Measurement of hydrogen leakage through stator windings into generator coolant water and oxygenation of the coolant water

#The present invention relates to methods of measuring hydrogen leakage through the stator windings of a generator into the generator coolant water and oxygenating the coolant water. Particularly, the present invention relates to methods of determining hydrogen leakage from a generator core environment into the coolant water flowing through the stator windings of the generator, as an indication of potential electrical insulator damage, while simultaneously oxygenating the coolant water.

#The stator bar windings of generators of a certain size are typically water-cooled. That is, water flows from an inlet coolant water header into flow passages within the hollow copper strand stator bars and then flows outwardly of the generator into an outlet coolant header for flow into a reservoir. The coolant water is supplied the windings via a closed loop system including a heat exchanger and a deionizer. Leaks in the stator windings of water-cooled generators, for example, at the brazed joints of the windings, can eventually cause insulation damage that can affect the reliability and longevity of the unit. Early detection of such water leakage enables strategic testing and repair to be scheduled during minor outages, avoiding costly surprise replacements and extended outages. If early insulation damage is not discovered, the problem can quickly compound itself, as stator components are subject to thermal shock, cycling, corrosion and mechanical vibrations. This harsh environment causes and exacerbates leaks at a variety of locations, though most commonly at series loops and other brazed connections.

#It will be appreciated that in water-cooled generators, a cooling core hydrogen environment is normally maintained at a higher pressure than the coolant water flowing through the stator windings. This pressure difference, combined with stator component permeability, particularly at the brazed joints, causes a slight, barely detectable, flow of hydrogen into the coolant water under normal operating conditions even in a leak-free generator. However, when leaks actually develop, the quantity of hydrogen flowing into the coolant water increases dramatically. By continuously or periodically monitoring the leakage flow of hydrogen into the coolant water, upward trending or step increases in the volume of hydrogen leakage can be used as a reliable indicator of water leaks and the potential for electrical insulation damage.

#FIG. 1 is a schematic representation of a known water cooling system for a generator; and

#FIG. 2 is a schematic representation of a water cooling system for a generator employing the methods of the present invention.

#Referring now to the drawings, particularly to FIG. 1, there is illustrated a water cooling circuit, generally designated 10, for a generator G. As well known, generator G includes a plurality of hollow stator bar windings through which water is circulated for cooling purposes. The cooling circuit through the stator bar windings is schematically illustrated at 12 and includes a water coolant inlet header 14 and a water coolant outlet header 16. The water coolant exiting the outlet header 16 is connected to a reservoir 18 by a water coolant return line 20. A water coolant supply line 22 supplies coolant water via pumps 24 and heat exchangers 26 to a water coolant inlet line 28 directly connected to the inlet header 14. A portion of the coolant water from the reservoir 18 flows through a deionizer 30 in a bypass line 32 which joins the water coolant supply line 22 downstream of heat exchangers 26. As a consequence of this arrangement, water coolant flows through the stator bar windings in a closed circuit.

#Typically, a vent line 33 is disposed at the top of the generator tapped into the water coolant return line 20, the opposite end being connected to the reservoir 18. Vent line 33 is used to break any vacuum in line 20. Additionally, a vent line 34 is connected between the reservoir 18 and a suitable vent opening 35 at the site of the generator for venting the gases above the coolant in the reservoir 18. The vent opening 35 is often spaced a substantial distance, on the order of hundreds of feet, from reservoir 18. While some oxygenation of the water as a result of air inlet through the vent 35 and vent line 34 into the reservoir does occur, the distance between the reservoir and the vent renders any substantial oxygenation of the coolant water at very modest levels. Moreover, if there is a net hydrogen leakage into the water coolant system, the venting of gases from the reservoir would afford substantially a one-way flow from the reservoir to the vent without a reverse flow air for water coolant oxygenation purposes. Additionally, systems of this type typically do not have any mechanisms for indicating potential stator bar winding insulation damage within the generator.

#Referring now to FIG. 2, wherein like parts are indicated by like reference numerals followed by the suffix "a," there is illustrated a closed-circuit water coolant system 10a for the generator Ga with the addition of a system for detecting hydrogen leakage from the stator bar winding core environment into the water coolant, e.g., hydrogen leakage through the brazed joints of the end loop connections of the windings. As indicated previously, detection of the magnitude of hydrogen leakage affords an indication of leakage of water from the stator bar windings and, hence, an indication of potential damage to the generator insulation. To accomplish this, the present invention provides an opening 40 to atmosphere at the water coolant exit or return line 20a from the outlet header. At the location of the opening 40, the coolant water flows downwardly to the reservoir 18a, creating a low-pressure area that induces an inward flow of air. That is, the flow of coolant water from the outlet header 16a to the reservoir 18a aspirates air through the opening 40 into the outlet flow line 20a. The opening may be provided in a line 42 having is a flow meter 44, a gate valve 46, a check valve 48 and a filter 50. The flow meter 44 measures the flow rate of air aspirated in the system and is coupled by an electrical output line 52 to a data acquisition and control unit 54. The opening 40 serves two purposes: (1) it increases the flow of gases from the reservoir through the vent line 34a such that the flow through the vent line will be unidirectional, enabling the percentage of H.sub.2 and thus the escaping hydrogen volume to be determined; and (2) the inflow of fresh air mixes with the coolant water as the water returns to the storage tank, thereby oxygenating the coolant water and providing sufficient oxygen levels to provide undesirable oxide formation on the interior surfaces of the windings. Also as illustrated in FIG. 2, a hydrogen analyzer 56 continuously samples the gas in vent line 34a exiting the system through vent 35a. Analyzer 56 measures the percentage of hydrogen in the air flowing through vent line 34a.