Current harmonic self-interleaving elimination control method and device for common direct current bus parallel grid-connected inverter

Abstract

The invention discloses a current harmonic self-interleaving elimination control method and device for a common DC bus parallel grid-connected inverter, and the device comprises two same grid-connected inverter cabinets which are connected in parallel, and each parallel branch of the two grid-connected inverter cabinets is provided with a sensing filtering unit, a hysteresis comparison unit, and a PWM modulation unit. The method comprises the following steps of: simultaneously inputting the same current simulation given quantity into parallel branches respectively connected with two same grid-connected inverter cabinets which are connected in parallel, independently carrying out hysteresis control on each parallel branch, and finally driving the two grid-connected power cabinets to output current through sensing, filtering, hysteresis comparison and PWM (Pulse-Width Modulation) links, switching harmonics of output currents of the two grid-connected power cabinets are naturally distributed in a staggered mode due to the self-stabilization effect. The method does not depend on any synchronous or communication mechanism, and the current on the parallel branches of the two grid-connected inverter cabinets is automatically staggered and connected in parallel, so that the total harmonic distortion of the total grid-connected current is reduced, and the reliability is ensured while the system design is simplified.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power generation, in particular to a current harmonic self-interleaving elimination control method and device for parallel grid-connected inverters with a common DC bus. BACKGROUND

[0002] New energy power generation systems widely use power electronic converters, and with the increase of unit capacity, the capacity of modern converters is also increasing. Currently, the capacity of wind power converters has reached 16MW, and the capacity of centralized photovoltaic converters can reach 9MW. Grid-connected converters are usually composed of multiple stages, and the inverter part directly connected to the grid, which converts DC voltage into three-phase AC voltage, can be collectively referred to as a grid-connected inverter.

[0003] Large-capacity grid-connected inverters often use a scheme of parallel connection of two identical but smaller power cabinets, and use a common DC bus to reduce the number of bus capacitors and ensure good electrical performance. If the two power cabinets are only independently modulated using traditional vector control PWM modulation methods, due to the asynchronous PWM carrier when the double-cabinet is parallel connected, it will cause many problems such as power quality, common-mode voltage, EMI, vibration, etc. Carrier synchronization and carrier phase-shifted PWM technology can overcome this shortcoming, but it requires microsecond-level fast synchronization between modular converters, such as through dedicated synchronization optical fibers, etc. But this fast synchronization mechanism often has low reliability, or needs more cost to ensure high reliability and performance. SUMMARY

[0004] The present application aims to solve the problem of high cost and insufficient reliability of the existing PWM carrier synchronization mechanism when the parallel grid-connected inverters with a common DC bus are connected in parallel, and proposes a current harmonic self-interleaving elimination control method and device for parallel grid-connected inverters with a common DC bus. By circuit design, the PWM carrier is synchronized when the double-cabinet is parallel connected, the output currents of the two grid-connected inverter cabinets are automatically interleaved in parallel, the total harmonic distortion of the total grid-connected current is reduced, the system design is simplified, and the reliability is ensured.

[0005] In order to achieve the above-mentioned application purposes, the technical solutions of the present application are as follows: The application discloses a current harmonic self-interleaving elimination control device of a common DC bus parallel grid-connected inverter, comprising two same grid-connected inverter cabinets arranged in parallel, characterized in that each of the two grid-connected inverter cabinets is connected with a parallel branch, and a sensing filtering unit, a hysteresis comparison unit and a PWM modulation unit are arranged on the two parallel branches; the signal input end of the hysteresis comparison unit on each parallel branch is connected with the sensing filtering unit, and the signal output end is connected with the PWM modulation unit; the signal input ends of the two hysteresis comparison units are also connected with a current reference signal generation unit, the current reference signal generation unit generates the same current reference signal and simultaneously inputs the same into the two parallel branches; the hysteresis comparison unit compares the actual output current of the grid-connected inverter cabinet after filtering by the sensing filtering unit with the current reference signal output by the grid-connected inverter cabinet, and then outputs corresponding PWM modulation signals through the PWM modulation unit to drive the switching devices in the corresponding grid-connected inverter cabinet to act.

[0006] Further, each grid-connected inverter cabinet comprises a DC bus capacitor, a power component and a grid-connected inductor, and the AC sides of the two grid-connected inverter cabinets are connected in parallel to a grid interface through grid-connected inductors with the same parameters.

[0007] Further, the power component comprises power electronic switching devices, which are connected with the PWM modulation unit signals on the corresponding parallel branches.

[0008] Further, the topology of the power component is a three-phase three-bridge-arm two-level topology or an NPC or ANPC three-level topology.

[0009] The application further discloses a current harmonic self-interleaving elimination control method of a common DC bus parallel grid-connected inverter, comprising the following steps: inputting the same current analog given quantity into the parallel branches connected with two same grid-connected inverter cabinets in parallel, independently performing hysteresis control on each parallel branch, and finally driving the output currents of the two grid-connected inverter cabinets through sensing filtering, hysteresis comparison and PWM modulation links; due to the self-stabilization effect, the switching harmonic waves of the output currents of the two grid-connected inverter cabinets are naturally interleaved and distributed, and the switching harmonic waves are mutually eliminated after being converged on the AC grid side, so that the total harmonic distortion of the total grid-connected current is reduced.

[0010] Further, the control law of the hysteresis comparison is as follows: when the output current of the controlled grid-connected inverter cabinet rises to the sum of the given current and the hysteresis current , the upper tube of the corresponding bridge arm of the switching device is turned off, the lower tube is turned on, the inductor current begins to drop, when the inductor current drops to , the upper tube of the corresponding bridge arm of the switching device is turned on and the lower tube is turned off; wherein is the hysteresis control bandwidth.

[0011] In conclusion, the application has the following advantages: 1. The current harmonic self-interleaving elimination control method and device provided by the application can automatically interleave the currents on the parallel branches of two grid-connected inverter cabinets without relying on any synchronization or communication mechanism, thereby realizing reduction of the total harmonic distortion of the total grid-connected current, simplifying system design while ensuring reliability. 2. The application can reduce the total harmonic distortion of the total grid-connected current, thereby reducing the filter capacitor in the grid-connected inverter cabinet, saving cost and reducing volume. 3. The control method is simple and easy to implement, and does not require complex control algorithms and parameter setting. 4. The application is not sensitive to the operating point and parameter changes of the converter, and can be applied to different parameter design application scenarios. 5. In the application, the AC side of the two grid-connected inverter cabinets is connected in parallel to the grid interface through inductors with the same parameters, and the inductors play a filtering role, which can further reduce the harmonic content of the output current of the grid-connected inverter cabinet and improve power quality. BRIEF DESCRIPTION OF DRAWINGS

[0012] The application will be further described in detail below in combination with the drawings and specific embodiments, in which: Figure 1 is a single-line diagram of a three-phase circuit and its control system, and the signal variables in the diagram are represented by ABC three-phase signals. Figure 2 is the current harmonic self-interleaving elimination control method and device of the grid-side converter of the wind turbine with common DC bus in embodiment 1. Figure 3 is the experimental current waveform diagram of embodiment 1. Figure 4 is the current harmonic self-interleaving elimination control method and device of the grid-side converter of the wind turbine with common DC bus in embodiment 1. Figure 5 is the experimental current waveform diagram of embodiment 1. DETAILED DESCRIPTION

[0013] In order to more clearly illustrate the application, the application will be further described below in combination with preferred embodiments and drawings. Those skilled in the art should understand that the specific description below is illustrative and not limiting, and should not limit the protection scope of the application.

[0014] The application provides a current harmonic self-interleaving elimination control device, as shown in Figure 1As shown, including two parallel same grid-connected inverter cabinet, two grid-connected inverter cabinet DC side share the same DC bus. Each grid-connected inverter cabinet is composed of DC bus capacitor, power components and grid-connected inductor. The output of the grid-connected inverter cabinet is connected to the power grid through the inductor, realizing the energy exchange between the inverter and the power grid. Preferably, the AC side of the two grid-connected inverter cabinets is connected to the power grid interface in parallel through inductors with the same parameters. The inductor plays a filtering role, which can further reduce the harmonic content of the output current and improve the power quality.

[0015] Each grid-connected inverter cabinet is connected with a parallel branch, and each parallel branch is connected with a hysteresis comparison unit, a PWM modulation unit and a sensing filter unit. The signal input end of the hysteresis comparison unit is connected with the sensing filter unit, and the signal output end is connected with the PWM modulation unit.

[0016] The signal input end of the two hysteresis comparison units is also connected with the current reference signal generation unit. The current reference signal generation unit is used to generate the same current analog given quantity, which is simultaneously input into the parallel branch of each grid-connected inverter cabinet, ensuring that the two grid-connected inverter cabinets work under the same reference signal, so that the fundamental amplitude of the load current is the same.

[0017] The sensing filter unit is arranged on the parallel branch of each grid-connected inverter cabinet, which is used to process and transmit the current sensor signal of the grid-connected inverter cabinet, to monitor the actual output current of the grid-connected inverter cabinet in real time, and to remove high-frequency noise through filtering processing to obtain a smooth feedback current signal. In this embodiment, a first-order or multi-stage low-pass filter is used for the filtering link, and the cutoff frequency of the filter can be adjusted to adapt to different application scenarios.

[0018] The hysteresis comparison unit is arranged on the parallel branch of each grid-connected inverter cabinet, which is used to compare the filtered actual current with the current reference signal. The hysteresis comparison unit generates a corresponding PWM modulation signal output to the PWM modulation unit according to the set hysteresis width, to drive the switching devices in the grid-connected inverter to act. Specifically, the control law of hysteresis comparison is: when the output current of the controlled grid-connected inverter rises to the sum of the given current and the hysteresis current , the upper tube of the corresponding bridge arm of the switching device is turned off, and the lower tube is turned on, and the inductor current will begin to decrease, and when it decreases to , the upper tube of the corresponding bridge arm of the switching device is turned on and the lower tube is turned off; wherein is the hysteresis control bandwidth. The hysteresis comparison unit has fast response characteristics, which can track the changes of the current reference signal in real time, ensuring accurate control of the output current of the grid-connected inverter.

[0019] The PWM modulation units on the two parallel branches are independent of each other without any synchronization mechanism, which ensures the simplicity and reliability of the system.

[0020] The power component is the core execution component of the device, responsible for receiving the control signal from the PWM modulation unit, and realizing the current output to the power grid through internal power electronic switching devices (such as IGBT or MOSFET). In the embodiment, the topology of the parallel power component can adopt a typical three-phase three-leg two-level topology or NPC or ANPC three-level topology.

[0021] Based on the current harmonic self-interleaving elimination control device described above, the embodiment further proposes a current harmonic self-interleaving elimination control method.

[0022] The complete control of the inverter usually includes inner loop current loop control and outer loop control, and the outer loop control targets of different inverters are different. The present application only focuses on the control of the inner loop current loop, and the control method is as follows: The same current analog given quantity is input into two parallel branches, each branch independently performs hysteresis control, and finally drives the output current of the parallel inverter cabinet through sensing and filtering, hysteresis comparison, PWM modulation and other links. There is no need for any form of signal synchronization mechanism between the two parallel inverter cabinets. The switching harmonics of their output currents will naturally form interleaved distribution due to the self-stabilizing effect, and through convergence on the AC grid side, they will be superimposed and eliminated, thereby significantly reducing the total harmonic distortion (THD) of the total grid current.

[0023] Embodiment 1 The embodiment takes the application in a wind turbine as an example. The current harmonic self-interleaving elimination control device structure of the wind turbine common DC bus and grid-side converter is as shown in Figure 2 , including a permanent magnet synchronous generator, a machine-side converter, a grid-side transformer, a grid-side converter, and a control device of the grid-side converter.

[0024] The permanent magnet synchronous generator is the energy source of the entire system, generating alternating current energy through wind power. The permanent magnet synchronous generator is connected to the machine-side converter to convert the generated alternating current into direct current. The machine-side converter is used to rectify the alternating current generated by the permanent magnet synchronous generator into direct current and transmit it to the grid-side converter through the DC bus. The machine-side converter usually adopts a three-phase full-bridge rectifier circuit to ensure efficient and stable energy conversion. The topology of the grid-side converter is a typical three-phase three-leg two-level topology. The grid-side transformer is used to step up the alternating current output by the grid-side converter to a voltage level suitable for grid access, and also serves as an electrical isolation function to ensure safe operation of the system.

[0025] The control device of the grid-side converter comprises a current reference signal generating unit, a hysteresis comparison unit, a PWM modulation unit, a sensing and filtering unit and the like. The functions of the units are described above. The parallel branch grid-side converter is the core component of the device, which receives the control signal from the PWM modulation unit and outputs current to the grid through the internal power electronic switching device IGBT.

[0026] Specifically, the grid-side converter comprises a first grid-side converter and a second grid-side converter in parallel, and each of the first grid-side converter and the second grid-side converter is provided with a DC bus capacitor and a grid-connected inductor. The AC sides of the two grid-connected inverter cabinets are connected in parallel to the grid interface through grid-connected inductors with the same parameters.

[0027] The hysteresis comparison unit, the PWM modulation unit and the sensing and filtering unit are arranged on the parallel branch of each of the first grid-side converter and the second grid-side converter. The sensing and filtering unit is connected to the output end of the grid-side current sensor, and comprises a current sensor arranged at the output end of the grid-side current sensor. The signal input end of the hysteresis comparison unit is connected to the sensing and filtering unit, and the signal output end is connected to the PWM modulation unit. The signal input end of the hysteresis comparison unit is also connected to the current reference signal generating unit, and the current reference signal generating unit generates the same current reference signal and inputs the signal to the parallel branch of each of the first grid-side converter and the second grid-side converter.

[0028] The filtering units on the first grid-side converter and the second grid-side converter are respectively marked as and , wherein and are filtering time constants. After filtering to remove high-frequency noise, the smooth feedback current signals are respectively represented as and . The hysteresis comparison unit compares the actual output current of the grid-connected inverter cabinet after filtering with the current reference signal output by the grid-connected inverter cabinet, and then outputs the corresponding PWM modulation signal through the PWM modulation unit to drive the switching device in the grid-connected inverter cabinet to act. The inner loop current loop control method based on the above control device is as follows: The same current analog given value is input to the parallel branches of the two grid-side converters. Each parallel branch independently performs hysteresis control, and finally drives the grid-connected inverter cabinets where the two grid-side converters are located to output current through the links of sensing and filtering, hysteresis comparison and PWM modulation. The two grid-connected inverter cabinets do not need any form of signal synchronization mechanism. The switching harmonics of their output currents will naturally form staggered distribution due to the self-stabilization effect, and through the AC grid-side convergence, they will be superimposed and eliminated, thereby significantly reducing the total harmonic distortion (THD) of the total grid current.

[0029] In this embodiment, the control law of hysteresis comparison is: when the output current of the controlled grid-side converter rises to the sum of the given current and the hysteresis current , the upper tube of the driving grid-side converter switch device corresponding to the bridge arm is turned off, and the lower tube is turned on, and the inductor current will start to decrease, when it decreases to , the upper tube of the driving switch device corresponding to the bridge arm is turned on and the lower tube is turned off; wherein is the hysteresis control bandwidth.

[0030] Figure 3 oscilloscope waveforms of the parallel A-phase currents (ia1 and ia2) of the two parallel branches of the grid-side inverter of the 2.5 MW wind power converter and the total current (ia1+ia2). As can be seen from the experimental results shown in Figure 3 : the fundamental wave amplitudes of the currents of the two parallel branches are the same, and each bears 1 / 2 of the total load current, indicating that the control method of the application achieves the basic expectation of parallel branch current control. The switching harmonic part of the parallel grid-connected inverter current successfully enters the self-interleaving state without any synchronization signal. Although the branch current of each grid-connected inverter has some ripples like ordinary converters, due to the harmonic cancellation effect, the total grid current is very smooth, with THD<5%. It shows that the current harmonic self-interleaving elimination control scheme of the application is effective.

[0031] Embodiment 2 This embodiment takes the application in a photovoltaic unit as an example, and the device structure is shown in Figure 4 , which includes a photovoltaic cell panel, a DC-DC unit, a grid-connected transformer, a photovoltaic inverter, and a control device of the photovoltaic inverter.

[0032] Among them, the photovoltaic cell panel is the energy source of the whole system, which generates direct current by absorbing solar energy. It is connected with the photovoltaic inverter to convert the generated direct current into alternating current. The DC-DC unit matches the output voltage of the photovoltaic cell panel with the DC bus voltage of the photovoltaic inverter, while realizing maximum power tracking. The topology of the photovoltaic inverter is a typical three-phase three-bridge two-level topology. The grid-connected transformer is used to step up the alternating current output by the photovoltaic inverter to a voltage level suitable for grid connection, while playing the role of electrical isolation to ensure the safe operation of the system.

[0033] The control device of the photovoltaic inverter includes a current reference signal generation unit, a hysteresis comparison unit, a PWM modulation unit, and a sensing filter unit, etc. The functions of each functional unit are referred to the foregoing description. The photovoltaic inverter, as the core executive component of the device, is used to receive the control signal from the PWM modulation unit and realize the current output to the grid through the internal power electronic switch device IGBT.

[0034] Specifically, the photovoltaic inverter comprises a first photovoltaic inverter and a second photovoltaic inverter in parallel, and each of the first photovoltaic inverter and the second photovoltaic inverter is located in a grid-connected inverter cabinet further comprising a DC bus capacitor and a grid-connected inductor, and the AC sides of the two grid-connected inverter cabinets are connected to a grid interface in parallel through grid-connected inductors with the same parameters.

[0035] The parallel branch of each of the first photovoltaic inverter and the second photovoltaic inverter is respectively provided with a hysteresis comparison unit, a PWM modulation unit and a sensing and filtering unit. The sensing and filtering unit is connected to the output end of the photovoltaic inverter and comprises a current sensor arranged at the output end of the photovoltaic inverter; the signal input end of the hysteresis comparison unit is connected to the sensing and filtering unit, and the signal output end is connected to the PWM modulation unit; the signal input end of the hysteresis comparison unit is also connected to a current reference signal generation unit, which generates the same current reference signal and simultaneously inputs the current reference signal into the parallel branch of each of the first photovoltaic inverter and the second photovoltaic inverter.

[0036] The sensing and filtering units of the first photovoltaic inverter and the second photovoltaic inverter are respectively marked as and , wherein and are filter time constants. After removing high-frequency noise through filtering, the smooth feedback current signals are respectively represented as and . The hysteresis comparison unit compares the actual output current of the grid-connected inverter cabinet after filtering with the current reference signal output by the grid-connected inverter cabinet, and then outputs the corresponding PWM modulation signal through the PWM modulation unit to drive the switching devices in the grid-connected inverter cabinet to act.

[0037] The inner loop current loop control method based on the above control device is as follows: The same current analog given quantity is simultaneously input into the parallel branches of the two photovoltaic inverters. Each parallel branch independently performs hysteresis control, and finally drives the grid-connected inverter cabinet where the two photovoltaic inverters are located to output current through the links of sensing and filtering, hysteresis comparison and PWM modulation. There is no need for any form of signal synchronization mechanism between the two grid-connected inverter cabinets, and the switching harmonics of the output currents of the two grid-connected inverter cabinets will naturally form staggered distribution due to the self-stabilization effect, and through the aggregation of the AC grid side, the switching harmonics of the two grid-connected inverter cabinets will be superimposed and eliminated, thereby significantly reducing the total harmonic distortion (THD) of the total grid-connected current.

[0038] In this embodiment, the control law of hysteresis comparison is: when the output current of the controlled photovoltaic inverter rises to the sum of the given current and the hysteresis current , the upper tube of the corresponding bridge arm of the photovoltaic inverter switching device is turned off, and the lower tube is turned on, and the inductor current begins to decrease, and when it decreases to When the voltage of the upper arm is higher than the voltage of the lower arm, the driving switch device is turned on, and the upper arm is turned on and the lower arm is turned off. The hysteresis control bandwidth is 1 / 2 of the normal control bandwidth.

[0039] Figure 5 The simulation waveforms (Ia1 / Ia2, Ib1 / Ib2, Ic1 / Ic2) of the parallel three-phase currents of the two parallel branches of the 500kW photovoltaic inverter are shown in the figure. Figure 5 As can be seen from the simulation results shown in the figure, Ia1 and Ia2 are the currents of the two parallel branches of phase A, and the other two phases are similar. The fundamental wave amplitudes of the two branches of the phase current are the same, and each bears 1 / 2 of the total current, which shows that the control method of the application achieves the basic expectation of parallel branch current control. The switching harmonic part of the parallel grid-connected inverter current successfully enters the self-interleaving state without any synchronization signal. Although the branch current of each grid-connected inverter has some ripple like ordinary converters, the total grid current is very smooth due to the harmonic cancellation effect, and the THD is less than 5%. It shows that the current harmonic self-interleaving elimination control method of the application is effective.

[0040] The above is only the preferred embodiment of the application, and does not limit the application in any form. Any simple modification or equivalent change made according to the technical essence of the application to the above embodiment falls within the protection scope of the application.

Claims

1. A current harmonic self-interleaving cancellation control device for a parallel grid-connected inverter with common DC bus, comprising two same grid-connected inverter cabinets arranged in parallel, characterized in that, Two grid-connected inverter cabinets are respectively connected with parallel branches, and a sensing filtering unit, a hysteresis comparison unit and a PWM modulation unit are arranged on the two parallel branches; the signal input end of the hysteresis comparison unit on each parallel branch is connected with the sensing filtering unit, and the signal output end is connected with the PWM modulation unit; the signal input ends of the two hysteresis comparison units are also connected with a current reference signal generating unit, and the current reference signal generating unit generates the same current reference signal and simultaneously inputs the same into the two parallel branches; the hysteresis comparison unit compares the actual output current of the grid-connected inverter cabinet after filtering by the sensing filtering unit with the current reference signal output by the grid-connected inverter cabinet, and then outputs the corresponding PWM modulation signal through the PWM modulation unit to drive the switching device in the corresponding grid-connected inverter cabinet to act.

2. The current harmonic self-interleaving cancellation control device of a co-DC bus parallel grid-connected inverter according to claim 1, characterized in that, Each grid-connected inverter cabinet comprises a DC bus capacitor, a power assembly and a grid-connected inductor, and the AC sides of the two grid-connected inverter cabinets are connected in parallel to the grid interface through grid-connected inductors with the same parameters.

3. The current harmonic self-interleaving cancellation control device of a parallel grid-connected inverter with common DC bus according to claim 2, characterized in that, The power assembly comprises a power electronic switching device, and the power electronic switching device is connected with the PWM modulation unit signal on the corresponding parallel branch.

4. The current harmonic self-interleaving cancellation control device of a parallel grid-connected inverter with common DC bus according to claim 2 or 3, characterized in that, The topology of the power assembly is a three-phase three-bridge-arm two-level topology or an NPC or ANPC three-level topology.

5. A current harmonic self-interleaving cancellation control method for a common DC bus parallel grid-connected inverter, the device according to any one of claims 1-4, characterized in that, It comprises: The same current analog given quantity is simultaneously input to the parallel branches respectively connected with the two same grid-connected inverter cabinets in parallel, each parallel branch independently performs hysteresis control, and finally drives the output current of the two grid-connected inverter cabinets through the sensing filtering, hysteresis comparison and PWM modulation links; due to the self-stabilization effect, the switching harmonics of the output currents of the two grid-connected inverter cabinets are naturally staggered and distributed, and after being converged through the AC grid-connected side, they are superimposed and eliminated with each other, thereby reducing the total harmonic distortion of the total grid-connected current.

6. The current harmonic self-interleaving cancellation control method of a co-DC bus parallel grid-connected inverter according to claim 5, characterized in that, The control law for hysteresis comparison is: when the output current of the controlled grid-connected inverter cabinet rises to the sum of the given current and the hysteresis current... When the driving switching device is activated, the upper transistor of the corresponding bridge arm is turned off, and the lower transistor is turned on, increasing the inductor current. It will begin to decline, and will now fall to... When the upper transistor of the driving switching device is turned on, the lower transistor is turned off; where This is the hysteresis control bandwidth.

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