Piedmont EMC, located in Hillsborough, North Carolina receives power delivery from an investor owned utility (IOU) via a radial 44 kV line. The 44 kV line serves two Piedmont EMC 44/12.47 kV substations before terminating at a 44/5 kV substation dedicated to a pipeline customer immediately adjacent to the substation.
When the either of the customer motors start without an SVC present, a sag exceeding 10% is observed on the 44 kV system at both of other substations which are serving residential loads. The 5 kV pipeline bus sags to approximately 0.83 per unit voltage during these motor startups. The voltage level is adequate for a prolonged start, but extends the duration of the voltage sags seen at the other two substations.
This customer has been operating since 1973, but the sag-related complaints grew as residential load increased at the two adjacent substations and as Piedmont EMC member dependence on digital electronics grew.
The initial effort to address motor starting sags used fast-switched capacitors. Classic capacitor problems were experienced like switching transients when capacitors came on-line, sags when VAR demand outstripped supplied capacitance as the capacitors were energized, and over-voltages prior to capacitor switching off. An excellent discussion of the alternatives subsequently considered for this application is contained a paper written by Ed Thomas .
Of all the solutions analyzed, the best solution to solve the voltage sag issue was the distribution SVC. Reduced-voltage soft-starting was considered, but its limited benefits and customer concerns about starting torque eliminated it as an alternative. Other alternatives that were considered and discarded included series capacitors.
Once a distribution voltage SVC was chosen, Piedmont EMC considered where to locate the SVC(s). Their careful analysis led to some highly pertinent observations.
- Locating an SVC on the 5 kV bus provided the best isolation due to the impedance between the 5 kV station and the other two Piedmont EMC substations. However, Piedmont EMC had no other 5 kV substations, and a 5 kV SVC would not be reusable.
- Placing the SVC directly on the 44 kV line was considered and discarded as being more expensive. Transformation would be required, and the total price of the SVC system could easily double.
- Placing the SVC on one or more 12.47 kV buses was studied. Simulations suggested that use of a single SVC system at the closest substation would yield results comparable with individual SVCs at each substation for about half the cost.
The chosen alternative is shown Below.
Piedmont EMC One-Line Diagram
The SVC selected for this application was rated at 8.4 MVAR, 3-phase, and it provides 7 levels of capacitive support in increments of 1240 kVAR. This step-size equates to a 3.2% step voltage change at nominal voltage, and was chosen using the GE Flicker Curve based on the frequency of the motor starts. Subsequent field measurements showed the effective voltage resolution of the SVC being below 3%, since VARs are applied in response to ramp voltage changes. Figure 4 shows the actual results of the starting of one of the pump motors with the SVC in-service.
First Pump Motor Start with SVC In-Service
The SVC unit was installed in 2004 and has performed reliably since then. In 2008, the SVC control was upgraded and it continued to operate well until 2016, when it was upgraded to the latest generation control system.
Piedmont EMC SVC in Substation
Piedmont’s careful consideration of alternative SVC locations resulted in a system with substantial residual value. By taking the responsibility for the SVC’s pad and wiring, they minimized their costs and developed an initial familiarity with the SVC. While AMSC provided startup services, and specialty maintenance (e.g.., controller upgrade), most of the regular SVC maintenance is performed by Piedmont EMC line crews.
Employing just one Instrumentation Technician, Piedmont elected to joint T-Star's remote monitoring and maintenance program in 2016. T-Star regularly accesses the SVC's data logging history, checks system performance, and determines when and if to adjust operating parameters. T-Star is able to diagnose any actual problems down to the component level for replacement by EMC personnel, and can diagnose many latent problems such as failing (but not yet completely failed) capacitors.