Control method of flexible load regulation transformer based on bidirectional capacity mutual aid

Abstract

The application discloses a control method of a flexible load-voltage-regulating transformer based on bidirectional capacity mutual aid, which comprises a Tm main transformer, a tapping switch group, a power electronic converter, K ck bypass switches and DC side energy storage units; a primary side winding of the Tm main transformer comprises a main winding, a converter energy-taking winding and a plurality of segmented windings; the main winding, the plurality of segmented windings, the tapping switch group, K ck bypass switches and a power grid constitute a main loop, AC sides of series side converters in the power electronic converter are respectively electrically connected to two ends of the K ck bypass switches, and AC sides of parallel side converters in the power electronic converter are respectively electrically connected to two ends of the converter energy-taking winding; a layered multi-stage control strategy is adopted to select a compensation mode of the power electronic converter as a series side independent voltage compensation mode, a parallel side independent reactive power compensation mode or a bidirectional capacity mutual aid compensation mode according to regulation requirements of a circuit system, and stepless compensation is performed on a primary side winding voltage of the main transformer.

Description

Technical Field

[0001] This invention relates to the field of transformer control, and more particularly to a control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance. Background Technology

[0002] With the increasing integration of renewable energy, electric vehicles, and various nonlinear loads into distribution networks, problems such as voltage fluctuations, voltage sags and dips, and reactive power imbalances are becoming increasingly prominent. Traditional on-load tap-changing transformers mainly rely on tap changer operation to achieve discrete voltage regulation. Although they can achieve steady-state voltage adjustment, they are difficult to achieve rapid and continuous dynamic compensation. Introducing small-capacity power electronic converters into on-load tap-changing transformers can form flexible on-load tap-changing transformers, achieving continuous voltage regulation and a certain degree of reactive power regulation. While existing flexible on-load tap-changing technologies can achieve continuous voltage regulation and reactive power compensation to some extent, they are limited by converter voltage levels, input power limits, DC-side voltage regulation requirements, and power flow path coupling relationships. They generally suffer from insufficient utilization of compensation capacity, low coordination between the two converters, inadequate consideration of the impact of tap changer operation, and unclear control hierarchy. Especially in scenarios where grid voltage fluctuations and reactive power compensation demands coexist, existing devices struggle to fully exploit the combined compensation potential of the series and parallel sides under fixed capacity constraints. This limits the overall compensation capability of the devices, making it difficult to meet the demands of new distribution networks for fast, continuous, coordinated, and high-utilization voltage regulation and compensation technologies. The main problems are as follows: First, there is a clear capacity competition between series-side voltage compensation and parallel-side reactive power compensation; second, the shared DC bus on both sides of the converter results in strong power coupling, leading to mutual constraints on compensation capabilities; and third, existing coordinated compensation methods do not adequately consider the impact of tap changer position changes, making it difficult to guarantee stable compensation effects under all operating conditions. Summary of the Invention

[0003] Purpose of the invention: In order to overcome the shortcomings of the prior art, the present invention provides a control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance. By adjusting the compensation voltage phase angle of the series-side converter and the compensation current phase angle of the parallel-side converter in the power electronic converter, bidirectional capacity mutual assistance between the series-side converter and the parallel-side converter is realized, thereby improving the compensation capability of the power electronic converter.

[0004] Technical Solution: To achieve the above objectives, the present invention provides a control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance, comprising a Tm main transformer, a tap changer group, a power electronic converter, and K... ck Bypass switch and DC-side energy storage unit; the primary winding of the Tm main transformer includes a main winding, a converter energy extraction winding, and several segmented windings; the main winding, several segmented windings, tap changer group, K ckThe bypass switch and the power grid form the main circuit. The AC side of the series converter in the power electronic converter is electrically connected to K respectively. ck The two ends of the bypass switch and the AC side of the parallel-side converter in the power electronic converter are electrically connected to the two ends of the converter's energy harvesting winding, respectively; the DC side of the series-side converter, the DC side of the parallel-side converter, and the power supply terminals of the DC-side energy storage unit are all connected in parallel to C. dc The two ends of the bus capacitor; determine whether the adjustment demand of the circuit system is voltage regulation demand, reactive power regulation demand, or simultaneous voltage and reactive power regulation demand, and adopt a hierarchical multi-level control strategy to select the compensation mode of the power electronic converter according to the adjustment demand of the circuit system, namely, series side independent voltage compensation mode, parallel side independent reactive power compensation mode, or bidirectional capacity mutual assistance compensation mode, including parallel side auxiliary series side voltage compensation mode and series side auxiliary parallel side reactive power compensation mode, to perform stepless compensation of the primary winding voltage of the main transformer.

[0005] Furthermore, in the hierarchical multi-level control strategy, when the regulation demand of the circuit system is voltage regulation demand, it is determined whether the voltage compensation exceeds the compensation range of the series-side converter; when the voltage compensation does not exceed the compensation range of the series-side converter, the power electronic converter adopts the series-side independent voltage compensation mode; when the voltage compensation exceeds the compensation range of the series-side converter, the power electronic converter adopts the parallel-side auxiliary series-side voltage compensation mode in the bidirectional capacity mutual compensation mode.

[0006] When the regulation demand of the circuit system is reactive power regulation demand, it is determined whether the reactive power compensation exceeds the compensation range of the parallel-side converter. When the reactive power compensation does not exceed the compensation range of the parallel-side converter, the power electronic converter adopts the parallel-side independent reactive power compensation mode. When the reactive power compensation exceeds the compensation range of the parallel-side converter, the power electronic converter adopts the series-side auxiliary parallel-side reactive power compensation mode in the bidirectional capacity mutual assistance compensation mode.

[0007] Furthermore, when the circuit system's regulation demand is a simultaneous demand for voltage and reactive power regulation, the priority of voltage regulation and reactive power regulation in the simultaneous demand for voltage and reactive power regulation is determined. If the priority of voltage regulation is high, the power electronic converter adopts the parallel side auxiliary series side voltage compensation mode in the bidirectional capacity mutual compensation mode. If the priority of reactive power regulation is high, the power electronic converter adopts the series side auxiliary parallel side reactive power compensation mode in the bidirectional capacity mutual compensation mode.

[0008] Furthermore, in the bidirectional capacity mutual compensation mode, the parallel-side auxiliary series-side voltage compensation mode adjusts the phase angle of the compensation current output on the AC side of the parallel-side converter, changes the current magnitude of the primary winding of the Tm main transformer, and increases the upper limit of the compensation voltage output on the series-side converter, thereby achieving voltage compensation by the parallel-side converter assisting the series-side converter; the calculation process is as follows:

[0009] ;

[0010] ;

[0011] ;

[0012] In the formula, To compensate for the voltage phase angle, To compensate for voltage, To compensate for the voltage amplitude; The power angle is given by [value], and k is the grid voltage sag factor. To compensate for the voltage of the primary winding, The voltage of the power grid before the voltage drop. The power factor angle, This is the minimum value of the power angle. To compensate for the current phase angle.

[0013] Furthermore, when the phase angle of the compensation current output on the AC side of the parallel-side converter... When the phase angle of the compensation current is less than 90°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually decrease, compensating voltage phase angle Gradually decrease; when the phase angle of the compensation current output on the AC side of the parallel-side converter is 90° < When the phase angle of the compensation current is less than 180°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually increase the compensation voltage phase angle Gradually increase.

[0014] Furthermore, in the bidirectional capacity mutual assistance compensation mode, the series-side auxiliary parallel-side reactive power compensation mode changes the primary winding voltage phase angle of the Tm main transformer by adjusting the phase angle of the compensation voltage output of the AC side of the series-side converter, thereby sharing part of the reactive power compensation and realizing the series-side converter assisting the parallel-side converter in reactive power compensation; the calculation process is as follows:

[0015] ;

[0016] ;

[0017] ;

[0018] In the formula, To compensate for the current phase angle, To compensate for the current, To compensate for the amplitude of the current; For power angle, This refers to the uncompensated primary winding current. To compensate for the load current referred to the primary winding, The power factor angle, This is the difference between the power factor angle and the power angle. ; To compensate for the voltage phase angle.

[0019] Furthermore, when the phase angle of the compensation voltage output on the AC side of the series converter... When the phase angle of the compensation voltage is less than 90°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually decrease; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° < When the phase angle of the compensation voltage is <90°+δ / 2, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually increase; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° + δ / 2 < When the phase angle of the compensation voltage is less than 180°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually increase, compensation current phase angle Gradually decrease.

[0020] Beneficial Effects: The control method of the flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance of the present invention enables the series-side converter to not only output in-phase or out-of-phase compensation voltage to achieve voltage stability, but also to change the phase angle relationship between the voltage and current of the primary winding of the main transformer by adjusting the phase of the compensation voltage, thereby sharing part of the reactive power compensation task and reducing the compensation current required by the parallel side. When the capacity of the parallel-side converter is close to the upper limit, the series-side converter participates in reactive power regulation, thereby expanding the reactive power support range of the whole machine. By adjusting the phase and amplitude of the input compensation current by the parallel-side converter, the equivalent current distribution of the primary winding of the main transformer is changed, thereby reducing the current burden of the series-side converter under fixed capacity constraints, reducing the equivalent current burden on the primary side, and thus increasing the upper limit of the compensation voltage that the series side can output, enabling it to output a higher amplitude compensation voltage; it is especially suitable for scenarios with large voltage fluctuations or when the series-side converter is close to the capacity upper limit. The bidirectional capacity mutual assistance compensation mode breaks through the functional boundaries of the traditional series side only handling voltage compensation and the parallel side only handling reactive power compensation, making fuller use of the total capacity of the power electronic converter. Connecting the DC-side energy storage unit directly to the DC side of the flexible on-load tap-changing transformer back-to-back converter creates a shorter energy exchange path within the device compared to configuring the DC-side energy storage unit externally. The DC-side energy storage unit can provide active power support to the series-side converter during grid voltage dips and absorb excess energy during grid voltage rises, thus reducing the burden on the parallel-side converter to maintain DC bus stability. Simultaneously, the DC-side energy storage unit effectively forms a small, unified power quality regulation unit within the flexible on-load tap-changing transformer, which helps improve system power density and reduce device footprint. Furthermore, it effectively weakens the strong coupling between the series and parallel sides caused by the shared DC bus, laying the foundation for capacity mutual compensation control. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a flexible on-load tap-changing transformer based on bidirectional capacity adjustment.

[0022] Figure 2 This is a flowchart of a hierarchical, multi-level control strategy.

[0023] Figure 3 A schematic diagram illustrating voltage compensation for the parallel-side converter in conjunction with the series-side converter.

[0024] Figure 4 This is a schematic diagram showing the situation where the compensation current of the parallel-side converter is perpendicular to the current of the primary winding of the main transformer.

[0025] Figure 5 To compensate for current phase angle respectively with , and The relationship curve.

[0026] Figure 6 To compensate for voltage phase angle respectively with , and The relationship curve.

[0027] Figure 7 Tap switch coefficient respectively with and The relationship curve. Detailed Implementation

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] like Figure 1 As shown, the control method based on a flexible on-load tap-changing transformer with bidirectional capacity mutual assistance includes a Tm main transformer 1, a tap changer group 2, a power electronic converter 3, and a K... ck Bypass switch and DC-side energy storage unit 4; the primary winding of the Tm main transformer 1 includes a main winding, a converter energy extraction winding, and several segmented windings; the main winding, several segmented windings, tap changer group 2, K ck The bypass switch and the power grid form the main circuit. The AC side of the series converter in the power electronic converter 3 is electrically connected to K respectively. ck The two ends of the bypass switch and the AC side of the parallel-side converter in the power electronic converter 3 are electrically connected to the two ends of the converter's energy extraction winding, respectively; the DC side of the series-side converter, the DC side of the parallel-side converter, and the power supply terminals of the DC-side energy storage unit 4 are all connected in parallel to C. dc The two ends of the bus capacitor.

[0030] The power electronic converter 3 adopts a back-to-back circuit structure of series-side converter and parallel-side converter; the introduction of back-to-back power electronic converters into the flexible on-load tap-changing transformer combines the coarse voltage regulation of the tap changer group with the fine voltage regulation of the series-side converter, which can transform the original stepped voltage regulation process of the traditional on-load tap-changing transformer into a continuous voltage regulation process; when the grid voltage fluctuates, the series-side converter can quickly output compensation voltage to prioritize transient support, and then the tap changer group completes the tap position switching, thus taking into account both voltage regulation speed and voltage regulation range.

[0031] The K ck The bypass switch is used to disconnect the circuit electronic converter from the main circuit when the circuit electronic converter 3 fails. When K ck When the bypass switch is open, the circuit electronic converter 3 is connected to the main circuit; when K ckWhen the bypass switch is closed, the circuit electronic converter 3 is connected to the main circuit; one end of the AC side of the series converter is electrically connected to one end of the capacitor C2 and K through the inductor L2. ck One end of the bypass switch is electrically connected to the other end of capacitor C2 and K on the AC side of the series converter. ck The other end of the bypass switch; one end of the AC side of the parallel-side converter is electrically connected to one end of the Nt converter's energy harvesting winding through the L1 inductor, and the other end of the AC side of the parallel-side converter is electrically connected to the other end of the Nt converter's energy harvesting winding.

[0032] One end of the Nm main winding is electrically connected to one end of the power grid, and the other end of the Nm main winding is electrically connected to one end of the first segmented winding among several segmented windings; the other end of any segmented winding among several segmented windings is electrically connected to one end of the next segmented winding; the other end of the Nm main winding and the other ends of the Np1~Npn segmented windings together serve as several taps, which are electrically connected to K through several tap switches K0~Kn in the tap changer voltage regulating unit 2. ck One end of the bypass switch, K ck The other end of the bypass switch is electrically connected to the other end of the power grid; the two ends of the secondary winding of the main transformer 1 are respectively connected to the positive and negative terminals of the power supply of the load.

[0033] The DC-side energy storage unit 4 connected to the DC side of the power electronic converter 3 can buffer the DC-side voltage power and provide active power to the load when necessary. Under different operating conditions, the DC-side energy storage unit 4 has different power flow paths with the series-side converter and the parallel-side converter. The connection of the DC-side energy storage unit 4 weakens the power coupling relationship between the two converters, thereby improving the compensation capability of the two converters. When the grid voltage fluctuates, the converter will control the series-side converter to output a compensation voltage for compensation. Specifically, when the grid voltage rises, the series-side converter outputs a compensation voltage in the opposite direction to the grid voltage. At this time, the series-side converter absorbs active power and stores it in the DC-side energy storage unit 4. When the grid voltage drops, the series-side converter outputs a compensation voltage in the same direction as the grid voltage. At this time, the series-side converter generates active power, which can be supported by the DC-side energy storage unit 4.

[0034] like Figure 2As shown, the control terminal determines whether the adjustment demand of the circuit system is voltage adjustment demand, reactive power adjustment demand, or simultaneous voltage and reactive power adjustment demand. A hierarchical multi-level control strategy is adopted to select the compensation mode of the power electronic converter 3 according to the adjustment demand of the circuit system: series-side independent voltage compensation mode, parallel-side independent reactive power compensation mode, or bidirectional capacity mutual compensation mode, specifically the parallel-side auxiliary series-side voltage compensation mode and the series-side auxiliary parallel-side reactive power compensation mode, to perform stepless compensation of the primary winding voltage of the main transformer 1. The control terminal can be any control module capable of achieving the above functions.

[0035] In the series-side independent voltage compensation mode of the flexible on-load tap-changing transformer topology, stepless voltage regulation of the primary winding voltage of the main transformer 1 in the main circuit is achieved by adjusting the compensation voltage output of the series-side converter and the tap position of tap changer group 2. Through the cooperation of the series-side converter and tap changer group 2, continuous voltage regulation of mechanical coarse adjustment and power electronic fine adjustment is realized. If the series-side converter undertakes more voltage compensation tasks, the compensation voltage... This will increase the capacity of the power electronic converter 3, thus occupying more capacity. If the compensation current phase angle of the parallel-side converter is adjusted, the current of the primary winding of the Tm main transformer is changed, and the upper limit of the output compensation voltage of the series-side converter is increased, a capacity mutual assistance channel between the parallel side and the series side is formed.

[0036] In the parallel-side independent reactive power compensation mode of the flexible on-load tap-changing transformer topology, reactive power compensation for the primary winding voltage of the main transformer 1 in the main circuit is achieved by adjusting the compensation current output by the parallel-side converter. The converter's energy extraction winding shares a magnetic core with the main winding and is mutually insulated; therefore, the current in the primary winding can be obtained by superimposing the current in the converter's energy extraction winding and the load-reduced current. If the parallel-side converter undertakes more reactive power compensation tasks, the compensation current... This will increase the capacity of the power electronic converter 3, thus requiring more power. If the reactive power is shared by adjusting the compensation voltage phase angle of the series-side converter, the compensation current can be reduced. This meets the demand, thus forming a capacity-sharing channel between the series side and the parallel side.

[0037] like Figure 2 As shown, in the hierarchical multi-level control strategy, when the adjustment demand of the circuit system is voltage adjustment demand, it is determined whether the voltage compensation exceeds the compensation range of the series-side converter. When the voltage compensation does not exceed the compensation range of the series-side converter, the power electronic converter 3 adopts the series-side independent voltage compensation mode. When the voltage compensation exceeds the compensation range of the series-side converter, the power electronic converter 3 adopts the parallel-side auxiliary series-side voltage compensation mode in the bidirectional capacity mutual compensation mode.

[0038] When the regulation demand of the circuit system is the reactive power regulation demand, it is determined whether the reactive power compensation exceeds the compensation range of the parallel-side converter. When the reactive power compensation does not exceed the compensation range of the parallel-side converter, the power electronic converter 3 adopts the parallel-side independent reactive power compensation mode. When the reactive power compensation exceeds the compensation range of the parallel-side converter, the power electronic converter 3 adopts the series-side auxiliary parallel-side reactive power compensation mode in the bidirectional capacity mutual assistance compensation mode.

[0039] When the circuit system requires simultaneous regulation of voltage and reactive power, the priority of voltage regulation and reactive power regulation in the simultaneous regulation of voltage and reactive power is determined. The priority can be set according to the actual needs. When the priority of voltage regulation is high, the power electronic converter 3 adopts the parallel side auxiliary series side voltage compensation mode in the bidirectional capacity mutual compensation mode. When the priority of reactive power regulation is high, the power electronic converter 3 adopts the series side auxiliary parallel side reactive power compensation mode in the bidirectional capacity mutual compensation mode.

[0040] When selecting the compensation mode for power electronic converter 3, it is also necessary to consider the current capacity margin of power electronic converter 3. For example, if the current capacity margin is 0, then K is required. ck When the bypass switch closes, the power electronic converter is taken out of the main circuit. When the current capacity margin is less than 15%, reactive power compensation is not possible; only voltage compensation is possible. When the current capacity margin is greater than or equal to 15% and less than 30%, voltage compensation takes precedence over reactive power compensation. If the regulation demand involves both voltage and reactive power regulation, voltage compensation is prioritized; if the regulation demand is solely reactive power regulation, reactive power compensation can be performed. When the current capacity margin is greater than or equal to 30%, the current capacity margin is not considered, and stepless compensation is performed according to the compensation mode selected by the hierarchical multi-level control strategy.

[0041] Both the series-side converter and the parallel-side converter employ a single closed-loop control method based on a quasi-proportional resonant controller to control the output compensation voltage or compensation current. When it is determined that the series-side independent voltage compensation mode or the parallel-side auxiliary series-side voltage compensation mode is to be used, the control terminal will generate a compensation voltage reference value. The signal is input to the series-side converter; the series-side converter controls the compensation voltage tracking and outputs the modulation signal. Modulation signal After system delay and other operations, the final compensation voltage is output. When determining whether to use the parallel-side independent voltage compensation mode or the series-side auxiliary parallel-side voltage compensation mode, the control terminal will generate a compensation current reference value. The signal is input to the parallel-side converter; the parallel-side converter controls the compensation current tracking and outputs the modulation signal. Modulation signal After system delay and other operations, the final compensation current is output. The system delay includes sampling delay, PWM delay, and discrete calculation delay.

[0042] A hierarchical multi-level control strategy is constructed based on a bidirectional capacity mutual compensation mode. Under different operating modes such as voltage regulation demand, reactive power regulation demand, or simultaneous voltage and reactive power regulation demand, the optimal compensation path is adaptively selected, improving the comprehensive utilization rate and engineering applicability of the flexible on-load tap-changing transformer, and enhancing its adaptability and operational flexibility under complex operating conditions. Through the combination of the bidirectional capacity mutual compensation mode and the hierarchical multi-level control strategy, the continuous voltage regulation capability and reactive power compensation capability of the flexible on-load tap-changing transformer can be improved simultaneously under fixed power electronic capacity constraints.

[0043] like Figure 3 As shown, the parallel-side auxiliary series-side voltage compensation mode in the bidirectional capacity mutual compensation mode changes the current magnitude of the primary winding of the Tm main transformer by adjusting the phase angle of the compensation current output on the AC side of the parallel-side converter. Under a fixed converter compensation capacity, this increases the upper limit of the compensation voltage output on the series-side converter, thus achieving voltage compensation by the parallel-side converter assisting the series-side converter. The calculation process is shown below:

[0044] ;

[0045] ;

[0046] ;

[0047] In the formula, To compensate for the voltage phase angle, To compensate for voltage, To compensate for the voltage amplitude; The power angle is represented by the grid voltage after the voltage drop. With the primary winding voltage after compensation The angle between them; k is the grid voltage drop factor. To compensate for the voltage of the primary winding, The voltage of the power grid before the voltage drop. The power factor angle is the phase difference angle between the voltage and the current. This is the minimum value of the power angle. To compensate for the current phase angle.

[0048] like Figure 3 As shown, The voltage of the power grid before the voltage drop. The voltage of the power grid after the voltage drop; To compensate for the previous winding voltage, To compensate for the voltage of the primary winding; This is the compensation voltage output from the series-side converter. To compensate for the voltage phase angle; This refers to the primary winding current that has not been compensated by the parallel-side converter. This refers to the primary winding current after compensation by the parallel-side converter; To compensate for the load current previously referred to the primary winding, To compensate for the load current referred to the primary winding, To compensate for the current phase angle; Figure 4 In the image, the circular shaded area on the left represents the compensation current output by the parallel-side converter. The adjustment range is further expressed as the compensation current phase angle. The adjustment range; the ring-shaped shaded area on the right represents the compensation voltage of the parallel-side converter auxiliary series-side converter. Afterwards, compensation voltage This increases the voltage regulation range.

[0049] like Figure 4 As shown, under a fixed converter compensation capacity, in order to achieve a higher compensation voltage output by the series-side converter, it is necessary to minimize the primary winding current of the Tm transformer. The primary winding current Compensation current from the output of the parallel-side converter The impact, when Perpendicular to At that time, the parallel-side converter compensation current The power angle has reached its minimum value. Also the minimum value, denoted as... ;based on Figure 5 As shown, the compensation current can be obtained. The relation is as follows:

[0050] ;

[0051] ;

[0052] Set the compensation current output of the parallel-side converter to the rated current, and then set the compensation current... From the relationship, the phase angle of the compensation current output of the parallel-side converter can be obtained. The expression is shown below; the calculation process is as follows:

[0053] ;

[0054] In the formula, This refers to the rated current output by the parallel-side converter.

[0055] The power angle is obtained by transforming the expression for the compensation current phase angle. Phase angle of the compensation current output of the parallel-side converter The relation is as follows:

[0056] ;

[0057] Depend on Figure 3 As shown, the phase angle of the compensation voltage output of the series-side converter can be obtained using the sine theorem. With power angle The relation is as follows:

[0058] ;

[0059] In the formula, k is the grid voltage sag coefficient. .

[0060] For the compensated voltage phase angle With power angle The relationship is transformed to obtain the phase angle of the compensation voltage output of the series converter. The expression; and according to Figure 3 The compensation voltage output of the series-side converter can be obtained from the relationship between the variables. The magnitude expression is shown below:

[0061] ;

[0062] .

[0063] like Figure 5 As shown, this is the phase angle of the compensation current. respectively with , and The relationship curves are represented by the phase angle of the compensation current. With power angle Relationship curve, compensation current phase angle With compensation voltage The relationship curve of amplitude, the phase angle of the compensation current Phase angle with compensation voltage The relationship curve; therefore, when the phase angle of the compensation current output on the AC side of the parallel-side converter is... When the phase angle of the compensation current is less than 90°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually decrease, compensating voltage phase angle Gradually decrease; when the phase angle of the compensation current output on the AC side of the parallel-side converter is 90° < When the phase angle of the compensation current is less than 180°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually increase the compensation voltage phase angle The value gradually increases. In summary, by reasonably controlling the phase angle of the compensation current output of the parallel-side converter, the upper limit of the compensation voltage output of the series-side converter can be increased, thereby enhancing the compensation capability of the series-side converter.

[0064] The series-side auxiliary parallel-side reactive power compensation mode in the bidirectional capacity mutual assistance compensation mode changes the phase angle of the primary winding voltage of the Tm main transformer by adjusting the phase angle of the compensation voltage output of the AC side of the series-side converter, thereby sharing part of the reactive power compensation and realizing the reactive power compensation of the series-side converter assisting the parallel-side converter; the calculation process is as follows:

[0065] ;

[0066] ;

[0067] ;

[0068] In the formula, To compensate for the current phase angle, To compensate for the current, To compensate for the amplitude of the current; For power angle, This refers to the uncompensated primary winding current. To compensate for the load current referred to the primary winding, The power factor angle, This is the difference between the power factor angle and the power angle. ; To compensate for the voltage phase angle.

[0069] When the power grid has a reactive power compensation requirement, and it is determined that the reactive power compensation exceeds the compensation range of the parallel-side converter, the amplitude of the primary winding voltage of the main transformer 1 before and after compensation must remain unchanged, and the amplitude of the load current referred to the primary winding must remain unchanged, as shown below:

[0070] ;

[0071] ;

[0072] When the output compensation voltage of the series-side converter is set to the rated voltage to maximize the reactive power compensation burden of the parallel-side converter, the compensation voltage output of the series-side converter can be obtained. Amplitude and phase angle of compensation voltage The expression is as follows:

[0073] ;

[0074] In the formula, This is the rated voltage output of the series-side converter; as shown in the above equation, when the grid voltage drops or rises, the compensation voltage phase angle can be changed. The power angle can be adjusted. This affects the compensation current required by the parallel-side converter.

[0075] For the compensated voltage phase angle The power angle is obtained by transforming the expression. Phase angle of the compensated voltage output of the series-side converter The relation is as follows:

[0076] ;

[0077] Similarly, the compensation current output by the parallel-side converter can be obtained. The amplitude expression and the phase angle of the compensation current output of the parallel-side converter. The expression is shown below:

[0078] ;

[0079] ;

[0080] like Figure 6 As shown, this is the compensation voltage phase angle. respectively with , and The relationship curves are respectively represented by the compensation voltage phase angle. With power angle Relationship curve, compensation voltage phase angle With compensation current The relationship curve of amplitude, compensation voltage phase angle Phase angle with compensation current The relationship curve; therefore, when the phase angle of the compensation voltage output on the AC side of the series converter is... When the phase angle of the compensation voltage is less than 90°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually decrease; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° < When the phase angle of the compensation voltage is <90°+δ / 2, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually increase; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° + δ / 2 < When the phase angle of the compensation voltage is less than 180°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually increase, compensation current phase angle Gradually decrease. In summary, by reasonably controlling the phase angle of the compensation voltage output of the series-side converter, the parallel-side converter can be assisted in adjusting the compensation current, thereby reducing the burden on the parallel-side converter.

[0081] Considering that tap changer operation alters the equivalent turns ratio of the main transformer, thereby affecting the phase angle range of the compensation voltage output of the series-side converter and the phase angle range of the compensation current output of the parallel-side converter; each tap changer in tap changer group 2 corresponds to a specific tap position, therefore a tap changer coefficient is set. This characterizes the tap changer's position; for example, if the tap changer is set to a 5% position with an adjustment range from -80% to 120%, then the tap changer coefficient... The transformation range is (-0.8, 1.2); compensation voltage and the voltage drop of the power grid Tap switch coefficients can be used This indicates that the power angle is obtained. and tap changer coefficient The expression is shown below:

[0082] ;

[0083] ;

[0084] ;

[0085] In the formula, The compensation voltage is represented by the output of the series-side converter. Variables converted to the second-order side This represents the variable representing the grid voltage after a voltage drop, referred to the secondary side. The turns ratio of the primary and secondary windings of the main transformer 1. This refers to the voltage of the load corresponding to the secondary side of the main transformer 1.

[0086] like Figure 7 As shown, this represents the tap changer coefficient. respectively with and The relationship curve; the power angle can be obtained through the above formula. and tap changer coefficient Relationship curve, compensation voltage phase angle and tap changer coefficient The relationship curve shows that when the tap changer connected to the primary side of the main transformer is in the middle position, the power angle... The phase angle of the compensated voltage output of the series-side converter This is the maximum value. When the tap changer connected to the primary side of the main transformer gradually moves up or down from the middle position, the power angle will change. The phase angle of the compensated voltage output of the series-side converter The tap changer coefficient gradually decreases, which in turn affects the effectiveness of the series-side converter in assisting the parallel-side converter for current compensation. Therefore, when using the series-side converter to assist the parallel-side converter for current compensation, the tap changer coefficient should be considered. To mitigate the impact of the series-side converter's current compensation effect on the parallel-side converter, a tap changer coefficient is introduced. The impact of tap changer operation on the capacity mutual compensation mode was considered, and the influence of tap changer operation on the power angle, compensation voltage phase angle, and compensation current phase angle was further analyzed. This enabled the hierarchical multi-level control strategy to match the actual operation process of the flexible on-load tap-changing transformer, thus making it more suitable for engineering application scenarios and improving the integrity and engineering adaptability of the control strategy. At the same time, it ensured that both the series-side auxiliary parallel-side and parallel-side auxiliary series-side capacity mutual compensation modes could remain calculable, controllable, and switchable under different tap positions.

[0087] This invention expands the original compensation mode where the series-side converter and the parallel-side converter operate independently into a bidirectional capacity mutual compensation mode where the series side assists the parallel side in reactive power compensation and the parallel side assists the series side in voltage compensation. Under the constraint of fixed converter capacity, the functional allocation relationship between the series and parallel sides is reconstructed to achieve a synergistic improvement in voltage compensation capability and reactive power compensation capability. Specifically, the series-side converter can reduce the demand for compensation current on the parallel-side converter by adjusting the phase of the compensation voltage to share part of the reactive power; the parallel-side converter can reduce the equivalent current burden on the primary side by adjusting the amplitude and phase of the compensation current, thereby increasing the upper limit of the compensation voltage output by the series-side converter.

[0088] This invention integrates a small-capacity power electronic converter with a traditional on-load tap-changing transformer. By increasing the power electronic device by only about 5% to 10% of the main transformer's rated capacity, it achieves multiple functions such as continuous voltage regulation, reactive power compensation, and enhanced compensation capability, resulting in high capacity utilization. Furthermore, in the event of a power electronic converter failure, it can be controlled via K... ck The bypass switch disconnects it from the main circuit without affecting the continued operation of the main transformer, thus combining good power supply reliability and engineering practical value; it also has good reliability, economy and engineering promotion prospects.

[0089] The above description is merely a preferred embodiment of the present invention. Those skilled in the art can make several modifications and optimizations based on the above disclosure without departing from the basic principles described above. These modifications and optimizations should be considered within the scope of protection as understood by the present invention.

Claims

1. A control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance, characterized in that: Includes Tm main transformer (1), tap changer group (2), power electronic converter (3), K ck Bypass switch and DC side energy storage unit (4); the primary winding of the Tm main transformer (1) includes a main winding, a converter energy extraction winding and several segmented windings; the main winding, several segmented windings, tap changer group (2), K ck The bypass switch and the power grid constitute the main circuit. The AC side of the series converter in the power electronic converter (3) is electrically connected to K respectively. ck The two ends of the bypass switch and the AC side of the parallel-side converter in the power electronic converter (3) are electrically connected to the two ends of the converter's energy extraction winding, respectively; the DC side of the series-side converter, the DC side of the parallel-side converter, and the power supply terminals of the DC-side energy storage unit (4) are all connected in parallel to C. dc The two ends of the bus capacitor; determine whether the adjustment demand of the circuit system is voltage adjustment demand, reactive power adjustment demand or voltage and reactive power adjustment demand, and adopt a hierarchical multi-level control strategy. According to the adjustment demand of the circuit system, select the compensation mode of the power electronic converter (3) as series side independent voltage compensation mode, parallel side independent reactive power compensation mode or bidirectional capacity mutual compensation mode, parallel side auxiliary series side voltage compensation mode and series side auxiliary parallel side reactive power compensation mode, and perform stepless compensation on the primary winding voltage of the main transformer (1).

2. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 1, characterized in that: In the hierarchical multi-level control strategy, when the adjustment demand of the circuit system is voltage adjustment demand, it is determined whether the voltage compensation exceeds the compensation range of the series-side converter; when the voltage compensation does not exceed the compensation range of the series-side converter, the power electronic converter (3) adopts the series-side independent voltage compensation mode; when the voltage compensation exceeds the compensation range of the series-side converter, the power electronic converter (3) adopts the parallel-side auxiliary series-side voltage compensation mode in the bidirectional capacity mutual compensation mode. When the regulation demand of the circuit system is the reactive power regulation demand, it is determined whether the reactive power compensation exceeds the compensation range of the parallel-side converter. When the reactive power compensation does not exceed the compensation range of the parallel-side converter, the power electronic converter (3) adopts the parallel-side independent reactive power compensation mode. When the reactive power compensation exceeds the compensation range of the parallel-side converter, the power electronic converter (3) adopts the series-side auxiliary parallel-side reactive power compensation mode in the bidirectional capacity mutual assistance compensation mode.

3. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 2, characterized in that: When the regulation demand of the circuit system is the simultaneous regulation demand of voltage and reactive power, the priority of voltage regulation and reactive power regulation in the simultaneous regulation demand of voltage and reactive power is determined. When the priority of voltage regulation is high, the power electronic converter (3) adopts the parallel side auxiliary series side voltage compensation mode in the bidirectional capacity mutual compensation mode. When the priority of reactive power regulation is high, the power electronic converter (3) adopts the series side auxiliary parallel side reactive power compensation mode in the bidirectional capacity mutual compensation mode.

4. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 2, characterized in that: The parallel-side auxiliary series-side voltage compensation mode in the bidirectional capacity mutual assistance compensation mode changes the current magnitude of the primary winding of the Tm main transformer by adjusting the phase angle of the compensation current output on the AC side of the parallel-side converter, thereby increasing the upper limit of the compensation voltage output on the series-side converter and achieving voltage compensation by the parallel-side converter assisting the series-side converter. The calculation process is as follows: ； ； ； In the formula, To compensate for the voltage phase angle, To compensate for voltage, To compensate for the voltage amplitude; The power angle is given by [value], and k is the grid voltage sag factor. To compensate for the voltage of the primary winding, The voltage of the power grid before the voltage drop. The power factor angle, This is the minimum value of the power angle. To compensate for the current phase angle.

5. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 4, characterized in that: When the phase angle of the compensation current output on the AC side of the parallel-side converter is... When the phase angle of the compensation current is less than 90°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually decrease, compensating voltage phase angle Gradually decrease; when the phase angle of the compensation current output on the AC side of the parallel-side converter is 90° < When the phase angle of the compensation current is less than 180°, As the voltage gradually increases, the amplitude of the compensation voltage output on the AC side of the series converter... Gradually increase the compensation voltage phase angle Gradually increase.

6. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 2, characterized in that: The series-side auxiliary parallel-side reactive power compensation mode in the bidirectional capacity mutual assistance compensation mode changes the phase angle of the primary winding voltage of the Tm main transformer by adjusting the phase angle of the compensation voltage output of the AC side of the series-side converter, thereby sharing part of the reactive power compensation and realizing the reactive power compensation of the series-side converter assisting the parallel-side converter; the calculation process is as follows: ； ； ； In the formula, To compensate for the current phase angle, To compensate for the current, To compensate for the amplitude of the current; For power angle, This refers to the uncompensated primary winding current. To compensate for the load current referred to the primary winding, The power factor angle, This is the difference between the power factor angle and the power angle. ; To compensate for the voltage phase angle.

7. The control method for a flexible on-load tap-changing transformer based on bidirectional capacity mutual assistance according to claim 6, characterized in that: When the phase angle of the compensation voltage output on the AC side of the series converter is... When the phase angle of the compensation voltage is less than 90°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually decrease; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° < When the phase angle of the compensation voltage is <90°+δ / 2, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually decrease, compensation current phase angle Gradually increase; when the phase angle of the compensation voltage output on the AC side of the series converter is 90° + δ / 2 < When the phase angle of the compensation voltage is less than 180°, As the current gradually increases, the amplitude of the compensation current output on the AC side of the parallel-side converter... Gradually increase, compensation current phase angle Gradually decrease.

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