WO2008040128A1

WO2008040128A1 - Method and device for suppressing vacuum switch restriking over-voltage

Info

Publication number: WO2008040128A1
Application number: PCT/CH2006/000542
Authority: WO
Inventors: Weidong Liu; Huigang Sun
Original assignee: Abb Research Ltd
Priority date: 2006-10-04
Filing date: 2006-10-04
Publication date: 2008-04-10

Abstract

What provided is a device to be connected in a circuit on both side conductors of a vacuum switch to suppress the restriking over-voltage occurring across the vacuum switch while interrupting an inductive load from the circuit, comprising: a conductor to be connected to the vacuum switch; a magnetic ring string (26) provided around the conductor, such that the ring string is coaxial with the conductor; and a resistor (27) coupled to the conductor, such that two ends of the resistor are connected to the conductor, and the ring string is located between the two connecting points. The particular advantage of the vacuum restriking over-voltage suppression approach proposed in the present invention is that it can limit the over-voltage both in steepness and amplitude, and it is simple in structure and reliable in its own insulation.

Description

Method and Device for Suppressing Vacuum Switch Restriking Over-voltage

Field of the Invention

The present invention relates to a device and a method for suppressing vacuum switch restriking over-voltage. More particularly, the present invention relates to a device and a method for suppressing restriking over-voltage generated when closing or interrupting a circuit by means of a vacuum switch in an electric power system, especially with voltage levels of 35kV and below.

Background of the Invention

Vacuum switch is widely used to close and interrupt a circuit in power systems, especially in power systems with voltage levels of 35kV and below.

When using a vacuum switch to close or interrupt a circuit, an electric transient can be excited in the circuit, stressing the insulation of the circuit a voltage higher than the rated operation voltage. The transient voltage generated due to the vacuum switch operation is called "vacuum switching over-voltage". Serious switching over-voltage may lead to insulation failure in power equipments.

The vacuum switching over-voltage has different forms in different circuit conditions. One kind of such a vacuum switching over-voltage is vacuum restriking over-voltage. When using a vacuum switch to disconnect an inductive load from the power system, there may appear multiple restriking across the vacuum gap of the switch. The generation mechanism of the multiple restriking will be described with reference to the circuit shown in Figure 1.

Figure 1 is a single-phase equivalent circuit showing the principle of restriking over-voltage while disconnecting an inductive load from a power system with a vacuum switch. The circuit comprises a power frequency power source 11 , a vacuum switch 12, an equivalent load inductance 13, an equivalent load capacitance 14, and a feed line 15 for coupling the load to the vacuum switch. The load is disconnected from the power source when the contacts of the vacuum switch are mechanically separated from each other.

At the instant the vacuum switch contacts are mechanically separated, there will be a vacuum arc striking across the vacuum gap. The arc sustains till the power frequency current is attenuated to zero, and then the circuit is electrically interrupted.

After electric interruption of the circuit, the power source 11 remains the power frequency alternating voltage, while the load side starts a free oscillation in the characteristic frequency dominated by the load inductance 13 and the load capacitance 14. This makes a voltage difference across the vacuum gap 12 called Recovery Voltage.

Those skilled in the art should understand that the mechanical separation of the switch contacts occurs at a random instant in a cycle of the power frequency alternating current and/or voltage, and it takes time for the contacts to be fully separated. Therefore, there are cases when the recovery voltage appears across the gap while the contact gap is not sufficiently separated to withstand the voltage, where breakdown of the vacuum gap will occur. The breakdown of the vacuum gap due to recovery voltage is known as restriking.

The restriking results in a steep traveling wave propagating throughout the circuit and thus leads to a high-frequency transient. The characteristic frequency of the transient depends on the distribution parameters of the circuit, mainly on the feed line length and the distribution capacitance of the load. This high frequency transient produces a series of high frequency zeros of the vacuum arc current at which arc extinction could happen again.

Thus, an alternate process could be started: arc extinction at a high frequency current zero; recovery voltage; restriking; high frequency transient; arc extinction at a high frequency vacuum arc current zero; and so on. This is known as multiple restriking.

Figure 2 is computer simulation of a multiple restriking process (involving 22 times of restriking) showing waveforms of the load voltage and the vacuum switch current.

When restriking occurs, a steep transient voltage can also be produced across the insulations of the circuit, which is known as restriking over-voltage. The higher the breakdown voltage of the vacuum gap is, the higher the amplitude of the restriking over-voltage. One of the important features of the restriking over-voltage is its high variation steepness. Steep over-voltage can make uneven voltage distribution across the insulation and make local over-stressing. When using vacuum switch to interrupt an inductive load such as a transformer, reactor, motor or an arc furnace, the steep restriking over-voltage can unevenly distribute along the windings, and over-stress the insulations between the coils.

Currently two approaches are known for suppressing the vacuum switching over-voltage, i.e. Metal Oxide Arrester (MOA) and RC Surge Absorber.

Metal Oxide Arrester is a nonlinear resistor, and is connected in parallel with the protected electric equipment to be interrupted. The Metal Oxide Arrester assumes a high impedance state to normal voltage and has no influence on the operation condition of the protected equipment. When over-voltage occurs, the Metal Oxide Arrester will change to a low impedance state and bypass most of the striking current and arrest the over-voltage thereby protecting the equipment. It can effectively limit the amplitude of the over-voltage. However, its effect on the over-voltage steepness is poor.

RC Surge Absorber, a resistor and a capacitor connected in series, is connected in parallel with the protected electric equipment to be interrupted. It can effectively suppress the switching over-voltage on both its amplitude and steepness. However, its application is not very popular presently because of its large size and oil insulation in capacitor.

Summary of the Invention

The present invention aims at solving the above mentioned problems, and provides an approach of suppression of the switching over-voltage occurred across the vacuum gap on both the amplitude and steepness while a vacuum switch is opened. The approach of the present invention is immune of the above mentioned disadvantages associated with the prior art approaches.

In one aspect of the present invention, there is provided a device for closing and opening a circuit including a first side and a second side, comprising a vacuum switch and at least one over-voltage suppressor connected in series with the vacuum switch, wherein the over-voltage suppressor comprises an inductor and a resistor connected in parallel with each other.

In an embodiment of the inventive device, the inductor is a string of conductive rings provided around a conductor in one or both of the first side and the second side of the circuit.

In a further embodiment of the inventive device, the rings are provided proximate to each other, and the resistor is formed as a case surrounding the rings. Of course, the rings can also be distributed along a feed line of the circuit.

The circuit employing the present invention can be a power frequency electric network, and the first side can be the public network, while the second side can be an inductive load including but not limited to transformer, reactor, motor or arc furnace. The first side and the second can also be two different parts of the public network respectively.

The material suitable for manufacturing the rings includes but is not limited to ferrite, amorphous cores, metallic magnetic powder core et al.

The dimensions of the ring string is to be determined based on certain conditions, including but not limited to the insulation, conduction, . and mechanical strength conditions of the vacuum switch and feed lines, as well as structural dimensions. On premise of satisfying these conditions, smaller inner radius and larger outer radius of the rings as well as longer strings will yield bigger inductance.

The resistance of the shunt resistor can be calculated and optimized on certain circuit conditions to match the functional requirements.

In another aspect of the present invention, there is provided a method for interrupting a circuit including a first side and a second side by use of a vacuum switch connected between the first side and the second side, comprising the steps of: leading the operation current through an over-voltage suppressor connected in series with the vacuum switch; and interrupting the circuit by separating the contacts of the vacuum switch.

In a preferred embodiment of the method of the present invention, said over-voltage suppresser comprises a magnetic ring string and a shunt resistor connected in parallel with each other, and the step of leading the operation current through an over-voltage suppressor includes leading the operation current through the magnetic ring string.

The method of the present invention can be widely used to protect equipments from restriking over-voltage.

The present invention finds wide applications in power systems especially with the voltage levels of 35kV and below.

Brief Description of the Drawings

Accompanying drawings are included to illustrate the principle and preferred embodiments of the present invention. Wherever possible, like reference numbers will be used to indicate same or similar features throughout the drawings, in which

Fig. 1 : A single-phase equivalent circuit of interrupting an inductive load with vacuum switch;

Fig. 2: Computer simulation of multiple restriking in inductive load interruption with vacuum switch;

Fig. 3: Using high frequency magnetic ring strings and shunt resistors to suppress vacuum switch restriking over-voltage in a single phase circuit;

Fig. 4: Using high frequency magnetic ring strings and shunt resistors to suppress vacuum switch restriking over-voltage in a three phase circuit;

Fig.5: A preferred configuration of an assembly of the magnetic ring string and the shunt resistor according to the present invention;

Fig. 6: Simulation test circuit of using high frequency magnetic ring string and shunt resistor to suppress vacuum switch restriking over-voltage;

Fig. 7: Magnetization curve of amorphous ring material FJ37 tested;

Fig. 8: Oscillographs of the test with no high frequency magnetic ring string and no shunt resistor;

Fig. 9: Oscillographs of the test with high frequency magnetic ring string but without shunt resistor;

Fig. 10: Oscillographs of the test with high frequency magnetic ring string and shunt resistor. Detailed Description of Implementation of the Invention

The present invention provides a simple, effective and reliable approach for suppression of vacuum switch restriking over-voltage. Now the approach and its principle will be described with reference to exemplary embodiments illustrated in the accompanying drawings which should be considered by no way as limitative.

Principle of suppressing vacuum switch restriking over-voltage

Providing high frequency magnetic ring strings on feed lines of a vacuum switch can increase the inductance (here after, magnetic ring string inductance) of the feed line. When traveling waves induced by restriking in a vacuum switch passes through the magnetic ring string, the steepness of the front edge of the wave will be reduced. The bigger the magnetic ring string inductance, the more the front edge will be reduced. On premise of satisfying the insulation, conduction, and mechanical strength conditions of the vacuum switch and feed lines, as well as structural dimensions, smaller inner radius and larger outer radius of the rings as well as longer strings will yield bigger inductance.

A shunt resistor paralleled with the ring string is to attenuate the high frequency transient produced by restriking. An extremely low resistance will bypass most transient current but with poor attenuation, while an extremely high resistance will bypass scarcely any transient current also with poor attenuation. For a specific circuit and ring string configuration, there exists an optimal resistor, which can be obtained through numerical calculation of which the principle and method is well known by those skilled in the art. The optimal resistor can also be obtained through tests.

Generally speaking, the dimension of the high frequency magnetic ring string should be determined according to the application condition. The shunt resistance depends on the equivalent parameters of the magnetic ring string.

Preferred embodiments

Figure 3 is a circuit arrangement showing a preferred embodiment of suppressing the vacuum switch restriking over-voltage while interrupting a single phase inductive load from a single phase power frequency source, in which 21 is the single-phase power frequency power source, 22 is the single-phase vacuum switch, 24 is the equivalent circuit capacitance including the load capacitance and feed line distribution capacitance, 23 is the equivalent load inductance, 25 is a feed line coupling the load to the power source, 26 is the high frequency magnetic ring string, and 27 is the shunt resistor.

In operation, when the vacuum switch 22 is opened to disconnect the load from the power source, the ring string 26 shows high resistance against the occurring restriking over-voltage and thus suppresses rising thereof. At the same time, part of the restriking current passes through the shunt resistor 27 due to said high resistance, and the amplitude thereof is attenuated by the resistor. As a result, the restriking over-voltage is suppressed. On the other hand, when the vacuum switch 22 is closed to connect the load to the power source, there also may occur high frequency transient in the circuit thereby producing over-voltage around the circuit, and the above configuration can accordingly suppress the over-voltage.

Figure 4 is a circuit arrangement showing another preferred embodiment of suppressing the vacuum switch restriking over-voltage while interrupting a three-phase inductive load from a three-phase power frequency source, in which "31 A", "31 B" and "31 C" are the three-phase power frequency power source, "32A", "32B" and "32C" the three-phase vacuum switch, "33A", "33B", "33C" the three-phase equivalent load inductance, and "34A", "34B" and "34C" the three-phase equivalent load capacitance, "35" feed line, "36" high frequency magnetic ring string, and "37" shunt resistor.

Similar to the operation of the single-phase system described above, the restriking over-voltage can be efficiently suppressed.

In practice, the material of the magnetic rings is an important factor for the effect of suppression. The magnetic ring material influences following aspects: (I) High frequency response characteristic. It determines whether the magnetic ring strings can make effective response to the steep traveling waves. (II) Magnetic saturation. Deep saturation of the magnetic ring leads to significantly decrease of the magnetic permeability and have to be avoided. (Ill) Magnetic permeability. On the premise of no deep saturation, magnetic materials with better magnetic permeability can achieve better effect of over-voltage steepness suppression. (IV) Power frequency loss. There should be no significant loss and temperature rising of the magnetic rings when the vacuum switch takes normal power frequency operation current.

High frequency magnetic materials currently available for the application include ferrite, amorphous cores, metallic magnetic powder core and et al.

Figure 5 shows a preferred construction of a high frequency magnetic ring string of the present invention. It consists of an internal conductor "41" which can be for example a conductor of the circuit connected to the vacuum switch, a high frequency magnetic ring string "42" and an external shunt resistor "43", which are of coaxial structure. The unit is connected in series with the vacuum switch. Normal power frequency operation current passes through the internal conductor "41", almost immune from the influence of the magnetic ring string "42" and the shunt resistor "43". In case of restriking of the vacuum switch, the internal conductor "41" shows a notable impedance against the steep traveling wave due to the high-frequency magnetic ring string "42". The steep traveling wave is shunted to the shunt resistor and is attenuated.

In the embodiments described above, magnetic ring strings are arranged on both side conductors of the vacuum switch. To simplify the solution, magnetic ring strings can also be arranged on only one side conductor of the vacuum switch, for example, the load side conductor. In this way, the restriking over-voltage can also be efficiently suppressed.

In the embodiments described above, the magnetic rings are arranged axially proximate to each other forming a ring string. In practice, the rings can also be distributed along conductors of the circuit, that is, the rings can be axially separated from each other, and the restriking over-voltage can also be efficiently suppressed.

Circuits employing the present invention can be of any voltage levels in principle, but the present invention is particularly advantageous for medium-voltage power systems with voltage level of 10-35kV. For a power system with voltage level of 35kV, a magnetic ring string with an inner diameter equal to or approximate with the feed line diameter, an outer diameter of 14mm larger than the inner diameter, and a total length of 760mm, together with a shunt resistor of 75 ohm can efficiently reduce the steepness of the rising over-voltage and attenuate the amplitude thereof. In practice, the outer diameter of the rings can be 5-100mm larger than the inner diameter thereof, preferably 10-50mm, more preferably 10-20mm, and the total length of the string can be 15-1000mm, preferably 100-800mm, and more preferably, 400-800mm. The shunt resistor can be 10-300ohm, preferably 20-150ohm, and more preferably 30-100ohm.

On the other hand, for a power system with voltage level of 1OkV, a magnetic ring string with an inner diameter equal to or approximate with the feed line diameter, an outer diameter of 10mm larger than the inner diameter, and a total length of 560mm, together with a shunt resistor of 55 ohm can efficiently reduce the steepness of the rising over-voltage and attenuate the amplitude thereof. In practice, the outer diameter of the rings can be 5-100mm larger than the inner diameter thereof, preferably 10-50mm, more preferably 10-20mm, and the total length of the string can be 15-1000mm, preferably 100-800mm, and more preferably, 400-800mm. The shunt resistor can be 5-300ohm, preferably 10-100ohm, and more preferably 15-75ohm.

The particular advantage of the vacuum restriking over-voltage suppression approach proposed in the present invention is that it can limit the over-voltage both in steepness and amplitude, and it is simple in structure and reliable in its own insulation.

Simulation Test

Simulation tests were performed to verify the effectiveness of the present invention. Figure 6 is the test circuit in which "51" is high voltage rectifying diode, "52" impulse capacitor (1μF, DC40kV), "53" vacuum contact gap of vacuum switch, "54" inductive coil (23mH, 1OkV), "55" impulse capacitor (1 nF, DC24kV), "56" high voltage probe (attenuation 1000:1 , bandwidth 75MHz, maximum input voltage 4OkV peak), "57" current shunt (1.25mΩ), "58" high frequency magnetic ring string (amorphous core FJ37, Magnetization curve as shown in Figure 7, inner diameter 26mm, outer diameter 40mm, total length 760mm), and "59" shunt resistor (75Ω).

The simulation was performed with only one striking of the vacuum gap. Charging the capacitor "52" till breakdown of the vacuum gap "53", a high frequency transient is produced in the circuit. Adjusting the vacuum gap to change the breakdown voltage, different amplitude of the transient can be produced. The measurement includes the voltage on the load coil "54" and the current through the vacuum gap "53".

The test was conducted in following circuit conditions: (i) testing with no high frequency magnetic ring string and no shunt resistor. The vacuum gap was broken down at the voltage of 11.5kV. The waveforms obtained is shown in Figure 8, in which, the peak transient voltage on the load is about 22.5kV, and the rising time of the transient front is about 180ns. (ii) Testing with high frequency magnetic ring string but without shunt resistor. The breakdown voltage was 13.5kV. The waveforms for this test is shown in Figure 9, in which, the peak transient voltage on the load is about 26kV, and the rising time of the transient front is about 640ns. (Hi) Testing with high frequency magnetic ring string and shunt resistor. The breakdown voltage was 12.7kV. The waveforms of the test is shown in Figure 10, in which, the peak transient voltage on the load is about 19kV, and the rising time of the transient front is about 600ns.

Comparing between the test results of the three conditions, the suppression effect of the high frequency magnetic ring string and the shunt resistor is remarkable. The magnetic string made a remarkable decrease of the transient steepness, and the shunt resistor made a remarkable decrease of the transient amplitude.

The restriking over-voltage of vacuum switch in inductive load interruption could not only be high in amplitude but also in steepness. When using vacuum switch to interrupt inductive loads such as transformer, reactor, motor or arc furnace, the steep restriking over-voltage can unevenly distribute along the windings, and over-stress the insulations between the coils. The present invention provides a method for suppression of the restriking over-voltage not only in amplitude but also in steepness. It is realized in the way that high frequency magnetic ring strings are put on the both side conductors of the vacuum switch and each magnetic ring string is paralleled with a shunt resistor. When the vacuum switch occurs restriking, the magnetic ring strings can slow down the transient front and the shunt resistors can make attenuation of amplitude. The extra advantage of the method is that it is simple in structure and reliable in its own insulation.

Although the present invention has been described with reference to specific embodiments, those skilled in the art could consider other variants and alternates based on the above description, which would therefore be included in the scope of the invention as stated in the following claims.

Claims

1. A method of protecting an equipment powered by an electric power source against restriking over-voltage occurring while interrupting the equipment from the power source with a switch, including a step of:

a. connecting a first circuit unit between the switch and the equipment, such that the switch, the first circuit unit, and the equipment are connected in a series configuration, wherein the first circuit unit comprises a first inductor and a first resistor connected in parallel with each other.

2. The method of claim 1 , further including a step of:

b. connecting a second circuit unit between the switch and the power source, such that the switch, the second circuit unit, and the power source are connected in a series configuration, wherein the second circuit unit comprises a second inductor and a second resistor connected in parallel with each other.

3. The method of claim 1 , wherein the first inductor is a first magnetic ring string composed of a plurality of magnetic rings.

4. The method of claim 2, wherein the first inductor is a first magnetic ring string composed of a plurality of magnetic rings, and the second inductor is a second magnetic ring string composed of a plurality of magnetic rings.

5. The method of claim 3, wherein the step "a" further includes putting the first magnetic ring string around the conductor coupling the switch to the equipment, such that the first magnetic ring string is coaxial with the conductor.

6. The method of claim 4, wherein the step "b" further includes putting the second magnetic ring string around the conductor coupling the switch to the power source, such that the second magnetic ring string is coaxial with the conductor.

7. The method of claim 5, wherein the step "a" further includes sheathing the first magnetic ring string with the first resistor, such that both ends of the first resistor contact the conductor.

8. The method of claim 6, wherein the step "b" further includes sheathing the second magnetic ring string with the second resistor, such that both ends of the second resistor contact the conductor.

9. The method of any one of the preceding claims, wherein the equipment is selected from a group of inductive loads including but not limited to: transformer, a reactor, a motor, an arc furnace, and any combination thereof.

10. The method of any one of the preceding claims, wherein the power supply is of the voltage level of 35kV or below.

11. A device for interrupting and/or connecting a circuit including a first side and a second, comprising a vacuum switch connected between the first side and the second side, and an over-voltage suppressor connected in series with the vacuum switch, characterized in that the over-voltage suppressor comprises an inductor and a resistor connected in parallel with each other.

12. The device of claim 11 , characterized in that the over-voltage suppressor is provided in the second side of the circuit.

13. The device of claim 12, further comprises an over-voltage suppressor provided in the first side of the circuit.

14. The device of any one of claims 11-13, characterized in that the inductor is a magnetic ring string provided on a conductor coupling the vacuum switch to the circuit such that the magnetic ring string is coaxial with the conductor.

15. The device of any one of claims 11-14, characterized in that the magnetic ring string comprises a plurality of magnetic rings axially proximate to each other, and the resistor is a shunt resistor formed as a case surrounding the rings.

16. The device of any one of claims 11-14, characterized in that the ring string comprises a plurality of magnetic rings or magnetic ring groups axially separated from each other, and the resistor is a plurality of shunt resistors formed as cases surrounding the individual rings and ring groups respectively.

17. The device of claim 15 or 16, characterized in that the magnetic rings are made of materials selected from a group including but not limited to: ferrite, amorphous material, metallic magnetic powder, and any combination thereof.

18. The device of any one of claims 11-17, wherein the first side is the public electric network, and the second side is an inductive load.

19. The device of claim 18, wherein the inductive load is selected from a group including but not limited to: a transformer, a reactor, a motor, an arc furnace, and any combination thereof.

20. The device of any one of the claims 11-19, wherein the inner diameter of the magnetic ring string is equal to or approximate with the conductor diameter, and the outer diameter of the magnetic ring string is 5-100mm larger than the inner diameter.

21. The device of claim 20, wherein the outer diameter of the magnetic ring string is 10-50mm larger than the inner diameter.

22. The device of claim 20, wherein the outer diameter of the magnetic ring string is 10-50mm larger than the inner diameter.

23. The device of claim 20, wherein the outer diameter of the magnetic ring string is 10-20mm larger than the inner diameter.

24. The device of any one of the claims 11-19, wherein the total length of the string can be 15-1000mm.

25. The device of claim 24, wherein the total length of the string can be 100-800mm.

26. The device of claim 24, wherein the total length of the string can be 400-800mm.

27. The device of any one of the claims 11-19, wherein the power supply is of the voltage level of 35kV, and the resistance of the resistor is 10-300ohm.

28. The device of claim 27, wherein the resistance of the resistor is 20-150ohm.

29. The device of claim 27, wherein the resistance of the resistor is 30-100ohm.

30. The device of any one of the claims 19-29, wherein the public electric network is a single-phase power supply with the voltage level of 35kV or below.

31.The device of any one of the claims 19-29, wherein the public electric network is a three-phase power supply with the voltage level of 35kV or below.

32. A device used to suppress restriking over-voltage occurred across a vacuum switch in a circuit, comprising: a conductor to be connected to the vacuum switch; a magnetic ring string provided around the conductor, such that the ring string is coaxial with the conductor; and a resistor coupled to the conductor, such that two ends of the resistor are connected to the conductor, and the ring string is located between the two connecting points.

33. The device of claim 32, wherein the magnetic ring string comprises a plurality of magnetic rings axially proximate to each other, and the resistor is a shunt resistor formed as a case surrounding the rings.

34. The device of claims 32, characterized in that the ring string comprises a plurality of magnetic rings or magnetic ring groups axially separated from each other, and the resistor is a plurality of shunt resistors formed as cases surrounding the individual rings and ring groups respectively.

35. The device of any one of claims 32 - 34, characterized in that the magnetic rings are made of materials selected from a group including but not limited to: ferrite, amorphous material, metallic magnetic powder, and any combination thereof.