I am working on grid connected Active Front End Converter - Inverter system to achieve unity power factor with reduced voltage and current THD. Some dc offset current has generated in spectrum analysis of grid current. What are the reasons of dc offset current? How to reduce/eliminate the dc offset current?
Many things can result in DC offset current components in grid-connected (AFE - Active Front End) converters:
• basic issues concerning inappropriate feed-back signal sensing and analog/digital signal processing (lack of DC offset calibration within digital control unit, absence of DC elimination modules on current feed-back paths, irregularities in software implementation of such filters, thermal impact on some sensors and other affected subsystems - in case the converter has to sustain very wide operating temperature range during long term (practically continuous) operation...
• issues related to more/less asymmetrical parameters of active and passive components within the converter's power circuit (voltage-drops, on-resistances and other characteristics, tolerances of filter components like input inductors, DC link capacitors, in some cases leakage inductance and other parameters related to interfacing transformer...)
• power switch issues caused by driver implementation and modulation techniques (dispersion of propagation delay during switching transients, incorrect synchronization of PWM carrier to the fundamental harmonic of grid voltage, intermodulation of some even harmonics
• inappropriate Phase Locked Loop algorithm issues - lack of DC offset suppression and susceptibility to DC component in grid voltage/current, and some times PLL induced injection of even harmonics in the current reference signals (caused by drawbacks of the algorithm, like basic EPLL...) and their intermodulation with the fundamental frequency current reference...
• lack of robustness of grid converter's applied control structure to severely distorted grid voltage by significant content of high order harmonics (e.g. caused by switching over-voltages due to operation of older thyristor-based converters as well as switching transients during start-up of such drives and other non-linear loads relatively close to the converter's point of load ...)
These are just a few basic groups of possible causes (origins) of DC current component, illustrating only top of the related "iceberg" (active DC component suppresion/elimination techniques related to AFE design - power topology/circuit selection and related control system design).
By the way (properly implemented) PR current controller is rather good choice for AFE's current controller.
Best regards
Nenad Težak, PhD EE
Control System Designer
KONČAR - Electrical Engineering Institute
The possible reasons for dc offset current are as follows;
1. Unbalance grid voltage
2. Switching of Active front end converter
The diagnosis depends highly on your grid topology, current sencing method and AFE control system, so much more information is required. It's not also clear what can you adjast in the system and what is to be done actually - a new AFE development, field application, etc.
Thank you Sir. I am working on single phase grid connected Active Front End Converter - Inverter system. In this topology, I am using Proportional Resonant (PR) current controller as the reference singnal is AC in nature.
Many things can result in DC offset current components in grid-connected (AFE - Active Front End) converters:
• basic issues concerning inappropriate feed-back signal sensing and analog/digital signal processing (lack of DC offset calibration within digital control unit, absence of DC elimination modules on current feed-back paths, irregularities in software implementation of such filters, thermal impact on some sensors and other affected subsystems - in case the converter has to sustain very wide operating temperature range during long term (practically continuous) operation...
• issues related to more/less asymmetrical parameters of active and passive components within the converter's power circuit (voltage-drops, on-resistances and other characteristics, tolerances of filter components like input inductors, DC link capacitors, in some cases leakage inductance and other parameters related to interfacing transformer...)
• power switch issues caused by driver implementation and modulation techniques (dispersion of propagation delay during switching transients, incorrect synchronization of PWM carrier to the fundamental harmonic of grid voltage, intermodulation of some even harmonics
• inappropriate Phase Locked Loop algorithm issues - lack of DC offset suppression and susceptibility to DC component in grid voltage/current, and some times PLL induced injection of even harmonics in the current reference signals (caused by drawbacks of the algorithm, like basic EPLL...) and their intermodulation with the fundamental frequency current reference...
• lack of robustness of grid converter's applied control structure to severely distorted grid voltage by significant content of high order harmonics (e.g. caused by switching over-voltages due to operation of older thyristor-based converters as well as switching transients during start-up of such drives and other non-linear loads relatively close to the converter's point of load ...)
These are just a few basic groups of possible causes (origins) of DC current component, illustrating only top of the related "iceberg" (active DC component suppresion/elimination techniques related to AFE design - power topology/circuit selection and related control system design).
By the way (properly implemented) PR current controller is rather good choice for AFE's current controller.
Best regards
Nenad Težak, PhD EE
Control System Designer
KONČAR - Electrical Engineering Institute
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