Hybrid flexible distribution transformer control strategy and system based on harmonic control

Abstract

The invention discloses a hybrid flexible distribution transformer control strategy and system based on harmonic control, and the control strategy comprises the steps: firstly determining a compensation voltage amplitude required for compensating a main transformer from a current output voltage to a system rated voltage, and then drawing the compensation voltage amplitude under the current compensation voltage amplitude; the method comprises the following steps: modulating a change curve of a total harmonic distortion coefficient along with a square wave compensation voltage amplitude when outputting a square wave compensation voltage by adopting step waves, and modulating the output square wave compensation voltage by adopting the step waves on the basis of a part which is smaller than or equal to a preset total harmonic distortion coefficient threshold value in the change curve, the current output voltage of the main transformer is compensated to the rated voltage of the system, so that the switching loss of a switching device and the overall power loss are reduced, the working efficiency of the converter is finally improved, and the comprehensive electric energy quality level of power supply and consumption is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and in particular relates to a hybrid flexible distribution transformer control strategy and system based on harmonic control. Background Art

[0002] Hybrid Distribution Transformer (HDT) is a new type of distribution transformer that improves the design of traditional distribution transformers and connects pulse width modulation (PWM) converters to them, which can make distribution transformers no longer limited to voltage level conversion and power transmission. Specifically, HDT not only has the basic functions of voltage level conversion and power transmission of traditional distribution transformers, but also can dynamically control its load voltage and grid current in real time, which has important application value for realizing active distribution networks.

[0003] In the existing research, the voltage compensation converter of the hybrid flexible distribution transformer adopts PWM modulation, and its switching frequency is generally several thousand hertz, which can effectively reduce the low-order harmonic content of the output voltage and filter the high-frequency harmonics caused by the higher switching frequency through the filter, but it also increases the switching frequency, resulting in large switching losses, resulting in large operating losses of the converter and low efficiency. If the switching frequency is reduced, the harmonics will be larger due to the small number of levels of the three-level. When excessive harmonic current flows through the transformer, it will significantly increase the power loss, causing the transformer to overheat, thereby shortening its service life. Therefore, there is a problem that it is difficult to balance the switching loss and harmonic content. Summary of the invention

[0004] The purpose of the present invention is to provide a hybrid flexible distribution transformer control strategy and system based on harmonic control that can reduce switching loss and overall power loss in response to the above-mentioned problems existing in the prior art.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] In a first aspect, the present invention provides a hybrid flexible distribution transformer control strategy based on harmonic control, the control strategy comprising:

[0007] S1. Calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage;

[0008] S2, plotting a curve of the total harmonic distortion coefficient versus the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude;

[0009] S3. Determine whether there is a portion in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold; if so, based on the portion in the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold, use step wave modulation to output a square wave compensation voltage, thereby compensating the main transformer from the current output voltage to the system rated voltage; if not, use PWM modulation to output an SPWM compensation voltage, thereby compensating the main transformer from the current output voltage to the system rated voltage.

[0010] In S2, a curve of the total harmonic distortion coefficient of the square wave compensation voltage output by step wave modulation versus the square wave compensation voltage amplitude is drawn according to the following steps under the current compensation voltage amplitude:

[0011] The first step is to calculate the square wave compensation voltage amplitude and pulse width at the current compensation voltage amplitude according to the following formula:

[0012] ;

[0013] In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer;

[0014] The output square wave compensation voltage amplitude should meet the following conditions:

[0015] ;

[0016] ;

[0017] In the above formula, is the DC bus voltage of the full-bridge power unit series converter; Indicates the lower limit of the bus voltage when the converter does not control rectification; is the transformation ratio of the voltage compensation winding; is the system rated voltage;

[0018] Step 2: Substitute the amplitude and pulse width of the square wave compensation voltage obtained in the first step into the following formula to calculate the total harmonic coefficient of the square wave compensation voltage:

[0019] ;

[0020] ;

[0021] In the above formula, Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; Indicates the total harmonic coefficient of the square wave compensation voltage;

[0022] The third step is to change the amplitude and pulse width of the square wave compensation voltage, so as to draw a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the amplitude of the square wave compensation voltage.

[0023] In S1, the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage is calculated according to the following formula:

[0024] ;

[0025] In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer.

[0026] In S3, based on the square wave compensation voltage amplitude corresponding to the minimum total harmonic coefficient in the variation curve, step wave modulation is performed to output the square wave compensation voltage.

[0027] The total harmonic distortion coefficient threshold is set to 5%.

[0028] In a second aspect, the present invention provides a hybrid flexible distribution transformer control system based on harmonic control, the control system comprising:

[0029] A compensation voltage calculation module is used to calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage;

[0030] A curve drawing module is used to draw a curve of the total harmonic distortion coefficient changing with the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude;

[0031] The strategy selection module is used to determine whether there is a part in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold; if so, the hybrid flexible distribution transformer is controlled to output a square wave compensation voltage based on the part in the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold, using step wave modulation to compensate the main transformer from the current output voltage to the system rated voltage; if not, the hybrid flexible distribution transformer is controlled to output an SPWM compensation voltage using PWM modulation to compensate the main transformer from the current output voltage to the system rated voltage.

[0032] The curve drawing module draws a curve of the total harmonic distortion coefficient of the square wave compensation voltage output by step wave modulation with the square wave compensation voltage amplitude under the current compensation voltage amplitude according to the following steps:

[0033] The first step is to calculate the square wave compensation voltage amplitude and pulse width at the current compensation voltage amplitude according to the following formula:

[0034] ;

[0035] In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer;

[0036] The output square wave compensation voltage amplitude should meet the following conditions:

[0037] ;

[0038] ;

[0039] In the above formula, is the DC bus voltage of the full-bridge power unit series converter; Indicates the lower limit of the bus voltage when the converter does not control rectification; is the transformation ratio of the voltage compensation winding; is the system rated voltage;

[0040] Step 2: Substitute the square wave compensation voltage amplitude and pulse width obtained in the first step into the following formula to calculate the total harmonic coefficient of the square wave compensation voltage:

[0041] ;

[0042] ;

[0043] In the above formula, Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; Indicates the total harmonic coefficient when the HDT transformer outputs square wave compensation voltage;

[0044] The third step is to change the amplitude and pulse width of the square wave compensation voltage, and draw a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the amplitude of the square wave compensation voltage.

[0045] The compensation voltage calculation module calculates the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage according to the following formula:

[0046] ;

[0047] In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer.

[0048] The strategy selection module controls the hybrid flexible distribution transformer to perform step wave modulation based on the square wave compensation voltage amplitude corresponding to the minimum total harmonic coefficient in the change curve to output a square wave compensation voltage.

[0049] The total harmonic distortion coefficient threshold is set to 5%.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] The present invention discloses a control strategy for a hybrid flexible distribution transformer based on harmonic control. The strategy first determines the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage, then calculates the total harmonic coefficient of the overall output voltage of the hybrid flexible transformer after the square wave compensation voltage is superimposed on the output voltage of the main transformer, and based on this, draws a curve of the total harmonic distortion coefficient changing with the square wave compensation voltage amplitude at the current compensation voltage amplitude. Based on the portion of the variation curve that is less than or equal to a preset total harmonic distortion coefficient threshold, the converter DC bus voltage of the hybrid flexible transformer is adjusted, and the square wave compensation voltage is output by step wave modulation to compensate the main transformer from the current output voltage to the system rated voltage, thereby achieving the purpose of reducing the switching loss of the switching device and the overall power loss, and ultimately improving the working efficiency of the converter and the comprehensive power quality level of the power supply and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The present invention is a flowchart of the method.

[0053] Figure 2 This is the HDT transformer that the present invention is directed to.

[0054] Figure 3 For test one curve.

[0055] Figure 4 For test 2 curve.

[0056] Figure 5 For test three curve.

[0057] Figure 6 The structure diagram of the system of the present invention is shown in FIG. DETAILED DESCRIPTION

[0058] The present invention is further described in detail below in conjunction with specific implementations and drawings.

[0059] like Figure 2As shown, the HDT transformer faced by the present invention is composed of a transformer body and a three-phase converter module. The transformer body is composed of a main transformer, a current compensation winding, and a voltage compensation winding. The three-phase converter module is composed of three single-phase full-bridge converters, which can realize functions such as three-phase voltage adjustment, reactive power compensation, and three-phase load imbalance compensation, thereby improving the power quality on the user side. The single-phase full-bridge converter is composed of a full-bridge power unit parallel converter and a full-bridge power unit series converter; specifically, the full-bridge power unit parallel converter is connected to the current compensation winding of the transformer body, the full-bridge power unit series converter is connected to the voltage compensation winding of the transformer body, and the full-bridge power unit parallel converter and the full-bridge power unit series converter share a DC bus capacitor Ces, and the DC bus capacitor Ces is used to control the DC bus voltage to be stable and realize bidirectional power flow.

[0060] The full-bridge power unit series converter outputs a single-phase AC voltage with adjustable amplitude and phase, and sends the single-phase AC voltage to the voltage compensation winding on the secondary side of the main transformer, thereby completing flexible voltage regulation. The full-bridge power unit parallel converter outputs a single-phase AC current with adjustable amplitude and phase, and sends the single-phase AC current to the secondary side of the main transformer, thereby achieving reactive power compensation and three-phase unbalanced compensation.

[0061] When the single-phase full-bridge converter works in step wave modulation mode, it outputs square wave voltage, and the switching frequency and switching loss in voltage compensation are smaller, which is more energy-saving; when working in SPWM modulation mode, it outputs sinusoidal wave voltage, which has lower harmonic characteristics and less power loss.

[0062] Embodiment 1:

[0063] See also Figure 1 , a hybrid flexible distribution transformer control strategy based on harmonic control, including:

[0064] S1. Calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage according to the following formula:

[0065] ;

[0066] In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer;

[0067] S2. Draw a curve of the total harmonic distortion coefficient versus the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude according to the following steps;

[0068] The first step is to calculate the amplitude and pulse width of the square wave compensation voltage according to the following formula:

[0069] ;

[0070] In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer;

[0071] The output square wave compensation voltage amplitude should meet the following conditions:

[0072] ;

[0073] ;

[0074] In the above formula, is the DC bus voltage of the full-bridge power unit series converter; Indicates the lower limit of the bus voltage when the converter does not control rectification; is the transformation ratio of the voltage compensation winding;

[0075] Step 2: Substitute the square wave compensation voltage amplitude and pulse width obtained in the first step into the following formula to calculate the total harmonic coefficient after the square wave voltage is compensated to the main transformer and superimposed on the output voltage of the main transformer:

[0076] ;

[0077] ;

[0078] ;

[0079] In the above formula, Indicates the amplitude of the nth harmonic component of the square wave compensation voltage; Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; It represents the total harmonic coefficient;

[0080] The third step is to change the amplitude and pulse width of the square wave compensation voltage, thereby drawing a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the amplitude of the square wave compensation voltage;

[0081] S3, determining whether there is a portion in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold;

[0082] If it exists, based on the portion of the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold, a step wave modulation is used to output a square wave compensation voltage to compensate the main transformer from the current output voltage to the system rated voltage; specifically, based on the amplitude of the square wave compensation voltage corresponding to the minimum total harmonic coefficient in the change curve, the amplitude and pulse width of the square wave compensation voltage are calculated;

[0083] If it does not exist, it means that any square wave amplitude and pulse width cannot meet the voltage compensation requirements. At this time, PWM modulation is used to output SPWM compensation voltage (that is, the control mode used by traditional HDT transformers) to compensate the main transformer from the current output voltage to the system rated voltage.

[0084] To verify the effectiveness of the control strategy described in the present invention, the following tests were conducted using a 100 kVA 10 kV hybrid flexible transformer prototype used on-site in a certain province and city.

[0085] The parameters are set as follows:

[0086] The rated voltage of the system is 220V; the input voltage of the transformer body is 10kV, the rated power of each phase is 33.3kW, and the maximum output current is 144A; the transformation ratio of the voltage compensation winding is 1:10, that is, the DC bus voltage required when the full-bridge power unit series converter outputs 20V voltage to the main transformer is 200V; since the uncontrolled rectifier voltage (that is, the minimum value of the DC bus voltage) is about 200V, when the HDT transformer outputs 220V, the minimum output voltage of the full-bridge power unit series converter is ±200V / 10=±20V, and since the conventional rated voltage of the DC bus is 750V (that is, the standard voltage used for DC transmission), the maximum output voltage of the full-bridge power unit series converter is ±750V / 10=±75V, based on which the adjustable range of the square wave output voltage is 20V-75V; refer to GB / T 14549-93 "Power Quality - Public Grid Harmonics" sets the total harmonic distortion coefficient threshold to 5%; the switching frequency of PWM modulation is set to 20kHz, the switching frequency of step wave modulation is set to 100Hz, and the operating frequency is set to 50Hz.

[0087] Test 1:

[0088] When the current output voltage of the main transformer is 190V, the calculation shows that the compensation voltage amplitude required to compensate the main transformer from the current output voltage of 190V to the system rated voltage of 220V is 30V; further drawing shows that Figure 3 The total harmonic distortion coefficient-square wave compensation voltage amplitude change curve shown in curve);

[0089] As you can see, Figure 3 In The curve has a part below 5%, that is, when the square wave compensation voltage amplitude Total harmonic distortion coefficient at around 20V-35V ≤5%, therefore, it is determined that the step wave modulation can be used to output the square wave compensation voltage, and the total harmonic distortion coefficient can be reduced while completing the voltage compensation. Control within 5%. Specifically, Figure 3 Total harmonic distortion coefficient in The minimum value is 2.7%, at which point the square wave compensation voltage amplitude The pulse width is 26V, the pulse width is 130°, and the corresponding DC bus voltage of the full-bridge power unit series converter is 260V (the DC bus voltage of 260V is higher than the uncontrolled rectifier voltage of 200V, which meets the requirements).

[0090] In summary, the full-bridge power unit series converter needs to control the DC bus voltage at 260V, and output a 26V square wave compensation voltage with a pulse width of 130° to the main transformer through a 1:10 voltage compensation winding (the output square wave compensation voltage is equivalent to a 30V sine wave voltage). After compensation, the overall output voltage of the HDT transformer reaches 220V, and the total harmonic distortion coefficient It is 2.7%, which meets the relevant standard requirements.

[0091] The switching losses of square wave compensation voltage and traditional PWM modulation are calculated as 100 Hz*260 V and 20 KHz*750 V respectively. It can be seen that the switching loss of conventional PWM modulation is 576.9 times the switching loss of the square wave compensation voltage of the present invention.

[0092] Test 2:

[0093] When the current output voltage of the main transformer is 170V, it can be calculated that the compensation voltage amplitude required to compensate the main transformer from the current output voltage of 170V to the system rated voltage of 220V is 50V; further drawing gives the following Figure 4 shown curve;

[0094] As you can see, Figure 4 In The curve has a part below 5%, that is, the square wave compensation voltage amplitude Total harmonic distortion coefficient at around 40.6V-46.3V ≤5%, therefore, it is determined that the step wave modulation can be used to output the square wave compensation voltage, and the total harmonic distortion coefficient can be reduced while completing the voltage compensation. Control within 5%. Specifically, Figure 4 Total harmonic distortion coefficient in The minimum value is 4.5%, at which point the square wave compensation voltage amplitude The pulse width is 43V, the pulse width is 132°, and the corresponding DC bus voltage is 430V (the DC bus voltage of 430V is 200V higher than the uncontrolled rectifier voltage, which meets the requirements).

[0095] In summary, the full-bridge power unit series converter needs to control the DC bus voltage at 430V, and output a 43V square wave compensation voltage to the main transformer through a 1:10 voltage compensation winding. The fundamental amplitude of the output square wave compensation voltage is 50V, and the square wave pulse width is 132°. After compensation, the overall output voltage of the HDT transformer reaches 220V, and the total harmonic distortion coefficient is It is 4.5%, which meets the relevant standard requirements.

[0096] The switching losses of square wave compensation voltage and traditional PWM modulation are calculated as 100Hz*430V and 20KHz*750V respectively. It can be seen that the switching loss of conventional PWM modulation is 348.9 times the switching loss of the square wave compensation voltage of the present invention.

[0097] Test three:

[0098] When the current output voltage of the main transformer is 160V, the compensation voltage amplitude required to compensate the main transformer from the current output voltage of 170V to the system rated voltage of 220V is calculated to be 60V; further drawing shows that Figure 5 shown curve;

[0099] As you can see, Figure 5 In The curve does not have a part below 5%, which means that it is impossible to maintain the total harmonic distortion coefficient while compensating the main transformer voltage by outputting a square wave voltage. ≤5%, so the traditional PWM modulation should be used to output the SPWM compensation voltage.

[0100] Embodiment 2:

[0101] See also Figure 6 , a hybrid flexible distribution transformer control system based on harmonic control, the control system includes: a compensation voltage calculation module, a curve drawing module, and a strategy selection module; the compensation voltage calculation module is used to calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage; the specific calculation formula is as follows:

[0102] ;

[0103] In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer;

[0104] The curve drawing module is used to draw a curve of the total harmonic distortion coefficient versus the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude; specifically, the following steps are performed:

[0105] The first step is to calculate the square wave compensation voltage amplitude and pulse width at the current compensation voltage amplitude according to the following formula:

[0106] ;

[0107] In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer;

[0108] Step 2: Substitute the square wave compensation voltage amplitude and pulse width obtained in the first step into the following formula to calculate the total harmonic coefficient of the square wave compensation voltage:

[0109] ;

[0110] ;

[0111] In the above formula, Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; Indicates the total harmonic coefficient when the HDT transformer outputs square wave compensation voltage;

[0112] The third step is to change the square wave compensation voltage amplitude and pulse width, and draw a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the square wave compensation voltage amplitude;

[0113] The strategy selection module is used to determine whether there is a part in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold; if so, the hybrid flexible distribution transformer is controlled to output a square wave compensation voltage using step wave modulation based on the part in the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold, thereby compensating the main transformer from the current output voltage to the system rated voltage; if not, the hybrid flexible distribution transformer is controlled to output an SPWM compensation voltage using PWM modulation, thereby compensating the main transformer from the current output voltage to the system rated voltage; specifically, step wave modulation is performed based on the square wave compensation voltage amplitude corresponding to the minimum total harmonic coefficient in the change curve to output a square wave compensation voltage; specifically, the total harmonic distortion coefficient threshold is set to 5%.

[0114] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0115] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0116] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0117] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0118] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A hybrid flexible distribution transformer control strategy based on harmonic control, characterized by: The control strategy includes: S1. Calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage; S2, plotting a curve of the total harmonic distortion coefficient versus the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude; S3. Determine whether there is a portion in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold value; if so, based on the portion in the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold value, use step wave modulation to output a square wave compensation voltage, thereby compensating the main transformer from the current output voltage to the system rated voltage; if not, use PWM modulation to output an SPWM compensation voltage, thereby compensating the main transformer from the current output voltage to the system rated voltage.

2. The hybrid flexible distribution transformer control strategy based on harmonic control according to claim 1 is characterized in that: In S2, a curve of the total harmonic distortion coefficient of the square wave compensation voltage output by step wave modulation versus the square wave compensation voltage amplitude is drawn according to the following steps under the current compensation voltage amplitude: The first step is to calculate the amplitude and pulse width of the square wave compensation voltage at the current compensation voltage amplitude according to the following formula: ； In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; The output square wave compensation voltage amplitude should meet the following conditions: ； ； In the above formula, is the DC bus voltage of the full-bridge power unit series converter; Indicates the lower limit of the bus voltage when the converter does not control rectification; is the transformation ratio of the voltage compensation winding; is the system rated voltage; Step 2: Substitute the amplitude and pulse width of the square wave compensation voltage obtained in the first step into the following formula to calculate the total harmonic coefficient of the square wave compensation voltage: ； ； In the above formula, Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; Indicates the total harmonic coefficient of the square wave compensation voltage; The third step is to change the amplitude and pulse width of the square wave compensation voltage, so as to draw a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the amplitude of the square wave compensation voltage.

3. The hybrid flexible distribution transformer control strategy based on harmonic control according to claim 1 or 2 is characterized in that: In S1, the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage is calculated according to the following formula: ； In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer.

4. The hybrid flexible distribution transformer control strategy based on harmonic control according to claim 1 or 2 is characterized in that: In S3, based on the square wave compensation voltage amplitude corresponding to the minimum total harmonic coefficient in the variation curve, step wave modulation is performed to output the square wave compensation voltage.

5. The hybrid flexible distribution transformer control strategy based on harmonic control according to claim 1 or 2, characterized in that: The total harmonic distortion coefficient threshold is set to 5%.

6. A hybrid flexible distribution transformer control system based on harmonic control, characterized in that: The control system comprises: A compensation voltage calculation module is used to calculate the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage; A curve drawing module is used to draw a curve of the total harmonic distortion coefficient changing with the square wave compensation voltage amplitude when the square wave compensation voltage is output by step wave modulation under the current compensation voltage amplitude; The strategy selection module is used to determine whether there is a part in the change curve that is less than or equal to a preset total harmonic distortion coefficient threshold; if so, the hybrid flexible distribution transformer is controlled to output a square wave compensation voltage based on the part in the change curve that is less than or equal to the preset total harmonic distortion coefficient threshold, using step wave modulation to compensate the main transformer from the current output voltage to the system rated voltage; if not, the hybrid flexible distribution transformer is controlled to output an SPWM compensation voltage using PWM modulation to compensate the main transformer from the current output voltage to the system rated voltage.

7. The hybrid flexible distribution transformer control system based on harmonic control according to claim 6 is characterized in that: The curve drawing module draws a curve of the total harmonic distortion coefficient of the square wave compensation voltage output by step wave modulation with the square wave compensation voltage amplitude under the current compensation voltage amplitude according to the following steps: The first step is to calculate the square wave compensation voltage amplitude and pulse width at the current compensation voltage amplitude according to the following formula: ； In the above formula, is the amplitude of the square wave compensation voltage; is the pulse width of the square wave compensation voltage; Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; The output square wave compensation voltage amplitude should meet the following conditions: ； ； In the above formula, is the DC bus voltage of the full-bridge power unit series converter; Indicates the lower limit of the bus voltage when the converter does not control rectification; is the transformation ratio of the voltage compensation winding; is the system rated voltage; Step 2: Substitute the amplitude and pulse width of the square wave compensation voltage obtained in the first step into the following formula to calculate the total harmonic coefficient of the square wave compensation voltage: ； ； In the above formula, Represents the square root of the sum of the squares of the amplitudes of the 2n+1th harmonic components of the square wave compensation voltage; Indicates the total harmonic coefficient when the HDT transformer outputs square wave compensation voltage; The third step is to change the amplitude and pulse width of the square wave compensation voltage, and draw a curve of the total harmonic distortion coefficient of the square wave compensation voltage versus the amplitude of the square wave compensation voltage.

8. The hybrid flexible distribution transformer control system based on harmonic control according to claim 6 or 7, characterized in that: The compensation voltage calculation module calculates the compensation voltage amplitude required to compensate the main transformer from the current output voltage to the system rated voltage according to the following formula: ； In the above formula, Indicates the compensation voltage amplitude required at the current output voltage of the main transformer; is the system rated voltage; is the current output voltage of the main transformer.

9. The hybrid flexible distribution transformer control system based on harmonic control according to claim 6 or 7, characterized in that: The strategy selection module controls the hybrid flexible distribution transformer to perform step wave modulation based on the square wave compensation voltage amplitude corresponding to the minimum total harmonic coefficient in the change curve to output a square wave compensation voltage.

10. The hybrid flexible distribution transformer control system based on harmonic control according to claim 6 or 7, characterized in that: The total harmonic distortion coefficient threshold is set to 5%.

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