|Renewable Energy and Grid Design
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
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
power is complemented to the
grid as synchronized
with the main power by an inverter.
of the discharged power from
will be negligible.
Table 1:Burden ratio
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
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
One of the solutions is the constant voltage
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
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
When the grid voltage is
the setting voltage ,the inverter
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
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
Figure B1:All renewable energy system