Alejandro R. Oliva

Switching Control Strategy to Minimize Dual Active Bridge Converter Losses

A switching control strategy to control the power flow and minimize the total power losses of the dual active bridge converter topology is proposed in this paper. The control strategy consists of driving the bridge with the largest DC voltage to generate a three-level pulsewidth-modulated (PWM) voltage waveform. This PWM is ruled by two manipulated variables: the phase shift between the primary and secondary transformer voltages and the modulation index. These variables are calculated using an algorithm that is deduced on the basis of particular calculation and analysis of converter losses, which are also presented in this paper. An experimental prototype was implemented to validate the theoretical analysis and feasibility of the proposal. The experimental results revealed that the overall efficiency of this converter can be improved up to 10% using the control strategy instead of the conventional one.

Modulation strategy to operate the dual active bridge DC-DC converter under soft switching in the whole operating range

A new modulation strategy that allows operating the dual active bridge (DAB) dc-dc converter under soft switching in the whole operating range is proposed. This strategy is ruled by imposing a certain modulation index in one of the two bridges and a phase shift between the transformer primary and secondary voltages. Moreover, the proposed algorithm reduces the reactive power and thus reducing the converter conduction losses. An experimental prototype was implemented and some experimental results are presented to validate the theoretical analysis. The experimental results reveal that the overall efficiency of the DAB topology can be improved up to 20% by implementing the proposed modulation strategy instead of the conventional one.

Digital control of a voltage-mode synchronous buck converter

A digital control algorithm capable of separately specifying the desired output voltage and transient response for a synchronous buck converter operating in voltage mode was developed. This algorithm is based on superimposing a small control signal onto a voltage reference at each switching cycle to cancel out the perturbations. A zero steady-state error in the output voltage can be obtained with the aid of additional dynamics to allow the controller to track a load change and update the reference to a new load state. The specifications of the control algorithm are achieved by pole placement using complete state feedback. The control algorithm was implemented on a digital signal processor (DSP)-controlled synchronous buck converter.

Extending the ZVS Operating Range of Dual Active Bridge High-Power DC-DC Converters

A switching control strategy to extend the soft-switching operating range of the dual active bridge (DAB) dc-dc converter under the zero-voltage-switching (ZVS) operating mode is proposed. The converter topology consists of two active bridges linked by a high-frequency transformer. One drawback of this strategy is that soft-switching is only possible in a restricted converter operating region. A novel pulse width modulation strategy to extend the conventional soft-switching operating mode region and its analysis are presented in this paper. Experimental results are given in order to validate the theoretical analysis and practical feasibility of the proposed strategy.

A PV dispersed generator: a power quality analysis within the IEEE 519

The use of environmentally clean photovoltaic (PV) dispersed generation will become more widespread in the future due to anticipated cost reductions in PV technology. This paper summarizes the results of a power quality (PQ) study performed on a PV generator in order to estimate the effects that inverter-interfaced PV dispersed generation might have upon the quality of electric power. Different interpretations of the harmonic distortion limits set in the IEEE 519-1992 standard are performed together with a comparison with the BC Hydros harmonic current limits. This paper also includes a statistical analysis of all measurements recorded with the help of two PQ monitors, an evaluation of the results from a connection/disconnection test, and harmonic simulation results.

Measurements of neutral currents and voltages on a distribution feeder

Neutral currents flow in four-wire distribution systems due to linear load unbalances and triplen harmonics associated with mainly the magnetizing currents of (distribution) transformers. It is known that high levels of neutral currents can lead to overloaded/burned neutral conductors, common-mode noise problems, derating of transformers and/or excessive voltage distortion. The proliferation of nonlinear loads which inject harmonic currents into the distribution system has brought more attention upon the levels of neutral currents and voltages found in distribution systems. To this end, this paper evaluates measurements taken on the neutral conductors of a distribution feeder at two different sites. The analysis of the measurements focuses on the magnitudes of the triplen-harmonic currents and voltages relative to their corresponding phase RMS values in order to separate the impact on the neutral conductors of nonlinear loads and linear load unbalances.

High-Performance Control of a DC–DC Z-Source Converter Used for an Excitation Field Driver

The electromechanical interface is a synchronous machine, because its field winding permits direct management of the magnetization during speed variations. For systems with a common dc-link for the drive and excitation converters, the efficiency is increased if the excitation drive has boosting capability. It is shown that with the proposed control strategy the Z-source converter is suitable for this application, becoming a better alternative than the typically used buck converter. The Z-source converter, in combination with the proposed multiloop control law, can achieve the desired voltage reference swing and high-performance tracking. An analytical comparison between the dominant losses of the buck topology, typically used in FESS, and the Z-source converter shows that the latter has higher efficiency for this application. The parameters of the converter prototype were experimentally identified and used to implement the proposed controller. The control strategy uses the two duty cycles as manipulated variables, one to allow tracking fast changes in the reference signal and the other to adapt the system to the slow changes. The combined action on both inputs contribute to the compensation of the nonminimum phase response of the Z-converter. Experimental results show the potential of the controller for tracking typical FESS application waveforms.

Power-quality monitoring of a PV generator

It is envisaged that the use of photovoltaic (PV) dispersed generation will become more widespread in the near future due to advances in PV technology which will lead to cost reductions. In order to gain an understanding of the effects that PV dispersed generation may have upon the quality of electric power, this paper presents a power quality study performed on a PV generator. The study includes measured waveforms, trends, a statistical analysis of the measurements and the results of a connection/disconnection test.

Harmonic distortion reduction in power inverters

The output voltage of PWM power inverters shows harmonic distortion due to several causes; the main ones are the modulation algorithm, nonlinearities due to the output filter, dead times, voltage drops across the switches and modulation of the dc bus voltage. The distortion is more evident when using low dc bus voltages. As a result, motors driven by these inverters have important torque pulsations. This work proposes to reduce the distortion produced by dead times and voltage drops across the switches, using a simple algorithm that recalculates the width of each PWM pulse, preserving the ideal area. The algorithm takes advantage of the fact that the dead times are not always necessary, but only when the load current changes its sign. By simulation, the THD was reduced from 18% to 0.29% in a single-phase inverter. The proposed algorithm only needs products and sums, so it is suitable for being implemented on a DSP with low processing load. Experimental results were obtained from a non-optimized laboratory prototype, showing a reduction of the THD from 17.9% to 0.59%.

Nonlinear analog controller for a buck converter: theory and experimental results

The design of a switching power supply (buck DC-DC converter) with regulated output voltage for a wide range of current consumption is considered. The control objective is to regulate the output voltage despite the existence of current load variations with bounded time derivative. A variable-structure technique is used to design a continuous time controller. A discrete and an analog version of the same controller are compared. Laboratory experiences are performed to obtain conclusions about the real performance of the proposed controller with satisfactory results.