Power distribution network voltage and frequency control method based on three-stage solid state transformer

Abstract

The application discloses a power distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer. The application scene of the control method is a three-stage solid-state transformer of a power distribution network, and the structure of the three-stage solid-state transformer comprises an input-stage AC-DC converter, an isolation-stage DC-DC converter and an output-stage DC-AC converter. The control method mainly comprises three parts: input-stage network-side voltage regulation control, isolation-stage single-voltage closed-loop control and output-stage network-side frequency regulation control. The controllability of input and output voltage and current of the three-stage solid-state transformer is fully utilized, and the regulation of active power and reactive power is separated: the input reactive current is controlled in the input stage, the input reactive power is regulated through the reactive current to achieve the purpose of regulating the network-side voltage; and the output voltage is controlled in the output stage, the active power consumption of the load is regulated through the voltage to support the network-side frequency. The control method is simple and convenient, does not need to increase auxiliary equipment, and can well regulate the network-side voltage and frequency.

Description

Technical Field

[0001] This invention belongs to the field of power electronics, specifically relating to a distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer. Background Technology

[0002] Voltage and frequency are important indicators of power quality, and the large-scale grid connection of intermittent renewable energy generation poses challenges to power quality. Renewable energy generation is intermittent and uncertain, unlike traditional thermal power generation which can be planned based on consumption. Moreover, renewable energy generation requires the use of power electronic converters to connect to the grid or to users. The significant characteristics of power electronic converters are rapid response and low inertia. Therefore, the grid connection of renewable energy generation will reduce the grid inertia to a certain extent, increase the frequency deviation of the grid after disturbances, and pose challenges to system stability and power quality.

[0003] Virtual synchronous generators (VSGs) are currently considered an ideal solution for grid connection of renewable energy generation. They improve upon the original three-phase inverter circuit control, enabling them to simulate the rotor mechanical characteristics and stator electrical characteristics of a synchronous generator. During system transients, they can actively contribute to system stability. However, the voltage and frequency regulation capabilities of VSGs are limited by distributed generation. To achieve better voltage and frequency regulation, energy storage systems need to be added, which undoubtedly increases costs.

[0004] Another approach is to start with the distribution network, involving users in maintaining power system stability. However, the power transformers in the distribution network, as one of the most important devices in the power system, have overly singular functions and cannot fully meet the new demands of modern power systems. Therefore, solid-state transformers (also known as power electronic transformers) based on power electronics technology have developed rapidly. Solid-state transformer topologies can be divided into three main categories: single-stage AC-AC structure, two-stage AC-DC-AC structure, and three-stage AC-DC-DC-AC structure. Currently, the most widely used is the three-stage structure, which is also the structure adopted in this invention. The three-stage solid-state transformer introduces a DC link in the middle, and each stage is independently controlled, thus enabling flexible control of the transformer's input and output voltage, current, and power, thereby playing a role in maintaining system stability. From a circuit structure perspective, the advantages of control flexibility and scalability also make the three-stage solid-state transformer the most widely used. Summary of the Invention

[0005] The purpose of this invention is to provide a voltage and frequency regulation control method for power distribution networks based on a three-stage solid-state transformer.

[0006] The technical solution to achieve the objective of this invention is a distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer. Its application is in distribution networks using a three-stage solid-state transformer, whose structure includes an input stage AC-DC converter, an isolation stage DC-DC converter, and an output stage DC-AC converter. The distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer consists of three parts: input stage grid-side voltage regulation control, isolation stage single-voltage closed-loop control, and output stage grid-side frequency regulation control.

[0007] The input-level grid-side voltage regulation and control scheme is as follows:

[0008] Step 1: Sample the three-phase voltage on the grid side and perform an abc / dq coordinate transformation on it to obtain the d-axis voltage component and q-axis voltage component of the three-phase voltage on the grid side in the rotating dq coordinate system;

[0009] Step 2: Sample the three-phase current on the grid side and perform an abc / dq coordinate transformation on it to obtain the d-axis current component and q-axis current component of the three-phase current on the grid side in the rotating dq coordinate system;

[0010] Step 3: Sample the input stage output voltage. The error between the sampled value and its command value is corrected by the PI controller to obtain the command value of the d-axis current, which constitutes the outer loop voltage control of the input stage.

[0011] Step 4: The error between the d-axis voltage sampled in Step 1 and the d-axis voltage correction command value is corrected by the PI controller to obtain the q-axis reactive current command value. The d-axis voltage correction command value is obtained by adding the q-axis reactive current command value to the d-axis voltage command value after feedback from the grid side droop coefficient, thus forming a closed-loop control of the reactive current command value.

[0012] Steps 5, 3, and 4 obtain the current command values ​​for the d-axis and q-axis, which are used as the input command values ​​for the inner current loop. The error between the command values ​​and the sampled values ​​in step 2 is corrected by the PI controller. At the same time, the dq-axis current sampled in step 2 is decoupled, and then added to the dq-axis voltage feedforward sampled in step 1. After dq / abc transformation, the modulation wave is obtained, which constitutes the inner current decoupling control of the input stage. Finally, the pulse signals of each switch in the input stage are obtained through SPWM modulation.

[0013] In this invention, the grid-side voltage regulation control can both compensate for reactive power and consume reactive power, corresponding to the grid-side voltage being adjusted upwards or downwards. Simultaneously, reactive power command value feedback is employed. When the grid-side voltage is lower than the nominal value, reactive power can be compensated more through reactive current command value feedback, thus increasing the grid-side voltage by a certain value. Similarly, when the grid-side voltage is higher than the nominal value, a solid-state transformer needs to consume a certain amount of reactive power to lower the grid-side voltage. Reactive current command value feedback can reduce the consumption of reactive power, again resulting in the grid-side voltage being slightly higher than the nominal value.

[0014] The isolation stage single-voltage closed-loop control scheme is as follows:

[0015] Step 1: Sample the DC voltage output of the isolation stage DC-DC converter. The error between the sampled value and its command value is corrected by the PI controller to obtain the phase shift angle.

[0016] Step 2 and Step 1 obtain the phase shift angle, and obtain the pulse signal of the isolation stage DC-DC converter switching transistor through phase shift modulation.

[0017] The isolation stage single-voltage closed-loop control uses a phase-shift angle to control the output voltage. This control method is simple and easy to implement, and is currently the most widely used control method. Compared to traditional power transformers, the isolation stage circuit of a solid-state transformer can isolate the grid-side AC system from the load-side AC system, so that fluctuations in grid-side frequency or voltage will not affect the load side.

[0018] The output stage grid-side frequency regulation control scheme is as follows:

[0019] Step 1: Sample the three-phase voltage output from the output stage and perform an abc / dq coordinate transformation on it to obtain the d-axis voltage component and q-axis voltage component of the load-side three-phase voltage in the rotating dq coordinate system;

[0020] Step 2: Sample the three-phase current output from the output stage and perform an abc / dq coordinate transformation on it to obtain the d-axis current component and q-axis current component of the load-side three-phase current in the rotating dq coordinate system;

[0021] Step 3: By changing the output voltage of the solid-state transformer, a small trapezoidal voltage disturbance is applied to the output voltage, and the sensitivity of the active power of the three-phase load to the voltage is measured during the disturbance ramp.

[0022] Step 4: Detect the grid-side frequency to obtain the frequency deviation. Set the droop characteristic of the solid-state transformer to obtain the active power deviation corresponding to the frequency deviation. Step 3: Know the three-phase load voltage sensitivity. Calculate the actual voltage that needs to be applied to the load to compensate for the frequency deviation using the formula. This constitutes the output voltage command value control.

[0023] The output voltage command values ​​obtained in steps 5 and 4 are fixed effective values. After three-phase conversion, they are transformed by abc / dq to obtain the d-axis and q-axis voltage command values. The error between the d-axis and q-axis voltages sampled in step 1 and their command values ​​is corrected by the PI controller to obtain the d-axis current command value and the q-axis current command value, which constitute the outer loop voltage control of the output stage.

[0024] Step 6: The error between the d-axis and q-axis current command values ​​and the sampled values ​​in Step 2 is corrected by the PI controller. At the same time, the dq-axis current sampled in Step 2 is decoupled and then added to the voltage feedforward. After dq / abc transformation, the modulation wave of the output stage circuit is obtained, which constitutes the inner loop current decoupling control of the output stage. Finally, the pulse signal of each switch in the output stage is obtained by SPWM modulation.

[0025] In this invention, the grid-side frequency regulation control can adjust the frequency both upwards and downwards, reducing the frequency deviation of the system when subjected to disturbances and improving the system's inertia and anti-interference capability to a certain extent. Simultaneously, the frequency regulation control is also closely related to the load's active power sensitivity to voltage and the proportion of the load controlled by the solid-state transformer. When the voltage change is constant, a higher voltage sensitivity indicates a greater power change, resulting in a wider active power regulation range in the system. Similarly, a larger proportion of solid-state transformers in the system leads to a wider active power regulation range, providing greater support for the grid-side frequency.

[0026] Compared to virtual synchronous machines, the method of this invention does not require energy storage support, separates the regulation of active and reactive power, makes full use of the current margin of the input and output stages of the power electronic transformer, and greatly improves the power electronic transformer's ability to regulate the frequency and voltage of the power grid. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0028] Figure 2 This is a structural diagram of the three-stage solid-state transformer of the present invention.

[0029] Figure 3 This is a simplified circuit diagram of the three-stage solid-state transformer of the present invention.

[0030] Figure 4 This is a block diagram of the input stage grid-side voltage regulation and control of the present invention.

[0031] Figure 5 This is a block diagram of the isolation stage single-voltage closed-loop control of the present invention.

[0032] Figure 6 This is a block diagram of the output stage network-side frequency adjustment control of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings.

[0034] Figure 1 This is a diagram showing the overall structure of the control method of the present invention. Figure 2 This is a structural diagram of a three-stage solid-state transformer. Based on its structure, it can be divided into an input stage, an isolation stage, and an output stage. Figure 1The main circuit section of the invention. Since the topology of a three-stage solid-state transformer is not fixed, to illustrate the specific implementation method of this invention in detail, the most common three-stage circuit topology will be used as an example to explain the implementation method, such as... Figure 3 As shown in the figure, all electrical quantities are labeled. The distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer consists of input-level grid-side voltage regulation control. Figure 4 ), isolation level single voltage closed-loop control ( Figure 5 ) and output stage grid-side frequency regulation control ( Figure 6 It consists of three parts, corresponding to Figure 1 The three-part control block diagram.

[0035] The input stage grid-side voltage regulation control process is as follows ( Figure 4 As shown):

[0036] Step 1: Sample the three-phase voltage e on the grid side a e b e c , for e a e b e c Performing an abc / dq coordinate transformation, we obtain the components e of the three-phase grid voltage in the rotating dq coordinate system. d e q , where ω s The angular frequency of the mesh side;

[0037]

[0038] Step 2: Sample the three-phase current i on the grid side a i b i c , for i a i b i c Performing an abc / dq coordinate transformation, we obtain the components i of the three-phase current on the grid side in the rotating dq coordinate system. d i q ;

[0039]

[0040] Step 3: Sample the output voltage u of the input stage rectifier circuit. dc1 and its instruction value u dc1_ref The error value, after being corrected by the PI controller, yields the command value i of the d-axis current. d_ref This constitutes the outer loop voltage control of the input stage, where the transfer function of the PI controller is as follows;

[0041]

[0042] Step 4, i q_refThe d-axis voltage command value V is obtained by feedback of the droop coefficient K. ref Sum of ΔV The corrected d-axis voltage command value:

[0043] V ref * =V ref +ΔV

[0044] The d-axis voltage e obtained in step 1 d The error value is corrected by the PI controller to obtain the command value i of the q-axis current. q_ref The above constitutes the reactive current command value i q_ref The closed-loop control, through feedback of the reactive current command value via the droop coefficient, can compensate for reactive power by increasing it to a certain value when the voltage drops, and reduce reactive power consumption by a certain value when the voltage rises.

[0045] The outputs of steps 5, 3, and 4 serve as the input command values ​​for the inner current loop. The error between the command value and the sampled value in step 2 is corrected by the PI controller. At the same time, the dq-axis current sampled in step 2 is decoupled and then added to the voltage feedforward. After dq / abc transformation, the modulation wave is obtained, which constitutes the inner current decoupling control of the input stage. Finally, the pulse signal of each switch in the input stage is obtained through SPWM modulation.

[0046]

[0047] The isolation stage single-voltage closed-loop control process is as follows ( Figure 5 As shown):

[0048] Step 1: Sample the DC output voltage u of the dual active bridge. dc2 u dc2 With the output DC voltage command value u dc2_ref The error value is corrected by the PI controller to obtain the phase shift angle.

[0049] Step 2: Given the phase shift angle in Step 1, the pulse signal of the dual active bridge switching transistors is obtained through phase shift modulation. The DC voltage is converted into a high-frequency square wave by the action of the input-side switching transistors, and coupled to the secondary side by the isolation stage transformer. Then, it is converted into DC voltage through the full-bridge circuit on the output side.

[0050] The output stage grid-side frequency regulation control process is as follows ( Figure 6 As shown):

[0051] Step 1: Sampling output stage outputs three-phase voltage u ia u ib u ic , for u ia u ib uic Perform an abc / dq coordinate transformation to obtain the components u of the three-phase voltage on the load side in the rotating dq coordinate system. id u iq , where ω i This is the angular frequency of the output voltage of the solid-state transformer. It is generally set to the rated angular frequency and is a fixed value. Changes in the grid-side frequency will not affect the output frequency.

[0052]

[0053] Step 2: Sampling output stage outputs three-phase current i ia i ib i ic , for i ia i ib i ic Perform an abc / dq coordinate transformation to obtain the components i of the three-phase current on the load side in the rotating dq coordinate system. id i iq ;

[0054]

[0055] Step 3: By changing the output voltage of the solid-state transformer, a small trapezoidal voltage disturbance is applied to the output voltage. During the disturbance ramp, the sensitivity of the active power of the three-phase load to the voltage is measured. The voltage sensitivity is defined as follows.

[0056]

[0057] Step 4: Detect the network-side frequency to obtain the frequency deviation Δω s The active power deviation ΔP corresponding to the frequency deviation is obtained by analyzing the system's droop characteristics, where:

[0058] ΔP=ΔP A +ΔP B +ΔP C

[0059] Given the three-phase load voltage sensitivity from step 3, the actual voltage V required to compensate for the frequency deviation can be calculated using the formula. i_ref This constitutes the output voltage command value control, and the calculation formula is as follows;

[0060]

[0061] The voltage command value V obtained in steps 5 and 4 i_ref After three-phase conversion and abc / dq transformation, the d-axis and q-axis voltage command values ​​u are obtained. id_ref u iq_ref The current command value i is obtained after correction by the PI controller. id_ref iiq_ref This constitutes the outer loop voltage control of the output stage;

[0062] Step 6: The error between the d-axis and q-axis current command values ​​and the sampled values ​​in Step 2 is corrected by the PI controller. At the same time, the dq-axis current sampled in Step 2 is decoupled and then added to the voltage feedforward. After dq / abc transformation, the modulation wave of the output stage circuit is obtained, which constitutes the inner loop current decoupling control of the output stage. Finally, the pulse signal of each switch in the output stage is obtained by SPWM modulation.

[0063]

[0064] The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations or modifications to the technical solutions of the present invention can be made, and all such equivalent transformations and modifications fall within the protection scope of the present invention.

Claims

1. A distribution network voltage and frequency regulation control method based on a three-stage solid-state transformer, characterized in that, The application scenario is a three-stage solid-state transformer for distribution networks. Its structure includes three parts: an input stage AC-DC converter, an isolation stage DC-DC converter, and an output stage DC-AC converter. The distribution network voltage and frequency regulation control method based on the three-stage solid-state transformer is characterized by the fact that its control method consists of input stage grid-side voltage regulation control, isolation stage single voltage closed-loop control, and output stage grid-side frequency regulation control. The input-level grid-side voltage regulation control includes input voltage and current sampling and output DC voltage sampling, reactive current command value closed-loop control, and AC-DC converter voltage and current dual closed-loop control. The isolation stage single-voltage closed-loop control includes output DC voltage sampling and phase shift angle control; The output stage grid-side frequency regulation control includes output voltage and current sampling, grid-side frequency detection, output voltage command value control, and DC-AC converter voltage and current dual closed-loop control. The output stage grid-side frequency regulation control controls the output voltage to control load consumption, thus supporting grid-side frequency changes. It includes the following control steps: Step 1: Sample the three-phase output voltage and perform an abc / dq coordinate transformation to obtain the d-axis and q-axis voltage components of the load-side three-phase voltage in the rotating dq coordinate system; Step 2: Sample the three-phase output current and perform an abc / dq coordinate transformation to obtain the d-axis and q-axis current components of the load-side three-phase current in the rotating dq coordinate system; Step 3: By changing the output voltage of the solid-state transformer, a small trapezoidal voltage disturbance is applied to the output voltage, and the sensitivity of the three-phase load active power to the voltage is measured during the disturbance ramp period; Step 4: Detect the grid-side frequency to obtain the frequency deviation, and obtain the corresponding active power deviation based on the droop characteristic of the solid-state transformer. Given the three-phase load voltage sensitivity, the actual voltage applied to the load to compensate for frequency deviation is obtained based on the three-phase load voltage sensitivity, forming the output voltage command value control; the voltage command values ​​obtained in steps 5 and 4 are fixed effective values, which are converted to three phases and then transformed by abc / dq to obtain the d-axis and q-axis voltage command values. The error between the d-axis and q-axis voltages sampled in step 1 and their command values ​​is corrected by the PI controller to obtain the d-axis current command value and q-axis current command value, forming the outer loop voltage control of the output stage; in step 6, the error between the d-axis and q-axis current command values ​​and the sampled values ​​in step 2 is corrected by the PI controller, and decoupled by combining the dq-axis current sampled in step 2, and then added to the voltage feedforward, and transformed by dq / abc to obtain the modulation wave of the output stage circuit, forming the inner loop current decoupling control of the output stage, and finally obtained by SPWM modulation to obtain the pulse signal of each switch in the output stage.

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