INTRODUCTION
The use of power electronics-based drive systems continues to rise annually, accompanied by a proliferation of nonlinear loads. This widespread adoption has led to a significant increase in network harmonic distortion, particularly in industrial plants. Concurrently, the continuous growth in energy demand has prompted transmission and distribution authorities to elevate power factor target values, aiming to enhance the capacity of the electrical grid.
As the resonance frequency of electrical installations approached fundamental harmonic frequencies, failures in power factor correction systems consisting solely of capacitors without detuned reactors increased. Excessive harmonic distortion in the grid and the potential for resonance between such capacitor-based power factor correction systems have necessitated the incorporation of detuned reactors in all such systems. It is particularly emphasized that power factor correction systems with only capacitors and no detuned reactors should be avoided, especially during the planning phase of new facilities.
In this context, the selection criteria for detuned reactors are of paramount importance. The utilization of reactors that are either designed and manufactured improperly or are of substandard quality can result in the system reverting to power factor correction systems comprising solely of capacitors. This, in turn, can lead to the recurrence of grid resonance challenges. To address these challenges, it is imperative to subject the reactors to rigorous evaluation during the selection process to ascertain their suitability and quality.
Detuned Reactor
HARMONICS
Harmonics are defined as distortions in the electrical current and voltage waveforms that arise as multiples of the fundamental frequency when nonlinear loads are connected to the power system.
RESONANCE
Industrial facilities are equipped with transformers, motors, and other inductive loads. The presence of these loads gives rise to an inductive reactance (XL) associated with the grid frequency (f) within the system. Additionally, capacitors employed in power factor correction systems contribute to a capacitive reactance (XC).
As demonstrated in the subsequent equations, both reactances are expressed as functions of frequency. The symbolization of inductance (L) and capacitance (C) is as follows:
XL inductive reactance (Ω) and XC capacitive reactance (Ω) are expressed as functions of frequency. These inductive and capacitive reactances form a continuous oscillating circuit, thereby determining the system’s total resonance frequency.
As demonstrated by the formula, there is a direct proportionality between capacitor current and frequency, whilst inductor current and frequency are inversely proportional. The addition of a power factor correction systems with only capacitors and no detuned reactors to a facility may result in the formation of oscillating circuits, which can potentially lead to resonance. To avert the occurrence of such resonance, it is imperative to employ detuned reactors, which are to be connected in series with the capacitors.
These detuned reactors are dry-type, iron-core reactors with copper or aluminum windings, and are utilised by means of connecting them in series with capacitors.
The initial step in selecting a detuned reactor involves verifying that the reactors adhere to the specifications outlined in the IEC 60076-6 standard.
The selection of detuned reactors is determined by several key technical parameters, including:
- Rated Voltage [Volt] Harmonic Voltage Spectrum
- Rated Frequency [Hertz] Linearity
- Rated Power [kVAr] Insulation Class and Ambient Temperature
- Reactor Factor [%p] Losses
- Detuning Frequency [Hertz] Vacuum Varnish Process
- Rated Voltage Level of the Capacitor Copper Terminals
Prior to defining these parameters in the context of detuned reactor design, it is essential to conduct necessary analyses and accurately determine the relevant data.
1. POWER QUALITY MEASUREMENT
Firstly, a power quality measurement must be conducted. This should be done using a Class A measurement device at the main distribution panel of the system to be installed. The results of this measurement should then be analysed thoroughly during the selection process for the detuned reactor.
The initial selection criteria for detuned reactors, namely the rated voltage and the rated frequency, must be determined based on these measurements.
It is imperative to note that the designed detuned power factor correction system should be configured to operate in parallel with the busbar. The system must be capable of operating independently of any active or passive harmonic filter and should function seamlessly in accordance with system parameters. This ensures uninterrupted operation of the power factor correction system, even if another filter is deactivated.
2. SERIES CAPACITOR:
Following the determination of the nominal voltage and frequency of the detuned reactor, the voltage level of the series capacitor must be calculated.
However, when a reactor is connected in series with the capacitor, the impedance of the detuned reactor causes the voltage at the capacitor terminals to be higher than the nominal grid voltage. Consequently, when calculating the capacitor voltage level, it is imperative to consider the following formula:
Important Note: When selecting the nominal voltage level of the capacitor, the permissible overvoltage values specified in IEC 60831 standard should not be taken into consideration. The nominal operating voltage indicated on the nameplate must be taken into account. These values do not represent the continuous operating voltage of the capacitor; rather, they signify the level of resistance to transient overvoltages.
For Example :
If the reactor factor p = 7% is selected in an installation with busbar voltage Un = 400V, then the voltage withstand of the selected capacitor must be above 430V. In addition, for the long life of the compensation installation, the capacitor voltage level should be selected by adding voltage tolerance to the 430V voltage level.
Assuming a nominal voltage of 400V and a detuning factor of 7%, the minimum required voltage withstand rating of the capacitor can be calculated as follows:
UC= 400 / (1−0,07)
UC= 430,1V
In addition, in order to ensure the long service life of the power factor correction system, it is necessary to take into account the grid voltage tolerance of the capacitor. By incorporating a 10% voltage tolerance, the ultimate minimum voltage level is delineated as such:
UC=430,1V+43V=473,1V
In this case, the selected capacitor must have a minimum voltage withstand rating of 473V.
As the reactor and capacitor are connected in series, the capacitor will draw the same current as that flowing through the reactor.
In the context of detuned power factor correction systems, the microfarad (𝜇𝐹) value of the capacitor is to be selected in accordance with the system’s resonance frequency and the inductance of the reactor. The capacitor value that is appropriate for the system is calculated on the basis of the reactor factor (p) and the corresponding resonance frequency (𝑓𝑟).
The design of detuned reactors typically incorporates detuning factors of 7% (189 Hz) or 14% (134 Hz). The rationale behind these values is to avoid critical resonance frequencies within the system and to ensure that harmonic currents remain within safe limits.
In the event of an erroneous microfarad value being selected for the capacitor, the system’s resonance frequency may be subject to alteration, which may, in turn, give rise to undesirable resonance effects.
3. REACTOR FACTOR:
Nonlinear loads, including but not limited to variable frequency drives and uninterruptible power supplies, are extensively utilised in industrial applications. The 6-pulse rectifier circuits located at the front-end of variable speed drives serve the function of harmonic current sources, thereby generating 5th, 7th, 11th, 13th, and higher order harmonics. These harmonics, produced by VFDs, are integer multiples of the fundamental frequency, resulting in components at 250Hz, 350Hz, 550Hz, 650Hz, and so on.When these harmonic currents flow through the network impedance, harmonic voltage distortions occur, leading to voltage components at 250Hz, 350Hz, 550Hz, 650Hz, and so on. In order to mitigate reactive energy penalties, industrial facilities frequently implement power factor correction systems in which capacitors are connected in parallel to the grid. However, these capacitor-only power factor correction systems have been observed to amplify harmonic currents generated by nonlinear loads, thereby causing instantaneous resonance issues. To circumvent such issues, a series-detuned circuit is often formed by connecting detuned reactors in series with the capacitors.
The most common nonlinear loads in industrial applications are variable frequency drives (VFDs) with a 6-diode rectifier front end. The input step containing six diodes results in the harmonic current components commencing from the fifth harmonic, thereby causing resonances to occur at 250Hz and above within the system.
In order to prevent resonance and avoid the amplification of harmonic current components, the detuned reactor detuning frequency is set below 250Hz, typically at 189Hz.
The selection of a detuned reactor with a detuning frequency of 189Hz ensures the elimination of unwanted resonance effects within the system, thereby preventing harmonic current amplification and reducing the harmonic voltage distortion ratio.
| Reactor Factor (p) and Harmonics | Resonance Frequency (fr) [For fn= 50Hz ] |
| 14% Detuned Reactor | 134Hz |
| 3rd Harmonic | 150Hz |
| 7% Detuned Reactor | 189Hz |
| 5th Harmonic | 250Hz |
Furthermore, 134Hz (14% reactor factor) detuned reactors are utilized in facilities with a significant presence of the 3rd harmonic.
4. HARMONIC VOLTAGE SPECTRUM :
The harmonic voltage spectrum and impedances govern the flow of each harmonic current through the detuned reactor across the entire harmonic range. Consequently, they dictate the detuned losses, heating, and linearity characteristics of the reactor.
In alternating current (AC) systems, the root mean square (RMS) value is representative of the equivalent direct current (DC) value that would engender the same power dissipation in a resistive load.
As evidenced by the above equations, the harmonic voltages across the entire spectrum of the supply network will flow through the detuned reactor as harmonic currents due to its impedance.
Consequently, when selecting and implementing detuned reactors, the harmonic spectrum and effective (RMS) current value must be taken into account to prevent core saturation and excessive heating.
5. LINEARITY:
The B-H curve, otherwise referred to as the hysteresis graph, is a representation of the relationship between magnetic field intensity and magnetic flux. This relationship is indicative of the ferromagnetic properties of the core material. In the event of the iron core of a detuned reactor reaching saturation, an alteration in its inductance value will be observed, with the potential to exert a detrimental effect on the system’s performance.
Consequently, a detuned reactor must be engineered to function within the linear region of the hysteresis curve. This ensures that the adverse effects of harmonic currents and system fluctuations on the reactor are minimised, thereby providing stable operation.
In the case of a detuned reactor with a 7% reactor factor, it is imperative that the reactor current is 1.8 times the nominal reactor current in order to remain within the linear region.
In the case of a detuned reactor with a 7% reactor factor, it is imperative that the reactor current is 1.8 times the nominal reactor current in order to remain within the linear region.
The linearity of the detuned reactor is of critical importance. It is imperative to note that even in circumstances where the nominal current is exceeded by 1.8 times under operating conditions, the inductance value stated on the nameplate should not deviate by more than 5%.
The proper functioning of the capacitor detuning circuit with the detuned reactor depends on ensuring that the detuned reactor does not enter saturation.
6. INSULATION CLASS AND AMBIANT TEMPERATURE
The permissible temperature rise of the detuned reactor remains within the limits defined by its insulation thermal class. The ambient temperature is a determining factor in the initial operating conditions of the reactor, while the insulation thermal class is the maximum temperature to which the reactor can withstand.
For example; in a T40/F reactor (40°C ambient temperature and F-class insulation)
T40: This indicates the maximum ambient operating temperature of the coil. T40 indicates that the coil can operate in an environment with an ambient temperature of up to 40°C, thereby ensuring that the detuned reactor functions safely under such conditions.
F-Class Insulation: Insulation materials utilised in electrical devices must demonstrate the capability to withstand specific temperature levels. The use of F-class insulation signifies that the insulation material can endure temperatures of up to 155°C, thereby ensuring the safe operation of the detuned reactor even under high-temperature conditions.
To clarify with an example, a reactor with an F (155°C) insulation class should be protected with a 120°C thermostat. However, it is recommended that the operating temperature does not exceed 100°C during use.
Additionally, the reactor should be equipped with a 120°C thermal switch (thermistor) that sends a shutdown signal to the control circuit if the temperature limit is exceeded.
7. LOSSES
From a broad viewpoint, power factor correction systems can be assessed within the framework of energy efficiency. Since detuned reactors are regarded as a component of the power factor correction system, they must exhibit low power losses. Otherwise, a scenario contradictory to the underlying nature of the process may arise.
One of the key factors in the selection of detuned reactors is energy losses, which can be classified into two primary categories: ohmic losses and hysteresis & eddy current losses. Ohmic losses are associated with the winding losses, while hysteresis and eddy current losses are collectively known as core losses within the system.
Hysteresis losses are directly proportional to the frequency, whereas eddy current losses are proportional to the square of the frequency. Consequently, in an environment with harmonics, core losses become significantly elevated. Furthermore, the presence of harmonics leads to an increased 𝐼rms value, resulting in increased winding losses as well. During the design of reactors, it is crucial to account for not only the fundamental components but also the entire harmonic spectrum.
In conclusion, detuned reactors must be designed with minimal losses and manufactured in accordance with IEC 60076-6 standards.
8. VACUUM VARNISH PROCESS
An important detail in this regard is the noise level. Reactors that are varnished under vacuum ensure noiseless performance while also preventing insulation failures caused by vibrations. After the vacuum varnishing process, the noise level should be measured using a sound level meter (dB meter).
9. COPPER TERMINALS
When selecting detuned reactors, the material composition of the terminals is a crucial consideration. While aluminum windings may be technically suitable, employing aluminum terminals can pose distinct drawbacks. Aluminum, being a softer metal, can lead to the weakening of connection points over time, resulting in loose connection failures that compromise the system’s reliability. Consequently, it is advisable to use copper terminals, which are a harder and more durable material compared to aluminum. Furthermore, nickel-plated aluminum terminals should be carefully evaluated, as they may prove misleading.
FINAL OUTCOME
In the contemporary era, the implementation of detuned reactor applications has become pervasive across numerous industrial facilities. However, it has been observed that a significant number of these reactors have been the cause of major issues in the field, largely due to inadequate quality or improper selection. Consequently, concerns regarding the reliability of detuned reactors have resurfaced, prompting businesses to revert to employing power factor correction systems that rely exclusively on capacitors. In reality, the fundamental cause of these issues is the utilisation of reactors that are either designed and manufactured improperly and substandard.
This study undertakes a comprehensive examination of the pivotal criteria to consider when selecting a detuned reactor, emphasising the significance of factors such as capacitor-reactor matching, frequency selection, linearity, temperature rise, insulation class, watt losses, thermal endurance, and material selection.
Additionally, conducting a power quality measurement in the facility prior to implementation, selecting the appropriate products, and requesting test certificates from the manufacturer are of paramount importance. Carefully examining the limit values specified in the aforementioned certificates and judiciously selecting them is essential for ensuring the reliable operation and long service life of the detuned reactors.
Didem Ergun Sezer
Electrical Engineer
Ergun Elektrik
