Page 48 - Solar Power in Building Design The Engineer's Complete Design Resource

18 SOLAR POWER SYSTEM PHYSICS

Islanding is a condition that occurs when the inverter continues to produce power
during a utility outage. Under such conditions the power produced by a PV system
becomes a safety hazard to utility workers who could be inadvertently exposed to haz-
ardous electric currents; as such, inverters are required to include anti-islanding con-
trol circuitry to cut the power to the inverter and disconnect it from the grid network.
Anti-islanding also prevents the inverter output power from getting out of phase
with the grid when the automatic safety interrupter reclosures reconnect the inverter
to the grid, which could result in high voltage spikes that can cause damage to the con-
version and utility equipment.

Institute of Electrical and Electronics Engineers The Institute of Electrical
and Electronics Engineers (IEEE) provides suggestions for customers and utilities
alike regarding the control of harmonic power and voltage flicker, which frequently
occur on utility buses, in its IEEE 929 guideline (not a standard), Recommended
Practice for Utility Interface of Photovoltaic (PV) Systems. Excessive harmonic
power flow and power fluctuation from utility buses can damage customers’ equip-
ment; therefore a number of states, including California, Delaware, New York, and
Ohio, specifically require that inverters be designed to operate under abnormal utility
power conditions.

Power limit conditions The maximum size of a photovoltaic power cogeneration
system is subject to limitations imposed by various states. Essentially most utilities are
concerned about large sources of private grid-connected power generation, since most
distribution systems are designed for unidirectional power flow. The addition of a large
power cogeneration system on the other hand creates bidirectional current flow con-
ditions on the grid, which in some instances can diminish utility network reliability.
However, it is well known that, in practice, small amounts of cogenerated power do
not usually create a grid disturbance significant enough to be a cause for concern. To
regulate the maximum size of a cogeneration system, a number of states have set
various limits and caps for systems that generate in excess of 100 kW of power.

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

PV Power cogeneration capacity In order to protect utility companies’ norm of
operation, a number of states have imposed a cap on the maximum amount of power
that can be generated by photovoltaic systems. For example, New Hampshire limits