RESIDUAL LIFE ASSESSMENT OF GENERATOR TRANSFORMERS IN

OLD HYDRO POWER PLANTS

Authored by: Sanjay Srivastava, Chief Engineer (HE&RM), Rakesh Kumar, Director (HE&RM),

R.K. Jayaswal, Dy. Director (HE&RM) & Amit Roy Singal, Asst. Director (HE&RM).

Presented by: Rakesh Kumar, Director (HE&RM), CEA

INTRODUCTION

• Renovation, Modernization, Uprating and Life Extension

(RMU&LE) has been recognized worldwide as a well proven

cost effective technique for improving the performance,

efficiency and reliability of old Hydro Power Plants (HPPs).

• Generator Transformer is one of the most important and costly

component of power generation next only to the generator.

• Any discussion on the topic of Life Extension of the generating

station will not be complete without discussing the ways and

means of Life Extension of the Generator Transformers.

AGEING OF GENERATOR TRANSFORMERS

• Ageing of solid insulation materials like Paper, Press Board,Wood based laminates etc.

• Ageing and degradation of Insulating Oil.

• Wear and tear of moving parts like Tap Changing Mechanism, Pressure Relief Device,

Monitoring equipments etc.

• Corrosion of metallic parts exposed to atmosphere.

In addition to the above, accelerated ageing taking place in the transformer on account of

the following abnormal operating conditions:

• Overloading of Transformers.

• Operation under overvoltage conditions.

• Impact of lightning/ switching surges from the system.

• Operation under short circuit conditions.

TESTS ON GENERATOR TRANSFORMERS• Dissolved Gas Analysis (DGA) of transformer oil.

• Monitoring of water content in transformer oil.

• Monitoring of Capacitance and Tan-Delta values of Condenser Bushing.

• Partial Discharge (PD) measurement.

• Winding Resistance Measurement

• Percentage Impedance Measurement

• Insulation Resistance Test

• Furan Analysis

• Magnetizing Current Test

• Sweep Frequency Response Analysis (SFRA)

• DIRANA Test

• Harmonics Measurement

DISSOLVED GAS ANALYSIS (DGA)

Number of gases are produced as by-products of the

ageing taking place in the insulating materials

1. Hydrogen 5. Acetylene

2. Methane 6. Carbon Dioxide

3. Ethane 7. Carbon Monoxide

4. Ethylene 8. Oxygen

MECHANISM OF GAS GENERATION IN TRANSFORMERS

Partial Discharge in OilLarge amount of H2 and little CH4

Arcing in Oil (Power Discharge)Large amount of H2 and C2H2

Oil Overheating

CH4, C2H6, C2H4 are formed in increasing order of severity

Traces of C2H2 in case of very high temperatures

Cellulose Overheating by Electric Fault

Large quantities of CO & CO2

DISSOLVED GAS ANALYSIS (DGA)Method recommended by CIGRE Task force

Ratio Value Indication

C2H2/C2H6

H2/ CH4

CO2/ CO

C2H2/ H2

>1

>10

>10

<3

>2 (with C2H2>30)

Discharge

Partial discharge

Cellulose overheating

Cellulose degradation

by electric fault

Fault gases diffusing

into main tank from

OLTC

Gas Concentration Ratios

DISSOLVED GAS ANALYSIS (DGA)Method recommended by IEC 60599 (1999)

Case Characteristic fault C2H2/C2H4 CH4/H2 C2H4/C2H6 CO2/CO C2H2/H2 O2/N2

PD Partial discharges (*) < 0.1 <0.2 - - -

D1 Low energy discharges >1 0.1-0.5 >1 - - -

D2 High energy discharges 0.6-2.5 0.1-1 >2 - - -

T1 Thermal fault (<300oC) (*) (*) <1 - - -

T2 Thermal fault (300,700oC) <0.1 >1 1,4 - - -

T3 Thermal fault (>700oC) <0.2 >1 >4 - - -

Cellulose (paper) degradation by

electric fault

- - - <3 - -

Fault gases diffusing into main tank

from OLTC

- - - - >2to3 -

Excessive consumption of oxygen as

a result of oil oxidation and/or paper

ageing

- - - - - <0.3

(*) Not significant

Note: In some countries, the ratio CH4/H2 is used instead of C2H4/C2H6.

Concentration ratio between gases

WATER CONTENT MEASUREMENT

• Hydrogen generated during ageing combines with the free oxygen

present in the oil and forms water. A part of this is absorbed by the

cellulose insulation and the balance remains as moisture in oil.

• As paper is highly hygroscopic in nature, when transformer is filled

with oil, it absorbs the moisture from oil, which affects its insulation

properties as well as reduces its life

System Voltage of Transformer kV Water content ppm, max.

Above Up to and including

- 72.5 20

72.5 245 15

245 420 10

420 800 10

TAN DELTA AND CAPACITANCE ON WINDINGS AND BUSHINGS

• This test is used to assess the condition of the oil and cellulose in terms of

moisture, insulation, etc.

• The advantage of this method is that it identifies the winding (HV or LV)

that has a possible problem.

• Dissipation factor of 0.5% is considered high whereas value of 1% is

considered hazardous for operation of transformer and may require its

replacement if completed its normal operating life.

• Capacitance value of bushing in service need to be compared with

factory test result value for deciding the extent of degradation.

WINDING RESISTANCE MEASUREMENT

• Transformer winding resistance measurement is carried out to

calculate the I2R losses and to calculate winding temperature

at the end of a temperature rise test.

• Winding resistance result should not vary by 5% with factory

test results or duplicate transformer result.

PERCENTAGE IMPEDANCE MEASUREMENT

• The percentage impedance of a transformer is the volt drop

on full load due to the winding resistance and leakage

reactance expressed as a percentage of the rated voltage.

• Test results must verify the nameplate rating. Percentage

error should be within 3%.

INSULATION RESISTANCE TEST

• Insulation resistance tests are made to determine insulation

resistance between individual windings to ground or

between individual windings.

• It shows the condition of insulation.

POLARIZATION INDEX TEST

• It is a ratio of the Insulation Resistance Measured for 10 minutes

to the insulation resistance value measured after 1 minute.

• PI is a good indication of winding contamination, moisture

ingress (leakage currents), and/or bulk insulation damage

(conduction currents).

• Acceptable PI for Power Transformers is greater than 1.5. For

Distribution Transformer, it should be at least 1.3.

DEGREE OF POLYMERISATION (DP)

• Remnant life of a transformer in service depends mainly on the

degree of degradation of the insulating materials

• DP is the average molecule size and closely relates to the

mechanical properties of paper

• Early and precise diagnosis of degradation not possible through

DGA.

• IEEE C57.91 loading guide mentions that one should be able to

operate a transformer well dried, oxygen free, and with 110 deg C

hotspot for 135,000 hours until 25% of retained tensile strength

(75% loss), or 150,000 hours until a DP of 200 is reached (80% loss

of tensile strength).

LIFE ASSESSMENT BY DP METHOD

DEGREE OF POLYMERISATION (DP)

• Test is invasive in nature.

• Sample can be taken only after taking the equipment out of

service.

• Taking paper sample is a tedious job.

• Collected sample may not be representative of entire insulation.

Limitations

FURAN ANALYSIS

• The concentration of furans in the mineral oil of a power transformer

is a good indicator of the condition of the cellulose materials such as

paper, pressboard and cotton tapes used for insulation.

• Not an oil quality test.

• Historically used as a major input for RLA of transformers.

• Furan analysis is used as a supplement along with DGA & DP

tests

• Limiting value of total furanic content is

Good Health : <3mg/Kg

Satisfactory health : 3-5 mg/Kg

Deteriorated health : >5mg/Kg

MAGNETIZING CURRENT TEST

• Magnetizing current test of Transformer is performed to locate

defects in the magnetic core structure, shifting of windings, failure

in turn-to-turn insulation or problem in tap changers.

• Useful in distinguishing between severe and less severe cases.

• An agreement to within 25% of the measured exciting current with

the previous test is usually considered satisfactory.

• If the measured exciting current value is 50% higher than the value

measured during pre-commissioning checks, then the winding

needs further analysis.

SWEEP FREQUENCY RESPONSE ANALYSIS (SFRA)

• This test passes a range of frequencies (between 10 Hz to 2 MHz)

through the transformer and then calculates the transfer function.

From these responses, the mechanical condition can be assessed.

• Reliable and sensitive method to evaluate the mechanical and

electrical integrity of transformers active part.

• FRA is also carried out periodically for detection of deformation of

smaller magnitude, when transformer experiences several short

circuit forces.

• Helps in monitoring health and condition assessment of

transformers.

DIELECTRIC RESPONSE ANALYSER (DIRANA) TEST

• DIRANA determines the water content in paper without the need of oil sampling.

• It combines Frequency Domain Spectroscopy (FDS) and Polarization Depolarization

Current (PDC+)

• The power dissipation factor of the insulation is measured over a wide frequency

ranging from the µHz to the kHz region. The resulting curve contains information

about the insulation condition.

• The very low frequencies contain information on moisture in the solid insulation, while

the position of the slope in the mid-range frequencies indicates the conductivity of the

liquid insulation.

• DIRANA is the only device which compensates the influence of ageing by-

products. Otherwise, this would lead to an overestimation of the water content in

aged transformers.

HARMONICS MEASUREMENT

• The harmful effects of harmonic voltages and currents on

transformer performance often go unnoticed until an actual failure

occurs.

• Application of non-sinusoidal excitation voltages to transformers

increase the iron losses in the magnetic core of the transformer.

• A more serious effect of harmonic loads served by transformers is

due to an increase in eddy current losses.

• The increase in transformer eddy current loss due to harmonics

has a significant effect on the operating temperature of the

transformer.

TYPICAL CASE STUDY

Insulation section

Insulation

Resistance

60sec (G)

Polarization

Index

Tan

(%)

Moisture content

(%)

HV Winding versus LV

Winding connected to

Grounded Tank

0.0724 0.98 11.016 5.11

HV Winding versus LV

Winding (Ungrounded)0.0907 0.99 11.474 5.40

LV Winding versus HV

Winding connected to

Grounded Tank

0.0367 1.00 9.786 5.65

Oil Test Parameters within permissible limits

DGA High concentration of ethylene

Furan Analysis 4.8 mg/Kg

Test Results

RLA Study on 5.888MVA, 6.6/66kV Generator Transformer in about 40 Year Old Hydro Power Plant

OBSERVATIONS ON TEST RESULTS• Insulation Resistance shows poor condition of insulation. PI values less than 1

are questionable.

• Limit of Tan Delta for aged transformer is 1%. Tan Delta values for this

transformer are very poor. Results largely exceed normal limits as per service

life and indicate over ageing of transformers.

• Moisture content shows that the transformer is in wet condition.

• The oil test results are normal and the oil parameters are within permissible

limits as per IS 1866:2000.

• The DGA results exhibits high concentration of ethylene gas indicating

possible overheating or thermal fault in the transformer.

• Furan analysis shows presence of high quantity (4.8 mg/Kg) of furanic

compounds.This shows poor health of paper insulation.

Thank You