An electrical generator is one of the most efficient large-scale machines. It converts mechanical to electrical energy with an efficiency coefficient of 99%. The remaining 1% can mainly be contributed to heat losses. Direct cooling is only necessary for larger generators with more than 250&nbsp;MVA where the cooling media is introduced via hollow conductors within the stator bars. Generators of approximately up to&nbsp;700 MVA nowadays use exclusively hydrogen (H2) gas as a cooling media due to its superior heat removal properties with very low friction losses and inertness within the stator cooling water chemistry compared to other gases. The H2 gas is not only cooling the stator directly but also the rotor and serves as the gas between stator and rotor to minimize the friction losses. Larger generators with more than 700&nbsp;MVA nowadays or older generators with more than 300&nbsp;MVA additionally introduced direct water cooling. The stator cooling water system (SCWS) is a closed loop equipped with filter(s), strainer(s), coolers, pumps and a demineralizer bypass to remove dissolved impurities. The high purity water is pumped through the copper hollow conductors to remove heat efficiently. H2 gas in these machines is still used for the rotor cooling was well as the gas between stator and rotor.The water chemistry of the stator cooling water is typically of neutral pH and has a conductivity of less than 0.1&nbsp;µS/cm. Two zones of the oxygen (O2) concentration have been established through the last 50 years, one at low dissolved O2 concentration with less than 20&nbsp;ppb, the other with high concentrations of more than 2&nbsp;ppm. The latter is the preferred option for originally equipped manufacturers (OEMs) like GE, Toshiba as well as one of the former Alstom incarnations called Alsthom.To remain at a high O2chemistry regime, air is introduced into the SCWS and monitored continuously. It is with outmost necessity to remove the carbon dioxide (CO2) from the injected air to prevent an acidic environment for the stator cooling water. Acidic conditions cause plugging of the copper hollow conductors, resulting in reduced cooling, reduced insulation lifetime, potential forced outages as well as irreversible damage to the generator.In addition to air, H2 is entering the SCWS via the Teflon®-hoses – a H2 permeable polymer -, connecting the collector ring to the stator bars as well as potential leaks. Due to these leaks, a stator leakage monitoring system should be in place to monitor the H2 leakage rate into the stator bars. It can also be used to regulate and monitor the air injection into the SCWS.Within the last couple of years, several nuclear and fossil power plants in the US suffered from a malfunction of the CO2 scrubber. This caused plugging of copper hollow conductors, resulting in several chemical online cleanings being necessary to not run into a forced outage and prevent severe damage to the generator.The second part of this publication is focusing on a solution to prevent and monitor the introduction of CO2 into the SCWS. The solution is not only monitoring the H2 leakage rate as well as the air injection, but is the first product on the market which also monitors the CO2 concentration of the air injected.This invention can not only be used for high O2 water chemistry but also to monitor the H2 leakage rate in low-oxygen stator cooling water systems, which is the preferred water chemistry in Westinghouse, Alstom and its predecessors (except Alsthom), and Siemens generator systems.

Stator Leakage Monitoring System in Water-Cooled Generators: Problems and Solutions

Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2021/volume/85109

ASME 2021 Power Conference (POWER2021)

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