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

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Imagination at work

VSC HVDC Transmission and Emerging Technologies in DC Grids Neil Kirby

EnergyTech2015 - Cleveland

30th November 2015

Summary

HVDC Technologies

HVDC Grid Control

HVDC Grid Protection

Future Converters

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

VDC_A VDC_B

Power flow A → B Power flow B → A

VDC_B IDC

Power flow A → B Power flow B → A

Voltage-Sourced

Converter (VSC) HVDC

Converter A Converter B

VDC_A VDC_B

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

EXPORT IMPORT

Converter B

PdcB PdcD

PdcC = Σ (PdcA , PdcB , PdcD)

Converter A

Converter C

Converter A

PdcA PdcC

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.

OPD OPC

Export

Import

-Idc IoA IoB IoD IoC

STATION A

STATION B

STATION C

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

DC Pole 1 Voltage

DC Pole 2 Voltage

0.0990 0.1000 0.1010 0.1020 0.1030 0.1040 0.1050 0.1060

DC Current

DC Pole 1 Voltage

DC Pole 2 Voltage

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

L Load

AC fault DC fault

Main : Graphs

0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...

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 ...

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 ...

Main : Graphs

0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...

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 ...

Main : Graphs

0.240 0.250 0.260 0.270 0.280 0.290 0.300 0.310 0.320 0.330 0.340 ...

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

Electronic

Switch

Ultra-fast

disconnectorPower

Electronic

Switch 1

Electronic

Switch 2

Hybrid DC breaker – operating principle

Ultra-fast

disconnector

Nominal Vdc

occurs

turns off

Ultra-fast disconnector

fully open; PE2 turns off

Ibreaker

Vbreaker

Ibreaker

Vbreaker

DC breaker – the importance of speed

Current

occurs

turns off

Ultra-fast disconnector

fully open; PE2 turns off

Clearing time Pre-clearing

Detection,

selectivity &

relay time

Voltage

rise time

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

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)

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)

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

HVDC Circuit Breaker

ANOTHER

DC GRID

HVDC Tomorrow DC Breakers used to separate out the DC network

Breaker

Breake

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

←Valve voltage

←Line-to-line voltage

We don’t just have to use sinewaves!

Modular Multi-level Converter – Full Bridge

T2 + T3

conducting

T1 + T3

T2 + T4

conducting

T1 + T4

conducting

Modular Multi-level Converter

Full-Bridge

Series Valve Alternate Arm Converter

Series Bridge Converter

Red, H-Bridge converts 80% of

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.kirby1@ge.com

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

Engineering