Arc Suppression Coil
The function of the arc extinguishing coil is to extinguish the arc, which is an inductive coil with an iron core. It is connected between the neutral point of the transformer (or generator) and the ground, forming an arc suppression coil grounding system. The transmission lines of the power system are grounded through arc suppression coils, which is a type of low current grounding system. During normal operation, there is no current passing through the arc suppression coil. When the power grid is struck by lightning or a single-phase arc grounding occurs, the neutral point potential will rise to the phase voltage. At this time, the inductive current flowing through the arc suppression coil and the capacitive fault current of the single-phase grounding cancel each other out, compensating for the fault current. The residual current after compensation becomes very small and insufficient to maintain the arc, thus extinguishing it on its own. In this way, the ground fault can be quickly eliminated without causing overvoltage.
When a short circuit fault occurs, the compensating capacitor current avoids the inductive coil generated by the arc, which is a type of low current grounding system.
Working principle:
The function of the arc suppression coil is to compensate for the capacitive current flowing through the fault point when a single-phase grounding fault occurs in the power grid. The arc suppression coil provides inductive current to reduce the current at the fault point to below 10A, which is beneficial for preventing reignition of arc light after zero crossing, achieving the purpose of arc suppression, reducing the probability of high amplitude overvoltage, and preventing the accident from further expanding. When the arc suppression coil is properly tuned, it can not only effectively reduce the probability of arc grounding overvoltage, but also effectively suppress the amplitude of overvoltage, while minimizing the thermal damage at the fault point and the voltage of the grounding grid.
From the perspective of utilizing the arc suppression coil, the smaller the absolute value of the detuning degree, the better, and it is best to be in a fully compensated state, that is, adjusted to the resonance point. However, during normal operation of the power grid, arc suppression coils with small detuning will generate various resonant overvoltages. For example, in a 6KV coal mine power grid, when the arc suppression coil is in a fully compensated state, the neutral point displacement voltage of the grid under normal steady-state operation is 10-25 times that of the uncompensated grid, which is commonly referred to as series resonant overvoltage. In addition, various operations of the power grid (such as the input of large motors, asynchronous closing of circuit breakers, etc.) may generate dangerous overvoltages. Therefore, during normal operation of the power grid or other faults other than single-phase grounding faults, the small detuning arc suppression coil brings not safety factors but hazards to the power grid. In summary, when there is no single-phase grounding fault in the power grid, it is desirable for the arc suppression coil to operate away from the resonance point. The arc suppression coil operating in a complete state usually incorporates damping resistors to suppress resonance overvoltage, and actual operating experience has shown good results.
Structural features:
The characteristics of the biased magnetic arc suppression coil structure are: electrically controlled continuously adjustable arc suppression coil, fully static structure, no moving parts inside, no contacts, large adjustment range, high reliability, and fast adjustment speed. The basic working principle of this coil is to apply a direct current excitation current to change the magnetic resistance of the iron core, thereby changing the reactance value of the arc suppression coil. It can adjust the inductance value at a speed of milliseconds with high voltage.
Control method:
The use of dynamic compensation fundamentally solves the contradiction between the series resonant overvoltage of the compensation system and the optimal compensation. As is well known, arc suppression coils have no benefits during normal operation of high-voltage power grids. If tuned to full compensation or near full compensation state at this time, series resonance overvoltage may occur, causing the neutral point voltage to rise. Various normal operations in the power grid and various faults outside of single-phase grounding may also generate dangerous overvoltage. So, during normal operation of the power grid, adjusting the arc suppression coil to track changes in the grid capacitance current is harmful or harmful. This is why the power department stipulates that "fixed arc suppression coils cannot work in full compensation or near full compensation state". Similar automatic compensation devices in China are all follow-up systems, which adjust the arc suppression coil to a fully compensated state before a ground fault occurs in the power grid, waiting for the occurrence of a ground fault. This is to avoid excessive series resonance overvoltage by connecting a damping resistor in series with the arc suppression coil, which limits the steady-state resonance overvoltage to an acceptable range and cannot solve the problem of transient resonance overvoltage. In addition, due to the power limitation of the resistor, it must be quickly cut off after a ground fault occurs, which undoubtedly adds an unsafe factor to the power grid. The biased magnetic arc suppression coil does not use the method of limiting the series resonance overvoltage, but adopts a dynamic compensation method that avoids the resonance point, which fundamentally prevents series resonance from occurring. That is, during normal operation of the power grid, no excitation current is applied, and the arc suppression coil is tuned to a state far away from the resonance point, but the size of the power grid capacitance current is detected in real time. When a single-phase grounding occurs in the power grid, the arc suppression coil is adjusted instantaneously (about 20ms) to implement optimal compensation.
Advantage features:
The single-phase grounding of the neutral point through the arc suppression coil in the power grid has the following characteristics:
(1) Like a neutral ungrounded power grid, the fault relative ground voltage is zero, and the non fault relative ground voltage rises to the line voltage, resulting in zero sequence voltage, which is greater than or equal to the phase voltage during normal operation of the power grid, as well as zero sequence current.
(2) The voltage at both ends of the arc suppression coil is zero sequence voltage, and the current of the arc suppression coil passes through the grounding fault point and the fault phase of the fault line, but not through the non fault line.
(3) If the system adopts a complete compensation method, the zero sequence current of both the faulty and non faulty lines is their own capacitance current to ground, and the direction of the capacitance current is from the bus to the line. Therefore, it is impossible to use the magnitude and direction of the steady-state current to determine the fault.
(4) When the system adopts overcompensation method, the zero sequence current flowing through the fault line is equal to the sum of the ground capacitance current and the residual current at the grounding point of the line. Its direction is the same as that of the zero sequence current of the non fault line, still pointing from the busbar to the line and with the same phase. Therefore, it is also impossible to use the difference in direction to distinguish between the fault line and the non fault line. Secondly, due to the small degree of overcompensation, it is also difficult to use the difference in zero sequence current magnitude to identify faulty lines like in a neutral ungrounded system.
Classification of arc suppression coils:
The air gap adjustment type belongs to the follow-up compensation system. The arc suppression coil belongs to a moving core structure, which achieves continuous adjustment of inductance by changing the magnetic resistance of the magnetic circuit by moving the iron core. However, its adjustment can only be carried out under low voltage or no voltage conditions, and the ratio of the upper and lower limits of its inductance adjustment range is 2.5 times. Under normal operation of the control system's power grid, adjust the arc suppression coil to near full compensation and connect a resistor of approximately 100 ohms in series with the arc suppression coil. Used to limit the series resonance overvoltage and keep the steady-state overvoltage value within the allowable range (phase voltage with a neutral point potential increase of less than 15%). When single-phase grounding occurs, the resistor must be short circuited within 0.2 seconds to achieve optimal compensation, otherwise there is a risk of explosion of the resistor. The main drawbacks of this product are four:
Due to its structure with up and down moving parts, the dynamic core arc suppression coil produces significant vibration noise when subjected to high voltage. Moreover, as the usage time increases, the internal looseness becomes more and more pronounced, resulting in increasing noise. The series resistance is about 3KW, 100 Ω. When the compensation current is 50A, a 250KW capacity resistor is required for long-term operation. Therefore, after grounding, the resistor must be quickly cut off, otherwise there is a risk of explosion. This affects the reliability of the entire device.
Due to the fact that even small changes in the air gap can cause significant changes in inductance, the accuracy of adjusting the air gap through mechanical components in motors is far from sufficient. The cost of using hydraulic adjustment is too high
During normal operation of the power grid, the arc suppression coil is in a fully compensated state or close to a fully compensated state. Although there is a series resonant resistor to limit the steady-state resonant overvoltage within the allowable range, various disturbances in the power grid (such as large motor switching, asynchronous closing, asynchronous closing, etc.) make the transient overvoltage hazard more serious.
After installing this product, the existing power directional single-phase grounding line selection device in the power grid cannot continue to be used.
This device belongs to a follow-up compensation system, and the only difference between it and the air gap type is that the dynamic core type arc suppression coil is replaced by an on load turn suppression coil. This type of arc suppression coil is modified from the original manual turn suppression coil by using an on load adjustment switch to change the number of turns of the working winding, in order to adjust the inductance. Its working mode is exactly the same as the air gap type, and it also uses series resistors to limit resonance overvoltage. Compared with the air gap type, this device eliminates the high noise of the arc suppression coil, but sacrifices the compensation effect. The arc suppression coil cannot be continuously adjusted and can only be adjusted in discrete stages, resulting in poor compensation effect. It also has the disadvantages of high overvoltage level, inability to use the original directional grounding selection device in the power grid, and the risk of explosion caused by the series connected resistor. In addition, this device is relatively messy and consists of four parts of equipment (grounding transformer, arc suppression coil, resistance box, control cabinet), making installation and construction more complex.
During normal operation of the power grid, the adjustable turn arc suppression coil measures the amplitude of the current flowing through the coil in real time to calculate the capacitance current to ground under the current mode of the power grid. Based on the pre-set minimum residual current value, the controller adjusts the on load voltage regulation tap to the required compensation gear. When a grounding fault occurs, compensate for the capacitance current during grounding to limit the residual current at the fault point within the set range.
Mainly, several sets of capacitors with controllable silicon (or vacuum switch) are connected in parallel on the secondary side of the arc suppression coil to adjust the capacitance impedance value of the secondary side capacitor. According to the principle of impedance conversion, adjusting the secondary side capacitance impedance value can achieve the requirement of changing the primary side inductance current.
The controllable silicon arc suppression coil is used to connect the primary winding of a high short-circuit impedance transformer as the working winding to the neutral point of the distribution network, and the secondary winding as the control winding, which is short circuited by two reverse connected controllable silicon. The conduction angle of the controllable silicon is adjusted from 0 to 180 °, so that the equivalent impedance of the controllable silicon changes between infinity and zero. The output compensation current can be continuously infinitely adjusted between zero and the rated value. Controllable silicon works in a capacitor free circuit connected in series with an inductor, and its operating conditions are free from the threat of anti peak voltage and the impact of sudden current changes, thus ensuring reliability. Its characteristics are as follows:
(1) By utilizing thyristor technology, the compensation current can be continuously and infinitely adjusted within the range of 0-100% of the rated current, achieving precise compensation over a large range and adapting to the different capacity requirements of the distribution network at different stages of development.
(2) By using short-circuit impedance as the working impedance, the volt ampere characteristics maintain excellent linearity within the range of 0-110% UN, thus enabling precise compensation.
(3) The arc suppression coil is adjustable and does not require the installation of damping resistors or series resonance, which not only improves the reliability of operation but also simplifies the equipment.
(4) After a single-phase grounding fault occurs, the arc suppression coil can output compensation current within 5ms at the fastest to suppress arc light and prevent phase to phase short circuit caused by air ionization caused by arc light; At the same time, it can effectively eliminate consecutive single-phase grounding faults with short intervals.
(5) The complete set of equipment has no transmission or rotating mechanism, high reliability, low noise, and simple operation and maintenance.
The biased magnetic arc suppression coil does not use the method of limiting series resonant overvoltage. The bias type arc suppression coil adopts a magnetized iron core segment arranged inside the AC coil. By changing the magnitude of the applied DC excitation current, the magnetic conductivity of the iron core is changed, thereby achieving the goal of changing the reactance value of the arc suppression coil. When the power grid is operating normally, no excitation current is applied, and the arc suppression coil is tuned to a state far away from the resonance point, but the size of the grid capacitance current is detected in real time. When the power grid is single-phase grounded, the arc suppression coil is adjusted instantaneously (about 20ms) to implement optimal compensation.
Product Standard:
GB/T 1094.6-2011 IEC 60076-6：2007
GB/T 1094.1-2011 IEC 60076-1：2011
GB/T 1094.3-2017 IEC 60076-3：2013
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