Page 217 - Sustainable On-Site CHP Systems Design, Construction, and Operations

190 De s i g n

A system bonding jumper is a connection between the grounding conductors of the
CHP generator and the grounded (neutral) conductor in the main switchgear, and is a
vital connection necessary so that ground fault current can return to the utility source.
If a system bonding jumper is not in place, a fault would have to travel via the grounding
electrode (i.e., earth), which is a very high-impedance path. If the impedance is too
high, the fault current may be so low that it would trip a circuit breaker and create a
potential (equipment and personnel) safety risk as well as an equipment damage risk
that is unnoticeable and will go unchecked. The reader is to not be confused with an
impedance grounding system as has been previously discussed above; the earth is at a
much higher impedance path than any resistor used in an impedance grounding
system. The size of the bonding jumper must be at least 12.5 percent of the equivalent
area of the phase conductors. For services up to approximately 600 A, Table 250.66 of
the NEC can be used as a shortcut to doing the calculation by hand.
The grounding electrode for a separately derived system shall be as near as possible
to the grounding electrode for the main system. According to the NEC, the preferred
grounding electrode to be used is either the metal water pipe or structural metal (both
specified in NEC Section 250.52). The purpose of this grounding electrode connection is
primarily to limit voltage imposed by lightning or surges, and to stabilize the voltage to
earth during normal operation. It also provides a path to earth for static dissipation.
Note that this electrode connection has virtually nothing to do with clearing faults
because of the high-impedance path noted above.
The size and type of the grounding electrode conductor is dependent upon whether or
not the CHP system has one generator (a single separately derived system, described in
NEC Section 250.30-A3) or multiple separately derived systems. In both cases, the con-
nection to the switchgear is the same point as where the system bonding jumper is
installed, and shall be sized as per NEC Table 250.66. However, where more than one
separately derived system is installed, a single grounding electrode conductor can be
tapped with a listed connector to each of the separate systems. Since each of the ground-
ing points for the separate systems are typically within the same room (and often within
the same switchgear cabinets where a tiebreaker is used), this can save the cost of a
large grounding electrode conductor being run potentially a long distance.

CHP Power Quality
The quality of the power delivered to a CHP facility is critical irrespective of the source.
Voltage transients, surges, sags, etc. have always been part of an electrical system; how-
ever, with the technological advances in the types of facility loads over the years, the
effects on system integrity caused by irregular voltage issues now more than ever have
greater impact on the overall facility systems and equipment. While a comprehensive
overview of power quality issues and solutions for these voltage irregularities and
other issues is beyond the scope of this book, an additional discussion of harmonics
and its relation to system grounding is important when thinking in terms of CHP systems
and follows below.
As previously discussed, impedance grounding of the generator neutral limits the
amount of fault current that will flow into the generator when a part winding ground
and/or arcing fault occurs. One of the impacts generator impedance grounding will
have is a likely minimized amount of damage to the generator. More importantly,
though, this resistance significantly attenuates the third harmonic distortion from the
generator and consequently to downstream electric loads.