Switching Principle and Standard Waveform Interpretation of on Load Tap Changer

Many engineers know to use instruments to measure on load tap changers, but they do not know how to interpret the test waveform when they see it. Understanding the switching principle is the foundation of understanding waveforms. Once this logic is explained clearly, waveform analysis becomes easy.

Switching mechanism of on load tap changer

The core issue with switching on load tap changers is that they cannot disconnect the load current (which can cause high-voltage arcs and damage the contacts), nor can they directly short-circuit two tap changers with different potentials (due to extremely high short-circuit currents). The solution is to "bridge transition" - connect a transition resistor in series between the old and new taps, allowing the current to flow through the transition resistor to the new tap first, and then disconnect the old tap.

This process operates in the order of a → g, with the key nodes being:

R0 (initial state): The switch has not yet been activated, and the instrument measures the sum of the winding resistance and the test line resistance, with a horizontal straight line waveform.

T1 (transition resistor 1 connected): The first transition resistor R1 is connected to the circuit, and the total resistance increases, causing the waveform step to jump upwards for a duration of T1.

T2 (bridging time): Transition resistor 1 and transition resistor 2 are both in the circuit, and the two taps are "bridged" through the transition resistor. The current is distributed in the two paths, and another step appears in the waveform, lasting for T2. At this stage, both taps are connected, and the new and old taps are connected through the transition resistor, which is the core stage of the switching process.

T3 (transition resistor 2 connected): The old tap has been disconnected, and only transition resistor R2 is in the circuit for a duration of T3.

T4 (total transition time): From the first transition resistor being connected to the complete end of the switch, T4 = T1+T2+T3。

T0 (three-phase synchronicity): Based on the phase that detects the switching action earliest, the time difference between the switching of the other two phases. The smaller T0, the more synchronous the three-phase switching, and the better the mechanical state.

Typical time of normal waveform

The switching time of different models of on load tap changers varies, and T4 is usually within the range of 20-80ms, which is considered normal. The transition resistance value is usually between a few ohms and several tens of ohms, subject to the manufacturer's information.

HZYA-3Z samples at a high speed of 20kHz with a resolution of approximately 0.05ms, accurately capturing the time nodes and resistance values of each stage. The waveform automatically displays and annotates various parameters.