Photovoltaic low-pass control strategy considering positive and negative sequence dynamic reactive current support and frequency multiplication power fluctuation suppression

Abstract

The invention discloses a photovoltaic low-pass control strategy considering positive and negative sequence dynamic reactive current support and frequency multiplication power fluctuation suppression, and the strategy comprises the steps: obtaining an expression of instantaneous power outputted by an inverter according to an instantaneous power theorem; through Park transformation, positive and negative sequence power grid voltage and current components under the synchronous rotating coordinate system are obtained, and then the relation between each power component output by an inverter and the positive and negative sequence voltage and current components is obtained; designing q-axis positive and negative sequence current instruction values after the fault by considering the requirement of positive and negative sequence dynamic reactive current support; under the condition that q-axis positive and negative sequence current instruction values after the fault are known, d-axis positive and negative sequence current candidate instruction values after the fault are calculated; in consideration of the through-current capability of the inverter, under the condition that a q-axis positive sequence current instruction value after a fault is known, a d-axis positive sequence current instruction value after amplitude limiting is obtained so as to adjust the d-axis positive sequence current instruction value after the fault, and then the d-axis positive sequence current instruction value after the fault is obtained; and taking the post-fault d-axis negative-sequence current candidate instruction value as a post-fault d-axis negative-sequence current instruction value. According to the invention, a reference basis is provided for low-voltage ride-through control of the photovoltaic electric field.

Description

Technical Field

[0001] The invention relates to a photovoltaic low-power wear-through control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubled power fluctuation suppression, and belongs to the technical field of photovoltaic grid-connected system fault control. Background Art

[0002] New energy is developing vigorously, and photovoltaic power generation, as the main force of new energy, is also growing rapidly in terms of capacity. In order to prevent the photovoltaic power station from being disconnected from the grid due to a rapid drop in the voltage at the grid connection point when a grid failure occurs, causing a secondary impact on the stable operation of the grid, the "Technical Regulations for Photovoltaic Power Station Access to the Power System" GB / T19964-2024 clearly states that photovoltaic power plants need to have the ability of low voltage ride through (LVRT) when a grid failure occurs.

[0003] The low-through control strategies of traditional photovoltaic inverters are to suppress negative sequence current, suppress active power and DC bus voltage double frequency fluctuations, and suppress reactive power double frequency fluctuations. However, it does not take into account the dynamic reactive current support capability required by photovoltaic grid-connected technical regulations. The standard clearly states that it is necessary to provide positive sequence reactive current support when a symmetrical fault occurs on the grid side, and when an asymmetrical fault occurs on the grid side, it is necessary to provide not only positive sequence dynamic reactive current support, but also negative sequence dynamic reactive current support.

[0004] Therefore, how to make the inverter provide the specified dynamic reactive current and suppress the frequency fluctuation of the inverter output power to provide a reference for the low voltage ride-through control of the photovoltaic field has become an urgent problem to be solved. Summary of the invention

[0005] The present invention provides a photovoltaic low-power wear control strategy that takes into account positive and negative sequence dynamic reactive current support and double frequency power fluctuation suppression. On the one hand, according to the requirements of positive and negative sequence dynamic reactive current support in the latest photovoltaic grid-connected technical regulations, the q-axis positive and negative sequence current command values ​​after the fault are designed; on the other hand, in order to simultaneously achieve the effect of suppressing the double frequency fluctuation of active power, the d-axis positive and negative sequence current candidate command values ​​after the fault are calculated, and further considering the maximum current carrying capacity of the inverter, the d-axis positive and negative sequence current candidate command values ​​after the fault are used when the limit is not reached, and the d-axis positive sequence current command value after the limit is used when the limit is reached; and then by changing the command value, the double frequency fluctuation of the inverter output power is suppressed.

[0006] The technical solution of the present invention is:

[0007] A photovoltaic low-power wear-through control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression includes the following steps:

[0008] Step 1: Based on the instantaneous power theorem, the expression of the instantaneous power output by the inverter is obtained; through Park transformation, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained, and then the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components is obtained;

[0009] Step 2: Considering the requirements of positive and negative sequence dynamic reactive current support, design the q-axis positive and negative sequence current command values ​​after the fault;

[0010] Step 3: When the positive and negative sequence current command values ​​of the q-axis after the fault are known, the candidate command values ​​of the positive and negative sequence current of the d-axis after the fault are calculated;

[0011] Step 4: Considering the current carrying capacity of the inverter, when the q-axis positive-sequence current command value after the fault is known, obtain the d-axis positive-sequence current command value after limiting to adjust the d-axis positive-sequence current command value after the fault, and then obtain the d-axis positive-sequence current command value after the fault; use the d-axis negative-sequence current command value after the fault as the d-axis negative-sequence current command value after the fault; use the q-axis positive and negative-sequence current command values ​​after the fault and the d-axis positive and negative-sequence current command values ​​after the fault as the complete command values ​​under the low-throughput strategy; complete the inverter low-throughput control by changing the command values.

[0012] Furthermore, the step 1 is specifically as follows:

[0013] When an asymmetric fault occurs in the power grid, the instantaneous power sent to the grid by the photovoltaic power station is expressed as:

[0014]

[0015] Where: P 瞬 , Q 瞬 The instantaneous active and reactive power sent to the grid by the photovoltaic power station, that is, the instantaneous power output by the inverter; P 0 , Q 0 P is the instantaneous active and reactive power DC component output by the inverter, S2 , Q S2 The inverter outputs sinusoidal active and reactive power components, P C2 , Q C2 is the cosine active and reactive power components output by the inverter; ω is the synchronous angular velocity, and t is the time;

[0016] When an asymmetric fault occurs in the power grid, the relationship between the power components output by the inverter and the positive and negative sequence voltage and current components is:

[0017]

[0018] Among them: P, N represent positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、eqN Represents the positive and negative sequence grid voltage components of the q axis; i dP 、i dN Represents the d-axis positive and negative sequence grid current components, i qP 、i qN Represents the q-axis positive and negative sequence grid current components.

[0019] Furthermore, the step 2 is specifically as follows:

[0020] Photovoltaic power stations should have the ability to support positive and negative sequence dynamic reactive current:

[0021]

[0022] Where: Δi tP Indicates the positive sequence dynamic reactive current increment injected into the grid by the photovoltaic power station; Δi tN Indicates the negative sequence reactive current increment absorbed from the power grid; K 2P , K 2N Indicates the positive and negative sequence current proportionality coefficient; U tP , U tN Indicates the per unit value of the positive and negative sequence components of the grid voltage; i N Indicates the rated current of the photovoltaic power station;

[0023] After taking into account the positive and negative sequence dynamic reactive current support capabilities, the specific expressions of the q-axis positive and negative sequence current command values ​​after the fault are established are as follows:

[0024]

[0025] Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault, i qref0 Indicates the q-axis positive sequence current command value before the fault.

[0026] Furthermore, the step three is specifically as follows:

[0027] Combined with the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components, after the q-axis positive and negative sequence current command values ​​after the fault are known, the candidate command values ​​of the d-axis positive and negative sequence current after the fault are calculated:

[0028]

[0029] Where: i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; P and N represent the positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; iqrefP 、i qrefN Indicates the q-axis positive and negative sequence current command values ​​after a fault.

[0030] Furthermore, the step 4 is specifically as follows:

[0031] When the q-axis positive sequence current command value after the fault is known, the expression of the d-axis positive sequence current command value after limiting is established, and the expression is as follows:

[0032]

[0033] Where: i drefPlimit Indicates the d-axis positive sequence current command value after being limited, I MAXP Represents the amplitude of the positive sequence current component, i qrefP Indicates the q-axis positive sequence current command value after a fault;

[0034] According to the d-axis positive sequence current command value after limiting and the selected d-axis positive and negative sequence current command values ​​after the fault, the d-axis positive sequence current command value after the fault is adjusted to obtain the d-axis positive sequence current command value after the fault; the q-axis positive and negative sequence current command values ​​after the fault and the d-axis positive and negative sequence current command values ​​after the fault are used as the complete command values ​​under the low wear strategy, and the expression is as follows:

[0035]

[0036] Where: i qrefN Indicates the q-axis negative sequence current command value after the fault; i drefP 、i drefN Indicates the positive and negative sequence current command value of the d-axis after the fault; i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; i qref0 Indicates the q-axis positive sequence current command value before the fault; Δi tP , Δi tN They respectively represent the positive-sequence dynamic reactive current increment injected into the grid by the PV power station and the negative-sequence reactive current increment absorbed from the grid.

[0037] Furthermore, the positive sequence current component amplitude expression is:

[0038]

[0039] Where: I MAXP Represents the amplitude of the positive sequence current component, I MAX Indicates the current amplitude output at the maximum current capacity of the inverter, I - represents the negative sequence current component, It is the absolute value of the phase difference between the positive sequence current and the negative sequence current.

[0040] The beneficial effect of the present invention is as follows: the present invention proposes a photovoltaic low-voltage ride-through strategy that takes into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression, which satisfies the effect of suppressing power frequency-doubling fluctuations while taking into account positive and negative sequence dynamic reactive current support. Therefore, compared with the traditional low-voltage ride-through control strategy that does not consider dynamic reactive current support, the present invention can effectively take into account positive and negative sequence dynamic reactive current support and suppress power frequency-doubling fluctuations to complete photovoltaic electric field low voltage ride-through control, providing a reference basis for photovoltaic electric field low voltage ride-through control. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The photovoltaic electric field low voltage ride-through control flow chart provided by the present invention;

[0042] Figure 2 Photovoltaic electric field equivalent topological structure diagram;

[0043] Figure 3 Photovoltaic field low voltage ride through control block diagram;

[0044] Figure 4 Schematic diagram of active power output from photovoltaic electric field ports under negative sequence low-through control;

[0045] Figure 5 Schematic diagram of reactive power output from photovoltaic electric field ports under negative sequence low-through control;

[0046] Figure 6 Schematic diagram of active power output from photovoltaic electric field ports under low wear control of the present invention;

[0047] Figure 7 Schematic diagram of reactive power output from a photovoltaic electric field port under low-breakdown control of the present invention. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. It should be noted that the embodiments in this application and the features in the embodiments can be combined with each other arbitrarily without conflict.

[0049] Example 1: Figure 1-7 As shown, according to a first aspect of an embodiment of the present invention, a photovoltaic low-through control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression is provided, comprising the following steps:

[0050] Step 1: Based on the instantaneous power theorem, the expression of the instantaneous power output by the inverter is obtained; through Park transformation, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained, and then the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components is obtained;

[0051] Step 2: Considering the requirements of positive and negative sequence dynamic reactive current support, design the q-axis positive and negative sequence current command values ​​after the fault;

[0052] Step 3: When the positive and negative sequence current command values ​​of the q-axis after the fault are known, the candidate command values ​​of the positive and negative sequence current of the d-axis after the fault are calculated;

[0053] Step 4: Considering the current carrying capacity of the inverter, when the q-axis positive-sequence current command value after the fault is known, obtain the d-axis positive-sequence current command value after limiting to adjust the d-axis positive-sequence current command value after the fault, and then obtain the d-axis positive-sequence current command value after the fault; use the d-axis negative-sequence current command value after the fault as the d-axis negative-sequence current command value after the fault; use the q-axis positive and negative-sequence current command values ​​after the fault and the d-axis positive and negative-sequence current command values ​​after the fault as the complete command values ​​under the low-throughput strategy; complete the inverter low-throughput control by changing the command values.

[0054] Furthermore, the step 1 is specifically as follows:

[0055] When an asymmetric fault occurs in the power grid, the instantaneous power sent to the grid by the photovoltaic power station is expressed as:

[0056]

[0057] Where: P 瞬 , Q 瞬 The instantaneous active and reactive power sent to the grid by the photovoltaic power station, that is, the instantaneous power output by the inverter; P 0 , Q 0 P is the instantaneous active and reactive power DC component output by the inverter, S2 , Q S2 The inverter outputs sinusoidal active and reactive power components, P C2 , Q C2 is the cosine active and reactive power components output by the inverter; ω is the synchronous angular velocity, and t is the time;

[0058] When an asymmetric fault occurs in the power grid, the relationship between the power components output by the inverter and the positive and negative sequence voltage and current components is:

[0059]

[0060] Among them: P, N represent positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、eqN Represents the positive and negative sequence grid voltage components of the q axis; i dP 、i dN Represents the d-axis positive and negative sequence grid current components, i qP 、i qN Represents the q-axis positive and negative sequence grid current components.

[0061] Furthermore, the step 2 is specifically as follows:

[0062] Photovoltaic power stations should have the ability to support positive and negative sequence dynamic reactive current:

[0063]

[0064] Where: Δi tP Indicates the positive sequence dynamic reactive current increment injected into the grid by the photovoltaic power station; Δi tN Indicates the negative sequence reactive current increment absorbed from the power grid; K 2P , K 2N Indicates the positive and negative sequence current proportionality coefficient; U tP , U tN Indicates the per unit value of the positive and negative sequence components of the grid voltage; i N Indicates the rated current of the photovoltaic power station;

[0065] After taking into account the positive and negative sequence dynamic reactive current support capabilities, the specific expressions of the q-axis positive and negative sequence current command values ​​after the fault are established are as follows:

[0066]

[0067] Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault, i qref0 Indicates the q-axis positive sequence current command value before the fault.

[0068] Furthermore, the step three is specifically as follows:

[0069] Combined with the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components, after the q-axis positive and negative sequence current command values ​​after the fault are known, the candidate command values ​​of the d-axis positive and negative sequence current after the fault are calculated:

[0070]

[0071] Where: i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; P and N represent the positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; iqrefP 、i qrefN Indicates the q-axis positive and negative sequence current command values ​​after a fault.

[0072] Furthermore, the step 4 is specifically as follows:

[0073] When the q-axis positive sequence current command value after the fault is known, the expression of the d-axis positive sequence current command value after limiting is established, and the expression is as follows:

[0074]

[0075] Where: i drefPlimit Indicates the d-axis positive sequence current command value after being limited, I MAXP Represents the amplitude of the positive sequence current component, i qrefP Indicates the q-axis positive sequence current command value after a fault;

[0076] According to the d-axis positive sequence current command value after limiting and the selected d-axis positive and negative sequence current command values ​​after the fault, the d-axis positive sequence current command value after the fault is adjusted to obtain the d-axis positive sequence current command value after the fault; the q-axis positive and negative sequence current command values ​​after the fault and the d-axis positive and negative sequence current command values ​​after the fault are used as the complete command values ​​under the low wear strategy, and the expression is as follows:

[0077]

[0078] Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault; i drefP 、i drefN Indicates the positive and negative sequence current command value of the d-axis after the fault; i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; i qref0 Indicates the q-axis positive sequence current command value before the fault; Δi tP , Δi tN They respectively represent the positive-sequence dynamic reactive current increment injected into the grid by the PV power station and the negative-sequence reactive current increment absorbed from the grid.

[0079] Furthermore, the positive sequence current component amplitude expression is:

[0080]

[0081] Where: I MAXP Represents the amplitude of the positive sequence current component, I MAX Indicates the current amplitude output at the maximum current capacity of the inverter, I - represents the negative sequence current component, It is the absolute value of the phase difference between the positive sequence current and the negative sequence current.

[0082] According to a second aspect of an embodiment of the present invention, a processor is provided, wherein the processor is used to run a program, wherein when the program is running, the photovoltaic low-breakthrough strategy taking into account the positive and negative sequence dynamic reactive current support and the double-frequency power fluctuation suppression as described above is executed.

[0083] Embodiment 2: A photovoltaic low-breakdown strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression is implemented in the following steps:

[0084] Taking the equivalent topological structure of a photovoltaic power station as an example, its equivalent topological diagram is as follows: Figure 2 As shown, assuming that a BC phase short circuit fault occurs in the system, the photovoltaic power station parameters are shown in Table 1:

[0085] Table 1 Specific parameters of grid-connected inverter

[0086]

[0087] In such Figure 2 A BC phase-to-phase short circuit fault occurs at point F shown in the figure. After the fault, the positive sequence voltage amplitude at the grid connection point drops to 0.7pu, from 0.48kV to 0.336kV, and the negative sequence voltage amplitude increases to 0.4pu, from 0kV to 0.192kV.

[0088] Combination Figure 3 Based on the above parameters, the following process is given according to the method of the present invention:

[0089] 1. According to the instantaneous power theorem, the expression of the instantaneous power output by the inverter is obtained; through Park transformation, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained, and then the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components is obtained, as follows:

[0090] When an asymmetric fault occurs in the power grid, due to the interaction between the positive and negative sequence voltages and currents, the inverter output power contains not only a DC component, but also a fluctuation of twice the grid frequency. Therefore, when an asymmetric fault occurs in the power grid, the instantaneous power sent to the grid by the photovoltaic power station is expressed as:

[0091]

[0092] Where: P 瞬 , Q 瞬 The instantaneous active and reactive power sent to the grid by the photovoltaic power station, that is, the instantaneous power output by the inverter; P 0 , Q 0 P is the DC component of the active and reactive instantaneous power output by the inverter, S2 , Q S2 The inverter outputs sinusoidal active and reactive power components, P C2 , QC2 is the cosine active and reactive power components output by the inverter; ω is the synchronous angular velocity, and t is the time.

[0093] When an asymmetric fault occurs in the power grid, the power components output by the inverter are expressed as follows:

[0094]

[0095] Where: P 0 , Q 0 P is the instantaneous active and reactive power DC component output by the inverter, S2 , Q S2 The inverter outputs sinusoidal active and reactive power components, P C2 , Q C2 The cosine active and reactive power components of the inverter output; P and N represent positive and negative sequences; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; i dP 、i dN Represents the d-axis positive and negative sequence grid current components, i qP 、i qN Represents the q-axis positive and negative sequence grid current components.

[0096] 2. Taking into account the requirements for positive and negative sequence dynamic reactive current support in the latest photovoltaic grid-connected technical regulations, the q-axis positive and negative sequence current command values ​​after the fault are designed, as follows:

[0097] Photovoltaic power stations should have the ability to support positive and negative sequence dynamic reactive current:

[0098]

[0099] Where: Δi tP , Δi tN They represent the positive sequence dynamic reactive current increment injected into the grid by the PV power station and the negative sequence reactive current increment absorbed from the grid respectively; K 2P , K 2N Indicates the positive and negative sequence current proportionality coefficients. The values ​​of the two coefficients should not be less than 1.0; U tP , U tN Indicates the per unit value of the positive and negative sequence components of the grid voltage; i N =0.48kA represents the rated current of the photovoltaic power station;

[0100] According to the photovoltaic electric field data and the voltage drop at the inverter port in Table 1, it can be seen that:

[0101]

[0102] Considering the current sampling direction of the control link, the actual value should be taken in the opposite direction, that is:

[0103]

[0104] After taking into account the positive and negative sequence dynamic reactive current support capability, the q-axis positive and negative sequence current command values ​​after the fault are obtained as follows:

[0105]

[0106] 3. Combined with the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components, after the q-axis positive and negative sequence current command values ​​after the fault are known, the candidate command values ​​of the d-axis positive and negative sequence current after the fault are calculated, as follows:

[0107] According to the obtained dq axis positive and negative sequence grid voltage components after the fault occurs, e dP =0.2549kV, e qP =0kV, e dN =0.0158kV, e qN =0.1424kV, combined with the q-axis positive and negative sequence current command values ​​after the fault, the d-axis positive and negative sequence current candidate command values ​​after the fault can be obtained:

[0108]

[0109] 4. Considering the inverter current capacity, after reaching the maximum current capacity of the inverter, the d-axis positive sequence current command value needs to be limited, and further used to obtain the d-axis positive sequence current command value after the fault; the q-axis positive and negative sequence current command values ​​after the fault and the d-axis positive and negative sequence current command values ​​after the fault are used as the complete command values ​​under the low wear strategy, and the inverter low wear control is completed by changing the command values. The details are as follows:

[0110] In the calculation of the d-axis positive sequence current command value after limiting, it is given as follows: I MAX =0.576kA represents the current amplitude output at the maximum current capacity of the inverter, I - =0.2365kA represents the negative sequence current component, rad is the absolute value of the phase difference between the positive sequence current and the negative sequence current; further, the amplitude of the positive sequence current component can be obtained: I MAXP =0.3944kA. After obtaining the positive sequence current component amplitude, the d axis positive sequence current command value is limited when the q axis positive sequence current command value after the fault is known, so as to obtain the d axis positive sequence current command value after limiting: i drefPlimit =-0.3787kA. Therefore, the final low-penetration strategy complete instruction value is as follows:

[0111]

[0112] In order to further illustrate the effectiveness of the present invention, the comparative simulation results of the present invention and the traditional control strategy are given: Figure 4-Figure 5 This is a schematic diagram of active and reactive power output from the photovoltaic electric field port under the traditional strategy of suppressing negative sequence low penetration control; Figure 6-Figure 7 Schematic diagram of active and reactive power output from the photovoltaic electric field port under low voltage ride-through control of the present invention; By comparison, it can be seen that the low voltage ride-through control strategy adopted by the present invention has a significant inhibitory effect on the double frequency fluctuation of active power, which not only meets the positive and negative sequence dynamic reactive current support capacity stipulated by the photovoltaic grid-connected technology, but also can be well combined with the suppression of double frequency power fluctuation of active power. Among them, the traditional control strategy is the negative sequence current low voltage ride-through control strategy.

[0113] Further, the principle of the present invention is given as follows:

[0114] The low-through control strategies of traditional photovoltaic inverters are to suppress negative-sequence current, suppress active power and DC bus voltage double frequency fluctuations, and suppress reactive power double frequency fluctuations. However, it does not take into account the negative-sequence dynamic reactive current support capability required by photovoltaic grid-connected technical regulations. The standard clearly states that it is necessary to provide positive-sequence reactive current support when a symmetrical fault occurs on the grid side, and when an asymmetrical fault occurs on the grid side, it is necessary not only to provide positive-sequence dynamic reactive current support, but also to provide negative-sequence dynamic reactive current support. When designing the low-through control strategy, the present invention takes into account the positive and negative sequence dynamic reactive current support capabilities of the photovoltaic field when a fault occurs in the power grid, and at the same time suppresses the double frequency fluctuation of the output power of the photovoltaic field. It studies the relationship between the output power and the voltage and current of the photovoltaic field when an asymmetrical fault occurs in the power grid, takes into account the positive and negative sequence dynamic reactive current support capabilities in the photovoltaic grid-connected technical regulations, and selects the optimal current command value in combination with the flow capacity of the photovoltaic inverter. By changing the command value, the low-through control of the inverter is completed. The present invention can enable the inverter to provide a specified dynamic reactive current and suppress the frequency fluctuation of the inverter output power, thereby providing a reference for the low voltage ride-through control of the photovoltaic electric field.

[0115] 1. According to the instantaneous power theorem, the expression of the instantaneous power output by the inverter is obtained; through the Park transformation, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained, and then the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components is obtained. The instantaneous power sent to the grid by the photovoltaic power station is expressed as:

[0116]

[0117] Where: P 瞬 , Q 瞬 P is the instantaneous power sent to the grid by the photovoltaic power station, that is, the instantaneous power output by the inverter; 0 , Q 0 is the instantaneous DC power component of the inverter output, PS2 , Q S2 is the inverter output sinusoidal power component, P C2 , Q C2 is the cosine power component output by the inverter; ω is the synchronous angular velocity, and t is the time.

[0118] By performing Parker transformation on the grid-side voltage and current, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained to represent the power components after the fault. When an asymmetric fault occurs in the grid, the relationship between the power components output by the inverter and the positive and negative sequence voltage and current components is:

[0119]

[0120] Where: P 0 , Q 0 is the instantaneous DC power component of the inverter output, P S2 , Q S2 is the inverter output sinusoidal power component, P C2 , Q C2 is the cosine power component of the inverter output; P and N represent positive and negative sequences; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; i dP 、i dN Represents the d-axis positive and negative sequence grid current components, i qP 、i qN Represents the positive and negative sequence grid current components of the q axis; i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command values ​​after a fault.

[0121] From the relationship between the power components output by the above inverter and the positive and negative sequence voltage and current components, it can be seen that when an asymmetric fault occurs in the power grid, the grid-connected inverter cannot control the six power components simultaneously through four independent current variables. Therefore, the four power components are usually controlled by four independent current variables. At this time, the three control objectives of suppressing negative sequence current, suppressing reactive double frequency fluctuations, and suppressing active double frequency fluctuations can be met respectively, as follows:

[0122]

[0123] In the formula, i drefP 、i qrefP 、i drefN 、i qrefN Indicates the inverter current command value; P 0ref , Q 0refIndicates the expected value of the inverter output power, k is the inverter control target gear switching instruction, k = 1 means the control target is to suppress the double frequency fluctuation of active power, k = -1 means the control target is to suppress the double frequency fluctuation of reactive power, k = 0 means the control target is to suppress the negative sequence current. 1 =(e dP ) 2 +(e qP ) 2 -k(e dN ) 2 -k(e qN ) 2 ,M 2 =(e dP ) 2 +(e qP ) 2 +k(e dN ) 2 +k(e qN ) 2 .

[0124] Changing the value of k will change the control target of the inverter accordingly. During low voltage ride-through, the current command values ​​under different control target modes are different. By adjusting the values ​​of two current commands, the values ​​of the other two current commands can be derived accordingly to change the effect of the low voltage ride-through strategy. Under the premise that two of the command values ​​are known, the other two command values ​​can be derived accordingly, so that all current command values ​​under the control target can be directly determined.

[0125] 2. Design the Q-axis positive and negative sequence current command values ​​after the fault, specifically:

[0126] When an asymmetric fault occurs in the power grid, the photovoltaic power station needs to inject a certain amount of positive-sequence reactive current into the power grid. Its value is the sum of the positive-sequence reactive current value and the positive-sequence reactive current increment value during normal operation. The negative-sequence reactive current absorbed from the power grid should be the difference between the negative-sequence reactive current increment values. The dynamic positive and negative-sequence reactive currents should respond to changes in voltage. The specific expressions are as follows:

[0127]

[0128] Where: Δi tP , Δi tN They represent the positive sequence dynamic reactive current increment injected into the grid by the PV power station and the negative sequence reactive current increment absorbed from the grid respectively; K 2P , K 2N Indicates the positive and negative sequence current proportional coefficients. The values ​​of the two coefficients should not be less than 1.0; U tP , U tN Indicates the per unit value of the positive and negative sequence components of the grid voltage; i N Indicates the rated current of the photovoltaic power station;

[0129] After taking into account the positive and negative sequence dynamic reactive current support capabilities, the specific expressions of the q-axis positive and negative sequence current command values ​​after the fault are established are as follows:

[0130]

[0131] Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault, i qref0 Indicates the q-axis positive sequence current command value before the fault, Δi tP , Δi tN They respectively represent the positive-sequence dynamic reactive current increment injected into the grid by the PV power station and the negative-sequence reactive current increment absorbed from the grid.

[0132] 3. In order to simultaneously suppress power frequency fluctuations, the candidate command values ​​of the d-axis positive and negative sequence currents after the fault are calculated when the command values ​​of the q-axis positive and negative sequence currents after the fault are known. Specifically, they are:

[0133] Combined with the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components (2), under the selected target of suppressing active power fluctuation, the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components P S2 With P C2 The value is 0. By transforming the formula, we can get the relationship between current and voltage when the value of k in formula (3) is k=1, P 0ref , Q 0ref As the connection quantity for current calculation, it is used to calculate the other two current command values. After the q-axis positive and negative sequence current command values ​​after the fault are known, the candidate command values ​​of the d-axis positive and negative sequence current after the fault can be calculated by formula (3):

[0134]

[0135] Where: i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; P and N represent the positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command values ​​after a fault.

[0136] 4. Considering the current carrying capacity of the inverter, when the q-axis positive sequence current command value after the fault is known, establish the expression of the d-axis positive sequence current command value after limiting; obtain the final dq-axis positive and negative sequence current command values, and complete the inverter low wear control by changing the command value, specifically:

[0137] After reaching the maximum current carrying capacity of the inverter, under the condition of the known q-axis positive sequence current command value after the fault, the expression of the d-axis positive sequence current command value after limiting is established, and the expression is as follows:

[0138]

[0139] Where: i drefPlimit Indicates the d-axis positive sequence current command value after being limited, I MAXP Represents the amplitude of the positive sequence current component, i qrefP Indicates the q-axis positive sequence current command value after the fault; I MAXP Represents the amplitude of the positive sequence current component, I MAX Indicates the current amplitude output at the maximum current capacity of the inverter, I - represents the negative sequence current component, It is the absolute value of the phase difference between the positive sequence current and the negative sequence current.

[0140] According to the d-axis positive sequence current command value after limiting and the selected d-axis positive and negative sequence current command values ​​after the fault, the d-axis positive sequence current command value after the fault is adjusted to obtain the d-axis positive sequence current command value after the fault; the q-axis positive and negative sequence current command values ​​after the fault and the d-axis positive and negative sequence current command values ​​after the fault are used as the complete command values ​​under the low wear strategy, and the expression is as follows:

[0141]

[0142] Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault; i drefP 、i drefN Indicates the positive and negative sequence current command value of the d-axis after the fault; i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; i qref0 Indicates the q-axis positive sequence current command value before the fault; Δi tP , Δi tN They respectively represent the positive-sequence dynamic reactive current increment injected into the grid by the PV power station and the negative-sequence reactive current increment absorbed from the grid.

[0143] The specific implementation modes of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above implementation modes, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression, characterized in that: The following steps are involved: Step 1: Based on the instantaneous power theorem, the expression of the instantaneous power output by the inverter is obtained; through Park transformation, the positive and negative sequence grid voltage and current components in the synchronous rotating coordinate system are obtained, and then the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components is obtained; Step 2: Considering the requirements of positive and negative sequence dynamic reactive current support, design the q-axis positive and negative sequence current command values ​​after the fault; Step 3: When the positive and negative sequence current command values ​​of the q-axis after the fault are known, the candidate command values ​​of the positive and negative sequence current of the d-axis after the fault are calculated; Step 4: Considering the current carrying capacity of the inverter, when the q-axis positive sequence current command value after the fault is known, the d-axis positive sequence current command value after limiting is obtained to adjust the d-axis positive sequence current command value after the fault, and then the d-axis positive sequence current command value after the fault is obtained; The post-fault d-axis negative-sequence current candidate command value is used as the post-fault d-axis negative-sequence current command value; The positive and negative sequence current command values ​​of the q-axis after the fault and the positive and negative sequence current command values ​​of the d-axis after the fault are used as complete command values ​​under the low wear-through strategy; the inverter low wear-through control is completed by changing the command values.

2. The photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression according to claim 1 is characterized in that: The step 1 is specifically as follows: When an asymmetric fault occurs in the power grid, the instantaneous power sent to the grid by the photovoltaic power station is expressed as: Where: P 瞬 , Q 瞬 is the instantaneous active and reactive power sent to the grid by the photovoltaic power station, that is, the instantaneous power output by the inverter; P0 and Q0 are the DC components of the instantaneous active and reactive power output by the inverter, P S2 , Q S2 The inverter outputs sinusoidal active and reactive power components, P C2 , Q C2 is the cosine active and reactive power components output by the inverter; ω is the synchronous angular velocity, and t is the time; When an asymmetric fault occurs in the power grid, the relationship between the power components output by the inverter and the positive and negative sequence voltage and current components is: Among them: P, N represent positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; i dP 、i dN Represents the d-axis positive and negative sequence grid current components, i qP 、i qN Represents the q-axis positive and negative sequence grid current components.

3. The photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression according to claim 1 is characterized in that: The step 2 is specifically as follows: Photovoltaic power stations should have the ability to support positive and negative sequence dynamic reactive current: Where: Δi tP Indicates the positive sequence dynamic reactive current increment injected into the grid by the photovoltaic power station; Δi tN Indicates the negative sequence reactive current increment absorbed from the power grid; K 2P , K 2N Indicates the positive and negative sequence current proportionality coefficient; U tP , U tN Indicates the per unit value of the positive and negative sequence components of the grid voltage; i N Indicates the rated current of the photovoltaic power station; After taking into account the positive and negative sequence dynamic reactive current support capabilities, the specific expressions of the q-axis positive and negative sequence current command values ​​after the fault are established are as follows: Where: i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command value after the fault, i qref0 Indicates the q-axis positive sequence current command value before the fault.

4. The photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression according to claim 1 is characterized in that: The step three is specifically as follows: Combined with the relationship between each power component output by the inverter and the positive and negative sequence voltage and current components, after the q-axis positive and negative sequence current command values ​​after the fault are known, the candidate command values ​​of the d-axis positive and negative sequence current after the fault are calculated: Where: i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; P and N represent the positive and negative sequence; e dP 、e dN represents the d-axis positive and negative sequence grid voltage components, e qP 、e qN Represents the positive and negative sequence grid voltage components of the q axis; i qrefP 、i qrefN Indicates the q-axis positive and negative sequence current command values ​​after a fault.

5. The photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression according to claim 1 is characterized in that: The step 4 is specifically as follows: When the q-axis positive sequence current command value after the fault is known, the expression of the d-axis positive sequence current command value after limiting is established, and the expression is as follows: Where: i drefPlimit Indicates the d-axis positive sequence current command value after being limited, I MAXP Represents the amplitude of the positive sequence current component, i qrefP Indicates the q-axis positive sequence current command value after a fault; According to the d-axis positive sequence current command value after limiting and the selected d-axis positive and negative sequence current command values ​​after the fault, the d-axis positive sequence current command value after the fault is adjusted to obtain the d-axis positive sequence current command value after the fault; the q-axis positive and negative sequence current command values ​​after the fault and the d-axis positive and negative sequence current command values ​​after the fault are used as the complete command values ​​under the low wear strategy, and the expression is as follows: Where: i qrefN Indicates the q-axis negative sequence current command value after the fault; i drefP 、i drefN Indicates the positive and negative sequence current command value of the d-axis after the fault; i drefP1 、i drefN1 Indicates the selected command value of the positive and negative sequence current of the d-axis after the fault; i qref0 Indicates the q-axis positive sequence current command value before the fault; Δi tP , Δi tN They respectively represent the positive-sequence dynamic reactive current increment injected into the grid by the PV power station and the negative-sequence reactive current increment absorbed from the grid.

6. The photovoltaic low-power wear control strategy taking into account positive and negative sequence dynamic reactive current support and frequency-doubling power fluctuation suppression according to claim 5 is characterized in that: The positive sequence current component amplitude expression is: Where: I MAXP Represents the amplitude of the positive sequence current component, I MAX Indicates the current amplitude output at the maximum current capacity of the inverter, I - represents the negative sequence current component, It is the absolute value of the phase difference between the positive sequence current and the negative sequence current.

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