T. Ise

Fundamental characteristics of DC microgrid for residential houses with cogeneration system in each house

DC microgrid is a novel power system using dc distribution in order to provide a super high quality power. This system is suitable for dc output type distributed generations and energy storages. The dc power can be converted to the proper ac or dc voltages for the consumers by converters placed near loads, and these converters do not require transformers. This distributed scheme of load side converters also contributes to provide the high quality power supplying. In this research, we assumed a dc microgrid applied for residential houses. All houses have a cogeneration system such as gas engine or fuel cell. The outputs of those distributed generations are connected to the dc distribution line, and the power from the generations can be shared among the houses. The hot water from distributed generations are used in each house. We constructed a small scale experimental in our laboratory, and examined the fundamental characteristics of the dc microgrid by the experimental system when it was connected to the bulk power system. Experimental results demonstrated that the system could supply high quality power to the loads stably against sudden load variations, voltage sags of the bulk power system and short circuits of the load.

Configuration and control of a DC microgrid for residential houses

DC microgrid is suitable for dc output type distributed generations and energy storages. In this paper, dc microgrid for residential houses is described. Electrical power through dc distribution line (± 170 V) can be converted into some proper ac or dc voltages by converters placed near loads. Each house has a cogeneration system such as gas engine or fuel cell. The outputs of those distributed generations are connected to dc distribution line, and the power from generations can be shared among residences. We constructed a small scale experimental system in our laboratory, and examined the fundamental characteristics of dc microgrid. The experimental results demonstrated that high quality power was supplied even when voltage sag occurred in the utility grid.

A configuration and control method of cascade H-bridge STATCOM

This paper shows a configuration and control method of cascade H-bridge STATCOM in three phase power system. The phase cluster of STATCOM consists of three H-bridge cells connected in series. The feature is that each H-bridge cell generates a different output voltage Vc, 2Vc, 4Vc. By this configuration, each phase cluster can generate 17-level output voltage. But, as well as the other cascade H-bridge multilevel converter, every H-bridge cell has isolated DC capacitor, and balancing problem of capacitor voltages exists. In this paper, a technique to balance the capacitor voltage of the cascade H-bridge STATCOM is proposed. Simulation results show the validity of proposed method.

Power modulation control of a three-phase to single-phase matrix converter for a gas engine cogeneration system

Application of a three-phase to single-phase matrix converter for a 1 kW household gas engine cogeneration system is proposed. The matrix converter connects a three-phase permanent magnet synchronous generator of the cogeneration system to single-phase utility. Application of the matrix converter has the following advantages: (a) the generator efficiency will be improved because power factor control of the generator with the matrix converter can reduce reactive current which increases copper losses in the windings and harmonic current which causes harmonic torques of the generator, (b) The converter efficiency will be also improved because the conducting losses of switching devices can be reduced by employing reverse-blocking (RB) devices such as RB-IGBTs. However, since the matrix converter has no energy storage component inside itself, power pulsation due to single-phase power appears directly in the three-phase side. In order to treat this power pulsation, we have proposed a novel method that realizes modulation of the instantaneous three- phase power of the generator synchronized with the single- phase power, and the power pulsation is absorbed with inertia of the rotor of the generator and a gas engine, which is large enough to regard the rotor speed as constant. The proposed method is demonstrated through numerical simulation and primary experiments using a motor/generator set and an IGBT matrix converter.

A study on a start-up method during a blackout of a doubly-fed induction generator applied to gas engine cogeneration system

A doubly-fed induction generator (DFIG), which is one of the variable speed generator, applied to a gas engine cogeneration system has been investigated. To operate during a blackout as an emergency power supply is one of important roles for the gas engine cogeneration system. However, the DFIG requires the initial excitation for startup during a blackout because DFIG has no excitation source. In this paper, the excitation method to generate a rated voltage at the primary side during a blackout, which is called “blackout start” has been proposed. In addition, a stand-alone operation following the blackout start has been investigated using both computer simulation and experiments with a gas engine simulator. Moreover, the proposed blackout start and stand-alone operation method has been verified through experiments using a real gas engine.

Enhanced stand-alone operating characteristics of an engine generator interconnected through the inverter using virtual synchronous generator control

In power systems, the engine generator plays an important role such as providing power at the time of power failure and at the stand-alone operating system. In using the engine generator for stand-alone system, there are upper and lower limits in the engine speed variation. As a result, if the engine speed exceeds the value, the engine generator comes to unstable and cannot go on the operation. So when we place a load on engine generator, we have to take the engine speed variation limit into consideration, as a result, the amount of the load come to be limited. In this research, we attempt to reduce the deviation of engine speed during the transient by using the energy storage system (ESS) which is used in the system of inverter control. In this paper, control scheme of ESS is shown and the simulation and experimental result are also shown with comparison between simulation and experiment as a result.

Effects of suppressing frequency fluctuations by parallel operation of virtual synchronous generator in microgrids

When a greatly varying load is connected to a weak power system, stability of the system becomes a problem. In these days, because the use of renewable energy is accelerating, not only the loads but also the distributed generators can be critical factors in destabilizing the system. A virtual synchronous generator (VSG) control allows a static inverter to behave similar to a synchronous generator (SG). Using this VSG technology, we can increase the introduction ratio of renewable sources, and thus, we can combine various types of power sources. Simultaneously, if the combination of governor and rotor inertia is represented in a first-order lag element, the VSG can suppress the frequency fluctuations in microgrids. In this paper, the oscillation mechanism in parallel operations of generators is clarified, and we present the effects of suppressing the frequency fluctuations using the VSG. The results are verified by simulation using EMTP-RV and in laboratory experiments.

Transient characteristics for load changes of a doubly-fed induction generator applied to gas engine cogeneration system in stand-alone operation

Application of a doubly-fed induction generator (DFIG), which is one of variable speed generators, to a gas engine cogeneration system has been investigated. To operate during a blackout as an emergency power supply is one of important roles for the gas engine cogeneration system. However, recent lean-burn and miller-cycle gas engine, which realizes high electrical efficiency, has relatively slow response performance to load changes compared with a diesel engine. Therefore in the case of conventional constant speed synchronous generator, the amount of load input at a time is limited to 30% of the rated power. On the other hand, DFIG is expected to increase the amount of load input at a time during the stand-alone operation because DFIG generates power at constant frequency irrespective of its rotational speed. In this paper, it is demonstrated experimentally that an increase in the gas engine speed resulted in an increase in the maximum amout of load input at a time using a real gas engine. In addition, experimental results also show that disconnection of load was successfully done. These results suggest that this system will improve the performance of emergency power supply.

A soft switching PFC converter operating discrete conduction mode with reduced losses of the auxiliary circuit

In this paper, a new family of zero-current-transition (ZCT) power factor correction (PFC) converters is introduced. The proposed converter operates in discrete conduction mode (DCM) and provides soft switching for the main switch and the auxiliary switch with a small inductor and capacitor even in high power / high voltage application. The auxiliary circuit of proposed converter operates for an enough short time compared with the switching period. Furthermore, the auxiliary circuit maintains an almost constant operation time during a power line cycle. As a result, the proposed converter can reduce the losses of the auxiliary circuit. In this study, a steady-state analysis of the proposed converter is described, and features of this converter are compared to ones of zero-voltage-transition (ZVT) converter in theory. These theoretical analyses are verified by a prototype converter.