Distributed Bus Protection Using Process Bus For A Large Transmission Substation At Duke Energy
Power buses are critical elements of a substation where multiple network elements, like lines, transformers, capacitor banks, etc., are interconnected. A fault on a bus affects multiple circuits simultaneously and usually results in extremely high fault currents. Without fast and reliable protection, bus faults can jeopardize personnel safety, damage equipment, and adversely impact system stability. High-speed bus protection with full redundancy is typically applied in transmission substations to protect these critical elements.
This paper will use a large 100 kV Duke Energy transmission substation as a case study for distributed bus protection using process bus. The paper will delve into the detailed design of distributed bus protection using IEC 61850-based process bus tailored to the transmission substation under study. The paper will include a discussion of Duke Energy’s study efforts in their pursuit of using digital substation technologies. The paper will provide the utility's perspective on the IEC 61850-based process bus, its potential advantages, and associated technical challenges. The technical insights derived from this study will aid Duke Energy in the decision-making process concerning the implementation of a process bus for large transmission substations.
The transmission substation at Duke Energy utilizes a double-bus configuration with manual disconnect switches, allowing each of the 16 breakers to be served from either bus. Due to the many breakers and the flexibility to manually adjust the bus connections, high-impedance bus differential relays are employed for bus protection. Currently, selector switches are included to allow a single differential zone to encompass both buses, simplifying maintenance and switching tasks. While legacy designs used electromechanical high-impedance bus differential relays, newer production standards are using high-impedance microprocessor relays. Since the substation requires full redundancy for bus protection, a comparison of fully redundant systems using the latest microprocessor designs will be included in this study. Component comparisons will be made between high-impedance microprocessor designs and equivalent IEC 61850-based process bus designs.
The process bus-based distributed bus relay supports high-speed, low-impedance bus differential elements. The bus relay is configured such that a single differential zone covers one bus during normal operation and both buses during maintenance. The distributed bus protection scheme uses 40 Process Interface Units, with 20 allocated to Main 1 and the remaining 20 to Main 2 bus relays. A comparative analysis of using IEC 61850-9-2 LE vs. IEC 61869-9 protocol for Sampled Values for distributed bus protection and its impact on process bus network design will be discussed in detail. Test results from process bus-based Process Interface Units and distributed bus relay will be presented to validate the performance of the protection scheme under N-1 contingency. The distributed bus protection design, comparative analysis, and test results provided in this paper will help utilities understand the potential benefits and technical challenges of implementing process bus-based bus protection systems.