A model is developed to determine the peak threshold current required to weld closed contacts of a circuit breaker or switch when passing fault currents. The model considers: 1) the total contact resistance of the closed contacts by including the expected constriction resistance as well as the resistance resulting from the high temperature at the contact region; 2) the effect of the “blow-off” force from the passage of the current through the region of contact; 3) the reduction of the hardness of the contact metal in the vicinity of the high temperature at the contact region; 4) the effect of forces on the contacts from the current path through the circuit breaker or switch design; 5) the effect of the contact design; 6) the short term passage of current; 7) the effect of multiple regions of contact; and 8) the effect of the passage of current from 1 to 4 s. Published experimental data of threshold welding current are compared to the analytical model.

It has become necessary to develop HVDC circuit breakers to protect against fault currents that may occur in HVDC grids. One possible HVDC circuit breaker design uses vacuum interrupters as the main DC current path and the DC current interrupter using a counter-pulse system to create an artificial current zero. As the counter-pulse has a relatively high -di/dt, the vacuum arc mode at the current zero is critical when withstanding the resulting Transient Interruption Voltages (TIVs). The fault current in the DC system can also increase rapidly to a very high value. In order to prevent serious damage to the DC grid, it is imperative that the vacuum interrupter open soon after the fault is detected and the counter pulse applied in order to limit its magnitude. This paper discusses the vacuum arc modes as the contacts open and the requirements to create a fully developed diffuse vacuum arc at the counter-pulse current zero. The vacuum interrupter also has to develop a large enough contact gap in order to withstand the expected recovery voltages (TIVs). To do this a high-speed opening mechanism is required. Thus, the vacuum interrupter will accelerate faster than it would have experienced in a HVAC vacuum circuit breaker. The paper presents the critical design parameters for the vacuum interrupter to operate in a HVDC vacuum circuit breaker and contrast them with vacuum interrupters used in HVAC circuit breakers.

Reliable bolted bus bar connections are necessary for the decades of life expected from them. This is especially true for bus bar systems in electric power stations where over 40 years life is the norm. A failure of a bolted connection can result in a substantial disruption of the power station's output and a major inconvenience to its customers. This paper discusses how best to make bolted connections between Al to Al, Al to Cu and Cu to Cu bus. The typical aging curve for a bolted joint is discussed. The advantages and disadvantages of infrared thermography and external joint temperature measurements as joint inspection tools are presented. The concept of bolted bus contact resistance is shown to be a reliable bolted joint inspection tool. It is shown that an increase of two times the original bolted joint's contact resistance is a reliable measure that a bolted connection is in the initial stages of failure. Experimental data for a poorly made bolted bus connection and well bolted busbar joints are presented to illustrate the importance of knowledge of this contact resistance increase in order to take timely corrective action. Finally, a straightforward method of regular inspection and maintenance of the bolted connection is presented.

Vacuum interrupters (VI's) can experience occasional late restrikes while capacitor switching. These late restrikes can occur at random up to 10 s after the interruption of the capacitor current. After interrupting the capacitor current, the open vacuum gap has a unidirectional recovery voltage impressed across it that is two or more times the circuit's peak value. It has been observed that pulses of emission current come from the cathode contact as each voltage wave reaches its peak value. This paper explores the effect of these emission currents as a possible cause of the late restrikes. After reviewing experimental emission current data, calculations of energy into the anode contact have been made. These show that the contact metal below the anode contact's surface can reach the melting point and beyond. When this occurs, it is possible that an eruption of anode metal vapor will pass into the contact gap. This will be ionized, initiate a break-down of the contact gap and result in the late restrike.

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 breaker's 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 interrupter's 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.

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 breaker's 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 interrupter's bellows. The magnitude of the applied closing force is an important parameter in a vacuum circuit breaker's 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.

A model is developed to calculate the threshold welding current for the two designs of large area vacuum interrupter contacts: the transverse magnetic field (TMF) contact and the axial magnetic field (AMF) contact. The model considers: (1) that large area, closed, vacuum interrupter contacts have more than one region, i.e. 'n', regions of contact; (2) the total contact resistance of the closed contacts by including the expected constriction resistance as well as the resistance resulting from the high temperature of the contact regions; (3) the effect of the 'blow-off' force from the passage of the current through the contact regions; (3) the attractive force generated by the magnetic field of the AMF contacts; and (4) the reduction of the hardness of the contact metal in the vicinity of the high temperature at the contact regions. A discussion of the model shows how it can be used as a design tool to analyze not only welded contact structures, but also to determine the contact force required to minimize the effect of welding during the passage of overload and short-time withstand currents.

Experimental data for the repulsion or blow-off force F B 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, F B = k· I 2 , 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.

An analysis is performed on data from an experiment to determine the closed contact force required to prevent the welding of closed, Cu-Cr contacts inside a vacuum interrupter passing 20kA (r.m.s) from 0.5s to 4s. It will be shown that the data can be explained if these closed contacts have more than one region of contact together with the heating of the contact's body during the passage of the current. The implication of this analysis for vacuum interrupters used in vacuum circuit breakers when required to stay closed during the passage of short circuit currents is discussed.