Page 71 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM COMPONENTS 47

■ Overcurrent relay
■ Ground trip or overcurrent relay
■ Overvoltage relay
■ Overfrequency relay
■ Underfrequency relay

Most inverters designed for PV applications are designed to allow simultaneous par-
alleling of multiple units. For instance, to support a 60-kW load, outputs of three 20-kW
inverters may be connected in parallel. Depending on the power-system requirements,
inverters can produce single- or three-phase power at any required voltage or current
capacity. Standard outputs available are single-phase 120 V ac and three-phase 120/208
and 277/480 V ac. In some instances, step-up transformers are used to convert the out-
put of 120/208-V ac inverters to higher voltages.

Input and output power distribution To protect inverters from stray spikes
resulting from lightning or other such high-energy events, dc inputs from PV arrays are
protected by fuses housed in junction boxes located in close proximity to the inverters.
Additionally, inverter dc input ports are protected by various types of semiconductor
devices that clip excessively high-voltage spikes resulting from lightning activity.
To prevent damage resulting from voltage reversal, each positive (+) output lead
within a PV cell is connected to a rectifier, a unidirectional (forward-biased) element.
AC output power from inverters is connected to the loads by means of electronic or
magnetic circuit breakers. These serve to protect the unit from external overcurrent
and short circuits.

Grid-connected inverters Earlier, I described the general function of inverters.
Here, I will review their interconnection to the grid, which requires a thorough under-
standing of safety regulations that are mandated by various state agencies. Essentially,
the goal of design safety standards for inverters used in grid-connected systems,
whether they are deployed in PV, wind turbine, fuel cell, or any other type of power
cogeneration system, is to have one unified set of guidelines and standards for the
entire country. Standard regulations for manufacturing inverters address such issues as
performance characteristics and grid-connectivity practices and are recommended by
a number of national test laboratories and regulatory agencies.
Underwriters Laboratories Standards For product safety, the industry in the
United States has worked with Underwriters Laboratories (UL) to develop UL1741,
Standard for Static Inverter and Charge Controller for Use in Independent Power
Systems, which has become the safety standard for inverters used in the United States.
Standard UL1741 covers many aspects of inverter design, including enclosures, printed
circuit board configuration, interconnectivity requirements such as the amount of
direct current the inverters can inject into the grid, total harmonic distortion (THD) of
the output current, inverter reaction to utility voltage spikes and variations, reset and
recovery from abnormal conditions, and reaction to islanding conditions when the util-
ity power is disconnected.