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

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 cannot only prevent the damage caused by circuit oscillation but also reduce the damping oscillation time by77.1%, 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.

Numerical Simulation of Properties of Charged Particles Initiated by Underwater Pulsed Discharge

Governing conservation equations for energy, momentum, mass and charge were deduced. Based upon these equations and the Saha equation, the particle density, temperature and pressure of the channel initiated by underwater pulsed discharge, are simulated. Influence of temperature and pressure on particles density is also analyzed. Some of the simulation results are in an agreement with experimental results. The results will be helpful in further understanding of the formation mechanism of underwater pulsed discharge plasma.

Underwater pulsed spark discharge influenced by the relative position between the top of a pin electrode and an insulating tube

Needle-to-plane geometry has been widely investigated and used in underwater pulsed discharges.The position relationship between the needle tip and insulation layer significantly affects the discharge patterns.We carried out experiments on underwater pulsed discharge with the needle tip protruding from,recessing into,and flushing with the insulating tube.The results are as follows.First,underwater pulsed discharge has a strong randomness under the experimental conditions.Different discharge patterns appeared under the same experimental environment.Second,recession into the insulator surface led to a higher probability of occurrence but a lower strength of spark discharge than protrusion.Third,between the needle tip protruding from and recessing into the insulation material,the average speed of propagation of underwater pulsed spark discharge decreased by an order of magnitude.The study shows that the optimum length of needle tip protruding from the insulation layer is 1 mm to obtain a strong underwater pulsed spark discharge.