1 Introduction Arc voltage is a parameter that can reflect the performance of the arc-extinguishing chamber of a high-voltage circuit breaker, and product designers have been very interested in it for many years. As early as more than 20 years ago, some people had already studied it [1]. The test system at that time consisted of a light oscilloscope and a low-resistance voltage divider. The low-resistance voltage divider affected the equivalence of the circuit and was only suitable for arc voltage testing of oil circuit breakers with high arc voltage. However, the arc voltage testing of vacuum circuit breakers and SF6 circuit breakers with low arc voltage, which are widely used now, has been rarely studied. This paper introduces a digital test system for arc voltage of high-voltage circuit breakers in a large-capacity synthetic test circuit. The synthetic test circuit is a single-phase grounded circuit, which is suitable for the breaking test of high-voltage circuit breakers of 40.5 kV and above. This paper uses this test system to measure the arc voltage of 40.5~252 kV high-voltage circuit breakers breaking short-circuit current and obtains the arc voltage waveform. Using this system, more valuable information can be collected from type tests. [b]2 Test System Composition[/b] The test system is shown in Figure 1. Sensor — 300 kV (dedicated voltage divider pair for arc voltage measurement) Isolation protection — including amplitude limiting protection device and isolation amplifier Digital fiber optic signal transmission system (referred to as digital fiber optic) — 12-bit resolution, sampling rate 10 MS/s Transient recorder — multi-channel, 256 k memory, IEEE488 interface Plotter — HP Colorpro CAD plotter [b]3 System Requirements and Features 3.1 System Safety[/b] As is well known, in arc voltage testing, the test system may withstand recovery voltages of several hundred kilovolts, and the safety of this measurement system is crucial. Therefore, a amplitude limiting protection device was specifically designed and researched, which, through effective cooperation with the voltage divider, successfully protected the safety of the measurement system. The limiting protection device has the following characteristics: (1) Response speed of 10-12s; (2) It can suppress both transient high-energy impulse voltage (duration of tens to hundreds of microseconds) and high-energy steady-state continuous voltage (duration of hundreds of milliseconds to tens of seconds); (3) Reliable and stable operation. Because the test system must meet the conditions for industrial operation, its reliability and stability requirements are very high and must be foolproof. Through hundreds of arc voltage tests on dozens of products in the large-capacity test station, it is proven that the device has good performance and stable and reliable operation; (4) High impedance and low leakage current. [b] 3.2 High common-mode rejection ratio of the sensor[/b] Due to the strong electromagnetic interference at the measurement site (the short-circuit current of the circuit breaker at the test site can reach 50 kA and the voltage can reach hundreds of kilovolts), the system is required to have a strong anti-common-mode interference capability. Arc voltage is typically in the tens to hundreds of volts range. After attenuation by the sensor, it is reduced to only a few hundred millivolts. Even a small amount of common-mode interference voltage superimposed on it will mask the arc voltage waveform, distorting the amplitude test and making it difficult to determine the arc initiation point. Therefore, a key point in arc voltage testing is to minimize common-mode interference. The rationality analysis of the sensor's common-mode rejection capability is as follows: Assume the sensor parameters are: Sensor 1: voltage division ratio K1, input voltage Uin1, output voltage Uout1; Sensor 2: voltage division ratio K2, input voltage Uin2, output voltage Uout2. Under the action of common-mode voltage UE, Uin1 = Uin2 = UE, then Uout1 = K1 UE, Uout2 = K2 UE. Therefore, the unbalanced output voltage ΔU = Uout1 - Uout2 = (K1 - K2)UE. Define the unbalance degree γE = ΔU/UE, then γE = K1 - K2, which is the difference between the two voltage division ratios. Under the differential-mode voltage Uin, considering the common-mode interference voltage UE, the sensor outputs are: Uout1 = K1(Uin/2 + UE), Uout2 = K2((-Uin/2) + UE). Therefore, the differential-mode output voltage is: Uout = Uout1 - Uout2 = (K1 + K2)Uin/2 + (K1 - K2)UE. Assuming the actual output voltage is K2Uin, the measurement error Uout - K2Uin = (K1 - K2)Uin/2 + γEUE = γE(Uin/2 + UE); the relative measurement error γ = (Uout - K2Uin) / (K2Uin) = γE(Uin/2 + UE) / (K2Uin), that is, γ = γE(UE / K2Uin + 1/2 K2). Assuming UE = 6000 V, Uin = 500 V, and K2 = 1/300, then γ = γE × 3750 → γE = γ/3750. The discussion is as follows: When γ = 3%, γE = 8 × 10⁻⁶, meaning the balanced voltage under 6 kV common-mode voltage is 48 mV; when γ = 2%, γE = 5.4 × 10⁻⁶, meaning the unbalanced voltage under 6 kV common-mode voltage is 32 mV. This means that under 6 kV common-mode voltage, if the unbalanced voltage of the voltage divider is controlled at 32 mV, the error it causes to the arc voltage test is only 2%. This implies that the design and manufacturing technology of the voltage divider is very difficult, requiring the difference in the voltage division ratio of the two voltage dividers to not exceed 5.4 × 10⁻⁶. [b]3.3 Digital Fiber Optic Signal Transmission System[/b] Due to the strong electromagnetic field at the measurement site, the secondary part of the measurement system uses a fully self-contained digital fiber optic cable enclosed in an EMC housing. This cable integrates data acquisition and opto-isolation, directly converting analog signals to digital signals at its front end. The digital signals are then transmitted via fiber optic cable to a transient recorder for storage and processing. The use of digital fiber optic cable significantly improves the system's common-mode interference immunity and the overall safety of the testing system. The technical parameters of the digital fiber optic system are as follows: Input differential impedance 1 MΩ, 50 pF Frequency band 5 MHz 3 dB Coupling method AC, DC or Ground Filtering Bessel filter Resolution 12-bit Sampling rate 10 MS/s Memory 256 k Inaccuracy ±9 LSB Nonlinearity ±3/4 LSB Triple shielding Battery powered for 16 h Range 100 mV～±100 V Transmission rate 240 MBit/s [b]4 System performance evaluation[/b] (1) Withstand voltage passed at 300 kV for 1 min. (2) Common mode rejection ratio measurement Table 1 shows the common mode rejection ratio measurement results, where: Uin is the common mode input voltage, 50Hz, RMS value, measured with an electrostatic voltmeter; Uout is the differential mode output voltage, measured with a digital multimeter. (3) Differential mode voltage divider ratio measurement Table 2 shows the differential mode voltage divider measurement results, where: Uin is the differential mode input voltage, 50Hz, effective value, standard AC voltage source; Uout is the differential mode output voltage, measured with a digital multimeter. [img=428,179]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gydq/2001-1/27-1.jpg[/img] (4) Bandwidth Table 3 shows the bandwidth measurement values ​​of the system, where: Uin is the differential mode input voltage, 50Hz, effective value, standard AC voltage source; F is the voltage frequency; Uout is the differential mode output voltage, measured with a digital storage oscilloscope. [b]5 Arc voltage measurement[/b] Since the system was put into trial operation, it has been working normally and the arc voltage of SF6 circuit breaker and vacuum circuit breaker interrupting short circuit current was tested. Example 1: Product model LW9-145/31.5, self-extinguishing arc interruption current 31.5 kA, the measured arc voltage waveform is shown in Figure 2. The waveform clearly shows the extinguishing peak and the arc initiation point, allowing for accurate determination of the arc burning time; the arc voltage waveform is smooth, and the trend conforms to the analysis of arc theory [2], allowing for quantitative determination of the arc voltage at each point. Example 2: Product model ZW-40.5/31.5, arc extinguishing method is cup-shaped longitudinal blowing; opening distance 20-22 mm; initial splitting velocity 1.6 m/s. The measured arc voltage waveform is shown in Figure 3. The measured arc voltage is simultaneously recorded with the short-circuit current waveform, allowing for clearer analysis of the state at each point on the arc voltage. From the waveform, the arc initiation point, arc extinguishing point, arc voltage at each point (tens of volts), and the arc-extending interference superimposed on the waveform are visible. **6 Conclusions** This system has good electromagnetic compatibility and can quantitatively measure the arc voltage when the circuit breaker interrupts the short-circuit current, meeting the requirements of industrial operation. During arc voltage testing, short-circuit current waveform testing can also be provided simultaneously, so the arc power can also be given in real time, providing a direct basis for arc research and circuit breaker performance analysis and improvement. It can also provide a reference for testing in other high-voltage, high-current applications. **References** [1] Peng Wenda. Measurement of Arc Voltage [J]. High Voltage Apparatus, 1975, No. 3 [2] Zhang Jierong et al. Principles and Applications of High Voltage Apparatus [M]. Beijing: Tsinghua University Press, 1989.