Tightening trip protection in spite of information delay
As power systems across the country operate closer and closer to maximum capacity, the strategies that control operation and protection must be enhanced, or blackouts and brownouts will become more common.
Protecting power system equipment requires high-speed decision making that is incompatible with delays of even tens of milliseconds. Protection decisions are weighted in favor of protecting the millions-of-dollars worth of equipment and long-term power capability over keeping the lights on in the immediate moment. This means that sometimes local and widespread blackouts may occur unnecessarily.
The phasor measurement unit (PMU) technology that was first built at Tech in the 1980s is becoming the global standard for power utilities to get wide-area, real-time data across their grid. However, there is a milliseconds-long delay, or latency, involved in getting remote or distant phasor measurements and relaying stations do not have the complete information for their decisions.
Led by the developers of the PMU, a power engineering research team is working with California utilities to overcome this issue and to determine optimum protection policies and settings for critically located relaying stations. The team is developing adaptive relaying technology with slow speed-protection in which the delay in remote phasor measurements is not an obstacle. Adaptive relaying systems use digital relays that can change their setting to adapt to changing system conditions, either automatically by sensing the proper parameters, or in response to a control signal.
The team is attacking the problem on four fronts. They are devising schemes for supervising back-up zones with remote phasor measurements so that back-up protection is not allowed to operate when it is not appropriate. “Instead of letting a single protection relay open a circuit at its first hint of trouble, we’re saying, ‘Let’s vote: if two relays say open, we’ll open,’” said ECE Department Head James Thorp, who is working on the project. “Using data-mining and other analysis, we’ve determined that this concept works remarkably well.”
The team is also devising algorithms and simulations to produce recommendations on load shedding, as well as techniques to adjust loss-of-field relay settings automatically. Finally, they are also designing a robust damping controller using remote phasor measurements to control dc-power lines to damp ionter-area oscillations. “All of these measures are within the capabilities of today’s technology,” said Thorp.
Virgilio Centeno is the principal investigator on the project. He and Thorp are joined by Arun Phadke, an emeritus university distinguished professor and Jaime De La Ree. The project is funded by the California Institute for Energy and the Environment (CIEE) and is being done in conjunction with Pacific Gas & Electric Company and Southern California Edison.