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Strathprints Institutional Repository

Emhemed, Abdullah and Burt, Graeme (2015) LVDC - redefining

electricity : enabling an LVDC last mile distribution network. In: IEC-BIS

International Conference on Low Voltage Direct Current, 2015-10-26 -

2015-10-28, Hotel The Ashok. ,

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Bureau of Indian Standards

LVDC-Redefining Electricity First International Conference on Low Voltage Direct Current

New Delhi, India,

26 & 27 October 2015

InternationalElectrotechnicalCommission

Enabling an LVDC last mile

distribution networkDr Abdullah Emhemed

Prof Graeme Burt

University of Strathclyde

Department of Electronic and Electrical Engineering

Technology and Innovation Centre

[email protected]

Tel: +44 141 4447274

Glasgow-United Kingdom

mailto:[email protected]

Overview

1. Issues with an existing LVAC last mile network

2. Moving to an LVDC last mile: motivations

3. Moving to an LVDC last mile: challenges

4. Protection challenges

5. Protection solution

6. Conclusions

NOP

11/0.4kV

MV

Su

pp

ly

http://corrupteddevelopment.com/

1. Issues with existing LVAC last mile

Operates at 60

-70% of their

limits with

losses 3%

Under pressure

to host more

renewables

Benefit the

least from

load diversity

Under pressure to

host more heat and

transport demand

Become a bottleneck to connect

further EV and heat pumps, power

flow could reach up to 130%-150%

in future UK system

Distribution

networks are

aging

Requires AC-DC conversion

for supplying DC loads and

hosting renewable

2. LVDC last mile: motivation

Load

Existing MV network

11kV/0.4kV

Load

variable speed ac source

Unipolaror bipolar

cable connections

Addressing the technical constraints

(enhanced voltage profile, limited impact

of the inductance, no skin effect, etc.)

Reduced losses and increased power capacity

Supporting better controls,

and easier to connect multiple

sources

flexibility in operation offers more flexible

market mechanism with better

stimulation of customers control demands

No phase balance and

synchronisation issues

Better control of peak demands

Potential benefits for DSOs

Suitable and powerful ICT platform

for integrating various smart grid

functionalities

Reduced fault level and allow the

use of lower current ratings and

smaller footprint

Release additional generation and

demand headroom, and defers

reinforcement

Selected projects of DC in distribution

ScottishPower £15m ANGLE に DC project

KEPCO aims to replace a number of existing AC rural MV distribution networks supplying light loads by LVDC networks to save up to 5% of the total operating cost

Real rural LVDC network as a part of Finnish national Smart Grids research programme

Potential benefits for DSOs


P. Muutinen Wデ ;ﾉくさE┝ヮWヴｷWﾐIWゲ from use of an LVDC system in public

electricity SｷゲデヴｷH┌デｷﾗﾐざ CIRED ヲヰヱン


Load

Existing MV network

11kV/0.4kV

Load

variable speed ac source

Unipolaror bipolar

cable connections

Reduced energy waste and losses by

reducing AC-DC conversion stages

More suitable for devices

generate and consume DC

(80% of todays load are DC)

Power converter interfaces are

more mature and their

associated costs are declining

Better control of energy leads to

better market services and consumers

cost savings

Potential benefits for end-users

IET LVDC Code of Practice

Section 4: Recognised standards of d.c. power distribution over

telecommunications cabling

Section 5: Proprietary d.c. power distribution over

telecommunications cabling

Section 6: Proprietary d.c. power distribution over proprietary cabling

Section 7: Proprietary d.c. power distribution over conventional single

phase a.c. power supply cabling

Section 8: Proprietary d.c. power distribution over conventional 3-

phase a.c. power supply cabling


If we are designing the first last mile today,

would be an AC or DC?

3. LVDC last mile: challenges

Very limited experience

Huge investment in AC

Interaction with existing AC grid

Lack of standards (topology, voltages, cable connections, etc.)

international systematic approach on LVDC not yet provided

LVDC benefits versus the cost

Existing LV protection is too simple and not capable of enabling the potential benefits afforded by LVDC last mile networks

4. Protection challenges

Characterisation of DC faults

DC protection for safety challenge

The requirement for high speed DC protection

Detecting and locating DC faults challenge

Protection against DC voltage disturbances

DC faults interruption challenge

Protection is top priority of any electrical distribution system

Does DC protection require more caution?

Characterisation of DC faults

0 0.002 0.004 0.006 0.008 0.01 0.0120

0.5

1

1.5

2

2.5

3

3.5x 10

4

Time (s)

Fau

lt c

urr

ent

(A)

Simulated fault current

Calculated fault current by IEC61660

Main DC bus

Rm Lm

Rca

Vg

Rsm Lsm LreRre

Common DC feeder

Isc

LfeRfe

Rb Lb

Battery

Vb

Capacitor Vca

Rca_l Lca_l

Rb_l Lb_l

Rm_l Lm_l

Vm

DC Motor

Ef

Rf Lf

XeqReq

Line

Smoothing reactor

IEC 61660 mathematical model

The traditional methods used for DC

fault calculations:

Static mathematical model

mathematical model implemented in

some ANSI/IEEE guidelines

IEC 61660-1 dynamic mathematical

model

ଵ ݐ ൌ ଵ Τష ഓభଵ Τష ഓభ Ͳ ݐ ݐଶ ݐ ൌ ͳ െ ூೖ షഓమ ூೖ ݐ ݐ

Q: Is IEC 61660 suitable for characterising

LVDC short circuit currents under all possible

system configurations?


0V30V200V 120V400V 60V1500V

Band I (ELVDC)Band II (LVDC)

Comparable safety

margin as for AC for

direct contact

(IEC60479) can be

provided in 2-wire &

3-wire systems

Dangerous and could kill

in case of direct contact

For <30V and in

normal dry

conditions for <60V,

basic protection is

not required for SELV

and PELV systems

Comparable safety margin

can be provided only with

3-wire system with

grounded middle point

ゅTｴW IEC ヲンEっWGヲ ┘ﾗヴﾆゲｴﾗヮがさDC SｷゲデヴｷH┌デｷﾗﾐゲ┞ゲデWﾏ ;ﾐS IﾗﾐゲWケ┌WﾐIWゲ aﾗヴ RCDゲざ

Protection for safety : RCDs are not widely and commercially available for DC

Protection for equipment: detecting, locating, and interrupting DC faults are challenging

How does DC influence the existing earthing systems?

L+

L-

PE

L+

L-

PE

0(N)

TN-S d.c. system

2-wire

system

3-wire

system

TN-C d.c. system

L+

L-PE

PEN

L+

L-

PE

L+

L-

0(N)

L+

L-

PE

TN-C-S d.c. system

L+

L-

PE

L-

PE

0(N)

L+

L-

PE

0(N)

L+

L-

PE

L+

TT d.c. system IT d.c. system

3-wire DC systems can have lower touch voltages


The requirement for high speed

4.8 5 5.2 5.4 5.6 5.8-1

0

1

2

3

4

Time (sec)D

C c

urren

t (k

A)

Fault3 (at 1km) Fault4 (at 2km)

Fault1(at 0km)

Fault2 (at 0.5km)

4.995 5 5.005 5.01 5.015 5.02 5.025-1

0

1

2

3

4

Time (sec)

DC

cu

rren

t (

kA

)

Fault1(at 0km)Fault2 (at 0.5km)

Fault3 (at 1km)

Fault4 (at 2km)

Transient DC

fault current

Steady state DC

fault current

2 - level

VSC

F2 F3

F4

0.75kV LVDC last mile

Load1

Load2

NOP

RMU

11/0.4 kV 4.5% 500kVA

11kV AC system

1km

2km

F1

CB1

CB2

Power electronics poor short circuit capability

DC current circulates in the converter and other

sensitive equipment

Protection systems need to be very fast to

- prevent a high transient and steady state DC currents from

damaging equipment

- prevent the main converters from losing control and unnecessary

tripping; and

- reduce the impact of post-fault power quality and stability

Detecting and locating DC faults

challenge

The natural small DC line impedances can lead to more

complexity for locating DC faults

Resistive faults hard to detect

Solutions

Using differential protection (time

synchronisation issue)

Using signal processing techniques-based

protection such as Travelling waves and Active

Impedance Estimation (AIE)

0 200 400 600 800 1000 1200

-100

-50

0

50

100

150

200

Time (micro sec)

Cu

rren

t (A

)

5 6 7 8 9 10 11 12

0

0.5

1

1.5

Time (sec)

DC

Vo

lta

ge (

kV

) Post-fsult4 DC voltage

Post-fault1 DC voltage



Protection against DC voltage

disturbances

Fast propagation of voltage disturbance

Sensitivity of AC/DC and DC/DC to voltage drops

Rapid DC voltage drop

Overvoltage on the DC side

caused by a line-to-earth fault on the AC side

Caused by the loss of the supply DC

neutral/earth of a bipolar DC system

Post-fault transient overvoltage due the

release of substantial energy stored in the

inductor ((ଵଶ ଶ) due slow protectionܫܮ

Voltage surge protection or fast protection that reduces the fault duration and

magnitude are required


DC arcs are more aggressive than AC

DC fault without zero crossing do not provide a

natural low point to extinguish the fault arc

More complex techniques such as increased arc

length and arc splitters are required

Mechanical breakers complied with IEC 60947-2

- LVAC Moulded Case CBs and Miniature CBs (magnetic

trip units to be adjusted for DC)

- DC CBs equipped with permanent magnetic

- DC CBs equipped with electronic trip units

- Lower DC current and voltage ratings compared to

equivalent AC due to the higher risk of fire in DC

DC solid state breakers

- 900 times faster than LVDC mechanical breaker

- On-state losses issueshttp://www.electronicproducts.com

Using different converter topologies

DC hybrid breakers????

- Full bridge DC/DC chopper

- full-bridge Modular Multi-level Converters


Protection

functions

Relays C IED1 IED2 IED3 IED4

Current directions 1 0 0 0 0

Trip function - - - -

Blocking reverse current -

Reclosing function - - - - -

Protection

functions

Relays C IED1 IED2 IED3 IED4

Current directions 1 1 0 0 0

Trip function - - - -

Blocking reverse current -

Reclosing function - - - -

dc-dc48-12V

ac-dc

ACCB

F1

F2

LVDC last mile

AC grid

AC/DCSSCB

F3C

SSCB

SSCB

SSCB

dc-dcICT links

Elect links

Smart DC home

Communication-assisted

Fault detection and locations are based on DC current

directions and magnitudes, and DC voltages

Using multiple IEDs

Using solid state circuit breakers for interrupting DC faults

Features

Multi-function DC protection for enabling an LVDC last

mile

5. Protection solution

LVDC testing facilities

DC

Po

wer

So

urc

e

Lo

ad1

SSCB1

SSCB2 SSCB4

SSCB3

Loa

d2

LR2 2

L LR2 2

L

LR L

LR L

LR2 2

L LR2 2

L

IED4

IED3

IED2

IED1

V V

VV

V

IED5

SSCB5

Communication link

2I

2V

3I

3V

1I

1V

1C

1F

3C

2C4I

4V

3F

5I

5V2F

Feeder1

Feeder2

PCC

LVDC experiment circuit schematic

Validating the concept

Validating the concept

Start

The SSCB remains

open

Reclosing function is

performed and loads are

energised

The fault is

temporary

End

Measure I (mag and

dir) & Vdc at each IED

The fault is located at the PCC, the

AACB is tripped and all downstream

µgenrators are tripped

The end user is faulted and its

local SSCB directly operates

The fault is on the main feeder, and its IED trips

the associated SSCB, and remotely trips all the

µgenrators connected to the feeder

The IED feeder with

(+) current direction

takes the lead

I & V within the

nominal limits

The converter

IED current

direction is (+)

and the feeder

IED is (-)

Any of the

customers IEDs

has a current

with (+) direction

YES NO

YES

YES

NO

NO YES

NO

Deployment of the Algorithm

using LabVIEW

Loads

Mimic LVDC

cablesCRIO: hardware-

software interface

Actual experiment setup

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700

-100

-50

0

50

100

150

200

Time (s)

Cur

rent

(A

)

The supply (grid) DCfault contributionwith protection

The supply (grid) DC faultcontribution without protection

The DC fault contributions fromfeeder 1 and 2 without protection

The DC fault contributions fromfeeder 1 and 2 with protection

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700-5

0

5

10

15

20

25

Time (s)

DC

vol

tage

s (V

)V1 at IED1

V2 at IED2

V4 at IED4

V5 at IED5

V3 at IED3

DC

Pow

er

Sourc

e

Loa

d1

SSCB1

SSCB2 SSCB4

SSCB3

Loa

d2

LR2 2

L LR2 2

L

LR L

LR L

LR2 2

L LR2 2

L

IED4

IED3

IED2

IED1

V V

VV

V

IED5

SSCB5

Communication link

2I

2V

3I

3V

1I

1V

1C

1F

3C

2C4I

4V

3F

5I

5V2F

Feeder1

Feeder2

PCC

Experimental results

100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 10000

50

100

150

200

250

300

350

400

Time (s)

Cur

rent

(A

)

Fault current for F3 fault



6. Conclusions

LVDC distribution systems have the potential to bring benefits

to future electricity grids

Replacing or energising an existing part of AC circuits using DC

is still at very early stages and limited by practical technical

solutions and lack of standards

The fund to LVDC projects is still limited to national levels

Protection and safety one of the major concerns

The developed fast acting DC protection has demonstrated

more resilient performance for future LVDC networks

Thank you & Q

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