🌱China’s HVDC transmission projects have developed rapidly after 2000, and the rapid development of HVDC transmission projects has led to a rapid increase in the demand for different types of power capacitors in converter stations. For a DC transmission project, the capacity of the power capacitors used is far from the capacity of the capacitors required by the AC system. For example: a DC project with a DC voltage equal to ±500 kV and a transmission power of 3000 MW, the total capacity of the required capacitors It has reached more than 7 million kilowatts (depletion), which almost accounts for 20% of the total market demand for high-voltage power capacitors in the country. What’s more, as my country’s high-voltage direct current transmission tends to develop in the direction of high voltage and large capacity, characteristics of capacitors the number of capacitors required will also increase exponentially.
🌿At present, characteristics of capacitors used in DC converter stations mainly include the following seven types: ① shunt capacitors installed in the AC field of the converter station; ② capacitors for filters installed in the AC field of the converter station; DC filter capacitors installed in the DC field; ⑤ capacitors for DC PLC noise filters (the capacitors have been basically replaced by ceramic sleeve type or silicone rubber type coupling capacitors); ⑥ neutral line impulse capacitors; ⑦ DC circuit breaker capacitors.
1. Parallel capacitors
☘️The DC shunt capacitor device is only used to compensate the reactive power required by the converter. It is different from the capacitors used in the ordinary AC power system in the following points:
🍀(1) The rated voltage is high, which includes the arithmetic sum of each harmonic voltage, that is, =+. Therefore, the calculation of the shunt capacitor used in the DC converter station is 20%~40% higher than that of the conventional AC power system capacitor without considering the harmonics. For example, the single-phase rated voltage of the AC 500kV shunt capacitor is as high as 357%. ~411 kV, sometimes in the design, considering the uneven voltage distribution between the series sections, it is also necessary to multiply the rated voltage of the capacitor unit by a redundancy factor of 1.02~1.05.
🎍(2) The rated working field strength of the capacitor unit is higher than that of the ordinary AC capacitor unit. Due to the high value, it is necessary to select a high dielectric field strength. Generally, domestic capacitors are selected to be 60 MV/m and above to ensure the reliability of operation.
🍂(3) The series reactance value is very small, usually the K value is less than 0.1%, and even in some projects, a shunt capacitor without series reactor at the tail end is used, that is, K=0. In China, basically, in the new DC projects after 2004, the capacitor switching switch adopts the synchronous opening and closing device (this book will be described in the following chapters), the closing inrush current is greatly reduced, and under normal circumstances, the capacitor will not be fused inside. wire damage.
💐(4) The capacitor used on the capacitor bridge arm of the converter station has more stringent requirements on the difference in capacitance value. The allowable deviation of the capacitance value of the conventional capacitor is -5%~+10% (the capacitance value of the DC capacitor is generally also followed), but some special Under such circumstance, if the capacitor of the switching switch of the phase selection closing controller was not installed in the early stage, and there were some deficiencies in the manufacture of the capacitor used for the DC converter station in the early stage of the same period, in practice, only the capacitance deviation of the capacitor used in the DC project was controlled. At 2%, the stable operation of the capacitor after switching can be ensured (and the improvement of the capacitor manufacturing quality of domestic capacitor manufacturers has also relaxed the limit of capacitance value deviation). The essence of controlling the capacitance value deviation of the capacitor is to ensure that each phase is connected in series Inter-segment voltage uniformity. Strict requirements for deviations make higher requirements for the design, manufacture and quality control of capacitors.
🌷(5) The structure of the capacitor tower of the DC converter station, the structure of 1 to 3 towers per phase can be selected according to the specific situation, and generally 1 to 2 towers are in the majority in China. Of course, if the floor space allows, use a multi-tower structure as much as possible to reduce the height of the capacitor tower and facilitate installation and operation and maintenance. This is also the difference between DC capacitors and AC compensation capacitors in the tower structure.
🌹(6) The annual damage rate of capacitors used in DC converter stations is much lower than that of ordinary AC capacitors. Because the power failure of capacitors has a certain impact on the operation of the DC system, the general bidding specifications have made mandatory requirements. The annual damage rate of capacitors is less than 0.2%, which puts forward higher requirements on the design and manufacturing process of DC capacitors.
🌺(7) The protection configuration is different from that of ordinary AC capacitors. The DC capacitor adopts bridge differential current protection, and the unit adopts internal fuse protection.
2. AC filter capacitors
In addition to providing reactive power for the converter, the AC filter capacitor is also used to filter out the various harmonics generated by the converter valve to the AC power grid. In addition, it has the same characteristics as shunt capacitors in terms of protection methods and protection methods. In addition, it has its own characteristics:
🌸(1) It needs to cooperate with reactors and resistors to form a filter that filters out specific frequency harmonics. Due to the need for tuning, there are stricter requirements on the tolerance and temperature characteristics of capacitors. Usually, the tolerance of capacitor units is controlled at ±2%, each phase tolerance is controlled at ±1%.
🌼(2) In addition to high-voltage capacitors, AC filter capacitors generally have low-voltage capacitors. The selection of rated voltage has its own characteristics. The largest of the following three requirements should be selected: ① Continuous working voltage value (fundamental voltage and The arithmetic sum of each harmonic voltage); ② the voltage value calculated according to the capacity sum of the fundamental wave and each harmonic; ③ the voltage value obtained by dividing the impulse withstand level value by 4.3.
🌻(3) When the capacitor unit on the low-voltage side of the AC filter cannot be protected by an internal fuse, the dielectric working field strength should be appropriately reduced in order to obtain higher operational reliability.
🌞(4) AC filter capacitors are noisy during operation. In general AC power systems, the noise level of capacitors will not attract people’s attention. However, AC filter capacitors operating in converter stations are not affected by the installation capacity. If it is large, the high-order harmonic current passing through is large, and the noise problem will be particularly prominent. In the bidding design specification for the new converter station, strict restrictions are put forward on the noise of the filter capacitor device. Therefore, the capacitor device adopts noise reduction measures and reduces the height of the capacitor tower to meet the requirements of noise reduction.
3. DC filter capacitors
🌝The DC filter capacitor is installed on the DC side to filter out the harmonics on the DC side. The demand for the DC filter capacitor itself is not high, but it has higher technical and production requirements than the capacitor used in the AC system of the converter station. , and even foreign countries have produced products with stable quality after many repeated improvements. At present, domestic manufacturers such as Xirong and Guirong are also capable of producing DC-side capacitors with performance that meets operating requirements. DC filter have the following characteristics of capacitors:
🌛The rated voltage is high, and its calculation formula is
In the formula ，
U—the maximum continuous operating voltage of the DC line, kV;
U—Each harmonic voltage on the DC side, kV;
K—voltage coefficient, the value is greater than or equal to 1.3.
⭐️According to formula (1-8), we take the conventional ±500kV DC converter station as an example, take 511kV, and the rated voltage of the 6/12th and 24/36th order DC filter capacitors will be as high as 836kV and 751kV, respectively.
🌟(2) The DC filter protection method, the capacitor unit adopts the internal fuse protection, and the capacitor bank adopts the bridge difference unbalanced current protection (the details will be introduced in the later chapters of this book).
✨(3) Voltage equalization measures: There are voltage equalization resistors and discharge resistors inside the capacitor unit, and the resistance value must consider the voltage equalization effect in the design. The current can also keep the voltage distribution basically uniform, and at the same time, the design can prevent overheating inside the DC characteristics of capacitors.
⚡️(4) The creepage distance of external insulation should be at least greater than 50mm/kV.
🌈(5) The design of the DC filter capacitor tower is different from that of the AC filter capacitor tower. In order to reduce the influence of the insulation leakage current on the voltage distribution, the DC filter capacitor tower can only adopt a single tower structure, and the capacitor unit can be installed flat on the tower. Decrease the height of the tower. Suspended installation can also be used if necessary (±80 kV DC converter stations have been put into operation in China, such as the Suidong converter station of the Chusui DC project, because the single tower of the DC filter capacitor is too high, the support type installation is unstable. , only the suspension installation method can be selected).
☀️(6) The low-voltage capacitor of the DC filter actually bears the AC voltage of each spectral wave during the operation, and its technical requirements are the same as those of the capacitor for the AC filter.
4.The AC converter transformer incoming line PLC capacitor
❄️At present, this type of capacitor is no longer designed and installed in many newly built converter stations. The early converter stations were designed to filter out the 30-500 kHz harmonics in the PLC frequency range on the AC bus, to prevent high-order harmonics generated by the converter. Harmonics cause noise interference to the PLC system on the AC side and the carrier communication on the connected and adjacent AC lines. Here is a brief introduction to its characteristics:
🍓(1) Its rated voltage is high, mainly including harmonic voltage. For example, the phase voltage of some conventional ±500 V DC engineering PLC capacitors in China is as high as 441 kV, which is 1.45 times the rated voltage of power frequency.
🍒(2) In terms of structure, if the capacitance of the PLC capacitor is small (generally below 1 nF), the column structure of the porcelain jacket is used, and when the capacitance is large (greater than 30 nF), the tower structure is used.
5. DC PIC characteristics of capacitors
🍑DC PLC capacitors include coupling capacitors for carrier communication of the DC side power line of the converter station (the new DC project is not used for carrier communication) and filter capacitors. Its essence is a low-pass filter. Its function is to filter out the harmonics in the frequency range of the DC line PLC and prevent noise interference to the adjacent line carrier communication. Its technical characteristics are the same as those of the AC PLC. Voltage equalization measures to offset the adverse effect of insulation leakage current on voltage distribution in the case of external pollution. The creepage distance of the external insulation is generally required to be greater than 50 mm/kV.
6. Neutral line impulse capacitors
🥭Also known as a surge capacitor, this capacitor protects the DC neutral and has the same characteristics as a DC filter capacitor.
7. DC circuit breaker characteristics of capacitors
🍍Before the DC circuit breaker breaks the current, the capacitor discharges to form a sinusoidal alternating current, and the principle of the AC circuit breaker can be used to cut off the DC current. This kind of capacitor has short working time, high operating overvoltage, and its demand is very small. Generally, it is specially designed according to the project.
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