MVDC FAULT PROTECTION FOR

SHIPBOARD POWER SYSTEMS Presenter: Xianyong Feng, PhD

Contributors: Dr. Angelo Gattozzi, Doug Wardell, Dr. Bob Hebner

Center for Electromechanics

The University of Texas at Austin

March 23, 2018 – ONR Control Workshop

Key Points

• Proposed a hybrid prot. method and an extra-fast fault

localization method for dc distribution systems

• Proposed a dc series fault detection/localization method

• Validated the approach in simulation and experiment

2

Outline

1. Overview

2. Methodology • Pole-to-pole dc short circuit fault

• Series dc arc fault

3. Simulation Study

4. Experimental Results

5. Conclusion

3

4

Motivation

• Power Equipment Damage and Human Safety

• Fire Hazards

5

Power System Fault Management

Detection

• Real-time monitoring

• Detect electrical abnormality

Localization

• Quickly and accurately localize fault

• Minimize system impact

Isolation

• Open protective devices

• Minimize load interruption

Restoration

• Quick recovery

• Restore interrupted loads to normal

6

DC System Protection Challenges

7

AC fault current

DC fault current

1. No fault current zero-crossing

2. Lower line impedances

3. High di/dt

4. Power electronic devices can not tolerate high fault current

5. Fast capacitor discharge

Overcurrent Protection

Advantage Disadvantage

1. Algorithm is simple

2. Need less sensors

1. Prot. coordination is difficult

8

Protection

unit

Protection

unit

Protection

unit

i1

i3

i2

...

...

Legend

Protective device

Current sensor

Current

measurement signal

Control command

Over-current protection

strategy

IF i1<ith, no downstream fault

IF i1>ith, downstream fault

A tripping command is generated

after a certain time delay

ith : preconfigured current threshold

Zone 1

Zone 2

Zone 3

Differential Protection

9

Differential

Protection Zone

Legend

Protective device

Current sensor

Protection

unit

i1

i2

i3

i4

i5

Current

measurement signal

Control command

Differential Protection

Strategy

IF i1+i2+i3+i4+i5<ith, no fault in zone

IF i1+i2+i3+i4+i5>ith, fault in zone

ith : preconfigured current threshold

Advantage Disadvantage

1. Reliable fault detection

2. Algorithm is simple

1. Need signal synchronization

2. May need comm. for long lines

Impedance Protection

10

Protection

unit

Protection

unit

Protection

unit

i1

i3

i2

...

...

Legend

Protective device

Current sensor

measurement signal

Control command

Impdeance Protection Strategy

Use current to detect downstream fault. If

a fault is detected, estimate impedance

IF Lestimated<Lth, fault in zone (internal fault)

IF Lestimated>Lth, downstream fault (external

fault)

Lestimated : estimated fault inductance

Lth : preconfigured inductance threshold

Zone 1

Zone 2

Zone 3

v3

v2

v1

Voltage sensor

Advantage Disadvantage

1. Good fault localization accuracy

2. Extra-fast fault localization

1. Need high sampling rate for voltage and current

2. Need fast controller

Converter Fault Current Limiting

1. Buck converter

2. Full-bridge MMC

3. Thyristor-based rectifier

11

Phase A

Cell1T1

T2

D1

D2uC1

Cell1

CellN

iSA

CellN

D3

D4

T3

T4

Full-bridge MMC

iSA

iSCiSB

S1

S2

S3

S4

S5

S6

Thyristor-based rectifier

=

Level 3 Level 4

Level 5

=~ =

DC UPS

...

...

Buck converter

12

Simplified MVDC Shipboard Power System

13

FCL

FCL

DP

DP

DPDPDPDP

OPOP

OPDP

OP

DP

OPOPDP

DP

DP

DP

DP

DPDP

OP

OP

OP

DP

DPDP

FCL

FCL

FCL

FCLFCL

FCL

Ref: N. Doerry and J. Amy, “The Road to MVDC,” Proc. of ASNE Intelligent Ships Symposium, Philadelphia PA, May 2015.

Short-Circuit Fault Detection and Localization

14

Differential

Protection Zone 1

Legend

Normally close device

Current sensorProtection

unit

i1

i2

i3

Current

measurement signal

Control command

Differential protection

strategy

IF i1+i2+i3<ith, no fault in zone

IF i1+i2+i3>ith, fault in zone

ith : preconfigured current threshold

...

Differential

Protection Zone 2Protection

unit

i4

Load

center

Normally open device

Protection

unit

Overcurrent

Protection Zone 3

Over-current protection

strategy

IF i4<ith, no downstream fault

IF i4>ith, downstream fault

A tripping command is generated

after a certain time delay

ith : preconfigured current threshold

Zone 1Zone 2

Zone 3

FCL

• Conceptual section of dc distribution system with a hybrid

protection system

Fast DC Fault Localization Algorithm

Inductance-based dc fault localization*

1. Estimate fault inductance with locally

measured v(t) and i(t)

2. Use estimated L to locate fault

=

L L L

Zone 1

(20 m)Zone 2

(65 m)Zone 4

(1.5 m)

=

~ =

Converter

with FCL

...

...

Fault 1 Fault 2

Fault 3...

L L L...

Zone 3 (10.2 m)...

...

Equivalent

inductance

Distance

level 1 level 2 level 3

L1

L2

L3

Line inductance distribution

RF

LRi

v

+

-

Equivalent fault circuit

* X. Feng, L. Qi, J. Pan, “A novel fault location method for dc distribution protection,” IEEE Trans. Industrial Applications, vol. 53, no. 3, 2017.

15

Fast DC Fault Localization Algorithm

16

Protection strategy design 1. Online moving-window least square method

2. Algorithm on micro-controllers Start

Go to next time

intervalFault detected? No

Read in (M+1) data points

v and i

)(/)(

)2(/)2(

)1(/)1(

MidtMdi

idtdi

idtdi

A

)(

)2(

)1(

Mv

v

v

B

BAAARR

L TT

F

1

Use estimated L to calculate the distance

from measurement point to fault

End

di/dt estimation

Flowchart of inductance-based fault

localization

Series Arc Fault Detection and Localization

• DC series arc faults

17

Series arc fault protection apparatus1,2

Flowchart of series arc fault detection

and localization1

1. X. Feng, et. al., “Converter-based dc distribution system protection,” IEEE IAS Annual Meeting, 2018, in review

2. Q. Xiong, et. al., “Arc fault detection and localization in photovoltaic systems using feature distribution maps of parallel capacitor currents,”

IEEE Journal of Photovoltaics, 2018, accepted

Arc inception

Series Arc Fault Detection and Localization

• More experimental test data are still required 1. Electrode distance

2. Separation speed

3. Separation acceleration

4. Electrode orientation

Vertical

Horizontal

5. Electrode material

Copper

Aluminum, etc.

6. Ambient conditions

Humidity

Pressure, etc.

18

Fabian M. Uriarte, et. al., “A dc arc model for series faults in low voltage microgrids,”

in IEEE Trans. Smart Grid, vol. 3, no. 4, pp. 2063-2070, Dec. 2012.

19

Simplified System Description

20

1. Two PGMs

‒ FCL in the converters

2. One propulsion load

‒ VFD + motor

3. One pulse load

‒ High di/dt

4. DC isolation devices

‒ Isolate faulted zone

5. Protection strategy1,2

‒ FCL + hybrid prot.

1. S. Strank, et. al., “Experimental test bed to de-risk the navy advanced development model,” Proc. of Electric Ship

Technology Symposium, Arlington, VA, Aug. 2017, pp. 352-358.

2. X. Feng, et. al., “Converter-based dc distribution system protection,” IEEE IAS Annual Meeting, 2018, in review

NC = Normally closed

FCL = Fault Current Limit

1 = Capacitor

2 = Mechanical Circuit Breakers

3 = Contactors

4 = Line Reactor

850 V, 1.2 MW

60 Hz, 3-phase

Lab Power

M

1.1 kV

2 MW

200 Hz

MISSION

LOAD

PMM

PGM

PGM

PCM

1.15 kV main dc bus

Equivalent

DC zonal

Load

PFN

Railgun

PROPULSON

LOAD=

=

==

850 V, 0.8 MW

60 Hz, 3-phase

Lab Power

2 2

3 3 333

1 & 4

1 & 4

Toshiba

NC NCNCNC

FCL

NCNC

NC

FCL

NC

3

NC

3

DP DP

DP

DP

DP

OP OP

OP

Circuit and Parameters

21

• Impact of fault resistance

on system behavior

• Differential prot. method

study

• Fault at the midpoint of

the main dc bus

1. Case 1: Rf = 20mΩ

2. Case 2 : Rf = 500mΩ

Rf – fault resistance

==

==

Reactor 1

Reactor 2PGM 2

PGM 1

Load 1

Load 2

0.6 MW

1 MW

0.39 mH0.39 mH

0.39 mH0.39 mH

5 mF

5 mF 0.5 mF

0.5 mF

9 mΩ

9 mΩ

80 µH

80 µH

Main

dc bus

950 V 60 Hz

3-Ph ac

950 V 60 Hz

3-Ph ac

1150 V dc

10 µH 2 mΩ

10 µH 2 mΩ

OP

OP

DP

DP + IP

DP

20 µH

20 µH

2 mΩ

2 mΩ

DP

DP

FCL

FCL

Reactor 1

0.39 mH0.39 mH

0.5 mF

9 mΩ80 µHPGM 1

Reactor 2

0.39 mH0.39 mH

0.5 mF

9 mΩ80 µHPGM 2 10 µH 2 mΩ

10 µH 2 mΩ

Equivalent Circuit during Fault

System Single Line Diagram

High Fault Resistance leads to faster settling time

22

Rf = 20 mΩ Rf = 500 mΩ

PGM current

PGM voltage

Current differential

Sensitivity Analysis

• Impact of fault resistance

• Criteria for current

differential threshold:

23

9.9 10 10.1 10.2 10.3 10.4 10.50

100

200

300

400

time (ms)

Cu

rre

nt (A

)

delta t = 2 us

delta t = 5 us

delta t = 10 us

delta t = 20 us

delta t = 50 us

Ithreshold > max(di/dt)·max(ΔT)

Current differential of the main dc bus zone

PGM fault currents with different Rf

ΔT - measurement time difference

of zonal boundary currents

Inductance-based Fault Localization HIL Test

Control-HIL test

1. Opal-RT simulator

‒ Simulate a dc system

‒ Convert v(t), i(t) to analog

‒ Read in switch status

2. Microcontroller

‒ Read in v(t), i(t) signals

‒ Execute prot. algorithm

‒ Send trip signal for fault

isolation

24

PXIe Real-Time/FPGA

HIL System Opal-RT Simulator

High speed

communication link

DC microgrid circuit in Opal-RTSimulated switching devices in NI PXI simulator

Control, Protection and Monitoring Hardwares

Digita

l/ana

log

I/O in

terfa

ce

Control Room

Time Sensitive Network

NI / TI controllers

Central series arc

fault localization

...

Local fault detection

and localization

Other functions

...

Central

controller

Inductance-based Fault Localization HIL Test

Control-HIL test results1

1. L estimation error < 9%

2. Fault detection/location time < 0.7 ms tripping signal

current signal

current signal

voltage signal

25

Estimation accuracy

1. X. Feng, et. al., “Fault inductance based protection for DC distribution systems,” Proc. of

IET Conference on Development of Power System Protection, March 2016, pp. 1-6.

26

LVDC Test Circuit (up to 250 V dc)

LVDC fault test circuit

DC power supply

30-kW converter with FCL

Downstream circuit

NI sbRio control board

=

Line impedance 1

Load Source 2Source 1

Line impedance 2

Current and

voltage sensors

Controller

=

7.7 mF 1.2 mF

27

Control Platform

• NI sbRIO-9606 (top-layer)

1. Processor speed: 400 MHz

2. Memory: 256 MB

3. FPGA: Xilinux Spartan-6 LX45

4. Comm. rate: 10-100 Mbps

• NI GPIC (middle-layer)

1. Digital/analog I/Os

2. FPGA and processor expansion I/Os

3. Half-bridge digital output Custom interface board

GPIC NI sbRIO-9606

28

Typical DC Fault Waveform

Switching spikes

Relay chattering

29

Single Source Fault Localization

*: measured inductance value

: estimated inductance value

Case # RF (mΩ) L (µH)

1 33 12.5

2 33 24

3 33 42

4 50 12.5

5 50 24

6 50 42

7 100 12.5

8 100 24

9 100 42

Case Definition*

*5 tests in each case

Test Circuit 1. Fault is on the load side

2. Sampling rate: 20 kHz

3. Data sample number: 5

30

Inductance estimation error

High sampling rate improves the accuracy

-30

-25

-20

-15

-10

-5

0

5

10

Test1 Test2 Test3 Test4 Test5

31 E

rror

[%]

Sampling rate: 20 kHz Sampling rate: 50 kHz

Two-Source Fault Test

Test Circuit 1. Two dc sources

2. Fault is on the load side

3. Sampling rate: 50 kHz

4. Data sample number: 5

In-zone fault Protection procedure: 1. Differential prot. accurately

detects in-zone fault

2. Activate fault localization

program

32

Current differential

Fault current

Two-Source Fault Localization

Test Scenario 1. Fault resistance: 50 mΩ

2. Source 1 fault inductance: 42 µH

3. Source 2 fault inductance: 24 µH

Test

#

Source 1 Source 2

Estimated L (µH) Error (%) Estimated L (µH) Error (%)

1 45.3 7.9% 21.5 -10.4%

2 41.6 -1.0% 25.3 5.4%

3 43.0 2.4% 22.3 -7.1%

4 45.3 7.9% 22.4 -6.7%

33

Summary

1. The inductance prot. uses local measurements only

‒ Fault detection and localization time < 0.7 ms

‒ L estimation error in control-HIL test < 9%

‒ L estimation error in hardware test < 12%

2. Differential protection can accurately detect in-zone

faults

3. Converter FCL effectively limits fault current

34

Ongoing Work: MW Fault Protection Test

Main elements

1. 3-ph 480-V ac

power source

2. Transformer

3. Controlled rectifier

4. Line reactor

5. Semikron power

converter modules

6. Cables

7. Fault resistor

8. Fault path switch

35

Controlled rectifier Line reactor

1.67-MVA 3-ph

converter module Cable

Fault

resistor

Fault path

switch

Transformer

Conclusion

1. Fault management is critical for power system safety

and reliability

2. The proposed dc prot. approach reduces fault clearing

time and system recovery time

3. The fast prot. method improves dc power system

resilience

36

Research Papers

1. S. Strank, et. al., “Experimental test bed to de-risk the navy advanced development

model,” Proc. of Electric Ship Technology Symposium, Arlington, VA, Aug. 2017, pp.

352-358.

2. X. Feng, et.al., “A novel fault location method for dc distribution protection,” IEEE

Trans. Industrial Applications, vol. 53, no. 3, May-June, 2017.

3. X. Feng, et. al., “Converter-based dc distribution system protection,” IEEE IAS Annual

Meeting, 2018, in review.

4. Q. Xiong, et. al., “Arc fault detection and localization in Photovoltaic systems using

feature distribution maps of parallel capacitor currents,” IEEE Journal of Photovoltaics,

2018, accepted

5. Q. Xiong, et. al., “Detecting and locating series arc fault in Photovoltaic system based

on time and frequency characteristics of capacitor current,” Solar Energy, 2018, in

review.

37

Thanks for your attention

Contact information: Xianyong Feng

Center for Electromechanics

The University of Texas at Austin

Email: x.feng@cem.utexas.edu

Phone: 512-232-1623

38