In the event of the failure to perform protective action in time when the equipment detects a fault, the safety of the equipment will be significantly impaired.
It is just like that the cause of disease of a patient is checked out, but the medication is not carried out in time, consequently, the patient does not live long.
The equipment has two kinds of protection actions. One is to push inverter protection, and the other is discharge protection.
(1) Push inverter protection
The three-phase full-control rectifier bridge is shown in the following diagram.
When the phase of the trigger pulse is ahead of the natural commutation angle, the Ud is a positive voltage. When the trigger pulse lags the natural commutation angle, the Ud is a negative value. When the Ud is positive, the load captures energy from the power grid, and when Ud is negative, the energy on the load is fed back to the power grid.
When the equipment fails, the trigger pulse lags behind the natural commutation angle so that Ud is negative and the residual energy of many energy storage elements (including the inductor and capacitor, etc) is fed back to the power grid to complete the push inverter protection. The higher the negative value of Ud is, the more the inverse protection is applied, so that the trigger pulse lags far behind the natural commutation angle. The deeper push inverseprotection requires the wider scope of pulse phase shift. The phase shifting scope of the rectification pulse of the equipment is 150°C. But for some high-frequency manufacturers, the scope is 120 ℃. Hence, the push inverse protection is not as reliable as the phase shift scope of 150 ℃.
(2) Discharge protection
For the push inverter protection as mentioned above, the fastest action time is 3.3ms. The reason can be seen from the figure below.
At t1, the corresponding trigger signal has been sent to the SCR. In this case, the corresponding SCR has been turned on. When the equipment detects a fault signalat t2, the removal of the trigger pulse is of no avail. It is because that the SCR is a semi-controlled device. It is impossible to turn off the SCR just by removal of the trigger pulse signal. The only way is to push the pulse to the inverter area at t3, and between t2 and t3, there will be DC voltage output. To this end, the company provides the unique discharge protection, that is, the SCR7 protective silicon is installed in the front of the inverter. In the event of a fault, the SCR7 trigger signal will be issued in addition to the push inverter protection, so that the energy in the electric reactor can be discharged. The turn-on time of the SCR is microseconds, which is much faster than 3.3ms protection time. At the same time, the front end of the SCR7 has a large reactance to ensure that the excessive large current is suppressed during SCR turn-on, thereby avoiding the protective silicon from burning. Therefore, there are only seven SCRs in the equipment rectifier cabinet of the company.
