Joan Salaet

Comparison between two methods of DQ transformation for single phase converters control. Application to a 3-level boost rectifier

The paper discusses the analysis and control of single-phase AC systems by means of rotating frames. Two methods for the D-Q transformation are compared in terms of computational requirements, transient response and sensibility. One of the transformation methods is based on sine and cosine composition, while the other uses a bi-phase transformation. The comparison has been done using simulation and an experimental prototype of a three-level NPC boost rectifier.

D-Q modeling and control of a single-phase three-level boost rectifier with power factor correction and neutral-point voltage balancing

This paper deals with the analysis, design and operation of a control system for a single-phase three-level rectifier with a neutral-point-clamped (NPC) topology. Usually the desired operating conditions for this type of converter are: unity displacement factor, output DC voltage regulation and neutral point voltage balancing. A d-q reference frame has been used in this work to model the rectifier behaviour in order to exploit the results obtained in the field of three-phase converters. In this way, a space vector modulation PWM method has been used, with the possibility of using redundant switching states to achieve charge balancing of the capacitors. The time assignment of each redundant switching state is accomplished by utilizing a closed-loop control system. Validity of the modeling and control strategies are confirmed by the transient and steady state simulation and experimental results.

Exact Linearization Nonlinear Neutral-Point Voltage Control for Single-Phase Three-Level NPC Converters

This letter proposes a new control scheme for the neutral-point (NP) balance in single-phase three-level NP-clamped converters. The control is addressed considering the plant under study, a nonlinear time-variant system. A quasi-exact linearization is applied allowing the application of classic control techniques. The described method is simple, general, and suitable for buck and boost topologies, as well as for inverter or rectifier operating modes under either linear or nonlinear loading. Correct operation is verified under simulated and experimental operation.

Optimal regulator with integral action and gain-scheduling for the comprehensive control of three-level NPC VSI

A new method to design a regulator for the complete large-signal control of the three-level neutral-point-clamped (NPC) VSI is presented in this work. For the regulator calculation, a comprehensive multivariable model of the NPC-VSI converter is used. The referred model is general and can be applied to generic types of sources, filters and loads. No information about the system is lost with this model. Optimal multivariable control law based on linear quadratic regulator (LQR) has been selected for the three-level VSI control strategy. This type of controller is essentially a proportional regulator, hence proper integral action has been added to the regulator, in order to cancel steady-state errors. Also, gain-scheduling control technique allow to extend the application of the controller from small-signal to large-signal operation. Both control techniques work concurrently in the regulator. Then, any state variable can be regulated using the proposed method, either in small and large-signal operation, including state variables related to the DC-link voltages. With the model and regulator presented, DC-link compensation is also achieved by means of the controller action. Subsequently, a specific switching strategy to control DC-link neutral point voltage is not required. A carrier-based PWM has been used for the switching strategy. A controller is designed to keep DC-link neutral point voltage balanced and to regulate the output voltage. The method can be used for any application, since the multivariable structure makes possible the control of any parameter. A NPC-VSI prototype has been realized and a PC-embedded DSP board is used for the controller implementation. Simulation and experimental results confirm the validity of the proposed controller design.

Control of three-level VSIs with a LQR-based gain-scheduling technique applied to DC-link neutral voltage and power regulation

A simple and effective method to design a regulator for the complete large-signal control of the three-level neutral-point-clamped (NPC) VSI is presented. For the regulator calculation, a comprehensive multivariable model of the NPC-VSI converter is used. The control strategy is based on linear quadratic regulator (LQR) and gain-scheduling control techniques. Any state variable can be regulated using the proposed method, either in small and large-signal operation, including state variables related to DC-link voltages. With the model and regulator presented, DC-link unbalance regulation is also achieved by means of the controller action instead of the switching strategy. A controller is designed to keep DC-link neutral point voltage balanced and to regulate the power delivered to the load. Simulation results confirm the validity of the proposed controller design. Steady-state and transient operation experimental results are obtained using a NPC-VSI prototype and a PC-embedded DSP board for control implementation.

SVM based control of a single-phase half bridge boost rectifier under power factor correction and balanced operation

A vector control based single-phase half-bridge boost converter circuit operating at a fixed high switching frequency is proposed in order to obtain a unity power factor (PF) and a balanced operation of the DC-link capacitors. In most cases this structure is controlled via a hysteresis comparator and a current sensor. Moreover, to prevent any imbalance in the voltages of the two DC-link capacitors it is necessary to use an additional voltage feedback. In the present paper a PWM switching technique which takes account of the phasorial nature of several electrical magnitudes in the converter is utilized. A mathematical model of the boost rectifier in the direct and quadrature axes is described and it is shown that this system is unstable and presents two control inputs. Then, to provide stability to the converter and to obtain the desired output DC voltage with a unity PF (though it can be set to another value) and voltage balance in the DC-link capacitors two feedback loops are implemented through both inputs. Analytical results are verified through simulation of the averaged circuit model. An experimental prototype is being made which will work from 90 V peak at the input to 300 V DC at the output.

A New Strategy for Decoupling Direct and Quadrature Currents in a Rotating Frame Current Regulator. Application to a Single-Phase Three-Level Boost Rectifier

A new Direct and Quadrature current decoupling strategy is presented which uses a filtering block placed between the current regulation and the modulation stages. This decoupling system is very easy to implement and it represents a clear improvement over the conventional feedforward method in relation to the closed loop system stability, the achieved current decoupling degree and the transient response speed. Simulation and experimental results have been obtained in the case of a single-phase three-level Neutral Point Clamped ac-dc converter which validate the effectiveness of this new technique.

A new neutral-point voltage control for single-phase three-level NPC converters

This paper proposes a new control scheme for the neutral-point balance in single-phase three level neutral-point clamped converters. The control is addressed, considering the plant under study a Non-Linear Time-Variant system and applying a quasi-exact linearization to it. This way it is possible to apply classic control techniques. The method depicted is simple, general and suitable to back and boost topologies as well as to inverter or rectifier operating modes. Correct operation is verified under both simulation environment and experimental operation. Experimental operation has been verified with linear and non-linear load.

Adaptive balancing of the neutral point voltage in a single-phase three-level rectifier

Keeping charge balance among its two capacitors is a very important issue in an ac-dc three- level Neutral-Point-Clamped (NPC) converter, apart from controlling the dc-link voltage and the input power factor. To this effect a new method for guaranteeing the voltage equalization of both capacitors in the case of a three-level ac-dc single-phase converter is presented that makes use of a nonlinear control loop and combinations of redundant switching functions. A set of simulations are carried out first with continuous and after with momentary imbalanced load in order to assess the good performance of this method. In the second case an adaptive gain of the control loop is revealed to be necessary for avoiding a possible transient current inrush during the balancing process. Simulations about this topic will be presented too.

Load voltage and output current control of three-level voltage source inverters

A load voltage and output current control for the three-level neutral-point-clamped (NPC) VSI is presented in this work. In usual operation, the regulator controls the voltage applied to the load. If an overcurrent occurs, the regulator will switch to current mode controller and the current will be driven to an acceptable value. A multivariable model of the NPC-VSI is used for the regulator calculation. The controller used in this paper is based on linear quadratic regulator (LQR) and gain-scheduling control techniques. With this control approach, any state variable can be regulated, either in small and large-signal operation, including state variables related to DC-link voltages. Then, a switching strategy to control DC-link neutral point voltage is not required. The controller is designed to keep DC-link neutral point voltage balanced and to regulate the load voltage or the output current. Simulations and experimental results agree and confirm the validity of the proposed controller