Surge current protection using superconductor

Surge current protection using superconductor

A Fault Current Limiter (FCL) is a device which limits the prospective fault current when a fault occurs (eg in a power transmission network). The term is generally applied to superconducting devices, whereas non-superconducting devices (such as simple inductors or variable resistors) are typically termed Fault Current Controllers. (For example, the ground fault circuit interrupter is commonly used in residential installations.)

Summary

Fault-current limiters using high temperature superconductors offer a solution to controlling fault-current levels on utility distribution and transmission networks. These fault-current limiters, unlike reactors or high-impedance transformers, will limit fault currents without adding impedance to the circuit during normal operation. Development of superconducting fault-current limiters is being pursued by several utilities and electrical manufacturers around the world, and commercial equipment is expected to be available by the turn of the century.

Fault-Current Problem

Electric power system designers often face fault-current problems when expanding existing buses. Larger transformers result in higher fault-duty levels, forcing the replacement of existing buswork and switchgear not rated for the new fault duty. Alternatively, the existing bus can be broken and served by two or more smaller transformers. Another alternative is use of a single, large, high-impedance transformer, resulting in degraded voltage regulation for all the customers on the bus. The classic tradeoff between fault control, bus capacity, and system stiffness has persisted for decades.

Other common system changes can result in a fault control problem:

·         in some areas, such as the United States, additional generation from cogenerators and independent power producers (IPPs) raises the fault duty throughout a system

·         older but still operational equipment gradually becomes underrated through system growth; some equipment, such as transformers in underground vaults or cables, can be very expensive to replace

·         customers request parallel services that enhance the reliability of their supply but raise the fault duty

Superconductive FCL

Superconductors offer a way to break through system design constraints by presenting an impedance to the electrical system that varies depending on operating conditions. Superconducting fault-current limiters normally operate with low impedance and are "invisible" components in the electrical system. In the event of a fault, the limiter inserts impedance into the circuit and limits the fault current. With current limiters, the utility can provide a low-impedance, stiff system with a low fault-current level, as Fig. shows.

Fig. ‘ Fault control with a fault-current limiter.

In Fig. , a large, low-impedance transformer is used to feed a bus. Normally, the FCL does not affect the circuit. In the event of a fault, the limiter develops an impedance of 0.2 per unit (Z = 20%), and the fault current I_SC is reduced to 7,400 A. Without the limiter, the fault current would be 37,000 A.

The development of high temperature superconductors (HTS) enables the development of economical fault-current limiters. Superconducting fault-current limiters were first studied over twenty years ago. The earliest designs used low temperature superconductors (LTS), materials that lose all resistance at temperatures a few degrees above absolute zero. LTS materials are generally cooled with liquid helium, a substance both expensive and difficult to handle. The discovery in 1986 of high temperature superconductors, which operate at higher temperatures and can be cooled by relatively inexpensive liquid nitrogen, renewed interest in superconducting fault-current limiters.

Think of Fault Current Limiters (FCLs) as advanced and large-scale surge protectors capable of protecting large portions of the electric power grid. Any homeowner knows that surge protectors limit the damaging currents that can harm plugged-in household devices. Simply put, FCLs can provide that same service for electric utilities. These specially designed devices allow for uninterrupted electrical service by limiting and regulating the amount of current moving through the transmission and distribution systems under abnormal conditions.
The emerging technology of FCLs has the potential to save money for utilities and increase efficiency for their customers by protecting equipment from damage and avoiding interruptions and outages. As the demand and sources for electricity rise, utilities are grappling with the challenge of more frequent and larger "fault currents."
Blackouts cost the U.S. economy somewhere between $104 billion and $164 billion annually, according to figures from the Electric Power Research Institute. EPRI's research, conducted in 2001, was compiled by the Consortium for Electric Infrastructure to Support a Digital Society (CEIDS) (PDF 581 KB). Utilities fear outages caused by fault currents could become more common as the demand for electricity continues to grow, especially in urban centers. The risk of larger fault currents also grows as more and more power sources are fed into the grid.
For instance, almost half of the states have mandates requiring the grid to handle significant boosts in the amount of energy from renewable sources such as solar and wind. These renewable energy sources are often connected in large numbers at specific locations in the electrical grid and can produce fault currents in excess of local limits.
Interest in advancing the use of FCLs is growing as utilities and the Department of Energy collaborate with manufacturers, national laboratories, and other stakeholders to modernize, expand and increase the capabilities of the nation's stretched-to-capacity electric grid. FCLs have the potential to play a pivotal role in transforming the current grid into the DOE's vision for a smart and more efficient grid.


During a ground fault, an FCL safely mitigates excess energy and prevents damage by switching to a high impedance state, which would normally affect utility transmission and distribution equipment.

Developed by ABB Corporate Research in Baden-Dättwil, Switzerland, the current limiter takes advantage of the unique ability of superconductors to transmit electricity without losses when cooled below a certain temperature and when the electric current is below a certain threshold level. When a short circuit occurs, the electrical current immediately rises above the critical value, which effectively 'shuts off' the current limiter's superconductivity. The resistance of the current limiter then instantly increases and reduces the current surge caused by the short circuit.

Superconductive Fault-Current Limiter Concepts

The Series Resistive Limiter

The simplest superconducting limiter concept, the series resistive limiter, exploits the nonlinear resistance of superconductors in a direct way. A superconductor is inserted in the circuit. For a full-load current of I_FL, the superconductor would be designed to have a critical current of 2I_FL or 3I_FL. During a fault, the fault current pushes the superconductor into a resistive state and resistance R appears in the circuit.

The superconductor in its resistive state can also be used as a trigger coil, pushing the bulk of the fault current through a resistor or inductor. The advantage of this configuration, shown in Fig.  is that it limits the energy that must be absorbed by the superconductor.

Fig.  Fault-current limiter with HTS trigger coil.

The fault-current limiter FCL normally is a short across the copper inductive or resistive element Z. During a fault, the resistance developed in the limiter shunts the current through Z, which absorbs most of the fault energy.

The trigger coil approach is appropriate for transmission line applications, where tens of megawatt-seconds would be absorbed in a series resistive limiter. The trigger coillconfiguration also allows an impedance of any phase angle, from purely resistive to almost purely inductive, to be inserted in the line.

The Inductive Limiter

Another concept uses a resistive limiter on a transformer secondary, with the primary in series in the circuit. This concept, illustrated in Fig.  yields a limiter suitable for high-current circuits (I_L > 1000 A). One phase of the limiter is shown. A copper winding W_Cu is inserted in the circuit and is coupled to an HTS winding W_HTS. During normal operation, a zero impedance is reflected to the primary. Resistance developed in the HTS winding during a fault is reflected to the primary and limits the fault.

The inductive limiter can be modeled as a transformer. The impedance of this limiter in the steady state is nearly zero, since the zero impedance of the secondary (HTS) winding is reflected to the primary. In the event of a fault, the large current in the circuit induces a large current in the secondary and the winding loses superconductivity. The resistance in the secondary is reflected into the circuit and limits the fault.

Fig.  Inductive fault-current limiter.

Inductive Fault Current Limiters for Grid Protection 50% reduction of fault currents – ¼ cycle reaction time – handles long duration faults and breaker reclosure attempts Fault tolerance is an increasingly important issue in power grid operation. Zenergy Power’s Fault Current Limiters protect power grids against damaging power surges caused by short circuits or lightning strikes while maintaining a disruption-free downstream power supply. Inductive Fault Current Limiters provide power grid operators with a new solution for grid reliability, cost-efficient grid expansion, and integration of distributed generation sources Inherently failsafe inductive limitation of fault currents without interruption in downstream power supply; Entirely passive and self-triggered Failsafe limitation of multiple faults; automatic recovery after each event Improved operational reliability in fully-stretched grids; Greatly reduced risk of large scale blackout due to cascading grid failures following a fault event Standard solution for cost-efficient grid expansion; Deferred replacement of substation equipment Simplified integration of distributed power generation into the network infrastructure.   Zenergy Power’s Fault Current Limiter has been put into regular operation in the United States’ power grid in March 2009. Southern California Edison is the first electric utility company in the US to use the device for protecting a distribution circuit of its medium voltage grid. 1.     Function 2.     Installation 3.     Economy   Inductive fault current limitation Fits in the grid without any retrofitting Efficient investment for grid expansion

Fault-Current Limiter Applications

Fault-current limiters can be applied in a number of distribution or transmission areas. Three main applications areas are

1.      . Fault-current limiter in the main position. The fault-current limiter FCL protects the entire bus

2.    Fault-current limiter in the feeder          position. The fault-current limiter FCL protects an individual circuit on the bus. Underrated equipment can be selectively protected as needed in this manner.

3      Fault-current limiter in the bus-tie      position. The two buses are tied, yet a faulted bus receives the full fault current of only one transformer.

. Fault-current limiter in the main position. The fault-current limiter FCL protects the entire bus.

The most direct application of a fault-current limiter is in the main position on a bus (Fig. ). Benefits of an FCL in this application include the following:

·         a larger transformer can be used to meet increased demand on a bus without breaker upgrades

·         a large, low impedance transformer can be used to maintain voltage regulation at the new power level

·         I²t damage to the transformer is limited

·         reduced fault-current flows in the high-voltage circuit that feeds the transformer, which minimizes the voltage dip on the upstream high-voltage bus during a fault on the medium-voltage bus

Fault-current limiter in the feeder position. The fault-current limiter FCL protects an individual circuit on the bus. Underrated equipment can be selectively protected as needed in this manner.

40MVA

TRANSFORMER

 LOAD

An FCL can also be used to protect individual loads on the bus (Fig.). The selective application of small and less expensive limiters can be used to protect old or overstressed equipment that is difficult to replace, such as underground cables or transformers in vaults.

. Fault-current limiter in the bus-tie position. The two buses are tied, yet a faulted bus receives the full fault current of only one transformer.

40MVA                                                                                                                                                                                                                                                TRANSFORMERS

                                                       

LOAD 

 An FCL can be used in the bus-tie position (Fig. ). Such a limiter would require only a small load current rating but would deliver the following benefits:

·         separate buses can be tied together without a large increase in the fault duty on either bus

·         during a fault, a large voltage drop across the limiter maintains voltage level on the unfaulted bus

HTS LEADS

HTS current leads represent the first large-scale application of high temperature superconductivity. This has occurred because even modest current density HTS material can be used to provide a significant reduction in the parasitic heat conducted into a cryogenic environment via the electrical leads used to provide current to the device. Fig.  illustrates a typical application.

Fig. . A conduction-cooled HTS magnet system used for magnetic separation, illustrating the use of HTS current leads to reduce heat load (LANL).

There are three classes of applications where HTS leads are seeing rapid introduction:

1.      high current LTS magnets cooled by liquid helium

2.      conduction-cooled LTS magnets

3.      conduction-cooled HTS magnets

HTS Lead Technologies

There are two basic technologies for HTS leads: bulk rods of ceramic superconductor, and metal matrix superconducting composites. Both have developed to the point that they are offered for commercial sale. There are advantages and disadvantages to each.

Bulk ceramic leads : are made by a variety of methods and of a number of different HTS materials, but the primary objectives are the same: to achieve a rugged ceramic structure with high critical current and low-resistance connections. The advantage of this approach is that the ceramics have intrinsically low thermal conductivity, so that leads may be made quite short for easier integration in the system. The disadvantages are that the ceramic rods are susceptible to breakage during installation, during operation, and (like the bulk structures used in fault-current limiters) during temperature excursions caused by driving the lead normal. In addition, it has proven difficult to provide very low resistance connections between the ceramic superconductor and the metallic connections at the ends of the leads. These disadvantages have been mitigated by careful system design in a number of magnets, and successful systems have been built using bulk leads.

Metal matrix composite leads : essentially use the powder-in-tube technology used for BSCCO wire to manufacture a wire or tape incorporating a low thermal conductivity metal or alloy in place of the customary silver matrix. This approach has employed only Bi-2223 superconductor. The advantages of metallic leads are intrinsic ruggedness, high tolerance to thermal excursions, and very low contact resistances. These advantages are to be balanced against the disadvantage of the somewhat higher thermal conductivity of the composite material, which requires that a longer lead assembly be used to achieve heat leaks comparable to bulk leads.

. Bulk HTS leads manufactured by Furukawa Electric.

. Metal matrix HTS leads manufactured by American Superconductor Corp.

 

QUENCHING OF SUPERCONDUCTORES

A superconductor when operated with in a certain temperature and external magnetic field range

      critical temperature –Tc

      critical magnetic field-Hc

 exhibits no electrical resistance  if the current flowing through it is below a certain threshold critical current level –Jc and is said to be in superconducting state

However if the current exceeds this critical current level the superconductor will undergo transition from its superconducting state to a normal resistive state takesplace .this transition is termed as quenching.

 

 

 

 

 

 

 

 

 Development

FCLs are under active development. In 2007, there were at least six national and international projects using magnesium diboride wire or YBCO tape, and two using BSCCO-2212 rods. Countries active in FCL development are Germany, the UK, the USA, Korea and China. In 2007, the US Department of Energy spent $29m on three FCL development projects.
Low temperature superconductors cannot be used for commercial FCLs as the AC losses at liquid helium temperatures mean that the cryogenic cooling cost makes the whole device uneconomic.
First applications for FCLs are likely to be in electric-drive ships: naval vessels, submarines and cruise ships. Many more FCLs will eventually be used to help control land-based electricity distribution and transmission systems.

World's highest-rated superconducting current limiter
1.2 MVA SCFCL in powerplant "Löntsch" (Nordostschweizerische Kraftwerke, Switzerland)
The new current limiter is the world's first superconducting device to go into service in a power plant. The prototype was installed in the Löntsch hydropower plant, near Glarus in central Switzerland, in September 1996 for long-term performance monitoring under actual operating conditions.

Japanese FCL Program

The driving factors for current limiters in Japan are somewhat different from those in the United States, given that IPPs and cogenerators are not as prevalent in Japan. Rather, the demand for power in Japanese metropolitan areas continues to grow because of economic growth and increased consumer use of electricity. In addition, industrial use of computers and other power-quality-sensitive equipment has forced the utilities to provide higher quality and more reliable power. The quite successful approach to improved power quality in Japan has been to increase connections between various power systems and to concentrate generation capacity in larger, more efficient units. Increasing interconnection does, however, increase the maximum fault current available at any point in the system, and this is rapidly leading to the need for breaker upgrades and system reconfigurations. Adding to the complexity of the situation in Japan is the limited room at substation sites, which can preclude breaker upgrades. The primary need, as expressed by management of the Tokyo Electric Power Company (TEPCO), is for a limiter for the nucleus of the Japanese transmission system, the 500 kV transmission grid.

In response to this real market pull there has been a series of programs to develop fault-current limiters using a variety of methods, with recent focus on superconducting limiters (Nakade 1994). Although FCLs are not a component of the NEDO budget, TEPCO has reported that it spends about ¥100 million per year (~$1 million) on this program, and some resistive FCL work is apparently included in the NEDO budget under the topic "Research of Superconducting Materials and Devices."

In the late 1980s, Seikei University manufactured a small-scale three-phase current-limiting reactor and demonstrated successful operation. This three-phase system introduces a large unbalanced reactance in the system to limit currents in the case of a single-phase short and quenches to introduce resistance in the circuit in the case of a three-phase fault.

Mitsubishi Electric Company (MELCO) has been participating in a MITI/NEDO FCL program since 1990. This is a resistive limiter approach using HTS films on a strontium titanate substrate that has demonstrated limiting of 400 A currents to 11.3 A. The Central Research Institute of the Electric Power Industry (CRIEPI) has developed the inductive limiter shown in Fig. 4.11 (Ichikawa and Okazaki 1995). This approach, similar to those of ABB and Siemens-Hydro Quebec, uses a cylinder of bulk BSCCO-2212 or BSCCO-2223 to separate a normal copper coil from an iron core. In normal operation, the field from the copper coil does not penetrate the superconductor; under fault conditions, however, the current induced in the superconductor is sufficient to drive it normal, and the magnetic field links the iron yoke. This greatly increases the inductance of the copper coil, thus providing current limiting. CRIEPI work has focused on ac magnetic shielding performance of bulk superconductors and their responses to fault currents. In addition, introduction of a "control ring" in the system to absorb some of the energy deposited during a fault has reduced the cooldown time of the shield following a faulted state.

. Schematic diagram of the CRIEPI inductive FCL (Ichikawa and Okazaki 1995).

The most extensive FCL program in Japan has been the collaboration between TEPCO and Toshiba. The long-term goal of this program is the development of a 500 kV limiter with a rated current of 8,000 A. Initial development has been focused on a distribution-level limiter designed for 6.6 kV.

As shown in Fig. , the FCL is formed by connecting four superconducting coils in a series-parallel configuration so the total inductance is minimized. One set of coils is used for each phase of the device, and limiting is accomplished by quenching the coils. The current version of the FCL shown in Fig. 4.13 uses a special low ac loss Nb-Ti conductor. Tests in a circuit with a nominal short circuit current of 25.8 kA have successfully demonstrated limiting to about 4,000 amps

Fig. 4.12. Configuration of coils in the TEPCO/Toshiba FCL (Nakade 1994, 34).

Fig. 4.13. Exterior view of the 6.6 kV 2,000 A-class current limiter. The coil is 420 mm in diameter and 640 mm long (Nakade 1994, 35).

Current limiting characteristics of Toshiba FCL shown in Fig. 4.13 (Nakade 1994, 35).

Recent work has included the introduction of HTS current leads to reduce the refrigeration load of the system to levels that can be handled by a 4 K Gifford McMahon refrigerator. Over three generations of the device, the heat leak has been reduced from 13.8 watts to 3.4 watts, which is nearing the required level.

Conclushion

Electric power disruptions cause hundreds of millions of dollars worth of economic loss every year to the world’s leading economies. Worldwide energy demand is increasing rapidly, requiring new solutions to dramatically improve the reliability of our energy supply. Fault Current Limiters are new devices, using the unique electrical properties of HTS to almost instantaneously protect power grids against short circuits and thereby prevent costly outages. They are a key member of a family of ultra-fast HTS devices and machines used for conditioning electrical power.

Benefits

In comparison to conventional technology, HTS fault current limitters provide

• Over 100 times faster response time
• 10 to 20 times shorter recovery time
• Time-adjustable response functions
• 1000 times the number of full-power protection cycles