PWM Inverter with Dual-Loop PI Controller: Implementation and Algorithm Analysis

The dual-loop PI controlled PWM inverter is a widely adopted control strategy in power electronics, primarily employed to enhance both dynamic response and steady-state performance of inverters. Its core principle involves the coordinated operation of two control loops—outer voltage loop and inner current loop—to achieve precise output regulation through independent voltage and current adjustments. The outer loop (voltage control loop) regulates the inverter's output voltage to track reference values. This loop's PI controller generates current reference signals based on voltage error (difference between reference voltage and actual output voltage), ensuring robust voltage regulation capabilities. In code implementation, this typically involves: - Sampling output voltage using ADC peripherals - Calculating error: V_error = V_ref - V_measured - PI controller computation: I_ref = Kp_v * V_error + Ki_v * ∫V_error dt - Anti-windup implementation to prevent integrator saturation The inner loop (current control loop) provides rapid response to current variations by adjusting PWM modulation signals through its PI controller, enabling inductor current to accurately track the reference current provided by the outer loop. This loop's design critically impacts system dynamic performance, including load disturbance rejection and harmonic suppression. Key implementation aspects include: - High-bandwidth current sampling (typically 10-100kHz) - Current error calculation: I_error = I_ref - I_measured - Fast PI computation: PWM_duty = Kp_i * I_error + Ki_i * ∫I_error dt - Duty cycle limitation and dead-time compensation Advantages of the dual-loop structure include: Enhanced Stability: The inner current loop suppresses sudden inductor current changes, reducing interference with the voltage loop Fast Dynamic Response: High-bandwidth design of the current loop enables rapid response to load variations Harmonic Suppression: Precise current control effectively reduces output waveform distortion through: - Real-time harmonic compensation algorithms - Selective harmonic elimination techniques - Adaptive filter implementations This control methodology is commonly applied in UPS systems, renewable energy grid-tied inverters, and motor drives. Parameter design must balance loop bandwidth and stability, typically optimized using frequency-domain analysis methods (Bode plots, Nyquist criteria) or experimental tuning approaches. Code implementation often incorporates: - Systematic gain scheduling based on operating points - Auto-tuning algorithms using relay feedback tests - Real-time stability margin monitoring - Fault detection and protection mechanisms