After evaluation of reliability in the previous chapters and its consideration as a converter figure of merit, in this and the next chapters, guidelines for improvement of reliability are presented. These methods are used in both design and operation process of the converter. The focus of this chapter is on the component stress reduction in the design process. Based on background of chapter two, reliability of a converter increases if it operates at a set point with low stress. It is assumed that the converter is under design process or operates without fault. The methods for reliability improvement in faulty converters are discussed in the next chapters. In this chapter, methods for reducing electric field are described at both system and printed circuit board level. Low temperature operating conditions for an electric power converter are described and tools for this goal are presented. Series connection for voltage sharing and parallel connection for current sharing is explained. Novel control methods of power converters for reducing the complexity and reliable operation are presented. Control of inrush current as a typical transient problem in electric power converters is presented. Methods for preventing the over stress condition on the components in faulty cases are described. Techniques for reducing mechanical and environmental stress are expressed. Mechanical dampers for preventing the high amplitude vibration and insulating colors against humidity are presented. Industrial and real samples are presented to demonstrate application of the proposed methods.
TopIntroduction: Stress On The Components
This chapter is starting chapter of the second part of this book. By now, we studied reliability calculation and methods of reliability testing. In the next five chapters of this book, we present the methods for reliability improvement.
Following the materials presented in Chapter 2 about mechanisms of fault in power converters, any method that reduces those failure factors can be considered as a technique for reliability improving.
The failure mechanism of a converter is started immediately after starting. In the beginning, the converter operates normally but under stress of failure factors:
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Power losses in the converter causes to temperature rise in various parts of the converter (Kaboli, Vahdati-Khajeh, Zolghadri, & Homaifar, 2005),
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Applied voltage causes to apply an electric field to insulators,
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Mechanical forces leads to vibrations
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Environmental factors
These factors acts from the beginning of converter application. In long term, they cause to age the converter and its failure. The time interval for changing a stress factor to a failure factor is directly related to value of stress. Higher stress leads to shorter time to failure and vice versa. Thus, the first method of reliability improvement, is reducing the stress factors on the converter. This is the subject of the current chapter. Figure 1 shows the state of this chapter in the flowchart of the book.
Figure 1. State of chapter 7 in the flowchart of the book
A cascading failure is a failure in a system of interconnected parts in which the failure of a part can trigger the failure of successive parts. Such a failure may happen in many types of systems, including power transmission, computer networking, finance and bridges.
Some of the methods for reliability improvement act in hardware level. Aim of these methods is usually reduction of hot spot temperature or reduction of electric field applied to devices.
Figures 2 to 4 show some methods for reduction of electrical field stress on equipments. Figure 2 shows application of insulator spacer between primary and secondary of a transformer. Figure 3 shows application of isolator base for inductors in high voltage application. Figure 4 shows a three phase transformer with spacing between high voltage windings.
Figure 2. Spacing between primary and secondary windings of a transformer
Figure 3. Isolator base for high voltage inductors
Figure 4. Spacing between different phases of a 3-phase transformer