What are the practical uses of transformers in power electronics?

1. Working Principle of the Spark Gap

The spark gap operates based on the principle of gas discharge. When a sufficiently high voltage is applied between two electrodes, the gas between the electrodes is ionized, forming a conductive channel, and thus spark discharge occurs. This process is similar to the discharge phenomenon that occurs between clouds and the ground during lightning. The ionization of the gas is due to the fact that the electric field strength is strong enough to enable the electrons in gas molecules to obtain sufficient energy to break free from the bondage of atoms or molecules, forming free electrons and ions. These free electrons and ions accelerate under the action of the electric field, collide with other gas molecules, generating more ionization processes, and ultimately leading to the breakdown of the gas and the formation of spark discharge.

According to Paschen's law, the breakdown voltage of a gas is a function of gas pressure, electrode spacing, and gas type. Given a specific gas type and pressure, there is a certain relationship between electrode spacing and breakdown voltage. Generally speaking, the larger the electrode spacing, the higher the breakdown voltage.

2. Basic Methods of Using the Spark Gap to Determine the Voltage

Calibrating the Spark Gap Device

Firstly, it is necessary to calibrate the spark gap using a known voltage. A standard voltage source, such as a high-precision DC or AC voltage generator, can be used and connected to the electrodes of the spark gap. Gradually increase the voltage until spark generation is observed, and record the voltage value and the corresponding electrode spacing at this time. For example, for a spark gap with air as the medium, when the electrode spacing is 1 mm, the breakdown voltage measured using the standard voltage source is 3 kV, thus obtaining a calibration data point.

By changing the electrode spacing and repeating the above process, a series of breakdown voltage data corresponding to different electrode spacings can be obtained, and the relationship curve between electrode spacing and breakdown voltage can be plotted. This provides a calibration basis for the subsequent measurement of an unknown voltage.

Measuring the Unknown Voltage

When determining an unknown voltage, connect the unknown voltage source to the calibrated spark gap device. Gradually increase the voltage until spark discharge is observed. Measure the electrode spacing at this time, and then according to the previously plotted calibration curve, look up the corresponding voltage value. This voltage value is approximately the unknown voltage. For example, when measuring the voltage of a high-voltage pulse, if spark generation is observed when the electrode spacing is 2 mm, and the corresponding voltage obtained from the calibration curve is 6 kV, then the voltage of the high-voltage pulse is determined to be approximately 6 kV.

3. Precautions and Sources of Error

Influence of Gas Conditions: The type, pressure, and humidity of the gas can have a significant impact on the breakdown voltage. For example, in a high-humidity environment, the increase in water vapor content in the air will lower the breakdown voltage of the gas. Therefore, during the measurement process, it is necessary to keep the gas conditions as stable as possible. If possible, it is best to conduct the measurement under standard atmospheric pressure and in a dry environment, or make corrections for changes in gas conditions.

Influence of Electrode Shape and Surface Condition: The shape (such as spherical, needle-shaped, flat-plate-shaped, etc.) and surface condition (such as roughness, presence of oxide layers, etc.) of the electrodes will also affect the breakdown voltage of the spark gap. Different shapes of electrodes will result in an uneven electric field distribution, thereby changing the breakdown voltage. For example, the needle-plate electrode structure has an electric field concentrated at the tip of the needle electrode, making it more prone to breakdown, and its breakdown voltage is relatively low. The roughness and oxide layers on the electrode surface may adsorb gas molecules or change the electric field distribution. Therefore, during the measurement process, it is necessary to ensure the consistency of electrode shape and surface condition, or take these factors into account and make corrections.

 Limitations of Measurement Precision: Measuring voltage using a spark gap is a relatively rough method, and its precision is limited by multiple factors. Besides the above-mentioned gas conditions and electrode factors, spark discharge itself is an instantaneous and somewhat random process that is difficult to precisely control and measure. Moreover, in high-voltage situations, multiple discharges or continuous arcs may occur, which will also affect the accuracy of the measurement results. Therefore, this method is usually used for a rough estimate of the voltage rather than for high-precision voltage measurement.