Transformers Testing

Transformer winding ohmmeters

Transformer ratio meters (TTR)

Loss tangent meter in dielectric oil TAND220A

OT-90: oil breakdown voltage tester

B100 Electronic Temperature Monitor (ETM)

C50 Bushing Monitor

Dielectric strength meter Dielectrotest

Insulating Oil Dielectric Loss & Resistivity Tester

Transformer Monitor E3

Distribution transformer tester ADOC-3

AE865C: mini oil degasifier

AEHV: vacuum oil purifier

Transformer testing instruments

Types of tests

DC tests

Measurement of winding resistance
OLTCs analysis
OLTC vibration testing
Heat-up test
Demagnetization process

AC tests

Measurement of the transformation ratio of power transformers
Verification of the transformation ratio of instrument transformers
Measurement of the excitation current
Phase angle measurement
Automatic vector group detection
Vector group detection
Magnetic equilibrium test
Magnetic balance test
Sweep frequency response analysis

Winding resistance measurement

Why is it important?

Damage to windings and connections
Malfunctioning of the OLTC contact
Defined by standards and guidelines

How it is performed

Inject DC current, measure voltage drop, calculate resistance R = U / I
Must be done for all phases and each tap position
Compare with reference measurement, on all phases

Kelvin's method (4-wire)

How is it done?

Dc current injection, measurement of voltage drop R = U / I
Time required to charge the windings and stabilize the current
A larger transformer requires higher currents and more time to saturate

Loading process

The current flowing through the coils causes a magnetic field that generates an opposing force (EMF) that slows down the charging process
At the beginning, the EMF is equal to the applied voltage and decreases as the current (Vs/R) increases

Loading

Loading time

Volt-seconds needed
A 100 kV winding at 50 Hz needs 450 Vs
A 100 kV winding at 60 Hz needs 375 Vs
Thus, the 50 Hz winding needs 75 s at 6 V
O ≈8 s at 60 V
Up to one minute for large transformers

Stabilization process

Core saturation
Depends on the time constant L/R
Eliminates the influence of inductance

How to saturate the core

A low current may not saturate the core in a reasonable time
The stabilization time depends on the time constant L / R
High current provides better saturation, which leads to less influence of L and ultimately shortens the test time
Higher magnetomotive force (FMM) = N x I - faster stabilization
Too high current can heat up the winding (IEC Itest<10%In, IEEE Itest<15%In)

Higher current => better saturation => less influence of L => shorter time

Too high currents heat up the winding (< 10% of In)

50 A - Test current Cooper heating

Measuring the test current

Test current

Measuring on the LV side can be time consuming

The LV side can have a very low resistance
Therefore large L / R time constant
Especially if the LV windings are delta connected, the current flows through all three phases
In large units, stabilization can take 15 to 60 minutes per phase
Sometimes, even 100 A may not be enough to significantly shorten the time

Use the high-voltage winding as an additional aid for faster saturation

LV winding measurement with HV winding support

The high voltage side has a much higher number of turns
More ampere-turns with less current
Increased MMF = (N1 + N2) ∙ I
MWF increase = (N1 + N2) ∙ I
Faster saturation

Test current

How is the test current determined?

At least 12 x Iex to saturate the core in a reasonable time
Iex is usually 2-5% of In (rated current)
Itest = 3% - 6% of In for most power transformers
Market offerings from 1 A to 100 A

Discharge

Safety

Inductive energy accumulated in the windings
Accidental open circuit

IEEE guide for field diagnostic testing of power transformers, regulators, and fluid-filled reactors, IEEE std C57152TM

In phase-to-phase comparison, the deviation of the resistance value should be less than 2%
CIGRE Working Group A234 - Guide for the maintenance of transformers
In comparison between phases, the deviation of the resistance value should be less than 2-3%
In comparison with the factory acceptance test results, the deviation of the resistance value should be less than 2-3%

What is a "good" result?

Tips from transformer manufacturers

VH deviation between phases - less than 03 - 05%

LV deviation less than 3-5%: more tolerance to error due to other circuit elements and smaller resistance values

The LV deviation between phases - less than 3-5%: more tolerance to error due to other circuit elements and smaller resistance values

The final decision is made based on the technical expertise of the test supervisors and their evaluation of the conditions

Transformer demagnetization

Why is it important?

The remanent magnetism can cause a variety of problems:

High amplitude inrush current at power transformer start-up
Malfunction of the protection relays
Erroneous results during electrical measurements on the transformer (FRA test, excitation current measurement, magnetic balance test, erroneous vector group detection)

Causes of transformer magnetization:

When disconnecting a transformer from service
Consequence of high fault currents
When disconnecting a transformer from service
Testing with direct current
After a time out of service

Comparison of maximum inrush current with and without residual flux

The user selects the initial current
At each subsequent step, the current decreases by 40% of the previous value
Demagnetization ends when a current of 5 mA is reached

How?

Demagnetization should be performed before AC tests, especially before FRA and excitation current testing, before AVGD, etc
If the test object is a three-phase (auto)transformer, demagnetization of all 3 phases should be performed
The initial demagnetization current should be as high as possible If the demagnetization is performed after a DC test on a transformer, such as winding resistance, the initial demagnetization current should have the same value as the current used for the DC test
Demagnetization should be performed from the side of the transformer that has the neutral point accessible If both the high voltage (HV) and low voltage (LV) sides have/do not have accessible neutral point, demagnetization should be performed from the HV side

When to demagnetize?

Before AC testing - to avoid bad results due to remanent magnetism
Before recommissioning a transformer - to avoid high inrush currents and incorrect operation of protective relays

AC test devices

Turn ratio measurement
Excitation current measurement
Phase angle measurement
Vector group detection
Magnetic balance test
SFRA

Turn-ratio measurement

Most important test
Detection of defects in the winding/core
Possible connection errors
Short-circuits
Standardized measurement
The pass/fail criterion for deviations from the turns ratio is ±05 %
Important for FAT to verify the nameplate

The ratio is measured by applying a voltage on the high-voltage side and measuring on the low-voltage side
Phase-to-phase with single-phase voltage
All phases at once with actual three-phase voltage
Any type of transformer can be tested, including special transformers, such as phase-shifting transformers, and transformers with non-standard vector group
Higher voltage: higher accuracy

Excitation current measurement

Why is it important

Short circuit detection
Laminated core shorts
Core delaminations
Delaminations of the core
Verification of the demagnetization process

How is it performed

The excitation current is the transformer primary no-load current or the magnetizing current (with open secondary) tested at reduced voltage
The results are compared with the values obtained with the same test voltage values
The results must follow a specific pattern based on the type of transformer construction

Vector group detection

Why is it important

Transformer vector group verification
During/after manufacturing
During/after repair
Searching for incorrect connections

How is it done?

Utilization of a proprietary algorithm and software solution loaded into the TRT
Fully automatic

Phase angle measurement

Why is it important?

It can indicate problems in the windings and/or magnetic core
Measurement of the displacement of transformers with specific vector groups (phase-shifting, rectifier, arc furnace and traction transformers)
Polarity verification of measuring transformers

How it is performed

Comparison of the AC voltage(s) applied on the HV side and the AC voltage(s) induced on the LV side

SFRA Measurement

SFRA measurement
Frequency response scanning analyzer
"Fingerprint" method
Used to evaluate the mechanical integrity of a transformer
Fingerprint" method

Core strain/movement
Core strain/motion
Faulty grounding
Faulty grounds
Short-circuited laminations
Core deformation/movement
Faulty grounding
Coil deformation (axial and radial)
Winding deformation (axial and radial)
Short-circuited coils or open windings
Shifting or deformation of the winding