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IEC 62747:2014 – Terminology for Voltage-Sourced Converters (VSC) for HVDC Systems
💡 Key Insight: VSC-HVDC technology has revolutionized long-distance power transmission and offshore wind integration. This standard provides the unified terminology essential for engineers, researchers, and operators to communicate precisely about these complex systems — from IGBT valve levels to multi-terminal HVDC networks.
1. Scope and Context
IEC 62747 defines standardized terms for self-commutated voltage-sourced converters used for transmission of power by high voltage direct current (HVDC). While written primarily for IGBT-based VSC systems, the terminology may also guide applications using other turn-on/turn-off semiconductor devices. Line-commutated and current-sourced converters are explicitly excluded — these are covered by separate standards (IEC 60633, IEC 60700 series).
The timing of this standard reflects the rapid commercial adoption of VSC-HVDC technology in the early 2010s, following breakthroughs in modular multilevel converter (MMC) topology that made VSC-HVDC economically viable for high-voltage, high-power applications. Prior to this standard, inconsistent terminology across manufacturers, research institutions, and project specifications created significant risk of misinterpretation.
✅ Industry Impact: VSC-HVDC is the enabling technology for offshore wind farm integration, cross-border power trading, and multi-terminal DC grids. Standardized terminology is the foundation for consistent engineering specifications, reliable project execution, and safe system operation.
2. VSC Topologies and Converter Architecture
2.1 Fundamental Topologies
The standard defines three fundamental VSC topologies based on voltage level count. Two-level converters switch between two discrete DC voltage levels, offering simplicity but producing higher harmonic content. Three-level converters (e.g., neutral-point-clamped) provide improved waveform quality. Multi-level converters switch between more than three discrete levels, offering the highest waveform quality and lowest losses but requiring complex control systems.
2.2 Modular Multilevel Converter (MMC)
The MMC represents the dominant topology in modern VSC-HVDC systems. Each VSC valve consists of a series connection of MMC building blocks (submodules or cells). A submodule contains a single IGBT-diode pair with its DC capacitor, while a cell contains multiple series-connected IGBT-diode pairs. The cascaded two-level converter (CTL) is a variant where each switch position comprises more than one IGBT-diode pair in series. The standard provides precise definitions for all these elements, ensuring consistent engineering communication.
| Topology | Voltage Levels | Key Characteristics | Typical Application |
|---|---|---|---|
| Two-level converter | 2 | Simple, higher harmonics, higher dv/dt | Low-voltage drives, early VSC-HVDC |
| Three-level converter (NPC) | 3 | Improved waveform, moderate losses | Medium-voltage applications |
| Modular Multilevel (MMC) | ≥ 3 (typically 200+) | Near-sinusoidal output, very low losses, modular | Modern HVDC, offshore wind |
| Cascaded Two-Level (CTL) | ≥ 3 | Series IGBT pairs per switch position | High-power HVDC applications |
2.3 Converter Valve Configuration
The standard meticulously defines the hierarchical structure: a VSC valve (switch type) consists of IGBT-diode pairs arranged to switch simultaneously; a VSC valve (controllable voltage source type) is a complete controllable voltage source assembly connecting one AC terminal to one DC terminal. A VSC valve level is the smallest indivisible functional unit — for series-connected IGBTs, one level equals one IGBT-diode pair; for MMC without series IGBTs, one level equals one submodule with auxiliaries.
3. Converter Units and System Integration
3.1 Converter Unit Configuration
A converter unit encompasses all equipment between the point of common coupling on the AC side and the point of common coupling on the DC side. It comprises one or more VSC units together with interface transformers, control equipment, protective and switching devices, and auxiliaries. The VSC unit includes three VSC phase units, storage capacitors, phase reactors, and associated control equipment.
| Component | Definition | Function |
|---|---|---|
| Converter unit | Full AC-DC conversion assembly | Complete conversion between AC and DC systems |
| VSC unit | Three VSC phase units + capacitors + reactors | Core power conversion stage |
| VSC phase unit | Connects two DC terminals to one AC terminal | Per-phase conversion |
| VSC valve (switch type) | Series IGBT-diode pairs, switched simultaneously | Controlled switching element |
| MMC building block | Self-contained 2-terminal voltage source + capacitor | Modular conversion unit |
3.2 Operating Conditions
The standard defines essential operating parameters: modulation index (ratio of converter AC voltage to DC voltage), firing angle, extinction angle, active and reactive power control modes, and valve stress parameters. The modulation index, corrected via Corrigendum 1, is defined as M = (2√2 Uc1) / (√3 Udc), where Uc1 is the fundamental converter voltage and Udc is the DC voltage.
⚠️ Engineering Note: The Corrigendum 1 (2015) corrected the modulation index formula. Engineers working with VSC systems should ensure they reference the corrected formula: M = (2√2 · Uc1) / (√3 · Udc). The pre-corrigendum formula could lead to significant miscalculations in converter operating point determination.
4. Engineering Design Insights
💡 Practical Takeaways for Engineers:
- Consistent notation saves millions: In multi-vendor HVDC projects, inconsistent terminology can lead to interface mismatches costing millions in rework. Mandate IEC 62747 terminology in all technical specifications and contracts.
- MMC scalability is key: The modular nature of MMC allows voltage scaling by adding submodules — from distribution level (10-30 submodules per arm) to ultra-high voltage (200+ submodules per arm). The standard’s definitions for submodule, cell, and valve level provide the vocabulary for this scalability.
- Distinguish VSC valve types: The standard’s distinction between “switch type” and “controllable voltage source type” valves is not academic — it reflects fundamentally different control philosophies. Switch type valves use PWM or nearest-level modulation; controllable voltage source type valves synthesize voltage waveforms directly by capacitor voltage balancing.
- Protection coordination: Understanding the standardized terminology for converter protection zones (valve, phase unit, VSC unit, converter unit) is essential for designing coordinated protection schemes that maintain selectivity across all fault locations.
- Multi-terminal implications: As HVDC grids evolve from point-to-point to multi-terminal configurations, the standardized definitions for DC side equipment (DC breakers, DC disconnectors, earth return transfer breakers) become increasingly critical for system design and operational coordination.
5. Frequently Asked Questions
Q1: Why differentiate between line-commutated (LCC) and voltage-sourced (VSC) converter standards?
LCC-HVDC (based on thyristors) and VSC-HVDC (based on IGBTs) have fundamentally different operating principles, failure modes, and application domains. LCC requires a strong AC grid for commutation, can suffer commutation failures, and consumes reactive power. VSC provides black-start capability, independent AC voltage control, and does not suffer commutation failures. Separate standards are necessary to address these distinct characteristics.
Q2: What is the significance of the modulation index in VSC operation?
The modulation index determines the utilization of DC voltage to produce AC voltage. Operating at a higher modulation index (closer to 1.0) improves DC voltage utilization and reduces converter losses, but leaves less margin for transient over-modulation during disturbances. The trade-off between efficiency and operating margin is a key engineering decision in VSC-HVDC design.
Q3: How does the MMC topology achieve its superior waveform quality?
By inserting or bypassing individual submodule capacitors in each arm, the MMC synthesizes a near-sinusoidal voltage waveform with very low harmonic distortion. With 200+ submodules per arm (typical for HVDC), the output voltage waveform approaches an ideal sine wave, eliminating the need for large AC harmonic filters and reducing the converter footprint significantly compared to two-level designs.
Q4: What are the key parameters for specifying a VSC valve?
Key parameters include: rated DC voltage, rated current (continuous and transient), maximum repetitive peak voltage, short-circuit withstand capability, switching frequency, and thermal cycling capability. The standard provides the vocabulary for specifying these parameters consistently across manufacturers and projects.
