Page 223 - Sustainable On-Site CHP Systems Design, Construction, and Operations

196 De s i g n

package and are meant to give utility engineers a general understanding of the pro-
posed interconnection. The interconnection documents will need to show the CHP gen-
erator, switchgear, utility feeders, conduit/conductor specifications, and control relays;
typically, the utility will want to see the same information that a contractor bidding
the job would need to construct the electrical installation correctly.
Depending on the interconnection agreement, the utility will also want to see speci-
fications for some of the electrical equipment. For example, when an automatic transfer
switch (ATS) is used, perhaps when the plant is used in a standby application, the utility
will want details on the transfer mechanism. If it is a closed transition ATS, where both
the utility and CHP generator are connected at the same time for a moment, the utility
will need to see specifications detailing the synchronization and protection schemes
between the two feeders. In a similar way, the utility will wish to see details on trans-
formers (such as impedances) and switchgear.
Another critical piece of information is a description of the CHP generating equip-
ment. The interconnection application will indicate the number (and prime mover type)
of generators being installed and model numbers. It will also ask for electrical informa-
tion such as
• Nameplate ratings (both kVA and kW). Often, the utility will want the gross and
net ratings, with net defined as the difference between gross rating and auxiliary
loads used to operate the generator. There may be a significant difference in
these values.
• Operating voltage and wiring configuration (single phase or three phase).
• Power factor rating (with adjustment range).
• Grounding system used (solidly grounded or impedance grounded).
• Short-circuit current produced by the generator. This is critical, and can be calculated
based on synchronous, transient and subtransient reactance (for a synchronous
generator) or locked rotor current (for induction generator).

Once the electrical designer has provided the utility with all the initial information
required, the application undergoes an initial review. In this stage, the utility reviews
the design with an eye to the interconnection agreement (nonexport, incidental export,
net metering). The certification of the equipment is reviewed. Voltage drop and short-
circuit current calculations are tested. During this time, an open line of communication
between the designer and utility is critical to ensure a smooth review process. If the
CHP generator is relatively small and the utility is satisfied with all of the design param-
eters, this may be the end of the review and the application would be approved.
However, there may be a further review required where the generator capacity is a
greater percentage of the facility load or the interconnection is more complex (such as
in a network system). The utility will do their own interconnection study, the cost of
which is usually borne by the CHP facility owner. It is critical that the time and cost for
this study is built into the project schedule and design budget. If it is not, then the
economic models used to justify the CHP facility in the first place may not be accurate.

Final Interconnection Acceptance and Start-Up
Of course, once a design has been approved and installation of the CHP generating
facility is underway, it is critical that the installation follows the approved design drawings