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RECOMMENDATIONS FOR TESTING OF SUPERCONDUCTING CABLES

Convener: D Lindsay (US)

Secretary: T Masuda (JP)

TUTORIAL B1.31 – TB 538

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Technical Brochure Outline

EXECUTIVE SUMMARY 5

1.0 Introduction 6

1.1 Background 6

1.2 Experience with Superconducting Cables 7

1.3 Cryogenic System Issues 8

1.4 Future Work 9

1.5 Definitions 9

2.0 Tests for Engineering Information 12

2.1 AC loss 12

2.2 Heat invasion of cryostat 12

2.3 Electrical parameters 13

2.4 Short circuit tests (Fault current) 13

•

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3.0 Type Tests on Cable Systems 14

3.1 Range of Approval 14

3.2 Summary of Type Tests 14

3.3 Type tests on complete cable systems 15

3.4 Type Tests on Cable 19

3.5 Type Tests on Accessories 19

4.0 Factory / Routine Tests 20

4.1 AC Voltage test for cable 20

4.2 AC Voltage test for accessory 21

4.3 Ic measurement test 21

4.4 Vacuum leak test 22

4.5 Pressure test 22

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5.0 After Laying Tests 23

5.1 General 23

5.2 Tests in warm conditions 23

5.3 Tests in cold conditions 24

5.4 Additional tests after commissioning 28

6.0 Bibliography / References 30

Annex A Abbreviations 31

Annex B Critical Current (Ic) Measurement 32

B.1 Critical Current Measurement Test 32

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Annex C Cryogenic System Considerations 34

C.1 Introduction 34

C.2 Refrigeration 34

C.3 Functional Requirements 36

C.4 Specification Guide for End User 42

C.5 Safety 43

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Prior Cigre TB on Superconducting Cables

1. TB 199, “Superconducting Cables. Impact on Network Structure and Control” (2002)

2. TB 229, “High Temperature Superconducting (HTS) Cable Systems” (2003)

3. TB 418, “Status of Development and Field Test Experience with High-Temperature Superconducting Power Equipment” (2010)

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Scope of this Brochure

• Provides recommendations for Type Testing and After Laying tests for AC superconducting cables

• “High Temperature” superconductors only. Excluded are “low temperature” materials.

• Voltages up to 150 (170) kV

• Included as an Annex is information and guidelines related to cryogenic system performance, specifications and safety.

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Key Definitions (1/2)

AC Loss For HTS cables, this term is used to describe the heat load generated by hysteretic losses associated with alternating current cycles in the superconducting materials. This heat must be accounted for in the thermal budget of the overall system design. HTS cables operating in direct current (DC) circuits do not generate AC losses.

Cold Dielectric (CD) A type of superconducting cable in which the dielectric material operates within the cryogenic environment.

Critical Current (Ic) The current in a superconducting material that results in an electric field of 1 µV/cm. For I>Ic the superconductor operates in a resistive (normal) state.

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Key Definitions (2/2)

Cryostat An apparatus designed to contain and thermally insulate a cryogenic environment.

High Temperature Superconductor (HTS) Superconducting materials that achieve the superconducting state at temperatures greater than 20 K (-253ºC). Typically, HTS materials are used in superconducting power applications that can be cooled with liquid cryogens. Most common is liquid nitrogen at 77 K (-196 ºC) due to its availability, cost and dielectric properties.

Low Temperature Superconductors (LTS) Superconducting materials that achieve the superconducting state at temperatures below 20 K (-253ºC). Examples of common LTS materials are Niobium-Titanium and Niobium-Tin alloys.

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Types of HTS Cables – Single Core Cable

• One electrical phase per cryostat

• HTS wires for phase conductor and concentric neutral of the cable

• May or may not include copper wires to shunt short-circuit currents to protect HTS wires

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Types of HTS Cables – Three Core Cable

• Three single-phase cable cores inside a common cryostat.

• HTS wires for phase conductor and concentric neutral of the core

• May or may not include copper wires to shunt short-circuit currents to protect HTS wires

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Types of HTS Cables – Triaxial Cable

• Three electrical phases are concentric on a common core inside a common cryostat

• Dielectric layers are phase-to-phase voltage rather than common phase-to-ground

• Cable screen/neutral may be copper or HTS wires

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Global Experience – HTS Installed in Power Grids

Country &

Utility

In-service

Date Voltage

Designed

Power

Level

Length Design

DK 2001 24 kV 30 m

Warm

Dielectric,

Single Core*

CN, China

Southern

Power Grid

2004 35 kV 120 MVA 33.5 m

Warm

Dielectric,

Single Core*

USA,

National

Grid

2006 34.5 kV 48 MVA 350 m Three Core

USA, AEP 2006 13.2 kV 69 MVA 200 m Triaxial

USA, LIPA 2008 138 kV 574 MVA 600 m Single Core

KR, KEPCO 2011 22.9 kV 50 MVA 500 m Three Core

JP TEPCO 2012 66 kV 200 MVA 250 m Three Core

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Pictures from Cigre TB418

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Global Experience – HTS Tested in Laboratories and Industrial Applications

Country Dates Voltage Designed

Power Level Length Design

JP Super-

GM 2005 77 kV 133MVA 500 m Single Core

US -

Carrollton 2000 - 2006 12.4 kV 27 MVA 30 m Single Core

KR - KEPRI 2007 22 kV 48 MVA 100 m Three Core

MX 2007 23 kV 80 MVA 100 m Three Core

DE / ES 2008 10 kV 17 MVA 30 m Single Core

DE / ES 2010 24 kV 133 MVA 30 m Single Core

Russia 2010 20 kV 104 MVA 200 m Single Core

KR 2011 154 kV 1000 MVA 100 m Single Core

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Section 2: Tests for Engineering Information

2.0 Tests for Engineering Information 12

2.1 AC loss 12

2.2 Heat invasion of cryostat 12

2.3 Electrical parameters 13

2.4 Short circuit tests (Fault current) 13

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2.1 AC Loss

• Resistive losses in HTS wires due to hysteretic losses of AC cycles

• Information needed to properly size cryo system

• Measured from 0 to 1.2x rated current

• Methods:

o Electrical

o Calorimetric – Vaporization of Liquid Cryogen

o Calorimetric – Liquid Cryogen Mass Flow

• AC losses in each phase of a Triaxial design may be different. Therefore each phase should be measured. Ideally they should be measured simultaneously

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2.2 Heat Invasion of Cryostat

• Heat leak into the cryostat is always present and represents significant load on cryogenic system

• Information used to properly size cryogenic system

• Methods:

o Calorimetric – Vaporization of Liquid Cryogen

o Calorimetric – Liquid Cryogen Mass Flow

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2.3 Electrical Parameters

• The inductance, capacitance and tan delta will be tested on a cable sample of a 1 m sample (or longer). The tests will be performed at rated cryogenic temperature.

• Cables with a Triaxial design will have different electrical parameters in each phase. The electrical parameters will be measured for each phase.

• Tan delta and capacitance will be determined at Uo to determine the dielectric losses.

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2.4 Short Circuit Tests

• Thermal management and mitigation during short circuit events is critical to system performance

• Fault Current Limiting Cables are in development

• Parameters to test:

o temperature and pressure reaction to limiting event (implying pressurized liquid nitrogen system)

o level and wave form of the limited current

o recovery time

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Section 3: Type Tests on Cable Systems

3.0 Type Tests on Cable Systems 14

3.1 Range of Approval 14

3.2 Summary of Type Tests 14

3.3 Type tests on complete cable systems 15

3.4 Type Tests on Cable 19

3.5 Type Tests on Accessories 19

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3.1 Range of Approval

• U, Uo and Um follow IEC 60183

• Type Tests are valid under the following conditions:

o The voltage group is not higher than that of the tested cable system (common Um)

o The nominal current is not larger than that of the tested cable

o The cable and the accessories have the same or similar constructions as that of the tested cable system(s)

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3.2 Summary of Type Tests

• Min cable length = 10m

• Min cable length between accessories = 5m

• Accessories shall be installed after the bending test on the cable

• Cable and accessories shall be assembled in the manner specified by the manufacturer's instructions

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3.3 Type Tests on Complete Cable Systems

Tests and Sequence of Tests

a) Bending test (See 3.3.3) on the cable and Ic measurement tests

b) Pressure test on the assembly (See 3.3.4)

c) Load cycle voltage test on the assembly (See 3.3.5)

d) AC voltage test on the assembly (See 3.3.6)

e) Lightning impulse voltage test followed by AC voltage test on the assembly (See 3.3.7)

f) Partial discharge tests on the assembly (See 3.3.8)

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Bending Test

• Bending performed at ambient temperature

• 3 times reverse bend without axial rotation

• Cable core(s) shall be inside cryostat or similar corrugated tube

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Load Cycle Voltage Test

• 20x load cycles

• 8 Hrs ON + 16 Hrs OFF

• Voltage == 2 Uo during whole test period

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Voltage Tests

Note factor for

Triaxial Cables

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Partial Discharge Test

NOTE: At the time of this writing, not all manufacturers have access to screened HV test rooms that also support the required cryogenic systems needed for HTS cables. Achieving 5 pC partial discharge sensitivity is difficult in these situations.

The test shall be performed in accordance with IEC 60840, the sensitivity being 5 pC or better. A test may be acceptable up to 10 pC, but samples of cable and accessory dielectric components must be tested to 5 pC sensitivity per section 3.4.2 and 3.5.1.

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3.4 Type Tests on Cables

• Bending Test

o Bending per test required prior to system assembly

o Min 95% retention of critical current of cable

• Partial Discharge on Cable (if 10 pC on System Level PD)

o Cable sample tested to max 5 pC

o Same voltage, temperature & pressure as system level test

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3.5 Type Tests on Accessories

• Partial Discharge on Accessory Components (if 10 pC on System Level PD)

o Cable sample tested to max 5 pC

o Same voltage, temperature & pressure as system level test

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Section 4: Factory / Routine Tests

4.0 Factory / Routine Tests 20

4.1 AC Voltage test for cable 20

4.2 AC Voltage test for accessory 21

4.3 Ic measurement test 21

4.4 Vacuum leak test 22

4.5 Pressure test 22

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4.1 AC Voltage Test for Cable

• One sample from each shipping reel shall be taken and subjected to the voltage test. In case several shipping reels are created by cutting cable length that was produced in the same manufacturing batch, the number of samples to be tested shall be such that one sample in each 500 meter section will be tested

• Min 1 meter sample

• 30 min at 2.5 Uo

• PD Test after withstand

o 1.75 Uo for MV

o 1.5 Uo for HV

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4.2 AC Voltage Test for Accessories

The main insulation parts of each prefabricated accessory shall undergo voltage and partial discharge test at ambient temperature according to either 1) or 2) below :

1) by using a host accessory into which a component of an accessory is substituted for test;

2) by using a simulated accessory rig in which the electrical stress environment of a main insulation component is reproduced. The test voltage shall be selected to obtain electrical stresses at least the same as those on the components in a complete accessory when subjected to the test voltages specified in 4.1.

• 30 min at 2.5 Uo

• PD Test after withstand

o 1.75 Uo for MV

o 1.5 Uo for HV

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4.3 Ic Measurement Test

• Critical current (Ic) of the cable cores in the test cable shall be measured for superconducting properties to check the damage of the superconducting wires due to bending. The measured Ic shall be more than 95 % of the designed Ic in consideration of LN2 temperature, magnetic field and so on. The measured Ic is defined as the current which generates 1 µV /cm voltage rise across the sample (See Annex B).

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4.4 Vacuum Leak Test

• The vacuum leak test at ambient temperature shall be carried out in accordance with local specification for all finished cables and components where the vacuum seal is made at the factory

• The test cable or components shall be evacuated by a vacuum pump system. The leak rate of tracer gas to the vacuum area through the outer and inner wall shall be detected by a helium leak detector (mass spectrometer). The final leak rate for a given test cable or components should be recognized as the sum of all individual leaks measured

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4.5 Pressure Test

• Pressure test shall be performed as a routine factory testing on specific components at request of customer or option of manufacturer. The test shall be carried out in accordance with local regulations for pressure equipment or engineering best practice

• Pressure test shall be carried out on components where pressurized gas or liquid must be contained. The components shall be made as a pressure vessel following local standards. The cable shall be pressurized with gas until the test pressure higher than predetermined value. The holding time shall follow to the local regulations

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Section 5: After Laying Tests

5.0 After Laying Tests 23

5.1 General 23

5.2 Tests in warm conditions 23

5.3 Tests in cold conditions 24

5.4 Additional tests after commissioning 28

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5.2 Tests in Warm Conditions

1. Vacuum Test

2. Pressure Test (According to Local Pressure Regulation Standards)

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5.3 Tests in Cold Conditions

1. DC Cricital Current (Ic) Test

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2. Voltage Test; options

a) AC Voltage (20-300 Hz)

b) DC Voltage

c) VLF

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3. Verify System Operational Control Parameters

o The cable system shall be in stabilized conditions and the cooling system shall be operating with the nominal mass flow and nominal supply temperature.

o The integration of control signals with the utility Supervisory Control and Data Acquisition (SCADA) system shall be verified. The selection of signals to be verified shall be at the discretion of the customer. As a minimum requirement those signals relevant for the cable system trip logic must be verified.

o The total temperature increase and the pressure drop of the cooling medium shall be measured. The measured values shall be in good agreement with the design values.

4. Dielectric Soak Test (optional if voltage test performed)

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5.4 Additional Tests after Commissioning

• Shielding Test

o applicable for single core cables only

o Measure of induced current from phase layer into neutral layer HTS wires

• A symmetrical current system shall be applied to the cable system by means of connecting the system to the grid. The current system shall be considered symmetric if the amplitude of each current of the three phases does not deviate by more than 5% from any of the other two phases current amplitudes and if the phase angle between the three currents is equal to 120°.

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Annex B – Critical Current Measurements

V

I 0

V

I 0

V-I Curve, HTS Only V-I Curve, HTS + Cu Component

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Annex C: Cryogenic System Considerations

Annex C Cryogenic System Considerations 34

C.1 Introduction 34

C.2 Refrigeration 34

C.3 Functional Requirements 36

C.4 Specification Guide for End User 42

C.5 Safety 43

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