Power distribution network flexible interconnection topology adopting frequency mixing modulation and control method

Abstract

The invention discloses a power distribution network flexible interconnection topology adopting frequency mixing modulation and a control method, and belongs to the technical field of power generation, power transformation or power distribution. The topology comprises a full power module, a first three-phase transformer, an intermediate frequency converter, a power frequency converter, a series APF and a second three-phase transformer, wherein an alternating current port of the full power module and an alternating current port of the series APF are connected in series between a first alternating current system and a second alternating current system; a high-voltage side winding of the first three-phase transformer is connected with an alternating-current port of the full-power module, and a low-voltage side winding of the first three-phase transformer is connected with an alternating-current port of the intermediate-frequency converter; a high-voltage side winding of the second three-phase transformer is connected with the second alternating current system, and a low-voltage side winding is connected with an alternating current port of the power frequency converter; the direct current ports of the intermediate frequency converter and the power frequency converter are connected through a direct current bus with a bus capacitor. According to the control method, frequency mixing modulation is adopted, the functions of fault isolation, dynamic power flow adjustment and the like are achieved, and the size and the manufacturing cost are reduced.

Description

Technical Field

[0001] The invention relates to an AC power distribution network flexible interconnection controller, and in particular discloses a distribution network flexible interconnection topology and a control method using mixed frequency modulation, belonging to the technical field of power generation, power transformation or power distribution. Background Art

[0002] As the characteristics of high efficiency, high energy density and high power electronics become increasingly prominent in the current AC distribution network, the research on power electronic equipment that solves problems such as uneven power flow distribution and fault voltage support has progressed rapidly, and has gradually developed into three mainstream topologies: series, parallel and series-parallel.

[0003] The single series topology has limited compensation for the circuit, and it is necessary to sacrifice one degree of control freedom to maintain the balance of the active power of the power electronic equipment itself. The parallel topology has to sacrifice the equipment construction cost and volume because it needs to carry full power. Series-parallel devices represented by the Unified Power Flow Controller (UPFC) have gradually been widely used for their flexible and comprehensive control. However, the voltage and current stress of the series port of the traditional UPFC topology is large, and it often undergoes multi-stage conversion of AC-DC-DC-AC. When applied to medium-voltage distribution networks, cascade structure conversion devices are often used, and the number of devices is large. Moreover, when a fault occurs, the traditional UPFC adopts a strategy of "avoiding" the fault to preserve the device, and cannot form effective fault isolation. The flexible interconnection device of the distribution network can form a certain fault isolation in the event of a fault based on the UPFC. In recent years, the research on the topology and control of the flexible interconnection device of the distribution network has developed rapidly.

[0004] A distribution network flexible interconnection device topology based on dual active bridge multiplexing is also applicable to AC medium voltage distribution networks, but the number of its internal high-frequency transformers is the same as the number of AC / DC modules, and in the event of a fault, effective fault isolation cannot be achieved through switch locking; a series high-voltage direct-hanging flexible AC ring closing device can also achieve the same function as a parallel flexible interconnection topology, and has a simple structure and is easy to implement in engineering, but the topology lacks a DC bus that is electrically isolated from the AC side, making it difficult to achieve the use scenario of multiple converter stations using the DC bus for energy mutual assistance. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and provide a distribution network flexible interconnection topology and control method using mixed frequency modulation. By optimizing the way in which high-frequency transformers and series APFs are connected to the AC feeder, mixed frequency modulation is used while retaining a DC bus that is electrically isolated from the AC system. While realizing the traditional series-parallel flexible interconnection topology voltage regulation, phase angle regulation, dynamic power flow regulation, reactive power compensation and other functions, the purpose of the invention is achieved by reducing the number of modules, reducing the size of the device and reducing the cost.

[0006] The present invention adopts the following technical solutions to achieve the above-mentioned invention object: In the first aspect, a flexible interconnection topology of a distribution network using mixed frequency modulation includes: a full-power module, a first three-phase transformer, a medium frequency converter, an industrial frequency converter, a series APF and a second three-phase transformer; the full-power module and the series APF are both composed of a plurality of cascaded full-bridge modules, and the AC port of the full-power module and the AC port of the series APF are connected in series between the first AC system and the second AC system; the first three-phase transformer, the high-voltage side winding of which is connected to the AC port of the full-power module, and the low-voltage side winding of which is connected to the AC port of the medium frequency converter; the second three-phase transformer, the high-voltage side winding of which is connected to the second AC system, and the low-voltage side winding of which is connected to the AC port of the industrial frequency converter; the DC ports of the medium frequency converter and the industrial frequency converter, the medium frequency converter and the industrial frequency converter are connected via a DC bus with bus capacitors.

[0007] Furthermore, in a flexible interconnection topology of a distribution network using mixed frequency modulation, a full-power module is connected in series with two AC system feeders to bear the current flowing through the feeders; the medium frequency converter and the industrial frequency converter together with the first three-phase transformer and the second three-phase transformer constitute an energy compensation circuit to maintain the capacitor voltage stability of the full-power module and realize bidirectional power flow.

[0008] Furthermore, in a flexible interconnection topology of a distribution network using mixed frequency modulation, when the medium frequency converter and the industrial frequency converter connected to the first three-phase transformer and the second three-phase transformer are both medium voltage, the medium frequency converter and the industrial frequency converter are both medium voltage converters, and when the medium frequency converter and the industrial frequency converter are both low voltage, the medium frequency converter and the industrial frequency converter are both low voltage converters, and the medium frequency converter and the industrial frequency converter only bear part of the power flowing between the two AC systems. The medium frequency converter and the industrial frequency converter can be implemented using a primary converter.

[0009] Furthermore, in a flexible interconnection topology of a distribution network using mixed frequency modulation, the series APF is connected in series with two AC system feeders and bears the current flowing through the feeders. A certain method is required to maintain the capacitor voltage stability of the full-bridge module.

[0010] Furthermore, in a flexible interconnection topology of a distribution network using mixed frequency modulation, there is a first filter inductor between the first AC system access port and the AC port of the full-power module, a second filter inductor between the low-voltage side of the first three-phase transformer and the AC port of the medium-frequency converter, and a third filter inductor between the low-voltage side of the second three-phase transformer and the AC port of the industrial frequency converter.

[0011] In the second aspect, the first aspect involves a control method for a flexible interconnection topology of a distribution network using mixing modulation, including: a full-power module mixing modulation strategy, a series APF mixing modulation strategy, a medium frequency converter mixing modulation strategy and an industrial frequency converter industrial frequency modulation strategy.

[0012] The mixed frequency modulation strategy of the full-power module utilizes mixed frequency modulation of industrial frequency superimposed on intermediate frequency. The industrial frequency modulation adopts a dual closed-loop control strategy of active and reactive power outer loop and current inner loop. The port voltage of the full-power module AC port connected to the feeder is modulated by industrial frequency. The power outer loop is used to enable the first AC system to operate at a power factor of 1. Open-loop modulation is used to perform intermediate frequency modulation on the port voltage of the full-power module AC port connected to the feeder, and the amplitude and phase of the intermediate frequency sinusoidal quantity are given.

[0013] The mixed-frequency modulation strategy of the series APF uses mixed-frequency modulation of industrial frequency superimposed on intermediate frequency for series compensation, and adopts a voltage single closed-loop control strategy to perform industrial frequency modulation on the port voltage of the series APF AC port connected to the feeder to control the stability of the capacitor voltage of the full-bridge module inside the APF. The industrial frequency quantity is zero in steady state; the current single-loop control is used to perform intermediate frequency modulation on the port voltage of the series APF AC port connected to the feeder. The intermediate frequency modulated voltage is used to offset the intermediate frequency component in the mixed-frequency modulation of the full-power module to prevent it from flowing into the AC system on both sides.

[0014] In the mixing modulation strategy of the intermediate frequency converter, the AC port uses mixing modulation of industrial frequency superimposed on intermediate frequency, and adopts current single-loop control to perform industrial frequency modulation on the AC port voltage of the intermediate frequency converter. The reference voltage of the industrial frequency modulation is composed of the opposite of the industrial frequency modulation voltage of the AC port of the full-power module and the industrial frequency modulation voltage correction amount, so that the industrial frequency current of the AC port of the intermediate frequency converter is 0; the voltage and current dual closed-loop control strategy is adopted to perform intermediate frequency modulation on the AC port voltage of the intermediate frequency converter to control the stability of the full-bridge module capacitor voltage of the full-power module.

[0015] In the power frequency modulation strategy of the power frequency converter, dual closed-loop control of DC bus voltage, reactive power and current is used to perform power frequency modulation on the AC port voltage of the power frequency converter. The stability of the DC bus voltage is maintained by adjusting the flow of active power, and sufficient reactive power support is provided to the power point, so that the second AC system can operate at a power factor of 1.

[0016] Furthermore, in a control method for a flexible interconnection topology of a distribution network using mixed frequency modulation, the intermediate frequency voltage modulation signal of the full power module, the intermediate frequency voltage modulation signal of the series APF, and the intermediate frequency voltage modulation signal of the intermediate frequency converter are all three-phase symmetrical modulation signals. Preferably, a three-phase positive sequence modulation signal is selected as the intermediate frequency voltage modulation signal.

[0017] The present invention adopts the above technical solution and has the following beneficial effects: (1) The flexible interconnection topology of the distribution network using mixed frequency modulation proposed in the present invention is different from the existing UPFC topology. By optimizing the full-power module and the way of connecting the APF in series to the feeder and adopting mixed frequency modulation of industrial frequency superimposed on intermediate frequency, the voltage and current stress of the module is reduced. While reducing the number of modules, the traditional UPFC voltage regulation, phase angle regulation, dynamic power flow regulation, reactive power compensation and other functions are retained, thereby improving the power density.

[0018] (2) Compared with other existing series-parallel flexible interconnection devices, the flexible interconnection topology of the distribution network using mixed frequency modulation proposed in the present invention can also realize dynamic continuous power flow control and power decoupling regulation, while reducing the use of high-frequency transformers and other passive components, retaining the DC bus that is electrically isolated from the AC system, facilitating the control of voltage and power, and can be applied to the construction of multiple converter stations through the DC bus energy mutual assistance usage scenario. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 This is a three-phase topological structure diagram of a flexible interconnected distribution network using mixed frequency modulation proposed in the present invention.

[0021] Figure 2 This is a control strategy block diagram of the full-power module proposed in the present invention.

[0022] Figure 3 This is a control strategy block diagram of the series APF proposed in the present invention.

[0023] Figure 4 This is a control strategy block diagram of the intermediate frequency converter proposed in the present invention.

[0024] Figure 5 This is a control strategy block diagram of the industrial frequency converter proposed in the present invention.

[0025] Description of the numbers in the figure: L f1 , the first filter inductor,L f2 , the second filter inductor, L f3 , the third filter inductor, T 1. The first three-phase transformer, T 2. The second three-phase transformer, C 1. DC bus capacitor. DETAILED DESCRIPTION

[0026] The technical solution of the invention is described in detail below in conjunction with the accompanying drawings. The technical solution in the implementation of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] The present invention provides a three-phase topological structure diagram of a flexible interconnected distribution network using mixed frequency modulation. Figure 1 As shown, specifically: the three-phase topology includes a full power module, a first three-phase transformer T 1. Medium frequency converter, industrial frequency converter, series APF and second three-phase transformer T 2. Among them, the full power module and the series APF are both composed of a plurality of full-bridge modules cascaded, and the full power module and the series APF are connected in series between the first AC system and the second AC system; the first three-phase transformer T 1, its high-voltage side winding is connected to the AC port of the full-power module, and its low-voltage side winding is connected to the AC port of the medium-frequency converter; the second three-phase transformer T 2. Its high-voltage side winding is connected to the second AC system, and its low-voltage side winding is connected to the AC port of the power frequency converter; the DC ports of the intermediate frequency converter and the power frequency converter, the intermediate frequency converter and the power frequency converter are connected through a DC bus capacitor. C 1 is connected to the DC bus of the first AC system access port. A first filter inductor is connected between the first AC system access port and the full power module AC port L f1 There is a second filter inductor between the low voltage side of the first three-phase transformer and the AC port of the medium frequency converter L f2 There is a third filter inductor between the low voltage side of the second three-phase transformer and the AC port of the power frequency converter. L f3 .

[0028] The control strategy block diagram of a full-power module in a flexible interconnection topology of a distribution network using mixed frequency modulation provided by the present invention is as follows: Figure 2 As shown, specifically: AC system 1 is connected to AC system 2 through a feeder. The voltage and current of the first AC system are u g1 , i 1. The voltage and current of the second AC system are u g2 , i 2. The AC port of the full power module is connected in series in the circuit, and the series port is recorded as u 1.

[0029] Port u 1 voltage adopts the mixing control of power frequency superimposed intermediate frequency, and its power frequency part adopts active and reactive power decoupling control. u g1 and i 1 The real-time active value is obtained by calculation P 1 and real-time reactive power value Q 1. Real-time active power value P 1 and active reference value P 1_ref The real-time reactive power value Q 1 and reactive power reference value Q 1_ref The difference between the real-time active and reactive power and the reference value is obtained by the PI controller. i 1d and i 1q Reference value i 1d_ref and i 1q_ref ,Will i 1d_ref and i 1q_ref and i 1d and i 1q The actual values ​​of are subtracted respectively. The control equation written by the circuit equation column can be calculated u Power frequency reference voltage on side 1 u 1_power , where its frequency modulation voltage u 1_med Adopt open loop control, frequency, phase and amplitude are given. u 1_power With the intermediate frequency modulation voltage u 1_med Superposition to get port u 1 modulation voltage u 1abc .

[0030] The control strategy block diagram of the series APF in the flexible interconnection topology of the distribution network using mixed frequency modulation provided by the present invention is as follows: Figure 3 As shown, specifically: The AC port of the series APF is connected in series in the line, and the series port is recorded as u 2. Port u 2 voltage is controlled by mixing power frequency and intermediate frequency, and the power frequency part is controlled by voltage single loop and is related to the current. i 1 In phase, the average value of the sampled APF full-bridge module capacitor voltage U APF_avg With reference value U APF_avg_ref After the difference is made, it is obtained by the PI controller u 2-port power frequency voltage reference amplitude u 2_ref , and then with the current i 1 by multiplying the phase u 2-port power frequency voltage reference value u 2_power Among them, the intermediate frequency part adopts current single loop control, the current i 1 The intermediate frequency component is filtered out by a high-pass filter, and the intermediate frequency current reference value is required i 1_MED_ref 0, filtering out the current of the intermediate frequency component i 1 is subtracted from the intermediate frequency current reference value and then passes through the PR controller to generate the intermediate frequency modulation voltage correction value. The intermediate frequency modulation voltage correction value is superimposed u The opposite of the intermediate frequency modulation voltage at port 1 u 1_med Get Port u 2 IF modulation voltage u 2_med . Power frequency modulation voltage u 2_power With the intermediate frequency modulation voltage u 2_med Superposition to get port u 2 modulation voltage u 2abc .

[0031] The control strategy block diagram of the medium frequency converter in the flexible interconnection topology of the distribution network using mixed frequency modulation provided by the present invention is as follows: Figure 4 As shown, specifically: The AC port of the intermediate frequency converter is connected to the low voltage side of the first three-phase transformer. The AC port of the intermediate frequency converter is u 3. Pair Port u The voltage of 3 is controlled by mixing the power frequency and the intermediate frequency, and the intermediate frequency part is controlled by voltage and current dual loops. The average value of the full-bridge capacitor voltage of the full-power module sampled U FPM_avg With reference value U FPM_avg_refAfter the difference is made, it is obtained by the PI controller u The 3-port intermediate frequency current reference amplitude is then multiplied by the phase of the given intermediate frequency voltage to obtain u 3-port intermediate frequency current reference value i' 3_ref , i' 3_ref With sampling u 3-port current i' 3 is subtracted and passed through the PR controller to obtain u 3-port intermediate frequency voltage correction, u The 3-port intermediate frequency voltage correction value is superimposed on the low-voltage winding voltage of the first three-phase transformer to obtain u 3-port IF modulation voltage u 3_med Among them, the power frequency part adopts current single loop control, the current i' 3. Filter out the power frequency component through a low-pass filter, and require the power frequency current reference value i' 3_power_ref 0, filtering out the power frequency component u 3-port current i' 3_power With the power frequency current reference value i' 3_power_ref After the difference is made, the PR controller generates the correction value of the power frequency modulation voltage at the AC port of the intermediate frequency converter. The correction value of the power frequency modulation voltage at the AC port of the intermediate frequency converter is superimposed. u The opposite number of the power frequency modulation voltage at port 1 u 1_power Get Port u 3. Power frequency modulation voltage u 3_power .port u 3. Power frequency modulation voltage u 3_power With the intermediate frequency modulation voltage u 3_med Superposition to get port u 3 modulation voltage u 3abc .

[0032] The control strategy block diagram of the power frequency converter in the flexible interconnection topology of the distribution network using mixed frequency modulation provided by the present invention is as follows: Figure 5 As shown, specifically: The AC port of the power frequency converter is connected to the low voltage side of the second three-phase transformer. The AC port of the power frequency converter is u 4. Pair Port u The voltage of 4 is controlled by decoupling the DC bus voltage and reactive power. u g2 The current and voltage are calculated to obtain the real-time reactive power value Q2. Real-time reactive power value Q 2 and reactive power reference value Q 2_ref The actual DC bus voltage U DC With the given DC bus voltage reference value U DC_ref The reactive power difference and DC bus voltage difference are obtained by PI controller. i 4d and i 4q Reference value i 4d_ref and i 4q_ref ,Will i 4d_ref and i 4q_ref Respectively i 4d and i 4q The actual value of is subtracted. The control equation written by the circuit equation column can be calculated u Reference voltage on side 4 u 4abc , k Indicates the transformation ratio of the second three-phase transformer.

[0033] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above specific embodiments. The above specific embodiments and the description in the specification are only for further illustrating the principles and preparation effects of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A flexible interconnection topology of a distribution network using mixed frequency modulation, characterized in that: include: A full power module, the AC port of which is connected in series to a feeder between the first AC system and the second AC system; A series APF, whose AC port is connected in series to a feeder between the first AC system and the second AC system; A first three-phase transformer, a high-voltage side winding of which is connected to an AC port of the full-power module; A second three-phase transformer, a high voltage side winding of which is connected to a second AC system; A medium frequency converter, whose AC port is connected to the low voltage side winding of the first three-phase transformer; and The industrial frequency converter has a DC port connected to the DC port of the medium frequency converter and an AC port connected to the low-voltage side winding of the second three-phase transformer.

2. A distribution network flexible interconnection topology using mixed frequency modulation according to claim 1, characterized in that: The DC port of the industrial frequency converter is connected to the DC port of the intermediate frequency converter via a DC bus with a bus capacitor.

3. A distribution network flexible interconnection topology using mixed frequency modulation according to claim 2, characterized in that: The full power module and the series APF are respectively composed of a plurality of cascaded full bridge modules.

4. A distribution network flexible interconnection topology using mixed frequency modulation according to claim 3, characterized in that: When one side of the first three-phase transformer connected to the medium frequency converter and one side of the second three-phase transformer connected to the industrial frequency converter are both medium voltage, the medium frequency converter and the industrial frequency converter are both medium voltage converters; when one side of the first three-phase transformer connected to the medium frequency converter and one side of the second three-phase transformer connected to the industrial frequency converter are both low voltage, the medium frequency converter and the industrial frequency converter are both low voltage converters.

5. A distribution network flexible interconnection topology using mixed frequency modulation according to claim 4, characterized in that: There is a first filter inductor between the first AC system access port and the full-power module AC port, a second filter inductor between the low-voltage side of the first three-phase transformer and the AC port of the medium-frequency converter, and a third filter inductor between the low-voltage side of the second three-phase transformer and the AC port of the industrial frequency converter.

6. A control method for a distribution network flexible interconnection topology using mixed frequency modulation according to any one of claims 1 to 5, characterized in that: include: The mixed frequency modulation strategy of the full-power module adopts a dual closed-loop control strategy of active and reactive power outer loop and current inner loop to perform industrial frequency modulation on the port voltage of the full-power module AC port connected to the feeder, and adopts an open-loop modulation strategy to perform intermediate frequency modulation on the port voltage of the full-power module AC port connected to the feeder; The mixed frequency modulation strategy of the series APF uses a voltage single closed-loop control strategy to perform power frequency modulation on the port voltage of the series APF AC port connected to the feeder, and uses a current single-loop control strategy to perform intermediate frequency modulation on the port voltage of the series APF AC port connected to the feeder; The mixed frequency modulation strategy of the intermediate frequency converter adopts a current single-loop control strategy to perform industrial frequency modulation on the AC port voltage of the intermediate frequency converter, and adopts a voltage and current double closed-loop control strategy to perform intermediate frequency modulation on the AC port voltage of the intermediate frequency converter; The power frequency modulation strategy of the power frequency converter adopts a dual closed-loop control strategy of DC bus voltage, reactive power and current to perform power frequency modulation on the AC port voltage of the power frequency converter.

7. A control method for a distribution network flexible interconnection topology using mixed frequency modulation according to claim 6, characterized in that: In the mixed frequency modulation strategy of the intermediate frequency converter, the reference voltage of the power frequency modulation is obtained by adding the inverse of the power frequency modulation voltage of the port where the full power module AC port is connected to the feeder and the power frequency modulation voltage correction amount of the intermediate frequency converter AC port.

8. A control method for a distribution network flexible interconnection topology using mixed frequency modulation according to claim 7, characterized in that: The intermediate frequency modulation voltage signal in the mixing modulation strategy of the full power module, the intermediate frequency modulation voltage signal in the mixing modulation strategy of the series APF, and the intermediate frequency modulation voltage signal in the mixing modulation strategy of the intermediate frequency converter are all three-phase symmetrical modulation signals.

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