Main Transformer Accidents and Light Gas Operation Issues

1. Accident Record (March 19, 2019)

At 16:13 on March 19, 2019, the monitoring background reported a light gas action of No. 3 main transformer. In accordance with the Code for Operation of Power Transformers (DL/T572-2010), operation and maintenance (O&M) personnel inspected the on-site condition of No. 3 main transformer.

On-site confirmation: The WBH non-electrical protection panel of No. 3 main transformer reported a Phase B light gas action of the transformer body, and the reset was ineffective. O&M personnel inspected the Phase B gas relay and gas sampling box of No. 3 main transformer, and conducted tests on the core and clamp grounding current of the transformer body.

At 16:36, the substation monitoring background reported a heavy gas action trip of No. 3 main transformer, with Phase B body on fire. The transformer’s fixed foam spray fire extinguishing system activated correctly (signal pictures available).

Measures for This Accident:

• Develop a light gas-to-trip transformation plan: Organize the compilation of technical transformation schemes, arrange subsequent power outage plans, and clarify O&M measures before transformation.

• Special inspection and transformation for in-service transformers: Conduct targeted inspections on operating transformers based on the fault cause, and formulate transformation measures.

2. Light Gas Alarm Handling Process

The Code for Operation of Power Transformers (DL/T572-2010) stipulates that transformer gas relays shall be equipped with two sets of contacts: light gas and heavy gas. Under normal operation, light gas is set to alarm mode, and heavy gas to trip mode. The typical handling process for transformer light gas alarms is as follows:

• When the gas protection signal is activated, immediately inspect the transformer to determine if it is caused by air accumulation, oil level drop, secondary circuit failure, or internal transformer faults.

• If gas exists in the gas relay, record the gas volume, observe the gas color and flammability, and collect gas and oil samples for chromatographic analysis.

• If the gas in the relay is colorless, odorless, non-flammable, and chromatographic analysis identifies it as air, the transformer may continue operation, and the air intake defect shall be eliminated promptly.

• If the gas is flammable or the dissolved gas analysis (DGA) result is abnormal, comprehensively judge whether to shut down the transformer.

New Anti-Accident Measures (9.2.3.6) stipulate: "If a transformer experiences two consecutive light gas alarms within one day, immediately apply for a power outage inspection; for transformers (reactors) withnon-forced oil circulation structure and no oil draining and nitrogen injection device, apply for immediate power outage inspection upon light gas alarm of the body."

Newly commissioned transformers or those undergoing oil treatment are prone to light gas alarms during initial operation. Forced oil circulation transformers are susceptible to air intake due to oil leakage in negative pressure areas; transformers with oil draining and nitrogen injection devices may have gas trapped in oil discharge pipelines—both scenarios can cause light gas alarms. A small amount of gas is normally released during transformer operation, but two consecutive light gas alarms within 24 hours indicate potential severe faults.

3. Statistical Analysis of Transformer Light Gas Alarms

Case 1: Light Gas Alarm at a Substation (July 7, 2015)

Fault Phenomenon: The substation monitoring background displayed "No. 3 main transformer body non-electrical protection alarm" and "Phase C light gas alarm of the body".Cause: On-site inspection found the oil level in the Phase C gas relay of No. 3 main transformer was low (gas volume exceeded 300ml; light gas alarm setting: 270±10ml) (Phases A and B were full). Based on inspection results and special meetings, the fault was caused by metallic foreign objects, which were judged to be introduced during transportation/installation (not from manufacturing), as monitoring could not cover the entire manufacturing process.Disposal: The faulty transformer was withdrawn and replaced with a standby phase. A maintenance workshop was built at the substation for on-site repair by the manufacturer; the repaired transformer was stored as a standby phase.

Case 2: Light Gas Alarm at a Substation (September 30, 2015)

Fault Phenomenon: The monitoring background reported "Phase C pressure surge alarm of #2 main transformer", "heavy gas trip signal", and "light gas alarm signal"; no electrical protection actions occurred.Cause: Communication gaps between the design institute and manufacturer led to excessive stress on the bushing top terminal; the bushing had weak sealing design; a negative pressure zone at the top of the bushing current-carrying conduit caused air/water inhalation after seal failure. The maximum horizontal offset between the lead T-connector and the high-voltage bushing reached 5.61m, leading to long-term lateral tension, deformation of the terminal and cover, seal failure, and internal discharge in the high-voltage winding due to air/water intake.Disposal: The faulty transformer was withdrawn and replaced with a standby phase. On-site repair was completed in 2016 after building a maintenance workshop, and the transformer was stored as a standby phase.

Case 3: Light Gas Alarm at a Substation (June 18, 2018)

Fault Phenomenon: Phase A light gas alarm of #1 main transformer at a substation.Cause: Oil and gas sampling tests showed non-flammable gas and normal oil data. The gas relay was vented on the same day, and the signal reset immediately. Long-term tracking of gas accumulation in the relay showed no new gas, confirming the alarm was caused by long-term air accumulation.

Case 4: Light Gas Alarm During Commissioning

Fault Phenomenon: Phase C light gas alarm of No. 3 main transformer during commissioning.Cause: Oil leakage from the transformer’s oil flow relay; the construction unit failed to fully vent gas after replacement. No acetylene was detected in the transformer.Disposal: Vent gas from the transformer.

Case 5: Light Gas Alarm at a Converter Station (November 20, 2018)

Fault Phenomenon: At 06:24:55, the OWS background of a converter station reported "Phase B light gas alarm of a converter transformer on a certain side"; at 06:40:57, "Phase B heavy gas protection of a converter transformer on a certain side" was reported, and the 01B/02B converter transformer circuit breakers tripped three-phase.Cause: Oil conservator bladder rupture. Oil entered the bladder, and a sudden temperature drop caused the oil-filled bladder to sink, blocking the oil pipeline and trapping gas, leading to gas relay action. The bladder ruptured due to long-term standby and accelerated aging in low temperatures.Disposal: On-site inspection confirmed a bladder tear (most oil in the conservator had entered the bladder). The bladder was replaced, and the converter transformer resumed operation.

Case 6: Light Gas Alarm at a Power Company (January 2, 2017)

Fault Phenomenon: The DC bipolar was in cold standby at a power company; at 20:37, a light gas alarm of the Pole 1 YYC converter transformer was reported, followed by a heavy gas trip at 20:42.Cause: On-site inspection found oil leakage from the No. 4 cooling cycle oil pump of the Pole 1 Y/Y C-phase converter transformer. Leakage stopped after disconnecting the pump power and closing valves at both ends. The root cause was poor material quality of bolts and pump end cover flanges, leading to severe corrosion, bolt breakage, pump body slippage, and massive oil leakage.Disposal: Replaced 4 submersible oil pumps of the Pole 1 YYC converter transformer, refilled oil, and conducted maintenance tests. Synchronously replaced or modified the bolt structure of 52 submersible oil pumps in 13 other converter transformers (including 2 standbys).

4. Explanation of Light Gas Set to Alarm or Trip

4.1 Introduction to Gas Relays

Gas relays operate by detecting gas generated from oil decomposition or oil surges caused by internal transformer faults, triggering light gas (alarm) or heavy gas (trip) contacts.

• Light Gas: Reflects minor faults (e.g., overload heating, local core heating, tank heating from magnetic leakage). Decomposed gas rises into the relay’s gas collection chamber, lowering the oil level and activating the light gas reed switch to send an alarm. Further oil level drop triggers heavy gas.

• Heavy Gas: Reflects severe faults (e.g., bushing grounding, inter-turn short circuit). Rapid gas generation pushes oil to impact a baffle, attracting the heavy gas reed switch via a magnet to trip the transformer.

4.2 Reasons for Light Gas Set to Alarm

• UHV AC Transformers: Each main and voltage-regulating transformer has only one gas relay; bushing risers connect to the body relay via gas collection pipes. Only one light gas alarm contact is available, set to alarm mode under normal operation (heavy gas to trip).

• Converter Transformers: Equipped with 1 gas relay (Siemens technology) or 7+ gas relays (ABB technology). Light gas is set to alarm mode (heavy gas to trip) under normal operation.

Converter transformer gas relays have only 1 or 2 light gas alarm contacts, prone to false alarms caused by water ingress in relay junction boxes, carrier gas from oil chromatography entering the tank, or air intake due to poor body sealing. No "two-out-of-three" anti-misoperation measures are available. If light gas is set to trip, false alarms could cause DC monopole (single valve group) trips, losing 1500MW or more power and threatening grid stability. Additionally, light gas provides a response window for minor faults (e.g., mild core/insulation heating) before heavy gas activation, improving equipment availability. Thus, the 18 Major Grid Anti-Accident Measures and Code for Operation of Power Transformers (DL/T572-2010) stipulate light gas shall only be set to alarm.

False Operation Cases:

• 2003: Water ingress in a converter transformer’s smoothing reactor gas relay caused bipolar block, losing 1281MW.

• 2019: A temporary outage occurred due to a fault in the contact circuit of a converter transformer’s gas relay.

4.3 Proposal to Modify Light Gas to Trip for UHV Transformers

Given the risk of sudden faults in UHV transformers threatening personnel safety, it is proposed to change light gas action from alarm to trip for UHV transformers, for the following reasons:

• Early Action in Severe Faults: Light gas may activate before heavy gas during sudden severe faults. Tripping on light gas can quickly isolate the faulty transformer, preventing major equipment damage or casualties. (e.g., 2016: A UHV shunt reactor exploded after multiple light gas alarms; 2017: Light gas activated 32 seconds before heavy gas during a converter transformer bushing fault.)

• Grid Robustness: The strengthened power grid can tolerate the loss of one valve group or transformer without stability issues.

• Reduced False Operation Risk: Enhanced management of non-electrical relays (e.g., installing rain covers, regular sampling inspections, circuit insulation checks) has significantly reduced false alarms. Statistics show no false light gas actions in converter stations (3 years) and substations (5 years); 6 recorded actions were caused by unvented gas during construction (not equipment faults).

Temporary Measure: During unstable operation of UHV transformers, set all light gas contacts (bushing risers, tap changer, body) of converter transformers, main transformers, and voltage-regulating transformers to trip mode to ensure personnel and equipment safety.

4.4 Implementation Plan for Light Gas to Trip Modification

• Converter Transformers: Modify software via DC control single-system withdrawal to change light gas signals to trip. No trip test is required after modification (signal circuits are verified annually); implement during planned outages (1 day).

• AC Transformers: Modify wiring on protection panels and conduct transmission tests (1 day).