應用於單相分散式發電系統之FPGA電流與電壓控制

Abstract

相較於單電感型濾波器，LCL濾波器由於其高階濾波特性具有較好的高頻諧波抑制能力，因此更適合應用在高功率、低切換頻率的併網型系統中，然而採用LCL濾波器將使得控制器的設計難度倍增，本文採用串級型多迴路電流控制架構，能有效降低LCL濾波器的諧振現象並達到低電流諧波失真度，具有設計、實現容易與系統高強韌性等特色。控制迴路由三個串聯型的控制器組成，分別對系統的各回授變數進行平均值控制，包含內電流迴路、內電壓迴路與外電流迴路，內迴路主司高頻的動態響應與增加系統的穩定性，外迴路負責穩態表現。由於本文針對高功率應用進行電流控制器設計，因此採用相對較低的系統開關頻率(12kHz)，而較低的開關頻率代表較大的換流器側電感電流漣波比(30%)，然而在該條件下採用傳統的單點同步取樣方式回授平均電流，其雜訊拒斥能力與平均值精準度表現較差。本文採用多重取樣策略解決在漣波電流較大條件下之取樣問題，該策略是在一個開關週期內以16倍頻率(192kHz)對電感電流進行快速取樣，並將取樣數值進行加總平均，該方法可有效提升回授訊號的準確性與雜訊拒斥能力。結合預測型電流控制以多組回授訊號作為模型運算基礎，使系統兼備快速的動態響應與低諧波失真度的電流控制效果。本文之設計方式除了應用在LCL併網電流控制系統之外亦適用於單電感併網電流控制系統與獨立模式穩壓系統，並且可達到較低的電流與電壓諧波失真度之控制效果。本文設計之LCL併網電流控制器可以單電感系統五分之一倍的總電感值達到相同的電流總諧波失真度(3.2%)。實驗部份採用Microsemi公司生產之SmartFusion混合訊號FPGA(A2F500)作為數位控制器的實現平台，並以一試作型3kW之單相全橋式換流器完成系統整合之測試與驗證。

Compared with the L-filter, the LCL-filter is more suitable for high-power, low-switching-frequency applications due to its better attenuation characteristics on high frequencies. However, control systems involving LCL-filters are inevitably more complicated. A cascaded multiple-loop control architecture is developed to not only damp the LCL resonance but also to achieve lower harmonic distortion of the grid-side currents. The method has advantages of easy design, implementation and system robustness. The control system consists of three cascaded control loops, an inner current, an inner voltage and an outer current loop, with the inner loop providing fast dynamic compensation for system disturbance and improving stability and the outer one ensuring precise steady-state performance. A relative low switching frequency (12kHz), which is used due to the current controller is designed for higher power applications, will leads to higher current ripple ratio (30%). However, by applying the conventional single-sampling strategy to feedback the average current under this condition, the performances of sampling accuracy and noise rejection will be poor. Therefore, a multiple-sampling strategy, which can increase the sampling accuracy with good noise rejection capability is developed to detect the average current under the condition of large ripple current. The average value is extracted by detecting the inductor current 16 times in one switching cycle (192kHz) and calculating the sum and average. By using predictive control technique with multiple feedback variables both good dynamic response and low total harmonic distortion can be achieved. The proposed design method can not only be applied to current regulation for grid-inverter with LCL-filter but also be used to the current regulation in grid-connected mode with L-filter and voltage regulation in stand-alone mode with lower current and voltage harmonic distortion. In this thesis, a 3.2% current THD of the LCL grid-inverter can be achieved with the total inductance reduced by 5 times smaller then the L-filter one. The FPGA controller has been realized by using a mixed-signal FPGA PSOC controller, the SmartFusion (A2F500), from Microsemi. The proposed method is validated by experiment with a prototype 3kW single-phase full-bridge inverter.