I. Introduction With the rapid growth of electricity supply and demand in China, the requirements for improving power quality and reducing transmission and transformation losses are becoming increasingly urgent. Using capacitor banks and reactors to achieve local reactive power balance is a major means of improving power quality and reducing line losses. Especially for 110kV and 35kV substations, as terminal substations in the power grid system, the task of directly providing high-quality power with stable voltage and reactive power balance to user loads is crucial. Currently, the reactive power compensation in most 110kV and 35kV substations is not ideal. In particular, although rural substations were originally designed with parallel capacitor banks, most cannot adapt to load changes and fail to achieve the desired compensation effect. During peak load periods, the power factor of the 10kV bus in some substations drops below 0.7. Therefore, it is very important to develop a new generation of highly reliable 10kV automatic reactive power compensation devices suitable for 110kV and 35kV substations that automatically track load changes for compensation while also automatically adjusting voltage. II. Necessity of Automatic Tracking Compensation 1. Load Changes Require Automatic Tracking Compensation Below is a typical daily load change curve for a substation: This is the daily load change curve for the 110kV Longshan substation in Zhangqiu City, Jinan. Most 110kV and 35kV substations have similar load changes. This substation originally had a manual switching device for a single capacitor bank. The switching switch used a traditional circuit breaker installed in the switch room (also called a capacitor outgoing switch), connected to the capacitors in the capacitor room via cables. Looking at the daily load change curve of this substation, the load change occurs in two peak cycles every 24 hours. A single capacitor bank obviously cannot track load changes and can only be manually switched according to the load situation. However, the traditional circuit breaker as the capacitor switching switch cannot meet the requirements of frequent operation. Frequent operation several times a day quickly leads to failure. Over time, operators become reluctant to perform frequent operations, leaving the substation in a long-term uncompensated state. 2. Substations that have achieved unattended operation need to convert capacitor banks to automatic compensation systems. Currently, most substations in China have installed integrated automation systems, achieving unattended operation. With this, parallel capacitor banks inevitably need to be converted to automatic compensation systems. The aforementioned 110kV Longshan substation installed integrated automation in 2002, achieving unattended operation. Simultaneously, the previously manually operated single-group capacitor banks were automatically switched using the integrated automation system. The switching switches are still the original capacitor output switches, using the integrated automation protection unit on the capacitor output switch cabinet as the control element and the power factor of the integrated automation system as the criterion for switching capacitors, thus achieving automatic switching of single-group capacitors. Obviously, load changes still cannot be tracked, the capacitor output switches are frequently in a faulty state, and operation is not normal. From a practical point of view, relying solely on integrated automation systems for automatic reactive power compensation is not ideal. III. Equipment Requirements for Achieving Automatic Reactive Power Tracking Compensation 1. Investment Cost Requirements Achieving automatic reactive power tracking compensation involves dividing the total compensation capacity of capacitors into multiple groups based on the load size, switching the number of capacitor groups according to load changes, and striving to achieve reactive power balance as much as possible. Compensation accuracy is directly proportional to the number of capacitor banks; the more capacitor banks, the higher the compensation accuracy, but the higher the equipment investment cost, the larger the footprint, and the higher the reliability requirements. To improve the compensation accuracy of the compensation device and achieve grouped tracking compensation, traditional equipment is difficult to promote due to investment costs and space limitations. 2. Requirements for capacitor switching switches: Automatically switching capacitors based on load changes is the most critical issue. Capacitor switching switches differ significantly from ordinary load switches. When switching on a capacitor, the switch contacts must withstand several times or even tens of times the rated current; when switching off a capacitor, the switch contacts withstand more than twice the rated voltage. Ordinary load switches do not exhibit these phenomena when closing or opening. Therefore, the requirements for capacitor switching switches are: ① The contact current-bearing capacity should be as large as possible, and the contact pressure should not be too small to withstand the impact of inrush current and reduce contact burn-out; ② The contact gap should not be too small to withstand overvoltage during opening and prevent reignition; ③ It should be suitable for frequent operation. High-voltage switches in automatic compensation devices operate several times or even more than ten times a day to switch capacitors according to load changes, which is much more frequent than high-voltage switches used for ordinary loads. From the above analysis, conventional circuit breaker switches cannot be used because they are not suitable for frequent operation. Although contactors are suitable for frequent operation, their contact pressure is only 1/10 or less of that of circuit breakers, resulting in poor current surge resistance and severe contact burn-out; the small contact gap makes reignition easy when switching capacitors, so they should be avoided as much as possible. The best switches for switching capacitors are circuit breakers suitable for frequent operation and with a low probability of reignition, or dedicated capacitor switching switches. 3. Capacitor and Reactor Requirements: There are no special requirements for capacitors in automatic compensation devices, mainly that they should be suitable for grouping according to capacity. The reactors in automatic compensation devices mainly suppress inrush current and harmonics. To reduce the overall size of the equipment, dry-type iron-core reactors are recommended. IV. Introduction to the DS3 Type 10kV Automatic Reactive Power Compensation Device The DS3 type 10kV automatic reactive power compensation device houses multiple switches within a standard switchgear enclosure. It divides capacitors into multiple groups, automatically tracking load changes and switching capacitor groups to achieve ideal reactive power compensation and on-site, real-time balance. Simultaneously, it automatically adjusts the on-load tap changer of the transformer based on changes in the 10kV bus voltage. This device is suitable for 35kV to 220kV substations, occupies a small area, and is simple to install. The device's significant features are as follows: 1. Small footprint and simple installation: Dividing capacitors into multiple groups to reduce the overall device footprint is a major challenge, which cannot be solved using traditional equipment and methods. This product adopts a compact structure, housing 5 dedicated vacuum circuit breaker switches, 5-channel microprocessor protection, 15 discharge coils, and a VQC integrated controller, etc., within a standard high-voltage switchgear enclosure. All primary and secondary electrical equipment, except for capacitors and reactors, is installed inside the device cabinet, as shown in the figure. When the compensation capacity is 10000kvar, it is divided into 5 groups for switching, and the entire device occupies only 6×1.2 square meters. During installation, only the primary input line and the secondary signal to be collected need to be connected. 2. Modular capacitor switching vacuum switch: The capacitor switching switch adopts a modular structure design, is small in size and can be combined arbitrarily. Five units can be installed at the bottom of a 1.2×1.2 square meter cabinet. The switch's arc-extinguishing chamber uses a vacuum circuit breaker arc-extinguishing chamber, which can withstand the impact of inrush current when switching capacitors for a long time. The large contact gap prevents reignition when switching capacitors. It features a specially designed operating mechanism for frequent operation, with a reliable operating life of 30,000 cycles. 3. Each capacitor group has microprocessor protection: Each group of capacitors is equipped with microprocessor protection against overvoltage, overcurrent, and three-phase unbalanced voltage. This can promptly detect internal capacitor faults, effectively prevent the expansion of internal faults, and prevent capacitors from developing from an internal fault state to an accident state, ensuring the safe operation of the capacitor bank. When any capacitor group fails, the microprocessor protection will trip that capacitor bank, while other capacitor banks continue to operate normally, without affecting the operation of the entire device. 5. The device features "four remote" functions (remote control, remote monitoring, and remote operation), with RS-232 and RS-485 communication interfaces for arbitrary connection. It can be integrated into substation integrated automation systems or dispatch automation systems to achieve these functions. 4. Tracking load changes and balancing locally in real time: The capacitors are divided into 5 groups, with continuous or cyclical switching to keep the power factor stable within a set range and minimize the impact on the power grid during capacitor switching. In the 110kV Longshan substation in Zhangqiu City, Jinan, the total capacitor capacity is 7500 kVAVR, divided into 5 groups of 1500 kVAVR each. The power factor setting is set at a lower limit of 0.94 and an upper limit of 0.98, with a reactive power setting of 1550 kVAVR. The following shows the capacitor switching status of this product's device over 24 hours: The device tracks load changes and switches capacitor groups, keeping the power factor stable within the range of 0.93-0.98, achieving a relatively ideal compensation effect. 5. Capacitor Fault Protection and Accident Protection: Capacitors are high-voltage equipment with high field strength. Frequent switching of capacitors can easily lead to breakdown faults, which can cause explosions and fires. Ensuring the reliability and safety of 10kV automatic reactive power compensation devices and preventing explosions and fires has always been a challenging issue. Timely detection of early capacitor faults and prompt disconnection of faulty capacitors are fundamental conditions for ensuring the safe operation of high-voltage reactive power compensation equipment and preventing explosions. However, detecting early capacitor faults is difficult using conventional current protection because a single high-voltage power capacitor consists of multiple small capacitor units connected in series. The rated voltage is distributed across these units. When a capacitor fails, one unit initially breaks down, but the current change is minimal, and conventional current protection will not activate. The breakdown of one unit increases the operating voltage of the remaining units, and the capacitor continues to operate, quickly leading to the breakdown of a second unit. This further increases the operating voltage of the remaining units, potentially causing bulging, cracking, and oil leakage. Continuing operation could very likely cause the capacitor to bulge, crack, leak oil, and eventually explode and catch fire. The DS3 type 10kV automatic reactive power compensation device is designed with two types of protection: fault protection and accident protection, as shown in the figure. Fault protection can promptly detect early breakdown faults in the capacitor, preventing the fault from developing into an accident. Accident protection can prevent the accident from escalating and spreading to the next higher level. The fault protection design has three protection levels: the first level is voltage imbalance protection, the second level is stage 1 overcurrent protection, and the third level is stage 2 overcurrent protection. When a small unit capacitor inside the capacitor experiences a breakdown short circuit (or open circuit), although the current changes very little, the zero-sequence voltage changes significantly. The zero-sequence voltage of each capacitor group is detected by an open delta connection formed by points (PTs) connected in parallel to each phase capacitor. It is unaffected by any external factors or power supply imbalances. It only occurs when the three-phase capacitance of the capacitor group is unbalanced. Therefore, this protection can accurately and promptly detect early capacitor faults, quickly disconnect the faulty capacitor, and prevent an explosion. The overcurrent setting values ​​for both stage 1 and stage 2 overcurrent protection are set at 1.25-1.55 times the rated current, and coordinated with the time setting value to make fault protection more reliable, ensuring safe operation of equipment, and enabling timely disconnection of capacitors in case of faults to prevent explosions and fires. The DS3 type 10kV automatic reactive power compensation device is a new generation of automatic reactive power compensation device developed after several years of operation and numerous improvements. Several years of operation have proven its high compensation accuracy, stable performance, and reliable protection, and it is currently in operation in nearly 400 substations.