Page 98 - Alternative Energy Systems in Building Design

74 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

sinusoidal output, most inverters employ electronic circuitry to filter a large number of
harmonics. Filter circuits consist of specially designed inductive and capacitor cir-
cuits that trap or block certain unwanted harmonics, the energy of which is dissipated
as heat. Some types of inverters, mainly of earlier design technology, make use of
inductor coils to produce sinusoidal wave shapes.
In general, dc-to-ac inverters are intricate electronic power conversion equipment
designed to convert dc to a single- or three-phase ac that replicates the regular electrical
services provided by utilities. Special electronics within inverters, in addition to con-
verting direct current to alternating current, are designed to regulate the output voltage,
frequency, and current under specified load conditions. As discussed in the following
sections, inverters also incorporate special electronics that allow them to automatically
synchronize with other inverters when connected in parallel. Most inverters, in addition
to PV module input power, accept auxiliary input power to from a standby generator,
which is used to provide power when battery voltage has dropped to a minimum level.
A special type of inverter, referred to as a grid-connected inverter, incorporates syn-
chronization circuitry that allows the production of sinusoidal waveforms in unison with
the electric service grid. When the inverter is connected to the electric service grid, it
can effectively act as an ac power-generation source. Grid-type inverters used in grid-
connected solar power systems are strictly regulated by utility agencies that provide
net metering.
Some inverters incorporate an internal ac transfer switch that is capable of accepting
output from an ac-type standby generator. In such designs, the inverters include special
electronics that transfer power from the generator to the load.
Standby Generators A standby generator consists of an engine-driven generator
that is used to provide auxiliary power during solar blackouts or when the battery
power discharge reaches a minimum level. The output of the generator is connected to
the auxiliary input of the inverter.
Engines that drive the motors operate with gasoline, diesel, natural gas, propane, or
any type of fuel. Fuel-tank size varies with operational requirements. Most emergency
generators incorporate under-chassis fuel tanks with sufficient storage capacity to
operate the generator for up to 48 hours. Detached tanks also can be designed to hold
much larger fuel reserves, which usually are located outside the engine room. In general,
large fuel tanks include special fuel-level monitoring and filtration systems. As an option,
the generators can be equipped with remote monitoring and annunciation panels that
indicate power-generation data and log and monitor the functional and dynamic
parameters of the engine, such as coolant temperature, oil pressure, and malfunctions.
Engines also incorporate special electronic circuitry to regulate generator output fre-
quency, voltage, and power under specified load conditions.

Hybrid System Operation As discussed previously, the dc output generated from
PV arrays and the output of the generator can be connected simultaneously to an
inverter. The ac output of the inverter, in turn, is connected to an ac load-distribution
panel, which provides power to various loads by means of ac-type overcurrent protec-
tion devices.