Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC TR 62672: Reliability and Availability Evaluation of HVDC Systems
High-voltage direct current (HVDC) transmission systems are critical infrastructure for long-distance power transfer, offshore wind integration, and interconnecting asynchronous grids. Unlike conventional AC systems, HVDC systems involve complex power electronic converters, sophisticated control systems, and unique failure modes that demand specialized reliability assessment methods. IEC TR 62672 provides a comprehensive framework for evaluating the reliability and availability of HVDC systems, establishing standardized metrics and methodologies that enable consistent performance benchmarking across the industry. This article examines the technical report’s approach and its practical implications for HVDC project engineers.
📋 1. Reliability Metrics and Performance Indicators
IEC TR 62672 defines a set of standardized reliability and availability metrics specifically tailored for HVDC systems. These metrics account for the unique operational characteristics of HVDC, including the distinction between forced outages, scheduled maintenance, and partial-capacity operation:
- Energy Availability (EA): The ratio of actual energy transmitted over a defined period to the maximum possible energy transmission, accounting for both full and partial outages.
- Forced Outage Rate (FOR): The probability that a converter station or HVDC system is in a forced outage state at a random point in time, calculated as forced outage hours divided by total operating hours plus forced outage hours.
- Planned Outage Rate (POR): Analogous metric for scheduled maintenance outages, which in HVDC systems are particularly significant due to the complexity of power electronic maintenance.
- Mean Time Between Forced Outages (MTBFO): A key reliability indicator that captures the average interval between unplanned outages requiring immediate corrective action.
- Mean Time To Repair (MTTR): The average time required to restore an HVDC system after a forced outage, including diagnostic time, spare part procurement, and repair execution.
💡 Engineering Insight: For HVDC systems, the traditional power industry metric of “availability” is often insufficient because it treats all outage states equally. IEC TR 62672 introduces the concept of partial availability factoring — recognizing that a bipolar HVDC system operating at 50% capacity (one pole out of two) may be quite acceptable from a system reliability perspective. The report recommends reporting both full and partial availability separately to provide a more meaningful picture of actual performance.
Key Reliability Parameters for HVDC Components
| Component | Typical Failure Mode | Impact on System | MTBF Range |
|---|---|---|---|
| Thyristor/IGBT valve | Short-circuit, gate drive failure | Pole or valve group outage | 20–50 years per valve |
| Converter transformer | Winding insulation failure, tap-changer fault | Pole or system outage (long repair) | 15–30 years |
| DC smoothing reactor | Insulation degradation, winding fault | Pole outage (long repair) | 20–40 years |
| AC/DC filter bank | Capacitor failure, tuning drift | Partial or full outage | 5–15 years |
| Control & protection system | Hardware failure, software bug | Valve group or pole outage | 3–10 years |
| DC switchgear | Arcing contact wear, mechanism failure | Sectional isolation | 10–25 years |
🔬 2. Evaluation Methodology and Data Requirements
IEC TR 62672 prescribes a systematic methodology for reliability and availability evaluation that covers the entire HVDC system lifecycle, from design through operation:
- System Definition and Boundary Setting: Clearly define the scope of the evaluation — whether it covers the entire HVDC link (both converter stations and the DC transmission line/cable), individual converter stations, or specific subsystems. The report emphasizes that boundary definition significantly influences all subsequent metrics.
- Data Collection and Classification: Outage events must be categorized by type (forced vs. planned), cause (internal vs. external), affected subsystem, and duration. The report recommends a minimum data collection period of three years for meaningful statistical analysis.
- Reliability Block Diagram (RBD) Modeling: Construct RBDs representing the functional architecture of the HVDC system, including series and parallel configurations, to calculate system-level reliability from component-level data.
- Markov State-Space Analysis: For systems with complex dependencies between failure modes and operating states (common in multi-terminal HVDC), Markov modeling is recommended to capture transitions between full-capacity, degraded, and outage states.
⚠️ Critical Consideration: One of the greatest challenges in HVDC reliability evaluation is the scarcity of long-term operational data. HVDC systems have relatively low volumes worldwide compared to AC systems, and individual converter stations can operate for years without a significant forced outage. IEC TR 62672 recommends combining manufacturer test data, field data from similar installations, and expert judgment through structured processes such as Delphi surveys. When data is limited, sensitivity analysis becomes essential to understand how uncertainty in input parameters affects the reliability predictions.
⚙️ 3. Engineering Applications and Design Optimization
The reliability and availability evaluation framework of IEC TR 62672 supports several critical engineering applications:
| Application | Methodology | Engineering Value |
|---|---|---|
| Spare parts optimization | MTBF/MTTR-based inventory modeling | Minimize stockout risk vs. carrying cost; critical for long-lead items like converter transformers |
| Maintenance strategy | Reliability-centered maintenance (RCM) analysis | Optimize planned outage scheduling; balance preventive vs. corrective maintenance costs |
| System design comparison | Availability comparison of alternative architectures | Quantify benefit of redundant converter groups, bypass switches, and valve redundancy |
| Performance guarantee | Availability warranty clauses in EPC contracts | Define liquidated damages thresholds; establish measurement and verification protocols |
| Life-cycle cost analysis | LCC incorporating reliability, O&M, and outage costs | Select design options that minimize total cost of ownership over 30–40 year plant life |
✅ Design Guidance: For bipolar HVDC systems, the standard provides compelling evidence that investing in pole independence (separate valve halls, independent cooling systems, segregated AC/DC switchyards) yields significantly higher system availability than shared-configuration designs. The marginal capital cost increase of 5–10% for independent pole design is typically offset by availability gains of 1–3%, which for a 3000 MW HVDC link can translate to tens of millions of dollars in avoided outage costs over the project lifetime.
🔴 Common Design Pitfall: Failing to account for common-cause failures (CCF) in redundant HVDC configurations. Even with fully redundant converter groups, if both groups share the same control power supply, cooling system, or protection scheme, a single failure can disable both groups simultaneously. IEC TR 62672 emphasizes that RBD models must incorporate CCF factors based on the degree of physical and functional separation between redundant paths. A CCF factor of just 2–5% can reduce the availability benefit of redundancy by 30–50%.
❓ Frequently Asked Questions
Q1: How does IEC TR 62672 differ from IEEE Std 1240 for HVDC reliability?
While IEEE Std 1240 provides general guide for HVDC reliability, IEC TR 62672 offers a more structured quantitative methodology with specific metrics (EA, FOR, MTBFO) and standardized data collection formats. The IEC report also places greater emphasis on partial availability states and provides detailed guidance for application to multi-terminal and VSC-HVDC systems, which were emerging technologies at the time of publication.
Q2: What availability levels are typical for modern HVDC systems?
Industry data referenced in IEC TR 62672 indicates that well-designed bipolar HVDC systems achieve energy availability of 95–98% excluding scheduled maintenance, and 92–96% including scheduled outages. Monopolar systems typically show 2–4% lower availability. VSC-HVDC systems, having fewer moving parts and more redundant valve configurations, are trending toward the higher end of these ranges.
Q3: How should reliability data for new HVDC technologies (e.g., VSC with MMC topology) be handled?
IEC TR 62672 recommends a Bayesian approach: start with prior distributions based on component-level test data and similar existing installations, then update the reliability estimates as operational experience accumulates. For modular multilevel converters (MMC), the report notes that the large number of submodules creates a statistical redundancy that should be modeled explicitly — the failure of individual submodules causes graceful degradation rather than complete pole outage.
Q4: Can IEC TR 62672 be applied to multi-terminal HVDC systems?
Yes, the report provides specific guidance for multi-terminal HVDC (MTDC) reliability evaluation. The key challenge is modeling the interdependencies between multiple converter stations sharing a common DC grid. The report recommends using Markov state-space models for MTDC systems, as RBD approaches become unwieldy with more than three terminals due to the combinatorial explosion of operating states.
