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Transcript
TESTING AND COMMISIONING THE POWER TRANSFORMER DET310 CHAPTER 4 THE POWER TRANSFORMER INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 3.1 INTRODUCTION Substation transformers can range from the size of a garbage can to the size of a small house; they can be equipped with a wide array of gauges, bushings, and other types of auxiliary equipment. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2 TRANSFORMER NAME PLATE The transformer nameplate contains most of the Important information that will be needed in the field . The nameplate should never be removed from the Transformer and should always be kept clean and legible. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 CONTINUEa) SERIAL NUMBER i) WEIGHT b) CLASS j) DIMENSION c) KVA RATING k) TAP CHANGER d) VOLTAGE RATING e) WINDING f) VEKTOR GROUPING g) PERCENTAGE IMPEDANCE h) INSULATING FLUIDS INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2.1 TRANSFORMER CLASS ONAN -Oil natural air natural ONAF - Oil natural air force (use fan) OFAF - Oil force air force ( use oil pump and fan) OFWF -Oil force water force (use oil pump and water cooling) ODAF – Oil force and air force Example Transformer Rating: 1000 KVA 550C ONAN, which stands for the capability to supply 1000KVA with a temperature rise of 550degres celcius with normal oil circulation (no fans or pumps). INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Hermetically sealed ONAN Conservator Type OFAF Conservator Type OFAF INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2.2 VA RATING Every transformer has a maximum output current that it can deliver at its standard output voltage. This VA rating (KVA or MVA for large power transformers) is dependent on the ambient temperature or cooling provided. Exceeding the VA rating will cause overheating of the core and windings and subsequent damage INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 ContinueThe capacity of a power transformer is defined by full load power, in kVA it can produce. S U ph I ph x10 3 x 3 Where S = Full load Power Uph= rated voltage at no load (V) Iph = rated output current (A) For Star Connection: I phase= Sx10 3 3xU ph Phase Voltage= U ph 3 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2.3 VOLTAGE RATING Transformer rated voltage is usually specified by the voltage ratio Example: 11/0.415kV ; Primary rated Voltage : 11kV Secondary rated Voltage: 0.415kV 4.2.4 WINDING EXAMPLE: PRIMARY : DELTA SECONDARY: STAR 4.2.5 VEKTOR GROUP The vector groups denotes denotes the way the transformers windings are connected: INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Continue: Example: Dy11 D= HV delta winding y= low voltage star winding 11=clock face (30 degrees lead) with reference to the HV 12 o’clock INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Continue: Winding falls into three main types: star, delta, and zigzag Can be categorised into four groups Group1: 0 degress phase shift: Yy0,Dd0,Dz0,Zd0 Group 2: 1800 deg phase shift: Yy6, Dd6, Zd6, Dz6 Group 3: -30 deg phase shift: Dy1, Yd1, Yz1, Zy1 Group 4: +30 deg phaseshift:Dy11,Yd11,Yz11,Zy11 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2.6: IMPEDANCE The impedance of transformers is express in percentage values to Enable fault level calculations to be carried out Example: 11/0.416kV Impedance: 4.75% Fault level: Transformer _ rating (V . A) x100% Sec. _ Voltagex Im pedance(%) INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.2.7 TRANSFORMER OIL Act as an insulation and thermal transfer agent Mineral hydrocarbon oil to BS 148 Sometimes synthetic liquids, called askarels (pyroclor) are used instead of oil as they are non-inflammable and non-explosive but very expensive INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING 4.2.8 TAP CHANGER 4.2.8 TAP CHANGER DET310 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 TAP CHANGER FUNCTION: ALLOWS THE NOMINAL VOLTAGE TO ADJUSTED TYPICALLY 2.5% AND 5% ABOVE AND BELOW NOMINAL VOLTAGE EXAMPLE: 11kV 5 taps Tap1: 5% below: 10.45 V Tap2: 2.5 % below 10.725 V Tap 3: Nominal Tap : 11kV Tap 4: 2.5 % above: 11.275 kV Tap 5: 5% above: 11.5kV INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.3 TRANSFORMER OIL 4.3.1 OIL DETERIORATION a) Oxidisation of Oil- Oil reacts with oxygen to form a hydrocarbon sludge - The sludge is deposited on the winding tank walls - Causing heat to develop. - Sludge may also block the cooling ducts. - Oxidisation will results in formation of hydrocarbon and acid - Causing corrosion of the various metals within the transformers INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.3.2 Thermal Decomposition of Oil - Heating of a Transformer will results of thermal decomposition - Will lead to breakdown of oil insulation - Prolonged operation above 98 deg reduces transformers life 4.3.3 Contamination - Particularly from moisture which enters through transformer breather INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.4 Oil System for Large Transformer a) The conservator - Oil reservoir and expansion vessel - Exhaust through breathing device-usually fitted with silica gel or drycol refrigeration dryer type b) Explosion Vent - Consists of a thin and relatively weak diaphragm -Under internal fault, the excess pressure in the tanks burst into explosion vent to release the pressure - Under normal circumstances, seals the transformer from the atmosphere INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 CONSERVATOR TANK BREATHER INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 RELIEVE VALVE INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.5 THE BUCHOLZ A mechanical relay connected through pipe and situated between main tank and conservator tank of a transformer. Only certain kVA transformer need to be installed with one, normally above 1MVA. It will be connected to the trip coil of the circuit breaker controlling the transformer or the installation INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Continue Principles of operation: Electrical faults cause violent generation of gases, which flows to the highest point, namely the conservator. This gases will Trapped inside the Bucholz compartment causing the level of oil to reduce thus causing the relay to initiate. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 continue”: Buchholz relay can be triggered by three operation 1) Air from atmosphere while filling the transformer oil 2) Mechanical failure 3) Insulation breakdown from oil 4) Winding insulation breakdown Gas formed usually consists of hydrogen, Acetylene and carbon monoxide. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Continue: 1) If the gas only contained less then 2% carbon monoxide, only breakdown in oil occurs. 2) If the gas contained more then 20%, the fault are due to both breakdown of oil and insulation INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6 COMMISSIONING TESTS 4.6.1 DATA COLLECTION - manufacture and date of manufacturer - serial Number - rating-normal and assisted - nominal voltages - full load current at nominal tap - percentage impedance - vektor group - tap changer type, range and no of tap INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6.2 General check and Inspection -Coordinating Gaps -Labelling -Earth Connections -Breathers -Valves in operational position -Tightness of fastening -Insulators undamaged and clean -No oil leaks -Correct oil levels in conservators INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6.3 Insulation Tests 1. Check Insulation between low voltage winding to earth with 1kV IR tester 2. Check Insulation between high voltage winding to earth with 5 kV IR tester. 3. Check Insulation between HV and LV winding with 5 kV IR tester Duration of all are one minutes Insulation Test 1kv and 5kV: Power Transformer Hv Side Lv Side E Bacaan 100 M,ohms keatas ARB INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6.4 Polarization Index Test. Carry out IR test between Low Voltage to Earth with 1kV IR tester Carry out IR test between HV to Earth with 5kV IR tester Carry out IR test between HV to LV with 5kV IR tester Duration of test is 10 minutes Divide the value obtained with value for 1 min test Polarization Index = R10/R1 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Between Insulation Resistance (MΩ) PI Accepted value Not less than 1.2 1 Minute 10 Minute Primary & Earth 4000MΩ 7000MΩ 1.75 Secondary & Earth 3500MΩ 8000MΩ 2.29 Primary & Secondary 5100MΩ 10000MΩ 2.00 INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6.5 Ratio Test 1) Inject 415V three phase supply to HV side of transformer with the LV side open circuited. 2)Measure the HV side and LV side voltage and calculate the ratio. Compare the ratio with the name plate value and ensure that the ratio is within the tolerance. 3)Repeat for all transformer taps. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 Primary Voltage AB: 200 BC: 200 CA: 200 Secondary output voltage Tap No Voltage an bn cn 4.32 4.32 4.32 1 11550 4.43 4.43 4.43 2 11275 4.54 4.54 4.54 3 11000 4.66 4.66 4.66 4 10725 4.78 4.78 4.78 5 10450 Example: a) At tap changer no 1: VAB = 200V (Delta) Vab VAB x Van 1 433 200 x 7.5V (line ) a 11550 7.5 4.32V ( phase) 3 Therefore the ratio is correct. INSUALTION TEST OF ELECTRICAL EQUIPMENT TESTING AND COMMISIONING DET310 4.6.7 Impedance test: Positive sequence impedance 1) Inject three phase supply to HV side of transformer with LV side short circuited as shown below. 2)With the transformer at nominal tap, measure and record the current and voltage at HV side. 3) Calculate the percentage impedance as given below: % Impedance Where Vm = measured voltage Im = measured current Vr = rated voltage Ir = rated current Compare the results with declared value on the nameplate. Repeat the above for transformer at lowest and highest tap positions.
