Amit Jha

COOLING, DRAINAGE & DEWATERING SYSTEMS

IN HYDRO POWER PLANTS

Necessity of Cooling System

- Relative motion between two parts / mediums generates heat due to frictional losses.

- Similarly electric current flowing in a conducting medium/ material, generates heat due to resistive (I2R) losses.

- Unless heat so generated is effectively removed from the affected parts/ medium, temperature of these parts/ medium may rise beyond permissible limits.

- Hence the necessity of having proper cooling arrangements which essentially involves transfer of heat from the affected component to a cooling medium which could be gaseous, liquid or solid form.

Optimizing Cooling Requirements / Arrangements

Cooling Process has to be tackled in following three distinct stages:

Minimization of Losses, Using Materials which can withstand higher temperatures ‘ Deploying effective and efficient cooling arrangements.

Minimization of Losses:

Increase generation Voltage to reduce current (I). Powerformers. As current (I) is a constant parameter for a specified MW rating and

generation Voltage, Resistance (R) can be reduced to lessen heat losses.For this purpose, multiple conductors having very low specific resistance per phase are used, to enhance the current carrying capacity. Standards specify limiting Current Density (A/mm2 ) for conductors.

Minimization of Losses:

Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field.The use of such superconducting materials as conductors in machines would result large savings in size of equipment, amount of conductor use, heating losses and energy required for cooling.

The Iron Losses in the Generator Stator and Rotor Core is reduced by selecting material with low reluctance.

The eddy currents formed by the changing magnetic flux in the core is also minimized by fabricating the core with stampings coated with insulating varnish etc.

One of the commonly used material for stampings is Silicon Steel.

Optimizing Cooling Requirements / Arrangements

Minimization of Losses:

The Bearing Surface is made of Babbit, a lining with very low coefficient of Friction (µ). Lower µ results in lower frictional losses, and therefore requires less cooling.

PTFE (Poly tetra flouro ethylene), commonly known as teflon, has been selected in countries like China and Russia for bearing surface.

Optimizing Cooling Requirements / Arrangements

Selection of Right Materials

Insulating Material - Materials which withstand higher temperatures are selected for insulation. Class F insulation is normally used in the Stator Windings.

Bearing Material - PTFE (Poly tetra flouro ethylene), commonly known as teflon has been selected in countries like China and Russia for bearing surface.

Optimizing Cooling Requirements / Arrangements

Effective Cooling Arrangements

The heat is generated in the components even after using optimized design, proper material selection and manufacturing. The same is required to be transferred effectively and efficiently from the affected Components to the Cooling medium to ensure safe operation of the Plant.

For this purpose, various cooling arrangements for such components have been adopted.

Principle of Operation of Cooling Arrangements

- Work on the Principle of heat transfer by maintaining appropriate temperature gradient between the affected (hot) component and the cooling medium.

- In Hydro-Stations primary cooling medium could be air, oil, water etc. As of now air and oil are primary cooling medium which in turn are cooled by water.

- The arrangements for maintaining various components within safe working temperatures constitute Cooling Water System.

Cooling in Generators

Source of Heat Generation in Electric Generators are:

- Winding (Stator and Rotor)

- Bearings (Thrust and Guide)

- Iron Losses.

- Resistance to Air Flow in the Generator ( Windage Losses)

Cooling of Generators

Cooling of Windings

Most Generators work with conventional air cooling. Heat produced during operation is removed by air circulated through ducts in the stator core. This air in turn is cooled by cold water flowing in Air Coolers mounted on the Stator. However, higher capacity / performance plants require more efficient mode of water cooling. Only 5% of Generators worldwide are equipped with water cooling.

Cooling of Generators

Cooling of Winding (Stator)

Some Manufacturers produce cooling system with stainless steel hollow conductors. Resorting to this quality material instead of conventional hollow copper conductors resulting in whole range of decisive advantages.

- Smaller number of hollow conductors for cooling- Higher Flow Speed- Faster heat removal- No increased losses

Cooling in Generators(Cross Sectional View depicting Air Flow)

Cooling in Generators

(Cross Section of a Hollow Conductor)The hollow conductors used for cooling are fully integrated into the bars and run parallel to the copper conductors. Chemically clean water is used as coolant, which as we know is electrically non-conductive.

Cooling of Generators

Bearings:

Hydro Generators have two kinds of Bearings namely:1. Thrust Bearing – This transfers the load of the Rotor,

Shaft/s and the runner through upper/ lower brackets to the Generator Foundation.

2. Guide Bearing – As the name suggests, it guide the shaft to rotate in the pre-determined shaft axis.

Cooling of these bearings is important for restricting the working temperature to the safe permissible limits, beyond which material deterioration, deformation and consequently unit outage would result.

Cooling of Bearings

Lube Oil acts as a lubricant between the stationary and the moving part of the machine, the lube oil is re-circulated after cooling with air/ water. This in turn, helps to reduce the temperature of the Bearing.

PTFE – Poly Tetra Floro Ethylene is being used widely in bearing surfaces in Russia and China, as it has high thermal capability, lower coefficient of friction and longer life.

Thrust Bearing Section

Cooling Tubes in Bearings

Cooling in Turbine Parts

Turbine Guide Bearings:Similar to the Generator Guide Bearing, the Turbine Guide Bearing also serves the purpose of guiding the shaft in a pre-desired axis.

Turbine Shaft Seals:In case of Reaction Turbines, Shaft Seals are provided to prevent water leakages from the Turbine Pit. Shaft Seals are provided with clean water under pressure, which in turn prevents leakage of water from Turbine to Top Cover / Turbine Guide Bearing.

Material of Shell & Tube Heat Exchangers

Carbon Steel, Stainless Steel, Cu-Ni 90:10, Titanium, incoloy and others.

Generator Coolers – Material CS, SS, Cu-Ni, Brass/ Tubes can be of Copper, Cu-Ni, SS, titanium and others.

Bearing Oil Coolers – Tube, fittings and headers can be of Cu-Ni, SS, admiralty brass, CS, Copper or any other as required.

Cooling of Transformers

AN Air Natural Cooled

ONANOil Natural and Air Natural

OFAF Coolers comes complete

With motor and fan assembly mounted and wired in a fan cabinet.

OFWF (Shell & tube type design)

ONAF (Normal convection design/ Tubes of Copper or Cupro-Nickel (Cu-Ni) etc.

ONAN Transformer

Air Cooled Motor

HVAC

Equipment that are air cooled e.g.

-Excitation Equipment and Other Electronic Panels

-Dry Type Transformers

-Motors for various Auxiliaries

They increase the air temperature by their cooling action. To maintain a safe and comfortable air temperature for the personnel and equipment in the Powerhouse, HVAC System is installed.

Types of Water Cooling Schemes

COOLING WATER SCHEME

SINGLE LOOP COOLING WATER

SCHEME

DOUBLE LOOP COOLING WATER

SCHEME

1. Single Loop CW Scheme

2. Double Loop CW Scheme

Single Loop Cooling Water Scheme

Double Loop Cooling Water Scheme

Cooling Water Scheme adopted for Malana II HEP

Open Loop Cooling Water Scheme

In this scheme, the Cooling Water consists of a single Loop of Circulating CW.

Water from the Source is pumped into the heat exchangers, and thereafter, the hot water is disposed in the tail race or river.

Advantages:

- Simple Scheme with Least Components.

Disadvantages:

- Erosion due to silt present in the water reduces the life of equipment.

Closed Loop Cooling Water Scheme

In this scheme, the Cooling Water consists of two Circuits of Circulating CW.

1. Primary Loop – The water from the penstock or tail race is fed to a heat exchanger (which cools

the Secondary Circuit Water) and is disposed in the tail race.

2. Secondary Loop – De-mineralized Water, is the working fluid of this system which is fed to the heat exchangers of Generator, Unit Bearings

and Transformers and its cooling is done by the Primary Circuit.

Closed Loop Cooling Water Scheme

Advantages:

- The Scheme provides higher reliability from silt damage, as Primary Circuits are made redundant by connecting multiple unit Tail Race.

Disadvantage:

- Higher Operational Costs, as pumps for both the circuits are required.

- Make up water tank is required to compensate the loss of DM Water due to leakage etc.

- Overall System Efficiency is Lower.

Silt Removal through Cyclone Separators

Cyclone separators are devices that utilize centrifugal forces and low pressure caused by spinning motion to separate materials of differing density, size, and shape.

Axial & Tangential Cyclone Separators

Heat Exchangers for Cooling Water SystemMalana – II HEP

Pumps for Cooling Water SystemMalana – II HEP

Drainage System

Drainage System comprise of two types:

1. Station Drainage – The Leakage water from the bushings of guide vanes, Shaft seals, MIV flanges, CW System etc. has to be evacuated out of the Powerhouse.

2. Structure Drainage – This is the seepage water from rocks / soil which enters the cavities such as an underground powerhouse due to gravity and high pressure of rock mass.

Drainage System

The Drainage System comprises of the following components:

1. Drainage Water Sump/ Pit – May be located at different Levels.

2. Drainage Water Pumps (Vertical Turbine or Submersible) with Starters.

3. Valves

4. Pipes and Fittings

5. Level Sensors.

Leakages in Turbine PitThree Methods of removing leaked water from Shaft Seal,

Guide Vane Bushes etc in Turbine Pit:

1. Gravitation Method: Leakage water is lead through embedded pipes by gravity drainage sump. This water is then pumped out to the tail race.

2. Ejection Method: The water is made to pass through a nozzle, which creates a low pressure area, thereby sucking the leaked water to Drainage sump, from where, it is pumped to the tail race.

3. Submersible Pump Unit: In case leakage water is not controllable with above said means, submersible pump sets are sometimes deployed for draining Turbine Top Cover.

Reaction Turbine Cross Section

Dewatering System

Dewatering System is of very high importance in respect to the Powerhouses equipped with Reaction Turbine, as the Runner Centerline is below the Tail Water Level.

The Water present in the draft tube is removed by directing the water into a Dewatering Pit, from where it is pumped out to the downstream of the Tail Race Gate, to avoid back flow.

Dewatering System

The Dewatering System comprises of the following components:

1. Dewatering Sump/ Pit – Is located at the lower most level of the draft tube/ turbine pit.

2. Dewatering Pumps (Choice as per Silt Content in Water)

3. Hand Operated Valves

4. Pipes and Fittings

5. Level Sensors.

Thank You

Air Cooled Transformer

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Air Cooled Electric Motor

FanCooling Fins

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