Page 216 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 189

Grounding System Selection
The following is an overview of some of the design considerations which may impact
the grounding system selection:

• Voltage. Proper generator grounding varies depending on the generator location
in the facility system and the voltage at which the generator will be tied to
the utility supply. Consider a typical situation—the utility service is medium
voltage, the utility transformer has a secondary voltage of 4.16 kV, and the
transformer has a low-impedance ground with a maximum ground fault current
of approximately 200 A. The facility generator is also at 4.16 kV and is paralleled
with the utility. In this scenario, the optimum configuration is for the CHP
generator neutral conductor to have a high-impedance ground, which will limit
the ground fault current to approximately 2 A. This fault current would be
sufficient to activate effectively an alarm condition while still permitting
continued operation until the faulted circuit is located and shutdown. In turn,
this avoids a total system outage.
• Harmonics. Most generators manufactured today which are used in CHP systems
are wye-connected. Since the third harmonic components of the phase currents
are additive in the neutral, they can best be attenuated in the neutral. To
accomplish this attenuation a grounding resistor helps perform this function.
These grounding resistors are typically rated for a particular voltage and current
and are provided within a separate enclosure. When harmonics are a critical
issue for a facility, generator neutrals should be impedance grounded when
paralleled with the utility service buses.
• Mode of operation. If the CHP generators are commonly not paralleled with
the utility source, there are other considerations which must be taken into
account for grounding purposes. First, if the distribution system feeds single
phase loads, which is typical for nonindustrial facilities, its neutral must be
solidly grounded so that it may be used as a circuit conductor, as required by
NEC 230-95. Second, if the generator is paralleled with the utility and its
neutral is high-impedance grounded, it cannot be used as a circuit conductor.
To allow for both possibilities, a common solution is to provide a bypass
device installed across the grounding resistor which shunts the impedance as
necessary.

Bonding Requirements
As a separately derived system, a CHP generator must be grounded and bonded as per
NEC Section 250.30. This section of the NEC has been revised most recently in 2008 due
to the complexity and importance of grounding and bonding these types of complex
and interrelated electrical systems. This section gives a summary of the components of
a well-grounded system—the system bonding jumper, the grounding electrode con-
ductor, and the grounding electrode. For more information, consult the NEC and perhaps
one of the many books written on this topic to further explain the grounding section
in detail.
Note that the summary below refers to a solidly or impedance grounded system.
There is a separate section (250.30B) to cover ungrounded systems; however, since this
type of grounding system is rare in CHP plants, it is not covered here.