1. Introduction To reduce reactive power transmitted in the power grid, decrease active power loss, and improve voltage quality, power supply companies generally install parallel compensation capacitor banks (hereinafter referred to as capacitor banks) in substations. A capacitor bank consists of capacitors, series reactors, surge arresters, circuit breakers, discharge coils, and corresponding control, protection, and instrumentation devices. Currently, most domestic capacitor manufacturers only produce capacitors; other equipment must be purchased externally, resulting in shortcomings in complete design and supply. Users must select appropriate capacitors and supporting equipment. Due to varying local conditions, there are significant differences in the selection and installation layout of capacitor banks. This article offers some analytical opinions on this matter. [b]2. Selection of Capacitor Capacity[/b] The configuration of capacitor bank capacity should achieve hierarchical and zoned balance of reactive power in the power grid, minimizing reactive power exchange between different voltage levels. Since the capacity of a capacitor bank is not continuously adjustable during operation, from the perspective of reducing the number of switching operations and improving the power factor, it is desirable for the capacitor bank to operate normally for most of the time without overcompensation. Analysis of substation load variations shows that substation load rates in the Xuzhou area are generally between 70% and 80%, with load levels above average for about two-thirds of the day. We calculate the annual average reactive load by dividing the annual reactive power consumption on the low-voltage side of the substation transformer by the annual operating time, and select capacitor bank capacity based on 90% of the annual average reactive load. In actual operation, since the rated voltage of the capacitor bank is generally 1.1 times the rated voltage of the power grid, while the voltage of the substation's low-voltage bus is generally controlled at 1 to 1.07 times the rated voltage of the power grid, the actual capacity of the capacitor bank needs to be reduced by 5.4% to 17.4%, thus ensuring that the capacitor bank can be put into operation for most of the time. For rural substations with significant seasonal load variations and substations expected to experience substantial load growth in the near future, the capacitor bank capacity can be appropriately increased, but the capacitor bank must be able to operate at reduced capacity. For aggregated and box-type capacitor banks, this requires intermediate capacity taps; for rack-type and semi-enclosed capacitor banks, this simply requires removing a few fuses. Simultaneously, series reactors equipped with harmonic amplification suppression functions are required to have intermediate capacity taps to ensure a constant reactance rate. Increasing the number of capacitor banks is beneficial for improving compensation effects, but it correspondingly increases equipment investment. Generally, capacitor banks in all 35-110kV substations are configured as one bank per transformer. From the perspective of reducing investment per unit kvar, the capacity of a single capacitor bank cannot be too small. Taking a 10kV all-film three-phase integrated capacitor bank as an example, let's compare the investment per unit kvar of 3600kvar and 1200kvar capacitor banks. Both types of capacitor banks are equipped with three single-phase discharge coils (each with a discharge capacity of 1700kvar), three zinc oxide surge arresters, 1% dry-type air-core series reactors, one set of vacuum switchgear, and a power cable length of 40m. Civil engineering and installation costs are calculated at 25% of the total equipment investment. See Table 1 for details of each investment. According to Table 1, the investment per unit kvar for the 3600kvar and 1200kvar capacitor banks is 65.5 yuan and 142.8 yuan, respectively. The unit kilovar investment of a 1200kvar capacitor bank has exceeded the unit kilovar investment of an automatically switched 10kV line pole-mounted capacitor bank. Therefore, the capacity of a single capacitor bank in a substation should not be less than 1200kvar. Table 1 Comparison of Capacitor Bank Investment Unit: 10,000 RMB Complete Equipment Vacuum Switchgear Cable Civil Engineering Installation Total Investment 3600kvar 13.17 4.5 1.2 4.72 23.59 1200kvar 8 4.5 1.2 3.43 17.13 [b]3 Selection of Capacitor Banks[/b] 3.1 Type Selection As for capacitor banks, the four main types commonly used in China are frame type, semi-enclosed type, integrated type, and box type, each with its own advantages and disadvantages. Frame type capacitor banks install single shell capacitors, fuses, etc. on a frame. The frame is made of hot-dip galvanized steel and is a traditional structural form. This type of product has the longest service life and rich operating experience. The advantages are a large safety distance, a small fault impact range, convenient maintenance, flexible capacity adjustment, and low cost per unit capacity. The disadvantages are a large footprint and a large workload for installation and maintenance. Except for substations in urban centers, the land occupation problem is easily solved for most substations. Therefore, this type of device will remain the dominant product for future widespread use. To reduce the footprint, a single-unit capacity full-film-shell capacitor can be selected. Semi-enclosed capacitor banks consist of two rows of single-shell capacitors laid horizontally, with the terminals facing inwards and the bottom outwards. The live parts of the capacitor are enclosed in metal, and the outer shell is grounded. First developed by ABB, it is widely used in Europe and America. Domestically, it is mainly produced by capacitor factories in Guilin and Jinzhou. The advantages are a compact structure, no need for isolation fences, a small footprint, flexible capacity adjustment, and low cost per unit capacity. The disadvantages are poor ventilation and heat dissipation after the live parts are enclosed, easy dust accumulation on the insulators, and condensation when the internal humidity is high, leading to flashover discharge. Several accidents have occurred during operation, and further improvements are needed; it is not suitable for widespread application at present. A multi-unit capacitor is made by connecting individual shell capacitors in series and parallel, then placing them in a large oil tank filled with insulating oil. It was first developed abroad by Nisshin Electric Corporation of Japan, and domestically, it was first successfully developed in 1985 by Heyang Power Capacitor Factory. Currently, various models from factories in Heyang, Xi'an, Jinzhou, and Wuxi have passed the two-level appraisal, and production has increased significantly year by year, accounting for 20% of the annual production of high-voltage parallel capacitors in 1996. Its advantages include compact structure, small footprint, fewer connections, and minimal installation and maintenance workload. To overcome the disadvantage of non-adjustable capacity, Wuxi Capacitor Factory developed adjustable-capacity multi-unit capacitors, divided into two categories based on capacity adjustment range: 50%/100% and 33.3%/66.7%/100%. Because the unit-shell capacitor is completely immersed in insulating oil, it prevents external insulation failure. The unit-shell capacitor is equipped with an internal fuse; if a few components fail, the fuse will cut off the fault, and the entire capacitor can continue to operate. The disadvantages are that the oil content is high, the large outer shell and oil tank are prone to oil leakage, and the time required for repair after failure is long and the unit capacity cost is high. There are two issues to note about the collective capacitor: (1) In order to avoid serious consequences when a phase-to-phase short circuit occurs in a large-capacity collective capacitor, the collective capacitor with a capacity of more than 5000kvar must be made into a three-phase split structure, that is, one unit per phase. (2) The creepage distance of the outer insulation of the bushing of the collective capacitor must be ≥3.5cm/kV (relative to the highest operating voltage of the system) to ensure its insulation strength. The box capacitor is a type of capacitor developed on the basis of the collective capacitor. The difference between it and the collective capacitor is that the internal unit capacitor has no shell and is directly immersed in insulating oil. The outer shell and large oil tank adopt a corrugated oil tank or a metal expansion tank, which is completely isolated from the external atmosphere. Compared with the collective capacitor, the shell volume and internal oil content are further reduced. Taking the 3000kvar product of Xi'an Power Capacitor Factory as an example, the shell volume of the box capacitor is reduced by 59.1% and the weight is reduced by 60.6% compared with the collective capacitor. Due to reduced material usage, the price is lower than that of modular capacitors. A disadvantage is that if an internal component fails and is tripped by the internal fuse, it can contaminate the insulating oil in the large oil tank. Only Japan produces and uses this type of capacitor abroad; unlike domestic products, it consists of a large assembly of components and lacks an internal fuse. Currently, factories in Heyang, Xi'an, and Jinzhou in China are producing it. This type of product represents the future development direction and can be selectively adopted gradually. In 1996, Guilin Capacitor Factory developed a modular, oil-free, self-healing capacitor, model BKMJJ, with a maximum three-phase capacity of 3000kvar. This capacitor consists of several units connected in series and parallel. Each unit is composed of several resin-encapsulated components connected in parallel and housed in a container with a discharge resistor. All live parts are covered by a flame-retardant ABS casing. The unit capacitors are combined horizontally or vertically according to their capacity. After trial operation on the grid, it passed evaluation in April 1998. This type of capacitor meets the requirements of urban substation equipment moving towards oil-free operation and can be gradually promoted and applied in urban power grid substations. 3.2 Circuit Breaker Selection The main requirements for capacitor bank circuit breakers are: no re-breakdown during opening and no significant bounce during closing. Currently, 6-10kV voltage levels mainly use oil-less circuit breakers and vacuum circuit breakers. Vacuum circuit breakers have the advantages of being resistant to frequent operation, requiring no maintenance of the arc-extinguishing chamber, and eliminating oil leakage problems. Therefore, there is a tendency to use only vacuum circuit breakers. We believe this issue should not be addressed with a one-size-fits-all approach. Vacuum circuit breakers have a problem with reignition after breaking, which has caused accidents multiple times during operation, even with imported vacuum circuit breakers. Oil-less circuit breakers, on the other hand, have the advantage of not reigniting when disconnecting capacitors. After the Northeast Electric Power Administration modified the contacts of oil-less circuit breakers, they can operate continuously for 1000 times without maintenance or oil changes, solving the problem of oil-less circuit breakers' inability to withstand frequent operation. Therefore, the circuit breakers for 6-10kV capacitor banks should be selected in accordance with the circuit breakers for substation outgoing lines, and it is not necessary to use vacuum circuit breakers for all of them. When ordering equipment, vacuum circuit breakers must undergo aging tests, while oil-less circuit breakers require modification of the contacts. For 35kV capacitor banks, SF6 circuit breakers are naturally the dominant choice. 3.3 Series Reactors: When designing capacitor banks, design departments generally configure 6% series reactors. This increases equipment investment and may not necessarily achieve the desired effect. According to the actual harmonic measurement results of 3 220kV substations, 24 110kV substations, and 18 35kV substations in a certain power grid, only 3 substations had high 3rd harmonic content, and 4 substations had high 5th harmonic content, requiring the configuration of 4.5% and 12% series reactors respectively to suppress harmonic amplification. Other substations had very low harmonic content and only needed to consider limiting the inrush current of the capacitor bank. Therefore, when configuring capacitor banks in substations, the background harmonic level of the power grid should be measured to determine the reactance of the series reactor. Oil-immersed series reactors are generally no longer used due to oil leakage and saturation problems. Dry-type air-core series reactors have been widely used due to their advantages of high mechanical strength, low noise and low maintenance. To reduce the floor space, three-phase stacked products can be used. During installation, attention must be paid to ensuring that the stacking order of the three phases is correct. If only limiting the inrush current of the capacitor combination is considered, a damped current limiter can be used, which consists of a damping resistor, a discharge gap and a small-capacity reactor. At the moment of closing, the reactor bears the full voltage. When the discharge gap breaks down, the damping resistor is connected to the circuit to limit the inrush current. After the inrush current decays, the voltage at the reactor terminals drops, the discharge gap extinguishes the arc, and the damping resistor is taken out of operation. 3.4 Discharge Coil The discharge coil is a necessary device to ensure the safety of equipment and personnel and must be configured. There are two issues to pay attention to regarding the discharge coil: (1) The discharge coil must be directly connected across the two ends of the capacitor, and cannot be connected across the two ends of the capacitor and the reactor in series. The latter wiring method cannot accurately reflect the unbalanced voltage generated after the internal fault of the capacitor, and it also prolongs the discharge time. (2) The structure of the internal discharge coil should not be adopted for the integrated capacitor. Because after the discharge coil is removed from the shell and installed on the top of the integrated capacitor tank, although the external wiring is simplified, the internal cross wiring is increased, which increases the number of fault points. There have been cases in operation where the integrated capacitor was taken out of operation due to the fault of the internal discharge coil. Given that the discharge coil itself is very cheap, from the perspective of improving the reliability of the integrated capacitor, the internal discharge coil can only be used when the reliability of the discharge coil is one to two orders of magnitude higher than that of the integrated capacitor. In order to facilitate the wiring layout, the manufacturer can be asked to install a bracket on the top or side wall of the integrated capacitor to place the discharge coil. 3.5 Surge Arresters Oil-less circuit breakers do not have the problem of reignition after the capacitor is disconnected, and generally do not need to be equipped with surge arresters. Vacuum circuit breakers must be equipped with surge arresters. Surge arresters for capacitor banks protect the capacitors and should be installed as close to the capacitors as possible. Because of past incidents where surge arresters installed inside switchgear exploded, causing power outages on the entire busbar, surge arresters should not be installed inside switchgear. A preferred wiring method is to install a single zinc oxide surge arrester at the capacitor neutral point. This method limits single-phase reignition overvoltages and ensures the arrester does not bear voltage during normal operation. 3.6 Regarding the use of all-film capacitors: All-film capacitors have advantages such as low loss, low heat generation, low temperature rise, small size, and light weight. Since domestic production began in 1986, the quality of domestically produced all-film capacitors has stabilized through continuous improvement, and their reliability is now better than some imported products. Production has increased significantly year by year since 1995, and several products have passed the certification of two ministries. Compared with advanced foreign products, the main difference lies in specific characteristics; material consumption is twice that of advanced foreign products. Even so, compared with film-paper composite dielectric products, the volume and weight are significantly reduced. Taking the 100kvar product from Guilin Capacitor Factory as an example: the volume of the all-film product is reduced by 31.2% and the weight by 44.4% compared to the film-paper composite dielectric product. Taking the 3000kvar product from Jinzhou Capacitor Factory as an example: the volume of the all-film product is reduced by 55% and the weight by 47.9% compared to the film-paper composite dielectric product. The use of all-film products in box-type capacitors eliminates the need for heat sinks. Recently, the capacitor manufacturing industry has formulated several measures to accelerate the development of domestically produced high-voltage all-film capacitors, which will further improve the quality of domestically produced high-voltage all-film capacitors. Therefore, all new capacitors should use all-film products, and benzyltoluene (M/DBT) and SAS-40 should be the preferred impregnating agents. [b]4 Conclusion[/b] The capacity of parallel compensation capacitor banks in substations is generally selected according to 90% of the annual average reactive load on the low-voltage side. In areas where land is not scarce, rack-type and aggregated products should be preferred. Urban central substations can gradually adopt dry-type or box-type capacitors. All new capacitors should use all-film products, and M/DBT or SAS-40 should be the preferred impregnating agent. [b]References[/b] 1 Fang Jinlan. Discussion on the current level of capacitors at home and abroad and related technological development. Power Capacitors, 1997(1) 2 Ni Xuefeng et al. Analysis on overvoltage protection mode of parallel capacitors. Power Capacitors, 1997(4) 3 Notification on the operation of parallel capacitors, Dispatch Network (1996) No. 136