installing the required facilities and the high ongoing monthly expenses. Latency is potentially another problem, but is tolerable for most DA applications.

Power Line Communications

In powerline communications, the high-voltage primary distribution wires act as the communications medium. System-wide implementation of power line communication facilities requires a major financial investment and commitment that usually cannot be justified solely for DA purposes. However, many utilities have already deployed power line communication systems for handling automatic metering, and DA systems can make use of this existing infrastructure.

With conventional power line carrier (PLC), a signal is injected into the primary lines via an interface at the distribution substation. The signal travels from the

main office to the substation on leased telephone line, point-to-point radio link, or other communications channel. The signal then passes through the PLC interface, and propagates down the primary wire into the DA end devices.

A disadvantage of PLC is that line capacitors attenuate the signals, resulting in some portions of the feeder that have no signal coverage. Special techniques, such as capacitor isolators, may be needed to eliminate this problem. Another problem is that PLC signals may be disrupted when a line fault occurs and during power line outages, making PLC unsuitable for automatic sectionalizing.

Communication via PLC is relatively slow. Data throughput ranges from 100 bps to 2 bits/hour, which may be too slow for some DA applications.

An emerging technology that utilizes

the power line as its communication media is broadband over power line (BPL). Unlike conventional power line carrier, the signal is injected at locations out on the distribution feeders themselves (not in the substation). The BPL signal is typically delivered to the feeder locations via optical fiber.

The strength of the injected signal is sufficient to reach customer locations up to a mile or so away without isolating the line capacitor banks. Because distribution feeders are usually much longer than a mile or two, several signal injection points are needed to reach customers across the feeder.

Adding DA applications to an existing BPL system that includes other utility applications (like automatic metering) would be technically feasible and cost-effective. However; implementing a BPL

Go to http://uae.hotims.com for more information.