Vehicle network interactive charging station adaptive fault ride-through method and device considering network voltage support

Abstract

The invention relates to the field of power system protection and control, in particular to a vehicle-network interactive charging station self-adaptive fault ride-through method and device considering power grid voltage support, and the method comprises the steps: collecting the voltage amplitude of a grid-connected point of a charging station at a main protection action moment, and if the voltage amplitude of the grid-connected point recovers to be more than 0.95 times of rated voltage, judging whether the grid-connected point recovers to be more than 0.95 times of rated voltage; active power control of the voltage source converter is switched into direct-current voltage control at the main protection action moment, the reactive power reference value is adjusted to be consistent with that before the fault, and the charging station recovers normal operation, otherwise, the discharging power reference value of the charging station is adjusted to be Pmax 2 at the main protection action moment, and the charging station recovers normal operation. And meanwhile, active power control of the voltage source converter is switched into direct-current voltage control, and a reactive power control reference value of the voltage source converter is adjusted to Qmax 2 until the voltage of the grid-connected point of the charging station is recovered to be more than 0.95 times of rated voltage. Corresponding control strategies are adopted according to different fault severity degrees, and the method can adapt to different fault severity degrees, different initial operation working conditions and the access number of the electric vehicles.

Description

Technical Field

[0001] The present invention relates to the field of power system protection and control, and particularly to an adaptive fault ride-through method and device for a vehicle-grid interactive charging station considering grid voltage support. Background Art

[0002] Vehicle-grid interaction refers to the interaction of energy and information between electric vehicles and the power grid through charging piles. Under vehicle-grid interaction, electric vehicles can reverse the electric energy in the battery to the power grid through charging equipment to provide power support. In the discharging mode, electric vehicles in the charging station generally output active power to the power grid through a two-stage structure of a voltage source converter and a dual-active full-bridge converter. Under a power grid short-circuit fault, through the flexible control of the voltage source converter, the charging station can output reactive power to support the grid voltage. However, after the power grid fault, the AC voltage drops, which may cause the active power output by the charging station in the discharging mode to decrease due to the current limit of the voltage source converter, resulting in unbalanced power on the DC side of the charging station and causing the DC voltage to rise, which may cause safety problems such as explosion of the DC bus capacitor.

[0003] Currently, under a power grid fault, the charging station generally directly disconnects from the grid by triggering the low-voltage protection on the AC side and reconnects to the grid after the voltage recovers. However, direct disconnection makes the charging station unable to provide support for the power grid during the fault duration, and since the power grid fault duration is very short, starting and stopping within a short time will damage the power electronic devices in the charging station and increase the operation and maintenance costs of the charging station. Currently, there is no fault ride-through technology for vehicle-grid interactive charging stations.

[0004] The structure of an electric vehicle charging station is similar to that of new energy with an inverter interface. There are already many technologies for fault ride-through of new energy with an inverter interface, mainly including using additional energy storage devices, supercapacitors, load shedding, power adjustment, etc. to avoid DC voltage over-limitation and control the output reactive power to support the power grid through the converter. The construction cost and operation and maintenance cost of additional energy storage and supercapacitors are relatively high, making it difficult to promote and apply them on a large scale in charging stations. Load shedding dissipates the surplus energy in the DC bus capacitor to avoid the generation of DC overvoltage, but load shedding switching may generate a large amount of heat in the charging station. Since charging stations are usually located in densely populated areas, load shedding switching is extremely likely to cause safety hazards such as fires, and transient current impacts are caused during load shedding switching, affecting the power electronic devices. Adjusting the power fed into the DC side can fundamentally suppress the rise of the DC voltage, but the existing technologies are based on the power balance after the fault, which may lead to unnecessary adjustment of the fed-in power, affecting the safety of the charging station and the support effect on the power grid. It is still difficult for the fault ride-through of vehicle-grid interactive charging stations to take into account DC voltage over-limitation, AC current over-limitation, and maximum support for the grid connection point voltage.

[0005] Therefore, how to achieve the adaptive fault ride-through of the vehicle-to-grid interactive charging station considering the grid voltage support has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] To solve the problem of fault ride-through of electric vehicle charging stations in the current discharge operation mode and provide a reference for maximizing the support of the grid connection point voltage while avoiding the over-limit of the DC voltage and AC current of the charging station, the present invention proposes an adaptive fault ride-through method for a vehicle-to-grid interactive charging station considering grid voltage support, including the following steps:

[0007] S1. Collect the voltage amplitude of the grid connection point of the charging station. When the voltage amplitude of the grid connection point is lower than 0.95 times the rated voltage, start the adaptive fault ride-through control of the charging station;

[0008] S2. Calculate the limit fault time of the charging station for fault ride-through according to the discharge power before the voltage dip of the grid connection point of the charging station;

[0009] S3. Compare the limit fault time with the main protection operation time of the grid. If the limit fault time is greater than the main protection operation time, execute step S4; if the limit fault time is less than the main protection operation time, execute step S7;

[0010] S4. Switch the voltage source converter of the charging station to constant power control, and set the active and reactive power reference values to the power operation point (P max1 , Q max1 ) that makes the grid connection point voltage maximum in the fault ride-through power domain of the vehicle-to-grid interactive charging station in the discharge operation mode, and execute step S5;

[0011] S5. Collect the voltage amplitude of the grid connection point of the charging station at the moment when the main protection should act. If the voltage amplitude of the grid connection point recovers to more than 0.95 times the rated voltage, switch the active power control of the voltage source converter to DC voltage control at the moment when the main protection should act, and adjust the reactive power reference value to be the same as before the fault, and the charging station resumes normal operation; otherwise, execute step S6;

[0012] S6. Adjust the discharge power reference value of the charging station to P max2 at the moment when the main protection should act, and at the same time switch the active power control of the voltage source converter to DC voltage control, and adjust the reactive power control reference value of the voltage source converter to Q max2 , until the voltage of the grid connection point of the charging station recovers to more than 0.95 times the rated voltage;

[0013] S7. The voltage source converter of the charging station maintains DC voltage control and reactive power control, and the reactive power and active power are set to the power operation point (P max2 , Q max2), until the grid connection point voltage of the charging station resumes to above 0.95 times the rated voltage.

[0014] The present invention also provides a vehicle-grid interactive charging station adaptive fault ride-through device considering grid voltage support, which is used to implement a vehicle-grid interactive charging station adaptive fault ride-through method considering grid voltage support, including:

[0015] An acquisition module, which collects the AC bus voltage, active power, and reactive power of the electric vehicle charging station before and after the grid fault;

[0016] A first calculation module, which is used to calculate the limit fault time and call the first comparison module;

[0017] A first comparison module, which is used to compare the limit fault time and the main protection action time. If the limit fault time is greater than the main protection action time, it calls the second calculation module; otherwise, it calls the third calculation module;

[0018] A second calculation module, which calculates the maximum active power and reactive power of the voltage source converter of the charging station when the limit fault time is greater than the main protection action time, and calls the first control module;

[0019] A first control module, the voltage source converter of the charging station is switched to constant power control, and the active power reference value and the reactive power reference value are respectively set to (P max1 , Q max1 ), and calls the second comparison module;

[0020] A second comparison module, which is used to compare the voltage amplitude at the grid connection point of the charging station at the moment when the main protection should act. If the grid connection point voltage resumes to above 0.95 times the rated voltage, it calls the second control module; otherwise, it calls the third control module;

[0021] A second control module, which switches the active power control of the voltage source converter to DC voltage control, and the reactive power reference value is the same as before the fault;

[0022] A third control module, which adjusts the discharge power reference value of the charging station to P max2 at the moment when the main protection should act, and at the same time switches the active power control of the voltage source converter to DC voltage control, and adjusts the reactive power control reference value of the voltage source converter to Q max2 ;

[0023] A third calculation module, which calculates the maximum active power and reactive power of the voltage source converter when the limit fault time is less than the main protection action time, and calls the fourth control module;

[0024] A fourth control module, the voltage source converter of the charging station maintains DC voltage control and reactive power control, and the reactive power reference value is set to Q max2, and adjust the charging station discharge power reference value to P max2 .

[0025] The present invention also provides a device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When executing the computer program, the processor implements an adaptive fault ride-through method for a vehicle-grid interactive charging station that takes into account grid voltage support.

[0026] In view of the current lack of vehicle-grid interactive charging station fault ride-through technology, and the fact that the fault ride-through technology of power electronic equipment such as new energy is difficult to apply to vehicle-grid interactive charging stations due to economical efficiency and neglect of the impact of active power on the grid connection point voltage. The present invention discloses a vehicle-grid interactive charging station adaptive fault ride-through control method, device and equipment considering grid voltage support. Compared with the prior art, the present invention has the following advantages:

[0027] 1. The present invention considers the impact of different fault durations on DC voltage, and can support the grid connection point voltage to the maximum extent while avoiding the safe crossing of the DC voltage and AC current of the charging station:

[0028] ① When the limit fault time is greater than the main protection action time, the present invention can avoid excessive increase in active current, and achieve the purpose of avoiding DC voltage and AC current exceeding the limit while maximally supporting the grid connection point voltage by only adjusting the power of the voltage source converter before the main protection action time; the prior art may cause the DC voltage to exceed the limit before the main protection action due to the significant reduction in active power, and only providing reactive power may not be able to effectively support the grid connection point voltage. Even if the main protection does not operate correctly to remove the fault, since the unbalanced power is eliminated at the moment of the main protection refusal to act, the present invention can always ensure the safety of the DC voltage and AC current of the charging station, and output active and reactive power according to the maximum voltage support point, which can also maximize the support of the charging station grid connection point voltage.

[0029] ② When the limit fault time is less than the main protection action time, the present invention quantifies the active and reactive power of the charging station required for the maximum voltage support within the power range of the voltage source converter limited by the AC current, and uses the DC voltage control of the voltage source converter and the adjustment of the discharge power reference value to avoid the DC voltage and AC current from exceeding the limit while maximally supporting the grid connection point voltage; if the existing technology is used, since the feasible domain of the charging station fault ride-through power does not exist at this time, the DC voltage will exceed the limit before the main protection is activated, and the grid connection point voltage support effect is poor.

[0030] 2. The present invention adopts corresponding control strategies according to different fault severities, and can adapt to different fault severities, different initial operating conditions, and the number of electric vehicles connected.

[0031] 3. The present invention only needs to add a calculation module and a switching link for controlling the reference value, without adding hardware, and has the advantages of simple principle, low cost, and high reliability. Description of the Drawings

[0032] Figure 1 It is a block diagram of the adaptive fault ride-through control method for a vehicle-grid interactive charging station considering grid voltage support according to an embodiment of the present invention;

[0033] Figure 2 It is a schematic structural diagram of the adaptive fault ride-through control device for a vehicle-grid interactive charging station considering grid voltage support according to an embodiment of the present invention;

[0034] Figure 3 It is the topology of an electric vehicle charging station adopted in an embodiment of the present invention;

[0035] Figure 4 It is a schematic diagram of electrical quantities of Scheme 1 according to an embodiment of the present invention;

[0036] Figure 5 It is a schematic diagram of electrical quantities of Scheme 2 according to an embodiment of the present invention;

[0037] Figure 6 It is a flowchart of an adaptive fault ride-through control method for a vehicle-grid interactive charging station considering grid voltage support according to the present invention. Detailed Embodiment

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] Please refer to Figure 1 , a block diagram of an adaptive fault ride-through control method for a vehicle-grid interactive charging station considering grid voltage support provided by an embodiment of the invention, which is applied to an electric vehicle charging station. The electric vehicle charging station at least includes a dual-active full-bridge converter and a voltage source converter. As Figure 6 , it includes the following steps:

[0040] S101. Collect the voltage amplitude at the grid connection point of the charging station. When the voltage amplitude at the grid connection point is lower than 0.95 times the rated voltage, start the adaptive fault ride-through control of the charging station and implement step S102;

[0041] S102. Calculate the limit fault time for the charging station to ride through the fault according to the discharge power before the voltage dip at the grid connection point of the charging station;

[0042] S103. Compare the ultimate fault time with the operation time of the main protection of the power grid. If the ultimate fault time is greater than the operation time of the main protection, execute S104; if the ultimate fault time is less than the operation time of the main protection, execute S107;

[0043] S104. Switch the voltage source converter of the charging station to constant power control, and set the active and reactive power reference values to (P max 2, Q max 2), and execute S105;

[0044] S105. Collect the voltage amplitude at the grid connection point of the charging station at the moment when the main protection should operate. If the voltage amplitude at the grid connection point recovers to more than 0.95 times the rated voltage, switch the active power control of the voltage source converter to DC voltage control at the moment when the main protection should operate, adjust the reactive power reference value to be the same as before the fault, and the charging station resumes normal operation; otherwise, execute S106;

[0045] S106. Adjust the discharge power reference value of the charging station to P max 2, and at the same time switch the active power control of the voltage source converter to DC voltage control. Adjust the reactive power control reference value of the voltage source converter to Q max 2 until the voltage at the grid connection point of the charging station recovers to more than 0.95 times the rated voltage;

[0046] S107. The voltage source converter of the charging station maintains DC voltage control and reactive power control. Set the reactive power reference value to Q max 2, and at the same time adjust the discharge power reference value of the charging station to P max 2 until the voltage at the grid connection point of the charging station recovers to more than 0.95 times the rated voltage.

[0047] In specific implementation, in step S101, some electrical parameter measuring instruments such as power analyzers and power quality analyzers can be used to measure the voltage, current, active power, reactive power and other parameters of the AC bus of the charging station in real time; for example, a resistive voltage divider or an isolated voltage sensor is used to collect the DC bus voltage of the DAB, and the specific limitation in this field is not made here.

[0048] In the embodiment of the present invention, in step S101, the preset voltage can be 0.95 p.u times the rated voltage. This preset voltage can intuitively reflect the voltage deviation situation. For example, the rated voltage of the bus of a certain charging station is 110 kV, and 0.95 p.u times the rated voltage is 110×0.95 = 104.5 kV. If the actual bus voltage is close to this value, it means that the voltage of the AC bus of the charging station has decreased to a certain extent, but it is still within the acceptable range; if the deviation is large, step S102 needs to be implemented to ensure the normal operation of the equipment.

[0049] In specific implementation, in step S102, the calculation method of the limit fault time for the charging station to ride through the fault is as follows:

[0050]

[0051] Where C is the DC capacitor of the charging station; U dc,lim is the upper limit of the DC voltage of the charging station; U * dc is the reference value of the DC voltage of the charging station; P s0 is the discharge power of the charging station before the fault; U f is the voltage at the grid connection point of the charging station under the fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter.

[0052] The embodiment of the present invention considers that when the limit fault time is greater than the main protection action time, the charging station maximally supports the voltage at the grid connection point under safe ride-through to avoid DC voltage and AC current over-limits, so as to improve the fault characteristics.

[0053] In specific implementation, in step S104, (P max 1, Q max 1) are the maximum active power and reactive power of the voltage source converter when the limit fault time is greater than the main protection action time, and its calculation method is as follows:

[0054] Obtain the combination of the active power and reactive power of the charging station with the maximum grid connection point voltage from the power boundary curve of the AC current limit, the lower boundary of the reactive power, and the DC voltage constraint boundary in the fault ride-through power feasible region;

[0055] According to the grid connection point voltages corresponding to the active power and reactive power combinations of each boundary, the combination of the active power and reactive power corresponding to the maximum grid connection point voltage is the power operation point (P max 1, Q max 1) obtained in the fault ride-through power domain of the charging station.

[0056] For the adaptive fault ride-through method of the vehicle-grid interactive charging station considering grid voltage support, the combination of the active power and reactive power of the charging station that maximizes the grid connection point voltage on the DC voltage constraint boundary is determined as follows:

[0057]

[0058] Where U dc,th is the upper limit of the DC voltage of the charging station; t1 is the main protection action time of the power grid; the parameters Q line,ab and Q a are calculated respectively as follows:

[0059]

[0060]

[0061] Among them, U f0 is the amplitude of the grid-connected point voltage of the charging station at the moment of fault; U g is the equivalent grid voltage; R f and X f are the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line, and the equivalent resistance and reactance of the power grid.

[0062] For the adaptive fault ride-through method of the vehicle-grid interactive charging station considering grid voltage support, the active and reactive power combinations of the charging station that make the grid-connected point voltage reach the maximum value at the lower boundary of reactive power are determined as follows:

[0063]

[0064] Among them, P x is the active power of the charging station that makes the grid-connected point voltage maximum when the reactive power is 0; mid{} represents the median operation; the parameters P b , P c are respectively expressed as:

[0065]

[0066] P c =U f0 K I I N

[0067] For the adaptive fault ride-through method of the vehicle-grid interactive charging station considering grid voltage support, the active and reactive power combinations of the charging station that make the grid-connected point voltage reach the maximum value on the power boundary curve limited by AC current are determined as follows:

[0068]

[0069] The above formula is Figure 1 Formula (24) in the above formula, where P curve , P a are calculated according to the following formula:

[0070]

[0071] Take (P g,f , Q g,f ) in the following formula as (P ref 1, Q ref 1), (P ref 2, Q ref 3) and (P ref 3, Q ref3), calculate the grid connection point voltages corresponding to different active and reactive power combinations on the boundary of the fault ride-through power feasible region:

[0072]

[0073] Among them, P g,f and Q g,f are the active and reactive powers output by the voltage source converter under the fault respectively.

[0074] In the embodiment of the present invention, when the ultimate fault time is less than the main protection action time, the charging station maximally supports the grid connection point voltage under the safe ride-through of avoiding the over-limitation of the DC voltage and AC current, so as to improve the fault characteristics.

[0075] Specifically, in step S6, the reference value P of the discharge power of the charging station max 2, and the reference value Q of the reactive power of the voltage source converter max 2 are the power operating points that make the grid connection point voltage the largest in the power domain of the charging station limited by the AC current. After the main protection fails to operate, it can avoid the over-limitation of the DC voltage and AC current of the charging station and maximally support the grid connection point voltage, and are calculated in the following manner:

[0076] S601. Calculate the active and reactive power combinations of the charging station that make the grid connection point voltage the largest on the power boundary curve of the power domain of the charging station limited by the AC current, the power boundary when the reactive power is zero, and the power boundary when the active power is zero respectively;

[0077] S602. Calculate the grid connection point voltages corresponding to the active and reactive power combinations of each boundary. The active and reactive power combination corresponding to the maximum value of the grid connection point voltage is the power operating point (P max 2, Q max 2) that makes the grid connection point voltage the largest in the power domain of the charging station limited by the AC current.

[0078] In step S6, the active and reactive power combinations on the power boundary when the active power is zero are determined in the following manner:

[0079]

[0080] Among them, Q y is the reactive power of the charging station that makes the grid connection point voltage the largest when the active power is 0; Q a1 is calculated in the following manner:

[0081]

[0082] The active and reactive power combinations on the power boundary when the reactive power is zero are determined in the following manner:

[0083]

[0084] The active and reactive power combinations on the power boundary curve are determined as follows:

[0085]

[0086] Please refer to Figure 2 , Figure 2 which is a schematic structural diagram of an adaptive fault ride-through control device for a vehicle-grid interactive charging station considering grid voltage support, including:

[0087] An acquisition module 201, which collects the AC bus voltage, active power, and reactive power of the electric vehicle charging station before and after the grid fault;

[0088] A first calculation module 202, which is used to calculate the limit fault time and call the first comparison module;

[0089] A first comparison module 203, which is used to compare the limit fault time with the main protection operation time. If the limit fault time is greater than the main protection operation time, it calls the second calculation module; otherwise, it calls the third calculation module;

[0090] A second calculation module 204, which calculates the maximum active power and reactive power of the voltage source converter of the charging station when the limit fault time is greater than the main protection operation time, and calls the first control module;

[0091] A first control module 205, which switches the voltage source converter of the charging station to constant power control, and sets the active power reference value and reactive power reference value to (P max 1, Q max 1) respectively, and calls the second comparison module;

[0092] A second comparison module 206, which is used to compare the voltage amplitude at the grid connection point of the charging station at the moment when the main protection should operate. If the voltage at the grid connection point recovers to more than 0.95 times the rated voltage, it calls the second control module; otherwise, it calls the third control module;

[0093] A second control module 207, which switches the active power control of the voltage source converter to DC voltage control, and the reactive power reference value is the same as before the fault;

[0094] A third control module 208, which adjusts the discharge power reference value of the charging station to P max 2 at the moment when the main protection should operate, and at the same time switches the active power control of the voltage source converter to DC voltage control, and adjusts the reactive power control reference value of the voltage source converter to Q max 2;

[0095] The third calculation module 209 calculates the maximum active power and reactive power of the voltage source converter when the ultimate fault time is less than the main protection operation time, and calls the fourth control module.

[0096] The fourth control module 2010 controls the DC voltage and reactive power of the voltage source converter in the charging station. The reactive power reference value is set to Q max 2, and at the same time adjusts the discharge power reference value of the charging station to P max 2.

[0097] Among them, (P max 1, Q max 1) are the maximum active power and reactive power of the voltage source converter when the ultimate fault time is greater than the main protection operation time; (P max 2, Q max 2) are the maximum active power and reactive power of the voltage source converter when the ultimate fault time is less than the main protection operation time.

[0098] In the embodiment of the present invention, through the combination of the acquisition module, multiple comparison modules, and multiple control modules, the relationship between the ultimate fault time and the main protection operation time is fully considered, so that the adaptive fault ride-through control of the charging station can always ensure the safety of the DC voltage and AC current of the charging station and support the grid connection point voltage to the maximum extent.

[0099] In the embodiment of the present invention, the adaptive fault ride-through control can always ensure the safety of the DC voltage and AC current of the charging station and support the grid connection point voltage to the maximum extent.

[0100] The control device includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements the fault ride-through control method for an electric vehicle charging station that takes into account reactive power support and charging power as described in the present invention.

[0101] In some embodiments, the memory may include non-permanent memory in a computer-readable medium, random access memory (RAM) and / or non-volatile memory in the form of, for example, read-only memory (ROM) or flash memory (flash RAM). The memory is an example of a computer-readable medium.

[0102] Such as Figure 3 , in the embodiment of the present invention, the charging station is connected to the distribution network through a 0.380 / 10 kV transformer. The equivalent resistance and inductance of the low-voltage side of the grid after conversion are 0.25 + j0.49 Ω. There are 4 electric vehicles in the charging station, the DC rated voltage is 800 V, and the outer-loop control parameters of the voltage source converter are k p = 2, k i = 50; the DAB parameters are k pv= 0.0005, k vi = 20, L r is 20e-6H, f s = 10000Hz, n = 800 / 420; Under normal operation, the active power of the charging station is 1 p.u., and the reactive power is set to 0. Set the system to have a three-phase short-circuit fault at 0.3 s. At the fault moment, the discharge powers of the electric vehicles are 180 kW, 190 kW, 210 kW, and 220 kW respectively. The maximum allowable coefficient of the AC current of the voltage source converter is 1.2, the upper limit of the DC voltage is 1.2 p.u., the operating time of the main protection of the distribution network is 0.4 s, and the operating time of the backup protection is 1 s. Set 3 comparison groups to verify the control method proposed in this paper: Method 1: The charging station adopts constant DC voltage control and reactive power control, and the reference value of reactive power is 0; Method 2: The charging station adopts the new energy fault ride-through method; Method 3: The charging station adopts the method proposed in the present invention.

[0103] The present invention gives a specific example 1:

[0104] At the moment of the fault, the voltage at the grid connection point of the charging station drops to 0.65 p.u., and the limit fault time is 112 ms, which is greater than the operating time of the main protection; in Method 3, the voltage source converter switches to active and reactive power control, and the reference values are 0.75 p.u. and 0.21 p.u. respectively; the grid connection point voltage, DC voltage, active and reactive powers of the voltage source converter, and DAB transmission power of the charging station under the 3 methods are as Figure 4 shown.

[0105] As Figure 4 (b) shows, the DC voltage does not exceed the limit during the fault under Method 1 and Method 3, while the DC voltage exceeds the limit about 50 ms after the fault occurs under Method 2. As Figure 4 (c) and (d) show, under Method 1, the active and reactive powers of the charging station during the fault are 0.78 p.u. and 0 p.u. respectively. Method 2 gives priority to reactive power control and outputs 0.33 p.u. of reactive power according to the voltage drop at the grid connection point, and the corresponding active power is 0.71 p.u. Method 3 outputs active and reactive powers of 0.75 p.u. and 0.21 p.u. respectively. As Figure 4 (a) shows, since the charging station only outputs active power under Method 1, the grid connection point voltage rises to 0.67 p.u.; in Method 2, since the charging station gives priority to reactive power control, the grid connection point voltage reaches 0.71 p.u.; in Method 3, the charging station outputs power according to the maximum voltage control point, and the grid connection point voltage is 0.75 p.u., and the voltage support effect is significantly improved compared with Method 1 and Method 2. Figure 4As shown in (e), the DAB transmission power basically remains at the pre-fault transmission power. The improvement of the voltage source converter control in Method 3 does not affect the discharge of the charging station during the fault, and can better support the grid connection point voltage while avoiding the over-limitation of the DC voltage and AC current.

[0106] The present invention gives a specific example 2

[0107] The fault conditions remain unchanged, but the main protection refuses to operate at 0.4 s, and the fault is removed by the backup protection at 1 s. The control strategies and reference values of Method 2 and Method 3 are the same as those in Case 1 before the main protection refuses to operate. After detecting that the fault has not been removed at 0.4 s, Method 3 resumes the DC voltage control of the voltage source converter, and at the same time adjusts the reference value of the discharge power of the charging station to 0.73 p.u and the reference value of the reactive power of the voltage source converter to 0.27 p.u. The DC voltage of the charging station, the grid connection point voltage, the active and reactive powers of the voltage source converter, and the DAB transmission power are as Figure 5 shown.

[0108] As Figure 5 (b) shows, the DC voltage exceeds the limit 30 ms after the main protection refuses to operate under Method 1. The DC voltage has exceeded the limit before the main protection operates under Method 2. In Method 3, the reference value of the discharge power of the charging station is adjusted at the moment when the main protection refuses to operate, so that the active power of the voltage source converter and the DAB transmission power are balanced again, and the DC voltage gradually recovers to the reference value after 0.4 s, avoiding the over-limitation of the DC voltage. As Figure 5 (c) and (d) show, in Method 1, due to maintaining the pre-fault control strategy, the active and reactive powers of the voltage source converter are 0.78 p.u and 0 respectively; in Method 2, the active and reactive powers of the voltage source converter are 0.71 p.u and 0.33 p.u respectively; in Method 3, the active and reactive powers of the voltage source converter change from 0.75 p.u and 0.21 p.u to 0.73 p.u and 0.27 p.u respectively. As Figure 5 (a) shows, under Method 1, during the fault, since the charging station only outputs active power, the grid voltage support effect is limited, and the grid connection point voltage is 0.68 p.u; in Method 2, since the charging station does not output power according to the maximum voltage control point, the grid connection point voltage is 0.71 p.u, and the voltage support effect is not obvious; in Method 3, the charging station outputs power according to the maximum voltage control point on the power range boundary limited by the AC current of the voltage source converter after the refusal to operate, and the grid connection point voltage rises from 0.75 p.u before the main protection refuses to operate to 0.77 p.u, and the voltage support effect is significantly improved compared with Method 1 and Method 2. As Figure 5 (e) shows, in Method 1 and Method 2, the DAB transmission power is not adjusted during the entire fault process, making Figure 5(b) DC voltage over-limit; in Method 3, when the DC voltage reaches the over-limit value, the active power of the voltage source converter is adjusted to 0.73 p.u. following the reference value of the charging station discharge power, avoiding Figure 5 (b) DC voltage over-limit. In Method 1 and Method 2, the voltage source converter is not operated at the maximum voltage support point within the fault ride-through power feasible region and cannot avoid DC voltage over-limit; Method 3 can also avoid DC voltage and AC current over-limit while better supporting the grid connection point voltage under the refusal of the main protection.

[0109] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An adaptive fault ride-through method for a vehicle-grid interactive charging station considering grid voltage support, characterized in that, It includes the following steps: S1. Collect the voltage amplitude at the grid connection point of the charging station. When the voltage amplitude at the grid connection point is lower than 0.95 times the rated voltage, start the adaptive fault ride-through control of the charging station; S2. Calculate the ultimate fault time for the charging station to ride through the fault according to the discharge power before the voltage dip at the grid connection point of the charging station; S3. Compare the ultimate fault time with the main protection operation time. If the ultimate fault time is greater than the main protection operation time, execute step S4; if the ultimate fault time is less than the main protection operation time, execute step S7; S4. Switch the voltage source converter of the charging station to constant power control, and set the active and reactive power reference values to the power operating point (P max1 , Q max1 ) in the vehicle-grid interactive charging station fault ride-through power domain in the discharge operation mode, and execute step S5; S5. Collect the voltage amplitude at the grid connection point of the charging station at the moment when the main protection should operate. If the voltage amplitude at the grid connection point recovers to more than 0.95 times the rated voltage, switch the active power control of the voltage source converter to DC voltage control at the moment when the main protection should operate, adjust the reactive power reference value to be the same as before the fault, and the charging station resumes normal operation; otherwise, execute step S6; S6. At the moment when the main protection should operate, adjust the discharge power reference value of the charging station to P max2 , and at the same time, switch the active power control of the voltage source converter to DC voltage control, and adjust the reactive power control reference value of the voltage source converter to Q max2 , until the grid connection point voltage of the charging station recovers to more than 0.95 times the rated voltage; S7. The voltage source converter of the charging station maintains DC voltage control and reactive power control. The reactive power and active power are set to the power operating point (P max2 , Q max2 ) that maximizes the grid connection point voltage in the power domain of the charging station limited by the AC current until the grid connection point voltage of the charging station recovers to more than 0.95 times the rated voltage.

2. The adaptive fault ride-through method for a vehicle-grid interactive charging station considering grid voltage support according to claim 1, wherein Calculating the ultimate fault time for the charging station to ride through the fault according to the discharge power before the voltage dip at the grid connection point of the charging station includes: Among them, T max is the limit fault time for the charging station to ride through faults; C is the DC capacitor of the charging station; U dc,lim is the upper limit of the DC voltage of the charging station; U * dc is the reference value of the DC voltage of the charging station; P s0 is the discharging power of the charging station before the fault; U f is the grid connection point voltage of the charging station under the fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter.

3. The method for adaptive fault ride-through of a vehicle-grid interactive charging station considering grid voltage support according to claim 1, wherein The power operating point (P max1 , Q max1 ) that maximizes the grid connection point voltage in the fault ride-through power domain of the vehicle-grid interactive charging station in the discharge operation mode is calculated as follows: S301. Calculate the active and reactive power combinations of the charging station that maximize the grid connection point voltage on the power boundary curve limited by the AC current, the lower boundary of the reactive power, and the DC voltage constraint boundary in the fault ride-through power feasible region respectively; S302. Calculate the grid connection point voltage corresponding to each combination of active and reactive power at the boundaries. The combination of active and reactive power corresponding to the maximum value of the grid connection point voltage is the power operating point (P max1 , Q max1 ) that maximizes the grid connection point voltage in the fault ride-through power domain of the charging station.

4. The method for adaptive fault ride-through of a vehicle-grid interactive charging station considering grid voltage support according to claim 3, characterized in that, The active and reactive power combinations of the charging station that maximize the grid connection point voltage on the DC voltage constraint boundary are determined as follows: Among them, (P ref1 , Q ref1 ) represents a combination of active and reactive power of the charging station that maximizes the grid connection point voltage; P s0 is the discharging power of the charging station before the fault; C is the DC capacitor of the charging station; U dc,th is the upper limit of the DC voltage of the charging station; U * dc is the reference value of the DC voltage of the charging station; t1 is the operation time of the main protection of the power grid; Q line,ab , Q a are two intermediate parameters; X f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line and the equivalent reactance of the power grid; R f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line and the equivalent resistance of the power grid; U g is the equivalent voltage of the power grid; U f0 is the amplitude of the grid connection point voltage of the charging station at the moment of the fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter.

5. The adaptive fault ride-through method for a vehicle-grid interactive charging station considering grid voltage support according to claim 3, characterized in that, The active and reactive power combinations of the charging station that maximize the grid connection point voltage on the lower boundary of the reactive power are determined as follows: P c = U f0 K I I N Among them, (P ref2 , Q ref2 ) is a set of active and reactive power combinations of the charging station that maximize the grid connection point voltage; P x is the active power of the charging station that maximizes the grid connection point voltage with the reactive power being 0; mid{} represents the median operation; P b , P c are intermediate parameters; P s0 is the discharging power of the charging station before the fault; C is the DC capacitance of the charging station; U dc,th is the upper limit of the DC voltage of the charging station; U * dc is the reference value of the DC voltage of the charging station; t1 is the operation time of the main protection of the power grid; U f0 is the amplitude of the grid connection point voltage of the charging station at the moment of the fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter.

6. The adaptive fault ride-through method for a vehicle-grid interactive charging station considering grid voltage support according to claim 3, wherein The active and reactive power combinations of the charging station that maximize the grid connection point voltage on the power boundary curve limited by the AC current are determined as follows: Among them, (P ref3 , Q ref3 ) is a set of active and reactive power combinations of the charging station that maximize the grid connection point voltage; P curve , P a are intermediate parameters; U f0 is the voltage amplitude at the grid connection point of the charging station at the moment of fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter; R f is the sum of the equivalent reactance of the charging station transformer, the transmission line impedance, and the equivalent resistance of the power grid; X f is the sum of the equivalent reactance of the charging station transformer, the transmission line impedance, and the equivalent reactance of the power grid; C is the DC capacitor of the charging station; U dc,th is the upper limit of the DC voltage of the charging station; U * dc is the reference value of the DC voltage of the charging station; t1 is the operating time of the main protection of the power grid.

7. The method for the adaptive fault ride-through of the vehicle-grid interactive charging station considering the grid voltage support according to claim 3, characterized in that When calculating the grid connection point voltages corresponding to different active and reactive power combinations on the boundary of the feasible region of the fault ride-through power, substitute the active and reactive power combinations of the charging station that maximize the grid connection point voltage, the active and reactive power combinations of the charging station that result in the maximum grid connection point voltage, and the active and reactive power combinations of the charging station that result in the maximum grid connection point voltage into the following formula (P g,f , Q g,f ) for calculation, that is: Among them, P g,f is the active power output by the voltage source converter under the fault; R f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line, and the equivalent resistance of the power grid; Q g,f is the reactive power output by the voltage source converter under the fault; X f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line, and the equivalent reactance of the power grid; U g is the equivalent voltage of the power grid.

8. The method for the adaptive fault ride-through of the vehicle-grid interactive charging station considering the grid voltage support according to claim 1, characterized in that Calculating until the power operating point that maximizes the grid connection point voltage in the charging station power domain limited by the AC current includes: S601. Calculate the active and reactive power combinations of the charging station that maximize the grid connection point voltage on the power boundary curve of the charging station power domain limited by the AC current, the power boundary when the reactive power is zero, and the power boundary when the active power is zero respectively; S602. Calculate the grid connection point voltage corresponding to the active and reactive power combinations of each boundary. The active and reactive power combination corresponding to the maximum value of the grid connection point voltage is the power operating point that maximizes the grid connection point voltage in the charging station power domain limited by the AC current.

9. The method for the adaptive fault ride-through of the vehicle-grid interactive charging station considering the grid voltage support according to claim 8, wherein The active and reactive power combinations on the power boundary when the active power is zero are determined as follows: P c = U f0 K I I N Among them, (P′ ref1 , Q′ ref1 ) is the active and reactive power combination on the power boundary when the active power is zero; Q y is the reactive power of the charging station that maximizes the grid connection point voltage when the active power is 0; U f0 is the amplitude of the grid connection point voltage of the charging station at the moment of the fault; K I is the maximum allowable AC current coefficient of the voltage source converter; I N is the rated AC current of the voltage source converter. The active and reactive power combinations on the power boundary when the reactive power is zero are determined as follows: Among them, (P′ ref2 , Q′ ref2 ) is the active and reactive power combination on the power boundary when the reactive power is zero; P x is the active power of the charging station that maximizes the grid connection point voltage when the reactive power is 0; The active and reactive power combinations on the power boundary curve are determined as follows: Among them, (P′ ref3 , Q′ ref3 ) is the active and reactive power combination on the power boundary curve; P curve is an intermediate parameter; P ref3 is the active power of the charging station that makes the grid connection point voltage reach the maximum value; X f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line, and the equivalent reactance of the power grid; R f is the sum of the equivalent reactance of the charging station transformer, the impedance of the transmission line, and the equivalent resistance of the power grid.

10. An adaptive fault ride-through device for a vehicle-grid interactive charging station considering grid voltage support, characterized in that, The vehicle-grid interactive charging station adaptive fault ride-through method considering grid voltage support described in claim 1 is implemented by including: An acquisition module that collects the AC bus voltage, active power, and reactive power of the electric vehicle charging station before and after the grid fault; A first calculation module for calculating the ultimate fault time and calling a first comparison module; A first comparison module for comparing the ultimate fault time with the main protection operation time. If the ultimate fault time is greater than the main protection operation time, call a second calculation module; otherwise, call a third calculation module; The second calculation module calculates the maximum active power and reactive power of the charging station voltage source converter when the ultimate fault time is greater than the main protection operation time, and calls the first control module; The first control module switches the voltage source converter of the charging station to constant power control, sets the active power reference value and the reactive power reference value to (P max1 , Q max1 ), respectively, and calls the second comparison module; The second comparison module is used to compare the voltage amplitude at the grid connection point of the charging station at the time when the main protection should operate. If the grid connection point voltage recovers to more than 0.95 times the rated voltage, it calls the second control module; otherwise, it calls the third control module; The second control module switches the active power control of the voltage source converter to DC voltage control, and the reactive power reference value is the same as before the fault; The third control module adjusts the discharge power reference value of the charging station to P at the moment when the main protection should act max2 , and simultaneously switches the active power control of the voltage source converter to DC voltage control, and adjusts the reactive power control reference value of the voltage source converter to Q max2 ; The third calculation module calculates the maximum active power and reactive power of the voltage source converter when the ultimate fault time is less than the main protection operation time, and calls the fourth control module; The fourth control module maintains the DC voltage control and reactive power control of the voltage source converter of the charging station. The reactive power reference value is set to Q max2 , and at the same time, the charging station discharge power reference value is adjusted to P max2 .

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