Leake is a 161/46/13.2-kV substation located close to the southern border of the Tennessee Valley Authority (TVA; Knoxville, Tennessee, U.S.) supply area in Mississippi (U.S.), serving the Central EPA distribution company at 46 kV and 13.2 kV. In 2001, the area peak load of 130 MW had load growth estimated at 4.1% per year.

Studies by TVA's bulk transmission planning group indicated the Leake area was susceptible to voltage problems. Possible remedial measures included a near-term plan to rebuild and convert the Leake-Sebastopol-Lake 46-kV line to 161 kV, and a long-term plan for a new 50- to 60-mile (80- to 97-km) 161-kV line from the Red Hills Generating Station to Leake Substation.

Discussions with American Superconductor (AMSC; Westborough, Massachusetts, U.S.) suggested a short-term solution to address the voltage problems in the area and to postpone some of the new transmission. AMSC proposed this might be an opportunity for a D-VAR, a power electronics-based dynamic reactive compensation system with subcycle response time (essentially a small Statcom). An alternative form of D-VAR, called D-SMES (which included energy storage) also was available. Because the D-VAR is trailer mounted, it could be quickly installed as a short-term solution and then relocated as desired.

AMSC assisted with studies to evaluate this short-term solution. Studies were performed using PSS/E on 2001, 2003 and 2010 summer peak load flow and stability base cases to determine whether the study area was susceptible to fast voltage collapse or slow voltage recovery. The study used steady-state load flow analysis and dynamic stability analysis. A voltage collapse criterion was adopted, requiring the transmission voltage to recover to at least 90% of nominal voltage within 0.5 seconds after clearing of a fault.

The analysis showed significant potential existed for severe voltage problems any time the load in the Leake area exceeded approximately 97 MW, or 77% of the predicted 2001 summer peak load level. The problems included steady-state low voltages that would have required load shedding of 33 MW following an outage of a single 161-kV line section, and also voltage collapse following a fault on the 161-kV system and subsequent breaker openings.

The most serious problem was found to occur for an outage of either the entire Philadelphia — Pearl River — Leake 161-kV line or any section of the line. Steady-state 161-kV voltages in the 60% to 80% range could occur, depending on load level and which of the various line sections was out of service. The analysis conducted determined that voltage collapse could occur following a fault on the Philadelphia — Pearl River — Leake 161-kV line. During the voltage collapse situation, area transmission and distribution voltages dropped to 40% to 70% and did not recover. Voltages in these ranges would result in loss of load, damage to customer equipment or both.

Based on TVA's 2001 load duration curves, the Leake area was at risk of either low steady-state voltage or voltage collapse approximately 12% of the time or 1070 hours per year. Additionally, it was expected the number of hours the line would be compromised would increase with load growth to 2278 hours by 2003.

A Solution to the Problem

AMSC proposed an 8 MVA D-VAR and an 8 MVAR distribution capacitor bank at the Leake Substation as a solution to solve immediate voltage problems and allow deferral of new transmission construction. Figure 1 shows the results of implementing the D-VAR solution after the outage of the Philadelphia — Pearl River — Leake 161-kV line. The D-VAR was able to effectively restore the voltages within the criterion.

The analysis demonstrated the D-VAR installation would reduce the number of hours the Leake area would be exposed to low voltage and voltage collapse from 1070 to 136 hours per year, a reduction of more than 87%. The completion of the Leake — Sebastopol — Lake 161-kV upgrade in 2003 and continued use of the D-VAR system was found to be sufficient to prevent all types of voltage problems from 2003 through 2007.

Further analysis showed an additional 8 MVA D-VAR device and a 15 MVAR 161-kV capacitor bank at Leake and Sebastopol would prevent voltage problems through 2010. This solution would allow at least a four-year deferment of construction of the new 161-kV line from Red Hills Generating Station to Leake Substation. The D-VAR solution, along with the upgrade of an existing distribution line, addressed all of TVA's voltage problems in the Leake area and allowed delayed construction of the new transmission line.

Implementation and Installation

TVA purchased the D-VAR solution for installation at the Leake Substation. Because studies showed only modest advantages from the SMES energy-storage option, this was not purchased. Figure 2 shows a one-line diagram of the solution in place. The D-VAR and associated capacitor bank was connected to the 13.2-kV bus at Leake via a transformer. Installation and commissioning were completed by July 2002. The solution also included an 8 MVAR distribution capacitor bank, which was to be controlled by the D-VAR. Figure 3 shows the installation of the D-VAR at Leake. Key features noted for the D-VAR include:

• It can control multiple capacitor banks, allowing future expansion of VAR capability.

• It is air-cooled, a factor believed to be significant in the reliability of the equipment.

TVA has considerable experience with reliability issues from water-cooled high-voltage power electronics. The equipment also offered a transient overload capability, providing up to 2.3 pu instantaneous output for up to 1 second. Although TVA has not studied application of this capability in detail, it is expected that it could be significant in mitigation of voltage collapse. The D-VAR includes an operation recorder, dial-up communication providing remote access to equipment and system condition, and dry contacts, which can be connected to an external recorder to confirm equipment operation. Its auxiliary power supply includes batteries, which can supply the internal control and computers for approximately 30 minutes. TVA has an independent power-quality recorder at Leake.

One lesson learned was that TVA's normal construction requirements did not allow full advantage to be taken of the temporary installation possible with a trailer-mounted device. Some aspects of the installation were based on permanent installation designs, with higher associated costs. Future installations will include additional planning to address this.

D-VAR has proven to provide good reliability in supporting voltages on the Leake area transmission system. One somewhat unexpected result has been the high number of operations — a consequence of the large number of voltage sags in the area. In the first 19 months since its installation in July 2002, it has responded to more than 500 events. Figure 4 highlights the events by month and also delineates three-phase sags from all other events. During this time, the D-VAR responded to and mitigated 71 three-phase sags.

Of the recorded events, 333 were characterized as shallow sags and 170 were sags greater than 10% of nominal. Of the 33 three-phase sags, 27 were characterized as deep events.

Since it was commissioned, the D-VAR has maintained a 99.66% availability rating. Availability was determined by dividing on-line time by the total possible hours, excluding planned/scheduled preventive maintenance, substation outages or time when the unit was placed off-line to facilitate other work in the substation. Total possible hours includes down time due to failure and response and repair time.

Two minor unexplained events have occurred with the D-VAR. At a relatively early stage of operation, the unit was found off-line with two phases of its 13.2-kV breaker open. This was probably due to a component failure elsewhere in the substation. More recently, following a prolonged local outage that exceeded the D-VAR's auxiliary battery supply life, the record of the D-VAR's operation at the start of the outage was not found, although it is believed that in all other respects it operated normally. In addition, based on operational experience, AMSC has modified the control settings of the D-VAR, including its automatic resetting. There have been no major outages required for repair of the equipment.

Conclusion

The reliability and characteristics of a D-VAR as a near-term solution for the Leake area have justified its selection as a cost-saving measure allowing delayed capital expenditure. It has deferred construction of a new 161-kV line for at least four years. Moreover, it has proven itself in the field by responding to more than 500 events, while maintaining a 99.66% availability rating. Until the long-term solution is in place, the D-VAR system will help TVA provide Leake area customers with reliable power. Because the D-VAR is trailer-mounted, the eventual construction of the new line will allow it to easily be reused at other locations.

Ian Grant is a senior engineer in TVA's transmission planning department.
isgrant@tva.com