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Renewable Energy and Grid Design

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Strong attention in wide area is focused on the problems of global warming and air pollution caused by burning fossil fuels. Therefore, renewable sources such as photovoltaics and wind power generators are increasing in the utility grids. In case these new sources are connected with utility grids by inverters, some unbalance of power would be occurred between the utility generator and the output of renewable energy and which sometimes leads to frequency fluctuation and voltage variation.

What the voltage fluctuation is caused by in the grid will be discussed here for going better.


A: Constant voltage inverter

Wind power sources are strongly expected to be used as one of the renewable energy particularly where the wind blows often and strong. But they are much frightful to the grid by the fluctuation due to the wind speed.
To improve the reliability of the power supply, a big storage connected with the wind power line, absorbs the surplus energy and discharges the stored energy back to the line when the wind blow is slow or stops. DC current discharged from the battery is converted to AC by an inverter, which is called as a virtual generator@shown in Figure A 1.
 Two generators which are the utility generator and the inverter should be synchronized in the grid to supply stable power to the consumers. But it's not easy in the conventional inverter.


Figure A1:Generator and Wind  (AC link)

In case most of the load power is supplied by a synchronous generator and small wind power is complemented to the grid as synchronized with the main power by an inverter. Effects of the discharged power from the inverter will be negligible.
But more wind generators are added to the grid ,the operation of the utility generator is strongly affected by the inverter and the grid voltage fluctuates depending on the reverse power coming back to the generator from the inverter.
One example is shown in table 1. When the load changes from 100Kw to 80 Kw suddenly. Generally out put of the inverter is almost same as 50 Kw. Then burden of the utility generator goes down from 50 Kw to 30 Kw(40% down).This rapid down of the burden causes the rotation speed of the generator rise up and the frequency also goes up.Subordinate inverter of the wind power is mutually affected by the main generator and the grid voltage is  fluctuating.

Table 1:Burden ratio
Load Utility generator Wind inverter
100Kw 50Kw 50Kw
80Kw 30Kw 50Kw
20%down 40%down almost same


One of the solutions is the constant voltage inverter.
Figure A2 shows a simulation result of the grid voltage in case of the conventional inverter.The inverter generates electric power proportional to the grid voltage and produces additional voltage to the grid voltage( shown as dotted line).
When the grid voltage is high , the conventional inverter adds more power to the grid which sometimes causes over voltage than the limited voltage(107V).


Figure A2 :In case of conventional  inverter

Figure A3 shows a simulation result in case of constant voltage inverter.
When the grid voltage is higher than a setting voltage of 104V,the constant voltage inverter do not discharge any current into the grid .
When the grid voltage is lower than the setting voltage ,the inverter discharge the power into the grid.

Figure A3 :Constant voltage inverter



B :DC link

Electric power is supplied by the both wind generator and the utility generator and those are integrated on the DC line as shown in Figure A2.

The generator is not necessary to synchronize with the grid frequency and voltage because it does not link directly to the grid.
The engine of the grid generator is operated likely to drive a car.If wind blows ,the engine rotates slowly as a idling mode which saves fuel.
And when wind goes down or stops,it is accelerated quickly in a short period.
Capacity of the battery is small enough to exchange two power sources.


Figure A2 : DC link

C: 100% renewable energy

All renewable sources such as solar, wind and biogas generator are integrated and supplied to the loads. Quick start of the methane gas generator could be possible to combine the many sources to obtain 100% renewable sources.


Figure B1:All renewable energy system


Biogas generator
http://www.env.go.jp/recycle/waste/biomass/whatisbiogass.html


Biogas energy
http://www.biofuelsenergyllc.com/projects.html



Symbols
Wind turbine generator Storage
Rectifier

Charge and discharge
Solar panel Inverter
Engine or synchronous generator Transformer