Page 199 - Applied Photovoltaics

under- or over-voltages for DC input and AC output (Standards Australia,
2002c).
x Power Quality. The harmonic content must be low, with the Australian
standard (Standards Australia, 2002b) specifying total harmonic distortion
(THD) limits of 5% for current 2% for voltage, to protect both loads and
utility equipment. The harmonic spectra are usually monitored up to about 50
harmonics, but inverters using high frequency commutation can produce
distortion outside that range. The waveform and power factor must be
acceptable to the utility. DC injection, which is inherently prevented by
inverters with line-frequency transformers but not by transformerless or high-
frequency transformer designs, would saturate the utility transformers and
cause outages. Hence, Standards Australia (2005) specifies that for a single-
phase inverter, the DC output current of the inverter must not exceed the
greater of 0.5% of its rated output current or 5 mA. The waveform must be
close to sinusoidal at 50 Hz (or 60 Hz in the USA), the frequency must be
within about 0.5 Hz of 50 Hz, while the acceptable power factor range is
typically 0.95 leading to 0.95 lagging. In Australia, the power factor must be
within the range 0.8 leading to 0.95 lagging.
x Compatibility with the array—The array’s maximum power voltage at
standard operating conditions must be compatible with the inverter nominal
DC input voltage. The maximum array open circuit voltage should also be
well within the inverter’s tolerable voltage range. Maximum power point
trackers are commonly included with grid-connected inverters to control the
operating voltage of the array (Schmid & Schmidt, 2003). Several different
tracking algorithms are in use, including ‘constant voltage’, ‘perturbation and
observation’, and ‘incremental conductance’, each with its particular
advantages and disadvantages (Kang et al., 2004).
x Electromagnetic Interference—This must be low enough to comply with
relevant local requirements.
x Lightning and voltage impulse protection. These must comply with local
rules.
x Presentation—Items to check include compliance with relevant electrical
codes, size, weight, construction and materials, protection against local
weather conditions, terminals, and instrumentation.

Inverter costs vary considerably and have been falling in recent years, but tend
towards 20% of the overall cost of systems smaller than 5 kW p or 10% for larger
systems.
10.2.3 On-site storage

On-site storage is not essential for grid-connected systems, since it is possible to sell
excess power to the grid during daylight and buy power at night. However, the
addition of storage to PV systems can greatly increase their value (Byrne et al.,
1993). Storage can be provided on site, typically via batteries or, for larger systems,
via pumped hydro, providing storage for peak period use.
In the longer term, such technologies as flywheels, fuel cells, underground caverns,
superconducting magnets, compressed air, ice or hydrogen may offer economical

186