Presentation TranscriptESP Rectifier transformer:ESP Rectifier transformer M.G.Morshad / ACM Transformer Mtce / TPS II

Principle of operation: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: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 increase 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 :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 :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 ESP: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 d1). 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 :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):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 :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 I 2 = (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 KV 2 = (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 V 1 = (KV 2 /R)*1000 37.28 74.55 186.38 186.38 260.93 260.93 Primary AC Current I 1 = I 2 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 KV 2 I 2 I 1 V 1 415 V supply

Specification - Stage II transformers: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 :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 :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

Open circuit test BHEL Transformer :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

Short circuit test BHEL Transformer :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 :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

Fault detection / Stage II :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

Specifications Stage I transformers: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: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 :Transformer connection / Stage I HF Choke b c a LV ACR HV m + HV Bushing Diode Stack 17 nos resistors, each 4M 182 K, 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 :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

Fault detection through meter readings (1):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 CircuitDC 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):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 :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 :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.