Thyristor Switching Module

A: A thyristor switching switch (also known as thyristor‑switched switch, English name Thyristor Switched Capacitor, abbreviated as TSC) is an electronic contact‑less power device module used for the rapid switching of power capacitors.
Take the ULHBt series as an example. The product is mainly composed of high‑power anti‑parallel high‑performance thyristor modules, isolation circuits, trigger circuits, synchronization circuits, protection circuits and drive circuits. It is also equipped with wiring terminals for controlling the switch on‑off state, with a control logic voltage of 0 V (off) and 12 V (on).
It realizes contact‑less switching of capacitors through power electronic technology. Featuring fast response speed, no inrush current, no electric arc, support for frequent operations and long service life, it serves as a core component for dynamic reactive power compensation.

A: The core component of a thyristor switching switch is an anti-parallel thyristor module (consisting of two thyristors connected in anti-parallel), whose conduction and off-state are controlled by a gate triggering signal:
(1)Turn-on process (zero-crossing voltage triggering): The synchronization circuit performs real-time monitoring of the voltage across the thyristor; when the voltage crosses zero, the triggering circuit generates a pulse signal to turn on the thyristor. Since the capacitor is connected at the zero-crossing point, the voltage across the capacitor is identical to the grid voltage; therefore, the theoretical inrush current is zero.
(2)Operating process: The thyristor remains continuously conducting, allowing current to flow through the thyristor module. Since the turn-on voltage drop of the thyristor is approximately 1–1.5 V, power dissipation and heat generation occur during operation; therefore, a heat sink must be used for heat dissipation.
(3)Disconnection process (current zero-crossing turn-off): Upon removal of the gate triggering signal, the thyristor turns off at the natural zero-crossing point of the AC current, achieving arc-free interruption. After disconnection, the capacitor discharges itself through a discharge resistor.
(4)The entire switching process is completed within 1–20 ms – significantly faster than that of a mechanical contactor.

A: Zero-crossing switching refers to the technique of switching a capacitor in at the zero-crossing point of the AC voltage and switching it out at the zero-crossing point of the AC current.
Zero-Current Principle:
(1)The magnitude of the inrush current during capacitor energization depends on the difference between the voltage across the capacitor at the moment of energization and the grid voltage.
(2)When energized at the voltage peak, the voltage difference is maximized, and the inrush current can reach dozens of times the rated current.
(3)When the capacitor is switched on at the voltage zero-crossing point, its pre-charging voltage is equal to the grid voltage, resulting in a zero voltage difference; consequently, the theoretical surge current is zero.
(4)In actual applications, due to detection and triggering delays, the inrush current is typically kept within three times the rated current – a value significantly lower than the dozens of times observed during contactor switching operations.
Zero-crossing switching is the core advantage that distinguishes thyristor switches from traditional contactors; it provides effective protection for capacitors, contactor contacts, and the power grid.