Toshihisa Shimizu

Topologies of single-phase inverters for small distributed power generators: an overview

This paper presents an overview of single-phase inverters developed for small distributed power generators. The functions of inverters in distributed power generation (DG) systems include dc-ac conversion, output power quality assurance, various protection mechanisms, and system controls. Unique requirements for small distributed power generation systems include low cost, high efficiency and tolerance for an extremely wide range of input voltage variations. These requirements have driven the inverter development toward simpler topologies and structures, lower component counts, and tighter modular design. Both single-stage and multiple-stage inverters have been developed for power conversion in DG systems. Single-stage inverters offer simple structure and low cost, but suffer from a limited range of input voltage variations and are often characterized by compromised system performance. On the other hand, multiple-stage inverters accept a wide range of input voltage variations, but suffer from high cost, complicated structure and low efficiency. Various circuit topologies are presented, compared, and evaluated against the requirements of power decoupling and dual-grounding, the capabilities for grid-connected or/and stand-alone operations, and specific DG applications in this paper, along with the identification of recent development trends of single-phase inverters for distributed power generators.

Flyback-Type Single-Phase Utility Interactive Inverter With Power Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System

In recent years, interest in natural energy has grown in response to increased concern for the environment. Many kinds of inverter circuits and their control schemes for photovoltaic (PV) power generation systems have been studied. A conventional system employs a PV array in which many PV modules are connected in series to obtain sufficient dc input voltage for generating ac utility line voltage from an inverter circuit. However, the total power generated from the PV array is sometimes decreased remarkably when only a few modules are partially covered by shadows, thereby decreasing inherent current generation, and preventing the generation current from attaining its maximum value on the array. To overcome this drawback, an ac module strategy has been proposed. In this system, a low-power dc-ac utility interactive inverter is individually mounted on each PV module and operates so as to generate the maximum power from its corresponding PV module. Especially in the case of a single-phase utility interactive inverter, an electrolytic capacitor of large capacitance has been connected on the dc input bus in order to decouple the power pulsation caused by single-phase power generation to the utility line. However, especially during the summer season, the ac module inverters have to operate under a very high atmospheric temperature, and hence the lifetime of the inverter is shortened, because the electrolytic capacitor has a drastically shortened life when used in a high-temperature environment. Of course, we may be able to use film capacitors instead of the electrolytic capacitors if we can pay for the extreme large volume of the inverter. However, this is not a realistic solution for ac module systems. This paper proposes a novel flyback-type utility interactive inverter circuit topology suitable for ac module systems when its lifetime under high atmospheric temperature is taken into account. A most distinctive feature of the proposed system is that the decoupling of power pulsation is executed by an additional circuit that enables employment of film capacitors with small capacitance not only for the dc input line but also for the decoupling circuit, and hence the additional circuit is expected to extend the lifetime of the inverter. The proposed inverter circuit also enables realization of small volume, lightweight, and stable ac current injection into the utility line. A control method suitable for the proposed inverter is also proposed. The effectiveness of the proposed inverter is verified thorough P-SIM simulation and experiments on a 100-W prototype

Generation control circuit for photovoltaic modules

Photovoltaic modules must generally be connected in series in order to produce the voltage required to efficiently drive an inverter. However, if even a very small part of photovoltaic module (PV module) is prevented from receiving light, the generation power of the PV module is decreased disproportionately. This greater than expected decrease occurs because PV modules which do not receive adequate light cannot operate on the normal operating point, but rather operate as loads. As a result, the total power from the PV modules is decreased if even only a small part of the PV modules are shaded. In the present paper, a novel circuit, referred to as the generation control circuit (GCC), which enables maximum power to be obtained from all of the PV modules even if some of the modules are prevented from receiving light. The proposed circuit enables the individual PV modules to operate effectively at the maximum power point tracking, irrespective of the series connected PV module system. In addition, the total generated power is shown experimentally to increase for the experimental set-up used in the present study.

A novel high-performance utility-interactive photovoltaic inverter system

This paper presents a novel photovoltaic inverter that cannot only synchronize a sinusoidal AC output current with a utility line voltage, but also control the power generation of each photovoltaic module in an array. The proposed inverter system is composed of a half-bridge inverter at the utility interface and a novel generation control circuit which compensates for reductions in the output power of the system that are attributable to variations in the generation conditions of respective photovoltaic modules. The generation control circuit allows each photovoltaic module to operate independently at peak capacity, simply by detecting the output power of the system. Furthermore, the generation control circuit attenuates low-frequency ripple voltage, which is caused by the half-bridge inverter, across the photovoltaic modules. Consequently, the output power of the system is increased due to the increase in average power generated by the photovoltaic modules. The effectiveness of the proposed inverter system is confirmed experimentally and by means of simulation.

A flyback-type single phase utility interactive inverter with low-frequency ripple current reduction on the DC input for an AC photovoltaic module system

In recent years, interest in natural energy has grown because of increased environmental concerns. Many kinds of inverter circuits and their control schemes for photovoltaic (PV) power generation systems have been studied. In a conventional system, the PV array in which many PV modules are connected in series is used to obtain sufficient DC-bus voltage for generating an AC utility line voltage from an inverter circuit. However, the total power generation of the PV array is sometimes decreased remarkably when a few modules are partially covered by shadows, thereby decreasing its inherent current generation, and preventing the generation current attaining its maximum value on the array. To overcome this drawback, an AC module strategy has been proposed. In this system, a small power DC-AC utility interactive inverter is mounted on each PV module individually. This inverter operates so as to generate the maximum power from its corresponding PV module. This paper proposes a novel flyback-type utility interactive inverter circuit suitable for AC module systems. The features of the proposed system are that it: (1) is small in volume and lightweight; (2) allows stable AC current injection into the utility line; (3) enables the stable parallel operation without AC current sharing control; and (4) enables the capacitance of the DC capacitor to be small. The effectiveness of the proposed system is clarified through simulation and experiments.

A Single-Phase Grid-Connected Inverter with a Power Decoupling Function

This paper presents a single-phase grid connected inverter with a power decoupling circuit. In the single-phase grid connected inverter, it is well known that a power pulsation with twice the grid frequency is contained in the input power. In a conventional inverter, electrolytic capacitors with large capacitance have been used in order to smooth the DC voltage. However, lifetime of those capacitors is shortened by the power pulsation with twice grid frequency. The authors have been studied a active power decoupling (APD) method that reduce the pulsating power on the input DC bus line, this enables to transfer the ripple energy appeared on the input DC capacitors into the energy in a small film capacitor on the additional circuit. Hence, extension of the lifetime of the inverter can be expected because the small film capacitor substitutes for the large electrolytic capacitors. Effectiveness of the proposed method is confirmed through simulation and experimental results.

A Practical Iron Loss Calculation for AC Filter Inductors Used in PWM Inverters

A novel iron loss calculation method is proposed based on an iron loss map for an AC filter inductor used in a pulsewidth-modulation (PWM) inverter. The iron loss map, previously reported by the authors, is created by measuring the dynamic minor loop on the B-H plane, and this is used for the calculation of inductor iron loss in the dc chopper circuit. However, in the case of an AC filter inductor used in a PWM inverter, the iron loss map cannot be directory applied to the iron loss calculation. In this paper, an iron loss calculation for an ac filter inductor is presented by expanding the loss map method. We describe the principle of expansion and the practical procedure for the calculation, which utilizes the loss map and a conventional circuit simulator. Iron loss for the ac filter inductor under some typical modulation methods of the PWM inverter are calculated and discussed with regard to the relation between the modulation methods and iron loss. The calculated results are verified using experimental results from a 500-W inverter setup. The inductor loss and conversion efficiency of the PWM inverter are measured for each modulation method.

A novel iron loss calculation method on power converters based on dynamic minor loop

The authors have reported a novel iron loss calculation method based on a loss-map of the magnetic materials. A distinctive feature of this method is that the iron loss on the inductors can easily be calculated in many kinds of converters dynamically. The dynamic measuring method of the dynamic minor-magnetic loop by using the buck-chopper circuit is presented. Next, some typical characteristics of the loss map derived from the dynamic minor loop measurement is discussed. Also, a novel iron loss calculation method of the ac filter inductor on the PWM inverter by using the loss-map method is described. Some experimental results shows that the iron loss obtained from this calculation method coincides well with the actual one

High power DC/DC converter using extreme close-coupled inductors aimed for electric vehicles

The DC-DC converter using coupled inductors is known as one of the topologies that achieve higher power density. This paper focused on this topology especially using extreme close-coupled inductors (ECCI). Following the elucidation of design guidelines for the coupled inductor theoretically, the superiority of the interleaved converter using ECCI in terms of reducing the size and losses in the magnetic components is demonstrated clearly through comparison with an interleaved converter using a set of looser-coupled inductors and a conventional single inductor converter. The effect of increasing a magnetic coupling factor of a set of coupled inductors from 0.980 to 0.999 is investigated by comparing it with other inductors of interleaved converters. The high power prototype converter with the coupling factor of 0.999 provides excellent performance with a time rating power density of 25.6 kW/liter.

Control Methods and Compensation Characteristics of a Series Active Filter for a Neutral Conductor

Due to the advance of information technologies, a large number of electronic products such as personal computers have been connected to power distribution systems in commercial buildings. Hence, voltage distortion on utility outlets and excessive neutral current on distribution lines have arisen and lead to a number of serious problems in the distribution system. Two control methods and the related compensation characteristics of a series active filter connected to the neutral conductor are presented in this paper. The distinct functions of the proposed active filter are the mitigation of the third-harmonic voltage and the neutral current in a three-phase four-wire distribution system in a building. The required power of the proposed active filter is less than 10% of that of the harmonic-producing loads. A control method of the dc capacitor voltage on the active filter circuit is also described. It is clarified through experiments that one of the two functions of the active filter can be realized selectively and the dc capacitor voltage of the active filter can be regulated to a desired value