Capacitors are undoubtedly one of the most fundamental passive components in electrical networks. They are extensively utilized in industrial applications to meet electricity tariff requirements and balance the power of inductive loads. Due to fluctuating load conditions, capacitors must be switched in and out of the circuit, with contactors employed for this switching process.
Automatic capacitor banks consist of multiple power factor correction steps, each of which is switched based on the load condition. Contactors are the most commonly used switching elements in these panels. The ideal switching point for a contactor when switching a capacitor is when the residual voltage across the capacitor equals the voltage level of the sinusoidal waveform. However, while this is theoretically possible, it does not occur in practice. Even if it happens by chance, it would only be in a single phase, as the ideal switching condition cannot be achieved in the other phases.
In stepped automatic capacitor banks, when steps are switched sequentially, very high inrush currents (also known as inrush or back-to-back currents) may arise. Capacitors inherently store energy, and during switching, the capacitors already present in the circuit feed the newly connected capacitor, resulting in extremely high discharge inrush currents. Current values up to 200 times the nominal current can be observed in stepped automatic capacitor banks. Under such operating conditions, the contactor’s contacts may melt, stick together, and significantly shorten the contactor’s lifespan.
The resistors in capacitor switching contactors limit the inrush current, which can reach up to 200 times the nominal value, down to approximately 70 times, ensuring safer system operation.
THE OPERATING PRINCIPLE OF CAPACITOR SWITCHING CONTACTOR
Switching On:
Capacitor switching contactors with resistors are designed to limit high inrush currents that arise when capacitive loads are switched on. In these contactors, pre-make contacts engage before the main contacts, activating a low-impedance resistor. This restricts the inrush current of the capacitors, preventing sudden current peaks and voltage transients.Once the main contacts are engaged, the pre-make contacts disengage, allowing the system to transition into normal operation. When the contactor is activated, the resistor is connected before the main contacts close. This ensures that the current flows through the pre-resistor before reaching the capacitor directly.The resistor limits the inrush current, mitigating sudden voltage drops and surge currents in the capacitors and the power grid. This stage not only reduces wear on the contactor contacts but also minimizes harmonic effects on the power system.
Principle Function Diagram :
Steady-State Operation:
Once the current passing through the resistor has been stabilised and limited, the main contacts are closed and the resistor is disconnected from the circuit. At this stage, the entire current flows directly through the main contacts. The switching on of the main contacts reduces power losses in the system, as the resistor only serves as a temporary transition element. This process ensures that the contactor operates with high efficiency and minimal losses.
Switching Off:
When the contactor is switched off, the main contacts open, disconnecting the capacitor from the circuit. At this moment, the resistor does not switch on, as the opening of the main contacts is sufficient to interrupt the load current.
THE STEPPED (MULTI STEP) COMPENSATION SWITCHING :
In a laboratory setting, the switching moment of a multi step stage was captured using an oscilloscope, which revealed the following current waveform patterns.
THE INDIVIDUAL (SINGLE STEP) COMPENSATION SWITCHING :
In a laboratory setting, the switching moment of a single stage was captured using an oscilloscope, which revealed the following current waveform patterns.
When both a contactor with resistor and a detuned reactor are employed in the power factor correction system, the K3-62K capacitor switching contactor has a lifespan of 300,000 switching cycles as stated in the catalog. Conversely, if a detuned reactor is not utilized, this lifespan is diminished to 150,000 switching cycles.
SWITCHING ON OPERATION WITH NORMAL CONTACTOR AND NO RESISTOR
- NORMAL CONTACTOR ( NO RESISTOR )
- NON-DETUNED PFC ( NO DE-TUNED REACTOR )
K3-62A Benedict Contactor
50kVar 400V (72A) Electronicon Capacitor
Vertical : 2000A per unit Horizantal : 0,625ms per unit
This analysis is valid solely within a laboratory environment and for a single capacitor step.
As observed from the oscilloscope recording, when a single step switches a 72A capacitor using a contactor without a pre-charge resistor, the peak current surpasses 4000A.
The measured inrush current value is applicable exclusively to a single-step capacitor. However, in power factor correction systems comprising multiple steps, the inrush current can escalate significantly. This is attributed to the phenomenon known as “back-to-back switching.”In this scenario, an already operational capacitor step provides power to the newly switched-in capacitor, leading to exceptionally high inrush currents. Specifically, as capacitor steps switch on , they supply one another through their charging currents, resulting in inrush currents that can reach levels up to 200 times the rated current (200In).
SWITCHING ON OPERATION WITH CAPACITOR SWITCHING CONTACTOR
- CAPACİTOR SWITHİNG CONTACTOR ( WİTH RESISTOR )
- DETUNED PFC ( WITH DE-TUNED REACTOR )
K3-62K Benedict Contactor
50kVar 400V (72A) Electronicon Capacitor
Vertical : 200A per unit Horizantal : 10ms per unit
This analysis is valid solely within a laboratory environment and for a single capacitor step.
The oscilloscope recording shows that when a single-step 72A capacitor with a detuned reactor is switched using a contactor with a resistor, the peak current is reduced to 200A.
Additionally, the detuned reactor acts as a filter against harmonics, resulting in a more distinct sinusoidal waveform.
The initial inrush current has been damped by the resistors and reduced to 200A due to the inertia of the detuned reactor. The measured inrush current value applies to a single-step capacitor. However, in power factor correction systems consisting of multiple steps, the detuned reactor will act as a filter, regulating the charging of the newly switched capacitor with the previously engaged capacitor step. As capacitor steps are switched on, they supply each other through their charging currents, and the detuned reactor will limit these currents.
The IEC 60947-4-1 Standard clearly specifies the usage categories of contactors, with the AC-6b Category, which includes capacitor switching contactor with resistor usage, being a standard requirement for power factor correction systems.
According to the IEC 60947-4-1 Standard, the contactor utilization categories can be listed as follows:
| Class | Application | Typical Load Type | Example Uses |
| AC-1 | Non-inductive or slightly inductive loads | Resistive loads | Heating systems, incandescent lamps |
| AC-2 | Starting and stopping slip-ring motors | Wound rotor motors | Slip-ring motor starters |
| AC-3 | Starting and stopping squirrel cage motors | Induction motors with low inrush current | Pumps, fans, compressors, conveyors |
| AC-4 | Plugging, inching, and reverse operation of motors | Induction motors with high inrush currents | Elevators, cranes, hoists, presses |
| AC-5a | Switching of discharge lamps | High-pressure sodium or mercury vapor lamps | Street lighting, industrial lighting |
| AC-5b | Switching of incandescent lamps | Resistive filament lamps | Household and commercial lighting |
| AC-6a | Switching of transformers | Transformers | Power transformers, isolation transformers |
| AC-6b | Switching of capacitors | Capacitor banks for power factor correction | Power factor correction panels, automatic capacitor switching |
| AC-7a | Non-inductive or slightly inductive loads in households | Resistive loads | Home heating, ovens, water heaters |
| AC-7b | Motor loads in household applications | Single-phase motors | Household appliances, washing machines |
FINAL OUTCOME
AC-6b class capacitor switching contactors with pre-resistor structures are specialized switching devices utilized in power factor correction systems. These contactors, developed in compliance with the IEC 60947-4-1 standard, are designed to regulate high inrush currents, particularly during capacitive load switching operations.
The pre-resistor mechanism prevents the current from directly reaching the contactor surface, instead diverting it through the resistor. This mitigates erosion and arc formation, thereby extending the electrical and mechanical lifespan of the contactors.
Consequently, AC-6b class capacitor switching contactors have become a critical component for enhancing the reliability and longevity of power factor correction systems.
Didem Ergun Sezer
Electrical Engineer
Ergun Elektrik A.Ş.
