Refresher Course on Distribution Transformers PDF

 Overview of MERALCO System 13.2/7.62kV 69kV 13.8kV 13.8kV 230kV 13.2/7.62kV Generation...

 Overview of MERALCO System 13.2/7.62kV 69kV 13.8kV 13.8kV 230kV 13.2/7.62kV Generation 115kV 13.8kV 4.16/2.4kV 4.8kV Transmission 34.5/20kV 6.24/3.6kV 13.2/7.62kV Sub-Transmission 13.8kV Distribution  OUTLINE  Transformer Nomenclature (Nameplate, Company Number)  Workshop  Transformer Taps  Transformer Polarity  Transformer Connections (Standards, Common Errors & Troubles)  Workshop  Connection for L-G Secondary System  Reminders on DT Installations  Typical MERALCO DT Primary H2 Primary H1 Bushing Bushing (At the Back) Secondary (X1, X3, X2, X4) KVA Bushings Rating Company Nameplate Number  DT Nameplate Sample Polarity KVA Rating (Subtractive) Primary Voltage Secondary Rating Voltage Rating (Dual Voltage) Voltage Rating per tap Schematic position Diagrams  DT Nameplate Sample Primary Voltage Rating KVA Rating Secondary Voltage Rating Polarity (Additive) Voltage Rating per tap Schematic position Diagrams  DT Nameplate Sample Company Number KVA Rating Secondary Voltage Rating Primary Voltage Rating Voltage Rating per tap position Schematic Diagrams  DT Nameplate Sample Primary Secondary KVA Rating Voltage Rating Voltage Rating Polarity (Additive)  Symbols Used in Transformer Voltage Ratings NAME SYMBOL APPLICATION EXAMPLES To separate the voltage - Dash - rating or ratings of separate windings. 34500 Grd.Y / 19920 V 240/120 V To separate voltage to be 13200 GRDY / 7620 V - Slant or Slash / applied or to be obtained from the same winding. 120 / 240 V To designate separate voltages which can be obtained by reconnecting 34500GRDY / 19920 V - Cross X the coils of a winding in 120/240 X 139/277 V series or multiple combinations Primary Voltage Ratings of DTs Used by MERALCO VOLTAGE RATING DESCRIPTION Single-bushing DTs for line-to-ground 34500Grd.Y/19920 connection on effectively grounded systems with line-to-line voltages of 34.5kV Two-bushing DTs for line-to-ground 3600/6240Y connection on system with line-to-line voltages of 6.24kV Two-bushing DTs for line-to-ground 2400/4160Y connection on system with line-to-line voltages of 4.16kV Two-bushing DTs for line-to-ground 4800/8300Y connection on system with line-to-line voltage of 8.3kV or for line-to-line connection on system with line-to-line voltage of 4.8kV. Primary Voltage Ratings of DTs Used by MERALCO VOLTAGE RATING DESCRIPTION For line-to-line connection on system of 13200 13.2kV line-to-line. Single-bushing DTs for line-to-ground 13200Grd.Y/7620 connection on effectively grounded systems with line-to-line voltages of 13.2kV. Two-bushing DTs for line-to-ground 7620/13200Y connection on system with line-to-line voltages of 13.2kV Without slash (/) – for line-to-line connection With slash (/) – for line-to-ground connection  DT Company Number Coding System of MERALCO I. PREFIXES (Primary Winding Rating) OLD SYSTEM NEW SYSTEM KV RATING W W 20 Z T 13.2, 13.8 Z Z 7.62 X X 3.6 Y Y 4.8 MY M 2.4  DT Company Number Coding System of MERALCO II. SUFFIXES (Secondary Winding Rating) CODE LETTER VOLTAGE RATING A 120/240 or 240/120 B 139/277 C 240/480 D (or Y) 139/277x120/240 E 240/139* F 480/277* * Three- G 240 phase DTs N 125/216*  Difference Between 120/240- & 240/120-Volt Ratings 120/240 2-section secondary winding which can be connected in parallel for output voltage 120V, in series for output voltage 240V, or in series for 3-wire service for (Available in 4- or 3-bushing 120/240V output voltage. transformers) (Memory Aid: 120-volt winding times 2) 240/120 Mid-tapped secondary suitable for 2-wire service at voltage 240V, or for 3-wire service. Cannot be connected for 2-wire service at voltage 120V. (Memory Aid: 240-volt winding divided by 2)  DT Company Number Sample B- 139/277 V Secondary W- 19.92 kV Primary  DT Company Number Sample A- 120/240 V Secondary M- 2.4 kV Primary  DT Company Number Sample D- Dual Voltage 120/240 X 139/277 V Secondary Z- 7.62 kV Primary  External Tap-Changers Operation 1. De-energize the DT 2. Loosen lock screw 3. Rotate switch to desired position 4. Tighten lock screw 5. Energize the DT Lock Screw Other Types of External Tap-Changers Internal View of a DT With an External Tap-Changer Tap- Changer Internal Tap-Changers Rotary Tap-Changers  Schematic Diagram of a Dual Voltage Transformer H1  H2   TAP 5 4 3 2 1 CHANGER      DUAL VOLTAGE PRIMARY TAP WINDING (SET AT INNER TAP) 139V 139V X1 X3 X2 X4 139/277 V  Schematic Diagram of a Dual Voltage Transformer H1  H2   TAP 5 4 3 2 1 CHANGER      DUAL VOLTAGE PRIMARY TAP WINDING (SET AT OUTER TAP) 120V 120V X1 X3 X2 X4 120/240 V Samples of DTs With Dual Voltage Tap Internal Tap changer Dual Voltage Taps Always refer to nameplate for actual ratings. Samples of DTs With Dual Voltage Tap Tap changer Dual Voltage Taps Changing the DUAL VOLTAGE TAP is done by transferring the connection of the primary lead Samples of DTs With Dual Voltage Tap Changing the DUAL VOLTAGE TAP is done by transferring the connection of the outer end of the curved copper strip Dual Voltage Taps Transformer Polarity Convention 200 kVA or smaller, and Additive 8.66 kV or below Single-phase transformers that do not meet either Subtractive one or both criteria  Terminal Markings of DTs I. ADDITIVE POLARITY H1 H2 H1 H2 H1 H2 X3 X1 X3 X1 X4 X1 X2 X2 X2 X3 ADDITIVE - X1 IS DIAGONALLY ACROSS FROM H1  Terminal Markings of DTs II. SUBTRACTIVE POLARITY H1 H2 H1 H2 H1 H2 X1 X3 X1 X3 X1 X4 X2 X2 X3 X2 SUBTRACTIVE - X1 IS ON SAME SIDE AS H1  Polarity of DTs EXAMPLES: 1) 100 kVA, 3.6 kV-240/120 V Additive 2) 250 kVA, 7.62 kV-240/120 V Subtractive 3) 25 kVA, 19.92 kV-240/120 V Subtractive 4) 167 kVA, 13.8 kV-240/120 V Subtractive All 20kV, 13.8kV or 13.2kV DTs are Subtractive  POLARITY TESTING OF A DT APPLY VOLTAGE HERE (240V or 120VAC ONLY) HV SIDE Temporary Shorting Vm Voltmeter Wire (Jumper) LV SIDE  If Vm reads more than the applied voltage - Additive  If Vm reads less than the applied voltage - Subtractive  Polarity of DTs X1 X3 X2 X4 Subtractive Polarity: 333 kVA, 3600/6240Y V - 120/240 V  Polarity of DTs X2 X1 X4 X3 Subtractive Polarity: 250 kVA, 34500Grd.Y/ 19920 V - 139/277 V Polarity of DTs X3 X2 X1 Additive Polarity: 25 kVA, 2400/4160Y V - 120/240 V  DT Connections Used By MERALCO System 1-Phase 1-Phase Open-Wye, Ungrd. Wye- Grd. Wye- Open-Delta- Delta- L-to-N L-to-L Open-Delta Delta Grd. Wye Open-Delta Delta 34.5 / 20 kV Wye Yes Yes Yes 13.2 / 7.62 kV Wye Yes Yes Yes Yes Yes Yes Yes 8.3 / 4.8 kV Wye Yes Yes Yes 6.24 / 3.6 kV Wye Yes Yes Yes 4.16 / 2.4 kV Wye Yes Yes Yes 13.8 kV Delta Yes Yes Yes 4.8 kV Delta Yes Yes Yes Note: The voltage on he leftmost column refer to the system voltage of the circuit & not on the voltage rating of the DT.  Single-phase Connection Line-to-Neutral Primary (Subtractive Polarity) A  N    H1 H2 X1 X3 X2 n   a  b  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V n N a Vbn = 120V A C PRIMARY SECONDARY Van = 120V  Single-phase Connection Line-to-Neutral Primary (Additive Polarity) A  N    H1 H2 X3 X1 X2 n   a  b  VOLTAGE VECTORS SECONDARY VOLTAGES B a Vab = 240V n N b Vbn = 120V A C PRIMARY SECONDARY Van = 120V  Single-phase Connection Line-to-Line Primary (Subtractive Polarity) A  B  H1 H2 X3 X1 X2 n  a  b  VOLTAGE VECTORS SECONDARY VOLTAGES B a Vab = 240V n A b Vbn = 120V C Van = 120V PRIMARY SECONDARY  Single-phase Connection Line-to-Line Primary (Additive Polarity) A  B  H1 H2 X1 X3 X2 n  a  b  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V n A a Vbn = 120V C Van = 120V PRIMARY SECONDARY  Open-Wye, Open-Delta Connection 2 Subtractive Polarity DTs A  B  C N    H1 H2 H1 H2 X3 X3 X1 X1 X2  X2 n   a b   c  VOLTAGE VECTORS SECONDARY VOLTAGES B b n Vab = 240V Van = 120V N a Vbc = 240V Vbn = 120V A C c Vca = 240V Vcn = 208V - Bastard PRIMARY SECONDARY Voltage  Open-Wye, Open-Delta Connection 2 Additive Polarity DTs A B  C  N    H1 H2 H1 H2 X1 X1 X3 X3 X2  X2 n   a b   c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 120V n N c Vbc = 240V Vbn = 120V A C a PRIMARY SECONDARY Vca = 240V Vcn = 208V Common Error in Open-Wye, Open-Delta Connections ERROR 1: X1 of Main DT was interlocked with X1 of Wing DT A B  C  N    H1 H2 H1 H2 X3 X1 X3 X1 X2 X2  n a    b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B c b Vab = 240V Van = 120V N n  Vbc = 240V Vbn = 120V A C a PRIMARY SECONDARY Vca = 416V Vcn = 317V Common Error in Open-Wye, Open-Delta Connections ERROR 2: H2 instead of H1 of one DT was connected to the primary A B   C N    H1 H2 H1 H2 X3 X1 X3 X1 X2  X2 n a    b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240 V Van = 120V N  n a Vbc = 416 V Vbn = 120V A C c PRIMARY SECONDARY Vca = 240 V Vcn = 317V Common Trouble in Open-Wye, Open-Delta Connections TROUBLE: H1 of both DTs energized from the same primary Detached jumper A  To B  Source C N    H1 H2 H1 H2 X3 X1 X3 X1 X2  X2 n a    b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b n Vab = 240 V Van = 120V N a Vbc = 480 V Vbn = 120V A C c PRIMARY SECONDARY Vca = 240 V Vcn = 360V  Wye-Delta Connection 3 Additive Polarity DTs A  B  C  N Floating Neutral  H1 H2 H1 H2 H1 H2 X1 X3 X1 X1 X3 X3 X2  X2  X2  n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 120V n N c Vbc = 240V Vbn = 120V A C a Vca = 240V Vcn = 208V - Bastard PRIMARY SECONDARY Voltage  Common Error in Wye-Delta Connections (Ex. 1 Subtractive & 2 additive DTs) ERROR: X1 of Wing 1 was connected to X1 of Main while X3 of Main was connected to X3 of Wing 2 A  B  C  Floating Neutral  H1 Wing 1 H2 H1 Main H2 H1 Wing 2 H2 X1 X1 X3 X1 X3 X3 C X2  X2  X2 C’ n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES c B a Vab = 240V Vc’a = 416V Van = 120V n N  Vbc = 416V Vbc’ = 240V Vbn = 120V b A C c’ Vca = 240V Vcc’ = 480V Vcn/Vc’n = 317V PRIMARY SECONDARY  Corrected Error in Wye-Delta Connection (Subtractive & 2 additive DTs) CORRECTION: H2 instead H1 of Main DT was tapped to the phase conductor. This results in the imaginary swapping of the X1 & X3 leads. A  B  C  Floating Neutral  H1 Wing 1 H2 H1 Main H2 H1 Wing 2 H2 H2 H1 X1 X1 X3 X1 X3 X3 X3 X1 X2  X2  X2 n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 120V n N c Vbc = 240V Vbn = 120V A C a Vca = 240V Vcn = 208V - Bastard PRIMARY SECONDARY Voltage  Wye-Wye Connection for 240-volt Service 3 Subtractive Polarity DTs A  B  C  N    H1 H2 H1 H2 H1 H2 X1 X4 139V 139V X4 X1 X4 X1  X3 X2   X3 X2   X3 X2    n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 139V N n Vbc = 240V Vbn = 139V A C a c Vcn = 139V Vca = 240V PRIMARY SECONDARY  Wye-Wye Connection for 480-volt Service 3 Subtractive Polarity DTs A  B  C  N    H1 H2 H1 H2 H1 H2 X1 X4 139V 139V X4 X1 X4 X1 X3 X2 X3 X2 X3 X2   n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 480V Van = 277V N n Vbc = 480V Vbn = 277V A C a c Vcn = 277V Vca = 480V PRIMARY SECONDARY  Wye-Wye Connection for 240-volt Service 3 Additive Polarity DTs A  B  C  N    H1 H2 H1 H2 H1 H2 X4 X1 X1 X4 X1 X4  X2 X3   X2 X3   X2 X3    n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 139V N n Vbc = 240V Vbn = 139V A C a c Vcn = 139V Vca = 240V PRIMARY SECONDARY  Wye-Wye Connection for 480-volt Service 3 Additive Polarity DTs A  B  C  N    H1 H2 H1 H2 H1 H2 X4 X1 X1 X4 X1 X4 X2 X3 X2 X3 X2 X3   n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 480V Van = 277V N n Vbc = 480V Vbn = 277V A C a c Vcn = 277V Vca = 480V PRIMARY SECONDARY  Open-Delta, Open-Delta Connection 2 Subtractive Polarity DTs A  B  C   H1 H2 H1 H2 X3 X1 X3 X1 X2  X2 n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B n b Vab = 240V Van = 120V A a 0º angular displacement Vbc = 240V Vbn = 120V C c Vca = 240V Vcn = 208V PRIMARY SECONDARY  Open-Delta, Open-Delta Connection 2 Additive Polarity DTs A  B  C   H1 H2 H1 H2 X1 X3 X1 X3 X2  X2 n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES c 180º angular B displacement Vab = 240V Van = 120V A a Vbc = 240V Vbn = 120V n C b Vca = 240V Vcn = 208V PRIMARY SECONDARY  Open-Delta, Open-Delta Connection of 2 Additive Polarity DTs A  B  C   H1 H2 H1 H2 X1 X3 X1 X3 X2 X2  n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B n b Vab = 240V Van = 120V A a 0º angular displacement Vbc = 240V Vbn = 120V C c Vca = 240V Vcn = 208V PRIMARY SECONDARY Common Error in Open-Delta, Open-Delta Connections ERROR: Secondary Interlock was Connected X3 to X3 A  B  C   H1 H2 H1 H2 X1 X3 X1 X3 X2 X2  n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES c B Vab = 240V Van = 120V A b Vbc = 240V Vbn = 120V n C a Vca = 416V Vcn = 317V PRIMARY SECONDARY  Delta-Delta Connection 3 Subtractive Polarity DTs A  B  C     H1 H2 H1 H2 H1 H2 X3 X1 X3 X3 X1 X1 X2  X2  X2  n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b n Vab = 240V Van = 120V A a Vbc = 240V Vbn = 120V c C Vca = 240V Vcn = 208V PRIMARY SECONDARY  Delta-Delta Connection 3 Additive Polarity DTs A  B  C     H1 H2 H1 H2 H1 H2 X1 X3 X1 X1 X3 X3 X2  X2  X2  n  a  b  c  VOLTAGE VECTORS SECONDARY VOLTAGES B b Vab = 240V Van = 120V A n c Vbc = 240V Vbn = 120V C a Vca = 240V Vcn = 208V PRIMARY SECONDARY SINGLE-PHASE 240V, LINE-TO-GROUND SECONDARY SYSTEM A  B C N   H1 H2 X3 X1 X2 a   b  INSULATED BARE GROUNDED PHASE CONDUCTOR PHASE CONDUCTOR OPEN-WYE, OPEN-DELTA CONNECTION FOR 3-PHASE, CORNER-GROUNDED DELTA SECONDARY A  B  C N    H1 H2 H1 H2 X1 X1 X3 X3 X2  X2 a   b  c  INSULATED PHASE BARE GROUNDED CONDUCTORS PHASE CONDUCTOR B b SECONDARY VOLTAGES N c Vab = 240V a Vbc = 240V A C Vca = 240V PRIMARY VECTORS SECONDARY VECTORS  WYE-DELTA CONNECTION FOR 3-PHASE, CORNER-GROUNDED DELTA SECONDARY A  B  C  N  H1 H2 H1 H2 H1 H2 X1 X3 X1 X1 X3 X3 X2  X2  X2 a  b  c   INSULATED PHASE BARE GROUNDED CONDUCTORS PHASE CONDUCTOR B b SECONDARY VOLTAGES N c Vab = 240V a Vbc = 240V A C Vca = 240V PRIMARY VECTORS SECONDARY VECTORS  Reminders on DT Installation A. New Installation  Inspect DT for any physical defects  Check specifications of withdrawn DT  Install as per safety & construction standards  Check for compliance to W.O. or F.O.  Before energizing:  Check for shorted/grounded service entrance conductors  Ensure customer main switch (CMS) is open & check for possible voltage presence  Energize DT  Check voltage at CMS  Accomplish Transformer Report (TR)  Reminders on DT Installation B. Replacement  Inspect DT for any physical defects  Check specifications of withdrawn DT  Eliminate all possible sources of power  Ground or short-circuit the secondary line or leads  Establish secondary phase markings/sketch existing connection  Install as per safety & construction standards  Before re-energizing:  Remove all temporary grounding wires  Check DT connection against sketch/drawing/phase markings  Release pressure by operating the pressure relief device  Energize DT  Check voltage & phase sequence at CMS (verify with customer)  Accomplish Transformer Report (TR) A Transformer Rated 13200 V A Transformer Rated 13200 Grd.Y/7620 V A Transformer Rated 7620/13200Y V  Available DT Banks at the Training Grounds 1) 3 – 15 kVA, 3600/6240Y – 120/240 Volts 2) 2 – 25 kVA, 34500Grd.Y/19920 – 240/120 Volts 3) 3 – 100 kVA, 34500Grd.Y/19920 – 120/240 x 139/277 Volts 4) 2 – 15 kVA, 34500Grd.Y/19920 – 240/120 Volts 5) 3- 10 kVA, 7620/13200Y – 120/240 Volts 6) 3- 15 kVA, 13200 – 120/240 Volts 7) 1-15 kVA, 13200Grd.Y/7620 - 240/120 Volts (Sub.) 1-10 kVA, 13200Grd.Y/7620 – 120/240 Volts (Add.)