Fund projects: 10,008,008 major projects of the National Science and Technology Support Program; MMC is a voltagesource topology applied to DC transmission of voltage source converters. This article introduces the topology of 0 and the corresponding working principle. Aiming at the characteristics of the new converter topology and its application in the field of HVDC transmission, a novel carrier-shifted modulation strategy suitable for modular multilevel converters is proposed. Compared with this strategy, this strategy has strong dynamic adjustment capabilities. Moreover, at the same time, the switching frequency also has good harmonic characteristics. According to the two principles of energy sharing and voltage balance of the submodules of the converter, combining the topology characteristics and modulation strategy of the converter, the capacitor voltage balance control strategy of the seed module is proposed to effectively ensure that the capacitor voltage of each submodule is in the same dynamic variation range. The simulation results verify the correctness and effectiveness of the proposed strategy. Technology; circulating current; capacitor voltage balance 0 attracts the world ’s power industry needs to adopt more flexible, economical and environmentally friendly transmission and distribution methods to meet the technical, economic and environmental challenges. Therefore, the demand for advanced power electronic devices has become increasingly urgent. Voltage source converter feeds DC transmission 0050 muscle 6converterhighollageDC, VSCHVDC6jfefi makes it play an active role in the field of renewable energy grid-connected passive network power supply city grid power supply asynchronous AC grid interconnection17, and has achieved great development in recent years . 8. Hejin, the core component of the 00 transmission system, its topology and modulation strategy have a great influence on the operating characteristics of 8, 0 and the effectiveness, economy and reliability of the 8 00 project The 8.0 that has been put into operation mostly adopts two-level, 50 or flat, 8 topology. The main problem of Calling 180 is the excessively high switching loss caused by excessively high switching frequency. The static dynamic voltage equalization and electromagnetic interference caused by the series connection are in addition. In addition, the switching consistency of the device is very high; level, 80 The main problems are the voltage equalization on the DC side and the effect of the third harmonic current on the neutral point on the DC side. One of the two topological structures also brings extremely difficult to the design layout and assembly of 0. In order to solve the above 80 question, Siemens has proposed a new type 8, suitable for 100 and 0, private bow modular multi-level converter 6. Adopt a block design, by adjusting the number of sub-modules in series Realize flexible changes in electrical and power levels, and can be extended to any level output, reducing electromagnetic interference and harmonic content of the output voltage, the output voltage is very smooth and close to the ideal sinusoidal waveform, so no large-capacity AC is required on the grid side Filter; the switching frequency of the switching device is low, and the switching loss is reduced accordingly; because the 0 topology disperses and stores energy in the capacitance of each sub-module of the bridge arm, the fault ride-through capability is improved. These characteristics of MMC improve the economic reliability and adaptability of HVDC transmission systems. It is said that the Ximen Guangshui built the world's 1st and only mining stream transmission tunnel 1 Cheng Dingyu 88301 North was called commercial operation in the United States in March 120, but at present, domestic and foreign academia is currently investing in MMC type voltage source converter DC transmission technology. Relatively few studies, key technologies such as modulation strategy capacitor voltage balance control fault protection have not been fully resolved, and in-depth research is urgently needed. The flutter structure only describes the external characteristics, advantages and disadvantages of the converter. Document 1314 proposes a multi-level space vector modulation method with high switching frequency suitable for motor speed regulation, but it is not suitable for the high-voltage large-capacity 8,100 transmission field that needs to reduce losses as much as possible. Reference 15 obtained the formula for the design of the capacitor voltage parameter, and proposed a capacitor voltage balance control strategy that introduces additional switching points, but did not propose a related MMC modulation method. The pre-charging process is also completed. The reference value of the capacitor voltage of the sub-module in the ratio balance strategy only considers the influence of the AC side voltage on it, and does not consider the influence of the change of the DC side voltage on it, so it is not suitable for 8, 10, Helai This paper proposes a novel type suitable for loose multi-level converters.5. It introduces the Fourier series expression of the output waveform at the time of 0% using this modulation strategy. This strategy has strong dynamic adjustment ability, and makes 40 at the lower switching frequency ll, too! VYl has good harmonic characteristics. The balance between the MMC sub-modules is difficult. In this paper, based on the two principles of energy sharing and voltage balance of sub-modules, combined with the converter topology and modulation strategy, a seed module capacitor voltage balance control strategy is proposed to ensure that the capacitor voltage of each sub-module is in the same level range. The capacitor voltage balance control strategy can work at a very low switching frequency, and the capacitor voltage of each sub-module does not need to be sorted, which reduces the number of switching times by 8 °, making the topology and operation of the modular 1 multi-level converter Mechanism Each bridge arm consists of several interconnected sub-modules with the same structure, SM reactors are connected in series, and the upper and lower bridge arms form a phase element. Root 10 block design, 6 bridge arms. There is symmetry, the electrical parameters of each sub-module and the reactance value of each bridge arm are the same. Significantly different from the previous 80 topology, the modular multilevel converter has no DC energy storage capacitor between the positive and negative poles on the DC side. The components at both ends of each sub-module are equipped with two switches. Through the action of two switches, Ding 1 and Ding 2, 1 can switch between the capacitor voltage and with two current directions at the same time. The specific switch state is described in 1. Among them, 1 turns on, 0 turns off. Since the three phase units in the converter have strict symmetry, take three phases as an example. The positive and negative busbars on the DC side of the household point and the point converter, their voltages relative to the reference neutral point are respectively. 2 and. 2., 1 and 2 are the controllable voltage source voltage of the upper and lower arms of the phase respectively as the voltage of the 3-phase AC output side. It can be obtained that the upper and lower two types in Equation 1 are added. It is known from Equation 1 that the output of the phase AC voltage is obtained by adjusting the number of sub-modules in which the upper and lower arms of the phase unit are in the input state; by Equation 2 It can be seen that the DC voltage is equal to the sum of the voltages of the upper and lower arms of the phase unit, that is, to ensure that the number of sub-modules put into the phase unit at any time is the same, these are necessary conditions for normal operation. Since the three phase units in PC0 have strict symmetry, and the upper and lower arms of the phase unit also have strict symmetry, the DC current 4 is equally divided among the three phase units, and the output of the three phases The current is also divided into 2 parts by the upper and lower arms. It can be obtained that the currents of the upper and lower arms of the 3-phase are based on the symmetry of the phase unit in 0. The 1-phase, the phase and the 3-phase have the same working mechanism as described above. To achieve high-performance and high-efficiency operation of the system, the modulation strategy of 2MMC must not only have an appropriate circuit topology as a basis, but also have a corresponding modulation strategy as a guarantee. For fire power multi-level converters, there are several modulation strategies such as step-wave pulse width modulation to eliminate specific sub-harmonic modulation of multi-carrier, technology multi-level voltage space vector control carrier phase shift pulse width modulation technology. Among them, although the staircase pulse width modulation algorithm is simple, but the output voltage is adjusted according to; 1 stream bus voltage or phase shift angle, dynamically adjust 1; difficult. The number of switches 1 is affected by the capacitor voltage balance control, and the loss is uncertain! 1 reduction; when eliminating certain points of the specific subharmonic modulation technical water meter, it is necessary to solve the nonlinear transcendental equation, the calculation is complicated, and the control is usually completed through the offline query method, and the dynamic characteristics are poor; The square number has a cubic relationship with the voltage space vector number. This technique will be limited when the number of levels is high. Therefore, in the field of 0, 00, where 0 requires a large number of levels, it is easy to implement, tearing 1 has more advantages, 1 this technology can achieve a higher equivalent at a lower switching frequency of the device The effect of switching frequency. Has good harmonic characteristics. 2 Carrier component. When = hungry to hungry = 2 3, the carrier harmonic 3 sideband harmonic component can be obtained. When = stress 7+, the mountain type 57 can be used, the output voltage of 05 sub-modules is linearly amplified by a multiple of a single sub-module; the equivalent switching frequency is increased by a huge amount. 3 is an even number, and carrier harmonics do not exist; when 7+ is an even number, sideband harmonics do not exist. Therefore, this technology can achieve a higher equivalent switching frequency at a lower switching frequency of the device, and has good harmonic characteristics. In the 1-phase unit of 0, the modulated waves of the two bridge arms under one phase are reversed to each other, so that the number of modules that are triggered by the upper and lower two bridge arms in each phase unit at any time is complementary to ensure that Each phase unit has a submodule input at any time. The reference of the phase angle of the modulated wave is 2. 3 Capacitance voltage balance control strategy 3.1 Control requirements In order to achieve the capacitance voltage balance of each sub-module in MMC, the method described in reference 16 to control and anticipate the system must continuously detect the capacitance voltage and the sub-module capacitance voltage in the same bridge arm Bridge arm current, then sort the sub-module capacitor voltage through software, and finally selectively put or switch the corresponding sub-module according to the bridge arm current. This method is simple to operate and more on the anvil, but in order to ensure the consistency of the capacitor voltage of each submodule, it is necessary to monitor and control the rotation of each submodule in a very short time, which will inevitably cause a switch in the submodule The unit is frequently turned on and off, which increases the switching loss of the converter. In response to this question, combined with the 88 stomach modulation method, a new module switching frequency is proposed. Then, the trigger state of the sub-modules is determined according to the change of the modulation wave, so as to adjust the charging and discharging time thereof, and then to achieve the balance of the capacitor voltage of each sub-module in the bridge arm. 3.2 The three phase units in the circulating inverter between the phase units are connected in parallel to the DC side bus. During operation, the DC voltage generated by each phase unit is difficult to maintain strict, so there is a circulating current flowing between the three phase units , 4 is the circulation principle inside the MMC converter. Since the structure of each phase unit of the 3 converter is symmetrical, the other phase units of the circulating current can be obtained by analogy. It can be drawn from Equation 89 that the circulating current only exists inside the converter, independent of the power supply or load connected to the outside of the converter. Although the reactance of the bridge arm can limit the size of the circulating current, the presence of circulating current between the phase units will still distort the current of the bridge arm and will also have a fixed impact on the capacitor voltage balance, so it is necessary to adopt an appropriate control strategy to Circulation is coordinated. 3.3 The control strategy consists of two parts. 1 can 1 even control. The role of this part is to ensure that the average value of the capacitance voltage of several sub-modules in each phase unit tracks its reference value, so that the energy is evenly distributed to these sub-modules. Control structure 5. The bad current is circulated in each module 1 and is affected by the current on the load side. An independent circulating current control inner loop can be designed to realize the regulation of the circulating current. The voltage outer loop adopts 1 regulator, which controls the average capacitor voltage in the phase unit to track the capacitor voltage reference value, and the output of the 1 regulator is used as the current reference; the current inner loop also uses 1 regulator to control the loop current to track the given loop current reference , The output of which is used as the capacitor voltage balance control adjustment. For example, when tweeting, the circulation reference value. ;increase. The independent current loop in the capacitor voltage control forces the actual value of the circulating current to track the reference value of the circulating current. Therefore, the feedback control of the loop control loop allows Chi to track its reference current without being affected by the load-side current, and flow to the converter; if 101 is negative, this is combined with 31 to the power flowing to the converter. Instead, watch out for 1 call, the inverter provides energy for the 1 flow side! 4. The capacitor discharge of the upper bridge is negative. If the ice is called negative, their combined power flows to the 1 side; at 13 o'clock, the power they cause also flows to the DC side. According to the above principle, the amount of voltage adjustment of the lower bridge arm capacitor is called. For 3.4, the modulation strategy combined with the capacitor voltage balance control quantity is based on the equivalent circuit of 3, and the reference value of the capacitor voltage of each sub-module can be obtained + the upper analysis arm 13, and now + the lower bridge arm can be seen from equation 1314, which is set in this article The reference value of capacitor voltage of each sub-module is not only affected by the AC side voltage of the converter, but also by the actual DC side voltage change. The reference pressure of literature 19 is set to a fixed value, which is significantly different from this article. Voltage balance control. The function of this part is to make the capacitance voltage of several sub-modules on the bridge arm track its reference value. The control structure, such as the voltage loop, uses a proportional regulator to control the capacitor voltage of the submodule to track the capacitor voltage reference value, and its output is the capacitor voltage balance control adjustment amount. Taking the upper bridge arm of the human phase as an example, since the capacitor voltage balance control is based on the current direction of the lower bridge arm, this is adjusted. The device should obtain energy from the DC side to charge the capacitor on the bridge arm. If 410 is a positive value, then the positive work combined with 41 is added to the voltage balance control of the capacitor in equation 1314, which can eventually participate in the carrier shift. The modulation wave amplitude of the phase modulation strategy, each bridge arm uses a different phase angle carrier and the amplitude of the sub-module modulation wave to compare, to generate a relatively independent condition group, the modulation signal, respectively drives the upper bridge arm, The power devices of the module determine whether they are put in or removed. The output voltages of the input sub-modules are superimposed to obtain the 440 bridge-arm stomach output voltage waveform. 2 The reference value of reactive power remains unchanged, and the DC voltage is adjusted from 10 to 9 at = 28. Result 13. By 810, when the reactive power of the system is reversed, the capacitor voltage of the bridge neutron module still maintains good consistency and can be stabilized in the same level range, so as to ensure that the DC bus voltage and the active power delivered by the system remain unchanged According to 3, when the DC voltage reference value changes, the module capacitor voltage is also dynamically adjusted, which can ensure the flexible adjustment of the DC bus voltage and the active power delivered by the system. From 9 and 12, we can see that the fluctuation range of the DC bus voltage is smaller than the fluctuation range of the capacitor voltage of a single submodule. This is because the modulated waves of the upper and lower bridge arms are inverse to each other, and the corresponding capacitor voltage waveforms of the bridge arm submodules are also inverse to each other. When several sub-modules in the upper and lower bridge arms are in the input state, the capacitor voltage waveforms are superimposed, and the fluctuations in the waveforms cancel each other out. The resulting fluctuation range of the current bus voltage is smaller than that of the individual sub-modules. . This feature can not only ensure the stability of the DC voltage but also reduce the harmonics on the DC side. 4 Simulation Research In the PSCAD simulation environment, build a two-level power transmission system based on CPSSPWM. Each phase unit is composed of 20 sub-modules, and there are 10 upper and lower bridge arms. The AC side system parameters are 6.12 rated AC line voltage on both sides, and the commutation inductance = 0.1 equivalent loss resistance = 0.02 DC side system parameter rated voltage. = 1 Inverter bridge arm reactance = 0. And = 5, the capacitor voltage reference of each sub-module is lkV. CPSPWM modulation frequency is 200Hz. In the simulation, fixed DC current is used for the rectifier. Voltage and constant reactive power control, the inverter side adopts constant active and constant AC voltage control. The simulation results of the rectifier side are taken as an example to introduce. 1 The DC current is constant, and the reference value of reactive power changes from 14, step to result 810 at 2-3 hours. The control strategy and capacitor voltage balance strategy, the designed control system realizes the fixed value control of DC voltage and the flexibility of transmission power Adjust. When the reactive power is adjusted, the effect on the active power and the DC voltage is small; when the DC voltage is adjusted, it has some effects on other quantities. 5. Conclusion strategy, this strategy can achieve the effect of higher equivalent switching frequency at lower device switching frequency, and has good harmonic characteristics. Combined with the 088 Li modulation strategy, a seed module capacitor voltage balance control strategy is proposed to ensure that the capacitor voltage of each sub-module is within the same cold level range and can maintain the consistency of the electric 1 change, thereby ensuring the stability of the DC voltage and the dynamics of the power Adjust. The switching frequency of the devices in the dry-throwing block is the same. 1 The lower the switching voltage, the capacitor voltage does not need to be sorted. This avoids the excessive number of 8-switches caused by the sorting, thereby reducing the noise, howling, 渮 湹, , 8, Tritium Ding 0, teach, wash, suck 1 level fluctuations, the range is controlled within 10, which shows that the capacitor voltage balance control strategy has good performance and can be dynamically adjusted. Acknowledgements Fou thank you Li Gengyin, Lu Pengfei, Li Guangkai, etc., supported by the Research Fund of North China Electric Power University. Development and prospect of light-weight HVDC transmission. Power System Automation, 2003, 27415. Zhang Guibin, Xu Zheng, Wang Guangzhu. Based on the steady-state modeling of the parent DC transmission system and its nonlinear control practices. Chinese Journal of Electrical Engineering, 2002, Hu Zhaoqing, Mao Chengxiong, Lu Jiming. A new optimized coordinated control application in light DC transmission. Chinese Journal of Electrical Engineering, 2005, Zhao Chengyong, Li Jinfeng, Li Guangkai. VSCHVDC control strategy based on independent adjustment of active power and reactive power J. Power System Automation, 2005, Yin Ming, Li Gengyin, Li Guangkai, etc. 30 00 continuous time state space models and their control strategy research exercises. Chinese Journal of Electrical Engineering, 2005, Luo Xiang, Tang Guangfu, Cha Kunpeng, etc. Test method of high-voltage DC transmission converter valve of voltage source converter. Power Grid Technology, 2010, 345 2 Take Xiang, Tang Guangfu, Cha Kunpeng synthetic experiment method in 80,00 converter valve short circuit current experiment. Power Grid Technology, 2010, 347913. Ding Champion, Ding Ming, Tang Guangfu, etc. New multi-level 0 sub-module capacitance parameters and voltage balancing strategy. Chinese Journal of Electrical Engineering, 2009, Ding Champion, Tang Guangfu, Ding Ming, etc. The topology mechanism and modulation strategy of the new multi-level voltage source converter module. Chinese Journal of Electrical Engineering, 2009, Li Jianlin, Zhao Dongli, Zhao Bin, etc. Carrier phase shift 8-cascade converter and its application in active power filter. China Electric Machinery Wu Hongyang, He Xiangning. Multi-level carrier, the essential relationship between stomach method and 8 stomach method and its application. Chinese Journal of Electrical Engineering, 2002, Zhao Xin, Zhao Xin 1985, male, master student, main research direction is HVDC transmission, new transmission and distribution technology and Zhao Chengyong 1964, male, professor, doctoral tutor, main research direction is HVDC transmission Modeling with system ruler 03, etc. 2, Shen Jiang Li Guangkai 1975, male, Ph.D., associate professor, the main research directions are high-voltage direct current transmission, new transmission and distribution technology, power quality analysis and control, etc. , Professor-level senior engineer, has long been engaged in AC and DC power systems and high voltage DC transmission Li Xiaolin 1963, male, senior engineer, master of engineering, engaged in the research of DC transmission and, 0 D 8. Chief Editor Wang Jianqiao
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