In the December 2008 issue of Transmission & Distribution World, Charles Newton wrote about “the T&D automation building blocks, the embryonic and emergent stages of what is today termed the smart or intelligent grid.” Newton's Straight Talk column stimulated consideration of the building blocks and requirements of the smart grid. During the last two decades, the building blocks have been developed within the deployment of distribution automation along the distribution feeder.

Among the essential components required for the smart distribution grid is feeder automation, or “true field automation” to use Newton's term. The line-post sensor is one of the foundational technologies of feeder automation that will open the on-ramp to the smart distribution grid and, thus, facilitate the managing of a new era of advanced distribution automation. Telemetry provides intelligence to determine the state of the feeder, monitor the quality of energy delivery along the feeder and promptly identify disturbances that impact reliability. The efficiencies promised by the emerging smart distribution grid are dependent on the telemetry of the grid.

Real-Time Operation, Real-Time Intelligence

The real-time operation of the smart distribution grid requires real-time intelligence from the distribution feeder. This feeder intelligence is needed in a reliable, repeatable and deterministic manner to support operational and switching decisions, optimize the efficiency of the smart distribution grid and promptly respond to system disturbances. Connected to an advanced remote terminal unit (RTU), the line-post sensor can provide the necessary real-time feeder intelligence.

The real-time operation goals for the smart grid demand a reliable solution for the recovery of distribution feeder intelligence. The line-post sensor has proven its resilience with wide area deployment in Alabama Power's distribution system. The integrity of the electric system is dependent on repeatable monitoring techniques, and the sensor outputs are not only repeatable between phases but also between sites along the distribution circuit. This repeatability ensures the development of real-time voltage and load profiles for analysis purposes.

From Intelligence to Decisions

The sensor provides two continuous ac outputs proportional to the line voltage and current. These outputs support deterministic reporting when connected to an advanced RTU using an efficient protocol for real-time communications. Integrating the sensors into the distribution automation deployment ensures the recovery of timely feeder intelligence for operational decisions, including the following:

  • Routine switching
  • System efficiency improvements
  • Circuit reconfiguration due to system disturbances.

Telemetry at discrete locations along the circuit facilitates switching decisions with improved certainty of the outcome of the decisions. The sensor outputs support phase-balancing activity, resulting in improved efficiency at the discrete monitoring locations along the circuit. The real-time reporting from the sensors allows operating personnel to respond to seasonal changes in system parameters. Finally, system disturbances are readily identified and promptly reported for immediate action by operating personnel. The isolation of faulted sections on the circuit can be performed based on the certainty of the actionable feeder intelligence reported from the sensor sites.

The line-post sensor facilitates collection of real-time intelligence. Real-time operations are improved with each discrete monitoring location. The real-time intelligence from the line-post sensors supports improved actionable decisions by the operating personnel.

Line-Post Sensor

The line-post sensor typically provides two low-ac energy, continuous-voltage outputs proportional to the line current and phase voltage at the point of application along the distribution feeder. The per-phase installation of sensors supports the typical power measurements for voltage, current, active power and reactive power on a per-phase basis to address the varying nature and frequent imbalance of the distribution loads along the radial distribution feeder. The direct ac input to the advanced RTU from the sensor also supports fault detection and power-quality measurements including harmonic content. The RTU reports the detection of fault current when the phase current input exceeds the fault current threshold setting and the duration of the fault current exceeds the time threshold. The RTU then calculates and reports the percent of total harmonic distortion for per-phase voltage and current while updating the power measurements.

Grid Operational Decisions

The fault detection intelligence is essential to making prompt operational decisions, isolating the faulted section and restoring service to the unfaulted sections of the distribution feeder. This operational feature can enable the self-healing nature of the smart distribution grid in the new era of advanced distribution automation. The line-post sensor supports operational decisions with pre-event load and post-event voltage data. Conditional switching decisions are made based on the actual measurements during a disturbance event. Thus, the asset utilization factor of the distribution feeder is increased even during near-peak system conditions.

The per-phase power measurements from the line-post sensors facilitate the real-time monitoring of losses along the distribution feeder and provide the necessary intelligence to reconfigure the smart distribution grid to minimize distribution system losses. The availability of the power measurements ensures the opportunity to dynamically minimize line losses throughout the annual loading cycle of the distribution feeder.

In urban service areas having many contiguous circuits, the circuit configuration during valley load conditions is expected to be different from the peak load conditions. Thus, frequent reconfiguration of the contiguous circuits is expected using distribution automation devices equipped with line-post sensors. For example, when actual measurements are captured during a 100°F (38°C) day and several 100°F days are expected thereafter, output from the line-post sensors provide the intelligence to minimize line losses and maximize the asset utilization of the distribution facilities during severe system loading events.

Multiple and Dissimilar Applications

The line-post sensor is applied in multiple and dissimilar applications along the distribution feeder, such as in distribution line monitoring, a switched capacitor bank, feeder voltage regulators and an automated distribution line switch. These dissimilar applications are coordinated with the source substation protection scheme, pole-mounted reclosers and sectionalizers along the distribution feeder. The pole-mounted recloser also may be automated, adding another remote view of the distribution feeder for fault detection and service restoration purposes.

The dissimilar applications use the line-post sensor to achieve their individual and unique design purposes; however, they become multipurpose with the integration of the line-post sensor to report feeder intelligence. As a result, each site along the distribution feeder becomes more than its functional purpose (e.g., remote-operated gang switch, switched capacitor bank, automated regulator). The multipurpose site also supports real-time operational decisions, fault detection, planning decisions and power-quality measurements. The feeder automation scheme, with its monitoring and reconfiguration capabilities, is greater than the sum of its parts. Thus, the goals and objectives of the smart distribution grid can begin to be realized.

In the early days of distribution automation deployment, distribution line monitoring was achieved using the 15-kV-class line-post sensors to recover feeder intelligence. The deployment of the line-post sensors in the feeder automation schemes can support the following at each site:

  • l Detection per phase

    • Voltage

    • Current

    • Megawatts

    • Megavolt-amperes-reactive

    • Megavolt-amperes (calculated)

    • Power factor (calculated)

    • Percent of total harmonic distortion

  • Fault detection.

While the line monitoring site does not affect the connectivity of the feeder, it does provide the complete suite of detection and measurements to facilitate feeder operations and respond to feeder disturbances. Intelligence from the line monitoring sites is frequently used for operational switching decisions using the nearby manual switches along the feeder. With the advent of the Electric Power Research Institute's Green Circuits initiative for the smart distribution grid, the line monitoring site is being deployed to recover the necessary feeder intelligence and ensure smart grid objectives are being achieved (e.g., circuit operation within regulatory constraints and increased asset utilization).

Switching Applications

The 15-kV- and 35-kV-class line-post sensors can be integrated with a remote-operated gang switch. This switch is normally closed, thus there is one set of sensors installed. For the normally open switch, one set of sensors with voltage and current detection is installed on one side of the switch and one set of voltage-only sensors is installed on the other side of the switch. This sensor configuration on the normally open switch supports the close-transition switching activity for contiguous feeders.

The line-post sensors also support distribution switched capacitor banks. Independent per-phase operation of each capacitor bank and the per-phase switching of the capacitors on zero-voltage crossing on closing is supported. The capacitors are switched to manage the volt-ampere-reactive load at the point of application along the distribution feeder. While doing so, the switched capacitor bank provides the same feeder intelligence, as mentioned previously, to support operational switching decisions of the nearby manual switches on the feeder source and load sides of the bank. Thus, the switched capacitor bank is used to manage the feeder's reactive load and is also an active participant in switching decisions for loss minimization and feeder reconfiguration during disturbance events.

Voltage-only sensing has been deployed to complement other line monitoring capable sites, such as switched capacitor banks and remote-operated gang switches, on the evaluation circuits in the Green Circuits initiative. A new optical sensor is being evaluated in a field trial by Alabama Power for use on the distribution system. The optical sensor is attached to the line conductor close to the pole but is not physically attached to the pole hardware for mounting purposes. Alabama Power's distribution design team reports promising early results from the field trial site.

To the On-Ramp and Beyond

The line-post sensor is quickly becoming an integral component of the smart distribution grid. The new era of advanced distribution automation is beginning to be realized with the installation of the line-post sensor. The sensor's continuous monitoring capability serves as the eyes and ears for distribution operating personnel. It provides the foundational technology to enable the self-healing nature of the smart distribution grid. Thus, the line-post sensor opens the on-ramp to the smart distribution grid.


George Larry Clark (glclark@southernco.com) is a principal engineer, power delivery for Alabama Power Co. (APC) and supports the deployment of the distribution SCADA, distribution automation and electronic mapboard technologies. He also supports the development of the integrated distribution management system and the smart grid strategy for APC and the Southern Company. During his 42 years at APC, Clark's responsibilities have included the operation and maintenance of transmission and distribution substations, and the planning, operation and maintenance of the electric distribution system. Clark received a BSEE degree from the University of South Alabama. A senior member of IEEE, Clark is the vice chairman of the IEEE PES Smart Distribution Working Group. He is a registered engineer in Alabama.

Companies mentioned:

Alabama Power Co. www.alabamapower.com

Electric Power Research Institute www.epri.com