A Modified Capacitor Current Feedback Active Damping Approach for Grid Connected Converters with an LCL Filter

Abstract

Capacitor current feedback active damping is extensively used in grid-connected converters with an LCL filter. However, systems tends to become unstable when the digital control delay is taken into account, especially in low switching frequencies. This paper discusses this issue by deriving a discrete model with a digital control delay and by presenting the stable region of an active damping loop from high to low switching frequencies. In order to overcome the disadvantage of capacitor current feedback active damping, this paper proposes a modified approach using grid current and converter current for feedback. This can expand the stable region and provide sufficient active damping whether in high or low switching frequencies. By applying the modified approach, the active damping loop can be simplified from fourth-order into second-order, and the design of the grid current loop can be simplified. The modified approach can work well when the grid impedance varies. Both the active damping performance and the dynamic performance of the current loop are verified by simulations and experimental results.

Voltage Feedforward Control with Time-Delay Compensation for Grid-Connected Converters

“…As a result, the one-step-delay control method is usually used for the converter control. This produces some delays between the calculation of the output-voltage reference of the converter and the updating of the PWM comparison value [16]. The updating frequency is related to the loading mode of the PWM module in the digital signal processor (DSP).…”

“…If m has been selected as an integer according to (16) in the z domain, the leading correction algorithm with the optimal leading step described by (15) can be expressed as:…”

“…However, for the traditional feedforward control used in the above papers, the sampled grid-voltage is directly added into the current regulator output without consideration of nonlinear factors, such as the signal conditioning, PWM control and sampling. In fact, a low-pass filter (LPF) in the grid-voltage filtering circuit will produce different phase lags at different frequency components, and the PWM control and sampler will also cause some delays [16]- [19]. Both of them reduce the performance of the feedforward control in terms of suppressing grid background harmonics.…”

Assessment of Grid-Side Filters for Three-Phase Current-Source Inverter PV Systems

“…This method consists in using the feedback of one of the filter variables and subtracting it to the converter reference to obtain an effect equivalent to that of a passive damping resistor. The feedback variable is most often the capacitor current [12]- [15] but other variables such as the capacitor voltage or grid-side current [14] and the grid-side or converter-side inductor voltage [8] can be used. In this work, the method is applied on the filter inductor voltages and a feedback of the grid currents is used so that no extra sensor is required.…”

“…This gap has been filled with filter-based damping [10], with virtual resistor capacitor current feedback [12], [13], [15], and with the lead-lag network method [18]. Also, a single-phase circuit [13], [14] or the approximation of an equivalent single-phase system [8], [11], [12] is used in most papers to tune the active damping feedback gain.…”