A fire started inside the excitation cabinet of a combustion turbine generator at a coastal peaking plant shortly after the unit reached full load. Within weeks, a second unit at the same plant produced smoke from the same location, and maintenance staff found heat damage at the same location on a third. No relay recorded a fault at the time of the fire. Damage at the identical joint on three units suggested a cause common to the design, installation, or environment.
Simplified Explanation
The generator's 13.8 kV neutral bus inside the cabinet joined a vertical aluminum busbar to a horizontal copper busbar with a bolted connection, covered by a plastic insulating boot. Over time the mating surfaces corroded. Corrosion product is a poor conductor, so the joint's resistance rose and it began to run hot. The heat accelerated the corrosion, and current began to flow through the steel bolts instead of the bus faces. Eventually the joint became hot enough to ignite the boot.
How it works
A bolted bus joint carries current through the contact area between the two mating surfaces. The bolts and washers supply the clamping force and are not meant to carry current. The likely failure sequence was:
- The aluminum and copper surfaces began to corrode at the joint, aided by moisture, a chloride-bearing coastal atmosphere, and the galvanic couple between aluminum and copper.
- Corrosion product built up between the mating surfaces and reduced the true contact area.
- The joint became a high resistance connection (HRC) and its temperature rose under normal load.
- Higher temperature accelerated corrosion, and repeated heating and cooling of steel, copper, and aluminum (which expand at different rates) opened the crevice further and let more moisture in.
- With the bus faces degraded, more current transferred through the bolt hardware, heating it and causing arcing at the washers.
- The joint temperature reached the ignition point of the insulating boot or nearby combustibles.
What the evidence looked like
- Bolts that carried current. Steel bolts and stainless conical washers showed melting and arc damage. Heating of the bolts indicates they were carrying significant current that should have passed through the bus faces.
- Flattened conical washers. The spring (conical) washers on the failed unit were flat, so they no longer maintained clamping force through thermal cycles. On a less-damaged unit they were still conical.
- Corrosion product on the mating surfaces. Laboratory examination found white aluminum oxide at the aluminum-copper interface and blue-green copper chloride deposits. Chlorine was detected under the insulation.
- Degraded plating. Tin plating was generally intact on the copper but compromised where corrosion was present on the aluminum. The equipment had been moved and reinstalled several times, which can damage plating.
- Inconsistent assembly. One unit used a copper spacer in the joint and another did not.
- Moisture path. The bus enclosure was not fully sealed, and insulation-resistance tests on the installed neutral bus failed acceptance values on most phase combinations.
The plasticized PVC boot released hydrogen chloride as it degraded thermally, which likely contributed to the local chlorine found on the parts. Some of the chemistry found after a fire can be a product of the fire itself.
What it means for owners
- It is recommended that owners inspect aluminum-to-copper bolted bus joints in generator, exciter, and neutral cabinets, especially in coastal or industrial atmospheres, and verify that the bar materials and plating are appropriate for the application.
- Specify plating and joint compound on both mating surfaces in accordance with the bus manufacturer and recognized practice (for example, USBR FIST Volume 3-3), and keep the joint preparation consistent across units.
- Replace bolt hardware that does not match the specified assembly. Use spring washers that keep clamping force through thermal cycles, and torque to the specified values.
- Add periodic inspection of booted joints for discoloration, flattened washers, and corrosion. An insulating boot can mask a hot joint during an infrared survey.
- Seal the cabinet and bus enclosure against moisture after confirming that sealing will not create a thermal management problem.
- Set relay clocks correctly and use the highest practical sampling rate. In this case, relay event files had incorrect dates and low sampling rates and could not confirm or exclude an electrical event at the time of the fire.
- Preserve evidence. In this case the owner returned the cables to service after testing, so they could not be examined later.
Standards referenced
- NETA MTS, Standard for Maintenance Testing Specifications (insulation-resistance acceptance values)
- USBR FIST Volume 3-3, Electrical Connections for Power Circuits (plating and joint compound)
- NFPA 921, Guide for Fire and Explosion Investigations (origin and cause methodology)
- Bus and bolt-assembly manufacturer installation and torque instructions
Details have been generalized to protect client confidentiality.