Introduction to the Generator II

9.3. Liquid Cooling

As the output rating of generators increases so the use of water cooling for heat removal in the stator becomes more effective. Stator Water Cooling is additional to the cooling still maintained by hydrogen circulation around the stator and through radial cooling passages within the stator.

In a water-cooled stator, demineralised water is circulated through dedicated pathways built into the stator laminations. The stator cooling circuit passes through the entire length of the stator and is connected to the stator water supply and return manifolds through insulated hoses (PTFE or other suitable plastic).

The stator water system usually consists of the following components:

Stator Water Pumps and Circuit Components

The stator water cooling system is critical for maintaining the generator within the limits of design operating temperatures and would normally demand redundancy in its pumps.

The stator water pumps are usually ac motor driven centrifugal pumps and are provided to circulate the stator water through the closed stator water system at a head pressure slightly lower than that of the hydrogen pressure contained within the generator frame.

This has the effect of allowing gas to leak into the stator water system rather than permitting stator water to migrate into the generator frame.

Small and insidious amounts of hydrogen gas entering the cooling water circuit may be detected by the incorporation of float chambers above the generator inlet and outlet manifolds and at the pump suction. Float operated alarm contacts are made when a set volume of gas displaces the water in the chamber. The frequency of alarms and the amount of gas vented from the system to clear the alarm aids in determining the size of the leak.

Filters are provided to remove any entrained particulate from the system.

Heat from the Stator Water System is dissipated to atmosphere through a heat exchanger. Usually the heat exchangers use lower quality cooling water as the cooling medium. The supply pressure of the auxiliary cooling water is lower than that of the stator water system to reduce the likelihood of stator water contamination if a leak in the heat exchanger occurs. 

Stator Water Flow and Differential Pressure Measurement

In order to monitor the stator water system’s performance it is common to include of flow measuring station in the supply line to the stator. If stator water flow falls momentarily an alarm is initiated and the standby pump is called into service, if the flow fails an alarm and trip is initiated.

A differential pressure monitoring station, located between the generator stator water inlet and outlet manifolds, serves to detect excessive leakage within the generator frame and initiate a trip of the generator. 

Stator Water Conductivity Monitoring

The quality of the demineralised water must be high to prevent current flow through the system. Metal pickup from the stator pipework can result in an increase in conductivity with time. The conductivity of the stator water is normally monitored and the provision of a side-stream, mixed bed, polishing column allows any dissolved solids to be removed and the conductivity to be maintained within design limits. 

Alternatives to Stator Water Cooling

Some modern generators are oil cooled. The stator winding, the core, busbars, terminals, and structural members of the stator are cooled with a fire-resistant dielectric liquid. The cooling medium, special oils, can be mixed and a fire-resistant quality can be applied to the mixture. The rotor winding has direct cooling with distillate.

The fire-resistant dielectric liquid and distillate circulation is provided by pumps, on a closed-loop system, with the cooling liquids being cooled in cooling liquid/water heat exchangers.

Small generators (70MW) have been developed (1970-2000) using high and low temperature superconductors in the stator and rotor windings. These winding have employed liquid and gaseous helium, liquid nitrogen and liquid argon as the cooling medium.

Figure 8:  Stator Water System