Page 219 - Sustainable On-Site CHP Systems Design, Construction, and Operations

192 De s i g n

distribution system. The transmission system operates at high voltages (typically 110 kV
and above) and, in the United States, is federally regulated. The second section is the
local distribution system, which is the medium voltage system that commonly provides
energy more directly to the customers. This local distribution system is the one which is
typically regulated by public utility commissions and this is the one which is intercon-
nected with CHP systems. For that reason, this discussion revolves around the distribu-
tion system. The reader is encouraged to further review transmission systems as they
are not discussed in further detail within this book.
There are two typical types of distribution system, each with their own particular
protection requirements. The most common distribution system type is called the radial
system. In broad terms, the radial system is similar in concept to a wheel with spokes. In
this configuration, the utility substation is the center of the wheel while the spokes are
the feeders which carry power to the customers. Typically, these feeders are expected to
carry energy in only one direction—from the utility to the customers—and they are
not joined downstream from the substation. Therefore, the distribution system likely
was designed by the utility assuming unidirectional power flow and not for power
flow from a CHP facility back to the substation. With the increased importance of CHP
systems and their applications today, both customers and utilities have had to review
ways in which to allow for bidirectional power over these initially designed and
installed radial systems. Such protection system opportunities are discussed in greater
detail below.
The other type of distribution system is called a network system, and is more
common in higher and more densely populated areas, which as expected have higher
load requirements. In this system type, there are multiple feeders originating in a sub-
station (much the same as with a radial system), but instead of single unidirectional
flow, they are joined together downstream from the substation in a variety of configura-
tions. This method of “loop feeding” via a network system improves reliability for utility
customers because it provides them with a secondary feeder in the event the primary
feeder is compromised. By definition, this network distribution allows for simple bidi-
rectional power flow and the distribution system design includes network protectors to
protect the local distribution system. Typical network protectors have relays, which are
not intended to be frequently operated, so protection relays on switchgear must act
quickly to isolate faults and to allow problems to be corrected before network protec-
tors are opened and reset.
Another factor which has impact on specific protection requirements is the agree-
ment for power export between the CHP facility and their serving utility. An agreement
like this is often dependent on the type and size of the CHP facility. For a small facility
with a prime mover like a fuel cell or microturbine, the most likely agreement is for
nonexport. In this case, the energy generated by the CHP facility is not expected to feed
into the utility distribution system since the generating capacity of the CHP generator
is only a small percentage of the overall facility load. For example, in California, if the
CHP generator capacity is no more than 25 percent of the rating of the facility’s service
equipment, the protections requirements of this type of system usually dictate one of
two probable solutions for use.
One solution is the use of a power monitor on the incoming utility power. The
utility will determine a set point for expected load based upon a percentage of typical
demand loads. If the load for the facility drops below the designated set point, the CHP
control system will be required to reduce its generation capacity or drop generation