Sami Pettersson

Practical Design and Implementation Procedure of an Interleaved Boost Converter Using SiC Diodes for PV Applications

The implementation of an interleaved boost converter (IBC) using SiC diodes for photovoltaic (PV) applications is presented in this paper. The converter consists of two switching cells sharing the PV panel output current. Their switching patterns are synchronized with 180° phase shift. Each switching cell has a SiC Schottky diode and a CoolMOS switching device. The SiC diodes provide zero reverse-recovery current ideally, which reduces the commutation losses of the switches. Such an advantage from the SiC diodes enables higher efficiency and higher power density of the converter system by reducing the requirement of the cooling system. This paper presents also an optimization study of the size and efficiency of the IBC. Based on 1) the steady-state characteristic of the topology; 2) the static and dynamic characteristics of the switching cells; 3) the loss model of the magnetic components; and 4) the cooling system design, the paper provides a set of design criteria, procedures, and experimental results for a 2.5 kW IBC prototype using SiC diodes.

Maximum power point searching method for partial shaded PV strings

This paper presents two methods aimed to search the global maximum power point (MPP) in conditions of multiple local MPPs. These conditions arise in cases where different photovoltaic (PV) panels in a string may be subject to different conditions of irradiance and/or temperature. The methods are referred as voltage-based global maximum power point searching (G-MPPS) method and current-based G-MPPS method. The former uses the voltage as the variable of control, in the sense that a voltage reference is generated, while the latter generates a current reference. These methods are based on the geometrical characteristics of the iv-characteristic curve. The idea behind the methods is to enlarge the increments of the voltage (or current), by using a set of auxiliary curves. These last are defined as the rectangular hyperboles defining subspaces of constant power. The result is a considerably faster searching process compared to the conventional sweep method. Simulations and experimental results are presented to assess the performance of the proposed method.

Control of single-phase inverter connected to the grid through an LCL filter

This paper presents a method to control a single-phase grid-connected LCL filter based power converter. The controller design is made in two steps, first, an active damping injection (ADI) mechanism is introduced to damp the resonance of the LCL filter and thus guarantee stability, and second, a harmonics compensation mechanism (HCM) is included, to cope with the harmonic distortion present in the grid voltage. Implementation of the ADI mechanism requires the design of a reduced-order observer (R-OBS), to estimate the non available states based on the measurable output. Simulation and experimental results are presented to assess the performance of the proposed controller.

Phase-locked loop for grid synchronization under unbalanced operation and harmonic distortion

This paper presents a synchronization algorithm aimed to provide an estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of an unbalanced and distorted three-phase reference signal. The proposed scheme does not require transformation of variables into the synchronous frame coordinates as conventional phase-locked loop (PLL) schemes. Therefore, it is not based on the phase angle detection. Instead, the angular frequency is detected and used for synchronization purposes. The design of the proposed scheme, referred as UH-PLL, is based on a complete description of a three-phase signal which involves both positive and negative sequences in stationary coordinates of the fundamental and harmonic components. Moreover, it includes an explicit harmonic compensation mechanism to alleviate the effect of harmonic distortion. Therefore, the proposed UH-PLL is intended to perform properly under severe unbalanced conditions, subject to distortion caused by low harmonics, and is robust against angular frequency variations, and sags and swells in the three-phase reference signal.

PV current sensorless MPPT for a single-phase PV inverter

This paper presents a maximum power point tracker (MPPT) for single-stage single-phase inverters. The proposed MPPT provides a solution to the issue of tracking maximum power point (MPP) of a photovoltaic (PV) module without measuring the PV current. This is of practical interest as the method permits the elimination of the PV current sensor, thus making a more economically attractive implementation. The design of the proposed MPPT is based on the direct-gradient descent method [1], and thus, it is referred as DG-MPPT-iless. It is aimed to have a rapid response on situations of rapid changes in irradiation and temperature. In the DG-MPPT-iless the information originally provided by the PV current sensor is reconstructed by estimators incorporated in the controller. Simulations and experiments have been carried out to test the dynamical performance of the proposed method.

Cascade three-phase PLL for unbalance and harmonic distortion operation (CSRF-PLL)

This paper presents a phase-locked loop (PLL) method aimed to provide an estimation of the phase angle and the amplitude of the fundamental positive sequence component of a three-phase reference signal subject to severe unbalance and high harmonic distortion. In addition the proposed scheme provides the estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of the reference signal. The proposed method is referred as CSRF-PLL and consists of a cascade interconnection of two recently reported PLL schemes, namely, UH-PLL reported in [1] and the conventional SRF-PLL reported in [2]. In the CSRF-PLL, the UH-PLL is used as a pre-filter to extract the positive-sequence component of the reference signal which is fed to the SRF-PLL. Recall that the proper operation of the SRF-PLL can only be guaranteed if its input is a pure sinusoidal and balanced signal, which in this case is represented by the fundamental positive sequence delivered by the UH-PLL. The SRF-PLL is used, in its turn, to estimate the fundamental frequency which is necessary for the proper operation of the UH-PLL. Simulations are provided to evaluate the performance of the proposed solution. Experimental evidence will be provided in the final version of the paper.

Multi-sampling maximum power point tracker (MS-MPPT) to compensate irradiation and temperature changes

This paper presents a method to track the maximum power point in a photovoltaic (PV) panel. The proposed method belong to the perturb and observe (P&O) based methods as the method intensionally introduce disturbances in the PV voltage to induce changes on the PV power. In this way, the slope of the pv-characteristic curve can be detected, which reveals the position of the maximum power point (MPP). The disturbance applied is usually a constant increment with varying direction, which is applied to the previous PV voltage reference (or actual PV voltage) to update the next PV voltage reference. Unfortunately, changes on irradiation and temperature may also cause changes in the PV power, which mix with the changes due to the intentional increment. This combination of effects confuses the detection of the slope of the pv-characteristic curve, which may entail a wrong decision of the next increment direction. Thus, conventional MPPT methods fail to track the MPP as they are not able to distinguish the effect of the intentional increment with respect to the effect due to irradiation and temperature changes. The proposed scheme uses a larger number of samplings and a combination of increments directions before taking a decision on the next direction of the increment. This allows to separate the effects caused by irradiation (or temperature) changes from the effects of the intentional increment. The proposed method can thus provide effective MPP tracking despite of changing conditions on irradiation and temperature. Simulation and experimental results of the proposed scheme are provided and compared to those of the conventional P&O under irradiation changes.

A combined method for anti-islanding in PV inverters

This paper presents a combined method to detect islanding conditions. The method combines two approaches in a two stage process, and is based on the injection of reactive current as a disturbance to detect islanding. In the first stage, the method tests the voltage rate of change and the frequency rate of change. Once an islanding situation is presumed in the first stage, then in the second stage, a positive feedback scheme is enabled to confirm the islanding condition. This combined detection method is more effective and precise, avoids false tripping out by reducing the sensitivity to other perturbations, and avoids instabilities as the positive feedback is not connected all time. Simulations are presented to assess the performance of the proposed combined detection method.

Current control of a three-phase inverter grid connected through an LCL filter

This paper presents a current controller for a transformerless three-phase two-level inverter grid connected by means of an LCL output filter. The proposed controller indirectly controls the grid-side current by controlling the inverter-side current. In this fairly simple controller, the damping injection is done by a simple proportional action of the measured inverter-side current, where the main issue is the design of the inverter-side current reference. It is shown that the inverter-side current has to be as distorted as necessary to guarantee a grid-side current of a pure balanced sinusoidal shape. The proposed controller considers positive and negative sequences of signals, and includes a harmonic compensation mechanism. Therefore, it is aimed to properly perform under unbalance and harmonic distortion. Simulations are presented to assess the performance of the proposed scheme.

Discrete phase-locked loop for three-phase systems

This paper presents a discrete phase-locked loop (PLL) method aimed to provide an estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of an unbalanced and distorted three-phase reference signal. The design of the proposed scheme is based on a discrete model of the generator of a three-phase signal. This model involves both positive and negative sequences of the fundamental component as well as low harmonic components. It is shown that the proposed method provides a more accurate response than discretized continuous time based PLL methods, especially in cases of digital implementation with low sampling frequency. The proposed method does not require transformation of variables into the synchronous frame coordinates as in most PLL schemes. It includes an explicit harmonic compensation mechanism to reduce the effect of harmonic distortion, and is robust against angular frequency variations, as well as sags and swells in the three-phase reference signal. Numerical results are provided to evaluate the performance of the proposed scheme. Experiments are under development at this moment, and the results will be provided in the final version of the paper.