Single-phase ground capacitance current

0 Introduction At present, most of China’s 10 kV distribution networks use an ungrounded neutral system, while some employ a resonant grounding system. Practice has shown that when both the scale of the power grid and the capacitive current during single-phase grounding are low, the advantage of this ungrounded system is that it allows continued operation for 2 hours after a single-phase grounding fault occurs, preventing power outages for users and thereby improving power supply reliability. However, as distribution networks expand and the number of cables and overhead lines increases, this approach exhibits its drawbacks. 　　(1) When a single-phase ground fault occurs in the distribution network, the ground arc cannot extinguish itself and will inevitably develop into an inter-phase short circuit, resulting in power outages for users and equipment damage. 　　(2) When intermittent arc grounding occurs, it induces high arc overvoltages, typically 3.5 times the line voltage or even higher. These overvoltages affect the entire power distribution network, causing devices with weak insulation to discharge and break down, which leads to equipment damage and serious outages. 　　(3) When someone accidentally touches a live part, the burns caused by the high current exacerbate the severity of the injury, potentially leading to death on the spot. 　　(4) Resonant overvoltage phenomena in distribution networks occur quite frequently. The amplitude of such ferroresonant overvoltages is not high, but they last for a long time, causing insulation flashover or arrester explosions due to low-frequency oscillations ； Or an overcurrent may occur in the transformer; in mild cases this will cause the TV fuse to blow, while in severe cases it can burn out the TV. When an unstable intermittent arc occurs, and the overvoltages resulting from repeated extinguishing and reignition coexist with ferroresonant overvoltages, it not only causes the TV to burn out but also leads to the destruction of all distribution equipment, a phenomenon known as \"chain failure\". A similar accident once occurred at the Chengwu Substation of the Heze Electric Power Bureau in Shandong, having a significant impact on the system’s power supply. Source: High-Voltage Switchgear Network (5) In areas where there is a significant conflict between overhead lines and trees, tripping and power outages due to inter-phase short circuits caused by single-phase faults occur frequently during windy or rainy weather. 　　Therefore, changing the operating mode of the distribution network neutral point and improving power supply reliability have become an urgent task for power supply authorities. 　　1 Current operating status of neutral point grounding through arc-suppression coils 1.1 Regulations regarding neutral point grounding through arc-suppression coils The \"Technical Code for Overvoltage Protection Design of Power Equipment\" stipulates that in power grids with a voltage level of 3–10 kV, arc-suppression coils should be installed when the current resulting from a single-phase ground fault exceeds 30 A. The power industry standard \"Overvoltage Protection and Insulation Coordination for AC Electrical Installations\" stipulates that arc suppression coils should be installed when the single-phase ground fault current in 10 kV overhead line systems is greater than 20 A, or when it is greater than 30 A in 10 kV cable line systems. The reason is that the arc can extinguish itself at this current level. Simulations conducted by Hengshan Power Automation Equipment Co., Ltd. using small animals as grounding sources have shown that in a hybrid grid consisting of cables and overhead lines, the arc cannot extinguish itself when the single-phase grounding capacitive current exceeds 11.75 A. Numerous studies conducted in China have shown that the upper limit for the capacitive current in single-phase ground faults should be set at 10 A, in order to improve the reliability of power supply in distribution networks. 　　Urban distribution networks have not used the neutral-point grounding method with arc-suppression coils for a long time, and the main reasons are: (1) Due to technical limitations, the magnitude of the capacitive current is not well known ； 　　(2) Subject to the regulations, some capacitive currents in the distribution networks that did not exceed 30 A did not attract attention ； 　　(3) The 10 kV distribution network has no neutral point; if compensation measures are to be employed, a dedicated artificial neutral point transformer is required, which makes implementation difficult. Source: High-Voltage Switching Network 　　1.2 Commonly Used Arc Suppression Coils 　　1.2.1 Adjustable-Turn Arc Suppression Coils 　　Adjustable-turn arc suppression coils are essentially core-type reactors; their magnetic circuit consists of a core with gaps, around which coils are wound. This type of arc-suppression coil changes its inductance by altering the number of turns in the windings; the inductance is proportional to the square of the number of turns N. The tap is adjusted using a no-load switch, which allows for discrete adjustments of the inductance. This type of arc-suppression coil is widely used due to its simple and mature manufacturing technology as well as high reliability. At present, all manually adjustable arc-suppression coils in China’s power systems are of the tap-type, and the same is true for the vast majority of such coils abroad. Replacing the no-load switch of this arc-suppression coil with a load-type switch enables adjustment while the circuit is energized, and the installation of a control device allows for automatic tuning; all automatically tracking compensation systems developed in China use tap-changing arc-suppression coils. 　　1.2.2 Arc-suppression coil with adjustable core air gap The working principle of this coil is to change the permeability by moving a movable core inserted into the coil, thereby altering the inductance of the coil. Theoretically, the inductance of such arc-suppression coils can be adjusted continuously, but in practice this is not feasible in engineering due to mechanical inertia and the control accuracy of motors. Its main drawback is a complex transmission mechanism and a high failure rate ； Slow response; the action time depends on the movement time of the movable core ； When adjusting the inductor at the rated voltage, noise is relatively high, and mechanical malfunction can sometimes occur due to dirt. Therefore, it is not widely used in power systems. 　　1.2.3 DC-polarized arc-suppression coils This type of coil operates on the same principle as saturated reactors; it utilizes the fact that the AC effective permeability of magnetic materials changes under the magnetizing effect of a DC control current, thereby altering the AC effective reactance and consequently changing the current in the AC circuit as well as the power in the load. The advantages of this arc-suppression coil are the absence of mechanical transmission components and a long service life; however, its adjustment range is limited, the control mechanism is complex, and the adjustment accuracy is low due to the non-linearity of the core’s magnetization curve. Furthermore, the manufacturing technology required for these coils is quite high; there are no mature products available at present, and they are rarely used abroad as well. Please log in to the High-Voltage Switchgear Network for more information. 1.3 Problems existing in manual arc-suppression coil grounding compensation systems. The method of grounding the neutral point through arc-suppression coils has been widely used in China’s 35 kV power grids for decades, with significant results; it is particularly effective in reducing the number of accidents and outages during thunderstorm seasons. However, the manual arc-suppression coil grounding compensation system has also revealed certain problems in long-term operation practice. (1) It is inconvenient to adjust. Since this arc-suppression coil is of the no-load adjustment type, it is necessary to shut it down in order to adjust the tap, which is both time-consuming and unsafe. Therefore, in actual operation, it is rare to make timely adjustments based on changes in the grid’s capacitive current. 　　(2) The operator finds it difficult to make adjustments. Due to the lack of devices for online, real-time measurement of capacitive current, after changes in grid parameters, the variation in capacitive current can only be determined manually (with very low accuracy), making it impossible to accurately determine the exact setting of the arc-suppression coil. 　　(3) Due to their inherent characteristics, manual grounding compensation devices can only operate in an over-compensation state in power grids (short-term under-compensation is also possible); they cannot operate in an under-compensation state for long periods, let alone in a fully compensated state. When an accident, trip, or change in reclosing parameters occurs in the power grid, the detuning degree cannot be controlled, resulting in it often operating at an unacceptable level of detuning and causing overvoltage in the event of a ground fault. 　　(4) The effect on suppressing arc overvoltage is poor. Studies both domestically and internationally have shown that only when the detuning degree does not exceed ±5% can the overvoltage be kept below 2.6 times the phase voltage. Due to the need to avoid the range of full compensation, the detuning degree of traditional arc-suppression coils generally has to be 15%–25% or even higher. Therefore, the arc-overvoltage multiplier is high, the operation time with a fault after grounding is long, posing a significant threat to the insulation of the equipment. Source: High-Voltage Switching Network (5) Operation under under-compensation conditions can lead to severe resonant overvoltages in the event of a wire break, posing a great threat to the insulation of the network. 　　(6) After a single-phase ground fault occurs in a cable line, it is necessary to eliminate the fault, a process that is complicated and results in unnecessary power outages for users. 　　(7) Currently, unattended substations are being widely adopted in urban distribution networks, which makes it more difficult to adjust non-automatic arc suppression coils and leads to a waste of manpower. This article is reprinted from ‘I Love Electrical Network’ www.52d7.cn. I Love Electrical – Creating the most professional electrical network! 2 Research on Automatic Tuning Ground Compensation Devices 2.1 The necessity of automatic tuning compensation Since manually adjusted arc-suppression coils do not utilize automatic tracking adjustments, they cannot always operate at their optimal setting; as a result, the compensatory function of these coils cannot be fully utilized. Table 1 lists the statistical data for different compensations. 　　Table 1 Statistical data for different power grids and different levels of operation and maintenance 　　 Compensation type, Ratio of the number of single-phase ground faults that cause equipment damage to the total number of single-phase ground faults, Average duration of single-phase ground faults/h: No compensation: 0.26–0.99, 0.80–6.10; Manual adjustment compensation: 0.07–0.25, 0.14–2.70; Automatic tracking compensation: 0.01–0.16, 0.01–1.40. Source: www.hvsi.cn 　As can be seen from Table 1, power grids equipped with automatic tracking compensation devices achieve the best compensation effects, **improving the reliability of power supply; this represents the development direction for 10 kV distribution networks in China. 　　2.2 The 2.2ZGTD series of automatic tracking compensation systems – arc-suppression coils in series with resistors for grounding compensation represent the latest research achievements in China. This approach offers the advantages of grid compensation, as well as the benefit of low-resistance grounding in terms of limiting the amplitude of overvoltages. It overcomes the inherent disadvantages of both methods, which is why it is widely used. At present, most domestic automatic tracking and compensation arc-suppression systems and their complete sets of equipment use an arc-suppression coil in series with a resistor for grounding. 　　2.2.1 Structure of the device The wiring of the primary equipment is shown in Figure 1 (without a neutral point). The complete set consists of a Z-type grounding transformer (not required when there is a neutral point in the system), a load-adjustable arc-suppression coil, a voltage-limiting and damping resistor bank, a microcomputer-based measurement and control unit, and a microcomputer-based line selection and protection device (which can be omitted). 　　 　　Figure 1: Wiring diagram of primary equipment 　　 (1) The grounded transformer with a twisted connection remains in an unloaded state during normal operation of the power grid; its zero-sequence impedance and no-load losses are very low. Its main functions are to provide an ideal artificial neutral point for connecting arc-suppression coils ； It can carry a load and also function as a station transformer ； As a component for regulating the asymmetric voltage of the system, it meets the requirements for automatic tuning. Source: www.hvsi.cn (2) The on-load adjustable arc-suppression coil is a type of tap-changing arc-suppression coil equipped with an on-load tap changer; it also comes with a TV and a TA. Single-phase load-switches are used in arc-suppression coils, and they operate easily under pre-debugged conditions (that is, during adjustment in the normal ungrounded state); they switch almost in an unloaded state, thus enjoying a long service life. Theoretically, for the automatic tuning of arc-suppression coils, it is best to use continuously adjustable arc-suppression coils; however, no ideal products currently exist. Tap-type arc-suppression coils with a sufficient number of taps are also available, as long as the degree of detuning when adjusting the coil remains within specified limits. Incidentally, for an arc-suppression coil with taps, the inductance value when each tap is in use is a fixed value, making it easier to calculate the detuning degree of the power grid. 　　(3) In an arc-suppression coil grounding system, the voltage-limiting damping resistor bank will experience series resonance when the inductive reactance of the arc-suppression coil is equal to the capacitive reactance of the capacitance to ground, resulting in overvoltage at the neutral point and equipment damage. Therefore, the regulations stipulate that in a power grid with the neutral point grounded through an arc-suppression coil, under normal operating conditions, the voltage displacement of the neutral point over an extended period shall not exceed 15% of the phase voltage; the arc-suppression coil must be in an over-compensation mode, with its detuning degree generally not exceeding 10%, and the residual current at the fault site shall not exceed 10 A. In this way, when the load is removed during operation, resonant overvoltage at the neutral point can be avoided. However, during actual measurement and adjustment, the detuning degree can reach 20%–30%, resulting in a larger residual current at the grounding point, which is unfavorable for arc extinction. In automatic tracking compensation systems, due to the high adjustment precision and operation near the resonance point, to prevent resonance overvoltage, a damping resistor bank is connected in series with the arc-suppression coil in the “ZGTD automatic tracking compensation device” in order to increase the damping rate of the circuit. This reduces the overvoltage at the neutral point under full compensation to below 15% of the phase voltage; therefore, this device can operate in states of over-compensation, under-compensation, and full compensation. To prevent the damping resistor from reducing the compensation capacity of the arc-suppression coil, the resistor is short-circuited in the event of a single-phase ground fault, which also avoids overheating of the resistor. The damping resistor is a ZX2 type sheet resistor, and its resistance value is set according to the system capacity, ensuring that the neutral point displacement voltage requirements are met. The short-circuit damping resistor uses two independent starting short-circuit circuits for the neutral point voltage and current. One set controls the AC contactor KM1 based on the neutral point voltage value; if this value exceeds the set threshold, the voltage relay activates, causing the AC contactor to close its contacts in order to short-circuit the damping resistor. The other set consists of the DC contactor KM2, an intermediate relay, and an overcurrent relay; when the current flowing through the arc-suppression coil due to a system ground fault exceeds the set value, the current relay activates, and through the intermediate relay, the DC contactor closes to short-circuit the damping resistor. The two sets of measures complement each other, ensuring reliable short-circuiting of the resistance. If equipped with a ground selection device, the damping resistor is short-circuited 0.5 seconds after grounding. Source: http://www.hvsi.cn (4) Microcomputer controller. The 8098 microcontroller, which offers good interference resistance, utilizes a unique real-time online measurement method to display various parameters of the power grid quickly, accurately, intuitively, and comprehensively. It allows for automatic or manual adjustment of the tap positions of the arc-suppression coil, ensuring that it operates in its optimal condition at all times. 　　(5) Microcomputer wire selection device. For a long time, research has been conducted on devices for identifying the location of single-phase ground faults in distribution networks, and ready-made products are now available on the market. The common methods include: adding second-harmonic signals ； Utilizing the grounded transient current ； Increase the active component of the grounding point ； Detecting the faulty branch using impulse voltage current ； Utilize the higher harmonic components in the ground current. In a compensation system in which the neutral point is grounded through an arc-suppression coil connected in series with a resistor, when a single-phase ground fault occurs and the resistance is not short-circuited, the active power component in the grounded circuit is greater compared to that in the normal circuits. Taking advantage of this characteristic, a microcomputer-based line selection device can collect the system’s zero-sequence voltage and the zero-sequence current of each circuit for analysis; the circuit with the highest active power component is identified as the grounded circuit. 　　2.2.2 Methods for measuring capacitive current online The key to achieving automatic tuning lies in rapidly and real-time measuring the capacitive reactance of the system’s capacitance with respect to ground, and calculating the capacitive current when the system is single-phase grounded. The main methods currently used in automatic tuning systems are the displacement voltage curve method, the neutral point displacement voltage (current) phase method, and the online real-time measurement method. The first two measurement methods require adjustment during the measurement process; this reduces the lifespan of the load switch, and the response time (the time required for the arc-suppression coil to be adjusted to the appropriate setting once the system’s capacitive current changes) cannot be very short. As a result, they are not effective in practical applications. The ZGTD series of devices employs online real-time capacitive current measurement; Figure 2 shows the zero-sequence equivalent network diagram of the system. When the device is put into operation, the unbalanced voltage E0 is first determined using a special method; thereafter, U phase, U0, and I0 are measured every 3 seconds. Thus, E0 = U0 + I0·XC, and IC = U phase/XC = U phase·I0/(E0 – U0). Please log in to: High Voltage Switchgear Network for more information. L represents the arc suppression coil ； XC is the system’s capacitive reactance to ground ； 　　E0 is the system unbalance voltage ； U0 represents the voltage at the neutral point in the system. Figure 2 shows the zero-sequence equivalent circuit. The advantage of this algorithm is that no adjustment of settings is required during measurement; the measurement interval is short (measurements are taken every 3 seconds), and the measurement accuracy is high. 　　2.2.3 Characteristics of the device 　　(1) A modular mechanism is used. The load-switch-controlled arc-suppression reactor, grounding transformer, voltage-limiting damping resistor bank, microcomputer-based measurement controller, and microcomputer-based grounding selection device are all independent components, which facilitates installation and maintenance. 　　(2) The three operating modes of over-compensation, under-compensation, and full compensation are all practical, and can be set or changed on-site as needed at any time. 　　(3) No gear shifting is required during measurement, offering advantages such as fast response speed, long service life of the load switch, and accurate tracking. 　　(4) Grounding the arc-suppression coil in series with resistance can limit the neutral point displacement voltage