Problems with the turbine ground brushes

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Siemens 53,000 kW condensing steam turbine air compressor unit, equipped with a reducer; steam flow rate of 150 t/h, pressure of 8.9 Mpa, temperature of 530°C. The high-pressure side of the turbine is fitted with grounding brushes. The initial service life of these brushes is around 6 months; after 4 years, their lifespan shortens to about 3 months. During the most recent inspection, all bearings of the air compressor showed signs of discharge corrosion. The bearings closest to the brushes were in better condition, while the further away they were from the brushes, the more severe the electrical erosion became (from closest to farthest: turbine – reducer – large gear box – air compressor rotor). The brushes were never replaced only when they were completely worn out; instead, they were replaced online as soon as they were nearly worn out. The compressor rotor is in good condition; the turbine rotor blades have slight scaling. Severe electrical erosion is present in the gear pairs of the reducer, the bearings of the reducer’s high-speed shaft, and all the bearings of the compressor. This is not the first time it has been disassembled; previous disassemblies did not result in such serious issues. I would like to ask: 1. Under what circumstances does the amount of charge generated by shaft system friction increase? 2. Theoretically, brushes never run out of wear; then why can’t the charge be conducted away smoothly? If the charge is released from locations other than the brushes due to a change in the conductivity of the oil film, are there any conductivity-related indicators in oil analysis? 3. In a turbine-compressor unit like this, where does the charge mainly arise from – friction on that side? Where should the location with the highest potential be? On the turbine side, is the potential higher on the high-pressure side or the low-pressure side? 4. Do any friends have examples of adding additional brushes in other locations for me to refer to?