三相四線市電併聯換流器中性線電流漣波抑制

Abstract

本論文提出應用於三相四線式併網換流器之中性線電流漣波抑制策略。無論三相系統是否平衡，因開關切換造成的高頻漣波電流依然會在中性線上。相較於三相系統不平衡所產生的中性線電流，因功率開關切換所造成的高頻電流漣波會往往被忽略。當有多組三項四線併網換流器會併接到相同電網時，會造成公用的中性線電流上的漣波疊加。並且因為各組併網換流器的切換頻率沒有同步，造成共用的中性線上的電流漣波難以估計。另一方面因為中性線會連接到直流端分離電容的中性點上，將導致分離式電容出現比預期更大的漣波電壓，影響電容的壽命，進而降低併網換流器的可靠度。 為了降低中性線上因開關切換造成的電流漣波，本論文提出中性線電流漣波抑制策略。在本論文中先利用雙重傅立葉級數的分析，推導出中性線電流漣波的數學模型。並且利用此數學模型提出的兩種三相換流器的載波相移策略，來達到降低中性線電流漣波的目的。本論文中首先將詳細說明中性線電流漣波的數學模型推導。然後提出固定載波相移策略與次序載波相移策略，並且說明兩種利用載波相移達成中性線電流漣波抑制策略。並藉由電腦模擬來驗證數學模型的準確性與電流漣波降低的可行性。最後實作一組三相5kVA 併網換流器的原型機，來驗證本論文所提出的中性線電流漣波抑制策略的正確性。實驗結果得到與傳統載波相比，固定載波相移策略將中性線電流漣波的均方根值降低了60.944%，峰對峰值降低了57.96%。次序載波策略將中性線電流漣波的均方根值降低了71.08%，峰對峰值降低了65.6%。

This thesis proposes two neutral line current ripple suppression strategies for threephase four-wire grid-tied inverters. Regardless of whether the three-phase system is balanced, high-frequency ripple current caused by switching will still appear on the neutral line. Compared to the neutral line current generated by the imbalance of the threephase system, the high-frequency current ripple caused by power switching is often overlooked. When multiple sets of three-phase four-wire grid-tied inverters are connected to the same grid, the ripple on the neutral line current will superimpose. Because the switching frequencies of each set of grid-connected inverters are not synchronized, the ripple current on the shared neutral line becomes difficult to estimate. On the other hand, since the neutral line is connected to the neutral point of the DC-side split capacitors, it will result in larger than expected ripple voltage on the split capacitors, affecting the lifetime of the capacitors and thus reducing the reliability of the grid-connected inverters. To reduce the current ripple on the neutral line caused by switching, this thesis proposes two neutral line current ripple suppression strategies. First, the double Fourier series analysis is used to derive the mathematical model of the neutral line current ripple. Using this mathematical model, two carrier phase shift strategies for three-phase inverters are proposed to reduce the neutral line current ripple. This thesis will first detail the derivation of the mathematical model of the neutral line current ripple. Then, the constant carrier phase shift (CCPS) strategy and the sequential carrier phase shift (SCPS) strategy are proposed, explaining how these two carrier phase shifting strategies achieve neutral line current ripple suppression. The accuracy of the mathematical model and the feasibility of reducing the current ripple are verified through computer simulations. Finally, a prototype of a three-phase 5kVA grid-tied inverter is implemented to validate the correctness of the proposed neutral line current ripple suppression strategies proposed in this thesis. Experimental results show that, compared to the conventional carrier strategy, the CCPS strategy reduces the RMS value of the neutral line current ripple by 60.944% and the peak-to-peak value by 57.96%. The SCPS strategy reduces the RMS value of the neutral line current ripple by 71.08% and the peak-to-peak value by 65.6%.