A filtering method and device for a twelve-pulse rectifier transformer

Abstract

The present invention discloses a filtering method and device for a twelve-pulse rectifier transformer, and relates to filtering technology for a power distribution system. The filtering method and device determine the tap positions of the star winding and the delta winding according to the withstand voltage and current conditions of the converter device, so as to optimize the capacity of the converter device, and then calculate the compensation current component to be injected according to the harmonic current component in the load current and the tap position, and inject the compensation current component into the tap of the secondary winding, while eliminating the harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer, and can achieve the optimal capacity without adding a matching transformer, thereby improving the voltage and current adaptability of the converter device and reducing the manufacturing cost and design difficulty of the twelve-pulse rectifier transformer.

Description

Technical Field

[0001] The present invention belongs to the filtering technology of power distribution system, and in particular to a filtering method and device of a twelve-pulse rectifier transformer based on harmonic magnetic potential balance and optimal capacity. Background Art

[0002] Twelve-pulse rectifier transformers are widely used in high-voltage DC transmission, high-power industrial rectification, urban rail power supply and other fields. Compared with six-pulse rectifiers, the harmonic content of its grid-side input current and DC-side output voltage is relatively low. Compared with eighteen-pulse and above multiple rectifiers, it is more economical and simple in structure. Compared with PWM rectifiers, it is more reliable and cheaper. However, for the entire power system, it still belongs to the category of power electronic nonlinear loads. The current harmonics generated will be transmitted to the power supply system through the power grid, which will not only increase the network loss of the power supply system, the malfunction and refusal of relay protection and automatic reclosing devices, the temperature rise, vibration, loss and noise of the motor, but also cause the user's voltage to be distorted and the reliability of power use to be reduced. Therefore, effectively reducing the harmonic content of the grid-side of the twelve-pulse rectifier transformer is not only in line with the "saving, clean and safe" energy development strategy for the sustainable and healthy development of the power industry, but also conducive to accelerating the construction of a clean, efficient, safe and sustainable modern energy system.

[0003] Passive filtering is the earliest and most traditional method for filtering harmonics in twelve-pulse rectifier transformers. It has a simple structure, high economy, reliable operation, and is the most widely used. However, it usually requires multiple groups of LC filters to be connected in parallel to filter out multiple harmonics of different frequencies. In addition, the passive branch is prone to resonance with the grid-side impedance, and it cannot fundamentally solve the problems of heating, vibration, and loss caused by harmonic currents to power supply equipment such as transformers.

[0004] In order to fundamentally solve the adverse effects of rectifier current harmonics on transformers, the inductive filtering technology adds a filter winding with an equivalent impedance close to zero to the rectifier transformer, and an external passive filter branch is connected to introduce the load harmonic current into the winding loop, thereby using the transformer ampere-turn balance to offset the load harmonic flux and avoid flowing into the primary side of the transformer, thereby achieving the purpose of harmonic shielding. This method can adapt to high-voltage and large-capacity systems and has been applied in HVDC, industrial rectification, wind farms and other fields. In order to reduce the space occupied by passive branches, this method subsequently integrates the inductor into the transformer to obtain magnetically integrated inductive filtering technology. However, in general, this method requires the addition of a filter winding that meets certain electromagnetic characteristics to the transformer core, which increases the manufacturing cost and design difficulty of the transformer. The external passive branch can only filter out harmonics of a specific frequency.

[0005] In order to make the filter device not affected by the grid-side impedance and to filter out the system harmonics in real time, active power filter technology and induction control filter technology developed based on induction filter technology came into being. The basic principle is to detect the harmonic components from the load current, and the converter generates the required compensation harmonic current components, so that the harmonic current or harmonic magnetic potential cancels each other out. In theory, this method can compensate for all the harmonic components of each frequency contained in the load current. However, when the induction control filter technology is used for filtering, the converter is directly connected to the load side of the rectifier transformer. The voltage on the load side is relatively high (such as 10kV), and the withstand voltage of the converter is relatively low (such as 6000V). A step-down transformer must be matched for step-down (that is, the converter is connected to the load side of the rectifier transformer through a step-down transformer). The converter can withstand the high voltage on the load side. When there is no step-down transformer, the capacity potential of the converter cannot be fully utilized, which increases the manufacturing cost and design difficulty of the rectifier transformer. Summary of the invention

[0006] In view of the problem in the prior art that the inductive control filtering method cannot fully utilize the capacity potential of the converter device, or a matching transformer must be added to fully utilize the capacity potential of the converter device, the present invention provides a filtering method and device for a twelve-pulse rectifier transformer, determines a suitable tap position of the rectifier transformer, injects a compensating current component from the tap through the converter device, and eliminates the grid-side harmonic current of the rectifier converter while achieving optimal capacity of the converter device, thereby reducing the manufacturing cost and design difficulty of the entire rectifier transformer.

[0007] The present invention solves the above technical problems through the following technical solutions: A filtering method for a twelve-pulse rectifier transformer comprises the following steps:

[0008] Step 1: Collect the load current and load voltage on the secondary side of the twelve-pulse rectifier transformer, and detect the harmonic current component in the load current;

[0009] Step 2: Determine the tap position of the secondary winding of the twelve-pulse rectifier transformer according to the withstand voltage and current conditions of the converter device, so as to optimize the capacity of the converter device;

[0010] Step 3: Calculate the compensating current component to be injected by the converter according to the harmonic current component in step 1 and the tap position in step 2;

[0011] Step 4: According to the compensating current component to be injected in step 3, the compensating current component is injected into the tap of the secondary winding through the current conversion device to eliminate the harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer.

[0012] The filtering method of the present invention determines the tap positions of the star winding and the delta winding according to the withstand voltage and current conditions of the converter device, so that the capacity of the converter device is optimized, and then calculates the compensation current component to be injected according to the harmonic current component in the load current and the tap position, and injects the compensation current component into the tap of the secondary winding. While eliminating the harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer, the optimal capacity can be achieved without adding a matching transformer, thereby improving the voltage and current adaptability of the converter device and reducing the manufacturing cost and design difficulty of the twelve-pulse rectifier transformer; the star winding tap and the delta winding tap on the secondary side of the rectifier transformer are respectively connected to a converter device, so that the port voltage of the converter device matches the tap port voltage of the winding, and the withstand voltage and current conditions are easily achieved, further reducing the design difficulty.

[0013] Furthermore, in step 1, the instantaneous reactive power theory is used to detect the harmonic current component in the load current.

[0014] Furthermore, in step 2, the converter device is a back-to-back converter. When the tap position is determined according to the withstand voltage and current conditions of the back-to-back converter, the voltage matching conditions on both sides of the back-to-back converter must also be met.

[0015] A back-to-back converter is used, the star winding of the back-to-back converter is connected to the tap of the star winding on the secondary side of the rectifier transformer, the delta winding of the back-to-back converter is connected to the tap of the delta winding on the secondary side of the rectifier transformer, the star winding and the delta winding of the back-to-back converter share a DC capacitor, which reduces the cost of the converter device, and the use of the back-to-back converter makes the structure of the entire rectifier transformer more compact.

[0016] Furthermore, the voltage matching condition on both sides of the back-to-back converter is:

[0017]

[0018] Among them, x is the ratio of the number of turns from the tap of the secondary side star winding of the rectifier transformer to the output terminal to the total number of turns of the star winding, and y is the ratio of the number of turns from the tap of the secondary side delta winding of the rectifier transformer to the output terminal to the total number of turns of the delta winding.

[0019] Since the star winding and delta winding of the back-to-back converter are respectively connected to the star winding tap and delta winding tap on the secondary side of the rectifier transformer, the port voltages of the star winding and delta winding of the back-to-back converter are matched, which is equivalent to matching the star winding tap port voltage and the delta winding tap port voltage on the secondary side of the rectifier transformer, that is, the number of winding turns on both sides of the star winding tap on the secondary side of the rectifier transformer is matched with the number of winding turns on both sides of the delta winding tap.

[0020] Furthermore, in step 3, the calculation expression of the compensation current component to be injected is:

[0021] and

[0022] Among them, i Ca1 、i Cb1 、i Cc1 are the compensation current components that the converter should inject into the A, B, and C phase taps of the star winding on the secondary side of the rectifier transformer, respectively. N1 and M1 are the coefficient matrices of the star winding on the secondary side of the rectifier transformer, i La1h 、i Lb1h i La1 、i Lb1 The harmonic current components, i La1 、i Lb1 are the load currents of phases A and B of the secondary star winding of the rectifier transformer, W A , W B are the turns of phases A and B of the primary winding of the rectifier transformer, W a1 , W b1 are the turns of phases A and B of the secondary star winding of the rectifier transformer, respectively; x is the ratio of the number of winding turns from the tap to the output terminal of the secondary star winding of the rectifier transformer to the total number of winding turns of the star winding;

[0023] and

[0024] Among them, i Ca2 、i Cb2 、i Cc2 are the compensation current components that the converter should inject into the A, B, and C phase taps of the triangle winding on the secondary side of the rectifier transformer, N2 and M2 are the coefficient matrices of the triangle winding on the secondary side of the rectifier transformer, i La2h 、i Lb2h i La2 、i Lb2 The harmonic current components, i La2 、i Lb2 are the load currents of phases A and B of the triangle winding on the secondary side of the rectifier transformer, W a2 , W b2 are the turns of phases A and B of the triangle winding on the secondary side of the rectifier transformer, respectively; y is the ratio of the number of turns from the tap to the output terminal of the triangle winding on the secondary side of the rectifier transformer to the total number of turns of the triangle winding.

[0025] Furthermore, in step 4, the specific operation of injecting the compensation current component is:

[0026] Step 4.1: The DC capacitor voltage of the converter device is passed through a PI controller to obtain an active d-axis current component;

[0027] Step 4.2: superimposing the compensation current component to be injected with the active d-axis current component to obtain an ideal compensation current component;

[0028] Step 4.3: taking the difference between the ideal compensation current component and the actual injected compensation current component as the input of the PR controller to obtain the output of the PR controller;

[0029] Step 4.4: Control the switch tube of the converter device according to the output of the PR controller in step 4.3 to inject the tap compensation current component of the secondary winding.

[0030] In order to stabilize the DC capacitor voltage of the converter, an active d-axis current component is superimposed on the compensation current component to be injected. The PR controller (proportional harmonic controller) is used to track the compensation current components of each frequency, thereby improving the injection accuracy of the actual injected compensation current component, making the actual injected compensation current component closer and closer to the ideal compensation current component, and making the filtering of harmonic current components more thorough.

[0031] Furthermore, the frequency domain transfer function expression of the PR controller is:

[0032]

[0033] Among them, K p is the proportional adjustment coefficient, K i is the integral adjustment coefficient, K Ih is the resonance coefficient, h is the resonance number, ω1 is the fundamental frequency angular frequency, ω PRh is the resonant frequency.

[0034] Accordingly, a filtering device for a twelve-pulse rectifier transformer includes:

[0035] A first current conversion device connected to the tap of the secondary star winding of the rectifier transformer, the first current conversion device being used to inject a first compensation current component into the tap of the secondary star winding of the rectifier transformer according to a first control instruction;

[0036] A second current conversion device connected to the secondary-side delta winding tap of the rectifier transformer, the second current conversion device being used to inject a second compensation current component into the secondary-side delta winding tap of the rectifier transformer according to a second control instruction;

[0037] The current and voltage acquisition unit is used to acquire the load current and load voltage of the star winding and the load current and load voltage of the delta winding on the secondary side of the rectifier transformer; acquire the DC capacitor voltage of the first converter and the DC capacitor voltage of the second converter; and acquire the first actual injected compensation current component and the second actual injected compensation current component respectively injected into the star winding tap and the delta winding tap by the first converter and the second converter;

[0038] A harmonic current component detection unit, used for obtaining a first harmonic current component of a star winding and a second harmonic current component of a delta winding on the secondary side of a rectifier transformer according to the load current and the load voltage;

[0039] A tap position determination unit is used to determine the ratio of the number of winding turns from the tap of the secondary side star winding of the rectifier transformer to the output terminal to the total number of winding turns of the star winding, and the ratio of the number of winding turns from the tap of the secondary side delta winding of the rectifier transformer to the output terminal to the total number of winding turns of the delta winding according to the withstand voltage and current conditions of the first converter and the second converter, so as to obtain the tap positions of the secondary side star winding and delta winding;

[0040] A compensation current component calculation unit to be injected is used to calculate the first compensation current component and the second compensation current component to be injected into the secondary side star winding tap and the delta winding tap respectively according to the first harmonic current component, the second harmonic current component and the tap position;

[0041] A PI control unit, used to obtain active d-axis current components of the first current conversion device and the second current conversion device according to the DC capacitor voltages of the first current conversion device and the second current conversion device respectively;

[0042] a comparison unit, used for superimposing the active d-axis current component of the first converter device with the first compensation current component to obtain a first ideal compensation current component, and superimposing the active d-axis current component of the second converter device with the second compensation current component to obtain a second ideal compensation current component; and for subtracting the first actual injected compensation current component from the first ideal compensation current component to obtain a first compensation difference, and subtracting the second actual injected compensation current component from the second ideal compensation current component to obtain a second compensation difference;

[0043] A PR control unit, configured to obtain a first output control amount and a second output control amount according to the first compensation difference and the second compensation difference, respectively;

[0044] A control unit is used to generate a first control instruction and a second control instruction according to a first output control quantity and a second output control quantity, so as to control the first converter and the second converter to respectively inject a compensating current component into the star winding tap and the delta winding tap on the secondary side of the rectifier transformer.

[0045] Furthermore, the first converter and the second converter are a back-to-back converter, the star winding of the back-to-back converter is connected to the star winding tap of the secondary side of the rectifier transformer, and the delta winding of the back-to-back converter is connected to the delta winding tap of the secondary side of the rectifier transformer.

[0046] Beneficial Effects

[0047] Compared with the prior art, the present invention proposes a filtering method and device for a twelve-pulse rectifier transformer. According to the withstand voltage and current conditions of the converter, the tap positions of the star winding and the delta winding are determined to optimize the capacity of the converter. Then, the compensation current component to be injected is calculated according to the harmonic current component in the load current and the tap position, and the compensation current component is injected into the tap of the secondary winding. While eliminating the harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer, the capacity can be optimized without adding a matching transformer, thereby improving the voltage and current adaptability of the converter and reducing the manufacturing cost and design difficulty of the twelve-pulse rectifier transformer. The star winding tap and the delta winding tap on the secondary side of the rectifier transformer are respectively connected to a converter, so that the port voltage of the converter matches the tap port voltage of the winding, which is easy to achieve the withstand voltage and current conditions, and further reduces the design difficulty.

[0048] The two independent current conversion devices in the present invention can be implemented by using a back-to-back current converter, which can save the cost of a DC capacitor, and the back-to-back current converter makes the structure of the entire filter device more compact.

[0049] The filtering method of the present invention uses a PR controller to track current components of various frequencies when injecting compensating current components, so that the actually injected compensating current components are infinitely close to the ideal compensating current components, thereby improving the injection accuracy of the actually injected compensating current components and making the filtering of harmonic current components more thorough. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solution of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only one embodiment of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 is a connection topology diagram of a back-to-back converter and a twelve-pulse rectifier transformer in an embodiment of the present invention;

[0052] Figure 2 1 is a voltage vector diagram of the star winding and the delta winding on the secondary side of the twelve-pulse rectifier transformer in an embodiment of the present invention, (a) is the voltage vector diagram of the star winding, and (b) is the voltage vector diagram of the delta winding;

[0053] Figure 3 1 is an equivalent circuit diagram of a triangle winding on the secondary side of a twelve-pulse rectifier transformer in an embodiment of the present invention, (a) is an equivalent circuit diagram of a triangle winding and a load current source, (b) is an equivalent circuit diagram of a triangle winding and a compensation current source;

[0054] Figure 4 is a generation diagram of an ideal compensation current component in an embodiment of the present invention;

[0055] Figure 5 1 is an equivalent circuit of the star winding of the back-to-back converter and the secondary star winding tap, and the secondary delta winding tap in the embodiment of the present invention, (a) is an equivalent circuit of the star winding of the back-to-back converter and the secondary star winding tap, (b) is an equivalent circuit of the delta winding of the back-to-back converter and the secondary delta winding tap;

[0056] Figure 6 is a PR control strategy diagram in an embodiment of the present invention;

[0057] Figure 7 is a control block diagram of an ideal compensation current component in an embodiment of the present invention;

[0058] Figure 8 is a Bode diagram of a current control open-loop transfer function in an embodiment of the present invention;

[0059] Among them, 1-back-to-back converter, 101-star winding Yy0 of back-to-back converter, 102-delta winding Yd11 of back-to-back converter, 2-primary side of twelve-pulse rectifier transformer, 3-star winding Yy0 of secondary side of twelve-pulse rectifier transformer, 4-delta winding Yd11 of secondary side of twelve-pulse rectifier transformer. DETAILED DESCRIPTION

[0060] The following is a clear and complete description of the technical solutions in the present invention in conjunction with the drawings in the embodiments of 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 of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0061] Taking the converter device as a back-to-back converter 1 as an example, a filtering method for a twelve-pulse rectifier transformer provided by the present invention comprises the following steps:

[0062] 1. Collect the load current and load voltage on the secondary side of the twelve-pulse rectifier transformer, and detect the harmonic current components in the load current.

[0063] like Figure 1As shown, the star winding of the back-to-back converter is connected to the tap of the star winding on the secondary side of the twelve-pulse rectifier transformer, and the delta winding of the back-to-back converter is connected to the tap of the delta winding on the secondary side of the twelve-pulse rectifier transformer. The star winding and delta winding on the secondary side of the twelve-pulse rectifier transformer use the same primary side.

[0064] It can be seen that the load current and load voltage on the secondary side of the twelve-pulse rectifier transformer include the load current and load voltage of the secondary side star winding, and the load current and load voltage of the secondary side delta winding. The instantaneous reactive power theory is used to detect the harmonic current components in the load current of the secondary side star winding and delta winding. The instantaneous reactive power theory is an existing technology for detecting harmonic current components. Please refer to "Harmonic Suppression and Reactive Power Compensation", published by Machinery Industry Press in 2016, written by Wang Zhaoan.

[0065] 2. According to the withstand voltage and current conditions of the back-to-back converter and the voltage matching conditions on both sides of the back-to-back converter, determine the tap position of the secondary winding of the twelve-pulse rectifier transformer to optimize the capacity of the back-to-back converter.

[0066] Figure 1 in,i A 、i B 、i C is the grid-side current of the twelve-pulse rectifier transformer, W A , W B , W C are the turns of the primary winding A, B, and C phases of the twelve-pulse rectifier transformer, respectively; L is the connection inductance of the star winding and the delta winding of the back-to-back converter connected to the tap of the star winding and the delta winding of the secondary side of the twelve-pulse rectifier transformer, respectively; i Ca1 、i Cb1 、i Cc1 are the compensating current components that should be injected into the taps of the A, B, and C phases of the secondary side star winding of the twelve-pulse rectifier transformer from the star winding of the back-to-back converter, i Ca2 、i Cb2 、i Cc2 They are the compensating current components that should be injected into the taps of the A, B, and C phases of the triangle winding of the back-to-back converter's secondary side triangle winding. The star winding and triangle winding of the back-to-back converter share a DC capacitor C, u dc is the DC voltage of the DC capacitor. Compared with two independent converters, the back-to-back converter saves the cost of a DC capacitor and has a more compact structure. a1 , W b1 , W c1 are the turns of the secondary side star winding Yy0 A, B, and C phases of the twelve-pulse rectifier transformer, respectively. a2, W b2 , W c2 are the turns of the A, B, and C phases of the secondary triangle winding Yd11 of the twelve-pulse rectifier transformer, and the ratio of the two is a1, b1, c1 are the output terminal symbols of the A, B, C phases of the secondary star winding Yy0 of the twelve-pulse rectifier transformer, u1, v1, w1 are the tap terminal symbols of the A, B, C phases of the secondary star winding Yy0 of the twelve-pulse rectifier transformer; a2, b2, c2 are the output terminal symbols of the A, B, C phases of the secondary delta winding Yd11 of the twelve-pulse rectifier transformer, u2, v2, w2 are the tap terminal symbols of the A, B, C phases of the secondary delta winding Yd11 of the twelve-pulse rectifier transformer; the ratio of the number of turns on both sides of the tap of the A, B, C phases of the secondary star winding Yy0 of the twelve-pulse rectifier transformer is x:1-x, x is the ratio of the number of turns from the tap of the secondary star winding of the twelve-pulse rectifier transformer to the output terminal of the star winding to the total number of turns of the star winding; the delta winding Yd11 of the secondary side of the twelve-pulse rectifier transformer The ratio of the number of winding turns on both sides of the A, B, and C phase taps is y:1-y, where y is the ratio of the number of winding turns from the tap of the secondary side triangle winding to the output terminal of the triangle winding to the total number of winding turns of the triangle winding. La1 、i Lb1 、i Lc1 are the load currents of phases A, B, and C of the secondary star winding of the twelve-pulse rectifier transformer, i La2 、i Lb2 、i Lc2 They are the load currents of the A, B, and C phases of the triangle winding on the secondary side of the twelve-pulse rectifier transformer; the output DC load voltage and DC load current of the twelve-pulse rectifier transformer are u Ldc 、i Ld , L load is the DC side load filter inductor, which is usually larger. load Represents the load resistance value, and S is the load switching switch.

[0067] according to Figure 1 The voltage vector diagram of the star winding and delta winding on the secondary side of the twelve-pulse rectifier transformer can be obtained, as shown in Figure 2 shown. Figure 2 In the figure, O1 is the neutral point of the secondary star winding Yy0 of the two-pulse rectifier transformer, and O2 is the virtual neutral point of the secondary delta winding Yd11 of the twelve-pulse rectifier transformer; Figure 2 In (a), the load voltage of the terminal of the secondary star winding Yy0 of the twelve-pulse rectifier transformer is taken as the base value. The per-unit value of the line voltage of the tap terminal of the secondary star winding Yy0 of the twelve-pulse rectifier transformer is:

[0068]

[0069] In formula (1), is the line voltage from terminal u1 to terminal v1, is the line voltage from terminal a1 to terminal b1, and Figure 2 In (b), in Δo2a2v2, the cosine theorem holds:

[0070]

[0071] Find:

[0072] Therefore, the per-unit value of the line voltage at the tap port of the secondary delta winding Yd11 of the twelve-pulse rectifier transformer can be obtained as follows:

[0073]

[0074] In formula (3), is the line voltage from terminal u2 to terminal v2, is the line voltage from terminal a2 to terminal b2, is the phase voltage from the virtual neutral point O2 to the terminal u2, It is the phase voltage from the virtual neutral point O2 to terminal a2.

[0075] It can be seen from equations (1) and (3) that, as the tap positions of the star winding Yy0 and the delta winding Yd11 on the secondary side of the twelve-pulse rectifier transformer change (i.e., the size of x and y changes), the corresponding tap port line voltage per unit value also changes. The tap port line voltage per unit value of the star winding Yy0 can be changed to any value between 0 and 1, while the minimum value of the tap port line voltage per unit value of the delta winding Yd11 is 0.5. Therefore, the line voltage per unit value of the filtering method of the present invention is adapted to a range of [0.5, 1]. Since the star winding and delta winding of the back-to-back converter are connected to the star winding tap and delta winding tap of the secondary side of the twelve-pulse rectifier transformer respectively, in order to match the port voltages of the star winding side and the delta winding side of the back-to-back converter, it can be equivalent to matching the port voltage of the star winding tap of the secondary side of the twelve-pulse rectifier transformer with the port voltage of the delta winding tap, that is, the number of turns on both sides of the star winding tap of the secondary side of the twelve-pulse rectifier transformer matches the number of turns on both sides of the delta winding tap, which needs to meet the following conditions:

[0076]

[0077] It can be seen from this that the tap positions of the star winding and the delta winding on the secondary side of the twelve-pulse rectifier transformer can actually be reflected by the number of winding turns on both sides of the star winding tap and the number of winding turns on both sides of the delta winding tap, that is, the tap position can be reflected by x and y.

[0078] When determining the tap position, while satisfying the voltage matching condition on both sides of the back-to-back converter, i.e., equation (4), it is also necessary to consider the voltage and current withstand conditions of the back-to-back converter and select a suitable tap position so that the capacity of the back-to-back converter is optimally utilized. The capacity of the back-to-back converter actually refers to its voltage and current withstand capabilities.

[0079] 3. Calculate the compensation current component that should be injected by the back-to-back converter according to the harmonic current component in step 1 and the tap position in step 2.

[0080] 3.1 Principle of magnetic potential balance on the secondary side of the star winding of the twelve-pulse rectifier transformer

[0081] Ignoring the magnetic potential balance error caused by the excitation current on the Yy0 side of the star winding, the current component i on the primary side induced by the secondary side star winding current of the twelve-pulse rectifier transformer is A1 、i B1 、i C1 The compensation current component to be injected into the star winding of the back-to-back converter and the load current of the A, B, and C phase star windings on the secondary side of the twelve-pulse rectifier transformer have the following magnetic potential balance equation:

[0082]

[0083] Phase shift simplification yields:

[0084]

[0085] Because I A1 +i B1 +i C1 =0, the above matrix equation is not the simplest form. The third matrix equation can be derived from the first two equations in the matrix. Therefore, the most simplified processing of equation (6) is as follows:

[0086]

[0087] By solving the determinants of the two coefficient matrices in equation (7), we can obtain:

[0088]

[0089] Therefore, both coefficient matrices are invertible. Equation (7) is also called the magnetic potential balance equation of the star winding Yy0 grid-side current, compensation current, and load current.

[0090] 3.2 Principle of magnetic potential balance on the secondary side of the triangle winding of the twelve-pulse rectifier transformer

[0091] The Yd11 side winding of the triangle winding adopts a triangle connection form, so the structure is relatively complex. In this embodiment, the circuit analysis of the secondary side triangle winding is performed using the superposition principle, and the output port load current and the compensation current injected by the tapped back-to-back converter are replaced by current sources respectively. The equivalent circuit of the Yd11 side winding of the triangle winding is as follows: Figure 3 As shown, it can be divided into two parts: (a) triangle winding and load current source, (b) triangle winding and compensation current source. Figure 3 in,i k11 、i k12 、i k21 、i k22 、i k31 、i k32 is the current flowing through each segment winding in the delta winding, i K1 、i K2 、i K3 is the secondary side delta winding current.

[0092] When the compensation current injected into the star winding of the back-to-back converter is 0, according to Figure 3 (a) and KCL's law, we have:

[0093]

[0094] The finishing includes:

[0095]

[0096] Similarly, ignoring the Yd11 excitation current error on the delta winding side, the current component i on the primary side induced by the delta winding current on the secondary side of the twelve-pulse rectifier transformer is A2 、i B2 、i C2 and the secondary side delta winding current i K1 、i K2 、i K3 The magnetic potential balance equation is:

[0097]

[0098] Substitute equation (9) into equation (10), and since i A2 +i B2 +i C2 =0, the equation can be further simplified to:

[0099]

[0100] The calculation of the coefficient matrix determinant of formula (11) is as follows. Since the result is not 0, it can be seen that the matrix is ​​invertible.

[0101]

[0102] When the compensation current injected into the delta winding of the back-to-back converter is 0, Figure 3 (b) and KCL's law, we have:

[0103]

[0104] and

[0105] Therefore, the above formula is reversible. At this time, the magnetic potential balance equation of the current component induced by the secondary side triangle winding current to the primary side and the current flowing through each segment winding in the triangle winding is:

[0106]

[0107] In formula (13), [A, B] represents the combination of matrix A and matrix B, and we have:

[0108]

[0109] By inverting formula (12) and substituting it into formula (13), we can get:

[0110]

[0111] In formula (14), the matrix C is the inverse matrix of the coefficient matrix in formula (12), as shown below. In formula (14), “·” represents a matrix multiplication operation.

[0112]

[0113] Arrange the above formula and perform the most simplified calculation to get:

[0114]

[0115] The calculation of the determinant of the coefficient matrix of formula (15) is as follows. Since the result is not 0, it can be seen that the matrix is ​​invertible.

[0116]

[0117] In summary, the magnetic potential balance equation of the grid-side current, compensation current, and load current of the secondary-side delta winding of the twelve-pulse rectifier transformer is shown in equation (16):

[0118]

[0119] 3.3 Filtering principle of harmonic magnetic potential balance

[0120] In the twelve-pulse rectifier transformer, if the primary and secondary transformation ratio of the star winding Yy0 is 1:1, the primary and secondary transformation ratio of the corresponding triangle winding Yd11 is At the same time, La1 The current waveform is expanded by Fourier series with the midpoint between the positive and negative half waves as the time zero point, and can be expanded as follows:

[0121]

[0122] In the above formula, I d is the load DC current. Since the secondary line voltage of Yd11 is 30 degrees ahead of Yy0, i La2 Also correspondingly ahead of i La1 30 degrees, can be pushed out La2 The Fourier series expansion is shown in formula (18):

[0123]

[0124] In the Yd11 connection, the fundamental wave and positive sequence harmonic components of the primary line current of the twelve-pulse rectifier transformer lag behind the fundamental wave and positive sequence harmonic components of the secondary line current by 30 degrees, while the negative sequence harmonic components of the primary line current lead the negative sequence harmonic components of the secondary line current by 30 degrees. A2 The Fourier series expansion is shown in formula (19):

[0125]

[0126] At this time, the 5th, 7th, 17th, 19th and other harmonic contents in the grid-side current cancel each other out, and only the 11th, 13th, 23rd, 25th and other harmonic components remain, as shown in formula (20). Therefore, in the present invention, only the 11th, 13th, 23rd, 25th and other harmonic components need to be compensated for when performing harmonic component compensation.

[0127]

[0128] From equations (7) and (16), it can be seen that the magnetic potential balance equations of the grid-side current, compensation current, and load current of the secondary side star winding Yy0 and the delta winding Yd11 have the same matrix form. Therefore, equations (7) and (16) can be summed, and then:

[0129]

[0130] In formula (21), i La11 with i Lb11 The current i La1 with i Lb1 The fundamental component in La1h with i Lb1h The current iLa1 with i Lb1 The 11th, 13th, 23rd, and 25th harmonic components in La21 with i Lb21 The current i La2 with i Lb2 The fundamental component in La2h with i Lb2h The current i La2 with i Lb2 The 11th, 13th, 23rd, and 25th harmonic components in the equation (7) are M1 and N1, respectively. La1 、i Lb1 With compensation current i Ca1 、i Cb1 The coefficient matrix of load current i is La2 、i Lb2 With compensation current i Ca2 、i Cb2 The coefficient matrix of i A 、i B Represents the A and B phase currents on the grid side of the twelve-pulse rectifier, including the current i induced by the star winding Yy0 and the delta winding Yd11 currents on the primary side A1 、i B1 with i A2 、i B2 Two parts.

[0131] If formula (21) satisfies formula (22):

[0132]

[0133] Then the grid-side current i of the twelve-pulse rectifier is A 、i B It only contains fundamental current components, and the harmonic components of 11, 13, 23, 25, etc. cancel each other out due to the magnetic potential balance with the converter compensation current component. At this time, the compensation current components that need to be injected into the secondary side star winding tap and delta winding tap of the back-to-back converter in the present invention are shown in equations (23) and (24):

[0134]

[0135]

[0136] According to equations (23) and (24), the compensation current components that should be injected by the star winding and delta winding of the back-to-back converter into the tap of the secondary star winding and delta winding respectively can be calculated. The compensation current components to be injected include the secondary star winding load current i La1 、i Lb1With the delta winding load current i La2 、i Lb2 The 11th, 13th, 23rd, 25th and other harmonic components.

[0137] 4. According to the compensation current component to be injected in step 3, the compensation current component is injected into the tap of the secondary star winding and the tap of the delta winding through the back-to-back converter to eliminate the harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer.

[0138] like Figure 4 As shown, the variables in the rectangular dashed box are the variables on the star winding side, and the variables in the elliptical dashed box are the variables on the delta winding side. Because the compensation current extraction method is the same, they share a block diagram. At the same time, in order to stabilize the DC capacitor voltage of the back-to-back converter, an active d-axis current component for stabilizing the DC capacitor voltage is superimposed in the back-to-back converter on the star winding side. This component is obtained by the output of the PI controller. The final output current with an asterisk is the ideal compensation current component on the secondary side of the star winding in this filtering method. And the ideal compensation current component on the secondary delta winding side

[0139] Figure 4 Medium, voltage The phase angle required for instantaneous reactive power theory detection of harmonic current components is obtained through the phase-locked loop PLL, i La1 、i Lb1 、i Lc1 and i La2 、i Lb2 、i Lc2 The 11th, 13th, 23rd, 25th, 35th, 37th harmonic current components in the synchronous coordinate change, and then after the filter LPF filtering and synchronous coordinate inverse transformation, are superimposed with the active d-axis current component to generate the ideal compensation current component. With the ideal compensation current component

[0140] Figure 1 The equivalent circuit of the star winding of the back-to-back converter and the secondary star winding tap and the secondary delta winding tap is as follows: Figure 5 As shown, with the DC capacitor C in the middle of the back-to-back converter as the boundary, the equivalent circuit is divided into the equivalent circuit of the star winding of the back-to-back converter and the tap of the secondary side star winding, as shown in Figure 5 (a), and the equivalent circuit of the delta winding of the back-to-back converter and the tapped delta winding of the secondary side, as shown in Figure 5 (b).

[0141] Figure 5 In (a), u u1o1 、uv1o1 、u w1o1 is the phase voltage of the secondary side star winding tap, u Ca1 、u Cb1 、u Cc1 is the port output voltage on the star winding side of the back-to-back converter, L and R are the inductance and resistance of the connected inductor. Figure 5 In (b), u u2v2 、u v2w2 、u w2u2 is the line voltage of the secondary delta winding tap, u u2o2 、u v2o2 、u w2o2 is the phase voltage of the line voltage for equivalent power conversion, u Ca2 、u Cb2 、u Cc2 It is the port output voltage on the delta winding side of the back-to-back converter.

[0142] according to Figure 5 And the KVL law is:

[0143]

[0144] After the above formula is transformed into the dq coordinate of the power (base) frequency synchronization, we get:

[0145]

[0146] In formula (26), i Cdi 、i Cqi To compensate for the d-axis and q-axis components of the current under synchronous coordinate transformation, w is the power frequency, L is the connection inductance, and u di 、u qi are the d-axis and q-axis components of the tap port phase voltage under synchronous coordinate transformation, u Cdi 、u Cqi are the d-axis and q-axis components of the phase voltage at the back-to-back converter port under synchronous coordinate transformation. It can be seen that the d-axis and q-axis components of the compensation current i Cdi 、i Cqi In the introduction of wLi Cqi 、wLi Cdi Decoupling term and feedforward term u di 、u qi The latter corresponds to u Cdi 、u Cqi Satisfies the linear relationship, and the control system becomes a linear single-input single-output system. The current component needs to be compensated Since the base frequency, 11, 13, 23, 25, 35, 37 and other sub-components contained therein are respectively transformed into DC, 10, 12, 22, 24, 34, 36 and other sub-components after the industrial frequency synchronous dq conversion, in order to accurately track the ideal current containing the AC component, the filtering method of the present invention introduces a PI+PR controller, wherein the PI is used for tracking the DC current component, and the PR controller is used for tracking the current components of each frequency, such as Figure 6 shown.

[0147] Taking into account the capacity, switching frequency, harmonic control requirements, etc., the filtering method of the present invention only compensates for the 11th, 13th, 23rd, 25th, 35th, and 37th harmonic load currents. Therefore, the frequency domain transfer function of the PR controller is:

[0148]

[0149] In the above formula, K p is the proportional adjustment coefficient, K i is the integral adjustment coefficient, K Ih is the resonance coefficient, h is the resonance number, ω1 is the fundamental frequency angular frequency, ω PRh is the resonant frequency; h = (10, 12, 22, 24, 34, 36), therefore, Figure 6 The ideal compensation current component control block diagram of the back-to-back converter Yy0 side and Yd11 side is as follows: Figure 7 shown. Figure 7 In, take K p =0.16, K i =0.1, K Ih =0.4, w PRh =5, the open-loop transfer function Bode diagram of the d-axis and q-axis components of the compensation current is shown in Figure 8 shown.

[0150] from Figure 8 It can be seen that the use of PI+PR controller can make the open-loop transfer function have larger gains at 0, 10, 12, 22, 24, 34, and 36 frequencies. These frequencies correspond to the 1, 11, 13, 23, 25, 35, and 37 components before the dq synchronous coordinate transformation of the compensation current. It can be seen from the phase-frequency characteristic curve that the minimum stability margin of the curve is 32.68 degrees, which means that the system can achieve error-free and stable tracking of the compensation current.

[0151] Accordingly, a filtering device for a twelve-pulse rectifier transformer includes:

[0152] A first current conversion device connected to the tap of the secondary star winding of the rectifier transformer, the first current conversion device being used to inject a first compensation current component into the tap of the secondary star winding of the rectifier transformer according to a first control instruction;

[0153] A second current conversion device connected to the tap of the delta winding on the secondary side of the rectifier transformer, the second current conversion device being used to inject a second compensation current component into the tap of the delta winding on the secondary side of the rectifier transformer according to a second control instruction;

[0154] The current and voltage acquisition unit is used to acquire the load current and load voltage of the star winding and the load current and load voltage of the delta winding on the secondary side of the rectifier transformer; acquire the DC capacitor voltage of the first converter and the DC capacitor voltage of the second converter; and acquire the first actual injected compensation current component and the second actual injected compensation current component respectively injected into the star winding tap and the delta winding tap by the first converter and the second converter;

[0155] A harmonic current component detection unit, used to obtain the first harmonic current component of the star winding and the second harmonic current component of the delta winding of the secondary side of the rectifier transformer according to the load current and the load voltage;

[0156] A tap position determination unit is used to determine the ratio of the number of winding turns from the tap of the secondary side star winding of the rectifier transformer to the output terminal to the total number of winding turns of the star winding, and the ratio of the number of winding turns from the tap of the secondary side delta winding of the rectifier transformer to the output terminal to the total number of winding turns of the delta winding according to the withstand voltage and current conditions of the first converter and the second converter, so as to obtain the tap positions of the secondary side star winding and delta winding;

[0157] A compensation current component calculation unit to be injected is used to calculate the first compensation current component and the second compensation current component to be injected into the secondary side star winding tap and the delta winding tap respectively according to the first harmonic current component, the second harmonic current component and the tap position;

[0158] A PI control unit, used to obtain active d-axis current components of the first current conversion device and the second current conversion device according to the DC capacitor voltages of the first current conversion device and the second current conversion device respectively;

[0159] A comparison unit, used for superimposing the active d-axis current component of the first converter device with the first compensation current component to obtain a first ideal compensation current component, and superimposing the active d-axis current component of the second converter device with the second compensation current component to obtain a second ideal compensation current component; and for subtracting the first actual injected compensation current component from the first ideal compensation current component to obtain a first compensation difference, and subtracting the second actual injected compensation current component from the second ideal compensation current component to obtain a second compensation difference;

[0160] A PR control unit, used to obtain a first output control amount and a second output control amount according to a first compensation difference and a second compensation difference respectively;

[0161] A control unit is used to generate a first control instruction and a second control instruction according to a first output control quantity and a second output control quantity, respectively, so as to control the first converter and the second converter to inject a compensation current component into the star winding tap and the delta winding tap of the secondary side of the rectifier transformer respectively.

[0162] The first converter and the second converter are back-to-back converters, the star winding of the back-to-back converter is connected to the tap of the star winding on the secondary side of the rectifier transformer, and the delta winding of the back-to-back converter is connected to the tap of the delta winding on the secondary side of the rectifier transformer. When the tap position determination unit determines the tap position according to the withstand voltage and current conditions of the back-to-back converter, it is also necessary to consider the matching conditions of the voltages on both sides of the back-to-back converter.

[0163] What is disclosed above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of changes or modifications within the technical scope disclosed in the present invention, which should be covered within the protection scope of the present invention.

Claims

1. A filtering method for a twelve-pulse rectifier transformer, characterized in that: The following steps are involved: Step 1: Collect the load current and load voltage on the secondary side of the twelve-pulse rectifier transformer, and detect the harmonic current component in the load current; Step 2: Determine the tap position of the secondary winding of the twelve-pulse rectifier transformer according to the withstand voltage and current conditions of the converter device, so as to optimize the capacity of the converter device; Step 3: Calculate the compensating current component to be injected by the converter according to the harmonic current component in step 1 and the tap position in step 2; Step 4: According to the compensating current component to be injected in step 3, the compensating current component is injected into the tap of each secondary winding through the current conversion device to achieve the elimination of harmonic currents of different specified frequencies on the grid side of the twelve-pulse rectifier transformer.

2. The filtering method according to claim 1, characterized in that: In step 1, the instantaneous reactive power theory is used to detect the harmonic current component in the load current.

3. The filtering method according to claim 1, characterized in that: In step 2, the converter device is a back-to-back converter. When the tap position is determined according to the withstand voltage and current conditions of the back-to-back converter, the voltage matching conditions on both sides of the back-to-back converter must also be met.

4. The filtering method according to claim 3, characterized in that: The voltage matching condition on both sides of the back-to-back converter is: Among them, x is the ratio of the number of turns from the tap of the secondary side star winding of the rectifier transformer to the output terminal to the total number of turns of the star winding, and y is the ratio of the number of turns from the tap of the secondary side delta winding of the rectifier transformer to the output terminal to the total number of turns of the delta winding.

5. The filtering method according to claim 1, characterized in that: In step 3, the calculation expression of the compensation current component to be injected is: and Among them, i Ca1 、i Cb1 、i Cc1 are the compensation current components that the converter should inject into the A, B, and C phase taps of the star winding on the secondary side of the rectifier transformer, respectively. N1 and M1 are the coefficient matrices of the star winding on the secondary side of the rectifier transformer, i La1h 、i Lb1h i La1 、i Lb1 The harmonic current component, i La1 、i Lb1 are the load currents of phases A and B of the secondary star winding of the rectifier transformer, W A , W B are the turns of phases A and B of the primary winding of the rectifier transformer, W a1 , W b1 are the turns of phases A and B of the secondary star winding of the rectifier transformer, respectively; x is the ratio of the number of winding turns from the tap to the output terminal of the secondary star winding of the rectifier transformer to the total number of winding turns of the star winding; and Among them, i Ca2 、i Cb2 、i Cc2 are the compensation current components that the converter should inject into the A, B, and C phase taps of the triangle winding on the secondary side of the rectifier transformer, respectively. N2 and M2 are the coefficient matrices of the triangle winding on the secondary side of the rectifier transformer, i La2h 、i Lb2h i La2 、i Lb2 The harmonic current component, i La2 、i Lb2 are the load currents of phases A and B of the triangle winding on the secondary side of the rectifier transformer, W a2 , W b2 are the turns of phases A and B of the triangle winding on the secondary side of the rectifier transformer, respectively; y is the ratio of the number of turns from the tap to the output terminal of the triangle winding on the secondary side of the rectifier transformer to the total number of turns of the triangle winding.

6. The filtering method according to claim 1, characterized in that: In step 4, the specific operation of injecting the compensation current component is: Step 4.1: The DC capacitor voltage of the converter device is passed through a PI controller to obtain an active d-axis current component; Step 4.2: superimposing the compensation current component to be injected with the active d-axis current component to obtain an ideal compensation current component; Step 4.3: taking the difference between the ideal compensation current component and the actual injected compensation current component as the input of the PR controller to obtain the output of the PR controller; Step 4.4: Control the switch tube of the converter device according to the output of the PR controller in step 4.3 to inject the tap compensation current component of the secondary winding.

7. The filtering method according to claim 6, characterized in that: The frequency domain transfer function expression of the PR controller is: Among them, K p is the proportional adjustment coefficient, K i is the integral adjustment coefficient, K Ih is the resonance coefficient, h is the resonance number, ω1 is the fundamental frequency angular frequency, ω PRh is the resonant frequency.

8. A filtering device for a twelve-pulse rectifier transformer, characterized in that: include: A first current conversion device connected to the tap of the secondary star winding of the rectifier transformer, the first current conversion device being used to inject a first compensation current component into the tap of the secondary star winding of the rectifier transformer according to a first control instruction; A second current conversion device connected to the secondary-side delta winding tap of the rectifier transformer, the second current conversion device being used to inject a second compensation current component into the secondary-side delta winding tap of the rectifier transformer according to a second control instruction; The current and voltage acquisition unit is used to acquire the load current and load voltage of the star winding on the secondary side of the rectifier transformer, and the load current and load voltage of the delta winding; and to acquire the DC capacitor voltage of the first converter device and the DC capacitor voltage of the second converter device; and collecting a first actual injected compensating current component and a second actual injected compensating current component respectively injected into the star winding tap and the delta winding tap by the first current conversion device and the second current conversion device; A harmonic current component detection unit, used for obtaining a first harmonic current component of a star winding and a second harmonic current component of a delta winding on the secondary side of a rectifier transformer according to the load current and the load voltage; A tap position determination unit is used to determine the ratio of the number of winding turns from the tap of the secondary side star winding of the rectifier transformer to the output terminal to the total number of winding turns of the star winding, and the ratio of the number of winding turns from the tap of the secondary side delta winding of the rectifier transformer to the output terminal to the total number of winding turns of the delta winding according to the withstand voltage and current conditions of the first converter and the second converter, so as to obtain the tap positions of the secondary side star winding and the delta winding; A compensation current component calculation unit to be injected is used to calculate the first compensation current component and the second compensation current component to be injected into the secondary side star winding tap and the delta winding tap respectively according to the first harmonic current component, the second harmonic current component and the tap position; A PI control unit, used to obtain active d-axis current components of the first current conversion device and the second current conversion device according to the DC capacitor voltages of the first current conversion device and the second current conversion device respectively; A comparison unit, configured to superimpose the active d-axis current component of the first converter device and the first compensation current component to obtain a first ideal compensation current component, and to superimpose the active d-axis current component of the second converter device and the second compensation current component to obtain a second ideal compensation current component; and used to obtain a first compensation difference by subtracting the first actual injection compensation current component from the first ideal compensation current component, and obtain a second compensation difference by subtracting the second actual injection compensation current component from the second ideal compensation current component; A PR control unit, configured to obtain a first output control amount and a second output control amount according to the first compensation difference and the second compensation difference, respectively; A control unit is used to generate a first control instruction and a second control instruction according to a first output control quantity and a second output control quantity, respectively, so as to control the first converter and the second converter to respectively inject a compensating current component into the star winding tap and the delta winding tap on the secondary side of the rectifier transformer.

9. The filtering device according to claim 8, characterized in that The first converter and the second converter are a back-to-back converter, the star winding of the back-to-back converter is connected to the star winding tap of the secondary side of the rectifier transformer, and the delta winding of the back-to-back converter is connected to the delta winding tap of the secondary side of the rectifier transformer.

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