Introduction to HVDC VSC HVDC Dr Radnya A Mukhedkar Group Leader, Senior Principal Engineer System Design GRID August 2010
The Voltage Sourced Converter Single Phase Alternating Voltage Output Steady DC Voltage Input + VSC P 2
The Voltage Sourced Converter Three Phase Alternating Voltage Output Steady DC Voltage Input + VSC P 3
AC/DC System Schematic Ideal Load Flow XTX XLIMB/2 IAC Line-to- Ground AC System Voltage Line-to- Ground Transformer Secondary Voltage Line-to- Ground Valve Voltage DC Voltage V1 V2 V3 I Im V 3 P V 3 sin( X ) V 1 V1 V Re Q V 3 cos( X ) V 1 V 1 P 4
VSC Synthesis of a Sine Wave Voltage Waveforms Simplest Possible Waveform Ideal Waveform Acceptable Approximation if Sufficient Steps are Used T 0557.3c P 5
The Voltage Sourced Converter Single Phase, 2-level Alternating Voltage Output Steady DC Voltage Input + VSC Neutral P 6
VSC: Three Main Classes Reduction in Harmonic Distortion + Increased Rating Complex Transformer + Simple Converters Simple Transformer + Complex, Multi-level Converters Simple Transformer + Simple Converters with PWM P 7
Complex Transformer + simple converters Output Voltage Simple VSC Simple VSC Output Voltage Resultant Output Voltage Output Voltage Simple VSC Simple VSC Output Voltage T 0811.1 P 8
Simple Transformer, Simple Converter + PWM Output Voltage Simple VSC Output Voltage, filtered P 9
Multi-Level Converter Output Voltage Multi-level VSC P 10
P 11 What is a multi-level converter? Total Flexibility
VSC Converter: phase arm Line-Neutral voltage (ideal) DC Transmission System ½U dc VSC Phase Unit AC Terminal +½U dc U ½U dc -½U dc P 12
Semiconductors for VSC Voltage-Sourced Converters require semiconductors which can carry current in both directions and withstand voltage in the positive direction 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 13
Basic 2-level inverter One phase arm Line-Neutral voltage DC Transmission System ½U dc VSC Valve V1 AC +½U dc U ½U dc VSC Valve V2 -½U dc =VSC Valve P 14
VSC Valves of the Controllable Voltage Source type Valve Voltage DC Transmission System ½U dc VSC Valve V1 AC +U dc U U(V1) U(V2) ½U dc VSC Valve V2 0 P 15
Circuit Types GRID
Neutral-point clamped inverter One phase arm (3 level) V1 Line-Neutral voltage DC Transmission System ½U dc V2 AC +½U dc U ½U dc V3 -½U dc V4 =Diode Valve =VSC Valve P 17
Neutral-point clamped inverter Three-phase circuit (3 level) V1 V1 V1 DC Transmission System ½U dc V2 AC V2 AC V2 AC ½U dc V3 V3 V3 V4 V4 V4 P 18
Neutral-point clamped inverter One phase arm (5 level) ¼U dc V1 V2 V3 +½U dc Line-Neutral voltage DC Transmission System ¼U dc ¼U dc V4 +¼U dc U AC -¼U dc V5 -½U dc V6 V7 =Diode Valve ¼U dc V8 =VSC Valve P 19
Flying Capacitor inverter One phase arm (3 level) V1 Line-Neutral voltage DC Transmission System ½U dc ½U dc V2 AC +½U dc U ½U dc V3 -½U dc V4 P 20
VSC with series-connected chain link modules a.k.a. Modular MultiLevel Converter Half-Link Full-Link Output Voltage Output Voltage P 21
VSC-HVDC 2 Basic Approaches Series-Connected IGBTs Multi-level circuit Conceptually simple circuit Low switching losses Requires PWM Easily scaleable High switching losses Virtually no harmonics Harmonic and EMC problems More complex controls from PWM + V + V + V + V - V - V - V - V = Chain-Link Module +½U dc +½U dc U U -½U dc -½U dc P 22
Valve Design GRID
VSC with series-connected half-chain links P A D1 IGBT1 + N D2 IGBT2 N Equivalent to: Valve Output Voltage Cannot electronically suppress faults on the DC side. Need to open the AC circuit breaker instead. P 24
VSC with series-connected full-chain links IGBT1 D1 P IGBT3 D3 A IGBT2 + Vdc IGBT4 B D2 N D4 Equivalent to: Valve Output Voltage Or Can suppress faults on the DC side by blocking the chain links (or putting them in reverse ) P 25
Circuit Topology Capacitor Power Module 3ν 2ν ν Module Line reactance (L) split ν 2ν 3ν α α α 1 π/2 π α π α 2 3 3 π π α 2 1 2π Becomes a means of protection M2 M3 M4 M5 M6 module high module high module high module high module high module high Overall Topology Stepped Waveform The number of modules = the number of devices in a conventional circuit P 26 Requires twice the number of devices
VSC Valves - Sub-module Components Inter Sub-module Connector VSC Sub-Module D1 IGBT1 R1 C1 IGBT SW1 T1 D2 IGBT2 Bypass Switch P 27
Main Components in Half Bridge IGBT (x2) Capacitor Half Bridge Power Module Circuit Bleed Resistor (x2) By-pass Switch P 28 Thyristor and Clamp Laminated Bus-Bar
Main Components in Full Bridge IGBT (x4) Capacitor Bleed Resistor (x2) By-pass Switch P 29 Laminated Bus-Bar
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