INTRODUCTION
The proliferation of power electronics-based drive systems and non-linear loads has led to a significant increase in background harmonic distortion in electrical grids, particularly in industrial settings. Concurrently, transmission and distribution authorities have raised target power factor (cosφ) values to enhance grid transmission capacity in response to rising energy demands.
Given the increasing proximity of resonance frequencies to fundamental harmonic frequencies in electrical installations, the utilization of conventional capacitors in power factor correction systems has led to a higher incidence of failures. The inherent risk of resonance between substantial harmonic distortion and conventional capacitors necessitates the incorporation of detuned reactors in all power factor correction systems. Therefore, the deployment of capacitor banks relying solely on conventional capacitors is discouraged, especially in new installations.
Therefore, the selection of an appropriately detuned reactor is of paramount importance. Employing reactors that are either suboptimally designed or do not adhere to established standards may cause the system to revert to the operational characteristics of a conventional power factor correction system, thereby precipitating the recurrence of resonance phenomena within the electrical grid. To mitigate these potential complications, meticulous attention must be paid to detuned reactor selection, with a focus on adherence to stringent quality and conformity benchmarks.
HARMONICS
Harmonics are current and voltage components that arise at integer multiples of the fundamental frequency when non-linear loads are connected to an electrical power system. The primary sources of non-linear loads include transformers, motors, welding machines, arc furnaces, variable speed drives, uninterruptible power supplies, and other equipment containing power electronics components. Harmonics cause deviations in the current and voltage waveforms from their ideal sinusoidal shape.
In recent years, there has been an increasing number of scientific studies on interharmonics. However, frequencies defined as harmonics are generally exact multiples of the fundamental frequency of the utility grid. Therefore, calculations and measurements typically consider frequencies between 100 Hz (2nd Harmonic) and 2.5 kHz (50th Harmonic).
RESONANCE
Industrial facilities often include inductive loads such as transformers, motors, and similar equipment. These loads introduce a XL inductive reactance (Ω), which depends on the grid frequency (f). In addition, capacitors used in power factor correction systems contribute a XC capacitive reactance (Ω) to the circuit.
As shown in the equations below, both reactances are functions of frequency. Inductance is denoted by “L” (in Henry), and capacitance by “C” (in Farad):
Both XL (inductive reactance) and XC (capacitive reactance) are frequency-dependent. These inductive and capacitive components have the potential to create a resonant circuit, where the overall system’s resonant frequency is defined as:
As the formulas indicate, capacitor current increases proportionally with frequency, while reactor current decreases inversely with frequency.
The introduction of capacitor banks, composed exclusively of capacitors, into a facility can potentially create resonant circuits, thereby predisposing the system to resonance phenomena. In order to mitigate resonance, it is imperative to connect detuned reactors in a series configuration with the capacitors.
- POWER QUALITY MEASUREMENT
Initially, a power quality measurement should be conducted utilizing a Class A measurement instrument at the main distribution panel input of the intended system. The outcomes of this measurement should undergo rigorous analysis during the selection of the detuned reactor.
The key parameters in selecting a detuned reactor include nominal voltage and frequency, both of which must be ascertained via empirical measurements.
Note: The projected detuned power factor correction system is to be engineered for parallel operation with the loads, functioning autonomously without dependence on other active or passive harmonic filters. This ensures continuous and stable operation of the PFC system, based on system parameters, even if an external filter is disabled.
The crucial parameters that must be identified include:
- Rated System Voltage [Volt]
- Rated Frequency [Hertz]
- Rated Power [kVAr]
- Reactor Factor [p%]
- Tuning Frequency [Hertz]
- SERIES CAPACITOR – DETUNED REACTOR MATCHING:
Once the nominal voltage and frequency of the detuned reactor are determined, the voltage rating of the series-connected capacitor must be calculated.
However, when a reactor is connected in series with a capacitor, the voltage appearing across the capacitor terminals will be higher than the nominal grid voltage due to the impedance of the detuned reactor. Therefore, the capacitor voltage should be calculated using the following formula:
Important Note: When selecting the nominal voltage rating of the capacitor, it’s crucial not to consider the overvoltage tolerance values defined in IEC 60831. Only the nominal operating voltage stated on the capacitor label should be taken into account, as the overvoltage values refer only to temporary, and not continuous, operating conditions.
For example :
In a system with a bar voltage of Un = 400V, if the reactor factor p=7% is selected, the voltage withstand capacity of the selected capacitor must be above 430V. In order to ensure the long service life of the power factor correction system, it is necessary to incorporate a voltage tolerance of +10% into the 430V voltage level, and to select the capacitor voltage level in accordance with this.
UC=430,1V+43V=473,1V
In this case, the voltage resistance of the selected capacitor must be a minimum of 473V.
It is evident that, due to the fact that the reactor and the capacitor are connected in series, the nominal current flowing through the reactor will be equivalent to the capacitor’s nominal current.
In the context of detuned power factor correction systems, the capacitance value of the capacitor is determined by the system’s resonance frequency and the inductance of the reactor. The determination of the correct capacitor value is contingent upon the knowledge of both the reactor factor and the corresponding resonance frequency.
The configuration of detuned reactors is generally undertaken with tuning values of 7% or 14% being the norm.
These settings have been developed to avoid critical resonance frequencies and to maintain harmonic currents at safe levels. In the event of an erroneous capacitance being selected, there is the possibility of a shift in the system’s resonance frequency, which could result in undesired resonance effects.
- REACTOR FACTOR :
In the industry, variable frequency drives (VFDs), UPS systems, and other non-linear loads are commonly used. The six-pulse diode rectifier circuits at the input stage of VFDs generate harmonics such as the 5th, 7th, 11th, and 13th (h = np±1), effectively acting as harmonic current sources.
The harmonics produced by VFDs are integer multiples of the fundamental frequency, including components such as 250 Hz, 350 Hz, 550 Hz, and 650 Hz. When these harmonic currents flow through the grid impedance, they cause harmonic voltage distortions, resulting in harmonic voltage components at similar frequencies.
To avoid reactive energy penalties, industrial facilities often install power factor correction systems designed only with capacitors in parallel with the grid. However, these systems can amplify the harmonic currents generated by non-linear loads and may trigger sudden resonance conditions.
To prevent these resonances, a series detuned reactor should be connected in series with the capacitors, creating a series tuning circuit.
Variable frequency drives with a six-diode input stage are among the most common non-linear loads in industrial settings. In these devices, harmonic current components typically start from the 5th harmonic. Consequently, system resonances tend to occur at frequencies of 250 Hz and higher.
To mitigate the amplification of resonances and harmonic current components and to avoid the influence of frequencies around 250 Hz, the detuned reactor’s resonance frequency is commonly set to 189 Hz. Selecting a detuned reactor with this tuning frequency helps prevent unwanted resonance effects, limits the amplitude of harmonic current components, and reduces voltage harmonic distortion.
| Reactor Factor (p) & Harmonics | Rezonance Frequency (fr) [for fn= 50Hz ] |
| 14% Detuned Reactor | 134Hz |
| 3rd Harmonic | 150Hz |
| 7% Detuned Reactor | 189Hz |
| 5th Harmonic | 250Hz |
In addition, 14% detuned reactors, which have a resonance frequency of 134 Hz, are preferred for systems where 3rd harmonics are dominant.
- SWITCHGEAR COMPONENTS IN DETUNED CAPACITOR BANK DESIGN:
- Capacitors IEC 60831
- Detuned Reactors IEC 60076-6
- NH Fuse Disconnectors IEC 60947-3
- Fuse Links IEC 60269
- Capacitor Contactors IEC 60947-4-1
- Reactive Power Control Relay
- Discharge Coils IEC 61558-2-20
- Fans IEC 60034-1
- Enclosure IEC 60439-1
All components intended for use in the detuned reactor capacitor bank must be checked to ensure they comply with the relevant quality standards.
The most critical point to consider in the design is the matching between the capacitor and the detuned reactor.
When selecting a detuned reactor, copper connection terminals should be preferred because aluminum terminals may lead to loose connections and oxidation issues.
4.a. Capacitors and Detuned Reactors:
In a detuned reactor capacitor bank, the voltage level at the capacitor terminals increases based on the detuned reactor factor(p). Therefore, both the nominal grid voltage and the reactor factor must be considered during selection. Due to the series-connected reactor, the actual voltage at the capacitor terminals will be higher than Un. To calculate this value, the formula Uc = Un ÷ (1 – p/100 %) is used. For example, if Un = 400 V and p = 7%, then Uc ≃ 430 V; adding a 10% voltage tolerance, a capacitor rated for at least 473 V should be selected.
According to the IEC 60831 standard, the capacitor’s nominal voltage must not be exceeded, and the allowed overvoltage values are applicable only for short-term endurance; these should not be considered for continuous operation.
To determine the resonance frequency (fr) of the detuned system, the following formulas are used: p=fₙ²/fᵣ² veya fᵣ= fₙ/√p; where typically p = 7% (fr ≈ 189 Hz) or p = 14% (fr ≈ 134 Hz) is selected to suppress the influence of high harmonics like 250 Hz.
In detuned reactor design, according to IEC 60076-6, the reactor inductance must be specified to achieve a detuning ratio between 7% and 14%. To avoid magnetic core saturation, the core cross-section and winding size must be appropriately selected, ensuring the air gap and magnetic properties remain within the linear region. Even if the current passing through the reactor reaches up to 1.8 times the capacitor’s nominal current, the inductance deviation should not exceed 5%.
Additionally, to minimize harmonic frequency components, hysteresis and eddy current losses, along with ohmic winding losses, must be minimized. To ensure vibration and noise control, vacuum varnishing should be applied, and copper terminals are preferred to improve long-term connection reliability.
4.b. NH Fuse Disconnectors and Fuse Links:
The NH fuse disconnectors used in detuned capacitor banks must comply with IEC 60947-3, and the NH fuse links must comply with IEC 60269. Detuned capacitor banks are typically installed adjacent to, or across from, the main distribution panel in electrical installations. In the single-line diagram, the capacitor bank is located downstream of the main distribution panel, which is fed by the power transformer.
As a result, the short-circuit power at the capacitor bank busbar is close to that of the transformer. Therefore, NH fuses with a breaking capacity of 100 kA should be used in the capacitor bank. Since fuse links may shatter during a short circuit, they must be installed inside NH fuse disconnect switches for personnel safety.
The reason NH fuses are used is not because of the continuous current, but because they have high short-circuit breaking capacities. Even if the nominal current of a single step in the capacitor bank is 16A or 25A, short-circuit protection should be implemented with NH00 fuse links and disconnect switches rather than with miniature circuit breakers.
4.c. Capcitor Contactors :
The contactors used in detuned capacitor banks must comply with IEC 60947-4-1. According to this standard, usage categories are clearly defined, and category AC-6b is mandatory for capacitor banks. The contactor should be selected one size above the nominal current of the capacitor step.
4.d. Reactive Power Control Relay:
Although commonly referred to as a “power factor controller,” this device does not operate like a relay but rather as a regulator, switching capacitor steps in and out gradually. As an electronic device, it should have a user-friendly interface and a large graphical display for ease of use. Fast and accurate step detection further simplifies operation. To meet power utility requirements (e.g., tan φ = 0.20 inductive and tan φ = 0.15 capacitive), the controller should have at least 12 steps, preferably 18. It should also be capable of measuring at least 5 mA as the minimum detection value.
An algorithm that avoids fixed step connections should be preferred to eliminate the drawbacks of static steps. To prolong step lifespan, an equal aging algorithm should be included.
For easy access and user convenience, the power factor controller should be mounted on the panel door of the power factor correction system.
4.e. Discharge Coil:
If the switching interval of capacitors is shorter than 8 seconds, the built-in discharge resistors may not be sufficient. In this case, additional discharge coils should be installed to ensure adequate discharge.
4.f. Capacitor Step Configuration :
Power factor controllers are not mechanical relays but digital regulators that switch capacitors in multiple smaller steps, allowing for more precise control during each switching event and achieving the targeted values (e.g., tan φ = 0.20 inductive or tan φ = 0.15 capacitive) with high resolution.
If one large-capacity step is used, contactors are forced to open and close frequently, leading to faster mechanical wear and reduced system life, often before the desired power factor is reached. In contrast, using smaller-capacity steps results in lower inrush currents, extending contactor lifespan and ensuring more stable operation.
It is recommended to use three-phase, three-terminal capacitor modules to simplify wiring and ease maintenance or replacement. In six-terminal capacitors, contactors are built into the module, which complicates the wiring diagram, extends service time, and makes replacements more difficult.
Moreover, keeping a fixed step permanently connected may leave a live point inside the capacitor bank even when the main capacitor switch is off—posing a serious safety hazard. Therefore, manual control buttons for capacitor steps should be eliminated from the front cover, and all step switching should be managed automatically by the power factor controller using regulator logic.
This approach ensures both operational safety and the precise, sustainable achievement of power factor targets.
- DESIGN
The detuned capacitor bank should be integrated within a panel frame with a height of 200 cm. During setup, the copper busbar must be affixed to the panel’s top using busbar supports. These supports should be arranged to avert busbar collisions during short circuits. The internal layout of components within the panel should be ordered vertically as follows: NH fuse disconnector , capacitor contactors, detuned reactors, and capacitors at the base. This configuration ensures the capacitors are shielded from heat emitted by the detuned reactors.
A widely used design in the market is the cassette type, where each step is mounted on a drawer. However, this design has serious drawbacks. The heat generated by the detuned reactors increases the temperature of the capacitors above them, accelerating their capacitance loss. This capacitance loss alters the tuning frequency between the reactor and the capacitor, leading to premature deactivation of the step.
Annual maintenance of cassette-type panels is also physically demanding. For example, removing the topmost cassette for servicing requires handling a 50 kg unit mounted at a height of 1.8 meters, a task that cannot be easily managed by a single person.
To overcome such issues, the panel should be designed to allow the current of each capacitor to be easily measured using a clamp ammeter, and for capacitors with capacitance loss to be replaced without difficulty. All wiring should be done with halogen-free flexible cables, using cross-sections appropriate for the fuse current ratings.
Even with the panel’s front cover open, direct contact with live parts must be prevented. Busbars and other energized areas should be covered with non-flexible fiberglass material of at least 5 mm thickness, which prevents contact without obstructing visibility. Warning labels must also be affixed to these areas.
When a detuned capacitor bank is situated opposite the main distribution panel in the electrical room and connected via cables, a discrete cable entry compartment should be incorporated into the capacitor bank. A singular connection to one entry point from the main distribution panel is adequate. Nevertheless, in the absence of a main busbar system within the capacitor bank, each output section in the main panel necessitates individual connection. Under these circumstances, parallel cabling and lugging mandate a distinct transposition study.
In light of the considerable inverse relationship between operating temperature and capacitor longevity, the capacitor bank should be designed to meet IP20 protection standards and incorporate effective ventilation mechanisms.
- FINAL OUTCOME
The detuned reactor capacitor banks designed and manufactured by Ergun Elektrik provide an efficient, durable, and maintenance-friendly solution. Their modular design, coupled with a robust cooling system and premium materials, positions these power factor correction systems as a resilient and economically viable option for electrical systems. Field applications indicate that detuned reactor capacitor banks play a crucial role in reducing reactive energy charges and mitigating harmonic distortion within the electrical grid.
Didem Ergun Sezer
Electrical Engineer
Ergun Elektrik
