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(SFRA-Sweep Frequency Response Analysis)
The measurement is carried out in accordance with IEC 60076-16: 2018. The test is repeated for each pair of HV-winding connections. e.g .: for 1U the impedance between 1U and 1V is measured.
The following applies to the other connections:
The HV-phases 1V and 1W are short-circuited and the star point of the LV is connected to the earth connection of the transformer. This variant must be used if the LV-star-point is grounded during operation. The reference value is thus 1U the phases.
1V and 1W are short-circuited, connected to the earth point and the star point of the LV is connected to the earth-connection of the transformer. This variant applies in order to see the difference in the case of a high-voltage earthing and thus results in the reference value for 1U the HV-phases.
1V and 1W are short-circuited and the star point of the LV is not connected. This variant must be used if the LV-star-point is not earthed during operation and thus forms the reference value for 1U. The measuring device stores the impedance value in 1000 points from 20 Hz to 2 MHz. These measured values <s></s>are measured at a later point in time and compared with the values of the initial measurement.
The DC resistance is measured between the phases on all winding systems using a resistor bridge. The winding temperature is also recorded
The voltage ratio and the vector group (phase shift) are measured at the nominal tap using a ratio measuring device. (Reference: specified vector group and tolerance of the voltage transmission
The transformer is short-circuited on the low-voltage side and fed in on the high-voltage side with the rated frequency and at least 50% of the rated current. The voltages, currents and losses are measured. (Reference: guaranteed value)
The transformer is fed in on the low-voltage side with the rated voltage and rated frequency. The other windings remain "open". The voltages, currents and losses are measured. (Reference value: guaranteed value)
The connections of the windings to be tested are short-circuited and connected to a single-phase AC voltage source. All other windings are short-circuited and earthed together with the core and all structural parts. The test voltage, the value of which depends on the specified insulation level, is applied for 60 seconds. The insulation of the auxiliary wiring is then checked with a voltage of 2kV for 60 seconds. (Remark: For transformers which have been in service, this level shall be reduced to 80%)
A voltage of 180% of the rated voltage and the frequency of 200 Hz is induced for 30 seconds. After the 30 seconds have elapsed, the voltage is reduced without interruption to 130% of the rated voltage and kept constant for 3 minutes. During the three-minute test period, the partial discharges are measured at specific time intervals
Der Transformator wird auf der Unterspannungsseite mit Bemessungsspannung und der Nennmessungsfrequenz angespeisst. Die anderen Wicklungen bleiben dabei "offen". Die Geräuschmessung erfolgt mit 10 Mikrophonen, die im Abstand von einem Meter zum Transformator und höchstens einem Meter untereinander positioniert worden sind. Aus 12 gemessenen sog. A-Werten wird der unkorrigierte Schalldruckpegel berechnet. Der A-bewertete Schallleistungspegel wird anschließend aus dem nicht korrigierten Schalldruckpegel und im Flächeninhalt der Messfläche berechnet.
The transformer is tested with a pulse voltage, the peak value of which is based on the specified insulation level. The test impulse voltage has the wave form 1.2 / 50µs (full wave) and has a negative polarity. The connections of the windings are “impulsed" one after another, with the other connections of this system being earthed via a shunt. All other windings are short-circuited and earthed together with the core and all structural parts. The test sequence for each winding connection to be "impulsed" consists of one 50 - 75% (reference) impulse and three consecutive 100% impulses.
The capacities of the windings to earth and between the windings as well as the loss factors of these are measured with the help of a capacitance measuring bridge. The ambient conditions prevailing during the measurement (temperature, relative humidity and pressure) are also determined. The measured capacity values, loss factors and the environmental parameters are documented