Non-traditional, small distributed generators, especially microturbines,
fuel cells and photovoltaic technologies, are being talked
about more frequently as an energy source for the next decade.
Most of these devices are asynchronously connected to the power
system through Static Power Converters (SPCs). These SPCs are
solid-state microprocessor-controlled thyristor devices that convert
DC or AC voltage at one frequency to 60 Hz system frequencies.
IEEE Standard 929 and UL 1741 provide guidance on SPC design.
Digital electronic control of the SPC regulates the device’s power
output and shuts down the machine when the utility system is
unavailable. These systems typically provide only a few cycles of
current for system faults. Some of the newest micro-turbines,
which are permanent magnet generators that generate power at
high frequency, have built-in anti-islanding protection to detect
when the generator is not operating in parallel with the utility. The
ability to verify the performance of this scheme through traditional
testing is difficult. Thus the utility must rely on factory tests of
the system. The need for traditional independent protection to
avoid system islanding is thus required by some utilities while
others rely on anti-islanding protection embedded in the
microprocessor control. Some inverter based systems can be
operated as voltage-source systems and thus can provide local
voltage control.