Lars Liljestrand

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.

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.

Medium voltage DC vacuum circuit breaker

The increased use of directed current (DC) as an enabler for the integration of renewable sources gains rising interest also for power transmission and distribution networks. Some preliminary installations showed some benefit of this technique compared with alternating current (AC) and pushed the development of switchgear for high (HV) and medium voltage (MV) DC grids. Several techniques able to provide DC interruption have been proposed in publications and are briefly described. The article focusses on actual development activities and tests of a medium voltage DC circuit breaker based upon the concept of current injection using a medium voltage vacuum interrupter.

A new hybrid medium voltage breaker for DC interruption or AC fault current limitation

A hybrid medium voltage breaker consisting of a power electronic switch (PE) by-passed by a parallel mechanical switch is presented in the paper. The main operation modes and the performances of a 12 kV apparatus with 25 kA short-circuit current capability are introduced. The results of the experimental tests confirm the electrical behaviors foreseen by the simulations during the design phase. The uses of the apparatus in MV applications as AC fault current limiter (FCL) and DC circuit breaker are presented and discussed.

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.

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.

DC power distribution: New opportunities and challenges

The benefits offered by the DC energy distribution in different applications raised the interests towards new power architectures and apparatus. The availability of the related LV and MV apparatus and protection schemes is in fact crucial to fully exploit the opportunities opened in the energy management for the smart grid. Experimental results of testing DC circuit breakers based upon current injection and solid-state technologies are presented and their effectiveness for DC distribution protection is discussed. The possibilities opened by local measurement and communication to support the operation of DC circuit breakers for DC protection as well as the design challenges and implementation issues are investigated.

Dependence of the Chopping Current Level of a Vacuum Interrupter on Parallel Capacitance

The distribution of chopping currents of a vacuum circuit breaker using CuCr25 contact material was measured in a laboratory circuit consisting of a three-phase 20-kV cable system and a dry-type distribution transformer rated 900 kVA. The cable length between the upstream feeding transformer and the circuit breaker was several hundreds of meters. An inductive load connected to the distribution transformer provided a 50-Hz current of 10 A rms. Under these conditions, the mean chopping current level was between 3.2 and 3.5 A and showed no significant dependence on the arcing time, which was varied between 2 and 8 ms. After addition of a surge capacitor of 130 nF between the transformer terminals and ground, which very much reduces the overvoltage during switching, the chopping current level increased to 5.6 to 7.7 A in dependence of the arcing time. In order to explain this, the interaction of the arc with the electric circuit in particular with the cables on both sides of the breaker has to be evaluated. If the cables are long, travelling waves need to be considered as well as all return current paths in a three-phase circuit. The coincidence of excited network oscillations with random arc instabilities may result in either momentary amplification or damping of such oscillations. An additionally installed capacitor serves as a source sustaining a momentary current reduction after an arc instability and allows the excitation of a subsequent arc instability leading to an increase of the chopping current with longer arcing times. With low capacitance on the load side, there is no interaction and therefore no influence from the arcing time.