Electromagnetic circuit interrupter

In one aspect, the present invention provides an electromagnetic circuit interrupter for use in a high voltage direct current (DC) aircraft power distribution system. The electromagnetic circuit interrupter comprises a contact mechanism operable to separate first and second electrical contacts by a first predetermined distance d1 for a predetermined time τ so as to sustain an arc when the contact mechanism is opened. The contact mechanism is further operable to separate the first and second electrical contacts by a second predetermined distance d2 after the predetermined time τ so as to extinguish the arc. The first predetermined distance d1 is less than said second predetermined distance d2. By deliberately sustaining the arc for a relatively long period of time, this aspect of the present invention is particularly useful for extending the operational lifetime of the contacts and thereby of the electromagnetic circuit interrupter itself.

FIELD OF INVENTION

The present invention relates generally to an electromagnetic circuit interrupter for a high voltage direct current (DC) aircraft power distribution system.

BACKGROUND OF THE INVENTION

Recent developments in aircraft power distributions have involved a move towards the use of high voltage DC power distribution systems so as to permit a weight reduction for wiring harnesses used to distribute electrical power within an aircraft.

However such high voltage DC systems give rise to additional problems when designing aircraft power distribution systems. The high DC voltages used can, for example, lead to a decreased component lifetime, particularly for electromagnetic switches used to interrupt circuitry from drawing power from the wiring harness. Such switches are preferred to solid state devices because of their higher power ratings and ability to resist the increased switching voltages. However, even these high power devices are not immune to the effects of contact sputtering caused by arcing of the switch contacts provided therein when such contacts are separated in order to break a circuit.

Various devices and techniques have therefore been developed in an attempt to enhance the lifetime of such switchable contacts by mitigating the effects caused by the inductive energy that is stored in the circuit and which causes arcing once the contacts are separated.

For example, various known techniques may employ conventional electromagnetic switches along with additional circuitry that is used to dissipate the inductive energy of the circuit so as to minimise the energy dissipated in the electromagnetic switches themselves [1-3]. Alternatively, various non-conventional electromagnetic switches have been produced which, for example, may seek to confine the physical position of arcs in an attempt to minimise contact erosion [4].

However, whilst such techniques can enhance the useful operational lifetime of electromagnetic switches, there is still a need in the art for high voltage DC electromagnetic circuit interrupters having a further enhanced operational lifetime, particularly when used for safety critical applications such as aircraft power distribution systems.

SUMMARY OF THE INVENTION

The present invention has thus been devised whilst bearing the above-mentioned drawbacks associated with conventional high voltage DC electromagnetic switching devices in mind.

According to one aspect of the present invention, there is thus provided an electromagnetic circuit interrupter for a high voltage DC aircraft power distribution system. The electromagnetic circuit interrupter comprises a contact mechanism operable to separate first and second electrical contacts by a first predetermined distance for a predetermined time so as to sustain an arc when the contact mechanism is opened. The contact mechanism is further operable to separate the first and second electrical contacts by a second predetermined, distance after the predetermined time so as to extinguish the arc. Additionally, the first predetermined distance is less than said second predetermined distance.

Such an electromagnetic circuit interrupter contrasts with conventional devices as it does not seek to open the contacts widely as soon as possible, but rather enables the contacts to be separated for a relatively long time (e.g. several milliseconds compared to prior art devices opening in microseconds) in order that an arc is produced and sustained for a relatively long period. This has the advantage that much of the inductive energy stored in a circuit can be dissipated during the predetermined time period before the contacts of the electromagnetic circuit interrupter become hot enough to melt. Subsequently, the contacts can be further or fully opened to break the circuit, the arc having been extinguished, thereby minimising or substantially eliminating any contact sputtering.

Hence, although the total switching time of the electromagnetic circuit interrupter is increased compared to conventional devices, the operational lifetime and reliability of the contacts can be greatly enhanced.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1Ashows an electromagnetic circuit interrupter100for a high voltage direct current (DC) aircraft power distribution system in accordance with various embodiments of the present invention in a closed contact position.

The electromagnetic circuit interrupter100comprises a first electrical contact120and a second electrical contact130hermetically sealed in a housing110. The first and second electrical contacts120,130are movable within the housing110between a closed position, an intermediate open contact position and a fully open contact position by activation of a contact mechanism102. These three positions are shown respectively inFIGS. 1A-1C. The housing110may contain a fill gas. In various embodiments, the fill gas may comprise one or more of: dry air, nitrogen, argon, neon, krypton etc. In various preferred embodiments, nitrogen or another inert gas or gas mixture may be used.

The first electrical contact120is formed with an electrically conductive projecting portion122which may be made of the same material as the main body of the first electrical contact120. Alternatively, the projecting portion122may be formed of dissimilar material, e.g. metal, from that of the main body of the first electrical contact120. Similarly, the second electrical contact130is formed with an electrically conductive projecting portion132which may be made of the same material as the main body of the second electrical contact130. Alternatively, the projecting portion132may be formed of dissimilar material, e.g. metal, from that of the main body of the second electrical contact130. The surfaces of the projecting portions122,132may be shaped or substantially flat.

In the closed contact position shown inFIG. 1A, the projecting portions122,132abut one another, or fit together depending upon their respective shapes, in order provide a low resistance electrical connection between the first and second electrical contacts120,130.

FIG. 1Bshows the electromagnetic circuit interrupter100in an intermediate open contact position. In the intermediate open contact position the contact mechanism102separates the surfaces of the projecting portions122,132by a first predetermined distance d1for a predetermined time τ. Various methods for determining the first predetermined distance d1and the predetermined time τ for embodiments of the invention are discussed further below.

When the first and second electrical contacts120,130are supplied with a high voltage DC potential difference therebetween, an arc150is sustained between the projecting portions122,132for a period substantially equal to the whole of the duration of the predetermined time τ. The arc150acts like a resistor in the circuit and dissipates stored inductive energy as heat energy causing the temperature of the proximal electrical contacts120,130to rise.

With fast (e.g. of the order of μS) full gap opening of contacts in conventional devices, the arc can heat the contacts up (through resistive I2R heating). This temperature rise may be enough to cause sputtering and intermittent restriking of the arc until enough inductive energy has been dissipated for this process to cease.

However by selecting the predetermined time τ and the first predetermined distance d1to ensure that the temperature rise of the electrical contacts120,130is limited to below the melting temperature of the materials from which they are formed, sputtering can be minimised and operational lifetime of the electromagnetic circuit interrupter100increased.

The various parameters chosen depend upon the exact current, voltage and power rating of the electromagnetic switch, the fill gas used, and the contact materials, hence the first predetermined distance d1, the second predetermined distance d2and the predetermined time τ vary according to the specific embodiment that is used.

One technique that can be applied to determine whether or not high voltage arcing will occur and/or various of the distance parameters involves finding the Paschen voltage for a particular electromagnetic circuit interrupter100embodiment.

For parallel conducting plates, Paschen found that the breakdown voltage Vb, (volts) could be described by the equation:

Vb=k1⁡(P,d)ln⁡(P,d)+k2Equation⁢⁢1
where P is the pressure of the gas between the two plates, d the separation distance between the two plates and k1and k2are constants dependant upon the specific gas or gas mixture used.

Differentiating Equation 1 and setting the derivative to zero, gives:
P·d=e(1−k2)Equation 2
which in turn enables the Paschen voltage VP=Vbminto found from Equation 1.

For example, for a high voltage application and so as to ensure arcing does actually occur, the operating high DC voltage of the electromagnetic circuit interrupter100must be greater than the Paschen voltage VPfor any particular gas and at any given temperature. For contacts in air at standard atmospheric pressure, for example, the following parameters may be selected: 1.5 mm<d1<2.5 mm with d2, for example, set such that d2≈3 mm.

FIG. 1Cshows the electromagnetic circuit interrupter100in a fully open contact position. In the fully open contact position the contact mechanism102separates the surfaces of the projecting portions122,132by a second predetermined distance d2(where d2>d1) until such a time as the electromagnetic circuit interrupter100is switched back to the closed contact position. When switching back from the fully open contact position to the closed contact position, the contact mechanism102rapidly and directly moves the first and second electrical contacts120,130together without any intermediate contact separation stages.

As the first and second electrical contacts120,130are fully opened from the intermediate open contact position, any arc150is rapidly extinguished. Additionally, since much of the stored inductive energy will already have been dissipated at this time, the arc150is highly unlikely to restrike and cause damage to the first and second electrical contacts120,130or the projecting portions122,132.

In various embodiments, the contact mechanism102may include one or more solenoid actuators and/or mechanical arrangements for moving the first and second electrical contacts120,130between the closed position, the intermediate open contact position and the fully open contact position. Various such embodiments would be readily envisaged by those skilled in the art of mechanical actuator design.

FIG. 2shows temporal I-V curves for a low voltage DC circuit interruption. The temporal I-V curves include a graphical depiction of a current (I) profile210and a graphical depiction of a voltage (V) profile220for a low voltage DC circuit interruption.

At time t=5 mS, the circuit is interrupted and the current profile210shows a steady decrease in the circuit current from about 200 Amps to about 40 Amps over a period of about 5 mS as the stored inductive energy dissipates as heat. A rapid current decrease to zero Amps is observed after about t=10 mS with the current dropping rapidly from about 40 Amps to zero during an interval of about 1 mS.

The voltage profile220shows how the potential between the contact electrodes varies over time. At t0, in this case equal to t=5 mS, circuit interruption begins and a potential of about 15 volts rapidly develops across the contact electrodes. At t0, the force holding the metallic electrodes together is reduced. This in turn increases the contact resistance resulting in increased heat. As the contact force is further reduced, the area over which current flows is reduced also increasing the contact temperature further. At the extreme limit, all of the circuit current passes through an infinitesimal surface area resulting in this area of the electrode melting and a controlled explosion occurs.

Metal vapour or particles thus sputter from the contact electrodes, and between t0and t1(about 1 mS later) conduction through metalised air occurs. At t1the electrode gap becomes vacuous in nature and a vacuum arc develops. The voltage profile of the vacuum arc follows the exponential curve shown increasing initially from about 15-20 volts at t1to about 48 volts at a time when the current profile210reaches zero Amps. During this time period, i.e. from about t=6 mS to about t=11 mS, the inductive energy

E=(12⁢LI2)
stored in the circuit is converted to heat within the are and some is also dissipated by the load connected to the circuit interrupter.

FIG. 3shows an I-V characteristic graph300for the low voltage arc produced inFIG. 2. The fill gas is nitrogen.FIG. 3shows that as the current in a circuit that is being interrupted reduces, the arc voltage rises (negative impedance). Once the current is reduced to zero the arc voltage also reduces to zero volts.

The arc voltage is also related to the gap over which the arc must traverse. If higher voltages are available and the circuit has enough energy stored, the arc may be drawn and higher arc voltages are observed.

FIG. 4shows various high voltage arc voltage waveforms402to420provided by operating various embodiments of the present invention. Voltage waveform402is substantially equivalent to the low voltage arc profile as perFIG. 3, described above.

The y-axis (Varc) is calibrated in volts. However, Varcis also indicative of the temperature of the arc (T2) relative to ambient temperature (T1), such that

Var⁢⁢c⁢α⁢⁢T2T1.
The x-axis (F(I)) is a function of the current flowing in the arc.

A predetermined time τ may thus be determined such that Tarc<Tmeltmin, where Tarcis the temperature generated by the arc and Tmeltminthe lowest melting temperature of the materials from which the first and second electrical contacts are made. For example, τ may be determined such that

T⁢ar⁢⁢c⁢<<Tmeltmin,e.g.⁢Tar⁢⁢c=Tmeltminα,
where α=2, 5, 10, 20, etc. to minimise contact sputtering and may be from about 1 mS to about 10 mS, for example.

An array of arc voltage waveforms possible in a circuit with higher voltages available is shown inFIG. 4. The second voltage waveform404has a profile equivalent to twice that of the low voltage arc profile of voltage waveform402. The third voltage waveform406has a profile equivalent to three times that of the low voltage arc profile of voltage waveform402. The fourth voltage waveform408has a profile equivalent to four times that of the low voltage arc profile of voltage waveform402. The fifth voltage waveform410has a profile equivalent to five times that of the low voltage arc profile of voltage waveform402. The sixth voltage waveform412has a profile equivalent to six times that of the low voltage arc profile of voltage waveform402. The seventh voltage waveform414has a profile equivalent to seven times that of the low voltage arc profile of voltage waveform402. The eighth voltage waveform416has a profile equivalent to eight times that of the low voltage arc profile of voltage waveform402. The ninth voltage waveform418has a profile equivalent to nine times that of the low voltage arc profile of voltage waveform402. The tenth voltage waveform420has a profile equivalent to ten times that of the low voltage arc profile of voltage waveform402.

Each of the voltage waveform curves402-420is related to a given arc gap. The voltage is directly proportional to the gap size. Therefore for a higher voltage arc to be realised a greater gap size must be provided. For example, the first predetermined distance d1may be defined as: d1=mλ, where m is a predetermined factor representing a number of mean free paths that make up the distance d between the first and second electrical contacts and λ is a DC low voltage arc gap substantially equal to one electron mean free path between first and second electrical contacts. The second predetermined distance d2may then be equal to a conventional gap distance for an equivalently rated conventional electromagnetic circuit breaker.

The mean free path λ may be defined such that:

λ=kTp⁢⁢σEquation⁢⁢3
k being Boltzmann's constant, T being the arc temperature (e.g. 15,000 Kelvin), p the pressure of the gas between the contacts, and σ gas specific cross sectional area.

In one embodiment, to interrupt a 270 volt circuit the following three stage process may be used in order to allow the circuit's inductive energy to be dissipated and prevent unwanted arc draw:1. Open the contacts to a distance about six to seven times the gap required for the low voltage arc402(e.g. m may lie in the range from about 6 to about 7). This provides an operating range for F(I) from about 8 to about 20 when Varc=270 volts, as can be seen inFIG. 4, and ensures an arc is sustained whilst also constraining the temperature rise of the contacts (proportional to Varc) to below the peak values seen for the curves412and414;2. Hold the contacts for a period of time τ for a given energy interruption capability, or until the current reaches zero Amps; and3. Open the contacts further to provide a dielectric withstand capability.

For example, using Equation 3 with p=101321 Pa; T=6000 K, and σ=πri2where riis the ionic radius for Nitrogen=30 nm, λ can be found. Multiples of λ can then be used to define the contact separation distances required. The contact predetermined opening time may be calculated by determining the time needed to dissipate an amount of energy ΔE, such that ΔE=Varc·I·t, according to a specific device rating.

The predetermined time τ may thus be chosen such that the inductive energy remaining in the circuit when the contacts are opened is not sufficient to increase the voltage across the contacts enough to enable the arc to restrike. An additional safety factor may be used such that Estored(τ)<Erearc, e.g. τ is chosen such that

Estored⁡(τ)=Erearcβ,
where Estored(t) is the amount of inductive energy remaining in the circuit at a time t after the contacts are separated and the circuit broken at time t=0, Erearcthe energy needed to cause the arc to restrike when the first and second electrical contacts are separated by the first predetermined distance d1, and β a safety factor greater than one (e.g. β=2).

Adopting such a release technique helps prevent the possibility of the arc re-striking should it be prematurely terminated. This contrasts with conventional devices in which if the metallic contacts are opened too fast, and the energy in the system is unable to sustain the original arc temperature, the arc quenches and current stops flowing. The still stored inductive energy in the system then increases the voltage across the contact gap until there is sufficient voltage available for breakdown to occur and thus re-strike the arc.

For example, in various embodiments of the present invention, the predetermined time τ may be from about 1 mS to about 15 mS, or more preferably from about 5 mS to about 8 in mS. In contrast, conventional electromagnetic devices often open contacts to break a circuit over a time period that is several orders of magnitude faster than such embodiments, e.g. of the order of microseconds or tens of microseconds.

Whilst various aspects and embodiments of the present invention have been described herein, those skilled in the art will also realise many embodiments of electromagnetic circuit interrupters falling within the scope of the claims may be made. Additionally, they will be aware that various techniques, both experimental and theoretical, may be used to determine certain operating parameters for such electromagnetic circuit interrupters, for example, in order to determine a first predetermined opening distance, a predetermined intermediate contact opening time and/or a second predetermined opening distance. Moreover, many versions of possible contact mechanism embodiments will also be apparent.

REFERENCES