2011-03-24_session1_carlbarker_hvdc as a bulk power transfer system
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GRID
Chief Engineer, Systems
Offshore Wind Training Seminar - March 2011
Session 1:
HVDC as a bulk power transfer system
Carl Barker
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HVDC as a bulk power transfer system March 2011 - P 3
Friends of the SuperGrid (FOSG)
Connection Capacity(GW)
Dogger Germany Offshore 10
Dogger Norfolk Bank 5
Dogger Firth of Forth 5
Dogger Norway 5
Germany Offshore - Munich 10
London Norfolk Bank 5
Norfolk Bank BelgiumOffshore
2
SuperNode
Belgium Offshore 2
Dogger - Hornsea 10
Germany Offshore 10
Norfolk Bank 5
Munich 10
Firth of Forth 5Figure 1: SuperGrid Phase 1
Figure 2: Interconnection and SuperNode Cap
Source: FOSG Position paper on the EC Communication for a European Infrastructure Package, Dec 2010
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Why DC Transmission?
HVDC transmission is the correct technology forbulk submarine energy transfer.
AC
DC
Charges and Discharges
Every Half Cycle
Only Charges the Cable Once
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Why DC Transmission?
Basic Structure of VSC Transmission System
t
Idc
t
ii
Iact
i
Iac
Idc= V1- V2R
DC transmissionline
Q1 Q2P
VSC VSC
Station 1 Station 2
V1 V2
IC1
Network
1
Network
2
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Agenda
Why DC Transmission? Page 3
Offshore Grids Page 7
DC Grid Control Page 14
DC Grid Protection Page 24
DC Grid Fault Clearance Page 34
DC Grid Standardisation Page 43
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Offshore DC Grids
Definition:
DC Grids Multiple converters connecting AC powernetworks to a DC power network
DC Grids Permit the economic transfer of power overburied cables reducing environmental impact
DC Grids Permits economic bulk transfer over large
distances DC Grids Reduce the number of AC/DC Conversions
therefore reduce losses
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132kV
400V400V400V
11kV11kV
400V400V400V
11kV11kV
DC Grid Configurations: Point-to-point System
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132kV
400V400V400V
11kV11kV
400V400V400V
11kV11kV
DC Grid Configurations: Meshed System
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132kV
400V400V400V
11kV11kV
400V400V400V
11kV11kV
DC Grid Configurations: Radial System
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HVDC connections
Grid connection
Cable +/-320kV DC
or similar
DC Converter
Station
HVAC cable
Onshore Offshore
MV array
cabling to
WTG
MV array
cabling to
WTG
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Platform Switchgear Arrangement
AC
DC
To Adjacent
Platform
To Shore
Station
To Adjacent
Platform
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Agenda
Why DC Transmission? Page 3
Offshore Grids Page 7
DC Grid Control Page 14
DC Grid Protection Page 24
DC Grid Fault Clearance Page 34
DC Grid Standardisation Page 43
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Two-terminal VSC Control
Basic Converter control of a two-terminal VSC
Converter 1 Converter 2
Converter 3
Converter 4
Slack bus
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DC Grid Control
Will a single utility / system owner be prepared to act asthe slack bus for all other interconnected systems?
Converter 1 Converter 2
Converter 3
Converter 4
Slack bus
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Comparison of AC and DC parameters
AC PARAMETER DC PARAMETERFrequency
Target DC Voltage
Vdc Voltage Change
))sin(V(
Voltage Change
V
Impedance of Connection
)X( Resistance of Connection
R
Real Power X
sinVV
Real Power
R
VV
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A typical VSC converter slope
characteristic
a*
Vdc
-Idc +Idc
IMPORT(A)
EXPORT(A)
VdcMAX
(A)
VdcMIN
(A)
LRSP
Vdc/IdcSlope(A)
-Idc
MAX(A)
+Idc
MAX(A)
-Idc IMPORT
LIMIT(A)
+Idc EXPORT
LIMIT(A)
VAa
b
c
d
d*
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A basic controller for DC converter
in a multi-terminal HVDC scheme
DispatchCentreConverter
Station
Controller
Porder
LRSP
Idcmax Limit
slopedc
dc
I
V
Vdc
MAX
V
X
Vorder
Limits
Vdc
MIN
Vorder
Idc measured
Iorder
Limits
Idcmin Limit
Idc measured
LRSP
Porder
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A two-terminal VSC Grid with power
flow from terminal A to terminal B
Vdc
-Idc +Idc
IMPORT(A)
EXPORT(A)
VdcMAX
(A)
VdcMIN
(A)
LRSP
OP
VA
VB
IMPORT
(B)
EXPORT
(B)
VdcMAX
(B)
VdcMIN
(B)
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A change in power demand compensated
by a new power dispatch
Vdc
+Idc
EXPORT
(A)
OP2
VA1
VB2
IMPORT
(B)
VdcMAX
(B)
VdcMIN
(B)
OP1
Vdc
+Idc
EXPORT
(A)
OP2
VA3
VB3
IMPORT
(B)
VdcMAX
(B)
VdcMIN
(B)
OP1
Constant
Pdc Line
OP3
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A three-terminal DC grid
OPB
Vdc
-Idc +Idc
LRSP
OPC OPA
IBIC IA = IB+ IC
IMPORT
(A)
EXPORT
(A)
IMPORT (B)
IMPORT (C- - )
EXPORT (B)
EXPORT (C- - )
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Voltage Optimiser
OPB
Vdc
-Idc +Idc
LRSP
OPC OPA
IBI
CIA
= IB+
IC
IMPORT
(A)
EXPORT
(A)
IMPORT (B)
IMPORT (C- - )
EXPORT (B)
EXPORT (C- - )
Steady-statetransmissionloss
minimisation
One converterterminal
determines thenew, higher,
LRSP
LRSP ramp can bestopped at anytime
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Agenda
Why DC Transmission? Page 3
Offshore Grids Page 7
DC Grid Control Page 14
DC Grid Protection Page 24
DC Grid Fault Clearance Page 34
DC Grid Standardisation Page 43
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DC Grid protection
Key issues
Multi-terminal DC cable systems are low inertia systems
A DC fault (voltage on one pole goes to zero) is experiencedsimultaneously throughout the system
Protection system must discriminate the faulted cable section
to allow rapid isolation by switchgear action
Multi-terminal system should return to stable operation, inminimum time with minimum loss of infrastructure
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Protection systems
AC
DC
To
Adjacent
Platform
To
Shore
Station
To
Adjacent
Platform
AC side transducers are conventionalelectromagnetic current transformers
DC side transducers are based on modern
fibre optic current measurement techniques
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Current Transducer Nxt Phase
Fibre optic
measurement head
Polymericinsulators toprotect fibre opticcables
Fibre optic connectionto matching unit
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Protection Zones
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
AC feeder from AC
collector platform
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Protection Zones
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
Transformer differential
zone
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Protection Zones
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
AC DC differential
zone
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Protection Zones
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
DC bus protection
zone
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Protection Zones
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
DC cable over-current
protection zone
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DC cable fault protection
DC bus protection DC bus protection
DC cable protection
Unbalance DCcurrent protection
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Agenda
Why DC Transmission? Page 3
Offshore Grids Page 7
DC Grid Control Page 14
DC Grid Protection Page 24
DC Grid Fault Clearance Page 34
DC Grid Standardisation Page 43
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Modular Multi-level Converter : Half
link
+ V
- V
+ V
- V
U
Module Output voltage
Lowest component count
Only one possibility of
output voltage polarity
No capability ofsuppressing DC-side faults
i i
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Option 1 Half-bridge converters +
Disconnects
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
= Mechanical Disconnect= AC Circuit Breaker
DC cable fault can not be cleared by half
bridge AC/DC converter
AC circuit breakers on all platforms open toclear the fault
Appropriate disconnects opened to isolatefaulted cable section
Complete multi-terminal scheme is re-started
M d l M l i l l C F ll
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Modular Multi-level Converter : Full
link
+ V
- V
+ V
- V
Same circuit as ALSTOMSTATCOM chain circuit Output DC voltage can be
either polarity Hence can connect as tapto LCC-HVDC link Can also suppress DC sidefaults
U
Module Output voltage
O i 2 F ll b id
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Option 2 Full-bridge converters +
Fast Switches
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
= Fast Isolating Switch= AC Circuit Breaker
DC cable fault can be cleared by full bridge
AC/DC converters
Appropriate fast (30 40ms) isolating switchesopened to isolate faulted cable section
Complete multi-terminal scheme is re-startedin 300 400ms
O ti 3 H lf b id t
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Option 3 Half-bridge converters +
Circuit Breakers
AC
DC
To Adjacent
Platform
To Shore
Station
To AdjacentPlatform
= DC Circuit Breaker= AC Circuit Breaker
DC cable fault can be cleared by the
appropriate DC circuit breaker
No AC/DC converter action is required
No interruption of power flow in the multi-terminal system, except faulted section
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DC Circuit Breaker
Half Bridge power electronic converter
Each pole is equivalent to 1/6th of the main AC/DC converter
Full DC fault current interruption capability
Full DC voltage withstand capability
Operating losses = 0.11% of station power per pole
Coordination is required between the over-current capability ofthe AC DC converter and the time required for the Breaker todetect and interrupt the fault current
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DC Circuit Breaker - Possibilities
There are no commercially available DC circuit breakers at this time,although R&D work is in progress. Possibilities include,
Vacuum
Plasma
Power electronic
Magnetic
Super-conducting
Hybrid of technologies
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TWENTIES project DC Breaker WP
Work package goal: specify and demonstrate the
critical component for multi-terminal grids, the DC
breaker
Candidate technologies:
Collaborative Activities
Mechanical switchHybrid switchPower electronic switch
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Agenda
Why DC Transmission? Page 3
Offshore Grids Page 7
DC Grid Control Page 14
DC Grid Protection Page 24
DC Grid Fault Clearance Page 34
DC Grid Standardisation Page 43
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Do We Need to Standardise?
Purpose of Standards
Support interoperability
Allowing interconnected systems to be built incrementally
and by different equipment suppliers, thus support
incremental investment plans and avoid stranded assets
Allow separation of cable and converter procurement thus
allowing buyers to take advantage of the increasing
number of HVDC cable manufacturers
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Functional Specifications
AC/DC Converters
HVDC Cables
DC Breakers DC-DC Converters
Dump Resistor
Equipment that should have a common functional specification
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Design Specification
Topology?
Symmetric Monopole Monopole Bipole
DC Voltage (nominal, steady-state and transient range)
Fault Current Contribution
Multi-terminal DC Protection
Multi-terminal DC control
Equipment that should be defined at the initial design stage
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DC Grid Standardisation Activities
International recommendations being created;
CENELEC - Four, five, six terminal grids Cigr B4-52 - Large pan-European grids
Cigr have just approved five further DC grid working groups;
B4-56 Guidelines for the preparation of connection agreements orGrid Codes for HVDC grids
B4-57 Guide for the development of models for HVDC converters in aHVDC grid
B4-58 Devices for load flow control and methodologies for directvoltage control in a meshed HVDC Grid
B4-59 Devices for load flow control and methodologies for directvoltage control in a meshed HVDC Grid
B4-60 Designing HVDC Grids for Optimal Reliability and Availabilityperformance
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