Prior examples address flicker mitigation for single loads.
REA Energy, a 5,000 member co-op headquartered in Indiana, Pennsylvania, had a different problem. Their problem was multiple coal mining customers located on a single circuit.
The two mining operations were located in the same area, approximately three miles apart. Each used continuous miners to push several AC motor powered cutting heads into coal seams to remove material, which is then carried from the mine face by integral conveyors. Profitability is proportional to output, and miners push the cutters as close to locked rotor as possible before withdrawing them from the face to regain speed.
For flicker mitigation purposes each site was modeled as a series of independent motor loads. The large motors (powering cutter heads) were modeled as moving from idle to near locked rotor conditions. The other motors (conveyors equipped with reduced voltage soft-starters) were modeled come on and off randomly. Steady state loads such as mine ventilation and pumping systems were ignored for flicker analysis, but were included for power flow, power factor, and harmonic analyses.
With one mine in-service, customer complaints began to increase. Then the second mining company requested service, and both companies had indicated plans to increase load. In addition, the REA had also received notice from a third mining operation that was planning to locate in the same area. All the mine operations would be served off of the same 12.47 kV circuit fed from a 46 kV substation.
Considering how to address the flicker, the REA was aware that typically these small mines in Appalachia last three to fifteen years. Even if reconductoring the 12.47 kV line would yield acceptable flicker mitigation, it would also result in a permanent high ampacity upgrade to a circuit that would have a maximum use of only seven years.
Choosing an SVC, the REA was able to reduce its initial costs and still have a reusable asset when mining ceased. During the planning process, the REA had two additional concerns surface: would each mine require an SVC and how the addition of a third mine on the circuit would affect the SVC(s) that would be purchased.
Since the circuit is not dedicated to the mining customers only, other customers can see flicker from mining operations. Analysis of the line impedance between mines showed that locating the SVC immediately to the source side of the first mine, and then correcting to levels below the IEC flicker table (Table A. 1, IEEE-1453), would leave flicker at the respective mine entrances at acceptable levels. Thus, one SVC could be used to treat all mining loads.
The flicker caused by the first two mines was addressed by using a 7-step SVC rated at 2.4 MVAR. A one line arrangement of the installation is shown in Figure 7.
The addition of the third mine would increase the maximum flicker levels and require a larger SVC than was required to serve two mines. If the third mine came on-line, this could be solved by designing the SVC for an in-field expansion. AMSC SVC installations are designed for field expandability: In this case, the system can be expanded to a 15-step SVC with a rating up to 6 MVAR by the addition of three static valves and suitable capacitors.
REA Energy One-Line Diagram
Since the third mine has not yet started, this SVC design proved useful. The REA did not invest in unneeded capacity.
In 2014, the two mines expanded. T-Star, working with the customer, was able to:
- Update to the next generation of controls (note: these are not the most current level of controls: they're not one generation back.)
- Add two 1200 kVAR vacuum-switched filter banks to provide base-load voltage support extending the range of the existing SVC.
In 2016, one of the two existing mines shut down. In 2017, the other existing mine announced plans to expand in order to mine the coal reserves that were to have been removed by the third mine.
REA's choice of an SVC was the prudent decision, ensuring that it's members (customers) received reliable, flicker-free electric service. The SVC was modified and reprogrammed several times during the opening, closing, and change in operations of the (originally three, now two) mines. When the second mine shuts down, in the foreseeable future, REA is assured that:
- It's provided good service at the lowest possible cost to its members.
- It's fully recovered what it spent from the mines over the life of the mines.
- It has a fully usable SVC that can be deployed for reuse, using its own personnel and a couple days of work.
- It hasn't "stranded" any assets running wires and building substations in an area that will never need them again.
Piedmont SVC - in Lieu of a large Substation