Improvement of circuit oscillation generated by underwater high voltage pulse discharges based on pulse power thyristor

Abstract

High voltage fracturing technology was widely used in the field of reservoir reconstruction due to its advantages of being clean, pollution-free, and high-efficiency. However, high-frequency circuit oscillation occurs during the underwater high voltage pulse discharge process, which brings security risks to the stability of the pulse fracturing system. In order to solve this problem, an underwater pulse power discharge system was established, the circuit oscillation generation conditions were analyzed and the circuit oscillation suppression method was proposed. Firstly, the system structure was introduced and the charging model of the energy storage capacitor was established by the state space average method. Next, the electrode high-voltage breakdown model was established through COMSOL software, the electrode breakdown process was analyzed according to the electron density distribution image, and the plasma channel impedance was estimated based on the conductivity simulation results. Then the underwater pulse power discharge process and the circuit oscillation generation condition were analyzed, and the circuit oscillation suppression strategy of using the thyristor to replace the gas spark switch was proposed. Finally, laboratory experiments were carried out to verify the precision of the theoretical model and the suppression effect of circuit oscillation. The experimental results show that the voltage variation of the energy storage capacitor, the impedance change of the pulse power discharge process, and the equivalent circuit in each discharge stage were consistent with the theoretical model. The proposed oscillation suppression strategy can not only prevent the damage caused by circuit oscillation but also reduce the damping oscillation time by 77.1%, which can greatly improve the stability of the system. This research has potential application value in the field of underwater pulse power discharge for reservoir reconstruction.

Failure mechanisms of the thyristor switch in pulsed arc electrohydraulic discharges

“…where R s and L S represent the equivalent resistance and inductance of the gas spark switch, while R E and L E represent the equivalent resistance and inductance of the discharge electrode, respectively. When the gas spark switch is used as the pulse power switch, the discharge process can be divided into four stages, namely the switch breakdown stage, electrode prebreakdown stage, discharge stage, and the damping oscillation stage [5]. After replacing the gas spark switch with the thyristor switch, the theoretical voltage and current waveforms and corresponding equivalent circuits of each discharge stage are depicted in figure 4.…”

“…Conventional electric pulse fracturing equipment employs the gas spark switch [3] as the pulse power switch, and the impreciseness of its reverse recovery time significantly affects the precision of electric pulse output frequency. The thyristor has the highest voltage withstand level and the largest output capacity [4], and if it is employed to substitute the gas spark switch in the pulse power supply, it can not only improve the accuracy of the electric pulse frequency output but also suppress the circuit oscillation [5].…”