A seamless loop power transfer device and a method for improving the waveform quality of the device

Abstract

The present invention provides a seamless closed-loop power transfer device and a method for improving the waveform quality of the device, including: when the device is put into use, determining the phase angle difference between two AC bus voltages, where different phase angle differences correspond to different compensation voltages; determining the equivalent winding value of the secondary winding of the transformer required, adjusting the series-parallel relationship of the secondary winding to output the required compensation voltage; during the process of the device exiting, changing the conduction angle of the thyristor, so that the DC voltage output by the rectifier gradually decreases, the inversion modulation ratio of the inverter remains unchanged, the AC voltage output by the inverter gradually decreases, and the compensation voltage gradually decreases. After the DC voltage drops to a preset value, then reducing the inversion modulation ratio to make the compensation voltage continue to decrease, so that the amplitude of the AC bus voltage after power transfer remains unchanged. The present invention enables the inversion modulation ratio of the inverter to be maintained at a stable level through the series-parallel design of the secondary winding of the transformer and the control of the thyristor conduction angle, improving the waveform quality of the output of the seamless closed-loop power transfer device.

Description

Technical Field

[0001] The present invention belongs to the field of seamless loop power transfer, and more specifically, relates to a seamless loop power transfer device and a method for improving the waveform quality of the device. Background Art

[0002] In the low-voltage distribution network, the method of power outage first and then switching is usually adopted during equipment maintenance, which causes a long power outage time for users. In recent years, with the increase of power consumption load, users have higher and higher requirements for power supply reliability. Therefore, in order to reduce the user's perception of power outage and reduce the impact of the input of standby power on the load, a new type of power transfer method, namely seamless loop power transfer, is proposed.

[0003] Seamless loop power transfer can be described as: injecting a voltage source between two low-voltage busbars to achieve flexible control of the loop power flow, providing zero voltage or zero current breaking conditions for the line switch, so as to achieve the purpose of shock-free load transfer. The device that realizes the seamless loop power transfer function is called a seamless loop device. This device provides an injected voltage source by a power electronic device and realizes the power flow control during the power transfer process. A transformer transforms the AC voltage output by the power electronic device to change the voltage level and injects a compensation voltage between the two low-voltage busbars.

[0004] Due to the wide application range of the seamless loop power transfer device, the seamless loop power transfer device is often used for different levels of compensation voltage and load current. When the seamless loop power transfer device is used for large-power transmission, if the switching frequency of the power electronic switch device is still maintained at a high level to ensure good output waveform of the device, high power loss will be generated. In order to reduce the power loss, the switching frequency should be reduced, while a lower switching frequency means a reduction in the waveform quality of the device output, seriously affecting the device performance. Since a reasonable inverter modulation ratio can significantly improve the waveform quality of the device inversion, in order to still make the device output a good waveform at a low switching frequency, it is necessary to maintain the inverter modulation ratio at a reasonable level in various situations so that the waveform of the power electronic device reaches the best at this modulation ratio level.

[0005] The inversion of the device is mainly reflected in the loop power transfer process and the withdrawal process of the device. Due to the different application conditions of the device, for example, the required compensated voltage and the required transferred load current are different. If a conventional double-winding transformer is used in the device, the inversion ratio cannot be maintained at a reasonable level under different working conditions. If a multi-winding transformer is used, although the modulation ratio can be at a reasonable level, the large power transmission makes the volume and weight of the transformer large, which is contrary to the mobility requirement in the on-site application of the seamless loop power transfer device, so it is still not feasible.

[0006] In addition, during the shutdown process of the device, the compensation voltage needs to be gradually reduced to zero. If the modulation ratio is directly reduced, the inverter modulation ratio will inevitably be reduced to a very small value during this process, resulting in poor waveform quality. This is not allowed for some applications with high requirements for power quality.

[0007] To solve the above contradiction, a device and method are needed to keep the inverter modulation ratio of the seamless closed-loop power transfer device at a certain preset level under the premise of ensuring good mobility of the device, so as to improve the output waveform quality of the device under various working conditions. Summary of the Invention

[0008] Aiming at the defects of the prior art, the purpose of the present invention is to provide a seamless closed-loop power transfer device and a method for improving the waveform quality of the device, aiming to solve the problem that the existing seamless closed-loop power transfer device cannot ensure good output waveform quality.

[0009] To achieve the above purpose, in the first aspect, the present invention provides a seamless closed-loop power transfer device, including: a rectifier, an inverter, and a transformer;

[0010] The rectifier is used to rectify the bus voltage into a corresponding DC voltage;

[0011] The DC side of the inverter is connected to the rectifier, and is used to invert the DC voltage rectified by the rectifier into an AC voltage and output it from the AC side of the inverter;

[0012] The primary winding of the transformer is connected to the AC side of the inverter, and the secondary winding is connected in series on the connection line between the two AC bus voltages, and is used to convert the AC voltage output from the AC side of the inverter into a corresponding compensation voltage and inject it between the two AC bus voltages; the secondary winding of the transformer is connected in series and parallel with multiple windings, and the equivalent winding value of the secondary winding can be adjusted by controlling the series and parallel relationship of the multiple windings;

[0013] When the seamless closed-loop power transfer device is put into use, the phase angle difference between the two AC bus voltages to be transferred is determined. Different phase angle differences correspond to different compensation voltages; the transformer determines the required compensation voltage according to the phase angle difference between the two AC bus voltages, and correspondingly determines the required equivalent winding value of the secondary winding, adjusts the series and parallel relationship of the secondary winding, and outputs the required compensation voltage;

[0014] The core component of the rectifier is a thyristor; during the process of the seamless closed-loop power transfer device exiting, the rectifier changes the conduction angle of the thyristor, causing the DC voltage output by the rectifier to gradually decrease. At this time, the inversion modulation ratio of the inverter remains unchanged, the AC voltage output by the inverter gradually decreases, the compensation voltage output by the transformer gradually decreases, and the phase of the compensation voltage gradually changes, so that the voltage amplitude on the AC bus voltage after power transfer remains unchanged; when the DC voltage output by the rectifier decreases to a preset value, the conduction angle of the thyristor is kept unchanged, and the inversion modulation ratio of the inverter gradually decreases, causing the compensation voltage to gradually decrease to a minimum value. When the compensation voltage decreases to the minimum value, the process of the seamless closed-loop power transfer device exiting ends; the series-parallel design of the secondary winding of the transformer and the control of the thyristor conduction angle keep the inversion modulation ratio of the inverter at a stable level, improving the quality of the output waveform of the seamless closed-loop power transfer device.

[0015] In an optional example, the device further includes: a PWM adjustment device;

[0016] The PWM adjustment device is used to determine the phase of the compensation voltage according to the voltage parameters before and after seamless closed-loop power transfer, so as to control the inverter using PWM technology, enabling the inverter to output the compensation voltage corresponding to the phase.

[0017] In an optional example, when the phase angle differences of the two AC bus voltages are 30°, 15°, and 7.5° under different working conditions, the turns ratios of the transformer should be 2:1, 4:1, and 8:1 respectively. Based on the turns ratio of the transformer, the series-parallel relationship of the secondary winding is changed to keep the turns ratio of the transformer at the level corresponding to the different phase angle differences.

[0018] In an optional example, the secondary winding of the transformer includes four identical sub-windings, and the sum of the windings in series of the four sub-windings is half of the primary winding;

[0019] When the turns ratio of the transformer is 2:1, the four sub-windings are in series;

[0020] When the turns ratio of the transformer is 4:1, the four sub-windings are connected in parallel in pairs and then in series;

[0021] When the turns ratio of the transformer is 8:1, the four sub-windings are in parallel.

[0022] In an optional example, the core component of the inverter is an IGBT.

[0023] In a second aspect, the present invention provides a method for improving the waveform quality of a seamless closed-loop power transfer device. The seamless closed-loop power transfer device includes a rectifier, an inverter, and a transformer. The DC side of the inverter is connected to the rectifier and is used to invert the DC voltage rectified by the rectifier into an AC voltage and output it from the AC side of the inverter. The primary winding of the transformer is connected to the AC side of the inverter, and the secondary winding is connected in series on the connection line between two AC bus voltages. The method includes the following steps:

[0024] When the seamless closed-loop power transfer device is put into use, determine the phase angle difference between the two AC bus voltages to be transferred. Different phase angle differences correspond to different compensation voltages. Among them, the compensation voltage is obtained by the transformer transforming the AC voltage output from the AC side of the inverter into the corresponding compensation voltage and injecting it between the two AC bus voltages. The secondary winding of the transformer is connected in series and parallel with multiple windings, and the equivalent winding value of the secondary winding can be adjusted by controlling the series and parallel relationship of the multiple windings.

[0025] Determine the required compensation voltage according to the phase angle difference between the two AC bus voltages, and correspondingly determine the required equivalent winding value of the secondary winding to adjust the series and parallel relationship of the secondary winding, so that the transformer outputs the required compensation voltage.

[0026] During the process of the seamless closed-loop power transfer device exiting, change the conduction angle of the thyristor so that the DC voltage output by the rectifier gradually decreases. At this time, the inversion modulation ratio of the inverter remains unchanged, the AC voltage output by the inverter gradually decreases, the compensation voltage output by the transformer gradually decreases, and the phase of the compensation voltage gradually changes, so that the voltage amplitude on the AC bus voltage after power transfer remains unchanged. The thyristor is the core component of the rectifier.

[0027] When the DC voltage output by the rectifier decreases to a preset value, keep the conduction angle of the thyristor unchanged and gradually decrease the inversion modulation ratio of the inverter so that the compensation voltage gradually decreases to a minimum value. When the compensation voltage decreases to the minimum value, the process of the seamless closed-loop power transfer device exiting ends.

[0028] Through the series and parallel design of the secondary winding of the transformer and the control of the conduction angle of the thyristor, the inversion modulation ratio of the inverter is maintained at a stable level, improving the waveform quality of the output of the seamless closed-loop power transfer device.

[0029] In an optional example, the phase of the compensation voltage gradually changes, which is specifically achieved by the following method:

[0030] Determine the phase of the compensation voltage according to the voltage parameters before and after seamless closed-loop power transfer, and use PWM technology to control the inverter so that the inverter outputs the compensation voltage corresponding to the phase.

[0031] In an alternative example, when the phase angle differences of the two AC bus voltages are 30°, 15°, and 7.5° under different operating conditions, the transformation ratios of the transformer should be 2:1, 4:1, and 8:1 respectively. Based on the transformation ratio of the transformer, the series-parallel relationship of the secondary winding is changed to maintain the transformation ratio of the transformer at the level corresponding to the different phase angle differences.

[0032] In an alternative example, the secondary winding of the transformer includes four identical sub-windings, and the sum of the windings in series of the four sub-windings is half of the primary winding;

[0033] When the transformation ratio of the transformer is 2:1, the four sub-windings are in series;

[0034] When the transformation ratio of the transformer is 4:1, the four sub-windings are connected in parallel in pairs and then in series;

[0035] When the transformation ratio of the transformer is 8:1, the four sub-windings are in parallel.

[0036] Generally speaking, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:

[0037] The present invention provides a seamless closed-loop power transfer device and a method for improving the waveform quality of the device, which can enable the seamless closed-loop power transfer device to adjust the modulation ratio of inversion according to different operating conditions. When the seamless closed-loop power transfer device is put into use, the transformation ratio of the transformer is changed through the series-parallel design of the secondary winding of the transformer, so that the inversion modulation ratio of the inverter remains unchanged to ensure that the seamless closed-loop power transfer device can output the corresponding compensation voltage; and during the process of the seamless closed-loop power transfer device exiting, the control of the conduction angle of the rectifier thyristor enables the inversion modulation ratio of the inverter to be maintained at a stable level and gradually reduces the compensation voltage. Through the device exit strategy provided by the present invention, the influence of the seamless closed-loop power transfer device on the device performance due to the deviation of the inversion modulation ratio from the reasonable level during the exit process can be effectively weakened. Since the modulation ratio of the inverter of the seamless closed-loop power transfer device in the present invention is maintained at a stable level throughout the entire working process, the seamless closed-loop power transfer device can output good-quality waveforms under various operating conditions.

[0038] The present invention provides a seamless closed-loop power transfer device and a method for improving the waveform quality of the device. The present invention can effectively reduce the volume and weight of the seamless closed-loop power transfer device, improve the on-site mobility of the seamless closed-loop power transfer device, and save the device cost to a certain extent. The present invention proposes a transformation form of the transformer in the seamless closed-loop power transfer device, which can enable the seamless closed-loop power transfer device to work flexibly under different operating conditions and expand the application scenarios of the seamless closed-loop power transfer device. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the application of the low-voltage distribution network seamless closed-loop power transfer device provided by the embodiment of the present invention.

[0040] Figure 2 Schematic diagram of low-voltage busbars and compensation voltage for closed-loop power transfer provided by an embodiment of the present invention.

[0041] Figure 3 Schematic diagram of the structure of a five-winding transformer provided by an embodiment of the present invention.

[0042] Figure 4 Schematic diagram of the structure of a certain phase of a multi-winding transformer after transformation provided by an embodiment of the present invention.

[0043] Figure 5 Schematic diagram of the application during the closed-loop phase of the device provided by an embodiment of the present invention.

[0044] Figure 6 Control flowchart for the withdrawal process of the seamless closed-loop power transfer device provided by an embodiment of the present invention. Detailed implementation manners

[0045] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0046] The output voltage is consistent with the grid voltage, so transformers with different turns ratios need to be used. Although conventional multi-winding transformers can meet the requirements of different compensation voltages, in the application scenarios of seamless closed-loop power transfer devices, large compensation voltages and large load currents do not appear simultaneously. If a conventional multi-winding transformer is used, the capacity utilization rate of the transformer will be reduced, indirectly resulting in an increase in the volume and weight of the seamless closed-loop power transfer device, and the mobility of the device will become worse.

[0047] Since the transformer turns ratio cannot be changed after the device is put into use to maintain the inverter modulation ratio at a certain level during the process of the compensation voltage gradually decreasing when the device is withdrawn. When the seamless closed-loop power transfer device is withdrawn, if it is necessary to control the compensation voltage output by the inverter to gradually become zero, the rectifier needs to act as an inverter during this process. Because a reasonable modulation ratio needs to be ensured during this process, the DC capacitor voltage is gradually reduced, and the DC capacitor discharges energy through the rectifier. If the device adopts a structure in which the rectifier module and the inverter module in pulse width modulation (PWM) mode are coupled through a DC capacitor, when the DC capacitor voltage drops to a relatively low level, the inverter modulation ratio of the rectifier will be far lower than the reasonable level, seriously reducing the output waveform quality and affecting the device performance.

[0048] Therefore, the present invention proposes a method that can ensure good output waveform quality and maintain mobility during the operation and withdrawal processes under various working conditions.

[0049] The method of the present invention enables the device to output a compensated voltage waveform with good quality under different degrees of compensated voltage during closed-loop power transfer. A multi-winding transformer with multiple turns ratios should be used. However, the conventional multi-winding transformer will increase the volume and weight of the device, making the on-site applicability of the device poor, which does not conform to the original design intention of the device. Therefore, the multi-winding transformer is transformed into a double-winding transformer form for use. Specifically: the secondary windings of the multi-winding transformer are connected through a control switch. For different working conditions, the on-off state of the secondary control switch is controlled to change the series-parallel relationship between the secondary windings of the multi-winding transformer. Finally, the multi-winding transformer is transformed into a double-winding transformer form for use (that is, there are only two output terminals after the transformation of the multi-winding on the secondary side of the transformer). The primary side of the transformer is connected to the AC side of the inverter responsible for outputting the compensated voltage in the seamless closed-loop power transfer device. After the multi-winding on the secondary side of the transformer is transformed into a double-winding form, it is connected in series on the tie line between the two buses during closed-loop power transfer.

[0050] When the method of the present invention is put into use, by obtaining the voltage difference between the two buses and the load current information to be transferred, the obtained measurement information is decided by the control center to judge the device working mode (control switch state) that best suits the current working condition, and corresponding operations are performed on the control switch. After the actual turns ratio of the transformer is determined, the control center adjusts the modulation ratio of the inverter up and down at a reasonable level by detecting the switch state of the transformer to adapt to the current working condition, so that the seamless closed-loop power transfer device can perform accurate voltage compensation according to the voltages on both sides of the closed-loop switch during the power transfer process to achieve turn-off under zero voltage condition and no impact on load transfer.

[0051] The method of the present invention enables the inverter modulation ratio of the inverter to be maintained at a preset optimal level during the device exit process. First, the DC capacitor voltage is gradually reduced to a smaller value, and then the inverter modulation ratio is gradually reduced to make the compensated voltage gradually decrease. During this process, the DC voltage is regulated by the rectifier. The core device of the rectifier adopts a thyristor in a phase-controlled form to avoid the inverter modulation ratio problem during the process of the rectifier regulating the capacitor voltage. Considering that the switching frequency of the IGBT in the inverter module is relatively high and the quality of the inverter waveform is relatively high, the core device of the inverter adopts an IGBT. Therefore, in the method, a rectifier with a thyristor as the core device and an inverter with an IGBT as the core device are coupled through a DC capacitor. The transformer connected to the inverter adopts a double-winding transformer form after the transformation of the multi-winding transformer.

[0052] The different working conditions depend on the different voltages of the two buses (compensated voltage) and the load current during closed-loop power transfer.

[0053] The rated voltage, turns ratio, capacity, etc. of the transformer adopted in the device are determined according to the actual on-site requirements.

[0054] During the process of the device being put into operation to being withdrawn, in order to avoid causing a large impact on the power transfer process, the transformation ratio of the transformer should not change.

[0055] The connection group of the transformer in the device is determined according to requirements.

[0056] Only the multi-windings of the transformer in the device with the same rated voltage can be connected in parallel, and only those with the same rated current can be connected in series.

[0057] The use of the device is not limited to low-voltage distribution networks.

[0058] The rectifier, inverter and the transformer connected thereto in the device are all designed modularly, and corresponding modules can be selected according to actual requirements during on-site application.

[0059] Embodiment 1

[0060] Figure 1 It is a schematic diagram of the application of the seamless closed-loop power transfer device for low-voltage distribution network provided by the embodiment of the present invention. Figure 1 During the shown closed-loop power transfer process, let be the voltage phasor of low-voltage bus 1, be the voltage phasor of low-voltage bus 2, and δ be the phase angle difference between the grid voltages. Refer to Figure 2 As shown, through calculation, the amplitude and phase angle of the compensation voltage can be obtained:

[0061]

[0062] Under general working conditions, it is considered that the voltage amplitudes of the two low-voltage buses are approximately equal, that is, At this time, there is It can be seen from this that generally, when U S can be approximated as a fixed value, the value of the compensation voltage phasor changes with the change of δ.

[0063] Usually 0° ≤ δ ≤ 30°. Let I represent the load current. In areas with dense loads in the city, δ is around 7.5°, let it be δ = 7.5°, I = 400 A; in general urban load areas, δ can be set to 15°, I = 200 A; in the urban fringe areas, δ is about 30°, and the load current is small, I can be set to 100 A.

[0064] When the phase angle differences of the low-voltage bus voltages are 30°, 15°, and 7.5° under different working conditions, the required values of the compensation voltage are respectively Therefore, for different operating conditions where the phase angle difference of the low-voltage bus voltage is 30°, 15°, and 7.5°, the transformer turns ratios should be 2:1, 4:1, and 8:1 respectively. When the inverter maintains the ideal modulation ratio, the rated voltage of the primary winding of the transformer is close to the grid voltage. When U1 = 1.04U s is set, the compensation voltages provided by the seamless closed-loop power transfer device are 0.52U s , 0.26U s , and 0.13U s respectively, meeting the compensation voltage requirements. A five-winding transformer with a turns ratio of 8:1:1:1:1 as shown in Figure 3 is used to transform the AC voltage output by the inverter, and different series-parallel combinations are adopted for the secondary windings of the five-winding transformer according to different operating conditions. Figure 3 The rated voltage-current relationships and switching states of different series-parallel combinations of the secondary windings of the transformer shown in Table 1. Taking phase a as an example, the transformation of the secondary winding of the multi-winding transformer is as shown in Figure 4 .

[0065] Thus, when the line voltage is 400V, U s = 240V. A double-winding transformer with a turns ratio of 2:1 has a rated voltage of 240V.

[0066] The capacity of the transformer mainly depends on the compensation voltage phasor and the load current that needs to be transferred during the closed-loop process. The capacity of the single-phase series transformer is equal to the product of the injected voltage and the current flowing through the series transformer, as shown in the following formula:

[0067]

[0068] Therefore, the capacity of the above double-winding transformer is 48 kVA; for the transformed transformer as shown in Figure 4 , from the rated voltage and rated current relationships of different series-parallel combinations of the secondary winding of the transformer shown in Table 1, the rated current of the primary side of the transformer is 50A, the rated voltage of the primary winding is 240V, and the capacity is 12 kVA, reducing the volume of the transformer. And when the multi-winding transformer is in the three schemes shown in Table 1, the available capacity of the transformer is always 12 kVA. When using a conventional double-winding transformer operating at δ = 15° or 7.5°, although the compensation voltage decreases, the current allowed to pass through the secondary side will not increase accordingly. Therefore, under the operating conditions of δ = 15° or 7.5°, the utilization rate of the transformer will decrease. It can be seen that using the method in the present invention to transform the transformer can effectively reduce the volume and weight of the device and enhance the mobility of the device for the common application scenarios of the seamless closed-loop power transfer device, and can effectively expand the application scenarios of the seamless closed-loop power transfer device.

[0069] Table 1

[0070]

[0071] Example 2

[0072] The working stages of the seamless loop - closing power transfer device are divided into: device access, loop - closing, power transfer, withdrawal, and removal. The loop - closing stage of the device will be specifically elaborated below in combination with the method in the present invention.

[0073] Set the three standard working conditions with the phase - angle differences of 30°, 15°, and 7.5° of the low - voltage bus voltage in Example 1 as the three working modes of the seamless loop - closing power transfer device.

[0074] After the device is put into operation, if the measured real - time phase - angle difference is δ = 5°, the control center judges the working mode of the seamless loop - closing power transfer device to which the current working condition belongs according to the real - time measurement data (it can be judged that it belongs to the working mode of δ = 7.5° at this time). The control center outputs corresponding instructions to the control switch of the transformer according to the corresponding working condition, so that the control switch of the transformer works in the working state of "parallel connection of four secondary windings" in Table 1. After stabilization, input the control - switch state to the control center. The control center combines the transformer turns ratio at this time with the required compensation voltage difference at δ = 5°, and then calculates the specific inverter modulation ratio at the ideal modulation - ratio level. The control center outputs corresponding control signals to the inverter according to the calculated modulation ratio, so that the output voltage of the inverter is equal to the required compensation voltage. (Since the working time of the seamless loop - closing power transfer device from access to removal is very short, it can be approximately considered that the operating condition will not change greatly in a short time.) Perform accurate voltage compensation, close the loop - closing switch, and complete the loop - closing. The schematic diagram of this process is shown in Figure 5 。

[0075] Example 3

[0076] Example 3 will specifically elaborate on the withdrawal stage of the device in combination with the method in the present invention.

[0077] When the device withdraws (at this time, the tie - line switch is disconnected, the loop - closing switch is closed, the transformer 2 to be repaired has been removed, and the power originally provided by the transformer 2 for the load on bus 2 is now provided by bus 1 through the branch where the loop - closing switch is located for the load on bus 2), the device needs to control the compensation voltage to gradually decrease to a minimum value close to zero (the setting of this minimum value needs to consider the control range of thyristors in the rectifier and maintain the modulation ratio for reducing the compensation voltage to the minimum value in the next stage at a reasonable level) to prepare for the removal of the device.

[0078] According to the method proposed by the present invention, the specific process of the device exiting is as follows: After the inverter diverts all the load current to the bus tie line (power transfer), the conduction angle of the thyristor is changed, and the thyristor rectifier is controlled to slowly decrease the DC capacitor voltage. At this time, the inversion modulation ratio of the inverter remains unchanged, the output voltage of the inverter gradually decreases, and the compensation voltage value on the secondary side of the transformer gradually decreases accordingly. The phase of the compensation voltage gradually changes to keep the voltage amplitude on bus 2 unchanged. When the DC voltage decreases to a preset small value, the conduction angle of the thyristor is kept unchanged, and the DC voltage is controlled to maintain at a small fixed value. Next, the modulation ratio of the inverter is gradually decreased at the optimal modulation ratio level, and the compensation voltage u on the secondary side of the transformer is slowly decreased c to a minimum value (the setting of this minimum value needs to consider other factors such as the device's own accuracy to make u c as close to zero as possible). When the compensation voltage is lower than or equal to the set minimum value, it can be considered that the compensation voltage is small enough, and the tie switch is closed. Thus, the device exit stage is completed. The control flow of the above device exit process is shown in Figure 6 .

[0079] It should be noted that the phase of the compensation voltage is determined according to the voltage parameters before and after seamless loop power transfer, and the inverter is controlled by PWM technology to make the inverter output the compensation voltage corresponding to the phase, so as to change the phase of the compensation voltage and keep the voltage amplitude on bus 2 unchanged.

[0080] Those skilled in the art can easily understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A seamless loop power transfer device, characterized in that, Including: A rectifier, an inverter and a transformer; The rectifier is used to rectify the bus voltage into a corresponding DC voltage; The DC side of the inverter is connected to the rectifier and is used to invert the DC voltage rectified by the rectifier into an AC voltage and output it from the AC side of the inverter; The primary winding of the transformer is connected to the AC side of the inverter, and the secondary winding is connected in series on the connection line between the two AC bus voltages and is used to convert the AC voltage output from the AC side of the inverter into a corresponding compensation voltage and inject it between the two AC bus voltages; the secondary winding of the transformer adopts a multi-winding series-parallel connection, and the equivalent winding value of the secondary winding can be adjusted by controlling the series-parallel relationship of the multi-windings; When the seamless closed-loop power transfer device is put into use, the phase angle difference between the two AC bus voltages to be transferred is determined. Different phase angle differences correspond to different compensation voltages; the transformer determines the required compensation voltage according to the phase angle difference between the two AC bus voltages, and correspondingly determines the required equivalent winding value of the secondary winding, adjusts the series-parallel relationship of the secondary winding, and outputs the required compensation voltage; The core component of the rectifier is a thyristor; during the process of the seamless closed-loop power transfer device exiting, the rectifier changes the conduction angle of the thyristor, so that the DC voltage output by the rectifier gradually decreases. At this time, the inversion modulation ratio of the inverter remains unchanged, the AC voltage output by the inverter gradually decreases, the compensation voltage output by the transformer gradually decreases, and the phase of the compensation voltage gradually changes, so that the voltage amplitude on the AC bus voltage after power transfer remains unchanged; when the DC voltage output by the rectifier decreases to a preset value, the conduction angle of the thyristor is kept unchanged, and the inversion modulation ratio of the inverter gradually decreases, so that the compensation voltage gradually decreases to a minimum value. When the compensation voltage decreases to the minimum value, the process of the seamless closed-loop power transfer device exiting ends; the series-parallel design of the secondary winding of the transformer and the control of the thyristor conduction angle keep the inversion modulation ratio of the inverter at a stable level, improving the quality of the output waveform of the seamless closed-loop power transfer device.

2. The seamless ring current transfer device according to claim 1, characterized in that, Also including: A PWM regulating device; The PWM regulating device is used to determine the phase of the compensation voltage according to the voltage parameters before and after seamless closed-loop power transfer, so as to control the inverter by using PWM technology and make the inverter output the compensation voltage corresponding to the phase.

3. The seamless loop current transfer device according to claim 1, wherein, When the phase angle differences between the two AC bus voltages are 30°, 15°, and 7.5° under different working conditions, the transformation ratios of the transformer should be 2:1, 4:1, and 8:1 respectively. Based on the transformation ratio of the transformer, the series-parallel relationship of the secondary winding is changed to keep the transformation ratio of the transformer at the level corresponding to the different phase angle differences.

4. The seamless loop current transfer device according to claim 3, characterized in that, The secondary winding of the transformer includes four identical sub-windings, and the sum of the windings in series of the four sub-windings is half of the primary winding; When the transformation ratio of the transformer is 2:1, the four sub-windings are connected in series; When the transformation ratio of the transformer is 4:1, the four sub-windings are connected in parallel in pairs and then in series; When the transformation ratio of the transformer is 8:1, the four sub-windings are connected in parallel.

5. The seamless loop current transfer device according to any one of claims 1 to 4, characterized in that, The core component of the inverter is an IGBT.

6. A method for improving the waveform quality of a seamless loop transfer device, the seamless loop transfer device comprising: Rectifiers, inverters, and transformers; the DC side of the inverter is connected to the rectifier and is used to invert the DC voltage rectified by the rectifier into an AC voltage and output it from the AC side of the inverter. The primary winding of the transformer is connected to the AC side of the inverter, and the secondary winding is connected in series on the connection line between the two AC bus voltages; it is characterized in that the method includes the following steps: When the seamless closed-loop power transfer device is put into use, determine the phase angle difference between the two AC bus voltages to be transferred. Different phase angle differences correspond to different compensation voltages; among them, the transformer transforms the AC voltage output from the AC side of the inverter into the corresponding compensation voltage and injects it between the two AC bus voltages; the secondary winding of the transformer is connected in series and parallel with multiple windings, and the equivalent winding value of the secondary winding can be adjusted by controlling the series and parallel relationship of the multiple windings; Determine the required compensation voltage according to the phase angle difference between the two AC bus voltages, and correspondingly determine the required equivalent winding value of the secondary winding to adjust the series and parallel relationship of the secondary winding so that the transformer outputs the required compensation voltage; During the process of the seamless closed-loop power transfer device exiting, change the conduction angle of the thyristor so that the DC voltage output by the rectifier gradually decreases. At this time, the inversion modulation ratio of the inverter remains unchanged, the AC voltage output by the inverter gradually decreases, the compensation voltage output by the transformer gradually decreases, and the phase of the compensation voltage gradually changes so that the voltage amplitude on the AC bus voltage after power transfer remains unchanged; the thyristor is the core component of the rectifier; When the DC voltage output by the rectifier decreases to a preset value, keep the conduction angle of the thyristor unchanged and gradually decrease the inversion modulation ratio of the inverter so that the compensation voltage gradually decreases to a minimum value. When the compensation voltage decreases to the minimum value, the process of the seamless closed-loop power transfer device exiting ends; Through the series and parallel design of the secondary winding of the transformer and the control of the conduction angle of the thyristor, the inversion modulation ratio of the inverter is maintained at a stable level, improving the quality of the output waveform of the seamless closed-loop power transfer device.

7. The method for improving the waveform quality of the seamless loop electrical transfer device according to claim 6, characterized in that, The phase of the compensation voltage gradually changes, which is specifically achieved by the following method: Determine the phase of the compensation voltage according to the voltage parameters before and after seamless closed-loop power transfer, and use PWM technology to control the inverter so that the inverter outputs the compensation voltage corresponding to the phase.

8. The method for improving the waveform quality of the seamless loop current transfer device according to claim 6, characterized in that, When the phase angle differences between the two AC bus voltages are 30°, 15°, and 7.5° under different working conditions, the turns ratios of the transformer should be 2:1, 4:1, and 8:1 respectively. Based on the turns ratio of the transformer, change the series and parallel relationship of the secondary winding so that the turns ratio of the transformer is maintained at the level corresponding to the different phase angle differences.

9. The method for improving the waveform quality of the seamless loop electrical transfer device according to claim 8, characterized in that, The secondary winding of the transformer includes four identical sub-windings, and the sum of the windings in series of the four sub-windings is half of the primary winding; When the turns ratio of the transformer is 2:1, the four sub-windings are connected in series; When the turns ratio of the transformer is 4:1, the four sub-windings are connected in parallel in pairs and then in series; When the turns ratio of the transformer is 8:1, the four sub-windings are connected in parallel.

Images