A high-frequency and high-voltage electrostatic precipitator power supply based on MMC

Abstract

The present invention discloses a high-frequency and high-voltage electrostatic dust removal power supply based on MMC, including a main circuit of a power supply system, a precharging strategy and a control strategy, and belongs to the technical field of environmental protection equipment. The quasi-square wave modulation adopted by the present invention can enable the power supply to achieve extremely high voltage gain without relying on the transformer ratio, greatly reduce the size of the transformer, and reduce the voltage change rate at the same time, thereby reducing the insulation difficulty of the intermediate isolation transformer. Based on the modulation strategy, a voltage equalization strategy is designed, which has a good sub-module voltage equalization effect.

Description

[0001] manual

[0002] A high-frequency and high-voltage electrostatic precipitator power supply based on MMC Technical Field

[0003] The present invention relates to a high-frequency and high-voltage electrostatic precipitator power supply based on MMC, and in particular to a high-frequency and high-voltage electrostatic precipitator power supply capable of achieving high voltage gain without a transformer, and belongs to the technical field of environmental protection equipment. Background Art

[0004] With the development of science and technology and the expansion of industrial production, the air pollution problem can no longer be underestimated. The main source of air pollutants is various industrial dusts discharged in large quantities due to industrial production. Electrostatic precipitators use high voltage to generate high-strength electric fields to ionize dust-containing gases, causing corona discharge and generating a large number of positive ions and electrons. Electrons collide with dust in the air to form negatively charged particles, which are then collected by the positively charged dust collecting electrode under the action of the electrostatic attraction of the space electric field. As the voltage increases, the greater the electric field strength, the more electrons are generated by air ionization, and the higher the dust removal efficiency.

[0005] Since the 21st century, with the maturity of high-frequency and high-voltage power supply technology, the original industrial frequency high-voltage power supply has gradually been replaced. Due to the better power quality, smaller power supply size and weight, and higher power density of high-frequency and high-voltage power supply, it has been widely used in the field of electrostatic precipitator.

[0006] The high-frequency and high-voltage electrostatic precipitator power supply commonly used in industry is highly dependent on the transformer ratio. The high-ratio transformer is expensive and bulky. Based on this technical problem, it is necessary to provide a high-frequency and high-voltage electrostatic precipitator power supply based on MMC that is independent of the transformer ratio to overcome the above defects. Summary of the invention

[0007] The present invention provides a high-frequency and high-voltage electrostatic precipitator power supply based on MMC, which can achieve high-gain conversion of voltage without relying on transformer ratio, and aims to improve the shortcomings of traditional high-frequency and high-voltage electrostatic precipitator power supplies, such as large volume and heavy weight.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A high-frequency and high-voltage electrostatic precipitator power supply based on MMC comprises a main circuit, a pre-charging strategy and a control strategy, wherein the control strategy comprises a quasi-square wave modulation strategy and a voltage equalization strategy.

[0010] Furthermore, the main circuit includes an inverter side and a rectifier side. The inverter side contains two bridge arms a and b composed of full-bridge sub-modules, wherein the upper bridge arm of bridge arm a and the lower bridge arm of bridge arm b are the first pair of cross bridge arms, and the other two are the second pair of cross bridge arms. The inverter side is directly connected to the low-voltage DC power supply, and the rectifier side contains two bridge arms c and d composed of half-bridge sub-modules, wherein the upper bridge arm of bridge arm c and the lower bridge arm of bridge arm d are the first pair of cross bridge arms, and the other two are the second pair of cross bridge arms. The rectifier side outputs the high voltage required for electrostatic dust removal, and the two sides are connected using a medium-frequency transformer with a transformation ratio of 1:1.

[0011] Furthermore, the specific method for pre-charging the submodules is as follows: a low-voltage DC power supply is used as a charging power supply on the inverter side, a current limiting resistor is connected, the rectifier side is disconnected, and the a and b bridge arms on the inverter side are charged simultaneously. The submodules with the same serial number in the two bridge arms are charged at all times, and the remaining submodules are removed. The capacitor voltage is measured in sequence in each cycle. When it is detected that the capacitor voltage reaches the expected value, the submodule is removed, and the subsequent submodules are put into operation in sequence until all submodules are charged. The current limiting resistor is disconnected, the rectifier side is connected, and the inverter side is put into operation; the inverter side outputs a quasi-square wave voltage as a charging power supply on the rectifier side, a current limiting resistor is connected, and when the voltage is positive, the submodules with the same serial number on the second pair of cross bridge arms on the rectifier side are charged simultaneously, and the remaining submodules are removed. When the voltage is negative, the submodules with the same serial number on the first pair of cross bridge arms on the rectifier side are charged simultaneously, and the remaining submodules are removed. The charging process is similar to the charging on the inverter side.

[0012] Furthermore, the signal generator sends a square wave with a duty cycle of 50% as a flag. For the first pair of cross bridge arms on the inverter side, when the flag is 1, the number of positive sub-modules in each bridge arm is calculated. If the bridge arm current is positive, the low voltage sub-module is prioritized, otherwise, the high voltage sub-module is prioritized. When the flag is 0, the number of negative sub-modules in each bridge arm is calculated. If the bridge arm current is positive, the high voltage sub-module is prioritized, otherwise, the low voltage sub-module is prioritized. For the other pair of cross bridge arms, The reaction of the flag bit is exactly opposite to the above; for the first pair of cross bridge arms on the rectifier side, no matter whether the flag bit is 1 or 0, the sub-modules are all in the positive input state, the only difference is the number of sub-modules put into operation. When the flag bit is 1, the number of sub-modules x put into operation in each bridge arm is calculated, and when the flag bit is 0, the number of sub-modules x-1 put into operation in each bridge arm is calculated. If the bridge arm current is positive, the low voltage sub-module is put into operation first, otherwise, the high voltage sub-module is put into operation first. The reaction of the other pair of cross bridge arms to the flag bit is exactly opposite to the above.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] Compared with the existing high-frequency and high-voltage electrostatic precipitator power supply, the high-frequency and high-voltage electrostatic precipitator power supply based on MMC proposed in the present invention can achieve voltage boosting without going through a high-ratio transformer. By reducing the voltage change rate through quasi-square wave modulation, the difficulty of transformer insulation design is reduced, and the volume and weight of the transformer are reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the circuit structure diagram of the high-frequency and high-voltage electrostatic precipitator power supply based on MMC.

[0016] Figure 2 This is the pre-charge flow chart.

[0017] Figure 3 The control strategy flow chart is shown in Figure 2.

[0018] Figure 4 This is the quasi-square wave voltage diagram output by the inverter side.

[0019] Figure 5 This is the voltage-sharing waveform when a submodule in the simulation works normally.

[0020] Figure 6 This is the capacitor voltage diagram when a submodule is pre-charged in the simulation.

[0021] Figure 7 is the DC voltage output by the rectifier side in the simulation. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0023] like Figure 1 As shown, the main circuit of the high-frequency and high-voltage electrostatic precipitator power supply based on MMC includes a rectifier side and an inverter side. The a and b bridge arms with the same number of full-bridge sub-modules constitute the inverter side, and the c and d bridge arms with the same number of half-bridge sub-modules constitute the rectifier side. It is stipulated that the upper bridge arm of the a bridge arm and the lower bridge arm of the b bridge arm are a pair of cross bridge arms on the inverter side, and the other two are another pair of cross bridge arms. The upper bridge arm of the c bridge arm and the lower bridge arm of the d bridge arm are a pair of cross bridge arms on the rectifier side, and the other two are another pair of cross bridge arms. The two sides are connected by a medium-frequency transformer with a transformation ratio of 1:1 to play an isolation role.

[0024] Since the initial voltage of the submodule capacitor is zero, the high-frequency and high-voltage electrostatic dust removal power supply needs to pre-charge the submodule before it is put into operation. The specific method is: first charge the inverter side, then cut off the rectifier side, connect the inverter side current limiting resistor, and charge the a and b bridge arms of the inverter side at the same time. Since the bridge arm parameters are exactly the same, the charging speed of the two bridge arms is exactly the same. The submodules with the same serial number in the two bridge arms are charged at the same time at all times, and the remaining submodules are cut off. After the two submodules are charged, they are cut off, and the next two submodules are put into use in sequence until all submodules are charged. When the voltage is positive, the second pair of cross bridge arms on the rectifier side is charged, and the submodules with the same serial number in the two half bridge arms are charged at the same time at all times, and the remaining submodules of the cross bridge arms and the other pair of cross bridge arms are cut off; when the voltage is negative, the submodules with the same serial number in the first pair of cross bridge arms on the rectifier side are charged, and the remaining submodules of the cross bridge arms and the other pair of cross bridge arms are cut off. The pre-charging flow chart is shown in FIG. Figure 2 As shown, the submodule pre-charging effect diagram is as follows Figure 6 It is worth mentioning that the expected value of the capacitor voltage of the inverter side submodule is U c That is, the low voltage DC power supply voltage value U dc , the expected value of the capacitor voltage of the rectifier side submodule is the amplitude of the quasi-square wave voltage U ac This is determined by the working method and will be described in detail below.

[0025] The modulation mode of the high-frequency and high-voltage electrostatic precipitator power supply enables it to achieve a large voltage gain without a transformer, where the inverter side is n / -(n-1) modulation and the rectifier side is x / (x-1) modulation. n / -(n-1) modulation means that when the inverter side is working, when the flag is 1, the first pair of cross bridge arms are positively input to n submodules, and the other pair of cross bridge arms are negatively input to (n-1) submodules, then the output voltage of bridge arm a is The output voltage of bridge arm b is The inverter side outputs an AC square wave voltage of u ac =u a -u b =(1-2n)U dc , at this time u ac =u a -u b =(2n-1)U dc x / (x-1) modulation means that when the flag is 1, the first pair of cross bridge arms are putting x submodules into operation, and the other pair of cross bridge arms are putting (x-1) submodules into operation, then the output voltage of the c bridge arm is The output voltage of the bridge arm D is again Therefore, at this time Uo=-(2x-1)u ac =(2x-1)(2n-1)U dc ; When the flag is 0, the two pairs of cross bridge arms are interchanged. Therefore, the high-ratio transformation of the low-voltage DC power supply is realized, and the output voltage U o like Figure 7 As shown, when the low voltage DC power supply is 210V, the output voltage U o Reached 42kV.

[0026] In order to reduce the difficulty of insulation design of the isolation transformer, the inverter side adopts quasi-square wave modulation, that is, u ac The waveform is as follows Figure 4 The quasi-square wave shown in the figure, the specific method is: when the flag bit is detected to change, the timer is reset and the timing is restarted, and the state of a submodule is changed every time interval t1. For the inverter side, when the flag bit becomes 1, the submodules of the first pair of cross bridge arms will be positively put into n, if the bridge arm current is positive at this time, then every time interval t1, a submodule with the lowest voltage will be positively put into, and the states of the other submodules will remain unchanged, if the bridge arm current is negative at this time, then a submodule with the highest voltage will be positively put into, and the states of the other submodules will remain unchanged, until the states of all submodules are changed; the submodules of the other pair of cross bridge arms will be negatively put into (n-1), if the bridge arm current is positive at this time, then every time interval t1, a submodule with the highest voltage will be negatively put into, and the states of the other submodules will remain unchanged, if the bridge arm current is negative at this time, then a submodule with the lowest voltage will be negatively put into, and the states of the other submodules will remain unchanged, until the states of all submodules are changed, at this time the AC voltage u ac Steps appear on the rising and falling edges of the voltage, reducing the voltage change rate.

[0027] After the step wave is generated, the voltage equalization calculation of the submodules must also be considered. The control strategy is as follows Figure 3 As shown in the figure, the overall idea is to determine the number of sub-modules and the charge and discharge status of each bridge arm according to the flag bit, time and current direction, and select the appropriate sub-module according to the sampling and sorting results of the sub-module capacitor voltage. The voltage balancing effect is as follows: Figure 5 As shown, it can be seen that the voltage of each sub-module fluctuates around the expected value, indicating that the voltage balancing strategy is feasible.

[0028] The present invention proposes a high-frequency and high-voltage electrostatic precipitator power supply based on MMC. The electrostatic precipitator power supply can achieve high-gain conversion of voltage without a high-ratio transformer, and only requires a medium-frequency transformer with a ratio of 1 to play an isolation role, which greatly reduces the volume, weight and cost of traditional high-frequency and high-voltage electrostatic precipitator power supplies; adopts a quasi-square wave modulation strategy to effectively reduce the voltage change rate, reduce the difficulty of transformer insulation design, and improve the shortcomings of traditional high-frequency and high-voltage electrostatic precipitator power supplies such as heavy weight and large footprint.

Claims

1. A high-frequency and high-voltage electrostatic precipitator power supply based on MMC, characterized in that: (1) The main circuit consists of four MMC bridge arms and an intermediate frequency isolation transformer; the two bridge arms a and b are the inverter side, connected to the DC power supply, and are composed of full-bridge sub-modules; the two bridge arms c and d are the inverter side, outputting the high voltage required for electrostatic dust removal, and are composed of half-bridge sub-modules; the midpoints of the two bridge arms a and b are connected to the primary side of the 1:1 intermediate frequency transformer, and the midpoints of the two bridge arms c and d are connected to the secondary side of the 1:1 intermediate frequency transformer; (2) The pre-charging strategy includes pre-charging the half-bridge sub-modules and full-bridge sub-modules that make up the four MMC bridge arms; (3) The control strategy includes a quasi-square wave modulation strategy and a corresponding voltage equalization strategy. The specific method is as follows: The flag bit is a square wave with a duty cycle of 50% emitted by the signal generator; For the upper bridge arm of the bridge arm a and the lower bridge arm of the bridge arm b on the inverter side, when the flag bit is 1, the number of positive sub-modules in each bridge arm is calculated, n. If the bridge arm current is positive, the low voltage sub-module is prioritized, otherwise, the high voltage sub-module is prioritized. When the flag bit is 0, the number of negative sub-modules in each bridge arm is calculated, n-1. If the bridge arm current is positive, the high voltage sub-module is prioritized, otherwise, the low voltage sub-module is prioritized. The reaction of the other pair of bridge arms to the flag bit is exactly opposite to the above, which is called n / -(n-1) modulation. For the upper bridge arm of the c bridge arm and the lower bridge arm of the b bridge arm on the rectifier side, no matter the flag bit is 1 or 0, the submodules are all in the positive input state. The only difference is the number of submodules put into operation. When the flag bit is 1, the number of submodules put into operation in each bridge arm is calculated as x. When the flag bit is 0, the number of submodules put into operation in each bridge arm is calculated as x-1. If the bridge arm current is positive, the low voltage submodule is put into operation first. Otherwise, the high voltage submodule is put into operation first. The reaction of the other pair of bridge arms to the flag bit is exactly opposite to the above, which is called x / (x-1) modulation.

2. According to claim 1, a high-frequency and high-voltage electrostatic precipitator power supply based on MMC is characterized in that: The specific method for precharging the submodule is as follows: the inverter side uses the DC side power supply as the charging power supply, disconnects the rectifier side loop, connects the current limiting resistor, and precharges the full-bridge submodule capacitor on the inverter side. The submodules with the same serial number in the two bridge arms a and b are in the charging state at the same time, and the other submodules are removed. The capacitor voltage is measured once per cycle. When the capacitor voltage reaches the expected value, the submodule is removed, and the subsequent submodules are put into operation in sequence until all submodules are charged, the current limiting resistor is disconnected, and the rectifier side is connected and put into operation; the rectifier side is output by the inverter side. The square wave voltage is used as the power supply for charging and the current limiting resistor is connected. When the voltage is positive, the sub-modules with the same serial number of the lower bridge arm of the c bridge arm and the upper bridge arm of the d bridge arm are charged at the same time, and the other sub-modules are removed. When the voltage is negative, the upper bridge arm of the c bridge arm and the lower bridge arm of the d bridge arm are charged at the same time, and the other sub-modules are removed. The capacitor voltage is measured once per cycle. When the capacitor voltage reaches the expected value, the sub-module is removed and the subsequent sub-modules are put into use in turn until all sub-modules are charged and the current limiting resistor is disconnected. At this point, the pre-charging is completed.

3. According to the MMC-based high-frequency and high-voltage electrostatic precipitator power supply according to claim 1, it is characterized in that: The method of generating quasi-square wave modulation is as follows: when a change in the flag bit is detected, the state of a sub-module is changed every time interval t1 according to the state that the sub-module should finally be in, until the states of all sub-modules are changed. At this time, the output square wave voltage on the inverter side has steps on the rising and falling edges.

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