IEEE Power & Energy Magazine - September/October 2014 - 80

Hardening the distribution network is the other approach
for preventing or mitigating the catastrophic effects of
weather-related disruptions.

Distributed Resources
Operation: Current Practices
this section describes the most common industry practices
today related to the operation of the distributed resources
described above. the focus of our discussion is on emergency operating conditions prior to, during, and following
a vLse. these are the conditions typically dealt with at the
emergency operations centers (eOcs) set up by utilities to
respond to vLses.

Standby Generators
Perhaps the most common application of distributed
resources during outage conditions is the use of standby
generators. within seconds of a utility outage, an automatic
transfer switch senses the power loss, commands the standby
generator to start, and then transfers the electrical load to
the standby generator to supply power to the circuits. after
utility power returns, the automatic transfer switch transfers
the electrical load back to the utility and signals the standby
generator to shut off. It then returns to standby mode, where
it awaits the next outage.
automatic standby generator systems may be required
by building codes for critical safety systems such as elevators in high-rise buildings, fire protection systems, standby
lighting, or medical and life support equipment. residential
standby generators are increasingly common, providing
backup electrical power to hvac systems, security systems,
and household appliances. current interconnection agreements do not allow customer-owned standby generators to
operate during normal conditions when the customer's load
is fed by the utility.

Distributed Generation (DG)
Utilities routinely use utility-owned DG, especially mobile
diesel generators, for support during restoration. Using customer or third-party-owned DG in the restoration process is
much less common.
In many U.s. utility service territories, customers can
install DG to offset their demand. with net metering (as is
well known in california, for example), a customer's excess
generation may feed back to the utility grid during normal
operation when output is higher than the customer's own load.
standard interconnection agreements generally require
that customer-owned DG be disconnected during an outage.
this is primarily for the protection of utility crews, who must
be certain that everything downstream of any breaker they
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open is deenergized, allowing them to perform work without risk of electrocution. there are control and anti-islanding mechanisms to automatically trip the DG during outage
conditions and avoid back-feeding the distribution grid. an
exception is when a customer disconnects from the utility grid
(islands) but has the capability to independently serve his own
load with DG, storage, and appropriate control.
these standard practices do not allow for the islanding
of a group of customers to enable them to share their DG
during an outage, for both technical and legal reasons. a
crucial concern is liability, since one customer's DG could
damage other customers' assets by introducing power quality problems such as excessive harmonics or voltage surges
or simply by failing to control voltage and frequency when
attempting to balance local generation and load. Further, the
direct trading of services among customers over utility distribution networks is normally prohibited, as current laws
governing safety, quality of service, asset ownership, and
economic transactions are based on the model of the utility
as an exclusive franchise.

Energy Storage
the current practices regarding customer energy storage
resources are similar to those described above for standby
generators and DG. their operation during outage conditions is strictly limited to supply-only loads on the customer's side of the meter, not back-feed power into the grid.
a number of U.s. utilities are experimenting with energy
storage systems to improve resilience. One example is the
Portland General electric smart Feeder demonstration project in downtown salem, Oregon, which aims to improve distribution reliability by leveraging distributed resources such
as energy storage, commercial DG, and Dr.

Microgrids
a microgrid, defined as a subset of the grid that can be
islanded (e.g., at the level of a university or corporate campus), can supply all or part of a customer's load during an
outage or in case of grid contingencies. again, the customer
may not back-feed into the grid or supply third-party loads
during outage conditions, due to safety and liability concerns. In particular, multicustomer microgrids, in which
several electrically adjacent customers are islanded together,
are not generally permitted because existing laws governing safety, quality of service, asset ownership, and economic
transactions generally are deficient.
september/october 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2014