Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 61975: High-Voltage Direct Current (HVDC) Systems — System Testing Requirements
✅ Standard at a Glance
IEC 61975, published in 2016 by IEC Technical Committee 22 (Power electronic systems and equipment), establishes a comprehensive framework for system testing of HVDC installations. The standard covers both Line-Commutated Converter (LCC) and Voltage-Sourced Converter (VSC) HVDC technologies, defining test procedures for factory acceptance testing (FAT), site commissioning testing, and performance validation. It ensures that HVDC systems meet specified performance, reliability, and interoperability requirements before commercial operation.
IEC 61975, published in 2016 by IEC Technical Committee 22 (Power electronic systems and equipment), establishes a comprehensive framework for system testing of HVDC installations. The standard covers both Line-Commutated Converter (LCC) and Voltage-Sourced Converter (VSC) HVDC technologies, defining test procedures for factory acceptance testing (FAT), site commissioning testing, and performance validation. It ensures that HVDC systems meet specified performance, reliability, and interoperability requirements before commercial operation.
🔌 1. Testing Framework and Acceptance Criteria
1.1 Classification of Tests
IEC 61975 organizes HVDC system testing into a structured hierarchy covering the entire project lifecycle from design validation through commissioning:
| Test Phase | Location | Scope | Key Tests |
|---|---|---|---|
| Type Tests (Design Validation) | Factory or high-power lab | One unit of each converter design | Valve dielectric test, operational test, valve cooling test, EMI measurement |
| Factory Acceptance Tests (FAT) | Factory (control system) | Control & protection cubicles, valve base electronics (VBE) |
Hardware-in-the-loop (HIL) simulation, control function verification, redundancy testing, fault scenario testing |
| Site Commissioning Tests | Installation site | Complete HVDC system | Energization, power transmission, fault ride-through, load rejection, power reversal, AC/DC system interaction |
| Performance Tests | Installation site | Complete HVDC system | Efficiency measurement, harmonic performance, availability demonstration, power quality verification |
💡 Engineering Insight
The most technically demanding part of IEC 61975 is the hardware-in-the-loop (HIL) testing requirement for control and protection systems. The standard mandates that the actual control cubicles (not replicated models) be connected to a real-time digital simulator (RTDS or similar) that models the complete AC and DC networks with a time step of 50 microseconds or less. The HIL model must include at least the converter transformers, AC filters, DC lines/cables, the AC network equivalent impedance at both terminals, and realistic fault initiation points. For a typical ±500 kV, 3000 MW LCC HVDC scheme, the HIL test campaign involves 500-800 individual test cases covering normal operation, AC and DC faults, control mode transitions, and protection system validation. The total testing duration typically spans 8-12 weeks.
The most technically demanding part of IEC 61975 is the hardware-in-the-loop (HIL) testing requirement for control and protection systems. The standard mandates that the actual control cubicles (not replicated models) be connected to a real-time digital simulator (RTDS or similar) that models the complete AC and DC networks with a time step of 50 microseconds or less. The HIL model must include at least the converter transformers, AC filters, DC lines/cables, the AC network equivalent impedance at both terminals, and realistic fault initiation points. For a typical ±500 kV, 3000 MW LCC HVDC scheme, the HIL test campaign involves 500-800 individual test cases covering normal operation, AC and DC faults, control mode transitions, and protection system validation. The total testing duration typically spans 8-12 weeks.
1.2 Performance Acceptance Criteria
IEC 61975 defines quantitative performance criteria that the HVDC system must meet during commissioning tests:
| Performance Parameter | LCC HVDC Requirement | VSC HVDC Requirement | Measurement Method |
|---|---|---|---|
| Power transmission accuracy | ±1.5% of rated power | ±1.0% of rated power | Calibrated wattmeter at both terminals |
| DC voltage regulation | ±2.0% at rated conditions | ±1.0% at rated conditions | DC voltage divider measurement |
| Converter losses | Within +5% of declared value | Within +5% of declared value | Calorimetric or electrical method |
| Fault recovery time (AC faults) | ≤100 ms after fault clearance | ≤50 ms after fault clearance | Digital fault recorder analysis |
| Harmonic performance (AC side) | THD ≤1.0% (telephone influence factor TIF ≤50) | THD ≤0.5% | Power quality analyzer (IEC 61000-4-30 Class A) |
| Step response rise time (power control) | ≤50 ms (10-90%) | ≤30 ms (10-90%) | Step change from 0.9 to 1.0 p.u. reference |
💡 2. Test Methods and Technical Requirements
2.1 AC Side and DC Side Testing
IEC 61975 divides commissioning tests into AC side and DC side categories, each with specific requirements:
AC side testing includes: AC filter bank energization and step response tests, converter transformer inrush current measurement and saturation detection verification, AC bus voltage control and reactive power capability verification, and AC fault ride-through testing at various fault distances and types (single-line-to-ground, three-phase, phase-to-phase). The standard requires that AC fault ride-through be demonstrated for faults at the converter bus and at key points along the connected AC network, with fault durations ranging from 50 ms to 500 ms.
DC side testing includes: DC cable/line energization from both ends, DC voltage ramp-up and ramp-down characteristics, DC fault clearing and recovery (for VSC systems with DC-side breakers), power reversal and power modulation tests, and DC ground electrode performance verification. For VSC-HVDC systems, the standard introduces specific DC fault handling tests to verify the converter’s ability to limit fault current and the DC breaker’s capability to isolate the faulted section within the specified interruption time (typically 3-5 ms for hybrid DC breakers).
⚠️ Design Warning
One of the most critical and frequently problematic tests in IEC 61975 is the load rejection test. The standard requires that the HVDC system withstand a sudden loss of full load (from 1.0 p.u. to near-zero power) without tripping or exceeding voltage limits. For LCC systems, this test can cause severe overvoltage on the DC side (up to 1.3-1.5 p.u.) because the converter continues to draw reactive power from the AC system but the DC power has collapsed. The control system must rapidly retract the firing angle to the minimum alpha limit while the AC filters generate excess reactive power. The standard requires that the AC bus overvoltage be limited to less than 1.2 p.u. and that the system re-stabilize within 200 ms. Engineers should note that this test carries inherent risks and should be conducted with protective settings temporarily widened to avoid unnecessary equipment stress.
One of the most critical and frequently problematic tests in IEC 61975 is the load rejection test. The standard requires that the HVDC system withstand a sudden loss of full load (from 1.0 p.u. to near-zero power) without tripping or exceeding voltage limits. For LCC systems, this test can cause severe overvoltage on the DC side (up to 1.3-1.5 p.u.) because the converter continues to draw reactive power from the AC system but the DC power has collapsed. The control system must rapidly retract the firing angle to the minimum alpha limit while the AC filters generate excess reactive power. The standard requires that the AC bus overvoltage be limited to less than 1.2 p.u. and that the system re-stabilize within 200 ms. Engineers should note that this test carries inherent risks and should be conducted with protective settings temporarily widened to avoid unnecessary equipment stress.
2.2 Electromagnetic Compatibility and Power Quality
IEC 61975 incorporates comprehensive EMC and power quality verification procedures. The standard requires measurement of conducted and radiated emissions from the converter station in accordance with IEC 61000-6-4 (industrial environment). Particular attention is given to the telephone interference factor (TIF) and psychometric weighting of harmonic voltages on the AC side, as HVDC converters are well-known sources of low-order characteristic harmonics (11th, 13th for 12-pulse LCC; switching frequency sidebands for VSC).
For VSC-HVDC systems, the standard introduces additional requirements for high-frequency harmonic measurement up to 150 kHz, addressing concerns about power-line carrier (PLC) interference and nearby communication system disturbance. The VSC switching frequency (typically 1-3 kHz for MMC topologies) generates sideband harmonics that can couple into adjacent metallic paths, requiring careful filter design and shielded busbar arrangements.
💻 3. Engineering Design Insights and Practical Considerations
3.1 Comparison of LCC and VSC Testing Requirements
IEC 61975 is notable for being one of the first IEC standards to provide unified testing guidance for both LCC and VSC HVDC technologies, while clearly differentiating the test requirements where the technologies diverge:
| Test Item | LCC HVDC | VSC HVDC | Key Difference |
|---|---|---|---|
| Commutation failure testing | Required (3-phase fault at inverter) | Not applicable (self-commutated) | VSC does not suffer commutation failures |
| Black-start capability | Not required (needs AC commutation voltage) | Required (can energize passive network) | VSC can operate into dead AC network |
| DC fault current contribution | Limited by firing angle control | Must be limited by converter control + DC breaker | VSC requires active DC fault management |
| Reactive power range | Consumes reactive power (typically 50-60% of P | Can generate/absorb (typically ±15-25% of P | VSC provides independent real/reactive control |
| Harmonic filter requirements | Large AC filters (15-25% of station cost) | Minimal AC filters (1-3% of station cost) | VSC has inherently lower harmonic generation |
✅ Testing Best Practice
Experience from recent HVDC projects (e.g., the China State Grid ±800 kV UHVDC links and the North Sea Wind Power Hub VSC projects) has demonstrated that a structured, phased approach to commissioning significantly reduces risks. The recommended sequence per IEC 61975 is: (1) component and subsystem tests, (2) control system HIL validation, (3) station auxiliary systems commissioning, (4) converter energization and no-load tests, (5) low-power operational tests (typically 1-5% of rated power through a back-to-back configuration), (6) progressive power ramp-up in 25% steps, and (7) full-power performance tests. Each phase must be formally signed off before proceeding to the next. This structured approach has been shown to reduce commissioning delays by 30-40% compared with ad-hoc testing methodologies.
Experience from recent HVDC projects (e.g., the China State Grid ±800 kV UHVDC links and the North Sea Wind Power Hub VSC projects) has demonstrated that a structured, phased approach to commissioning significantly reduces risks. The recommended sequence per IEC 61975 is: (1) component and subsystem tests, (2) control system HIL validation, (3) station auxiliary systems commissioning, (4) converter energization and no-load tests, (5) low-power operational tests (typically 1-5% of rated power through a back-to-back configuration), (6) progressive power ramp-up in 25% steps, and (7) full-power performance tests. Each phase must be formally signed off before proceeding to the next. This structured approach has been shown to reduce commissioning delays by 30-40% compared with ad-hoc testing methodologies.
3.2 Documentation and Reporting
IEC 61975 requires comprehensive documentation of all test results, including test procedures (pre-approved by the purchaser), raw measurement data (time-stamped and unprocessed), analysis results with uncertainty calculations, and formal test certificates. The standard specifies that all critical measurements (power, voltage, current, losses) must be traceable to international standards, and the measurement uncertainty must be reported alongside the measured values.
For dispute resolution, IEC 61975 recommends that both the supplier and purchaser agree on the test procedures and acceptance criteria before testing begins, and that a test witness from each party signs off on each test as it is completed. Any deviations from the agreed procedures must be documented as non-conformance reports (NCRs) with root cause analysis and corrective action plans.
❓ Frequently Asked Questions
❔ How does IEC 61975 address the testing of multi-terminal HVDC systems?
The 2016 edition of IEC 61975 introduces specific provisions for multi-terminal HVDC (MTDC) systems, including DC grid testing. Key additional requirements include: DC-side fault isolation verification at each terminal, power flow control accuracy in multi-terminal configurations, DC voltage droop control testing, and communication latency measurement between terminals. For MTDC systems with more than three terminals, the standard recommends supplementary real-time simulation studies covering all credible N-1 contingencies before proceeding to site testing.
❔ What are the required testing durations for availability demonstration?
IEC 61975 specifies a minimum reliability demonstration period (typically 30 days continuous operation at rated power with no unscheduled outages) and an availability calculation based on IEC 61710 (power system availability). The standard requires that the demonstrated availability during the test period must equal or exceed the contractual availability guarantee, typically 98-99% for modern LCC systems and 97-98% for VSC systems (reflecting the higher complexity of VSC valve construction). Any forced outages during the demonstration period reset the clock — the 30-day period restarts from zero after each forced outage.
❔ Can the standard be applied to back-to-back HVDC systems?
Yes. IEC 61975 applies equally to back-to-back HVDC systems (where both converters are in the same station with no DC transmission line). The test procedures are essentially the same, with the simplification that DC line/cable tests are replaced by DC busbar tests between the two converters. Back-to-back systems are typically tested in a circulating power configuration where the rectifier and inverter are connected in a closed loop, allowing full-power testing with minimal energy consumption from the AC network.
❔ What are the requirements for testing during extreme ambient conditions?
The standard requires that the HVDC system demonstrate its specified performance across the full range of expected ambient conditions, including: high ambient temperature (typically up to 45-50 °C for air-cooled valves), low temperature (down to -25 °C for outdoor equipment), high humidity (up to 95% non-condensing), and altitude corrections for installations above 1000 m. For tropical installations, additional solar radiation and monsoon rain tests may be required by the purchaser. Performance derating due to extreme conditions is permitted within agreed limits, but the system must demonstrate stable operation at all specified ambient conditions.
