The DC circuit-breaker for low-voltage grids : design, implementation and analysis of DC circuit-breaker with countercurrent injection principle

Virdag, Muhammad-Ali-Khan; de Doncker, Rik W. (Thesis advisor); Bauer, Pavol (Thesis advisor)

1. Auflage. - Aachen : E.ON Energy Research Center (2021, 2022)
Book, Dissertation / PhD Thesis

In: E.On Energy Research Center : PGS, Power Generation and Storage Systems 96
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2021

Abstract

Faults in DC systems are difficult to interrupt compared to their AC counterparts due to absence of natural current-zero (CZ) crossing in DC current. This puts forward the need of enhanced arc chambers inside the circuit-breaker to cope with DC arc. An approach of connecting multiple AC circuit-breakers in series to extinguish DC arc leads to an increase in size and cost of circuit-breaker. Furthermore, rate of rise of fault current in voltage source converter (VSC) based DC microgrids is very high due to the absence of high inductance transformers. This is further aggravated by the DC link capacitors inside DC-DC converters, which discharge immediately in case of a fault. As a result, there is steep increase in the fault current with a rate of rise being dependent on the equivalent impedance of the system. To cope with this fast-rising fault current in DC systems, a circuit-breaker has to interrupt the fault in a very short duration of time. It is therefore important, to come up with alternate methods for the extinction of electric arc inside DC circuit-breaker. One such method to have minimum arcing during turn-off process is to artificially create zero crossing in the DC fault current using power-electronic circuitry. This method is investigated in the thesis. In this thesis, at first a prototype of hybrid DC circuit-breaker topology based on counter-current injection principle to perform zero current switching (ZCS) is developed and analyzed. The topology allows use of a mechanical circuit-breaker assisted with a power electronic based auxiliary circuit (RLC auxiliary circuit and RCD snubber circuit) for fast switching of DC fault currents at minimal arcing. By using thyristors instead of mechanical circuit-breakers to enforce switching at current zero (CZ) crossing, the work focuses on the modeling of different circuit elements for DC circuit-breaker to interrupt a fault current up to 100A with prospective fault current up to 10kA in 380VDC grids. In addition, an enhanced design of a capacitor charging circuit for the DC circuit-breaker is proposed and implemented. The proposed design, on one hand, uses only one semiconductor device for the charging circuit compared to two semiconductor switches that are generally used in the charging circuits. On the other hand, it reduces the complexity of control and power circuit. To reduce the overvoltage transient during turn-off of DC circuit-breaker, an RCD snubber circuit is designed that reduces the overvoltage created by surplus counter-current flowing through thyristor. Dedicated gate drive circuitry, that offers galvanic isolation for MOSFET based switches, was also designed and implemented during the thesis. The work investigates the parameters and boundary conditions that result in successful and unsuccessful interruption of the DC circuit-breaker. The peak value of counter-current in auxiliary circuit should be higher than the fault current to create zero crossing in the fault current flowing through thyristor, a necessary condition for thyristor to go in off-state in the absence of firing signal. Apart from that, thyristor should remain under negative voltage for a certain amount of time (tq) to ensure successful turn-off. Series RLC elements in the auxiliary circuit have an impact both on the peak value of countercurrent and the negative voltage across thyristor. This relationship is analyzed and identified in the thesis. Furthermore, the turn-on time duration for the capacitor charging circuit and the auxiliary circuit with RLC elements govern the circuit-breaker performance in terms of transients of current through voltage across the thyristor. The prototype was tested with different types of loads and corresponding behavior was analyzed. Capacitive load delivers high fault current compared to inductive and resistive loads; however, this topology of hybrid DC circuit-breaker offers fast turn-off to capacitive loads. A test bench was built to test the developed DC circuit-breaker prototype, with a current of up to 100A and a source voltage of 380VDC.