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New Fault Current Limiters for Utility Substations – Design, Analysis, Construction, and Testing By Frank Darmann Tim Beales Australian Superconductors A Wholly Owned Division of SC Power Systems, Inc. USA 1 TechCon® 2003 Asia-Pacific 2 TechCon® 2003 Asia-Pacific New Fault Current Limiters in Utility Substations – Design, Analysis, Construction, and Testing Frank Darmann, PhD Tim Beales, PhD Australian Superconductors A Wholly Owned Subsidiary of SC Power Systems, Inc. USA Abstract The fault current on the low voltage (LV) side of a sub-station is dependent on the transformer impedance, the number of transformers connected to the LV bus, and the incoming fault level on the high voltage (HV) side. Switchgear and other plant on the 22 and 11 kV distribution side typically have a fault rating of 500 and 250 MVA respectively. Due to the increase of incoming fault levels, or the installation of another transformer, fault levels at the LV bus can sometimes exceed these ratings. Typical standard solutions to this problem include upgrading the LV switchgear, installing Neutral earthing resistors (NERs), or operating the LV bus in a “split bus mode”. A less common solution is to install passive series limiting reactors or fault current limiters (FCLs), which can reduce the fault current but at the expense of regulation and not insignificant energy losses. In this paper, a new generation of passive series fault current limiter will be described which present none of the disadvantages of conventional FCLs. The improved properties include a negligible reactance during normal conditions, high impedance during fault conditions, very low copper loss, removal of the transient and DC component, and limitation of the steady state fault current. These advantages are achieved by employing three phase saturated core fault current limiters with the saturating current provided by a single high efficiency, high current density DC superconducting coil. These superconducting fault current limiters (SCFCL) are still passive devices and do not suffer from switching delays. Introduction The need to limit faults in Australia's electricity supply system is a necessity from both practical and regulatory requirements. A EU study has estimated that about 150 faults per year per 100 km of line occur in distribution lines in Europe. There are many reasons for wishing to limit fault currents at the zone substation level. These include: 1. To meet the regulatory requirements, 2. To assist in the parallel operation of existing transformers to improve reliability without resorting to use of high impedance transformers, 3. To reduce expenditure on system upgrades allowing existing switchgear to remain in circuit, 3 TechCon® 2003 Asia-Pacific 4. To extend the lifetime of substation equipment, including the circuit breakers and transformers, 5. To enable flexibility in substation upgrades, 6. To increase safety, reliability, and improve voltage sag levels for upstream customers. 7. To prevent saturation of current transformers during faults Superconducting FCLs have an automatic fault sensing response, are self-triggering, have a very short response period of less than 1 ms, and in the normal operating conditions have negligible reactive voltage drop and energy loss. Superconducting FCLs can be designed to be sited at several key locations within a network. There are several designs that can employ superconductors to limit fault currents: (i) the resistive model that uses a single superconductor resistive element; (ii) the resistive shunt model that uses a resistive superconductor element with a resistive shunt; (iii) the inductive shunt model that uses a resistive superconductor element with an inductive shunt; (iv) the screenedmagnetic core inductive model that uses a cylinder made of superconductor surrounding a magnetic core; (v) the shorted-secondary inductive model that uses a short-circuit superconductor winding or ring surrounding a magnetic core; (vi) the electronic-switching inductive model that uses a superconductor coil in association with a power electronic circuit; and (vii) the saturated magnetic core inductive model that uses a superconductor winding and a magnetic core. The superconducting FCL design being developed by AS is a saturable inductor-type. The operating principle is an iron core that is wound with two coils, one normal AC and the other superconducting DC. The HTS DC winding is energized sufficiently to saturate the iron core with magnetic flux. Consequently, when current flows in the AC winding attached in series to the load AC system, it will not significantly change the magnetic flux, and therefore the impedance to the AC current is very low. However, should a fault occur in the system upstream from the FCL, the AC current rises and takes the iron core out of saturation. As the magnetic flux has now changed, the impedance of the AC winding increases and the fault current can be limited. Australian Superconductors first looked at the generic properties of superconducting FCLs in medium and high voltage networks, and began to focus exclusively on the saturated magnetic core inductive model. The saturable FCL design being developed by AS has a number of advantages over other superconducting FCL designs: (i) it is designed to use HTS tape as the DC winding, so there are miniscule operational losses; (ii) the saturable FCL design uses HTS tape which is the most robust, practicable and reliable conductor compared to other potential HTS current limiting elements; (iii) there can be no superconducting quench with this design; (iv) the superconductor is not connected to the rated AC system current and therefore, the HTS need not be rated the same as the power system current levels; (v) the required impedance to limit the fault current can be easily achieved by controlling the number of AC 4 TechCon® 2003 Asia-Pacific winding turns; (vi) the fact that the fault current capability is essentially determined by the NA value of the DC coil, means that a series of fault current levels may be accommodated by this design using an off-load tap changer-type arrangement; (vii) the HTS cryogenic system is very simple, since the high voltages and currents of the power system do not pass through it; and (viii) in the unlikely event that the HTS FCL should fail for any reason, the normal power supply will not be cut off. The operational and design details of the Australian Superconductors SC-FCL working prototype will be given in a later section. Conventional Reactor Designs Conventional series reactors are very similar to transformers, in that they use paper-insulated windings with copper conductors. The steel core however, is not continuous, but is built with an air gap. This forms a high reluctance magnetic flux path, which increases the terminal impedance of the copper windings. There are four basic types of conventional series reactors used to provide a high impedance: (i) those with disk or helical windings that are cast in concrete with an air core; (ii) those with their coils immersed in oil with an iron core that contains an air gap; (iii) those with their coils immersed in oil with an eddy current induced magnetic shield and no iron core; and (iv) those with their coils immersed in oil with an electromagnetic shield and no iron core. Ideally, series current limiting reactors would not contain an iron core, since high fault currents saturate the iron and this tends to reduce the terminal impedance. This feature reduces the fault current level that can be limited. Hence, to circumvent this problem, reactor Designs (iii) and (iv) listed above have no iron core. In the iron-free type of reactor, mechanical and thermal considerations determine the magnitude of the fault current that can be limited, rather than any magnetic considerations. The type of reactor cast in concrete (Design (i)) completely eliminates iron from the device. It consists of helical or disk type windings of copper conductor supported by symmetrical concrete posts arranged around a circle. The posts have no steel re-enforcement. The relatively high heat capacity of concrete provides excellent temperature control. The disadvantage of this design is that there is a considerable external magnetic field, and this poses a health and safety hazard to personnel working near to the device. In addition, the concrete has to be cast without any defects, and the reactor must be housed in a room of nonmagnetic materials and so, for example, a concrete structure with steel re-enforcement is not acceptable. The same considerations do not apply to the oil-filled reactors of Designs (ii) to (iv). Owing to the iron content of these designs, the magnetic field is constrained to move along the lowest reluctance path, which will be through the iron core. Several types of design exist. One particular three-phase design, invented by ABB and manufactured under licence by Alstom in Australia, uses three long cylindrical stacks (one per phase) of alternating transformer steel and non-magnetic slate/epoxy matting acting as separator between the laminations to provide the "air” gap. This arrangement can be shielded with aluminium to limit the external field, or 5 TechCon® 2003 Asia-Pacific be clad in low-loss iron laminations to prevent any external field. The air gaps make up no more than about 1% of the magnetic flux path, but this is sufficient to reduce the flux density of the device to a level such that, even at fault currents of 10 to 12 times the normal rated current, the core remains unsaturated, and the reactance only drops by a few percent compared to the steady state reactance. This report will show, however, that even for very small reactors, the copper losses associated with these types of reactors are about two orders of magnitude greater than an HTS FCL is. Owing to the complexity of the circuit equations (Equations A1-A4 in Appendix 1), and the need to study the effect of faults at various “points-on-wave”, the only viable option was to use electromagnetic transient (EMT) analysis using the Power Systems Computer Aided Design (PSCAD) software package [1]. PSCAD is an industry standard electromagnetic transients simulation program used by engineers, scientists, utilities, consultants, research, and academic institutions all around the world. PSCAD is a graphical user interface used to set up the circuit to be analysed, and EMTDC is the solution engine. It is used in the planning, operation, and design of power systems, and in the teaching and research into power systems, power electronic systems, and their control. The capability of the package is evident from many publications that exist in the public domain in relation to the studies carried out. The package is being continuously refined and improved by a team of researchers, engineers, and software experts. The Power Quality Centre at the University of Wollongong has access to the latest PSCAD software developed in Canada. Placement of SC-FCLs in T&D Networks The most appropriate location of SC-FCLs was identified to be on the secondary side of each transformer circuit, as shown in Figure 1, such that each transformer in a substation is individually protected. Each FCL is rated at the maximum load current of each transformer circuit. Other placement possibilities exist, notably, the high voltage side of the transformers at the sub-station or on the low voltage side of the transformers at the terminal station. These two options will effectively limit the incoming fault level, however, the complexity of the SCFCL design would also increase owing to the higher voltage level required. Therefore, in the short term, we considered a low voltage FCL design (operating at 22 or 11 kV) and the connection scheme shown in Figure 1 to be the most applicable solution for rapid implementation in the Australian grid. 6 TechCon® 2003 Asia-Pacific Xl Xl Figure 1. Individual transformer protection using SC-FCLs Principles of the Australian Superconductors Designed SC- FCL Previous work in academia has shown that the design of a single-phase DC saturated superconducting FCL requires two separate closed iron cores as shown in Figure 3. A separate DC HTS winding that is wound in the form of a solenoidal coil energizes each core independently. The cores are both saturated to a designated field intensity value, Hdc, in the opposite sense, according to the standard dot notation used in the industry, as shown in Figure 2. Typically, Hdc will have a value between 9,000 and 20,000 Am-1. The DC current flows out of the page in the positive cycle saturated core, and into the page in the negative cycle saturated core. The corresponding points on the DC magnetisation curve of the cores are denoted by (Bdc, Hdc) and (-Bdc, -Hdc), respectively. In the above concept, the superconducting DC windings will be maintained at a constant temperature of 77 K (-196 ºC) as they are immersed in liquid nitrogen, and the whole assembly is contained in an isothermal vacuum-jacketed cryostat, with its outer walls at room temperature. The ampere-turns required for each HTS DC coil to achieve the required value of Hdc in a wound core (i.e no air gaps) is given approximately by NI = 2(2w + 2h)Hdc (1) where N is the number of total DC turns, I is the DC excitation current, w is the average core width in the plane of the paper of Figure 2, h is the average core height in the plane of the paper of Figure 2, and Hdc is the required field intensity of the saturated core. 7 TechCon® 2003 Asia-Pacific dc AC linkage windings, n turns DC saturating windings, N turns Figure 2 Schematic of single phase SC-FCL layout The AC windings in the above model are then arranged in such a manner that the AC flux arising from each coil is in the opposite sense to the DC core magnetisation of each. The instantaneous inductance may be found from the differential or incremental permeability, diff μdiff = (dB/dH) | Bdc = ΔB/ ΔH | Bdc (2) where ΔB and ΔH are the maximum extents of the minor hysteresis loop at the DC bias points, Bdc and Hdc. Appendix 1 details the derivation of an equation approximating diff. The instantaneous impedance presented to the network may be expressed in phasor notation as Z = R + 2πf(n2A/l)μdiffj (3) where R is the resistance of the AC coils, f = frequency of operation (50 Hz), l = mean flux path length of the iron core assuming no air gaps, j = square root of -1 (the imaginary number), and n = the number of turns of the AC winding. The resistance of the AC coils, R, is small compared to the imaginary part of the impedance during a fault. For an effective HTS fault current limiter, the normal operating inductance of the core must be small, so as not to impose any unnecessary regulation of the line or impedance to the current flow. This is normally achieved by ensuring that the DC flux density in the iron core, Bdc is 1.9 – 2.2 T, and thereby ensuring that μdiff is approximately zero. The DC field is chosen such that an oscillatory fault current of peak value, If, will increase the differential permeability to a large value during a fault condition. The size of the cores, DC 8 TechCon® 2003 Asia-Pacific current, and DC turns are calculated based on the fault level and the permeability of the iron so that nIf(max)/l = Hdc (4) and nIf(min)/l = Hdc - Hdc(sat) (5) where n is the number of AC turns, l is the length of the wound core magnetic circuit, Hdc is the DC field intensity of the DC coil, Hdc(sat) is the field intensity required to saturate the core, If(max) is the maximum fault current that the HTS FCL is required to limit, and If(min) is the minimum fault current that the HTS FCL is required to limit. To limit the fault current, the number of AC linkage windings is chosen such that n = NI/If (6) Owing to the oscillatory nature of the fault current, two separated cores, as shown in Figure 2, are required to provide different senses of differential permeability to the AC windings and thereby limit the fault in both positive and negative cycles of the fault current. A three-phase FCL to the above concept would therefore require six saturated cores and would require six separate HTS DC windings. Australian Superconductors three-phase superconducting FCL Figure 3 shows a schematic representation of a three-phase SC-FCL design patented by Australian Superconductors that takes the basic design concepts given above, and builds in added advantages for commercial devices. Figure 3. highlights the placement of the core cross-sections in relation to the single superconducting winding used and the AC copper linkage turns. This design maximises the phase-to-phase clearance between the copper linkage coils. One design advantage is the use of a single superconducting coil to saturate all the iron cores to the required design value. The iron core laminations chosen for the SC-FCL are Kawasaki type 35RG155 laminations. Since there is only a small steady state perturbation in the hysteresis loop about the DC position, expensive low loss core steels are not required, and thicker, cheaper laminations than those used in transformers can be employed. The incremental permeability of the 35RG155 laminations was fitted to a closed-form curve to allow the magnetisation properties to be used in the PSCAD modelling software package. Figure 4 shows a linear depiction of the magnetisation curve, and a graphical explanation of the operating principals of a superconducting FCL. At 50 Hz and under normal operating conditions (i.e., no fault), the magnetisation response of the iron core will oscillate about the operating point shown in Figure 4. In this condition, μdiff 9 TechCon® 2003 Asia-Pacific w d Basic footprint = (3.9.d+2.w)^2 Iron core leg, diameter: d, window: w x h AC coils Cryostat HTS coil and support Footprint Figure 3: Schematic of an SC Power/Australian Superconductor’s 3-phase SC-FCL. Figure 4. Magnetisation properties of core steel laminations, 35RG155 10 TechCon® 2003 Asia-Pacific is approximately that of air (the FCL is essentially an air core inductor, μdiff = 4π x 10-7), and the terminal reactance of the device is essentially zero owing to the very few ac turns. In a fault condition, the magnetisation of the iron will decrease to a point where μdiff has a maximum value of μdiff = 0.1275, (five orders of magnitude greater) and the terminal inductance increases to a value of a few henries, depending on the specific FCL design used. Essentially, the FCL behaves like a current-controlled reactance. A plot of the terminal inductance versus terminal current for one such FCL design is shown in Figure 4. Each peak is produced by each half of the iron core. In this case, the inductance was designed to have a maximum at a fault current of 2.5 kA to effectively clip the fault current to this value. The two peaks effectively form a high impedance barrier to the fault current. Terminal inductance (i) 10.0000 1.0000 0.1000 Fault conditions 0.0100 0.0010 Normal operating conditions 0.0001 0.0000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000 Terminal current (i, amps) Figure 5 Typical plot of the SC-FCL instantaneous terminal inductance versus instantaneous terminal current of a SC-FCL This particular design would limit the fault current to 2.5 kA. The steady state inductance of this design is negligible, owing to the fact that the incremental permeability at the DC operating point is approximately 4x10-7. In addition, the resistance of the DC linkage coils is also small compared to the power rating. Hence, during normal operating conditions, the FCL is invisible to the network, and requires no extra voltage regulation to fluctuations in normal load levels. 11 TechCon® 2003 Asia-Pacific Advantages of DC saturated SC- FCLs over series limiting reactors The traditional approach to limit fault currents has been to employ higher reactance transformers, and to install series limiting reactors. Series reactors impose significant losses at the substation, which can amount to hundreds of kW that are generated 24 hours a day over the life of the substation. A conventional series reactor, for example with an inductance of 0.4 mH and rated at 11 kV/2.4 kA would have a copper loss component of 100 kW, and would waste 877 MWh annually in lost energy. At a price of $20/MWh, this represents a cost of $17.5k per annum, or $0.5 M over 30 years. A SC-FCL can reduce fault current levels without introducing a high impedance during steady state (non-fault) operation. This study has calculated that the overall steady state losses of a SC-FCL (i.e including DC current coils) are below 10% of the losses of a conventional reactor. The advantages of SC FCLs over conventional reactors are: 1. The steady state reactance of the Australian Superconductors FCL design is negligible. 2. The copper loss of the Australian Superconductors FCL design is low, approximately 10 kW. 3. The windings of the Australian Superconductors FCL design are significantly smaller in size owing to the high current density of the superconductor windings (10,000 A/cm2 compared to 350 A/cm2). 4. The superconductor windings of the Australian Superconductors FCL design are isothermal, so there is no long-term degradation in the insulation. 5. There is no oil in the of the Australian Superconductors FCL design, which reduces the risk of fire, lowers maintenance, and reduces the need for civil works. 6. The coolant used for superconducting windings of the Australian Superconductors FCL design is liquid nitrogen, which is environmentally benign. 7. The fault level of the Australian Superconductors FCL design can be adjusted by tuning the DC current to the If value required. 8. The Australian Superconductors FCL design has flexible dimensions. 9. The Australian Superconductors FCL design is able to completely clip initial transients. 10. The lifetime of the main portion of the HTS FCL is unlimited, owing to its cryogenic operating temperature, which essentially preserves the windings. The copper coils can be replaced or upgraded at a later date, because they are relatively small and cheap. 11. HTS FCLs are future proof. Whereas conventional reactors are rated for a single line current and a single fault current level, HTS FCLs do not suffer this limitation. Future increases in the fault current level can be easily accommodated: increases in the rated line current can be accommodated by changing the AC coils, and the original DC HTS coils and iron core can remain. 12 TechCon® 2003 Asia-Pacific Performance Study of Australian Superconductors SC-FCL Design at Specific Australian Utility Sites Site 1: Substation ST1 Substation ST1 has two Delta: Star, 45 MVA 132/11 kV transformers each with an impedance of 67 % on a 100 MVA base. The total primary line current is rated at 400 A, and the secondary line current is rated at 4,800 A. The substation occupies an area of 100 x 82 m2 and is located in a high population growth area. The load is fed from 14 radial feeders off the site to a mixture of domestic and industrial users, with the property lines of residential houses abutting the once isolated site. The only area available for a superconducting FCL is on a blue-metal area of 5.58 x 7.06 m2 adjacent to, and between the existing transformers. The majority of the free land on site is unsuitable for the installation of power equipment owing to the significant induction from the overhead 132 kV lines that traverse it. At this site, a single transformer is used to supply the load, in which case the contribution to the fault impedance is the full 67%. However, both transformers may be required to operate in parallel for approximately three months of the year and in this case, the steady state fault current level is 272 MVA (14.3 kA line current) for a bolted, three-phase to ground short circuit. To reduce the fault level, a split bus arrangement is employed. A SC-FCL was design to limit the fault level to 250 MVA (13.1 kA) on a solid bus arrangement. The electrical layout of the ST1 substation was modelled using PSCAD software. This confirmed the transient nature of the faults at the ST1 substation, and confirmed the behaviour of the SC-FCL under a variety of fault conditions. The worst case of a three-phase to ground bolted short circuit coinciding with a voltage zero on one phase was used to demonstrate the behaviour of the FCL. However, the behaviour of the FCL under other types of fault were also examined, such as phase to phase and phase to ground faults. Figure 6 shows the PSCAD network lay out for ST1 with both transformers in service. Design and Space Considerations of the SC-FCL at ST1 Figure 7 shows the 5.58 x 7.06 m2 available footprint area for a superconducting FCL at the ST1 substation. This is an area located between the two transformers. In addition, the height is limited to approximately 3.0 m. A similar space is available for a second superconducting FCL to the left of the transformer location. A superconducting FCL was designed to fit into the required volume using Australian Superconductors' design. The FCL dimensions are 2.2 x 2.2 x 2.5 m3. However, if there was a need to limit the height, for example, for transport or other clearance reasons, then the FCL could be redesigned, while still meeting all the electrical requirements. As an example, an alternative footprint is shown in Figure 6, which has dimensions of 4.0 x 4.0 x 1.2 m3. Both designs have an approximate copper winding loss of 6 kW. Scope is also available for designing a very small footprint SC-FCL (1.4 x 1.4 m) but which has a correspondingly greater height (4.2 m). 13 TechCon® 2003 Asia-Pacific To cable box 5580 2500 7060 2200 5580 Figure 6. ST1. Volume options, which will enclose the SC-FCL, and the available space between the existing transformers. All dimensions are in mm. Electromagnetic Transients Analysis at ST1 Using PSCAD/EMTDC The PSCAD software package was used to model the performance of the ST1 substation under electromagnetic transients (EMT) (i.e., faults). It was confirmed that the most severe (i.e., highest amplitude) transients in the current occurred when the fault coincided with a voltage zero crossing on one of the phases. Owing to the symmetry of the system, all faults were synchronised to a zero crossing of the Phase A voltage waveform. We proposed to locate a three-phase FCL in each of the 11kV/2.4kA circuits at ST1, as shown in the PSCAD diagram, Figure 7. With no FCL in service, the fault current in each circuit is nominally 7.15 kA rms, which gives a fault current of 14.3 kA rms in the bus bar. To meet the requirements of the Integral Energy engineers, the Australian Superconductors’ superconducting FCL design had to be able to limit the fault current in each circuit to 6.55 kA rms. There were two design options for the FCL that were considered, the final design choice would depend on the specification of the installed switchgear and circuit breakers: (i) FCL Design 1 was configured to limit the steady state rms current waveform per circuit to 6.55 kA rms, and substantially reduce the transient peak; and (ii) FCL Design 2 was configured to completely limit the fault current waveform to below 9.3 kA peak during the transient and steady state operating conditions. 14 TechCon® 2003 Asia-Pacific PSCAD model of the Rooty Hill sub-station 132 kV 11 kV 14.3 kA rms 7.15 kA rms 13.1 kA rms A B #1 Ia1 B B LFCL1 + Ib1 Ib #2 C C Ic1 LFCL2 + Ic 0.3 C C 132.0 11.0 + A A 45 [MVA] 0.3 Ia LFCL3 3 Phase RMS Ea(11kVbus) A B A C #1 132.0 #2 11.0 Ia2 FAULTS C ABC->G B Ib2 LFCL4 + C Ic2 LFCL5 + faultcontrol B 45 [MVA] + A Faultcontrol A 0.021095 0.3 B Va 6.55 kA rms Timed Breaker Logic Closed@t0 LFCL6 7.15 kA rms 1.192 0.00285 1.192 0.00285 1.192 0.00285 6.55 kA rms Figure 7. PSCAD circuit diagram for ST1 showing the existing fault current (272 MVA) and the required fault current (250 MVA). ST1 SC- FCL Design 1 Figure 8 shows the current waveform in phase A both before and after a fault, and with and without a SC-FCL Design of type 1. Only one phase is shown for clarity, as the other phase current waveforms are similar. The peak transient is reduced from 15 kA to 12 kA and the steady state fault level is limited to 6.55 kA rms (9.3 kA peak) as required. ST1 SC- FCL Design 2 Figure 9 shows the waveforms of the Phase A current for a SC-FCL of design type 2 at substation ST1 compared to the normal situation (without SC-FCL) and an appropriately rated series reactor (0.126 ). Both the transient and steady state waveform are limited by the SC-FCL, but the series reactor only reduces the steady state response. The advantage of the SC-FCL is that the transient is clipped as well as the steady state response, and this cannot be achieved with a conventional series reactor. To ensure the suitability of the FCL under different types of faults at different point on wave timings, FCL Design 2 was used in an EMT analysis on faults at a voltage zero of the single phase to ground type and phase to phase type. In addition, the analysis was also carried out for a line to ground fault at a voltage maximum, and a three phase to ground fault at a current maximum was also checked with very similar results. 15 TechCon® 2003 Asia-Pacific Figure 8 Fault current on phase A with and without a SC-FCL of design type 1 at ST1 (Three-phase fault occurs at a voltage zero crossing on phase A) Figure 9 Clipped fault current - with SC-FCL of design type 2. 16 TechCon® 2003 Asia-Pacific Site 2: Substation ST2 Substation ST2 has three Star: Star, 20 MVA 66/22 kV transformers each with an impedance of 10 % on a 20 MVA base. The primary line current is 175 A, and the secondary line current is rated at 525 A per transformer. The substation occupies an area of 86 x 40 m2 and was nominated because of its high fault level (three transformers are already installed) and its strategic importance. The 22 kV load is fed from 13 feeders off the site to a mixture of resistor (NER) has been installed to reduce the current for line to ground faults. The 22 kV fault currents are summarised as follows: 1. Three phase = 12.3 kA (4.1 kA per phase), 2. Phase to ground = 2.3 kA with an 8 Ω NER installed, 3. Phase to ground = 13.0 kA without an NER installed. Design of a SC- FCL for ST2 As an exercise to develop an alternative to the installed NER, a SC-FCL was designed to limit the fault current to 2.5 kA per circuit. In this case, the design, which completely clips the fault current, was the preferred option. The PSCAD software was used to set up a network diagram of the ST2 substation site to determine the transient response to various types of faults, both with and without a SC-FCL. Figure 10 shows the PSCAD circuit diagram. The assumptions made about the ST2 substation were as follows: 1. 2. 3. 4. 5. The lagging load power factor = 0.8. The X/R ratio of the 66 kV generation = 20. The steady state load current per circuit = 0.42 kA (i.e. 1.25 kA / 3). All three transformers are in circuit. All fault analyses were carried out with the fault occurring on the 22 kV bus at the substation without including any extra line impedance. 6. No NER was included in the circuit. It was confirmed that the worst-case transients were produced when the fault occurred on a voltage zero (phase to ground) on one of the three phases. Several protection regimes were looked at, and the one presented in this report is the one considered to be the best solution. This was to insert a three-phase superconducting FCL into each circuit on the secondary side of each 20 MVA transformer, as shown in Figure 10. A three-phase superconducting FCL was designed to reduce the steady state fault current in each circuit to 2.5 kA peak. The dimensions of the SC-FCL are approximately 2.0 x 2.0 x 2.4 m2. 17 TechCon® 2003 Asia-Pacific Va A A 0.00646 0.3 B B #1 A Ia1 + B B Ib1 LFCL1 + Ib C C Ic1 LFCL2 + Ic A 20 [MVA] #2 0.3 C C 66.0 22.0 0.3 Ia LFCL3 3 Phase RMS Ea(22kVbus) Faultcontrol A B B 20 [MVA] #1 C #2 66.0 22.0 A Ia2 + B Ib2 LFCL4 + C Ic2 LFCL5 + FAULTS C ABC->G faultcontrol A Timed Breaker Logic Closed@t0 LFCL6 A B C 20 [MVA] #1 #2 66.0 22.0 8.129 0.0194 8.129 0.0194 8.129 0.0194 + A Ia3 B LFCL7 + Ib3 C Ic3 LFCL8 + LFCL9 Figure 10 PSCAD circuit diagram of the Heatherton site Electromagnetic Transients Analysis at ST2 Using PSCAD Figure 11 shows the results of the transient analysis of the ST2 substation for a three-phase to ground fault on the 22 kV bus without a superconducting FCL present. The circuit load current before the fault was 0.42 kA rms, and the maximum transient fault current in each transformer circuit was 10.6 kA occurring at t = 0.21 s, half a wave cycle, after the fault occurred. A 5-kA DC offset in Phase A is clearly visible, and the steady state fault current is 6.2 kA peak or 4.4 kA rms. Figure 12 shows the results of the transient analysis of ST2 substation with a three-phase superconducting FCL inserted into each transformer circuit between the secondary and the bus. The maximum fault current was limited to 2.5 kA both in the transient and in the steady state. The ST2 substation FCL design was chosen such that the transient and the steady state fault were both limited to a maximum peak current of 2.5 kA. However, other FCL configurations could have been employed. For example, it is possible to limit the transient current to any given figure (say, 3.5 kA), and also limit the steady state to a given figure (say, 2.5 kA). 18 TechCon® 2003 Asia-Pacific Figure 11. Transient fault currents per circuit at ST2 for a three phase to ground bolted short circuit. Figure 12 Steady state three phase fault response of Heatherton with SC-FCL. 19 TechCon® 2003 Asia-Pacific Transient analysis of the same circuit was undertaken for the following types of faults with very similar results to those presented. 1. 2. 3. 4. Single phase to ground, voltage zero crossing on that phase. Single phase to ground, current maximum crossing on that phase. Phase to phase, voltage zero crossing on one of those phases. Three phase to ground, current maximum crossing. It was estimated that to achieve the same reduction in steady state fault current, a fixed conventional reactance of 2.04 would be required in each phase of each transformer circuit to reduce the final steady state rms current (in each circuit) to 2.5 kA. The response of the ST2 substation to a bolted three phase fault was analysed using PSCAD analysis with each of the FCL’s replaced by fixed 2.04 Ω reactors (one in each phase of each transformer circuit). Although the steady state rms fault current is indeed reduced to 2.5 kA (Figure 13), the initial peak remains at 6 kA. The additional advantages of the SC-FCL include the ability to clip the transient to 2.5 kA, and reduce the DC component by half. Figure 13 Phase currents per circuit at ST2 for a three phase to ground fault occurring at a voltage zero with 2.04 conventional series reactors installed. 20 TechCon® 2003 Asia-Pacific The Australian Superconductors Prototype SC-FCL A working 1 MVA single-phase prototype SC-FCL has been built and tested at the Australian superconductors plant. Figure 14 shows the essential components. 440 mm 650 mm Figure 14. The Australian Superconductors prototype single phase SC-FCL showing 2 separate iron cores with copper linkage coils (nac), and two cryogenic vessels holding the SC coils (N) For demonstration purposes and for confirming the mathematical model of operation, the steady state current is set to 40 A rms through a 2 3.4 kW resistive load. A 6 V battery and 0.2 resistor provides a current of 30 A dc through the superconducting coils. A contactor and push button operated trip coil provide a user operated short circuit across the 2 resistive load. A sealed quick-lag type circuit breaker serves as a visual demonstration to the operation of the FCL. The CB time lag characteristics are such that a fault current above 400A will trip the coil within half a cycle (0.01 s) and a fault current below 400A will take more than 2 s to trip. Prototype Simulations and Measurements Figure 15 shows the results of the PSCAD/EMTDC simulations with and without the prototype FCL in circuit for a number of different AC, linkage turns. Figure 16 shows the measured fault current with and without the SC-FCL in circuit. The load is resistive and hence the steady state fault current is achieved within 2 – 3 cycles. As can be seen, the SCFCL virtually eliminates the transient fault current from 2.4 kA to 0.4 kA. Figure 17 shows the steady state fault current in comparison to the simulation. The disagreement in the shape of the current waveform is due to the approximation used for the incremental permeability (Equation A1). 21 TechCon® 2003 Asia-Pacific Figure 15 Results of PSCAD simulations with various ac linkage turns (nac) Figure 16 Measured fault currents with and without a SC-FCL in circuit 22 TechCon® 2003 Asia-Pacific Figure 17 A comparison of the steady state fault current with the SC-FCL in circuit. Simulated and measured waveforms shown. Terminal Inductance of the SC-FCL Figure 18 shows the instantaneous terminal inductance (i.e on the AC side) as a function of time, and superimposed on the fault current waveform. These curves offer insight into the working principal of the SC-FCL and how the regulation problems imposed by series reactors are overcome. Firstly, in the steady state, the terminal impedance is negligible which is a direct consequence of the saturated iron core. When a fault condition occurs, the terminal impedance increases to 188 Ohms at the set fault current limit (NI/nac), which effectively clips it and prevents further increases. 23 TechCon® 2003 Asia-Pacific Figure 18 Instantaneous terminal inductance of the SC-FCL in relation to the fault current Cooling Considerations for the Superconducting Coil The DC superconducting coils used to energise the magnetic circuit will be immersed in liquid nitrogen under a slight positive pressure of approximately 0.1-0.6 bar, depending on the specific design parameters. Immersing the coils in liquid nitrogen simply cools the coils to a temperature below their superconducting transition temperature and so maintains the coils in a zero resistance state. In the DC mode, there is zero resistance to the passage of an electrical current. Liquid nitrogen is available in tonnage daily tonnage quantities throughout Australia from several companies, the largest being BOC Gases, Linde, and Air Liquide. The liquid nitrogen is kept in a large thermos-type flask called a vacuum-insulated cryostat, because it will tend to evaporate owing to heat ingress from the ambient. The cryostats proposed for the Australian Superconductors design are to be manufactured of stainless steel, and the vacuum insulation can keep the liquid nitrogen evaporation rate to a very small, finite value. The actual evaporation rate will depend on the cryostat design. However, for the two FCL designs considered in this report, the approximate evaporation rate is 20 litres of liquid 24 TechCon® 2003 Asia-Pacific nitrogen per day. The Australian Superconductors FCL design can employ two techniques that can address the liquid nitrogen evaporation problem. Total Loss System Owing to the fact that gaseous nitrogen is environmentally benign, and liquid nitrogen is available for as little as 40 cents a litre, it can be economical to allow the nitrogen gas to simply vent to the atmosphere, and simply replenish the liquid from a large capacity (≥1,000litre) storage vessel. The cost of replacing this liquid nitrogen lost would be in the range of $8 a day. Automatic re-fill systems, which detect the level of the liquid nitrogen to an accuracy of 1% and can initiate a solenoid valve to re-fill the cryostat, are available for under $4,000. Owing to the relatively low cost of this type of replenishment system, a large redundancy can be cheaply built in. In addition, sufficient liquid reserve can be built into the cryostat to cover for up to two weeks or more, by making it artificially larger than necessary. Re-liquefaction Using a Cryocooler An alternative cooling arrangement is a closed cycle system where no nitrogen gas is allowed to escape. In this arrangement, a “cold head”, which resembles a copper block, is inductioncooled by a cryocooler refrigeration unit, and is used to reliquefy the nitrogen gas directly in the cryostat. The cold head sits directly above the liquid nitrogen pool in the FCL cryostat. The cold head and is fed with cold, pressurised helium gas, which acts as the refrigerant (or working fluid) to maintain the cold head at a temperature between 66 and 70 K (-207 and 203 ºC). The nitrogen gas that has evaporated from the pool of liquid in the FCL cryostat condenses on the surface of the cold head, where it exchanges its latent heat and recondenses back into liquid nitrogen. Once the droplets of condensed nitrogen reach a given size, they simply fall back down into the cryostat under the influence of gravity. A small cryocooler of only 2 kW capacity would be able to cope with a liquid nitrogen evaporation rate of 10 litres a day, and a 5.5 kW capacity cryocooler would be able to reliquefy a nitrogen evaporation rate of 40 litres a day. This is because, in terms of their cooling power, cryocoolers rapidly increase in efficiency with increasing size of the cryocooler. This fact almost single-handedly dictates why superconducting devices become much more economically competitive as the system power requirements increase. The size of a cryocooler re-liquefaction system has a volume of 1 x 0.8 x 0.7 m3, and weighs 200 kg. The cold head inserted into the cryostat is essentially cylindrical in shape with a diameter of 100 mm and a length of 500 mm. A length of about 250 mm protrudes outside the FCL cryostat. A typical system to reliquefy 40 litres of liquid nitrogen a day would cost approximately $70,000 to purchase, ship, and install, and would cost an average of $2,000 per year to maintain. The maintenance would consist of replacement of some gasket and O-ring seals every 8,000 hours, and the replacement of a cryocooler part (the regenerator) every 36,000 hours. The maintenance time is expected to be approximately 2 hours per year on average. 25 TechCon® 2003 Asia-Pacific Maintenance requires a shut down of the cryocooler, but not of the FCL device, as the liquid nitrogen buffer within the cryostat will keep the superconducting coils cold. Conclusions It was shown in this report that superconducting FCLs can reduce the fault level at substations, enabling a previously split-bar operation to be made solid, as in the case of ST1, or displacing the need for a neutral earthing resistor, as in the case of ST2. It was also shown that unlike conventional reactors, SC- FCLs can completely clip the fault current transient to a predetermined level. In addition, a relatively small conventional reactor at ST1 would consume about 100 kW of energy, whereas the AS superconducting FCL would use only about 6 kW. Superconducting FCLs are also future proof, in that their rating can be upgraded by replacement of an item with a small capital value (i.e., the AC linkage coils) compared to cost of replacing the entire device. It was also shown that SC-FCLs could be made sufficiently small to fit in the available space at the substations located at ST1 and ST2. The provision for cooling the DC coils of the superconducting FCL is cheap, of low risk, and an existing solution exists, which is reliable and proven in the field. In particular, the most economical cooling technique is to use a simple storage vessel containing liquid nitrogen, and this can be topped-up using an auto re-filling scheme. Sufficient redundancy can easily be added to the cooling system to allow for maintenance periods. Biography Specialising in high power superconducting equipment, Frank Darmann has carried out projects for the Ministry of Energy and Utilities, the Electricity Supply Association of Australia, and has built an efficient transformer from high temperature superconductors. He is currently testing a prototype saturable fault current limiter, measuring the high voltage breakdown of materials in liquid nitrogen, designing high voltage bushings for use in liquid nitrogen, and designing high power superconducting transformers using FEM techniques. Frank Darmann, PhD graduated from Monash University with honours degrees in Physics (H, 1992) specialising in electro-magnetics, and Electrical Engineering (H, 1995) where he specialised in power engineering and communications. His Engineering honours thesis was on the subject of embedded generation in utilities. He completed his doctoral thesis in 2002 from the University of Wollongong in Electrical Engineering while working at Australian Superconductors. He has presented his findings at over a dozen overseas and local conferences to experts in the field of superconductivity. Professor Tim Beales (PhD, BSc) is Manager Director of Australian Superconductors a wholly owned subsidiary of SC Power systems, Inc. USA. 26 TechCon® 2003 Asia-Pacific Appendix 1. Derivation of a Model for the Terminal Inductance of a DC Saturated SC-FCL using Laminations of type: Kawasaki 35RG155. Step 1. Using a graphical method, the derivative of the magnetisation curve (B-H curve) of the specified laminations was linearised over the range of H from zero to 30,000 A/m with an adjustable step size to suit the curve – very dense near H = 0 (step size = 1 A/m) and very sparsely when saturated near H = 1000 (step size = 1000 A/m). The piece wise linear curve was then fitted to an equation using public domain software. The fitted expression is given by Equation A1. dB e a . H c . eb / H dH (A1) Where a = 4.6377358, b= -12.751086, c = -2.6760766 Strictly speaking, this is not the incremental permeability, diff, in Equation 2; however, it does serve as a good approximation and is suitable for predicting the overall behaviour of the SC-FCL. Equations A2 and A3 describe the inductance of a coil in an iron circuit which is biased by a separate source. L n 2 A dB . l dH (A2) Where n = ac coupling turns, A = iron core cross sectional area, l = magnetic length, N = DC ampere turns, and where: 1 H ( N I dc ni) l (A3) where i = ac current, Idc = dc current As can be appreciated, expressions A2 and A3 will lead to a terminal inductance which is a strong function of the instantaneous current, I, that is, L = f (i). Equation A4 gives the full analytical expression for the terminal inductance of the SC-FCL derived in this work. L(i ) n2 A l NI ni c bl bl NI ni c . exp ( ) ( ) . exp a . e . l l ( NI ni) ( NI ni) 27 TechCon® 2003 Asia-Pacific (A4) Where: L is the overall inductance as a function of the instantaneous value of the current in the copper coils, i = i (t) is the instantaneous value of the current in the copper coils, n = Turns in each of the copper linkage coils, N = Turns in the DC HTS coil, A is the area of the iron core circuit, l = length of the wound magnetic circuit, e = exponential function ~ 2.71828182 References [1] Report of the SCENET Working Group, “Power Applications of Superconductivity - a Roadmap for Europe”. [2] The developer of the PSCAD software package is the Manitoba HVDC Research Centre (a wholly owned subsidiary of Manitoba Hydro, Canada) of Cree Crescent, Winnipeg, Manitoba, Canada R3J 3W1, http://www.hvdc.ca). A personal edition of PSCAD/EMTDC with limited node capabilities can be downloaded from http://www.pscad.com for evaluation. 28 TechCon® 2003 Asia-Pacific