Neil Kirby: VSC HVDC Transmission and Emerging Technologies in DC Grids

Imagination at work
VSC HVDC Transmission and
Emerging Technologies in DC Grids
Neil Kirby
EnergyTech2015 - Cleveland
30th November 2015

© 2015 GeneralElectric Company- All rights reserved
Summary
HVDC Technologies
HVDC Grid Control
HVDC Grid Protection
Future Converters
2

Two HVDC technologies
Line Commutated Converters
LCC – HVDC/UHVDC
Voltage Source Converters
VSC – HVDC
 Rating limited today : ~1GW
 More Versatile Control of MW / MVar
 Linear Bi-directional control
 Functionality suitable for “DC grid”
 Good for weak AC systems
 Higher Losses 1.0-1.2%
 Higher Power Ratings : up to 10GW
 Longer History : In service since 1980’s
 Best for Overhead Line Transmission
 Asynchronous Interconnections
 Need AC Filters => Bigger Switchyard
 Lower losses < 0.8%
Uses Thyristors Uses Transistors
All HVDC Systems Need:
Power Converters
HVDC Control & Protection
Cooling Plant
Power Transformers
AC Switchyards & Protection
DC Switchyards & Protection (Excl BTB) Auxiliary
Power Supplies
Buildings
Extensive Network Analysis

VSC versus LCC HVDC
Line-Commutated
Converter (LCC) HVDC
Converter A Converter B
RDC
VDC_A VDC_B
IDC
Power flow A → BPower flow B → A
VDC_A
VDC_B
IDC
Power flow A → BPower flow B → A
VDC_A
VDC_B
VDC_A
VDC_B
IDC
Voltage-Sourced
Converter (VSC) HVDC
Converter A Converter B
RDC
VDC_A VDC_B
IDC
VDC_A
VDC_B
IDC

Control of HVDC Grids
Many possible methods of controlling power flow in HVDC Grids
Two such methods proposed are:
• Slack Bus Control
• Droop Control
……. and many variations or both!
Market requires that HVDC Grids be multi-vendor …
… AND different control modes will be required for different
types of AC system…
…… AND different control modes might be used within a Grid
without adverse interaction…..

• Converter A changes from Import (Rectifier) to Export (Inverter)
• Converter C (“Slack” converter) forced from Export (Inverter) to Import (Rectifier)
Converter D
Vdc
+Pdc
EXPORT IMPORT
OPB
-Pdc
Converter B
PdcBPdcD
PdcC = Σ (PdcA , PdcB , PdcD)
OPA
Converter A
PdcA
Converter C
OPC
PdcC
OPD
Converter A
OPA
PdcAPdcC
OPC
Converter C
Control of HVDC Grids – Slack Bus Control

Control of HVDC Grids - Droop Characteristic
All converters contribute to the “slack bus”.
“Sharing of change” by converters determined by relative droop settings of each converter.
DC power at each converter can be corrected/changed by either:
– Re-dispatch from a central Grid Controller (requires telecommunication).
– Local control at the converter (autonomous control) e.g., constant power
– or AC frequency control.
LRSP
Vdc
OPA
Vdc
OPB
OPD
OPC
OPB
Export
+Idc
Import
-Idc IoA IoBIoDIoC

STATION A
STATION B
STATION C
LINK
Controller
Control of HVDC Grids - LINK controller
“LINK” Controller
• Operator control interface
• DC power flow solver
• Validation of power flow before
initiating change
• Automatic drift correction

DC Grid Protection
- Zones of protection
Keep the protections associated with Zones 4 and 5
physically separate in order to permit for future multi-vendor
upgrade to multi-terminal

Converter DC Side Faults
1. Faults across high
impedance ground
= High voltage
2. Fault across the converter
= High current
(pu)
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
DC Pole 1 Voltage
DC Pole 2 Voltage
(kA)
20.0
0.00
0.0990 0.1000 0.1010 0.1020 0.1030 0.1040 0.1050 0.1060
DC Current
(pu)
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
DC Pole 1 Voltage
DC Pole 2 Voltage
(kA)
20.0
0.00
0.0990 0.1000 0.1010 0.1020 0.1030 0.1040 0.1050 0.1060
DC Current

Clearance of DC Side Faults - Today
Voltage Source
Converters use the
mechanical AC breaker
as the Primary Protection
Line Commutated
Converters use the
power electronics as
the Primary Protection

Clearance of DC Side Faults - Tomorrow
Half-Bridge Voltage Source
Converters can use a hybrid
DC breaker as the Primary
Protection
Full-Bridge Voltage source
Converters use the power
electronics as the Primary
Protection

Protection requirements – AC vs. DC faults
Load
V
I
L
Load
V
I
L
AC fault DC fault
Main : Graphs
150
175
200
Vbreakerdc
Main : Graphs
0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...
...
...
0
25
50
75
100
125
150
175
200
y
Vsystemdc
Main : Graphs
0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...
...
...
0.0
1.0
2.0
3.0
4.0
5.0
y
Idc
Main : Graphs
0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...
...
...
-150
-100
-50
0
50
100
150
y
Vsystem
Main : Graphs
0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...
...
...
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
y
Iac
Main : Graphs
50
100
150
Vbreaker
• Natural zero crossings
• Magnitude decays over time
• Typically higher inductance
• No zero crossings
• Magnitude quickly raises over time
• Typically lower inductance

Review of options for HVDC circuit breaker
Topology Illustration Conclusions
Direct interruption using
arc voltage
Widely used at up to 2-3 kV
(e.g. railways) but
impossible for HVDC
Passive resonant
current zero creation
OK for HVDC load switching
but much too slow for fault
clearing
Active resonant current
zero creation
1. With standard circuit
breakers: much too slow
2. With vacuum switches:
high contact erosion
Solid state
Very fast but
Very high losses (MW range)
Hybrid
Fast enough and low losses.
Best solution.
Circuit Breaker
Circuit Breaker
Circuit Breaker
+
Power
Electronic
Switch
Ultra-fast
disconnector
Power
Electronic
Switch 1
Power
Electronic
Switch 2

Hybrid DC breaker – operating principle
PE2
PE1
Ultra-fast
disconnector
time
time
Nominal Vdc
Fault
occurs
PE1
turns off
Ultra-fast disconnector
fully open; PE2 turns off
Ibreaker
Vbreaker
Ibreaker
Vbreaker

DC breaker – the importance of speed
Current
time
Fault
occurs
PE1
turns off
Ultra-fast disconnector
fully open; PE2 turns off
Clearing time
Pre-clearing
time
Detection,
selectivity &
relay time
Voltage
rise time
I0
t1
Ipeak = I0 + di/dtfault . t1
Depends on
system strength
Depends on
breaker design
and protection
philosophy
To minimise the peak current rating of the breaker:
1. Detection, selectivity & relaying time to be as short as possible
2. Pre-clearing time to be as short as possible

DC breaker – the effect of inductance
Current
time
I0
Additional inductance is a double-edged sword!
• It helps you on the way up…
• But makes life harder on the way down again
Without added
system inductance
With additional
inductance
Best of both worlds?
Needs something
smarter than just a
plain inductor

The Grid Solutions Hybrid DC Breaker
Power Electronics switch 1 (commutation
module)
PE2
PE1
Ultra-fast
disconnector
• Multiple IGBTs in parallel
• Two inverse-series per commutation module
• Number of commutation blocks required depends on DC voltage
• Number of IGBTs in parallel is more than is needed for thermal
reasons alone
− Lowest possible losses
− Simplification of cooling
• Natural convection air cooling only
− No forced cooling
− No phase change media
− Nothing to leak

The Grid Solutions hybrid DC breaker
Power electronics switch 2 (auxiliary branch)
PE2
PE1
Ultra-fast
disconnector
• Well-known solution: multiple IGBTs in series (and inverse-series)
• Alstom considered this, but ultimately chose a different solution for
the demonstrator
• Novel solution based on thyristors
• Very robust and capable of very high currents
• Number of time-delaying branches can be modified
based on fault level and operating voltage
First time-delaying branch
Second time-delaying branch
Arming branch

The Grid Solutions DC breaker demonstrator
Built and tested at Grid Solutions’ switchgear research
facility in Villeurbanne, near Lyon, France
Key ratings:
• Rated voltage: 120 kV
• Rated direct current: 1500 A
• Overload current in closed state > 3000 A for 1 minute
Test programme agreed with and witnessed by RTE
• Dielectric tests between the terminals of an (open) breaker
• Continuous and short-time current through a (closed) breaker
• Interruption tests
Part of EU FP7 “Twenties” project

TO
ANOTHER
DC GRID
HVDC Tomorrow
DC Breakers used to separate out the DC network
DC/AC
Breaker
DC/AC
DC/AC
DC/AC
AC/DC
AC/DC
Breake
r
DC Sub-Network
AC/DC
AC/DC
AC/DC
DC Sub-Network

Where are we going?

Modular Multi-level Converter
DC Pole to Pole Fault:-
T2 Diode Conducts
Fault current uncontrolled
Fault current can only be
stopped by
a) AC breaker
b) DC breaker
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
T1
C
T2
+T1
C
T2
+T1
C
T2
+

←Valve voltage
←Line-to-line voltage
We don’t just have to use sinewaves!

Modular Multi-level Converter – Full Bridge
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
S
M
T
1
C
T
2
+
Vm
T
3
T
4
+Vc
Vm
-Vc
T
1
C
T
2
+
T
3
T
4
T2 + T3
conducting
T
1
C
T
2
+
T
3
T
4
T1 + T3
OR
T2 + T4
conducting
T
3
T
4
T
1
C
T
2
+
T1 + T4
conducting

T1
C
T2
+
Modular Multi-level Converter

Full-Bridge
Series Valve
Alternate Arm Converter

Series Bridge Converter
 Red, H-Bridge converts 80% of
power
 Switching losses are minimised
by, Yellow, half-bridge chain-links
providing zero voltage soft-
switching
 6th harmonic voltage cleaned by a
few full-bridge, blue, chain-links on
H-bridge output
Benefits
 Low footprint (over HB-MMC) as only
one HB Chain-link valve across the
DC rail
 Cost savings over HB-MMC
Half-Bridge
Full-Bridge
Series Valve

Controlled Transition Bridge
 Parallel converter style
approach
 Allows switching losses to be
managed by Chain-links
 Reduces filtering requirements
over LCC
 Maintains high current
capability
 Chain-link capacitor small
 Complex control requirements
Half-Bridge
Full-Bridge

Neil Kirby: VSC HVDC Transmission and Emerging Technologies in DC Grids

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