在動態和瞬態操作下評估微電網的電池儲 能和太陽能發電源的可靠度

Reliability evaluation of battery-energy-storage and solar-energy-generating sources in microgrids under dynamic and transient operation.

範氏庄(Thi-Trang Pham)

指導教授 ： 郭財吉

共同指導教授 ： 黃博滄(Po-Tsang Huang)

中原大學/電機資訊學院/工業與系統工程研究所/博士(2021年)

https://doi.org/10.6840/cycu202101034

摘要

微電網主要是提供本地負載供電，其中包含分佈式發電機和儲能係統。分佈式發電機主要來源為可再生能源，例如太陽能發電系統、風力渦輪機發電系統。聚合電池儲能系統為具有多個電池儲能裝置的聚合系統，為常被使用以提高微電網中可再生能源供電的可靠度。聚合電池儲能系統用於控制源負載功率平衡，使微電網能夠以高穩定性和可靠度操作，為不同的客戶供電。為了展示聚合電池儲能系統在微電網中的重要性，本研究的第一個貢獻是分析在微電網不同動態操作情況下聚合電池儲能系統的可靠度性能。具體而言，本研究利用馬可夫模型的分析方法以評估整個聚合電池儲能系統的操作可靠性。除聚合電池儲能系統外，關鍵組件的使用時間相關故障率、電壓波動和功率損耗相關故障率 (VF-PL DFR) 諸如雙向直流/交流，直流/直流轉換器、直流/交流逆變器、開關和保護裝置、電池模塊和電池充電器/控制器等也被制定並納入可靠度評估。根據聚合電池儲能系統和光伏 (PV) 發電系統的微電網的不同動態操作情況，聚合電池儲能系統的功率損耗相關故障率可能會受到不同的影響。本研究分析了微電網隨機動態操作場景，包括：負載功率變化、光伏電源間歇不穩定運行、微電網並網和離網操作模式、聚合電池儲能系統的充放電狀態。模擬測試結果被提出和討論，以驗證微電網中 聚合電池儲能系統 的操作可靠度在很大程度上取決於其不同的動態操作策略以及施加的電壓過應力。

另一方面，直流（直流）微電網是一種新興技術，可有效利用光伏發電系統和電池儲能係統等直流電源。在直流微電網的離網（或孤島）模式下，可再生能源的操作，例如 光伏發電系統和儲能係統應得到更多關注，使直流微電網能夠滿足各種負載需求的供電連續性，調度可再生能源的間歇輸出功率，並應對故障類型。這些可能會導致 可再生能源和能源儲存系統的性能可靠性降低。因此，本文的第二個貢獻是在動態和瞬態操作考慮下對孤島直流微電網的光伏發電系統進行可靠度分析。目的是闡明離網直流微電網中光伏發電系統的動態電壓變化故障率和故障電流變化故障率的計算。動態電壓變化故障率主要取決於動態操作條件，例如光伏功率波動和負載功率變化，而 故障電流變化故障率 表示由於直流微電網的瞬態操作條件（例如極對極和極對接地故障。然後綜合考慮使用的時變故障率、功率損耗和溫度相關故障率、動態電壓變化故障率 和故障電流變化故障率 來評估孤島直流微電網中光伏發電源的系統級和組件級可靠性。馬爾可夫狀態轉移圖和察普曼－科莫高洛夫方程式被推導出並應用於光伏系統可靠度評估。實驗結果表明，光伏發電系統直流-直流功率變換器的可靠度指標受孤島直流微電網的動態和暫態操作影響最大。此外，光伏系統的 動態電壓變化故障率 大多小於其 故障電流變化故障率，但由於這些情況在孤島直流微電網中更頻繁地重複出現，光伏發電機組的系統級可靠度會因動態情況而顯著降低。此外，由於直流 微電網 的動態和瞬態操作，光伏發電系統的平均故障時間和平均故障間隔時間可能會顯著降低。基於光伏電池的直流微電網通常在農村/當地能源社區中以離網/孤島模式操作。對於這種離網操作模式，直流微電網頻繁重複的動態操作場景會降低光伏系統和電池儲能係統中功率轉換器的可靠度如光伏系統的間歇輸出功率，負載功率的隨機波動。

事實上，離網直流微電網光伏發電系統和負載系統的動態操作會導致電池能源儲存系統雙向功率變換器的可靠度有所下降，因為電池儲能電源承受不同的充電/放電水平 提供適當的源負載功率平衡。此外，離網直流微電網的瞬態操作場景會顯著影響光伏系統和 電池能源儲存系統 功率轉換器的可靠性。為了使上述假設更清楚，本論文的第三個貢獻是在當地能源社區動態和瞬態操作考慮下，對基於離網光伏電池的直流微電網中的總功率轉換單元進行了可靠度分析。總功率轉換單元 包含光伏發電系統的升壓轉換器、電池能源儲存系統 的雙向轉換器和直流負載系統的降壓轉換器。主要目的是提供解釋在離網直流微電網中分別從動態和瞬態操作條件計算 總功率轉換單元 的動態電壓相關故障率和故障電流相關故障率。然後，結合有用時間相關故障率、動態電壓變化故障率和故障電流相關故障率 來評估直流微電網中 總功率轉換單元 的系統級和組件級可靠度。馬爾可夫狀態轉移圖應用於 總功率轉換單元 的可靠性評估。實驗結果表明，與 總功率轉換單元 中的升壓或降壓轉換器相比，雙向功率轉換器的可靠度受動態和瞬態操作的影響更大。此外，總功率轉換單元 的 動態電壓變化故障率 幾乎小於其 故障電流相關故障率，但是由於在孤島直流微電網中更頻繁地重複這些情況，動態功率變化情況可能會顯著降低 總功率轉換單元 的系統級可靠度。總功率轉換單元的平均失效前時間和平均失效間隔時間 值可能會因離網直流微電網的動態和瞬態操作而顯著降低。

關鍵字

電池儲能係統 ； 轉換器 ； 動態操作 ； 故障分析 ； 逆變器 ； 微電網 ； 光伏系統 ； 可靠度 ； 瞬態操作

並列摘要

Distributed generators (DGs) and energy storage systems (ESSs) are mostly integrated into a microgrid (MG) to supply power for local loads. One of the most common distributed generators is the solar-energy-generation system, while the common ESS is an aggregate system with multiple battery-energy-storage devices which is used to improve the power-supply reliability from renewable energy sources in the MG and is defined as an aggregate battery energy storage system (ABESS). The ABESS is used to control the source-load power balance so that the microgrid can operate at a high-reliability level to supply electricity for different customers. To demonstrate the significance of the ABESS in the MG, the first contribution of this dissertation is to analyze the reliability performance of the ABESS under different dynamic-operation cases of the microgrid. More clearly, a novel analytic approach based on Markov-chain models is proposed to assess the reliability of the whole ABESS. Along with the used-time-dependent failure rates, the voltage-fluctuation and power-loss dependent failure rates (VF-PL DFR) of critical components of the ABESS such as bidirectional converters, DC/AC power inverters, switching and protective devices, battery modules, and battery charger/controller are also formulated and incorporated in the reliability evaluation process. According to different dynamic-operation cases of a microgrid with the ABESS and photovoltaic (PV) generation systems, the VF-PL DFR of the ABESS can significantly fluctuate. Random dynamic-operation scenarios of the MG are implemented including the on-grid and off-grid operation modes of the microgrid, the change in load power, the intermittent and unstable operation of PV sources, and the discharge and charge states of the ABESS. Simulation testing results are shown and discussed to validate that the performance reliability of the ABESS in the microgrid significantly depends on its different dynamic-operation strategies along with the applied voltage overstress.

On the other hand, a direct-current (DC) microgrid is an emerging technology deployed to effectively utilize DC-power sources such as photovoltaic generation systems, and battery energy storage systems. At the off-grid (or islanded) mode of the DC microgrid, the operation of renewable energy sources (RESs), e.g. the PV-generating system, and the ESSs should be paid more attention to so that the DC microgrid could get the continuity of power supply to various load demands, dispatch the intermittent output power of RESs, and cope with fault types. These could lead to a decrease in the performance reliability of RESs and ESSs. Thus, the second contribution of this dissertation is to perform the reliability analysis of PV-generating systems in the islanded DC microgrid under dynamic and transient operation considerations. The purpose is to shed light on the calculation of the dynamic-voltage-varying failure rate (DVVFR) and the fault-current-varying failure rate (FCVFR) of a PV-generating system in the off-grid DC microgrid. The DVVFR mostly depends on dynamic operation conditions such as the PV power fluctuation and the load power change whereas the FCVFR represents the failure probability due to transient operation conditions of the DC microgrid such as pole-to-pole (P-P) and pole-to-ground (P-G) faults. A comprehensive consideration of the used-time-varying failure rate (TVFR), the power-loss and temperature-dependent failure rate, the DVVFR, and the FCVFR has been then performed to evaluate the system-level and component-level reliability of PV-generating sources in the islanded DC microgrid. Markov-state transition diagrams and Chapman–Kolmogorov equations are derived and applied for the PV-system reliability assessment. Experimental results reveal that the reliability index of the DC-DC power converter in the PV-generating system is the most affected by the dynamic and transient operation of the islanded DC microgrid. In addition, the DVVFR of the PV system is mostly smaller than its FCVFR, however, the system-level reliability of the PV-generating unit can be significantly reduced by the dynamic cases due to more frequent repetition of these cases in the islanded DC microgrid. Moreover, the mean-time-to-failure (MTTF) and mean time between failures (MTBF) of the PV-generating system could be dramatically decreased because of the dynamic and transient operation of the DC MG.

The PV-battery-based DC microgrid normally operates at the off-grid/islanded mode in a rural/local energy community (LEC). For this off-grid operation mode, the reliability of power converters in both the PV system and the battery energy storage system could be reduced by frequently-repeated dynamic operation scenarios of the DC microgrid such as the intermittent output power of the PV system, the random fluctuation in load power. Indeed, the dynamic operation of the PV-generating system and load system in the off-grid DC microgrid could lead to a certain decrease in the reliability of the bidirectional power converter of the BESS because of withstanding different charging/discharging levels of the battery storage source to provide the proper source-load power balance. Moreover, transient operation scenarios of the off-grid DC microgrid can significantly impact the reliability of power converters of both the PV system and the BESS. To make the above assumptions more clearly, the third contribution of this dissertation is to do the reliability analysis of an aggregate power conversion unit (APCU) in the off-grid PV-battery-based DC microgrid under dynamic and transient operation considerations in the local energy community. The APCU contains the boost converters of the PV-generating system, the bidirectional converter of the BESS, and the buck converter of the DC-load system. The main objective is to shed light on calculations of the dynamic-voltage-dependent failure rate (DVDFR) and the fault-current-dependent failure rate (FCDFR) of the APCU from dynamic and transient operation conditions respectively in the off-grid DC microgrid. Then, the combination of the useful-time-dependent failure rate (UTDFR), the DVDFR, and the FCDFR is proposed to evaluate the system-level and component-level reliability of the APCU in the DC microgrid. The Markov-state transition diagram is applied for the APCU’s reliability assessment. Experimental results show that the reliability of the bidirectional power converter is more affected by the dynamic and transient operation than that of the boost or buck converters in the APCU. In addition, the DVDFR of the APCU is almost smaller than its FCDFR, however, the system-level reliability of the APCU could be significantly reduced by dynamic power-change cases due to more frequent repetition of these cases in the islanded DC microgrid. The MTTF and MTBF values of the APCU could be dramatically decreased by the dynamic and transient operation of the off-grid DC microgrid.

並列關鍵字

Battery energy storage systems ； converters ； dynamic operation ； fault analysis ； inverters ； microgrid ； photovoltaic system ； reliability ； transient operation

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