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IEC 62751-2-2014: Power Losses in VSC Valves for HVDC Systems – Modular Multilevel Converters
This standard specifies a detailed methodology for calculating power losses in voltage sourced converter (VSC) valves employing modular multilevel converter (MMC) topology for HVDC systems, covering conduction losses, switching losses, DC capacitor losses, snubber losses, and valve electronics consumption.
1. MMC Valve Loss Categorization and Calculation Principles
IEC 62751-2-2014 addresses the unique loss mechanisms in modular multilevel converter (MMC) valves, which differ fundamentally from two-level VSC valves covered in IEC 62751-1. In MMC topology, each valve consists of multiple building blocks (submodules), typically in half-bridge or full-bridge configuration, each containing its own DC capacitor. This modular structure creates distinct loss distribution patterns that must be carefully analyzed.
The standard categorizes valve losses into five principal groups:
Conduction losses dominate in MMC valves and arise from the on-state voltage drops across IGBTs and anti-parallel diodes during current conduction. The standard provides detailed formulas for calculating these losses based on device characteristics (VCE(sat), VF) and the conduction angle determined by the modulation strategy. For MMC building blocks, conduction losses are calculated per submodule and aggregated across all inserted modules in each valve branch.
| Loss Category | Primary Components | Typical Contribution (%) | Key Parameters |
|---|---|---|---|
| Conduction losses | IGBT VCE(sat), diode VF | 55–70 | Valve current, modulation index, submodule count |
| Switching losses | IGBT Eon, Eoff, diode Erec | 20–30 | Switching frequency, DC voltage, junction temperature |
| DC capacitor losses | ESR of capacitor bank | 3–8 | Capacitor ripple current, ESR, submodule voltage |
| Snubber losses | RCD snubber circuits | 1–3 | Snubber capacitor, switching frequency |
| Valve electronics | Gate drivers, control circuits | 1–5 | Off-state voltage, auxiliary power supply topology |
Unlike two-level converters where switching losses dominate, MMC valves typically have conduction losses as the largest component due to the higher number of semiconductor devices in the conduction path at any instant. The valve current flows through all inserted submodules within a branch simultaneously, making conduction loss optimization critical for MMC efficiency.
2. Detailed Loss Calculation Methodology
IGBT conduction losses are calculated by integrating the instantaneous collector-emitter voltage (VCE) multiplied by the collector current (IC) over the conduction period. The standard accounts for the temperature dependency of VCE(sat), typically modeled as VCE(Tj) = VCE0 + rC · IC + kT · (Tj − 25 °C). Similarly, diode conduction losses use the forward voltage characteristics VF(Tj) = VF0 + rF · IF.
Switching losses are calculated using device switching energy curves (Eon, Eoff for IGBT; Erec for diodes) provided by the manufacturer. The standard specifies that these values must be corrected for the actual DC voltage, current, and gate resistance at the operating point. For MMC valves, the switching frequency per device can be significantly lower than the equivalent two-level converter due to the multilevel voltage waveform, which is a key efficiency advantage.
DC voltage-dependent losses account for leakage currents through the semiconductor devices in their off-state and the discharge resistors across the submodule capacitors. These losses are proportional to the square of the DC voltage and are particularly relevant during low-load or standby operation.
Engineering Insight: The third harmonic injection technique, described in Annex A, can significantly increase the AC voltage utilization of MMC converters without increasing the DC voltage. By injecting a third harmonic component into the modulation reference, the effective modulation index can be increased by up to 15 %, reducing the valve current for the same power transfer and consequently reducing conduction losses by 5–10 %.
3. Total Valve Losses Per HVDC Substation
The standard requires that total valve losses be reported per HVDC substation under specified operating conditions, including full load, partial load (typically 50 % and 75 %), and no-load (standby) operation. The calculation must account for:
- All six valve branches (three phases, two arms per phase) in the MMC configuration
- The circulating current within each phase that flows between the upper and lower arms, which contributes additional conduction losses beyond the fundamental AC current component
- The arm inductor losses, which are part of the valve assembly and subject to the same loss reporting requirements
- The auxiliary power consumption of valve electronics, including gate driver units, submodule controllers, and fiber-optic communication interfaces
The standard specifies an informative annex with a complete worked example demonstrating the loss calculation for a hypothetical MMC valve with five submodules per building block, illustrating the step-by-step process from device data to total station losses.
| Operating Condition | Conduction Losses (MW) | Switching Losses (MW) | Other Losses (MW) | Total per Station (MW) |
|---|---|---|---|---|
| Full load (100 %) | 4.2 | 1.8 | 0.7 | 6.7 |
| Partial load (75 %) | 3.1 | 1.5 | 0.6 | 5.2 |
| Partial load (50 %) | 2.0 | 1.1 | 0.5 | 3.6 |
| No load (standby) | — | — | 0.15 | 0.15 |
A common engineering oversight: When aggregating submodule losses to total valve losses, the circulating current contribution should not be neglected. Even though it does not contribute to active power transfer, the circulating current (typically 10–20 % of rated arm current) flows through all semiconductor devices in the arm and can increase total conduction losses by 3–7 %. Accurate loss models must include second-harmonic circulating current suppression control in the modulation strategy.
4. FAQs
Q: How does MMC loss distribution differ from two-level VSC?
A: MMC valves have lower switching losses per device (due to lower effective switching frequency) but higher conduction losses (more devices in the conduction path). Overall system efficiency at rated power is typically comparable, but MMC shows better partial-load efficiency due to the ability to bypass submodules at light load.
Q: What is the significance of the submodule capacitor voltage ripple on losses?
A: Capacitor voltage ripple (typically 5–10 % of nominal) affects the DC voltage-dependent losses and creates additional stress on semiconductor devices. Higher ripple increases the peak blocking voltage requirement and may increase switching losses. The standard recommends including a 10 % voltage margin in loss calculations.
Q: Can this standard be applied to HVDC systems with full-bridge submodules?
A: Yes, Annex A.6.2 specifically addresses full-bridge MMC building blocks, which have twice the number of semiconductor devices per submodule compared to half-bridge configuration. The conduction loss calculation is extended to account for the additional devices in the current path during both active and reactive power operation.
Q: How are valve electronics power losses measured or estimated?
A: Two methods are specified: power supply from off-state voltage across each IGBT (calculating losses from the leakage current and voltage divider network) and power supply from the DC capacitor (measuring the quiescent power consumption of each submodule controller). Typical submodule electronics consume 15–50 W, including gate drive, optical communication, and protection circuits.
