Patent Grant
9111698
This application claims priority under 35 U.S.C. §119 to French application 11/59557 filed in France on Oct. 21, 2011, the entire content of which is hereby incorporated by reference in its entirety.
The disclosure relates to protecting electrical equipment or installations such as protecting electrical equipment against voltage surges, due to lightning impact, for example.
It is known how to ensure protection of an electric installation against voltage surges by means of devices including at least one component for protection against voltage surges, in particular one or several varistors and/or one or several spark-gaps. Such devices are currently designated by the term of lightning arrester. For single-phase installations, a varistor can be connected between the phase and the neutral while a spark-gap is connected between the neutral and the ground. For three-phase installations, varistors can be between the different phases and/or between each phase and the neutral and a spark-gap between the neutral and the ground. For electric installations operating with a DC current, for example for installations of photovoltaic generators, resort is also made to varistors and optionally spark-gaps.
The use of a spark-gap as a protective device against voltage surges may pose a problem for dealing with the follow current of the spark-gap. Indeed, because of priming of the spark-gap, a current may continue to flow through the primed spark-gap and this even after the end of the transient voltage surge. This current is sustained by the voltage source of the electric installation to be protected. This current then corresponds to a follow current which is desirably cut off by breaking the arc formed in the spark-gap. This problem of cutting off the follow current is notably posed in the case of an electric installation operating under a DC current such as an installation for photovoltaic generation of electricity.
Exemplary embodiments of the present disclosure are related to lightning arresters for which different breaker systems can be used.
For example, in the case when an arc only forms between two electrodes at the end of the life of the varistors, there exist single-use breaker systems including mechanical short-circuiting of the arc and then dealing with the short-circuit current with a fuse.
In another example, a spark-gap can be used as a lightning arrester, arcs are repeatedly formed between the electrodes of the spark-gap, preventing the use of single-use breaker systems which are unsuitable. The cutting-off of arcs which repeatedly form, moreover corresponds to a need for other pieces of equipment for which the purpose is to cut off a current as a result of a fault, or any external action. Multiple-use breaker systems can be used both for pieces of equipment such as contactors, circuit breakers or switches and for lightning arresters with spark-gaps.
The systems disclosed herein are based on enlarging the distance between the electrodes between which the arc forms or on separating the arc into a multiplicity of arcs. In both cases, the cutting-off of the arc is achieved by raising the so-called arc voltage to a sufficiently high value so that the voltage source is no longer capable of maintaining this arc voltage. Thus, when the voltage of the source is high, the multiple-use breaker systems should allow an all the greater enlargement of the distance between the electrodes or an all the greater separation into a multiplicity of arcs. For high operating voltages which may be encountered in photovoltaic installations, for example between 500 and 1,000V or even up to 1,500V because of the DC nature of the current, adaptation of the previous systems to the cutting-off of such voltage levels may lead to significant dimensional constraints. Now lightning arrester devices can be contained in casings said to be “mountable” on a DIN rail. These casings do not exceed a width of 17.5 mm and a length of 92 mm, and are then too small for being able to meet such dimensional constraints.
Therefore there exists a need for a method for cutting off an electric arc with which the bulkiness of the devices applying it, may be less significant.
Exemplary embodiments of the present disclosure are directed to a method for cutting off an electric arc which forms between two main electrodes, the method including displacing the electric arc formed towards an electrode located in an intermediate position between both main electrodes, separating the electric arc formed into two secondary electric arcs between the main electrodes and the intermediate electrode, a semiconductor switch normally open, connecting the intermediate electrode to one of the main electrodes, closing the semiconductor switch for extinguishing the secondary electric arc between both electrodes which are connected by the semiconductor switch, opening the semiconductor switch in order to extinguish the other secondary electric arc.
In an exemplary embodiment disclosed herein, the method includes a time-out after separation of the arc formed into two second electric arcs in order to prevent an arc from being re-formed between both main electrodes upon closing the semiconductor switch.
In another exemplary embodiment, the method includes a time-out after closing the semiconductor switch in order to prevent the extinguished arc from being re-formed between the intermediate electrode and one of the main electrodes upon opening the semiconductor switch.
In another exemplary embodiment, a method for protecting an electric installation against transient voltage surges is disclosed, the method applying the cutting-off of an electric arc according to the previous cut-off method when a transient voltage surge occurs in the electric installation to be protected causing the formation of a first electric arc between both main electrodes, the main electrodes being connected to the electric installation to be protected.
According to the present disclosure, an exemplary electric installation is connected to a low voltage electricity distribution network.
According to another exemplary embodiment, an electric installation operates under a DC current, such as an installation for photovoltaic generation of electricity.
An exemplary embodiment of the present disclosure is directed to a protection device for protecting an electric installation against transient voltage surges, including two terminals for connecting the device to the electric installation to be protected, a first main electrode and a second main electrode, each main electrode being connected to respectively one of the connection terminals, an electrode located in an intermediate position between the first main electrode and the second main electrode, a semiconductor switch, normally open, connecting the intermediate electrode to the first main electrode, a circuit for controlling the semiconductor switch, the control circuit being provided in order to successively ensure closing of the switch, and then opening of the switch, after an electric arc formed between the main electrodes has been divided into two arcs via the intermediate electrode.
According to an exemplary embodiment, the semiconductor switch is an insulated gate bipolar transistor or a field effect transistor with a metal-oxide gate.
According to another exemplary embodiment, the control circuit ensures a time-out between the division of the electric arc into two arcs via the intermediate electrode and the closing of the switch and/or between the closing of the switch and the opening of the switch.
According to another exemplary embodiment, the electrodes are fixed, both main electrodes being positioned facing each other from a first side to a second side and forming a spark-gap; and the intermediate electrode partly extending between both main electrodes from the second side.
According to an exemplary embodiment, the device includes a unit for triggering an arc between the main electrodes if a transient voltage surge occurs on the electric installation to be protected, the triggering unit including an electrode for triggering an arc from the first side of the main electrodes.
According to an exemplary embodiment, the intermediate electrode has a wedge-shaped end portion on the side where the intermediate electrode extends between both main electrodes.
According to the present disclosure, the exemplary protection device includes a magnet positioned in order to displace, in the direction from the first side to the second side, an electric arc which forms between the main electrodes of the spark-gap and/or the main electrodes being divergent from the first side to the second side.
According to an exemplary embodiment, the device includes an additional connection terminal and an additional spark-gap formed by two additional electrodes, one of the additional electrodes being connected to the additional terminal and the other one of the additional electrodes being connected to one of the two terminals for connecting the device to the electric installation.
According to an alternative, the device is specially designed for applying the previous method.
An exemplary method for cutting off an electric arc being formed between two main electrodes is disclosed, the method comprising: displacing the formed electric arc towards an electrode located in an intermediate position between both main electrodes; separating the electric arc formed into two secondary electric arcs between the main electrodes and the intermediate electrode, a semiconductor switch, normally open, connecting the intermediate electrode to one of the main electrodes; closing the semiconductor switch for extinguishing the secondary electric arc between both electrodes which are connected by the semiconductor switch; and opening the semiconductor switch in order to extinguish the other secondary electric arc.
An exemplary device for protecting an electric installation against transient voltage surges is disclosed, comprising: two terminals for connecting the device to the electric installation to be protected; a first main electrode and a second main electrode, each main electrode being connected to respectively one of the connection terminals; an electrode located in an intermediate position between the first main electrode and the second main electrode; a semiconductor switch, normally open, connecting the intermediate electrode to the first main electrode; a circuit for controlling the semiconductor switch, the control circuit being provided in order to successively ensure closing of the switch, and then opening of the switch, after division of an electric arc formed between the main electrodes into two arcs by the intermediate electrode.
Other features and advantages of the disclosure will become apparent upon reading the detailed description which follows, of embodiments of the disclosure, given only as an example and with reference to the drawings wherein:
Exemplary embodiments of the present disclosure relates to a method for cutting off an electric arc. The method is applied for a first main electrode and a second main electrode between which an electric arc to be broken, may form as a result of a fault, of an external action or of an external event, such as lightning impact or the separation of mobile contact in a mechanical switch.
A semiconductor switch connects the intermediate electrode to the first main electrode. A semiconductor switch is a switch formed by superposition of layers of doped semiconductors. Thus, a semiconductor switch corresponds to a switch for which the closed or open nature is allowed by a semiconductor operating in a switching mode, either letting through the current or interrupting it. Unlike a mechanical switch, the semiconductor switch is without any mobile contact or mobile mechanical part, the movement of which achieves the switching between the closed condition and the open condition and ensures interruption of the current by the distance separating the mobile contact and the fixed contact. The semiconductor switch then ensures interruption of the current without causing a creation of an arc unlike a mechanical switch. The semiconductor switch may be a bipolar transistor with a gate (better known as a “Insulated Gate Bipolar Transistor” abbreviated as “IGBT”) or a field effect transistor with a metal oxide gate (better known as a “Metal Oxide Semiconductor Field Effect Transistor” abbreviated as “MOSFET” or “MOS”).
The exemplary cut-off method as disclosed herein is applied after forming a first electric arc 62 between both main electrodes 24 and 28. The first electric arc 62 between both main electrodes 24 and 28 is also designated by the term of “electric arc formed” 62. As a result of the formation of the first electric arc 62, the method includes the displacement of the electric arc 62. According to
When the electric arc 62 is displaced as far as the level of the intermediate electrode 26, the exemplary method includes separation of the first electric arc 62 into two second electric arcs 64 and 68. Each of the two second electric arcs 64 and 68 is also designated by the term of “secondary electric” arc 64 or 68. The intermediate electrode 26 can have a floating potential. The second electric arc 64 is formed between the first main electrode 24 and the intermediate electrode 26 while the second electric arc 68 is formed between the second main electrode 28 and the intermediate electrode. The steps of the method before separation of the arc 62 into arcs 64 and 68 corresponds to the phase referenced as 32. As shown in
After the separation of the arc 62 into arcs 64 and 68, the second arcs may also be displaced in the direction from the first side P to the second side D (from left to right according to
The exemplary method then includes the closing of the semiconductor switch in order to extinguish the second electric arc 64 between the intermediate electrode 26 and the first electrode 24. Closing of the switch actually causes a short-circuit of the arc 64 by setting the first main electrode 24 and the intermediate electrode 26 to the same potential. Because of the short-circuit, the current flowing in the arc 64 entirely flows into the switch which causes extinction of the arc 64. This step of the method corresponds to the phase referenced as 36.
After extinction of the arc 64, the method includes the opening of the semiconductor switch in order to extinguish the other second arc 68. Indeed, opening the switch causes insulation of the intermediate electrode 26 relatively to the first main electrode 24. As these electrodes 24 and 26 are no longer connected either by the switch or by the arc 64 extinguished beforehand, the follow current drained by the arc 68 can no longer flow as far as the main electrode 24 except if it recreates an electric arc. For this, the voltage between the intermediate electrode 26 and the main electrode 24 should be greater than the breakdown voltage of the air gap which separates these electrodes 24 and 26. It is helpful to note that the breakdown voltage of an air gap is greater than the voltage for maintaining an already formed arc and crossing this same gap. If the voltage of the source can be sufficient for maintaining an arc initially formed between 24 and 26, the source voltage is insufficient for allowing breakdown of this same air gap, e.g., insufficient for allowing formation of a new arc between 24 and 26. The distance between the electrodes 24 and 26 is selected accordingly. Thus, the opening of the switch causes extinction of the arc 68. This step of the method corresponds to the phase referenced as 38.
After application of the exemplary method, the follow current is completely cut off because of the extinction of both arcs 64 and 68. The cutting-off of an arc provided by the exemplary method is achieved without increasing the voltage for maintaining the arc in the spark-gap, unlike the spark-gap of the prior art. Thus according to this method, it is no longer necessary to cause the arc-maintaining voltage to exceed the voltage of the source by fractionating many times the arc or by increasing the size of the arc. In an exemplary embodiment, the method disclosed herein may therefore be applied in a spark-gap having insulation distances between these different electrodes which are sufficient for preventing the formation of a new arc with the voltage of the source of the installation. As the voltage for forming a new arc is much higher than the voltage for maintaining an already formed arc, the exemplary method of the present disclosure allows reduction in the distances between the electrodes of the spark-gap. The spark-gap applying the method may have limited bulkiness while ensuring cutting-off of the electric arc maintained by high voltage sources.
The positioning of the electrodes according to
In exemplary embodiments having mobile electrodes, the exemplary method of the present disclosure also allows reduction in the maximum separation distances of the electrodes between them. The exemplary protection device with mobile electrodes applying the disclosed method may itself also have limited bulkiness while ensuring the cutting-off of the electric arc maintained by high voltage sources.
The exemplary cutting-off method already discussed may be particularly advantageous when it is applied in a more general method for protecting an electric installation against transient voltage surges.
In the field of protection against transient voltage surges, for example due to lightning impact, provision may be made for positioning a spark-gap at the terminals of an electric installation, as a lightning arrester. The formation of an electric arc in the spark-gap during the transient voltage surge gives the possibility of limiting the voltage at the terminals of the electric installation to be protected. However, at the end of the transient voltage surge event, this electric arc can be maintained by the voltage source of the electric installations to be protected. This maintaining of the arc perturbs return to normal operation of the installation. The application of the previous cutting-off method in a method for protection against voltage surges then gives the possibility of cutting off the current immediately even for high source voltages while limiting the bulkiness of the protection device applying the protection method.
Exemplary embodiments of the present disclosure allows protection of electric installations, such as electric installations connected to a low voltage electric distribution network.
According to a known technique, an electric installation connected to a low voltage electricity distribution network can include a low voltage electric installation with an assigned effective voltage up to 1,000V AC or up to 1,500V DC except for pieces of electric equipment of very low voltage. Very low voltage pieces of electric equipment may be defined as pieces of equipment having an effective assigned voltage of less than 12V AC or DC. Thus the electric installation to be protected may be an electric installation operating with a voltage in a range between 12V and 1,000V AC and between 12V and 1,500V DC. Such very low voltage pieces of electric equipment are exemplary directly connected to a low voltage electricity network. In other words, the method for protecting an electric installation connected to a low voltage electricity network as described herein is distinguished from a known method for protecting microelectronic components against voltage surges.
Among the electric installations connected to a low voltage electricity distribution network, the exemplary protection method is applied to electric installations operating with DC current, for example for an installation for photovoltaic generation of electricity. The application of the cutting-off method in a method for protecting an installation against voltage surges notably gives the possibility of cutting off follow currents maintained by a DC voltage source of 1,500V such as in installations for photovoltaic generation of electricity.
The origin of the times shown in
During phase 32, an arc is formed because of the voltage surge at the terminals of the main electrodes 24 and 28 of the spark-gap 20. The voltage at the terminals of the main electrodes is illustrated by the curve 50. During such a voltage surge, the spark-gap limits the voltage 50 to the voltage for priming the arc in the spark-gap. This arc allows a current 40 to flow between the main electrodes 24 and 28. At the beginning of phase 32, this current 40 then corresponds to a lightning current 48 which is the major portion of the current associated with the transient voltage surge. This lightning current 48 is positive or negative depending on the polarity of the transient voltage surge, lightning may for example be with a positive or negative discharge. After the transient voltage surge, the current 40 and the voltage 50 drop. The formed electric arc 62 may be maintained and may drain the follow current provided by the voltage source of the electric installation to be protected. The current 40 then corresponds to the follow current 42 and the voltage 50 corresponds to the voltage for maintaining the arc 62 between the main electrodes 24 and 28.
During the transient voltage surge and then during the flow of the follow current, the arc 62 is displaced towards the intermediate electrode 26. In an exemplary embodiment, the electrodes 24 and 28 can be divergent on the side D of the intermediate electrode 26, the displacement of the arc towards the intermediate electrode 26 causes an increase in the voltage of the arc after the transient voltage surge. Indeed, the voltage of the arc depends on the length of the arc on the one hand and on its number of feet, here two, on the other hand: one at the electrode 24 and the other one at the electrode 28. This increase in the voltage 50 continues with the displacement of the arc 62 until the arc 62 is separated into two arcs 64 and 68 by the intermediate electrode 26.
Phase 34 is then entered. Upon separation of the arc 62 into two arcs 64 and 68, the voltage 50 at the terminals of the main electrodes 24 and 28 suddenly increase because of the increase in the number of arc feet which passes from 2 to 4: i.e. two feet for each of the arcs 64 and 68. The separation of the arc 62 into two arcs 64 and 68 also corresponds to the occurrence of a voltage 52 between the intermediate electrode 26 and the electrode 24. When the device is symmetrical along the plane of the intermediate electrode 26, the voltage 52 corresponds to half the voltage 54 between the electrodes 24 and 28. This voltage 52 is maintained until closure of the semiconducting switch. However the voltage 52 may slightly increase with the voltage 54 because the arcs 64 and 68 continue to be displaced between electrodes diverging towards the side D.
Upon closing the semiconductor switch, phase 36 is entered. Closing of the semiconductor switch causes formation of a short-circuit between the electrode 26 and the electrode 24. The current 46 flowing through the switch corresponds to the currents having been drained previously by the short-circuited arc 64, i.e. the current 46 corresponds to the follow current 42. The voltage 52 between the intermediate electrode 26 and the electrode 24 is cancelled out and the arc 64 is broken. The voltage 50 between the electrode 24 and 28 is then decreased and passes from voltage 54 to voltage 56.
In an exemplary a time-out between the separation of the first arc 62 and the closure of the semiconductor switch can be provided. Such a time-out corresponds to the duration of the phase 34. With this time-out it is possible to make sure that upon closing the switch, the current drained by the spark-gap actually corresponds to a follow current 42 and no longer to a lightning current 48. Thus the possibility of the lightning current 48 of flowing through the semiconductor switch which would damage the semiconductors of the switch, is avoided. Moreover, independently of the use of the cutting-off method with a purpose of protection against voltage surges, timing out the duration of the phase 34 contributes to preventing an arc from being reformed between both main electrodes 24 and 28 upon closing the semiconductor switch. Indeed, the duration of this time-out may be selected so as to make sure that before the closing of the switch, the air initially ionized by the first arc 62 is deionized.
Subsequently to the closing of the semiconductor switch, phase 38 is entered, by opening this same switch. The current 46 flowing through the switch is then zero and the follow current 42 can no longer flow between the intermediate electrode 26 and the main electrode 24. This causes extinction of the arc 68, the voltage between the main electrodes then becomes equal to the voltage of the electric installation source and the current 40 flowing through the spark-gap is zero. The follow current 42 is therefore cut off. A time-out may be provided between the closing and opening of the semiconductor switch in order to prevent the arc from reforming between the intermediate electrode 26 and the first main electrode 24 upon opening the semiconductor switch. The duration of this time-out may be selected so as to make sure that, before opening the switch, the air initially ionized by the arc 64 is deionized. Such a time-out corresponds to the duration of the phase 36.
An exemplary embodiment of the present disclosure relates to a device for protecting an installation against transient voltage surges. The exemplary protection device includes two terminals for connecting the device to the electric installation to be protected. With reference to
The exemplary protection device further includes the intermediate electrode 26 located in an intermediate position between the main electrodes 24 and 28. When the main electrodes form the spark-gap 20, the intermediate electrode partly extends between both main electrodes from the second side D. The device includes the normally open semiconductor switch and connecting the intermediate electrode 26 to the first main electrode 24.
The exemplary protection device further includes a circuit 78 for controlling the semiconductor switch. The assembly formed by the semiconductor switch and the control circuit 78 is referenced as 70 in
Generally, the exemplary protection device may be specially designed for applying one of the embodiments of the previous methods.
Thus, in the exemplary protection device of the present disclosure, the control circuit 78 may ensure the time-out before closing the switch and/or between the closing of the switch and the opening of the switch. Referring back to
After the moment when the IGBT becomes conducting, the capacitor C1 maintains the control voltage and charges a capacitor C2 via a resistor R4. Depending on the calibration of C2 and R2, the desired charging time for C2 is obtained allowing the intended time-out of phase 36. When the capacitor C2 voltage reaches the reverse voltage of a Zener diode DZ2, the Zener diode DZ2 becomes conducting. This causes a voltage to be applied to the terminals of a resistor R5, allowing switching of the thyristor T2 into the conducting state. The IGBT is then short-circuited and the IGBT passes from the conducting state to the blocked state. The follow current is cut off by the opening of the IGBT and the arc 68 is extinguished. In other words, the portion 74 of the control circuit 78 ensures control upon opening the IGBT. Phase 38 is entered.
Following the extinction of the arc 68, the capacitors C1 and C2 are respectively discharged into the resistors R3 and R6.
In an exemplary embodiment of the present disclosure varistor V1 is provided for ensuring the protection of the IGBT while suppressing the possible lightning current peak associated with the voltage surge in the case when there is still a voltage surge at the moment when the arc 62 is separated into arcs 64 and 68. Generally, in all the embodiments described earlier, the positioning of the intermediate electrode 26 on the side D of the main electrode may be adjusted in order to ensure an intended time-out for the duration of phase 32. The time-out of the phase 32 may thus correspond to a sufficiently long duration so that the voltage surge episode, for example due to lightning impact, is finished before the beginning of phase 34.
Referring back to
According to another exemplary embodiment of the disclosure, the semiconductor switch may include a plurality of IGBTs positioned in parallel relatively to each other, for example two IGBTs in parallel. Such a parallel arrangement of IGBTs, allows the thereby formed semiconductor switch to drain a larger follow current intensity as compared with the semiconductor switch including a single IGBT. Such an embodiment is particularly advantageous for uses of the exemplary protection device disclosed herein relating to the protection of photovoltaic installations which may provide high intensity currents, such as an intensity of more than 1,000 A. According to this exemplary embodiment, the control circuit 78 illustrated in
In the exemplary circuit illustrated in
Depending on the exemplary embodiment selected for the control circuit 78, any other means for limiting the intensity of the current flowing through the control circuit 78 may be provided for limiting such an intensity to a level below the threshold intensity of the current for maintaining the arc 68 in the exemplary protection device. According to an exemplary embodiment, the selection of the means for limiting the intensity results from a compromise between the limitations of the intensity of the control circuit and obtaining a level for this intensity which is sufficient for operating the control circuit of the semiconductor switch.
According to another exemplary embodiment, the device may include a magnet positioned for displacing the electric arc 62 from the first side P to the second side D. The magnet may correspond to an assembly of opposite poles of distinct permanent magnets.
In an exemplary protection device having no magnet, the electric arc formed in the device moves under the effect of its own energy. The higher the intensity of the current drained by the arc, the more the displacement of the arc is facilitated. When the intensity of the current drained by the arc is too low, the arc 62 may have difficulties in moving under the sole effect of its own energy. For certain electric installations notably for installations for photovoltaic generation of electricity, the follow currents may assume very low values. Indeed, the follow current of an installation for photovoltaic generation of electricity may have several values between a quasi zero value (night time) and a maximum value (daytime without any clouds). These low follow current values such as currents of the order of 0.5 A, may not be sufficient for operating cut-off systems exclusively based on the displacement of the arc under its own energy. The use of the magnet in the device 90 then gives the possibility of facilitating the displacement of the arc 62 even in the case of a low follow current intensity. Such an embodiment of the device 90 gives the possibility of obtaining a device for protecting an electric installation against voltage surges, independently of the value of the follow current. In another exemplary embodiment of the present disclosure, the main electrodes 24 and 28 of the device may be divergent from the first side P to the second side D, as illustrated in
In yet another exemplary embodiment, the intermediate electrode 26 may have a wedge-shaped end portion on the side where the intermediate electrode 26 extends between both electrodes 24 and 28. The wedge-shaped end of the intermediate electrode is then the end of the electrode which is the closest to the side D of the main electrode 24 and 28. According to
When the electric installation to be protected is an installation operating with DC current, the two active conductors are the conductor with polarity + and the conductor with polarity − respectively. It is estimated that in 60% of the installations of this type, the polarities + and − are floating relatively to the ground. For the remaining installations where one of the active conductors is connected to the ground, it is estimated that it is the conductor with polarity + which is connected to the ground in 95% of the cases. Thus, upon using the device 90 in the Y protection mode, the terminals 98 and 198 can be connected to the conductors with polarity − and + respectively, while the terminal 94 may be connected to the ground. According to this connection diagram, for the large majority of installations operating with DC current, the spark-gap 20 with the intermediate electrode 26 is connected between the ground and an active connector not connected to the ground. This allows the device 90 to ensure efficient Y protection with a follow current which is cut off for the large majority of installations operating with DC current.
In the case of a single-phase electric installation operating with AC current, one of the two protected active conductors may be the phase while the other one of the two protected active conductors may be the neutral.
In a symmetrical embodiment of device 90 as illustrated in
Still with reference to
The device 90 in the different embodiments described earlier may include a unit for triggering an arc between the main electrodes 24 and 28, or 124 and 128 if need be.
Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
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