Reliability prediction in power electronic converters is of paramount importance for converter manufacturers and operators. Conventional approaches employ generic data provided in handbooks for random chance failure probability prediction within useful lifetime. However, the wearout failures affect the long-term performance of the converters. Therefore, this paper proposes a comprehensive approach for estimating the converter reliability within useful lifetime and wear-out period. Moreover, this paper proposes a wear-out failure prediction approach based on a structural reliability concept. The proposed approach can quickly predict the converter wear-out behavior unlike conventional Monte Carlo based techniques. Hence, it facilitates reliability modeling and evaluation in large-scale power electronic based power systems with huge number of components. The proposed comprehensive failure function over the useful lifetime and wear-out phase can be used for optimal design and manufacturing by identifying the failure prone components and end-of-life prediction. Moreover, the proposed reliability model can be used for optimal decisionmaking in design, planning, operation and maintenance of modern power electronic based power systems. The proposed methodology is exemplified for a photovoltaic inverter by predicting its failure characteristics.

In an ac microgrid, a common frequency exists for coordinating active power sharing among droop-controlled sources. A common frequency is absent in a dc microgrid, leaving only the dc source voltages for coordinating active power sharing. That causes sharing error and poorer voltage regulation in dc microgrids, which in most cases, are solved by a secondary control layer reinforced by an extensive communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes an alternative droop scheme for low-voltage dc (LVDC) microgrid with both primary power sharing and secondary voltage regulation merged. The main idea is to introduce a non-zero unifying frequency and a second power term to each dc source by modulating its converter with both a dc and a small ac signal. Two droop expressions can then be written for the proposed scheme, instead of the single expression found in the conventional droop scheme. The first expression is for regulating the ac frequency and active power generated, while the second is for relating the dc voltage to the second power term. The outcomes are better active power sharing and average voltage regulation in the dc microgrid, coordinated by the common injected ac frequency. These expectations have been validated by results obtained from simulations.

Power converters are one of the failure sources in modern power systems, and hence driver of maintenance and downtime costs, which should be reduced by reliable design, control and operation of converters. This paper proposes a power sharing control strategy for evenly distributing the thermal stresses among dc converters in dc microgrids, and consequently enhancing the overall system reliability. The aim of this paper is to extend the aging process of failure prone converters by adjusting their loadings. The proposed approach employs the prior experienced thermal damages on the converter's fragile components in order to adjust its contribution on demand supply. According to the proposed strategy, the higher the thermal stress on a converter is, the lower the power it will supply. As a result, the overall system reliability will be improved. A numerical case study on a dc microgrid is presented to illustrate the effectiveness of the proposed power sharing strategy. Moreover, experimental tests are provided to demonstrate the applicability of the reliability-oriented power sharing method.

This paper aims to incorporate the reliability model of power electronic converters into power system reliability analysis. The converter reliability has widely been explored in device-and converter-levels according to physics of failure analysis. However, optimal decision-makings for design, planning, operation and maintenance of power electronic converters require system-level reliability modeling of power electronic-based power systems. Therefore, this paper proposes a procedure to evaluate the reliability of power electronic based power systems from the device-level up to the system-level. Furthermore, the impact of converter failure rates including random chance and wear-out failures on power system performance in different applications such as wind turbine and electronic transmission lines is illustrated. Moreover, due to a high calculation burden raised by the physics of failure analysis for large scale power electronic systems, this paper explores the required accuracy for reliability modeling of converters in different applications. Numerical case studies are provided employing modified versions of the Roy Billinton Test System (RBTS). The analysis shows the converter failures may affect the overall system performance depending on its application. Therefore, an accurate converter reliability model is, in some cases, required for reliability assessment and management in modern power systems.

In a classical ac Micro-Grid (MG), a common frequency exists for coordinating active power sharing among droop-controlled sources. Like the frequency droop method, a voltage based droop approach has been employed to control the converters in dc MGs. However, voltage variation due to the droop gains and line resistances causes poor power sharing and voltage regulation in dc MG, which in most cases are solved by a secondary controller using a communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes a new droop scheme to control dc sources by introducing a small ac voltage superimposed onto the output dc voltage of converters. Therefore, dc sources can be coordinated together with the frequency of the ac voltage, without any communication network like Synchronous Generators (SGs) in conventional power systems. Small signal stability analysis as well as mathematical calculations are presented to demonstrate the analogy between the proposed strategy and frequency-based droop approach of the SGs. The effectiveness of the proposed control system is evaluated by simulations and verified by experiments.

Renewable energy resources are becoming the dominating element in power systems. Along with de-carbonization, they transform power systems into a more distributed, autonomous, bottom-up style one. We speak of Smart Grid and Microgrid when distributed energy resources take over. While being a means to improve technical and financial efficiency, planning, operations, and carbon footprint, it is these new technologies that also introduce new challenges. Reliability is one of them, deserving a new way of describing and assessing system and component reliability. This paper introduces a new reliability framework that covers these new elements in modern power systems. It can be seen that reliability assessment of modern power systems also requires introducing local reliability concepts as well as incorporating different electromagnetic/mechanical stability issues.

Mission profiles such as environmental and operational conditions together with the system structure including energy resources, grid and converter topologies induce stress on different converters and thereby play a significant role on power electronic systems reliability. Temperature swing and maximum temperature are two of the critical stressors on the most failure prone components of converters, i.e., capacitors and power semiconductors. Temperature related stressors generate electro-thermal stress on these components ultimately triggering high potential failure mechanisms. Failure of any component may cause converter outage and system shutdown. This paper explores the reliability performance of different converters operating in a power system and indicates the failure prone converters from wear out perspective. It provides a system-level reliability insight for design, control and operation of multi-converter system by extending the mission profile-based reliability estimation approach. The analysis is provided for a dc microgrid due to the increasing interest that dc systems have been gaining in recent years; however, it can be applied for reliability studies in any multi-converter system. The outcomes can be worthwhile for maintenance and risk management as well as security assessment in modern power systems.

Conventional droop methods for load sharing control in Low Voltage Direct Current (LVDC) microgrids suffer from poor power sharing and voltage regulation, especially in the case when operating many dc sources with long feeders. Hence, the communication based approaches are employed to improve the load sharing accuracy and voltage regulation. To avoid using such an infrastructure and the corresponding effects on the reliability and stability, an adaptive droop controller based on a superimposed frequency is proposed in this paper. Load sharing accuracy is improved by adapting the droop gains utilizing an introduced ac-power. The secondary controller locally estimates and compensates the voltage drop due to the droop controller. The proposed power sharing approach can properly control the load sharing and voltage regulation without utilizing any extra communication system. The effectiveness of the proposed control system is verified by simulations and experimental tests.