Transients

Power transfer in transformers can occur from either the high voltage or low voltage side.
The power being transferred generally takes the shape of a sine wave.
In an ideal scenario, all sine waves would be perfectly transmitted from the primary to the secondary side. However, real-world conditions often do not permit this.

Definition of Transients: These are brief events that can adversely affect the sine wave quality of voltage. They are momentary interruptions that can last anywhere from nanoseconds to a few milliseconds.
Transients can result from:
- Atmospheric phenomena (e.g., lightning strikes)
- Switching operations in power grids
- Faults and other electrical events.
Not Related to Harmonics: Transients are distinct from harmonics. Harmonics represent permanent deviations in waveforms, while transients are temporary perturbations.

Transients can be caused by atmospheric events, such as lightning or other generated disturbances in the power systems (e.g., short circuits).
The characteristics of transients include:
- Switching Transients: Caused by operations like opening/closing circuit breakers or connecting/disconnecting loads. These can introduce ripple effects in the voltage waveform.
- Lightning Transients: These can encompass frequency components ranging from 10 kHz to 3 MHz. Lightning surges can exceed the voltage ratings for which transformers are designed (example: 550 kV lightning pulses).
Inrush Current: This is a type of transient current experienced when a transformer is switched on for the first time.

The IEEE Standards play a crucial role in specifying how switching impulse tests should be conducted for transformers. These standards are agreed upon by manufacturers and utility operators.
Under the standards, transformers are designed with additional robustness to withstand transients like lightning and switching impulses, far exceeding their rated operational voltages (e.g., designed for 138 kV but tested for significantly higher voltages).

Full Wave Impulse: According to IEEE standards, 1.2/50 microsecond duration full wave standards are defined for testing.
Chopped Wave Tests: These are repeat tests conducted to assess the transformer’s resilience to fast transient events, occurring in the range of 10 kHz to 100 MHz.
Each of these tests simulates the various high-frequency transients transformers might experience due to both operational and natural causes.

Transformers are composed of:
- Core
- Windings: The windings exhibit both resistive properties (due to copper) and inductive properties (due to their coil structure).
Capacitive effects are also significant between winding turns and discs, affecting overall behavior under transient testing conditions.

Testing transformers against transient conditions involves complex interactions between inductances, capacitances, and resistance, which cannot be easily calculated manually. Mathematical and computational models are required to analyze these phenomena due to complexity.
One such tool developed is Winding Transient Tool (Wind Turn), used to simulate the electrical stresses and analyze if the transformer would withstand various impulse tests.

To avoid insulation breakage during tests, transformers must achieve a linear voltage distribution along the winding turns when subjected to impulse tests. Non-linear distributions can lead to higher stress on particular turns or discs, risking insulation failure.
Factors affecting voltage distribution include:
- Series Capacitance (CS): Increasing series capacitance helps in achieving more uniform voltage distribution during tests.
- Interleaving Turns: By strategically arranging winding turns, designers can create longer paths for impulse voltage, thereby improving the linearity of voltage distribution.

Interleaving winding turns is time-consuming and adds to manufacturing complexity but may be necessary for transformers designed for high impulse levels (e.g., 765 kV transformers that need to withstand impulses of 2 million volts).
It is crucial for engineers to carefully design transformers to ensure that they can efficiently handle transient events without risking insulation breakdown or consequential failures.

Engineers need to constantly consider both high-frequency and low-frequency tests to ensure the reliability and safety of transformers under transient conditions. The implementation of advanced software like Wind Turn aids in calculating the appropriate configurations to withstand potential electrical stresses.