WO2014201475A1

WO2014201475A1 - Ring main unit

Info

Publication number: WO2014201475A1
Application number: PCT/VN2014/000002
Authority: WO
Inventors: Viet Thong Ho
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-06-10
Filing date: 2014-06-02
Publication date: 2014-12-18
Anticipated expiration: 2015-12-10

Abstract

The invention relates to an electrical three-phase medium voltage ring main unit, wich has a sealed shell containing insulating gas and three terminals, of which: two input terminals for electrical-connection to both sides "a" and "b" of power source, and one output terminal for electrical-connection to one branch. Inside the said sealed shell, there are twoo switches; protector and earthing switch, the first switch is a load-break switch electrical-connected directly between two said input terminals, and the second switch is a transfer-switch electrical- connected between one output terminal and the twoo input terminals, all switches and circuit breakers or fuses are so arranged vertically. The ring main unit according to the invention is smaller than the known ring main unit and low-cost manufacturing.

Description

RING MAIN UNIT

Backround of the invention

This invention related to a completely insulated equipment to be used to connect between two ring-main feeders of the three-phase medium-voltage loop and one transformer feeder of the distribution substations or to the other circuit. In particular, this invention related to a novel ring main unit.

Prior art

In general, the three feeders ring main unit including: switches; fuses or circiut-breaker (protector); earthing switch, and all said components are integrated in a sealed shell containing SF6 insulating gas (sulfur hexafluoride), in wich: two ring- main feeders for connection to both sides "a" and "b" of the medium-voltage loop network and one transformer feeder.

Normally, the three feeders ring main unit has 5 functions as follows: i. "Closed" / "open" two sides "a" and "b" of power source; ii. "Closed" / "open" to the transformer's branch; iii. Automatically interrupts when overload or fault occurs on tranformer's branch; iv. Grounding of outputs; v. Transfer source of transformer's branch from one side to the other side of source. It is called shortly as: "Transfer source".

The functionality of the ring main unit is done by switchs, the protector and earthing switch arranged on the electrical circuit in sealed shell and connected between three terminals of three feeders. In this invention, it is necessary to distinguish between the two types of components wich are "switch" and "protector" based on their function and definitions specifically as follows:

"Switch" is the medium voltage electrical switchgear having three functions corresponding to its three operating statuses such as: "close" / "open" / "earth"; or "close source a" / "open "/" close source b"; or "close a source"/ "open-earth 7 "close b source".

Switch may be the disconnector capable of "close" / "open" without current or small current, or the load-break switch capable of "close" / "open" with rated current (200; 400; 630 A), or the transfer-switch that switches a load between two "a" and "b" side of source, or the switch-breaker capable "close" / "open" with short circuit current (16; 20 kA) such as circuit breaker of ring main unit RM6 - Schneider Electric. All kinds of switch may be different in ability of current, but have one common thing wich is the "earth" position. In this invention, circuit breaker included "earth" position is called "switch-breaker".

"Protector" functions automatic interruption in case of overload or short circuit, eg: fuses or circuit breakers. Fuse wire is automatically disconnected based on the phenomenon of cut-off of the fusible wire when the flowing current exceeds a pre-determined value. Circuit breakers includes interruptor and control relays to be combined with current transformer. Relays will send signals to the interruptor for automatic interruption automatically in case of overload or short circuit. It is necessary to differentiate between the circuit breaker as the "protector" and the switch-breaker as the "switch", particulaly: the circuit breaker has only two operating statuses "close"/ "open", whereas, the switch-breaker has three operating statuses "close" / "open" / "earth".

The ring main unit with the built-in switch-breaker combined with relays and current transformers have the ability to automatically interrupt in case of overload or fault without additional fuse or breaker contacts, called as "breaker ring main unit".

In principle: in an electrical circuit there is only one switch in order to connect or separate the two its ends. Practically, there is the same problem inside all types of ring main unit available in the market: there are two switches to "close" / "open" one electrical circuit which is linked between the two terminals that connect with both sides "a" and "b" of power loop, meaning that: all the known ring main unit has a redundant switch.

Why is there this situation? Because the two switches mentioned above are not only used to "close" / "open" a circuit but also to transmit power supply to the transformer's branch from two sides "a" and "b" of power source.

The transfer source is used in two cases, as follows:

• The first case: due to malfunction at one side of the power supply, it should be grounded and isolated for repair. Transformer's branch should be energized at the remaining power supply.

• The second case: one side of the power supply is faulted, should be grounded and isolated for repair. Transformer's branh should be transferred to the remaining power to continue the power supply.

Because the first case is just assumption which rarely happens, so it can be ignored .

To evaluate the second case, more conditions should be taken into

consideration:

1- According to regulations by the Electricity of Vietnam, number of the event allows for medium voltage underground cable is 3.6 cases, per 100 km in a year.

2- In urban areas with the load growth, more new substations are built among the existing substations. The average distance from the existing substation to a new substations is 500 ÷ 800 meters calculated on the basis of,the length of underground cable.

3- Depreciation life of the device (such as RMU) is a minimum of 7 years and a maximum of 10 years, see Circular No. 203/2009/TT-BTC, dated 20/10/2009 issued by Ministry of Finance (Vietnam). 4- The maximum time to unplug or convert the cable terminal of ring main unit is 15 minutes.

Based on this information, the determined probability of an cable fault on section between two ring main unit per year is 1.8 ÷ 2.8%, which means (theoretically): in the range from 34.7 to 55.5 years, one time of source transfer is needed. The number of source transfer required for one position of ring main unit is very small.

It can meet the requirements of power source transfer with different solutions, as follows:

• The first solution: chose the ring main unit with source transfer function, same as currently applied. So, about 5 units needed to replace for one place due to expiration, there is only one unit used to operate one time to transfer power source, 4 other units are installed without the use to source transfer.

• The second solution: Chose the ring main unit without source transfer function. When source transfer source is required, cable terminals will be changed or unplugged . Blackout period will be extended as required for changing or unplugging the cable terminals, 15 minutes at maximum.

Today, thanks to the technology of inspection on insulated medium voltage cables, the weaknesses in insulation of underground cable will be detected early and therefore, repair is planned. This will help reducing the probability of the cable problem. Futhurmore, the arrangement of power cut at the proper time of appropriate making the outage extended about 15 minutes will not affect to the customers.

The switch inside the ring main unit usually is the load-break switch with the hundred times of "close'Vopen" at rated current. Both switches are used for source transfer function which is only one time in operation during about from 34.7 to 55.5 years. This fact causes waste, and more particularly, the redundancy of a switch increases the size of the ring main unit. Currently, few land is available for building more substations. There is one type of substation occuppying least space compared to the existing and widely used substations which is called as "one-pole substation". Ring main unit of one-pole substation is usually allocated in independent place, e.g: in public places such as sidewalks and transformer of this type of substation is mounted on a pillar. With that layout solution, ring main unit can be accessed from at least two different sides to assemble the cable terminals.

Solution of the one-pole substation has been granted authority certificate in Vietnam.

Most of the existing types of known ring main units are 1,021 mm (Safe Ring - ABB) to 1,130 mm (RM6 - Schneider) wide. Particularly, the ring main unit 8DJ10 of Siemens is 710 mm wide, known as the smallest among the existing types. However, its size still affects travelling if being installed on the sidewalk.

The size of RMU depends on the number and layout of the switches inside. All types of three feeder RMUs have three switches, most of which are arranged horizontally, and there are three cable compartment corresponding with three switches. All three cable compartment are arranged on the front of RMU, they can be accessed to assemble cable terminals only from front side of the RMU.

The existence of two switches used for source transfer function and the allocation of three switches in horizontal orientation as mentioned above are the cause for increased size of the existing ring main units.

Therefore, it is necessary to reduce the size of the ring main unit by changing the solution for source transfer function, particularly by removing a redundant switch as mentioned above and re-arranging switches and cable compartment to be consistent with the independently allocated ring main unit.

Summary of the invention

It is an object of the invention to provide an three feeder ring main unit, in which, the number of switches is less than known three feeder ring main unit. It is another object of the invention to provide an three feeder ring main unit which is smaller than known three feeder ring main unit.

It is another object of the invention to provide an three feeder ring main unit having the structure in accordance with the conditions that it be located independently or almost independently.

To achieve the the above objects and other object, according to one embodiment of the invention there is provided a three feeder ring main unit having enough five functions same as known three feeder ring main unit, but it has only one transfer-switch instead of two switches, which are used in the known ring main unit for the transfer source. The above ring main unit having reverse switch is called the " transfer-switch ring main unit".

The transfer-switch ring main unit has a sealed shell containing insulated gas with three terminals, two inputs for connection to both sides "a" and "b" of the power source and one output for connection to the branch. In sealed shell there are two switches, the first switch having three position es "close" / "open" / "earth" is connected directly between the two inputs terminals, and the second switch is transfer-switch that having three position es: " close a source" / "open" / "closed b source " is connected directly between one output terminal and the two inputs terminals.

The transfer-switch ring main unit also includes the breaker or the fuse for protection, which is connected between the moving contact of the transfer-switch and the output terminal, and includes two earthing switch, in which: the first earthing switch is arranged in electrical circuit which is connected directly between the fixed contact of said first switch and one of the two intput terminals, the second earthing switch is arranged in electrical circuit which is connected directly to the output terminal.

The transfer-switch ring main unit is used for all five functions wich is equivalent to the known ring main unit, but only two switches, mean that: reducing one switch in comparison with known ring main unit, thereby reducing the size and simpler structure. According to other embodiment of the invention there is provided a novel ring main unit is the same functions as known ring main unit without the transfer source, hence, it is called "no-transfer ring main unit".

The no-transfer ring main unit is same as the transfer-switch ring main unit, except that: the second switch is a load-break switch having three position es "close" / "open" / "earth", and it is connected between one output terminal and one of the two inputs terminals.

Thus, for the no-transfer ring main unit, the output terminal is connected to one of the two input terminals when the second switch is closed, and connected to remain input terminal when both the first and the second switches are closed. The no-transfer ring main unit is the most simple device among ring main unit, that is proposed in this invention.

According to further embodiment of the invention there is a "transfer-breaker ring main unit", in wich: the second switch is a switch-breaker. Beside the two mentioned switches, the transfer breaker ring main unit do not use more circuit breaker or fuse to protect the branch, thus, number of switches and protection unit required is least among all types of ring main units proposed in this invention

This invention also proposes a transfer ring main unit is only using to connect a loop and other branch without protection. This device is the same as transfer ring main unit mentioned above, but exclude the protection.

More particularly, this invention also proposes a ring main unit, inside wich: all switches and circuit breakers or fuses are so arranged verticaly, that its width is equal to the width needed for fitting one terminal of the conventional ring main unit, mean that: reduced by about three times compared with the conventional ring main unit.

It should be noted that the outputs and the corresponding cable compartments are arranged in various sides, in accordance with conditions under which ring main unit are arranged independently or almost independently. Thus, the ring main unit proposed in this invention meets all the mentioned purposes.

Brief description of the drawings

The purposes, characteristics, benefits, aspects mentioned above and others of the invention will become easily understandable for the experts in this sector if looking at the detailed description based on the attached drawings below:

FIG. 1 Shows the known ring main unit equiped with a circuit breaker.

FIG. 2 Shows the known ring main unit equiped with a fuse.

FIG. 3 Shows the " transfer-switch ring main unit " equiped with a circuit breaker.

FIG. 4 Shows the " transfer-switch ring main unit " equiped with a fuse.

FIG. 4' Shows the same device in FIG 4 but different kind of switch.

FIG. 5 Shows the "no-transfer source ring main unit".

FIG. 6 Shows the "transfer-breaker ring main unit".

FIG. 7 Shows the layout of ring main unit in a loop.

FIG. 8 Shows the " transfer-switch ring main unit " without protection.

Detailed description of the preferred embodiments

In this invention, the drawing of the medium- voltage three-phase equipment is expressed in the form of one phase, the same parts of the remaining two phases are arranged in parallel with the phase described in the drawings.

FIG. 1 and FIG. 2 show two types of the known three feeders ring main unit, their components having the same function will have the same symbols and the same description. The known ring main unit include at least: a sealed shell (100) containing gas insulated, on which there are two inputs terminals (101 and 102) connecting to the two sides "a" and "b"of the power source, and one output terminal (103) connecting to the branch; two switches (111, 112) are two load-break switches having three states "close" / "open" / "earth" in serial connection with each other and with the two input terminals (101) and (102); switch (113) having three states "close" / "open" / "earth" connect in serial with the circuit breaker (1 14 - FIG. 1) or the fuse (114a- FIG. 2) and with output terminal (103), the switch (1 13) may be a load-break switch or disconnector corresponding to the protector is a circuit breaker or a fuse; in FIG. 1, circuit breaker (114) driven by protection relays (115) combination with current transformers (116) located on the branch circuit.

The ring main unit described in FIG. 1 and FIG. 2 have enough five functions mentioned in the "background of the invention". All the switches, the breaker and the earthing switch that shown in this drawing and the next drawings in this invention, are shown at the "open" position.

General characteristics of the two types of known ring main unit shown in FIG. 1 and FIG. 2 is:

• The first: There are three switches (111, 1 12 and 1 13) inside a sealed shell (100), in which: there are two switches (1 1 1) and (1 12) in serial connection on one electrical circuit connected between two input terminals (101) and (102).

• The second: All the three switches (11 1, 112 and 113) are arranged horizontally.

FIG. 3 Showing "transfer-switch ring main unit" equipped with a circuit breaker, is proposed in this invention. One sealed shell (200) is filled with insulating gas, including: two switches (21 1 and 212), the circuit breaker 213, and earthing switch 221.

Sealed shell (200) has three terminals (201, 202, 203). The two input terminals 201 and 202 are positioned at the lower part of the shell (200) and can be in the same direction: on the side, or in two different directions: on the front and back.

The output terminal 203 is arranged in a different position with two inputs 201 and 202, so that can be accessed from different direction to install the cable terminal, such as the top of the shell 200 and may be on same side of the two intput terminals 201 and 202, or on the other side with them.

The first switch 211 is a load breaker switch having three position "close" / "open" / "earth" and corresponding contacts: the moving contact and the fixed contact are one by one in direct connection with one of two inputs 201 and 202, and the earth contact is in direct connection with the shell.

The second switch 212 is a transfer- switch having three states: " close a source" / "open - earth" / "closed b source" and corresponding contacts: two fixed contacts and one moving contact, each of the two fixed contacts is one by one connected directly to either input terminals 201 and 202.

The circuit breaker 213 is connected electricaly between the moving contact of the transfer-switch 212 and the output terminal 203, the relay 215 is connected to current transformers 216 mounted on the branch, the function of relay 215 is to send the signal to interrupt the circuit breaker 213 when the current of the branch exceeds a predetermined current value. The earthing switch 221 is arranged on circuit connected to input 202.

The functionas of the ring main unit in the FIG. 3 are described as follows: i. Closing or opening two sides "a" and "b" of power source are done by switch 21 1. In working mode that the two sides "a" and "b" of power source are connected: the switch 21 1 is in "close" position. When the two sides of the source are required to be isolated: the switch 21 1 is moved from "close"" to "open" position and power source connected to the input terminal 201 is isolated from power source corresponding to input terminal 202. The switch 21 1 can be operated in the condition of rated load, the fixed contact of switch 21 1 is denoted as "." (Large dots) and "-" together in FIG. 3, wich is the symbol of the fixed contact of the load break switch. ii. Closing or opening the branch is done by transfer-switch 212 to interlock the circuit breaker 213 under the conditions: the transfer-switch 212 will operate only when the circuit breaker 213 is in "open" state. When the transfer-switch 212 is in state of "close a source" and circuit breaker 213 is in state of "closed": the branch is connected to one side "a" of the power source corresponding with input terminal 201. In this case, the transfer-switch 212 are locked and inactive. iii. The function of automatic interruption in case of overloading or transformer malfunctioning is done by the circuit breaker 213 and the relay 215 connected to the current transformer 216. "Close" status of circuit breaker 213 is attached to a compressed spring to accumulate energy (not shown). When an overloading or malfunctioning in the transformer happens, the current on the branch is reflected in current transformer 216 and exceeds a predetermined current value, so relay 215 will send the signal to release the energy of spring which will make the circuit breaker automatically moving from "close" to "open" status and then transformer branch is isolated from the power source. When problems have been solved and need to be re-energized, circuit breaker 213 is operated from "open" to "close" status. The fix contact of circuit breaker 213 is denoted by "x". iv. The function of grounding the terminals. To ground the input 201, load breack switch 211 is moved from "open" to "earth" position. Grounding input 202 is done by earth switch 221 : moving from "open" to "earth" position, the earth switch 221 can be interlocked under the condition that it can be operated only when the circuit breaker 211 is in "open" position. For grounding output 203, the following tasks will be performed in sequence: 1 - Operating the circuit breaker 213 from "close" to "open" position; 2 - Operating the transfer switch 212 from "close a source" or "close b source" state to "open-earth" position; 3 - Moving the circuit breaker 213 from "open" to "close" position as initial situation. When this process completes, output 203 is connected to ground. When required to move back to the energisation status, the circuit breaker 213 and the transfer switch 212 are operated in reverse order. v. The function of the source transfer. When needed to transfer power supply for the branch from the "a" to the "b" source, the following operation sequence will be followed: 1- Move the circuit breaker 213 from "close" to "open" position; 2 - Operate the transfer switch 212 from "close a source" to "open-earth" state and then to "close b source" state; 3 - Move the circuit breaker 213 from "open" to "close" position, branch will be then connected to the power source corresponding to input terminal 202 at the end of the operation and at the same time, the operation of transfer-switch 212 is locked. It will be easiest for the transfer- switch 212 in in no- load operation, if it is a disconnector. In FIG. 3, the fixed contact of transfer-switch

212 is denoted by which is a symbol the fixed contact of the disconnector.

Device on FIG. 3 has enough five functions as same as the above known ring main unit. The operation of the switches is done by the operator or by remotely controlled motor.

Characteristics of the device circuit shown in FIG. 3 are:

• Firstly, there are two switches inside a sealed shell 200 and only one switch 21 1 is directly connected between the two input terminals 201, 202 for connection to both sides "a" and "b" of power source.

• Secondly: all components including switches 211, 212 and circuit breaker

213 are arranged vertically.

Because only one switch 21 1 is connected between the two input terminals 201 and 202 for connection to both sides "a" and "b" of power source, the device shown in FIG. 3 has one switch less than the known ring main unit. More particularly, thanks to the vertical alignment of all the switches and protector, the device mentioned in this invention is about one third as wide as the conventional ring main unit.

FIG. 4 shows a transfer-switch ring main unit with fuse protection, that is proposed in this invention. The device shown in FIG. 4 is completely identical to the device shown in FIG. 3 in term of the circuit between the inputs 201 and 202. The difference is that the load transfer-switch 212 has three positons: "close a source" / "open" / "close b source" in combination with the fuse 213a connected to the output terminal 203, and the earthing switch 222 is arranged in the circuit connected directly to the output terminal 203. The operation to perform the functions of the device combined between load transfer- switch and fuse are as follows: i. The function to "close" / "open" two sides of the power source is done by switch 211 : same as described in FIG. 3. ii. The function to "close'V'open" a branch to either side of power source is done by transfer switch 212a. The transfer switch 212a can be operated in load condition. The fixed contract of the transfer-switch 212a is denoted by "." and "-" together, as the symbol of the load break switch. iii. The function of automatic interruption in case of overloading or fault is done by fuse 213a in combination with load transfer-switch 212a. The load transfer- switch 212a will automatically move from "close" to "open" position when at least one phase of the fuse 213a is interrupted by the current via the fuse exceeding its rated value. iv. The function of grounding the outputs: grounding the input terminals 202 and 201 hasbeen indicated at the FIG. 3. For grounding output terminal 203, the earth switch 222 is interconnected with transfer-switch 212a and only moves from "open" to "earth" position when transfer switch 212a is already in "open" position. v. The function of source tranfer is done by the transfer-switch 212a: in the state of "close a source" or "close b source" - the branch is connected to either side "a" or "b" of the power source corresponding with input terminal 201 or 202, and in "open" position- the branch is isolated from both sides of the power source.

FIG. 4' shows a device proposed by this inventionsimilar to the device described in FIG. 4, but with different types of switches. The switches 21 and 212a' in FIG. 4' replace switches 21 1 and 212a in FIG.4. Fuctions of these two types of switch are similar to each other but the motion of the blade is different. In FIG. 4', the rotation axis is located at midpoint of the blade, for example, Siemens 's switches. In FIG. 4, the rotation axis is located at one end of the blade, for example, ABB's switches. The moving contacts of the switches 21 Γ and 212a' include both contacts to be interconnected. The moving contacts of the switches 211 and 212a is one end of the blade. The fixed contacts of the switches 21 Γ and 212a' is denoted by "." and "-" used together as the^' symbol of the load break switch.

So, various types of "transfer-switch ring main unit" (shown in FIG. 3, FIG. 4 and FIG. 4') have the same functions as the known ring main unit (shown in the FIG. 1 and FIG. 2), but there are only two switches inside, meaning that one switch less than the known types.

A device proposed in this invention and called as "no- transfer source ring main unit", is shown on FIG. 5.

Components of the no-transfer source ring main unit are similar to the components of the device shown in FIG. 3: the sealed shell 200; two inputs 201 and 202 to be connected to the power source and an output 203 to be connected to the branch; the switch 21 1 with three positions: "close" / "open" / "earth"; an earth switch 221.

Those components of the no-transfer source ring main unit different from the components of the device shown in FIG. 3 are: the switch 212b which is the load break switch with three positions "close" / "open" / "earth" and has the moving contact, the fixed contact and the earth contact, in which the fixed contact is connected to the moving contact of the switch 211 and connected directly to the input terminal 201, the fuse 213a is connected between the moving contact of switch 212b and the output terminal 203, the switch 212b in coordination with fuse 213a to automatically switch from "close" to "open" position when at least one phase of fuse is interrupted. Fuse tubes are arranged outside the sealed shell 200 for easy replacement, earth switch 222 is arranged in circuit connected directly to the output terminal 203 and is interlock with the switch 212b.

The operation to perform the following functions: i. "Closing" / "opening" two sides of the power source is done by switch 21 1 : same as described in FIG. 3. ii. "closing" / "opening" the transformer branch is done by the switch 212b: in "close" position, the transformer branch is connected to one side of the power source that is connected to input terminal 201 and connected to the input 202 if the switch 21 1 is in "close" position. When the load switches 212b is in "open" position, the branch is isolated from both side of the power source. iii. Function to automatically switch in case of overload or fault at transformers is done by 213a fuse in coordination with the switch 212b. The switch 212b will automatically move from "close" to "open" position when at least one phase of the fuse 213a is interrupted. iv. The function of earth: the earthing of input terminals 201 and 202 are same as indicated in FIG. 3. The earthing of output terminal 203 is done by moving the switch 212b from "open" to "earth" position, the earth switch 222 is interconnected with the switch 212b and it will move from "open" to "earth" position in concurrence with the switch 212b.

The branch corresponding to output terminal 203 is connected to only one side of the power source corresponding to input terminal 201. The output terminal 203 will be connected to the input terminal 202 when the switch 21 1 is "closed". As analyzed in the section "Technical Status of the invention", the source transfer function is rarely used and replaced by changing or removing the cable terminal. Thus, the device proposed in FIG. 5 is a new type of ring main unit and is the most simple device proposed in this invention.

There is the device called "transfer-breaker ring main unit" proposed in this invention and shown in FIG. 6 where the circuit between two input terminals 201 and 202 is same as the type proposed.

A switch of this device is transfer-breaker 212c, two fixed contacts of which are connected directly to the two input terminals 201 and 202, and one moving contact is connected directly to the output 203.

The fixed contact of transfer-breaker 212c capable of arc extinguishment is able to "close" / "open" at the short-circuit current and denoted by "x", similar to the symbol of circuit breaker inside the known ring main unit such as RM6 Schneider Electric. The transfer-breaker 212c is controlled by relay 215 combined with current transformer 216. The earth switch 222 is equiped with interlock with the transfer- breaker 212c that only operates when the transfer-breaker 212c is in "open" position. Therefore, the transfer-breaker ring main unit does not require additional protectors such as the circuit breaker 213 or fuse 213a to be arranged in the equipment mentioned above.

The "transfer-breaker ring main unit" have the same functions like the known ring main unit, but least switches and protectors are required compared to other types of device proposed by this invention.

It should be noted that the switches, fuses and earthing switch described in this invention are similar to or is the structure being used to manufacture the types of conventional ring main units and because they have been very popular to to the experts in the sector of electrical equipment, so more detailed description on these parts is not necessary.

FIG. 7 shows a diagram of the electrical medium voltage loop including transformers denoted from 401 to 408 and the cable sections denoted from 301 to 308 are connected to each other through the ring main units with the sealed shell denoted by 100 and 200. Two transformers 402 and 406 are connected to two different types of ring main units that proposed in this invention.

For easy understanding, the ring main unit is shown in the most simple way including switches 111, 1 12 and 1 13 are arranged inside a sealed shell 100 - for the known devices; a switch 211 and a transfer-switch 212 are arranged in a sealed shell 200 - for the "transfer-switch ring main unit" of this invention; and the switch 21 1 and 212b are arranged in a sealed shell 200 - for the "no-transfer source ring main unit" of this invention. Each ring main unit is connected to the two sides "a" and "b" of the power source and to the branch 1 to energize the transformer.

In FIG. 7, one can see clearly the difference in the structure of the device in this invention from the conventional ring main unit: Only one switch is connected between the two input terminals "a" and "b" of the device according to this invention while there are two switches between the two inputs of the conventional device.

In the mode of closed loop, the switches 111, 112 and 212 of all the devices are in "close" position, the loop is energized by two busbars a and b. In operation mode of open loop, a switch at any point of the loop is open. Assuming that the switch in "open" position is located outside the positions in FIG. 7, so all the switches 111, 112 and 212 in FIG. 7 are in "close" position. The transformers 401 ÷ 404 are energized from the "a" busbar and the transformers 405 ÷ 408 are energized from the "b" busbar.

Under normal conditions, there is no difference in working ability between the device proposed in this invention and the conventional device.

Let's compare a transfer-switch ring main unit proposed in this invention to a conventional ring main unit in case of fault at cable section 303. When the cable section 303 is faulty, both switches 211 and 11 1 at both ends of cable section 303 need to move to "open" position and then to "earth" position (not shown) to isolate the faulty section from cable network. At unit of the loop where the switch was "open" in the open loop mode, the switch will be moved to "close" position. The loop is interrupted at the cable section 303.

The transfer-switch 212 moves to "close a source" position to supply power to transformer 402 from "a" busbar through cable 302. At the location of transformer 403, the switches 112 and 1 13 are in "close" position to supply power to transformers 403 from "b" busbar via cable section 304. When the isolation of transformer 402 and 403 is required, the transfer-switch 212 and switch 1 13 are moved from "close" to "open" position. The ability to connect from the two sides of the power supply "a" and "b" with the branch 1 to energize two transformers 402 and 403 is the same. Thus, the functions of the two above mentioned ring main units are completely equivalent to each other.

In cases a fault occurrs at cable section 305, let's compare the no-transfer source ring main unit proposed by this invention including a sealed shell 200 and the switches 211 and 212b to a conventional ring main unit. When the cable section 305 is faulty, both switches 21 1 and 1 1 1 at both ends of cable section 303 need to move to "open", then "earth" (not shown) position to isolate fault section from cable network. At unit of the loop where the switchs is "open" in the open loop mode, the switch will be moved to "close" position and the loop is interrupted at the cable section 305.

The transformer 405 is powered from "a" side of power source by the switches 1 12 and 1 13 in "close" position, while the transformer 406 is not energized because the switch 21 1 in "open" position to insulate section cable 305 can not be connected to the cable section 306.

The power is supplied to the transformer 406 as follows: removing the cable terminal at "b" terminal of cable section 305, then moving the switch 21 1 to "close" position, the transformer 406 is powered from the busbar "b" through section cable 306. The maximum time required for this operation is 15 minutes.

Thus, using the no-transfer source ring main unit does not affect the power supply of the transformer in case the power cable is faulty and one side of power source needs to be isolated for repair.

FIG. 8 shows the "transfer-switch ring main unit" to create the branch for other circuits and require no protection function. Components of this device is same as the one mentioned in FIG. 4 but without protector. So no more description is needed.

In case all three cable compartments are aligned horizontally on the front side, the ring main unit is as wide as the total width of three cable compartments. This solution is necessary under the condition that the device is accessible only from the front for the cable assembly. The conventional ring main unit is designed for this solution.

In FIGS.3 ÷ 6, all switches and protector are aligned vertically, ie: the width and the depth of the sealed shell 200 is the size required to accommodate a switch or a protecter in the gas insulated environment. The two dimensions mentioned above do not depend on the number of switches and protecters inside the sealed shell 200, and one of the two dimensions mentioned above should be equal to the width of one cable copartment of the conventional ring main unit, meaning that: the width of the embodiment according to this invention is only 1/3 of the known ring main unit.

The above solution to align switches and protecter vertically is not restricted by the layout of the cable chamber because the input and output terminals are located in different side or different parts of the sealed shell, as shown in the drawings of this invention.

Although the invention was described in detail by the preferred embodiments based on the accompanying drawings, but it should be noted that the changes and other modifications are obvious to experts in the field this technique. Any changes and modifications thereupon must is understood that within the scope of the invention points determined by the confirmation requirements below.

Claims

1. An electrical three-phase medium voltage ring main unit (RMU) having switches; protector and earthing switch integrated inside a sealed shell containing insulating gas, said sealed shell has three terminals, of which: two input terminals for electrical-connection to both sides "a" and "b" of power source and one output terminal for electrical-connection to one branch, characterized in that: inside the said sealed shell, there is one (first) switch electrical-connected directly between two said input terminals.

2. The ring main unit as claimed in claim 1, characterized in that: inside the said sealed shell, there is the second switch electrical-connected between the said output terminal and at least one of the two said input terminals.

3. The ring main unit as claimed in claim 2, characterized in that: there is the earth switch placed in the circuit connected directly to the fixed contact of the first switch.

4. The ring main unit as claimed in claim 2 or 3, characterized in that: the second switch is the three positions transfer-switch: "closed a source" / "open" / "closed b source" electrical-connected between one said output terminal and twoo said output terminals.

5. The ring main unit as claimed in claim 4, characterized in that: the "open" position of said transfer-switch is associated with the contact that connected to a sealed shell.

6. The ring main unit as claimed in claim 2 or 3, characterized in that: the second switch is a load-break switch having three positions "close"/ "open"/ "earth", said load-break switch is connected between one input terminal and the output terminal.

7. The ring main unit as claimed in any said claim, characterized in that: at least, one of the twoo said switches capable of breaking and making at the short-circuit currents.

8. The ring main unit as claimed in any claims 2 to 6, characterized in that: the second switch is coordinated with a fuse or circuit breaker.

9. The ring main unit as claimed in claim 8, characterized in that: an earth switch is placed at the elctrical circuit connected directly to the said output terminal.

10. The ring main unit as claimed in any said claim, characterized in that: all the said switches and fuses or circuit breakers are arranged vertically.