GRID

Chief Engineer, Systems

Offshore Wind Training Seminar - March 2011

Session 1:

HVDC as a bulk power transfer system

Carl Barker

HVDC as a bulk power transfer system – March 2011 - P 2

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

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 – Belgium Offshore

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 Capacities

Source: FOSG Position paper on the EC Communication for a European Infrastructure Package, Dec 2010

HVDC as a bulk power transfer system – March 2011 - P 4

Why DC Transmission?

HVDC transmission is the correct technology for bulk submarine energy transfer.

AC

DC

Charges and Discharges

Every Half Cycle

Only Charges the Cable Once

HVDC as a bulk power transfer system – March 2011 - P 5

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

HVDC as a bulk power transfer system – March 2011 - P 6

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

HVDC as a bulk power transfer system – March 2011 - P 7

Offshore “DC Grids”

Definition:

• “DC Grids” – Multiple converters connecting AC power networks to a DC power network

• “DC Grids” – Permit the economic transfer of power over buried 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

HVDC as a bulk power transfer system – March 2011 - P 8

132kV

400V400V400V

11kV11kV

400V400V400V

11kV11kV

DC Grid Configurations: Point-to-point System

HVDC as a bulk power transfer system – March 2011 - P 9

132kV

400V400V400V

11kV11kV

400V400V400V

11kV11kV

DC Grid Configurations: Meshed System

HVDC as a bulk power transfer system – March 2011 - P 10

132kV

400V400V400V

11kV11kV

400V400V400V

11kV11kV

DC Grid Configurations: Radial System

HVDC as a bulk power transfer system – March 2011 - P 11

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

HVDC as a bulk power transfer system – March 2011 - P 12

Platform Switchgear Arrangement

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

HVDC as a bulk power transfer system – March 2011 - P 13

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

HVDC as a bulk power transfer system – March 2011 - P 14

Two-terminal VSC Control

• Basic Converter control of a two-terminal VSC

Converter 1 Converter 2

Converter 3

Converter 4

Slack bus

HVDC as a bulk power transfer system – March 2011 - P 15

DC Grid Control

• Will a single utility / system owner be prepared to act as the slack bus for all other interconnected systems?

Converter 1 Converter 2

Converter 3

Converter 4

Slack bus

HVDC as a bulk power transfer system – March 2011 - P 16

Comparison of AC and DC parameters

AC PARAMETER DC PARAMETER

Frequency

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

HVDC as a bulk power transfer system – March 2011 - P 17

A typical VSC converter slope

characteristic

a*

Vdc

-Idc +Idc

IMPORT

(A)

EXPORT

(A)

Vdc

MAX

(A)

Vdc

MIN

(A)

LRSP

Vdc/Idc

Slope(A)

-Idc

MAX(A)

+Idc

MAX(A)

-Idc IMPORT

LIMIT(A)

+Idc EXPORT

LIMIT(A)

ΔVA a

b

c

d

d*

HVDC as a bulk power transfer system – March 2011 - P 18

A basic controller for DC converter

in a multi-terminal HVDC scheme

Dispatch

Centre

Converter

Station

Controller

Porder

LRSP

Idcmax Limit

slopedc

dc

I

V

Vdc

MAX

ΔV

∑

X

Vorder

Limits

Vdc

MIN

Vorder

Idc measured

Iorder

Limits

Idcmin Limit

∑

Idc measured

LRSP

Porder

HVDC as a bulk power transfer system – March 2011 - P 19

A two-terminal VSC Grid with power

flow from terminal A to terminal B

Vdc

-Idc +Idc

IMPORT

(A───)

EXPORT

(A───)

Vdc

MAX

(A)

Vdc

MIN

(A)

LRSP OP

ΔVA

ΔVB

IMPORT

(B─ ─ ─)

EXPORT

(B─ ─ ─)

Vdc

MAX

(B)

Vdc

MIN

(B)

HVDC as a bulk power transfer system – March 2011 - P 20

A change in power demand compensated

by a new power dispatch

Vdc

-Idc +Idc

IMPORT

(A───)

EXPORT

(A───)

Vdc

MAX

(A)

Vdc

MIN

(A)

LRSP

OP2

ΔVA1

ΔVB2

IMPORT

(B─ ─ ─)

EXPORT

(B─ ─ ─)

Vdc

MAX

(B)

Vdc

MIN

(B)

OP1

Vdc

-Idc +Idc

IMPORT

(A───)

EXPORT

(A───)

Vdc

MAX

(A)

Vdc

MIN

(A)

LRSP

OP2

ΔVA3

ΔVB3

IMPORT

(B─ ─ ─)

EXPORT

(B─ ─ ─)

Vdc

MAX

(B)

Vdc

MIN

(B)

OP1

Constant

Pdc Line

OP3

HVDC as a bulk power transfer system – March 2011 - P 21

A three-terminal DC grid

OPB

Vdc

-Idc +Idc

LRSP

OPC OPA

IB IC IA = IB+ IC

IMPORT

(A───)

EXPORT

(A───)

IMPORT (B─ ─ ─)

IMPORT (C— - - —)

EXPORT (B─ ─ ─)

EXPORT (C— - - —)

HVDC as a bulk power transfer system – March 2011 - P 22

Voltage Optimiser

OPB

Vdc

-Idc +Idc

LRSP

OPC OPA

IB IC IA = IB+ IC

IMPORT

(A───)

EXPORT

(A───)

IMPORT (B─ ─ ─)

IMPORT (C— - - —)

EXPORT (B─ ─ ─)

EXPORT (C— - - —)

• Steady-state

transmission

loss

minimisation

• One converter

terminal

determines the

new, higher,

LRSP

• LRSP ramp can be

stopped at any

time

“Autonomous Converter Control in a Multi-Terminal HVDC System”

Authors: Carl Barker Robert Whitehouse, Alstom Grid, UK

8th International Conference on AC and DC Power Transmission, IET, London, 2010

HVDC as a bulk power transfer system – March 2011 - P 23

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

HVDC as a bulk power transfer system – March 2011 - P 24

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 experienced simultaneously 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, in minimum time with minimum loss of infrastructure

HVDC as a bulk power transfer system – March 2011 - P 25

Protection systems

AC

DC

To

Adjacent

Platform

To

Shore

Station

To

Adjacent

Platform

= Current

Transducers

AC side transducers are conventional

electromagnetic current transformers

DC side transducers are based on modern

fibre optic current measurement techniques

HVDC as a bulk power transfer system – March 2011 - P 26

Current Transducer – Nxt Phase

Fibre optic

measurement head

Polymeric

insulators to

protect fibre optic

cables

Fibre optic connection

to matching unit

HVDC as a bulk power transfer system – March 2011 - P 27

Protection Zones

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

PlatformAC feeder from AC

collector platform

HVDC as a bulk power transfer system – March 2011 - P 28

Protection Zones

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

PlatformTransformer differential

zone

HVDC as a bulk power transfer system – March 2011 - P 29

Protection Zones

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

PlatformAC – DC differential

zone

HVDC as a bulk power transfer system – March 2011 - P 30

Protection Zones

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

DC bus protection

zone

HVDC as a bulk power transfer system – March 2011 - P 31

Protection Zones

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

DC cable over-current

protection zone

HVDC as a bulk power transfer system – March 2011 - P 32

DC cable fault protection

DC bus protection DC bus protection

DC cable protection

Unbalance DC

current protection

HVDC as a bulk power transfer system – March 2011 - P 33

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

HVDC as a bulk power transfer system – March 2011 - P 34

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 of

suppressing DC-side faults

HVDC as a bulk power transfer system – March 2011 - P 35

Option 1 “Half-bridge converters +

Disconnects

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

= 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 to

clear the fault

Appropriate disconnects opened to isolate

faulted cable section

Complete multi-terminal scheme is re-started

HVDC as a bulk power transfer system – March 2011 - P 36

Modular Multi-level Converter : Full

link

+ V

- V

+ V

- V

• Same circuit as ALSTOM

STATCOM chain circuit

• Output DC voltage can be

either polarity

• Hence can connect as tap

to LCC-HVDC link

• Can also suppress DC side

faults

U

Module Output voltage

HVDC as a bulk power transfer system – March 2011 - P 37

Option 2 “Full-bridge” converters +

Fast Switches

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

= Fast Isolating Switch= AC Circuit Breaker

DC cable fault can be cleared by “full – bridge”

AC/DC converters

Appropriate fast (30 – 40ms) isolating switches

opened to isolate faulted cable section

Complete multi-terminal scheme is re-started

in 300 – 400ms

HVDC as a bulk power transfer system – March 2011 - P 38

Option 3 “Half-bridge” converters +

Circuit Breakers

AC

DC

To Adjacent

Platform

To Shore

Station

To Adjacent

Platform

= 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

HVDC as a bulk power transfer system – March 2011 - P 39

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 of

the AC – DC converter and the time required for the “Breaker” to

detect and interrupt the fault current

HVDC as a bulk power transfer system – March 2011 - P 40

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

HVDC as a bulk power transfer system – March 2011 - P 41

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

HVDC as a bulk power transfer system – March 2011 - P 42

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

HVDC as a bulk power transfer system – March 2011 - P 43

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

HVDC as a bulk power transfer system – March 2011 - P 44

Functional Specifications

• AC/DC Converters

• HVDC Cables

• DC Breakers

• DC-DC Converters

• Dump Resistor

Equipment that should have a common functional specification

HVDC as a bulk power transfer system – March 2011 - P 45

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

HVDC as a bulk power transfer system – March 2011 - P 46

DC Grid Standardisation Activities

• International recommendations being created;

− CENELEC - Four, five, six terminal grids− Cigrè B4-52 - Large pan-European grids

• Cigrè have just approved five further DC grid working groups;

− B4-56 Guidelines for the preparation of “connection agreements” or “Grid Codes” for HVDC grids

− B4-57 Guide for the development of models for HVDC converters in a HVDC grid

− B4-58 Devices for load flow control and methodologies for direct voltage control in a meshed HVDC Grid

− B4-59 Devices for load flow control and methodologies for direct voltage control in a meshed HVDC Grid

− B4-60 Designing HVDC Grids for Optimal Reliability and Availability performance

www.alstom.com