LVDC-Redefining Electricity
First International Conference on Low Voltage Direct Current
International
Electrotechnical
Commission
Bureau of
Indian Standards
New Delhi, India,
26 & 27 October 2015
Enabling an LVDC last mile
distribution network
Dr Abdullah Emhemed
Prof Graeme Burt
University of Strathclyde
Department of Electronic and Electrical Engineering
Technology and Innovation Centre
Abdullah.emhemed@strath.ac.uk
Tel: +44 141 4447274
Glasgow-United Kingdom
Institute for Energy and Environment
Core disciplines
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Power System Analysis
Power System Simulation
Power System Economics
Energy Markets
Active Network Management
Machines & Power Electronics
Control, Protection & Monitoring
Wind Energy Systems
Renewables
Dielectric Materials/Pulsed Power
HV Technology/UHF Diagnostics
Energy System Modelling
Research portfolio: £40m
Overview
1.
2.
3.
4.
5.
6.
Existing LVAC last mile network: issues
Moving to an LVDC last mile: motivations
Moving to an LVDC last mile: challenges
Protection challenges
Protection solution
Conclusions
1. Existing LVAC last mile: issues
Under pressure
to host more
renewables
Requires AC-DC conversion
for supplying DC loads and
hosting renewable
Distribution
networks are
aging
MV Supply
Operates at 60
-70% of their
limits with
losses 3%
11/0.4kV
NOP
Under pressure to
host more heat and
transport demand
Benefits the
least from
load diversity
Become a bottleneck to connect
further EV and heat pumps, power
flow could reach up to 130%-150%
in future UK system
http://corrupteddevelopment.com/
2. LVDC last mile: motivation
Potential benefits for DSOs
Existing MV network
Addressing the technical constraints
11kV/0.4kV
Reduced losses
Better utilisation of
existing assets
Better control of peak demands
Reduced fault level
Unipolar
or bipolar
cable
connections
Enhanced voltage profile
Load
Releases additional
generation and
demand headroom
More efficient for renewables
Easier to connect multiple
sources, and no phase balance
and synchronisation issues
Powerful ICT platform for
integrating various smart grid
functionalities
Load
variable
speed ac
source
Defers reinforcement
Offers more flexible market mechanism
with better stimulation of customers
http://eur-lex.europa.eu/legal-content
2. LVDC last mile: motivation
Potential benefits for end-users
Existing MV network
75% of EU houses are
energy inefficient
11kV/0.4kV
Reduced energy waste and losses
More suitable for devices
generate and consume DC
(80% of todays load are DC)
Better control of energy leads to
better market services and consumers
cost savings
1% in EU energy savings
cuts gas import by 2.6%
Unipolar
or bipolar
cable
connections
Load
Power converter interfaces are
becoming more mature and their
associated costs are declining
Load
variable
speed ac
source
2. LVDC last mile: motivation
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
Verification of existing a.c. power supply
Capability assessment
Implementation
Co-existence issues
Labelling
Certification Protocol
Cabling
Systems
Thermal considerations
Practical impact of thermal effects
Controlling injected power
Cabling acceptance and troubleshooting testing
3. LVDC last mile: challenges
Data
centres
Buildings
Very limited experience
Huge investment in AC
Interaction with existing AC grid
Lack of standards (topology, voltages, cable
LVDC Last
mile
connections, interference, and etc.)
International systematic approach on LVDC not yet
provided
LVDC benefits versus the cost
Existing LV protection solutions are too simple and
not capable of enabling the potential benefits
afforded by LVDC last mile networks
-
-
Standards
EMerge Alliance Data/Telecom Centre
Standard: 380Vdc
EMerge Alliance Occupied Space Standard:
24Vdc
IET Code of Practice (CoP) for the
application of LED lighting systems
IET CoP for LVDC power distribution in
Buildings
No standards are available
4. Protection challenges
Protection is top priority of any electrical
distribution system
Does DC protection require more caution?
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
Characterisation of DC faults
The traditional methods used for DC
fault characterisations:
 Static mathematical model
 Mathematical model implemented in
some ANSI/IEEE guidelines
 IEC 61660-1 dynamic mathematical
model
Xeq
Line
Rre Lre
𝑖2 𝑡 = 𝑖𝑝
Vg
3.5 x 10
Fault current (A)
Lb
𝜏1
1 − 𝑒 −𝑡𝑝
𝜏1
0 ≤ 𝑡 ≤ 𝑡𝑝
𝑝
𝐼𝑘 −𝑡−𝑡
𝐼𝑘
1−
𝑒 𝜏2 + 𝑡 ≥ 𝑡𝑝
𝑖𝑝
𝑖𝑝
4
Rb_l Lb_l
Vb
5000
Simulated fault current
Calculated fault current by IEC61660
3
Rb
1 − 𝑒 −𝑡
Main DC bus
Fault current (A)
Req
Smoothing
reactor
Rsm Lsm
𝑖1 𝑡 = 𝑖𝑝
2.5
2
1.5
1
Battery
4000
3000
Fault2 (F2)
Simulated fault current
Calculated fault current by IEC61660
Fault3 (F3)
Fault4 (F4)
2000
1000
0.5
Rca
Rca_l Lca_l
Vca
Common DC feeder
Rfe
Capacitor
Lf
Rm Lm
Rm_l Lm_l
Vm
Ef
0.002
0.004
0.006
Time (s)
0.008
0.01
0.012
0
0
0.005
0.01
0.015
0.02
0.025
0.03
Time (s)
Lfe
Isc
Rf
0
0
DC Motor
IEC 61660 mathematical model
Q: Is IEC 61660 suitable for characterising
LVDC short circuit currents under all possible
system configurations?
Emhemed, A.; Burt, G., “The effectiveness of using IEC61660 for characterising short-circuit currents of
future low voltage DC distribution networks”, CIRED 2013
DC protection for safety challenge
Band II (LVDC)
1500V
200V
400V
Dangerous and could kill
in case of direct contact
Band I (ELVDC)
Comparable safety margin
can be provided only with
3-wire system with
grounded middle point
120V
60V
Comparable safety
margin as for AC for
direct contact
(IEC60479) can be
provided in 2-wire &
3-wire systems
30V
For <30V and in
normal dry
conditions for <60V,
basic protection is
not required for SELV
and PELV systems
Protection for safety : RCDs are not widely and commercially available for DC
Protection for equipment: detecting, locating, and interrupting DC faults are challenging
*The IEC 23E/WG2 workshop, “DC distribution system and consequences for RCDs”
0V
The requirement for high speed protection
RMU
CB1
11/0.4kV
4.5% 500kVA
F2
F3
Load1
1km
Transient DC
fault current
2-level
VSC
NOP
F1
Steady state DC
fault current
2km
Load2
Power electronics have poor short circuit capability
DC current circulates in the converter
Protection systems need to be fast enough to
-
prevent a high transient and steady state DC currents from
damaging equipment
prevent the main converters from losing control and unnecessary
tripping
reduce the impact of post-fault power quality and stability
Looking for < 5ms
4
DC current (kA)
0.75kV LVDC last mile
Fault1(at 0km)
Fault2 (at 0.5km)
3
Fault3 (at 1km)
2
Fault4 (at 2km)
1
0
-1
4.995
5
5.005
5.01
5.015
Time (sec)
5.02
4
Fault1(at 0km)
DC current (kA)
11kV AC system
CB2
F4
3
2
Fault2 (at 0.5km)
Fault3 (at 1km)
Fault4 (at 2km)
1
0
-1
4.8
5
5.2
5.4
Time (sec)
5.6
5.8
5.025
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
200
150
Current (A)
Using differential protection (time
synchronisation issue)
100
50
0
-50
-100
0
200
400
600
Time (micro sec)
Using signal processing techniques-based
protection such as Travelling waves and Active
Impedance Estimation (AIE)
800
1000
1200
Protection against DC voltage
disturbances
Rapid DC voltage drop
Fast propagation of voltage disturbance
Sensitivity of AC/DC and DC/DC to voltage drops
Overvoltages on DC side
2
1.5
DC Voltage (kV)
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
1
inductor ( 𝐿𝐼2 )
Post-fault1 DC voltage
Post-fsult4 DC voltage
1
Post-fault3 DC voltage
0.5
Post-fault2 DC voltage
0
5
6
7
8
9
Time (sec)
10
Voltage surge protection or fast protection that reduces the fault duration and
magnitude are required
11
12
DC faults interruption challenge
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
DC faults interruption challenge
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 issues
Using different converter topologies
- Full bridge DC/DC chopper
- full-bridge Modular Multi-level Converters
DC hybrid breakers?
http://www.electronicproducts.com
5. Protection solution
Multi-function DC protection scheme
F2
SSCB
ACCB
Protection
functions
AC/DC
C
AC grid
F3
F1
Relays
C
IED1
IED2
IED3
IED4
Current directions
1
0
0
0
0
-
-
-
-
-
-
-
-
Trip function
SSCB
dc-dc
ac-dc
SSCB
ICT links
Elect links
48-12V
Blocking reverse current
-
Reclosing function
-
dc-dc
SSCB
Features
LVDC last mile
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
Protection
functions
Relays
C
IED1
IED2
IED3
IED4
Current directions
1
1
0
0
0
-
-
-
-
-
-
Trip function
-
Blocking reverse current
-
Reclosing function
-
Validating the concept
LVDC testing facilities
Validating the concept
SSCB2
SSCB4
RL
L
V
V
I4
I2
IED2
V2
SSCB1
V4
RL
F3
L
Feeder1
I1
IED1
SSCB3
V1
RL
F1
2
L
2
RL
2
SSCB5
L
2
V
V
I3
I5
IED5
IED3
V3
V5
F2
Communication link
RL
2
L
2
RL
2
L
2
Feeder2
LVDC experiment circuit schematic
C3
Load2
DC Power
Source
V
C1
C2
IED4
Load1
PCC
Validating the concept
Start
Measure I (mag and
dir) & Vdc at each IED
YES
Loads
Mimic LVDC
cables
CRIO: hardwaresoftware interface
I & V within the
nominal limits
NO
The converter
IED current
direction is (+)
and the feeder
IED is (-)
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
YES
YES
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 SSCB remains
open
NO
The fault is
temporary
YES
NO
The IED feeder with
(+) current direction
takes the lead
Any of the
customers IEDs
has a current
with (+) direction
NO
Reclosing function is
performed and loads are
energised
End
Deployment of the Algorithm
using LabVIEW
Actual experiment setup
Experimental results
Current (A)
400
PCC
SSCB2
Fault current for F1 fault
300
Fault current for F2 fault
250
Fault current for F3 fault
200
150
SSCB4
RL
350
100
L
50
V
V
IED2
IED4
V2
V4
RL
C1
SSCB3
V1
RL
F1
2
L
2
300
350
RL
2
SSCB5
L
2
I5
IED5
IED3
V5
F2
450
500
550
600
650
700
C3
750
800
850
900
950
1000
The supply (grid) DC
fault contribution
with protection
50
0
The DC fault contributions from
feeder 1 and 2 with protection
-50
V
I3
400
The supply (grid) DC fault
contribution without protection
100
V
V3
250
150
Feeder1
I1
IED1
200
200
F3
L
150
Time (s)
The DC fault contributions from
feeder 1 and 2 without protection
-100
Load2
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
450
500
550
600
650
700
Time (s)
25
V4 at IED4
20
Communication link
RL
2
L
2
RL
2
L
2
Feeder2
DC voltages (V)
DC Power
Source
V
0
100
Current (A)
SSCB1
C2
Load1
I4
I2
15
V5 at IED5
10
V1 at IED1
5
V2 at IED2
V3 at IED3
0
-5
0
50
100
150
200
250
300
350
400
Time (s)
6. Conclusions
LVDC last mile systems
have the potential to bring benefits to future electricity grids, and
mainly increased power capacity and controllability
are still at very early stages and limited by practical technical solutions
and lack of standards and business cases
fund opportunities are still very limited
protection and safety are still one of the major concerns
The developed fast acting DC protection has demonstrated more
resilient performance for future LVDC networks by offering:
Fast detecting and
locating DC faults
Good level of
selectivity
Fast interrupting DC
faults at low level
Fast reclosing
function
Thank you & Q