Xiaofei Yao

Axial Magnetic Field Strength Needed for a 126-kV Single-Break Vacuum Circuit Breaker During Asymmetrical Current Switching

The objective of this paper is to investigate how strong an axial magnetic field (AMF) is needed in test duty T100a for a 126-kV single-break vacuum circuit breaker (VCB) when interrupting a rated short circuit current of 40-kA rms in light of arcing contact gap, arc duration and phase shift effect. The AMF is generated by a pair of 2/3 coil-type AMF contacts. First, the accuracy of a 3-D finite element simulation of AMF is validated by an AMF measurement at a contact gap 30 mm and the error is <; 10%. After that test duty T100a of the 126-kV single-break VCB are carried out 18 times with arc duration from 3.3 to 19 ms at 40-kA rms. With a given opening displacement curve, the contact gap as arc extinguishes within a range from 12 to 50 mm corresponding to the arc duration from 3.3 to 19 ms. Then the AMF distribution of the 126-kV single-break VCB at each contact gap at which arc extinguished is simulated to analyze the current interrupting results. The results show that there are successful interruptions at the short-circuit current 40-kA rms in test duties T100a, with the shortest arcing contact gap window from 18 to 23 mm and the longest arcing contact gap of 44 mm. The shortest arcing contact gap window corresponds to the maximum AMF at the intermediate plane of the contact gap per kilo-ampere of the breaking peak current from 9.3 to 8.5 mT/kA. In addition, the longest arcing contact gap corresponds to the value of 6.1 mT/kA. The phase shift time at the center of intermediate plane of the contact gap ranged from 1.36 to 0.75 ms corresponding to an increase of arcing contact gap from 12 to 50 mm in the tests.

Mechanical reliability of a 126 kV single-break vacuum circuit breaker

The IEC 62271-100 international standard states that the mechanical endurance test is a mandatory test for a circuit breaker (CB) to verify its mechanical performance. The objective of this paper is to demonstrate the failure modes of three mechanical components and to show how they can be improved to extend the mechanical endurance of a prototype 126 kV single-break vacuum circuit breaker (VCB). The three mechanical components are: a pin (stainless steel: 20CrNiMo) contained in a contact spring component, a joint bearing (stainless steel: 1Cr18Ni9Ti), and an inter-locking pawl (steel: Q235). All three components showed low-cycle metallic fatigue failure modes, which presented as deformation and metallic crack initiation and propagation. During mechanical endurance operations (i.e., closing and opening operations under no load conditions), the pin in the contact spring component fractured after approximately 178 operations; the joint bearing deformed greatly and fractured after about 1,500 operations; and the inter-locking pawl fractured after approximately 2,100 operations. Based on the theory of multi-body system dynamics and the S-N characteristics of the metal material applied in each case, the failure modes and fatigue lives of the three failed components and improved versions of their geometries were analyzed, respectively. Both the simulation results and the experimental results for the mechanical endurances of the improved component geometries offered greater confidence in the mechanical reliability of the 126 kV VCB: after 1,500 operations, the improved pin worked well, with no deformation or initial cracking; after 2,000 operations, small deformations were observed on both terminals, but with no initial cracking; and after 2,000 operations, the joint bearing and the inter-locking pawl both continued to work well, with no deformation or initial cracking.

Development of a 126kV single-break vacuum circuit breaker and type test

High-voltage vacuum circuit breakers in transmission voltage, as their environmental friendly characteristics, are drawing more and more interests all over the world. The objective of this paper is to present experimental results of type-test for a 126kV/40kA-2500A single-break vacuum circuit breaker based on a standard of IEC 62271-100. Two kinds of test circuits were adopted in type tests for the 126kV/40kA-2500A single-break vacuum circuit breaker. One is direct test circuit for test duties such as out of phase making and breaking test duties OP1 and OP2. The other is synthetic circuit for test duties, such as terminal fault test, capacitive current switching test, short-line fault test, out-of -phase test, double earth test and electrical endurance test. Type-test results, for the 126kV vacuum circuit breaker, presented in this paper are as follows: Dielectric test, 230kV 1min power frequency voltage withstand test and lightning impulse voltage withstand test, with 15 times 550kV in both of polarities; Short time withstand current with 4s duration and peak withstand current test with 100kA; Terminal fault test, including test duties: T10, T30, T60, T100s, T100a; Out of phase making and breaking test, including OP1 and OP2; Short-line fault test L90 and L75; Double-earth fault test as well as capacitive current switching tests of class C1 with test duties of LC1, CC1 and LC2, CC2; Electrical endurance tests; And assessment of condition after test with a 80%of dielectric test voltage; Temperature rise test under rated current of 2500A; Mechanical endurance test of class M1. In summary, the 126kV/40kA-2500A single-break circuit breaker passed the type tests.

Development and Type Test of a Single-Break 126-kV/40-kA–2500-A Vacuum Circuit Breaker

The objective of this paper is to present a 126-kV/40-kA-2500-A single-break vacuum circuit breaker (VCB) and its type test results based on the IEC 62271-100: 2006 and GB1984-2003 standards. A single-break 126-kV vacuum interrupter (VI), with a pair of 2/3 coil-type contacts, is used in the 126-kV VCB. The external insulation of the VI is provided by SF6 at 0.1 MPa. A spring-type operating mechanism is used to provide a closing velocity of 1.15 m/s, which is an average velocity for 4 mm before the moving contact mates with the stationary contact. The opening velocity within 20 mm, which is 1/3 of a full contact stroke of 60 mm, is 3.5 m/s. Two kinds of test circuits are used in the type tests to verify the making and breaking capabilities of this 126-kV VCB prototype. One is a direct test circuit for test duty cycles, such as out-of-phase making and breaking test duties, that is, OP1 and OP2; capacitive current switching test of class C1, that is, CC1/LC1 and CC2/LC2. The other is a synthetic circuit, used for tests, such as the terminal fault test, the short-line fault test, the double earth fault test, and the electrical endurance test of class E2. The dielectric tests of a 230-kV/1 min power frequency voltage withstand test and a 550-kV lightning impulse voltage withstand test were used to verify the dielectric withstand performance of the VCB. Experiment results presented in this paper showed that the 126-kV/40 kA-2500 A single-break VCB passed the type tests according to IEC 62271-100 and GB1984-2003.

Determination of opening velocities for vacuum circuit breakers at transmission voltage

The objective of this paper is to propose a quantitative method to determine opening velocities for VCBs at transmission voltage. A couple of 2/3 coil-type axial magnetic field (AMF) contacts, which are used in a 126 kV vacuum interrupter, are arranged in a demountable vacuum chamber. The movable contact is opening with three different kinds of opening travel-curves. Each curve is adjusted by two opening velocities, v1 and v2, respectively. The v1 is an average opening velocity within 1/3 full contact gap, which is with 1.8 m/s, 2.4 m/s and 2.7 m/s, respectively. The v2 is an average opening velocity within 2/3 full contact gap, which is with 2.4 m/s, 2.7 m/s, and 3.0 m/s, respectively. The test current ranges from 4 kA to 40 kA, stepped by 4 kA. When a VCB is used to interrupt a short-circuit current, the preferred opening velocity is determined by reducing the arcing interval tI for the intense arc mode and increasing the arcing interval tII until the formation of high-current anode mode (HAM). The preferred v1 is higher than a threshold value v1_th, at which there is a peak critical contact gap dI_peak, for the evolving from intense arc mode into diffuse arc mode. The preferred value v2 is lower than another threshold value v2_th, at which there also exists a peak critical contact gap dII_peak, for the evolving from diffuse arc mode into the footpoint mode and anode spot mode.

Opening velocity characteristics for vacuum interrupters with cup-type axial magnetic field contacts

The objective of this paper is to experimentally determine an opening velocity for vacuum interrupters, with cup-type axial magnetic field (AMF) contacts. The opening velocity characteristics of the VCB includes an initial opening velocity v₁ and an average opening velocity v₂. The initial opening velocity v₁ is defined as an average velocity in 1/2 full contact gap of 20 mm. The average opening v₂ is defined as an average velocity of full contact gap. Experiment result shows that a higher v₁ always decreases the duration for the intense arc mode. However, a higher v₂ tends to result in an earlier formation of anode spot mode. The preferred initial opening velocity v₁ should higher than a threshold velocity v_{1_th}, at which the peak critical contact gap d_{I_th} is achieved, for the intense arc mode evolving into diffuse arc mode. The preferred average opening velocity v₂ should be at a threshold velocity v_{2_th}, at which the maximum critical contact gap d_{II_th} is achieved, for the diffuse arc mode evolving into the large-gap active-anode mode. The lower of v₂ below the v_{2_th} the better, as the large-gap active-anode mode would be avoided.

Research on mechanical reliability of a permanent magnetic actuator for a 126kV vacuum circuit breaker

The objective of this paper is to investigate mechanical reliability of a permanent magnetic actuator (PMA) for a 126kV vacuum circuit breaker. Impact characteristics and fatigue life of the collision components in actuator were analyzed by experiment and simulation. 3D finite element model of impact components were built up. Dynamic impact force between armature and static magnetic conductive yoke were calculated by explicit method. It was also verified by PVDF piezoelectric sensors in the experiments in opening operation. Based on the impact simulation and operating test results, failure mode and fatigue life of the static magnetic conductive yoke and armature were calculated. Experiment results showed that maximum impact force in opening and closing operations of PMA were 892kN and 198kN. Simulation results indicated that the maximum stress in impact process is 871kN, which is verified by experiments. In addition, fatigue mode of impact component was low-cycle crack fatigue. The outside part and inside part of opening static magnetic conductive yoke had appeared failure after 3990 and 3027 operations respectively by using oil damper. Their mechanical lives met the requirement of IEC standard. When the impact velocity was lower than 1.3m/s, the fatigue lives of impact components for opening could meet the IEC standard requirement of class M1.

Anode mode diagram: A determination of opening displacement curve for a 126kV vacuum circuit breaker

The objective of this paper is to experimentally determine an opening displacement curve based on the vacuum arc anode mode diagram of a 126 kV single-break vacuum circuit breaker. The experiments were carried out in a LC discharging circuit. In this experiment, a pair of 2/3 coil-type AMF contacts, with a diameter of 100 mm and a contact material of Cu50Cr50, were installed in a demountable vacuum chamber. Experimental results show that there were two significant contact gaps 20(±2) mm and 40(±2) mm corresponding to the minimum arcing region and the maximum arcing region respectively. The opening velocity of 3.5 m/s in the minimum arcing region was appreciable as the vacuum arc can transfer out of the intense arc mode region very soon. While in the maximum arcing region, the opening velocity of 3.0 m/s was appreciable as the duration of the opening displacement maintained in the footpoint mode region and the anode spot mode region can be relatively shorter.

An influence of an ambient magnetic field induced by a nearby parallel conductor on high-current vacuum arcs

Vacuum arcs are well influenced by a nearby ambient magnetic field. This effect may cause a burn-out of the shield in a vacuum interrupter or even cause a failure interruption. The objective of this paper is to determine an influence of a nearby ambient magnetic field, induced from a conductor parallel assembled to a vacuum interrupter, on the high-current vacuum arc characteristics in the VI. A copper rod conductor is assembled in parallel but electrically connected in series with a test vacuum interrupter. The distance between the conductor rod axis and the test vacuum interrupter axis was controlled as 60 mm, 100 mm and 140 mm, respectively. In the vacuum interrupter, cup type axial magnetic field electrodes were used, which has a contact material CuCr30 and the contact diameter was 42 mm. Contact gap was set to 6mm and 12 mm, respectively, in order to compare an influence of the ambient magnetic field on the high-current vacuum arc behaviors. Arc current is provided by a single-frequency LC discharging circuit, which provides an arcing time of 10.4 ms. And the arc current increased from 3.1 kA to 24.8 kA, which stepped by 3.5 kA. Experimental results showed that with a stronger ambient magnetic field, the arcing energy variation was higher and the arc voltage noise lasted a longer duration. As the ambient magnetic field increased from the minimum case (No copper rod) to the case of axial distance 140 mm, 100 mm and the strongest case 60mm axial distance, the arc energy variation range increased from 0.66 kJ to 1.25 kJ, 1.36 kJ and 1.78 kJ, respectively, at 10.3 kA with the 12 mm contact gap distance. Correspondingly, the arc energy variation range increased from 0.82 kJ to 0.75 kJ, 0.84 kJ and 1.07 kJ at 17.8 kA with the 6mm contact gap distance. And the arc-voltage noise duration increased from 2.6 ms to 4.5 ms at 18.1 kA with the 12 mm contact gap distance, and the duration increased from 2 ms to 3.8 ms at 17.8 kA with the 6mm contact gap distance.

Determination of equivalent capacitors in a double-break vacuum circuit breaker based on an impulse-voltage method

The stray parameters of a double-break vacuum circuit breaker (VCB) have a significant influence on the voltage distribution of the two breaks. The stray parameters are equivalent to two main equivalent capacitors. The two main equivalent capacitors are the equivalent break capacitor (C1) and the equivalent capacitor between the middle point of the two breaks and earth (C2). The objective of this paper is to determine the capacitance of C1 and C2 based on an impulse-voltage method. A 40.5 kV double-break VCB was studied in the experiment. An 8.5 kV pulse was applied on the double-break VCB. The voltage curves on each break were measured by two high voltage probes. A simulation circuit was set up. The parameters were tuned in the simulation circuit to match the simulated output curve with the measured output curve. The capacitance of C1 and C2 were obtained. For the test VCB, C1 was determined as 16 pF and C2 was determined as 27 pF. C1 was agreed with a measurement by a precise LCR instrument. The impulse-voltage method needed only a few shots to determine approximate C1 and C2 without using precise and expensive instruments.