A method and system for active magnetic biasing of a hybrid distribution transformer

Abstract

The application also discloses a hybrid power distribution transformer magnetic bias active suppression method and system. Two sets of HDT control strategies are designed according to the magnetic bias suppression mechanism before and after no-load closing, the HDT is judged to be closed based on the network side current, and the smooth switching of the two sets of control strategies is realized accordingly; the excitation currents of the HDT main transformer and the isolation transformer are calculated based on the magnetic potential balance principle, and the nonlinear feedback of the excitation currents of the main transformer and the isolation transformer is carried out, so as to realize the magnetic bias suppression on the basis of ensuring the HDT network side current and the load voltage control function. The system does not need to add complex and expensive core magnetic flux sensors, and does not need to use a complex magnetic flux observation algorithm, and can eliminate the DC magnetic bias of the HDT under any working condition without affecting the basic control function of the HDT.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of transformers, and particularly relates to a hybrid distribution transformer magnetic bias active suppression method and system. BACKGROUND

[0002] A hybrid distribution transformer (HDT) is a new type of distribution transformer, and its basic feature is to integrate a traditional power frequency transformer and a back-to-back voltage source converter. Among many configuration schemes, a converter high-low cross connection type HDT constructed by means of an isolation transformer and a three-winding main transformer has outstanding advantages. Specifically, in this HDT scheme, the transformer ratio selection range is large, the circuit parameter selection range is wide, and the device selection is flexible and convenient. However, in actual operation, the cores of the two transformers of this HDT will inevitably be subjected to DC magnetic bias; when the magnetic bias is relatively serious, it will often lead to magnetic saturation; and after the core is saturated, the excitation inductance will be sharply reduced, thereby causing a large excitation inrush current and further causing the HDT to be unable to operate normally.

[0003] Because the closing time of the traditional mechanical circuit breaker is very dispersed, the residual magnetism of the core is difficult to accurately measure; thus, no-load closing cannot be performed at the theoretically expected best time, which leads to the fact that the magnetic bias of the HDT main transformer cannot be avoided and the excitation inrush current cannot be eliminated. In the distribution network, although the relay protection device can withstand a certain range of excitation inrush current without causing protection misoperation, the excitation inrush current contains a large amount of harmonic and DC components, and because the number of distribution transformers is large and the overall closing frequency is high, the pollution brought to the entire distribution network system cannot be ignored.

[0004] For the isolation transformer of the HDT, the terminal voltage is the compensation voltage, and the polarity and amplitude of the compensation voltage will change with the fluctuation of the grid voltage, which makes the core of the isolation transformer more prone to DC magnetic bias, thereby causing the excitation inrush current. If no measures are taken, only the core cross section of the isolation transformer can be as large as possible to withstand the DC magnetic bias, but this will inevitably waste a large amount of copper and iron materials.

[0005] For the above problems, the existing HDT control strategy rarely involves the magnetic saturation problem caused by the DC magnetic bias, which becomes a bottleneck problem that hinders the normal grid-connected operation of the HDT. In the HDT, two transformers are connected to respective converters in a back-to-back manner, one of which is called a current compensation converter, which is connected to the control winding of the main transformer and is mainly used to eliminate the harmful currents such as harmonics, reactive power, and asymmetry in the load current to the harm to the distribution network. The other is called a voltage compensation converter, which is connected to the isolation transformer through the valve side winding and is mainly used to eliminate the adverse effects of voltage fluctuation and asymmetry in the distribution network on power supply to the load.

[0006] In fact, the control function of the two converters of the HDT is far more than the above-mentioned voltage and current regulation. By adding a new control strategy, it is expected to further regulate the core flux of the two transformers. The existing research gives a bias magnetic suppression strategy based on flux linkage tracking. However, the flux linkage tracking must feedback the core flux linkage, and the DC component of the flux linkage cannot be accurately and quickly obtained by observation methods such as end voltage integration. Although the core flux sensor can be installed in each core column to obtain the core flux linkage, the core flux sensor is expensive and difficult to install in the transformer core. In addition, the newly added flux sensor needs to be matched with the corresponding A / D conditioning circuit and control circuit acquisition interface. Therefore, the addition of flux measurement feedback will greatly increase the manufacturing difficulty of the HDT, thereby greatly reducing the cost performance of the HDT. SUMMARY

[0007] The purpose of the present application is to overcome the above-mentioned shortcomings of the prior art, and to provide a hybrid distribution transformer bias magnetic active suppression method and system to solve the problem of lack of suppression measures for the bias magnetic field of the main transformer of the hybrid distribution transformer HDT. The method of the present application can pre-establish a steady-state magnetic flux before closing, and realize bias magnetic suppression through superimposed excitation current nonlinear feedback after closing, so that it is not necessary to install a flux linkage sensor, and the DC bias magnetic field can be suppressed without affecting the original control function of the HDT, thereby effectively overcoming the bottleneck problem of HDT magnetic saturation.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0009] A hybrid distribution transformer bias magnetic active suppression method, comprising the following steps:

[0010] Step 1: determining whether the HDT is closed based on the grid-side current, if not, executing step 2; if yes, executing step 3;

[0011] Step 2: controlling the HDT current compensation converter through the main transformer pre-magnetization strategy and the main transformer excitation current superimposed feedback strategy, and controlling the HDT voltage compensation converter through the isolation transformer pre-magnetization strategy and the main transformer and isolation transformer excitation current superimposed feedback strategy;

[0012] The output of the HDT current compensation converter stabilizes the load voltage, and limits the zero sequence circulating current inside the grid-side winding when the connection is in a delta form; the output of the HDT voltage compensation converter compensates the grid-side voltage fluctuation;

[0013] Step 3: controlling the HDT current compensation converter through the grid-side current control strategy and the main transformer excitation current superimposed feedback strategy, and controlling the HDT voltage compensation converter through the load voltage control strategy and the main transformer and isolation transformer excitation current superimposed feedback strategy;

[0014] The output of the HDT current compensation converter makes the grid-side current sinusoidal symmetrical, and the output of the HDT voltage compensation converter stabilizes the load voltage.

[0015] Further improvement of the present application is that:

[0016] Preferably, in step 1, the grid-side current of the main transformer is measured, the sum of the three-phase absolute values of the grid-side current is obtained through the grid-side current, when the sum is greater than a set threshold value, the HDT is in the closed state, and when the sum is less than 0, the HDT is in the open state.

[0017] Preferably, in step 2, the pre-magnetization strategy of the main transformer is that the d-axis load voltage, the q-axis load voltage, the 0-axis load voltage of the secondary winding and the 0-axis component of the grid-side current are input into the synchronous rotating coordinate, and the deviations are obtained after being subtracted from the respective reference signals, and the respective deviations are input into the PI controllers of the respective axes, and the outputs of the d-axis PI controller and the q-axis controller are taken as the d-axis and q-axis output instructions of the total controller; the sum of the output of the 0-axis PI controller of the load voltage and the output of the 0-axis PI controller of the grid-side current is taken as the 0-axis output instruction.

[0018] Preferably, in step 2, the pre-magnetization strategy of the isolation transformer is that the d-axis load voltage, the q-axis load voltage and the 0-axis load voltage of the grid-side winding are obtained, and the deviations are obtained after being subtracted from the respective reference signals, and the respective deviations are input into the PI controllers of the respective axes, and the output values of the PI controllers of the respective axes are input into the HDT voltage compensator.

[0019] Preferably, in step 3, the grid-side current control strategy is that the d-axis grid-side current, the q-axis grid-side current and the 0-axis grid-side current of the main transformer are obtained, the deviations are obtained after being subtracted from the respective reference signals, and the respective deviations are input into the PI controllers of the respective axes, and the outputs of the PI controllers of the respective axes are input into the HDT current compensation converter.

[0020] Preferably, in step 3, the load voltage control strategy is that the d-axis load voltage, the q-axis load voltage and the 0-axis load voltage of the secondary winding are input into the synchronous rotating coordinate, and the deviations are obtained after being subtracted from the respective reference signals, and the respective deviations are input into the PI controllers of the respective axes, and the output values of the PI controllers of the respective axes are input into the HDT voltage compensator.

[0021] Preferably, in steps 2 and 3, the main transformer excitation current superimposed feedback strategy is that the excitation currents of the respective phases of the main transformer are subjected to sign root operation, the respective root operation results are multiplied by feedback coefficients, and each product is subtracted from the original modulation signal of the respective phase to obtain the output value of the HDT current compensation converter.

[0022] Preferably, in step 2 and step 3, the main transformer and the isolation transformer excitation current superposition feedback strategy is: obtaining the main transformer and the isolation transformer excitation current of each phase, multiplying the main transformer excitation current of each phase with the feedback coefficient K im1t , multiplying the isolation transformer excitation current of each phase with the feedback coefficient K im2 , obtaining the final modulation signal of each phase by superimposing the two roots with the original modulation signal of each phase respectively, and inputting the final modulation signal into the HDT voltage compensation transformer.

[0023] Preferably, in step 2, the feedback coefficient K im1t is approximately 0; in step 3, the feedback coefficient K im1t is approximately 0.

[0024] A hybrid distribution transformer magnetic active suppression system comprises:

[0025] A judging module is used for judging whether the HDT is closed based on the grid-side current, if not, implementing the non-closed module; if yes, implementing the closed module;

[0026] The non-closed module is used for controlling the HDT current compensation transformer through the main transformer pre-magnetization strategy and the main transformer excitation current superposition feedback strategy, and controlling the HDT voltage compensation transformer through the isolation transformer pre-magnetization strategy and the main transformer and the isolation transformer excitation current superposition feedback strategy;

[0027] The output of the HDT current compensation transformer is used for stabilizing the load voltage and limiting the grid-side zero sequence current; the output of the HDT voltage compensation transformer is used for compensating the grid-side voltage fluctuation;

[0028] The closed module is used for controlling the HDT current compensation transformer through the grid-side current control strategy and the main transformer excitation current superposition feedback strategy, and controlling the HDT voltage compensation transformer through the load voltage control strategy and the main transformer and the isolation transformer excitation current superposition feedback strategy;

[0029] The output of the HDT current compensation transformer is used for making the grid-side current sinusoidal and symmetrical, and the output of the HDT voltage compensation transformer is used for stabilizing the load voltage.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] The application discloses a hybrid distribution transformer magnetic bias active suppression method, which studies an HDT magnetic bias suppression method based on working condition detection and excitation current feedforward on the basis of existing equipment. The method indirectly calculates excitation currents of an HDT main transformer and an isolation transformer according to a magnetic potential balance principle by detecting currents of a primary winding, a secondary winding, a control winding and a valve side winding of the HDT. Different HDT control strategies suitable for different working conditions are constructed according to different magnetic bias suppression mechanisms before and after no-load closing, and the HDT control strategies can be smoothly switched. On this basis, a proportional controller is used to perform nonlinear feedback on the excitation currents of the main transformer and the isolation transformer, and the excitation currents are superimposed on control instructions of an original current and a voltage compensation converter. Then, the DC magnetic bias of the HDT is suppressed on the basis of realizing HDT grid current and load voltage control, so that the harm of magnetic saturation and excitation inrush current to the HDT is eliminated. The purpose of the active control of the core magnetic chain of the HDT is to suppress magnetic saturation, so that it is no longer necessary to make the core magnetic chain track the reference signal in real time and accurately. For the transformer, the excitation current is close to 0 before the core magnetic flux is saturated, and the excitation current sharply increases only when the core magnetic flux exceeds the saturation point and enters the saturation section. Therefore, the DC magnetic bias can be limited by feeding back the excitation current. Compared with the core magnetic flux, the excitation current of the application can be approximately calculated based on the magnetic potential balance principle from the winding currents, and the winding currents originally need to be measured when improving the comprehensive performance of the HDT voltage and current control system. Therefore, the excitation current feedback does not need to increase the investment of new sensors and conditioning circuits, is simple to realize, and is more practical.

[0032] The application further discloses a hybrid distribution transformer magnetic bias suppression system, which designs two sets of HDT control strategies according to the magnetic bias suppression mechanisms before and after no-load closing, judges whether the HDT is closed based on grid side current, and realizes smooth switching of the two sets of control strategies. The excitation currents of the main transformer and the isolation transformer are calculated based on the magnetic potential balance principle, nonlinear feedback is performed on the excitation currents of the main transformer and the isolation transformer, and the excitation currents are superimposed on original control instructions of the HDT converter, so that the magnetic bias suppression of the HDT is realized on the basis of ensuring the HDT grid side current and load voltage control function. The system does not need to add complex and expensive core flux sensors, and does not need to use a complex flux observation algorithm, and can eliminate the DC magnetic bias of the HDT under any working condition without affecting the basic control function of the HDT. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 HDT main circuit configuration scheme.

[0034] Figure 2 Synchronous signal generation and required coordinate transformation voltage and current.

[0035] Figure 3 Current compensation converter control strategy.

[0036] Figure 4 HDT DC bus voltage outer loop control strategy.

[0037] Figure 5 Voltage compensation converter control strategy. DETAILED DESCRIPTION

[0038] The application will be described in further detail below with reference to the drawings:

[0039] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application; the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance; in addition, unless otherwise explicitly specified and limited, the terms "mounting", "connection" and "connection" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection; it can be directly connected, or indirectly connected through an intermediate medium; it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0040] One of the embodiments of the present application discloses a hybrid distribution transformer magnetic bias active suppression method based on working condition switching and excitation current superposition feedback, two sets of HDT control strategies are designed according to the magnetic bias suppression mechanism before and after no-load closing, whether the HDT is closed is judged based on the network side current, and the smooth switching of the two control strategies is realized; the excitation currents of the HDT main transformer and the isolation transformer are calculated based on the magnetic potential balance principle, and the nonlinear feedback of the excitation currents of the main transformer and the isolation transformer is carried out, which is superimposed on the original control instruction of the HDT converter, so as to realize the magnetic bias suppression on the basis of ensuring the HDT network side current and load voltage control function. Specifically, the method comprises the following steps:

[0041] Step 1, judging whether the HDT is closed based on the network side current.

[0042] By measuring the network side current of the main transformer, the three-phase absolute sum of the network side current can be obtained, when the three-phase absolute sum of the current is greater than the set threshold value (the threshold value is a very small threshold value slightly greater than 0), it is determined that the network side circuit breaker is in the closed state, and when the three-phase absolute sum of the current is less than 0, the network side circuit breaker is in the open state.

[0043] Step 2, if HDT is not closed, before closing. The HDT current compensation converter is used to stabilize the load voltage and limit the zero sequence circulating current in the network side winding of the delta connection, and the HDT voltage compensation converter is used to compensate the network voltage fluctuation. The main transformer pre-magnetization strategy CS2 and the main transformer excitation current superimposed feedback strategy CS3 control the current compensation converter CV p At the same time, the isolation transformer pre-magnetization strategy CS5 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV t At the same time, the isolation transformer pre-magnetization strategy CS5 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV t At the same time, the isolation transformer pre-magnetization strategy CS5 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV

[0044] Specifically, the main transformer pre-magnetization strategy CS2 is that, for the current compensation converter, the d / q / 0-axis load voltage and the 0-axis network side current are fed back in the synchronous rotating coordinate, based on which, the output of the load voltage PI controller is taken as the d / q-axis output instruction, and the sum of the output of the load voltage 0-axis PI controller and the output of the network side current 0-axis PI controller is taken as the 0-axis output instruction; accordingly, on the one hand, the load voltage zero sequence is stabilized, and on the other hand, the network side winding zero sequence circulating current is avoided from being too large; the controller output is applied to the HDT current compensation converter, combined with the nonlinear feedback of the main transformer excitation current, so as to establish a stable power grid synchronous magnetic chain for the HDT main transformer before closing, and this process is the main transformer excitation current superimposed feedback strategy CS3. For the voltage compensation converter, the isolation transformer pre-magnetization strategy CS5 is that the d / q / 0-axis output voltage of the isolation transformer is fed back in the synchronous rotating coordinate, and the output of the PI controller thereof is applied to the HDT voltage compensation converter, combined with the main transformer and isolation transformer excitation current superimposed feedback strategy CS6, i.e. the nonlinear feedback of the main transformer and isolation transformer excitation current, so as to establish a stable magnetic chain for the HDT isolation transformer before closing.

[0045] Wherein, the 0-axis output instruction is equal to the sum of the output of the load voltage 0-axis PI controller and the output of the network side current 0-axis PI controller. This setting mode stabilizes the load voltage zero sequence on the one hand, and avoids the network side winding zero sequence circulating current from being too large on the other hand.

[0046] Step 3, if HDT is closed, after no-load closing, the HDT current compensation converter is used to realize the sinusoidal symmetric control of the network side current, and the HDT voltage compensation converter is used to stabilize the load voltage. The network side current control strategy CS1 and the main transformer excitation current superimposed feedback strategy CS3 control the current compensation converter CV p At the same time, the load voltage control strategy CS4 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV t At the same time, the load voltage control strategy CS4 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV t At the same time, the load voltage control strategy CS4 and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV

[0047] For the current compensation converter, the grid-side current control strategy CS1 is to superimpose the main transformer excitation current feedback item after each phase original control instruction in the stationary coordinate, specifically, to feed back the d / q / 0-axis grid-side current, and the PI controller output directly acts on the HDT current compensation converter, combined with the variable excitation current superimposed feedback strategy CS3, i.e. the main transformer excitation current nonlinear feedback, thereby realizing the grid-side current sinusoidal symmetric control after closing and overcoming the main transformer DC bias magnetization. For the voltage compensation converter of the HDT, the load voltage control strategy CS4 is to superimpose the main transformer excitation current and the isolation transformer excitation current feedback item after each phase original control instruction; specifically, to feed back the d / q / 0-axis load voltage, and the PI controller output acts on the HDT voltage compensation converter, and the main transformer and isolation transformer excitation current superimposed feedback strategy CS6 is combined with the main transformer and isolation transformer excitation current nonlinear feedback, thereby stabilizing the HDT load voltage after closing and avoiding the HDT isolation transformer from being magnetically saturated.

[0048] Preferably, the main transformer per-phase excitation current is calculated according to the magnetic potential balance principle from the primary winding, secondary winding and control winding currents of the main transformer, and the isolation transformer per-phase excitation current is calculated according to the magnetic potential balance principle from the valve-side winding and grid-side winding currents of the isolation transformer.

[0049] Specifically, to realize the main transformer and isolation transformer excitation current nonlinear superimposed feedback, first, the sign square root of the excitation current is calculated, then multiplied by the proportional coefficient, and finally superimposed into each phase original control instruction of the HDT. For the current compensation converter of the HDT, the main transformer excitation current feedback item needs to be superimposed after each phase original control instruction in the stationary coordinate; for the voltage compensation converter of the HDT, the main transformer excitation current and the isolation transformer excitation current feedback item needs to be superimposed after each phase original control instruction. The main transformer per-phase excitation current is calculated according to the magnetic potential balance principle from the primary winding, secondary winding and control winding currents of the main transformer, and the isolation transformer per-phase excitation current is calculated according to the magnetic potential balance principle from the valve-side winding and grid-side winding currents of the isolation transformer.

[0050] The following further illustrates in combination with specific embodiments:

[0051] Embodiment

[0052] Reference Figure 1 In this embodiment, the main circuit configuration scheme of the hybrid distribution transformer is as shown in Figure 1 The device includes a main transformer T m , an isolation transformer T se , a voltage compensation converter CV t , and a current compensation converter CV p . The main transformer T m includes a primary winding (W1a ,W 1b ,W 1c ), secondary winding (W 2a ,W 2b ,W 2c ), control winding (W 3a ,W 3b ,W 3c ), isolation transformer T se Including grid side winding (W 5a ,W 5b ,W 5c ), valve side winding (W 4a ,W 4b ,W 4c ). Primary winding (W 1a ,W 1b ,W 1c ) and grid-side winding (W 5a ,W 5b ,W 5c ) are connected in series and then connected in a triangle through the grid side circuit breaker (S A ,S B ,S C ) is connected to the 10kV side of the distribution network. Secondary winding (W 2a ,W 2b ,W 2c ) The output side is connected to the RC filter, and the star neutral point is connected to supply power to the load, and the control winding (W 3a ,W 3b ,W 3c ) and valve side winding (W 4a ,W 4b ,W 4c ) Connect to CV according to the star neutral line lead-out method p and CV t .

[0053] In order to achieve magnetic bias suppression, HDT needs to install 16 current sensors to collect Figure 1 Main transformer T m The grid-side current (i Psa ,i Psb ,i Psc ), secondary winding current (i 2a ,i 2b ,i 2c ), control winding current (i 3a ,i 3b ,i 3c ) and isolation transformer T se The valve side winding current (i 4a ,i 4b ,i 4c ). It is also necessary to install 14 voltage sensors to collectFigure 1 The grid side voltage of HDT (u sa ,u sb ,u sc )、Load voltage(u 2a ,u 2b ,u 2c ), grid-side winding voltage (u 5a ,u 5b ,u 5c ) and DC bus capacitor voltage (u Ds ,u Dx ).

[0054] In this embodiment, the method for actively suppressing magnetic bias in the hybrid distribution transformer includes the following steps:

[0055] Step 1: Determine whether the HDT is closed based on the grid-side current.

[0056] In this step, the main transformer T can be obtained from the measured currents of each winding. m The excitation current (i m1a ,i m1b ,i m1c ) and isolation transformer T se The excitation current (i m2a ,i m2b ,i m2c ), as shown in formula (1).

[0057]

[0058] Where K 13 For the primary winding (W 1a ,W 1b ,W 1c ) and control winding (W 3a ,W 3b ,W 3c ) ratio, K 23 For the secondary winding (W 2a ,W 2b ,W 2c ) and control winding (W 3a ,W 3b ,W 3c ) ratio. K 54 For the grid side winding (W 5a ,W 5b ,W 5c ) and valve side winding (W 4a ,W 4b ,W 4c ) ratio.

[0059] For judgment Figure 1 The grid side circuit breaker (SA ,S B ,S C ) is closed, by measuring the main transformer T m The grid-side current (i Psa ,i Psb ,i Psc ), combined with formula (2), the sum of the absolute values ​​of the three-phase line currents on the grid side is obtained and recorded as i sm .

[0060] i sm =|i Psa -i Psc |+|i Psb -i Psa |+|i Psc -i Psb | (2)

[0061] Obviously, when i sm >i syz (i syz is a very small threshold slightly greater than 0), it can be determined Figure 1 Grid side circuit breaker (S A ,S B ,S C ) is in the closed state, and when i sm <0, it can be determined Figure 1 Grid side circuit breaker (S A ,S B ,S C ) is in disconnected state.

[0062] The bias magnetic suppression strategy based on working mode switching and excitation current superposition feedback of the present invention can be found in Figure 2 、 Figure 3 、 Figure 4 and Figure 5 . Figure 2 、 Figure 3 and Figure 5 The required coordinate transformation matrix is ​​shown in equations (3) and (4). Figure 2 The synchronous phase signal θ is used by the grid side voltage (u sa ,u sb ,u sc ) is phase-locked.

[0063]

[0064]

[0065] See also Figure 3 and Figure 5 It can be seen that the HDT bias suppression strategy based on industrial control switching and excitation current superposition feedback of the present invention is respectively aimed at the current compensation converter CVp With voltage compensation converter CV t .

[0066] Step 2, if HDT is not closed, before closing, use HDT current compensation converter to stabilize load voltage and limit the network side zero sequence current, use HDT voltage compensation converter to compensate the network side voltage fluctuation. The specific operation is: then S CVp With S CVt are connected to their contacts ②, main transformer pre-magnetization strategy CS2 and main transformer excitation current superposition feedback strategy CS3 control current compensation converter CV p implementation control; at the same time, isolation transformer pre-magnetization strategy CS5 and main transformer and isolation transformer excitation current superposition feedback strategy CS6 control voltage compensation converter CV t implementation control. At this time, CV p and CV t Before no-load closing, pre-T m and T se Establish a steady-state flux linkage synchronized with the grid voltage.

[0067] The implementation principle of the strategy is: if T m and T se The excitation current is 0, which means that T m and T se The magnetic bias is not serious, and both are not saturated, so the excitation current feedback loop described by CS3 and CS6 will not produce control effect, at this time only CS2 and CS5 produce control effect, T m and T se The pre-magnetization is normal. If the excitation current of T m and T se is significantly greater than 0, it means that the transformer has a large degree of magnetic bias and is obviously saturated. At this time, CS3 and CS6 will produce obvious control effect, and the control signals produced by the two will be superimposed in the original modulation signal (m pa0 , m pb0 , m pc0 ) and (m ta0 , m tb0 , m tc0 ), so as to actively correct the flux linkage of T m and T se , so as to ensure that the magnetic bias of T m and T se is quickly reduced below the saturation point, thereby ensuring that the flux linkage of T m and T se is always a steady-state flux linkage synchronized with the grid.

[0068] Before no-load closing, CS6 (i m1a , i m1b , im1c ) on T m The magnetic bias suppression effect of T im1t is weak, and if the feedback coefficient K se is too large, it will worsen the pre-magnetization effect of T im1t Therefore, preferably, K CVt needs to be adjusted according to the state of S CVt , and when S im1t turns on the contact ②, K sm should be approximately 0.

[0069] 2) If it is determined through i CVp that HDT is in the process of closing, the HDT current compensation converter is used to realize the sinusoidal symmetric control of the grid-side current, and the HDT voltage compensation converter is used to stabilize the load voltage. The specific operation is as follows: S CVt and S p both turn on their contact ①, the grid-side current control strategy CS1 and the main transformer excitation current superimposed feedback strategy CS3 control the current compensation converter CV t , and at the same time, the load voltage control strategy CS4 and the main transformer and the excitation current superimposed feedback strategy CS6 control the voltage compensation converter CV p . At this time, CV t completes the sinusoidal, unit power factor and symmetric control of the grid current, and CV m completes the symmetric and sinusoidal stabilization control of the load voltage. At the same time, the excitation current superimposed feedback can correct the core flux in real time, thereby eliminating the DC bias of T se and T m core.

[0070] The implementation principle of the strategy is as follows: if the excitation current of T se and T m is 0, it indicates that the bias of T se and T p is not serious, and both are not saturated, so the excitation current feedback loop of CS3 and CS6 will not produce control effect, at this time, only CS1 and CS4 control CV t and CV m respectively, and the grid-side current of HDT after closing is always controlled to be a sinusoidal symmetric wave, and the load voltage is controlled to be a stable and symmetric sinusoidal wave. If the excitation current of T se and T m is obviously greater than 0, it indicates that the transformer is biased and saturated. At this time, CS3 and CS6 will produce obvious control effect, thereby actively correcting the flux of T se and T m to ensure that the bias of T se and T m is rapidly reduced below the saturation point, thereby ensuring that Tse It is not saturated, and the grid-side current and load voltage control functions of the HDT are not affected.

[0071] After no-load closing, CS3 switches to T m The bias suppression effect is very weak. If the feedback coefficient K im1 Too large will worsen the grid-side current control effect. Therefore, it is preferred that K im1 Still need to follow S CVp The state is adjusted when S CVp When contact ① is connected, K im1 It should be approximately taken as 0.

[0072] After no-load closing, it is actually CV t With CS6 se With T m At the same time, a bias magnetic suppression effect is produced. Specifically, when (i m1a ,i m1b ,i m1c ) is significantly greater than 0, CS6 will superimpose a control effect, thereby affecting T se The output voltage of the grid is sa ,u sb ,u sc ) cannot be changed actively, so through T se Can indirectly affect T m The terminal voltage of the core further affects the magnetic flux of the core, thereby eliminating T m DC bias magnetism. And when (i m2a ,i m2b ,i m2c ) is significantly greater than 0, CS6 will directly affect T se The magnetic flux linkage is reduced, thereby eliminating the bias magnetism.

[0073] Among them, for the current compensation converter CV p The control strategy includes the grid-side current control strategy CS1, the main transformer pre-magnetization strategy CS2, the closing condition judgment and logic switching switch S CVp And the main transformer excitation current superposition feedback strategy CS3.

[0074] The CS1 needs to feedback the d / q / 0 axis grid side current (i Psd ,i Psq ,i Ps0 ), and find (i Psd ,i Psq ,i Ps0 ) and their respective reference signals (i Psdref ,i Psqreg ,i Ps0refThe deviation is used as the input of each axis PI controller. The inverted output of each axis PI controller is used as the output of the next stage S CVp The reference signal of CS1 is i Psdref with i Ps0ref From Figure 4 The output of the HDT DC bus voltage outer loop control is shown in Figure 1. Psdref From the DC bus voltage stability control strategy CS uD , and the reference signal i Ps0ref The DC bus split capacitor voltage deviation suppression strategy CS uDd .

[0075] The CS2 needs to feedback the d / q / 0 axis load voltage (u 2d ,u 2q ,u 20 ) and the grid-side current 0-axis component i Ps0 , and find (u 2d ,u 2q ,u 20 ,i Ps0 ) and their respective reference signals (u 2dref ,u 2qreg ,u 20ref ,i Ps0ref The output of each axis PI controller is used as the input of the subsequent stage S CVp Input, which is the same as u 20 with i Ps0 The relevant PI controller outputs are summed and used as the 0-axis output.

[0076] According to the S CVp The state of each axis of CS1 and CS2 is used as the CV under the synchronous coordinate p The original modulated signal (m pd ,m pq ,m p0 ), for (m pd ,m pq ,m p0 ) to perform inverse coordinate transformation to generate CV p Each phase original modulation signal (m pa0 ,m pb0 ,m pc0 ).

[0077] The CS3 needs to feed back the excitation current of each phase of the main transformer (i m1a ,i m1b ,i m1c ), for (i m1a ,i m1b ,im1c ) after symbol root extraction, and multiplying it by the feedback coefficient K im1 pa0 pb0 pc0 pa pb pc , control CVp.

[0078] The control strategy for the voltage compensation converter CV t includes a load voltage control strategy CS4, an isolation transformer pre-magnetization strategy CS5, a closing condition judgment and logic switch S CVt and a main transformer and isolation transformer excitation current superimposed feedback strategy CS6.

[0079] The CS4 needs to feed back d / q / 0-axis load voltages (u 2d , u 2q , u 20 ), and calculate the deviation of (u 2d , u 2q , u 20 ) from respective reference signals (u 2dref , u 2qref , u 20ref ), and take the deviation as the input of the respective axis PI controller. The output of the respective axis PI controller after inversion is taken as the input of the subsequent S CVt .

[0080] The CS5 needs to feed back d / q / 0-axis load voltages (u 5d , u 5q , u 50 ), and calculate the deviation of (u 5d , u 5q , u 50 ) from respective reference signals (u 5dref , u 5qref , u 50ref ), and take the deviation as the input of the respective axis PI controller. The output of the respective axis PI controller is taken as the input of the subsequent S CVt , and the CS5 reference signal (u 5dref , u 5qref , u 50ref ) is calculated by formula (5).

[0081]

[0082] In the formula, U p1N is the rated line voltage of the distribution network 10kV side.

[0083] According to the S​​​​​​CVt The state of each axis of CS4 and CS5 is used as the CV under the synchronous coordinate t The original modulated signal (m td ,m tq ,m t0 ), for (m td ,m tq ,m t0 ) to perform inverse coordinate transformation to generate CV t Each phase original modulation signal (m ta0 ,m tb0 ,m tc0 ).

[0084] The CS6 needs to feed back the excitation current of each phase of the main transformer and the isolation transformer (i m1a ,i m1b ,i m1c ) and (i m2a ,i m2b ,i m2c ). Right (i m1a ,i m1b ,i m1c ) and (i m2a ,i m2b ,i m2c ) multiplied by the feedback coefficient K im1t With K im2 , and then perform symbol rooting respectively, and then combine its output with the original modulation signal (m ta0 ,m tb0 ,m tc0 ) phases are superimposed to obtain the final modulation signal (m ta ,m tb ,m tc ).

[0085] according to Figure 3 and Figure 5 The control strategy shown can ultimately generate CV p and CV t The modulation signal (m pa ,m pb ,m pc ) and (m ta ,m tb ,m tc ), using bipolar SPWM to generate CV p and CV t The driving pulse (p p1 ,p p4 ,p p3 ,p p6 ,p p5 ,p p2 ) and (p t1 ,pt4 p t3 p t6 p t5 p t2 ). The current compensation converter CV p and the voltage compensation converter CV t execute the magnetic bias suppression strategy.

[0086] The above description is merely that of the preferred embodiments of the application and is not to be taken in a limiting sense but is made merely for the purpose of providing some preferred embodiments of the application. Any modification of the base scope of the application in light of the other approaches set forth in any attached claims and their equivalents is to be included within the scope of the application.

Claims

1. A method for actively suppressing magnetic bias in a hybrid distribution transformer, characterized in that: The following steps are involved: Step 1: Determine whether the HDT is closed based on the grid-side current. If not, proceed to step 2; if closed, proceed to step 3. Step 2: Control the HDT current compensation converter through the main transformer pre-magnetization strategy and the main transformer excitation current superposition feedback strategy, and control the HDT voltage compensation converter through the isolation transformer pre-magnetization strategy and the main transformer and isolation transformer excitation current superposition feedback strategy; The output of the HDT current compensation converter stabilizes the load voltage and limits the zero-sequence circulating current inside the grid-side winding when connected in a delta configuration. The output of the HDT voltage compensation converter compensates for grid-side voltage fluctuations. Step 3: Control the HDT current compensation converter through the grid-side current control strategy and the main transformer excitation current superposition feedback strategy, and control the HDT voltage compensation converter through the load voltage control strategy and the main transformer and isolation transformer excitation current superposition feedback strategy; The output of the HDT current compensation converter makes the grid-side current sinusoidally symmetrical, and the output of the HDT voltage compensation converter stabilizes the load voltage.

2. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 1, the grid-side current of the main transformer is measured, and the sum of the absolute values ​​of the three-phase grid-side current is obtained through the grid-side current. When the sum is greater than the set threshold, the HDT is in the closed state, and when the sum is less than 0, the HDT is in the open state.

3. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 2, the main transformer pre-magnetization strategy is: input the d-axis load voltage, q-axis load voltage, 0-axis load voltage and grid-side current 0-axis component of the secondary winding into the synchronous rotating coordinate, subtract them from their respective reference signals to obtain deviations, input each deviation into each axis PI controller, and use the outputs of the d-axis PI controller and the q-axis controller as the d-axis and q-axis output instructions of the overall controller; and use the sum of the output of the load voltage 0-axis PI controller and the output of the grid-side current 0-axis PI controller as the 0-axis output instruction.

4. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 2, the pre-magnetization strategy of the isolation transformer is: obtain the d-axis load voltage, q-axis load voltage and 0-axis load voltage of the grid-side winding, subtract them from their respective reference signals to obtain deviations, input each deviation into each axis PI controller, and input the output value of each axis PI controller into the HDT voltage compensation converter.

5. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 3, the grid-side current control strategy is: obtain the d-axis grid-side current, q-axis grid-side current and 0-axis grid-side current of the main transformer, subtract them from their respective reference signals to obtain deviations, input each deviation into each axis PI controller, and input the output of each axis PI controller into the HDT current compensation converter.

6. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 3, the load voltage control strategy is as follows: input the d-axis load voltage, q-axis load voltage and 0-axis load voltage of the secondary winding in the synchronous rotating coordinates, subtract them from their respective reference signals to obtain deviations, input each deviation into the PI controller of each axis, and input the output value of the PI controller of each axis into the HDT voltage compensation converter.

7. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In steps 2 and 3, the main transformer excitation current superposition feedback strategy is: after taking the sign root of the excitation current of each phase of the main transformer, multiply the root results by the feedback coefficient, and subtract each product from the original modulation signal of each phase to obtain the output value of the HDT current compensation converter.

8. The method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 1, characterized in that: In step 2 and step 3, the main transformer and isolation transformer excitation current superposition feedback strategy is: obtain the main transformer excitation current of each phase and the isolation transformer excitation current of each phase, and compare the main transformer excitation current of each phase with the feedback coefficient K im1t Multiply the excitation current of each phase of the isolation transformer and the feedback coefficient K im2 Multiply; after taking the roots of the two products respectively, the two root values ​​are superimposed with the original modulation signals of each phase to obtain the final modulation signals of each phase, and the final modulation signals are input to the HDT voltage compensation converter.

9. A method for actively suppressing magnetic bias in a hybrid distribution transformer according to claim 7 or 8, characterized in that: In step 2, the feedback coefficient K im1t Approximately take it as 0; in step 3, the feedback coefficient K im1t Approximately taken as 0.

10. A hybrid distribution transformer active bias suppression system, characterized in that: include: A judgment module is used to judge whether the HDT is closed based on the grid-side current. If not, the unclosed module is implemented; If closing, implement the closing module; The unclosed module is used to control the HDT current compensation converter through the main transformer pre-magnetization strategy and the main transformer excitation current superposition feedback strategy, and to control the HDT voltage compensation converter through the isolation transformer pre-magnetization strategy and the main transformer and isolation transformer excitation current superposition feedback strategy; The output of the HDT current compensation converter stabilizes the load voltage and limits the grid-side zero-sequence current; the output of the HDT voltage compensation converter compensates for grid-side voltage fluctuations; The closing module is used to control the HDT current compensation converter through the grid-side current control strategy and the main transformer excitation current superposition feedback strategy, and to control the HDT voltage compensation converter through the load voltage control strategy and the main transformer and isolation transformer excitation current superposition feedback strategy; The output of the HDT current compensation converter makes the grid-side current sinusoidally symmetrical, and the output of the HDT voltage compensation converter stabilizes the load voltage.

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