Page 83 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM CONFIGURATION AND CLASSIFICATIONS 53

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 pro-
vide net metering.
Some inverters incorporate an internal ac transfer switch that is capable of accept-
ing an 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 sizes vary with the operational requirements. Most emer-
gency generators incorporate underchassis fuel tanks with sufficient fuel storage capac-
ity to operate the generator up to 48 hours. Detached tanks could also be designed to
hold much larger fuel reserves, which are usually 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 the generator out-
put frequency, voltage, and power under specified load conditions.
Hybrid system operation As previously discussed, the dc output generated from
the PV arrays and the output of the generator can be simultaneously connected to an
inverter. The ac output of the inverter is in turn connected to an ac load distribution
panel, which provides power to various loads by means of ac-type overcurrent protec-
tion devices.
In all instances, solar power design engineers must ensure that all chassis of
equipment and PV arrays, including stanchions and pedestals, are connected togeth-
er via appropriate grounding conductors that are connected to a single-point service
ground bus bar, usually located within the vicinity of the main electrical service
switchgear.
In grid-connected systems, switching of ac power from the standby generator and
the inverter to the service bus or the connected load is accomplished by internal or
external automatic transfer switches.
Standby power generators must always comply with the National Electrical Code
requirements outlined in the following articles:

■ Electrical Service Requirement, NEC 230
■ General Grounding Requirements, NEC 250
■ Generator Installation Requirements, NEC 445
■ Emergency Power System Safety Installation and Maintenance Requirements,
NEC 700