Jesper Magnusson

Separation of the Energy Absorption and Overvoltage Protection in Solid-State Breakers by the Use of Parallel Varistors

Hybrid and solid-state breakers offer new possibilities in the power grid by enabling faster switching, and by simplifying dc breaking. However, they consists of expensive power electronic components that are sensitive to overvoltage transients and require energy absorbing elements mounted in parallel. At turn-off, the rapidly decreasing current in the power electronic switch and the presence of an inherent stray inductance leads to hazardous overvoltage transients across the breaker. This paper investigates the possibility to split the overvoltage protection and energy absorption into two separate components. By optimizing the voltage ratio between two varistors, one can dimension a small electronics varistor for overvoltage protection and a large power electronics varistor for energy absorption. With this setup the power electronics varistor is allowed to be in a circuit with a large stray inductance and can thus be placed further away without causing an uncontrolled overvoltage. It is shown both in circuit simulations as well as in a small-scale experiment that if the voltage ratio between the two varistors is large enough, the inner varistor only has to absorb 1-2% of the system energy.

Comparison of Two Ultra-Fast Actuator Concepts

In this paper, two different types of ultra-fast electromechanical actuators are compared using a multi-physical finite element simulation model that has been experimentally validated. They are equipped with a single-sided Thomson coil (TC) and a double-sided drive coil (DSC), respectively. The former consists of a spirally-wound flat coil with a copper armature on top, while the latter consists of two mirrored spiral coils that are connected in series. Initially, the geometry and construction of each of the actuating schemes are discussed. Subsequently, the theory behind the two force generation principles are described. Furthermore, the current, magnetic flux densities, accelerations, and induced stresses are analyzed. Moreover, mechanical loadability simulations are performed to study the impact on the requirements of the charging unit, the sensitivity of the parameters, and evaluate the degree of influence on the performance of both drives. Finally, it is confirmed that although the DSC is mechanically more complex, it has a greater efficiency than that of the TC.

Electric to Mechanical Energy Conversion of Linear Ultrafast Electromechanical Actuators Based on Stroke Requirements

The operational efficiency of ultra fast actuators used as drives in high voltage direct current breakers are at best 5 %. To boost their efficiency, the design of the energizing circuit is crucial. A multi-physics finite element method (FEM) model coupled with a SPICE circuit model that is able to predict the performance of the actuator with an accuracy of at least 95 % has been developed and verified experimentally. Several variants of prototypes and models have been simulated, built, and tested. It was shown that one of the main problems leading to low efficiencies is the stroke of the drive. However, there is a possibility to increase the efficiency of the electric to mechanical energy conversion process of the studied Thomson (TC) and double sided coils (DSC) to a maximum of 54 % and 88 % respectively if their stroke is minimized. This can be done at the expense of increasing the complexity and the cost of the contact system by designing a switch with several series connected contacts that is encapsulated in a medium with a high dielectric strength. Another proposed solution is to design a current pulse with a rise time that is considerably shorter than the mechanical response time of the system. Parametric variations of capacitances and charging voltages show that the TC and the DSC can achieve efficiencies up to 15 % and 23 % respectively. Regardless of the chosen method, the DSC has a superior efficiency compared to a TC.

On the Design of Ultra-Fast Electromechanical Actuators: A Comprehensive Multi-Physical Simulation Model

In this paper, a simulation of an ultra-fast electromechanical drive was performed by using a two-dimensional axi-symmetric multi-physical finite element model. The aim of this paper is to primarily show that the following model can be used to simulate and design those actuators with good accuracy, secondly, to study the behavior and sensitivity of the system and thirdly, to demonstrate the potential of the model for industrial applications. The simulation model is coupled to a circuit and solves for the electro-magnetic, thermal, and mechanical dynamics utilizing a moving mesh. The actuator under study is composed of a spiral-shaped coil and a disk-shaped 3mm thick copper armature on top. Two numerical studies of such an actuator powered by 2640 J capacitor banks were performed. It is shown that forces up to 38 kN can be achieved in the range of 200 μs. To add credibility, a benchmark prototype was built to validate this experimentally with the use of a high speed camera and image motion analysis.

Optimal design of a medium voltage hybrid fault current limiter

The connection of distributed generation increases the short circuit power which in turn might exceed the ratings of the installed circuit breakers. A solution is to limit the available short circuit power by increasing the grid impedance, but since there is a constant strive for lower losses and higher power transfer capabilities, this is not desired. The application of a fault current limiter (FCL) that can limit the current before the first peak enables a power system with high short circuit power and low short circuit current. This can increase the stability of the grid and reduce the requirements of other equipment. This work presents a simulation model to be used as an aid in the design of a hybrid FCL for a 12 kV AC system. The proposed model combines a transient analysis circuit model with an optimization module to obtain multiple sets of possible design parameters. The design is not straight forward since there is a trade-off between several of the design parameters.

EMTP modeling of hybrid HVDC breakers

HVDC meshed systems offer several advantages in front of their HVAC counterparts. However, the lack of fast, low-losses and reliable circuit breaker technology has postponed their implementation. A recently proposed hybrid breaker design can alter this situation. The new hybrid HVDC breaker offer a fast low-loss operation that can be easily adapted to high-power high-voltage DC applications. This paper presents the implementation and testing of an HVDC breaker model based on the new design. Simulation results are included to illustrate the operation of the new device when running as a fault-current limiter.

Impact on voltage rise of PV generation in future swedish urban areas with high PV penetration

There have been a large amount of statements from different countries, claiming that the integration of photovoltaic generation in the distribution grids can eventually impact the power quality and pose challenges for the distribution system operator. In Sweden, the level of penetration of small scale distributed generation is still low and no such problems have been observed. This study is conducted to investigate the voltage levels in an urban distribution grid when the level of photovoltaic generation is increased. The study is done by modeling the Swedish urban area by PSCAD. The aspects of the model include network design of a real distribution grid, everyday load, photovoltaic generation based on real data, photovoltaic penetrations at different levels and considers the current regulations in Sweden. The results indicate that there are no problems with overvoltages even with a high penetration of photovoltaic generation. Instead the risk of over-current through the installed cables seems to be a greater limitation. The loading of the distribution transformers is decreased due to the mix of commercial and domestic loads in the local grid.

On the use of metal oxide varistors as a snubber circuit in solid-state breakers

When solid-state switches are used in DC-breaker topologies, the turn-off operation can cause transient over-voltages that might harm the semiconductor itself. The over-voltage is caused by the combination of the very rapid current decrease of a solid-state switch and an undesired stray inductance in the parallel MOV-branch. The authors have proposed a possible solution where a smaller MOV is connected close to the solid-state switch to limit the over-voltage. This way, the over-voltage protection can be separated from the energy absorption task of the MOV. A small scale test set-up has been used to show that the peak voltage across the breaker is fully determined by the inner MOV. It is also shown that the performance can be increased by changing the U-I-characteristics of the outer MOV by adding several components in parallel.

Spectroscopic investigations of the ablated species from the polymers exposed to electric arcs in air

Polymeric walls have been widely used in the last decades to improve the arc interruption process in electrical switching applications. This improvement is achieved by the evaporation (ablation) of the polymeric walls due to the highly energetic radiation generated by the electrical arcs. This experimental study deals with polymeric walls that are exposed to the electrical arcs generated between a 5 mm air gap with prospective current of 1.4 kA. In this paper, two different techniques are discussed aiming at the identification of the dominant ablated species produced during the arc interruption process, namely Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analysis coupled with Fourier transform infrared analysis of evolved gases (EGA). In addition, the morphological and chemical changes on the surface of the exposed polymeric walls are analyzed by microscopical techniques.

Experimental study of the current commutation in hybrid DC-breakers

To interrupt a current in a DC system where the feeding voltage is higher than a few kilovolt is a challenge. One popular solution today is the hybrid DC-breaker where a semiconductor is bypassed by a mechanical switch in normal operation to decrease the on-state losses. This paper presents an experimental study of the commutation from the mechanical switch to a parallel diode and IGBT. Three sets of tests were performed with six different current levels between 50 and 450 A. The test circuit consists of a capacitor bank as an energy source, a current limiting inductor, and the breaker. As the pre-charged capacitor bank is discharged through the current limiting inductor, the current will rise linearly, similar to the fault current in a stiff DC system. It was seen that with a very low stray inductance in the commutation loop, the commutation occurs in some ten microseconds just after the contact separation. Due to the small contact separation, the contacts might regain metallic contact again, causing the commutation to stop until the contacts separate again and the arc reignites. Longer arcing times were studied by introducing a larger inductance in the commutation loop. For currents of some hundred amperes, the arc voltage is fairly independent of the current level, but increases with increasing contact separation resulting in an increasing current derivative and a faster commutation. Finally the commutation and interruption of a current in a hybrid DC-breaker with an IGBT was demonstrated.