A method for suppressing magnetizing inrush current of a hybrid transformer and a hybrid transformer

Abstract

The present invention relates to a method for suppressing excitation inrush current of a hybrid transformer, comprising the following steps: step 1, detecting the voltage and current on the high-voltage input side of the transformer through a sensor, and obtaining the voltage phase on the high-voltage input side through a phase-locked loop; step 2, when the voltage fluctuation on the high-voltage input side causes magnetic circuit saturation and causes excitation inrush current, the current signal on the high-voltage input side is fed back to the second converter, and the second converter generates a compensation voltage adjustment instruction through a control loop; step 3, according to the compensation voltage adjustment instruction, the second converter outputs a series compensation voltage, controls the power flow and adjusts the power factor, and realizes excitation inrush current suppression. Compared with the prior art, the present invention has the advantages of adjusting the input voltage to keep the output voltage stable when the voltage of the power transformer drops or rises temporarily; ensuring that the inrush current is quickly attenuated during the excitation inrush current stage of the transformer, and ensuring the safe and reliable operation of the power transformer.

Description

Technical Field

[0001] The present invention relates to the field of electric power systems, and in particular to a method for suppressing magnetizing inrush current of a hybrid transformer and the hybrid transformer. Background Art

[0002] Power transformers are an important part of the power system, and their working status directly affects the quality of power. The access of a large number of complex sources and loads such as distributed power sources and power electronic equipment causes a series of problems such as voltage drops and harmonic currents during the operation of power transformers. Similarly, in scenarios such as transformer no-load closing, fault maintenance reclosing, and input voltage phase angle drift, the residual magnetism and transient voltage components of the transformer itself will cause the transformer core to saturate, generate large excitation currents, and endanger the stable operation of the transformer itself.

[0003] The transformer itself lacks effective adjustment means and cannot effectively deal with the problem of voltage instability under complex source and load access conditions; and the solutions for transformer excitation inrush current mainly include the control of the transformer closing angle and the identification of the transformer residual magnetism, but the above solutions lack feasibility. For example, in the prior art, the Chinese patent publication number CN115021234A discloses a method for suppressing transformer excitation inrush current under a power grid simulator. This method measures multiple physical quantities, including the real-time magnetic flux when the drop is completed, the maximum saturation magnetic flux of the transformer, and the voltage angular velocity and voltage effective value of the inverter command voltage. This method is difficult to implement in practice. Therefore, it is urgent to propose an effective transformer voltage stabilization scheme and an effective and feasible excitation saturation inrush current suppression scheme to ensure the safe and reliable operation of power transformers. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a method for suppressing the excitation inrush current of a hybrid transformer and a hybrid transformer.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] The present invention provides a method for suppressing excitation inrush current of a hybrid transformer, comprising the following steps:

[0007] Step 1: Detect the voltage and current on the high-voltage input side of the transformer through a sensor, and obtain the voltage phase on the high-voltage input side through a phase-locked loop;

[0008] Step 2: When the voltage fluctuation on the high-voltage input side causes the magnetic circuit to be saturated and causes the excitation inrush current, the current signal on the high-voltage input side is fed back to the second converter, and the second converter generates a compensation voltage adjustment instruction through a control loop;

[0009] Step 3: According to the compensation voltage adjustment instruction, the second converter outputs the series compensation voltage, controls the power flow, adjusts the power factor, and realizes the excitation inrush current suppression.

[0010] Furthermore, obtaining the voltage phase of the high-voltage input side specifically includes: inputting the voltage of the high-voltage input side into a generalized second-order integrator to construct a vertical component with a constant voltage amplitude and a phase angle lagging by 90 degrees; outputting the voltage amplitude and the vertical vector to an αβ to dq coordinate system converter; converting V q Input it into the PI controller and control it to zero to achieve phase angle tracking of the input voltage.

[0011] Furthermore, the second converter adopts a single closed-loop voltage control strategy, which specifically includes: feeding back the high-voltage input side current signal to the second converter to obtain an output current I FB , the output current I FB After proportional feedback and then passing through a low-pass filter, the second converter outputs a series compensation voltage reference value, which is added to the voltage reference value generated in the steady state as the output voltage reference value of the second converter in the excitation saturation stage. The voltage reference value is subtracted from the input voltage V2, and the difference is used as the input of the PI controller. After modulation by the PI controller, the PWM signal of the second converter is output.

[0012] Furthermore, the first converter is connected in parallel to the low-voltage output side of the transformer to obtain active and reactive power from the AC side, the AC voltage on the AC side is rectified to a DC voltage, and a stable DC side voltage is provided to the second converter through the first converter.

[0013] Furthermore, the first converter adopts a voltage and current dual-loop control strategy, including: collecting the low-voltage output side V1 and the current I sh , and phase-lock to obtain the voltage phase sin(wt1); collect the DC side voltage, and convert the DC side voltage V dc With the DC side voltage reference value V dcref The difference is used as the input of the PI controller. The input of the PI controller is multiplied by the voltage phase sin(wt1) to obtain the output current reference value of the first converter. The current reference value is multiplied by the current I sh The difference is input into the PI controller, and after being modulated by the PI controller, a PWM signal of the first converter is output.

[0014] Optionally, when the voltage on the high-voltage input side of the transformer is in a steady state, the high-voltage input side voltage is constructed as an orthogonal component through a generalized second-order integrator, and the voltage phase angle sin(wt2) is transformed by dq to obtain the input voltage amplitude; the voltage reference value is subtracted from the voltage amplitude, and multiplied with the voltage phase sin(wt2) after the limiting stage as the second converter output series compensation voltage reference value, the series compensation voltage reference value is subtracted from the input high-voltage input side voltage, the difference is used as the PI controller input, and after modulation by the PI controller, the PWM signal of the second converter is output.

[0015] In a second aspect, the present invention provides a hybrid transformer, comprising a single-phase transformer and a back-to-back converter, wherein the back-to-back converter comprises a first converter and a second converter, wherein the DC side of the first converter is connected to the DC side of the second converter, the input side of the first converter is connected in parallel to a low-voltage power supply side, and the AC output side of the second converter is connected in series to a high-voltage power supply side through a coupling transformer, and the hybrid transformer is used to implement any one of the above methods.

[0016] Furthermore, the first converter and the second converter adopt a full-bridge structure.

[0017] Furthermore, the voltage of the low-voltage power supply side is 400V.

[0018] Furthermore, the voltage of the high voltage power supply side is 10 kV.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The excitation saturation inrush current suppression method provided by the present invention can ensure that when the voltage of the power transformer drops or rises temporarily, the input voltage is adjusted to keep the output voltage stable; it can ensure that during the transformer excitation inrush current stage, the inrush current is rapidly attenuated to ensure the safe and reliable operation of the power transformer.

[0021] 2. The hybrid transformer structure of the present invention connects the converter to the power transformer through a coupling transformer, which can achieve relevant control functions while having the advantages of low voltage and current requirements for power electronic devices. On the basis of safe and reliable operation of the power transformer, it also has certain economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the method steps of the present invention;

[0023] Figure 2 This is the structural diagram of the hybrid transformer system;

[0024] Figure 3 The schematic diagram of the excitation saturation flow of the power transformer, where (a) is the magnetic circuit saturation process, and (b) is the inrush current process under the condition of magnetic circuit saturation;

[0025] Figure 4 It is a schematic diagram of the phase-locked loop structure and dq decomposition;

[0026] Figure 5 is a schematic diagram of the control strategy of the first converter;

[0027] Figure 6 is a schematic diagram of the control strategy of the second converter;

[0028] Figure 7 The simulation result of input voltage regulation on the 10kV power supply side in the embodiment of the present invention;

[0029] Figure 8 This is the simulation result of the excitation saturation inrush current suppression on the 10kV power supply side in the embodiment of the present invention;

[0030] Fig. 9 The simulation results of the output voltage and current of the first converter and the second converter in the embodiment of the present invention are shown in FIG. 1 , wherein (a) is the output current of the first converter, (b) is the DC side voltage, and (c) is the output voltage and current of the second converter. DETAILED DESCRIPTION

[0031] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0032] Example 1

[0033] The present invention proposes a hybrid transformer, such as Figure 2 As shown, it includes a single-phase transformer and a back-to-back converter. The single-phase transformer is connected to the high-voltage input side 10kV power supply side and the low-voltage output side 400V power supply side. The power electronic converter is a single-phase back-to-back converter, wherein the first converter input side is connected in parallel to the 400V power supply side, the second converter DC side is directly connected to the first converter DC side, and the second converter AC output side is connected in series to the 10kV power supply side through a coupling transformer.

[0034] Among them, the first converter and the second converter are adapted to single-phase scenarios and both adopt a full-bridge structure.

[0035] The hybrid transformer can suppress the excitation inrush current caused by input voltage variation.

[0036] Specifically, the first converter rectifier controls the input DC side reference voltage. The first converter control strategy includes a voltage outer loop and a current inner loop. The second converter inverts the output series compensation voltage, adjusts the input voltage and suppresses the inrush current. The second converter control strategy is a single closed-loop voltage control strategy. The second converter control strategy can be used to control the excitation inrush current after the magnetic circuit is saturated in the hybrid transformer.

[0037] Example 2

[0038] This embodiment provides a method for suppressing the excitation inrush current caused by the change of transformer input voltage. Figure 3 As shown, Figure 3 (a) is the magnetic circuit saturation process. As the transformer input voltage changes sinusoidally, the transformer core flux changes sinusoidally. When the transformer input voltage amplitude or phase angle changes, the flux will be biased, causing the transformer to change from operating in the linear region to operating in the nonlinear region at the peak of the flux. At this time, the flux changes little, the converter current increases greatly, and an excitation surge current appears.

[0039] Figure 3 (b) is the inrush current process under excitation saturation, the parameters are as follows Figure 3 As shown in (b), the voltage and current equation of the circuit is:

[0040]

[0041] Φ=Φ s +Φ σ +Φ m ,R=R s +R σ , L=L s +L σ +L m

[0042]

[0043] Rearranging the above equation:

[0044]

[0045]

[0046] Among them, U s is the input voltage, Φ s is the inductance flux corresponding to the line impedance, Φ σ is the transformer leakage flux, Φ m is the transformer magnetic flux, Φ is the total flux in the circuit, R is the total resistance in the circuit, R s is the line resistance, R σis the transformer resistance, L is the total inductance in the circuit, L σ ,L s ,L m They are transformer leakage inductance, line inductance, and magnetizing inductance Φ r is the magnetic flux of the magnetic circuit before the input voltage changes, U m is the amplitude of the input voltage before the change, is the equivalent impedance angle, ω is the input voltage angular velocity, U m0 is the amplitude of the input voltage after the change, α and α0 are the phase angles before and after the transformer input voltage changes, and the other parameters are as follows Figure 3 (b) as shown.

[0047] It can be seen from the above flux equation that when the voltage at the transformer input changes, the flux contains a steady-state component and an attenuation component related to the flux before the change. The amplitude of the attenuation component is related to the initial flux, and the attenuation speed is related to the line impedance characteristics. When the resistance is higher, the flux decays faster, and the transformer exits the nonlinear region faster accordingly.

[0048] Based on this principle, the method for suppressing the excitation inrush current provided in this embodiment is mainly implemented by two converters, such as Figure 2 As shown, the first converter and the second converter adopt the back-to-back converter form. The first converter is connected in parallel to the 400V power supply side of the transformer low-voltage output side, works in rectification mode, obtains active and reactive power from the AC side, and stabilizes the DC side voltage; the second converter is connected in series to the 10kV power supply side of the high-voltage input side, works in inverter mode, obtains stable DC voltage input through the DC side connected to the first converter, and outputs series compensation voltage according to the control scheme under different high-voltage input side working conditions, and adjusts the active and reactive power of the high-voltage input side to achieve the purpose of stabilizing the AC voltage of the high-voltage input side and suppressing the excitation inrush current.

[0049] Specifically, Figure 1 As shown, the method comprises the following steps:

[0050] Step 1: Detect the voltage and current on the high-voltage input side of the transformer through a sensor, and obtain the voltage phase on the high-voltage input side through a phase-locked loop.

[0051] Specifically, Figure 4 As shown in the figure, to obtain the phase angle of the single-phase sampled voltage, the sampled voltage is first input into the generalized second-order integrator (SOGI) to construct a vertical component with constant amplitude and 90-degree phase angle lag; secondly, the original sampled voltage and the constructed vertical vector are output to the αβ to dq coordinate system converter, and the transformation relationship is as follows:

[0052]

[0053] Among them, V dis the d-axis component in the synchronous rotating coordinate system (at this time, the rotating coordinate system is synchronized with the grid phase, and the d-axis is the voltage amplitude), V q is the q-axis component in the synchronously rotating coordinate system.

[0054] Finally, V q Input it into the PI controller and control it to zero, thereby achieving phase angle tracking of the input voltage.

[0055] Step 2: When the voltage fluctuation on the high-voltage input side causes the magnetic circuit to saturate and thus causes the excitation inrush current problem, the current signal I on the high-voltage input side is fed back. FB To the second converter, the second converter generates a compensation voltage adjustment instruction through a control loop.

[0056] Specifically, the normal operation of the second converter also requires the first converter to provide a stable DC voltage. Figure 5 As shown, the first converter control adopts the voltage and current dual-loop control strategy: collecting the 400V power supply side voltage V1 and current I sh , and phase-lock to obtain the voltage phase sin(wt1); collect the DC side voltage, and convert the DC side voltage V dc With the DC side voltage reference value V dcref The difference is used as the input of the PI controller. The input of the PI controller is multiplied by the voltage phase sin(wt1) to obtain the output current reference value of the first converter. The current reference value is multiplied by the current I sh The difference is input into the PI controller, and after being modulated by the PI controller, the PWM signal of the first converter is output. With the help of this control scheme, the DC side voltage can be controlled to be stable through the first converter, so that the second converter can output a stable and controllable series compensation voltage.

[0057] like Figure 6 As shown, the second converter control strategy is a single closed-loop voltage control strategy: collecting the 10kV side power supply voltage V2 and current I FB , and phase-locked to obtain the voltage phase sin(wt2).

[0058] In steady state, this embodiment is controlled by the second converter control strategy. First, the 10kV side voltage is constructed as an orthogonal component through a generalized second-order integrator, and the voltage phase angle sin(wt2) obtained by the phase-locked loop is transformed into a dq value to obtain the input voltage amplitude; the voltage reference value is subtracted from the voltage amplitude, and multiplied with the voltage phase sin(wt2) after the limiting stage as the output series compensation voltage reference value of the second converter, the reference value is subtracted from the input voltage V2, and the difference is used as the PI controller input, and after the PI controller is modulated, the output is the PWM signal of the second converter.

[0059] When the second converter outputs an instruction, the on and off of the converter switch tube must meet specific conditions. The PWM signal is the switch tube switch signal output by the control strategy according to the instruction value. The input of the control loop is the compensation voltage adjustment instruction, and the output is the modulation wave. The modulation wave is pulse width modulated (PWM) to obtain the PWM signal, which can control the switch tube to open and close after the drive.

[0060] In this embodiment, when the transformer excitation saturation occurs, the sampled second converter output current I FB After proportional feedback, the output voltage reference value of the second converter that needs to be adjusted is obtained after passing through a low-pass filter; the voltage reference value is added to the voltage reference value generated in the steady state as the output voltage reference value of the second converter in the excitation saturation stage, the voltage reference value is subtracted from the input voltage V2, the difference is used as the input of the PI controller, and after modulation by the PI controller, the PWM signal of the second converter is output.

[0061] Step 3: The second converter outputs a series compensation voltage according to the adjusted voltage command, adjusts the power factor by controlling the power flow, and thereby achieves inrush current suppression.

[0062] In order to verify the effectiveness of the method provided in this embodiment, this embodiment conducted a simulation experiment. In the simulation experiment, the DC side reference voltage V dcref The reference value of the input voltage on the power supply side is 2000V, 10kV ref The voltage of the transformer is 10kV, the load is 1000kVA, the rated capacity of the transformer is 1200kVA, the rated voltage of the primary side is 15kV, and the rated voltage of the secondary side is 500V. During the simulation, the voltage of the 10kV power supply side drops from 10kV to 9100V at 0.1s, the voltage regulation function of the second converter is put into operation at 0.15s, and the input voltage phase angle jumps at 0.3s, and the converter inrush current suppression function is put into operation. The simulation results are as follows: Figure 7 , Fig. 9 shown. Figure 8 For a separate simulation study, the current comparison simulation results of the 10kV power supply side with and without the converter-assisted inrush current suppression scheme after the phase angle jump at 1s are studied.

[0063] like Figure 7 As shown, at 0.1s, a voltage drop occurs, and the input voltage drops from 10kV to 9100V. At 0.15s, the voltage regulation function of the second converter is put into operation. At this time, the second converter tracks the input voltage, outputs the difference between the input voltage and the input voltage reference value, compensates for the voltage drop, and supports the input voltage on the 10kV power supply side from 9100V to 10kV.

[0064] like Figure 8As shown, at 1s, the input voltage phase angle of the 10kV power supply side jumps, and the transformer flux changes from attenuation to accumulation, and the transformer changes from operating in the linear region to operating in the nonlinear region. At this time, when the flux increases slightly, the current increases greatly, and the inrush current is more than 3 times the steady-state current. The excitation saturation is attenuated by the line resistance, and the attenuation time is long. When the second converter is introduced, the second converter detects the current on the 10kV power supply side and outputs a series compensation voltage with the same phase and opposite direction. This is equivalent to connecting a virtual resistor in series in the line, thereby consuming the energy stored in the excitation inductance, greatly reducing the attenuation time constant, and quickly attenuating the excitation inrush current. Figure 8 In the figure, the red line is the surge current without the converter, and the yellow line is the surge current when the converter is working. The introduction of the second converter reduces the surge current amplitude and quickly attenuates the surge current within 1 to 2 power frequency cycles.

[0065] like Fig. 9 As shown, Fig. 9 (a) is the output current of the first converter. After each output voltage action timing, after a phase-locked period, the converter outputs a current that is in phase or in the opposite direction of the parallel input voltage. At this time, the power factor of the first converter is 1, and only the active power is adjusted to stabilize the DC side capacitor voltage. Fig. 9 (b) is the DC side voltage. After each output voltage action, the DC side voltage is stabilized after a short adjustment. Fig. 9 (c) is the output voltage and current of the second converter. In the voltage regulation stage (0.15s), the second converter outputs an adjustable series compensation voltage to stabilize the input voltage. In the surge suppression stage (0.3s), the second converter outputs a larger voltage and current. At this time, the second converter quickly adjusts the active power to suppress the transformer excitation saturation surge current.

[0066] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for suppressing magnetizing inrush current of a hybrid transformer, characterized in that: The hybrid transformer includes a single-phase transformer and a back-to-back converter, the back-to-back converter includes a first converter and a second converter, the DC side of the first converter is connected to the DC side of the second converter, the input side of the first converter is connected in parallel to the low-voltage power supply side, and the AC output side of the second converter is connected in series to the high-voltage power supply side through a coupling transformer. The method includes the following steps: Step 1: Detect the voltage and current on the high-voltage input side of the transformer through a sensor, and obtain the voltage phase on the high-voltage input side through a phase-locked loop; Step 2: When the voltage fluctuation on the high-voltage input side causes the magnetic circuit to be saturated and causes the excitation inrush current, the current signal on the high-voltage input side is fed back to the second converter, and the second converter generates a compensation voltage adjustment instruction through a control loop; Step 3: According to the compensation voltage adjustment instruction, the second converter outputs the series compensation voltage, controls the power flow, adjusts the power factor, and realizes the excitation inrush current suppression.

2. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 1, characterized in that: The specific steps of obtaining the voltage phase of the high-voltage input side include: inputting the voltage of the high-voltage input side into a generalized second-order integrator to construct a vertical component with a constant voltage amplitude and a phase angle lagging by 90 degrees; outputting the voltage amplitude and the vertical vector to an αβ to dq coordinate system converter; converting V q Input it into the PI controller and control it to zero to achieve phase angle tracking of the input voltage.

3. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 1, characterized in that: The second converter adopts a single closed-loop voltage control strategy, which specifically includes: feeding back the high-voltage input side current signal to the second converter to obtain an output current I FB , the output current I FB After proportional feedback and then passing through a low-pass filter, the second converter outputs a series compensation voltage reference value, which is added to the voltage reference value generated in the steady state as the output voltage reference value of the second converter in the excitation saturation stage. The voltage reference value is subtracted from the input voltage V2, and the difference is used as the input of the PI controller. After modulation by the PI controller, the PWM signal of the second converter is output.

4. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 3, characterized in that: The first converter is connected in parallel to the low-voltage output side of the transformer, active and reactive power is obtained from the AC side, the AC voltage on the AC side is rectified to a DC voltage, and a stable DC side voltage is provided to the second converter through the first converter.

5. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 4, characterized in that: The first converter adopts a voltage and current dual-loop control strategy, including: collecting the low-voltage output side V1 and the current I sh , and phase-lock to obtain the voltage phase sin(wt1); collect the DC side voltage, and convert the DC side voltage V dc With the DC side voltage reference value V dcref The difference is used as the input of the PI controller. The input of the PI controller is multiplied by the voltage phase sin(wt1) to obtain the output current reference value of the first converter. The current reference value is multiplied by the current I sh The difference is input into the PI controller, and after being modulated by the PI controller, a PWM signal of the first converter is output.

6. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 3, characterized in that: When the voltage on the high-voltage input side of the transformer is in a steady state, the high-voltage input side voltage is constructed as an orthogonal component through a generalized second-order integrator, and the voltage phase angle sin (wt2) is transformed by dq to obtain the input voltage amplitude; the voltage reference value is subtracted from the voltage amplitude, and multiplied with the voltage phase sin (wt2) after the limiting stage as the second converter output series compensation voltage reference value, the series compensation voltage reference value is subtracted from the input high-voltage input side voltage, and the difference is used as the PI controller input, which is modulated by the PI controller to output the PWM signal of the second converter.

7. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 1, characterized in that: The first converter and the second converter adopt a full-bridge structure.

8. A method for suppressing magnetizing inrush current of a hybrid transformer according to claim 1 or 7, characterized in that: The voltage of the low-voltage power supply side is 400V.

9. The method for suppressing magnetizing inrush current of a hybrid transformer according to claim 1 or 7, characterized in that: The voltage of the high voltage power supply side is 10 kV.

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