Page 49 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM COMPONENTS 19

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, which
result in power surges. Additional power surges could also result from switching
capacitor banks used for power factor correction and from power conversion equip-
ment or 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, IEEE has developed national recom-
mended guidelines for inverter manufacturers to provide appropriate surge protection.
A series of tests devised to verify IEEE recommendations for surge immunity are per-
formed by UL as part of equipment approval.

PV system testing and maintenance log Some states, such as 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 sys-
tems installed in the state of Texas a log must be maintained of all maintenance
performed.

EXAMPLE OF A UL1741 INVERTER

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 pro-
tection fuses, provides accumulated dc output to the inverter. At its dc input the invert-
er is equipped with an automatic current fault isolation circuit, a dc surge protector,
and a dc backfeed protection interrupter. In addition to the preceding, the inverter has
special electronic circuitry that constantly monitors ground faults and provides instant
fault isolation. Upon conversion of direct current to alternating current, the internal
electronics of the inverter provide precise voltage and frequency synchronization with
the grid. Figure 1.9 depicts view of inverter electronics.
An integrated isolation transformer within the inverter provides complete noise iso-
lation and filtering of the ac output power. A night isolation ac contactor disconnects
the inverter at night or during heavy cloud conditions. The output of the inverter also
includes an ac surge isolator and a manual circuit breaker that can disconnect the
equipment from the grid.
A microprocessor-based control system within the inverter includes, in addition to
waveform envelope construction and filtering algorithms, a number of program sub-
sets that perform anti-islanding, voltage, and frequency control.