The document describes the operation and components of an ESP rectifier transformer. It discusses: - How the SCR controller varies the conduction angle to control the voltage and current delivered to the ESP fields based on feedback signals. - Issues that can occur in ESP fields like back corona caused by high resistive dust layers and field shorts caused by low resistive dust layers. - Key components of the rectifier transformer like the SCR assembly, transformer unit, rectifier bridge, resistance board, insulating oil, and current limiting reactor. - Operating parameters that are set depending on conditions like the uni pulse/semi pulse mode, peak mode, charge ratio, and spark control rate to maintain field energization without

1. ESP Rectifier transformer
M. G. Morshad , ADGM / Electrical
TPS II ( 7 x 210MW) NLC India Ltd

2. Principle of operation
• Electrodes at high voltage create a corona effect (ionized atmosphere)
surrounding them.
• This charges the passing particles. Once charged, particles are subject to
a transverse electrostatic force that pulls them toward the collecting plates.
• Plates are periodically “rapped” (vibrated) to make the collected particles
fall down into a receiver hopper.

3. Back corona
-
+ + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + +
Positively
charged
collecting
plates
- - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - -
High resistive
dust particles
Negatively
charged dust
particles
Negatively charged
emitting electrodes
Spark between layers
of dust particles
In case of high resistive dust ( dry dust) , dust layer creates an insulation between the
positively charged collecting plate and negatively charged dust particles.
 In such condition, spark / arc within the layer of dust particle is formed with the decrease of
KV (DC). This phenomena is known as BACK CORONA.
As a result of spark / arc formation , field current (mA ) gets increased with substantial
decrease in field voltage KV (DC).
To avoid back corona, field voltage KV(DC) has to be reduced sufficiently, but such
measures finally reduces the collection efficiency of the field

4. Field short
In case of low resistive dust ( wet dust), dust layer gets positively charged.
In such condition whenever the gap between positively charged dust particles & negatively
charged electrodes gets reduced due to accumulation of dust layer , spark ( that extinguishes with
the reduction of applied voltage ) or arc (that does not extinguish with the reduction of applied voltage )
gets emitted from emitting electrode to the collecting plates causing shorting of fields.
As a result of field shorting , field voltage KV (DC) gets collapsed with drawing of high field
current (mA ) between emitting electrode and the collecting plates.
This may cause the failure of HV winding if transformer is not switched off immediately after
field short.
-
+ + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + +
Negatively charged
emitting electrodes
Spark between layers
of dust particles
+ + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + +
+
+ + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + +

5. Voltage - current characteristics
KV ( DC)
mA ( DC)
Back Corona Zone
Operating Zone
Field Short
0
Operating Zone : With the increase of field voltage [KV (DC)], field current (mA) increases linearly
and no spark is emitted.
Back Corona zone : Spark starts emitting causing decrease in field voltage KV(DC) with high
increase in field current (mA)
Field short : Spark persist continuously causing field voltage KV(DC) to become zero with maximum
flow of field current (mA)

6. Parameters affect the performance of ESP
1. Gas Temperature :
Normally ESP is designed to operate in the temperature range 180- 200 Deg C. At higher temperature, the
quality of insulation deteriorate and flash over voltage limit decreases. In such condition operating voltage
has to be brought down to avoid back corona that results in lower dust collecting efficiency . At temperature
below the acid dew point, deposition of acid in the structure leads to faster corrosion .
2. Moisture content :
Moisture content has a large influence on the performance of ESP. Moisture increases the ionization
tendency and decreases the resistivity of the dust particles. As an effect of these factors dust collection
efficiency increases with reduced back corona tendency .
3.Dust particle size:
The collecting efficiency increases with increase in particle size since the larger particles receive charge
more quickly and attains migration velocity. (Migration velocity is proportional to diameter when d>1pm and
is independent when d<1μm). Hence , collection efficiency decreases with the increase of fineness of the
dust particles’.
4. Dust resistively:
Dust resistivity increases with the increase of dryness of dust and quality of fuel. At higher dust resistivity ,
internal spark over between two layers of dust takes place as a result of potential difference created by the
high resistance of dust. This phenomena is called Back Corona. Once the back corona starts , field intensity
( KV DC) start reducing with increase of field current . This reduces the collecting efficiency of the ESP.
5. Rapping frequency :
Whenever the electrode surface is subjected to rapping shock, re-entertainment of particles takes place in
the main flow path and carried away by the gas causing increase in emission level . To reduce the re-
entertainment to a minimum level, it must be allowed to form a layer of significant thickness of dust so that
when it is dislodged by rapping, the layer breaks into agglomerate masses, sufficiently large to fall into the
hopper before, being carried out by the moving gas stream into the outlet duct. Secondly rapping frequency
is to be set to optimum level for each field in accordance with the concentration and type of dust entering the
field to minimize penetration.

7. Components of rectifier transformers
Microprocessor based
Voltage controller
Control panel –
1. Analog Meters for quick visual indication of operating level.
2. Electronic Controller for Controlling power (KV and mA)
delivered to the TR.
3. SCR Module with Heat Sinks under control of the Electronic
Controller for Phase Control of the AC Feed to the TR.
4. Circuit Breaker, Contactor and Misc. control and interlock
relays Phase control Thyristor,
Transformer unit –
1. Series reactor,
2. Transformer coil,
3. Rectifier bridge,
4. Resistor assembly
5. HF choke.
6. Insulating oil
7. LV & HV Bushing
8. Control feed back terminals

8. SCR controller
The single phase input voltage of 480 V is applied across the SCR assembly which is
made up of two SCRs connected in a reverse polarity parallel configuration .
The primary voltage of transformer is controlled by changing the conduction angle of
the SCR with the help of feed back signal ( mA & KV) taken from DC side of the
secondary
The SCRs are protected from dv/dt damage by a resistor capacitor snubber network
consist of metal oxide varistor and fuse to protect against over current conditions.

9. Transformer Unit
Rectifier
Bridge
+ -DC Volt / Current
AC Input = 415 V,200A
AC Out put = 53570 V, 1.40 A
Voltage
Ratio
1:143.42
Output DC Voltage = 53570 x (1.414/1.08) = 70KV
Output DC current = 1.40 / 1.414= 0.990 A
Output KWr = V x I = 70 x0.990 = 69.3KW
Input KVA = 373.5 x 200 = 75KVA
Input KW = 75KVA x1 = 75KW
 Single phase transformer with higher voltage
ratio is used to create high voltage at secondary
terminal
 Transformer is operated with 2 phase supply for
achieving natural commutation of rectifier
bridge.
 Transformer KVA is controlled by controlling the
voltage & current with the help of SCR controller
 Heat generation during operation is equal to
(Input 75 KW – Output 69.3 KW ) 5.7 KW is
carried away by the silicon oil to maintain the
temperature.
 Gas generation due to high temperature is
detected by BUCHOLTZ relay
PRIMARY ( LV) SECONDARY ( HV)
KVA Volt Amps Volt Amps DC mA
30 373.5 80 53570 0.56 400
45 373.5 120 53570 0.84 600
60 373.5 160 53570 1.12 800
75 373.5 200 53570 1.40 1000
90 373.5 240 53570 1.68 1200
105 373.5 280 53570 1.96 1400
Various capacity of transformer
CLR

10. Current Limiting Reactor
Rectifier
unit
L
V
H
V
Series
Reactor 373.5 V
Supply
 Impedance (Z) = V/I (AC Resistance)
 Z = V/I = L x 2πf Ohms
 % Z = [(L x 2πf x I) / V ] x 100 Ohms
 L = (V x %Z) x (I x 2πf ) Henry
CLR reactance (L) = 1.5 mH
CLR resistance ( R ) =9 mΏ
 Impedance of CLR =24.64%
TR impedance = 10%
System impedance = 34 .64%
 A system impedance of 50% limits the
maximum AC current to twice the rated
current. At 33% the limit is three times
the rated current
1. The primary purpose of the CLR is to limit the surge current that is produced due to
generation of spark & arc in the ESP field . The typical CLR value is selected in such a way
that it can limit the current surge, within a 8.3 msec (Line ½ Cycle) to approximately 2 – 4
limitation is required because the SCR controller cannot respond (turn off) until the end of
the line cycle.
2. The secondary purposed of the CLR is provide a means for decreasing mA and KV ripple
on the DC Power delivered to the ESP. The reduced Ripple results in increased average KV
levels, and increased ESP performance (efficiency)
415 V
Supply

11. Rectifier Diode assembly
•
 The diode assembly is used to convert the high voltage AC output of the transformer to a DC signal.
 The diode assembly is made up of a series string of many diode junctions. This series string of diodes
should be capable of blocking at least twice the peak output voltage of the T/R. i.e. (2 x 70 KVp = 150 KV)
 For obtaining high blocking voltage special measures are employed to assure proper voltage sharing.
 Improper voltage sharing is caused by variation of the reverse leakage of individual diode junctions.
This variation results in an uneven distribution of the PIV among the diodes. In such case the diodes will
fail in a “domino” fashion

12. HF Choke
 High Frequency Choke / Air Core Reactor (ACR) is an electric coil that is connected between the rectifier
bridge and HV bushing for protecting the TR Rectifier Bridge from high frequency, high voltage spikes and
disturbances that occur within the sparking ESP.
 ACRs used in modern T/Rs are rated from 20 to 50 (mH) and must be capable of withstanding up to 2
times peak rated voltage.
 As the ESP sparks and arcs, the full ESP voltage will be impressed across the ACR.
ACR design must provide sufficient layer insulation and clearance to accommodate such voltage.
 Typical failure mode for ACRs is a spark over of the layer insulation. Since the ACR is physically much
smaller than the transformer secondary coil, it is subject to extreme voltage stress.
 In the event of any contamination of the dielectric fluid, the ACR is often the first component to fail.

13. Resistance board assembly
- feed back signal for control & monitoring
1. mA feed back
This signal is used for Control and monitoring.
The mA Feed back is implemented by a power resistance of approximately 10 Ohms. It is used to
provide a 10 Volt DC signal that will correspond to a 1000 ma TR output. Other resistor values may be
used for other ratios.
The ma feed back is electrically connected between the Positive (+) leg of the HV Bridge and Earth
Ground.
The Resistor must be of High Reliability Rating and also backed up by a protective HV Device.
If this component fails the rated High Voltage is imposed upon this feed back wire.
2. KV Feed back signal
The KV signal is implemented through use of a high voltage divider with a typical ratio 8,000 to 1.
The ratio uses an 80 Meg Ohm resistor on the high end and a 10K Ohm resistor on the low end, thus
producing a feedback of 8 KV per volt. 120 Meg dividers are sometimes used for higher voltage TR’s.
Typical KV Feed Back systems are not frequency compensated, yet provide a reasonable representation
of the ESP signal.
One of the most important feedback signals is secondary voltage or kV. Although this is one of the most
commonly inaccurate signals found in many installations, inaccuracies in KV feed back can be calculated
by the following formula : KV ( DC ) = (Primary current x Primary Voltage X 700) / Secondary mA
 Like the ma Feed Back.. This signal must be protected since the full output voltage of the TR can be
imposed on this wire

14. Insulating oil
The Dielectric Fluid is used to provide cooling for the TR internal components as well as to provide high
voltage insulation.
Mineral Oil, Silicone Oil and R-Temp Oil are fluids used.
PCB fluid - Askeral (Prior to 1970) ,Silicon Fluid , R-Temp type fluids are having higher fire point and
therefore they are used where fire is of greater concern.
 Silicon fluid is more viscous than mineral oil at temperatures above 10 Deg c and therefore requires
additional radiator or bigger tank for proper cooling.
 Silicone fluid has a greater affinity for water absorption compared to Mineral oil
(Water saturation point for Mineral oil is approx 70 ppm while silicone can be as high as 200 ppm).
 Silicone fluid can maintain a higher dielectric properties at high water concentrations.
 The solid insulation of silicon oil filled transformer gets contaminated with water easily since the
Silicone fluid has a greater affinity for water absorption and about 90% of the water present in oil is
absorbed by the insulation due to natural migration of moisture. Hence HOC is required frequently in
silicon fluid for keeping the solid insulation dry.
 Decomposition of Silicon fluid due to internal arcing generates gases and carbon particles. When
Bucholtz relay is actuated by gases, carbon particles contaminated the oil as well as gets attracted to the
transformer windings which finally causes the failure of the transformer due to insulation failure. Hence to
remove the contamination from the oil , proper oil filtration or total oil replacement is required before
installing the transformer after repair and rectification.

15. Characteristics of silicon oil

16. Principle of operation
Depending upon gas temperature, dust
resistivity and gas velocity following
parameters are set-
1. uni pulse mode/ Semi pulse mode ,
2. peak mode ,
3. charge ratio,
4. spark control rate ( S & T)
5. secondary DC current limit
mA
Time
Current limit
Max Current at which spark occur
20ms Blocking time
S
T
 With switching on primary , SCR
controller increase the conduction angle
depending upon the DC feed back signal
(mA & KV) till it reaches the set current.
 During the current rise whenever the
secondary encounters with sparks which is
detected by low voltage and high current
DC feed back signal , SCR controller
immediately stops conduction.
 SCR controller restarts conduction after
20 ms with slop less than 5%
 This process goes on continuously to
keep the field in energized condition with
negative polarity without any spark.

17. Uni pulse / Semi pulse mode
In uni pulse mode of operation fields are in continuous charging state with all half
cycle of sinusoidal input ( Charge ratio = 1). This results in imposing of high peak
voltage and high average current on the field which causes higher power
consumption, lower dust collecting efficiency due to frequent occurrence of Back
Corona Effect in the field.
In semi pulse mode of operation fields are in intermittent charging state with only
preset half cycle of sinusoidal input ( Charge ratio >1). This results in imposing of
high peak voltage and lower average current on the field which causes Lower
power consumption, Higher dust collecting efficiency due to complete avoiding of
Back Corona Effect in the field.
V
I
V
I
SCR controller Rectifier Uni pulse
Semi pulse
Sinusoidal input

18. Charge Ratio
To avoid back corona , optimization of field voltage KV (DC) is needed and It is achieved by
increasing the time gap between the consecutive voltage pulse which is denoted as charge
ratio.
For higher dust resistivity, higher charge ratio is required so that field voltage is imposed
after a sufficient interval to avoid back corona
 To maintain the sufficient average field current for increasing collection efficiency , field
current is to be set at 200% for charge ratio more than 1
 Power consumption reduces with the increase of charge ratio
 For setting field current at 200% , HV coil is frequently exposed to high current that may
lead to failure of coil.
Since lignite ash is low resistive dust ( Wet dust), system can be set for charge ratio between
1 & 3 .
Uni pulse mode
Semi pulse mode
Charge Ratio 1
Charge Ratio 3
Charge Ratio 5
Semi pulse mode
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8

19. spark control rate ( S & T control)
The spark rate is determined by the settings of S-control and T-control.
 Suppose T-Control is set at 20% , the time required by the rectifier to reach the rated
current after a spark, from zero current will be 2 minutes.
 Suppose S-Control is set 5% of the rated current, the time from S-Control break point to
next spark will then be 5% of the T-Control time (5% of 2 minutes), that is 6 seconds.
 If we do not account for the thyristor block time (20mS) then 6 seconds is the statistical
interval between sparks in the ESP.
 S-Control & T-Control are affected neither by the absolute value of current nor of the
voltage at which a spark occurs, the spark rate is constant.
5%
95%
S
T= 6 sec

20. Field current setting
Formula Field I Field II Field III Field IV Field V Field VI
Secondary DC Current mA 100.00 200.00 500.00 500.00 700.00 700.00
Secondary AC Current I2 = (mA x 1.4141)/1000 0.14 0.28 0.71 0.71 0.99 0.99
Secondary DC Voltage KVp = (70 x mA)/1000 7.00 14.00 35.00 35.00 49.00 49.00
Secondary AC Voltage KV2 = (KVp x 1.08)/1.414 5.35 10.69 26.73 26.73 37.42 37.42
Out Put KW Kwo = (mAxKVp)/1000 0.70 2.80 17.50 17.50 34.30 34.30
Trfo voltage ratio R 143.42 143.42 143.42 143.42 143.42 143.42
Primary AC Voltage V1 = (KV2/R)*1000 37.28 74.55 186.38 186.38 260.93 260.93
Primary AC Current I1 = I2 x K 20.28 40.56 101.41 101.41 141.97 141.97
In Put KW Kwi = (V1 x I1)/100 0.76 3.02 18.90 18.90 37.04 37.04
Trfo Loss KW loss = (Kwi - KWo) 0.06 0.22 1.40 1.40 2.74 2.74
L
V
H
V
CLR
HFC
mA
KVp
+ Positive
- Negative
KV2
I2I1
V1
415 V
supply

21. Specification - Stage II transformers
Name Rectifier Transformer
Supply Voltage 415 V AC two phase
Make BHEL
Location Stage II ESP roof top
Capacity 75 KVA
Rated primary Voltage ( LV) 373.5 V
Rated primary current () 200.8 A
Rated secondary voltage (HV) 53570 V
Rated secondary current (HV) 1.4 A
Voltage ratio 143.42
Oil Capacity 400 Liters ( 2 Barrels)
Type of oil Silicon oil
Total weight including oil 1300 Kg

22. Location - stage II transformers
5A
1A
2A
3A
4A
6A
11A
7A
8 A
9 A
10A
12 A
5B
1B
2B
3B
4B
6B
11B
7B
8 B
9 B
10B
12 B
Clean gases to chimney
Dusty gases from RAPH

23. Transformer connection / Stage II
HF Choke
H.V
Resistance
a1
av
a3
LV
ACR
HV
AR
AS2
AS1
A2
A1
Protection
diode
Diode
Stack
Terminal / Parts Purpose
a3 - av AC series Reactor to restrict primary current incase of shorted secondary ( Resistance 9.32 m Ohms)
av- a1 winding terminal ( Resistance 14.6 m Ohms)
Internal Terminal HV winging terminal (Resistance 454 Ohms)
a3 – a1 Two phase AC input terminal (Resistance 24.84 m Ohms)
A1 Negative terminal to create negative potential in the fields
A2 Positive terminal earthling point to create positive potential in the structure
AS2 – AR DC feed back voltage measuring terminal
HF Choke To reduce sparking rate at HV terminal ( Inductance 50mH, 6.74 Ohms)
Diode Stack Full wave bridge rectifier for converting AC to DC
H.V resistance Voltage divider
Protection diode To protect the bridge from reverse biasing

24. Open circuit test – BHEL Transformer
Voltage Applied on LV
terminals Using Variac (Volt)
Magnetizing current measured
on LV terminals (Amps)
DC feed Back voltage measured
between AS2&AR (V)
50 0.116 20.20
100 0.176 41.00
150 0.190 58.20
200 0.280 77.20
250 0.490 96.50
300 2.460 116.00
350 3.110 133.00
374 4.240 140.50
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
0 50 100 150 200 250 300 350 400
Magnetizing current measured
on LV terminals (Amps)
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
0 100 200 300 400
DC feed Back voltage measured
between AS2&AR (V)

25. Short circuit test – BHEL Transformer
Voltage Applied on LV
terminals Using Variac
(Volt)
Current measured on
LV terminals
(Amps)
DC feed Back Current
measured between
AS2 & AS1 (mA)
DC Current measured
on HV terminals (A)
20 36.00 0.220 0.101
40 67.00 0.400 0.183
60 98.00 0.580 0.230
80 131.00 0.770 0.320
100 171.00 1000.000 0.420
120 199.00 1140.000 0.500
130 206.00 1160.000 0.510
0.00
50.00
100.00
150.00
200.00
250.00
0 20 40 60 80 100 120 140
AC Current measured on LV
terminals (Amps)
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0 50 100 150
DC Current measured on HV
terminals (Amps)

26. Acceptance test / Stage II
Parameters Value
IR Value
Minimum 200 M Ohm
HV – E , ( 2.5 KV Megger),
HV – ( 2.5 KV Megger),
LV – E ( 0.5 KV Megger)
LV Winding
resistance 14- 15 m Ohms
AC Reactor
resistance 9 – 9.5 m Ohms
Combined
resistance 24 – 25 m Ohms
Magnetizing
current test
Voltage current As2 – AR
50 Volt 108 mA 19 V DC
100 volt 170 mA 39 V DC
150 volt 200 mA 59 V DC
200 Volt 0.26 A 79 V DC
250 Volt 0.46 A 99 V DC
300 Volt 1.25 A 118 V DC
350 Volt 2.81 A 136 V DC
400 Volt 4.0 A 145 V DC
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
0.00 200.00 400.00 600.00Amps
Volts
Amps
Amps

27. Fault detection / Stage II
Parameters Value
Two phase Input AC voltage 110 to 120 Volt
Primary current 0.2 to 0.3 Amps
Secondary Voltage 33 KV
Secondary current Zero
OCC test at local – Keeping A1 open
Parameters Value
Two phase Input AC voltage 110 to 120 Volt
Primary current 14 – 15 Amps
Secondary Voltage 33 KV
Secondary current 100 mA
Load test at local – Keeping A1 close
Fault
detection
Actuations of Buchholtz
relay BOTTOM FLOAT
Actuations of Buchholtz
relay TOP FLOAT
Causes
•Internal short circuit between turns
•Short Circuit between phase & earth
Phase to phase short circuit
Insulation break down
Causes
Low oil level
Air accumulation
Fault in core lamination
Break down in core blot Insulation
Local over heating in the winding
Wrong connection

28. Specifications – Stage I transformers
Make MERLIN GERIN ( France)
Location ESP I,II,II
Population / Unit 24 Nos
Total Population 3 x 24 = 72 Nos
Capacity 75 KVA
% impedance 8%
Primary rated current 181 amps (AC)
Voltage Ratio 415 V / 54000V
Output voltage 75Kv(DC)
Output current 0.13 Amps (DC)
DC out put 59 KW
Primary fuse rating 250 amps / 500 Volt
Protection DGPT 2000 ( Gas emission, internal pressure &
Temperature)
Total weight of one transformer 900 Kg
Oil weight per transformer 290 Kg
Type of oil used HUILE OIL ( Askarel)

29. Location – stage I transformers
A5
A1
A2
A3
A4
A6
B5
B1
B2
B3
B4
B6
C5
C1
C2
C3
C4
C6
D5
D1
D2
D3
D4
D6
Dusty gases from RAPH
Clean gases to chimney

30. Transformer connection / Stage I
HF
Choke
b
c
a
LV
ACR
HV
m
+
HV Bushing
Diode
Stack
17 nos
resistors,
each
4MΩ
182 KΩ,
¼ W
resistors
Spark detector
Terminal / Parts Purpose
a - c AC series Reactor to restrict primary current incase of shorted secondary ( Resistance 11.2 m Ohms)
c- b LV winding terminal ( Resistance 18.8 m Ohms)
a-b Two phase AC input terminal (Resistance 29.5 m Ohms)
+ Grounding point of HV DC terminal earthling point to create positive potential in the structure
m Spark detector terminals

31. Open circuit test – Stage I transformer
Voltage applied
between (a-b)
Current through
primary winding
50 Volt 89.2 m A
100 Volt 148.2 m A
150 Volt 0.19A
200 Volt 0.27 A
225 Volt 0.34 A
250 Volt 0.42 A
275 Volt 0.57 A
300 Volt 0.77 A
325 Volt 1.04 A

32. Fault detection through meter readings (1)
Primary side Secondary side 1. Check if controller is responding to sparking. If it
is, use a scope to verify that sparks/arcs are
occurring. Run T/R with precipitator disconnected to
verify that T/R is not sparking internally.
2. Check for open SCR fuses.
3. Verify that SCRs are firing.
4. Check for open CLR.
5. Check for proper operation of controller power
components - circuit breaker, contactor
No power to T/R set
Primary side Secondary side
Short Circuit—DC Side
1. Run T/R set with HV bushing disconnected from
the precipitator.
a. If no current flows the short is in the precipitator.
b. If current still flows the short is in the T/R set.
2. If precipitator is shorted, check electrodes and
insulators for shorts.
3. If T/R is shorted, check HV bushing and external
switch (if applicable) for shorts

33. Fault detection through meter readings (2)
1. Megger diodes for shorts.
2. Run T/R without diodes. If AAC still high,
transformer is bad.
Primary side Secondary side
Short Circuit T/R set
Primary side Secondary side
1. Run T/R set with HV bushing grounded
externally.
a. If current flows, precipitator field is open.
b. If no current flows, T/R is open.
2. If precipitator is open, check all HV
connections to electrodes.
3. If T/R is open, megger unit. Check for open
diodes or connections in T/R tank
Open circuit

34. Failure sequence
In cases of severe arcs or shorted field, the current may instantly rise to twice rating but quickly
reduced by the controller to safe level and this instant over current is permitted to continue with every
automatic switching on , excessive heat is generated in the HV winding & diodes stack .
As a result of heat the solder that fastens
the diodes to the PC board to melt away
and causes arcing between the diode lead
and the PC board.
Actuation of B’ Relay
Instant arching causes generation of gas
Arcing results in the breakdown of the dielectric fluid.
Continuous arching causes generation of carbon
particles
Carbon particle gets accumulated in HV windings
HV winding gets shorted
As a result of heat the HV winging joints gets melted.
inter winding arcing HV winding gets opened

35. Measures to be taken for avoiding frequent failure of
transformer
1. Transformer must be switched off whenever it encounter with field short.
2. Whenever transformer gets failed due to internal arc , Transformer shall be
filled with new oil after rectification.
3. Since silicon oil is highly hygroscopic, periodical oil circulation is required to
avoid moisture absorption in solid insulation which may lead to failure of
transformer due to weakness in solid insulation.
4. Availability of feed back signal ( mA & KV) must be ensured before putting the
transformer in service since wrong feed back may lead to spurious power input
( Voltage & current ) to the Transformer due to malfunction of thyristor controller.
5. Ensure cleanliness of field and ash level in hopper before switching on the
transformer for avoiding switching on of transformer with field short.
6. Set charge ratio 1 for repaired transformer and 3 for non repaired transformer
for achieving current setting according to the physical condition of the
transformer.