Page 74 - Alternative Energy Systems in Building Design

50 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

UTILITY-SIDE DISCONNECTS AND ISOLATION TRANSFORMERS

In some states, such as California, Delaware, Florida, New Hampshire, Ohio, and Virginia,
utility regulators require that visible and accessible disconnect switches be installed
outside for grid service isolation. It should be noted that several states, including
California, require that customers open the disconnect switches once every 4 years to
check that the inverters are performing the required anti-islanding.
In other states, such as New Mexico and New York, grid-isolation transformers are
required to reduce the noise created by private customers that could be superimposed
on the grid. This requirement, however, is not a regulation that is mandated by the UL
or the Federal Communication Commission (FCC).

PV power cogeneration capacity In order to protect utility companies’ norms of
operation, a number of states have imposed a cap on the maximum amount of power that
can be generated by PV systems. For example, New Hampshire limits the maximum to
0.05 percent and Colorado to 1 percent of the monthly grid network peak demand.

Inverter capability to withstand surges In most instances, power distribution
is undertaken through a network of overhead lines that are constantly exposed to
climatic disturbances, such as lightning, that result in power surges. Additional power
surges also could result from switching capacitor banks used for power factor correction,
power conversion equipment, and during load shedding and switching. The resulting
power surges, if not clamped, could seriously damage inverter equipment by breaking
down conductor insulation and electronic devices.
To prevent damage caused by utility spikes, the IEEE has developed nationally rec-
ommended guidelines for inverter manufacturers to provide appropriate surge protection.
A series of tests devised to verify IEEE recommendations for surge immunity are
performed by the UL as part of equipment approval.

PV system testing and maintenance log Some states, including California,
Vermont, and Texas, require that comprehensive commissioning testing be performed on
PV system integrators to certify that the system is operating in accordance with expected
design and performance conditions. It is interesting to note that for PV systems installed
in the state of Texas, a log must be maintained of all maintenance performed.

Examples of UL1741 inverter specification The following is an example of a
UL1741-approved inverter manufactured by SatCon, Canada. An optional combiner
box, which includes a set of special ceramic overcurrent protection fuses, provides
accumulated dc output to the inverter. At its dc input, the inverter is equipped with an
automatic current fault isolation circuit, a dc surge protector, and a dc backfeed pro-
tection interrupter. In addition to the preceding, the inverter has special electronic cir-
cuitry that constantly monitors ground faults and provides instant fault isolation. On
conversion of direct current to alternating current, the internal electronics of the inverter
provide precise voltage and frequency synchronization with the grid.