Retrofitting vs. building for DGA
For a new transformer project, the clean path is to reserve the oil sampling port and installation space at the equipment tendering stage and commission the online dissolved gas analysis (DGA) monitor together with the main transformer. For the far more common case — a transformer already in service — the retrofit is done live.
The whitepaper describes in-service retrofit as a live installation: the oil circuit connects through the transformer’s existing oil sampling or filling valve, the system consumes no oil and does not alter oil flow, and installation does not interrupt main-transformer operation. Retrofit projects in China must also be validated against the criteria of GB/T 7252 and the online-monitor specification DL/T 722.
The closed-loop oil circuit: take, degas, return
The constraint that governs everything is the oil circuit. The design principle is a closed-loop bypass: oil is drawn from the sampling port, passed through the degassing module, and returned to the transformer — sample → degas → return. Four rules define a safe retrofit oil circuit:
- No oil consumption — no stage may exhibit net oil take-up; the oil returns to the tank.
- No flow disturbance — the transformer side should perceive only a stable, controllable bypass oil flow; the device must not alter the oil-flow distribution inside the transformer.
- No air or moisture ingress — the circuit is sealed, with a bubble trap and venting design at the inlet so free gas does not reach the degassing chamber and cause measurement spikes.
- Live-installation safety — the work respects live-line-work rules, and a leak-tightness check follows connection.
Sampling-port selection
Before any connection, the site survey settles three things: the sampling port, the cabinet location, and the communication path. The whitepaper’s guidance for the sampling port is specific:
- Locate it on the lower return-oil side of the transformer, where oil flow is sufficient and dead-oil zones are avoided — this ensures sample representativeness and response speed.
- Check the valve-port diameter and oil level, which determine the natural differential pressure of the oil circuit; adjust installation height or add auxiliary means as needed.
- Verify on-site explosion-proof and fire-protection zoning, plus lightning-protection and grounding requirements, before fixing the cabinet position and communication route.
Commissioning sequence
Commissioning is treated as a system-engineering task, in four steps:
- Connection and safety — connect the closed-loop circuit with bubble trap and flow control, perform a full purge and line flush to clear historical oil samples and bubbles, then a leak-tightness check.
- Power-on and self-test — verify the laser, photoacoustic cell, microphone, and temperature control, and record the background-noise baseline.
- Calibration — calibrate the full gas path and all detection channels with standard gas.
- Integration — verify the communication point tables item by item (MODBUS, IEC 61850 MMS/GOOSE, IEC 60870-5-104, DNP3.0, and so on) and test the multi-level alarm reporting and interlock logic.
Field calibration and offline laboratory cross-check
The credibility of online data is established by comparing it with laboratory analysis, not by trusting a spec sheet. During early service, the recommendation is to take offline oil samples simultaneously for laboratory analysis — sampling per IEC 60567 and comparing per the interpretation criteria of IEC 60599. Formal operation begins only after online and laboratory results confirm consistency.
It helps to know the yardstick: the total uncertainty of laboratory analysis is on the order of ±15%. Online and offline measurements in the same order of uncertainty are considered consistent; the goal is a stable systematic deviation over time, which is what makes trend and gas-generation-rate judgment meaningful.
Operation and calibration after commissioning
A laser photoacoustic spectroscopy (L-PAS) system such as the DGA-900 has no carrier gas and no routine consumables, so the ongoing burden is modest, but it is not zero:
| O&M item | Recommended period | Content |
|---|---|---|
| Standard-gas calibration | 6–12 months | Verify per-channel linearity and drift; correct if out of tolerance |
| Metrological traceability | Annually or per procedure | Trace values to national metrology standards; third-party verification when necessary |
| Degassing and oil-circuit inspection | 6–12 months | Flow, seal integrity, membrane condition (the membrane is long-term stable, not a routine consumable) |
| Remote inspection | 1–3 months | Data completeness, alarm records, self-diagnostic results |
The 1–3 month trial acceptance
Before acceptance is signed, a 1–3 month trial operation period is recommended. During the trial, the operator watches trend curves, data completeness, and offline-comparison deviations; acceptance is signed after the agreed indicators are met. This trial is the practical proof that the retrofit did not disturb the transformer and that the data is trustworthy enough to drive maintenance decisions.
PAS DGA for energized-transformer retrofits
PAS DGA’s DGA-900 9-gas plus moisture monitor is designed for live installation: a closed-loop oil circuit, no oil consumption, no flow disturbance, and standard-gas calibration rather than consumables. For deployment planning, the considerations in how to select an online DGA monitor apply, and IEC 60599 vs. IEEE C57.104 explains how the acceptance criteria are anchored to standards.
Contact PAS DGA for a site survey and a retrofit commissioning plan for your in-service transformers.