Hans Schellekens

Compact high-voltage vacuum circuit breaker, a feasibility study

This study focuses on a compact vacuum circuit breaker (CB) for nominal voltage of 72.5 kV, nominal current of 2000 A and short circuit current of 31.5 kA. The dielectric conception of the vacuum interrupter (VI) is based on a multiple floating shield configuration which reduces the risk of total breakdown due to partial breakdown between the contacts and shield. An optimised electrical field distribution on the contact surface with the maximum field pointing towards the floating shield is arranged and hence reducing the breakdown effective area of the contacts facing each. All tests have been performed on a specially designed single-phase circuit breaker pole-unit equipped with a low energy spring type mechanism. Interruption performance was confirmed in direct tests. The circuit-breaker pole was proved to be restrike-free with respect to cable charging class C2. Dielectric tests in new condition and after interruption tests confirmed the announced ratings. These tests confirm the feasibility of this HV-CB with 25% more compact VTs based on the outlined dielectric conception.

Capacitor bank switching with vacuum circuit breakers

After interruption of a capacitive current sometimes Non Sustained Disruptive Discharges (NSDDpsilas) are observed when using vacuum circuit breakers. According to network calculations, NSDDpsilas generate significant over-voltage on the terminal of the capacitor bank to earth and across the circuit breaker (CB) terminals. In a full scale experiment at 12 kV on an 8 MVAr capacitor bank artificial NSDDpsilas are produced using a TVG. The over-voltage depends on the momentary voltage difference across the breaker and, hence, varies with the phase angle. The over-voltage on the terminal of the capacitor bank to earth varies from 1.5 to 4.2 pu, and to maximum 5.2 pu. across the CB terminals. The effectiveness of surge arresters on the limitation of the over-voltages is measured. The over-voltages are limited to 2.2 pu. on the terminals of the capacitor bank to earth and 3.2 pu. across the CB terminals. A single surge arrester on the neutral point of the capacitor bank yields the same level of protection and, therefore, is a cost effective alternative.

Anode Temperature and Plasma Sheath Dynamics of High Current Vacuum Arc After Current Zero

This paper addresses the behavior of a high-current vacuum arc after current zero (CZ). Langmuir probes operating in the electron saturation current mode are used to detect the time of arrival of the cathode sheath at the probe position. The probe current is measured just before and after the CZ followed by transient recovery voltage both with and without external axial magnetic field (AMF). For an AMF-stabilized arc, the plasma sheath expanded radially with respect to the electrode axis, whereas for an arc with no AMF, the sheath boundary expanded rather spherically away from the anode. The anode temperature immediately after the CZ is estimated using the optical pyrometry method. Thermal radiation of a hot sample is recorded using a HSFC four-channel high-speed camera. Each camera channel is equipped with an interference filter to record radiation at a particular wavelength (600, 700, 775, and 825 nm). The camera channels are preliminarily calibrated. The calibration sample is a molybdenum crucible with a known spectral emissivity.

Continuous vacuum monitoring in vacuum circuit breakers

Vacuum presence monitoring is a routine maintenance procedure carried out on vacuum circuit breakers, in which the CB is isolated from the electrical network. The presence of the vacuum is verified by applying a voltage across the open contacts of the vacuum interrupters VIs higher than the breakdown voltage of such contacts in air. This procedure has a 50 years proven record and has found world wide acceptance. Today other methods are proposed to monitor the vacuum presence. These methods can be divided in 3 additional classes discriminated by the pressure range in which they are active. Each class has a clear advantage not shared by the others. Vacuum pressure monitors using high vacuum pressure gauges can give an early warning of an upcoming failure. Monitors using low vacuum pressure gauges detect the dielectric failure caused by vacuum loss as it comes. Monitors using barometric gauges clearly indicate the end of life of the VI. Today only these 3 systems can be used to monitor the pressure in a continuous way. The continuous vacuum monitor significantly decreases the probability to operate a VCB with a leaky VI, hence increasing the availability of the VI on the VCB for the network operator to 99.9999%. The continuous vacuum monitor based on low pressure vacuum gauges is considered an optimal engineering choice as it does not impact the design of the VI.

Vacuum Interrupters: Design optimisation requires reliable experimental data

Vacuum interrupter technology is mature with respect to manufacturing quality, reliability and performance. One would expect that this leads to a convergence in terms of conception resulting in a dominating design principle. Yet still a large number of designs and dimensions coexist in the market. Numerical calculation techniques are used to optimise the geometry in order to reduce the overall dimension of the VI. This approach requires criteria for maximum allowable electric field or current density values. Yet a look at the state of the art shows that the maximum allowable electric field may vary at least by a factor of 2 and current density on interruption by 20% depending in a subtle way on process parameters. Mastering these details is the drive to more compact designs and new applications.

3-D Simulation of High-Current Vacuum Arcs Under Combined Effect of Actual Magnetic Field and External Transverse Magnetic Field

Based on a steady-state 3-D magnetohydrodynamic (MHD) model, the high-current vacuum arc (HCVA) under combined effect of actual magnetic field (MF) and external transverse MF (ETMF) is simulated. The actual MF is generated by cup-type axial magnetic field contact system commonly used in commercial vacuum circuit breakers. The ETMF may cause the deflection of arc column, which is the main reason of the contact deflected erosion. According to some experimental results, the electron temperature in HCVA is assumed to be uniform and equal to 3 eV. Therefore, the MHD model is simplified by neglecting the electron energy equation to improve the simulation efficiency. With the three conservation equations (mass, momentum, and energy) of ion flow coupling solved, the spatial distributions of some flow parameters can be obtained. The influence of all three components of the MF is inserted by solving the magnetic transport equations sequentially. Proper boundary conditions are set on the cathode and anode side, which separated the cathode spots mixing region and anode sheath region from computation domain, respectively. Under the influence of the ETMF, the deflection of the plasma flow can be predicted, which may be helpful to understand the mechanism of the contact deflected erosion.

Dielectric strength of vacuum interrupters influence of manufacturing process

A test method based on combined measurement of voltage breakdown value and position is used to measure the microscopic electric field enhancement factor on copper and stainless steel electrodes in vacuum. This method is then used to evaluate the effectiveness of an industrial cleaning process. Process parameters are identified that positively influence the dielectric strength. An improvement of 15% is obtained.

Vacuum switch-disconnectors : 1. Dielectric behaviour

This study on vacuum switch-disconnectors addresses two major questions: dielectric behaviour of the VI and the reliability in a practical situation. In the experimental study, the ageing of a specially designed vacuum switch-disconnector is studied. Several internal design parameters (shape of the shields, contacts and contact gap) are varied. The ageing of VIpsilas is simulated by no-load switching, by load current switching as well as short-circuit closing; the dielectric behaviour is analysed by applying a lightning impulse voltage after each operation. A total of 200 operations are performed on each single VI out of a series of 10 interrupters. It is demonstrated that : (1) no-load switching and nominal current switching donpsilat change the dielectric behaviour; the breakdown probability can be described by a Gaussian distribution with as main characteristic that the standard deviation is about 16% of the mean breakdown value. (2) Closing in on short circuit leads to a temporary reduction of the breakdown voltage; a single interruption of the load current permits to restore the original breakdown strength.

Visualization of late breakdown in vacuum

Experimental observations of late breakdowns in a vacuum circuit breaker were made using an intensified charge coupled device camera, (ICCD). These late restrikes can cause arc reignition and interruption failure. This type of failure is rare and can be initiated by a number of mechanisms. This paper illustrates the method used to observe this furtive phenomenon. Key images of the phenomenon occurring during breakdown initiation are shown.

Vacuum disconnectors an application study

The ageing of vacuum interrupters with combined functions of disconnector and circuit-breaker or switch is studied for compact shielded solid insulated switchgear. Different prototypes are manufactured to investigate the influence of contact materials and contact designs. A test program is presented that realistically represents the ageing as described in the standards. The feasibility to create vacuum disconnecting circuit-breakers, vacuum switch-disconnectors and vacuum earthing switches that respond reliably to the requirements of the standards is demonstrated. Most of the tested designs comply with the current IEC62271-1, −100, −103 and −102 requirements for combined disconnect functions. It is shown that all combined disconnector functions can be realised with suitably designed VIs in a compact and economical way even for environmental friendly, but dielectrically very constraint, shielded solid insulated switchgear.