New technology streamlines operations and improves customer service.
Lincoln, Nebraska, U.S., is located near the top end of Tornado Alley-famous for the violence and frequency of its thunderstorms. Maintaining reliable electrical service in this environment is a particularly demanding task. To contend with the fury of these storms, automatic isolation of faults and reclosing circuits are major applications to include in substation integration designs. After reviewing the cost and limited capabilities of its conventional alarm and remote terminal unit (RTU) systems, Lincoln Electric System (LES) was motivated to implement new technologies that would improve reliability and make the utility more competitive.
Based on the limitations of its current system, plus the potential cost of installing remote terminal units, LES initiated a programmable logic controller (PLC)-based control and alarm strategy in 1996. Rather than use an existing substation to test the new technology, this publicly owned non-profit company selected a new distribution substation as the demonstration site. Factors such as the size of the site, minimum risk, and timing made the new substation a good selection - particularly since LES knew it had two major 115-kV ring bus substations scheduled to be built in the near future.
Without a critical deadline, the new substation (NW 48th & Adams) offered LES personnel the long anticipated opportunity to learn the capabilities of a PLC/PC system in substation integration applications-knowledge that could be put to use on larger projects in the future. After nearly a year of interviewing consultants, LES contracted with NovaTech, Lenexa, Kansas, U.S., to design the measurement, communication and control functions for the new substation integration system. LES required a contractor that had extensive field experience with PLC-based control strategies, including PLC programming, to integrate the new substation with existing systems and practices.
Scope of Project The NW 48th & Adams substation has one 35-kV supply feed, one 20 MVA transformer and three 12-kV distribution feeders. Schweitzer Engineering Laboratories (SEL) relays are used for protection and a Beck-with tap changer control for tap changing under load (TCUL). Bitronics MultiComm digital power meters are used to measure secondary power and to display these values at each breaker panel. The meters also communicate these measurements over a local area network (LAN) for use by the supervisory control and data acquisition (SCADA) system, PLC and the human machine interface (HMI) devices. The substation is connected to the existing SCADA system, which uses CDC Type I protocol. Local control was implemented using a personal computer with Wonderware In Touch software integrated with the LAN communication system. TCUL was implemented in the PLC in addition to normal control, which can be initiated locally or by SCADA. NovaTech implemented these concepts using Modicon hardware and NovaTech software.
All communication devices are connected via a Modbus Plus LAN. The Bitronics meters, the PLC and the HMI have native Modbus Plus ports, which allow them to be connected directly to the LAN. Devices that do not provide a direct Modbus Plus connection must use an interface device, in this case the Modicon Programmable BridgeMux. NovaTech has a protocol conversion library for the BridgeMux that permits connecting many devices to the Modbus Plus LAN and to SCADA. In the NW 48th & Adams substation, the SEL relays and the SCADA interface required the use of the BridgeMux interface. Figure 1 illustrates the simplified communications system.
SCADA The SCADA BridgeMux maintains a database of all the points being monitored by the SCADA master. The database is constantly refreshed with information supplied by each device on the LAN. The Bitronics' meters provide the electrical measurements, and the PLC and relay bridge provide the status information. SCADA commands are sent to the PLC over the LAN.
The SCADA BridgeMux also provides a diagnostic port that is available locally or remotely using a dial-up telephone. The diagnostic port provides communications statistics and the communications messages (to and from the Master) in ASCII format. This capability reduces, if not eliminates, the need for a test set to troubleshoot the RTU. Other diagnostic tools are also available to pinpoint problems in the substation communication system.
Programmable Logic Controllers The PLC provides status inputs and executes commands sent by SCADA or the local HMI. The PLC uses Modicon high power output cards to provide sufficient voltage and current to switch substation devices, eliminating the need for interposing relays. The PLC is also configured to provide additional SCADA data types such as sequence of events (SOE) and momentary change detect (MCD). The SOE card, designed by Monahan Engineering, is capable of 1 msec resolution and can record up to 2000 events per card. The MCD data types are obtained using standard status inputs with NovaTech program modules in a PLC ladder logic program. The MCDs are detected and reported to the SCADA BridgeMux and, in turn, to the remote SCADA Master.
Once a PLC-based system is in place, additional automation is cost-effective. For example, dynamic voltage control (the TCUL) is accomplished through the PLC using simple ladder logic. The accuracy and speed of voltage measurements provided directly over the LAN by the Bitronics MultiComm meters is used to maintain stable voltage control via the transformer tap changer. The Bitronics meters supplied the voltages, currents and power factor values to the PLC, as well as the panel display.
Human Machine Interface The human machine interface (HMI) is a personal computer connected to the Modbus Plus LAN that uses a Wonderware In Touch graphical interface package. The graphical illustrations are designed to emulate the SCADA control center screens. The similarity in colors, shapes and data presented on the screens at the control center and the substation permit effective communication between system operators and technicians in the field.
The HMI has a relational database (Microsoft Access) that stores substation data for later retrieval. The database allows the user to group data for detailed review. Lincoln Electric System's preventive maintenance activities are then prioritized based on this analysis.
Dial-up Communication The protective relays communicate via the Relay Bridge, as required, with the SCADA system and local HMI. The SCADA information does not routinely include event logs or oscillographic data, since this information may be contained in files much too large to transport via SCADA. However, these files are transmitted over the LAN to another interface Bridge-Mux using NovaTech's NovaHost product for retrieval by dial-up telephone on an as-needed basis. This data transfer is completed without interfering with the normal data flow to and from SCADA.
Expanding Capabilities Since building NW 48th & Adams, four other substations have implemented the PLC/PC based automated control systems. Four additional systems, for both transmission and distribution substations, are currently in the engineering/planning process. The major application at these substations is automatic fault isolation and restoration.
If there is a permanent fault on the line, the circuit breakers open the line immediately. The PLC program senses the change and begins a methodical procedure to restore service by closing the breaker(s). If the breakers hold, service is immediately restored to all lines. If the breakers open again, only the line with the fault is left open. This isolates the loss of service to the smallest number of customers possible. Once the fault is cleared, the PLC is instructed to restore service and breakers are closed.
The PLC-based system allows control philosophy changes and service changes to be quickly implemented via PLC program changes at minimal cost and without wiring changes. The performance and flexibility of the PLC technology will continue to make Lincoln Electric more competitive in an increasingly competitive marketplace.
Lessons Learned LES intentionally bought somewhat more capability than it immediately needed, anticipating the need for future expansion. This excess capability has allowed the utility to add additional control strategies as needed at little additional cost.
To no one's surprise, the implementation of advanced technologies presented a learning curve for engineering, operations and maintenance personnel. Doug Buhrman, senior engineer in the System Protection Department, was instrumental in the project's success. Working closely with NovaTech, he trained LES personnel on use of the new equipment and software. Fortunately, excitement ran high about the project and LES personnel quickly developed a good understanding of the new technology.
Leo Fendrick graduated from the University of Nebraska in 1971 with the BS in electrical engineering. He immediately joined Lincoln Electric System and is transmission and substation protection engineering supervisor. He is a member of the IEEE.