Erik D. Taylor

The repulsion or blow-off force between closed contacts carrying current

Experimental data for the repulsion or blow-off force FB between closed contacts carrying current were compiled in order to derive an empirical relationship for this force. The data cover a period of 65 years, and includes data from contacts in both air and in vacuum with currents ranging from 1–200 kA peak. Plotting the blow-off force vs. the current reveals a simple relationship independent of the contact material, FB = k· I2, where i is the peak current and k is a constant. This relationship provides a quick estimate of the blow-off force in practical designs for overload and fault currents. It can help guide the selection of the proper contact force to prevent welding, and is very useful in the development of theories calculating the welding current for closed contacts carrying current.

Long-term vacuum integrity of vacuum interrupters

Vacuum interrupters (VIs) are the primary switching technology for medium voltage (1-52kV) electrical power systems. The extensive field service of VIs is now raising interest in determining the end of life for these devices, with the primary concern being the loss of vacuum. The manufacturing method of the VIs can affect the calculated long-term life. In particular, there are three main design groups to evaluate. The key differences between these three groups are the processing temperature and permeability of the housing to hydrogen and helium. Many potential failure modes occur independently of the field application or lack thereof, and can be evaluated based data from VIs in long-term storage or after operations, such as seen during development and certification testing. The field returns history from VI manufacturers also provided critical information on the VI lifetime. These various factors can be combined to identify risk factors that could alter the estimated life.

The effect of the axial magnetic field structure on the threshold welding current for closed axial magnetic field vacuum interrupter contacts

There are two designs of large area, vacuum interrupter contacts: the transverse magnetic field (TMF) contact and the axial magnetic field (AMF) contact. These contacts are required to perform a wide variety of roles within vacuum circuit breakers. One duty is to pass short-circuit currents with the vacuum interrupters contacts closed for a period of time (1 to 4 seconds), after which the circuit breakers mechanism must be able to open the contacts. Thus, the possibility of contact welding must be minimized. The flow of current through practical contacts generates a repulsive blow-off force, which has to be balanced by a closing force from the circuit breaker mechanism plus the force from atmospheric pressure acting on the vacuum interrupters bellows. The axial magnetic field (AMF) vacuum interrupters have an additional attractive force because of the parallel currents flowing in the two AMF coils behind the contacts faces. This force is calculated using three- dimensional finite element analysis (FEA) for three practical AMF designs using a contact diameter of 62 mm and a current of 31.5 kA (45.5 kA peak). The extra attractive forces are then combined with the other forces acting on the closed vacuum interrupter contacts to calculate the threshold welding current: the current above which the contacts will form a weld. Calculations of the total closing force compares the difference in the threshold welding current between the three AMF contact designs.

Guest Editorial Special Issue on Vacuum Discharge Plasmas (ISDEIV-PS)-2017

In the Special Issue, we are very pleased to present the expanded versions of selected papers from the 27th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV). It was held at the Xi’an Jiaotong-Liverpool International Conference, Suzhou, China, from September 18–23, 2016.

Model for the welding of axial magnetic field vacuum interrupter contacts

Vacuum interrupters are required to perform a wide variety of roles within vacuum circuit breakers. One duty is to pass short-circuit currents with the vacuum interrupters contacts closed for a period of time (1 to 4 seconds), after which the circuit breakers mechanism must be able to open the contacts, Thus the possibility of contact welding must be minimized. The flow of current through practical contacts generates a repulsive blow-off force, which has to be balanced by a closing force from the circuit breaker mechanism plus the force from atmospheric pressure acting on the vacuum interrupters bellows. The magnitude of the applied closing force is an important parameter in a vacuum circuit breakers design. Axial magnetic field (AMF) vacuum interrupters have an additional attractive force because of the parallel currents flowing in the two AMF coils. This force is calculated using three-dimensional finite element analysis (FEA) for practical AMF designs using contact diameters ranging from 62-100 mm. These results are then compared to two dimensional FEA models and analytic formulas, including the effect of the current frequency on the results (DC vs. 50 Hz). These attractive forces can then be combined with the other forces acting on the closed vacuum interrupter contacts to calculate the threshold welding current: the current above which the contacts will form a weld. Calculations of the total closing force compare the difference in the threshold welding current between AMF and other VI contact designs.

Performance of axial magnetic field vacuum interrupters at various transient recovery voltages and contact gaps

The primary application of vacuum interrupters (VI) is to interrupt short-circuit currents during faults on electrical power systems. One common VI design uses an axial magnetic field (AMF) to force the arc to remain diffuse, thereby allowing the interruption of very high short-circuit currents. In this work, VIs with 62 mm diameter AMF contacts were short-circuit tested in a synthetic circuit at rated voltages ranging from 17.5-38kV, and with contact gaps ranging from 7-19 mm. Despite the wide range of test parameters, the key parameter determining the success of the interruption attempt was the transferred charge during arcing Q. For Q <; 275 A · s, the VIs interrupted with a high degree of success at the first current zero; and for Q > 275 A · s the interruption performance dropped sharply, with a narrow transition region. This performance was observed over the full range of transient recovery voltages (TRV) and contact gaps tested, which represent most of the medium voltage application range. These results suggest that the transferred charge during arcing is a key characteristic for AMF contact designs.

Performance of vacuum interrupters in electrical power systems with an effectively earthed neutral

Circuit breakers using vacuum interrupters (VI) are often tested according to the IEC standard 62271-100. This standard specifies a series of current interruption tests to verify the operation of the circuit breaker under a variety of different failure conditions. The IEC 62271-100 standard will be updated at the end of 2016 to include additional new tests for medium voltage circuit breakers, designed to verify successful interruption in electrical power systems with an effectively earthed neutral. In the current version of the standard, three-phase tests are performed in the non-effectively earthed neutral case. This situation is referred to as kpp = 1.5, where kpp is the first-pole-to-clear factor. In the updated standard, additional three-phase tests are performed with an effectively earthed neutral, referred to as kpp = 1.3. This setup leads to higher stress on the second phase to clear. In order to evaluate the effect of the new tests, two different VI designs, in regular commercial use which previously passed the 62271-100 standard, were tested according to the new requirements. One VI style was an axial magnetic field (AMF) design for 40.5kV/31.5kA, and the other a radial magnetic field (RMF) design for 17.5kV/31.5kA. Both VI designs were first tested with symmetric and asymmetric close/open operations for kpp = 1.5. The VIs were then tested with symmetric operations (T100s) and a large number of asymmetric operations (T100a) with kpp = 1.3. Both VI types passed these test duties, which greatly exceeded the requirements of the updated IEC 62271-100. This performance indicates that well-designed VIs that pass the current version of IEC 62271-100 will also be able to pass the new kpp = 1.3 tests for systems with an effectively earthed neutral.

Interruption performance at frequency 50 or 60Hz for generator breaker equipped with vacuum interrupters

Vacuum interrupters are serving worldwide in distribution circuits, meeting the electrical and mechanical requirements specified in the IEC and/or ANSI standards for low and medium voltage applications. Generator circuits require special generator circuit-breakers and are tested according to IEC 62271-37.013/ANSI/IEEE C37.013. Traditionally, generator circuit-breakers have been very large devices based on air- or SF6- blast interruption technology. Over the last 35 years, the short circuit interruption performance of vacuum interrupters has been dramatically increased due to continuous development, especially of contact system design and contact materials. These improvements allow the application of vacuum interrupter technology to generator circuit-breakers. Three phase vacuum circuit-breakers are available for this application in generator circuits at 50 and 60 Hz. To fulfil the required short circuit interruption ability, the test has to be performed at 50Hz for IEC markets and at 60Hz / 50Hz for ANSI markets. When the short circuit interruption ability is tested at 50Hz, the results can be transferred to 60Hz because the arcing time is reduced, while the di/dt is slightly steeper at current zero. Four main factors related to vacuum technology are investigated: The influence of the arcing time duration before current zero (CZ), the transferred charge I x dt, di/dt steepness at CZ, and finally the transient recovery voltage (TRV).

Effect of arc time on the current breaking capability of vacuum interrupters

Vacuum interrupters on medium voltage power systems need to interrupt short-circuit currents over a wide range of arc times. Synthetic testing allows the careful study of the interruption performance over arc times ranging from 0 to 13.3ms for a 50Hz system. Comparing the interruption performance as a function of arc time observed in single- and three-phase direct testing and single-phase synthetic testing gives good agreement. This agreement extends over different rating points, different contact designs, and different arc control methods. Additional tests compared synthetic tests with and without re-ignition for arc times greater than 10ms.

Application of research in field emitter arrays to the breakdown of contacts in vacuum

Field emitter arrays (FEAs) consist of micrometer-sized sites on a surface in vacuum designed to drive the field emission of electrons. Large arrays of these devices are envisioned for applications in video displays and x-ray generation. Research in this area is relevant for studies in field emission and breakdown on contacts in vacuum and vacuum interrupters, particularly for the application of vacuum interrupters to high-voltage (HV) electrical systems. The detailed knowledge of the actual surface structure and conditions in FEAs can exceed what is feasible for macroscopic contacts, giving the opportunity for more detailed comparison of field emission/breakdown theories to experiments. The first part of this work examines the applicability of FEA research to macroscopic contacts in vacuum. The second part compares the field enhancement factor, emission area, non-uniformity of emission sites, and influence of particles between these two arrangements.