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IEC 62927: Voltage Sourced Converter (VSC) Valves for HVDC Power Transmission
Electrical type and production tests for self-commutated converter valves in VSC-HVDC systems
IEC 62927, published in 2017 with Corrigendum 1 in 2017, specifies electrical type and production tests for self-commutated converter valves used in voltage sourced converter (VSC) systems for high-voltage direct current (HVDC) power transmission. Developed by IEC TC 22 (Power Electronic Systems and Equipment) Subcommittee 22F (Power Electronics for Electrical Transmission and Distribution Systems), this standard fills a critical testing framework gap as VSC-HVDC technology has become the dominant choice for offshore wind integration, multi-terminal DC networks, and long-distance submarine power transmission. Unlike line-commutated converter (LCC) HVDC systems, VSC-HVDC can independently control active and reactive power, operate in weak AC networks, and even supply passive loads, making it the technology of choice for modern HVDC projects worldwide.
IEC 62927 applies to self-commutated valves, typically using Insulated Gate Bipolar Transistor (IGBT) technology, which form the building blocks of VSC-HVDC converters. The standard covers both two-level and modular multilevel converter (MMC) topologies, with voltage ratings typically spanning from +/- 80 kV to +/- 525 kV. The MMC topology, which dominates modern installations, uses hundreds of submodules per valve arm, each requiring coordinated insulation coordination and switching control. The standard provides the framework for verifying valve design adequacy through a structured series of dielectric, operational, and production tests conducted under conditions representative of the most severe stresses expected in service.
Test Classification and Scope
The standard divides tests into three main categories: dielectric tests, operational tests, and production tests. Dielectric tests verify the valve insulation system ability to withstand various overvoltage stresses, including AC voltage, DC voltage, lightning impulse, and switching impulse. Operational tests verify the valve ability to conduct and switch current under specified conditions, including minimum DC voltage tests, maximum operating current tests, and fault current tests. Production tests are performed on each valve or submodule to verify manufacturing quality, including partial discharge measurement, control and monitoring system functional checks, and coolant system pressure tests.
| Test Category | Test Type | Purpose |
|---|---|---|
| Dielectric Tests | AC voltage withstand | Verify insulation under AC stress (valve-to-ground) |
| DC voltage withstand | Verify insulation under DC stress (valve-to-ground) | |
| Lightning impulse test | Verify insulation coordination for atmospheric surges | |
| Switching impulse test | Verify insulation for internal switching transients | |
| Operational Tests | Minimum DC voltage test | Verify valve turn-on at lowest operating voltage |
| Maximum current turn-off | Verify safe turn-off at peak operating current | |
| Fault current test | Verify valve withstand during DC-side faults | |
| Production Tests | Partial discharge measurement | Detect insulation defects during manufacturing |
| Control system functional test | Verify gate drive and monitoring functionality | |
| Coolant system pressure test | Verify cooling system integrity |
The dielectric test requirements are specified for each valve terminal (DC terminal, AC terminal, and valve-to-ground). The test levels are derived from the valve rated voltage (Uv) using multiplication factors specified in the standard. For example, the DC voltage withstand test is performed at 1.85 x Uv for 60 minutes, while the lightning impulse withstand level is 1.3 times the valve protective level (VPL). The AC voltage test uses 1.2 x the maximum AC voltage across the valve for 60 seconds. These test levels ensure that the valve insulation coordination aligns with the overall HVDC converter station insulation design as specified in IEC 60071-1 and IEC 60071-2.
For VSC valves using the modular multilevel converter topology, the dielectric stress distribution across submodules is not uniform during transient conditions. The standard requires that the insulation design consider the worst-case voltage distribution, which may concentrate stress on submodules closest to the AC or DC terminal depending on the transient type. Electromagnetic transient (EMT) simulations are typically used to determine the voltage distribution across the valve during each test condition, and the test parameters must be adjusted accordingly to ensure all submodules are adequately stressed.
Operational and Production Test Details
The operational tests are designed to verify the valve capability under realistic operating conditions. The minimum DC voltage test verifies that the valve can be turned on (submodules inserted) at the lowest DC voltage expected during operation, typically 10-20% of rated voltage. This is critical for black-start capability and low-power operation. The maximum current turn-off test verifies that the valve can safely interrupt the maximum expected operating current, typically 1.2 times the rated current, while absorbing the energy in the snubber circuits and submodule capacitors. The fault current test subjects the valve to a worst-case DC-side fault, typically a pole-to-pole or pole-to-ground fault at the DC bus, verifying that the valve can withstand the fault current until the DC breaker or converter protection operates.
| Test | Condition | Pass Criteria |
|---|---|---|
| Minimum DC voltage turn-on | 0.1-0.2 x Uv DC | All submodules turn on within specified time |
| Maximum current turn-off | 1.2 x Irated | Voltage across valve within design limits |
| Fault current withstand | Peak fault current per system study | No valve damage, protection operates correctly |
| Load cycle test | Rated power, thermal cycling | Temperature within limits, no degradation |
| Minimum extinction voltage | Minimum AC system voltage | Stable commutation, no commutation failure |
Production testing of each submodule ensures consistent manufacturing quality. The partial discharge (PD) test is particularly important — each submodule must have PD levels below specified limits (typically 10 pC at 1.2 x rated voltage) to ensure long-term insulation reliability. The PD test is sensitive to manufacturing defects such as voids in the insulation, delamination, or contaminated surfaces. The control system functional test verifies that the submodule control board, gate drive units, and monitoring sensors operate correctly across the full voltage and temperature range. The coolant system pressure test at 1.5 times the rated pressure verifies that no leaks exist in the cooling system, which is critical for removing the heat generated by IGBT switching losses (typically 1-2% of the rated power per submodule).
Modern VSC-HVDC valves using press-pack IGBTs and MMC topology with full-bridge submodules can achieve valve efficiencies exceeding 99.5%, with switching losses below 1% of rated power. The modular structure enables fault-tolerant operation — a single submodule failure can be bypassed without shutting down the converter, achieving availability figures above 99.9% for multi-terminal HVDC systems. This level of reliability is essential for offshore wind applications where access for maintenance is limited to weather windows and the cost of unplanned downtime can exceed $1 million per day for a 1 GW installation.
Engineering Design Insights for VSC Valve Systems
From a system design perspective, the thermal management of VSC valves is one of the most critical engineering challenges. Each IGBT module in a VSC valve dissipates between 1-3 kW of heat during full-load operation, and a complete valve assembly with hundreds of submodules can generate several megawatts of thermal power. The cooling system design directly impacts the valve current rating and reliability. Deionized water cooling systems with stainless steel piping and plate heat exchangers are standard for modern VSC-HVDC installations, achieving thermal resistances below 0.02 K/W per IGBT module. The coolant flow rate, inlet temperature, and pressure must be precisely controlled to maintain IGBT junction temperatures below 125 deg C under all operating conditions including temporary overloads of up to 1.1 per unit for several seconds.
Electromagnetic compatibility is another critical design consideration. VSC valves generate high-frequency electromagnetic emissions due to the switching transitions of IGBTs, with dV/dt rates of 1-5 kV/microsecond and di/dt rates of 1-10 kA/microsecond. The valve hall must be designed as a Faraday cage with electromagnetic shielding effectiveness of at least 60 dB at frequencies up to 30 MHz. The valve reactor (phase reactor) at the AC side of each valve arm must be designed for the combined fundamental and switching frequency currents, with the core and winding design optimized for minimal losses at both the fundamental (50/60 Hz) and switching (typically 1-5 kHz for MMC) frequencies. The standard recommends that the valve design include comprehensive EMC testing to verify compliance with applicable limits and ensure compatibility with control and protection equipment located within the same station.
| Parameter | +/- 320 kV System | +/- 525 kV System |
|---|---|---|
| Rated DC voltage | 320 kV | 525 kV |
| Rated power | 1000-1200 MW | 2000-3000 MW |
| Submodules per arm | 200-250 | 350-450 |
| IGBT module rating | 4.5 kV / 3 kA | 4.5 kV / 3 kA |
| Valve efficiency | > 99.3% | > 99.5% |
| Cooling system | Deionized water | Deionized water |
| Seismic withstand | 0.3 g horizontal | 0.5 g horizontal |
Q1: What is the difference between IEC 62927 and IEC 62501 (LCC valve testing)?
A: IEC 62927 is specifically for self-commutated VSC valves (IGBT-based), while IEC 62501 covers line-commutated converter (LCC) valves (thyristor-based). VSC valves have fundamentally different operational characteristics including self-turn-off capability, higher switching frequencies, and different fault response. The dielectric and operational test requirements reflect these differences.
A: IEC 62927 is specifically for self-commutated VSC valves (IGBT-based), while IEC 62501 covers line-commutated converter (LCC) valves (thyristor-based). VSC valves have fundamentally different operational characteristics including self-turn-off capability, higher switching frequencies, and different fault response. The dielectric and operational test requirements reflect these differences.
Q2: Are the tests in IEC 62927 applicable to both two-level and MMC topologies?
A: Yes, the standard is designed to be topology-neutral. However, the test implementation differs — for MMC valves, individual submodule tests are combined with valve-level tests to verify the complete system. The standard provides guidance on adapting test procedures for different topologies while maintaining equivalent stress levels.
A: Yes, the standard is designed to be topology-neutral. However, the test implementation differs — for MMC valves, individual submodule tests are combined with valve-level tests to verify the complete system. The standard provides guidance on adapting test procedures for different topologies while maintaining equivalent stress levels.
Q3: How often must production tests be performed?
A: Production tests are performed on every submodule or valve section during manufacturing. Routine production testing ensures consistent quality. The standard specifies that 100% of submodules must pass the production test sequence before installation.
A: Production tests are performed on every submodule or valve section during manufacturing. Routine production testing ensures consistent quality. The standard specifies that 100% of submodules must pass the production test sequence before installation.
Q4: What is the significance of the minimum DC voltage turn-on test?
A: This test validates the valve ability to start the converter from a de-energized state (black-start capability), which is essential for island grids and offshore wind platforms where the VSC must establish the AC grid voltage before the wind turbines can synchronize. The test also verifies that the submodule capacitor charging circuits function correctly at the lowest expected DC voltage, ensuring reliable self-powering of the gate drive units from the submodule capacitor voltage. Without this capability, the VSC may not be able to initiate operation after a complete station blackout.
A: This test validates the valve ability to start the converter from a de-energized state (black-start capability), which is essential for island grids and offshore wind platforms where the VSC must establish the AC grid voltage before the wind turbines can synchronize. The test also verifies that the submodule capacitor charging circuits function correctly at the lowest expected DC voltage, ensuring reliable self-powering of the gate drive units from the submodule capacitor voltage. Without this capability, the VSC may not be able to initiate operation after a complete station blackout.
