PP&L Develops Cost-competitive Substation SCADA Remote
By Carlton H. Rinehimer and Paul Grabowski, PP&L
Pennsylvania Power & Light Co. (PP&L), Allentown, Pa., has developed a hybrid SCADA system for installation in area supply substations scattered throughout central and eastern Pennsylvania. The hybrid system in each station consists of a centrally located RTU that is connected to analog meters monitoring power equipment within the substation. A LAN connects the meters to the RTU. The new system is directly replacing a variety of commercial RTUs installed 20 years ago. The new RTUs are also being added to substations that have not had SCADA systems and they will be included in new substations.
Developed entirely by PP&L substation engineering personnel, the RTU is housed in a commercially available cabinet and assembled by a local shop. The cost of the new RTU is competitive with the latest commercially available units. At the heart of the PP&L RTU is an off-the-shelf, general purpose, compact programmable logic controller (PLC) comprised of commercially available, modular components. PP&L chose to develop a standard RTU of its own design to simplify SCADA selection, troubleshooting and repair. The new device assures a continuity of SCADA features, construction and software throughout all area supply substations, whether fully developed or not. The device is programmed in ladder logic. Functions formerly done by electromechanical relays, such as simultaneous tripping and closing, DC voltage monitoring and under-frequency trip and alarm time delays are now performed through PLC interface cards and software ladder logic.
One Design for all Substations
PP&L utilizes a fully developed 138 kV to 69 kV area supply substation having two primary circuits, two transformers and eight secondary circuits. The SCADA system for such a station requires 19 discrete status and control input/output (I/O) points, 39 discrete alarm points and 46 analog points for electrical information. The new RTU features a single Modicon 984-245 compact PLC to service these points and provide the operating logic. Modicon automation products are manufactured by Schneider Automation Inc., North Andover, Mass., a unit of Groupe Schneider S.A., France.
For reasons of security, reliability, cost and space, few analog points are brought directly into the new RTU. Instead, a multifunction digital meter is added to the control cabinet of each 12 kV circuit breaker. The meter monitors three-phase currents and bus voltages, calculates megawatts and MVARs, performs A/D conversions, locally displays values and transmits values over a 1.0 Modicon Mbps Modbus Plus LAN to the new RTU. The meter is manufactured by Bitronics Inc., a Modicon ModConnect Partner, in Lehigh Valley, Pa.
Every RTU made by PP&L includes sufficient Modicon I/O modules to cover the required status, control and alarm points of a fully developed substation`s SCADA system. Several hot spare modules are included as well. Excess I/O point capacity remaining in a partially developed substation provides additional spare points. Both operating and spare modules are monitored by the SCADA system and will alarm if determined to be unhealthy. The system will also alarm if any module is missing from its slot or if the wrong module is installed in a slot.
Identical Wiring Featured
The physical layout of the PLC racks in every RTU cabinet mimics PP&L`s standard area supply substation layout. Substation sides are designated left and right when viewed from the high voltage end. Likewise, the racks in the cabinet are designated left and right (top and bottom in the cabinet) when viewed from the left (or PLC central processing unit) end. Furthermore, each rack is dedicated to a specific function.
Within a rack, each module is dedicated to a specific function. Note that the status and control racks contain an additional hot-spare output module for future control functions. Two four-point analog input modules are allotted for analog systems or for analog data independent of the Modbus Plus LAN. One analog module is dedicated to dc monitoring through resistor blocks.
Within a module, each I/O point is dedicated to a specific device present in a fully developed substation. All points, whether they will be used or not, are shop-wired from the modules to terminal block positions, and all are shop-labeled at both ends. During RTU installation, field wiring is simply connected to the correct block positions.
If a new 12 kV breaker is added to a yard, no modification to the PLC is necessary. The digital meter is simply connected to the LAN, the field wiring connected to the correct RTU terminal blocks and the SCADA software in the central office updated to display the new points.
One Program Covers all Applications
A single ladder logic program has been prepared by PP&L for shop loading into the RTUs. The program is for substations utilizing 24, 48 and 125 V dc control voltages. PP&L has also developed and is building an identical SCADA RTU for its regional and bulk power non-PLC substations. This RTU may have a second Modicon PLC dedicated to alarm functions. The alarm PLC passes alarms over the Modbus Plus LAN to the SCADA PLC and to I/O modules interfaced to a local alarm display. The SCADA PLC ladder logic program is tailored to each substation`s specific requirements. However, the benefits gained from this RTU are similar to those gained from the area supply substation unit.
As more powerful and capable PLCs are introduced, PP&L will have a clear migration path for upgrading RTUs already in the field and for designing and building less expensive, more compact and more advanced RTUs in the future.
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The new SCADA RTU, based on a Modicon PLC, was developed and built by PP&L.
The scrolling display of the three-phase multifunction digital meter is normally set to amps.
Co-authors Carlton Rinehimer (left) and Paul Grabowski inspect a 12 kV line breaker.