Intermediate frequency furnace inverter overcurrent and overvoltage protection principle

1) The reason for the overcurrent generated at the inverter terminal is as follows;
(1) Fluctuations in the load during operation cause overcurrent. Induction furnaces fluctuate greatly during the smelting process, especially in the initial stage of smelting, the parameters change more intensely, often causing overcurrent.
(2) In-service bridge inverter, two pairs of bridge arm thyristor exchange loss error, inverter failure, short circuit current caused.
(3) Sudden interruption of the thyristor trigger pulse of the bridge inverter in operation, causing the short circuit current caused by the short-circuit of the bridge arm diagonal thyristor.
There are also various overcurrents caused by various reasons. It is not economical and reliable to use the fast fuse protection for the overcurrent on the inverter side.
2) The reason for the overvoltage generated at the inverter is as follows;
(1) (Intermediate frequency voltage Uc=1.1Ud/cosɑ. Since the lead angle ɑ is too large, the intermediate frequency voltage Uc is too high when the rectified voltage Ud is constant.

(2) The timing of the inverter trigger pulse is determined by the intersection of the Uc and -Ic signals, if the Uc signal from the voltage transformer is suddenly interrupted. Then the intersection will be determined by the Ic signal, and ɑ will increase rapidly, causing the intermediate frequency voltage Uc to be too high.
(3) The furnace induction coil suddenly opens to cause an overvoltage.
(4) Peak overvoltage generated when the thyristor is turned on and off.
3 The overcurrent and overvoltage of the inverter are protected by pulse blocking method.
1) The following briefly explains the operation principle of pulse blocking.
(1) When the DC output terminal is short-circuited in the three-phase bridge full-controlled rectifier circuit, the rectification trigger pulse is quickly moved to the minimum ß angle (for example, ß=30o, equivalent to ɑ=150o) to generate a reverse voltage. Quickly turn off the rectified thyristor and cut off the short-circuit current. This method of quickly moving the rectification trigger pulse to the blocking position by the rectifying device is called pulse blocking. Pulse blocking can not be simply understood as removing the pulse. In fact, the three-phase bridge full-controlled rectifier circuit works in the inverter state. (The inverter mentioned here and the intermediate frequency inverter we are talking about are completely two concepts, and cannot be confused.)
(2) When the inverter circuit is short-circuited, the current increases rapidly. At this time, there is a DC reactor. The voltage close to the rectified output is added to both ends of the reactor Ld, and Ld will generate strong magnetic energy. If the rectifier circuit is not blocked immediately, the magnetic energy will not be released, and the hazard is immeasurable. It must be that the rectifier bridge is in the inverter state to feed this energy back to the AC power supply grid. Over time, Ua will gradually decrease to zero, and the current will be reduced to zero accordingly. The thyristor will be rectified and turned off to achieve the purpose of protection.
(3) In the actual protection link of the system, the overcurrent signal is taken from the three through-current current transformers at the AC input end, the three current transformers are connected in a star connection, and then three 5/0.1 transformers are transformed. The star is connected to the K1, K2, and K3 of the main control board, and is rectified by the three-phase bridge and taken out as a voltage. The overvoltage signal is taken from the intermediate frequency voltage transformer and is taken out after single-phase bridge rectification (see Figure 11). The dynamic end position of the potentiometer W7 determines the overcurrent protection setting value. 4 Comparator LM339 is divided into two groups of over-current and over-voltage tasks. The 5th corner is connected with a reference voltage. The current signal output from the potentiometer's moving end is connected to 6 angles. At this time, compared with the 5th angle, when the current signal voltage is greater than ( Take the negative voltage) reference voltage, the comparator 2 angle will output high potential, divided into four output, one way makes U3B negative input terminal 6 corner potential higher than the positive input terminal 7 corner potential, 1 angle output low potential, lock U3A, Ensure that the 2 corner output is high. One way through the triode drive relay to drive the door panel indicator as an external indication, one way into the control loop to quickly move the rectification trigger pulse to the minimum ß position is equivalent to ɑ = 150o. The rectification trigger pulse is in the blocked state, (the rectification trigger integrated block TC787C The 4th angle is the trigger phase shift control end, the low electric displacement phase, the high potential blockade. The position of the dynamic terminal of the same potentiometer W8 determines the protection setting value of the overvoltage. Once an overcurrent or overvoltage occurs, it is protected by a circuit to achieve pulse blocking. Under normal circumstances, the overcurrent and overvoltage values ​​are set at 1.2 times the rated operating current voltage.

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