GB2489015A

GB2489015A - Combination Fault Current Limiter

Info

Publication number: GB2489015A
Application number: GB1104372.6A
Authority: GB
Inventors: Philip Michael Sargent
Original assignee: DIBORIDE CONDUCTORS Ltd
Current assignee: DIBORIDE CONDUCTORS Ltd
Priority date: 2011-03-15
Filing date: 2011-03-15
Publication date: 2012-09-19
Anticipated expiration: 2031-03-15
Also published as: GB201104372D0; GB2489015B

Abstract

The Combination Fault Current Limiter (CFCL) controls the fault energy transmitted to the network within a small range irrespective of the size of the fault that it is exposed to. A first superconducting fault current limiter limits a fault current to a level greater than the nominal current, and after a period of time more than one full AC cycle (or more than 5 ms for DC), a second fault current limiter limits the current further to a non-zero value. The period of time may be varied according to the intensity of the fault. The CFCL is able to react quickly to limit fault energy but still allow a circuit breaker to recognise the fault.

Description

Thermal Fault Current Limiter

TECHNICAL FIELD

This invention relates to a superconducting device for limiting the fault current in electrical power networks.

BACKGROUND OF INVENTION

When a fault or short-circuit occurs in an electric power system, a short and intense current flows. This is termed the "fault current" and it may be many times the usual operating current; usually up to 20x but sometimes even more. These fault currents damage equipment and cabling. They can arise from lightning strikes, or from failure of equipment. A superconducting Fault Current Limiter (FCL) uses an inherent property of a superconducting material to limit the fault current to a much lower level, typically 2 to lOx times the operating current. Some FCLs currently in development only limit the current to 80% of the prospective fault current.

The damage that is caused by fault currents can be considered to be of three types: electrodynamic effects (proportional to the square of the current), thermal or energy effects (proportional to the duration of the fault times the square of the current), and arc energy effects (proportional to the product of the voltage, the current and the duration of the fault).

Superconducting Fault Current Limiters (FCLs) have been devised using many different operating principles: with or without semiconductor switches, with or without magnetic fields to aid triggering superconducting to normal quenches. One design uses a superconducting coil simply to magnetically saturate an iron core, the fault limiting effect arising from core de-saturation and not from superconductor quenching at all. All the other types fundamentally rely on a superconductor quenching from a superconducting to a normal state. A resistive FCL is the simplest of this latter "quenching" FCL type.

An example of a simple resistive FCL is in US patent 5,761,017 "High Temperature Superconductor Element for a Fault Current Limiter" filed 15 June 1995 which also reviews many different designs of FCL. Dalessandro et al. wrote a paper describing magnesium diboride FCLs published in Trans. Appl. Supercon. 17 (2) June 2007 1776-1779 which was presented at the ASC 2006 conference. A particularly clearly written description of the prior art is given by Theva in their US patent 7,760,067 (filed 2 Mar.2006).

The "range" of fault current for an FCL is the ratio of the highest fault current for which an FCL is designed to the lowest fault current which causes the same FCL to trip. These are currents as limited by the FCL, they are not prospective fault currents.

The "fault energy" for an FCL is defined for a particular fault duration: typically for a purely resistive FCL this is either 6 or 10 full cycles of AC or 50 or lOOms for DC. This is the duration typically before a circuit breaker trips. The fault energy is the integral with time of the square of the fault current as it changes during the fault duration.

The "range of fault energy" is the ratio of fault energies for the same two extreme cases: the lowest which causes the FCL to trip to the highest for which it is designed to cope with.

The "prospective" fault current is that which would have occurred for a particular fault in the network if the FCL had not been present. The prospective fault current is much higher than the nominal current the network is designed for, and much larger than the actual current before the fault. Introducing an FCL always limits the actual current to less than the prospective current.

SUMMARY OF INVENTION

The present invention is a combination FCL (cFCL) which limits the current in two or more steps. First, it limits the fault current to a level greater than that nominally carried but less than the prospective fault current. Second, after a period of time, it limits the current more severely to a lower level.

The purpose of the first limitation is to prevent severe damage to equipment from the prospective fault current, but to still admit a large and noticeable fault current such that other devices in the electrical network can detect that a fault has occurred. The second stage limits the current further to prevent damage to the FCL itself, and to further reduce the damaging effect of the fault on other equipment in the network, and to enable lower-rated components to be used in the network.

This invention applies to all types of quenching superconducting fault current limiter: resistive or inductive or any combination or mixture thereof. The invention applies to alternating and to direct current (AC and DC) networks.

According to the invention there is provided a combination fault current limiter (cFCL) as defined in the accompanying claims.

DETAIL OF INVENTION

Because of the three different types of impact (damage) caused by faults, an advantage of the combination FCL (cFCL) is that if the detector systems use one type of impact (electrodynamic, thermal or arc), which must be permitted because those detectors are already in place or for some other reason, the impacts from the other types can be severely reduced without affecting the detector sensitivity.

Another advantage is that because the fault current is reduced to such a low level after the second or subsequent limitation, circuit breakers in series with the combination FCL (cFCL) can be much cheaper and smaller as they have to break only a smaller current. This is particularly true for DC networks where circuit breakers are significantly more expensive as there is no "current zero" or "voltage zero" at which they can choose to break the circuit.

If it can be arranged in a cFCL that the delay is roughly inversely proportional to the square of the fault current (as in the Alternative Embodiment below), then the cFCL always lets through approximately the same fault energy irrespective of the size of the fault. Since many types of fault detector are sensitive to the fault energy, this alternative embodiment makes the design and selection of fault detection devices in the network easier and potentially cheaper, and their operation more robust.

BRIEF DESCRIPTION OF THE DRAWINGS

No drawings supplied.

EXAMPLE OF A SPECIFIC EMBODIMENT

Two resistive FCLs are connected electrically in series: one fast-acting FCL designed to limit when it sees a current more than 2x the nominal level and to insert a resistance which limits the current to 5x nominal, and one slower-acting FCL designed to limit when it sees a current of 2x nominal and to insert a resistance which limits the current to 0.lx nominal.

For this embodiment, the network has an impedance of 1 Ohm, the nominal current is 50Hz 100A rms and the prospective fault current is 1000A rms.

When constructed from magnesium diboride wire with a critical current for 50Hz of 100A rms, the first FCL is made from 2 parallel wires and transitions rapidly, within the first half-cycle of 50Hz, when it reaches 200A and the second FCL is made from 2 parallel wires and starts to transition at 200A but which does not transition entirely until several cycles after the fault initiates.

In order to reduce the fault current by half from 1000A to 500A (5x nominal), the first FCL must double the impedance of the system, so it must have a normal resistance of 1 Ohm. For magnesium diboride wire with a normal resistance (at 40K) of approximately 0.5 Ohm/m, this means that the first FCL is made from 2 parallel wires, each 4m long (or proportionally longer if the wire has a lower resistance per unit length) wound non-inductively.

The second FCL is required to drop the current from 500A to 1OA (0.1 nominal), so the impedance of the whole system must increase by a factor of 50 from 2 Ohms (network plus first FCL) to 100 Ohms. Therefore the second FCL must have a resistance of 98 Ohms. One wire of length 49m has a resistance for 98 Ohms, so for 2 wires in parallel the length of each wire must be 2x as long, so the second FCL is made from 2 parallel wires each 98m long. It may be noted that this is substantially less than the 600m estimated in US patent 5,761,017 for high temperature superconductor (i.e. YBCO or BSCCO) FCLs.

The second, slower acting FCL is made using a more thermally conducting encapsulant material (for example, as described in US patent 5,761,017) to delay heating and tripping whereas the first, fast FCL heats essentially adiabatically; or alternatively the second FCL is maintained at a lower temperature such that it takes longer to heat up and to transition.

This embodiment also operates effectively on a DC network, though the nominal and trip current levels will he somewhat different because of the differences in heat generation and diffusion between DC and AC systems.

EXAMPLE OF AN ALTERNATIVE EMBODIMENT

This embodiment is similar to the first, except that the second resistive FCL has a transition current which is above that which the first FCL limits to. Thus the second FCL does not see a current large enough to cause it to trip instantly. However, the conductors of the two FCLs are linked thermally so that the heat generated in the tripped, first FCL is sufficient, in combination with the limited fault current, to make the second FCL trip. This thermal coupling is created by non-inductively co-winding the second FCL with the first, such that the superconducting wires of the two FCLs are in close thermal proximity while electrically insulated from one another (except at the end where they are connected in series). The thermal conductivity and specific heat of the material supporting and separating the two FCLs is designed such that the heat pulse from the quench of the first FCL causes the second FCL to quench at the designed number of cycles or milliseconds or seconds later.

"Thermal proximity" is determined by the aggregate thermal diffusivity (diffusivity is related to both conductivity and specific heat capacity) of the combination of materials and the duration of the quenching process.

COMPARISON WITH PREVIOUS PATENTS

There are patents which describe complex FCL designs using several elements which superficially appear to he similar to the current invention. Here we describe how this invention differs very significantly from these other patents.

Much of the complexity of these inventions stem from the need to cope with variable and inhomogeneous superconductors (BSCCO) or with tape which needs significant stabilising conductive metal coatings or externally-applied heating or magnetic fields to prevent hot spots (YBCO). Magnesium diboride wire is unlike HTS materials in that it is robust and electrically conductive and these refinements are not necessary, e.g. see the paper by Dalessandro referred to earlier.

Patent US 7,359,164 B2 (Xing Yuan) is a recent example of a series of patents and inventions produced by the "Matrix FCL Project" coordinated by SuperPower Ltd. This resistive FCL project began with a solid BSCCO bar element and then changed to an YBCO tape element, hut retaining many earlier design decisions.

The Yuan patent describes an FCL with two sub-elements: a trigger and a current limiter. The current limiter is a resistive FCL in itself, hut it is caused to quench from the superconducting state by a magnetic field created by the trigger. If the trigger (also superconductive FCL) is highly engineered or selected to be uniform and to trip at a precise current, then the current limiter element can be made of inhomogeneous or variable i.e. cheaper, superconductor material.

In Yuan's patent the two elements are in series, as in our invention, but they are designed to trip instantly and as soon as possible one after the other: within the same half-cycle of alternating current; as Yuan says: "during the initial rise of the fault current". Yuan's intention is to have no delay at all (if possible) between the tripping of the two FCL elements in series whereas our invention has a deliberate delay. This is clear from col.5 lines 65-67 and col.6 lines 1-6 of his patent: the triggering of one FCL component by the other happens magnetically, i.e. nearly instantly (at the speed of light) between two co-wound inductors.

Patent US 7,359,165 B2 (Hiroshi Kubota) of the same date at the Yuan patent, also describes two FCLs in series. As in our invention and in the Yuan invention, FCL(1) trips first, and then FCL(2). The Kubota invention uses heat rather than magnetism to trip FCL(2). The first idea is to use a separate heater intimately connected to FCL(2) which is connected in parallel with FCL(l) and therefore only starts heating once FCL(1) has tripped. The second idea is to use the heat generated in FCL(1) itself to trigger FCL(2).

The first idea has the heater intimately close to FCL(2) so the time delay for the heat to conduct is minimal. Thus FCL(2) trips during the initial rise of the fault current (as in the Yuan invention). Kubota says "there are as few materials as possible between" the heater and FCL(2) (col.6, lines 8-9).

Kubota also states that FCL(2) has a higher trigger current than FCL(1) (col.3, lines48- 56), a concept which could also be useful to our invention but which is not necessary.

In his case it is strictly necessary as both his FCL(1) and FCL(2) are fast-acting.

Kubota states that it is helpful if FCL(1) is more sensitive and transitions quickly (col.6, lines 35-40) so by implication FCL(2) in his invention should be slower-acting, as in our invention. However, there is no sensible and intentional delay before FCL(2) triggers, he states that he wants the entire limiting to happen in a period of time of the order of a millisecond (col.6, lines 49-51) and that the heat diffusion distance between the heater and FCL(2) should be designed to keep this time delay so short. Delays of lms and 0.Sms are used in a detailed description (col.9, lines 41- 45). The importance of very high speed for the triggering in his FCL(2) is further stated in col.10, lines50-62.

Kubota's second idea, that the heat generated in FCL(1) after it has gone normal is used to trigger FCL(2) is strictly as an addition to the first idea (col.6, lines 47-49; col.11, lines 52-53, claim 7). In our invention (alternative embodiment) this heat from FCL(1) is the primary and only external heat applied to FCL(2).

Patent EP 1383 178 Al (ABB) describes a single resistive FCL constructed from alternating high and low critical current regions. This produces a continuous time-varying resistance which increases monotonically as the quench propagates from the "constrictions" (which have a lower critical current and quench first) into the rest of the YBCO superconductor. The time taken for the first phase of quench in the constrictions (phase 1) is of the order of 5 microseconds, after which the quench propagates into the rest of the superconductor. The purpose of the constrictions is to force a uniform quench over the whole device as thin-film YBCO has slightly variable critical current along its length and is not robust against hot-spots and burn-out (unlike round-wire MgB2, because of both physical section and materials property differences). In this ABB invention, the long term current (more than 5 microseconds, more than 25 microseconds, or several milliseconds -[Figli) is all within the duration of the first peak of an AC cycle: i.e. very short duration. This "long term" current is lower than the peak current and "may be adjusted independently", eventually down to 20% of the nominal current (para 45). This patent does not say that the long term current was to be below or above the nominal current of the device. Neither does this patent specify that the duration in Phase 2 should be long enough to enable other protection systems to operate: this invention has Phase 2 as short as possible to enable a uniform quench.

The ABB patent is to be contrasted with the current invention where there is a long sensible delay (more than l000x longer: more than S ms) between the two quench regimes in the two FCLs to allow protection systems to operate.

US patent 3,925,707 (Westinghouse) describes a resistive FCL limiting immediately to a current "much less than the rated load current" for high DC voltages (100kv). This dates from 1973 envisaging the use of ten kilometres of NbTi wire and introduces the concept of a shunt resistor in an FCL. This invention shows that the transient behaviour caused by the induction of the external system leads to a current minimum and that a circuit breaker can be designed to operate at the moment of that minimum. The invention does not permit any period of fault current long enough or high enough to trigger other protection systems. The invention requires "mechanical actuators" (circuit breakers) operating a fixed time after the fault occurs.

DDR patent 119924 (Schroth) describes an FCL operating as a DC magnet protection system. It describes several sections of superconductor which quench at different times, creating a monotonically increasing resistance in three steps. The purpose of the invention is to approximate the behaviour of a resistance increasing as the square of the time from the start of the fault: an increasing resistance in a fast, controlled manner. It does not provide a period of high current during which other protection systems operate as this rising resistance is the magnet protection system.

US patent 2008/0165455 Al (Isfort and Wolf) describes a resistive PCL specifically made only of high temperature superconductors (HTS), i.e. the copper-oxide family of superconductors. It describes a thermally conductive and electrically insulating solid encapsulant designed to prevent hot spots and burn-out damage in the HTS material. The whole is immersed in a "cooling bath" i.e. a liquid cryogen. A number of such units are connected in series, but they are designed to operate simultaneously, not sequentially. The entire FCL is intended to limit fault currents to a" value close to the nominal current": not more than 2 -6x and preferably 2 -3x the nominal current. The good thermal conduction and coolant bath is intended to allow the FCL to recover to the superconducting state while carrying a current equal to the nominal current (para [0113]) 50 that an external circuit breaker is not needed. This is in contrast with the current invention which can limit currents to a very small percentage of the nominal current.

Claims

Claims Thermal Fault Current Limiter 1. A combination Fault Current Limiter (cFCL) comprising a first superconducting Fault Current Limiter (FCL) and a second superconducting FCL, which limits a fault current to a level greater than the nominal current carried by the cFCL, and then after a period of time more than one full cycle (AC) or more than Sms (DC) abruptly decreases the current further but not to zero.
2. A device of claim 1 where the first limitation is between 110% of the nominal current and 90% of the prospective fault current.
3. A device of claim 1 where the second limitation reduces current to above the nominal current level.
4. A device of claim 1 where the current is successively decreased in one or more further stages after the second current decrease.
5. A device of claim 4 where one of the successive limitations reduces current to below the nominal current level.
6. A device of claim 1 where the second limitation reduces current to below the nominal current level.
7. A device of claim 5 or 6 where the current is eventually reduced to less than 50% of the nominal current level.
8. A device of claim 5 or 6 where the current is eventually reduced to less than 20% of the nominal current level.
9. A device of claim S or 6 where the current is eventually reduced to less than 10% of the nominal current level.
10. A device of claim 5 or 6 where the current is eventually reduced to less than 1% of the nominal current level.
11. A device of claim 1 where the cFCL comprises two or more resistive FCLs connected in series.
12. A device of claim 1 where the period of lime before the second or final current reduction is varied according to the intensity of the fault.
13. A device of claim 12 such that the range of fault energy as passed by the combination FCL is controlled to less than 50% of the range it would be if the second (and subsequent if present) FCL were replaced with a circuit breaker operating after six cycles (AC) or SOms (DC).
14. A device of claim 1 where the cFCL comprises two or more superconducting quenching FCLs in series.
15. A device of claim 1 where the cFCL comprises at least one superconductor quenching FCL in series with a saturating-core FCL.Amendments to the claims have been filed as follows: Claims Thermal Fault Current Limiter 1. A combination Fault Current Limiter (cFCL) comprising a first superconducting Fault Current Limiter (FCL) and a second superconducting FCL, which limits a fault current to a level greater than the nominal current carried by the cFCL, and then after a period of lime more than one full cycle (AC) or more than Sms (DC) abruptly decreases the current further but not to zero.2. A device of claim 1 where the first limitation is between 110% of the nominal current and 90% of the prospective fault current.3. A device of claim 1 where the second limitation reduces current to above the nominal current level.4. A device of claim 1 where the current is successively decreased in one or more further stages after the second current decrease.5. A device of claim 4 where one of the successive limitations reduces current to below the nominal current level.6. A device of claim 1 where the second limitation reduces current to below the nominal current level.7. A device of claim 5 or 6 where the current is eventually reduced to less than 50% of the nominal current level.8. A device of claim 5 or 6 where the current is eventually reduced to less than 20% 0 of the nominal current level.0 9. A device of claim 5 or 6 where the current is eventually reduced to less than 10% CY) of the nominal current level.10. A device of claim 5 or 6 where the current is eventually reduced to less than 1% of the nominal current level.11. A device of claim 1 where the cFCL comprises two or more resistive FCLs connected in series.12. A device of claim 1 where the period of time before the second or final current reduction is varied according to the intensity of the fault thus making the cFCL always let through approximately the same fault energy irrespective of the size of the fault.13. A device of claim 12 such that the range of fault energy as passed by the combination FCL is controlled to less than 50% of the range it would be if the second (and subsequent if present) FCL were replaced with a circuit breaker operating after six cycles (AC) or SOms (DC).14. A device of claim 1 where the cFCL comprises two or more superconducting quenching FCLs in series.15. A device of claim 1 where the cFCL comprises at least one superconductor quenching FCL in series with a saturating-core FCL.