By Ed Sullivan, freelance writer
After almost 25 years of service, the generator and transformer protection systems at the hydropower plant of a western U.S. dam and reservoir were upgraded, including the replacement of electromechanical relays with microprocessor-based relays from Schweitzer Engineering Laboratories Inc. (SEL).
The dam is an earth-and-rock-fill structure, more than 600 feet high and 1,500 feet wide at the crest. Located above ground at the dam site, the power plant houses two 150 MW turbine hydrogenerators. The power generated at this peaking plant supports the requirements of the nearby area, with the remaining energy marketed through the grid to customers outside the area.
Major components of the upgrade project included two SEL-387E relays installed for comprehensive protection, metering and event reporting of the power plant transformers. Generator protection is provided by two SEL-300G generator relays, which also provide important metering and events reporting/recording capabilities plus thermal protection in conjunction with resistance temperature detectors (RTDs).
Historically, the system used analog transducers (12 per hydrogenerator) to convert RTD measurements into scaled analog quantities. These transducers required annual calibration, using considerable technician time in the process.
 The hotspot was at the endcaps of the generator stator winding; the heat radiated toward the RTDs that are located above. |
Due to maintenance requirements and some heating issues on one generator, a more economical and reliable RTD technology was considered. The initial approach was to take advantage of the capability of acquiring and transmitting temperature data with an SEL 2600A RTD module.
“The customer chose the SEL-2600A RTD module because it requires no annual calibration or maintenance and results in a simplification of RTD wiring,” says Greg Rauch, SEL application engineer. “A single fiber-optic interconnection between the SEL-2600A and SEL-300G relay provides all needed temperature data for the alarm and protection capabilities within the SEL 300G relay.”
There were heating issues with one of the hydrogenerators. Up until the SEL relay installation, alarm and protection functions were only on one RTD for each generator.
“The customer later found that the RTD used for this purpose was not located at the hot spot of the generator,” says Rauch. The hot spot was at the endcaps of the generator stator winding, and the heat radiated toward the RTDs located above. There were no data gathering capabilities with the relay-connected RTD, so no information was available as to what was happening with that RTD.
 The dam is an earth-and-rock-fill structure more than 600 feet high and 1,500 feet wide at the crest. |
Rauch says the customer considered changing the RTD being monitored by the existing relay, but decided instead to install the SEL technology, which can give alarms and tripping on all RTDs simultaneously.
The original SEL-2600 RTD module was installed and tested. However, the RTD-connecting cables were not shielded, and the existing RTDs had a much lower signal-to-noise ratio tolerance; therefore, the module could not overcome the noise from the RTD wiring while the generator was under load.
To deal with the noise problem, SEL developed a new RTD module, the SEL 2600A. Designed for use with transformers, generators, electric motors and other power system apparatus, the SEL 2600A operates with four types of RTD inputs, including 100-ohm platinum, 100-ohm nickel, 120-ohm nickel and 10 ohm copper. The SEL 2600A is a replacement for the SEL 2600 and provides enhanced noise rejection technology. Fiber-optic links eliminate expensive cable pulls and provide electrical noise immunity and ground isolation between devices.
Another benefit of the installation is that a single fiber-optic strand runs to the control room where the generator protection relay is located and handles data that come from the SEL-2600A. Rauch says that eliminates several control wires, which are susceptible to electromagnetic interference and other noise sources. “Because this product eliminates the use of transducers altogether, it frees additional space in cable trays and wiring junction boxes.”
The SEL-2600A temperature monitoring scheme at the dam and power plant is used for alarm and trip functions in each SEL 300G. Temperature data are passed to the plant data control system via Modbus communications from the SEL 300G to the plant remote terminal unit. Stator temperature data are available locally and at the main control center that is more than 60 miles away.
After several years of use, the new power plant protection system is operational and error-free. Thermal data are now transmitted from 12 different RTDs in each hydrogenerator. The RTDs are virtually maintenance-free, which frees up technical support personnel. The project engineer felt that the project was a highly worthwhile experience.
The SEL-2600A is compatible with several SEL devices, including the SEL-701 and SEL-749M motor protection relays. It may also be used with SEL fiber-optic transceivers to connect the SEL 387, the SEL-300G, the SEL-421 and SEL-451 protection, automation and control systems, and the SEL-2032 and SEL 2030 communications processors.
Author: Ed Sullivan is a freelance writer based in Hermosa Beach, Calif.