GRID

August 2010

Dr Radnya A Mukhedkar

Group Leader, Senior Principal Engineer

System Design

Introduction to HVDC

VSC HVDC

P 2

The Voltage Sourced ConverterSingle Phase

VSCSteady DC

Voltage Input

+

Alternating Voltage Output

P 3

The Voltage Sourced ConverterThree Phase

Steady DC Voltage

Input

+

Alternating Voltage Output

VSC

P 4

AC/DC System Schematic – Ideal Load Flow

DC Voltage

Line-to- Ground

Valve Voltage

Line-to- GroundTransformer

Secondary Voltage

Line-to- GroundAC System Voltage

V1 V2 V3

IAC

XTX XLIMB /2

I

V 3

V1

VRe

Im1

3V

X

)sin(VP

VX

V)cos(VQ

VX

)sin(P

113V

X

V)cos(VQ

P 5

Voltage Waveforms

T 0557.3c

Acceptable Approximation ifSufficient Steps are Used

VSC Synthesis of a Sine Wave

Ideal Waveform

Simplest Possible Waveform

P 6

The Voltage Sourced ConverterSingle Phase, 2-level

Steady DC Voltage

Input

+

Alternating Voltage Output

Neutral

VSC

P 7

VSC: Three Main Classes

Complex Transformer

+

Simple Converters

Simple Transformer

+

Complex,

“Multi-level” Converters

Reduction in Harmonic Distortion

+

Increased Rating

Simple Transformer

+

Simple Converters

with PWM

P 8T 0811.1

Complex Transformer + simple converters

SimpleVSC

SimpleVSC

SimpleVSC

SimpleVSC

Output Voltage

Output Voltage

Output Voltage

Output Voltage

Resultant Output Voltage

P 9

Simple Transformer, Simple Converter + PWM

Output Voltage

Simple VSC

, filteredOutput Voltage

P 10

Multi-Level Converter

Output Voltage

Multi-level VSC

P 11

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

Volts

Time

Volts

What is a multi-level converter?

Total Flexibility

P 12

VSC Converter: phase arm

½Udc

½Udc

DC Transmission

SystemAC

Terminal

VSC Phase Unit

U

+½Udc

-½Udc

Line-Neutral voltage (ideal)

P 13

Semiconductors for VSC

Voltage-Sourced Converters require semiconductors which can carry current in both directions and withstand voltage in the positivedirection

The following types of device have the appropriate properties:

Thyristor derivatives:

− GTO: Gate Turn-Off thyristor− GCT: Gate Commutated Thyristor (= a GTO with a better gate

drive)− IGCT: Integrated Gate Commutated Thyristor (=a GCT with the

gate drive “integrated” into the semiconductor package)

Transistor derivatives:

− BJT: Bipolar Junction Transistor (only for low power and low frequency)

− MOSFET: Metal-Oxide Semiconductor Field Effect Transistor (only for low power)

− IGBT: Insulated Gate Bipolar Transistor− IEGT: Injection Enhanced Gate Transistor – similar to an IGBT

Do not confuse IGBT and IGCT!!

P 14

Basic 2-level inverterOne phase arm

U

+½Udc

-½Udc

Line-Neutral voltage

½Udc

½Udc

DC Transmission

System

=VSC Valve

VSC Valve

V1

VSC Valve

V2

AC

P 15

VSC Valves of the ‘Controllable Voltage Source’ type

½Udc

½Udc

DC Transmission

System U

+Udc

Valve Voltage

VSC Valve

V1

VSC Valve

V2

AC

0

U(V1) U(V2)

GRID

Circuit Types

P 17

Neutral-point clamped inverterOne phase arm (3 level)

½Udc

½Udc

DC Transmission

System

=VSC Valve=Diode Valve

U

+½Udc

-½Udc

Line-Neutral voltageV1

V2

V3

V4

AC

P 18

Neutral-point clamped inverterThree-phase circuit (3 level)

½Udc

½Udc

DC Transmission

System

V1

V2

V3

V4

AC

V1

V2

V3

V4

AC

V1

V2

V3

V4

AC

P 19

Neutral-point clamped inverterOne phase arm (5 level)

¼Udc

¼Udc

DC Transmission

System

U

Line-Neutral voltage

¼Udc

¼Udc

=VSC Valve

=Diode Valve

+¼Udc

-¼Udc

+½Udc

-½Udc

V1

V2

V3

V4

V5

V6

V7

V8

AC

P 20

Flying Capacitor inverterOne phase arm (3 level)

½Udc

½Udc

DC Transmission

System U

+½Udc

-½Udc

Line-Neutral voltageV1

V2

V3

V4

½Udc

AC

P 21

VSC with series-connected chain link modulesa.k.a. Modular MultiLevel Converter

“Half-Link” “Full-Link”

Output Voltage Output Voltage

P 22

VSC-HVDC2 Basic Approaches

Series-Connected IGBTs

� Conceptually simple circuit� Requires PWM� High switching losses� Harmonic and EMC problems

from PWM

Multi-level circuit

� Low switching losses� Easily “scaleable”� Virtually no harmonics� More complex controls

+ V

- V

+ V

- V

+ V

- V

+ V

- V

= “Chain-Link” Module

U

+½Udc

-½Udc

U

+½Udc

-½Udc

GRID

Valve Design

P 24

VSC with series-connected half-chain links

+

IGBT2D2

IGBT1D1

A

N

P

N

Valve Output VoltageEquivalent to:

Cannot electronically suppress faults on the DC side. Need to open the AC circuit breaker instead.

P 25

VSC with series-connected full-chain links

+

IGBT1

IGBT2

IGBT3

IGBT4

D1 D3

D2 D4N

P

A BVdc

Valve Output VoltageEquivalent to:

Can suppress faults on the DC side by blocking the chain links (or putting them “in reverse”)

Or

P 26

Circuit Topology

Line reactance (L) split

Becomes a means of protection

� The number of modules = the number of devices in a conventional circuit

� Requires twice the number of devices

Capacitor

Module

Stepped WaveformOverall Topology

Power Module

M2

M3

M4

M5

M6

Timeπ/2α

1 π−α1

α2

α3

π−α2

π−α3

π

2ν

3ν

−2ν

−3ν

ν

−ν2π

module high

module high

module high

module high

module high

module high

P 27

VSC Valves - Sub-module Components

VSC Sub-Module

Inter Sub-module Connector

IGBTIGBT1

IGBT2

R1

C1

D1

D2

T1

SW1

Bypass Switch

P 28

Main Components in ‘Half Bridge’

Half Bridge Power Module Circuit

IGBT (x2)

Capacitor

Bleed Resistor (x2)

Laminated Bus-Bar

Thyristor and Clamp

By-pass Switch

P 29

IGBT (x4)

Capacitor

Bleed Resistor (x2)

Laminated Bus-Bar

By-pass Switch

Main Components in ‘Full Bridge’

GRID

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