EPE Association: NORpie 2002 - Topic 03: CONTROL OF CONVERTERS

NORpie 2002 - Topic 03: CONTROL OF CONVERTERS
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   Comparative Evaluation of Control Techniques for a Three-Phase Three-Switch Buck-Type AC-to-DC PWM Converter System
By T. Nussbaumer; J. W. Kolar [View]
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Abstract: In this paper the cascade output voltage control of a unidirectional three-phase three-switch buck-type unity power factor PWM rectifier is discussed. Two control structures, i.e. an inner DC side inductor current control and an inner AC side input current and input filter capacitor voltage control are discussed. Guidelines for the control design which is nbased on a dynamically equivalent DC-to-DC converter model are given. Small signal transfer functions are derived by state space averaging. Finally, a comparative evaluation of the dynamics and of the realization effort of the control schemes is given and the continuation of the research is discussed.

   Load Sharing for Parallel Inverters without Communication
By T. Skjellnes, A. Skjellnes, L. E. Norum [View]
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Abstract: By connecting several UPS inverters in parallel, the reliability or capacity of a UPS system can be improved. To make this parallelization possible, the inverters need some form of load sharing facility. This paper presents a local controller which can be used to connect inverters in parallel. The controller regulates their individual voltage responses, using voltage and frequency droops to share loads and stay synchronized. All measurements are made locally. Simulations and experimental results are presented.

   Control Aspects of a LCL Grid-Connected Green Power Inverter
By A. B. Kjaer; G. K. Andersen; C. Klumpner; F. Blaabjerg [View]
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Abstract: A full-bridge inverter for interfacing the utility grid is developed for using in a Green Power Inverter application. The inverter is feed from an arbitrary green power source (fuel cell, photovoltaic, small wind turbine, etc.) through a rectifier into the dc-link. In order to maintain a sinusoidal grid current with low harmonic distortion and a high power factor, the inverter is controlled to emulate a negative resistance towards the grid. The size of the emulated resistor is determined by the dc-link voltage controller, which tries to maintain a constant dc-link voltage. This is however not possible, while the power into the dc-link is constant and the power out of the dc-link is a second powered sinusoidal with an amplitude of two times the average power. For that reason a small ripple is present. In order to lower the transmitted high frequency current ripple, due to the operation of the inverter, a LCL filter is inserted between the grid and the inverter. It is shown that the LCL filter may be regarded as an inductor for frequencies slightly below the filters resonant frequency; hence the control of the filter becomes easy. On the other side, the z-plane poles for the filter, even with parasitic resistances, lie closer to the border of the unit-circle. Adding a resistance in parallel with the outer inductor in the LCL filter shows to improve the stability on the cost of a little higher loss. A 1 kW Green Power Inverter was designed and implemented in the laboratory. The result shows that the LCL filter is stable when the damping-resistor is added. The total harmonic current distortion was measured below 4,0 the power factor is better than 0,99 for an input power above 300W.

   Open Loop Control of the Current-Source Active Power Filter
By M. Salo; H. Tuusa [View]
[Download]

Abstract: This paper present a new control system for a current source active power filter. The harmonic current compensation is realized using only a feedforward control of the load currents. The LC filter resonance of the converter is damped in an open loop manner using the dynamic equations of the supply filter. The benefits of the proposed control system are that it is simple and straightforward, the number of measurements and sensors can be minimized and also that the LC filter size can be optimized according to proper harmonic distortion level of the supply currents without care about the stability issues of the closed-loop system. This usually leds to reduction of teh filter size. The control system also includes the calculation delay compensation of teh digital control system. The simulation results show effective current harmonic compensating performance of the ninlinear loads.

   Suggested Stabilizing Loops for Prevention of Core Saturation Instability in HVDC Systems
By I. Norheim; T. Undeland [View]
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Abstract: Different stabilizing loops have been inserted into a modified CIGRE HVDC benchmark model which is more vulnerable for core saturation instability than the original CIGRE HVDC benchmark model. By using the simulation program PSCAD/EMTDC version 3 it is demonstrated that these may prevent development of core saturation instability. Of special interest it is shown how FFT functions may improve stabilizing loops.

NORpie 2002 - Topic 03: CONTROL OF CONVERTERS
You are here: EPE Documents > 05 - EPE Supported Conference Proceedings > NORpie - Proceedings > NORpie 2002 > NORpie 2002 - Topic 03: CONTROL OF CONVERTERS
	[return to parent folder]

	Comparative Evaluation of Control Techniques for a Three-Phase Three-Switch Buck-Type AC-to-DC PWM Converter System By T. Nussbaumer; J. W. Kolar	[View] [Download]
Abstract: In this paper the cascade output voltage control of a unidirectional three-phase three-switch buck-type unity power factor PWM rectifier is discussed. Two control structures, i.e. an inner DC side inductor current control and an inner AC side input current and input filter capacitor voltage control are discussed. Guidelines for the control design which is nbased on a dynamically equivalent DC-to-DC converter model are given. Small signal transfer functions are derived by state space averaging. Finally, a comparative evaluation of the dynamics and of the realization effort of the control schemes is given and the continuation of the research is discussed.

	Load Sharing for Parallel Inverters without Communication By T. Skjellnes, A. Skjellnes, L. E. Norum	[View] [Download]
Abstract: By connecting several UPS inverters in parallel, the reliability or capacity of a UPS system can be improved. To make this parallelization possible, the inverters need some form of load sharing facility. This paper presents a local controller which can be used to connect inverters in parallel. The controller regulates their individual voltage responses, using voltage and frequency droops to share loads and stay synchronized. All measurements are made locally. Simulations and experimental results are presented.

	Control Aspects of a LCL Grid-Connected Green Power Inverter By A. B. Kjaer; G. K. Andersen; C. Klumpner; F. Blaabjerg	[View] [Download]
Abstract: A full-bridge inverter for interfacing the utility grid is developed for using in a Green Power Inverter application. The inverter is feed from an arbitrary green power source (fuel cell, photovoltaic, small wind turbine, etc.) through a rectifier into the dc-link. In order to maintain a sinusoidal grid current with low harmonic distortion and a high power factor, the inverter is controlled to emulate a negative resistance towards the grid. The size of the emulated resistor is determined by the dc-link voltage controller, which tries to maintain a constant dc-link voltage. This is however not possible, while the power into the dc-link is constant and the power out of the dc-link is a second powered sinusoidal with an amplitude of two times the average power. For that reason a small ripple is present. In order to lower the transmitted high frequency current ripple, due to the operation of the inverter, a LCL filter is inserted between the grid and the inverter. It is shown that the LCL filter may be regarded as an inductor for frequencies slightly below the filters resonant frequency; hence the control of the filter becomes easy. On the other side, the z-plane poles for the filter, even with parasitic resistances, lie closer to the border of the unit-circle. Adding a resistance in parallel with the outer inductor in the LCL filter shows to improve the stability on the cost of a little higher loss. A 1 kW Green Power Inverter was designed and implemented in the laboratory. The result shows that the LCL filter is stable when the damping-resistor is added. The total harmonic current distortion was measured below 4,0 the power factor is better than 0,99 for an input power above 300W.

	Open Loop Control of the Current-Source Active Power Filter By M. Salo; H. Tuusa	[View] [Download]
Abstract: This paper present a new control system for a current source active power filter. The harmonic current compensation is realized using only a feedforward control of the load currents. The LC filter resonance of the converter is damped in an open loop manner using the dynamic equations of the supply filter. The benefits of the proposed control system are that it is simple and straightforward, the number of measurements and sensors can be minimized and also that the LC filter size can be optimized according to proper harmonic distortion level of the supply currents without care about the stability issues of the closed-loop system. This usually leds to reduction of teh filter size. The control system also includes the calculation delay compensation of teh digital control system. The simulation results show effective current harmonic compensating performance of the ninlinear loads.

	Suggested Stabilizing Loops for Prevention of Core Saturation Instability in HVDC Systems By I. Norheim; T. Undeland	[View] [Download]
Abstract: Different stabilizing loops have been inserted into a modified CIGRE HVDC benchmark model which is more vulnerable for core saturation instability than the original CIGRE HVDC benchmark model. By using the simulation program PSCAD/EMTDC version 3 it is demonstrated that these may prevent development of core saturation instability. Of special interest it is shown how FFT functions may improve stabilizing loops.