Page 218 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 191

Interestingly, generator stator windings are not truly connected in parallel with the
axis of the generator shaft, but rather are set at an angle to the generator shaft. This
approach is known as the generator pitch. Utility generators are usually 2/3 pitch, which
has been shown to produce minimum third-harmonic distortion in the generator output.
However, generators used in typical industrial applications such as central plants are
rarely 2/3 pitch and therefore can produce a significant third-harmonic component of a
magnitude that is related to and depends upon the pitch of the individual generator.
Careful attention to generator neutral grounding and winding pitch can produce
outputs having a “clean” sine wave with very little harmonic distortion. As an example;
if a 0.73 pitch generator was operated in parallel with a utility service wye-connected
transformer, with both neutrals solidly grounded, the sine wave shape would most
likely show objectionable third-harmonic distortion. However, if a 2/3-pitch generator
was operated in parallel with a utility service wye-connected transformer with the
transformer neutral solidly grounded and the generator neutral impedance grounded,
the generator will produce a very good sine wave shape showing very little third-harmonic
distortion. Since impedance grounded systems introduce other challenges, as noted
earlier, the system designer must balance these challenges with reasonable expectations
for harmonic distortion in the CHP facility.

Interconnection Rules and Standards
Perhaps the most critical reason that the electrical design for a CHP facility is different
from other and more simplistic electrical design applications is due to the requirements
for interconnection with the serving utility. Most electrical utilities will have rules and
design/construction standards in place which regulate interconnections with the power
grid. The rules and standards are in place to ensure that power distribution to the
public is not compromised by smaller energy producers such as CHP facilities or alter-
native energy source providers like a photovoltaic array. For example, in the state of
California, the California Public Utilities Commission (CPUC) has a specific rule (Rule 21)
which all regulated public utilities must follow. Many of the smaller municipal utilities
within the state have adopted similar rules, and it can be said that most state-regulated
and municipal-run power utility providers in almost every jurisdiction will have some
level of rules and requirements focusing on interconnectivity. These rules often include
a very rigorous application and certification process which are discussed in general
terms here. While a detailed description of this process is not included within this
chapter, there are typically guidebooks or online information for the application process
available from each local public utility commission and/or through each local utility.
Protection Requirement Considerations
When on-site generation and utility systems are operated in parallel, there are many
factors such as point of connection, grounding, synchronization, protective relaying,
and system isolation that must be considered to provide safe and efficient operation. All
of these functions in one way or another fall under the loose but very important term of
“protection.” Without system protection on the electrical systems, both the utility and
CHP systems and its downstream loads are subject to unsafe operation and potentially
very harmful situations.
In general terms, the electric grid can be divided into two distinct sections. The
first section is the transmission system, which is the backbone of the national electric