power systems are generally classified according to their
functional and operational requirements, their component
configurations, and how the equipment is connected to other
power sources and electrical loads. The two principle
classifications are grid-connected or utility-interactive
systems and stand-alone systems. Photovoltaic systems can be
designed to provide DC and/or AC power service, can operate
interconnected with or independent of the utility grid, and can
be connected with other energy sources and energy storage
systems.1.7.1 Grid-Connected (Utility-Interactive) PV Systems.
or utility-interactive PV systems are designed to operate in
parallel with and interconnected with the electric utility grid.
The primary component in grid-connected PV systems is the
inverter, or power conditioning unit (PCU). The PCU converts the
DC power produced by the PV array into AC power consistent with
the voltage and power quality requirements of the utility grid,
and automatically stops supplying power to the grid when the
utility grid is not energized. A bi-directional interface is
made between the PV system AC output circuits and the electric
utility network, typically at an on-site distribution panel or
service entrance. This allows the AC power produced by the PV
system to either supply on-site electrical loads, or to back
feed the grid when the PV system output is greater than the
on-site load demand. At night and during other periods when the
electrical loads are greater than the PV system output, the
balance of power required by the loads is received from the
electric utility This safety feature is required in all
grid-connected PV systems, and ensures that the PV system will
not continue to operate and feed back onto the utility grid when
the grid is down for service or repair.
Diagram of grid-connected photovoltaic system
Stand-alone PV systems are designed to operate
independent of the electric utility grid, and are generally
designed and sized to supply certain DC and/or AC electrical
loads. These types of systems may be powered by a PV array only,
or may use wind, an engine-generator or utility power as an
auxiliary power source in what is called a PV-hybrid system. The
simplest type of stand-alone PV system is a direct-coupled
system, where the DC output of a PV module or array is directly
connected to a DC load (Figure 5). Since there is no electrical
energy storage (batteries) in direct-coupled systems, the load
only operates during sunlight hours, making these designs
suitable for common applications such as ventilation fans, water
pumps, and small circulation pumps for solar thermal water
heating systems. Matching the impedance of the electrical load
to the maximum power output of the PV array is a critical part
of designing well-performing direct-coupled system. For certain
loads such as positive-displacement water pumps, a type of
electronic DC-DC converter, called a maximum power point tracker
(MPPT) is used between the array and load to help better utilize
the available array maximum power output.
Direct-coupled PV system.
many stand-alone PV systems, batteries are used for energy
storage. Figure 6 shows a diagram of a typical stand-alone PV
system powering DC and AC loads. Figure 7 shows how a typical PV
hybrid system might be configured.
of stand-alone PV system with battery storage powering DC and AC
of photovoltaic hybrid system.