Power angle synchronization-based droop inverter low-voltage ride-through control method and system

Abstract

The invention discloses a droop inverter low-voltage ride-through control method based on power angle synchronization. The method comprises the following steps: S1, determining a fault of a power distribution network; s2, obtaining components of an inverter output voltage vector in an alpha-beta static coordinate system when the power distribution network fails and is normal, and establishing a cross term; s3, controlling a cross term to be equal to 0 to obtain a frequency correction amount; adjusting the output power of the inverter according to the frequency correction; monitoring the output current and voltage of the inverter; s4, when the output current is higher than the limit value, obtaining a first voltage amplitude according to the output current in combination with the Ohm law; s5, subtracting the previous correction voltage from the first voltage amplitude, and obtaining a voltage correction value through a PI controller; adding the voltage correction and the inverter output voltage to obtain a correction voltage; s6, circulating the step S5; when the output current is less than or equal to a limit value, ending circulation; and S7, if the voltage of the power distribution network returns to normal, determining that the fault is removed. The invention further discloses a droop inverter low-voltage ride-through control system based on power angle synchronization.

Description

Technical Field

[0001] The present invention relates to the field of power grid technology, and in particular to a droop inverter low voltage ride through control method and system based on power angle synchronization. Background Art

[0002] The droop inverter can provide voltage and frequency support for the power grid system and realize power distribution without communication. It is widely used in the grid connection of new energy. The stable operation of the droop inverter after a power grid failure is mainly affected by two aspects: transient synchronization instability and output overcurrent.

[0003] In the prior art, some scholars have provided solutions to deal with the problem of transient synchronous instability. For example, Solution 1: To achieve power angle stability before and after the fault by indirectly changing the active power reference value of the droop inverter; Solution 2: To control the input and output power balance by optimizing the active-frequency equation to avoid transient instability; Solution 3: To analyze the transient stability of the droop inverter by the phase plane method, and prove that the first-order characteristics of the active loop enable the inverter to have better transient performance. Some scholars have also provided solutions to deal with the problem of output overcurrent. For example, Solution 4: After the current exceeds the safety threshold, switch the grid-forming inverter to the grid-following type to suppress the overcurrent; Solution 5: Add a limiter to the current inner loop to limit the current reference value, thereby suppressing the inverter output current; Solution 6: Use virtual impedance to limit the current. However, Solution 1 and Solution 2 ignore the impact of the reactive loop; Solution 3 ignores the overcurrent problem of the droop inverter; Solution 4 has the risk of losing synchronization; Solution 5 will cause the power loop to saturate and then power oscillation; Solution 6 reduces the stability of the inverter to a certain extent. The existing technology generally analyzes power angle stability and output current limiting as separate issues, without considering the transient synchronization instability problem of the droop inverter during a fault and the output overcurrent problem as an interrelated whole. However, power angle stability control can limit the inverter output current to a certain extent, and inappropriate current limiting measures will often reduce the power angle stability.

[0004] In summary, a new technical solution is urgently needed to solve the technical problem of how to design a control strategy that can take into account both the transient synchronization instability problem and the output overcurrent problem of the droop inverter during a fault. Summary of the invention

[0005] The present invention provides a droop inverter low voltage ride through control method and system based on power angle synchronization, which is used to solve the technical problem of how to design a control strategy that can take into account the transient synchronization instability problem and output overcurrent problem of the droop inverter during a fault.

[0006] To achieve the above object, the present invention provides a droop inverter low voltage ride through control method based on power angle synchronization, comprising the following steps:

[0007] S1. Monitor the voltage of the distribution network; when the voltage of the distribution network is lower than the preset voltage limit, it is determined that a fault occurs in the distribution network.

[0008] S2, respectively obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u mFα u mβ -u mα u mFβ .

[0009] S3. Control the cross term to be equal to 0 through the PI controller to obtain the frequency correction; adjust the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control; monitor the output current and output voltage of the droop inverter in real time.

[0010] S4. When the output current is higher than the preset current limit, a first voltage amplitude is obtained according to the output current combined with Ohm's law in an AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter during a distribution network fault condition.

[0011] S5. Subtract the previous correction voltage from the first voltage amplitude and obtain a voltage correction value through a PI controller; add the voltage correction value to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter.

[0012] S6, loop S5; when the output current is less than or equal to the preset current limit, the loop ends and the fault current limiting control is completed.

[0013] S7. Determine whether the distribution network voltage has returned to normal. If so, determine that the distribution network fault has been cleared.

[0014] Preferably, obtain u mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ include:

[0015] u mα 、u mβ 、u mFα and u mFβ It can be expressed as:

[0016]

[0017] Among them, U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails; U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network; Indicates the actual phase angle of the droop inverter under normal operating conditions of the distribution network. Indicates the actual phase angle of the droop inverter during distribution network fault conditions.

[0018] Preferably, establish the cross term u mFα u mβ -u mα u mFβ include:

[0019] and It can be expressed as:

[0020]

[0021] Among them, f 0 and f F They represent the output voltage frequency of the droop inverter under normal and fault conditions of the distribution network respectively; t represents time; δ 0 and δ F They represent the power angle of the droop inverter under normal and fault conditions of the distribution network respectively.

[0022] According to u mα 、u mβ 、u mFα 、u mFβ , and The expression of the cross term u mFα u mβ -u mα u mFβ About U m , U mF , δ 0 and δ F The relationship is:

[0023] u mFα u mβ -u mα u mFβ =U m U mF sin[2π(f 0 -f F )t+δ 0 -δ F ]

[0024] Preferably, controlling the cross term to be equal to 0 through a PI controller to obtain a frequency correction; adjusting the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control includes:

[0025] In the short period of time after the distribution network voltage drops, the output frequency fluctuation of the grid-connected droop inverter is small, so the frequency difference Δf=f 0 -f F is smaller, then the cross term u mFα u mβ -u mα u mFβ is equivalent to:

[0026] u mFα u mβ -u mα u mFβ =U m U mF sin(δ 0 -δ F )

[0027] At the power angle difference δ 0 -δ F When the cross term u is small, mFα u mβ -u mα u mFβ Further equivalent to:

[0028] u mFα u mβ -u mα u mFβ =U m U mF (δ 0 -δ F )

[0029] U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network, U m Not 0; U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails, U mF Not 0; when the grid voltage drops to 0, it is equivalent to the droop inverter island operation. mF Still not 0; if you need to implement δ 0 -δ F = 0, then the cross term u needs to be controlled mFα u mβ -u mα u mFβ is equal to 0.

[0030] Therefore, the cross term u is controlled by the PI controller mFα u mβ -umα u mFβ When the frequency correction value is equal to 0, the frequency correction value is obtained, and the output power of the droop inverter is corrected by adding the frequency correction value to the output frequency of the droop inverter, so as to achieve power angle stability.

[0031] Preferably, the frequency correction amount can be expressed as:

[0032] Δf=(k fp +k fi / s)(u mFα u mβ -u mα u mFβ )

[0033] Wherein, Δf represents the frequency correction; k fp and k fi They represent the frequency adjustment proportional coefficient and integral coefficient in the PI controller respectively; s represents the complex frequency domain variable.

[0034] Preferably, obtaining the first voltage amplitude according to the output current combined with Ohm's law in an AC circuit includes:

[0035] According to Ohm's law in AC circuit, the output current I is obtained mF The expression can be expressed as:

[0036]

[0037] Among them, U gF Indicates the grid side voltage when the distribution network is faulty; X g Indicates line impedance.

[0038] Assume that the output current of the droop inverter is I under normal operating conditions of the distribution network mN To ensure that the droop inverter can be safely connected to the grid during the fault phase, the preset current limit is AI mN , 1.2≤A≤1.5; when the output current I mF Equal to the preset current limit AI mN When , the expression of the output current is transformed to obtain the expression of the first voltage amplitude:

[0039]

[0040] Preferably, the voltage correction value is obtained by using a PI controller after subtracting the previous correction voltage from the first voltage amplitude, and the voltage correction value includes:

[0041] The voltage correction can be expressed as:

[0042] ΔU=(k up +k ui / s)(U mF -E* )

[0043] Among them, ΔU represents the voltage correction; k up and k ui They represent the voltage regulation proportional coefficient and integral coefficient in the PI controller respectively; E * Indicates the correction voltage of the droop inverter.

[0044] Preferably, the preset voltage limit includes:

[0045] Assume that the grid side voltage amplitude is U under normal operating conditions g , then the preset voltage limit is BU g , 0.9≤B≤1.

[0046] Preferably, determining whether the distribution network voltage has returned to normal includes:

[0047] Determine whether the distribution network voltage is greater than or equal to a preset voltage limit, and if so, determine that the distribution network voltage has returned to normal.

[0048] The present invention also provides a droop inverter low voltage ride-through control system based on power angle synchronization, which is used in the method of the present invention. The system includes a first module, a second module, a third module, a fourth module, a fifth module, a sixth module and a seventh module.

[0049] The first module is used to monitor the voltage of the distribution network; when the voltage of the distribution network is lower than a preset voltage limit, it is determined that a fault occurs in the distribution network.

[0050] The second module is used to obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions respectively. mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u mFα u mβ -u mα u mFβ .

[0051] The third module is used to control the cross term to be equal to 0 through the PI controller to obtain the frequency correction; adjust the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control.

[0052] The fourth module is used to monitor the output current and output voltage of the droop inverter in real time; when the output current is higher than the preset current limit, the first voltage amplitude is obtained according to the output current combined with Ohm's law in the AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter during the distribution network fault condition.

[0053] The fifth module is used to obtain a voltage correction value by subtracting the previous correction voltage from the first voltage amplitude through a PI controller; the voltage correction value is added to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter.

[0054] The sixth module is used to send a loop instruction to the fifth module; when the output current is less than or equal to the preset current limit, a loop end instruction is sent to the fifth module to complete the fault current limiting control.

[0055] The seventh module is used to determine whether the distribution network voltage has returned to normal, and if so, determine that the distribution network fault has been removed.

[0056] The present invention has the following beneficial effects:

[0057] The droop inverter low voltage ride through control method based on power angle synchronization of the present invention takes into account the inverter power angle instability and output overcurrent problems under fault conditions. mFa u mb -u ma u mFb And the cross term is controlled to be equal to 0 by the PI controller, and the frequency correction is obtained. The output power of the droop inverter is adjusted according to the frequency correction, so that the method of the present invention can achieve power angle stability control. After completing the power angle stability control, if the output current of the droop inverter is higher than the preset current limit, the first voltage amplitude is obtained according to the output current of the droop inverter combined with Ohm's law under the AC circuit, and the voltage correction is obtained by subtracting the previous correction voltage from the first voltage amplitude through the PI controller; the output voltage of the droop inverter is adjusted according to the voltage correction to obtain the correction voltage, and the output voltage of the droop inverter is corrected cyclically, and the cycle is terminated when the output current is less than or equal to the preset current limit, so that the method of the present invention can achieve fault current limiting control. The method of the present invention takes into account the transient synchronization instability problem and the output overcurrent problem of the droop inverter in the fault, so that the method of the present invention can ensure that the system can stably connect to the grid and operate after the fault.

[0058] The droop inverter low voltage ride-through control system based on power angle synchronization of the present invention is used in the method of the present invention and has the same technical effect as the method of the present invention.

[0059] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0061] Figure 1 It is a schematic diagram of a method flow of a preferred embodiment of the present invention.

[0062] Figure 2 It is a schematic diagram of the grid-connected circuit structure of a droop inverter according to a preferred embodiment of the present invention.

[0063] Figure 3 Schematic diagram of the control structure of a conventional droop inverter according to a preferred embodiment of the present invention.

[0064] Figure 4 It is a power angle stabilization control diagram of a preferred embodiment of the present invention.

[0065] Figure 5 It is a fault current limiting control diagram of a preferred embodiment of the present invention.

[0066] Figure 6 This is a waveform diagram of the output of a conventional droop inverter when a Class I fault occurs in the distribution network of a preferred embodiment of the present invention.

[0067] Figure 7 It is a waveform diagram of the inverter output based on power angle stabilization control when a distribution network Type I fault occurs in a preferred embodiment of the present invention.

[0068] Figure 8 This is a waveform diagram of the output of a conventional droop inverter during a Class II fault in a distribution network according to a preferred embodiment of the present invention.

[0069] Fig. 9 It is a waveform diagram of the inverter output based on power angle stabilization control when a distribution network Type II fault occurs in a preferred embodiment of the present invention.

[0070] Fig.10 It is a waveform diagram of the inverter output based on the comprehensive control of the present invention when the distribution network type II fault occurs in the preferred embodiment of the present invention. DETAILED DESCRIPTION

[0071] The embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0072] See also Figure 1 In a preferred embodiment of the present invention, a droop inverter low voltage ride through control method based on power angle synchronization is provided, comprising the following steps:

[0073] S1. Monitor the voltage of the distribution network; when the voltage of the distribution network is lower than the preset voltage limit, it is determined that a fault occurs in the distribution network.

[0074] In a preferred embodiment of the present invention, the preset voltage limit value includes:

[0075] Assume that the grid side voltage amplitude is U under normal operating conditions g , then the preset voltage limit is BU g , 0.9≤B≤1.

[0076] S2, respectively obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u mFα u mβ -u mα u mFβ .

[0077] In a preferred embodiment of the present invention, obtaining u mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ include:

[0078] u mα 、u mβ 、u mFα and u mFβ It can be expressed as:

[0079]

[0080] Among them, U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails; U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network; Indicates the actual phase angle of the droop inverter under normal operating conditions of the distribution network. Indicates the actual phase angle of the droop inverter during distribution network fault conditions.

[0081] In the preferred embodiment of the present invention, the cross term u is established mFα u mβ -u mα u mFβ include:

[0082] and It can be expressed as:

[0083]

[0084] Among them, f 0 and f F They represent the output voltage frequency of the droop inverter under normal and fault conditions of the distribution network respectively; t represents time; δ 0 and δ F They represent the power angle of the droop inverter under normal and fault conditions of the distribution network respectively.

[0085] According to u mα 、u mβ 、u mFα 、u mFβ , and The expression of the cross term u mFα u mβ -u mα u mFβ About U m , U mF ,δ 0 and δ F The relationship is:

[0086] u mFα u mβ -u mα u mFβ =U m U mF sin[2π(f 0 -f F )t+δ 0 -δ F ]

[0087] S3. Control the cross term to be equal to 0 through the PI controller to obtain the frequency correction; adjust the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control; monitor the output current and output voltage of the droop inverter in real time.

[0088] In a preferred embodiment of the present invention, the cross term is controlled to be equal to 0 by a PI controller to obtain a frequency correction amount; the output power of the droop inverter is adjusted according to the frequency correction amount to complete the power angle stabilization control, which includes:

[0089] In the short period of time after the distribution network voltage drops, the output frequency fluctuation of the grid-connected droop inverter is small, so the frequency difference Δf=f 0 -f F is smaller, then the cross term u mFα u mβ -u mα u mFβ is equivalent to:

[0090] u mFα u mβ -u mα u mFβ =U m U mF sin(δ 0 -δ F )

[0091] At the power angle difference δ 0 -δ F When the cross term u is small, mFα u mβ -u mα u mFβ Further equivalent to:

[0092] u mFα u mβ -u mα u mFβ =U m U mF (δ 0 -δ F )

[0093] U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network, U m Not 0; U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails, U mF Not 0; when the grid voltage drops to 0, it is equivalent to the droop inverter island operation. mF Still not 0; if you need to implement δ 0 -δ F = 0, then the cross term u needs to be controlled mFα u mβ -u mα u mFβ is equal to 0.

[0094] Therefore, the cross term u is controlled by the PI controller mFα u mβ -u mα u mFβ When the frequency correction value is equal to 0, the frequency correction value is obtained, and the output power of the droop inverter is corrected by adding the frequency correction value to the output frequency of the droop inverter, so as to achieve power angle stability.

[0095] In a preferred embodiment of the present invention, the frequency correction amount can be expressed as:

[0096] Δf=(k fp +k fi / s)(u mFα u mβ -umα u mFβ )

[0097] Wherein, Δf represents the frequency correction; k fp and k fi They represent the frequency adjustment proportional coefficient and integral coefficient in the PI controller respectively; s represents the complex frequency domain variable.

[0098] S4. When the output current is higher than the preset current limit, a first voltage amplitude is obtained according to the output current combined with Ohm's law in an AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter during a distribution network fault condition.

[0099] In a preferred embodiment of the present invention, obtaining the first voltage amplitude according to the output current combined with Ohm's law in an AC circuit includes:

[0100] According to Ohm's law in AC circuit, the output current I is obtained mF The expression can be expressed as:

[0101]

[0102] Among them, U gF Indicates the grid side voltage when the distribution network is faulty; X g Indicates line impedance.

[0103] Assume that the output current of the droop inverter is I under normal operating conditions of the distribution network mN To ensure that the droop inverter can be safely connected to the grid during the fault phase, the preset current limit is AI mN , 1.2≤A≤1.5; when the output current I mF Equal to the preset current limit AI mN When , the expression of the output current is transformed to obtain the expression of the first voltage amplitude:

[0104]

[0105] S5. Subtract the previous correction voltage from the first voltage amplitude and obtain a voltage correction value through a PI controller; add the voltage correction value to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter.

[0106] In a preferred embodiment of the present invention, the voltage correction amount is obtained by using a PI controller after subtracting the previous correction voltage from the first voltage amplitude, and the voltage correction amount includes:

[0107] The voltage correction can be expressed as:

[0108] ΔU=(k up +k ui / s)(U mF-E * )

[0109] Among them, ΔU represents the voltage correction; k up and k ui They represent the voltage regulation proportional coefficient and integral coefficient in the PI controller respectively; E * Indicates the correction voltage of the droop inverter.

[0110] S6, loop S5; when the output current is less than or equal to the preset current limit, the loop ends and the fault current limiting control is completed.

[0111] S7. Determine whether the distribution network voltage has returned to normal. If so, determine that the distribution network fault has been cleared.

[0112] In a preferred embodiment of the present invention, determining whether the voltage of the power distribution network has returned to normal includes:

[0113] Determine whether the distribution network voltage is greater than or equal to a preset voltage limit, and if so, determine that the distribution network voltage has returned to normal.

[0114] The droop inverter low voltage ride through control method based on power angle synchronization of the present invention takes into account the inverter power angle instability and output overcurrent problems under fault conditions. mFa u mb -u ma u mFb And the cross term is controlled to be equal to 0 by the PI controller, and the frequency correction is obtained. The output power of the droop inverter is adjusted according to the frequency correction, so that the method of the present invention can achieve power angle stability control. After completing the power angle stability control, if the output current of the droop inverter is higher than the preset current limit, the first voltage amplitude is obtained according to the output current of the droop inverter combined with Ohm's law under the AC circuit, and the voltage correction is obtained by subtracting the previous correction voltage from the first voltage amplitude through the PI controller; the output voltage of the droop inverter is adjusted according to the voltage correction to obtain the correction voltage, and the output voltage of the droop inverter is corrected cyclically, and the cycle is terminated when the output current is less than or equal to the preset current limit, so that the method of the present invention can achieve fault current limiting control. The method of the present invention takes into account the transient synchronization instability problem and the output overcurrent problem of the droop inverter in the fault, so that the method of the present invention can ensure that the system can stably connect to the grid and operate after the fault.

[0115] In a preferred embodiment of the present invention, a droop inverter low voltage ride-through control system based on power angle synchronization is also provided, which is used in the method of the present invention. The system includes a first module, a second module, a third module, a fourth module, a fifth module, a sixth module and a seventh module.

[0116] The first module is used to monitor the voltage of the distribution network; when the voltage of the distribution network is lower than a preset voltage limit, it is determined that a fault occurs in the distribution network.

[0117] The second module is used to obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions respectively. mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u m F α u mβ -u mα u mFβ .

[0118] The third module is used to control the cross term to be equal to 0 through the PI controller to obtain the frequency correction; adjust the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control.

[0119] The fourth module is used to monitor the output current and output voltage of the droop inverter in real time; when the output current is higher than the preset current limit, the first voltage amplitude is obtained according to the output current combined with Ohm's law in the AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter during the distribution network fault condition.

[0120] The fifth module is used to obtain a voltage correction value by subtracting the previous correction voltage from the first voltage amplitude through a PI controller; the voltage correction value is added to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter.

[0121] The sixth module is used to send a loop instruction to the fifth module; when the output current is less than or equal to the preset current limit, a loop end instruction is sent to the fifth module to complete the fault current limiting control.

[0122] The seventh module is used to determine whether the distribution network voltage has returned to normal, and if so, determine that the distribution network fault has been removed.

[0123] The droop inverter low voltage ride through control system based on power angle synchronization of the present invention is used in the method of the present invention and has the same beneficial effects as the method of the present invention.

[0124] Verification part:

[0125] To verify the effectiveness of the present invention, a simulation platform was built on MATLAB / Simulink. Figure 2 The droop inverter simulation model is shown in Figure 2. Figure 2 Middle,U dc is the DC side voltage of the inverter; U g is the voltage amplitude on the grid side; L g is the line inductance; Lf and C f are filter inductance and filter capacitance respectively; Load is the load of the distribution network; u m and i m are the droop inverter output voltage and current vectors, and the amplitudes of the droop inverter output voltage and current are U m and I m The main simulation parameters are shown in Table 1.

[0126] Table 1 Simulation parameters

[0127]

[0128] The mathematical expressions of the active-frequency control equation and reactive-voltage control equation of the traditional droop inverter can be expressed as:

[0129]

[0130] Among them, P ref and Q ref Respectively represent the active and reactive power reference values ​​of the droop inverter; P e and Q e Respectively represent the actual output value of the inverter active and reactive power; ω N and U N They represent the virtual speed rating and inverter rated voltage amplitude respectively; k p and k q Respectively represent the active and reactive droop coefficients; ω and U * They represent the virtual speed actual value and voltage command value respectively.

[0131] The control process of the traditional droop inverter can be found in Figure 3 , δ represents the virtual power angle.

[0132] The power angle stabilization control process of the method of the present invention can be found in Figure 4 .exist Figure 4 In, f N Indicates rated frequency, 50Hz.

[0133] The fault current limiting control process of the method of the present invention can be found in Figure 5 .exist Figure 5 Middle,U N Indicates the rated voltage amplitude, 311V; U * Indicates the output voltage of the droop inverter.

[0134] Distribution network faults can be divided into two categories according to the degree of voltage drop:

[0135] Type I fault: The voltage drop is not large and there is a power balance point in the system.

[0136] Type Ⅱ fault: The voltage drops significantly, and there is no power balance point in the system.

[0137] When the grid-side voltage drops to 0.6 p.u. and a Type Ⅰ fault occurs, the output waveform of the traditional droop inverter is as Figure 6 shown. Due to the existence of a power balance point, the inverter can operate stably under the fault. During the fault, the output power angle of the inverter is 1.9 p.u., the output voltage amplitude is 0.94 p.u., and the output current amplitude is 1.65 p.u., exceeding the safe range, which will threaten the safe operation of the inverter connected to the grid. Figure 7 is the output waveform of the inverter based on the power angle stability control of the present invention. During the fault, the output power angle of the inverter is maintained near the rated value, the output current amplitude is 1.15 p.u., and the output voltage amplitude is 0.95 p.u. Compared with the traditional droop control, after adding the power angle stability control, the output current amplitude of the inverter is suppressed, which is consistent with the theoretical analysis.

[0138] When the grid-side voltage drops to 0.2 p.u. and a Type Ⅱ fault occurs, the output waveform of the traditional droop inverter is as Figure 8 shown. Due to the non-existence of a power balance point, the inverter will become unstable under the fault, and the power angle, voltage, current, etc. will exhibit periodic oscillations. Among them, the maximum output current amplitude is 2.32 p.u., which seriously threatens the safe operation of the inverter connected to the grid. Fig. 9 is the output waveform of the inverter based on the power angle stability control of the present invention. At this time, the output power angle of the inverter is stabilized near the rated value, but the output current amplitude reaches 1.84 p.u., exceeding the safety threshold, still threatening the safe operation of the inverter. It can be seen that when the grid-side voltage drops significantly, it is difficult to ensure the safe operation of the inverter only by power angle stability control. To further limit the output current amplitude of the inverter during the fault, after adding an additional fault current limiting control on the basis of the power angle stability control, the output waveform of the inverter is as Fig.10 shown. Under the comprehensive control of the method of the present invention, during the fault, the output power angle of the inverter is stably the rated value, and the output current amplitude is 1.5 p.u., meeting the requirements of safe grid-connected operation.

[0139] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A droop inverter low voltage ride through control method based on power angle synchronization, characterized in that: The following steps are involved: S1. Monitoring the voltage of the distribution network; when the voltage of the distribution network is lower than a preset voltage limit, determining that a fault occurs in the distribution network; S2, respectively obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u mFα u mβ -u mα u mFβ ; S3, controlling the cross term to be equal to 0 through a PI controller to obtain a frequency correction; adjusting the output power of the droop inverter according to the frequency correction to complete power angle stabilization control; Real-time monitoring of the output current and output voltage of the droop inverter; S4. When the output current is higher than the preset current limit, a first voltage amplitude is obtained according to the output current combined with Ohm's law in an AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter in a distribution network fault condition; S5, subtracting the previous correction voltage from the first voltage amplitude and obtaining a voltage correction value through a PI controller; adding the voltage correction value to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter; S6, loop S5; When the output current is less than or equal to the preset current limit, the cycle ends and the fault current limiting control is completed; S7. Determine whether the distribution network voltage has returned to normal. If so, determine that the distribution network fault has been cleared.

2. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 1, characterized in that: The acquisition u mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ include: u mα 、u mβ 、u mFα and u mFβ It can be expressed as: Among them, U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails; U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network; Indicates the actual phase angle of the droop inverter under normal operating conditions of the distribution network. Indicates the actual phase angle of the droop inverter during distribution network fault conditions.

3. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 2, characterized in that: The establishment of the cross term u mFα u mβ -u mα u mFβ include: and It can be expressed as: Among them, f0 and f F They represent the output voltage frequency of the droop inverter under normal and fault conditions of the distribution network respectively; t represents time; δ0 and δ F They represent the power angle of the droop inverter in normal and fault conditions of the distribution network respectively; According to u mα 、u mβ 、u mFα 、u mFβ , and The expression of the cross term u mFα u mβ -u mα u mFβ About U m , U mF , δ0 and δ F The relationship is: in mFα in mβ -in mα in mFβ =U m IN mF sin[2π(f0-f F )t+δ0-δ F ]。 4. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 3, characterized in that: The step of controlling the cross term to be equal to 0 through a PI controller to obtain a frequency correction; and adjusting the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control comprises: In the short period of time after the distribution network voltage drops, the output frequency fluctuation of the grid-connected droop inverter is small, so the frequency difference f0-f F is smaller, then the cross term u mFα u mβ -u mα u mFβ is equivalent to: in mFα in mβ -in mα in mFβ =U m IN mF sin(δ0-δ F ) At the power angle difference δ0-δ F When the cross term u is small, mFα u mβ -u mα u mFβ Further equivalent to: in mFα in mβ -in mα in mFβ =U m IN mF (δ0-δ F ) U m Indicates the output voltage amplitude of the droop inverter under normal operating conditions of the distribution network, U m Not 0; U mF Indicates the output voltage amplitude of the droop inverter when the distribution network fails, U mF Not 0; when the grid voltage drops to 0, it is equivalent to the droop inverter island operation. mF Still not 0; if you need to achieve δ0-δ F =0, then the cross term u needs to be controlled mFα u mβ -u mα u mFβ is equal to 0; Therefore, the cross term u is controlled by a PI controller mFα u mβ -u mα u mFβ The frequency correction amount is equal to 0, and the frequency correction amount is obtained. The frequency correction amount is added to the output frequency of the droop inverter to correct the output power of the droop inverter, so as to achieve power angle stability.

5. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 4, characterized in that: The frequency correction can be expressed as: Δf=(k fp +k fi / s)(u mFα u mβ -u mα u mFβ ) Wherein, Δf represents the frequency correction; k fp and k fi They represent the frequency adjustment proportional coefficient and integral coefficient in the PI controller respectively; s represents the complex frequency domain variable.

6. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 5, characterized in that: Obtaining a first voltage amplitude according to the output current combined with Ohm's law in an AC circuit includes: According to Ohm's law in AC circuit, the output current I is obtained mF The expression can be expressed as: Among them, U gF Indicates the grid side voltage when the distribution network is faulty; X g Indicates line impedance; Assume that the output current of the droop inverter is I under normal operating conditions of the distribution network mN To ensure that the droop inverter can be safely connected to the grid during the fault phase, the preset current limit is AI mN , 1.2≤A≤1.5; when the output current I mF Equal to the preset current limit AI mN When the expression of the output current is transformed, the expression of the first voltage amplitude is obtained:

7. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 6, characterized in that: Subtracting the previous correction voltage from the first voltage amplitude and obtaining the voltage correction amount through the PI controller includes: The voltage correction amount can be expressed as: ΔU=(k up +k ui / s)(U mF -E * ) Where ΔU represents the voltage correction; k up and k ui They represent the voltage regulation proportional coefficient and integral coefficient in the PI controller respectively; E * Indicates the correction voltage of the droop inverter.

8. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 7, characterized in that: The preset voltage limit includes: Assume that the grid side voltage amplitude is U under normal operating conditions g , then the preset voltage limit is BU g , 0.9≤B≤1.

9. The low voltage ride through control method of a droop inverter based on power angle synchronization according to claim 8, characterized in that: The determining whether the power distribution network voltage returns to normal includes: Determine whether the distribution network voltage is greater than or equal to the preset voltage limit, and if so, determine that the distribution network voltage has returned to normal.

10. A droop inverter low voltage ride through control system based on power angle synchronization, used in the method according to any one of claims 1 to 9, characterized in that: The system comprises a first module, a second module, a third module, a fourth module, a fifth module, a sixth module and a seventh module; The first module is used to monitor the voltage of the distribution network; when the voltage of the distribution network is lower than a preset voltage limit, it is determined that a fault occurs in the distribution network; The second module is used to obtain the output voltage vector u of the droop inverter under distribution network fault and normal working conditions respectively. mF and u m ; Get u respectively mF and u m The component u in the αβ stationary coordinate system mFα and u mFβ and u mα and u mβ , and establish the cross term u mFα u mβ -u mα u mFβ ; The third module is used to control the cross term to be equal to 0 through the PI controller to obtain a frequency correction; adjust the output power of the droop inverter according to the frequency correction to complete the power angle stabilization control; The fourth module is used to monitor the output current and output voltage of the droop inverter in real time; when the output current is higher than the preset current limit, a first voltage amplitude is obtained according to the output current combined with Ohm's law under the AC circuit; the first voltage amplitude includes the output voltage amplitude of the droop inverter under the distribution network fault condition; The fifth module is used to obtain a voltage correction value by subtracting the previous correction voltage from the first voltage amplitude through a PI controller; the voltage correction value is added to the output voltage of the droop inverter to obtain a correction voltage; the initial value of the correction voltage is the real-time output voltage of the droop inverter; The sixth module is used to send a cycle instruction to the fifth module; when the output current is less than or equal to the preset current limit, send a cycle end instruction to the fifth module to complete the fault current limiting control; The seventh module is used to determine whether the distribution network voltage has returned to normal, and if so, determine that the distribution network fault has been removed.

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