Building a Better Way

Teams From Two Universities Create an Ultrafast Mechanical Disconnect Switch for Distribution System Protection

By Tushar Damle, Jonathan Goldman, Lukas Graber and Chunmeng Xu, Georgia Institute of Technology, and Matthew Bosworth and Michael Steurer, Florida State University

An estimated 7.6 million people lost power in Florida, Georgia and North Carolina during Hurricane Irma. The outages lasted from a few hours to several days, depending on the location. Other less devastating weather events also resulted in a loss of power to a few hundred thousand people in Pennsylvania, New York and Michigan, all in 2017. Utilities across the country are challenged by weather events and disasters that create power disruptions, collectively costing them and their customers billions of dollars a year. Restoring power to customers as quickly and efficiently as possible–or, better yet, reducing the number of customers losing power–is a top priority for all utilities.

The switch paddle of fast mechanical disconnect switch

A grid that is highly interconnected can restore power to affected areas faster, but interconnecting substations increases the fault current level, exposing substation equipment to higher levels of stress. This means many utilities must upgrade their substation equipment to meet the requirements for higher fault current ratings. In addition, increasing penetrations of renewables contributes to higher levels of fault currents, making utilities reluctant to encourage home owners and small businesses to connect renewable power sources to their grids. Technologies and devices that limit the fault current enable more resilient grids with a greater share of renewables.

CAD model of the complete switch assembly (BELOW) — patent pending

“Fault current limiting devices reduce the stress on grid components, allowing us to keep them working in parallel,” said Sergo Sagareli, a senior engineer at Consolidated Edison of New York, which is working on an initiative to improve current limiting devices.

Hybrid Circuit Breaker-based Solution

A hybrid circuit breaker-based solution is being developed to enable higher fault currents and thus improve grid resiliency. The hybrid circuit breaker-based solution, first proposed by ABB, combines solid state switches with a fast mechanical disconnect switch that can limit the fault current to two to three times the nominal current and protect the substation equipment that would otherwise not be rated to the higher fault current levels.

In a hybrid circuit breaker, the load current flows through the mechanical switch where losses are low, which is shown as A in the figure above. Upon fault, the current is redirected to the solid-state switch by the opening of the current commutation switch, which is the auxiliary breaker that is shown as B in the figure. The mechanical switch subsequently opens at zero current and the solid-state switch, C in the figure, clears the fault well before reaching the prospecting peak current. Transient switching energy is absorbed in another parallel device, which is shown as D in the figure. The total time taken to clear the fault is expected to be less than one eighth of a power cycle (< 2 milliseconds), which limits the fault current.

Typical hybrid circuit breaker topology with the mechanical switch (yellow)

The enabling technology for a hybrid circuit breaker is a fast mechanical disconnect switch, capable of opening in a few milliseconds or less while also carrying high continuous current during normal operation. A team from the Georgia Tech Plasma and Dielectrics Lab, in collaboration with the Center for Advanced Power Systems (CAPS) at Florida State University and Georgia Tech VentureLab, are working to develop such a fast mechanical disconnect switch. The fast mechanical disconnect switch is an improved version of the so-called fault isolation device, developed by CAPS and North Carolina State University (NCSU) for the FREEDM Systems Center.

The mechanical switch uses a piezoelectric actuator integrated inside a vacuum switching chamber (top of figure at left). Two ceramic bushings act as the power terminals on top of the grounded chamber. The actuator is controlled from signals outside the chamber though a multi-pin vacuum feedthrough. The actuator has an elliptic shell that amplifies the mechanical response of the piezoelectric stack. The elliptic shell is housed in a vacuum-compatible polymeric frame to which the outer conductors and moving contact tabs are attached. The switch (bottom of figure at left) uses contacts of optimized geometry and material to maximize voltage withstand capabilities when the switch is in the open position and to minimize losses during current conduction. Arc quenching capabilities are not required because the hybrid topology guarantees contact opening at zero current.

“A cost-effective, hybrid combination of the low loss ultra-fast mechanical switch with series commutating switch, in parallel with the interrupt capabilities of a solid-state switch to limit and clear fault currents, would have great potential as a replacement for traditional breakers and fault interrupting switches,” said John Schaffer, consultant and former general manager of G&W Electric Co.’s system protection division. Schaffer, is one of several industry experts involved in the initiative to create and prove the ultra-fast mechanical switch.

The original design developed for the FREEDM Systems Center was optimized for 15 kV distribution grids and rated for 100 amp RMS (ARMS) current with 200 ARMS short-term overcurrent capability. Upgrades to 600 ARMS continuous current rating are currently being implemented. Contact opening time has been demonstrated to be less than 1 millisecond. Experiments have confirmed that the separated contacts can currently withstand 18kVRMS continuously. Further improvements will aim at increasing the voltage withstand capabilities without sacrificing speed. The disconnect switch will be tested at CAPS’s power hardware-in-the-loop testbed. The testbed includes several power electronic converters rated up to 5 MW (variable voltage source inverters and modular multilevel cell converters) and a real time digital simulator (RTDS). The switch will be tested for its ability to clear faults in both AC and DC distribution systems as a part of a hybrid circuit breaker.

The disconnect switch also has potential applications in protection of DC distribution systems, either as a part of a hybrid circuit breaker or a standalone disconnect switch. The disconnect switch can be used as a hybrid circuit breaker for the protection of DC microgrids and energy storage systems, where the lack of a natural zero crossing during short circuit necessitates fault current limitation. In addition, the disconnect switch can be used as a stand-alone protection device for all-electric ships proposed by the U.S. Navy. Such ships take advantage of the rapid response of power electronics for fault protection without the penalty of having to shut down major segments of the electrical system to isolate a fault. This necessitates using fast disconnect switches in the load carrying branches, in their “breakerless” distribution systems.

The fast mechanical disconnect switch being developed by Georgia Tech and Florida State University has applications in multiple areas because it limits the fault current and enables rapid reconfiguration of the system after clearing the fault. It can enable a greater share of renewable energy sources in the grid and help in faster restoration of power to areas affected by natural disasters. Furthermore, it can increase life expectancy of substation equipment by implanting controlled switching schemes. This project is funded in part by the National Science Foundation.| PGI

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