What Is the Purpose of Capacitors in Parallel?

Shunt capacitor, originally called a phase-shift capacitor. It is mainly used to compensate the reactive power of the inductive load of the power system to improve the power factor, improve the voltage quality, and reduce line losses. The single-phase shunt capacitor is mainly composed of a core, a casing, and an outgoing structure. A metal foil (as an electrode plate) is stacked with an insulation paper or a plastic film and wound, and a number of components, insulation pieces and fasteners are press-fitted to form a capacitor core, and impregnated with an insulating oil. The leads of the capacitor plate are led in series and in parallel to the outgoing connection piece at the lower end of the outgoing porcelain sleeve. The metal case of the capacitor is filled with insulating dielectric oil.

The installation of shunt capacitors in substations is an effective measure to improve voltage quality and reduce power loss. ^[1]

The load on the power grid changes at all times, and the parallel capacitors need to be frequently switched on and off. During the process of the breaker opening and closing the parallel capacitors, operating overvoltages will inevitably occur, which may damage the parallel capacitors and affect the normal operation of the power grid. ^[2]

Shunt capacitors will amplify harmonics, which may cause damage to electrical equipment. ^[3]

Commonly used shunt capacitors can be divided into a variety of types such as single iron shell type, box type, collective type, semi-closed type, dry type and inflatable type according to their structure.

Single iron-shelled shunt capacitor

This type of capacitor has a large amount and a wide range. A single unit generally has a capacity of 50, 100, 200, 334kvar, etc. Now there are products with larger capacities (such as 500kvar and above). Generally, products with a capacity of 100kvar or more have a Fuse. Once this kind of product is damaged, users can quickly replace it with spare parts and get the device back in time. Therefore, the use rate of this kind of product is high. In addition, external fuses can be configured to provide relatively complete protection. At present, 220kV, especially 330kV and above voltage grade substations mostly use a single iron-shell shunt capacitor. There are also more and more people in order to improve the rust and corrosion resistance of capacitors, requiring stainless steel plates instead of ordinary steel plates to produce capacitors. Even so, some also spray anti-ultraviolet paint on the surface; such a protective layer can prevent rust and corrosion, and greatly reduce the negative effect of ultraviolet radiation on the temperature rise of the capacitor, thereby extending the service life of the capacitor.

Among this type of capacitor, China has been mainly for internal fuse capacitors for two or three decades, that is, a small fuse is installed on each component of the capacitor. In recent years, non-fuse capacitors have appeared. They are capacitors with neither internal fuses nor external fuses. Before the 1970s, domestically produced all-paper capacitors and early paper-film composite capacitors were white. At that time, the internal fuse was still in the research stage, and it was impossible to use it in products. The special external fuse for protecting capacitors was also from 1980. Development has only begun since the beginning of this year. After the internal components of the capacitor break down, they are all protected by electromagnetic relays, so the capacitors at that time were completely non-fuse capacitors. The subsequent application of internal and external fuses has made China's non-fuse capacitors disappear for about 30 years. Although there have always been capacitors without internal fuses, they are only allowed to use external fuses.

Non-fuse full film capacitors have a new meaning that is different from the previous. It has passed the transistor relay and integrated circuit relay stages and directly entered the era of microcomputer protection. There are three ways to connect internal components of non-fuse capacitors in China:

(1) The traditional dominant device is connected in parallel and then in series. The number of internal parallel components is relatively small, and it is not appropriate to configure a small-capacity capacitor with an internal fuse (for example, less than 10kvar). This wiring method has been used.

(2) The internal components are connected in series and then in parallel, that is, a wiring method that has recently been re-advocated.

(3) Internal components have both serial and parallel components, but different from the above two wiring methods, there are parallel and parallel in the string, which is a hybrid connection method. There is no uniform format for such a connection, which needs to be determined based on the requirements for the capacity and protection of a single unit during design.

This type of capacitor is not suitable for a complete set of 10kV capacitors. Although the wiring method of the serial and parallel components is relatively better among the three, its single capacity should not be larger than 100kvar. The advantages of non-fuse capacitors are simple structure, low loss and low manufacturing cost.

Box-type shunt capacitor

The shape of this capacitor is similar to that of small and medium-sized transformers. The interior is a single capacitor core with an iron case removed. Several capacitors are connected in series and parallel according to the design requirements. The single unit of this product has a large capacity (500kvar and above). After damaged components appear inside, once the carbon black is precipitated and diffused, it is basically impossible to repair.

Collective shunt capacitor

According to its structure, this capacitor has two types: semi-sealed and fully sealed. The oil storage tank with a drying filter is the former, whether or not there is an oil seal at the entrance; the oil storage tank without the oil tank and using other methods to compensate for the change in oil level cold and hot inside the box belongs to the latter. An electric capacity adjustment product developed at present, the operation practice shows that it is not very reliable. Its movable contact is in the oil. Over time, it is prone to poor contact, which may cause local overheating. In addition, it will produce momentarily when transferring between two terminals. Phase problems may cause trouble, so you can use the switch to manually adjust the capacity after power off.

The capacitor has outstanding advantages and disadvantages. Its main advantages are easy installation, low maintenance workload, and saving space. The disadvantage is mainly inconvenience to the user. Although its maintenance workload is small, it is not intuitive to observe it, and it cannot relax its attention to capacity changes; especially in places with harmonics, its capacity Changes must always be watched. As the operating time goes on, the internal fuse may gradually move, which will cause a three-phase capacitance imbalance. This fault is difficult to repair on site, and it takes time to return to the factory for repair, which affects the operation rate of the capacitor. Furthermore, the change in the percentage of the series reactance of the parallel compensation device caused by this will be far from a predetermined target when it reaches a certain level, and even cause trouble. In particular, a parallel compensation device with a 4.5% reactance percentage should be selected in advance. Once this percentage declines and approaches 4%, reliable countermeasures should be taken. What's more noteworthy is that there is a "dead zone" for ground protection when the lower end (in oil) of the capacitor's high-voltage outlet bushing is flashed or broken down to ground. There are no targeted measures in the "Code for Design of Shunt Capacitor Devices" (GB 50227-1995) and relevant national industry standards; once such an accident occurs, it can only be developed after the components have been damaged and an unbalanced voltage or current has appeared. In order to force backup relay protection action. Operational practice shows that such accidents have occurred, and they are all malignant accidents. Therefore, when putting into operation of such products, consideration should be given to prevent this problem. In fact, the cause of this type of accident is the failure of insulation to ground, which is caused by blind spots in protection. The backup protection action is that the accident has been enlarged, causing the collective capacitor to be severely damaged, and a remedy after an unbalanced voltage or current has been generated. The existing protection cannot prevent such a vicious accident.

Statistics of actual operation of parallel compensation devices in recent years show that the annual damage rate of collective capacitors is about 4 times that of a single iron-shell capacitor, and it is higher in some areas; coupled with factors such as inability to repair on site, in recent years The market share of the products showed a clear downward trend.

Semi-enclosed shunt capacitor

The semi-closed shunt capacitor is an assembly made by placing a single capacitor bushing against a bushing on a special steel frame, and then closing the conductive part (the ground potential part is not closed). Can be laid in multiple layers and developed to high altitudes to save floor space. This kind of product has high requirements for the dipping process of the capacitor unit. It is best to install an external fuse, otherwise it is difficult to ensure safe operation. This type of product is produced by ABB, and their products are available in the subtropical regions of China, and have been operating safely for more than 10 years. There were some problems in the early days of domestic production, and some people suggested that the use of the product should be banned, but the improved product has a safe operation record of more than 10 years.

Dry-type shunt capacitor

This capacitor is a high-voltage capacitor that is transplanted from low-voltage metallized film technology. Several components are connected in series and in parallel, so it still has self-healing characteristics and is in line with the development direction of oil-free products. Oil-free capacitors do not burn as expected, and the polypropylene-based film inside the capacitor will still catch fire when conditions are available. In addition, self-healing capacitors are not foolproof, and can reliably heal themselves after each partial breakdown. Practice has proven that there is a probability of not "self-healing" (that is, self-healing failure), so this product must be designed with effective fire prevention measures and special protection measures to ensure safe operation.

Inflatable shunt capacitor

This capacitor is currently coexisting with oil and gas, that is, the oil in the box of the integrated product is replaced with gas, and the single iron shell product inside is still oil-immersed. Since the thermal conductivity of gas is not as good as that of liquid, special measures must be taken in this regard for this type of product to ensure reliable heat dissipation. Heat pipe technology is a commonly used one. However, the actual performance of this type of product is not satisfactory; one of the reasons is that the gas leakage cannot be automatically alarmed in time, and at the same time, a trip signal must be sent to the circuit breaker in order to timely remove the capacitor to prevent the gas leakage from causing the insulation level to drop and cause malignancy accident.