by Kerry Diehl, American Superconductor Corp.
Every day, employees of large and small U.S. electric utilities pick up the phone to hear customers say, “We’re putting in a new mining operation,” or, “We’re building a large new recycling facility at the edge of town.” New service or expansion requests such as these challenge grid operators trying to balance economic development, growth and quality-of-service issues. This is particularly true where a distribution network is aging, heavily used and therefore susceptible to voltage disturbances caused by loads with rapidly fluctuating active and reactive power requirements.
Traditional disruptive loads such as arc furnaces, rolling mill drives and auto crushers receive a lot of press coverage, but most power quality (PQ) problems are caused by a diverse group of smaller loads with rapid fluctuations in power. These “small” loads——such as large induction motors, welders, power regulators, sawmills, water treatment plants, pumps, compressors and cranes——can cause flicker, the most common voltage disturbance reported by utility customers. Large electric utilities and smaller rural electric operations alike use modular static VAR compensator (SVC) installations as a solution to such load-induced voltage disturbances. Using an SVC, system operators can eliminate voltage sags and flicker associated with the large, problematic electrical loads typically created by new or expanded loads without making larger, permanent fixed investment in the power system.
SVC solutions provide customer benefits as well, including a more stable voltage supply and higher power factor to end users. In many cases, the availability of an SVC permits end customers to further expand their facilities without impacting customers on the same circuit.
Conventional solutions to mitigate voltage sag and flicker deal with changing circuit characteristics by increasing the fault current at the point of common coupling. Depending on the circumstances, this can be accomplished by a combination of re-conductoring, building out a new circuit or generally upgrading the power system on the line side of a customer and then changing all upstream relay and protection settings. Aside from expenses associated with fixed-asset upgrades on behalf of a single customer, there is the risk that such an investment becomes useless if the customer ceases operation.
Applying an SVC can eliminate the sag and flicker, achieving results at least comparable to conventional solutions, but with two advantages. First, the typical cost of an SVC is significantly lower. More important, an SVC can be relocated if load requirements change. It can be field-modified, including expansion, to meet changing circumstances. The ability to relocate these solutions lowers investment risk dramatically and provides system operators with a mobile asset that can be used and reused to address changing load requirements on a utility system.
SVCs use thyristor switch technology to respond to voltage changes cycle by cycle. This high-speed response allows SVCs to offset the effect problematic loads have on power systems. Using totally passive, solid-state, modular designs, SVCs operate as needed and without mechanical wear and tear.
SVCs: Resolving real-world problems
REA Energy Cooperative Inc., which provides electric service to more than 24,000 members in seven rural western Pennsylvania counties, installed two American Superconductor SVC smart switches to resolve flicker problems at coal mines in the service area. The REA installations are for mining operations with loads in the 1 MW to 5 MW range, with each taking service from a 12 kV distribution feeder that is shared with other customers. Routine operation of the mining equipment caused flicker and other disturbances for customers on the same circuit. Power system operators frequently encounter this scenario: a single industrial operation requiring service.
REA considered many options, including line reinforcements, reconductoring, converting to higher-distribution circuit voltage, providing a dedicated circuit as well as a series capacitor solution. None approached the ease, favorable first and operating cost and technical advantages of a modular, drop-in SVC solution with remote-monitoring capability and harmonic filters. Given the success of these two SVC installations, REA has modified its connection policies for large loads under similar load-to-fault current conditions. REA now mandates that these customers install an SVC or comparable mitigation as a condition of the connection agreement. REA is considering adding harmonic filters in new SVC installations to enlarge potential PQ benefits.
PacifiCorp., one of the largest electric utilities in the western U.S. and serving more than 1.6 million customers across six states, has already overseen installation of four SVC solutions, including one operating at transmission-level voltage. PacifiCorp, which has more than 6,000 employees, operates some 61,000 miles of distribution line and thus SVC installations in remote areas of the utility’s grid are of particular value to providing voltage support to its large customers. Customers in these remote areas, sometimes at the physical end of distribution circuits, are especially susceptible to larger voltage fluctuations and frequently experience lower than normal voltage levels under everyday, steady conditions.
PaciCorp’s first experience with an AMSC distribution voltage SVC was at an auto-shredding facility connected at 138 kV, which was near another facility operating an electric arc furnace. The auto shredder, which employed a 5,000 horsepower (HP) wound rotor induction motor, suffered from low voltage and flicker. The installation of a pad mounted 8.4 MVAR SVC at this location provided virtually complete mitigation of the previously serious PQ issue.
PaciCorp adopted an additional SVC solution to mitigate voltage disturbances stemming from a geothermal power plant in its Milford, Utah, service area. The plant has 90 percent or greater availability and, when in operation, provides voltage support for the area. When the plant was not operating, PaciCorp received numerous complaints from local customers about flicker caused by the largest hog feed mill in the western United States. It provides feed for tens of thousands of animals by operating two 700 HP pelletizers. Although the large AC motors have “soft starters,” the loading on the running motors varies widely and can approach locked rotor current. The net result was that when the geothermal plant was idle, the grid was weak and the pelletizers caused flicker.
To resolve this, PacifiCorp installed a remotely monitored, pole-mount 3.1 MVAR SVC that was deployed in two days for a fully installed project cost of less than $300,000. If an SVC solution had not been available for this location, the only option would have been to upgrade the local transmission system at a cost of some $10 million.
In summary, SVCs have evolved from a standard transmission system tool to a proven distribution-voltage PQ enhancer. Large and small electric distribution companies, and large electric users, are encountering distribution SVCs with increasing frequency and learning that they are a most cost-effective means to place large, problematic loads on distribution circuits.
Kerry Diehl is director of SVC products at American Superconductor Corp. in West Mifflin, Penn. E-mail him at firstname.lastname@example.org.