A comprehensive power quality compensation device and method for distribution network transformer

Abstract

The present invention discloses a comprehensive power quality compensation device and method for distribution network transformers. By installing a full-power current converter (VSC) and an isolated series transformer on either the high-voltage or low-voltage side of a distribution transformer, the device dynamically adjusts output voltage and unbalanced voltage, load unbalanced current, harmonic currents, and reactive power compensation. This device comprehensively addresses common power quality issues in distribution networks, such as voltage and current imbalance, harmonic currents, and reactive current. It integrates the functions of multiple traditional power quality management devices, reduces installation space and equipment investment costs, and offers excellent economic benefits.

Description

Technical Field

[0001] The present invention relates to the field of power distribution networks, and in particular to a power quality comprehensive compensation device for a transformer in a distribution network and a method thereof. Background Art

[0002] Industrial distribution network transformers present a variety of power quality issues. Single-phase power supply at the user end can cause voltage and current imbalance in three-phase transformers, increasing transformer losses and shortening their lifespan. In industrial applications with three-phase power supply, voltage imbalance can directly impact power quality and, in severe cases, affect equipment operation. On the user side of distribution network transformers, motors and other equipment generate harmonics and reactive power. Harmonics cause voltage and current distortion, increasing losses and impacting equipment operation. Reactive power increases losses and reduces power factor.

[0003] Traditional power quality management equipment generally solves specific power quality problems separately, such as solving reactive power compensation problems (SVG) or harmonic problems (APF). Power quality management often requires a large investment and lacks comprehensive and economical methods and equipment to solve power quality problems. As distribution networks have increasingly higher requirements for power quality, the role of comprehensive and economical methods and equipment to solve power quality problems is particularly important. Summary of the Invention

[0004] To solve the above problems, the present invention provides a comprehensive power quality compensation device for distribution network transformers. The device dynamically adjusts the output voltage and unbalanced voltage, unbalanced current of the load, harmonic current, and realizes reactive power compensation by installing a full-power current converter (VSC) and an isolated series transformer on the high-voltage side or the low-voltage side of the distribution transformer.

[0005] Preferably, the full-power converter includes: a series converter arranged on the distribution network transformer side, used to solve voltage imbalance and voltage fluctuation problems; and a parallel converter arranged on the load side, used to solve current imbalance, harmonic control and reactive power compensation problems.

[0006] Preferably, the secondary side of the isolation series transformer is connected in series between the output of the distribution transformer and the load, the series converter is connected to the primary side of the isolation series transformer, and the parallel converter is connected in parallel with the load side.

[0007] To solve the above problems, the present invention also provides a comprehensive compensation method for power quality of distribution network transformers. The method dynamically adjusts the output voltage and unbalanced voltage, unbalanced current of the load, harmonic current and reactive power compensation by installing equipment on the high-voltage side or low-voltage side of the distribution transformer. The method is characterized in that the method is implemented by a full-power converter (VSC) and an isolated series transformer. The isolated series transformer is connected in series between the output of the distribution transformer and the load. The series converter of the full-power converter is connected to the primary side of the isolated series transformer, and the parallel converter of the full-power converter is connected in parallel with the load side. The series converter on the grid side is used to solve the problems of unbalanced voltage and voltage fluctuation, and the parallel converter on the load side is used to solve the problems of unbalanced current, reactive power compensation and harmonic current.

[0008] Preferably, solving the problem of voltage imbalance and voltage fluctuation of the distribution network transformer specifically includes the following steps: detecting the terminal voltage of the distribution transformer, and extracting the positive and negative sequence components of the voltage, controlling the negative sequence voltage to 0, and controlling the positive sequence voltage to the rated value, thereby solving the voltage imbalance and voltage fluctuation problems.

[0009] Preferably, solving the problem of current imbalance on the load side specifically includes the following steps: detecting the current on the load side, extracting the positive and negative sequence components of the current, controlling the negative sequence current to 0, and solving the current imbalance problem.

[0010] Preferably, solving the reactive power compensation problem of the distribution network specifically includes the following steps: detecting the voltage and current at the transformer end, controlling the power factor of the grid to 1 by dynamically compensating the reactive current, and solving the reactive compensation problem in real time and linearly.

[0011] Preferably, solving the harmonic problem of the distribution network transformer specifically includes the following steps: detecting the voltage at the load end, obtaining the synchronous phase through the PLL phase-locked loop, and extracting the harmonic current component by detecting the current on the load side, controlling the harmonic current to 0, and realizing the harmonic control of the current.

[0012] Preferably, the closed-loop control of the series converter solves the voltage imbalance and power supply fluctuation problems by the following steps:

[0013] (1) Obtaining the grid phase θ through the phase-locked loop PLL s ;

[0014] (2) Extract the positive and negative sequence components of the voltage, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0. Closed-loop control is performed on the dq axis respectively. According to needs, the controller can be a PI controller or other types of controllers.

[0015] Preferably, the closed-loop control of the parallel converter solves the current imbalance and harmonic problems, which is achieved by the following steps:

[0016] (1) Obtaining the grid phase θ through the phase-locked loop PLL s ;

[0017] (2) Extract the positive and negative sequence components of the load current, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0. Closed-loop control is performed on the dq axis respectively. The controller can be a PI controller or other types of controllers as needed;

[0018] (3) By extracting each harmonic component and performing the corresponding abc / dq transformation, the dq value of each harmonic current is obtained. The dq given value of the harmonic component is 0. Closed-loop control is performed on the dq axis respectively. The controller can be a PI controller or other types of controllers as needed;

[0019] (4) Collect the voltage and current of the distribution network transformer, calculate the power factor, and perform closed-loop control by setting the power factor to 1 to achieve real-time adjustment of the reactive component. The controller can be a PI controller or other types of controllers.

[0020] (5) In order to improve the stability and dynamic response performance of the converter, a current loop is added. The voltage command from the current loop is modulated by SVPWM to obtain the corresponding PWM signal, which is then amplified through isolation to drive the corresponding IGBT module.

[0021] The present invention has the following advantages:

[0022] (1) The present invention provides a comprehensive solution to the power quality management of distribution network transformers. It solves the voltage imbalance and reactive power compensation problems by connecting a series converter to the grid transformer, and solves the current imbalance and harmonic problems by connecting a load in parallel to the converter. It can comprehensively compensate for the voltage and current imbalance, harmonic control, and reactive power compensation of the distribution network, thereby reducing the investment cost of power quality management.

[0023] (2) The present invention detects the voltage and current at the transformer end or the load end, and adaptively realizes voltage balance control by adjusting the positive and negative sequence components of the voltage and current, which has strong real-time performance and robustness;

[0024] (3) The present invention detects the voltage and current at the transformer end, controls the power factor of the power grid by dynamically compensating for reactive current, and solves the reactive power problem in real time and linearly.

[0025] (4) The present invention detects the current on the load side, extracts the harmonic current components, and controls the harmonic current to be 0, thereby significantly eliminating specific harmonics. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1The specific structure diagram of a distribution network transformer comprehensive compensation device of the present invention is shown.

[0027] Figure 2 A topological diagram of a comprehensive compensation device for a distribution network transformer according to the present invention is shown;

[0028] Figure 3 A control block diagram of a comprehensive compensation device for a distribution network transformer according to the present invention is shown; DETAILED DESCRIPTION

[0029] Figure 1 The detailed structure diagram of a distribution network transformer comprehensive compensation device is shown, which includes an isolation series transformer and a full power converter (VSC).

[0030] The series-side structure features an isolated series transformer with its secondary side connected in series between the grid and the load, while the primary side of the isolated series transformer is connected to a VSC series converter. The parallel-side structure features a VSC shunt converter connected in parallel with the load. The series converter on the distribution network transformer side addresses voltage imbalance and voltage fluctuation, while the load-side shunt converter addresses current imbalance, reactive power compensation, and harmonic control.

[0031] Figure 2 This figure shows a topological diagram of a comprehensive compensation device for distribution network transformers according to the present invention, and provides specific implementation methods for comprehensive compensation control of distribution network transformer voltage and current imbalance, harmonic control, and reactive power compensation. The series-side structure features an isolated series transformer secondary connected in series between the grid and the load, with the isolated series transformer primary connected to a series converter of a full-power converter (VSC). The parallel-side structure features a parallel converter of the full-power converter (VSC) connected in parallel to the load.

[0032] The converter connected to the load side is called a shunt converter and is primarily used to control the VSC's DC voltage stability, three-phase current imbalance, and harmonic current compensation. The converter connected to the primary side of the isolated series transformer is called a series converter and is primarily used to control the load's supply voltage, three-phase voltage imbalance, and reactive power compensation.

[0033] Due to the use of AC / DC / AC conversion, the grid side and the load side are completely isolated, and the load voltage can be controlled at any set point through voltage closed-loop control.

[0034] The isolated series transformer is connected in series between the output of the distribution transformer and the load, the series converter of the full-power converter is connected to the primary side of the isolated series transformer, and the parallel converter of the full-power converter is connected in parallel with the load side. The series converter on the grid side is used to solve the problems of unbalanced voltage and voltage fluctuation, and the parallel converter on the load side is used to solve the problems of unbalanced current, reactive power compensation and / or harmonic current.

[0035] Solving the voltage imbalance and voltage fluctuation problems of distribution network transformers specifically includes the following steps: detecting the voltage at the distribution transformer terminal, and extracting the positive and negative sequence components of the voltage, controlling the negative sequence voltage to 0, and controlling the positive sequence voltage to the rated value, thereby solving the voltage imbalance and voltage fluctuation problems.

[0036] Solving the current imbalance problem on the load side specifically includes the following steps: detecting the current on the load side, extracting the positive and negative sequence components of the current, controlling the negative sequence current to 0, and solving the current imbalance problem.

[0037] Solving the reactive power compensation problem in the distribution network specifically includes the following steps: detecting the voltage and current at the transformer end, controlling the power factor of the grid to 1 by dynamically compensating the reactive current, and solving the reactive power compensation problem in real time and linearly.

[0038] Solving the harmonic problem of distribution network transformers specifically includes the following steps: detecting the voltage at the load end, obtaining the synchronous phase through the PLL phase-locked loop, and extracting the harmonic current components by detecting the current on the load side, controlling the harmonic current to 0, and realizing current harmonic control.

[0039] By adopting closed-loop control of the VSC, unbalanced voltage, unbalanced current, reactive power compensation and harmonic control can be adjusted in real time.

[0040] The positive and negative sequence components of voltage and current are extracted by generalized second-order integration method.

[0041] The specific formula of the generalized second-order integral method is as follows:

[0042]

[0043]

[0044] Where: v is the input signal, v' is the input signal, H d (s) is the bandpass filter transfer function, H q (s) is the low-pass filter transfer function, ω is the filter center angular frequency, and k is the damping ratio.

[0045] When the input voltage or current signal angular frequency is consistent with the filter center angular frequency, the input signal v passes through H d (s), yielding an output voltage signal v' with an angular frequency of ω. The output signal qv' has the same amplitude as v', but lags its phase by 90°, meaning that the output signals v' and qv' are orthogonal. This allows the generalized second-order integration method to be used to extract the positive and negative sequence signals of voltage or current.

[0046] See also Figure 2, the closed-loop control of the series converter solves the voltage imbalance, voltage fluctuation and reactive power compensation problems, and is achieved through the following steps:

[0047] (1) Obtaining the grid phase θ through the phase-locked loop PLL s ;

[0048] (2) Extract the positive and negative sequence components of the voltage, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0. Closed-loop control is performed on the dq axis respectively. According to needs, the controller can be a PI controller or other types of controllers.

[0049] See also Figure 2 The closed-loop control of the parallel converter solves the current imbalance, harmonics and reactive power compensation problems, which can be achieved through the following steps:

[0050] (1) Obtaining the grid phase θ through the phase-locked loop PLL s ;

[0051] (2) Extract the positive and negative sequence components of the load current, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0. Closed-loop control is performed on the dq axis respectively. The controller can be a PI controller or other types of controllers as needed;

[0052] (3) By extracting each harmonic component and performing the corresponding abc / dq transformation, the dq value of each harmonic current is obtained. The dq given value of the harmonic component is 0. Closed-loop control is performed on the dq axis respectively. The controller can be a PI controller or other types of controllers as needed;

[0053] (4) Collect the voltage and current of the distribution network transformer, calculate the power factor, and perform closed-loop control by setting the power factor to 1 to achieve real-time adjustment of the reactive component. The controller can be a PI controller or other types of controllers;

[0054] (5) In order to improve the stability and dynamic response performance of the converter, a current loop is generally added. The voltage command from the current loop is modulated by SVPWM to obtain the corresponding PWM signal, which is then amplified through isolation to drive the corresponding IGBT module.

[0055] This can not only meet the high-quality power supply quality requirements of the load, but also achieve good energy-saving effects.

[0056] Figure 3 A specific control block diagram of an embodiment of the present invention using a full power converter (VSC) as a comprehensive compensation device for distribution network transformers is shown.

[0057] Both the grid-side series converter and the load-side shunt converter employ a vector control strategy. Key functional modules include a PLL (phase-locked loop) for detecting the grid-side voltage phase, a PI controller (PI is used as an example; other controller types can be used depending on actual needs), a coordinate transformation module (abc / dq transformation and dq / abc conversion), and a comparator. Vector control ensures that all given quantities (including DC voltage, grid-side power, the positive- and negative-sequence d- and q-axis components of the grid-side voltage, and the positive- and negative-sequence d- and q-axis components of the load-side current) are maintained at their set values.

[0058] Taking the load-side parallel converter as an example, the following is explained:

[0059] (1) Collecting the DC voltage U of VSC dc , grid side voltage U abcl and load side three-phase current I abcl .

[0060] (2) Using the phase-locked loop PLL and voltage U abcl Get the grid synchronization angle θ. Based on θ, the three-phase current I abcl Transformed to the dq axis, the positive sequence components are I dlpos and I qlpos , the negative sequence components are I dlnes and I qlnes .

[0061] (3) The given value of DC voltage is Will and the actual voltage U dc After comparison, the d-axis current setting is obtained through the PI controller, and the q-axis current setting is 0; the d-axis and q-axis current settings and the actual positive sequence current analysis I dlpos , I qlpos By comparison, the d-axis and q-axis voltages output by the converter are obtained by adding the relevant coupling terms through the PI controller.

[0062] (4) The current setting of the negative sequence converter d axis and q axis is 0, and the current setting and actual positive sequence current analysis I dlpos , I qlpos By comparison, the three-phase current unbalance compensation component is obtained through the PI controller.

[0063] (5) The positive and negative sequence d-axis and q-axis harmonics are extracted through harmonic current, and the harmonic current compensation components are calculated.

[0064] (6) The given value of the inverter output voltage is directly controlled by the IGBT of the converter after PWM conditioning to achieve closed-loop control.

[0065] The grid-side series converter also adopts a similar vector control strategy, which will not be described here.

[0066] The above description further details the present invention in conjunction with specific preferred embodiments. The specific implementation of the present invention should not be construed as being limited to these descriptions. In particular, connecting the device in series with the high-voltage side of a distribution transformer is essentially consistent with the present invention. For those skilled in the art, equivalent substitutions or obvious variations, provided they maintain the same performance or application without departing from the spirit of the present invention, should be considered within the scope of the present invention.

Claims

1. A comprehensive power quality compensation device for a distribution network transformer. The device dynamically adjusts output voltage and unbalanced voltage, load unbalanced current, harmonic current, and reactive power compensation by installing a full-power converter and an isolated series transformer on the high-voltage or low-voltage side of the distribution transformer. The secondary side of the isolated series transformer is connected in series between the grid and the load, and the primary side is connected to the full-power converter. The grid side and the load side are completely isolated, and the load voltage is controlled at any set point through a voltage closed loop. The full-power converter includes a series converter arranged on the transformer side of the distribution network to solve voltage imbalance and fluctuation problems, and a parallel converter arranged on the load side to solve current imbalance, harmonic control and reactive power compensation problems; The secondary side of the isolation series transformer is connected in series between the output of the distribution transformer and the load, the series converter is connected to the primary side of the isolation series transformer, and the parallel converter is connected in parallel with the load side; The series converter and the parallel converter maintain respective given quantities at set values ​​through vector control, wherein: The vector control strategy of the parallel converter is: (1) Collecting the DC voltage of the full power converter U dc , grid side voltage U abcl and load side three-phase current I abcl ; (2) Using the phase-locked loop (PLL) and voltage U abcl Get the grid synchronization angle θ, and based on θ, convert the three-phase current I abcl Transformed to the dq axis, the positive sequence components are I dlpos and I qlpos , the negative sequence components are I dlnes and I qlnes ; (3) The given value of DC voltage is U* dc ,Will U* dc and actual voltage U dc After comparison, the d-axis current setting is obtained through the PI controller, the q-axis current setting is 0, the d-axis and q-axis current settings and the actual positive sequence current components I dlpos 、 I qlpos By comparison, the d-axis and q-axis voltages output by the converter are obtained by adding the relevant coupling terms to the PI controller; (4) The current setting of the negative sequence converter d axis and q axis is 0, the current setting and the actual positive sequence current component I dlpos 、 I qlpos By comparison, the three-phase unbalanced current compensation component is obtained through the PI controller; (5) The positive and negative sequence d-axis and q-axis harmonics are extracted through harmonic current and the harmonic current compensation components are calculated; (6) The given value of the inverter output voltage is directly controlled by the IGBT of the converter after PWM conditioning to achieve closed-loop control.

2. A method for comprehensive power quality compensation of distribution network transformers, wherein the method installs equipment on the high-voltage side or low-voltage side of the distribution transformer to dynamically adjust the output voltage and unbalanced voltage, unbalanced current of the load, harmonic current, and reactive power compensation, characterized in that: The comprehensive compensation method for power quality of distribution network transformer is realized by full power converter and isolated series transformer. The isolation series transformer is connected in series between the output of the distribution transformer and the load. The series converter of the full-power converter is connected to the primary side of the isolation series transformer. The parallel converter of the full-power converter is connected in parallel with the load side. The series converter on the grid side is used to solve the problems of unbalanced voltage and voltage fluctuation. The parallel converter on the load side is used to solve the problems of unbalanced current, reactive power compensation and harmonic current. The full-power converter includes a series converter and a parallel converter, and both the series converter and the parallel converter adopt a vector control strategy. The series converter and the parallel converter include a PLL phase-locked loop, a PI controller, a coordinate transformation module, and a comparator for detecting the grid side voltage phase. The series converter and the parallel converter maintain respective given quantities at set values ​​through vector control, wherein the vector control strategy of the parallel converter is: (1) Collecting the DC voltage of the full power converter U dc , grid side voltage U abcl and load side three-phase current I abcl ; (2) Using the phase-locked loop (PLL) and voltage U abcl Get the grid synchronization angle θ, and based on θ, convert the three-phase current I abcl Transformed to the dq axis, the positive sequence components are I dlpos and I qlpos , the negative sequence components are I dlnes and I qlnes ; (3) The given value of DC voltage is U* dc ,Will U* dc and actual voltage U dc After comparison, the d-axis current setting is obtained through the PI controller, the q-axis current setting is 0, the d-axis and q-axis current settings and the actual positive sequence current components I dlpos 、 I qlpos By comparison, the d-axis and q-axis voltages output by the converter are obtained by adding the relevant coupling terms to the PI controller; (4) The current setting of the negative sequence converter d axis and q axis is 0, the current setting and the actual positive sequence current component I dlpos 、 I qlpos By comparison, the three-phase unbalanced current compensation component is obtained through the PI controller; (5) The positive and negative sequence d-axis and q-axis harmonics are extracted through harmonic current and the harmonic current compensation components are calculated; (6) The given value of the inverter output voltage is directly controlled by the IGBT of the converter after PWM conditioning to achieve closed-loop control.

3. The method for comprehensive compensation of power quality of distribution network transformer according to claim 2, characterized in that: Solving the voltage imbalance and voltage fluctuation problems of distribution network transformers specifically includes the following steps: detecting the voltage at the distribution transformer terminal, and extracting the positive and negative sequence components of the voltage, controlling the negative sequence voltage to 0 and the positive sequence voltage to the rated value, thereby solving the voltage imbalance and voltage fluctuation problems.

4. The method for comprehensive compensation of power quality of distribution network transformer according to claim 2, characterized in that: Solving the problem of current imbalance on the load side specifically includes the following steps: detecting the current on the load side, extracting the positive and negative sequence components of the current, controlling the negative sequence current to 0, and solving the current imbalance problem.

5. The method for comprehensive compensation of power quality of distribution network transformer according to claim 2, characterized in that: Solving the reactive power compensation problem in the distribution network specifically includes the following steps: detecting the voltage and current at the transformer end, controlling the power factor of the grid to 1 by dynamically compensating the reactive current, and solving the reactive power compensation problem in real time and linearly.

6. The method for comprehensive compensation of power quality of distribution network transformer according to claim 2, characterized in that: Solving the harmonic problem of distribution network transformers specifically includes the following steps: detecting the voltage at the load end, obtaining the synchronous phase through the PLL phase-locked loop, and extracting the harmonic current components by detecting the current on the load side, controlling the harmonic current to 0, and realizing current harmonic control.

7. The method for comprehensive compensation of power quality of distribution network transformer according to any one of claims 2 to 6, characterized in that: The closed-loop control of the series converter solves the voltage imbalance and voltage fluctuation problems by the following steps: (1) Obtain the grid phase θs through the phase-locked loop PLL; (2) Extract the positive and negative sequence components of the voltage, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0, and closed-loop control is performed on the dq axis respectively.

8. The method for comprehensive compensation of power quality of distribution network transformer according to any one of claims 2 to 6, characterized in that: The closed-loop control of parallel converters solves the current imbalance and harmonic problems and is achieved through the following steps: (1) Obtain the grid phase θs through the phase-locked loop PLL; (2) Extract the positive and negative sequence components of the load current, perform abc / dq transformation on the negative sequence component, and obtain the dq value of the negative sequence component. The dq given values ​​of the negative sequence component are all 0, and closed-loop control is performed on the dq axis respectively; (3) By extracting each harmonic component and performing the corresponding abc / dq transformation, the dq value of each harmonic current is obtained. The dq given value of the harmonic component is 0, and closed-loop control is performed on the dq axis respectively; (4) Collect the voltage and current of the distribution network transformer, calculate the power factor, and perform closed-loop control by setting the power factor to 1 to achieve real-time adjustment of the reactive component; (5) In order to improve the stability and dynamic response performance of the converter, a current loop is added. The voltage command from the current loop is modulated by SVPWM to obtain the corresponding PWM signal, which is then driven by isolation amplification to drive the corresponding IGBT module.

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