Analysis and design of fault-tolerant scheme for dc-dc and DC-ACPower conversion stages in solid-state transformer

Abstract

Power electronic converters are extensively used in grids and microgrids. They can be com bined with renewable energy sources for a variety of applications. Inverters, DC-DC converters, and rectifiers are frequently used in solid-state transformers (SST) and two-stage single-phase inverters or rectifiers. SSTs have progressed swiftly since they possess characteristics such as medium-frequency (MF) isolation stage, connection to medium voltage (MV), controllability, size and weight constraints, and improved power conversion efficiency. The SST and two-stage single phase inverters are difficult to build and operate reliably since they include semiconductor com ponents, gate drivers, cooling system, heat sink, control circuit, and other auxiliary circuits. As a result, the system’s overall reliability is ensured by improving the reliability of each individual converter. Dual active bridge converter (DAB) with galvanic isolation is a type of DC-DC converter that is commonly used in a variety of applications such as SST, electric car battery charger, UPS, fast charging station, renewable energy system, traction application, etc. The continuous operation of the converter is vital in telecommunications, renewable energy systems, and machine-critical applications. The fault tolerance feature of the converter allows the network to be used even after unexpected failures. Semiconductor device failures in power converters are classified as either open-circuit (OC) or short-circuit (SC). SC faults are catastrophic and must be isolated as soon as possible. While the long-term current/voltage stress produced by OC failures can destroy switches and other related components. As a result, the objective of continuous converter operation and the rated output voltage demands the implementation of a fault-tolerant (FT) technique. An FT method to support the continuous operation of a DAB converter is proposed which includes a fault tolerant capacitor (Cf) with appropriate control of duty and phase shift. Rated voltage is achieved in the post-fault correction for primary or secondary-side faults. Also, the transition from faulty condition to post-fault correction is smooth and within the defined limits. A fault-tolerant converter is one that can continue to work even after a sudden failure. The self-reliant feature, on the other hand, refers to utilising existing hardware (no new power circuit) to achieve fault-tolerance for the respective fault. The self-reliance of a converter is determined by the type of converter, the location of the faulty switch, and the type of fault (OC/SC fault). A single primary switch failure results in half the pre-fault voltage and less power at the output. A secondary side SC failure results in minimal voltage and power, resulting in discontinuous con verter operation. The SRC behaviour in the post-fault scenario differs when more than one switch fault occurs on any side. This thesis proposes the self-reliant characteristic for single and two switches open/short-circuit fault on the primary and secondary side of SRC. A post-fault correc tion that maintains operating continuity and the rated output voltage at the load is presented for various fault conditions. A fault-tolerant capacitor is used in conjunction with control parame ter variation to carry out post-fault correction for a single switch and different combinations of two switch faults. For more than one switch fault, the fault-tolerant and self-reliant feature of the converter is dependent on the combination of faulty switches. The development in the field of smart transformer essentially requires a robust and reliable inverter. A fault-tolerant inverter keeps the converter running even after sudden failures, ensuring the converter’s reliability against various faults. A semiconductor switch failure in an inverter can cause the majority of circuit faults, followed by electrolytic capacitor failures. Semiconductor failures can cause switch/leg short-circuits, open-circuits, and single/multiple switch faults. These issues necessitate the use of additional components to isolate and repair the damaged switch. Redundancy in form of switches, legs or modules in inverter functioning is frequently used for a variety of unique goals such as ripple compensation, fault tolerance, etc. Ultimately, the number of redundant components grows as a result of multiple goals being achieved within a unit. This thesis presents a method to maximize the use of the redundant leg. A new transition technique is designed to enhance the benefits of the redundant leg by properly transitioning the converter operation from pre-fault to post-fault correction (fault-tolerant mode). The redundant leg in the pre-fault situation serves to extend the lifespan of the DC-link capacitor by ensuring 0-100% compensation of the second harmonic ripple (SHR) in the DC-link current. The same leg is used to achieve fault tolerance in post-fault repair when adopting the suggested post-fault correction approach. This thesis investigates several fault-tolerant techniques for DC-DC converters and inverters which can be used in solid-state transformers and DAB inverters. All fault-tolerant techniques are empirically tested and validated on the designed hardware prototypes.