DC Micro Grid System - Sumitomo Electric …

1 Electric WIRE & CABLE, ENERGY132 DC Micro grid System1. IntroductionHow can we increase the amount of photovoltaic (PV)generation? From this viewpoint, we are overviewing elec-tric facilities from power plants to Electric appliances in de-mand sites. PV modules generate DC Electric power. Thepower should be converted to AC that is synchronized withcommercial grids to be transmitted and distributed to de-mand sites. To reduce energy dissipation through thetransmission, the power is sent near the demand site afterbeing raised the Electric voltage to 66 kV or higher. Thepower is transformed to 100 V and provided to residentialoutlets after multi-processed reduction in voltage at substa-tions and pole-mounted transformers. Therefore, weshould consider how we can establish efficient transmissionand distribution systems for PV generation in addition tocost, efficiency and lifetime for generation facilities, if weutilize the power source as facilities for PV generation often stayidle as well as generation facilities themselves, because theydo not yield electricity during night and poor weather.

2 Ifcontribution from solar power were much smaller thantransfer capability, existing facilities could take care of understand this problem easily, we assume a huge PVfarm comparable to a nuclear power plant with a gigawattage class output. PV generation, which has poor yieldfor its footprint, needs vast ground to generate such a bigpower. Consequently, the generation facilities must be setup in sites far from consuming regions. Transmission facil-ities must have enough large capacity for maximum cur-rent which can be generated under the best weathercondition. They do not work during off-generating timesuch as at night and under poor sunshine. If PV plants sup-plied constant huge power as dam type hydraulic or nu-clear plants, we would make choice of a far-reachingtransmission System that connects distant sources and aconsuming power storage devices, such as batteries, canabsorb fluctuation of PV generation and equalize powertransmission.

3 However, this scheme reduces capacity oftransmission facilities and requires rather huge additionalcost for the huge accumulators. Therefore, until drasticallyreduced cost is available for storage devices, we cannotadopt this method. Then, put gas turbines together, withwhich we are able to adjust output power rather combined plant can absorb the fluctuation of PV gen-eration, and consequently, improve the operation ratio fortransmissions. However, it requires a parallel establishedthermal power plant comparable to the PV, which is aroundabout way for our initial goal, the introduction of alarge amount of mentioned above, large scale PV plants in remotesites have a serious problem on economic efficiency. Weneed a new power System that enables the introduction ofa massive amount of distributed PV units in demand article proposes DC Micro grid systems as an optionfor such a Purpose and Architecture of the DC Micro grid SystemFollowing three terms are briefly summarized pur-poses of the DC Micro grid System .

4 (1) Increase the introduction of distributed PV units.(2) Reduce energy dissipation and facility costs resultingfrom AC/DC conversion by integrating the junctionbetween a commercial grid and DC bus which connectsPV units and accumulators.(3) Supply power to loads via regular distribution lines(not exclusive lines for emergency) even during theblackout of commercial 1shows a schematic view of the DC Micro gridsystem. This System utilizes a DC bus as its backbone anddistributes power to a community that consists of severaldozens or a hundred of households in a residential area. A350 V DC bus is installed instead of 200 V / 100 V lines inconventional AC distribution systems and connected witha high voltage commercial grid through the intermediaryof a bidirectional AC/DC converter.

5 All the PV units in thecommunity are linked with the DC bus through DC/DCA DC Micro grid System has been proposed as a power network that enables the introduction of a large amount of solarenergy using distributed photovoltaic generation units. To test the feasibility of the System , we have developed ademonstration facility consisting of silicon photovoltaic (Si-PV) units, copper indium gallium (di)selenide photovoltaic(CIGS-PV) units, concentrator photovoltaic (CPV) units, an aerogenerator, and a redox flow battery. The redox flowbattery, a key component for supply-demand adjustment in the Micro grid System , successfully balanced supply anddemand in the grid by its rapid charge-discharge ability even under the fluctuating condition of power generation : Micro grid , DC distribution network, redox flow batteryDC Micro grid SystemNaoki AyAi*, Toshiya HisAdA, Toshikazu sHibATA, Hidekazu MiyosHi, Takashi iwAsAkiand ken-ichi kiTAyAMAconverters.

6 These converters always track the maximumpower point of the DC power sources which fluctuates de-pending on the intensity of solar radiation. Conventionalappliances can be used as they are if an inverter is installedin each house to change the DC power into 200 V / 100 VAC power, but DC power feeding will spread widely be-cause of its high efficiency, once safe and compact gears,such as breakers and outlets, are standardized in the fu-ture. Storage batteries of the community are also linked tothe DC bus. The DC-based distribution System reduces fa-cility costs and energy dissipation associated with AC/DCconversion because the PV units and battery are DC con-nected and most of the current energy-saving appliancesoperate on DC due to the progress of inverter is why we should push ahead with the DC System doesn t require long transmission lines toconvey solar power from remote areas because the PV unitshave been distributed in the demand area.

7 Power sourcesand loads are closely located to each other in a excess and deficiency of power are variable factorswhich should be compensated for a good balance betweensupply and demand. The compensation System , which con-sists of storage batteries and a bidirectional power con-verter, keeps a good power balance in the community byabsorbing short term power fluctuations. Since long termfluctuations, such as those between day and night, are alsosmoothed by the battery System , the Micro grid systemseems to be a small source or load for the outer wide-areagrid. Consequently, this scheme reduces the cost for thestabilization of commercial state of charge (SOC) of the storage battery alwaysindicates the time integral of difference between supplyand demand in the DC Micro grid System .

8 The SOC be-comes full with excess power, whereas it reaches the lowerlimit in deficiency. The amount and direction of the powerflow from a commercial grid is controlled according to theSOC, and power supply is maintained in the Micro , power supply to the Micro grid might be regu-lated to stabilize the power flow of the commercial , information about the situation of the commer-cial grid is essential for the operation of the Micro grid sys-tem. The current regulation on grid connection requiresprotective relays that decouple the inverter at the link pointwhen they detect abnormal voltage or frequency(1). In ad-dition to this function, the inverter should be equippedwith a power flow regulator that controls the purchasingand sales of currents according to the control signal fromthe commercial grid .

9 This scheme requires bidirectionalcommunication between Micro girds and the commercialgrid, which also enables the automatic calculation by ex-changing information on purchased and sold power be-tween the Micro grid and the commercial grid . We can setthe rate for purchasing and selling power in detail by uti-lizing the bidirectional communication. When power sup-ply is tight in the commercial grid , power consumption issuppressed by raising both purchasing and selling the other hand, when power supply is sufficient, powerconsumption is encouraged by abating the rate. Thus, theautomatic calculation System of the Electric rate functionsas a huge power trading market. The operation of the com-mercial grid is committed to a free market process exceptfor the tight situation that needs central the above mentioned cooperative control with thecommercial grid , the DC Micro grid needs to have a goodautonomous adjustability.

10 A storage battery with a large ca-pacity can easily respond to changes in supply and demandbut requires large footprint and high cost. For example,think of the situation where the SOC of the storage batteryis full but the excessive power cannot be sold to the outergrid, and we have no choice but to suppress the generationof PV. To avoid such an unfavorable situation, we shoulduse loads before the SOC becomes full, while suspendingthe use of the loads that can wait during low SOC. Targetsfor the demand control are air conditioners, lights, boilers,laundry machines, dishwashers, EV chargers, elevators, andwater supply pumps of buildings and DC Micro grid is also resistant to disasters. Evenunder conditions where Electric power and fuel are notsupplied from outside, we can have Electric power the time of the power failure of the commercial grid ,the DC Micro grid works as an independent power sourcethat is disconnected from the commercial grid .