Nitrogen Blanket on CHP Auxiliary Transformers: Continuous Rapidox 2100 Monitoring

On a CHP plant, diagnostic attention usually focuses on the unit’s main power transformer. Yet auxiliary transformers — feeding BFP, ID fans, turbine oil pumps, lighting and MV switchgear — outnumber main units, and one failure can trip the same block. Continuous nitrogen blanket monitoring is a job for Rapidox 2100 from the Rapidox catalogue; portable checks use Rapidox 1100.

Many auxiliary tanks run with a nitrogen blanket over oil to slow oxidation. The blanket works only while oxygen in the gas phase stays low. Air ingress through regulator, dryer or flange is a common cause of “surprise” bad DGA on a “small” transformer.

Why auxiliary transformers drop out of DGA programmes

Factor	Main transformer	Auxiliary
Online DGA	Often installed	Rare
Lab frequency	3–12 months	“When we can”
Nitrogen blanket	Yes	Yes, but no O₂ monitoring
Failure impact	Planned repair	BFP/fan trip, unit stop

Without an oxygen trend, maintenance learns about oil degradation from the lab with lag, while a neighbour cooling pump may already show vibration alarms.

Monitoring architecture

Level 1 — portable walk-down (Rapidox 1100)

For fleets of 6–20 auxiliary transformers:

• monthly O₂ at nitrogen line or headspace valve;
• log blanket pressure and oil temperature;
• if O₂ exceeds nameplate limit — check dryer, regulator, flange leaks.

Suitable where budget does not allow a fixed channel per tank.

Level 2 — continuous control (Rapidox 2100)

For critical station-service units (turbine oil pumps, BFP, header pumps):

• Rapidox 2100 on blanket bypass line;
• O₂ alarm/warning in DCS;
• interlock with “dryer off” and “low N₂ pressure”.

Zirconia sensor in 2100 is designed for process duty without frequent electrochemical cell replacement.

Typical faults and gas signatures

1. Empty N₂ bottle — pressure falls, O₂ approaches atmospheric.
2. Saturated dryer — moisture in blanket, faster oxidation; DGA later shows CO/CO₂ rise.
3. Conservator membrane crack — daily O₂ trend while visual inspection looks normal.
4. Commissioning with air — blanket filled with air instead of N₂; O₂ ≈ 21% from day one.

In all cases an early O₂ trend costs less than emergency auxiliary replacement and auxiliary drive downtime.

Link to main transformer and DGA

Auxiliary units share station resources:

• common DGA lab — headspace Rapidox 3100 speeds sample acceptance;
• same transformer yard — walk-down rules as in bay monitoring.

Scope

For CHP, thermal and hydro plants with oil-filled auxiliary transformers. Oil & gas substations with different nitrogen schemes are out of scope.

Conclusion

A nitrogen blanket without oxygen control is a false sense of security. Rapidox 2100 for critical station-service units and Rapidox 1100 for walk-downs close the gap between scheduled DGA and real operation. Full line at Rapidox.

Request a consultation for your auxiliary transformer fleet.