Upper and lower valve groups in an HVDC arrangement can operate at different potentials to earth. Their transformer phases may share similar apparent power and AC ratios while requiring different insulation interfaces. A common spare must cover the intended installed duty, not merely the nominal AC rating. Apply the HVDC transformer specification framework of IEC/IEEE 60076-57-129 to the stated liquid-immersed equipment [1].
Define the actual bridge arrangement and pole references. Draw the node potential map for each group at normal operation and the relevant qualification or abnormal states. Identify the DC offset and AC waveform on every valve-side lead and bushing.
Compare the installed interfaces
| Compatibility item | Upper-group evidence | Lower-group evidence |
|---|---|---|
| Winding-to-earth potential | Node map and required history | Node map and required history |
| Bushing duty | Combined waveform and component specification | Combined waveform and component specification |
| Oil-end leads and screens | Geometry, clearances and grading arrangement | Geometry, clearances and grading arrangement |
| AC ratio and impedance | Actual tap and winding-pair data | Actual tap and winding-pair data |
| Physical replacement | Phase terminal and cooling/control interfaces | Phase terminal and cooling/control interfaces |
Distinguish a common design from a convertible design
A spare may be built to cover both groups without modification, or it may require specified changes to bushings, screens or leads. These are different replacement strategies. For a convertible unit, define the configuration change and the inspection or tests needed afterward.
Do not assume that increasing one bushing voltage rating automatically covers the transformer winding-to-earth path. The installed screen geometry and internal insulation remain part of the assessment. Likewise, a matching potential map does not establish impedance or bus-interface compatibility.
Screen the spare against both earth-potential envelopes
Assume a 50 Hz sinusoidal valve-terminal AC component of 50 kV RMS, with DC offsets of +100 kV and +300 kV in the two positions. The respective positive crests are 100 + √2 × 50 = 170.71 kV and 300 + √2 × 50 = 370.71 kV. The second is 2.17 times the first despite identical AC voltage. A spare screened only against the lower envelope therefore needs the upper-position insulation review before interchangeability is claimed; waveform duration and polarity history remain separate qualification inputs.
Keep the replacement case traceable
Identify the spare configuration and drawing revision accepted for each group. Record any operating restrictions and the evidence supporting them. Review protection, cooling and instrument interfaces with the same configuration.
If common-spare eligibility remains uncertain, name the missing field map, waveform or installation evidence. Equal MVA is a useful initial comparison; it cannot answer the insulation question that differs between valve-group positions. Use the actual LCC or MMC connection for each replacement assessment.
Retain conductivity and polarization history in the insulation review [2].
References
- IEC/IEEE, IEC/IEEE 60076-57-129:2017, Power transformers – Part 57-129: Transformers for HVDC applications.
- CIGRE JWG A2/B4.28, Technical Brochure 406, HVDC Converter Transformers – Design review, test procedures, ageing evaluation and reliability in service, 2010.