Southern California Edison gains experience installing a full-scale 15-kV class superconducting fault current limiter on its Circuit of the Future.
In recent years, as a result of interconnected energy sources and new transmission lines, ever-increasing fault current levels have created an urgent need to limit fault current to safe, manageable values without degrading the robustness and stability of the power system. For this reason, Southern California Edison (SCE) provided a test bed to validate a fault current level and obtain experience with fault current limiter (FCL) devices.
SCE has a long tradition of collaborating with industry, government and academia in the development and demonstration of equipment that solves power system problems. The utility promotes the advancement of power system technologies to benefit customer service and improve operational efficiency.
Operating from March 2009 through October 2010, SCE's FCL demonstration project became the first high-temperature superconducting (HTS) FCL installation in commercial service in the United States. The field demonstration was cosponsored by the California Energy Commission (CEC) and U.S. Department of Energy (DOE) under the oversight of the California Institute of Energy and Environment (CIEE).
Site Selection and Switching
After reviewing several potential locations for the FCL demonstration site, the Avanti Circuit — also known as the Circuit of the Future — was selected. It is one of several 12-kV feeders from the Shandin Substation.
The Avanti Circuit is unique because it is designed with enhanced automation normally found on transmission circuits, not distribution circuits. For example, a fault can be detected and isolated with the remaining healthy load supplied from alternate circuits.
Other reasons for selecting this circuit for the FCL demonstration is that the available fault current of the Avanti Circuit can reach values that will test the FCL, and the Shandin Substation had sufficient space to accommodate the installation. Also, Circuit of the Future component fault current ratings are sufficiently high, so if the FCL were taken out of service, there would be no safety or reliability consequences.
The FCL installed on SCE's system was a full-scale engineering prototype based on a biased, saturable core using a HTS DC coil. This unit is the product of Zenergy Power from a six-year effort partially funded by the CEC and DOE.
To help ensure service reliability to all customer load during the demonstration period, an innovative bypass/isolation switch providing three connection possibilities was conceptualized by SCE and custom built by G&W Electric Co., a circuit breaker supplier. The bypass/isolation switch can connect the FCL into the source, applying 12 kV to the FCL; connect the FCL in series with the source and load for normal operation; or completely isolate the FCL.
Other Site Issues
SCE selected elbow terminations to interface the high-voltage power terminations, allowing the FCL to be easily connected to the circuit through the bypass switch. Deadbreak, premolded cable connectors penetrating the FCL enclosure 3 ft (0.9 m) above ground level were installed for the 12-kV cable connections. This facilitated the physical connection and disconnection of the FCL and also eliminated cable splicing.
The connectors are 25-kV voltage class, 125-kV basic insulation level (BIL), and current rated at 900 A continuous and 25 kA momentary. Stainless-steel flanges were specified for these connectors to reduce induced eddy currents. The flanges and cable shields were grounded to the FCL's common grounding circuit, which, in turn, was connected to the substation's ground mat.
The SCE and Zenergy team worked closely to define the auxiliary power needs for the FCL. Since 120/240-V, three-phase delta was available on site, the low-voltage components were designed for this. The AC auxiliary power load at a typical medium-voltage substation is about 40 kVA to 75 kVA. To meet the 100-kVA requirement for the FCL, the station power rating was upgraded, allowing the FCL load to operate without affecting the substation's normal load. A 100-A, three-phase circuit breaker was installed as a dedicated source for FCL power.
A local city ordinance allows no more than 55 dB of total sound energy to be emitted during the daytime and 45 dB at night as measured at the perimeter of the substation. Sound measurements with the unit on and off were performed. To comply with the stated limits, the FCL was sound insulated to better contain the sound emission of the compressors and cold heads. The FCL enclosure also was oriented 30 degrees off parallel with respect to the bypass switch to reduce the noise projected to the closest neighbors.
As there are no industry-accepted standards for FCL testing, the National Electric Energy Testing Research and Applications Center (NEETRAC) in close collaboration with several of its member utilities, including SCE, developed a detailed FCL test program based on IEEE and International Council on Large Electric Systems (CIGRÉ) standards for transformers and reactors.
The Avanti FCL was rated at 15 kV, 1,200-A rms and designed to limit a 23-kA rms symmetric fault by at least 20%, with less than 1% voltage drop at maximum load current. The FCL was first subjected to heat runs at a load current of 750 A and full DC bias current to verify the maximum temperature rise of the AC coils and HV terminations.
A comprehensive battery of high-power tests were then conducted at BC Hydro's Powertech laboratories. A total of 65 separate tests were performed, including 32 full-power fault tests with first-peak fault current levels up to 59 kA, all at rated voltage. Fault tests included individual fault events of 20 cycles duration to 30 cycles duration, multiple fault events in rapid sequence (to simulate recloser operation) and extended fault events of up to 82 cycles duration, simulating primary protection failure scenarios. The measured performance agreed closely with calculations in all cases.
The FCL also was tested under full lightning impulse at Powertech's high-voltage laboratory. Under the first round of tests, flashovers occurred between the high-voltage jumper cables connecting the AC coils. All jumpers were replaced and rearranged with more separation. Other high-voltage enhancements made to pass the more-severe chopped impulse tests included the application of heat-shrink insulation over the exposed terminations; larger lugs and wall openings; molding mastic rubber to cavities, sharp objects and connectors; and solid connection of all ground leads to a common ground point.
The following high-voltage switching surge overvoltage acceptance testing was conducted at SCE laboratories:
55-kV peak (1.2 × 50 µsec) wave
One 110-kV peak (1.2 × 50 µsec) wave
One 60-kV peak chopped (at 2 µsec) wave
Two 120-kV peak chopped (at 2 µsec) waves
Two 120-kV peak (1.2 × 50 µsec) waves (within 10 minutes after the last chopped wave).
SCE's commissioning tests at the site consisted of coil resistance, power factor and megger measurements of all phases. These measurements were performed as a safety and reliability check every time work was conducted in the high-voltage compartment to verify the high-voltage insulation had not been disrupted. The change of the FCL's insertion inductance as a function of the DC bias current also was measured by SCE by reducing the DC bias current from its full operational value to zero.
Operational Event: Resonance
During testing, the FCL experienced a loss of DC bias current caused by an unexpected reset of the programmable automation controller triggered by random access memory (RAM) overflow. After being reset, the controller came up in an idle or off mode. This shutdown occurred at a time when the circuit load current was approximately 120 A. The FCL remained in the circuit for approximately 45 minutes to 50 minutes, producing an initial 300-V rise in line voltage, followed by a 400-V drop in line voltage on the 12-kV circuit bus.
During this period, two large automatic capacitor banks operated to regulate the voltage on the circuit according to their programmed limits. The insertion of the FCL's high nonlinear inductance, due to the absence of DC bias current, generated some harmonics, some of which caused a minor resonance effect in combination with the shunt capacitors. The FCL was bypassed after 50 minutes, and the circuit returned to normal operation. There were no negative consequences; however, as a result of this event, the bypass switch was always set in its automatic mode whenever the FCL was in service.
Operational Event: Cryogenic Shutdown
Only once, during a hot summer week, did the FCL experience venting of the cryogenics fluid and require replenishment of the liquid nitrogen. The site reached an ambient temperature of 108°F (42°C) in July 2009, causing power to the compressor compartment's heating, ventilation and air conditioning (HVAC) to trip. The refrigeration systems of the device had operated almost continuously since its installation.
The solution was to upgrade the 3-ton HVAC to a 5-ton unit rated at 125°F (52°C) ambient working temperature. An extension to the existing compressors' enclosure was added to provide sufficient airflow to the three faces of the heat-exchanger coils. A shade structure for the 5-ton HVAC and the compressors' enclosure also was built. No changes were needed for the station power and light supply, and there were no further HVAC interruptions throughout the summer on even hotter days.
Operational Event: Multiple-Fault Event
Ten months after installation, the FCL experienced its first in-service fault. Measured data confirmed the event evolved from a phase-to-phase fault, to a three-phase fault, to a temporary recovery, to a phase-to-phase fault, to another three-phase fault, ending with an open line and clearing. This multi-fault event occurred over a 3-sec period.
The sequence initiated when phases A and B of the overhead circuit came together during high winds. This phase-to-phase fault lasted about 250 msec, when it evolved into a three-phase fault, lasting about half a second. The air insulation between the phases recovered its dielectric strength for about 1 sec, but then the conductors of phases A and B again flashed over to each other for about three-fourths of a second.
The arc was about to be extinguished when phase C flashed, resulting in a three-phase fault. The event ended about a quarter of a second later when the phase B conductor opened, dropped to the ground and the circuit's protection cleared this solid ground fault. The FCL limited the fault current throughout the various faults and sent the corresponding monitoring and control signals.
Operational Event: Auxiliary Power Loss
The FCL experienced three outages of substation auxiliary power due to faults on either the high-voltage side of the substation or the transmission lines. The FCL is equipped with a DC uninterruptible power supply, which allowed it to ride through the outages without any downtime. It sent the correct alarms and bypass commands.
In one of these events where auxiliary power was lost, a fault on the 115-kV high-voltage side caused a voltage dip and a 2-minute 12-kV outage. As required, the FCL's controller immediately issued a bypass command, sent out alerts and initiated an orderly shutdown of the DC bias current and cooling systems. When AC power resumed, the cooling system compressor was restarted and the cryogenic parameters returned to normal, with the HTS coil ready for re-energization.
The project established a superconducting FCL could be engineered and qualified to a detailed operational performance specification, defined by SCE. The operational and maintenance considerations associated with FCL use in commercial service were established through first-hand experience. This may be a basis for widespread adoption of HTS FCLs. Also demonstrated was successful current limiting and recovery from a real in-service fault event. The device operated as designed with all systems responding and recovering as expected.
SCE and Zenergy Power learned valuable lessons about the deployment of this new technology. In particular, the team learned to analyze the circuit being protected for potential resonance behavior, ensure adequate air conditioning of the compressors and control electronics and that the device can safely ride through loss of station power.
Ardalan Kamiab (firstname.lastname@example.org) is project manager of the Irvine smart grid demonstration project at Southern California Edison (SCE). Previously, he was the project manager responsible for implementing the design, construction and operation of the Circuit of the Future at SCE. He also has specialized in the area of power quality related to distribution systems and customer facilities. As an SCE senior field engineer, Kamiab was the lead for distribution substation annual load planning for two key zones of the SCE system. He has extensive experience in substation engineering and automation using electronic relaying and communication schemes in transmission, distribution and hydro generation systems.
California Energy Commission www.energy.ca.gov
Department of Energy www.energy.gov
G&W Electric www.gwelec.com
Southern California Edison www.sce.com
Zenergy Power www.zenergypower.com