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We can offer a wide range of off-line and on-line measurement on power transformers
Needed instruments
Mega-ohmmeter
Available instruments
Megger MIT 525
Reference standard
IEEE Std 57.113
Description
IR is defined as the relationship between the voltage applied to the insulation and the current absorbed by it. Because the IR value is strongly dependent on temperature, the ratio between IR at 10 minutes and IR at 1 minute (from voltage application) is calculated, this ratio is the polarization index (PI) which allow to obtain results less dependent on temperature.
The measurement is performed by applying to the insulation object of the test a negative continuous voltage (amplitude dependent on the rated voltage of the winding) and measuring its absorbed current. Other windings are grounded. The voltage application lasts 1 minute (IR only) or 10 minutes (IR and PI), in the second case RI is detected every minute.
The test is performed by applying 5000 V to each winding, grounding the others, and between windings.
This measurement is performed by applying 500 V also to the core and armature insulation towards ground.
Needed instruments
Measurement bridge, current booster
Available instruments
Tettex 2880G + 5287
Reference standard
–
Description
The short-circuit inductance / reactance measurement is performed in order to highlight possible winding deformations.
The measurement is made by properly connecting the power supply bridge and the power booster to the windings.
The short-circuit inductances of all windings are measured.
Needed instruments
Ratio-meter
Available instruments
Tettex TTR 2796
Reference standard
CEI 14-4
Description
With this measure, the transformation ratios, polarity of the windings and vector group are determined.
The measurement is performed at the same time on the three phases, connecting the instrument to the winding terminals.
Needed instruments
Milli-ohmmeter
Available instruments
IRIS Power (Adwel) DWR10
Reference standard
CEI 14-4
Description
The measurement of the ohmic winding resistance is intended to verify the good state of the winding connections to the bushings, and if available, tap changers connections.
The measurement is performed using classic volt-amp method, performed with an integrated instrument, which directly supplies the resistance value and discharge the winding after completing the measure.
Needed instruments
Measurement bridge, HVAC supply
Available instruments
Doble M4100
Omicron CPC80 + CP TD1
Tettex Midas 2880G
Reference standard
CEI 14-4
Description
These tests allow the evaluation of aging of insulating materials and their moisture content. The capacity of the entire winding also allows to highlight any deformations of the winding itself.
The measurement is carried out by means of an AC power bridge, connected to the phase to be measured. Bushings can be separately measured when equipped with a capacitive tap. The measurement is performed on all windings and between windings.
Needed instruments
SFRA instrument
Available instruments
Doble M5400
Reference standard
CEI 14-4
Description
The SFRA measurement consists of injecting a variable frequency sinewave signal at each winding and obtaining its “response”. This results in a graph showing the response dBs vs frequency.
This measure, when compared with the same measure previously performed on the same machine, allows to diagnose the presence of shorts between turns and deformations of the winding, core displacements and core ground problems.
The measurement is performed with a special instrument, time to time connected to the winding to be measured.
Needed instruments
Strumento di misura SP
Available instruments
Techimp Portable PDCheck 2.4
Reference standard
IEEE Std 57.113, IEC 60270
Description
Partial Discharges (PDs) are electrical discharges located inside or on the insulation surfaces, which only partially bridges the insulation distance.
In power transformers the presence of Partial Discharges can occur both in Insulating Oil (Corona) and in the impregnated paper inside the tank, one of the most delicate points are in any case bushings, where the presence of PD is often linked to rather rapid failure mechanisms.
The exposure of transformers to overvoltages may trigger PD phenomena that may not be extinguished when the transient is finished.
An on-line SP measure, to be repeated annually, if no defects are detected, can help in preventing long outages the machine.
The graph that compresses most information from PD measurement is the Pattern (Amplitude Vs Phase), which identifies the type detected phenomena.
Off-line measurements are also possible, by measuring through the capacitive taps of bushings. External HV supply is needed.
Needed instruments
PD instrument, PD calibrator
Available instruments
Techimp Portable PDCheck 2.4
Reference standard
IEEE Std 57.113, IEC 60270
Description
The presence on a power transformer of permanently installed sensors allows Partial Discharges (PD) to be measured at any time during machine service.
W.E.S.T. Power Srl offers its customers supply of sensors, all their accessories and installation service.
Sensor installation requires a working day during which the transformer will be out of service.
Installation consists of the following elements:
