Soft-Switching Converter With HF Transformer for Grid ... - IEEE Xplore

12 downloads 659 Views 2MB Size Report
Grid-Connected Photovoltaic Systems. Mario Cacciato, Member, IEEE, Alfio Consoli, Fellow, IEEE, Rosario Attanasio, and Francesco Gennaro. Abstract—In this ...
1678

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 5, MAY 2010

Soft-Switching Converter With HF Transformer for Grid-Connected Photovoltaic Systems Mario Cacciato, Member, IEEE, Alfio Consoli, Fellow, IEEE, Rosario Attanasio, and Francesco Gennaro

Abstract—In this paper, the design, realization, and performance evaluation of a single-phase 3-kW dc/ac power converter, using an active-bridge dc/dc converter and a full-bridge dc/ac, are introduced, presenting a novel solution on the industrial scenario for the considered application. Control algorithms, including the maximum power point tracking, paralleling to the grid, and converter switching signals, are digitally implemented on a standard microcontroller. Index Terms—Distributed generation systems, phase-shift modulation, photovoltaic (PV) power systems, synchronous phase-locked loop (PLL).

I. I NTRODUCTION

O

VER THE last few years, the interest in photovoltaic (PV) applications has grown exponentially. By the end of 2006, the total installed capacity of PV systems around the world had reached more than 6500-MWp power. Compared to the 1200 MWp globally installed at the end of 2000, a growth at an average annual rate of more than 35% has been seen. As PV systems need an interface based on power electronic converters to be connected with the grid or single load, the PV market has become appealing for many power electronics manufacturers [1]. In fact, a PV generator exhibits nonlinear voltage–current characteristics, and its maximum power point varies with solar radiation and temperature. In order to suitably connect the PV generator to the grid, single or multistage inverters are used, addressing many specifications as high efficiency and large input voltage range. Recent studies on the PV technical and economic targets report that, in order to meet the goal of reducing the energy cost for PV systems to $0.06/kWh by 2020, the PV inverter prices will need to decline to $0.25–0.30/Wp by 2020 [2]. Improvements in design, technology, and manufacturing of PV inverters are needed to achieve price and performance targets. This paper aims to outline the development and evaluation of a converter architecture which is new for PV applications with the aim of achieving significant reduction of production costs and high efficiency. Other peculiar characteristics of the proposed converter are integration level, galvanic isolation, and wide input voltage range.

Manuscript received January 13, 2009; revised September 2, 2009. First published September 22, 2009; current version published April 14, 2010. M. Cacciato and A. Consoli are with the Department of Electrical, Electronic and System Engineering, University of Catania, 95125 Catania, Italy (e-mail: [email protected]; [email protected]). R. Attanasio and F. Gennaro are with STMicroelectronics, 95100 Catania, Italy (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2032201

A prototype has been realized, and a fully digital control algorithm, including power management for grid-connected operations and maximum power point tracking (MPPT) algorithm, is implemented on a dedicated control board, equipped with last-generation 32-b microprocessor. II. S TATE OF THE A RT OF C OMMERCIAL PV C ONVERTERS In addition to a PV generator, a typical PV grid-connected system installation for residential and small commercial applications includes a dc/ac converter, and a battery pack in the case of stand-alone operation is also required. Off-theshelf dc/ac converters, in the power range of 1–5 kW, can be classified according to the following factors: the number of power processing stages, the use of galvanic isolation either with a high-frequency (HF) or low-frequency (LF) transformer, and the choice of decoupling stage placement between the inverter and energy source [3], [4]. During the last two decades, the topology choice of the inverter manufacturer has been based on a single-stage fullbridge (FB) inverter, connected to a centralized PV generator characterized by series and parallel connection of several PV modules. Although this is one of the simplest ways to interface a PV array to the grid, problems caused by partial shading and possible mismatch between module and cell characteristics have pushed the inverter manufactures and researchers to study alternative topologies. Nowadays, string or multistring configurations of PV modules and dual-stage dc/ac topologies are known as an efficient solution for both power conversion and energy management. In fact, module operations are optimized under different conditions, ensuring maximum energy yield. As a consequence, system efficiency has been improved from 85%–90% of earlier products to nearly 95% of the most recent ones. Presently, some companies claim 98% efficiency for their inverters, obtained with transformerless topologies. Moreover, not only are transformers responsible for 3%–5% of total losses but they also negatively account for weight, cost, and component number. However, by providing the galvanic isolation required by some countries’ standards, HF transformers are often used in modern designs, being a good compromise in terms of cost, efficiency, and safety constraints. System modularity is another desired feature, characterizing new inverters. As installation data show that 2–3-kW inverters dominate the market, new modules are generally designed above 2-kW output power. Recently, some companies have also introduced a “master–slave” concept between modules in order to optimize system efficiency even under low-radiation-level operation.

0278-0046/$26.00 © 2010 IEEE

CACCIATO et al.: SOFT-SWITCHING CONVERTER WITH HF TRANSFORMER FOR GRID-CONNECTED PV SYSTEMS

Fig. 1.

1679

Converter scheme.

It has to be considered that proper integration of the converter system in the specific environment of the selected application can only be ensured by control. Control actions must allow interface and communication of the PV converter with the power grid and the user. They also must comply with modularity, reliability, and compatibility demands. Therefore, the control scheme of an efficient PV converter has to include an MPPT algorithm that is responsible for setting the PV field bias point in order to extract the maximum energy in all irradiation conditions; a grid synchronization control (GSC) algorithm that is responsible for the initialization of the system, grid angle estimation, and anti-islanding monitoring; and a power management control (PMC) algorithm that is responsible for the current injection into the grid. While performing all such tasks, the converter system proposed in this paper will introduce some new aspects that will be detailed in the following. DSP microcontrollers are widely used to perform all control actions, owing to their good performance with complex mathematical operations and multiple pulsewidth modulation (PWM) outputs for drive signal generation. Over the last few years, many different inverter topologies have been designed and their performance investigated for grid connection in the power range of 1–5 kW. Among non-isolated configurations, the cascade connection of a buck, boost, or buck–boost converter and an FB or half-bridge (HB) inverter seems to be very common, while the cascade connection of a current source FB dc/dc converter and an FB or HB inverter is often used when galvanic isolation and high conversion ratios are required [5]. In both cases, decoupling capacitors for LF voltage ripple can be placed either on the low- or highvoltage side. HF link topologies are also suitable for this application [6] but are not largely employed due to control issues and considerable component count. Moreover, decoupling capacitors can only be placed on the low-voltage side. Another class of converters, whose performance evaluation has been recently started by manufacturers, is represented by zero-voltage-transition phase-shift converters. Among those, the FB phase-shift converter is already used in telecom and server applications, where high power density and high efficiency are mandatory. Active bridges, first introduced in [7], also exploit the phase-shift concept in order to achieve zero-voltage switching (ZVS) for the power devices [8]–[10].

III. P ROPOSED C ONVERTER The scheme of the proposed converter is shown in Fig. 1. It is composed of an input bridge (M 1–M 4) connected to an active bridge through an HF transformer, and an FB inverter used to generate a controlled 230-Vrms 50-Hz sinusoidal voltage. The active bridge, connected to the secondary of the transformer, is used to achieve ZVS operation for both input and output devices of the dc/dc converter (Fig. 2). Moreover, the current stress on the secondary-side switches is reduced. The transformer provides galvanic isolation between the PV array and the ac output voltage, improving the overall safety of the system. The leakage inductance of the transformer, which is typically considered an unintended parasitic component with a negative impact on the power converter, is used in such a topology as a power transfer element, thus eliminating the device overvoltage and the need of snubber circuits. Proper phase-shift control between the input bridge legs and the output bridge legs allows one to shape the transformer current, thus achieving ZVS for all the power devices and voltage step-up from a minimum input of 150 V to a regulated 450-V dc. The operating principle of the converter is based on the phase shift between the secondary leg (devices M 6 and M 7) and the first leg of the input bridge (devices M 1 and M 4). The input and rectifier bridges generate two square waveforms, respectively, across the primary and secondary windings of the transformer, shifted by the angle δ. The effect of the primary voltage V trasfprim and secondary voltage reflected to the primary V  trasfsec is to shape the inductance current according to the ratio (V trasfprim /V  trasfsec ) [11]. Due to symmetry during the two halves of the switching period, the current expression can be written as   Vbus Vdc + θ n   Vbus π 2 Vbus n ·δ+ Vdc − n , 0≤θ≤δ − 2ωs Llk   1 Vbus iLlk (θ) = Vdc − (θ − δ) ωs Llk n   2Vdc ·δ+ Vbus n − Vdc π , δ ≤θ≤π + 2ωs Llk

iLlk (θ) =

1 ωs Llk

(1)

(2)

1680

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 5, MAY 2010

Fig. 2. DC/DC converter main waveforms.

and the power transferred to the output is Pav

  2 Vdc δ = dδ 1 − ωs Llk π

(3)

where Llk is the leakage inductance, Vdc is the input voltage, Vbus is the dc-link voltage, ωs = 2π · fs , δ is the phase-shift angle, and d = ((Vbus /n)/Vdc ). In order to achieve ZVS for

all the power devices of the dc/dc converter, the following equations must be satisfied:   Vbus π 2 Vbus n · δ + Vdc − n