24-07-2012, 02:48 PM
FLYING WINDMILLS OR GENRATOR TECHNOLOGY
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ABSTRACT
Arc Fault Circuit Interrupting (AFCI) technology has been proposed as a means to improve aircraft wiring system safety. Arc Fault Circuit Breakers that provide supplemental protection against arc fault conditions in addition to the thermal overload protection provided by present generation circuit breakers are being developed, and prototypes have undergone flight-testing.
Although AFCI technology has been in-service in many household applications in the United States for a number of years, the challenges associated with adapting this technology to operate in aircraft electrical systems are significant. Complicating factors include; higher and variable AC line frequencies, the need for DC protection, and lack of ground return wires required for GFI type protection used in most household AFCI technologies.
Whereas households in the US operate at 120 VAC, 60 Hz, aircraft electrical systems operate at 115 VAC, 400 Hz. Also, aircraft electrical systems incorporate both single and three-phase circuits. In addition to AC power, aircraft electrical systems also utilize 28 VDC power. Furthermore, aircraft OEMs have begun to incorporate 270 VDC power and variable frequency power into aircraft electrical systems, with frequency ranges from 200 to 800 Hz. AFCI detection algorithms that rely on the frequency characteristics of a waveform may be affected by aerospace variable frequency systems, complicating the design of arc fault technology.
In total, it is readily apparent that the aircraft electrical system presents a much more complex engineering challenge for arc fault technology than the household application. Fortunately, a significant amount of development work has been accomplished that leads us to believe these challenges can be overcome.
This paper will address the methods for determining and distinguishing between arcing characteristics and normal steady state and transient load conditions on aircraft, and the requirements of arc fault detection algorithms to address these conditions. The paper will discuss methods being implemented to provide robust circuit protection against arc fault conditions on aircraft and the desired product performance attributes of arc fault circuit breakers.
1.INTRODUCTION
Definition: An arc-fault circuit interrupter (AFCI) is a device intended to provide protection from the effects of arc-faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc- fault is detected.
Arc Fault Circuit Interrupting (AFCI) technology has been proposed as a means to improve aircraft wiring system safety. Arc Fault Circuit Breakers that provide supplemental protection against arc fault conditions in addition to the thermal overload protection provided by present generation circuit breakers are being developed, and prototypes have undergone flight-testing.
Although AFCI technology has been in-service in many household applications in the United States for a number of years, the challenges associated with adapting this technology to operate in aircraft electrical systems are significant. Complicating factors include; higher and variable AC line frequencies, the need for DC protection, and lack of ground return wires required for GFI type protection used in most household AFCI technologies.
Aircraft circuit breakers have historically been the best available protection for aerospace wiring. Today’s design standards are based on technologies that are 40 years old. In aircraft breakers the protection is provided two ways. Overload currents operate either a bimetal trip latch or hydraulic damped magnetic plunger. The “instantaneous trip “ feature is a high current magnetic trip action found on some aircraft breakers. The time to trip during an overload is determined by the time it takes to heat a bimetal to the temperature that delatches the breaker.
The more current that heats the bimetal, the shorter the time it takes to trip the breaker. A hydraulic-magnetic style of breaker contains a magnetic slug sealed in fluid which moves to a trip position in response to the square of the current. These circuit interruption devices are selected by aircraft design engineers to protect the aircraft wiring from overheating or melting. During arcing faults these currents are often small, short in duration and well below the overcurrent time protection curve designed into these breakers. Recent events have brought these limitations in design and function to the forefront. “Electrical arcing failure “as the ignition source, has been suspected in several recent airline disasters.
It is readily apparent that the aircraft electrical system presents a much more complex engineering challenge for arc fault technology than the household application. Fortunately, a significant amount of development work has been accomplished that leads us to believe these challenges can be overcome.
This paper will address the methods for determining and distinguishing between arcing characteristics and normal steady state and transient load conditions on aircraft, and the requirements of arc fault detection algorithms to address these conditions. The report will discuss methods being implemented to provide robust circuit protection against arc fault conditions on aircraft and the desired product performance attributes of arc fault circuit breakers.
1.1 Arc Fault Circuit Interruption Requirements for Aircraft Applications
The issue of aircraft wiring safety has received widespread attention in recent years, highlighted by the unfortunate TWA 800 and Swissair 111 tragedies. As a result of these incidents and other concerns, the issue of wiring safety has been taken up by OEMs, regulatory agencies, the military, and is being addressed as part of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC).
According to industry sources, there is at least one “smoke-in-the-cockpit” incident per week in the United States on commercial aircraft. These can result in unscheduled landings and compromise aircraft safety. The military also has extensive documentation of arcing issues affecting its fleet of planes. Many of these smoke incidents as well as numerous unseen conditions in cargo holds and electronics bays are the result of wiring faults.
As a result of these incidents, a number of recommendations have been made to improve aircraft wiring safety. Included among these is the development of arc fault circuit breakers for enhanced wiring system protection.
2.WIRING SAFETY
Aircraft wiring safety is not a new issue. Thermal circuit breakers were originally developed to protect the wire insulation on aircraft from damage due to overheating conditions caused by excessive over-current conditions. These devices have performed admirably for over 50 years. However, there are other conditions and factors (outlined below) that can damage aircraft wiring. These conditions can manifest themselves in arcing events that cannot be protected solely by thermal devices since the arcing currents do not reach the thresholds for the thermal circuit breaker curves [see graph below].
Aromatic polyimide wire insulation material was identified in the 1980s as being susceptible to arc tracking under certain conditions, and as a result was removed from use on most new-production aircraft. However, there are still many aircraft in service utilizing this type of wire insulation in some applications. Other application conditions relating to aging of wire insulation, environmental factors, chemical exposure, location of wiring bundles and maintenance procedures also contribute to create potential safety conditions that can affect any type of aircraft wiring, whether on commercial or military aircraft. The photograph on the following page vividly shows the extent of damage that can be caused by arcing events. These conditions require a supplemental protection means to improve the overall level of aircraft wiring safety.
A relatively new technology – arc fault circuit interruption (AFCI) technology has been developed to provide this supplemental protection. AFCI technology monitors the electrical circuit for arcing events that are indicative of potentially unsafe wiring conditions that could result in fires or loss of electrical circuit functionality. When coupled with an interruption mechanism, AFCI technology can be utilized to enhance aircraft wiring safety.
Ideally, all circuits on an aircraft ought to be protected with AFCI technology to ensure complete system integrity. Wire bundles on aircraft typically contain wires from many different circuits bundled together – sometime as few as three or four to well over 50 wires in a bundle. Wires for separate circuits are closely coupled and wire bundles are often routed adjacent to each other, resulting in a condition where a catastrophic fault of one wire could damage or impair multiple circuits in common or adjacent wire bundles. Thus, one unprotected circuit could compromise an entire wire bundle(s) of protected circuits.
2.1 Challenges Associated with Arc Fault Circuit Breakers
Arc Fault Circuit Interrupter (AFCI) technology was originally developed for household applications in the 1990s, and has been proposed to improve aircraft wiring safety via retrofit on in-service aircraft and installation in new-production aircraft. Arc fault circuit breakers would replace traditional thermal circuit breakers, providing a dual-function device that augments the traditional over-current protection with electronic arc fault protection packaged in one circuit breaker device. This arc fault circuit breaker would only be slightly larger than today’s devices, able to be retrofit into present aircraft circuit breaker panels and installed on new production aircraft. AFCI technology can also be incorporated into solid state power controllers (SSPC’s) or remote power controllers (RPC’s) for applications on new aircraft that utilize these devices.
Although AFCI technology has been in-service in many household applications in the United States for a few years, the challenges associated with adapting this technology to operate in aircraft electrical systems are significant. Complicating factors include higher and variable AC line frequencies, the need for DC protection, and lack of ground return wires required for GFI type protection used in most household AFCI technologies. Aircraft electrical systems also present a harsh EMI environment not found in the household application.
Whereas households in the US operate at 120 VAC, 60 Hz, aircraft electrical systems operate at 115 VAC, 400 Hz. Aircraft electrical systems also incorporate both single and three-phase circuits. In addition to AC power, aircraft electrical systems also utilize 28 VDC power. Furthermore, aircraft OEMs have begun to incorporate 270 VDC power and variable frequency power into aircraft electrical systems, with frequency ranges from 200 to 800 Hz. AFCI detection algorithms that rely on the frequency characteristics of a waveform may be affected by aerospace variable frequency systems, complicating the design of arc fault technology.
There are also marked differences in the power quality on aircraft platforms [Boeing 737-Classic compared with Boeing 777] and between manufacturers [Boeing compared with McDonnell Douglas compared with Airbus], and power quality has improved as electrical system design has improved and matured on newer generation aircraft. Differences in electrical system design between aircraft platforms can also greatly affect device performance.
In total, it is readily apparent that the aircraft electrical system presents a much more complex engineering challenge for arc fault technology than the household application. Fortunately, a significant amount of development work has been accomplished that has demonstrated that these challenges can be overcome.