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This post was last edited by chenjinfeng on 2020-2-10 at 16:43. The two-stage inter-phase overcurrent device protects against inter-phase short-circuit faults in the wires, insulators, and bushings between the capacitor bank and the circuit breaker; it can also serve as a backup protection for internal faults in the capacitors. Current quick-break protection is generally not installed in the capacitor bank circuit, as such protection must take into account the inrush current during the switching of the capacitor bank as well as the current resulting from external discharges; as a result, its protection range and effectiveness cannot be fully utilized. Principles and functions of overvoltage protection: Due to factors such as changes in system load, the system voltage also changes frequently. The reactive power output by the capacitor and the internal active power loss are proportional to the square of the voltage across its terminals, that is, Qc=ωCU2 and P=ωCU2tanδ. When the operating voltage is too high, the active losses inside the enclosure increase rapidly. When the heat generated within the capacitor exceeds the amount of heat that can be dissipated into the surrounding air through the capacitor’s cooling mechanism, thermal equilibrium is disrupted, the temperature rises, ionization increases, which leads to the aging of the dielectric and a reduction in its lifespan. In addition to causing the capacitor casing to expand, the development of thermal breakdown leads to localized breakdowns, which can easily result in the capacitor exploding. Therefore, the capacitor must be equipped with adequate power-frequency overvoltage protection to ensure that it operates within the maximum allowable voltage and within the specified time frame. **The standards specify that the maximum allowable duration of power-frequency overvoltage for capacitors is: they can operate continuously at 1.1 times the rated voltage ； At 1.15 times the rated voltage, it can operate for 30 minutes every 24 hours ； At 1.2 times the rated voltage, it is 5 minutes ； At 1.3 times the rated voltage, it is 1 min. To ensure that the overvoltage component can return reliably after an instantaneous overvoltage occurs, it is advisable to give it a high return coefficient, which can be set at 0.95 (>0.98). The overvoltage component voltage is taken from the busbar PT. To prevent false operation of the overvoltage protection in the case of single-phase grounding of the busbar, line voltage is used. Since the voltage is taken from the busbar PT, to prevent the capacitors from being switched off accidentally due to excessively high busbar voltage when they are not in operation, a circuit breaker closed condition criterion is included in the overvoltage protection element. The principle and function of low-voltage protection: Considering the characteristics of capacitors themselves, if protection for capacitors in operation is suddenly lost, it causes no damage to the capacitors themselves. However, the following consequences may occur: ① When the power supply side of the substation is disconnected, an accident causes a trip, or the voltage drops sharply, if the capacitors are still connected to the busbar, then once the power supply is restored or a backup power source comes online, the busbar voltage rises rapidly. Before the residual voltage across the capacitors drops to 10% of their rated voltage, it is possible for the capacitors to be subjected to a voltage higher than 1.1 times their rated value, resulting in damage. ②When power is restored to the substation, closing the transformer with capacitors connected may cause resonant overvoltage, leading to damage to the capacitors. ③In the early stages of power restoration at a substation, if the transformer has not yet been loaded or the load is low, the bus voltage may be high, which can also cause overvoltage in the capacitors. Therefore, a low-voltage protection is added, and its operating time should be shorter than that of the reclosing mechanism for the upstream power supply feed or the BZT. When the transformer protection or other protections cause the busbar feed circuit breaker to trip, the capacitors should also be tripped simultaneously; however, if a low-voltage protection is installed with an appropriately set operating time, it may not be necessary to trip the capacitors as well. The low-voltage protection voltage is derived from the busbar PT; to prevent the capacitors from being disconnected accidentally due to excessively low busbar voltage when they are not in operation, a circuit breaker closed condition criterion is included in the low-voltage circuit. To prevent false activation of the low-voltage protection in the event of a PT disconnection, line voltage is used, which is calculated by software ； And it is possible to choose whether to use flow blocking through a control word. Overload protection. Overloading of capacitor banks is caused by system overvoltage and higher harmonics. According to national standards, capacitors should be able to operate for extended periods at an effective current level of 1.3 times the rated current; for capacitors with the greatest deviation in capacitance value, the allowable overcurrent level can reach 1.43 times the rated current. Since, per regulations, capacitor banks must be equipped with overvoltage protection to address the steady-state increase in bus voltage, and since large-capacity capacitor banks generally require series reactors to suppress high harmonics, overload protection need not be installed. Only when the system has a high content of higher harmonics ； Or, after the capacitor bank is put into operation and actual measurements show that the current in its circuit exceeds the allowable value, then an overload protection device is installed, which operates with a time delay to send a signal. To match the overload characteristics of capacitors, a reverse-time characteristic is advisable. Principle and function of single-phase ground protection: Whether a single-phase ground protection device should be installed in a parallel capacitor bank depends on the grounding method of the power grid in which the capacitor bank is located. For ungrounded systems, where the neutral point of the capacitor bank is not directly grounded either, whether the capacitor bank is placed on insulating supports or on the ground, it does not constitute part of the system’s natural capacitance; therefore, there is no need to install single-phase grounding protection. At present, in neutral-point non-directly grounded systems in China, the wiring of parallel capacitor banks is usually Y-wiring or double Y-wiring. Parallel capacitor banks in high-voltage, ultra-high-voltage, and ultra-high-voltage systems with directly grounded neutral points, or on the AC side of DC transmission systems, generally adopt a Y0 connection. If single-phase ground protection is to be added, the principle and implementation are the same as those described in 11. of \"Principles of Line Protection and Software Instructions\". The unbalance protection is used to detect internal faults in capacitor banks. Large-capacity parallel capacitor banks are composed of many individual capacitors connected in series and in parallel (usually first in parallel and then in series). If one capacitor fails, it is disconnected by its dedicated fuse, with little impact on the entire capacitor bank, as capacitors have a certain capacity for overload, and appropriate margins are usually built in when selecting equipment during design. However, when multiple capacitors fail and are removed, it may cause the capacitors that remain in operation to experience severe overloading or overvoltage (after the capacitors are removed, the capacitive reactance of the faulty section increases, leading to an endpoint voltage that can exceed 1.1 times the rated voltage), thereby damaging them. Therefore, protective measures are necessary, with unbalance protection being commonly used. The principle of protection is to detect the difference (current or voltage) between the healthy and faulty parts within a group of capacitors. Depending on the wiring configuration of the capacitor bank (triangle, star, and double-star), the methods for implementing unbalance protection also vary. Common protection methods include: zero-sequence voltage protection (open delta voltage protection), neutral-point imbalance voltage or current protection, voltage differential protection, and bridge differential current protection. The so-called zero-sequence current balance protection for capacitor banks involves installing zero-sequence current transformers and zero-sequence current relays on the connection line of the neutral points of two sets of capacitors connected in star configuration. In this way, when a capacitor in one phase fails during operation, a zero-sequence current is generated at the neutral point; the zero-sequence current transformer then activates the zero-sequence current relay, causing the switch to trip and thereby disconnecting the capacitor bank to prevent the fault from spreading further. Discharge coils are suitable for power systems of 35 kV and below; they are connected in parallel with high-voltage parallel capacitor banks to allow the residual charge in the capacitors to be discharged rapidly after they are removed from the power system, so that the residual voltage of the capacitors reaches the specified value within a set time. They come equipped with a secondary coil, which enables line monitoring. Discharge coils are commonly used discharge components in capacitor cabinets; sometimes, discharge PTs are used instead of discharge coils. Whether to use discharge coils or voltage transformers for discharging capacitors depends on the capacity of the capacitors – generally, voltage transformers are sufficient for small-capacity capacitors, while discharge coils are necessary for large-capacity capacitors. When the capacitor loses power, the discharge coil acts as a discharge load to quickly release the residual charge at both ends of the capacitor. Standardly, for high-voltage capacitors, it is required that their terminal voltage be reduced to less than 50V within 5 seconds after power is cut off. During operation, the discharge coil is used as a voltage transformer, its secondary winding being connected in an open delta configuration to provide protection against internal faults in the capacitor bank (a PT on the busbar cannot be used for this purpose). We often talk about the open delta protection and unbalanced voltage protection for capacitor banks; zero-sequence unbalance protection is actually this type of protection. This type of protection is widely used in 10KV single-Y connected capacitor banks.