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Overcurrent and overvoltage protection

A short circuit in the medium-frequency power supply load or a failure in the inverter switch can cause a short-circuit current to flow through the rectifier circuit, posing a threat to the thyristors in both the rectifier and the inverter. Therefore, it is essential to install a protective circuit. When a short-circuit current occurs, the firing pulse of the rectifier circuit will switch from the rectification zone to the active zone with a millisecond-level timing, causing the firing angle to rapidly transition from less than 90 degrees to greater than 90 degrees. The magnetic field energy stored in the reactor will then be released into the power grid via the rectifier circuit. If the short-circuit surge current exceeds the capacity of the rectifier circuit or its internal circuitry, the firing pulse of the rectifier circuit will trigger the fuse to provide protection.

As shown in the operational steps, under normal operating conditions, the DC current id and voltage ud of the intermediate-frequency power supply are both within the normal range. Suppose a device failure occurs at time t1, causing the thyristor in the inverter to break down and effectively short-circuit the lower arm directly to the DC side. At this point, the DC current id does not change significantly due to the effect of the reactor, but the current rises rapidly. At time t2, the DC current id reaches the threshold value for overcurrent protection, triggering the overcurrent protection mechanism. When thyristors v6 and v2 return the device to its normal operating state, the firing angle quickly shifts to below 150°—similarly, when v6 and v2 restore the device to normal operation, v4 and v3 also come into play. However, during the transition of the firing angle from 120° to 150°, v4 and v3 remain untriggered. Once the overcurrent protection is activated, as the firing angle α increases, the line voltage Ubc begins to drop. By time t3, Ubc reaches zero, and at time t3, the DC voltage Ud turns negative. Under the action of reverse electromotive force, the current starts to decrease from its peak value; the rectifier circuit switches from rectification mode to active inversion mode, and the inductor Ld, acting as a DC source, releases energy back into the power grid. As the energy stored in Ld diminishes, the DC current id gradually decreases and falls below the safe limit voltage after time t4. By time t6, the induced electromotive force across Ld can no longer overcome the current generated by the grid voltage, and thus active inversion comes to an end.

 

During the aforementioned protection actions, the overcurrent protection system can reduce the DC voltage below the safe limit within half a power-frequency cycle, typically without causing damage to the smelting equipment. The principle behind overvoltage protection is identical to that of overcurrent protection. By acquiring the medium-frequency electrical energy signal at the load end, when the value exceeds the allowable threshold, the firing angle α will rapidly shift to 150° and immediately cease operation. A thyristor is a current-controlled device, and its likelihood of being damaged by overvoltage is significantly higher than its likelihood of being damaged by overcurrent.

 

Therefore, the most commonly used protective measure for thyristors is the RC snubber circuit. This protection primarily leverages the characteristic that the voltage across a capacitor cannot change abruptly to absorb transient overvoltages. By connecting a resistor in series, some of the energy generated by the overvoltage can be dissipated, thereby suppressing resonance within the circuit.

 

 

 

 

 

 

Conclusion

 

The medium-frequency furnace melting alarm and protection system ensures the furnace’s cooling and structural safety through a water-cooling system. It monitors the electrical system’s insulation against ground via a grounding protection system, preventing high-voltage discharge to ground. Moreover, the overvoltage and overcurrent protection system safeguards the main circuit components from overload and breakdown, thereby ensuring their safety. These three protection systems not only guarantee the safe operation of the medium-frequency furnace but also further enhance its operational efficiency.