Technical Articles
Capacitor switching always remain as one of the challenging operations. Capacitor draws very high inrush current for a very short duration. Due to this, life of all the devices in the circuit like capacitor, contactor, short circuit protection device (SCPD) and cables are affected. They all are subjected to thermal & electrical stress, each time the capacitor is switched on & off. Various switching options are available like Power Contactors (AC3 duty), Capacitor Duty Contactors (AC6b duty) and Thyristor Switching Module (Static switch). In spite of good options of AC6b contactors available, switching capacitor through AC3 contactor is very popular even now. In this article, we are going to compare the switching performances of AC3 and AC6b duty contactors, with and without 7% series reactors through simulation. Even though the static switch (TSM) can be configured to switch with almost zero inrush current, we are not going to consider in this study because its actual application is for highly fluctuating loads.
The switching of capacitor banks in Automatic Power Factor Correction (APFC) panels results in huge inrush current, at the moment of switching. During the transient phase, the capacitor almost behaves like short circuit. Even though it remains for a few milli seconds, it is dangerous because the peak values can reach up to 400 times the rated current along with high frequency oscillations. The magnitude, frequency and duration of the inrush current depends upon below factors:
1. Rate of change of voltage (difference in voltage) at the point of switching
2. The inductance in the network (including series reactors, cables, switchgear, transformer)
3. The transformer power rating and % impedance (%Z)
4. Method used for power factor correction
- fixed capacitor bank
- multi-stage capacitor bank with steps of equal ratings
- multi-stage capacitor bank with steps of unequal ratings
5. In multi-stage capacitor bank, the nos. and rating of steps already switched on
In most of the installations, the multi-stage capacitor banks are used with steps of unequal ratings. In majority of the industries, higher kVAr ratings would be on for most of the time and less frequently switched, as they compensate the base load of the industry. For varying loads, fine-tuning of power factor correction would be carried out by smaller rated capacitors. In such cases, the value of peak inrush current would be far higher and hence the smaller capacitors will be heavily stressed.
It may look simple to calculate the capacitor rated current and select the switching device to be able to carry rated capacitor current. Nevertheless, the contactor selection must be done considering below two criteria:
Even today, majority of the users employ power contactors (AC3 duty) to switch capacitors in APFC panels. In order to ensure the contactor withstands both the inrush current and capacitor current along with harmonics, they simply derate the contactor to 2 to 2.5 times the rated current, as a rule of thumb. For example, in order to switch 100 kVAr capacitor, they select 225 A (MNX 225) AC3 rated contactor.
Use of AC3 duty contactors, derated as per thumb rule, is generally not recommended, as they do not limit the inrush current. This increases the probability of failure of capacitors due to damage of metal spray layer (as shown in fig) because of high inrush current magnitude & frequency. If MCCBs are used as SCPD, their contacts would bounce due to high inrush current and may get damaged (pitting & carbon formation). The high inrush current may result in nuisance operation of fuses as well. Such issues in SCPDs are very well documented in various sites.
Just de-rating AC3 contactor does not help to improve the situation, as there is no effect on inrush current.
Some users believe that when detuned reactors (7% or 14%) are used along with capacitors, the inrush current would reduce. It is true to some extent, but the time duration of high frequency oscillation is actually longer than switching pure capacitors. This also increases the peak transient voltages and the same is applied across the capacitors. This causes high voltage stress to the capacitors and the dielectric may fail prematurely.
The next best option is use of capacitor duty contactors with pre- closing resistors (as shown in fig). Though they do not have a point-on wave control, they are very effective in controlling inrush currents due to the presence of pre-closing resistors. The resistance value is generally in the range of 2 to 6 Ohms, with 4 ohms being typical. Considering a system voltage of 440 V and a 25 kVAr capacitor, the rated current is 32 A. Even when switched at the peak of the voltage and assuming that capacitor behaves as a complete short circuit, the current would be limited to a peak of 155 (440 x 1.414 / 4), which is just 3.5 times the rated peak current (32 x 1.414). In practical cases the actual inrush current could be much lower considering other circuit impedances and the probability that the switching could happen at any point on wave, including voltage zero.
AC6B duty contactors also help damp the switching transient oscillation due to the insertion of a ohmic resistance in the circuit during switching instant. The presence of this resistance effectively damps the transient and reduces the duration of oscillation.
Simulation:
- First waveform represents supply voltage
- Second one represents Capacitor current and
- Third one is the Voltage across capacitor terminals
1. Single capacitor bank with AC3 and no series reactor, switching at peak voltage
Ipeak = 450 A
Vpeak across the capacitor = 700 V
2. Single capacitor bank with AC3 and with 7% series reactor, switching at peak voltage
Ipeak = 220 A
Vpeak across the capacitor = 750 V
3. Single capacitor bank with AC6B and no series reactor, switching at peak voltage
Ipeak = 190 A
Vpeak across the capacitor = 480 V
4. Single capacitor bank with AC6B and with 7% series reactor, switching at peak voltage
Ipeak = 115 A
Vpeak across the capacitor = 520 V
5. 10X parallel capacitor bank with AC3 and no series reactor, switching at peak voltage
Ipeak = 4000 A
Vpeak across the capacitor = 415 V
6. 10X parallel capacitor bank with AC3 and with 7% series reactor, switching at peak voltage
Ipeak = 200 A
Vpeak across the capacitor = 600 V
7. 10X parallel capacitor bank with AC6B and no series reactor, switching at peak voltage
Ipeak = 1400 A
Vpeak across the capacitor = 400 V
8. 10X parallel capacitor bank with AC6B and with 7% series reactor, switching at peak voltage
Ipeak = 140 A
Vpeak across the capacitor = 565 V
Summary:
Below table is the summary of above simulation, where the peak inrush current values and peak transient voltages across capacitors are shown:
| AC3 contactor | AC6b contactor | ||
| 10x parallel banks | With 7% reactor | Ip = 200 A | Ip = 140 A |
| Vp = 600 V | Vp = 565 V | ||
| w/o reactor | Ip = 4000 A | Ip = 1400 A | |
| Vp = 415 V | Vp = 400 V | ||
| Single bank | With 7% reactor | Ip = 220 A | Ip = 115 A |
| Vp = 750 V | Vp = 520 V | ||
| w/o reactor | Ip = 450 A | Ip = 190 A | |
| Vp = 700 V | Vp = 480 V |
What is important is not just the peak value, but duration of peak and duration of oscillations. Below table represents the duration of some:
| Parameters | AC3 duty | AC6b duty | AC3 duty + reactor | AC6b duty + reactor |
|---|---|---|---|---|
| Peak current | 450 A | 190 A | 220 A | 115 A |
| Peak voltage | 700 V | 480 V | 750 V | 520 V |
| Duration of oscillation (current) | Up to 150 ms | Up to 130 ms | Up to 500 ms | Up to 400 ms |
| Duration of oscillation (voltage) | Up to 110 ms | Up to 90 ms | Up to 400 ms | Up to 200 ms |
From the above tables it can be inferred that:
As the results indicate, reducing inrush current by introducing resistance in the capacitor circuit is the best method, whether with or without reactors. Usage of derated Power Contactors (AC3) will just withstand the inrush current. From cost point of view, derated AC3 contactors will be more expensive than AC6b equivalent. Addition of inductance with AC3 contactor results in increased oscillations and can cause failure of capacitor & contactor itself. Trouble free operation of all the components in APFC panels are very important because the user may end up paying huge penalty, in case power factor is not maintained above the prescribed limits. Proper switching with capacitor duty contactor is one of the ways of ensuring that.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
Assistance Required?
Select an option to Contact Us
Contact Us 
