Isolate Communications from a Ground Potential Rise (GPR)
by Jack Duckworth, P.E.
& Ernest M. Duckworth, Jr., P.E.
In a deregulated power market, the successful power companies not only
have effective power system operation and maintenance organizations,
but also reliable wire-line communication networks when fiber is not
cost effective. Very reliable wire-line communications networks,
require a properly isolated network to avoid the effects of Ground
Potential Rise (GPR).
There are many different
kinds of problems that may jeopardize the integrity of wire-line
communications. However, the most overlooked or ignored problem is
that of the GPR which may only last for a fraction of a second, but
damages or destroys wire-line communications equipment at substations,
powerplants, and switchyards throughout most of the United States.
A GPR results when the earth
and grounding system develops a potential from a lightning strike or a
power line fault. Since all communications circuits are grounded and
have a very limited resistance to a high-voltage ground spike, a GPR
event causes a high-voltage current to jump normal communications
isolation and energize the communications circuits as it seeks a path
to a lower voltage potential ground through the communications system.
Standard protection for
communication circuits, i.e., a shunting gas tube or solid state surge
arrestor will fire backwards when subjected to a GPR and will provide
a path for a high-voltage current to enter the communications
equipment rather than blocking the current flow. The only method to
protect wire-line communications equipment is to provide total
isolation from ground by employing isolation equipment.
Isolation of communications
facilities costs money, and the cost must be compared to the costs, as
well as the legal consequences, of doing nothing. However, with rare
exception, the cost of isolation is a small fraction of the costs that
may be incurred from doing nothing. There are three major costs which
need to be considered when evaluating the need to isolate
communications equipment from GPR. The first is the direct cost of
having to replace destroyed or damaged equipment. The second is the
direct cost of increased power operations and lost revenue from the
failure of real-time communications with critical metering and
switching equipment. The third is the direct cost of a total blackout,
or the death of a worker because the communications circuit
There are detailed and well
established guidelines that discuss the technicalities of calculating
GPR as well as how to adequately isolate against its effects. This
information may be found in IEEE Std. 487-2000.
There are two protection
philosophies that existed in the regulated power market, which must be
carefully reexamined in the context of the new deregulated power
market. The first philosophy is the ALet it fail, because the
probability of failure is small@ or the ALet it fail@ philosophy. The
second philosophy is the AProtect it", because although the
probability is small, the cost of failure is too high@, or the
AProtect it@ philosophy.
The successful market player
in this new age will be the company that adopts the AProtect it@
philosophy, where the benefits of protection exceed the cost of
protection. This player will insulate themselves from being the
responsible company to a blackout. The unsuccessful power companies
will be those who maintain the ALet it fail@ philosophy without
evaluating the risks and costs of not adequately protecting their
vital lines of communication and will most likely be responsible for a
Operations and maintenance
costs in the new free market will be much more highly scrutinized than
they ever were in the past. The cost of failing to provide needed
communications protection will not be acceptable by a nation used to
total reliability in the old regulated power market.
In the deregulated power
market, costs are being scrutinized like never before. It is one thing
to gamble that the cost of replacing communications equipment damaged
by GPR will be lower than the cost of protecting that equipment.
However, it is a whole different matter to gamble that the increased
costs for system operations, due to the loss of real time information,
will be less than the cost of communications protection. It is quite
evident that the electric companies using isolation equipment on their
wire-line communication facilities will be better prepared to compete
head-on in the new age of the deregulated power market and be less
likely to be blamed for the coming blackouts.