Distribution Automation / Substation Automation, Metering

Rural Co-op Builds on Flexible PLC System to Control Capacitor Banks

Issue 11 and Volume 18.

by Mack Wainwright, Central Alabama Electric Cooperative

Central Alabama Electric Cooperative (CAEC) between Birmingham and Montgomery primarily serves a residential member base of more than 41,500 across a 10-county service territory.

Although the area’s growth has leveled off since 2008 as a result of the economic downturn, CAEC has continued to invest in smart grid technologies to ensure higher reliability and systemwide cost management efficiencies when the expected growth resumes. The modernization program included better peak management though power factor optimization.

Query Result
Query Result

CAEC has employed various voltage and reactive power control capabilities to stabilize the distribution system for reliability support in meeting peak demand. More recently, there has been an increasing interest in enhancing and extending these control capabilities with forms of real-time optimization.

To do this, CAEC created a plan to optimize power factor with new and existing assets in the distribution network. Forty-eight new capacitor banks were added and 27 existing units have been upgraded for remote operation. Capacitor bank data flowing through the TWACS advance metering infrastructure (AMI) from Aclara populates an ESRI geospatial information system (GIS) and is seen in a Futura mapping system. The plan also calls for developing a MultiSpeak data interface between the CGA supervisory control and data acquisition (SCADA), formerly QEI, substation automation and the TWACS AMI system to manage open and close commands.

Legacy voltage and reactive power control capabilities, along with the recent CAEC initiatives, require voltage measurements at key locations in the network. There are a wide range of configuration options; however, CAEC wants to leverage the AMI as a natural outgrowth of these considerations. Advanced meters can provide a low-cost addition to existing, dedicated voltage-monitoring equipment and can provide voltage measurement samples at frequent reporting intervals through the existing network.

Interoperability

Instead of using preprogrammed capacitor banks to locally control voltage levels and reactive power flows, CAEC saw the value of coordinating them in a more comprehensive, systemwide fashion. Interoperability is an absolute necessity, particularly when sharing data between operational and enterprise systems.

CAEC considered both wired and wireless fixed-base AMI options and went with a power line carrier (PLC) system because of the vastness of its territory and pre-existing infrastructure. The PLC system also works well in the power factor management application with the capacitor banks.

A simple SQL script was written for each capacitor bank to communicate open/closed status, voltage readings, current on neutral and voltage readings from down line meters to the AMI headend. The script is written:

  SELECT a.CSTSTATE,
a.CMDSENTDT AS DateRead, a.SERIALNUMBER,
a.CSTVOLTAGE AS Volts,
a.CSTNEUTRALCURRENT AS Amp
FROM CSTRESPONSELOG a
WHERE a.CMDSENTDT=
   (SELECT MAX(CSTRESPONSELOG.CMDSENTDT)
   FROM CSTRESPONSELOG WHERE
   CSTRESPONSELOG.SERIALNUMBER =
   a.SERIALNUMBER)

When performing a query of the system tags, the user sees the following record, minus the callouts:

With an off-the-shelf tool (Import EXT), the same data is imported into the mapping system using a common field to create each feature class within the geo-database. Symbols were created to represent open, closed, and nonresponding cap banks and banks with blown fuses.

Import EXT
Import ExT

Import EXT also allows for categorizing voltages to display high, low, and normal voltages.

All of this information is displayed on the map, where a cap bank on a feeder is indicated as open on the first map and closed on the second.

AMI a Handy Conduit

Experiences with numerous distribution automation and field-area network implementations have brought to light two important factors that affect power:

1) The need to select the right communications infrastructure or field-area network to support voltage measurement and control capabilities; and
2) The value of AMI as the foundation for linking data as additional input for feeder voltage measurements to complement existing, dedicated voltage-monitoring equipment.

Historically, the control of voltage and reactive power (VAR) flow has been an important basic element in electric power measurement. Tariffs regarding customer service include minimum voltage levels and the need to control reactive power and meet power quality requirements.

Import EXT Map
Import EXT Map

Given how voltage levels drop toward the end of long distribution feeders, utilities have had to supply power at a marginally higher voltage level from the substation than the required minimum. These buffers of higher voltage supply levels ensure that customers at the end of long feeders receive good electrical service.

As real-time monitoring capabilities improve and sensors become more economical, utilities can lower their voltage buffer and associated costs while maintaining or even improving upon their compliance with the required minimum voltage and power quality levels.

With the Aclara AMI, CAEC can employ the same communications infrastructure to switch capacitor banks. With this flexibility in mind, utilities that combine distribution automation and AMI can create synergies that help lower their total cost of ownership for smart grid-related deployments. The primary purposes of AMI and smart metering are to lower expenses by automating meter reading and providing real-time customer usage data, enabling time-of-use scenarios or other applications such as prepay. The same data and infrastructure provide a lower-cost data source for more detailed voltage information. In this scenario, the type of AMI network helps determine whether a lower-cost voltage solution is the better choice.

The communications infrastructure needs to be sufficiently robust, scalable and interoperable to ensure resistance to obsolescence and ongoing positive return on investment for the overall solution. This need is particularly important, given the changes in communications requirements and intensity that can occur as utilities evolve their capabilities to support the more complex and communications-intensive grid operations of the future.

Communications Plan: Implementing AMI

In supporting power factor through the AMI, employing a flexible communications network is critical. A good communications implementation plan uses channelization of the network to avoid creating data collisions when AMI voltage data from meters is run along with the regular network traffic. CAEC is highly satisfied with the progress it has made with the greater control from its PLC-based AMI for power factor optimization and anticipates achieving higher performance once the MultiSpeak interface to the SCADA system is completed.

Mack Wainwright is a GIS and metering supervisor at Central Alabama Electric Cooperative in Prattville, Ala.

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