Lightning impulse is an important overvoltage form that affects the insulation safety of power equipment. As a key insulating and cooling medium in oil-immersed transformers and other equipment, insulating oil's streamer initiation, propagation, and breakdown behavior under strong impulse electric fields directly affect equipment insulation design and operational reliability. How can we "visualize" this rapid discharge process, which spans nanoseconds to microseconds in duration and covers spatial scales from tens of micrometers to centimeters? This paper introduces a high-speed synchronous diagnostic platform for recording streamer morphology and its electrical responses.

Liquid Dielectric Impulse Discharge Observation Platform

This platform, jointly established by the State Key Laboratory of Power Transmission Equipment Technology at Chongqing University (CQU) and Shuangjiang Energy, is a high-speed imaging and synchronous diagnostic system designed for impulse discharge research in liquid dielectrics. The platform consists of impulse voltage generation, optical illumination, ultra-high-speed imaging, synchronous triggering, and data acquisition and image analysis modules. It can record the key processes of streamer initiation, propagation, and breakdown in insulating oil under lightning impulse voltage, while synchronously capturing voltage and pulse current signals. In a single impulse test, the system can acquire multiple time-sequenced images with nanosecond-level exposure, thereby reducing motion blur caused by rapid streamer movement and improving the resolution of initial streamer stages and branch details. Combined with image processing and electrical signal analysis, characteristic parameters such as streamer length, propagation velocity, branch count, channel morphology, and breakdown time can be extracted, providing experimental evidence for evaluating the impulse discharge performance of different insulating oil and additive systems.

Core Capabilities: Three Major Units in Synergy

From the perspective of experimental implementation, the platform can be further categorized into three major experimental units: impulse voltage generation and discharge test unit, background illumination unit, and synchronous imaging unit.

Impulse Voltage Generation and Discharge Test Unit

The core of the system consists of an impulse voltage generator and a precisely designed discharge oil tank (Figures 1 and 2). The impulse voltage generator delivers a maximum discharge voltage of up to 600 kV and can generate standard lightning impulse waveforms of 1.2/50 μs. An oscilloscope with a maximum sampling rate of 40 GS/s and a pulse current sensor with a nominal bandwidth of 250 MHz are used to synchronously record impulse voltage waveforms and pulse current signals generated during discharge. The internal effective dimensions of the discharge oil tank are 150 × 150 × 200 mm, with an adjustable needle-plate electrode configuration (Figure 3) to establish a typical highly non-uniform electric field. The needle tip radius of curvature is approximately 50 μm, creating a strong local electric field near the tip to promote streamer initiation.

Background Illumination

To "freeze" streamer morphology under extremely short exposure times, two types of dedicated background light sources are incorporated into the system:

• Collimated parallel background light source: Used with high-speed cameras to capture the overall morphology of streamer development at larger scales.

• High-power uniform diffuse background light source (rated 300 W): Used in conjunction with an ultra-high-speed framing camera with exposure times as short as hundreds of nanoseconds, meeting the brightness requirements for nanosecond-level transient image capture.

Synchronous Imaging Unit

The platform's standout capability lies in its ultra-high-speed imaging performance. It is equipped with a pyramidal beam-splitting ultra-high-speed framing camera. The incident image is distributed to multiple imaging channels through a pyramidal beam-splitting structure, with each channel sequentially exposed at set time delays. The camera features 4 imaging channels, each capable of recording 2 images, allowing up to 8 time-sequenced images to be obtained from a single discharge event. The minimum single-frame exposure time/gate width reaches 1.2 ns, sufficient to capture the fine morphology of the initial streamer development stage. The system extracts a pre-trigger pulse from the induced current signal during impulse generation. After delay control, it triggers both the oscilloscope and the ultra-high-speed camera, achieving temporal synchronization between electrical signals and streamer images.

Research Mission: Unlocking the Secrets of Insulating Oil Discharge

The ultimate goal of this platform is to support insulating oil performance evaluation and discharge mechanism research. The following presents the results of positive and negative polarity lightning impulse tests on ester-based insulating oils using this system:

• Positive streamers: Initiate near the needle tip and propagate toward the plate electrode under applied voltage. The number of branches increases progressively, exhibiting an overall tree-like morphology. As breakdown approaches, some branches gradually form dominant channels and ultimately bridge the electrode gap.

• Negative streamers: Also initiate near the needle tip but differ from positive streamers in terms of branch morphology, radial extent, and channel evolution. During development, multiple branches extend alternately before gradually forming a dominant channel in the bridging direction. The main channel formed during the breakdown stage is more continuous, with clearer channel boundaries and propagation paths.

The synchronized acquisition of high-temporal-resolution images with voltage and current signals provides important experimental evidence for analyzing streamer initiation, propagation, branch evolution, and breakdown under different impulse polarities.