PLECS. Subsequently, it is experimentally validated using a laboratory prototype.

   Stability Analysis of Current Digital Controllers for LCL Filters Connected to the Grid using State Feedback
By Ivan Jorge Gabe; Jorge Rodrigo Massing; Vinicius F. Montagner; Humberto Pinheiro [View]
[Download]

Abstract: This paper addresses the stability of discrete controlled grid connected voltage source inverters with LCL-filter usually found in wind power generation systems. First a theorem that relates the controllability of the discrete dynamic equation with the LCL-filter parameters and the sampling frequency is derived. Then, a robust partial state feedback design on the linear matrix inequalities framework guarantee the stability damping the LCL-filter resonance for a large set of grid conditions without requiring tuning procedures. Finally, an internal model controller is added to ensure asymptotic reference tracking and disturbance rejection, therefore reducing significantly the impact of grid background voltage distortion on the line currents. Simulation and experimental results are presented to support the theoretical analysis carried out and to demonstrate the system performance.

EPE 2007 - Subtopic 06-1 - LS: Converter control, current/voltage control
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2007 - Conference > EPE 2007 - Topic 06: 'Modulation strategies and specific control methods for static converter' > EPE 2007 - Subtopic 06-1 - LS: Converter control, current/voltage control
	[return to parent folder]

	Control of Grid Connected AC-DC Converters with Minimized DC Link Capacitance under Unbalanced Grid Voltage Condition By J. George Hwang; Peter W. Lehn; Manfred Winkelnkemper	[View] [Download]
Abstract: Due to the limited lifetime of electrolytic capacitors, use of film capacitors is gaining popularity in AC-DC converter applications. In order to constrain both cost and converter volume while using film capacitors, small DC link capacitance values must be employed. Two challenges are associated with converters that have small DC link capacitors. First, such converters are highly susceptible to transient over/under voltages on the DC link. Secondly, even minor grid voltage imbalance can cause large 2nd harmonic DC link voltage ripple to appear, possibly interfering with the load. This paper details a control scheme that overcomes both these problems. A feedforward matrix is employed to generate a set of reference current commands that are processed by high bandwidth stationary frame current controllers. A novel DC link voltage controller combines load feedforward with a resonant term to reduce transient over/under voltages and improve immunity to 2nd harmonic voltage ripple under unbalanced grid conditions. Experimental results demonstrate the efficacy of the proposed converter controller.

	Four leg parallel Z-source inverter based DG systems to enhance the grid performance under unbalanced conditions By C. J. Gajanayake; R. Teodorescu; F. Blaabjerg; D. M. Vilathgamuwa; P. C. Loh	[View] [Download]
Abstract: This paper presents a DG system based on four-leg parallel Z-source inverters in integrating a renewable generation system into the grid. Particularly, four-leg distribution schemes give flexibility into the DG system by supporting other functions of power distribution like control of zero sequence components and unbalance mitigation. To increase the capacity and to have redundancy, a parallel structure for the Z-source inverter is proposed. The emphasis is given to component count and the modular structure, thereby reducing the cost while achieving the system reliability. A modulation method is proposed based on interleaved carriers to reduce the output current ripple. Separate controllers are designed in stationary reference frame for the AC side of each inverter. The AC side controller is designed using a combination of p+resonance and negative feedback time delay. The combined controller would deliver good reference tracking and harmonic rejection properties. Another controller is designed for the DC side Z-source impedance network to mitigate the fluctuations in the renewable source. The whole system is driven from a higher level controller that would generate current references to operate the total system in two operating modes, to deliver specified power and to control the unbalances and zero sequence. Proposed control method is tested with simulation results obtained using Matlab/Simulink and PLECS. Subsequently, it is experimentally validated using a laboratory prototype.

	Stability Analysis of Current Digital Controllers for LCL Filters Connected to the Grid using State Feedback By Ivan Jorge Gabe; Jorge Rodrigo Massing; Vinicius F. Montagner; Humberto Pinheiro	[View] [Download]
Abstract: This paper addresses the stability of discrete controlled grid connected voltage source inverters with LCL-filter usually found in wind power generation systems. First a theorem that relates the controllability of the discrete dynamic equation with the LCL-filter parameters and the sampling frequency is derived. Then, a robust partial state feedback design on the linear matrix inequalities framework guarantee the stability damping the LCL-filter resonance for a large set of grid conditions without requiring tuning procedures. Finally, an internal model controller is added to ensure asymptotic reference tracking and disturbance rejection, therefore reducing significantly the impact of grid background voltage distortion on the line currents. Simulation and experimental results are presented to support the theoretical analysis carried out and to demonstrate the system performance.