Abstract: Traditional power capacitor testing is usually conducted offline with the power off, affecting production. Existing online diagnostic technologies for power capacitors focus on measuring capacitance and dielectric loss angle, with results lagging behind fault occurrence and yielding unsatisfactory results. This paper mainly focuses on common faults of high-voltage power capacitors in power systems, analyzing the mechanism of discharge phenomena in these faults. It concludes that various power capacitor faults are usually accompanied by discharge phenomena, indicating that discharge phenomena are the initial signs of faults. The feasibility of online monitoring of discharge phenomena in high-voltage power capacitors is analyzed, a method for processing discharge signals is found, and a complete theoretical design for a new online monitoring technology for power capacitor discharge is presented. Keywords: Power capacitor; Discharge phenomenon; Online monitoring; Spectrum analysis Power capacitors are widely used in power systems and are among the most important working components. The two main types of power capacitors are parallel capacitors and coupling capacitors. Parallel capacitor devices, as a crucial reactive power source, play a decisive role in improving power system structure and power quality. Coupling capacitors mainly undertake power system filtering, carrier wave, and high-frequency protection tasks. In addition to enduring long-term operating voltage, power capacitors are also subject to various internal and external overvoltages during operation, leading to gradual aging. Under operating conditions, power capacitors are directly connected to transmission lines. Conventional testing methods require shutting down the entire transmission line, impacting production. Furthermore, preventative testing of power capacitors is frequently missed due to power outages, posing a safety hazard. Statistics on online power capacitor accidents show that partial discharge is a common initial symptom of accidents, subsequently developing into short-circuit faults in some components. Currently, online testing of power capacitors typically involves monitoring the current (I, C, and tanσ) flowing through the capacitors under high operating voltage to determine fault status. However, changes in capacitance and dielectric loss angle are delayed results that occur after discharge accumulates to a certain extent. Direct monitoring of discharge phenomena allows for earlier and more timely detection and prevention of accidents. Therefore, monitoring partial discharge in power capacitors is an effective way to prevent accidents. However, there are few reports on online monitoring of partial discharge in power capacitors. This paper proposes a new online monitoring technology for power capacitor discharge, which can be used to diagnose its working condition. 1. Analysis of Power Capacitor Discharge Phenomena 1.1 Parallel Capacitors Parallel capacitors are sealed on the outside and connected to the three phases of the busbar through the outgoing terminals, with one terminal grounded. The internal structure consists of insulating paper, aluminum foil, and capacitor oil forming a series capacitor element, which cannot be observed from the outside. Fault phenomena of parallel capacitors include oil leakage, bulging, flashover of the casing, fuse blowing, and explosion. Almost all faults of parallel capacitors are accompanied by discharge phenomena. Oil leakage from the porcelain bushing and casing. Due to oil leakage, the inside of the bushing becomes damp, the insulation resistance decreases, the oil level drops, and the upper part of the element is easily dampened, leading to breakdown and discharge. Among all faults of parallel capacitors, bulging accounts for the largest proportion. It is normal for the oil tank to expand and contract with temperature changes, but when partial discharge occurs inside, the insulating oil produces a large amount of gas, which will deform the tank wall and form obvious bulging. A bulging capacitor cannot be repaired and must be replaced. Due to poor insulation and severe surface contamination, insulation breakdown and surface discharge occur under conditions of internal and external overvoltages and system resonance in the power grid, leading to flashover damage to the porcelain bushing. Fuse blowing originates from discharge within the capacitor's internal components, resulting in fault breakdown. Capacitor explosion occurs when a fault breakdown of an internal component causes a through-circuit between the capacitor's electrodes. Other capacitors operating in parallel will discharge onto the faulty capacitor; if the energy injected into the capacitor exceeds the explosion energy the casing can withstand, the capacitor will explode. Besides contamination and moisture during operation, the causes of parallel capacitor accidents are also related to the capacitor's structure and manufacturing quality. A comprehensive analysis is as follows: 1) Under high field strength, capacitor element breakdown often occurs at electrode edges, corners, and the contact points between leads and plates, as well as where elements fold. These areas have high electric field strength and current density, making them prone to partial discharge and overheating that burns the insulation. Appropriate isolation measures and reasonable structural design should be adopted during manufacturing. 2) Overvoltage caused by excessively high operating voltage or switch reignition will also produce partial discharge. The insulation between the electrodes and the tank is generally high. Manufacturing processes and component quality, such as poor insulation material quality and impure capacitor oil, are the main causes of this type of discharge. 3) Poor sealing. Coupling capacitors are fully sealed electrical appliances. If the seal is poor, water may enter and cause damage during operation. Poor sealing often manifests as oil leakage during operation. Coupling capacitors with long-term oil leakage will not only experience water and moisture ingress due to reduced internal pressure, but will also experience discharge failure due to reduced oil volume and leakage at the top. 4) Excessive operating voltage of power capacitors generates a large amount of loss and damages insulation. Excessively high operating temperature and the addition of harmonics can also induce capacitor discharge. The former damages insulation, while the latter increases the operating voltage. 1.2 Coupling Capacitors Most coupling capacitor accidents occur in rainy and polluted weather. The accident phenomenon is surface discharge causing flashover. Internal discharge accumulation may cause breakdown and short circuit, fuse blowing, or even the capacitor itself bursting. Its fault phenomenon analysis is similar to that of parallel capacitors. Besides the issues of dirt and moisture during operation, the causes of accidents are also related to the capacitor's own structure and manufacturing quality. A comprehensive analysis is as follows: 1) AC and DC discharge mechanisms differ, resulting in different equivalent circuits. DC discharge is more prone to breakdown. Appropriate isolation measures and reasonable structural design should be adopted during manufacturing. 2) Improper insulation design at the capacitor core location during manufacturing can lead to excessive electric field strength at the core's sharp corners during operation, easily causing discharge. Poor drying of the capacitor core, residual moisture, or prolonged exposure to air after winding can also create hidden dangers. 3) Regarding poor sealing: Coupling capacitors are fully sealed electrical appliances. If the seal is poor, water and moisture may enter during operation, leading to damage. Each coupling capacitor is equipped with an expander and undergoes pre-shipment inspection. Poor sealing often manifests as oil leakage during operation. Coupling capacitors with long-term oil leakage, in addition to reduced internal pressure leading to water and moisture ingress, may also experience discharge failure due to reduced oil volume and upper oil leakage. 4) Unreasonable processes and defects during manufacturing, as well as damage caused during handling, can also become hidden dangers for coupling capacitor accidents. Based on existing experience, potential hazards include defects in the manufacturing process of the clamping plate, insufficient aromatic hydrocarbon content in the capacitor oil, component soldering failure, and component misalignment. These defects can easily induce discharge phenomena. Conclusion: External discharge of a power capacitor is caused by the high voltage of the electrodes or busbar discharging to the casing, while internal discharge is caused by discharge between the plates. Internal discharge of the capacitor is unobservable. 2. Design of Online Monitoring Technology The structural block diagram of this technology is shown in Figure 1, consisting of three main parts: a sensing device, a signal processing device, and a computer analysis system. The core idea of ​​the technology is to clarify the mechanism of fault discharge in power capacitors. Surface discharge is caused by the high voltage of the internal electrodes or busbar discharging to the capacitor casing, while internal discharge is caused by discharge between the capacitor plates. Surface discharge of the capacitor can be observed as flashover, while there is currently no mature method to detect internal discharge. Surface discharge signals flashover is rapid and at a high frequency, while internal discharge is relatively slow and at a lower frequency. Fault formation is a gradual accumulation process, leading to heating and insulation breakdown. The lowest potential point of the entire power capacitor device is located on its grounding wire; therefore, most signals from both surface and internal discharges of the capacitor should travel through this grounding wire. Therefore, a wideband Rokowski coil is used as a sensor to capture the discharge signal from the capacitor's ground wire. This signal is then fed into a spectrum analyzer to obtain a spectrum diagram. Due to the significant frequency difference between the internal and external discharge signals, comparison with standard spectrum analysis data in a computer database can determine whether the capacitor is discharging internally. The discharge intensity and signal identification can be measured using appropriate energy sensors and noise reduction methods. Regarding the Rokowski sensing coil: A Rokowski coil is formed by connecting the two ends of an air-core solenoid coil and winding it around a current-carrying conductor. Arc discharge generates a wide-bandgap electromagnetic wave, a portion of which propagates to the grounding lead of the measuring terminal. Because the grounding lead of the measuring terminal has a very low potential to ground, any flashover discharge signal from any part of the coupling capacitor will flow to the ground through it. By winding the Rokowski coil around this grounding lead, the discharge signal can be accurately measured. The Rokowski coil has a wide frequency response (up to 30MHz), making it suitable for online acquisition of discharge signals from various high-voltage equipment. The monitoring instrument is used to determine the average peak energy received by the test coil. By monitoring the radio frequency (RF) current, the degree of discharge of the coupling capacitor can be determined, and then a decision can be made on how to handle it. The RF monitoring device配套 with the Rokowski coil consists of an RF amplifier, a logarithmic amplifier, a detector, and a peak hold circuit. The signal acquired by the Rokowski coil is amplified by RF and logarithmic signals before being sent to the detector, and then peak held before being sent to the A/D converter. All of these circuit components can use standard integrated circuit chips. The center frequency of the RF monitoring device depends on the application and background RF noise. It can detect the discharge arc signal of the parallel capacitor through a sensing device to identify the discharge phenomenon of the parallel capacitor, but it cannot determine the location of the discharge. However, this is sufficient for the purpose of measuring only the discharge intensity of the coupling capacitor. After the signal passes through the radio frequency monitoring device, it is sent to the A/D converter and then analyzed by the spectrum analyzer to obtain the frequency components of the extracted signal. The computer compares the spectrum analysis data with the standard spectrum data in the database. The newly emerging frequency components determine whether the internal electric field is within the normal range, whether the internal field strength needs to be calculated and analyzed, and then decide whether further processing, such as power outage maintenance, is required. The technology uses software based on the finite element method to analyze and calculate the internal electric field strength of the power capacitor that requires further processing. The software is implemented in C language. 3. Conclusion Since almost all fault precursors in power capacitors are accompanied by discharge, surface discharge can be observed, but internal discharge is difficult to monitor, which brings difficulties to the operation and maintenance of capacitors. The proposed method for online monitoring of the discharge phenomenon of parallel capacitors is a positive and desirable measure to prevent accidents before they occur. The technology is theoretically sound and feasible, the components are easy to implement, and the perspective is novel. The launch of mature products is currently underway.