CASE STUDIES: FAILURE ANALYSIS

OF SHORT CIRCUIT GENERATORS

Presenter

Arun Kumar Datta Engineering Officer

Operation & Maintenance Division, Switchgear Testing & Development Station

Central Power Research Institute (CPRI), Bhopal, India.

POWER SYSTEM

GENERATION

TRANSMISSION DISTRIBUTION

GENERATION

POWER SYSTEM EQUIPMENTS

• GENERATOR

• BUS DUCT

• CIRCUIT BREAKER

• TRANSFORMER

• DISCONNECTOR

• CT, PT, RELAY

• LINE CONDUCTOR

• TOWER, etc.

FAULTS IN POWER SYSTEM • SINGLE LINE TO GROUND

• DOUBLE LINE TO GROUND

• THREE PHASE

• LINE TO LINE

• SIMULTANEOUS

• OPEN CIRCUIT

Equipments connected in the complex system are prone to

see a few short circuits in its lifetime. Hence these

equipments should be subjected to short circuit test, prior

to finalize their design. CPRI laboratories at Bhopal

create this short circuit condition and supply the fault

power to the equipment under test.

WHY TESTING?

Short circuit test at CPRI laboratory Conventional Short circuit test

SHORT CIRCUIT

• High power flow from the appended equipment till the fault point.

• Huge wastage of energy in the form of heat and force.

• Instability in the grid may cause failure, etc.

SHORT CIRCUIT CURRENT & POWER

If V is the rated voltage applied and Z is the transformer impedance referred

to the primary side (for simplicity the source & external circuit impedances

are neglected) then, in secondary short circuited condition, current drawn

from the source side will be

Isc = V = V = I x 100 = Rated current x 100

Z %Z x V %Z %age impedance

100 I

Short circuit Power in VA = V x Isc.

Example: A 1-ph, 13.2MVA, 132kV/33kV, 100A(pri) transformer with %age

impedance 5, will draw

100x100 = 2000A in the primary under secondary

5 short circuited condition.

Short circuit power drawn

= 132000 x 2000 = 264x106 VA or 264MVA

Where as the test transformer was only of 13.2 MVA rated.

SHORT CIRCUIT FORCES

7

BROKEN INSULATOR

8

SHORT CIRCUIT FORCES

IMPACT OF SHORT CIRCUIT

IMPACT OF SHORT CIRCUIT

IMPACT OF SHORT CIRCUIT

IMPACT OF SHORT CIRCUIT

IMPACT OF SHORT CIRCUIT

SOURCES OF SHORT CIRCUIT POWER

Power requirement during short circuit test is very high. This power can be drawn directly from grid or from S.C. generator.

100MVA ONLINE TESTING STN.

• Drawing power from 132kV grid and further stepping down .

• Grid is loaded during test.

SOURCES OF SHORT CIRCUIT POWER

1250MVA SHORT CIRCUIT TESTING STATION

• Two specially designed 3 phase Alternators capable of supplying required fault power to test equipment (ratings 1500MVA, 12.5kV, 69kA, 3000rpm, 50Hz each).

• The supply grid is not loaded during the short circuit period.

SHORT CIRCUIT GENERATOR

CONSTRUCTIONAL FEATURES(STATOR)

SHORT CIRCUIT GENERATOR

CONSTRUCTIONAL FEATURES(ROTOR)

SHORT CIRCUIT GENERATOR Vs. CONVENTIONAL

SHORT CIRCUIT GENERATOR CONVENTIONAL GENERATOR

Short time rating Continuous rating

Extra high mechanical strength for

repeated short circuit forces

No extra mechanical reinforcement

is required

High conductor current density As per rated current

Extra care for insulation Standard insulation level

Normally motor as prime mover Turbine as prime mover

Forced air cooling Normally H2 cooled

Flexible winding configuration In a fixed configuration

Required more (33%) damper winding No extra amount of damper winding

Specially designed foundation to absorb

the jerks.

Standard foundation design.

LABORATORIES OF CPRI BHOPAL

• 1250MVA SHORT CIRCUIT TESTING STATION.

• 100MVA ONLINE TESTING STATION.

• SUPPLEMENTARY TEST LABORATORY.

• ENERGY METER TEST LABORATORY.

• CALIBRATION LABORATORY.

• OIL TEST LABORATORY

CPRI PROFILE

• The Central Power Research Institute commonly known as CPRI, is the leading provider of Global Certification services to Utilities and Manufacturers of Electrical equipment over 51 yrs. • CPRI laboratories are accredited as per International Standard

ISO/IEC: 17025 norms by National Accreditation Board for Laboratories (NABL).

• Accredited by Bureau of Indian Standards (BIS). • CPRI laboratories Accredited by ASTA INTERTEK, UK. • Approved by UL-USA, CSA-Canada. • Member of Short Circuit Testing Liaison (STL). • Serve as nodal centre for applied research in electric power

engineering and assist utilities / industries in product development. • CPRI laboratories are at par with any other international laboratory

engaged in testing and certification of power system apparatus of manufacturers and entrepreneurs.

• It is the fourth largest electrical testing laboratory in the world.

BANGALORE 1960

Central Research and Testing Laboratory &

Head Quarters

1960

Switchgear Testing and Development

Station

NOIDA

1992

Regional Testing Laboratory

NAGPUR

1993

Thermal Research Centre

HYDERABAD 1993

UHV Research Laboratory

CPRI NETWORK

BHOPAL

KOLKATA

GUWAHATI

Regional Testing

Laboratory

• Generator-1 was running at its rated speed i.e. 3000 rpm.

• Stator output voltage was set as 12kV for a station test.

• During rising side of the voltage at 8kV, rotor earth fault

protection relay operated and field breaker tripped.

• Vibration was also high for the excited period.

• Vibration and sound was normal at 3000rpm under un-

excited condition.

FAILURE CASE STUDY – I

Shot No. If (Amps) Vt (kV) Remark

1 115 2 --

2 250 5 --

3 840 8* Rotor earth fault relay operated

& abnormal vibration noticed

High vibration was due to increase in field current

i.e. 840 Amps at 8kV whereas; nominal current is only 720 Amps for 12kV.

As the rpm decreased below 1800 rpm, the rotor earth fault relay got reset.

* The plant was set for 12 kV but it was tripped at 8 kV.

Alternator has a field system supplied

with direct current and insulated from

the machine frame. A single ground fault

in the rotor circuit is not dangerous for

the machine but it must be detected and

eliminated before a second fault can

appear which would have catastrophic

for the machine.

A low voltage supply is injected from the

secondary winding of a transformer

through a capacitor. This capacitor also

blocks the DC from the field supply. In

case of earth fault in the rotor circuit,

current in the transformer secondary

winding passes through the operating coil

of the relay, thereby activating the trip

circuit.

OPERATING PRINCIPLE OF ROTOR E/F RELAY(G-1)

FAULT DIAGNOSIS AT STANDSTILL

Stator winding resistance

Terminal Design value at 75°C Measured value at 32°C Remark

U-X-U’-X’ 1.228 mΩ 1.6230 mΩ No

remarkable

change V-Y-V’-Y’ 1.228 mΩ 1.6260 mΩ

W-Z-W’-Z’ 1.228 mΩ 1.6078 mΩ

Recurrent Surge Oscillograph (RSO)

Rotor winding resistance

Terminal Design value at

75°C

Measured value at

32°C

Remark

Between slip rings 152 mΩ 149 mΩ No remarkable

change

‘A’ - Transmitted ‘B’ - Reflected CONFUSED??

FAULT DIAGNOSIS AT SPINNING

IMPEDANCE OF ROTOR WINDING

RPM App. volts Current meas. Impedance Impedance change

0 10.18 2.70 3.77 --

2000 10.08 3.30 3.05 -19.09%

3000 15.18 5.20 2.92 -22.54%

IR VALUE OF ROTOR WINDING

RPM Megger value at 250V, 1min

0 331 MΩ

2000 175 MΩ

3000 0.000 MΩ

Recurrent Surge Oscillograph (RSO) (with special attachment)

Before 2000 RPM After 2000 RPM

FINAL DIAGNOSIS: THE FLYING EARTH FAULT

REPAIR WORK

• The flying earth fault of this rotor was a typical condition which

couldn’t be pin pointed accurately at stand still condition.

• The machine was 50 yrs old with class ‘B’ insulation, which got

deteriorated over the years.

• A complete rewind with upgraded insulation was the only full proof

method of addressing the problem.

• Any other patch work would have involved various degrees of risk.

• Disassembling of rotor was carried out with removing of retaining

rings, dismantling of slip rings, CC bolts and D leads.

• Rotor coils were of continuous type.

Rotor coil Retaining ring removal

REPAIR WORK

A few slot wedges had shown hair line cracks of about 10 mm length during DP test. Wedges (both

ends) were strengthened with brazing alloy as a preventive action for future. Rotor body was sand

blasted and thereafter applied insulating paint on it.

Rotor rewinding was done using same copper with new class ‘F’ insulation imported from

Switzerland. After assembling of all pressing tools on rotor, polymerization cycle was performed.

New insulation Polymerization process

Rewinding process Wedge with hair line crack

TESTS ON RE-WOUNDED ROTOR

Measurements after repair

Insulation Resistance Winding resistance High Voltage test

Test voltage: 500 VDC

After 15s: 430 MΩ

After 60s: 886 MΩ

133.16 mΩ at 33°C Test voltage: 1700 VAC for 1

min. Withstood,

Leakage current: 188.8mA

Vibration measurement

Surge comparison test

•Over speed test at 10 % above nominal speed was carried for two minutes.

•Rotor balancing done at 3000 rpm and vibration values were taken.

•Surge comparison tests were performed during and after balancing.

From inner slip ring From outer slip ring

TESTS ON STATOR

Electromagnetic Core Imperfection Detection

(ELCID) test • This test is carried out on the stator winding of

generator to detect imperfections, faults and hot

spots in the stator core.

• Core laminations are coated with a thin layer of

electrical insulation to prevent Eddy currents

being induced between them by rotating magnetic

field.

• ELCID test was conducted by passing an exciting

current of 13.45 Amps through 6 turns of looped

wire around the stator frame.

• Test results showed that inter-laminar insulation of

the stator core is healthy.

Wedge mapping test by electronic wedge

tightness detector • Wedge mapping test revealed that about 19.5% of

the wedges were loose and about 19.6% of the

wedges were slightly loose. Re-wedging of the

stator winding was carried out.

ELCID

WEDGE MAPPING

GENERATOR RE-COMMISSIONING • The re-wounded rotor was threaded inside the stator bore and shaft line

was aligned.

• All the required pre-commissioning tests were carried out.

• Open circuit characteristic of the generator was plotted after first

excitation.

• A 12kV VCB was under test.

• One complete test duty and one shot for

second test duty were completed without

any problem.

• During second shot for second test duty,

when the voltage was on rising side after

the excitation request, the ‘Rotor earth

fault’ indication appeared and Excitation

circuit breaker (DCCB) tripped.

• At the time of tripping the voltage was

around 4 kV.

• No abnormal vibrations and other

parameters were recorded/observed

during or prior to that incident.

FAILURE CASE STUDY – II

This static relay applies a low frequency voltage (4.75Hz, 24V p-p) to the

field winding of the machine under supervision through a relay coupler.

The measurement of the rotor IR value is achieved by measurement of

the current flowing through a shunt inside the relay. This value is

continuously compared with a preset value and activates after a set time

delay, whenever it crosses the limit.

The relay coupler is a LC circuit which allows the passage of 4.75 Hz

and blocks all other frequencies.

OPERATING PRINCIPLE OF ROTOR E/F RELAY(G-2)

FAULT DIAGNOSIS

• Rotor insulation resistance value between slip ring to earth: zero.

• Resistance of the rotor winding was checked and was found very

high comparing to its value during commissioning (33.71 mΩ).

Such high resistance indicated the opening of winding and

conduction through the rotor body.

• The D-lead connections were also inspected and found intact.

• To check the condition of the rotor winding RSO test was

conducted which confirmed the rotor winding insulation failure.

• After opening of retaining rings, it was noticed that the melted

copper had pierced the insulation under the retaining ring. When

the insulation sheet was removed, voids were observed on the coils

& between the copper strands at the brazed junction.

End ring Damper windings Insulation puncture Voids

ROOT CAUSE ANALYSIS

• This rotor was manufactured in England.

Voids presented at the brazed junction

during manufacturing process has made

stress concentration during current flow.

• Opening of the turn at the brazed junction

level.

• When the conducting surface was too small,

some copper strands melted.

• Some short circuit occurred between the

turns which had melted copper stands.

• The melted copper flew towards the outer

diameter and pierced the insulation under

the retaining rings.

• DP test was made on the retaining rings. No

crack was discovered. They are not damaged

by the electric flash because the earth fault

was made with the dampers.

Stress concentration at voids

REPAIR WORK

• Disassembled the complete rotor.

• Replaced the copper strands those were submitted to high

stresses.

• Changed the type of brazing material.

• Suppressed the stress concentration by filling all the voids

between the copper strands at the brazed junction level.

• The rotor was rewound with new Class F insulation (slot liners

and the top packers).

• After assembling of all pressing tools, rotor coil insulation

polymerization cycle was performed at 100°C.

Repairing of damaged strands Rewinding work

TESTS ON RE-WOUNDED ROTOR

• Over speed at 10 % above nominal speed was carried for two minutes.

• Rotor was balanced at 3000 rpm and vibration values were:

NDE : 7.067 microns (pk-pk) & DE : 5.115 microns (pk-pk).

• During balancing, RSO test was performed in running at nominal speed.

Measurements after repair

Insulation

Resistance

Winding

resistance

High Voltage test

Test voltage: 500

VDC

After 15s: 1.64 GΩ

After 60s: 3.0 GΩ

38.6 mΩ at

31°C

Test voltage: 1500

VAC for 1 min.

Withstood,

Leakage current:

105mA RSO during balancing

TESTS ON STATOR

• ELCID test and Wedge mapping were conducted.

• Test results showed a healthy core and tight wedges.

Hence no work was required to be done on the stator

except complete cleaning.

GENERATOR RE-COMMISSIONING

• Re-commissioning work was carried out and OCC plot

was taken.

• OCC was identical to the earlier characteristics.

• Timely activation of automatic relays saved the plant from

other damages.

• But no system can be called full proof until unless constant

vigil is maintained. People working with any system are the

best judges for it.

• Both the generators were put into service after satisfactory

commissioning. They are operating absolutely trouble free.

• The generator-2 has been Up-graded as Motor-less along

with this reformation process.

• Reliability of the short circuit testing has increased

manifold and it is available for more numbers of equipment

testing.

CONCLUSION

Intellectual solve problems, genius prevent them

-Albert Einstein.

..THANKS FOR YOUR ATTENTION..

ARUN KUMAR DATTA

akdbpl@rediffmail.com

Ph: 09425371370

Questions