Harmonic-reducing multi-electric-heater series high-power stepless power regulating control method and system

Abstract

The application discloses a multi-electric-heater series high-power stepless power regulation control method and system capable of reducing harmonics, each electric heater is configured with an independent controller and a bidirectional thyristor switch module, a controller of one of the electric heaters is selected as a master controller, and the rest of the controllers are slave controllers; the master controller divides one power regulation total period into n continuous and mutually non-overlapping sub-time periods equal to the number of the electric heaters, each sub-time period contains a plurality of power grid periods; the power regulation opening degree is divided into n continuous and equally divided opening degree intervals, in each opening degree interval, a composite power regulation strategy of staggered zero-crossing and phase shift compensation is performed on the n electric heaters. The application can effectively disperse the harmonic currents generated by each unit in the time domain, avoid the superposition effect of the same frequency and the same phase, thereby reducing the system THD, and can ensure that the system can realize high-precision stepless regulation in the entire power range, and realize the simultaneous consideration of power regulation precision and power quality.

Description

Technical Field

[0001] This invention belongs to the field of electrothermal power control, specifically relating to a method and system for controlling the series connection of multiple electric heaters with high power stepless adjustment to reduce harmonics. Background Technology

[0002] In large-scale industrial electric heating systems (such as molten salt energy storage, high-temperature chemical reactions, and metal smelting), the performance requirements for electric heating devices are becoming increasingly stringent. Typically, three core indicators must be met simultaneously: megawatt-level high-power output capability, high-precision stepless power regulation (resolution up to 0.1%), and low harmonic pollution to the power grid. These three factors together constitute the key technological threshold for modern, efficient, and green electric heating systems.

[0003] To achieve high power output, using a single electric heater would subject critical components in the main circuit, such as thyristors, cables, and insulators, to extremely high voltage and current stresses. This not only significantly increases equipment costs but also introduces engineering risks such as heat dissipation difficulties, insulation failure, and decreased reliability. Therefore, in engineering practice, multiple electric heaters can be combined and operated to distribute the total power demand across multiple sub-heating units, thereby effectively reducing the electrical stress on individual circuits and improving system safety and economy.

[0004] However, the combined operation of multiple electric heaters introduces new control challenges. Taking four electric heaters as an example, when the total system power regulation command is 20%, each heater needs to bear about 5% of the rated power output. Under this low power condition, if all heaters use zero-crossing power regulation (i.e., adjusting the average power by controlling the number of full half-waves on), although harmonic generation can be avoided, the zero-crossing power regulation at low duty cycles manifests as long-term off and short-term pulsed on, resulting in discrete pulsed current waveforms for each heater. If multiple devices are not strictly synchronized or the loads are slightly different, the current pulses will overlap unevenly in time, causing drastic fluctuations in the total input current, leading to problems such as grid voltage flicker and control system instability. Conversely, if phase-shifting power regulation is used throughout (i.e., continuously controlling the effective value of the output voltage by adjusting the thyristor conduction angle), although high-precision stepless power regulation at the 0.1% level can be achieved, the current waveform is severely distorted at small conduction angles (such as corresponding to 5% power), and is rich in low-frequency harmonics such as the 3rd, 5th, and 7th. Since each heater conducts at the same time with the same phase angle, its harmonic currents are superimposed in phase on the grid side, causing the total harmonic distortion (THD) of the system to rise sharply, seriously interfering with other electrical equipment, and even triggering protection tripping.

[0005] Currently, another approach involves connecting multiple electric heaters in parallel or independently to the power grid, each equipped with the same phase-shifting power controller. These controllers receive the total power command, distribute it proportionally, and then independently execute phase-shifting control. While this method can achieve high power and high-precision power regulation, it has the following significant drawbacks: ① Severe harmonic concentration and superposition: All heaters are turned on at the same phase angle within the same power grid cycle, and the resulting harmonic currents are in the same phase. They are superimposed and amplified at the point of common coupling, resulting in persistently high THD. ② It is impossible to balance power regulation accuracy and power quality: If zero-crossing power regulation is used to reduce harmonics, continuous adjustment at the 0.1% level cannot be achieved in the low power range. The power regulation is stepped and it is difficult to meet the requirements of precise temperature control. ③ Lack of active harmonic suppression mechanism: Existing control strategies only focus on power distribution and do not decouple multiple harmonic sources from the time or phase dimensions, thus failing to weaken harmonic generation from the source.

[0006] In summary, existing technologies face a fundamental contradiction in high-power electric heating systems: the inability to simultaneously achieve "high-precision stepless power regulation," "low harmonic injection," and "rational device withstand voltage." A new control method is urgently needed to overcome this challenge. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention proposes a method and system for controlling the series connection of multiple electric heaters with high power stepless adjustment to reduce harmonics. The specific technical solution is as follows: A method for controlling the series connection of multiple electric heaters to reduce harmonics and achieve high-power stepless power regulation is proposed. Each electric heater is equipped with an independent controller and a bidirectional thyristor switch module. The controller of one electric heater is selected as the master controller, and the remaining controllers act as slave controllers. The method includes: The main controller divides a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, which is equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer; the power regulation opening degree is divided into n opening degree intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%; Within each opening interval, different zero-crossing and phase-shifting power regulation strategies are applied to the n electric heaters, specifically as follows: Within the opening range of 0~(100 / n)%, each of the n electric heaters selects one sub-time period from the n sub-time periods for phase shifting power adjustment, and does not adjust power in other sub-time periods, and the sub-time periods for phase shifting power adjustment of the n electric heaters do not overlap. Within the opening range of (100 / n)% to 2*(100 / n)%, n electric heaters each select one sub-time period from n sub-time periods for zero-crossing power adjustment and one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment and the sub-time periods for zero-crossing power adjustment of the n electric heaters do not overlap. The power opening of the zero-crossing power adjustment sub-time period is (100 / n)%, and the power opening of the phase-shifting power adjustment sub-time period is 0 to (100 / n)%, and the sum of the two reaches a power opening of (100 / n)% to 2*(100 / n)%. Similarly, within the opening range of i*(100 / n)%~(i+1)*(100 / n)%, n electric heaters each select i sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment by the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment by the n electric heaters is the same. The sum of the power openings of the zero-crossing power adjustment sub-time periods is i*(100 / n)%, and the power opening of the phase-shifting power adjustment sub-time periods is 0~(100 / n)%, and the sum of the two reaches a power opening of i*(100 / n)%~(i+1)*(100 / n)%. Within the opening range of (n-1)*(100 / n)%~100%, n electric heaters select n-1 sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. At the same time, the sub-time periods for phase-shifting power adjustment of the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment of the n electric heaters is the same. The sum of the power opening of all sub-time periods for zero-crossing power adjustment is (n-1)*(100 / n)%, and the power opening of the sub-time period for phase-shifting power adjustment is 0~(100 / n)%, and the sum of the two reaches the power opening of (n-1)*(100 / n)%~100%.

[0008] Furthermore, the electric heater is connected in the circuit in a three-phase three-wire manner as a resistive load; the controller outputs control signals by acquiring line voltage.

[0009] Furthermore, the main controller uses an external interrupt to align the clocks of the controllers of multiple electric heaters.

[0010] Furthermore, when zero-crossing power regulation is connected to phase-shifting power regulation, one-third of the grid cycle T of the zero-crossing power regulation mode is turned off in advance, thereby turning off the zero-crossing power regulation of one phase to avoid the superposition of current peaks with the phase-shifting current waveform.

[0011] A multi-electric heater series high-power stepless power regulation control system for reducing harmonics is used to realize a multi-electric heater series high-power stepless power regulation control method for reducing harmonics; the system includes an upper-level power regulation parameter sending module, a main controller, n-1 slave controllers, and a power supply isolated thyristor drive module; Both the master controller and the slave controller include a line voltage acquisition module, a startup time alignment module, and a thyristor control signal output module. The main controller also includes a calculation and decision module and a parameter allocation module; The upper-level power adjustment parameter sending module is used to obtain the power opening parameter and send the power opening parameter to the main controller; The main controller's calculation and decision module is used to divide a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer; the power regulation opening degree is divided into n opening degree intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%; and within each opening degree interval, different zero-crossing and phase-shifting power regulation strategies are designed for the n electric heaters; The parameter allocation module of the main controller is used to allocate parameters related to the power adjustment strategy for each electric heater according to the output of the calculation and decision module, and send them to the slave controller. The line voltage acquisition module is used to acquire the line voltage of the corresponding three-phase power supply for the electric heater; The startup time alignment module is used to align the startup times of the master controller and the slave controller; The thyristor control signal output module is used to output a thyristor trigger control signal after the alignment start time; The power supply isolated thyristor driver module is used to receive the thyristor control signal, amplify it, and then drive the bidirectional thyristor switch module.

[0012] The beneficial effects of this invention are as follows: 1. To address the problem that existing series-connected electric heaters perform phase-shifting power regulation at the same time with the same phase angle, resulting in the superposition of harmonic currents (especially low-frequency harmonics such as the 3rd, 5th, and 7th harmonics) within the same time period, causing a sharp increase in THD, this invention adopts a composite strategy of "time-shifted zero-crossing - phase-shifting compensation" to strictly stagger the phase-shifting power regulation operations of each electric heater to be executed in different sub-time periods. Through this time-shifting mechanism, the harmonic currents generated by each unit are effectively dispersed in the time domain, avoiding the superposition effect of the same frequency and phase, thereby reducing the system THD and greatly improving the power quality of the power grid.

[0013] 2. To address the limitations of existing methods in simultaneously achieving high-precision stepless power regulation across the entire range, this invention employs harmonic-free zero-crossing power regulation to provide the base power for most sub-time periods, while using phase-shifting power regulation for fine compensation in a staggered sub-time period. This ingeniously combines the advantages of both power regulation methods, ensuring that the system can achieve high-precision, truly stepless regulation at the 0.1% level throughout the entire 0%–100% power range, while suppressing harmonics to an extremely low level. Thus, it simultaneously achieves both power regulation accuracy and power quality. Attached Figure Description

[0014] Figure 1 This is a schematic flowchart of a multi-electric heater series high-power stepless power regulation control method for reducing harmonics according to an embodiment of the present invention.

[0015] Figure 2 This is a schematic diagram of the power adjustment method of a single electric heater according to an embodiment of the present invention.

[0016] Figure 3 This is a schematic diagram of zero-crossing power regulation and phase-shifting power regulation at a power regulation ratio of 50% in an embodiment of the present invention.

[0017] Figure 4 This is a diagram illustrating the series connection of multiple electric heaters under high power conditions in an embodiment of the present invention.

[0018] Figure 5 This is a power regulation mode distribution diagram for four sub-time periods when the power opening degree is 12.5% ​​in an embodiment of the present invention.

[0019] Figure 6 This is a power regulation mode distribution diagram for four sub-time periods when the power opening degree is 37.5% in an embodiment of the present invention.

[0020] Figure 7 This is a power regulation mode distribution diagram for four sub-time periods when the power opening degree is 62.5% in an embodiment of the present invention.

[0021] Figure 8 This is a power regulation mode distribution diagram for four sub-time periods when the power opening degree is 87.5% in an embodiment of the present invention.

[0022] Figure 9 This is a transformation diagram of the total current distortion rate at different firing angles in the phase-shift power regulation embodiment of the present invention.

[0023] Figure 10 This is a transformation diagram of each harmonic under different firing angles of phase-shift power modulation in an embodiment of the present invention.

[0024] Figure 11This is a waveform diagram of single-phase shift power regulation when the power regulation opening degree is 33.33% in an embodiment of the present invention.

[0025] Figure 12 The waveform diagram of the combined power regulation of off-time zero-crossing power regulation and off-time phase-shifting power regulation in an embodiment of the present invention is shown when the power regulation opening is 33.33%.

[0026] Figure 13 This is a waveform diagram of power regulation with time-lapse zero-crossing and phase-shift compensation when the power regulation opening is 33.33% in an embodiment of the present invention.

[0027] Figure 14 This is a comparison diagram of the current waveforms when the zero-crossing power adjustment is connected to the phase-shifting power adjustment in an embodiment of the present invention, showing whether the current is turned off in advance. Detailed Implementation

[0028] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The purpose and effects of the present invention will become clearer. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0029] On one hand, embodiments of the present invention provide a high-power stepless power regulation control method for multiple electric heaters connected in series to reduce harmonics. n electric heaters are electrically connected in series to form a high-power heating unit whose total power is the sum of the power of each individual electric heater. Each electric heater is equipped with an independent controller and a bidirectional thyristor switch module. Each electric heater is connected in the circuit using a three-phase three-wire system as a resistive load. The controller of one electric heater is selected as the master controller, and the remaining controllers are slave controllers. Figure 1 As shown, the method includes: The main controller divides a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, which is equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer, typically ranging from 10 to 100, and n ranges from 2 to 10. The power regulation opening is divided into n opening intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%. Within each opening interval, different zero-crossing and phase-shifting power regulation strategies are applied to the n electric heaters, specifically as follows: Within the total power adjustment ratio P∈0~(100 / n)% opening range, each of the n electric heaters selects one sub-time period from the n sub-time periods for phase-shift power adjustment, and no power adjustment is performed in other sub-time periods. Furthermore, the sub-time periods for phase-shift power adjustment by the n electric heaters do not overlap to prevent current superposition and harmonics. The power opening is changed by adjusting the phase-shift trigger angle; a trigger angle of 0° corresponds to an opening of (100 / n)%, and a trigger angle of 180° corresponds to an opening of 0. Since the phase-shift power adjustment ratio will not exceed (100 / n)%, the harmonics are relatively small. The power distribution of the n electric heaters in the n sub-time periods is shown in equations (1)~(3), where P... i (t) represents the power distribution of the i-th electric heater over n sub-time periods. α The trigger angle for phase-shift power regulation. P phase ( α This is a phase-shifting power regulation strategy. P zero Zero-crossing power adjustment strategy: Within the total power adjustment ratio P∈(100 / n)%~2*(100 / n)% opening range, each of the n electric heaters selects one sub-time period from the n sub-time periods for zero-crossing power adjustment and selects one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment by the n electric heaters do not overlap, and the sub-time periods for zero-crossing power adjustment by the n electric heaters do not overlap. The power opening of the zero-crossing power adjustment sub-time period is (100 / n)%, and the power opening of the phase-shifting power adjustment sub-time period is 0~(100 / n)%, and the sum of the two reaches a power opening of (100 / n)%~2*(100 / n)%. The power opening is adjusted by changing the firing angle during the phase-shifting power adjustment sub-time period. Since the phase-shifting power adjustment ratio will not exceed (100 / n)%, the harmonics are relatively small. The power distribution of the n electric heaters in the n sub-time periods is shown in equations (4)~(6). Similarly, within the opening range of i*(100 / n)%~(i+1)*(100 / n)%, n electric heaters each select i sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment by the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment by the n electric heaters is the same. The sum of the power openings of the zero-crossing power adjustment sub-time periods is i*(100 / n)%, and the power opening of the phase-shifting power adjustment sub-time periods is 0~(100 / n)%, and the sum of the two reaches a power opening of i*(100 / n)%~(i+1)*(100 / n)%. During the phase-shifting power adjustment sub-time period, the power opening is adjusted by changing the firing angle. ···· Within the opening range of (n-1)*(100 / n)%~100%, n electric heaters select n-1 sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. At the same time, the sub-time periods for phase-shifting power adjustment of the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment of the n electric heaters is the same. The sum of the power opening of all sub-time periods for zero-crossing power adjustment is (n-1)*(100 / n)%, and the power opening of the sub-time period for phase-shifting power adjustment is 0~(100 / n)%, and the sum of the two reaches the power opening of (n-1)*(100 / n)%~100%. Because each electric heater has n-1 (≥2) sub-time periods for zero-crossing power adjustment, it is required that the number of zero-crossing power adjustment sub-time periods superimposed in each sub-time period be consistent. For example, if an electric heater needs to perform zero-crossing power adjustment in n-1 sub-time periods, then each sub-time period can superimpose a maximum of n-1 zero-crossing power adjustments, plus one phase-shifting power adjustment to reduce current oscillation and harmonics. The power distribution of n electric heaters in n sub-time periods is shown in equations (7) to (9): Because a three-phase three-wire system is used, there is no neutral wire connected, therefore The expression is as follows: On the other hand, the present invention also provides a multi-electric heater series high-power stepless power regulation control system for reducing harmonics, used to realize a multi-electric heater series high-power stepless power regulation control method for reducing harmonics; the system includes an upper-level power regulation parameter sending module, a main controller, n-1 slave controllers, and a power supply isolated thyristor drive module.

[0030] Among them, the upper-level power regulation parameter sending module is used to obtain the power opening parameter and send the power opening parameter to the main controller; Both the master controller and the slave controller include a line voltage acquisition module, a startup time alignment module, and a thyristor control signal output module. The master controller also includes a calculation and decision module and a parameter allocation module.

[0031] The main controller's calculation and decision module is used to divide a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, which are equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer; the power regulation opening degree is divided into n opening degree intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%; and within each opening degree interval, different zero-crossing and phase-shifting power regulation strategies are designed for the n electric heaters.

[0032] The parameter allocation module of the main controller is used to allocate parameters related to the power adjustment strategy for each electric heater according to the output of the calculation and decision module, and send the parameters to the slave controller via RS485.

[0033] The line voltage acquisition module is used to acquire the line voltage of the corresponding three-phase power supply for the electric heater; The startup time alignment module is used to align the startup times of the master controller and slave controllers; The thyristor control signal output module is used to output the thyristor trigger control signal after the alignment start time.

[0034] The power supply isolated thyristor driver module is used to receive the thyristor control signal, amplify it, and then drive the bidirectional thyristor switch module.

[0035] The host system typically uses a DCS or Android device to predict the load / generation of the power grid and calculate the total power regulation ratio P. P is then sent to the main controller, which connects to the slave controller via an RS485 line. Control parameters are sent to the slave controller via the Modbus_RTU protocol. These parameters include: the number of sub-time periods, the length of each sub-time period (kT, where k represents the number of grid cycles), the power regulation modes for the n sub-time periods, the trigger angle for the phase-shifting power regulation mode, the power regulation ready signal, and the power regulation start signal. After receiving the parameters, the slave controller enables the power regulation ready signal and then waits for the power regulation start signal from the main controller.

[0036] The combined power of multiple electric heaters requires consistent initial power adjustment times. This invention uses hardwiring to connect multiple heaters together, and the master controller initiates a hardwiring level transition to trigger an external interrupt in the slave controller, causing both the master and slave controllers to begin power adjustment simultaneously. This method controls the power adjustment time alignment accuracy to within 100µs.

[0037] The following specific embodiment demonstrates the effectiveness of the method and system of the present invention.

[0038] This embodiment is designed to match the 6-10 kV high voltage at the generator outlet, employing a resistance-type high-voltage heating method. The connection method is a three-phase three-wire star connection. Power consumption is adjusted by controlling the switching on and off of the three-phase thyristor array. This embodiment achieves zero-crossing power regulation and phase-shifting power regulation through the timing of the switching on and off of the thyristors on the three-phase lines. Figure 2 As shown.

[0039] Zero-crossing power regulation uses a fixed-cycle method for adjustment. Its core logic is to adjust the proportion of the thyristor's conduction cycles at the zero-crossing point within a fixed time period to achieve power regulation. For example, within a fixed 10-second cycle, if the load is turned on for 5 seconds and off for 5 seconds of that voltage cycle, 50% power output can be achieved. Phase-shifting power regulation is a flexible and precise power control technology. Its core principle is to adjust the conduction angle of the thyristor (i.e., the phase deviating from the voltage zero-crossing point) to change the proportion of the load's conduction time within each voltage cycle, thereby achieving continuous power regulation.

[0040] like Figure 3 The diagram shows 50% zero-crossing power adjustment and phase-shifting power adjustment. This embodiment uses four electric heaters as an example to illustrate the connection method for multiple electric heaters. Figure 4 As shown, controller #1, acting as the master controller, receives the total power parameters from the upper-level DCS. After calculating the parameters, controller #1 distributes the control parameters to slave controllers #2, #3, and #4. The master controller distributes the total power evenly to achieve the effect of graded heating of the molten salt. For example, when the total power adjustment ratio P = 45%, each electric heater operates at 45% of full power during the power adjustment cycle (n sub-time periods). The n electric heaters are combined to achieve 45% of the full power of the n control cabinets, but the power adjustment mode of each electric heater is different in each sub-time period.

[0041] Here, we take four electric heaters (n=4, n ranges being 0~25%, 25~50%, 50~75%, 75~100%) as an example, and the specific power adjustment modes are as follows: When P∈(0, 25%), phase-shifting power regulation is performed in one sub-time period, and no power regulation is performed in other sub-time periods. The power distribution of the four electric heaters in the four sub-time periods is as follows: When P∈(25%, 50%), one sub-time period is used for phase-shift power regulation, and another sub-time period is used for zero-crossing power regulation. The power distribution of the four electric heaters in the four sub-time periods is as follows: When P∈(50%, 75%), one sub-time period is used for phase-shift power regulation, and two sub-time periods are used for zero-crossing power regulation. The power distribution of the four electric heaters in the four sub-time periods is as follows: When P∈(75%, 100%), one sub-time period is used for phase-shift power adjustment, and three sub-time periods are used for zero-crossing power adjustment. The power distribution of the four electric heaters in the four sub-time periods is as follows: The percentage of zero-crossing power adjustment is determined by the number of sub-time periods, with zero-crossing power adjustment accounting for 25% in a single sub-time period. Because a three-phase three-wire system is used, there is no neutral wire connected. P phase The expression for (α) is as follows: Taking four electric heaters (n=4) with a full power of P=6.25MW as an example, experiments were conducted on four power adjustment ratio ranges, and the following waveforms were obtained. Figure 5 The power adjustment mode of four electric heaters in one power adjustment cycle (four sub-time periods) when P=12.5%. Figure 6 The power adjustment mode of four electric heaters in one power adjustment cycle (four sub-time periods) when P=37.5%. Figure 7 The power adjustment mode of four electric heaters in one power adjustment cycle (four sub-time periods) when P=62.5%. Figure 8 The power adjustment mode of four electric heaters in one power adjustment cycle (four sub-time periods) when P=87.5%.

[0042] Harmonic analysis was performed on the superimposed current waveform of the main incoming line cabinet. Harmonic content analysis was conducted on the waveforms at each conduction angle under a single electric heater, and the results are as follows: Figure 9 and Figure 10 As shown, the range with the largest effective value of harmonic current is concentrated between 60° and 90°, and is relatively uniform. Most of the harmonic current is distributed in the 5th, 7th, 11th, and 13th harmonics, and the harmonic content has a trapezoidal distribution. This places a requirement on the harmonic containment capability of the power generation equipment at the power take-off end of the electric heater.

[0043] The following analysis compares the harmonic reduction effect of the present invention with the "time-shifted zero-crossing-phase-compensation" scheme of the present invention by comparing it with two other combined power adjustment methods. Analysis of the harmonic content of a single electric heater shows that the harmonic content is highest in the 60°~90° phase-shifted power adjustment range. Therefore, a total power percentage of 33.33% is selected as an example, because the firing angle corresponding to 33.33% is 86°, which falls within the 60°~90° range, the range with the highest harmonic content. Figure 11 The waveform for single phase-shift power modulation at a ratio of 33.33% is shown. Figure 12 The waveform diagram shows the combination of time-shifted zero-crossing power adjustment and time-shifted phase-shifting power adjustment at a ratio of 33.33%. Figure 13The waveform diagram of the time-shifting zero-crossing-phase-shifting compensation power regulation at a ratio of 33.33% is shown in Table 1. As can be seen from Table 1, the harmonic content is highest with single phase-shifting power regulation. The combination of time-shifting zero-crossing power regulation and time-shifting phase-shifting power regulation significantly reduces the harmonic content by avoiding the firing angle (60°~90°) of the maximum harmonic range. However, the combined harmonic content of the four electric heaters' phase-shifting power regulation is not as effective as that of single phase-shifting power regulation. Although the time-shifting zero-crossing-phase-shifting compensation power regulation method of this invention does not avoid the firing angle (60°~90°) of the maximum harmonic range, it uses time-shifting phase shifting to reduce the superposition of phase-shifting waveforms, resulting in a significant reduction in harmonic content. Therefore, it is proven that the combination of time-shifting zero-crossing-phase-shifting compensation power regulation method of this invention can effectively reduce the harmonic content.

[0044] Table 1 In this embodiment, zero-crossing power regulation starts timing from the zero-crossing point of phase C, and the sub-time period is set as a multiple of the grid cycle T. The zero-crossing point of phase C remains at the end of the sub-time period. When zero-crossing power regulation is connected to phase-shifting power regulation, to prevent current distortion caused by the zero-crossing triggering of the other two phases, the zero-crossing triggering of the other two phases is turned off one-third of the grid cycle after the zero-crossing point of phase C. The specific effect is as follows: Figure 14 As shown, to avoid distortion of the first current wave in the phase-shifting power regulation current waveform when it is not turned off in advance, it will be superimposed with the zero-crossing power regulation current of other cabinets, causing a spike in the total incoming cabinet current.

[0045] It will be understood by those skilled in the art that the above descriptions are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for controlling the series connection of multiple electric heaters with high power stepless adjustment to reduce harmonics, characterized in that, Each electric heater is equipped with an independent controller and a bidirectional thyristor switch module. The controller of one electric heater is selected as the master controller, and the remaining controllers act as slave controllers. The method includes: The main controller divides a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, which is equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer; the power regulation opening degree is divided into n opening degree intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%; Within each opening interval, different zero-crossing and phase-shifting power regulation strategies are applied to the n electric heaters, specifically as follows: Within the opening range of 0~(100 / n)%, each of the n electric heaters selects one sub-time period from the n sub-time periods for phase shifting power adjustment, and does not adjust power in other sub-time periods, and the sub-time periods for phase shifting power adjustment of the n electric heaters do not overlap. Within the opening range of (100 / n)% to 2*(100 / n)%, n electric heaters each select one sub-time period from n sub-time periods for zero-crossing power adjustment and one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment and the sub-time periods for zero-crossing power adjustment of the n electric heaters do not overlap. The power opening of the zero-crossing power adjustment sub-time period is (100 / n)%, and the power opening of the phase-shifting power adjustment sub-time period is 0 to (100 / n)%, and the sum of the two reaches a power opening of (100 / n)% to 2*(100 / n)%. Similarly, within the opening range of i*(100 / n)%~(i+1)*(100 / n)%, n electric heaters each select i sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. Simultaneously, the sub-time periods for phase-shifting power adjustment by the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment by the n electric heaters is the same. The sum of the power openings of the zero-crossing power adjustment sub-time periods is i*(100 / n)%, and the power opening of the phase-shifting power adjustment sub-time periods is 0~(100 / n)%, and the sum of the two reaches a power opening of i*(100 / n)%~(i+1)*(100 / n)%. Within the opening range of (n-1)*(100 / n)%~100%, n electric heaters select n-1 sub-time periods from n sub-time periods for zero-crossing power adjustment and select one sub-time period for phase-shifting power adjustment. At the same time, the sub-time periods for phase-shifting power adjustment of the n electric heaters do not overlap, and the number of overlapping sub-time periods for zero-crossing power adjustment of the n electric heaters is the same. The sum of the power opening of all sub-time periods for zero-crossing power adjustment is (n-1)*(100 / n)%, and the power opening of the sub-time period for phase-shifting power adjustment is 0~(100 / n)%, and the sum of the two reaches the power opening of (n-1)*(100 / n)%~100%.

2. The method for reducing harmonics using a series-connected high-power stepless power regulation control of multiple electric heaters as described in claim 1, characterized in that, The electric heater is connected to the circuit in a three-phase three-wire manner as a resistive load; the controller outputs control signals by acquiring line voltage.

3. The method for reducing harmonics using a series-connected high-power stepless power regulation control of multiple electric heaters as described in claim 1, characterized in that, The main controller uses an external interrupt to align the clocks of the controllers for multiple electric heaters.

4. The method for reducing harmonics using a series-connected high-power stepless power regulation control of multiple electric heaters as described in claim 1, characterized in that, When zero-crossing power regulation is connected to phase-shifting power regulation, one-third of the grid cycle T of the zero-crossing power regulation mode is turned off in advance, thereby turning off the zero-crossing power regulation of one phase to avoid current spikes generated by superposition with the phase-shifting current waveform.

5. A multi-electric heater series high-power stepless power regulation control system for reducing harmonics, characterized in that, The system is used to implement the harmonic reduction multi-electric heater series high-power stepless power regulation control method according to any one of claims 1 to 4; the system includes an upper-level power regulation parameter sending module, a main controller, n-1 slave controllers, and a power supply isolated thyristor drive module. Both the master controller and the slave controller include a line voltage acquisition module, a startup time alignment module, and a thyristor control signal output module. The main controller also includes a calculation and decision module and a parameter allocation module; The upper-level power adjustment parameter sending module is used to obtain the power opening parameter and send the power opening parameter to the main controller; The main controller's calculation and decision module is used to divide a total power adjustment cycle into n consecutive and non-overlapping sub-time periods t1, t2, ..., t3, equal to the number of electric heaters n. i ···t n Each sub-time period contains k power grid cycles T, where k is a positive integer; the power regulation opening degree is divided into n opening degree intervals, namely 0~(100 / n)%, (100 / n)%~2*(100 / n)%, ..., (n-1)*(100 / n)%~100%; and within each opening degree interval, different zero-crossing and phase-shifting power regulation strategies are designed for the n electric heaters; The parameter allocation module of the main controller is used to allocate parameters related to the power adjustment strategy for each electric heater according to the output of the calculation and decision module, and send them to the slave controller. The line voltage acquisition module is used to acquire the line voltage of the corresponding three-phase power supply for the electric heater; The startup time alignment module is used to align the startup times of the master controller and the slave controller; The thyristor control signal output module is used to output a thyristor trigger control signal after the alignment start time; The power supply isolated thyristor driver module is used to receive the thyristor control signal, amplify it, and then drive the bidirectional thyristor switch module.

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