In March this year Southern California Edison completed the installation of Zenergy’s proprietary HTS fault current limiter into its electricity grid in Southern California, a national first for the technology. It is also the beginning of a larger programme of investigation into the value of such devices for support of the grid nationwide.
The Zenergy HTS prototype is designed to help prevent blackouts in electricity transmission systems and is the first such device ever to be installed, activated and operated in the United States grid. Following successful testing for medium voltage power grids in the USA at the end of 2008, implementation in Californian MV grids was placed under a California Energy Commission project with the result that the FCL is now online and operating in real-time to provide protection to the distribution circuit of the power grid. Its maker Zenergy is an HTS specialist with a couple of world firsts already under its belt – first to install an HTS hydro-generator (for E.ON’s grid, due in 2009) and first with an HTS applications for offshore wind power.
The firewall for power grids
An FCL is a series device installed in the electricity supply line, and acts like a firewall. Zenergy’s HTS fault current limiter, like all FCLs, is designed to protect power grid equipment against damaging power surges caused, for example, by short circuits or lightning strikes, and prevents the tripping of major plant at such times. The advantage of HTS switching lies in the inherent qualities of an HT superconductor. As the current rises in a fault condition a point is reached when superconductivity switches off and DC resistance is restored, automatically limiting the current flow. This happens more or less instantly, at any rate sufficiently quickly to prevent a transient overcurrent, significantly reducing the risk of power grid failures and interruptions to power supplies compared to a standard FCL. Zenergy’s product is of the saturated core type – an HTS coil wound on an iron core – and utilises the extremely high magnetising current available and the wide range of permeability of magnetic materials. A high permeability core allows a low impedance during normal operation and a very high impedance during fault current events.
Once the fault current subsides, the FCL again allows standard levels of current to flow, protecting the system automatically without human intervention. It is entirely a passive device and needs no resetting or post-operation maintenance.
These properties of speed and automatic resetting will be even more applicable to the ‘smarter’ grids of the future which will make great demands on the reliability, speed and repeatability of equipment to maintain supply and improve the resilience and flexibility that constantly changing and reversible flows will render necessary.
The full list of benefits is said to include:
• suppression of overloads, while the downstream power supply is maintained without any interruption, and at its regular strength
• improved operational reliability of fully stretched grids greatly reducing the risk of large scale blackout due to cascading grid failures following a local defect
• cost-efficient protection of transformers and other power grid equipment against power surge damage
• simplified integration of an increasing number of decentralised power generation systems into the network infrastructure.
In a smart market
The significance of being the first company to install and operate such a device into the US electricity grid is particularly apparent in the light of the severe problems that utility companies are facing in coping with increasing power demands from consumers while attempting to satisfy the publicly stated demand for distributed power generation (including wind and solar power) from government authorities. Combined, these factors are leading to an increased occurrence of electrical blackouts which recent research estimates to be costing commercial businesses in the United States over US$100 billion per annum.
As is widely reported, these escalating costs, along with security concerns, efficiency demands and increased renewable energy production are creating a determined effort to bring about the evolution of a national `smart grid’ in the United States. And not just in the USA – the global market for FCLs alone is expected to be worth up to US$ 5 bn per annum as it approaches maturity.
This ongoing investment would represent a significant modernisation of the existing grid system and the adoption of a number of devices from different technology areas, FCLs being prominent among them, that will lead to a more demand managed, efficient and stable electricity distribution system. Recent estimates prepared for the US Department of Energy have shown that a reduction in greenhouse gases equivalent to taking 53 million (working) cars of the road could be achieved by just a 5% improvement in overall grid efficiency. These efficiencies will also drive substantial cost savings.
This being so, it is no surprise that Zenergy has been invited to work together with a number of the utility companies in the US throughout the development of its FCL, and has also participated on projects funded by the Department of Energy, California Energy Commission and the Department of Homeland Security.
Testing fault current controllers
The future success and operational power of supergrids will be derived in large part from grid control devices like fast acting FCLs, and in pursuit of the best technology, trials of three prototype FCL installations are planned by SCE, to be carried out at its Circuit of the Future test bed. Of the three units on trial, one is a solid state device developed by EPRI, while the other two are both high temperature superconducting devices, made by Zenergy and American Superconductor respectively. The programme is already underway and is expected to take five years in all. Phase I will come to completion in 2010, Phase II two years later.
Fault currents are beginning to exceed the capabilities of circuit breakers to safely and reliably interrupt the faults. This can result in catastrophic breaker failure and exposure of transformers and other critical equipment to damaging fault currents. An alternative (though not fully satisfactory) solution is to split substation buses to redistribute the fault currents to safe levels; this approach negatively impacts system reliability and flexibility of operation. A better alternative is to protect under-rated circuit breakers so they can continue in use.
The use of FCLs suggests several likely benefits. They can greatly reduce the destructive forces of high fault currents on transformers and other equipment. They limit the necessary modifications to existing protection systems and the need to install higher-capacity breakers than already exist. They can more readily accommodate fault current contributions from new generation, especially renewables plants that come on-line in transmission-constrained areas. And they avoid SF6 issues with high-capacity breakers.
SCE’s project is intended to evaluate the current technology status of FCCs, develop test protocols and testing plans for each prototype technology, and develop evaluation criteria – performance, cost, maintenance issues, manufacturing requirements, installation issues, expected reliability, and others.
Controlled (factory or lab) and ‘real-world’ (at the SCE Avanti substation) field testing will be performed on what are regarded as the three leading technologies (EPRI and SC Power in Phase 1, AMSC in Phase 2) followed by an assessment of the potential for migration of the technology to transmission-level applications.
The technologies
EPRI’s solid state fault current limiter (SSFCL) is based on a new generation of advanced thyristor technology. Testing is scheduled for mid-2009. Zenergy’s HTS fault current limiter uses HTS elements in combination with a saturable iron core. Testing started in late 2008. The third device, an FCC being made by a collaboration of Siemens and American Superconductor, also an HTS design, is still the subject of negotiations between Siemens/AS and the DoE on project scope, schedule and funding. Testing is provisionally scheduled for 2010.
At the end of 2007 Zenergy successfully fault-tested its prototype FCL, and in 2008 entered a period of extensive modelling, computer simulation, and experimental verification when it designed and built its first CEC FCL, designed its second FCL under DoE funding, and identified a prospective host utility for the working prototype.
During 2009 it intends to assemble the subsystems for its second distribution class DoE FCL, third Party test the DoE FCL, install and operate the 26kV FCL in the electrical grid, start design work on a transmission class FCL, and evaluate commercially available 2G wire for suitability in a saturated core FCL design.
Funding
The scale of official interest can be gauged from the DoE’s willingness to support the technology with grants. In 2007 Zenergy received $11 million to support its five year programme to design, build, and test a transmission class saturable core reactor FCL, with the intermediate goal of achieving a viable distribution-class unit with a core wound with 1G wire.
Meanwhile it has had $0.5m from the California Energy Commission to install an HTS grid stability device into their grid. It is that installation that has just been completed.
Smart FCL for New York
Zenergy Power has now been contracted by Con Edison to build and test a single phase FCL for installation in New York City’s power grid.
Con Edison has authorised the project in the expectation that it would be applicable to a number of substations within the utility’s electrical systems. Zenergy expects to deliver the prototype by the end of August.
Pat Duggan, project manager and FCL specialist at Con Edison, commented ‘Fault current limiters will be an essential element of the smart grid to maintain reliability and improve its resilience and flexibility. This is especially important as the load grows, including the move to electricity as a preferred source for new uses such as plug-in hybrid vehicles.’
The tests are to ensure the device can protect equipment from fault currents in the utility’s
13.8 kV feeder system. Zenergy expects to extend its designs to other FCLs for higher voltage lines of up to 138kV and beyond.