Setting new limits

Controlling fault currents is an essential part of maintaining grid stability and power quality. Superconducting fault current limiters could help to boost power quality, and reduce utilities’ costs.

Staff Report

Maintaining power quality in the grid is one of the greatest challenges that utilities face today. With the advent of sophisticated automation systems in manufacturing and industry, end users are increasingly aware of the effect that grid disturbances can have on their operations. A short dip in the supply voltage means downtime for the operation and can even cause damage to sensitive equipment.

Disturbances in the grid system can arise from a number of causes (see sidebar) but utilities must maintain voltage and VAr support within the constraints of their existing systems. Not only must they protect the end users, but they must also ensure that the grid equipment itself is protected from damage caused by faults.

A healthy electrical system is therefore one with a high fault current tolerance. A typical line-to-line fault will create current levels up to ten times higher than the rated current, putting high levels of stress on grid components. Each fault experienced by utility equipment reduces its operating lifetime, and its ability to withstand further faults. In addition, equipment that is approaching the end of its useful life can fail earlier than planned if it experiences a fault due to the extra energy it experiences. For example, an aging transformer with windings that are already loose and sagging from years of service and fault energy dissipation will have a greater chance of failing during a fault condition. This is because the windings must dissipate the energy of the fault, which manifests itself in large stresses on the equipment.

Figure 1. FCL devices can be installed in a variety of grid locations to fulfil a number of utility requirements
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The present methods employed to limit the total energy of a fault vary from utility to utility, with conventional reactors and circuit breakers being a popular choice. The methods that are used to combat fault currents are:

  • Splitting the grid to provide interconnections through higher voltage links
  • Increasing the short circuit withstand capability by replacing the substations
  • Limiting the short circuit current by operational methods, such as sectionalizing and sequential network tripping using a sophisticated control system
  • Using conventional reactors.

However, while these methods help to limit the fault, power quality can still be affected and equipment may still need to be rated for a relatively high fault.

Limiting fault currents

Finding a more effective way of limiting faults has been a widely discussed topic around the world. Effective fault current limiters would enable a utility to reduce the energy generated by a fault before the circuit breakers operate, i.e., limit the fault current between the time the fault occurs and the time that the circuit breaker becomes operational. This substantially reduces the amount of energy that needs to be dissipated by the utility equipment. By limiting fault currents, utility equipment can be more reliable, require less maintenance, and can be specified with a lower tolerance to faults. These benefits translate directly into lower initial capital costs and long-term operating expenses.

According to Dr. Jàƒ¼rgen Kellers of German high temperature superconducting (HTS) specialists Trithor GmbH, one fault limiting product available on the market is an Is limiter. “This consists of an explosive-type fuse which disrupts the current path when it senses an overcurrent,” says Kellers. “However, it is an active device used mainly in industrial grids because it is not totally fail-safe. Utility grids need fail-safe systems otherwise equipment would have to be rated to cope with the full force of a fault.”

One technology with the potential to limit fault currents to just two to three times the nominal current rating is a fault current limiter (FCL) based on high temperature superconducting (HTS) materials. Trithor is currently developing and testing a saturated core-type FCL, which uses the company’s HTS coil product based on high performance HTS wire.

Trithor’s FCL device consists of an AC coil in the grid that acts as the reactor (or choke). It is filled with a pre-magnetised iron circuit. In addition, there is a DC bias coil that drives the iron into saturation so the choke is filled with saturated iron. If an excessive current is applied then the magnetic force is large enough to drive the iron out of saturation, thus increasing the inductance of the system and increasing the impedance to the current.

Figure 2. Trace of current versus time for a fault, with and without FCL
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“The material used in FCLs have an inherent ability to limit currents, making them fail-safe and giving them a fundamental advantage over conventional technologies,” notes Kellers.

“Superconducting material has zero resistance up to a certain current level and then becomes the perfect resistor. In addition, superconducting wires have a high current density, so the devices are compact, and their zero resistance means that magnetizing the iron is easy.”

There are many reasons why utilities would require FCL devices in their grids. These include:

  • To meet regulatory requirements
  • To assist in the parallel operation of existing transformers to improve reliability without resorting to the use of high impedance transformers
  • To increase cost savings by reduced expenditure in system upgrades
  • To extend the lifetime of substation equipment, including the circuit breakers and transformers
  • To increase safety, reliability, and improve voltage sag levels for upstream and downstream customers.

Wind in the grid

Kellers points to the increasingly complex nature of grid systems such as the UCTE as a perfect example of the need for more accomplished fault limiting systems. “The UCTE grid and its underlying medium voltage grid used to be well-structured, divided into generation, transmission, distribution and supply. However, there is now generation at the distribution level and consumers at the high voltage level, so the structure is more complicated.

“Wind power is one example of generation coming on to the grid at the low and medium voltage levels: wind farms are usually located in rural areas where the grid is weak. Wind turbines will create fault currents and traditionally you would need a new high voltage line to overcome the problem. However, an FCL device could control the problem where wind is connected to a medium voltage line.”

FCL devices could also overcome power quality problems for industrial consumers, and, says Kellers, change the paradigms of grid planning. “Traditionally a utility engineer could design a grid to be ‘stiff’ or ‘soft’, depending on technical and economic needs. However, with FCL he can have the best of both worlds: he can design a grid with high short circuit power under normal operation (giving good grid stability as well as the ability to connect consumers with high and fluctuating loads at a tolerable level of disturbance to other consumers) while making sure there are low fault currents under short circuit conditions.”

Figure 3. Ricor and the University of Bar-Ilan are to install a saturated core FCL in the IEC grid
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At the moment, the principal market for FCL systems is in the distribution networks, where they would be installed in the feeder positions of transformers and bus positions where two buses are joined. There is also a large potential market in the high voltage sector because this would mean large savings for utilities. Further development of the technology is required for such applications, however.

FCL technology is close to commercialization, says Kellers. Australian Superconductor successfully lab tested the first HTS saturated core FCL as a 1 MVA single phase device in 2002. Nexans installed an FCL device in the RWE grid in Germany in 2004, testing it for a year, while a consortium of the Israeli company Ricor along with the university of Bar-Ilan has just completed the first saturated core FCL and is preparing to undertake a utility test of the device in the medium voltage grid of the Israeli Electricity Company. Trithor, too, is on the cusp of a major development, and is planning in the coming weeks to announce a demonstration project using its saturated core FCL.

Network faults

Electrical faults can occur at any time in the electrical network. A common cause is tree branches falling on overhead lines as a result of lightning strikes or strong winds, and results in a line-to-line fault or a line-to-ground fault if the line falls and touches the ground. Another common cause is the inadvertent severing of an underground cable during civil works.

The power released by a network fault is measured in MW and is the multiple of the fault current (measured in kiloamperes) and the nominal steady state line voltage of the system (measured in kV). Typically, on low voltage distribution networks, faults are in the range of 250-500 MW, and can be up to 2000 MW on high voltage transmission networks. This energy is about ten times greater than the level that typical distribution and transmission equipment is designed to safely handle under normal operating conditions. Therefore, circuit breakers are used to isolate a fault, and prevent the passage of the fault current further into a network. However, the action of a circuit breaker is not instantaneous, and all equipment in the path of a potential fault current (e.g. transformers, switchboards, cables, and the circuit breakers themselves) must be rated to withstand a specified overcurrent for a set period before the circuit breakers can operate (typically between 0.06 and 0.2 s).

Circuit breakers are expensive pieces of equipment. They have a limited fault rating, for example, 250 MW. This rating is essentially a measure of the fault current withstand multiplied by the nominal line voltage. If a fault develops above this level, and the circuit breaker is called to interrupt this fault, severe damage is likely. Hence, fault currents are designed or circuit breakers are selected to be lower than the maximum fault level that could eventuate.

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