The development of a Roebel cable based 1 MVA HTS transformer

Transcription

1 The development of a Roebel cable based 1 MVA HTS transformer Neil Glasson 11 October 2011 Mike Staines 1, Mohinder Pannu 2, N. J. Long 1, Rod Badcock 1, Nathan Allpress 1, Logan Ward 1 1 Industrial Research Limited, New Zealand 2 Wilson Transformer Pty Ltd, Melbourne

2 Outline Introduction Design overview Roebel cable Short circuit fault High voltage insulation Heat transfer Cryostat design Summary

3 Project Partners

4 Design parameters Parameter Primary Voltage Secondary Voltage Maximum Operating Temperature Target Rating Primary Connection Secondary Connection LV Winding LV Rated current Value 11,000 V 415 V 70 K, liquid nitrogen cooling 1 MVA Delta Wye 20 turns 15/5 Roebel cable per phase (20 turn single layer solenoid winding) 1390 A rms HV Winding 918 turns of 4 mm YBCO wire per phase (24 double pancakes of turns each) HV Rated current 30 A rms

5 HV winding Uses 4 mm Superpower tape I/I c ~ 25% Polyimide wrap insulation 24 double pancakes No encapsulation due to concerns of Poorer heat transfer Voids lowering withstand voltage Potential I c degradation

6 LV winding Single layer solenoid on fibre glass composite former Liquid nitrogen in contact with cable surface YBCO Roebel Cable L = 20 m 15/5 (15 strands, 5 mm width) Self field Ic ~ K No strand or cable insulation Flux deflectors will be added First transformer cable configured for end-to-end Ic test

7 Core Warm core with cruciform, 6-stepped section of high grade core steel No-load loss of about 750 W At 20:1 cooling penalty, cold bore is not feasible

8 YBCO Roebel Cable Roebel cable or Continuously Transposed Cable (CTC) is useful for Forming a high current capacity conductor 100 s to 1000 s of Amps (even 10,000s at low T) Reducing AC losses rule of thumb magnetisation losses scale with strand width Cables are labelled with the convention # of strands / strand width We are making two designs 15/5 and 10/2 Roebel strand Strands wound together and geometric parameters

9 Roebel Cable Manufacture Punch tool and frame (a) (b) Punching Tape de-spool (c) (d) Set-up for automated multi-strand Roebel strand production. Control systems Tape re-spool Formation of Roebel punched strands in 40 mm and 12 mm wide feedstock material. (a) 4 x 5 mm strands in 40 mm wide material, (b) 1 x 5 mm wide strand in 12 mm wide material, (c) 10 x 2 mm strands in 40 mm wide material, (d) 3 x 2 mm wide strands in 12 mm wide material. Winding Automated planetary wind system for 15/5 cable Capable of winding several hundred continuous metres of cable.

10 Field (T) Correlation Wire qualification For Roebel we require 2D uniformity - Scan wire magnetically (penetrated or remnant field) - Quantify uniformity using statistical correlation with an ideal magnetic profile Correl( X, Y) ( x ( x x) x)( y 2 ( y y) y) Where Correl 1 X is a dataset representing calculated field Y{y 1 y j } is magnetic data across tape T T 200mm (a) (a) (b) We use continuous scanning of the Remnant magnetic field to assess tape quality (a) tape with a known defect, and (b) tape with only small scale variability. Some wire is extremely good! Example Profiles cor_0.99 cor_ cor_0.75 Correlation along a length of YBCO wire, a minimum Correl can be specified for input wire Wire Position (mm) Position (m)

11 Roebel cable performance Measured end-to-end Ic of cable is 1400 A 77 K. Design calls for 1390 A rms = 1970 A peak. Ic tests on strand samples at 70 K and subsequent analysis predicts 2500 A cable Ic. Ipeak/Ic = 0.79 This is yet to be confirmed by experiment on a cable at 70 K. Use of load line method to predict cable critical current Described by Staines et al, The development of a Roebel cable based 1 MVA HTS transformer, to be published in Supercond. Sci. Technol. 24 (2011)

12 Short circuit fault handling 2 second short circuit withstand is a common requirement of conventional transformer standards. What is a realistic target for HTS? HTS lacks the resistive conductor cross sectional area to carry a short circuit for 2 seconds. What is a realistic short circuit duration for an HTS transformer? Determine fault current limiting performance. HTS under short circuit exhibits highly non-linear response how will this limit fault current? Cable strand has exposed cut edges. Roebel strand is punched exposing edges of conductor. Does this pose a risk of conductor damage due to ingress of LN2 and then subsequent heating in a short circuit?

13 Short circuit simulation Adiabatic model assumes no heat transfer from the conductor into liquid nitrogen. Assuming instantaneous shift of current into 40 micron thickness of copper (20 micron each side, Ω= 12.5 x 10-3 / m per 77 K). Simulation incorporates temperature dependence of both resistance and thermal capacity of the conductor.

14 T p, T s (K) Simulation output LV winding will reach 350 K at t = 200 ms T p, T s vs. Time Impedance doubles from 0.05 pu to 0.1 pu initially then up to 0.5 pu over 200ms (due to change in resistance as temperature increases) T s T P Time (s) Tp=Temperature of primary winding Ts= Temperature of secondary winding

15 Simulation output Fault limited peak current = 14 ka (about 35% of the peak that would occur if limited only by the 0.05 pu leakage reactance). Cable will offer significant current limiting during a short circuit fault. pu = per unit, actual value divided by base value.

16 Short circuit strand testing 200 ms DC pulse, peaking at >15 V/m in strand (transformer short circuit voltage = 12 V/m). T increased to 300 K in 60 ms There was no damage (no change in Ic). Repeat test after 3 week soak in LN2. V 2A V 2B V 2C V 2D V 2E V 2F V 1 Current Shunt -ve Terminal +ve Terminal Conclusion 200 ms duration short circuit is OK and there is no evidence of damage due to nitrogen ingress into the exposed cut edge of the cable strands.

17 Impulse response in HV winding 21 kv peak voltage From VOLNA Proprietary modelling software modelling response to standard 95 kv impulse

18 High voltage insulation 95 kv impulse response modelling HV winding must withstand 550 V from turn-to-turn. (21 kv / 38 turns per double pancake) Commercial 25 micron polyimide wrapped insulation was tested to confirm suitability. Two wrapped strands held together and stressed until 77 K. 20 tests analysed according to IEC 62539* Insulation will survive 2 kv. * Guide for the statistical analysis of electrical insulation breakdown data

19 Heat transfer experiment Fix power dissipation in strand and measured temperature rise of cable winding in LN2 Used conductor with Tc < 65 K. Copper layer used as both resistive heater and temperature sensor. Tested at 68 K and 77 K, both at atmospheric pressure.

20 Typical heat transfer regimes Note: hysteresis in transitions from one regime to the other and the shape of transition from convection to nucleate. Van Sciver Helium Cryogenics

21 Power Dissipated [W/m] 10 Heat transfer results Heat Transfer - Dependence on Bath Temperature 1 Nucleate boiling Assume 1 W/m of strand heating due to AC loss Convective heat transfer Conductor Temperature Rise, dt [K] Sub cooled results are at 68 K and atmospheric pressure. Inner, middle and outer refer to position in Roebel cable strand stack inner being against the winding former, outer being directly exposed to liquid nitrogen. 3 77K Outer 77K Middle 77K Inner Subcooled Outer Subcooled Middle Subcooled Inner Sub cooling necessary to avoid nucleate boiling. Convective cooling keeps T < 3 K. Location of strand has minimal impact.

22 Desired conditions inside the transformer cryostat The windings must be held in the temperature range 65K to 70K in order to achieve the required performance of the superconducting cable. It is desirable to ensure that the evolution of gas in the windings is minimized. Gas bubbles degrade the dielectric performance of the LN2 as well as risking an accumulation of gas that might thermally insulate regions of the windings and give rise to hot spots. S.M.Baek et al, Electrical Breakdown Properties of Liquid Nitrogen for Electrical Insulation Design of Pancake Coil Type HTS Transformer. IEEE Trans. Appl. Supercond. Vol 13, No.2, June 2003

23 Nitrogen phase diagram Temperature range Max. pressure Operation here makes pressure vessel design easier Operation at elevated pressure allows significant temperature increase before phase change. Vapour and liquid only co-exist on the saturation line Atmospheric pressure

24 Cryostat layout Common cryostat for all three phases is best Smaller footprint and reduced heat load due to fewer electrical bushings and reduced shell area. Simplified nitrogen circulation system only one inlet and one outlet required. Three individual cryostats - easiest to manufacture Cryostat pressure design is easier Less risk with a simple cylindrical structure Compromise with three individual eccentric cryostats Simple cylindrical shells Reduced transformer length and nitrogen volume X 1.5 X 1.3 X

25 Cryostat lid arrangement Foam insulation plug Gas in equilibrium due to temperature profile through foam Liquid return Electrical bushing Bulk liquid at 65K 70K

26 Cryostat work in progress Single phase cryostat base cold shell and mould Cryostat cold shell test vessel

27 Summary Roebel cable described high current capacity/low AC loss. Tests and analysis predict cable Ic of 2500 A at 70 K. Short circuit fault across the Roebel cable winding can be tolerated if disconnected within 200 ms. Standard copper stabilizer provides significant fault current limiting - Simulation predicts short circuit fault limited to 35% of the prospective fault current Standard 25 micron spiral wrapped polyimide insulation will withstand 2 kv turn-to-turn. 1 W/m continuous dissipation in sub-cooled LN2 will be below nucleate boiling regime. Cable temperature will be up to 3 K warmer than bulk LN2.

28 Summary cont... Cryostat design discussed. The target normal operating temperature in the cryostat is in the range 65 K to 70 K. There is benefit in operating cryostats at elevated pressure. A foam plug beneath the lid will allow operation at elevated pressure while maintaining a gas/liquid transition at some point between the lid and the bulk liquid region.

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