An LCC converter-transformer specification cannot be transferred to an MMC station by matching DC power or AC winding voltage alone. Rebuild the connection, valve-winding-to-earth stress and disturbance schedule from the selected converter. IEC/IEEE 60076-57-129 covers liquid-immersed HVDC converter transformers, including back-to-back applications; it separates this equipment from industrial converter-transformer applications. [1]
Translate the converter topology into winding duty
LCC bridges depend on AC commutation and commonly use phase-shifted transformer groups. MMC operation depends on its submodules, modulation and control. The resulting transformer requirements must follow the actual terminal connections and waveforms. The HVDC review distinguishes LCC and VSC transformer AC/DC stresses and explicitly notes that some VSC alternatives also have substantial DC stress. [2]
| Requirement | LCC evidence to request | MMC evidence to request |
|---|---|---|
| Connection and phase displacement | Bridge-group and winding-terminal map | Selected converter/transformer connection; phase shift only where required by that design |
| Winding heating | Valve-winding spectra before group cancellation, taps and load cases | Transformer-port spectra including modulation and operating modes |
| Earth-referenced insulation | AC plus DC potential at each valve group, including reversal history | Actual common-mode, earthing and fault-induced potential |
| Disturbance schedule | Commutation-related current/voltage events and clearing | Submodule-dependent fault paths, controls and clearing |
Compare equal AC ratings with different earth potentials
Assumptions: two proposed station models give one valve-winding terminal 100 kV RMS sinusoidal AC at 50 Hz. In the LCC case its potential to earth is uL = 200 + 100√2 sin(2π50t) kV. In the particular grounded MMC case it is uM = 100√2 sin(2π50t) kV. These are assumed node waveforms, not universal topology ratings.
The LCC terminal ranges from +58.6 kV to +341.4 kV, with a +200 kV mean. The MMC terminal ranges from −141.4 kV to +141.4 kV, with zero mean. Thus equal AC RMS values coexist with a 2.414-fold difference in maximum absolute earth potential and different sustained DC stress.
Write separate acceptance inputs for the two designs
For this LCC case retain the combined AC/DC earth-stress requirement and the relevant polarity history. For this MMC case require its specified common-mode and temporary fault waveforms before setting insulation tests; its zero normal-state mean cannot establish zero fault-state DC stress. Assign test levels and procedures from the applicable contract and verified requirements, rather than using the calculated operating peaks as test voltages.
Keep both designs’ thermal, mechanical and dielectric verification, but replace copied bridge-group or commutation clauses with the mechanisms that exist in the selected station. Require transformer-terminal evidence: MMC arm currents and cancellation at an LCC grid terminal do not directly supply the individual winding’s current spectrum.
References
- IEC/IEEE, IEC/IEEE 60076-57-129:2017, Power transformers — Part 57-129: Transformers for HVDC applications.
- A. Alassi, S. Bañales, O. Ellabban, G. Adam and C. MacIver, HVDC Transmission: Technology Review, Market Trends and Future Outlook, Renewable and Sustainable Energy Reviews 112, 2019, sections 3.1–3.2. DOI: 10.1016/j.rser.2019.04.062.