Power electronic transformer high-voltage port DC bus voltage control method

Abstract

The invention discloses a power electronic transformer high-voltage port direct-current bus voltage control method, and relates to the technical field of power electronic conversion. For a power electronic transformer circuit composed of a cascaded H-bridge converter, a DC / DC converter and a DC / AC converter, power grid voltage phase information is obtained through sub-module DC bus voltage of the cascaded H-bridge converter of the power electronic transformer, and active power transmission characteristics between a high-voltage AC port and a cascaded H-bridge are regulated and controlled based on the phase information. The DC bus voltage of the sub-module is kept stable; meanwhile, the excitation flux linkage amplitude is calculated by combining the voltage effective value and the reactive power of the high-voltage alternating-current port, the reactive power transmission characteristic between the two is regulated and controlled, the stability of the system under the weak power grid is effectively improved, and the method has certain engineering application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic transformer control, and in particular to a DC bus voltage control method for a high-voltage port of a power electronic transformer. BACKGROUND

[0002] With the large-scale access of new energy power generation, energy storage devices, and electric vehicles, the power system is accelerating the evolution to an AC-DC hybrid form. As the core hub connecting the power grid, power source, load, and energy storage, the control performance of the power electronic transformer directly affects the stability of the system. However, the high proportion of new energy grid connection leads to the continuous weakening of the grid strength, and the voltage fluctuation and background harmonics are intensified, which poses a severe challenge to the grid connection adaptability of the electronic transformer.

[0003] The current power electronic transformer generally adopts a feedforward decoupling control strategy based on a phase-locked loop, which exposes serious defects in a weak grid: first, the phase-locked loop is easily affected by the change of grid impedance, causing phase jitter and triggering subsynchronous / ultrasynchronous oscillation; second, the limited bandwidth of the phase-locked loop leads to a lag in dynamic response, making it difficult to track grid fault transients; third, the sub-module voltage balancing control is strongly coupled with the phase-locked loop, and the voltage balancing accuracy deteriorates when the voltage is distorted.

[0004] Although some research attempts to use virtual synchronous machine control or wideband phase-locked loop for improvement, the virtual synchronous machine control sacrifices the dynamic response speed, and the wideband phase-locked loop lacks anti-harmonic interference ability, neither of which can break through the coupling architecture of phase detection and DC voltage control. Therefore, there is an urgent need to develop an electronic transformer control method that does not rely on a phase-locked loop and can quickly maintain the stability of the high-voltage port DC bus voltage in a weak grid. SUMMARY

[0005] In view of the defects in the prior art, the present application provides a DC bus voltage control method for a high-voltage port of a power electronic transformer, which aims to obtain phase information through the DC bus voltage of a cascaded H-bridge converter, control the active power transmission between the high-voltage AC port and the cascaded H-bridge converter, and stabilize the output DC bus voltage of the cascaded H-bridge converter through phase control. At the same time, the amplitude of the excitation flux is obtained according to the effective value of the high-voltage AC port voltage and the reactive power, and the reactive power transmission between the high-voltage AC port and the cascaded H-bridge converter is controlled to improve the stability of the system in a weak grid.

[0006] To solve the above technical problems, the present application provides a DC bus voltage control method for a high-voltage port of a power electronic transformer, which is used for a traditional power electronic transformer, uses the DC bus voltage of a cascaded H-bridge sub-module to obtain the grid voltage phase and maintain the stability of the DC bus voltage, and the specific process is as follows:

[0007] Step 1: Sample the DC bus voltage of all sub-modules of the cascaded H-bridge, calculate the average value, and compare the average value with the preset DC bus voltage expected value to obtain the voltage deviation;

[0008] Step 2: Based on the voltage deviation, calculate the angular frequency ω through voltage closed-loop control, and then calculate the voltage phase θ of the high-voltage AC power grid through angular frequency closed-loop control;

[0009] Step 3: Sample the high-voltage AC port voltage of the cascaded H-bridge to obtain its effective value, compare it with the reference value, and then obtain the adjustment amount through droop closed-loop control;

[0010] Step 4: Superimpose the adjustment amount, the preset converter output reactive power reference value, and the converter output reactive power sample value calculated by sampling the high-voltage AC port voltage and current of the cascaded H-bridge converter, and obtain the excitation flux amplitude through integral control, and then sum it with the excitation flux feedforward value to obtain the modulation wave amplitude E m ;

[0011] Step 5: Based on the phase θ and the modulation wave amplitude E m , generate a three-phase modulation wave, output a driving pulse after modulation, and control the switching action of the cascaded H-bridge sub-modules to stabilize the DC bus voltage.

[0012] The control method uses the output DC voltage of all sub-modules of the cascaded H-bridge to obtain the voltage phase information of the high-voltage AC power grid without relying on a phase-locked loop; based on the phase information, the power transmission between the cascaded H-bridge sub-modules and the high-voltage AC power grid is adjusted to balance the power of the two and maintain the stability of the DC bus voltage at the output end of the cascaded H-bridge sub-modules.

[0013] The power transmission between the cascaded H-bridge converter and the high-voltage AC port based on the phase θ includes the following two cases:

[0014] When the high-voltage AC port transmits active power to the cascaded H-bridge converter, if the DC / AC converter output power increases, the DC bus voltage of the cascaded H-bridge sub-module decreases, the angular frequency ω increases through voltage closed-loop control, which causes the difference between the phase angle of the converter modulation wave and the phase angle of the high-voltage AC port voltage to increase, thereby increasing the active power input by the converter, charging the DC bus capacitor of the sub-module, and restoring the voltage to a stable value;

[0015] When the cascaded H-bridge converter transmits active power to the high-voltage AC port, if the DC / AC converter input power decreases, the DC bus voltage of the cascaded H-bridge sub-module decreases, the angular frequency ω decreases through voltage closed-loop control, which causes the difference between the phase angle of the converter modulation wave and the phase angle of the high-voltage AC port voltage to decrease, thereby decreasing the active power output by the converter, charging the DC bus capacitor of the sub-module, and restoring the voltage to a stable value.

[0016] The high-voltage port direct-current bus voltage control method provided by the application is applied to a traditional power electronic transformer, and the power electronic transformer is composed of a cascaded H-bridge converter, a DC / DC converter and a DC / AC converter;

[0017] The cascaded H-bridge converter is connected to a high-voltage alternating-current power grid through a filter inductor and is composed of a plurality of H-bridge sub-modules; the H-bridge sub-module is composed of four power electronic switching devices and a direct-current bus capacitor; the input end of the H-bridge sub-module is the midpoint of the bridge arm of the switching device, and the output end is the upper and lower ends of the direct-current bus capacitor; the number of the sub-modules in the cascaded H-bridge depends on the voltage level of the high-voltage alternating-current port and the withstand voltage value of the power electronic switching device;

[0018] The DC / DC converter is composed of a plurality of isolated DC / DC converters; the input end of each isolated DC / DC converter is connected to the output end of the corresponding sub-module of the cascaded H-bridge; the output ends of the isolated DC / DC converters are connected in parallel to form a direct-current bus, which is the output end of the DC / DC converter and is connected to the input end of the DC / AC converter;

[0019] The isolated DC / DC converter comprises an input direct-current capacitor, a DC / AC H-bridge, a high-frequency transformer, an AC / DC H-bridge and an output direct-current capacitor;

[0020] The DC / AC converter comprises a direct-current capacitor, six power electronic switching devices and a three-phase filter inductor; the six power electronic switching devices form three two-level bridge arms, and each two-level bridge arm is formed by two power electronic switching devices connected in series; the input end of the DC / AC converter is connected in parallel to the direct-current capacitor and the three two-level bridge arms contained in the DC / AC converter, and the output end is connected to the midpoints of the three two-level bridge arms through the three-phase filter inductor.

[0021] Compared with the prior art, the application has the following advantages:

[0022] 1) The phase information of the high-voltage alternating-current port voltage is obtained through the cascaded H-bridge sub-module direct-current bus voltage, which realizes independent synchronization and grid connection without a phase-locked loop, avoids the stability problem caused by using a phase-locked loop to obtain phase information, realizes stable operation of the power electronic transformer under a weak power grid, improves the inertia and frequency support capability of the high-voltage alternating-current port power system, and enhances the stability of the system.

[0023] 2) The phase information is obtained through the direct-current bus voltage control of the application for control, which simplifies the control structure, is conducive to circuit analysis and system model establishment, and facilitates the design of control parameters and system main circuit parameters. Other features and advantages of the application will be described in detail in the subsequent specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The schematic diagram of power electronic transformer circuit structure in the embodiment of the present application;

[0025] Figure 2 The control block diagram of power electronic transformer DC bus voltage control in the embodiment of the present application;

[0026] Figure 3 The voltage waveform diagram of medium-high voltage AC port of power electronic transformer adopting feed-forward decoupling control based on phase-locked loop synchronization in the embodiment of the present application;

[0027] Figure 4 The current waveform diagram of medium-high voltage AC port of power electronic transformer adopting feed-forward decoupling control based on phase-locked loop synchronization in the embodiment of the present application;

[0028] Figure 5 The voltage waveform diagram of medium-low voltage AC port of power electronic transformer adopting feed-forward decoupling control based on phase-locked loop synchronization in the embodiment of the present application;

[0029] Figure 6 The current waveform diagram of medium-low voltage AC port of power electronic transformer adopting feed-forward decoupling control based on phase-locked loop synchronization in the embodiment of the present application;

[0030] Figure 7 The output DC bus voltage waveform diagram of cascaded H-bridge sub-module of power electronic transformer adopting feed-forward decoupling control based on phase-locked loop synchronization in the embodiment of the present application;

[0031] Figure 8 The voltage waveform diagram of medium-high voltage AC port of power electronic transformer adopting DC bus voltage control in the embodiment of the present application;

[0032] Figure 9 The current waveform diagram of medium-high voltage AC port of power electronic transformer adopting DC bus voltage control in the embodiment of the present application;

[0033] Figure 10 The voltage waveform diagram of medium-low voltage AC port of power electronic transformer adopting DC bus voltage control in the embodiment of the present application;

[0034] Figure 11 The current waveform diagram of medium-low voltage AC port of power electronic transformer adopting DC bus voltage control in the embodiment of the present application;

[0035] Figure 12 The output DC bus voltage waveform diagram of cascaded H-bridge sub-module of power electronic transformer adopting DC bus voltage control in the embodiment of the present application. DETAILED DESCRIPTION

[0036] In order to make the objects, advantages and features of the present application more apparent, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be emphasized that the above drawings and the following description are only exemplary and are not intended to limit the scope of the present application and its applications.

[0037] The present application provides a DC bus voltage control method for power electronic transformer, the circuit thereof is shown in Figure 1 , which comprises a cascaded H-bridge converter, a DC / DC converter and a DC / AC converter; the cascaded H-bridge converter is connected with a high-voltage AC power grid, the DC output port of the cascaded H-bridge sub-module is connected with the input end of the DC / DC converter, and the output end of the DC / DC converter is connected with the input end of the DC / AC converter. The DC bus voltage control method for power electronic transformer is applicable to the above-mentioned circuit, as shown in Figure 2 , that is, the average value V dc of the DC bus voltage of all sub-modules of the cascaded H-bridge is collected, the angular frequency and the DC bus voltage are represented by a certain mapping relationship according to the DC bus voltage reference value V dcref and the grid angular frequency reference value ω n , the mapping relationship is represented by G DVC , and the real-time angular frequency can be obtained, such as:

[0038] ω-ω n =G DVS (V dc -V dcref ) (1)

[0039] The structure of the mapping relationship is selected as:

[0040]

[0041] Wherein k dvs represents the mapping coefficient.

[0042] The phase θ is obtained by integrating the calculated angular frequency ω, and the amplitude of the excitation flux is calculated by combining the voltage signal and the reactive power (reference value and sampling value) of the high-voltage AC port, and finally the relationship between the internal electromotive force and the voltage and the reactive power is obtained as:

[0043]

[0044] Wherein, the obtained internal electromotive force is the modulation wave amplitude E m , the modulation wave amplitude reference value is E0, Q set and Q e are the reactive power reference value and the sampling value, respectively, and U refU and D are the reference and sampled voltage signal values ​​at the high-voltage AC port, respectively. q Where is the damping coefficient, and K is the inertial constant.

[0045] Finally, the phase θ is used as the amplitude E of the modulated wave. m The expressions for the three-phase modulation waves are calculated, driving pulses are generated, the output power of the cascaded H-bridge converter is controlled, and the DC bus voltage of the submodule is kept stable.

[0046] In one possible embodiment, Figure 1 This is a schematic diagram of the circuit structure of a power electronic transformer. The high-voltage AC port is connected to a cascaded H-bridge converter via a filter inductor. The cascaded H-bridge converter consists of n H-bridge submodules. The output ports of the H-bridge submodules are connected to the input ports of a DC / DC converter. The output ports of the DC / DC converters are connected in parallel to form a common DC bus, which can be used as a low-voltage DC output port. The input ports of the DC / AC converter are connected to the common DC bus, and the output port is a low-voltage AC port.

[0047] To illustrate the feasibility and effectiveness of this invention, this embodiment uses a 10kV, 1MW power electronic transformer as an example for simulation:

[0048] When a power electronic transformer using traditional phase-locked loop (PLL) feedforward decoupling control is operating, the voltage and current waveforms at the high-voltage AC port, the voltage and current waveforms at the low-voltage AC port, and the DC bus voltage waveform output from the cascaded H-bridge submodule are as follows: Figures 3-7 As shown in the figure. During the period from 0 to 0.2s, it operates with a 200kW load, and at 0.2s, it slowly introduces an 800kW load, and at 0.6s, it removes the 800kW load.

[0049] The voltage and current waveforms at the high-voltage AC ports are stable with high sinusoidal intensity, and the current waveform changes with load switching. The voltage and current waveforms at the low-voltage AC ports are also stable with high sinusoidal intensity; the output voltage fluctuates during load switching but quickly recovers to the rated voltage. The current waveform changes with load switching. The DC bus voltage waveform output from the cascaded H-bridge submodules is stable; however, under high load power, the submodule capacitor voltage fluctuates significantly, and the DC bus voltage exhibits second-harmonic frequency fluctuations.

[0050] When a method for controlling the DC bus voltage at the high-voltage port of a power electronic transformer is adopted, the voltage and current waveforms at the high-voltage AC port, the voltage and current waveforms at the low-voltage AC port, and the DC bus voltage waveform output from the cascaded H-bridge submodule are as follows: Figures 8-12 As shown, it operates with a 600kW load from 0 to 1 second, slowly adds a 400kW load at 1 second, and removes the 400kW load at 1.6 seconds.

[0051] The high-voltage AC port voltage and current waveforms are stable, and the current waveform changes with the switching of the load; the low-voltage AC port voltage and current waveforms are stable, and the waveform is high in sinusoidal degree, the output voltage fluctuates when the load is switched, and the current waveform changes with the switching of the load. Similarly, the cascade H-bridge sub-module output DC bus voltage waveform is stable, and when the load power is large, the sub-module capacitor voltage fluctuates greatly, and there is a double-frequency fluctuation.

[0052] The above embodiments have described the technical solutions of the present application in detail. Apparently, the present application is not limited to the described embodiments. Based on the embodiments in the present application, those skilled in the art can make various changes, but any change equivalent or similar to the present application belongs to the protection scope of the present application.

[0053] The contents not described in detail in the specification belong to the prior art known to those skilled in the art.

[0054] Finally, it should be pointed out that: the above only describes the preferred examples of the present application, and is not used to limit the present application, although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for controlling the voltage of a high-voltage port DC bus of a power electronic transformer, characterized in that, The method is used for a traditional power electronic transformer, utilizes a cascaded H-bridge submodule DC bus voltage to obtain a power grid voltage phase and maintain a high voltage port DC bus voltage stable, and the specific process is as follows: Step 1: sampling DC bus voltages of all cascaded H-bridge submodules, calculating an average value of the DC bus voltages, and comparing the average value with a preset DC bus voltage expected value to obtain a voltage deviation; Step 2: based on the voltage deviation, calculating an angular frequency ω through voltage closed-loop control, and then calculating a high voltage AC grid voltage phase θ through angular frequency closed-loop control; Step 3: sampling a high voltage AC port voltage of the cascaded H-bridge, obtaining an effective value of the high voltage AC port voltage, comparing the effective value with a reference value, and obtaining an adjustment amount through droop closed-loop control after the comparison; Step 4: The modulation amount, the preset transformer output reactive power reference value and the transformer output reactive power sample value calculated by sampling the high-voltage alternating current port voltage and current of the cascaded H-bridge converter are superimposed, integrated control is performed to obtain the excitation flux linkage amplitude, and then the excitation flux linkage feedforward value is summed to obtain the modulation wave amplitude E m ; Step 5: based on the phase θ and the modulation amplitude E m A three-phase modulation wave is generated, and after modulation, an output driving pulse is generated to control the switching action of the cascade H-bridge sub-module, so that the high-voltage port DC bus voltage is stable.

2. The control method according to claim 1, characterized by, The output end DC voltages of all cascaded H-bridge submodules are utilized to obtain voltage phase information of a high voltage AC grid, without needing to rely on a phase-locked loop; Based on the phase information, power transmission between the cascaded H-bridge submodules and the high voltage AC grid is adjusted, so that the power of the cascaded H-bridge submodules and the high voltage AC grid is balanced, and the DC bus voltage of the output end of the cascaded H-bridge submodules is maintained stable.

3. The control method according to claim 2, characterized by, The power transmission between the cascaded H-bridge converter and the high voltage AC port based on the phase θ includes the following two cases: When the high voltage AC port transmits active power to the cascaded H-bridge converter, if the DC / AC converter output power increases, the DC bus voltage of the cascaded H-bridge submodule decreases, the angular frequency ω increases through voltage closed-loop control, the difference between the phase angle of the converter modulation wave and the voltage phase angle of the high voltage AC port increases, so that the active power input by the converter increases, the DC bus capacitor of the submodule is charged, and the voltage returns to the stable value; When the cascaded H-bridge converter transmits active power to the high voltage AC port, if the DC / AC converter input power decreases, the DC bus voltage of the cascaded H-bridge submodule decreases, the angular frequency ω decreases through voltage closed-loop control, the difference between the phase angle of the converter modulation wave and the voltage phase angle of the high voltage AC port decreases, so that the active power output by the converter decreases, the DC bus capacitor of the submodule is charged, and the voltage returns to the stable value.

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