Introduction to the Generator II

The generator is internally cooled by gaseous hydrogen and demineralised stator water, separate heat exchangers being provided for the hydrogen and stator water systems. In order to maintain the hydrogen pressure within the generator casing shaft seals are made gas-tight by pumping oil through them. This arrangement is known as the seal oil system.

Hydrogen gas is maintained within the generator casing at a pressure of 310 kPa and is circulated within the generator by fans mounted on each end of the generator shaft. The hydrogen circulates primarily through the rotor windings, and also around the rotor, stator, casing and rotor-stator gas gap, and through the four internal gas-water heat exchangers, located at each of the four corners of the generator outer frame.

The temperature of the hydrogen gas is maintained constant within the generator by automatically regulating the flow of cooling water through the heat exchangers. Cooling water is supplied from the Unit Cooling Water System and circulates through the hydrogen coolers via inlet and outlet couplings at the base of the coolers.

The hydrogen coolers comprise a bank of water cooled tubes, vertically installed between two water boxes by means of a tube plate. The complete assembly is housed in a rectangular shell. A liquid leakage drain is fitted at the base of each cooler and connected, by means of suitable pipework, to the generator stator fluid leakage detector so as to provide information regarding leakage of cooling water within the system.

The two basic types of temperature detecting devices used within the generator are resistance temperature detectors and thermocouples. Resistance temperature detectors are installed adjacent to the hydrogen gas coolers for detecting the temperature of the circulating hydrogen gas within the generator. Compensating leads bring the output of the detecting devices through conduits and hydrogen gas tight glands to an external terminal board for connection to recorders located in the UCR.

Twelve resistance temperature detectors (RTD’s) are installed within the stator windings between selected conductor bar assemblies of each phase, for detecting the temperature in areas of maximum temperature rise during normal generator operation.

Ten RTD elements are installed adjacent to the hydrogen gas inlet and outlet passages of the hydrogen gas coolers for monitoring the temperature of the circulating cooling medium within the generator assembly.

A thermocouple is installed in the cooling water outlet of each stator bar for monitoring the temperature of the outlet stator cooling water passing from each manifold.

A thermocouple is installed within each end of the rotor bearing close to the metal lining of the bearings to facilitate accurate monitoring of the temperature at the bearing surfaces. The thermocouple leads are brought out through the generator outer end shields and are connected to terminal boards positioned at the ends of the stator outer frame.

The supply of hydrogen to the generator is maintained by a separate control system, which includes pressure regulators and gas controllers. The system includes a means of supplying carbon dioxide to the generator casing in order to purge it during hydrogen filling and emptying operations.

Due to the combustible nature of a hydrogen-air mixture within the range of 4 to 74% hydrogen, precautions are taken to prevent such a mixture occurring. High purity hydrogen does not support combustion and, as long as the purity is above 95%, there is no danger of explosion.

A stator gas analyser is connected to the generator to measure the purity of the gas, which is normally held between 97 and 99%. A drop in purity below 95% causes an “H₂ PURITY LOW” alarm to be initiated in the UCR and a signal to be sent to the data logger.

The hydrogen gas is dried by a drying unit mounted outside the generator, the drying agent being activated alumina, which must be periodically reactivated. 

Seal Oil System

The seal oil system for maintaining the generator rotor shaft gas-tight is generally located on the ground floor below the generator. The system is normally automatically controlled but can be manually.

Owing to the importance of the seal oil system and the risk of explosion associated with its failure, comprehensive back-up facilities are provided in the even of failure due to a loss of AC power. If a failure occurs, an alarm will be raised in the UCR. The oil used by the seal oil system is taken from the turbine oil system.

To retain the hydrogen gas within the generator, radial type seal assemblies which utilise an oil film sealing technique are mounted on the end shields at each end of the stator. The seal assemblies are installed inboard of the rotor bearings.

The seal assemblies form an annular space around the sealing land faces on the generator rotor, and oil under pressure is admitted to the annular space to prevent the escape of hydrogen or the ingress of air.

The seal assemblies comprise two adjacent seal rings. One seal ring faces inside the stator to the hydrogen gas and the other outwards to the air. The two seal rings are located in a seal casing and are separated axially but held together circumferentially by springs which hold them against the casing sides, assisted by seal oil pressure.

Seal oil pressure is regulated to 50kPa above the hydrogen gas pressure within the generator. In each seal assembly, oil passes through the clearance between the two seal rings, and also between the seal rings and the rotor shaft. The oil discharges partly inside to the hydrogen gas, and partly outwards to the air side, finally returning to the main oil tank via the seal oil system. 

Stator Cooling

As opposed to the rotor conductors, which are cooled by hydrogen gas, the stator conductors are cooled by a separate stator cooling water system. The cooling water is contained in a storage tank and pumped through the hollow conductors of the stator. It enters and leaves the conductors near the turbine end and passes from the upper to the lower part of the stator at the generator collector end.

The temperature of the cooled water entering the stator is maintained at 46°C, cooled by one of two external water-water heat exchangers. Only one heat exchanger is in service at any one time.

There are two stator cooling water pumps. The pumps are normally operated automatically but can be controlled manually from the UCR or the stator water control cubicle, located on the GWOC cubicle. A drop of 69kPa in the stator water pressure causes the stand-by pump to start.

The conductivity of the stator cooling water is maintained by a resin type de-ioniser. A rise in the electrolytic conductivity of the demineralised water used in the stator cooling system will cause an alarm to be raised in the UCR