Page 222 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 195

time. This is known as a synchronization relay. Often the utility will require the syn-
chronization time is 30 seconds or more before the generator can reconnect after a utility
disturbance. This will allow the utility system (reclosers, sectionalizers) to reset to their
normal operating state. Sectionalizers often will not reset and operate correctly without
a long enough loss of voltage.
The other CHP system condition that a utility is typically very concerned with is a
condition known as “islanding.” In this condition, the CHP generator is still providing
energy to the facility when the utility has lost power. For a large facility where the CHP
generator capacity is quite a bit smaller than the facility load, this is not necessarily a
problem, provided maintenance workers at the facility understand their facility still
is partially energized and account for this. However, if the CHP generator is also
exporting power into the utility distribution system, it is creating an energized “island”
for the utility, surrounded by nonenergized distribution. This is a potentially dangerous
situation for utility maintenance workers.
The utility will want assurances that the CHP facility design has protection against
islanding. There are a number of ways of providing this protective function, including
passive methods like voltage relays (undervoltage, ANSI #27 and overvoltage, ANSI #59)
or frequency relays (ANSI #81). However, while it is expected the generating facility volt-
age or frequency will drop slightly whenever the utility connection is no longer present;
however it isn’t an absolute guarantee. Active protection methods may include meas-
urement of the response to a small disturbance added into the system by the CHP
facility. If the utility is connected, the system impedance will be much smaller (near
zero) than the impedance of the CHP facility in a stand-alone situation, and the impact
of the disturbance will change significantly when utility is disconnected.
There are other system protection and coordination issues which are dependent upon
the generator size, utility connection type, and grounding type. For example, improper pro-
tective device coordination can be a concern, particularly when it leads to nuisance fuse
blowing (which can turn a temporary fault into a permanent outage). Voltage issues within
a CHP facility may cause utility sectionalizers to operate incorrectly, or contribute to a volt-
age regulation problem. Solving these potential issues requires project-specific relaying
solutions on a case-by-case basis. The final section of this chapter includes a sample riser
diagram with typical relays and discusses how these relays help solve some typical issues.
Regardless of how or where the on-site generators are connected into the system,
safety must always be a first consideration for both personnel and equipment, and must
include positive means of locking out either the utility or the CHP system in order to
provide safe access to the circuits by facilities and maintenance personnel. Therefore,
padlocking provisions on the tie-point disconnect which are made accessible to the
utility company personnel is an absolute essential safety requirement. Most of the utility
companies will also require a ground and test device for medium-voltage switchgear to
assure that it is grounded before anyone attempts to service the equipment.

Interconnection Process Overview
The process which the electrical systems designer must follow to meet utility rules for
interconnection will vary by locale. However, there are some standards that apply to
any facility design regardless of utility and jurisdiction. An integral part of the application
process is providing a drawing package consisting of an electrical single-line drawing,
equipment layout plans, grounding plans, site plan, and protective relaying specifications.
Of course, the power and controls single-line drawings are the heart of this documentation