Recurrent Surge Oscillograph (RSO) for Rotor

Winding Shorts Detection

P.G.S. Kumar V. Raghavendra Rao
(retd.AGM, BHEL), Director (Technical) Manager (Testing)
MRTG Power Diagnostic Tests Pvt. Ltd., MRTG Power Diagnostic Tests Pvt. Ltd.,
Hyderabad ,India Hyderabad ,India
pgsk51@yahoo.com sarirag@yahoo.com

Abstract— Detection of inter turn shorts in rotor windings of result. Detection and location of inter turn shorts is the key
high voltage turbine generators (TG) has emerged as one of the approach in condition monitoring of TGs which enable pro
important diagnostic measures in condition monitor (CM) for active plans of rectification to minimize further deterioration.
uninterrupted power generation. Conventional test methods
cannot identify the location of defects. Recurrent Surge This paper deals with Recurrent Surge Oscillograph (RSO)
Oscillograph (RSO), an off-line novel technique method has test method to effectively detect and locate inter turn shorts of
widely supported the quality of decisions of diagnosticians to
rotor windings. Traditional test methods and disadvantages are
locate the shorts in rotor windings.
This paper begins with traditional techniques in use to observe
narrated. Effects of inter turn shorted field windings are
inter turn shorts of the field windings. Principle of time domain mentioned. Owing to the benefits of the novel approach, RSO
reflectometer ( TDR) is presented. Methodology of applying RSO test has been viewed as mandatory for condition assessment
technique for rotor windings is detailed. Wave patterns for diagnosis (CAD) of TGs in recent years. Time Domain
possible inter turn shorts with a demo delay unit are depicted. Reflectometry, the principle behind RSO is briefed.
Case study on faulty windings of TG is presented. This novel Waveforms of various faults are depicted by delay unit demo.
detection technique benefits the customers to avoid sudden Case study on 15MW TG is presented and rectification
outages. methods to isolate the fault is recommended.

Keywords- Excitation winding, inter turn shorts, recurrent surge II.TRADITIONAL DETECTION OF INTER TURN SHORTS
oscillograph(RSO), time domain reflectometry Assessing the condition of field winding which is hidden in
I.INTRODUCTION rotor slots and end bells is a challenging domain. Incorrect
decisions may result into power plant shut down for long time.
EXCITATION winding (Fig.1) of turbine generators (TG)
is a highly stressed component and prone to inter turn Traditional methods to observe inter turn shorts of field
shorts during service life. Operating the rotors with shorted windings are
turns 1) Open circuit (OC) and load current (LC) test
2) Ohmic resistance measurement
3) Impedance measurements in static and running
condition
4) Pole and turn to turn volt drop test
5) Vibration analysis
6) Flux probe analyzer

In residual life analysis (RLA) and condition monitor (CM)

diagnosis studies, above methods are being used as off-line
and flux probe is recommended for on-line detection of shorts.

In OC and LC tests, an increase in excitation energy for the

Figure.1 Typical excitation winding of 2 pole category TG same output voltage and load current is indication of shorts in
the field winding. Vibration behavior is different. Up rise of
may limit the loads due to higher excitation energy, abnormal DC current in rotor circuit will be the result if the shorts are
vibration behavior, bowed rotors and more thermal >5%. Location of the defects are not in the per view of this
dissipation. Catastrophic effects such as earth faults and performance test.
bearing damages will happen. Huge power generation loss,
halt of economic growth and financial crisis are the ultimate

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 A rotor slot contains 10 to 15 turns and no. of coils are 6 to
7 per pole. The order of resistance is in hundreds of milli
ohms. One turn short due to damaged insulation may result in
no variation in resistance. DC winding resistance
measurement will not provide data on inter turn shorts.

Impedance method is practiced as off line test at standstill

and running condition with no excitation. Low AC variable
voltage is fed to the winding and current is recorded. The
calculated impedance is studied over the prevailed signatures
to assess the extent of shorts. 50 to 500HZ power supply is Fig.2 Cut section of Field slot
used for magnification. Accuracy and sensitivity of the Connections of coils to form N and S poles are depicted in
measurements are highly variable vowing to rotor position Fig.3
whether it is stand alone, inside the stator, placed on steel
rollers nearer to ground level, running speed etc. Associated
tests shall be conducted to frame the decisions.

Flux probe test requires pre installation of flux probes in the

stator slots. A shut down of the power plant and threading out
of rotor is necessary for assembly of the sensor. It will scan
and measure each slot magnetic flux while in rotation. Display
of the flux pattern on oscilloscope will reveal the condition of
turns in the slots. It is a costly proposition to provide flux
probe system. Fig.3 Scheme of Connections

Equivalent circuit (Fig.4) of the two pole field winding for

None of the above examinations solely can significantly high frequency travelling wave is presented with distributed
decide the shorted turn status. Probability of confirmation of resistance, inductance and capacitance elements along the
the shorts increases significantly by other related tests. Course circuit.
of action for rectification involves disassembly and
reassembly of rotor, removal of retaining rings and major
rewinding etc which costs lakhs of rupees.

Recurrent surge oscillograph (RSO) test will confirm

positively the inter turn shorts and also possible location on
percentage basis. It is widely recognized by the generator
community. Power plant customers are insisting to conduct
this diagnosis tests for condition assessment.
III.RECURRENT SURGE OSCILLOGRAPH (RSO)
RSO test technique is used to capture on dual channel
oscilloscope, the reflected waves of the input rectangular pulse Fig.4 RLC circuit for field winding
travelled in the field winding simultaneously at the two slip Calculation of magnitudes of the distributed electrical
rings. It is a non destructive off line method to locate inter elements are difficult due to variable geometry
turn faults of windings. Principle of time deflection configuration of machine size and design. Measurements
reflectometry (TDR) has been extended to visualize the inter are carried with travelling waves to find the anomalies of
turn short defects in TG excitation windings. windings.
IV.EXCITATION WINDING OF TG V.TIME DOMAIN REFLECTOMETRY
Slot fill is of Cl.F insulation system with pre moulded slot High frequency disturbances travel as waves on
liners of single length. Top insert are of epoxy glass laminated transmission lines upto the end of the line. Part of the wave is
strips with suitable ventilation ducts for effective cooling paths. reflected back and some amount of energy is transmitted
Current carrying turns are separated by cl. F flexible epoxy further. Abrupt change in impedance of line will result into
resin glass content which are bonded by special technological reflected and transmitted voltage and current signals of varied
process. Contents of the slot are wedged with extruded copper magnitudes. Principle of travelling waves is utilized to rotor
alloy profiles of high mechanical strength as shown in the windings of TGs for shorts detection.
Fig.2. Connections of coils to form N and S poles are depicted
in Fig.3

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 VI.TRAVELLING AND REFLECTION WAVES
Voltage of incident wave is
e  EU t 
A transmission line possess two wires of distributed Z1  Z ; Z 2  LS
resistance, capacitance and inductance all along its length. A
Coefficient of reflection 
LS  Z 
high frequency surge wave when applied at one end of the line
travels at approximately speed of light to the other end
LS  Z 
through the characteristic impedance of the symmetrical R-L- Voltage of the reflected wave,
  Z 
e '  e   1  2 exp   t  EU t 
C network.

Z0 
L   L 
C Exponential decay of the reflected wave exists at the junction.
Where Z 0 = Characteristic impedance of transmission line VII.APPLICATION OF RSO FOR SHORTS DETECTION IN WINDINGS
L= Inducatnce per unit length Application of time domain reflectometer (TDR) for rotor
C= Capacitance per unit length windings inter turn shorts detection is referred as recurrent
surge oscillograph ( RSO).
For an open ended transmission lines, the incident surge
current will reflect back in succession to the sending end as a A signal generator produces step voltage of 5 to 10 volts
reflected wave. For a length of one mile cable line, with a magnitude of high rise wave front. This high frequency
propagation velocity of 66% of speed of light, a time of 8.146 rectangular pulse is directed into the characteristic impedance
 sec s. is needed for signal to travel from one end to dead of the distributed R-L-C circuit of the rotor winding through
end. Double the time ( 16.292  sec . ) is required to complete the slip rings against earthed shaft in succession( Fig. 5).
the circuit for the reflected signal. Time domain reflectometer
(TDR), a special instrument is used to observe the transit time
of the reflected pulse which is useful to determine the length
of the conductors. This art is used to determine shorts in
cables and now it is adopted to TG field windings to locate
hidden inter turn shorts and earth faults.

The travelling wave gets disturbed at faulty junctions due to

sudden change in impedance of the line. Voltage magnitudes
will differ from case to case of impedance values as below.
Fig.5 Circuit connection of RSO
 For an open circuit of long lines with surge impedance Z
Incident pulsating wave will travel at one slip ring into the
Voltage of incident wave is
e  EU t  field circuit with speed of light and reaches other slip ring in a
healthy field winding with no inter turn shorts and earth faults.
Z1  Z ; Z 2   Reflection wave will travel back from open end of the other
Z 2  Z1  slip ring to the input slip ring point. Both the traces are
Coefficient of reflection   1 identical and the summation after inversion will be a straight
Z 2  Z1  line with no ripples. Healthy winding will be symmetrical
Voltage of the reflected wave, e  e  e  EU t 
' electrically and travelling time of transient pulse injected at
both the slip rings will be same. It results into identical
 For a line ending with resistance same as surge reflections as shown in the fig.6 if superimposed a single trace
impedance Z of the line is visible.

Voltage of incident wave is

e  EU t 
Z1  Z ; Z 2  R  Z
Coefficient of reflection  
R  Z   0
 R2  Z 
Voltage of the reflected wave, e  e  0
'

No reflected wave present at the junction in this case.

 For a long line with sudden change in impedance L

Fig.6Two traces & Super imposed straight trace of healthy field winding

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 pole category. 15MW,11KV TG rotor was excited and as the
Resulting reflected signals at the shorted turns due to sudden DC energy was increasing, vibration levels were shooting up.
change in impedance travels back to the front end of the Generator was loaded and vibration were at alarming position.
winding. It can be observed on duel channel oscilloscope as Excitation energy was not matching with the protocols and
two different waves with kinks(Fig.7). A comparison between field winding was drawing higher excitation current . Machine
the super imposed incident and reflected waves will be used could not be loaded to rated capacity due to excess energy
for finding out the abnormal contacts of turns. levels.
A shut down had been taken and condition assessment
diagnosis tests were conducted. RSO waveforms ( fig.10 )
revealed inter turns shorts in the winding overhang zone
beneath end bells.

Fig.7 Trace with kink of shorted field winding

VIII. DEMO WITH DELAY UNIT

RLC network with 10 sections with a characteristic impedance
of 100 ohms is used as delay unit. Terminations of each
section is for simulating the shorts and observing the wave Fig.10 Waveform for shorted turns
patterns on digital oscilloscope.
Retaining rings were removed and shorted turns ( fig.11) were
noticed between the coils.

Fig.8 Waveform with inter turn short

Fig.11 Typical shorted turns of rotor winding

After correction, RSO repeated and single trace waveform was

observed on the screen which approved the repair procedure.
X. CONCLUSION
Aim of this paper is to familiarize the RSO technique for
application engineers of power plants. It is found that RSO test
waveforms are specific over conventional tests to recognize
Fig.9 Waveform with earth fault inter turn shorts of field windings of 2 pole version TGs. It is
low cost and small voltage diagnostic test to assess the
Effect of inter turn shorts and earth faults are depicted in the condition of windings during manufacture and at sites.
fig. 8 & 9. Reflected waveforms will possess approximate percentage
IX.CASE STUDY location of the fault which none of the traditional tests
provide. Safety to personnel and equipment is ensured as the
This case study illustrates the difference in waveform input voltages levels are of the order of 10 to 15 volts.
pattern for healthy and inter turn shorted rotor windings of 2 Appropriate action plans for rectification of inter turn shorts is

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possible with this method. It has been recognized a condition REFERENCES
based diagnostic tool by asset managers of power plants. [1] Isidor Kerszenbaum ,Clyde Maughan, ―Utilization of Repetitive
Maintenance staff will be relieved of surprise shut down of the Surge Oscillograph (RSO) in the Detection of Rotor Shorted Turns
machine by introducing RSO test as mandatory diagnostic test in Large Turbine Generators ‖, 2011 Insulation Conference
in service. The case study on one of the TG by RSO identified [2] ―Generator Rotor Shorted Turn Analyzer for Turbo- Generator DC
–Field Windings‖, Instruction Manual,Sumatron, Inc.
the fault location and rectification has been carried effectively.
[3] ―Generator Field Winding Shorted Turns:Observed Conditions
and Causes‖, DJ Albright and DR Albright,Generatortech Inc.
[4] ―High Voltage Engineering‖, MS Naidu,V Kamaraju
[5] Test Report of Condition Assessment and Diagnosis on 15MW
TG,MRTG Power Diagnostic Tests Pvt. Ltd.