Superconducting Fault Current Limiters:Concepts, design methods and applications

Antonio MorandiDEI Guglielmo MarconiDep. of Electrical, Electronic andInformation Engineering

University of Bologna, Italy

Tuesday, November 27, 2018

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• The fault current problem

• FCL technologies

• Resistive SFCLConceptEffect on the grid – case study

• Saturated type SFCL

• The state of the art of SFCL

Outline

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… faulthappens !

4

a poly-phase fault produces an overcurrent, which in turn produces

• Damage of components

• Outage or even black out

The ultimate effect of a fault is an economic damage for both theoperator and the customer

• Voltage disturbance

Network operators are required toensure appropriate power quality

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Short circuit power

XccU +

cc

2n

cc

2

cc 3X

V

X

US

U = Vn / 3

[ MVA ]

Strong grid: Scc Xcc

Weak grid: Scc Xcc

Scc : power delivered by the grid in short circuit condition

• Short circuit power changes form point to point

SccA > SccBA B

• Short circuit power increases with the level of interconnection

A A’ A’’

SccA < SccA’ < SccA’’

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Advantages of the strong grid – Normal operation

XccUn +

load

I

Un

Umin

I

Scc =

Scc

U

U

U = Un Xcc I Imax’ Imax’’

• Reduced dependence of the voltage on the load (high voltage quality)

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• Wider porpagation of voltage distrurbances

A’

UA

t

U

B’

A

B UB

tf

UAUB

tf

t

U

Disadvantages of the strong grid – During fault• Increased stress for the components

XccUn +

load

I = Un / Xcc

• Thermal stress

• Mechanical stressF = k I 2

I

Q = R I 2t

short circuitpower

norm

alco

nditi

onfa

ult

• poor persistentvoltage quality

• poor stability

• high persistentvoltage quality

• high stability

• low vulnerability• high transient

voltage quality

• high vulnerability• poor transient

voltage quality

For obtaining high network’s performance both in normal condition and during thefault the shift form high to low short circuit power is required

Strong gridWeak grid

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short circuitimpedance

9

For obtaining high network’s performance both innormal condition and during the fault a condition-based increase of the impedance is required

Fault current limiter (FCL): a device with anegligible impedance in normal operation whichis able to switch to a high impedance state in caseof extra current (fault)impe

danc

e

current

For obtaining high network’s performance both in normal condition and during thefault a fault current limiter is required

FCLZX

VS

cc

2n

cc

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Pyrotechnic (ABB Is-limiter) Solid State Superconducting

Advantagesavailable since 1955thous. of units installed

Drawbacksno self restoration, non fail safesafety concerns (explosion)not available for high voltage

Advantages“popular technology”

Drawbacksreliability, losses, cost

Advantageseffectiveness, reliability

Drawbackscooling, cost,“a technology for special purposes”

Under R&D worldwide

Fault current limiting technologies

Widely proved on fieldAn existing technology

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Superconducting Fault Current Limiter (SFCL)

• Resistive type SFCL

• Magnetic Shield Type SFCL(and SMART COIL)

• Saturated core type SFCL

• Rectifier type SFCL

+ many variants

resistance

inductance

Quench based

Quench free

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Resistive Type Superconducting fault Current Limiters (R-SFCL)

conventional shunt reactor

mechanical switch

+

A resistive shunt could be used in principle but theinductive one is preferred due to dissipation

non inducting SC winding• bifilar helical windings (1GHTS/bulks, MgB2)• bifilar pancakes (2G HTS)• alternate pancakes (2G HTS)

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• STEP 1. A “reversible” SC/Normal transition is first induced below Tc due to overcurrent

1

cc

0SC

SCNS

SCNSeq

eq

,

,)(

,)(,

,

n

TJ

J

TJ

ETJ

TJT

TJTTJ

JTJE

Brandt, 1999Duron et. al., 2004

• STEP 2. A “stable” SC/Normal transition is finally obtained due to heating

Limiting mechanism

• An immediate increaseof the resistance isobtained above Jc

creep

normal stateT < Tc

• The over-temperature depends onthe total resistance and hence on theamount of conductor

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• Lower dissipation during the fault• Reduced temperature increase at the end of

the fault• Quick recovery time• High allowable residual voltage during the

fault

A long length of conductor is desirable …..

A fast mechanical switch is added in series with thesuperconductor in order to withstand long faultduration without excessive overtemperature andallow safe and complete recovery

but

• High capital cost• High AC losses (high operating cost)

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A case study

Nominal Voltage, Vn 15 kVrmsMinumum Residual Voltage, Vr 70 % Vn (14 kVrms)Nominal current, In 480 Arms (12.5 MVA)Opening time of circuit breaker, toCB 120 msTime delay for opening command, tdCB

Is1 = 630 Arms I Is2 = 1400 Arms 800 ms tdCB 0 msIs2 = 1400 Arms I tdCB = 0 ms

Reclosing time of circuit breaker, trCB 400 ms

This is to take intoaccount temporaryovercurrents whichroutinely occurs in thegrid

12 km0.5 km

0.5 km 1.5 km

4 km

Subt

ram

issio

n 1 km40 MVAXcc = 0.87

8 MVA4 MVA

6 MVA4 MVA

2 MVA4 MVA

10 MVA

A

15 kV

132 kV

F G

D

E

BC

FCL

sensitivecustomers

disturbingcustomers

overheadrural feeder

A typical distribution gridsupplying a mix of industrial,commercial, residential andrural loads

Main characteristics and settings of the protectionsRfeeder = 0.27 / kmXfeeder = 0.35 / km

Standards EN61000-4-1 and EN61000-4-34specify the residual voltage VR on equipmentduring a disturbance

Tolerant (VR 40 % )

Sensitive (VR 70 % )

The residual voltage during the fault at allbus of the network is below the threshold ofeven tolerant equipment

Peak fault current: 21.4 kA

Total thermal let-through: 12.7*106 A2s(this is close to the allowable limit)

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• Appropriate limiting effect

• Appropriate protection from voltagedisturbances to costumers not directlyaffected by the fault

• No interference with existingprotections

The need to provide appropriate protection from voltage disturbance (70 % residualvoltage) is usually the stricter and sets the actual limit on the minimum possibleimpedance of the device

Syst

em le

vel

Devi

ce le

vel

• No damage must occur to the deviceduring the fault. The temperature mustnot overcame a given limit (usually 300 K )at the end of the fault

Design Criteria for resistive SFCL

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3and25.1 s

sccsc I

XX

VII

shuntq XR

)(~~37.0L

0maxcondcond

oCBcond TTc

tV

37.0

3

VV

XX

X

shuntcc

shunt

ccshunt XX

CBoCBrrecovery ttt

very demanding

Voltage disturbance is prevented for healthycustomers.

Both the peak current and the thermal let-through are greatly reduced

unlimited limitedPeak current 21.4 kA 8.8 kA 61 %Thermallet-through 12.7*106 A2s 2.2*106 A2s 83 %

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Effect of the SFCL on the grid

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Advantages• Immediate and fail safe operation• Compact size• Negligible impedance in normal condition• High impedance gain

Critical aspects• Recovery time• AC operation of the SC - AC loss• Hot spots during light overcurrent• Direct exposure of the SC component to high voltage

To resume on resistive type SFCL :

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Superconducting Fault Current Limiter (SFCL)

• Resistive type SFCL

• Magnetic Shield Type SFCL(and SMART COIL)

• Saturated core type SFCL

• Rectifier type SFCL

+ many variants

resistance

inductance

Quench based

Quench free

H

HSC

IFCL

Copper winding

SCwinding

DC currentsource

B = 0 r (Htot) Htot

Saturated core type SFCL

Htot = HSC H

Magnetic field of the core

Magnetic permeability of the steel

Inductance of the copper winding

FeFe

Sl

NNL

2

SCr0

2

)HH(μμ

By controlling the total magnetic fieldof the core it is possible to control theinductance of the copper winding

Htot < Hsatr >> 1L is high

Htot > Hsatr = 1L is low

B

Htot

Hsat21

H

HSC

IFCL Copper winding

SCwinding

DCcurrentsource

Hsat HSC

Htot = HSC H

A strong field is applied by means of the SC winding so as to saturate the coreA small inductance is obtained for the copper winding (Htot > Hsat, r = 1)

If a positive current circulates in the copper winding the field in the core is weakenedA small inductance is obtained for the copper winding as far as HSC H > Hsat

If the currents increases further the total magnetic field goes below saturation and theinductance becomes high

B

Htot

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Hsat HSC

Htot = HSC H

B

Htot

Hsat

Imax Imin

I

L ( I ) (ideal)

Lhigh

Llow

• If due to positive overcurrent thecore reaches the saturation in theopposite direction the inductancebecomes low again

• If the current of the copper coil isnegative saturation is strengthened.No increase of impedance is obtained

(half wave limiter)

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H

HSC

H

HSC

IFCL

Copperwinding 1

SC winding

DC currentsource

Copperwinding 2

• For three-phase arrangement one common SCwinding is used to saturate all the six cores

• In order to obtain the increase of impedancefor both positive and negative overcurrentstwo half-wave limiters with opposite windingdirections are placed in series

• One common SC winding is used to saturateboth cores

full-wave limiter

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• The Saturated core type SFCL does not exploit the SC/normal transition

• Superconductor is only used for saturating the cores

• In principle a normal conducting coil could be used for saturating the cores

• A very large amount of Ampereturns is needed for saturating the cores

• Continuous operation is required (7/7-24/24)

• Joule losses and the amount of copper would make the device unfeasiblein practice

• Superconductivity is the enabling technology for this type of limiter

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Advantages• Immediate and fail safe operation• DC operation of the superconductor (no AC losses)• No direct exposure of the SC component to high voltage• (Virtually) immediate recovery

Critical aspects• Low and narrow impedance gain• Non negligible impedance in normal operation• Very large size• Losses in the copper coil• Overvoltage on the SC winding during the fault and possible

demagnetization of the core during the fault

To resume on saturated core type SFCL :

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The state of the art of FCL

INNOPOWER2011BSCCO

RSE2011BSCCO

KEPRI2011YBCONEXANS

2009BSCCO Bulk

AMAT2016YBCO

SIEMENS2016YBCO ZENERGY

2012BSCCO

ASG2017MgB2

Complete demonstrators submitted to laboratory and/or field tests

NEXANS2009BSCCO Bulk

NEXANS2015YBCO

AMAT2013YBCO

NEXANS2009YBCO

INNOPOWER2009BSCCO

NEXANS2015/AMPACITYYBCO

KEPC2019YBCO

China, fundedproject2019 ?YBCO

DC FCLFAST GRID – EU2021YBCO

SUPEROX2019YBCO

China, funded project2019 ?YBCO

Volta

ge, k

V

Current, kA

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Some Achievements

Siemens Applied Materials

Resistive type, YBCO12 kV, 815 AInstalled 3/2016Augsburg, Germany

Resistive type, YBCO115 kV, 550 AInstalled 7/2016Thialand Saturated core type, YBCO

220 kV, 800 AInstalled 2012Tianjin, China

Innopower

M. Noe, EUCAS 2017 Short Course , Power Applications–Fault Current Limiters

Rated voltage 36kV

Continuous normal current 800 Arms

Maximum normal current 1400A / 15 minutes

Unlimited peak fault current 20.9 kApeak / 7.8 kArms

Peak limited current 13.0 kApeak / 4.8 kArms

Fault duration Up to 3 seconds

Maximum allowable voltage drop 600V rms

A 36 kV / 800 A saturated core SFCL for real grid installation

Successfully tested at IPH in Berlin in October 2016• 5 faults with 200 ms duration• 1 fault with 3 s duration

To be installed in Substation for a 3 year trial

A range of SFCLs of similar design is available fromASG Power Systems for service at 75 kV and 145 kV

Developed by ASG Power Systems(formerly by Applied Supercond. )

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Thank you for your attentionantonio.morandi@unibo.it