Electric energy quality controller structure of photovoltaic power generation compensation direct current bus

Abstract

The invention discloses an electric energy quality controller structure of a photovoltaic power generation compensation direct current bus. In an existing UPQC topological structure, direct-current bus integrated photovoltaic power generation is adopted to control voltage stability of a direct-current bus, so that a new UPQC topological structure is constructed, a photovoltaic module PV1 and a photovoltaic module PV2 are newly added to the new UPQC topological structure, and the photovoltaic module PV1 is connected to the load side of a power distribution network in parallel to provide electric energy for an alternating-current load; the photovoltaic module PV2 is connected to the direct current bus side through the boost module, directly charges the direct current bus capacitor, provides electric energy required by voltage compensation for the SAPF, and supplies power to the alternating current load through the PAPF when the power distribution network is powered off. The method has the beneficial effect that the effective compensation of the voltage energy of the direct-current bus during grid connection and islanding of the direct-current bus is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field related to microgrids, and in particular to a power quality controller structure for a photovoltaic power generation compensation DC bus. Background Art

[0002] As the proportion of microgrids in distribution networks continues to increase, the power generated by microgrids is volatile, which causes fluctuations in grid voltage. A large number of power electronic equipment connected to distribution networks also generates a large number of harmonics. Therefore, the power quality of distribution networks containing microgrids has received increasing attention. As an effective power quality management device, the unified power quality controller UPQC has pointed out the research direction for the comprehensive management of power quality in distribution networks. Since UPQC is a comprehensive power quality control equipment, it uses a series-parallel filter method to achieve comprehensive management of electric energy. The series-parallel filters (SAPF and PAPF) complete the bidirectional conversion of electric energy by sharing the DC bus. However, the energy coupling between the DC bus reduces the stability of the DC bus voltage control, directly affects the energy exchange between the two, and affects the compensation performance of UPQC. Therefore, it is of great significance to achieve a stable control strategy for the DC side voltage.

[0003] In recent years, although many research results have been achieved in the study of UPQC DC side voltage control, the PI feedback control technology is still the main control strategy. However, most of the current methods based on PI feedback controller parameter optimization still have many problems, especially the overall structure of UPQC is still limited to the existing energy compensation method of SAPF and PAPF sharing the DC bus, resulting in the instantaneous voltage shock generated when connected to the grid, which seriously affects the stability of the bus. Summary of the invention

[0004] The present invention aims to overcome the above-mentioned deficiencies in the prior art and provides a power quality controller structure for a photovoltaic power generation compensated DC bus that can ensure effective compensation of the DC bus voltage energy when the DC bus is grid-connected or islanded.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A power quality controller structure for photovoltaic power generation compensating a DC bus adopts DC bus integrated photovoltaic power generation in an existing UPQC topology to control the DC bus voltage stability, thereby constructing a new UPQC topology, wherein the new UPQC topology newly adds photovoltaic modules PV1 and PV2, wherein the photovoltaic module PV1 is connected in parallel to the load side of the distribution network to provide power for the AC load; the photovoltaic module PV2 is connected to the DC bus side through a boost module to directly charge the DC bus capacitor, providing the power required for voltage compensation for the SAPF, and supplying power to the AC load through the PAPF when the distribution network is powered off.

[0007] Aiming at the problems existing in the structure of current power quality controller and the optimization problem of PI feedback for DC bus voltage stabilization, the present invention proposes a power quality controller structure for photovoltaic power generation compensating DC bus to ensure effective compensation of DC bus voltage energy when the DC bus is grid-connected or islanded.

[0008] As a preference, for UPQC operation involving a microgrid, if the grid voltage u s Normally, PAPF improves the power factor of the distribution network through reactive power compensation; if the grid voltage u s If a drop occurs, SAPF outputs the corresponding compensation voltage u1 to ensure that the load voltage u L When operating within the standard range, PAPF needs to absorb active power from the grid to maintain the DC bus voltage u dc Constant; Assume that the equivalent parallel resistance of the DC bus end loss is R dc , the load is a series resistance load R L and series inductive load L L , the load current is i L , the load voltage is u L , then the mathematical model expression is as shown in formula (1):

[0009]

[0010] In the formula, i Ld Represents the load current i L The real part, i Lq Represents the load current i L The imaginary part of Ld and u Lq Similarly, they represent the load voltage u L The real and imaginary parts of

[0011] The UPQC DC bus voltage u dc The stable control is to adjust the active current injected into the grid and adopt PI control. The DC bus capacitor is affected by the UPQC compensation output voltage u1. The DC bus capacitor current expression is (2):

[0012]

[0013] Where V 1dc is the DC voltage output by SAPF, the expression of SAPF output current = I1sin(wt+I2), I1 is the amplitude of SAPF output current, L1 is the inductance on the output side of SAPF, R1 is the resistance on the output side of SAPF, and I2 is the phase of SAPF output current;

[0014] On the DC bus side of the SAPF, according to Kirchhoff's current law, it is as follows (3):

[0015]

[0016] In the formula, C dc is the DC bus capacitance, R s is the equivalent resistance of the power grid;

[0017] Since the grid current will not change transiently, according to the small signal model theory, assuming that i dc1 ,u 1cd ,u dc If there is a small signal disturbance near the steady-state operating point, then there is expression (4):

[0018]

[0019] Among them, I dc1 、u 1cd0 、u dc0 is the steady-state component of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. The other three variables are the transient small disturbance components of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. Substituting equation (4) into equations (2) and (3), the small signal equation of the SAPF DC side is as follows (5):

[0020]

[0021] Then the relationship between the UPQC DC bus capacitor voltage and the SAPF output compensation voltage can be obtained as follows:

[0022]

[0023] Where, T dc =0.5C dc R s , K dc =3I1 R s / 4u dc0 , K dc is the DC bus proportional constant, T dc is the DC bus proportional period, S is the Fourier S transform;

[0024] It can be seen from equation (6) that the DC bus capacitor voltage is affected by the output voltage.

[0025] Preferably, the specific framework structure of the photovoltaic module PV2 connected to the DC bus side through the boost module is as follows: photovoltaic module PV2, boost module, voltage and current acquisition module, drive module, DSP processor, battery and DC bus capacitor, the DSP processor is used as the main control system, the drive module adopts an IGBT module, wherein the photovoltaic module PV2, boost module, battery and DC bus capacitor are connected in sequence, the voltage and current acquisition module is connected to the input and output ends of the boost module to collect voltage and current and send them to the DSP processor, the DSP processor controls the drive module according to the collected voltage and current to feedback adjust the boost module.

[0026] As a preferred embodiment, the boost module adopts a Boost circuit, and the Boost circuit input voltage U pv With output voltage U o The relationship is as follows (7):

[0027] U pv ·D·T+(U pv -U0)·(1-D·T)=0 (7)

[0028] In the formula, the Boost circuit contains a switch tube, T is the switching period of the switch tube, and D is the duty cycle. The duty cycle is the ratio of the switch tube conduction time to the working period, and the value is between (0, 1). It can be sorted out as follows:

[0029]

[0030] To realize impedance transformation for Boost circuit, we can get the relationship between input and output voltage: U o =U L (1-D), according to the theory that the power before and after the transformation is equal, we have I o =I L / (1-D), where U L is the equivalent voltage, I L is the equivalent current, and the equivalent resistance R is connected in series eq As shown in formula (8):

[0031]

[0032] Where I0 is the output current, R L is the parallel equivalent resistance;

[0033] From formula (8), we can see that when the duty cycle D is adjusted, the Boost circuit is connected in series with an equivalent resistance R eq The internal resistance R of the photovoltaic panel of the photovoltaic module PV2 pv When the impedances are matched, the maximum output power of the photovoltaic cell is as shown in formula (9):

[0034]

[0035] The beneficial effect of the present invention is to ensure effective compensation of the voltage energy of the DC bus when the DC bus is grid-connected or islanded. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a topological diagram of the UPQC of the present invention;

[0037] Figure 2 is the UPQC DC bus voltage U dc PI control chart;

[0038] Figure 3 It is the photovoltaic power generation control block diagram of UPQC DC bus voltage;

[0039] Figure 4 This is the schematic diagram of the Boost circuit;

[0040] Figure 5 It is the equivalent diagram of the Boost circuit after impedance transformation. DETAILED DESCRIPTION

[0041] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0042] A photovoltaic power generation compensated DC bus power quality controller structure adopts DC bus integrated photovoltaic power generation to control DC bus voltage stability in the existing UPQC topology, thereby constructing a new UPQC topology to realize UPQC's multiple functions of providing DC side and important AC load power, such as Figure 1 As shown in the figure, the new UPQC topology structure adds photovoltaic modules PV1 and PV2. The photovoltaic module PV1 is connected in parallel to the load side of the distribution network to provide power for the AC load; the photovoltaic module PV2 is connected to the DC bus side through the boost module to directly charge the DC bus capacitor and provide the power required for voltage compensation for the SAPF. When the distribution network is powered off, the AC load can be powered through the PAPF. Therefore, this topology structure improves the versatility of UPQC. The entire topology Figure 1 In fact, it is an AC-DC conversion and inversion process. In the two conversion processes, a series filter SAPF and a parallel filter PAPF are added respectively, and a photovoltaic module PV2 is added to the DC bus and a Boost boost module is added to adjust the DC bus. In addition, a photovoltaic module PV1 is connected to the load side.

[0043] For UPQC operation involving microgrids, if the grid voltage u s Normally, PAPF improves the power factor of the distribution network through reactive power compensation; if the grid voltage u s When a drop occurs, SAPF outputs the corresponding compensation voltage u c, ensure that the load voltage u L When operating within the standard range, PAPF needs to absorb active power from the grid to maintain the DC bus voltage u dc Constant; Assume that the equivalent parallel resistance of the DC bus end loss is R dc , the load is a series resistance load R L and series inductive load L L , the load current is i L , the load voltage is u L , then the mathematical model expression is as shown in formula (1):

[0044]

[0045] In the formula, i Ld Represents the load current i L The real part, i Lq Represents the load current i L The imaginary part of Ld and u Lq Similarly, they represent the load voltage u L The real and imaginary parts of

[0046] The UPQC DC bus voltage u dc The stable control is usually to adjust the active current injected into the power grid. Currently, PI control is used, such as Figure 2 As shown in the figure, a ,i b ,i c For three-phase input, i ah ,i bh ,i ch For three-phase output, C 32 and C 23 For three-phase to two-phase conversion and corresponding inversion, C wt With C wt -1 is the wavelet transform and inverse wavelet transform, U dcf and U dcr are the command value and effective value of the DC bus voltage, PI refers to the PI control algorithm, and sin and -cos represent the output of a set of sine and cosine waveform signal generators.

[0047] according to Figure 2 As shown, the DC bus capacitor is affected by the UPQC compensation output voltage u1, and the DC bus capacitor current expression is (2):

[0048]

[0049] Where V 1dcis the DC voltage output by SAPF, the expression of SAPF output current = I1sin(wt+I2), I1 is the amplitude of SAPF output current, L1 is the inductance on the output side of SAPF, R1 is the resistance on the output side of SAPF, and I2 is the phase of SAPF output current;

[0050] On the DC bus side of the SAPF, according to Kirchhoff's current law, it is as follows (3):

[0051]

[0052] In the formula, C dc is the DC bus capacitance, R s is the equivalent resistance of the power grid;

[0053] Since the grid current will not change transiently, according to the small signal model theory, assuming that i dc1 ,u 1cd ,u dc If there is a small signal disturbance near the steady-state operating point, then there is expression (4):

[0054]

[0055] In the formula, u 1cd is the DC voltage output by SAPF. The above formula can be simply expressed as A=A0+A1, where A0 refers to the initial value and A1 is the disturbance value of the wavelet signal. Therefore, I dc1 is the steady-state component of the DC bus capacitor current, is the transient wavelet disturbance component of the DC bus capacitor current, and the other two equations are the same. That is, in the formula, I dc1 、u 1cd0 、u dc0 is the steady-state component of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. The other three variables are the transient small disturbance components of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. Substituting equation (4) into equations (2) and (3), the small signal equation of the SAPF DC side is as follows (5):

[0056]

[0057] Then the relationship between the UPQC DC bus capacitor voltage and the SAPF output compensation voltage can be obtained as follows:

[0058]

[0059] Where, T dc =0.5C dc R s , K dc =3I1 R s / 4u dc0 , K dc is the DC bus proportional constant, T dc is the DC bus proportional period, S is the Fourier S transform;

[0060] It can be seen from equation (6) that the DC bus capacitor voltage is affected by the output voltage, and the energy coupling between the SAPF and PAPF also reduces the stability of the DC bus voltage control. In addition, the DC bus voltage will deviate significantly from the reference in the transient process caused by load connection and removal, load voltage drop and surge, etc., which has a significant impact on the compensation control of UPQC.

[0061] In view of the above situation, if Figure 3 As shown in the figure, the specific framework structure of the photovoltaic module PV2 connected to the DC bus side through the boost module is as follows: photovoltaic module PV2, boost module, voltage and current acquisition module, drive module, DSP processor, battery and DC bus capacitor. The DSP processor is used as the main control system, and the drive module uses an IGBT module. The photovoltaic module PV2, boost module, battery and DC bus capacitor are connected in sequence. The voltage and current acquisition module is connected to the input and output ends of the boost module to collect voltage and current and send them to the DSP processor. The DSP processor controls the drive module to feedback and adjust the boost module according to the collected voltage and current. Among them, the DSP minimum system is the DSP processor, which is actually a single-chip microcomputer used to adjust the Boost boost module. The DSP minimum system is also connected to the power circuit and peripheral circuit.

[0062] The boost module uses a Boost circuit, and its schematic diagram is as follows Figure 4 shown. Figure 4 In the figure, VT is a switch tube, which is controlled by the driving module. When it is turned on, the current of the photovoltaic module PV2 flows back through the switch tube and cannot flow to RL, and the Boost module stops working; when it is turned off, the Boost module works. VD is a diode, which plays a reverse cutoff role. L is an inductor, which plays a role in the dead time generated when the switch tube is turned on and off, and can play a role in continuous current. Figure 4 The Boost circuit input voltage U can be obtained pv With output voltage U o The relationship is as follows (7):

[0063] U pv ·D·T+(U pv -U0)·(1-D·T)=0 (7)

[0064] In the formula, the Boost circuit contains a switch tube, T is the switching period of the switch tube, and D is the duty cycle. The duty cycle is the ratio of the switch tube conduction time to the working period, and the value is between (0, 1). It can be sorted out as follows:

[0065]

[0066] To achieve impedance transformation for Boost circuit, Figure 5 As shown. The equivalent of the boost circuit after impedance transformation Figure 5 In the figure, the upper figure is the equivalent circuit after the impedance of the circuit diagram is combined, and the lower figure is the circuit diagram in the initial state.

[0067] From the relationship between input and output voltage, we can get: U o =U L (1-D), according to the theory that the power before and after the transformation is equal, we have I o =I L / (1-D), where U L is the equivalent voltage, I L is the equivalent current, and the equivalent resistance R is connected in series eq As shown in formula (8):

[0068]

[0069] Where I0 is the output current, R L is the parallel equivalent resistance;

[0070] Note: In Thevenin's theorem, any two-terminal network containing a source circuit can be equivalent to a voltage source in series with a resistor, that is, a real voltage source. L is the voltage of the equivalent voltage source, I L is the current of the equivalent current source, R eq is the equivalent series resistance, R L is the parallel equivalent resistance, the equivalent voltage source U L With the series equivalent resistance R eq Correspondingly, the equivalent current source I L With the parallel equivalent resistance R L correspond.

[0071] From formula (8), we can see that when the duty cycle D is adjusted, the Boost circuit equivalent input impedance R eq The internal resistance R of the photovoltaic panel of the photovoltaic module PV2 pv When the impedances are matched, the maximum output power of the photovoltaic cell is as shown in formula (9):

[0072]

[0073] The core of the topological structure of this scheme is: a photovoltaic power generation unit (photovoltaic module PV2) with a Boost boost module is connected in parallel to the DC bus, and the output end of the Boost boost module is connected in parallel to the DC bus, controlled by the drive module, and the drive module is controlled by the DSP minimum system (DSP processor). The DSP minimum system is connected to a voltage and current acquisition module, which simultaneously acquires the voltage and current of the photovoltaic power generation unit and the DC bus, and compares the voltage and current at both ends in real time. When the voltage of the DC bus drops and flickers, the DSP minimum system can compare the imbalance between the photovoltaic power generation unit and the DC bus voltage, and then control the switch tube of the Boost boost module to cut off, so that the circuit of the Boost boost module is turned on, and the voltage of the photovoltaic power generation unit can compensate the DC bus, and at the same time compensate the DC bus capacitor, and the DC bus capacitor can play the role of peak elimination and valley filling. When the DSP minimum system collects and compares the voltage of the photovoltaic power generation unit and the DC bus to reach a balance, the switch tube of the Boost boost module is controlled to be turned on, and the circuit of the Boost boost module is short-circuited, and the Boost boost module does not work.

[0074] In summary, the present invention aims at the problems existing in the structure of the current power quality controller and the DC bus voltage stabilization PI feedback optimization problem, and proposes a power quality controller structure for photovoltaic power generation compensation DC bus to ensure effective compensation of the DC bus voltage energy when the DC bus is grid-connected or islanded.

Claims

1. A power quality controller structure for a photovoltaic power generation compensation DC bus, characterized in that In the existing UPQC topology, DC bus integrated photovoltaic power generation is used to control the DC bus voltage stability, thereby constructing a new UPQC topology. The new UPQC topology adds photovoltaic modules PV1 and PV2. The photovoltaic module PV1 is connected in parallel to the load side of the distribution network to provide power for the AC load; the photovoltaic module PV2 is connected to the DC bus side through a boost module to directly charge the DC bus capacitor and provide the SAPF with the power required for voltage compensation. When the distribution network is powered off, the AC load is powered through the PAPF.

2. The power quality controller structure of a photovoltaic power generation compensation DC bus according to claim 1 is characterized in that: For UPQC operation involving microgrids, if the grid voltage u s Normally, PAPF improves the power factor of the distribution network through reactive power compensation; if the grid voltage u s If a drop occurs, SAPF outputs the corresponding compensation voltage u1 to ensure that the load voltage u L When operating within the standard range, PAPF needs to absorb active power from the grid to maintain the DC bus voltage u dc Constant; Assume that the equivalent parallel resistance of the DC bus end loss is R dc , the load is a series resistance load R L and series inductive load L L , the load current is i L , the load voltage is u L , then the mathematical model expression is as shown in formula (1): In the formula, i Ld Represents the load current i L The real part, i Lq Represents the load current i L The imaginary part of Ld and u Lq Similarly, they represent the load voltage u L The real and imaginary parts of The UPQC DC bus voltage u dc The stable control is to adjust the active current injected into the grid and adopt PI control. The DC bus capacitor is affected by the UPQC compensation output voltage u1. The DC bus capacitor current expression is (2): Where V 1dc is the DC voltage output by SAPF, the expression of SAPF output current = I1sin(wt+I2), I1 is the amplitude of SAPF output current, L1 is the inductance on the output side of SAPF, R1 is the resistance on the output side of SAPF, and I2 is the phase of SAPF output current; On the DC bus side of the SAPF, according to Kirchhoff's current law, it is as follows (3): In the formula, C dc is the DC bus capacitance, R s is the equivalent resistance of the power grid; Since the grid current will not change transiently, according to the small signal model theory, assuming that i dc1 ,u 1cd ,u dc If there is a small signal disturbance near the steady-state operating point, then there is expression (4): Among them, I dc1 、u 1cd0 、u dc0 is the steady-state component of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. The other three variables are the transient small disturbance components of the DC bus capacitor current, SAPF output DC voltage, and DC bus voltage. Substituting equation (4) into equations (2) and (3), the small signal equation of the SAPF DC side is as follows (5): Then the relationship between the UPQC DC bus capacitor voltage and the SAPF output compensation voltage can be obtained as follows: Where, T dc =0.5C dc R s , K dc =3I1 R s / 4u dc0 , K dc is the DC bus proportional constant, T dc is the DC bus proportional period, S is the Fourier S transform; It can be seen from equation (6) that the DC bus capacitor voltage is affected by the output voltage.

3. The power quality controller structure of a photovoltaic power generation compensation DC bus according to claim 2 is characterized in that: The specific framework structure of the photovoltaic module PV2 connected to the DC bus side through the boost module is as follows: photovoltaic module PV2, boost module, voltage and current acquisition module, drive module, DSP processor, battery and DC bus capacitor. The DSP processor is used as the main control system, and the drive module adopts an IGBT module. The photovoltaic module PV2, boost module, battery and DC bus capacitor are connected in sequence. The voltage and current acquisition module is connected to the input and output ends of the boost module to collect voltage and current and send them to the DSP processor. The DSP processor controls the drive module according to the collected voltage and current to feedback adjust the boost module.

4. The power quality controller structure of a photovoltaic power generation compensation DC bus according to claim 3 is characterized in that: The boost module uses a Boost circuit, and the Boost circuit input voltage U pv With output voltage U o The relationship is as follows (7): U pv ·D·T+(U pv -U0)·(1-D·T)=0 (7) In the formula, the Boost circuit contains a switch tube, T is the switching period of the switch tube, and D is the duty cycle. The duty cycle is the ratio of the switch tube conduction time to the working period, and the value is between (0, 1). It can be sorted out as follows: To realize impedance transformation for Boost circuit, we can get the relationship between input and output voltage: U o =U L (1-D), according to the theory that the power before and after the transformation is equal, we have I o =I L / (1-D), where U L is the equivalent voltage, I L is the equivalent current, and the equivalent resistance R is connected in series eq As shown in formula (8): Where I0 is the output current, R L is the parallel equivalent resistance; From formula (8), we can see that when the duty cycle D is adjusted, the Boost circuit is connected in series with an equivalent resistance R eq The internal resistance R of the photovoltaic panel of the photovoltaic module PV2 pv When the impedances are matched, the maximum output power of the photovoltaic cell is as shown in formula (9):

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