ESP Rectifier transformer

M.G.Morshad / ACM Transformer Mtce / TPS II

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.

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

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

+ + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + +

+

+ + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + +

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)

Parameters affect the performance of ESP1. Gas Temperature : Normally ESP is designed to operate in the temperature range 180- 200 Deg C. At higher temperature, thequality of insulation deteriorate and flash over voltage limit decreases. In such condition operating voltagehas to be brought down to avoid back corona that results in lower dust collecting efficiency . At

temperaturebelow 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.

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 oil7.LV & HV Bushing8.Control feed back terminals

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.

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 = 75KVAInput 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 relayVarious capacity of transformer

CLR

Current Limiting Reactor

Rectifier unit

LV

HV

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 mHCLR 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

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

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.

Resistance board assembly - feed back signal for control & monitoring

1. mA feed backThis 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

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.

Characteristics of silicon oil

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.

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

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

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

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

LV

HV

CLR

HFC

mA

KVp

+ Positive

- Negative

KV2

I2I1

V1415 V supply

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

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

Transformer connection / Stage II

HF Choke

H.VResistance

a1

av

a3

LV

ACR

HV

AR

AS2

AS1A2

A1

Protection diode

DiodeStack

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 terminalHF Choke To reduce sparking rate at HV terminal ( Inductance 50mH, 6.74 Ohms)Diode Stack Full wave bridge rectifier for converting AC to DCH.V resistance Voltage divider Protection diode To protect the bridge from reverse biasing

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 20.20100 0.176 41.00150 0.190 58.20200 0.280 77.20250 0.490 96.50300 2.460 116.00350 3.110 133.00374 4.240 140.50

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

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 DC100 volt 170 mA 39 V DC150 volt 200 mA 59 V DC200 Volt 0.26 A 79 V DC250 Volt 0.46 A 99 V DC 300 Volt 1.25 A 118 V DC

350 Volt 2.81 A 136 V DC400 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

0.00 200.00 400.00 600.00Am

psVolts

Amps

Amps

Fault detection / Stage II

Parameters ValueTwo phase Input AC voltage 110 to 120 VoltPrimary current 0.2 to 0.3 AmpsSecondary Voltage 33 KVSecondary current Zero

OCC test at local – Keeping A1 open

Parameters ValueTwo phase Input AC voltage 110 to 120 VoltPrimary current 14 – 15 AmpsSecondary Voltage 33 KVSecondary 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 circuitInsulation break down

CausesLow oil level Air accumulation Fault in core lamination Break down in core blot Insulation Local over heating in the winding Wrong connection

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)

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

Transformer connection / Stage I

HF Choke

b

c

a

LV

ACR

HV

m

+

HV Bushing

DiodeStack

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

Open circuit test – Stage I transformer

Voltage applied between (a-b)

Current through primary winding

50 Volt 89.2 m A100 Volt 148.2 m A150 Volt 0.19A200 Volt 0.27 A225 Volt 0.34 A250 Volt 0.42 A275 Volt 0.57 A300 Volt 0.77 A325 Volt 1.04 A

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

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

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

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.