After Oil immersed transformer be installed, we should do testing works as follows:
1. Telemetry of insulation resistance:
(1) Telemetry project, high pressure to low pressure and ground (shell), low pressure to high pressure and ground (shell)
(2)Select 2500V megohm meter, the appearance of megohm meter should be good, complete shell, flexible shake handle, no long resistance pointer, no glass damage.
(3) Open circuit test of megawatt-hour meter, separate two pens, shake the handle of megawatt-hour meter up to 120 y/min, the needle pointing to infinite (∞) is better. Short circuit test: shake the handle of megawatt-hour meter, overlap two than instantaneous, the needle pointing to “0” (zero), indicating that megawatt-hour meter is normal.
(4) Qualified value: at 20℃+, the new transformer is not less than 450 MΩ, and the operation is not less than 300 MΩ. This value should not be reduced by 30% compared with the previous value.
(5)Absorption ratio R60/R12 should be 1.3 times at 10-30℃
2. DC resistance measurement:
It can measure the welding quality of transformer internal conductor and lead, whether the parallel branch connection is correct, whether there is interlayer short circuit or internal break, tap-changer, whether the contact between bushing and lead is good, etc.
3. Measurement method:
bridge can be used to measure, direct reading, high accuracy, no bridge can use voltage drop method, voltage drop method to measure DC resistance wiring a) measurement of small resistance b) measurement of large resistance 1 – measured coil 2 – knife gate 3 – accumulator 4 – voltmeter 5 – ammeter DC resistance calculation formula: U – voltmeter reading (mv) I – ammeter reading (A) with tap switch Transformers of 1.5% shall be measured at all tap positions during handover or overhaul. When a three-phase transformer has a neutral point due to the outlet line, the resistance of each phase line diagram should be measured. When there is no neutral point outlet line, the resistance of the line can be measured.
When measuring, all non-tested line diagrams should be open and not short-connected. When measuring, we must wait for the current to stabilize before reading. We should pay attention to personal safety.
4. Judgment criteria:
The difference between DC resistance of each phase diagram should not be greater than 2% of the three average values, and the relative change should not be greater than 2% compared with previous measurements. In order to compare with the measured values in the factory or in the past, the DC resistance values should be converted to the same temperature values.
The formula is as follows: R1=R2*(T+Δt1）/（T+Δt2）. For copper conductors, the T value is 235; for aluminium conductors, the T value is 225. Formula: R1 – converted DC resistance value at t℃, R2 – current temperature resistance value, Δt1 and Δt2 represent the corresponding temperature (℃) value of conductor respectively.
For example, if you know that the resistance of a copper conductor at 30℃ is R30, then the resistance of the conductor at 60℃ is: R60 = R30* (235+60)/(235+30)= R30*295/265 = 1.113*R30.
Oil immersed transformer test Failure reasons:
(1)Poor contact of tap changer
(2)The coil or lead is badly welded and broken.
(3)Poor connection between conductive rod and lead of casing
(4)Interturn or interlayer short circuit of coil.
5.Oil immersed transformer Group test:
(1) Polarity measurement by single-phase transformer. The purpose of three-phase transformer measurement group is to make correct connection and judge whether the transformer can operate in parallel.
(2) Polarity measurement: DC and AC measurements can be used. In addition, DC measurements are introduced: DC test wiring chooses 2-4V batteries and zero position in the central DC voltmeter. At the moment of closing, the meter needle swings in the positive direction, while at the moment of opening, the meter needle swings in the negative direction, which reduces polarity. On the contrary, add polarity.
(3) There are DC method and AC method to determine the connection group of three-phase transformer.
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Oil immersed transformer trial operation:
1. Transformers can be tested only after all test items are qualified.
2. Before commissioning, a comprehensive inspection of transformers should also be carried out.
3. The transformer is subjected to five impact tests (closing tests).
4. No-load operation time is related to transformer capacity, generally no less than 24 hours.
5. After no-load operation time is completed, the transformer will be reloaded.
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