A Presentation On Cooling And Testing Of Turbo Generators At

BHEL, Haridwar

BySuhani Varshney

2008 UEE 152

Situated in the foot hills of the Shivalik range in Haridwar district of Uttarakhand state

BHEL Haridwar was founded in 1960 with Russian collaboration

Plant is divided into two sections one is Heavy Electrical Equipment Plant(HEEP) and the other one is Central Foundry Forge Shop(CFFP)

About BHEL Haridwar

HEEP CLASSIFICATION

EXECUTIVES SUPERVISORS TECHNICIANS ARTISANS CLERICAL STAFF

CLASSIFICATION OF EMPLOYEES

The two pole generator uses direct water cooling for the stator winding, phase connectors and bushings.

It uses direct Hydrogen cooling for Rotor windings.

The losses in the remaining generator components, such as iron losses, winding losses and stray losses are also dissipated through Hydrogen.

Cooling Processes Involved

The Direct cooling essentially eliminates hot spots and differential temperature between adjacent components which may result in mechanical stresses.

Advantage of Using Direct Cooling

The rotor winding consists of several series connected coils inserted into longitudinal slots of rotor body.

The coils are wound so that two poles, north and south are obtained.

All the conductors having identical copper and cooling duct cross section.

Cooling ducts are arranged so as to allow the cooling gas (hydrogen) to flow through the rotor body to cause its uniform cooling.

Rotor Coil Design

Cooling Duct Arrangement

Rotor Fans The cooling air in generator is

circulated by two axial flow fans located on the rotor shaft at each end.

To augment the cooling of the rotor winding, the pressure established by the fan works in conjunction with the air expelled from the discharge parts along the rotor.

The blades of the fan have threaded roots for being screwed into rotor shaft.

The blades are drop forged with aluminum alloy, the threaded root fastening permit angle to be changed.

The stator bars, phase connectors and bushings are designed for direct water cooling.

The stator bars are arranged with one hollow strand and two solid strand which ensure minimum heat removal capacity and minimum loses.

At the bar ends , all the solid strands are jointly brazed into a connecting sleeve.

The electrical connection between top and bottom bars is made by a bolted connection at the connecting sleeve.

Stator Bar Design

The hollow strands are connected into a water box from which the cooling water enters and exists via Teflon insulating hoses connected to annular manifolds.

The water manifolds are insulated from the stator frame, permitting the insulation resistance of the water filled winding to be measured.

During operation, the water manifolds are grounded.

Hydrogen is circulated in the Generator interior in a closed circuit by a multistage axial flow fan arranged on the Rotor at the turbine end.

The hot gas is drawn by the fan from the air gap

And then delivered to the coolers where it is re cooled and then divided into three flow paths after each cooler.

Hydrogen Cooling Circuit

Flow Path 1: directed into the rotor at the turbine end and below the fan hub for cooling of the turbine end half of the rotor.

Flow Path 2: directed from the coolers to the individual frame compartments for cooling of the stator core.

Flow Path 3: directed to the stator end winding space at the exciter end through guide ducts in the frame for cooling of the exciter end half of the rotor and of the core end portion.

The Three Flow Paths

Hydrogen Coolers The hydrogen cooler is a

shell and tube type heat exchanger which cools Hydrogen in the generator.

The heat removed from Hydrogen is dissipated through the cooling water.

The cooling water flows through the tubes while Hydrogen is passed around the finned tubes.

Hydrogen coolers are subdivided into identical sections which are vertically mounted in the turbine end stator end shield.

The treated water is used for cooling of the stator windings, phase connectors and bushings.

The primary water is circulated in a closed circuit and dissipates the absorbed heat to the secondary cooling water in the primary water cooler.

This circuit is divided into two Flow Paths.

Primary Cooling Water Circuit

Flow Path 1: Passes to a water manifold on the exciter end of the generator and from there to the stator bars via insulated holes.

Flow Path 2 : Cools the phase connectors consisting of thick walled copper tubes through which the cooling water is circulated.

The Two Flow Paths

TESTING OF TURBO GENERATORS

Mechanical heat run Blocked Rotor test No load test OCC and SCC High Voltage test Inter strand test Nitrogen test Thermal Shock test

ROUTINE TESTS ON TURBO GENERATOR

The machine is rolled and run at rated speed after ensuring the bearing oil and kept at rated speed for stabilization of bearing temperatures.

The vibrations are measured at rated speed on both the bearing housings in Horizontal,

Vertical and Axial directions. The temperature of stator is monitored by

monitoring RTDs embedded in core, tooth and winding. The vibrations should be less than 5 microns and noise level should be in between 75-90 Db

MECHANICAL RUN AND MEASUREMENT OF VIBRATIONS AT RATED SPEED:

The machine is prepared for short circuit characteristic using current transformers and

shorting the terminals. The machine is run at rated speed and drive

motor input voltage and current are noted and machine is excited gradually in steps, at 20%, 40%,60%,80%,100% rated current of the machine.

The excitation is reduced and cut off. The speed is reduced and the machine is cooled at

lower speed.

Blocked Rotor Test

The temperature are checked from machine RTD’s. The machine is stopped when it is sufficiently cooled down.

The stator winding temperature should be less than 60 C.

From the Blocked Rotor test, we will get copper losses.

The short circuit characteristics is plotted from SCC results by selecting X-axis as field current and Y-axis as % rated current.

The machine is run at rated speed and the motor input voltage and current are noted and machine is excited gradually in steps, cat 20%,40%,60%,80%,90%,95%,100%,105%,110% and 120% of rated voltage of machine.

At 100% rated voltage the following parameters are noted:

Shaft voltage, checking of phase sequence, bearing vibration, RTD’s readings.

No Load Test

The temperatures are checked from machine RTD’s. The machine is stopped when it is

sufficiently cooled down. The stator core temperatures to be less than 60 C.

From the Open Circuit test, we will get Iron losses.

The Open Circuit Characteristics is plotted on a graph paper from OCC results by selecting

X-axis as field current and Y-axis as % rated voltage.

A variable 50 HZ A.C voltage of single phase is applied across the input leads and readings

of voltage and current are noted down from 50v-200 v in steps of 50V. Impedance is measured by using the

formula: Z = V/IWhere Z = impedance in ohms; V = voltage in volts; I = current in amps Rotor Impedance is measured at standstill

and at rated speed of the machine.

Measurement of Rotor Impedance

The High Voltage is applied to windings by increasing gradually to required value and

maintained for one minute and reduced gradually to minimum. The transformer is switched off and winding

is discharged to earth by shorting the terminal to earth using earthing rod connected to earthen wire.

The test is conducted on all the phases and rotor winding separately.

High Voltage Test

When High Voltage test is done on one phase winding, all other phase windings, rotor winding, instrumentation cables and stator body is earthed.

High Voltage test levels: Stator winding = (2 Ut +1) KV Rotor winding = (10 * Up) V Where Ut = Rated voltage of the machine under test Up = Excitation voltage

This test is basically done to check any short circuit between two consecutive conductors of a bar.

For this test all the bare conductors at both the ends are separated from each other, then a live wire is connected to a conductor and received from a consecutive conductor to light a lamp.

If the lamp lights up this implies that some short circuit is present between two conductors due to improper insulation.

Inter Strand Test

As the name suggests, Nitrogen is the main constituent of this test, is used to check any leakage in the bars, by checking the brazing.

Nitrogen at a pressure of 10Kg/Cm 2 is forced from on end of the bar and other end of the bar is blocked then the pressure of 10Kg/Cm 2, so that a constant pressure is maintained and then both the ends are dipped in water.

If bubbles appear, it implies that the brazing is week.

Nitrogen Test

In this test, hot water and cold water is passed through the Rotor winding alternatively, Hot water at a temperature of 90 and cold water at 27 degree Celsius.

The above process is repeated 26 times so as to check if the generator can withstand the working conditions of rotor rotating at a speed of 3000 rpm.

Thermal Shock Test

Cooling of Rotor windings is done with hydrogen gas and that of stator, phase connectors and bushings is done with DM water.(Direct cooling)

To ensure that all functional requirements are fulfilled, and to estimate the performance of

generator, the TURBO GENERATORS are required to undergo some tests. To determine the efficiency of TG, blocked rotor and no load

tests are performed which determine the Cu and Iron losses respectively.

There are also several tests performed on TG like mechanical test, water test, inter strand test , etc.

There are also several tests which are performed at the generating station before the installation of TG

Conclusion