Power Transformers Testing Booklet For True Engineers

Power Transformer Measurements

This booklet serves as a supportive guide, providing a summary of applied electrical measurements on power transformers. These electrical tests are crucial for assessing and confirming the parameters and dielectric strength of the transformer. It also conveys the manufacturer’s commitment to internal quality assurance and the ambition to create a valuable product.

While certain calculations are essential for the preparation and analysis of measurements, we are not addressing the aspects of scientific methodology or mathematical explanations. The primary objective is to convey information and provide formal elucidations through fundamental formulas.

The first few chapters address relevant Standards and dielectric integrity, followed by sections detailing each measurement, so explaining the testing of high voltage devices, including power transformers.

Be advised that all depicted circuits are illustrative and intended solely for enhanced comprehension. The relevant measuring circuits for the transformer’s tailored vector group will be included in the final subchapter of each section, where applicable.

Recommended Test Sequence

In light of the IEC and IEEE guidelines and recommendations, as well as our own experience, we propose the following test protocol:

1. Ratio, polarity and phase displacement
2. Resistance measurement
3. Current Transformer tests
4. Dielectric test:
   1. Switching impulse test (if required)
   2. Lightning impulse test
   3. Separate source AC voltage test
   4. Measurement of Insulation capacitances & loss factor (tan )
   5. Insulation resistance measurement
   6. Induced voltage test (including Partial discharge measurement)
5. Sound level measurement
6. No-load test (followed by sound level test, if specified)
7. Load loss and impedance
8. Zero-sequence impedance test (if specified)

Unlike the standard practices of IEC and IEEE, we consider insulation resistance measurement a routine test, necessitating compliance for each individual transformer.

Why Transformer Fails?

A transformer is expected to encounter multiple short circuits throughout its operational lifespan; but, eventually, one such incident may induce minor winding displacement, significantly impairing the transformer’s capacity to withstand subsequent short circuits. As the transformer ages, its components degrade, resulting in an elevated likelihood of failure.

Multiple factors contribute to degradation, as detailed here.

1. Paper insulation deterioration

The lifespan of transformer paper insulation is finite and is influenced by temperature, oxidative, and moisture factors. Upon reaching the conclusion of its anticipated lifespan, the mechanical integrity of the paper insulation significantly declines, yet its electrical integrity remains adequate.

2. Core and winding movements

The core, windings, and turns may shift due to short circuit stresses, vibrations, transport jolts, and the relaxing of clamping pressure accumulated over the transformer’s lifespan.

3. Tap changer

The moving components and electrical connections in the tap-changer degrade over time.

4. Auxiliary components

Auxiliary components, like as bushings, degrade; for instance, bushing gaskets may leak, resulting in moisture intrusion and insulation degradation.

5. Gaskets

The primary gaskets and piping gaskets are leaking, permitting moisture to enter the transformer and causing oil to seep from it.

6. Rust

Rust induces degradation of ferrous materials, particularly in humid environments.

Voltage stresses & dielectric integrity

Throughout its operational lifespan, transformers encounter various voltage stressors that may arise during both normal and abnormal functioning. Over-voltages are often classified into three categories:

1. Lightning over-voltages (A) – with a duration in the order of microseconds
2. Switching over-voltages (B) – with a duration in the order of a fraction of a second
3. Over-voltages (C) – with a duration in the order of seconds to minutes

Figure 1 – Types of over-voltages

Dielectric tests aim to confirm the integrity of transformers in the event of over-voltages, as previously outlined. The various categories of over-voltages have also been addressed in a testing protocol. The specific test code for an object, as dictated by a Standard, is essentially contingent upon the object’s dimensions and rated voltages.

Test voltages are predominantly sinusoidal alternating currents. DC voltages are typically pertinent just to valve transformers, such as HVDC transformers.

With the advancement of electronic diagnostic equipment, other tests have been incorporated, such as the measuring of partial discharges, which has become essential in contemporary practice.

Lightning over-voltages

Lightning over-voltages result from atmospheric air discharges. The magnitude of the over-voltage in the grid is dependent upon the lightning current and the impedance at the striking site. Figure 2 illustrates the characteristic waveform of a lightning over-voltage.

This wave is unipolar, exhibiting a singular polarity, and propagates linearly from the point of voltage application. It ascends to a zenith within few microseconds (wave front, characterized by a pronounced surge) and diminishes to zero within approximately one hundred microseconds (wave tail). The propagating wave is distorted and attenuated due to line resistance and corona discharge.

The installation of those protective measures may, conversely, induce a significant voltage breakdown, observable as a truncated lightning impulse at the transformer terminal.

Figure 2 – Lightning over-voltages

Where:

• FW = full in tail wave
• CW = chopped wave
• FOW = in front chopped wave

Switching over-voltages

Switching operations induce transient phenomena, which may result in over-voltages. The shape, duration, and amplitude are contingent upon the grid’s setup and the point of switching operation associated with the sinusoidal wave.

Figure 3 illustrates an instance of over-voltage occurring during a switching operation in an overhead wire.

a) Configuration of network
b) equivalent diagram
c) oscillogram of switching impulse voltage

Figure 3 – Switching over-voltages

Temporary over-voltages

Temporary operational and non-operating overvoltages arise from the following factors:

• Load rejection: over-voltage of 1,1 to 1,4 pu (several seconds),
• Single-phase short-circuit: over-voltage of 1,2 to 1,7 pu (depending on neutral point configuration),
• Ferro resonance (saw-tooth oscillations),
• Ferranti-effect, or
• Other resonance oscillations.

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