Transformer Testing & Maintenance
The company offers a complete Transformer Maintenance Service from Power Factor Testing to Oil Treatment. The company uses specific testing and diagnostic techniques and tools to assess the condition of oil-filled and dry power transformers. These processes are often above and beyond the routine maintenance work required to keep the transformer operational.
Insulation Power Factor/Dissipation Factor Testing
The purpose of this test is to determine whether or not there is contamination of the windings and the insulation system, and to determine a power factor for the overall insulation, including bushings, oil, and windings. It is a measure of the ratio of the power (I squared R) losses to the volt-amperes applied during the test. The power factor obtained is a measure of watts lost in the total transformer insulation system including the bushings. The power factor should not exceed 0.5% at 20degrees C. Correction of test results can be done automatically on the test set. The watts loss should not exceed one-half of one percent of the total power input (volt-amps) from the test. The values obtained at each test are compared to previous tests and baseline factory tests, and a trend can be established as the insulation system ages. It should be remembered that power factor or dissipation factor is a measure of insulation dielectric power loss, and is not a direct measure of dielectric strength. Conditions which cause abnormal power loss usually also cause reduction of dielectric strength. The power-factor values are independent of insulation area or thickness, and increase only with an increase of contamination by moisture, other foreign matter or ionisation, and therefore are easier to interpret than insulation resistance values, which additionally depend on insulation area and thickness.
The purpose of this test is to determine whether or not there is contamination of the windings and the insulation system, and to determine a power factor for the overall insulation, including bushings, oil, and windings. It is a measure of the ratio of the power (I squared R) losses to the volt-amperes applied during the test. The power factor obtained is a measure of watts lost in the total transformer insulation system including the bushings. The power factor should not exceed 0.5% at 20degrees C. Correction of test results can be done automatically on the test set. The watts loss should not exceed one-half of one percent of the total power input (volt-amps) from the test. The values obtained at each test are compared to previous tests and baseline factory tests, and a trend can be established as the insulation system ages. It should be remembered that power factor or dissipation factor is a measure of insulation dielectric power loss, and is not a direct measure of dielectric strength. Conditions which cause abnormal power loss usually also cause reduction of dielectric strength. The power-factor values are independent of insulation area or thickness, and increase only with an increase of contamination by moisture, other foreign matter or ionisation, and therefore are easier to interpret than insulation resistance values, which additionally depend on insulation area and thickness.
Capacitance Tests
This test measures and records the capacitance (including bushings) between the high and low voltage windings, between the high voltage winding and the tank (ground), and between the low voltage winding and the tank (ground). Changes in these values as the transformer ages and other events occur, (such as nearby lightning strikes or through faults), indicate winding deformation and structural problems such as displaced wedging and winding supports.
Excitation Current Test
The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems. On three-phase transformers, results are also compared between phases. This test measures the current needed to magnetize the core and generate the magnetic field in the windings.
For bushings having a potential tap, both the capacitance between the top of the bushing and the bottom tap (normally called C1) and the capacitance between the tap and ground (normally called C2) are measured. To determine bushing losses, power factor tests are also performed. Bushings without a potential tap are normally tested from the bushing top conductor to ground. These test results are compared with factory tests and/or prior tests to determine deterioration. About 90% of bushing failures may be attributed to moisture ingress evidenced by an increasing power factor.
This test measures and records the capacitance (including bushings) between the high and low voltage windings, between the high voltage winding and the tank (ground), and between the low voltage winding and the tank (ground). Changes in these values as the transformer ages and other events occur, (such as nearby lightning strikes or through faults), indicate winding deformation and structural problems such as displaced wedging and winding supports.
Excitation Current Test
The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems. On three-phase transformers, results are also compared between phases. This test measures the current needed to magnetize the core and generate the magnetic field in the windings.
For bushings having a potential tap, both the capacitance between the top of the bushing and the bottom tap (normally called C1) and the capacitance between the tap and ground (normally called C2) are measured. To determine bushing losses, power factor tests are also performed. Bushings without a potential tap are normally tested from the bushing top conductor to ground. These test results are compared with factory tests and/or prior tests to determine deterioration. About 90% of bushing failures may be attributed to moisture ingress evidenced by an increasing power factor.
(Sweep) Frequency Response Analysis (SFRA/FRA)
For (S)FRA the company uses an Omicron FRAnalyzer, the FRAnalyzer is a sweep frequency response analyzer for power transformer core and winding diagnosis. Its concept – universal hardware controlled by software running on a computer – makes the FRAnalyzer an efficient and flexible solution for the diagnosis of power transformer windings and magnetic cores. The FRAnalyzer evaluates the frequency response of the transformer windings by using the sweep frequency response analysis (SFRA) in the frequency domain. A sinusoidal voltage with constant amplitude and variable discrete frequencies is applied to the winding under test and the frequency of the input signal is successively increased. The amplitude and phase of the output signal is measured against the frequency and the output-to input amplitude ratio and the phase shift between the output and input signals are evaluated.
The FRAnalyzer measures the frequency response of the transformer windings in a wide frequency range and compares it with that in a healthy condition. From the frequency response deviations, many different types of defects in the transformer winding and magnetic core can be diagnosed.
These include:
For (S)FRA the company uses an Omicron FRAnalyzer, the FRAnalyzer is a sweep frequency response analyzer for power transformer core and winding diagnosis. Its concept – universal hardware controlled by software running on a computer – makes the FRAnalyzer an efficient and flexible solution for the diagnosis of power transformer windings and magnetic cores. The FRAnalyzer evaluates the frequency response of the transformer windings by using the sweep frequency response analysis (SFRA) in the frequency domain. A sinusoidal voltage with constant amplitude and variable discrete frequencies is applied to the winding under test and the frequency of the input signal is successively increased. The amplitude and phase of the output signal is measured against the frequency and the output-to input amplitude ratio and the phase shift between the output and input signals are evaluated.
The FRAnalyzer measures the frequency response of the transformer windings in a wide frequency range and compares it with that in a healthy condition. From the frequency response deviations, many different types of defects in the transformer winding and magnetic core can be diagnosed.
These include:
- Coil deformation – axial and radial
- Faulty core grounds
- Partial winding collapse
- Hoop buckling
- Broken or loose clamps
- Shorted turns and open windings
- Core deformation
Transformer Turns Ratio
Transformer Turns Ratio is the ratio of the number of turns in the high voltage winding to that in the low voltage winding. Our TTR300 Series of three-phase transformer turns ratio test set is designed to measure the turns ratio of power, instrument, and distribution transformers in a substation or manufacturing environment. The instruments are ideal for use by power transformer manufacturers. Their unique testing procedures and storage capability allows an operator to set up and test difficult three-phase transformers in a fraction of the time than it used to take with other TTRs.
Transformer Turns Ratio is the ratio of the number of turns in the high voltage winding to that in the low voltage winding. Our TTR300 Series of three-phase transformer turns ratio test set is designed to measure the turns ratio of power, instrument, and distribution transformers in a substation or manufacturing environment. The instruments are ideal for use by power transformer manufacturers. Their unique testing procedures and storage capability allows an operator to set up and test difficult three-phase transformers in a fraction of the time than it used to take with other TTRs.
Phase Angle Deviation
The phase angle deviation is the relationship between the voltage signal applied to the high voltage winding and the voltage signal extracted from the low voltage winding. The phase deviation between the high and low side of a transformer is generally very small. However, the phase deviation can change significantly if there is deterioration or damage in the transformer core. Our test equipment can measure this phase relationship with the resolution necessary to detect a problem.
Winding Resistance
Measuring the DC resistance of transformer windings with a High current micro-Ohm Meter will aid in identifying problems such as shorted or open windings, as well as loose connections. Our test equipment can measure the DC resistance of both single phase and three phase transformer windings.
The phase angle deviation is the relationship between the voltage signal applied to the high voltage winding and the voltage signal extracted from the low voltage winding. The phase deviation between the high and low side of a transformer is generally very small. However, the phase deviation can change significantly if there is deterioration or damage in the transformer core. Our test equipment can measure this phase relationship with the resolution necessary to detect a problem.
Winding Resistance
Measuring the DC resistance of transformer windings with a High current micro-Ohm Meter will aid in identifying problems such as shorted or open windings, as well as loose connections. Our test equipment can measure the DC resistance of both single phase and three phase transformer windings.
Temperature Probes
The Hart Scientific 9140 Mid-Range Field Calibrator may be used as a portable instrument or bench
top temperature calibrator for calibrating thermocouple and RTD temperature probes. The 9140 is small enough to use in the field, and accurate enough to use in the lab.
The temperature is accurately controlled by Hart’s hybrid analog/digital controller. The controller uses a precision platinum RTD as a sensor and controls the well temperature with a solid state relay (triac) driven heater. The LED front panel continuously shows the current well temperature. The temperature may be easily set with the control buttons to any desired temperature Within the specified range.
Dry type Transformers
The increase in the popularity of dry type transformers in recent years means that is no longer possible to use traditional Dissolved Gas Analysis to monitor the condition of these transformers on site or to assess their condition after a major fault or lightning strike on the system. However, our state of the art test equipment allows the condition monitoring of dry type transformers comparable to that of oil filled transformers.
The Hart Scientific 9140 Mid-Range Field Calibrator may be used as a portable instrument or bench
top temperature calibrator for calibrating thermocouple and RTD temperature probes. The 9140 is small enough to use in the field, and accurate enough to use in the lab.
The temperature is accurately controlled by Hart’s hybrid analog/digital controller. The controller uses a precision platinum RTD as a sensor and controls the well temperature with a solid state relay (triac) driven heater. The LED front panel continuously shows the current well temperature. The temperature may be easily set with the control buttons to any desired temperature Within the specified range.
Dry type Transformers
The increase in the popularity of dry type transformers in recent years means that is no longer possible to use traditional Dissolved Gas Analysis to monitor the condition of these transformers on site or to assess their condition after a major fault or lightning strike on the system. However, our state of the art test equipment allows the condition monitoring of dry type transformers comparable to that of oil filled transformers.