Page 215 - Sustainable On-Site CHP Systems Design, Construction, and Operations

188 De s i g n

If cooling systems are not well designed, or switchgear built that doesn’t allow for
proper air circulation, temperature gradients can cause moisture similar to the way
warm weather fronts and cold weather fronts cause precipitation where they collide.

Grounding Considerations
A CHP generator serves a facility with energy separate from a utility and has no direct
electrical connection to the supply conductors originating from the facility. By this
definition, it is considered a separately derived system by the National Electrical Code
(NEC). This has a number of implications for grounding and bonding the CHP gen-
erator and plant with the facility electrical distribution system and these will be dis-
cussed in greater detail later in this section. Before that, however, we will discuss the
various grounding system types and provide guidance on selection of which grounding
system to choose.

Grounding System Types
The most common type of grounding system is a solidly grounded wye system. Typi-
cally, the primary feed is delta-connected and the secondary is wye-connected, and
there is a solid connection between the ground on the primary and the center of the wye
on the secondary. In this configuration, the grounded (neutral) conductor in the secondary
carries single phase or unbalanced three phase current. This grounding system is very
common because it works in applications where both three-phase motor loads and
277 V per single phase lighting loads are needed.
In a fault condition, the available short-circuit current between a phase conductor
and ground is dependent on the impedance of the distribution system. In addition to
the conductors making up the pathway between the fault and the ground source, the
total impedance would also include the impedance of the primary generation source.
Another common grounding method is impedance grounding, which is similar in
concept to the solidly grounded wye system in that there is a connection between the
ground on the primary and the center of the wye on the secondary. However, in this
case, an impedance source (i.e., a resistor) is inserted in this connection pathway. If the
resistor is large, it is considered a “high-impedance” grounded system and the fault to
ground will be a low current value, so low in some cases that an overcurrent device will
not open. When a small resistor is used, it is a “low-impedance” grounded system. In
this case, the ground fault will be large enough to trip an overcurrent device, but the
value will still be low enough for a relaying scheme typical in CHP switchgear to handle.
An alternate to a grounded system is an ungrounded system. In this case, there is
no physical connection between the phase conductors and ground. Similarly to an
impedance grounded system, the fault current is very limited in a sustained line-to-
ground fault, and overcurrent protective devices do not automatically trip. Hence, a
characteristic shared by both of these systems is that a facility can continue to run
through a ground fault. However, while impedance grounding has other advantages
as discussed below, an ungrounded system is only recommended when it is critical
that ground faults do not shut down a continuous process, such as in an industrial
facility. Maintenance staff must be quick to clear these faults before they cause sus-
tained overvoltage in associated phase conductors (which can cause insulation dam-
age). For most CHP facilities (e.g., hospitals or campus environments), ungrounded
systems are not recommended.