HK1165623A - Switching device - Google Patents

Description

Switching device

Technical Field

The present invention relates to a switchgear including a gas earthing/disconnecting unit that performs earthing/disconnecting by gas insulation.

Background

A power distribution facility is provided with a closed distribution board (referred to as switchgear) which houses a vacuum interrupter for interrupting a load current or an accident current, a circuit breaker and an earthing switch for ensuring safety of an operator when performing maintenance and inspection of a load, a detection device for a system voltage or a system current, and a protection relay and the like.

Among such switchgear apparatuses, there is a combination type gas insulated switchgear apparatus including a three-position ground disconnecting switch having a rotor as a contact and a vacuum valve in a metal container, and the metal container is filled with SF6 gas as an insulating gas (see, for example, patent document 1).

Patent document

Patent document 1: japanese laid-open patent publication No. 6-12948

In the above-described combination type gas insulated switchgear, since the vacuum valve and the three-position ground disconnecting portion are provided inside the metal container in which SF6 gas is sealed, a highly airtight epoxy bushing is required as a separate component for connecting to, for example, a high-voltage cable. Further, an O-ring and an O-ring groove for maintaining airtightness between the sleeve and the metal container are required, which causes a problem that the number of parts increases and the structure becomes complicated.

Further, since SF6 gas having a high global warming potential is used as an insulating medium, there is a problem of low environmental compatibility.

Further, in the case of gas insulation, the problem that the rotor-switching method is difficult to be increased in size because of the high insulating performance of SF6 gas is involved, but in the case of gas insulation having lower insulating performance than SF6 gas, the rotor-switching method is involved, which increases the size of the breaking portion and increases the size of the entire device.

In the case of gas insulation, when an arc occurs due to, for example, the operation of the breaking unit, the temperature of the shielding gas in the breaking unit rapidly increases, but after the arc disappears, the temperature of the shielding gas decreases to, for example, the vicinity of the outside air temperature. Therefore, in order to ensure the reliability of the device, there is a problem that it is necessary to prevent dust from entering the disconnection portion and dew condensation from occurring.

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a switchgear capable of achieving miniaturization and ensuring high reliability even when a gas insulation system is employed in a disconnecting/grounding switch unit.

In order to achieve the above object, a first aspect of the present invention includes: a chassis divided by a grounding metal plate into a bus bar region located at the center, a switch region located below the bus bar region, a cable region located on the back side, and a control region located on the front side; and a gas-insulated earthing and disconnecting switch and an operating device thereof, which are arranged in the switch area; the opening/closing area portion is formed as a box body of a quasi-sealed structure which is separable from the casing and allows air to pass therethrough but prevents oil and moisture particles from passing therethrough.

In the second invention, the box of the quasi-sealed structure according to the first invention is provided with a filter member made of a porous membrane that allows air to pass between the inside and the outside but prevents oil and moisture particles from passing through.

In the third aspect of the present invention, in the second aspect of the present invention, the case of the quasi-hermetic structure includes an air intake/exhaust port to which the filter member is attached on a rear surface side, and the cable region portion is communicable with air inside and outside.

A fourth aspect of the present invention is the moisture absorbing/releasing member according to any one of the first to third aspects of the present invention, wherein the moisture absorbing/releasing member absorbs moisture when the internal humidity rises and releases moisture when the humidity falls.

In the fifth aspect of the present invention, in the fourth aspect of the present invention, the moisture absorbing/releasing member is provided inside a side member of the case constituting the quasi-hermetic structure.

A sixth aspect of the present invention is any one of the first to fifth aspects, wherein the bus bar region, the cable region, and the control region are independent of each other, but air communication between the regions is established.

The present invention has an effect that, according to the present invention, since the casing accommodating the gas insulated disconnection/grounding switch is formed into a quasi-sealed structure which allows air to pass therethrough but prevents oil and moisture particles from passing therethrough, the occurrence of dust contamination and condensation can be prevented. As a result, even if the gas insulation system is adopted, a switchgear that can be downsized and that can ensure high reliability can be provided.

Drawings

Fig. 1 is a side view, partially in section, showing one embodiment of the switchgear of the present invention.

Fig. 2 is a vertical cross-sectional view of a switch unit constituting one embodiment of the switchgear of the present invention, showing its on state.

Fig. 3 is a vertical cross-sectional view for explaining an operation in a cut-off state of the switch unit shown in fig. 2.

Fig. 4 is a vertical cross-sectional view for explaining an operation in the open state of the switch unit shown in fig. 2.

Fig. 5 is a vertical sectional view for explaining an operation in the pre-grounding state of the switch unit shown in fig. 2.

Fig. 6 is a vertical cross-sectional view for explaining an operation of the switch unit shown in fig. 2 in a grounding state.

Fig. 7 is a rear view showing a switch unit constituting one embodiment of the switchgear of the present invention.

Fig. 8 is a partial perspective side view showing a switch unit constituting one embodiment of the switchgear of the present invention.

In the figure:

1-cabinet, 1 a-bus bar area part, 1B-switch area part, 1 c-cable area part, 1 d-control area part, 2-grounded metal plate, 3-bus bar connection sleeve, 4-vacuum valve (vacuum breaker), 5-grounded breaking part (gas grounded switch), 6-switch unit, 7-operating device, 8-cable connection sleeve, 12A-fixed side electrode, 12B-movable side electrode, 14-arc shield, 15-bellows, 17-vacuum valve gas insulated operating lever, 20-sleeve side fixed electrode, 21-grounded side fixed electrode, 22-flexible conductor, 23-middle fixed electrode, 24-grounded breaking part movable electrode, 25-grounded breaking part gas insulated operating lever, 70-air suction and exhaust port, 71-filter, 72-moisture absorption and release component and 100-box body.

Detailed Description

An embodiment of a switchgear according to the present invention will be described below with reference to the drawings.

Fig. 1 and 2 are views showing an embodiment of a switchgear of the present invention, fig. 1 is a partially sectional side view showing an embodiment of a switchgear of the present invention, and fig. 2 is a front sectional view showing a switch unit constituting an embodiment of a switchgear of the present invention, and is a view showing an on state.

In fig. 1, a casing 1 of a switchgear is divided into a bus bar region 1a, a switch region 1b, a cable region 1c, and a control region 1d by a grounding metal plate disposed inside. A front door 1e that can be opened on one side is provided on the front side (right side in fig. 1) of the housing 1, and a detachable rear panel 1f is provided on the rear side (left side in fig. 1) of the housing 1.

The bus bar region 1a is disposed on the upper side toward the center in the depth direction (the left-right direction in fig. 1) of the enclosure 1, the switch region 1b is disposed below the bus bar region 1a, and the cable region 1c is disposed on the rear side (the left side in fig. 1) of the enclosure 1. The control area 1d is disposed on the upper side of the rear surface of the front door 1e, and is located at a position facing the bus bar area.

Each region is basically formed by a plate-shaped bottom plate 2a and a top plate 2b made of a grounded metal plate, and left and right side plates that close a gap between the bottom plate 2a and the top plate 2 b.

A grounding metal plate 2c is provided at the boundary between the upper part of the cable region 1c and the bus bar region 1 a. A three-phase bus bar (not shown) is disposed in the bus bar region 1a in parallel with the front surface of the housing 1 (in the figure, in the direction perpendicular to the paper surface) via a bus bar connecting sleeve 3 as a connecting member.

In this example, the switch area portion 1b has a box structure in which a switch unit 6 including a vacuum interrupter (vacuum circuit breaker) 4 and a ground breaking portion (gas ground switch) 5 is assembled with an operation device 7 in a housing to form a quasi-hermetic structure, and is removed from the box at the time of replacement of parts and maintenance. The detailed description thereof will be described later.

In the cable region portion 1c, disposed are: a cable connecting bushing 8 connected to the fixed contact of the vacuum interrupter (vacuum circuit breaker) 4 and introduced into the cable region 1c, a cable connector attachable to the cable connecting bushing 8, and a cable (not shown) connected to the cable connecting bushing 8 via the cable connector.

A grounding metal plate 2d is provided at the boundary between the upper part of the bus bar region 1a and the control region 1 d. In the control area portion 1d, a control portion including a capacitor, a protective relay, and the like, which are not shown, is provided on the back surface side of the door 1e or the like on the front surface side.

As described above, although the bus bar region 1a, the cable region 1c, and the control region 1d are independent of each other, air for cooling each device in each region communicates with each other and communicates toward the discharge port, not shown, of the enclosure 1.

Next, a detailed configuration of the switch unit 6 constituting one embodiment of the switchgear of the present invention will be described with reference to fig. 2. As shown in fig. 2, the switch unit 6 includes: the vacuum interrupter 4, the earthing and disconnecting switch 5, the bus bar connecting bushing 3, and the cable connecting bushing 8 are integrally molded with a solid insulator 30 such as epoxy in the grounded metal container 31.

The vacuum valve 4 includes a fixed-side electrode 12A, a movable-side electrode 12B, a fixed-side holder 13A connected to the fixed-side electrode 12A, a movable-side holder 13B connected to the movable-side electrode 12B, and an arc shielding member 14 for protecting the ceramic insulating sleeve from an arc, in a vacuum vessel including the fixed-side ceramic insulating cylinder 10A, the movable-side ceramic insulating cylinder 10B, the fixed-side end plate 11A, and the movable-side end plate 11B. The fixed-side holder 13A is connected to the sleeve center conductor 8A for cable connection, and can supply power to the load side. Further, a bellows 15 for moving the movable-side holder 13B is disposed on the movable side. The vacuum valve 4 maintains the vacuum inside by a bellows 15 connected to the movable-side end plate 11B and the movable-side holder 13B, and switches between an on state and an off state by allowing the movable-side electrode 12B and the movable-side holder 13B to move in the axial direction.

The ground breaking unit 5 includes: the sleeve-side fixed electrode 20 connected to the bus bar side via the bus bar connection sleeve center conductor 3A, the ground-side fixed electrode 21 at the ground potential, and the intermediate fixed electrode 23 located therebetween and electrically connected to the movable-side holder 13B on the vacuum valve 4 side via the flexible conductor 22 are gas-insulated inside. The fixed electrodes are all arranged linearly and have the same inner diameter. The movable electrode 24 of the earthing and disconnecting switch can be switched to three positions of closing, disconnecting, and earthing by linearly moving the earthing and disconnecting switch in the earthing and disconnecting switch 5 with respect to each fixed electrode. The movable electrode 24 of the earthing and disconnecting switch is connected to an earthing and disconnecting switch gas-insulated operating rod 25 connected to the operating mechanism of the operating device 7 shown in fig. 1. Further, since the movable electrode 24 of the earthing and disconnecting switch is configured to have a portion in contact with each of the fixed contacts by the spring contact 26, the movable electrode 24 of the earthing and disconnecting switch is not hindered from moving, and reliable contact can be achieved by the elastic force.

The switch unit 6 of fig. 2 is constituted by: a vacuum valve 4 having at least a pair of contacts capable of being opened and closed; a ground disconnection unit 5; a cable connection sleeve 8 connected to the fixed-side electrode 12A and the fixed-side holder 13A of the vacuum valve 4; a bus bar connection bushing 3 connected to a bushing-side fixed electrode 20 of the earthing and disconnecting switch 5; a flexible conductor 22 for connecting the intermediate fixed electrode 23 of the earthing and disconnecting switch 5 to the movable side of the vacuum interrupter 4; a vacuum valve gas-insulated operating rod 17 mechanically connected to the movable-side electrode 12B and the movable-side holder 13B of the vacuum valve 4; a gas-insulated operating rod 25 for the earthing and disconnecting switch mechanically connected to the movable electrode 24 for the earthing and disconnecting switch; the earthing and disconnecting switch 5 includes: a movable electrode 24 of the earthing and disconnecting switch provided in the atmosphere and linearly displaceable at three positions; a sleeve-side fixed electrode 20 and an intermediate fixed electrode 23 which are electrically conducted via the movable electrode 24 of the earthing and disconnecting switch when the movable electrode 24 of the earthing and disconnecting switch is in the closed position; and a grounding side fixed electrode 21 which is electrically connected to the intermediate fixed electrode 23 via the grounding/disconnecting portion movable electrode 24 when the grounding/disconnecting portion movable electrode 24 is at the grounding position; the vacuum interrupter 4, the ground breaking portion bushing-side fixed electrode 20, the bus bar connection bushing 3, and the cable connection bushing 8 are integrally cast from a solid insulator 30.

The bus bar connecting sleeve 3 is configured by covering the periphery of the bus bar connecting sleeve central conductor 3A with a solid insulator 30, and the cable connecting sleeve 8 is configured by covering the periphery of the cable connecting sleeve central conductor 8A with a solid insulator 30.

Next, the operation of the switch unit 6 constituting one embodiment of the switchgear of the present invention will be described with reference to fig. 3 to 6. Fig. 3 is a vertical cross-sectional view for explaining an operation in a disconnected state of the switch unit shown in fig. 2. Fig. 4 is a vertical cross-sectional view for explaining an operation in the open state of the switch unit shown in fig. 2. Fig. 5 is a vertical sectional view for explaining an operation in the pre-grounding state of the switch unit shown in fig. 2. Fig. 6 is a vertical cross-sectional view for explaining an operation of the switch unit shown in fig. 2 in a grounding state. In fig. 3 to 6, the same reference numerals as those shown in fig. 1 and 2 denote the same parts or equivalent parts, and therefore, the description of the parts will be omitted.

Fig. 3 shows the off state of the switch unit 6. In the on state of the switch unit 6 shown in fig. 2, the load current is passed from the bus bar connecting bushing 3 to the cable connecting bushing 8 via the earthing and disconnecting switch 5 and the vacuum valve 4. In this state, when a short circuit occurs on the load side connected to the sleeve 8 for cable connection and a large fault current flows, the vacuum valve 4 is shut off to shut off the fault current as shown in fig. 3.

Fig. 4 shows the open state of the switching unit 6. The movable electrode 24 of the earthing and disconnecting switch and the sleeve-side fixed electrode 20, and the movable electrode 24 of the earthing and disconnecting switch and the sleeve-side fixed electrode 21 are not electrically conducted, and the earthing and disconnecting switch is turned into the disconnecting state by driving the movable electrode 24 of the earthing and disconnecting switch to the lower side of the paper surface to a position where the gap between the movable electrode 24 of the earthing and disconnecting switch and the sleeve-side fixed electrode 20 is large and the gap between the movable electrode 24 of the earthing and disconnecting switch and the earthing-side fixed electrode 21 is small.

In this case, the bus bar connecting bushing 3 and the cable connecting bushing 8 are disconnected at two points of the vacuum valve 4 and the earthing and disconnecting switch 5, and therefore reliability is high. Further, by designing the withstand voltage between the movable electrode 24 of the earthing and disconnecting switch and the earthing side fixed electrode 21 to be lower than the withstand voltage between the fixed side electrode 12A and the movable side electrode 12B of the vacuum valve 4, the structure can be made to have high reliability with priority on earthing.

Fig. 5 shows a state of the switch unit 6 at an early stage of grounding. As shown in the drawing, the movable electrode 24 of the earthing and disconnecting switch is driven downward in the drawing to a position where the movable electrode 24 of the earthing and disconnecting switch is in contact with the earthing side fixed electrode 21, whereby the movable electrode 24 of the earthing and disconnecting switch, the flexible conductor 22 and the movable side electrode 12B are brought to the earthing potential, and a potential difference from the load side is applied between the electrodes in the vacuum valve 4.

Fig. 6 shows a grounded state of the switch unit 6. From the state of the switch unit 6 shown in fig. 5, when the vacuum interrupter 4 is further turned on, the cable connecting sleeve 8 is grounded via the vacuum interrupter 4, the flexible conductor 22, the intermediate fixed electrode 23, the movable electrode 24 of the earthing and disconnecting switch, and the earthing side fixed electrode 21. At this time, even if the cable connection bushing 8 is in a charged state, the final closing operation is performed by the vacuum valve 4, and therefore, the short-circuit current closing capacity is not required for the earthing and disconnecting switch 5.

In order to return the switch unit 6 from the grounded state to the on state again, the inside of the vacuum interrupter 4 is brought into the off state, and then the movable electrode 12B is turned on in the vacuum interrupter 4 after moving the movable electrode 24 of the ground disconnecting portion to bring the spring contact 26 into contact with the sleeve-side fixed electrode 20.

Next, the configuration of the switching region section 1b including the switching unit 6 will be described with reference to fig. 1, 7, and 8. Fig. 7 is a rear view showing a switch unit constituting one embodiment of the switchgear of the present invention. Fig. 8 is a partial perspective side view showing a switch unit constituting one embodiment of the switchgear of the present invention. In fig. 7 and 8, the same reference numerals as those in fig. 1 to 6 denote the same or corresponding portions, and the description thereof will be omitted.

In the present example, although the switch area section 1b has been described as a box structure in which the switch unit 6 including the vacuum interrupter (vacuum circuit breaker) 4, the ground disconnecting section (gas ground switch) 5, and the operation device 7 are assembled to the housing to form a quasi-sealed structure, the switch unit 6 has a split-phase structure, three switch units are arranged in parallel perpendicularly to the front surface of the housing 1, and a shield layer (not shown) is disposed between the respective phases, thereby suppressing the occurrence of a short-circuit fault between the phases (see fig. 7).

A driving unit 40 for the vacuum valve 4 and a driving unit 41 for the ground breaking unit 5, which have vertical driving directions (vertical directions in fig. 1 and 8), are disposed on the front side of the housing 1 of the proximity switch unit 6. These driving portions 40 and 41 are connected to the vacuum valve gas-insulated operating rod 17 and the ground disconnection portion gas-insulated operating rod 25 via operating mechanisms 42 and 43, respectively. The driving portions 40 and 41 and the operating mechanisms 42 and 43 constitute the operating device 7.

As shown in fig. 7 and 8, the devices such as the opening and closing unit 6, the driving units 40 and 41, and the operating mechanisms 42 and 43 are disposed in a case 100 formed in a quasi-sealed structure having a filter 71.

The case 100 constituting the opening/closing area portion 1b includes: a bottom member 51; side members 52, 52 provided on the left and right sides of the bottom member 51, respectively; an upper back member 53a and a lower back member 53b which vertically connect the side members 52, 52 on the back; a front member 54 provided on the front surface of the bottom member 51; and a ceiling member 55 forming the top. Each of the members 51, 52, 53, 54, and 55 is formed of a grounded metal plate, and is tightly clamped and fixed via a spacer or the like by a bolt, a nut, or the like.

The side members 52 and 52 are constituted by a first side member 52a corresponding to the side portions of the vacuum valve 4 and the earthing and disconnecting switch 5, and a second side member 52b corresponding to the driving units 40 and 41.

As shown in fig. 7 and 8, the lower back surface member 53b of the casing 100 constituting the opening/closing region 1b is provided with the intake/exhaust port 70. A filter 71 is provided inside the case 100 of the air inlet/outlet 70. Since the filter 71 is a porous membrane member that allows air to pass through from the inside to the outside but prevents oil and moisture particles from passing through, the opening/closing area 1b communicates with the cable area 1b only through the air intake/exhaust port 70 and the filter 71, and does not communicate with the bus bar area 1a and the control area 1d, which are other areas. The porous membrane constituting the filter 71 has many fine through holes, and allows air to pass therethrough, but prevents oil and moisture particles from passing therethrough. That is, the opening/closing region 1b is formed as a quasi-sealed structure that allows air to pass therethrough but blocks oil and moisture particles from passing therethrough, and when the pressure in the opening/closing region 1b is negative, air is introduced from the cable region 1c through the filter 71, but the entry of foreign matter such as oil, moisture, and dust is prevented.

In order to protect the switch unit 6, which is a high-voltage device disposed in the switch area portion 1b, from dust and the like, the filter 71 has a pore diameter of 0.1 μm or less than 0.1 μm. Although air can enter and exit the switching area portion 1b through the filter 71, the filter does not allow dust to pass therethrough, so that the switching unit 6 and the operation device 7 are protected from dust. The filter 71 is a filter that allows only air to pass therethrough, and examples thereof include gorex (GORE-TEX).

Further, moisture absorbing/releasing members 72, 72 are provided inside the first side members 52a, 52a of the casing 100 constituting the opening/closing region portion 1 b. The moisture absorbing/releasing member 72 preferably has a high moisture absorbing property and a high moisture absorbing speed.

Next, the operation of one embodiment of the switchgear of the present invention will be described.

The filter 71 prevents dew condensation in the case 100 by circulating the air inside and outside the case 100 of the quasi-hermetic structure. As a result, the reliability of the ground disconnection portion (gas ground switch) 5 in the case 100 can be ensured.

In addition, the moisture absorbing and releasing member 72 rapidly absorbs moisture when the humidity inside the cabinet 100 rises, and rapidly releases moisture when the humidity drops and dries, thereby controlling the relative humidity variation caused by the temperature variation within a certain range. As a result, for example, condensation caused by a rapid temperature change in the case 100 can be prevented.

That is, the dew condensation in the casing 100 constituting the opening/closing region 1b is a phenomenon in which the excessive water vapor which cannot be accumulated in the air becomes water droplets due to cooling of the high-temperature air having a large amount of saturated water vapor, and occurs, for example, in the gas-insulated operating rod 25 or the like constituting the ground breaking portion of the switch unit 6 and the inner wall of the casing 100, and in order to prevent such dew condensation, it is necessary to circulate clean internal and external air not containing dust or the like, or to rapidly remove the excessive water vapor.

According to the above-described one embodiment of the switchgear of the present invention, since the housing 100 accommodating the ground disconnecting portion (gas ground switch) 5 of the switch unit 6 is made a quasi-sealed structure, it is possible to prevent the contamination of dust. As a result, even if gas insulation is employed, a switchgear that can be made compact and that can ensure high reliability can be provided.

Further, according to the above-described one embodiment of the switchgear of the present invention, since the housing 100 is made in a quasi-sealed structure by the air intake/exhaust port 70 to which the filter 71 that allows air to pass but blocks oil and moisture particles from passing is attached, it is possible to cool heat generated in the housing 100 by air flowing in from the cable region 1c while preventing entry of dust. As a result, even if gas insulation is employed, a switchgear that can be made compact and that can ensure high reliability can be provided.

Further, according to the above-described one embodiment of the switchgear of the present invention, since the moisture absorbing/releasing member 72 for controlling the humidity at the time of an abrupt temperature change is provided in the casing 100, it is possible to prevent dew condensation due to an abrupt temperature change of the switching region portion 1 b. As a result, even if gas insulation is employed, a switchgear that can be made compact and that can ensure high reliability can be provided.

In the above-described embodiment, the suction/exhaust port 70 of the casing 100 is provided in the lower back surface member 53b, but the present invention is not limited thereto. Or may be provided at the upper back member 53a or the like. In the above-described embodiment, the moisture absorbing/releasing member 72 is provided inside the first side members 52a, 52a of the casing 100, but the present invention is not limited thereto. It is possible to provide the inside of the case 100 with any arrangement.

In the above embodiment, the example in which the moisture absorbing/releasing member 72 is provided in the casing 100 and the filter 71 is provided in the air inlet/outlet 70 of the casing 100 has been described, but the moisture absorbing/releasing member 72 may be omitted when used in an environment where the temperature and humidity of the casing 100 do not change rapidly.

Although not described in detail herein, for example, the configuration of the interlock device between the operators is shown as a good example for implementation, and the invention is not limited to this.

Claims (5)

1. A switchgear is characterized by comprising:

a chassis;

a bus area part divided by a grounding metal plate and positioned at the central part in the case;

a cable area part located on the back side of the case;

a control area part located on the front side of the case; and

a box body which is located below the bus bar region part and is provided to be separable from the cabinet, is configured as a quasi-seal structure which allows air to pass therethrough but prevents oil and moisture particles from passing therethrough, and is configured as a switching region part,

the switch area part comprises a circuit breaking part for cutting off current, a gas switch part for switching circuits by adopting gas insulation and an operating device thereof;

an air suction and exhaust port provided in the case of the opening/closing area; and

and a filter member which is attached to the air intake/exhaust port and is composed of a porous membrane member which allows air to pass therethrough from the inside to the outside but prevents oil and moisture particles from passing therethrough.

2. The switchgear as claimed in claim 1,

the case of the quasi-hermetic structure has a suction/discharge port to which the filter member is attached on the back surface side, and the cable region portion is communicable with the air inside and outside.

3. The switchgear as claimed in claim 1 or 2,

the case of the quasi-sealed structure is provided with a moisture absorbing/releasing member that absorbs moisture when the internal humidity rises and releases moisture when the humidity falls.

4. The switchgear as claimed in claim 3,

the moisture absorbing/releasing member is provided on the inner side of the side member of the case constituting the quasi-sealed structure.

5. The switchgear as claimed in any of claims 1 to 4,

the bus bar region, the cable region, and the control region are independent of each other, but the regions are in air communication with each other.