Selection of MCCB for capacitor circuit

When selecting MCCB for a capacitor circuit, attention shall be paid to the two points, the circuit opening and closing

points, and harmonic current as stated below.

Leading current circuit opening surge (at circuit opening)

When a capacitor circuit as shown in Fig. 5. 7 is opened at the time t1 shown in Fig. 5. 8, the circuit is broken at the zero point t2 of leading current i. After this, the voltage on the power supply side will change as shown by the Vt curve. However, on the load side, since the voltage is kept at Vc owing to the electrical charge of the capacitor, a potential difference between MCCB contacts will occur as the voltage difference between Vc and Vt, the potential difference will be approx. twice the supply voltage peak value Em at t3 approx. 1/2 cycle after t2, and, if the contacts have not opened sufficiently, reignition of arc will occur. Then, the electrical charge of the capacitor will be discharged by the damped oscillation from the voltage magnitude of 4Em on the oscillation circuit determined by the reactance on the electric circuit and the capacitor capacity. After the arc is extinguished, Vc will be maintained at –Em again, and the potential difference between contacts, the difference between Vc and Vt, will increase. While this is repeated, the contacts will sufficiently open, reignition will not occur, and the circuit will open. Mitsubishi MCCB have extremely high contact opening speed and will rarely repeat reignition. However, note that some MCCB do not have a quick-make/ quick-break mechanism. ON such MCCB, if the capacitor capacity is small, the electrical charge is not discharged until the oscillating current is sufficiently attenuated. Therefore, if the arc is extinguished near the peak value in the reverse direction to the oscillation voltage, the capacitor voltage, as shown in Fig. 5. 9, will be maintained near -3Em at the first reignition and will gradually increase to 5Em at the second ignition and -7Em at the third reignition, thereby leading to damage to the capacitor. Therefore, it is necessary to use MCCB with a quick-make/ quick-break mechanism.

Selection of MCCB in consideration of inrush current (at circuit closing)

When the supply voltage is V (v), capacitor capacity is C (F), frequency is f (Hz) and current is I (A), the relationship with kVA capacity P is determined as shown below.

When the capacitor circuit is closed, the capacitor electrical charge q = CV appropriate to the voltage instantaneous value V in the closing phase must be supplied instantaneously. To supply the electrical charge, large inrush current will flow. Assume that a circuit containing a capacitor has constants as shown in Fig. 5. 10. 1 and the circuit is closed when the voltage V reaches the supply voltage peak value V = Em.

According to the transient phenomenon theory, the flowing current is determined as shown below.

The change in is plotted in Fig. 5. 10. 2, and the maximum value of current im is determined as shown below.

Although the voltage V is not constant, it is allowed to consider V to be equal to Em until the transient phenomenon disappears because t0 is significantly small. Since the transit time is regarded as about 2Ƭ0, for the capacitor circuit, it is necessary to select MCCB having such an magnetic tripping current that MCCB does not operate at the passing current of im × 2Ƭ0.

Then, calculate the time in the following case.

In the case of MCCB for circuit of 3-phase, 200V, 50 Hz, 150 kvar capacitor: According to calculation, C = 1.1943 3 10-2 (F), and I = 433 (A). To estimate R and L of the circuit, the circuit short circuit current is assumed to be approx. 100 times the circuit capacity, 50000A.

Since the transit time is approx. 2t0, select MCCB having an unlatching time of 0.001 s at a current of 6600A. If Model

NF630-SW is selected, since its relay time at 10000A is 0.0029 s, it will not operate at the passing current shown above even if the unlatching time is shorter than this time. However, to prevent abnormal wear or adhesion of MCCB contacts due to large passing current and to ensure the safety against unnecessary operation, the magnetic tripping

current should be set larger than

The rating to ensure that the magnetic tripping current is larger than 4700A is 500A. In Table 5. 14, applicable MCCB rated current values are shown. If the short-circuit capacity of the circuit is remarkably larger than the rated breaking capacity, it is necessary to examine which model to be selected in accordance with the above example of calculation because MCCB may operate not only for protection against short circuit, but also owing to inrush current applied when the circuit is closed. The above selection procedures apply in case where one capacitor bank is used and a reactor is not used. If 1 to 6 capacitor banks and a 6% reactor are used, see Table 5. 14.

1. The rated current of the circuit breaker to be selected is approx. 150% of the rated current of capacitor.
2. When capacitor banks are switched according to the change in power factor, separately install electromagnetic contactors to open and close the circuit.
3. To select the rated current of circuit breaker for main line, determine the sum of the capacitor capacities on the branch circuits, and find the appropriate rated current in the column of the number of banks “1” in the above table.
4. The values at frequencies of 50 Hz and 60 Hz are shown.

Selection in consideration of harmonic current

 Since capacitors have the property of expanding voltage distortion to several times higher current distortion, if there is a device applying a thyristor which may cause distortion in the voltage waveform near the capacitor, care must be taken in selecting MCCB. It has been reported that the current distortion reached 360% although the voltage distortion was about 19%. If there is a voltage distortion source near the capacitor and the current distortion is large, select a thermal magnetic MCCB for capacitor circuit.