Page 100 - Alternative Energy Systems in Building Design

76 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

New electric meters make use of digital electronic technology that registers power
measurement by solid-state current- and voltage-sensing devices that convert analog
measured values into binary values that are displayed on the meter bezels by liquid-
crystal display (LCD) readouts. In general, conventional meters only display power
consumption; that is, the meter counting mechanism is unidirectional.
Net Metering The essential difference between a grid-connected system and a stand-
alone system is that inverters, which are connected to the main electrical service, must
have an inherent line-frequency-synchronization capability to deliver the excess power
to the grid.
Net meters, unlike conventional meters, have the ability to record consumed or gen-
erated power in an exclusive summation format; that is, the recorded power registra-
tion is the net amount of power consumed—the total power used minus the amount of
power that is produced by the solar power cogeneration system. Net meters are sup-
plied and installed by utility companies that provide grid-connection service systems.
Net-metered solar power cogenerators are subject to specific contractual agreements
and are subsidized by state and municipal governmental agencies. The major agencies
that undertake distribution of the State of California’s renewable energy rebate funds
for various projects are the California Energy Commission (CEC), Southern California
Edison, Southern California Gas (Sempra Power), and San Diego Gas and Electric
(SG&E), as well as principal municipalities, such as the Los Angeles Department of
Water and Power. When designing net-metering solar power cogeneration systems,
designers and their clients must familiarize themselves with the CEC rebate fund require-
ments. Essential to any solar power implementation is the preliminary design and
economic feasibility study needed for project cost justification and return-on-investment
(ROI) analysis. The first step of the study usually entails close coordination with the
architect in charge and the electrical engineering consultant. A preliminary PV array
layout and a computer-aided shading study are essential for providing the required
foundation for the design. Based on the shading study, the solar power engineer must
undertake an econometrics study to verify the validity of and justification for the
investment. On completion of the study, the solar engineer must assist the client in
completing the required CEC rebate application forms and submit them to the service
agency responsible for the energy cogeneration program.

Grid-Connection Isolation Transformer To prevent spurious noise transfer
from the grid to the solar power system electronics, a Δy isolation transformer is placed
between the main service switchgear disconnects and the inverters. The delta winding
of the isolation transformer, which is connected to the service bus, circulates noise
harmonics in the winding and dissipates the energy as heat.
Isolation transformers are also used to convert or match the inverter output voltages
to the grid. Most often, in commercial installations, inverter output voltages range from
208 to 230 V (three phase), which must be connected to an electric service grid that
supplies 277/480-V power. Some inverter manufacturers incorporate output isolation
transformers as an integral part of the inverter system, which eliminates the use of
external transformation and ensures noise isolation.