Turbo generator cooling

An efficient cooling and ventilating system is needed to keep the temperature of the windings and other components of the generator resulting due to various losses, below certain limits depending on the insulation.

Cooling in ac generator

Dissipation in generators appears as heat which must be removed. This heat appears in the armature conductors, field-winding conductors, stator core, rotor surface, and other structural elements of themachine. Cooling of armature and field conductors may be direct or indirect; the difference is directcontact of the cooling medium with the conductor or contact through electrical insulation.

Dissipation:a loss of energy from a dynamic system.Dynamic - Motion

Losses in an ac generator

The three losses found in an AC generator are: -

1.Internal voltage drops due to the internal resistance and impedance of the generator

2.Hysteresis losses

3.Mechanical losses

Cooling methods of turbogenerators

1.Stator winding-indirectly air cooledRotor winding-directly air cooled

2.stator winding-indirectly hydrogen cooledRotor winding-directly hydrogen cooled

3.stator winding-directly water cooledRotor winding-directly hydrogen cooled

Air cooled turbo generator

In this,stator winding is indirectly air cooled whereas the rotor winding and stator core is directly air cooled.This type of cooling is applicable for rating of 30 MW to 60 MW generators.

Hydrogen cooled turbogenerator

A hydrogen-cooled turbo generator is a turbo generator with gaseous hydrogen as a coolant.

Advantages:

1.Reduction in size of machine2.Increase in life of machine

Coolant-An agent that produces cooling.

Hydrogen cooling systemAlthough hydrogen is a very useful medium for cooling the generator internal com-ponents, it is very dangerous if not handled correctly. A dedicated system to handle thesupply and control of the hydrogen atmosphere inside the generator is required. Sincehydrogen is used at generator casing pressures up to 90 psig, the generator is also con-sidered a pressure vessel. This requires various sealing arrangements to keep the hy-drogen inside the machine.

Supply of the hydrogen to the generator is generally provided by an on-site hydro-gen manufacturing plant, or purchased in a pressure container and replenished periodically.

Degas-to remove gasPurge-to clean thoroughly

In addition to the hydrogen, a separate supply system is required for CO2 to purgethe generator of hydrogen during filling and degassing. CO2 is used because it is inertand will not react with the hydrogen. If the hydrogen in the generator were to bepurged with air, this would encroach upon both the upper and lower explosive limitsdue to the combustible nature of a hydrogen/oxygen mixture.

Hydrogen at high purity (above 90%) will not support combustion, and at this level there is no danger of explosion since the explosive range of a hydrogen/oxygen mixture is 4 to 75% hydrogen inair. To prevent the possibility of an explosive mixture when filling the generator withhydrogen for operation, air is first purged from the generator by CO2, and the CO2 isthen purged by hydrogen. When degassing the generator for shutdown, hydrogen isfirst displaced by CO2 and then the CO2 is purged by air. This way, no explosive mix-ture of hydrogen and oxygen can occur. In some rare cases, other inert gases have beenused, such as argon.

During operation, a gas pressure regulator automatically maintains the generatorcasing hydrogen pressure at a preset (rated) value. If hydrogen leaks occur, the pres-sure regulator admits additional hydrogen from the supply system until the predeter-mined pressure is restored. There is always a certain amount of expected leakage intothe seal oil, through minute leaks, permeation through the stator winding hoses, and soforth, but most generators should be capable of continuous operation below 500 cubicfeet per day loss. If the loss increases to 1500 cubic feet per day, the source of the leakshould be investigated immediately and corrected.

A hydrogen gas analyzer is usually present to monitor the hydrogen purity, whichshould be maintained above 97%. Dew-point monitoring is sometimes provided tocontrol the level of moisture inside the generator. The dew point is generally maintained below 10C and should not be allowed to rise above 0C at generator casing Pressure.

Dew point:The dew point is the temperature at which the air will become completely saturated.

Dew Point

the temperature at which air becomes saturated by water vapor when cooled at a given moisture content and a constant pressure.

Saturation is the point at which a solution of a substance can dissolve no more of that substance and additional amounts of it will appear as a separate phase

Inside the generator, the hydrogen picks up heat from the various components as itflows over and through such components as the stator core vents and rotor winding.Then it is routed to pass through heat exchangers inside the generator, where the hy-drogen leaving the cooler outlet side has been reduced in temperature to complete an-other cycle of heat pickup as it goes through the same generator components again.

Seal oil system

Seal-to close completelyJuncture-a place where things joinRating-A number, letter, or other mark that refers to the ability of something

1.To keep the hydrogen inside the generator,various places in the generator must be sealed to prevent hydrogen leakage to atmosphere.

2.One of the most difficult seals to make is the juncture between the stator and the rotating shaft of the rotor. This is done by a set of hydrogen seals at both ends of the machine.

3.The seals may be of the journal (ring) type orthe thrust-collar type, one thing both arrangements have in common is the requirementof high-pressure oil in the seal to do the actual sealing. The equipment that providesthe oil to do this is called the seal-oil system.

Stator cooling water system

The stator cooling water system (SCW) is used to provide a source of demineralizedwater to the generator stator winding for direct cooling of the stator winding and asso-ciated components. The SCW is generally used in machines rated at or above 300MVA. Most SCW systems are provided as package units, mounted on a single platform, which includes all of the SCW system components. All components of the sys-tem are generally made from stainless steel or copper materials.