Monitoring conditions in or about a.c. powerlines, in both overhead and underground and primary and secondary applications, is a useful practice for electric utility companies in order to anticipate outages which occur due to faulty equipment and overloads on a.c. powerlines and which result in loss of service for potentially a large number of customers. The potential for an outage and for loss of the greatest number of customers is increased during peak periods when power usage is at a maximum and delivery of continuous power is most critical. Outages caused by faulty and overloaded lines, transformers and other equipment are expensive to repair, dangerous for utility company employees, and costly to the electric utility company in terms of income lost for lost service and in terms of damage to the utility's reputation. The effects of an unexpected outage as a result of a faulty or overloaded powerline are exacerbated if the powerline is underground.
Thus, a.c. powerline sensors which sense electrical conditions, such as power, voltage and current are very useful to electric utility companies in monitoring a.c. powerlines and associated equipment, such as transformers and switches, in order to better anticipate the likelihood of an unexpected outage occurring due to faulty and overloaded equipment. If the electric utility companies are able to monitor the conditions on the powerlines, they are better able to perform maintenance on and replacement of powerlines which are likely to become de-energized as a result of an overload or fault, thereby lowering the number of unexpected outages.
In order to perform this monitoring most efficiently, typically a communications link between each sensor on the system being monitored and a remote base station is established. This allows the utility company to monitor all of its sensors in one remote location instead of having to individually check each sensor in situ. One method of establishing a communications link is achieved by transmitting signals to a local ground station by means of, for example, an FM radio link. The signals are then transmitted to a remote central monitoring location via, e.g. radio, land lines or satellite channels. See U.S. Pat. No. 4,786,862 to Sieron. This type of communication link is complex, expensive and requires use of a significant amount of hardware.
A better approach involves utilizing the powerline being monitored to transmit high frequency communications signals between the sensors and the base station. This is accomplished by making a direct electrical connection between the sensors and the powerline and the base station and the powerline. The direct electrical connection, however, requires that an invasive electrical connection be made to the power circuit that is being monitored. This type of installation is expensive for the utility company as it requires a significant number of man hours to perform the installation, is potentially dangerous for the installer and can cause a service interruption for the customer. Due to these limitations, powerline communications have not been widely used in the electric utility industry for communications with powerline sensors.