How can harmonic distortion be controlled in large plastic extrusion plants?
The limitations of traditional passive solutions
In the plastics processing industry, the continuity of extrusion processes depends significantly on the quality of the electrical power supply.
Large 1300 A AC drives operating at 690 V frequently use 6-pulse rectifiers. To limit the harmonic distortion generated by this configuration, three-phase line reactors are normally installed in series with the rectifier modules.
This solution represents a first level of harmonic mitigation, but its effectiveness is closely linked to the operating conditions of the plant.
Under maximum extruder load conditions, the residual harmonic content can reach THDi values between 30% and 35%. During partial load phases, such as maintaining the temperature of the extrusion heads or during mould changes, the fundamental current decreases and the relative attenuation provided by the reactor becomes less effective.
Under these conditions, increases in harmonic distortion, overheating of the 690 V lines and voltage drops at the MV/LV transformer substation may occur.
Why do variable harmonics represent a critical issue for the plant?
With a nominal current of 1300 A, a harmonic distortion level of 30–35% means that more than 400 A of harmonic currents are injected into the plant’s internal electrical network.
Although the harmonic frequencies generated remain substantially constant, their amplitude continuously varies according to the operating conditions of the extruder.
This variability makes traditional passive compensation systems, designed for relatively stable operating conditions, less effective.
How does the modular AHF architecture developed by IREM work?
To effectively manage variable harmonic distortion, IREM has developed a modular parallel AHF (Active Harmonic Filter) solution designed for 690 V industrial applications.
The system operates as a controlled current source: through real-time harmonic analysis algorithms based on Fast Fourier Transform (FFT), the control electronics identify the harmonic content present on the network and command the inverters to generate compensation currents equal in magnitude and opposite in phase to the unwanted components.
In this way, harmonics are neutralized directly at the point of connection, maintaining THDi within the limits required by the application.
What advantages does the modular parallel architecture offer?
AHF modules operate on a common busbar system, uniformly distributing the compensation contribution. In a configuration consisting of four 100 A modules, more than 400 A of harmonic currents can be neutralized without concentrating thermal stress on a single unit.
Centralized injection on the busbar system
AHF modules operate on a common busbar system, evenly distributing the compensation load.
In a typical configuration, more than 400 A of harmonic currents are compensated through four 100 A modules, ensuring balanced thermal stress distribution across the power devices.
Selective module isolation
Each module is connected to the main system through its own switch-disconnector equipped with high breaking capacity fuses.
In the event of a fault, the affected module is selectively isolated without interfering with the operation of the 690 V line.
Operational continuity and maintenance without plant shutdown
The modular structure allows a single module to be isolated and replaced while keeping the filtering system operational at reduced capacity.
This feature enables maintenance or repair activities to be carried out without stopping the extruder and without interrupting the production process.
What regulatory and energy efficiency benefits can be achieved?
The integration of IREM AHF active filters with the reactors already installed upstream of the rectifier makes it possible to leverage the benefits of the original equipment solution and reduce THDi to values below 8%, ensuring compliance with the requirements of the most stringent international standards, including IEEE 519.
From an energy perspective, the reduction of harmonic currents results in:
- lower Joule-effect losses in conductors;
- reduction of additional losses in the MV/LV transformer;
- lower thermal stress on 690 V cables;
- reduced risk of nuisance tripping of protection devices;
- improved reliability of the entire power supply system.
The adoption of a modular AHF solution therefore improves the plant’s Power Quality, increasing the overall efficiency of the extrusion process and ensuring greater operational continuity.
