Apparatus and method for reactive power compensation and harmonic suppression of a photovoltaic grid-connected system

Abstract

The present application relates to a kind of reactive compensation and harmonic suppression of photovoltaic grid-connected system device and method, comprising: variable unit, for realizing maximum power tracking control;Inverter unit, for the current circuit of fixed voltage generation of output with grid voltage same frequency, same phase output;Filter unit, for filtering the high-frequency burr caused by switching device on-off to current circuit;Resonance unit, for the passive filter of current circuit after filtering high-frequency burr;Signal acquisition module, for collecting the voltage, current signal of output side circuit as electric signal;Signal conditioning module, for realizing the detection of reactive and harmonic current, and the reactive and harmonic current that need to be compensated are calculated;And with photovoltaic array active component synthesis unified control composite instruction current, form current inner loop control, control the output of inverter unit to obtain the required grid current.The present application can filter the high-frequency burr caused by switching device on-off, effectively overcome the capacity limit of filter unit.

Description

Technical Field

[0001] This invention belongs to the field of electrical equipment technology, and relates to a device and method for reactive power compensation and harmonic suppression, particularly a device and method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system. Background Technology

[0002] In existing technologies, photovoltaic (PV) grid-connected power generation only provides active power to the grid, that is, inverting the DC power of the solar photovoltaic array into AC power and feeding it to the grid while ensuring a high power factor. The reactive power of the load is generally provided by the grid. With the widespread application of PV power generation technology, implementing reactive power compensation and harmonic suppression technologies during the PV grid-connected inverter process can better adapt to the grid connection needs of the power system, and is of great significance for reducing the burden on the grid and improving power supply quality.

[0003] Currently, one of the key technologies for reactive power compensation and harmonic suppression in photovoltaic grid-connected systems is the detection of reactive current and harmonic current. For example, the device and method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system, as proposed in announcement number CN102074965B, controls the output current of the inverter unit, thereby causing the current on the load to track the command current in a sawtooth wave shape, thus canceling harmonic current and reactive current, achieving the purpose of harmonic suppression and reactive power compensation. It has a simple structure, real-time detection and control, and a fast current response.

[0004] However, the above solution still has shortcomings. It requires a load circuit (nonlinear load) to achieve harmonic suppression and reactive power compensation. Therefore, the control structure needs to be further optimized. In addition, the voltage and circuit data collected by the current acquisition module and connected to the inverter may have glitches caused by the switching devices during the on-off control.

[0005] A search revealed no prior art that is identical or similar to this invention. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and to propose a device and method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system. This device can filter out high-frequency glitches caused by the switching of switching devices, effectively overcome the capacity limitation of the filter unit, and effectively reduce the system cost.

[0007] The present invention solves its practical problem by adopting the following technical solution:

[0008] A device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system, comprising:

[0009] The conversion unit is used to effectively output a fixed voltage from the voltage generated by the photovoltaic array in order to achieve maximum power point tracking control;

[0010] An inverter unit is a circuit used to generate a current output from a fixed output voltage that is in phase and frequency with the mains voltage.

[0011] The filtering unit is used to filter out high-frequency glitches caused by the switching devices switching on and off to the current circuit.

[0012] The resonant unit is used to passively filter the current circuit after filtering out high-frequency glitches, serving as the output-side circuit.

[0013] The signal acquisition module is used to acquire the voltage and current signals of the output circuit as electrical signals.

[0014] The signal conditioning module is used to detect reactive and harmonic currents based on electrical signals, calculate the reactive and harmonic currents that need to be compensated, and synthesize them with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current.

[0015] Furthermore, the reactive power compensation and harmonic suppression device of the photovoltaic grid-connected system also includes a load for connecting to the output side circuit. The conversion unit is based on a Boost DC-DC converter, and the inverter unit is based on a full-bridge inverter circuit. The conversion unit and the inverter unit are connected through a DC bus.

[0016] Furthermore, the filter unit includes three capacitors that are respectively connected to the A, B, and C phase lines of the current circuit.

[0017] Furthermore, the resonant unit includes three resonant branches consisting of a capacitor, an inductor, and a capacitor, which are respectively connected to the A, B, and C phase lines of the current circuit.

[0018] Furthermore, a coupling transformer is connected between the filtering unit and the resonant unit.

[0019] Furthermore, the signal acquisition module is based on an AC voltage transformer, and the signal conditioning module includes a microcontroller and a processing module, with the AC voltage transformer and processing module connected to the microcontroller.

[0020] Furthermore, the microcontroller samples two of the A, B, and C phases of the current circuit based on the AC voltage transformer and sends them to the arithmetic module. The arithmetic module collects the instantaneous values ​​of the two voltage signals and constructs sine and cosine signals with the same frequency and phase as the grid voltage by combining trigonometric function calculations. After passing through the mean filter, it obtains a three-phase symmetrical unit sine signal and finally synthesizes a unified control composite command current.

[0021] Furthermore, the detection of reactive and harmonic currents includes the difference between the three-phase load current and the fundamental active current component. The fundamental active current is obtained by combining the MPPT unit with the AVR unit to obtain the fundamental active current with the same frequency and phase as the grid voltage. The MPPT unit and the AVR unit are connected to the photovoltaic array to generate the output circuit.

[0022] A method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system includes:

[0023] The voltage generated by the photovoltaic array is effectively output as a fixed voltage to achieve maximum power point tracking control;

[0024] A circuit that generates a current output from a fixed output voltage in the same frequency and phase as the mains voltage;

[0025] Filter out high-frequency glitches caused by the switching devices switching on and off to supply current to the circuit;

[0026] The current circuit after filtering out high-frequency glitches is passively filtered and used as the output side circuit.

[0027] The voltage and current signals of the output circuit are collected as electrical signals;

[0028] Based on the electrical signal, reactive and harmonic currents are detected and the reactive and harmonic currents that need to be compensated are calculated. Then, the reactive and harmonic currents are combined with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current.

[0029] Furthermore, the specific method for detecting reactive and harmonic currents based on electrical signals, calculating the reactive and harmonic currents that need compensation, and synthesizing them with the active component of the photovoltaic array to form a unified control composite command current is as follows:

[0030] The instantaneous expression for the three-phase grid voltage is as follows:

[0031] Where V is the instantaneous maximum value, and ωt+Φ is the phase;

[0032] For two phases of the three-phase system, sample the voltage signals of phases A and B, and obtain the following results based on trigonometric relationships:

[0033]

[0034] Among them: U m Voltage amplitude;

[0035]

[0036] The calculations for M and N are as follows:

[0037]

[0038] Combining equations (2) and (3), the unit sine and cosine signals with the same frequency and phase as the A-phase grid voltage can be obtained as follows:

[0039]

[0040] The required three-phase reference voltage signal can be constructed based on the trigonometric function relationship and equation (4);

[0041]

[0042] To detect reactive and harmonic currents, simply disconnect the Gq branch in the algorithm, perform single-operation on the Gp branch, and then subtract the obtained fundamental active current component from the three-phase load current. After obtaining the DC component of Gp, directly subtract it from the photovoltaic active DC component Ipv to calculate the command current. The formula can be expressed as:

[0043]

[0044] In the formula, the first part is the compensation command current for reactive power and harmonics, and the second term is the fundamental active current of the photovoltaic grid connection.

[0045] Advantages and beneficial effects of the present invention:

[0046] 1. This invention proposes a device and method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system. The signal conditioning module includes a computation module, which improves dynamic response capability through a mean filter. Furthermore, the overall structure of the device is simplified, effectively filtering out high-frequency glitches caused by the switching of devices, overcoming the capacity limitations of the filtering unit, and significantly reducing system cost. Digital signal processing is also used to reduce detection errors introduced by analog circuits.

[0047] 2. The device of the present invention not only realizes the photovoltaic grid-connected power generation function, but also has a significant effect on improving the harmonics and reactive current generated by nonlinear loads, thereby improving the power supply quality and capacity of the power grid. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the circuit structure and control block diagram of the device of the present invention;

[0049] Figure 2 This is a block diagram illustrating the principle of the signal acquisition module and the signal conditioning module of the present invention.

[0050] Figure 3 This is a schematic diagram of the synthesized instruction current of the computing module of the present invention. Detailed Implementation

[0051] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings:

[0052] A device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system, such as... Figure 1 and Figure 2 As shown, it includes:

[0053] The conversion unit is used to effectively output a fixed voltage from the voltage generated by the photovoltaic array in order to achieve maximum power point tracking control;

[0054] An inverter unit is a circuit used to generate a current output from a fixed output voltage that is in phase and frequency with the mains voltage.

[0055] The filtering unit is used to filter out high-frequency glitches caused by the switching devices switching on and off to the current circuit.

[0056] The resonant unit is used to passively filter the current circuit after filtering out high-frequency glitches, serving as the output-side circuit.

[0057] The signal acquisition module is used to acquire the voltage and current signals of the output circuit as electrical signals.

[0058] The signal conditioning module is used to detect reactive and harmonic currents based on electrical signals, calculate the reactive and harmonic currents that need to be compensated, and synthesize them with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current.

[0059] In this embodiment, the reactive power compensation and harmonic suppression device of the photovoltaic grid-connected system further includes a load for connecting to the output side circuit. The conversion unit is based on a Boost DC-DC converter, and the inverter unit is based on a full-bridge inverter circuit. The conversion unit and the inverter unit are connected through a DC bus.

[0060] In this embodiment, the filtering unit includes three capacitors that are respectively connected to the A, B, and C phase lines of the current circuit.

[0061] In this embodiment, the resonant unit includes a resonant branch consisting of three capacitors, an inductor, and a capacitor, which are respectively connected to the A, B, and C phase lines of the current circuit.

[0062] In this embodiment, a coupling transformer is connected between the filtering unit and the resonant unit.

[0063] In this embodiment, the signal acquisition module is based on an AC voltage transformer, and the signal conditioning module includes a microcontroller and a processing module, with the AC voltage transformer and processing module connected to the microcontroller.

[0064] In this embodiment, the microcontroller samples two of the A, B, and C phases of the current circuit based on the AC voltage transformer and sends them to the arithmetic module. The arithmetic module collects the instantaneous values ​​of the two voltage signals, combines them with trigonometric function calculations to construct sine and cosine signals that are in phase and frequency with the grid voltage, passes them through a mean filter, and then obtains a three-phase symmetrical unit sine signal. Finally, it synthesizes a composite command current for unified control.

[0065] In this embodiment, the detection of reactive and harmonic currents includes the difference between the three-phase load current and the fundamental active current component. The fundamental active current is obtained by combining the MPPT unit with the AVR unit to obtain the fundamental active current with the same frequency and phase as the grid voltage. The MPPT unit and the AVR unit are connected to the output circuit of the photovoltaic array.

[0066] A method for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system includes:

[0067] The voltage generated by the photovoltaic array is effectively output as a fixed voltage to achieve maximum power point tracking control;

[0068] A circuit that generates a current output from a fixed output voltage in the same frequency and phase as the mains voltage;

[0069] Filter out high-frequency glitches caused by the switching devices switching on and off to supply current to the circuit;

[0070] The current circuit after filtering out high-frequency glitches is passively filtered and used as the output side circuit.

[0071] The voltage and current signals of the output circuit are collected as electrical signals;

[0072] Based on the electrical signal, reactive and harmonic currents are detected and the reactive and harmonic currents that need to be compensated are calculated. Then, the reactive and harmonic currents are combined with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current.

[0073] The specific method for detecting reactive and harmonic currents based on electrical signals, calculating the reactive and harmonic currents that need compensation, and synthesizing them with the active component of the photovoltaic array to form a unified control composite command current is as follows:

[0074] The instantaneous expression for the three-phase grid voltage is as follows:

[0075] Where V is the instantaneous maximum value, and ωt+Φ is the phase;

[0076] For two phases of the three-phase system, sample the voltage signals of phases A and B, and obtain the following results based on trigonometric relationships:

[0077]

[0078] Among them: U m Voltage amplitude;

[0079]

[0080] The calculations for M and N are as follows:

[0081]

[0082] Combining equations (2) and (3), the unit sine and cosine signals with the same frequency and phase as the A-phase grid voltage can be obtained as follows:

[0083]

[0084] The required three-phase reference voltage signal can be constructed based on the trigonometric function relationship and equation (4);

[0085]

[0086] To detect reactive and harmonic currents, simply disconnect the Gq branch in the algorithm, perform single-operation on the Gp branch, and then subtract the obtained fundamental active current component from the three-phase load current. After obtaining the DC component of Gp, directly subtract it from the photovoltaic active DC component Ipv to calculate the command current. The formula can be expressed as:

[0087]

[0088] In the formula, the first part is the compensation command current for reactive power and harmonics, and the second term is the fundamental active current of the photovoltaic grid connection.

[0089] In this invention, such as Figure 1 As shown, the photovoltaic grid-connected system provides active power to the grid. Specifically, it generates electricity through a photovoltaic array, and the resulting DC power passes through a Boost-type DC-DC converter (comprising inductor L1, switch S, and diode D1) to achieve maximum power point tracking. The subsequent stage is a full-bridge inverter circuit, which generates current in phase and frequency matching the grid voltage. Both are connected via DC bus C2. The full-bridge inverter circuit outputs three-phase AC power, which is then filtered by a filter unit composed of capacitors C4-C6 through inductors L2-L4 to remove high-frequency glitches caused by the switching of the devices. The power is then transmitted to a resonant unit via a coupling transformer TB. This resonant unit consists of capacitors C6-C8, inductors L5-L7, and capacitors C9-C11, forming a secondary passive filter branch. Inductors L5-L7 and capacitors C6-C8 resonate in series at the fundamental frequency, allowing the inverter to withstand only a small fundamental voltage. This effectively overcomes the capacity limitation of the filter unit and significantly reduces system costs.

[0090] In this invention, the DC-side voltage U1 of the full-bridge inverter circuit is used as a reference value for the active power component by the difference between the voltage detected on the DC side and the reference voltage Ux, which is then processed by the VCR unit. The MPPT unit is a maximum power point tracking control unit, which determines the maximum power point operating voltage of the photovoltaic array, while the VCR unit is a voltage regulation control unit.

[0091] In this invention, a signal acquisition module collects the instantaneous values ​​of two voltage signals, while a signal conditioning module combines trigonometric function operations to construct sine and cosine signals that are in phase and frequency with the grid voltage, thereby obtaining the required three-phase symmetrical unit sine signal. The device eliminates analog circuits such as phase-locked loops and sine / cosine generators, reducing detection errors caused by analog circuits through digital signal processing and saving hardware development costs.

[0092] In this invention, the instantaneous expression of the three-phase grid voltage is:

[0093] Where V is the instantaneous maximum value, and ωt+Φ is the phase;

[0094] For two phases of the three-phase system, sample the voltage signals of phases A and B, and obtain the following results based on trigonometric relationships:

[0095]

[0096] Among them: U m Voltage amplitude;

[0097]

[0098] The calculations for M and N are as follows:

[0099]

[0100] Combining equations (2) and (3), the unit sine and cosine signals with the same frequency and phase as the A-phase grid voltage can be obtained as follows:

[0101]

[0102] The required three-phase reference voltage signal can be constructed based on the trigonometric function relationship and equation (4);

[0103]

[0104] To detect reactive and harmonic currents, simply disconnect the Gq branch in the algorithm, perform single-operation on the Gp branch, and then subtract the obtained fundamental active current component from the three-phase load current. After obtaining the DC component of Gp, directly subtract it from the photovoltaic active DC component Ipv to calculate the command current. The formula can be expressed as:

[0105]

[0106] In the formula, the first part is the compensation command current for reactive power and harmonics, and the second term is the fundamental active current of the photovoltaic grid connection.

[0107] The signal conditioning module of this invention includes a computation module, which improves dynamic response capability through a mean filter. The overall structure of the device is simplified, effectively filtering out high-frequency glitches caused by the switching of switching devices, overcoming the capacity limitation of the filtering unit, and effectively reducing system cost. It also reduces detection errors caused by analog circuits by using digital signal processing.

[0108] It should be emphasized that the embodiments described in this invention are illustrative rather than limiting. Therefore, this invention includes, but is not limited to, the embodiments described in the specific implementation. Any other implementations derived by those skilled in the art based on the technical solutions of this invention are also within the scope of protection of this invention.

Claims

1. A device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system, characterized in that: include: The conversion unit is used to effectively output a fixed voltage from the voltage generated by the photovoltaic array in order to achieve maximum power point tracking control; An inverter unit is a circuit used to generate a current output from a fixed output voltage that is in phase and frequency with the mains voltage. The filtering unit is used to filter out high-frequency glitches caused by the switching devices switching on and off to the current circuit. The resonant unit is used to passively filter the current circuit after filtering out high-frequency glitches, serving as the output-side circuit. The signal acquisition module is used to acquire the voltage and current signals of the output circuit as electrical signals. The signal conditioning module is used to detect reactive and harmonic currents based on electrical signals, calculate the reactive and harmonic currents that need to be compensated, and synthesize them with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current. The signal acquisition module is based on an AC voltage transformer, and the signal conditioning module includes a microcontroller and a processing module, wherein the AC voltage transformer and the processing module are connected to the microcontroller. The microcontroller samples two of the A, B, and C phases of the current circuit based on the AC voltage transformer and sends them to the arithmetic module. The arithmetic module collects the instantaneous values ​​of the two voltage signals and constructs sine and cosine signals with the same frequency and phase as the grid voltage by combining trigonometric function calculations. After passing through the mean filter, it obtains a three-phase symmetrical unit sine signal and finally synthesizes a unified control composite command current. The detection of reactive and harmonic currents includes the difference between the three-phase load current and the fundamental active current component. The fundamental active current is obtained by combining the MPPT unit with the AVR unit to obtain the fundamental active current with the same frequency and phase as the grid voltage. The MPPT unit and the AVR unit are connected to the output circuit of the photovoltaic array.

2. The device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system according to claim 1, characterized in that: It also includes a load for connecting to the output-side circuit. The conversion unit is based on a Boost DC-DC converter, and the inverter unit is based on a full-bridge inverter circuit. The conversion unit and the inverter unit are connected via a DC bus.

3. The device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system according to claim 1, characterized in that: The filtering unit includes three capacitors that are respectively connected to the A, B, and C phase lines of the current circuit.

4. The device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system according to claim 1, characterized in that: The resonant unit includes three resonant branches consisting of a capacitor, an inductor, and a capacitor, which are respectively connected to the A, B, and C phase lines of the current circuit.

5. The device for reactive power compensation and harmonic suppression in a photovoltaic grid-connected system according to claim 1, characterized in that: A coupling transformer is connected between the filtering unit and the resonant unit.

6. The method for implementing a reactive power compensation and harmonic suppression device for a photovoltaic grid-connected system as described in any one of claims 1-5, characterized in that: include: The voltage generated by the photovoltaic array is effectively output as a fixed voltage to achieve maximum power point tracking control; A circuit that generates a current output from a fixed output voltage in the same frequency and phase as the mains voltage; Filter out high-frequency glitches caused by the switching devices switching on and off to supply current to the circuit; The current circuit after filtering out high-frequency glitches is passively filtered and used as the output side circuit. The voltage and current signals of the output circuit are collected as electrical signals; Based on the electrical signal, reactive and harmonic currents are detected and the reactive and harmonic currents that need to be compensated are calculated. Then, the reactive and harmonic currents are combined with the active component of the photovoltaic array to form a unified control composite command current, forming an inner current loop control, which controls the output of the inverter unit to obtain the required grid current.

7. The method for implementing a reactive power compensation and harmonic suppression device for a photovoltaic grid-connected system according to claim 6, characterized in that: The specific method for detecting reactive and harmonic currents based on electrical signals, calculating the reactive and harmonic currents that need compensation, and synthesizing them with the active component of the photovoltaic array to form a unified control composite command current is as follows: The instantaneous expression for the three-phase grid voltage is as follows: (1) Where V is the instantaneous maximum value, and ωt+Φ is the phase; For two phases of the three-phase system, sample the voltage signals of phases A and B, and obtain the following results based on trigonometric relationships: （2） Among them: U m Voltage amplitude; ； The calculations for M and N are as follows: （3） Combining equations (2) and (3), the unit sine and cosine signals with the same frequency and phase as the A-phase grid voltage can be obtained as follows: （4）； The required three-phase reference voltage signal can be constructed based on the trigonometric function relationship and equation (4); （5）； To detect reactive and harmonic currents, simply disconnect the Gq branch in the algorithm, perform single-operation on the Gp branch, and then subtract the obtained fundamental active current component from the three-phase load current. After obtaining the DC component of Gp, directly subtract it from the photovoltaic active DC component Ipv to calculate the command current. The formula can be expressed as: （6） In the formula, the first part is the compensation command current for reactive power and harmonics, and the second term is the fundamental active current of the photovoltaic grid connection.

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