Surge arrester and gas-insulated electric apparatus

A surge arrester according to an embodiment of the present invention includes a switching unit connected to a gas-insulated electric equipment in which insulating gas is sealed, and switching a limited voltage of the surge arrester into a limited voltage smaller than a low-temperature critical voltage indicating a withstand voltage generating a dielectric breakdown when the insulating gas is liquefied.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-136781, filed on Jun. 20, 2011; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a surge arrester and a gas-insulated electric apparatus.

BACKGROUND

Conventionally, gas-insulated electric equipment such as a gas-insulated switchgear (GIS), a gas-insulated bus (GIB), and so on in which insulating gas is sealed are provided at an electric power system. In the gas-insulated electric equipment, a center conductor is stored in a grounded metal tank in which the insulating gas is sealed, and insulation between the metal tank and the center conductor is maintained.

However, in case of a gas-insulated electric equipment using SF6gas as the insulating gas, liquefaction of the sealed SF6gas occurs and internal sealed gas pressure is lowered under a low-temperature surrounding environment lower than −25° C., and therefore, a problem in which insulation performance deteriorates occurs. Accordingly, the gas-insulated electric equipment is placed indoor where it does not become low-temperature to avoid the liquefaction of the SF6gas when the gas-insulated electric equipment is applied under the low-temperature surrounding environment.

However, the gas-insulated electric equipment is necessary to be connected at outdoor to secure an insulation distance in the atmosphere when the gas-insulated electric equipment is necessary to be connected in the atmosphere to be connected to an overhead power line and a voltage transformer circuit. In this case, a part of the gas-insulated electric equipment leading to an air connection part is placed under the low-temperature surrounding environment.

In general, the sealed insulating gas is not liquefied when the conductor stored in the metal tank of the gas-insulated electric equipment is conducted because it is heated by heat generation of the conductor. On the other hand, a gas pressure of the gas-insulated electric equipment is lowered to approximately an atmospheric pressure because the liquefaction of the SF6gas sealed at a high-pressure exceeding the atmospheric pressure occurs in accordance with lowering of a surrounding temperature when it is not conducted. It is impossible to maintain the insulation ability for a high impulse voltage such as a thunderstorm when the pressure of the insulating gas is lowered to approximately the atmospheric pressure.

Accordingly, in order to prevent that the insulation performance deteriorates caused by the liquefaction of the insulating gas under the low-temperature surrounding environment, a measure such that the gas-insulated electric equipment is heated by an external electric heat source and so on is proposed.

However, it is necessary to be constantly conducted to heat and operational electric power is lost in the conventional method heating by the external electric power source. In addition, there is a possibility in which reliability of a whole of the equipment deteriorates because a problem in which the liquefaction of the insulating gas occurs cannot be solved at a trouble time of the external electric power source.

DETAILED DESCRIPTION

A surge arrester in an embodiment of the present invention includes a switching unit connected to a gas-insulated electric equipment in which insulating gas is sealed, and switching a limited voltage of the surge arrester into a limited voltage smaller than a low-temperature critical voltage indicating a withstand voltage generating a dielectric breakdown when the insulating gas is liquefied.

The surge arrester and a gas-insulated electric apparatus according to an embodiment of the present invention are described with reference to the drawings.

First Embodiment

A gas-insulated electric apparatus according to a first embodiment is described by usingFIG. 1. Here, the gas-insulated electric apparatus includes a gas-insulated electric equipment and a zinc oxide surge arrester, and the gas-insulated electric equipment is a gas-insulated bus1in which SF6gas is sealed.

FIG. 1illustrates a configuration chart of the gas-insulated bus1connected to an overhead power line3and a surge arrester2. Here, the gas-insulated bus1is connected to the overhead power line3via a connection line4. The zinc oxide surge arrester2is connected to the connection line4via a connection line5.

Next, a configuration of the zinc oxide surge arrester2is described by usingFIG. 2. The zinc oxide surge arrester2includes a first zinc oxide element (high-temperature surge arresting unit)201, a second zinc oxide element (low-temperature surge arresting unit)202, a disconnecting switch203and a control unit204(switching unit).

The first and second zinc oxide elements201,202are both made up by zinc oxide and have predetermined limited voltages. The first and second zinc oxide elements201,202start discharging when an impulse voltage higher than the limited voltage such as a thunderstorm is applied. Accordingly, a voltage at the limited voltage or more is not applied to the gas-insulated bus1. As illustrated inFIG. 2, the first and second zinc oxide elements201,202are disposed in parallel with each other.

The first zinc oxide element201is connected to the gas-insulated bus1via plural connection lines4,5(refer toFIG. 1). Besides, the second zinc oxide element202, which is provided side by side with the first zinc oxide element201, is connected to the gas-insulated bus1via the disconnecting switch203and the plural connection lines4,5. Besides, a limited voltage V1of the first zinc oxide element201and a limited voltage V2of the second zinc oxide element202are different, and the limited voltages V1, V2are respectively determined by expressions (1), (2).

In the expressions (1), (2), a reference symbol Va represents a voltage value of a commercial frequency system applied to the gas-insulated bus1, the overhead power line3, the connection line4, and so on. A reference symbol Vb represents a critical voltage in which insulation can be maintained at a normal time when the SF6gas filled in the gas-insulated bus1is not liquefied (hereinafter, the Vb is called as a high-temperature critical voltage). A reference symbol Vc represents a critical voltage in which the insulation can be maintained when the SF6gas filled in the gas-insulated bus1is liquefied caused by the low-temperature surrounding environment. Therefore, the Vc is normally a value smaller than the Vb (hereinafter, the Vc is called as a low-temperature critical voltage).

For example, when the commercial frequency system voltage value Va is at 500 kV, a gas pressure of the SF6gas is maintained at a high-pressure of approximately 0.4 MPa because the SF6gas sealed in the gas-insulated bus1is not liquefied when the center conductor stored in the gas-insulated bus1is conducted. Accordingly, the critical voltage capable of maintaining the insulation in the gas-insulated bus1at the gas pressure of 0.4 MPa is set to be the high-temperature critical voltage Vb.

Further, when the electricity to the center conductor is stopped and the SF6gas is liquefied under the surrounding environment of −50° C., the gas pressure is lowered to a low-pressure of approximately 0.1 MPa. Accordingly, the critical voltage capable of maintaining the insulation in the gas-insulated bus1at the gas pressure of 0.1 MPa is set to be the low-temperature critical voltage Vc.
Va<V1<Vb  (1)
Va<V2<Vc  (2)

The disconnecting switch203is connected to the connection line5. It is constantly in an open state, but it is closed and becomes a closed state when a close instruction is input by the control unit204.

The control unit204outputs the close instruction to the disconnecting switch203when it detects that the gas pressure in the gas-insulated bus1becomes smaller than a threshold value set in advance.

Next, operations when the high impulse voltage of 1000 kV is applied caused by a thunderbolt and so on to the overhead power line3are described when the limited voltage V1of the first zinc oxide element201is set to be 1400 kV, and the limited voltage V2of the second zinc oxide element202is set to be 600 kV. Here, it is described while dividing into a case when the SF6gas sealed in the gas-insulated bus1is liquefied and a case when the SF6gas is not liquefied.

When the SF6gas is not liquefied, the disconnecting switch203is in the open state, and therefore, the first zinc oxide element201is electrically connected to the gas-insulated bus1, and the second zinc oxide element202is not in a state electrically connected to the gas-insulated bus1. Accordingly, when the thunderbolt falls on the overhead power line3and the high impulse voltage of 1000 kV is applied to the gas-insulated bus1via the connection line4, the discharge does not occur at the first zinc oxide element201because the limited voltage V1of the first zinc oxide element201is 1400 kV. Accordingly, the high impulse voltage of 1000 kV is applied to the gas-insulated bus1, but the high-temperature critical voltage Vb is larger than 1400 kV being the limited voltage V1of the first zinc oxide element201, and therefore, a dielectric breakdown does not occur in the gas-insulated bus1.

When the SF6gas is liquefied and the gas pressure in the gas-insulated bus1becomes smaller than the threshold value set in advance, the control unit204outputs the close instruction to the disconnecting switch203to make the disconnecting switch203at the closed state. The second zinc oxide element202is thereby electrically connected to the gas-insulated bus1. Therefore, when the thunderbolt falls on the overhead power line3and the high impulse voltage of 1000 kV is applied to the second zinc oxide element202via the connection line4, the discharge occurs at the second zinc oxide element202because the limited voltage V2of the second zinc oxide element202is 600 kV. Accordingly, the dielectric breakdown does not occur in the gas-insulated bus1because the high impulse voltage is not applied to the gas-insulated bus1.

As stated above, according to the present embodiment, the limited voltage V2of the second zinc oxide element202is smaller than the low-temperature critical voltage Vc, and the second zinc oxide element202is connected to the gas-insulated bus1via the disconnecting switch203. When the SF6gas is liquefied by the low-temperature surrounding environment, the control unit204turns the disconnecting switch203electrically connected to the gas-insulated bus1from the open state (off state) to the closed state (on state) to electrically connect the gas-insulated bus1and the second zinc oxide element202. Thereby, the control unit204switches the limited voltage of the zinc oxide surge arrester2from the limited voltage V1higher than the low-temperature critical voltage Vc to the limited voltage V2smaller than the low-temperature critical voltage Vc. Accordingly, the dielectric breakdown does not occur in the gas-insulated bus1even when the SF6gas is liquefied caused by the low-temperature surrounding environment.

Besides, the external electric power source is not used, and therefore, operational electric power loss does not occur, and reliability of the gas-insulated bus1can be improved.

Note that the control unit204detects that the gas pressure in the gas-insulated bus1becomes smaller than a threshold voltage set in advance and outputs the close instruction to the disconnecting switch203in this embodiment, but it is not limited thereto. The control unit204may detect that the electricity is stopped by a current transformer and so on provided at the gas-insulated bus1, and outputs the close instruction. Namely, it may be constituted such that the above-stated switching operation is performed when a flowing current value becomes lower than a threshold value set in advance while the conductor stored in the gas-insulated electric equipment is conducted.

Besides, the control unit204may be provided at the gas-insulated bus1, or may be substituted by a supervisory control device, a protection and control device, a PC and so on provided at a distance, and it is possible to output the close instruction by operating the control unit204by a user. Further, the disconnecting switch203and the control unit204may be connected via a network, and it is possible to control the open and close states of the disconnecting switch203from remote location by using the network.

Second Embodiment

A gas-insulated electric equipment of a second embodiment is described by usingFIG. 3. Here, the gas-insulated electric equipment is a gas-insulated bus in which the SF6gas is sealed, andFIG. 3illustrates a configuration chart of the gas-insulated bus connected to a gas-insulated transformer.

Different points of the present embodiment from the first embodiment are that the overhead power line3is substituted by a gas-insulated transformer6, and the first zinc oxide element201is provided at the gas-insulated transformer6as illustrated inFIG. 3. The same reference symbols are used to designate the same elements as the first embodiment, and descriptions thereof are not given.

The first zinc oxide element201provided at the gas-insulated transformer6is electrically connected to the connection line4via a connection line7.

Operations of the present embodiment are similar to the first embodiment, and therefore, detailed descriptions are not given. In the present embodiment, when the control unit204judges that the gas pressure of the SF6gas sealed in the gas-insulated bus1becomes smaller than the threshold value set in advance, the control unit204closes the disconnecting switch203and makes it at the closed state, and thereby, the second zinc oxide element202of which limited voltage V2is low is electrically connected to the gas-insulated transformer6as same as the first embodiment. Namely, the zinc oxide surge arrester2is switched from the limited voltage V1of the first zinc oxide element201which is higher than the low-temperature critical voltage Vc to the limited voltage V2of the second zinc oxide element202which is smaller than the low-temperature critical voltage Vc. Accordingly, the high impulse voltage is not applied to the gas-insulated bus1even under the low-temperature surrounding environment in which the SF6gas is liquefied, and therefore, the dielectric breakdown does not occur in the gas-insulated bus1.

According to the present embodiment, it is possible to obtain the similar effect as the first embodiment also as for the gas-insulated bus1connected to the gas-insulated transformer6.

Third Embodiment

A gas-insulated electric equipment according to a third embodiment is described by usingFIG. 4. Here, the gas-insulated electric equipment is a gas-insulated bus in which the SF6gas is sealed, andFIG. 4illustrates a configuration chart of the gas-insulated bus connected to an overhead power line.

As illustrated inFIG. 4, the gas-insulated bus1is electrically connected to the overhead power line3via the connection line4. The zinc oxide surge arrester2is electrically connected to the connection line4via the connection line5. The present embodiment and the first embodiment are different in a configuration of the zinc oxide surge arrester2.

The configuration of the zinc oxide surge arrester2of the present embodiment is described by usingFIG. 5. The zinc oxide surge arrester2includes the zinc oxide element201, the control unit204, a conducting terminal205, and a ground system206.

The zinc oxide element201is divided into two sections of an upper section2011and a lower section2012. The conducting terminal205is provided between the two sections2011,2012.

The control unit204electrically connects the grounded ground system206to the conducting terminal205when the control unit204detects that the gas pressure in the gas-insulated bus1becomes smaller than the threshold value set in advance. The ground system206and the conducting terminal205are electrically connected, and thereby, the lower section2012of the zinc oxide element201is practically ignored. Accordingly, the limited voltage V2when the ground system206and the conducting terminal205are electrically connected becomes lower than the limited voltage V1when they are not electrically connected in the zinc oxide element201.

Here, the limited voltage V1under a state in which the ground system206is not connected to the conducting terminal205and the limited voltage V2under a connected state are respectively adjusted to be within ranges represented by the expressions (1), (2) in the first embodiment, in the zinc oxide element201. Specifically, lengths of the upper section2011and the lower section2012are adjusted to be within the above-stated ranges in the zinc oxide element201.

Next, operations when the high impulse voltage of 1000 kV is applied caused by the thunderbolt and so on to the overhead power line3are described when the limited voltage V1before connection is set to be 1400 kV, and the limited voltage V2after connection is set to be 600 kV in the zinc oxide element201. Here, it is described while dividing into a case when the SF6gas sealed in the gas-insulated bus1is liquefied and a case when it is not liquefied.

When the SF6gas is not liquefied, the conducting terminal205and the ground system206are not electrically connected with each other, and therefore, the limited voltage of the zinc oxide element201is 1400 kV (V1). So, the discharge does not occur at the zinc oxide element201even if the high impulse voltage of 1000 kV is applied to the zinc oxide element201caused by the thunderbolt to the overhead power line3. Accordingly, the high impulse voltage of 1000 kV is applied to the gas-insulated bus1via the connection line4, but the dielectric breakdown does not occur in the gas-insulated bus1because the high-temperature critical voltage Vb is larger than 1400 kV.

When the SF6gas is liquefied and the control unit204judges that the gas pressure of the SF6gas in the gas-insulated bus1becomes smaller than the threshold value set in advance, the control unit204electrically connects the conducting terminal205and the ground system206. So, the limited voltage of the zinc oxide element201decreases from 1400 kV (V1) to 600 kV (V2). If the high impulse voltage of 1000 kV is applied to the zinc oxide element201via the connection line4caused by the thunderbolt to the overhead power line3, the discharge occurs at the zinc oxide element201. Accordingly, the high impulse voltage is not applied to the gas-insulated bus1, and therefore, the dielectric breakdown does not occur in the gas-insulated bus1.

According to the present embodiment, it is not necessary to provide two zinc oxide elements, and therefore, materials and costs can be reduced in addition to the effect of the first embodiment.

Note that in the above-stated first to third embodiments, it is constituted by using the zinc oxide surge arrester using the zinc oxide element as the surge arrester, but the similar effect can be obtained by using a gap-type surge arrester.

According to the embodiments of the present invention, it becomes possible to provide a surge arrester and a gas-insulated electric apparatus suppressing occurrence of dielectric breakdown without using an external electric heat source even when they are provided under a low-temperature surrounding environment.