CN118554782A - Power converter and high-frequency circulation suppression method - Google Patents
Power converter and high-frequency circulation suppression method Download PDFInfo
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- CN118554782A CN118554782A CN202311732831.3A CN202311732831A CN118554782A CN 118554782 A CN118554782 A CN 118554782A CN 202311732831 A CN202311732831 A CN 202311732831A CN 118554782 A CN118554782 A CN 118554782A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
- H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
- H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
- H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
- H02H9/021—Current limitation using saturable reactors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—ELECTRIC POWER NETWORKS; CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/01—Arrangements for reducing harmonics or ripples
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—ELECTRIC POWER NETWORKS; CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/38—Arrangements for feeding a single network from two or more generators or sources in parallel; Arrangements for feeding already energised networks from additional generators or sources in parallel
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
- H02M1/0038—Circuits or arrangements for suppressing, e.g. by masking incorrect turn-on or turn-off signals, e.g. due to current spikes in current mode control
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/32—Means for protecting converters other than automatic disconnection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
- H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
- H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
- H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/539—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency
- H02M7/5395—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency by pulse-width modulation
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—ELECTRIC POWER NETWORKS; CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2101/00—Supply or distribution of decentralised, dispersed or local electric power generation
- H02J2101/20—Dispersed power generation using renewable energy sources
- H02J2101/22—Solar energy
- H02J2101/24—Photovoltaics
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Inverter Devices (AREA)
Abstract
The application provides a power converter and a parallel operation loop current inhibition method. The power converter converts direct current from the photovoltaic module into alternating current and transmits the alternating current to the power grid, the power converter comprises an inverter circuit, a filter, a first current sensor, a second current sensor and a controller, the filter comprises a first inductor, a filter capacitor and a second inductor, the first inductor is connected with the filter capacitor in series and then connected with the inverter circuit in parallel, a series connection point of the first inductor and the filter capacitor is connected with the second inductor, the first current sensor is used for detecting current flowing through the first inductor, and the second current sensor is used for detecting current flowing through the filter capacitor. The controller is used for responding to the difference current which is larger than a first threshold value and controls the on and off of a switching tube in the inverter circuit so as to reduce the current flowing through the second inductor, wherein the difference current is the difference value between the current flowing through the first inductor and the current flowing through the filter capacitor.
Description
Technical Field
The application relates to the field of photovoltaic power generation, in particular to a power converter and a high-frequency circulation suppression method.
Background
In recent years, with the increasing prominence of energy and environmental problems, renewable energy sources mainly including solar energy and wind energy have been developed. Photovoltaic power generation systems have been rapidly developed as one of the important solutions for renewable energy sources. In order to increase reliability and expandability of a photovoltaic power station, a mode of connecting a plurality of inverters in parallel is generally adopted in engineering. Meanwhile, in order to suppress higher harmonics of the ac power output from the inverter, a filter is generally provided in the inverter. In practical application, a single inverter can meet the grid-connected requirement during grid-connected operation. However, when a plurality of inverters are operated in parallel at the same time, resonance circulation is likely to occur between filters of the plurality of inverters. The components of the circulating current can roughly comprise a fundamental circulating current and a harmonic circulating current, wherein the high-frequency circulating current can cause overheating of devices such as an inductor, a capacitor and the like when the devices flow through, and the power generation efficiency and the safety of the inverter are seriously affected. Therefore, it is important to timely and effectively detect and suppress the high-frequency loop current between the parallel inverters to become the inverter.
Disclosure of Invention
In order to timely and accurately identify and inhibit high-frequency circulation, the application provides a power converter and a method for detecting and inhibiting the high-frequency circulation.
In a first aspect, the present application provides a power converter, where the power converter is configured to convert dc power from a photovoltaic module into ac power and transmit the ac power to a power grid, the power converter includes an inverter circuit, a filter, a first current sensor, a second current sensor, and a controller, the filter is connected to an ac output of the inverter circuit, the filter includes a first inductor and a filter capacitor, the first inductor is connected in series with the filter capacitor and then connected in parallel with the inverter circuit, the first current sensor is configured to detect a current flowing through the first inductor, and the second current sensor is configured to detect a current flowing through the filter capacitor. The controller is used for responding to the high-frequency current to be larger than a set threshold value, controlling the switching tube in the inverter circuit to be turned on and off so as to reduce the current flowing through the filter, wherein the high-frequency current is the current for filtering the power frequency component in the difference current, the difference current is the current value obtained by subtracting the current flowing through the filter capacitor from the current flowing through the first inductor, and the power frequency component is the current with the frequency of power frequency in the difference current. The application takes the high-frequency current as an object, directly characterizes the size of the high-frequency loop current passing through the filter, further avoids the high-frequency loop current identification error caused by time delay, and improves the real-time performance and accuracy of the high-frequency loop current identification.
In a possible embodiment, the power converter further comprises a subtracting circuit, one end of the subtracting circuit is connected to the first current sensor and the second current sensor, the other end of the subtracting circuit is connected to the controller, and the subtracting circuit is configured to output a signal representing the difference current to the controller. Specifically, the subtracting circuit receives a signal characterizing the current i-L1 flowing through the first inductor and a signal characterizing the current i-C1 flowing through the filter capacitor from the first current sensor and the second current sensor, respectively. On the basis, the subtracting circuit obtains a signal used for representing the difference current through difference operation. By arranging the subtracting circuit, the operation pressure of the controller can be reduced, and the model selection requirement of the controller is reduced.
In one possible embodiment, the power converter further comprises a high-pass filter circuit connected between the subtracting circuit and the controller, the high-pass filter circuit being configured to output a signal indicative of the high-frequency current to the controller. Through setting up the high pass filter circuit, shift out the process of high pass filter the controller, can alleviate the operating pressure of controller, and then reduced the selection type requirement of controller.
In a possible embodiment, the filter further comprises a second inductor, one end of the second inductor is connected to the first inductor and the filter capacitor, and the other end of the second inductor is used for being connected to the power grid. When the filter comprises a second inductance, the filter is an LCL type filter. That is, the detection logic of the high frequency loop current in the present application can be applied to the LCL type filter.
In one possible embodiment, the first current sensor is one of a current hall, a current sampling resistor and a current sampling transformer, and the second current sensor is one of a current hall, a current sampling resistor and a current sampling transformer.
In one possible implementation, the power converter is a single-phase power converter, or the power converter is a three-phase power converter.
In one possible implementation, the inverter circuit includes a T-type three-level leg, or the inverter circuit includes an I-type three-level leg.
In a second aspect, the present application provides a method for suppressing parallel operation circulation, where the method includes: and controlling the switching on and off of a switching tube in the inverter circuit to reduce the circulation current flowing through the filter in response to the high-frequency current being greater than the set threshold. The filter is connected with an alternating current output end of the inverter circuit, the filter comprises a first inductor and a filter capacitor, the first inductor is connected with the filter capacitor in series and then connected with the inverter circuit in parallel, the high-frequency current is a current for filtering out an industrial frequency component in a differential current, the differential current is a current value obtained by subtracting the current flowing through the filter capacitor from the current flowing through the first inductor, and the industrial frequency component is a current with the industrial frequency in the differential current. The application takes the high-frequency current as an object, directly characterizes the size of the high-frequency loop current passing through the filter, further avoids the high-frequency loop current identification error caused by time delay, and improves the real-time performance of the high-frequency loop current identification.
In a third aspect, the present application provides a power supply system, including the power converter of any one of the first aspects, wherein dc input ends of the plurality of power converters are used for being connected to a photovoltaic module or an energy storage battery, and ac output ends of the plurality of power converters are connected in parallel and then are used for being connected to a power grid.
In general, the inverter provided by the application acquires the high-frequency current in the circulating current through sampling i-L1 and i-C1 and through difference value operation and high-pass filtering treatment, so that the size of the high-frequency circulating current flowing through the filter is timely and accurately acquired, and meanwhile, the inverter judges whether to start a high-frequency circulating current inhibition algorithm or not based on the size of the high-frequency circulating current, so that the high-frequency circulating current is effectively eliminated.
Drawings
FIG. 1 is a schematic diagram of a networking of a photovoltaic power plant;
FIG. 2 is a topological structure diagram of two power converters operating in parallel;
Fig. 3 is a control structure diagram of a single inverter according to the present application;
fig. 4 is a control structure diagram of still another single inverter according to the present application;
fig. 5 is a control structure diagram of still another single inverter according to the present application;
Figure 6 is a control flow diagram of loop suppression provided by the present application.
Detailed Description
Referring to fig. 1, fig. 1 is a schematic diagram of a networking of a photovoltaic power station, in which a photovoltaic module converts solar energy into electric energy by a photovoltaic effect and outputs direct current to an inverter, and the inverter converts the direct current output by the photovoltaic module into low-voltage alternating current (illustratively, 800V) and further delivers the low-voltage alternating current to a box-type substation. After the low-voltage alternating current output by the inverter is converted into medium-voltage alternating current (35 kV in an example) by the box-type transformer substation, the medium-voltage alternating current is continuously conveyed to a booster station (power grid) to realize grid-connected power generation of the photovoltaic power station.
With the gradual increase of the duty ratio of the photovoltaic in the power grid, the high permeability of the photovoltaic brings great impact to the power grid, and a series of problems are brought to the stability of the voltage of the power grid, the quality of electric energy and the operation control. For this reason, an energy storage device is usually added to a photovoltaic power generation system, and a stable current source is provided for a power grid through the charge and discharge characteristics of the energy storage device. Meanwhile, through the arrangement of the energy storage device, the light rejection phenomenon of the photovoltaic power generation system can be relieved, and the overall power generation capacity of the photovoltaic power station is improved.
With continued reference to fig. 1, the photovoltaic power station further includes an energy storage battery, which is connected to the box-type substation through an energy storage converter. When the voltage of the power grid fluctuates, the energy storage battery can transmit stable direct current to the energy storage converter, the energy storage converter converts the direct current from the energy storage battery into alternating current and outputs electric energy to the power grid through the box-type transformer substation, and then active support and reactive support are provided for the power grid. In addition, when the power generation of the photovoltaic module is excessive, the generated electric energy of the photovoltaic module can also charge the energy storage battery, so that the waste of the electric energy is avoided.
It is worth mentioning that in photovoltaic power plants, the inverter and the energy storage converter may be collectively referred to as a power converter. Unlike an inverter, an energy storage converter is able to convert ac power from an energy storage battery into dc power for charging the energy storage battery in addition to converting dc power from the energy storage battery into ac power for delivery to a power grid. That is, the energy storage converter may enable bi-directional energy conversion between direct current and alternating current.
In the following, the present application will be described by taking an inverter as an example, and the loop current detection scheme provided by the present application will be specifically described with reference to fig. 2 to 6. It should be noted that the loop current detection scheme provided by the application can also be applied to an energy storage converter, and the application is not limited to this.
In order to better understand the technical scheme provided by the application, a mechanism for generating the circulating current under the parallel connection scene of a plurality of inverters is introduced. As mentioned above, in order to improve the reliability and scalability of photovoltaic power plants, it is common in engineering to connect ac output terminals of multiple inverters in parallel and to connect to a grid through a grid-connected common point (Point of Common Coupling, PCC). However, researchers find that when N inverters are operated in parallel, the equivalent grid impedance of a single inverter in the system is equivalent to N times the actual grid impedance. That is, when a plurality of inverters are connected in series, the equivalent impedance of the connection line between each inverter and the power grid increases, and the ac power generated by the inverter is more difficult to be smoothly transmitted to the power grid than when a single inverter is connected in series. Therefore, when the inverters are operated in parallel, only a part of the alternating current generated by the inverters through power conversion can be smoothly transmitted to the power grid, and another part of the alternating current forms a current loop between the filters of the inverters, and the part of the current is called circulating current. In a practical scenario, the circulating current can be decomposed into fundamental wave circulating current and harmonic wave circulating current, wherein the fundamental wave circulating current corresponds to a current component with the same frequency as the power grid frequency in the circulating current, and the harmonic wave circulating current corresponds to a current component with different frequency from the power grid frequency in the circulating current.
Further, during the operation of the inverters, because the phases of the triangular carriers are inconsistent, parameters of the inverters are different, and the like, switching tubes in the inverters running in parallel cannot be synchronously turned on and off, so that the impedance of the filter in each inverter to the high-frequency ripple wave generated when the switching tube in the inverter acts is reduced, the high-frequency ripple wave generated by each inverter is superposed, and finally high-frequency circulation is formed. It should be noted that the high-frequency circulating current refers to a current component in the circulating current with a frequency near an operating frequency of a switching tube in the inverter, and the switching tube refers to a switching tube included in an inverter circuit in the inverter. In addition, because the filter comprises more inductors, high-frequency circulating current can be converted into heat and vibration when flowing through the inductors, so that the inductors are overheated, the service life of the inverter is shortened, and even faults and burnout of the inverter can be caused when the high-frequency circulating current is serious. Here, the high frequency circulation belongs to one of the above harmonic circulation.
Referring to fig. 2, the present application takes a scenario in which two inverters are operated in parallel as an example, and illustrates the flow direction of the circulation between the two inverters. As shown in fig. 2, the dc input terminals of the first inverter 10 and the second inverter 20 are used for connecting a dc source such as a photovoltaic module or an energy storage battery, and the first inverter 10 is connected in parallel with the ac output terminal of the second inverter 20 and is connected to a power grid through an ac bus. Taking the first inverter 10 as an example, the first inverter 10 includes a bus capacitor, an inverter circuit 11, and a filter. The dc input end of the inverter circuit 11 is connected to a bus capacitor, the ac output end of the inverter circuit 11 is connected to a filter, the bus capacitor is used for smoothing the bus voltage, the inverter circuit 11 includes a plurality of switching transistors for implementing conversion from dc to ac, and the filter is used for filtering noise and harmonic waves of the ac output by the inverter circuit 11, so as to improve the quality and stability of the ac output by the first inverter 10.
Further, the filter includes a first inductor L1, a second inductor L2, and a filter capacitor. In fig. 2, since the first inverter 10 is a three-phase inverter, three inductors near the inverter side are collectively referred to as a first inductor L1, three inductors near the grid side are collectively referred to as a second inductor L2, and the first capacitor C1, the second capacitor C2, and the third capacitor C3 are collectively referred to as a filter capacitor C. Taking a filter on an a-phase output line in the first inverter 10 as an example, one end of the third capacitor C3 is connected to a midpoint of the bus capacitor, the other end of the third capacitor C3 is connected to the first inductor L1 and the second inductor L2, one end of the first inductor L1 is connected to the inverter circuit 11, and the other end of the first inductor L1 is connected to the second inductor L2.
It should be noted that the inverter circuit 11 may be directly connected to the dc bus or indirectly connected to the dc bus, and in the same manner, the inverter circuit 11 may be directly connected to the filter or indirectly connected to the filter. In addition, since the second inverter 20 has the same structure as the first inverter 10, a description thereof will be omitted.
Further, when the two inverters are operated in parallel, part of the ac a outputted from the first inverter 10 flows to the second inductor L2 'and the filter capacitor C' of the filter in the second inverter 20 through the ac a bus, and finally, the current returns to the filter of the first inverter 10 through the ac B bus and the ac C bus to form a circulating current. It should be noted that fig. 2 is only an illustration of the circulation, and the circulation current loop may be different from that shown in fig. 2 in the present application in a practical scenario.
Further, with the first inverter 10, when there is a high-frequency loop current in the loop current between the first inverter 10 and the second inverter 20, since the impedance of the first inductance L1 in the filter to the high-frequency loop current is large, the high-frequency loop current generally can only flow through the second inductance L2 of the filter and the filter capacitance. Similarly, with the second inverter 20, when there is a high-frequency circulating current in the circulating current between the first inverter 10 and the second inverter 20, the high-frequency circulating current generally flows only through the second inductance L2 'of the filter and the filter capacitor C'. As described above, since the inductance element through which the high-frequency circulation flows will be overheated, the second inductance L2 is easily overheated for the filter of the first inverter 10, severely affecting the normal operation of the second inductance L2. Therefore, when a plurality of inverters are operated in parallel, it is important to detect the high-frequency circulating current in the circulating current in time in addition to the whole circulating current.
There are generally two solutions to the problem of how to effectively detect whether high frequency circulation occurs between a plurality of inverters operating in parallel. Taking the first inverter 10 in fig. 2 as an example, in the first embodiment, a temperature detection circuit may be directly disposed near the second inductor L2 to monitor the temperature of the second inductor L2 in real time, and if the temperature of the second inductor L2 exceeds a set threshold value, it is indicated that a high-frequency circulation is generated between the first inverter 10 and the second inverter 20. However, the temperature detection circuit is not high in detection accuracy, is prone to delay, and cannot normally and effectively detect whether high-frequency circulation occurs between a plurality of inverters connected in parallel. In the second scheme, whether high-frequency circulation current occurs between a plurality of parallel inverters can be identified according to the standard deviation of the power grid voltage and the voltages at two ends of the filter capacitor C in the filter. However, due to the fluctuation of the power grid voltage, under some working conditions, the second scheme cannot timely and accurately detect whether high-frequency circulation current occurs among the plurality of inverters connected in parallel. Meanwhile, since the voltage component corresponding to the high-frequency circulation is far smaller than the voltage of the alternating current output by the inverter, the signal-to-noise ratio is extremely low when the controller of the inverter calculates the magnitude of the high-frequency circulation, and it is difficult to accurately calculate the magnitude of the high-frequency circulation according to the voltage component corresponding to the high-frequency circulation.
In contrast, the present application proposes a scheme for detecting whether a high-frequency circulation occurs between parallel inverters, which has simple control logic and requires fewer hardware components, and will be described with reference to fig. 3 to 6.
Example 1:
Referring to fig. 3, fig. 3 is a block diagram of a single inverter loop current control provided by the present application. Specifically, the direct current input end of the inverter is used for being connected with a direct current source such as an energy storage battery or a photovoltaic module, and the alternating current output end of the inverter is used for being connected with a power grid. The inverter includes an inverter circuit, a filter, and a controller. The inverter circuit is used for converting direct current from a direct current source into alternating current and outputting the alternating current to the power grid through the filter, and the filter is used for filtering harmonic components of the alternating current output by the inverter circuit so as to improve the quality of the alternating current output by the inverter. When the filter is an LCL type filter, the filter comprises a first inductor L1, a first capacitor C1 and a second inductor L2, wherein the first inductor L1 and the first capacitor C1 are connected in series and then connected with the inverter circuit in parallel, one end of the second inductor L2 is connected with the first inductor L1 and the first capacitor C1, and the other end of the second inductor L2 is connected with the power grid.
Further, the inverter further includes a first current sensor CT1 and a second current sensor CT2. The first current sensor CT1 is connected in series with the first inductor L1 for detecting a current flowing through the first inductor L1, i.e. i-L1, and the second current sensor CT2 is connected in series with the first capacitor C1 for detecting a current flowing through the first capacitor C1, i.e. i-C1.
It should be noted that, in practical application, the first current sensor CT1 may be a current hall, a current sampling resistor, or a current sampling transformer. Similarly, in practical application, the second current sensor CT2 may be a current hall, a current sampling resistor, or a current sampling transformer.
Further, the inverter further includes a subtracting circuit and a high pass filter circuit. One end of the subtracting circuit is connected with the first current sensor CT1 and the second current sensor CT2, and the other end of the subtracting circuit is connected with the controller through the high-pass filter circuit. Specifically, during inverter operation, the first current sensor CT1 and the second current sensor CT2 transmit a signal representative of i-L1 and a signal representative of i-C1, respectively, to a subtracting circuit. The subtracting circuit performs difference operation on the signal representing i-L1 and the signal representing i-C1 to obtain a difference current signal and transmits the difference current signal to the high-pass filter circuit. The high-pass filter circuit filters the power frequency component signal in the difference current signal to obtain a high-frequency current signal and transmits the high-frequency current signal to the controller. After receiving the high-frequency current signal, the controller judges whether the high-frequency current is larger than a set threshold value, and if the high-frequency current is larger than the set threshold value, the controller starts a high-frequency circulation suppression algorithm to reduce the current flowing through the filter.
The differential current is the difference between i-L1 and i-C1, the differential current signal is used for representing the differential current, the high-frequency current is the current for filtering the power frequency component in the differential current, the power frequency component is the current with the frequency of power frequency (the working frequency of the power grid) in the differential current, the high-frequency current signal is used for representing the high-frequency current, and the power frequency component signal is used for representing the power frequency component.
It is worth mentioning that when the controller starts the high-frequency circulation suppression algorithm, the controller can suppress circulation between the plurality of parallel inverters by changing the output characteristics of the inverter circuit. Illustratively, the controller controls the switching on and off of the switching tubes in the inverter circuit by varying the pulse width modulation signal (Pulse Width Modulation, PWM), thereby varying the output characteristics of the inverter circuit. In addition, because the voltage level that the controller can withstand is low, the controller can only receive signals for characterizing the differential current and the high-frequency current, and the signals can completely characterize the related information of the differential current and the high-frequency current though the voltage level is low.
As described above, when a plurality of inverters are operated in parallel, since the high-frequency impedance of the first inductance L1 is large, when a high-frequency loop current is generated between the plurality of inverters connected in parallel, the high-frequency loop current generally passes through only the filter capacitance C1 and the second inductance L2. In addition, the high-frequency loop current is very easy to cause overheat of the second inductor L2, and the safety and the efficiency of the inverter are affected, so that the current i-L2 flowing through the second inductor L2 is taken as the object, and the magnitude of the high-frequency loop current among a plurality of parallel inverters can be intuitively shown. However, since the second inductor L2 is located on the connection line between the inverter circuit and the power grid, the i-L2 is usually larger, and if the current sensor is provided for the second inductor L2 to directly detect the size of the i-L2, the requirement for selecting the current sensor is higher, and the hardware cost is increased. In this regard, applicants have found that during inverter operation, i-L1 may be equivalent to the sum of i-C1 and i-L2. Therefore, the application indirectly represents i-L2 by the difference current (i.e. the difference between i-L1 and i-C1), thereby avoiding directly setting a current Hall for the second inductor L2 and reducing the hardware cost.
In addition, compared with the method for identifying whether high-frequency circulation occurs between a plurality of parallel inverters according to the standard deviation of the power grid voltage and the voltages at two ends of the filter capacitor, the method for detecting the high-frequency circulation directly takes the difference current as an object and further judges whether circulation occurs between the parallel inverters according to the high-frequency current in the difference current, so that the process of converting a voltage signal into a current signal by a controller is omitted, and the real-time performance and the accuracy of high-frequency circulation detection are improved.
Example 2:
Referring to fig. 4, fig. 4 is a block diagram of still another single inverter loop current control provided by the present application. Specifically, the direct current input end of the inverter is used for being connected with a direct current source such as an energy storage battery or a photovoltaic module, and the alternating current output end of the inverter is used for being connected with a power grid. The inverter includes an inverter circuit, a filter, and a controller. The inverter circuit is used for converting direct current from a direct current source into alternating current and outputting the alternating current to the power grid through the filter, and the filter is used for filtering harmonic components of the alternating current output by the inverter circuit so as to improve the quality of the alternating current output by the inverter. When the filter is an LCL type filter, the filter comprises a first inductor L1, a first capacitor C1 and a second inductor L2, wherein the first inductor L1 and the first capacitor C1 are connected in series and then connected with the inverter circuit in parallel, one end of the second inductor L2 is connected with the first inductor L1 and the first capacitor C1, and the other end of the second inductor L2 is connected with the power grid.
Further, the inverter further includes a first current sensor CT1 and a second current sensor CT2. The first current sensor CT1 is connected in series with the first inductor L1 for detecting a current flowing through the first inductor L1, i.e. i-L1, and the second current sensor CT2 is connected in series with the first capacitor C1 for detecting a current flowing through the first capacitor C1, i.e. i-C1.
It should be noted that, in practical application, the first current sensor CT1 may be a current hall, a current sampling resistor, or a current sampling transformer. Similarly, in practical application, the second current sensor CT2 may be a current hall, a current sampling resistor, or a current sampling transformer.
Further, the inverter further includes a subtracting circuit. Specifically, one end of the subtracting circuit is connected to the first current sensor CT1 and the second current sensor CT2, and the other end of the subtracting circuit is connected to the controller. Specifically, during inverter operation, the first current sensor CT1 and the second current sensor CT2 transmit a signal representative of i-L1 and a signal representative of i-C1, respectively, to a subtracting circuit. The subtracting circuit performs difference operation on the signal representing i-L1 and the signal representing i-C1 to obtain a difference current signal and transmits the difference current signal to the controller. After the controller receives the difference current signal, the software carries out high-pass filtering processing on the difference current signal to obtain the related information of the high-frequency current in the difference current. Or after the controller receives the difference current signal, the controller can also perform fast Fourier operation (Fast Fourier Transform, FFT) on the difference current signal through software so as to extract the related information of the high-frequency current in the difference current. Based on this, the controller determines whether the high frequency current is greater than a set threshold, and if the high frequency current is greater than the set threshold, the controller starts a high frequency loop current suppression algorithm to reduce the current flowing through the filter. The difference current is the difference between i-L1 and i-C1, the difference current signal is used for representing the difference current, the high-frequency current is the current for filtering the power frequency component in the difference current, and the power frequency component is the current with the frequency of power frequency in the difference current.
It is worth mentioning that when the controller starts the high-frequency circulation suppression algorithm, the controller can suppress circulation between the plurality of parallel inverters by changing the output characteristics of the inverter circuit. Illustratively, the controller controls the on and off of the switching tubes in the inverter circuit by changing the PWM signal, thereby changing the output characteristics of the inverter circuit. In addition, because the voltage level that the controller can withstand is low, the controller can only receive a signal that characterizes the differential current, which, although at a low voltage level, can more fully characterize the information about the differential current.
By the arrangement, a current sensor arranged for the second inductor L2 can be omitted, the arrangement of a high-pass filter circuit can be omitted, and hardware cost is further reduced.
Example 3:
Referring to fig. 5, a block diagram of loop current control of a single inverter according to the present application is shown in fig. 5. Specifically, the direct current input end of the inverter is used for being connected with a direct current source such as an energy storage battery or a photovoltaic module, and the alternating current input end of the inverter is used for being connected with a power grid. The inverter includes an inverter circuit, a filter, and a controller. The inverter circuit is used for converting direct current from a direct current source into alternating current and outputting the alternating current to the power grid through the filter, and the filter is used for filtering harmonic components of the alternating current output by the inverter circuit so as to improve the quality of the alternating current output by the inverter. When the filter is an LCL type filter, the filter comprises a first inductor L1, a first capacitor C1 and a second inductor L2, wherein the first inductor L1 and the first capacitor C1 are connected in series and then connected with the inverter circuit in parallel, one end of the second inductor L2 is connected with the first inductor L1 and the first capacitor C1, and the other end of the second inductor L2 is connected with the power grid.
Further, the inverter further includes a first current sensor CT1 and a second current sensor CT2. The first current sensor CT1 is connected in series with the first inductor L1 for detecting a current flowing through the first inductor L1, i.e. i-L1, and the second current sensor CT2 is connected in series with the first capacitor C1 for detecting a current flowing through the first capacitor C1, i.e. i-C1.
It should be noted that, in practical application, the first current sensor CT1 may be a current hall, a current sampling resistor, or a current sampling transformer. Similarly, in practical application, the second current sensor CT2 may be a current hall, a current sampling resistor, or a current sampling transformer.
Further, the first current sensor CT1 and the second current sensor CT2 are connected to a controller. The controller performs difference operation on the signals representing i-L1 and the signals representing i-C1 through software, and further obtains relevant information of difference current. And then, the controller continues to carry out high-pass filtering processing on the related information of the difference current through software so as to obtain the related information of the high-frequency current in the difference current. Or after the controller receives the related information of the differential current, the controller can also perform FFT on the differential current signal through software so as to extract the related information of the high-frequency current in the differential current. On the basis, the controller judges whether the high-frequency current is larger than a set threshold value, and if the high-frequency current is larger than the set threshold value, the controller starts a high-frequency circulation suppression algorithm to reduce the current flowing through the filter. The difference current is the difference between i-L1 and i-C1, the high-frequency current is the current for filtering the power frequency component in the difference current, and the power frequency component is the current with the frequency of power frequency in the difference current.
By the arrangement, a current sensor arranged for the second inductor L2 can be omitted, meanwhile, a subtracting circuit and a high-pass filter circuit can be omitted, and hardware cost is further reduced.
Example 4:
Referring to fig. 6, fig. 5 is a diagram of a method for suppressing high-frequency loop current provided by the present application. The method can be applied to power conversion devices such as photovoltaic inverters or energy storage converters and executed by a controller of the power conversion devices. Taking a photovoltaic inverter as an example, the photovoltaic inverter comprises an inverter circuit, a filter and a controller, wherein the inverter circuit is used for converting direct current from a direct current source into alternating current and outputting the alternating current to a power grid through the filter, and the filter is used for filtering harmonic components of the alternating current output by the inverter circuit so as to improve the quality of the alternating current output by the inverter. When the filter is an LCL type filter, the filter comprises a first inductor L1, a first capacitor C1 and a second inductor L2, wherein the first inductor L1 and the first capacitor C1 are connected in series and then connected with the inverter circuit in parallel, one end of the second inductor L2 is connected with the first inductor L1 and the first capacitor C1, and the other end of the second inductor L2 is connected with a power grid. When the inverter is running, the control is for:
S101: judging whether the high-frequency circulation is greater than a set threshold, if so, proceeding to step S102;
S102: starting a loop current suppression algorithm;
S103: reducing the high frequency current.
The high-frequency current is a current for filtering out an industrial frequency component in the differential current, the differential current is a difference between a current flowing through the first inductor L1 and a current flowing through the first capacitor C1, and the industrial frequency component is a current with an industrial frequency in the differential current.
It is worth mentioning that when the controller starts the high-frequency circulation suppression algorithm, the controller can suppress circulation between the plurality of parallel inverters by changing the output characteristics of the inverter circuit. Illustratively, the controller controls the switching on and off of the switching tubes in the inverter circuit by varying the pulse width modulation signal (Pulse Width Modulation, PWM), thereby varying the output characteristics of the inverter circuit.
It should be noted that, for convenience of description, the inverters shown in the above embodiments 1 to 3 are all single-phase inverters. In practical application, the technical scheme provided by the application can be applied to a three-phase inverter. For the three-phase inverter, a first current sensor CT1 and a second current sensor CT2 may be disposed on each filter corresponding to each alternating current, where the first current sensor CT1 and the second current sensor CT2 are respectively configured to detect currents flowing through the first inductor L1 and the second inductor L2.
In addition, in order to better demonstrate the technical principle of the present application, the above embodiments 1 to 3 take the LCL filter as an example, and describe the high-frequency loop current suppression method provided by the present application in detail. In practical applications, the LCL filter may also be replaced by an LC filter, which the present application is not limited to. In particular, the arrangement of the second inductance L2 is dispensed with in the LC filter compared to the LCL filter. When high-frequency circulation occurs among the inverters running in parallel, the high-frequency current can still flow through the filter capacitor and devices between the first inductor L1 and the power grid although the impedance of the first inductor L1 to the high-frequency current is large, and the running safety and efficiency of the inverters are also affected.
In addition, in the inverter provided by the application, the bridge arm of the inverter circuit can be in a T-type three-level or I-type three-level topology and the like, and the application is not limited to the topology.
In general, the inverter provided by the application acquires the high-frequency current in the circulating current through sampling i-L1 and i-C1 and through difference value operation and high-pass filtering treatment, so that the size of the high-frequency circulating current flowing through the filter is timely and accurately acquired, and meanwhile, the inverter judges whether to start a high-frequency circulating current inhibition algorithm or not based on the size of the high-frequency circulating current, so that the high-frequency circulating current is effectively eliminated.
The foregoing is merely illustrative of the present application, and the present application is not limited thereto, and any person skilled in the art will readily recognize that variations or substitutions are within the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (9)
1. The power converter is characterized by comprising an inverter circuit, a filter, a first current sensor, a second current sensor and a controller, wherein the inverter circuit is used for converting direct current from a photovoltaic module or an energy storage battery into alternating current and transmitting the alternating current to a power grid through the filter, the filter comprises a first inductor and a filter capacitor, the first inductor is connected in series with the filter capacitor and then connected in parallel with the inverter circuit, the first current sensor is used for detecting current flowing through the first inductor, and the second current sensor is used for detecting current flowing through the filter capacitor;
the controller is used for responding to the high-frequency current to be larger than a set threshold value, and controlling the on and off of a switching tube in the inverter circuit so as to reduce the high-frequency current flowing through the filter, wherein the high-frequency current is a current for filtering out an industrial frequency component in a difference current, the difference current is a current value obtained by subtracting a current flowing through the filter capacitor from a current flowing through the first inductor, and the industrial frequency component is a current with the industrial frequency in the difference current.
2. The power converter of claim 1, further comprising a subtracting circuit having one end connected to the first current sensor and the second current sensor and another end connected to the controller, the subtracting circuit configured to output a signal to the controller that characterizes the differential current.
3. A power converter according to claim 1 or 2, further comprising a high pass filter circuit connected between the subtracting circuit and the controller, the high pass filter circuit being adapted to output a signal representative of the high frequency current to the controller.
4. A power converter according to any of claims 1-3, characterized in that the filter further comprises a second inductance, one end of which is connected to the first inductance and to the filter capacitance, and the other end of which is intended to be connected to the power grid.
5. The power converter of any of claims 1-4, wherein the first current sensor is one of a current hall, a current sampling resistor, and a current sampling transformer, and the second current sensor is one of a current hall, a current sampling resistor, and a current sampling transformer.
6. The power converter of any of claims 1-5, wherein the power converter is a single-phase power converter or the power converter is a three-phase power converter.
7. The power converter of any of claims 1-6, wherein the inverter circuit comprises a T-type three-level leg or the inverter circuit comprises an I-type three-level leg.
8. A parallel operation loop suppression method, characterized in that the method comprises:
and responding to the high-frequency current to be larger than a set threshold value, controlling the switching tube in the inverter circuit to be turned on and off so as to reduce the high-frequency current flowing through the filter, wherein the filter is connected with an alternating current output end of the inverter circuit and comprises a first inductor and a filter capacitor, the first inductor is connected with the filter capacitor in series and then connected with the inverter circuit in parallel, the high-frequency current is a current for filtering out an industrial frequency component in a differential current, the differential current is a current value obtained by subtracting the current flowing through the filter capacitor from the current flowing through the first inductor, and the industrial frequency component is the current with the industrial frequency in the differential current.
9. A power supply system, characterized in that the power supply system comprises a plurality of power converters according to any one of claims 1-7, wherein the direct current input ends of the power converters are used for being connected with a photovoltaic module or an energy storage battery, and the alternating current output ends of the power converters are connected in parallel and then are used for being connected with a power grid.
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