Non-linear Loads: Connected devices such as computers and variable frequency drives draw non-sinusoidal currents, introducing odd-order harmonics (3rd, 5th, 7th, etc.) into the system.

High-Frequency Switching: The UPS’s power conversion stages (rectifiers and inverters) use pulse-width modulation (PWM) with high switching frequencies, generating high-frequency harmonics (above 2kHz) that can propagate through the grid.

Single-Tuned Filters: A series combination of a capacitor and inductor tuned to a specific harmonic (e.g., 5th harmonic at 250Hz for 50Hz grids). They effectively attenuate the target harmonic by up to 90%.

High-Pass Filters: Consist of capacitors, inductors, and resistors configured to attenuate all harmonics above a certain frequency (e.g., 300Hz). They are particularly useful for high-frequency UPS systems, where switching harmonics dominate.

Advantages: Low cost, high reliability, and simple design make them suitable for applications with stable harmonic profiles.

Limitations: Fixed tuning (prone to detuning due to component aging), bulkiness, and potential resonance with the grid impedance, which can amplify certain harmonics if not properly designed.

Detection: Sensors measure the load current, and a digital signal processor (DSP) extracts harmonic components using fast Fourier transform (FFT) or adaptive filtering algorithms.

Injection: A voltage-source inverter (VSI) generates a compensation current with the same magnitude but opposite phase to the detected harmonics, neutralizing them before they reach the grid.

Shunt APFs: Connected in parallel with the UPS output, they compensate for current harmonics by diverting harmonic currents away from the grid.

Series APFs: Inserted in series with the grid, they mitigate voltage harmonics by injecting harmonic voltages that cancel grid distortions.

Hybrid APFs: Combine active and passive filters, leveraging the low cost of passive filters for low-order harmonics and APFs for dynamic compensation of high-order harmonics. This reduces APF capacity requirements by 50–70%.

Dynamic Response: Adjusts to varying harmonic loads within milliseconds, ideal for high-frequency UPS serving fluctuating loads (e.g., data centers with variable server activity).

Broadband Suppression: Effective for harmonics from 50Hz to 10kHz, covering both load-generated and switching harmonics.

No Resonance Risk: Unlike passive filters, APFs do not interact with grid impedance, eliminating resonance issues.

Reduced Switching Losses: Lower voltage stress per switch allows higher switching frequencies without increasing harmonics.

Lower THD: 3-level inverters typically achieve THD < 3% without additional filters, compared to 5–8% for conventional 2-level inverters.

Real-Time Optimization: MPC predicts the inverter’s future states and selects the switching state that minimizes harmonic distortion, adapting to load changes in microseconds.

Harmonic Injection: Strategic injection of specific harmonics (e.g., 3rd harmonic) into the PWM reference signal reduces the required DC bus voltage and lowers low-order harmonics.

Adaptive Harmonic Cancellation: Algorithms learn and adapt to harmonic patterns, improving suppression efficiency over time.

Phase-Locked Loops (PLLs): Advanced PLLs with harmonic rejection capabilities ensure stable grid synchronization, even in distorted environments.

Power Factor Correction (PFC): Achieves near-unity power factor and THD < 5% by controlling the rectifier to mimic a resistive load.

Bidirectional Operation: Enables energy recovery during UPS discharge, reducing harmonic generation during battery mode.

Interleaving: Phase-shifted switching of rectifier modules cancels out certain harmonics (e.g., 5th and 7th) through destructive interference.

Increased Frequency: Higher switching frequencies (50kHz+) in multi-phase designs reduce harmonic content in the audible range and simplify filtering.

Common-Mode Filters: Use ferrites and capacitors to attenuate common-mode currents, preventing interference with nearby electronics.

Differential-Mode Filters: Reduce differential-mode high-frequency harmonics, ensuring compliance with CISPR 22 (radiated emissions) and CISPR 11 (industrial equipment).

Harmonic Analyzers: Embedded sensors measure THD and harmonic spectra, triggering filter activation or rectifier adjustments.

Grid Impedance Detection: Algorithms monitor grid impedance to avoid resonance with passive filters, ensuring stable operation under varying grid conditions.