Why does the capacitor current over zero cut-off produce overvoltage?

The overvoltage will occur after the capacitor is cut off. The general explanation for this problem is that the arc re ignition phenomenon occurs at the switch contact during the process of switching, which leads to the overvoltage of the capacitor. The overvoltage caused by arc reburning needs to be combined by many factors, including contact breaking time, system inductance, reburning time and other factors. The mechanism analysis of the overvoltage is very complex, so we will not discuss it here.

The capacitor controlled by thyristor will also produce overvoltage after removal. This phenomenon cannot be explained by arc reburning, because the thyristor will not produce arc reburning. Thyristor can only be turned off when the current is over zero. Therefore, the capacitor controlled by thyristor must be cut off under the condition of zero crossing current. Therefore, we discuss the overvoltage phenomenon with thyristor controlled capacitor.

For thyristor controlled single-phase capacitors, there is no overvoltage after removal. Because the voltage and current of the capacitor have 90 degrees phase difference, the moment when the capacitor current is zero is the highest voltage. Therefore, we can determine that the voltage of the single-phase capacitor controlled by thyristor is equal to the peak value of the power supply voltage after the switch off, that is, 1.414 times of the effective value of the power supply voltage, which is within the voltage withstand range of the capacitor.

The three-phase capacitors controlled by thyristors are different. The removal process of three-phase capacitor is much more complicated than that of single-phase capacitor. The three-phase power capacitor controlled by thyristor will have abnormal high voltage after the removal, which may pose a threat to the safety of the capacitor. The following is an example of the structure shown in Figure 1 to introduce the removal process of three-phase delta connected capacitor: set the effective value of system voltage u=400v, and cancel the trigger signal of thyristor when phase a voltage is close to the positive peak value, so the SCR A is turned off when the voltage of phase a equals to the positive peak value (the current of the SCR is zero at this time)

UA=400×1.414÷1.732=327V

UB=-400×1.414÷1.732÷2=-163V

UC=-400×1.414÷1.732÷2=-163V

According to the above three formulas, we can determine the voltage on the capacitor:

Positive at cab voltage = 327 + 163 = 490v a

CCA voltage = 327 + 163 = 490v A is positive

CBC voltage = 0

After SCR is off, the circuit structure becomes the form of capacitor cab and CCA after series connection and then parallel with CBC. When the on-line voltage UBC is equal to the positive peak value (the current of SCRB and SCRC is zero at the same time), SCRB and SCRC are off at the same time. During the process from SCR off to SCRB and SCRC simultaneously off, the voltage of capacitor CBC is charged from 0 to line voltage peak, and the b-terminal is positive. The current and voltage of cab are in series with CCA, and the current and voltage of cab are discharged in reverse direction. The current and voltage of CCA belong to continuous charging. The sum of the discharge voltage of cab and charging voltage of CCA is equal to the peak value of line voltage. Therefore, we can determine the voltage on the capacitor:

Cab voltage = 490-566 ÷ 2 = 207v A is positive

CCA voltage = 490 + 566 ÷ 2 = 773v A is positive

CBC voltage = 400 × 1.414 = 566v positive at b-end

For 400V voltage capacitor, the peak voltage normally bears is 566v, while the abnormal high voltage on CCA reaches 773v, which is 1.366 times of the normal peak value. This abnormal high voltage can only be discharged slowly by the discharge resistance inside the capacitor, which will last for decades.

The problem is not only that, before the abnormal residual voltage of capacitor CCA is not discharged to less than the line voltage peak, it is impossible to put into operation again if the voltage over zero input condition cannot be met, that is, SCRA and SCRC can not be triggered again. However, SCRA and SCRB can be triggered again. After SCR and SCRB trigger, the voltage of CCA will drop, and then SCRC can trigger to make the capacitor put into normal operation again. At the next cut-off, abnormal high voltage will reappear. For the dynamic reactive power compensation device with frequent switching, frequent switching operation will make the capacitor often in the abnormal high voltage state, which will have a serious impact on the capacitor life.

Through the above analysis, we can see that although thyristor control capacitor can realize current zero crossing cut-off, it will cause abnormal residual voltage in capacitor. The abnormal residual voltage is far higher than the normal peak voltage of capacitor operation, so it will cause damage to capacitor and the degree of damage is unpredictable. The above analysis process is the triangle connection of capacitor. Because triangle connection has the equivalence with star connection, it can be determined that overvoltage will also occur when the star connected capacitor is cut off when the current is over zero. The following is an example of the structure shown in Figure 2 to introduce the removal process of three-phase star connected capacitor: set the effective value of system voltage u=400v, and cancel the trigger signal of thyristor when phase a voltage is close to the positive peak value, so the SCR a will turn off when the voltage of phase a equals to the positive peak value (the current of the SCR is zero at this time), at this moment:

UA=400×1.414÷1.732=327V

UB=-400×1.414÷1.732÷2=-163V

UC=-400×1.414÷1.732÷2=-163V

According to the above three formulas, we can determine the voltage on the capacitor:

Cab voltage = positive at 327v a end

CCA voltage = -163v b-terminal is negative

CBC voltage = -163v C end is negative

After the SCR is off, the circuit structure becomes the form of capacitor CB and CC in series. When the on-line voltage UBC is equal to the positive peak value (the current of SCRB and SCRC is zero at the same time), SCRB and SCRC are off at the same time. During the process from SCR off to SCRB and SCRC simultaneously off, the voltage of capacitor CA is unchanged, and the current and CC are consistent in series, while the current and voltage of CB are in reverse direction, the current and voltage of CC belong to continuous charging. The sum of discharge voltage of CB and charging voltage of CC is equal to the peak value of line voltage. Therefore, we can determine the voltage on the capacitor:

CA voltage = positive at 327v a end

CB voltage = -163+566 ÷ 2 = 120V positive at B end

CC voltage = -163-566 ÷ 2=-446v C end is negative

Since the voltage withstand voltage of star connected capacitor is designed as 230V, the peak voltage of normal operation is only 327v, which shows that the abnormal residual voltage is far higher than the normal peak voltage of capacitor.