Published in IET Power Electronics Received on 11th December 2011 Revised on 18th June 2012 doi: 10.1049/iet-pel.2011.0470 ISSN 1755-4535 Low-frequency current ripple reduction in front-end boost converter with single-phase inverter load A.Ale Ahmad 1 A. Abrishamifar 1 S. Samadi 2 1 Electrical and Electronic Engineering, Iran University of Science and Technology Q1 2 Electrical and Electronic Engineering, Research Institute of Food Science and Technology E-mail: alahmad_ah@yahoo.com Abstract: The low-frequency current ripple that always appears at the input of the single-phase DC/AC inverters decreases the lifetime of DC voltage sources, such as fuel cells and chemical batteries. In this study, based on series and parallel feedback theory, a proportional-integral Q2 controller is designed for the front-end boost converter in two-stage power converters. This controller increases the output impedance of the boost converter, which reduces the low-frequency current ripple at the input of this two-stage converter. Since the designed controller corrupts the dynamic response of the boost converter, the DC-link voltage severely over/undershoots in step load conditions. Overcoming this issue by employing a non-linear gain in the forward path is shown. By applying this proposed technique, the output voltage over/undershoot stays in an acceptable range. Therefore both the low-frequency input current ripple and the DC-link over/undershoot problems disappear simultaneously without employing any additional equipment, especially a bulky DC capacitor. The simulation and experimental results for a 2.5 kW prototype confirm the performance of the proposed idea. 1 Introduction Today, two-stage power converters (Fig. 1) that include a boost converter and an inverter are widely used in uninterruptible power supplies and distributed generation of electric power such as fuel cells, solar cells and wind turbines [1, 2]. In this structure, the inverter delivers AC power to consumers or the AC grid and the boost converter provides a proper DC voltage for the inverter. Here, the compatibility of the input characteristics of the boost converter and the DC sources is an important issue. This compatibility affects the efficiency, lifetime and power delivery from DC sources, especially for fuel cells [3, 4] and chemical batteries [5–7]. Therefore the input characteristic of the boost converter should be designed to be compatible with the specifications of the DC sources. These sources suffer extremely from low-frequency current ripple. Moreover, this ripple causes solar cells to not work at their maximum power point tracking. The low-frequency ripple current arises from the input behaviour of the inverter, and it is small in three-phase inverters. However, the low-frequency current ripple is very large at the input of single-phase inverters and its frequency is twice the inverter output-voltage frequency. However, the ripple should not be allowed to pass through DC sources. A large-size DC capacitor at the input of the single-phase inverter can easily absorb most of this ripple before it propagates back. However, given the low-frequency contents of the current ripple, the capacitor should be bulky and expensive. To reduce the low-frequency current ripple, Pereira et al. [8], Saito and Matsui [9] and Ma et al. [10] applied an active filter that does not require a bulky DC capacitor at the input of the inverter. Despite good performance of the active filter, it requires extra switching devices, an inductor and a capacitor, which increases the size of the total converter and reduce total efficiency. In two-stage power converters, the back propagation of the current ripple can be cancelled by adding a high-speed current loop control to the existing voltage loop control of the boost converter [11]. This approach is very efficient; however, in practice, a large DC capacitor is required for energy buffering [12]. Moreover, because the bandwidth of the voltage loop is very narrow and it cannot control the over/ undershoot of the output voltage in step load condition, then the boost converter requires a bulky DC capacitor to damp the voltage over/undershoot. Under the assumption that the boost converter has large output impedance, this paper (Section 2) calculates the minimum required DC capacitor for the input of the connected single-phase (50 Hz) inverter. The large output impedance of the converter prevents the propagation of the low-frequency current ripple to the input of the converter. In Sections 3 and 4, by employing the series and parallel feedback theory, a dual-loop controller with a proportional- integral (PI) compensator is designed for the boost converter. The PI compensator is tuned to increase the output impedance of the converter at the frequency of the current ripple. As the response time of this controller is very slow, an undesirable over/undershoot of the output voltage will occur in a step load condition. To overcome this problem, we proposed adding a non-linear gain in the forward path of the voltage loop, which is discussed in IET Power Electron., pp. 1–8 1 doi: 10.1049/iet-pel.2011.0470 & The Institution of Engineering and Technology 2012 Techset Composition Ltd, Salisbury Doc: H:/Iee/PEL/Articles/Pagination/PEL20110470.3d www.ietdl.org