Grid-Connected System - Wind and Solar Power Systems - page 230

13
Grid-Conne cted System
The wind and photovoltaic power systems have made a successful transition
from small stand-alone sites to large grid-connected systems. The utility
interconnection brings a new dimension in the renewable power economy
by pooling the temporal excess or the shortfall in the renewable power with
the connecting grid. This improves the overall economy and the load avail-
ability of the renewable plant; the two important factors of any power sys-
tem. The grid supplies power to the site loads when needed, or absorbs the
excess power from the site when available. One kWh meter is used to record
the power delivered to the grid, and another kWh meter is used to record the
power drawn from the grid. The two meters are generally priced differently.
Figure 13-1 is a typical circuit diagram of the grid-connected photovoltaic
power system. It interfaces with the local utility lines at the output side of
the inverter as shown. A battery is often added to meet short term load peaks.
In the United States, the Environmental Protection Agency sponsors grid-
connected pv programs in urban areas where wind towers would be imprac-
tical. In recent years, large building-integrated photovoltaic installations have
made significant advances by adding the grid-interconnection in the system
design. Figure 13-2 shows the building-integrated pv system on the roof of
the Northeastern University Student Center in Boston, MA. The project was
part of the EPA PV DSP Program. The system produces 18 kW pv power and
is connected to the grid. In addition, it collects sufficient research data using
numerous instruments and computer data loggers. The vital data are sampled
every 10 seconds, and then are averaged and stored every 10 minutes. The
incoming data includes information about the air temperature and wind
speed. The performance parameters include the DC voltage and current gen-
erated by the pv roof, and the AC power on the inverter output side.
In the United Kingdom, a 390 square meter building-integrated pv system
has been in operation since 1995 at the University of Northumbria, Newcastle
( Figure 13-3 ) . The system produces 33,000 kWh electricity per year and is
connected to the grid. The pv panels are made of monocrystalline cells with
the photoconversion efficiency of 14.5 percent.
On the wind side, most grid-connected systems are large utility-scale
power plants. A typical equipment layout in such plants is shown in
Figure 13-4 . The wind generator output is at 480 volts AC, which is raised