Aircraft DC Power Quality Characteristics of a PCTRU

Abstract

An overview of aircraft DC power quality specifications reveals that only minor changes have occurred in recent years within industry standards. Current and future advanced electronic aircraft are requiring significant power quality improvements due to increased use of digital and COTS (commercial off the shelf) systems. Certain electronic systems do not function properly due to various types of electrical disturbances. Some systems shutdown, fault or exhibit operational delays due to power interruptions or “blackout” conditions. Undervoltage or “brownout” conditions also cause this effect. Some electronic systems exhibit critical faults that can affect safety or mission success due to overvoltage conditions. Additional effects of high voltage spikes or overvoltage transients are known to reduce the life of utilization equipment [1], which is directly related to the health of the aircraft’s electronic system and creates an economic burden. These disturbances are described as high voltage transients, low voltage transients or brownouts, power interruptions and voltage modulation. Another type of perceived perturbation on the DC bus is caused by ripple voltage. All of these are fundamental DC power quality characteristics that can induce problems in electronic systems. These disturbances are caused by various portions of the entire electrical power system consisting of AC generators, transmission lines, power converters, contactors, electrical power distribution controls and AC/DC loads from utilization equipment. Presented here is a power supply system described as a Passively Controlled Transformer Rectifier Unit (PCTRU) that preserves the desired reliability of a traditional transformer rectifier unit (TRU) while providing stability to the aircraft 28 VDC bus and mitigating risks caused by these various electrical disturbances. INTRODUCTION One major AC to DC power conversion topology utilized for aerospace applications is a simple Transformer Rectifier Unit (i.e. TR or TRU) [2]. These designs represent the most reliable power supply topology, which continually meet the first goal of an aerospace DC electrical power system objective-reliability. Significant power quality improvements can be achieved with the basic TRU as described in detail here. However, complex solutions with feedback control (i.e. regulated) topologies that forcefully control power also provide power quality improvements, but these benefits are diminished to an extent by additional failure modes, lower reliability, increased electromagnetic interference for emissions and susceptibility and increased cost. If the DC power supply fails resulting in “loss of power”, power quality is the least concern. Moreover, if the additional fault is “high steady state output voltage”, the more complex solution may cause additional harm to the aircraft electrical system and utilization equipment. These conditions led to the development of a passively controlled power supply that mitigates these risks while preserving reliability.