Over / under-voltage compensation judgment and flexible treatment method for distributed photovoltaic system based on instant line current direction

Abstract

The invention relates to the field of power equipment and grid connection, in particular to a distributed photovoltaic over / under-voltage compensation judgment and flexible treatment method based on an instant line current direction. A voltage compensation mode is adopted, namely, a high-speed and reliable digital processor is adopted as a main controller, a high-power silicon controlled rectifier is adopted as an execution device, a compensation transformer is adopted as a voltage adjusting element, the line current direction is detected, and when current flows from a load side to a network side, the voltage adjusting device has a voltage reduction function, and the voltage can be reduced by 15%; when the line current flows from the grid side to the load side, the device bypasses or operates in the boost mode, and the boost capacity is also 15%. Switching is fast, the time is not longer than 20 ms, local power factor compensation is achieved, and the functions of protection and bypass are achieved; the voltage adjusting device has a single-phase voltage adjusting device and a three-phase voltage adjusting device, and the capacity and the output voltage range can be selected.

Description

Technical Field

[0001] The present invention relates to the field of power equipment and grid connection, and in particular to a method for over / under voltage compensation judgment and flexible management of distributed photovoltaics based on instantaneous line current direction. Background Art

[0002] With the promotion of distributed photovoltaics, the voltage of the distribution network has increased. At the same time, the line voltage cannot be controlled at the substation, causing the line voltage to exceed the upper limit and the voltage to be unqualified.

[0003] Currently, there are not many technologies for handling this type of high voltage. Basically, the high voltage is reduced by adjusting the inverter program, that is, controlling the power generation and limiting the online current. Users generally do not cooperate with this method.

[0004] Some substations adjust the output voltage to the minimum value to lower the line voltage. However, due to irregular fluctuations in power generation, this cannot completely solve the high voltage problem. Furthermore, when photovoltaic power generation suddenly stops, the transformer voltage cannot be adjusted in time, resulting in low line voltage and unqualified low voltage. This also places a heavy workload on substation operators, increases the number of transformer on-load switch voltage adjustments, and increases the transformer maintenance workload.

[0005] Limitations of existing technical solutions: Inverter power generation control defects: Economic contradictions: Forced power generation restrictions result in an average annual power generation loss of 15-20%, and users have strong resistance; Insufficient regulation accuracy: Most inverters only support ±5% power regulation, which cannot precisely match voltage requirements; Communication delay: In centralized control mode, the data collection-decision-execution link delay is more than 30 seconds; Substation voltage regulation strategy defects: Static voltage regulation failure: Fixed lowest gear operation causes PV absorption capacity to drop by 30%; Increased equipment loss: Statistics from a provincial power grid show that frequent voltage regulation shortens transformer life by 20%; Voltage oscillation risk: In 2022, a sudden drop in photovoltaic output caused a voltage drop in a certain location, with the lowest voltage reaching 350V and lasting for 17 minutes; Search keyword groups: distributed and photovoltaic and power distribution and direction and judgment and governance and power grid and high voltage and low voltage; Analysis of English translated phrases: Distributed and Photovoltaic and Distribution and Direction and Judgment and Management and Grid and High Voltage and Low Voltage; On May 22, 2025, an abstract search for "distributed and photovoltaic and power distribution and direction and judgment and management and power grid and high voltage and low voltage" was conducted on China National Knowledge Infrastructure (CNKI), but no relevant literature was found.

[0006] Search at the United States Patent and Trademark Office: On May 22, 2025, a search was conducted on the United States Patent and Trademark Office website for “Distributed with Photovoltaicwith Distribution with Direction with Judgment with Management with Grid withHigh Voltage with Low Voltage,” but no relevant literature was found; the search URL was https: / / ppubs.uspto.gov / pubwebapp / .

[0007] WIPO search: On May 22, 2025, a search was conducted on WIPO's https: / / patentscope2.wipo.int / for "Distributed and Photovoltaic and Distribution and Direction and Judgment and Management and Grid and High Voltage and Low Voltage", but no relevant documents were found.

[0008] Japan Patent Office search: On May 22, 2025, a search was conducted on the Japan Patent Office website https: / / www.j-platpat.inpit.go.jp / for "Distributed and Photovoltaic and Distribution and Direction and Judgment and Management and Grid and High Voltage and Low Voltage", but no relevant documents were found. Summary of the Invention

[0009] Purpose of the invention: To provide a better method for distributed photovoltaic over / undervoltage compensation judgment and flexible management based on the instantaneous line current direction. The specific purpose can be seen in the multiple substantial technical effects of the specific implementation part.

[0010] In order to achieve the above purpose, the present invention adopts the following technical solutions: This distributed photovoltaic system uses a voltage compensation method to determine over / undervoltage compensation and provide flexible management based on the real-time line current direction. It employs a high-speed, reliable digital processor as the main controller, a high-power thyristor as the actuator, and a compensation transformer as the voltage regulator. The system detects the line current direction. When current flows from the load side to the grid side, the line indicates overvoltage. The voltage regulator operates in step-down mode, reducing the voltage by 15%. When current flows from the grid side to the load side, the line indicates undervoltage and operates in step-up mode, also with a 15% boost capability. The system features fast switching times of less than 20ms, local power factor compensation, and protection and bypass functions. The voltage regulator is available in single-phase and three-phase versions, with selectable capacity and output voltage ranges. High voltage management for distributed photovoltaic systems utilizes a method that addresses both 10kV and 380V lines, with different management solutions designed for different line conditions.

[0011] A further technical solution of the present invention is: Through the analysis of the formation mechanism of low voltage and high voltage in the line, and a large amount of research, a method of determining whether the compensation mode is step-up or step-down through complex parameter detection and calculation was found. It was found that as long as the direction of the line current is detected, it can be determined whether the voltage compensation required by the line is step-up or step-down. A control method and device for controlling the working mode of the compensation equipment after line current detection was developed; the high-power thyristor used for voltage control adopts solid-state relays, adopts optocoupler isolation, has an RC spike absorption circuit, and has a fast fuse to protect the solid-state relay. The relay is in the control circuit, and there are no power electronic devices in the main circuit, which can ensure that there is no power outage in the power supply circuit during the adjustment process.

[0012] A further technical solution of the present invention is: The system monitors the effective values ​​of current and voltage in real time, uses the AMR processor to switch the thyristor at zero crossing to generate corresponding compensation voltage and current, and performs compensation in phases to stabilize the output voltage within ±3% of the set value, and realize three-phase voltage imbalance and power factor compensation.

[0013] A further technical solution of the present invention is: When managing distributed photovoltaic high voltage, it is also necessary to ensure that the line does not suffer from low voltage due to the management of high voltage.

[0014] A further technical solution of the present invention is: It includes low voltage control strategies when there is no photovoltaic power generation and high voltage control strategies when there is photovoltaic power generation and current is sent up; Calculation of low voltage control when there is no photovoltaic power generation: According to the circuit diagram, divide the line into several sections and calculate the resistance R, reactance X, and impedance value of each section according to different wire specifications; Calculate the current distribution along the line based on the load distribution and transformer load rate; Establish a calculation model to calculate the voltage value along the line; Based on the existing power factor value of the line, calculate the reactive power compensation required to compensate to a power factor of 0.98, using 400kvar per unit as the compensation unit and distribute it in the line; Calculate the line current distribution after reactive power compensation, and use this current to calculate the voltage distribution along the line. When the voltage is around 9.5kV, add a 10kV power electronic compensation substation to increase the voltage to 10.5kV. At this point, the layout of the low-voltage management intelligent reactive power compensator and the 10kV power electronic compensation substation has been completed; Photovoltaic power generation, when there is current transmission, high voltage control strategy: Based on the photovoltaic distribution and the maximum substation current during power transmission, the voltage distribution along the line is calculated using the formula U=E+I·Z. Based on the location and capacity of the low voltage control equipment, the 10kV power electronic compensation substation is designed to reduce the voltage to 9.5kV. Calculations are performed to determine whether the voltage of the entire line is within the acceptable range. If low-voltage equipment is used as high-voltage control equipment and cannot completely control the high voltage, a 10kV power electronic compensation substation can be installed at a suitable location based on the low-voltage control equipment to achieve the high-voltage control goal.

[0015] A further technical solution of the present invention is: High voltage control strategies include: 10kV line current has no reverse transmission, or some line segments have reverse transmission current control solutions; In this case, the 10kV line voltage does not exceed the upper limit, and only the low-voltage lines in some substations have high voltages exceeding the upper limit; This type of situation cannot be managed on the 10kV line, but must be managed on the low-voltage line where there is an over-limit transformer. By collecting low-voltage line parameters, including wire diameter, length, load distribution, total photovoltaic power generation and other data, and through calculation, a low-voltage flexible voltage management device is added where the voltage exceeds 415V. If the voltage increases by more than 10%, multiple low-voltage flexible voltage management devices are required to manage the voltage simultaneously. The location and capacity are determined based on simulation calculations.

[0016] A further technical solution of the present invention is: High voltage management strategies include: 10kV line current reverse management solution: In this type of line, the 10kV line voltage exceeds the limit. If this is managed on the low-voltage line, equipment would need to be installed on each substation, which is very troublesome and requires a large investment. Instead, a 10kV flexible voltage management device and an intelligent flexible power quality management device can be installed on the 10kV line to manage the voltage. This solution can reduce the number of devices, lower equipment investment, and shorten the installation workload.

[0017] Due to the intermittent nature of photovoltaic power generation and the randomness of 10kV line current direction changes, to ensure line voltage compliance, the equipment must promptly adjust the control method of the 10kV flexible voltage management device when the line current direction changes, while also taking into account low line voltage management. When the current reverses, reactive power may shift from inductive to capacitive, requiring power quality compensation devices to track and compensate for the varying reactive power properties.

[0018] By collecting 10kV line drawings, load distribution, photovoltaic reverse transmission capacity, and line load when there is no photovoltaic power generation, a simulation model is built. Through simulation calculations, the line compensation plan is determined, taking into account both high voltage and low voltage management.

[0019] A further technical solution of the present invention is: High voltage control strategies include: 10kV line high voltage control plus some low voltage line control solutions; In order to ensure that the 10kV voltage is within the range of ±7%, the voltage of some substations in the 10kV line will be around 10.5kV. When the substation near this point has photovoltaic power transmission, the low-voltage line will also have high voltage; or the substations at other points have large photovoltaic power generation capacity and large reverse current. Even if the substation voltage is low, it will cause high voltage for some users on the line. In this case, the 10kV line is first treated to ensure that the entire 10kV line operates within the qualified voltage range. At the same time, treatment is carried out on the voltage lines that need treatment to ensure that the voltage for all users is qualified.

[0020] The present invention adopting the above technical solution has the following beneficial effects compared with the existing technology: it adopts power electronics technology and flexible compensation technology to automatically compensate for voltage and line reactive current. By instantly detecting the direction of line current, the equipment control mode can be selected, and the equipment can be instantly controlled to operate in the correct mode and respond quickly to make the line voltage qualified, including high voltage control and low voltage control, at the same time.

[0021] The equipment adopts a fully automatic maintenance-free design to minimize line maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to further illustrate the present invention, the following is further described with reference to the accompanying drawings: Figure 1 is the line voltage and current flow diagram; among them: when the current is reversed, due to the huge capacity of the large power grid, the transformer outlet voltage will not change due to the change of the line voltage, that is, the E value remains basically unchanged, and the line end voltage U=E+I·Z Therefore, when the line reverse current is larger, the line impedance Z is larger, and the terminal voltage is higher.

[0023] Figure 2 = is the line voltage and current flow diagram; where: due to the uncertainty and timeliness of distributed photovoltaic power generation, when photovoltaic power generation is not or partially generated, the power generation is less than the power consumption of the line, and the current flows from the substation to the load side, which will cause the terminal voltage to drop. Figure 2 .

[0024] At this time, the line end voltage U=E+I·Z Therefore, when managing high voltage in distributed photovoltaic systems, it is necessary to ensure that low voltage in the lines does not occur due to the management of high voltage.

[0025] Figure 3 This is the circuit diagram for low voltage control calculation when there is no photovoltaic power generation; Figure 4 For photovoltaic power generation, when there is current transmission, the circuit diagram of high voltage control calculation; Figure 5 For: low voltage line high voltage control equipment and installation; Figure 6 This is a schematic diagram of the installation of 10kV line management equipment; Figure 7 It is an AC-DC converter structure; Figure 8 It is a DC-AC converter structure; Figure 9 It is a structure with high-power thyristor as the core component; Figure 10 The following is a connection diagram of a three-stage bypass system for both methods; Figure 11 This is the schematic diagram of the patented device that simultaneously solves the problems of 10kV line power factor, three-phase imbalance, compensation of capacitive and inductive loads, and elimination of high-order harmonics below 19th; Figure 12 This is the double-pole installation engineering drawing of the intelligent flexible low-voltage voltage adjustment device; Figure 13 is the schematic diagram; DETAILED DESCRIPTION

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0027] Mechanism of high voltage generation As the primary carrier of power transmission, the actual transmission line model is not an ideal conductor that ignores any impedance effects. Instead, it should be a distributed parameter model that takes into account multiple power parameters such as resistance (R), reactance (X), conductance (G), and susceptance (B). The specific values ​​of these distributed parameters will be affected to a certain extent by factors such as the conductor model and the transmission environment.

[0028] The capacity of a single inverter in a distributed photovoltaic system is relatively small. The inverter control method basically controls the power generation, and the voltage basically follows the grid voltage. When the power generation is greater than the power consumption of the line, the current direction changes and the energy is sent to a higher-level grid through the line. Figure 1 . When the current is reversed, due to the huge capacity of the large power grid, the transformer outlet voltage will not change due to the change of the line voltage, that is, the E value remains basically unchanged, while the line end voltage U=E+I·Z Therefore, when the line reverse current is larger, the line impedance Z is larger, and the terminal voltage is higher.

[0029] Due to the uncertainty and timeliness of distributed photovoltaic power generation, when photovoltaic power generation is not or partially, the power generation is less than the power consumption of the line, and the current flows from the substation to the load side, which will cause the terminal voltage to drop. Figure 2 .

[0030] At this time, the line end voltage U=E+I·Z Therefore, when managing high voltage in distributed photovoltaic systems, it is necessary to ensure that low voltage in the lines does not occur due to the management of high voltage.

[0031] A calculation model is established based on information such as the line diagram, conductor model, corresponding length, load distribution, photovoltaic distribution, maximum active power without photovoltaic power generation, and maximum reverse active power during photovoltaic power generation.

[0032] refer to Figure 3 : Calculation of low voltage control when there is no photovoltaic power generation: According to the circuit diagram, divide the line into several sections and calculate the resistance R, reactance X, and impedance value of each section according to different wire specifications.

[0033] According to the load distribution and transformer load rate, the current distribution along the line is calculated.

[0034] Establish a calculation model to calculate the voltage values ​​along the line.

[0035] According to the existing power factor value of the line, the reactive compensation required to compensate to a power factor of 0.98 is calculated, with each 400kvar unit as the compensation unit and distributed in the line.

[0036] Calculate the line current distribution after reactive power compensation, and use this current to calculate the voltage distribution along the line. When the voltage is around 9.5kV, add a 10kV power electronic compensation substation to increase the voltage to 10.5kV.

[0037] At this point, the layout of the low-voltage management intelligent reactive power compensator and the 10kV power electronic compensation substation has been completed.

[0038] refer to Figure 4 , Photovoltaic power generation, when there is current uplink, high voltage control calculation Based on the photovoltaic distribution and the maximum current of the substation when power is transmitted, the voltage distribution value along the line is calculated using U=E+I*Z.

[0039] Based on the location and capacity of the low-voltage control equipment, the 10kV power electronic compensation substation is calculated with the goal of reducing the voltage to 9.5kV to determine whether the voltage of the entire line is within the qualified range.

[0040] If low-voltage equipment is used as high-voltage control equipment and cannot completely control the high voltage, a 10kV power electronic compensation substation can be installed at a suitable location based on the low-voltage control equipment to achieve the high-voltage control goal.

[0041] High voltage control solution Since high line voltage is caused by the reverse transmission of current in the line, the voltage increase caused by the distributed parameters of the line causes the line voltage to increase. Therefore, the high voltage of the line containing distributed photovoltaics must be controlled through line voltage control.

[0042] There are several situations that need to be managed separately and with different solutions.

[0043] There is no reverse current in the 10kV line, or there is a solution to control the reverse current in some line segments.

[0044] In this case, the 10kV line voltage does not exceed the upper limit, and only the low-voltage lines in some substations have high voltages that exceed the upper limit.

[0045] This type of situation cannot be managed on the 10kV line, but must be managed on the low-voltage line where there is an over-limit transformer.

[0046] By collecting low-voltage line parameters, including wire diameter, length, load distribution, total photovoltaic power generation and other data, and through calculation, a low-voltage flexible voltage management device is added where the voltage exceeds 415V. If the voltage increases by more than 10%, multiple low-voltage flexible voltage management devices are required to manage the voltage simultaneously. The location and capacity are determined based on simulation calculations.

[0047] Schematic diagram of voltage management solution installation, such as Figure 5 : High voltage control solution Since high line voltage is caused by the reverse transmission of current in the line, the voltage increase caused by the distributed parameters of the line causes the line voltage to increase. Therefore, the high voltage of the line containing distributed photovoltaics must be controlled through line voltage control.

[0048] There are several situations that need to be managed separately and with different solutions.

[0049] There is no reverse current in the 10kV line, or there is a solution to control the reverse current in some line segments.

[0050] In this case, the 10kV line voltage does not exceed the upper limit, and only the low-voltage lines in some substations have high voltages that exceed the upper limit.

[0051] This type of situation cannot be managed on the 10kV line, but must be managed on the low-voltage line where there is an over-limit transformer.

[0052] By collecting low-voltage line parameters, including wire diameter, length, load distribution, total photovoltaic power generation and other data, and through calculation, a low-voltage flexible voltage management device is added where the voltage exceeds 415V. If the voltage increases by more than 10%, multiple low-voltage flexible voltage management devices are required to manage the voltage simultaneously. The location and capacity are determined based on simulation calculations.

[0053] Schematic diagram of voltage management solution installation, such as Figure 5 : 10kV line current reverse control solution.

[0054] In this type of line, the 10kV line voltage exceeds the limit. If this is managed on the low-voltage line, equipment would need to be installed on each substation, which is very troublesome and requires a large investment. Instead, a 10kV flexible voltage management device and an intelligent flexible power quality management device can be installed on the 10kV line to manage the voltage. This solution can reduce the number of devices, lower equipment investment, and shorten the installation workload.

[0055] Due to the intermittent nature of photovoltaic power generation and the randomness of 10kV line current direction changes, to ensure line voltage compliance, the equipment must promptly adjust the control method of the 10kV flexible voltage management device when the line current direction changes, while also taking into account low line voltage management. When the current reverses, reactive power may shift from inductive to capacitive, requiring power quality compensation devices to track and compensate for the varying reactive power properties.

[0056] By collecting 10kV line drawings, load distribution, photovoltaic reverse transmission capacity, and line load when there is no photovoltaic power generation, a simulation model is built. Through simulation calculations, the line compensation plan is determined, taking into account both high voltage and low voltage management.

[0057] 10kV line management equipment installation diagram Figure 6 : 10kV line high voltage management plus part of the low voltage line management plan.

[0058] In order to ensure that the 10kV voltage is within the range of ±7%, some substations in the 10kV line will have a voltage around 10.5kV. When there is photovoltaic power transmission from the substation near this point, the low-voltage line will also have a high voltage; or the substations at other points have a large photovoltaic power generation capacity and a large reverse current. Even if the substation voltage is low, it will cause high voltage to some users of the line.

[0059] In this case, we first treated the 10kV line to ensure that the entire 10kV line operated within the acceptable voltage range. Simultaneously, we treated the remaining sub-voltage lines to ensure that all users maintained acceptable voltages. This solution not only ensured that voltages at all points on the line remained within acceptable limits, but also reduced investment and construction workload.

[0060] Key technical points: 1. Mechanism of high voltage generated by the circuit Analysis reveals that the mechanism for high voltage generation in distributed photovoltaic systems is current reverse flow, generated by the distributed conductors. Therefore, to control high voltage on a line, the only way is to lower the line voltage at appropriate locations along the line. If current reverse flow occurs on a 10kV line, the 10kV line must be controlled; controlling the low-voltage line alone will not achieve high voltage control.

[0061] 2. Controlling high voltage cannot cause low voltage in the line Photovoltaic power generation is not continuous and random. If high-voltage equipment is not managed and grid current changes cannot be tracked in a timely manner, it will cause low voltage on the line. Low voltage on the line is also not allowed.

[0062] Therefore, the management equipment must be flexible, and it is best to be able to manage high voltage and low voltage at the same time, so as to ensure that the voltage of the line is qualified under any circumstances and reduce investment.

[0063] 3. Instantly judge the direction of line current and realize instant switching of compensation equipment.

[0064] When there is no photovoltaic power generation on the line, the line voltage decreases along the line, and most of the line reactive power is inductive. Therefore, the power electronic compensation substation requires voltage boosting and reactive power compensation requires capacitive reactive power.

[0065] During photovoltaic power generation and power reverse transmission, the voltage along the line is boosted, and most of the line reactive power is capacitive. Therefore, power electronic compensation substations require voltage reduction, and reactive power compensation requires inductive compensation.

[0066] If the compensation is wrong, it will cause more serious consequences. Therefore, as the photovoltaic power generation changes, the compensation amount and compensation method must change quickly, preferably within 60ms.

[0067] The basis of control is the direction of the line current. Therefore, accurate and real-time detection of the line current direction and real-time control of the operation mode of the compensation equipment in the line are the key to the system.

[0068] 4. The particularity of the equipment Power electronic compensation substations must be able to both step up and step down voltages, and the switching needs to be fast and accurate.

[0069] The reactive power compensator can generate both inductive and capacitive reactive power, and the conversion between the two must be fast and accurate.

[0070] 5. Comprehensiveness of voltage management For some lines, only part of the low-voltage line power is sent to the 10kV line. The 10kV line consumes the power locally, and the 10kV line power is not sent. This type of situation only deals with the low-voltage line with high voltage.

[0071] To transmit power up a 10kV line, the 10kV line must be regulated. However, at several substations near the front end of the 10kV power electronic compensation substation, if the photovoltaic power generation is large, even if the 10kV line voltage is regulated, high voltage will still exist at the end of these low-voltage lines. Voltage regulation can be performed on these low-voltage lines to ensure that the voltage of the entire line is within the specified range.

[0072] Equipment Introduction Flexible compensation equipment mainly includes: 10kV intelligent flexible voltage adjustment device, 10kV power quality compensator, 380V / 220V intelligent flexible low voltage adjustment device.

[0073] 1. 10kV intelligent flexible voltage regulator (10kV power electronic compensation substation) 1.1 Equipment structure with IGBT as core component The converter of the 10kV power electronic compensation substation adopts the two-level IGBT full-bridge mode to realize AC-DC-AC power conversion and transformation, which includes two parts: rectifier and inverter.

[0074] AC-DC converter structure is as follows Figure 7 As shown, the three-phase input rated AC voltage is 440V. After passing through the fuse, filter, LCL circuit, and soft start circuit, the three-phase voltage is converted into DC voltage through the IGBT rectifier bridge. The DC rated voltage is 750V. The DC bus is equipped with support capacitors and fuses, and output to the inverter.

[0075] The structure of DC-AC converter is as follows Figure 8 As shown, the DC bus converts the DC voltage into AC voltage through the IGBT inverter, and outputs it through the fuse, LC circuit, fast bypass switch, and filter. The three-phase AC voltage rating is 0.4kV.

[0076] The fast bypass switch, also known as TBS, consists of a parallel pair of positive and negative thyristors, a contactor, and a lightning arrester. When the grid-side winding of the series transformer bears the system short-circuit current or the converter fails, the TBS will close quickly to protect the converter equipment.

[0077] AC-DC-AC converter technical features: (1) 10kV / 1.25kV AC power conversion is achieved. The converter adopts a three-phase full-bridge AC / DC structure with low output harmonics and high output power. It is inserted into the 10kV power supply line through a series transformer and can effectively compensate for the voltage drop of the 10kV line. (2) It has multiple protection functions. In the event of a 10kV line short circuit fault, a converter input source fault, or a converter internal fault, the fast bypass TBS on the low-voltage side closes, and the 10kV bypass switch BK1 on the high-voltage side closes, effectively protecting the converter equipment. When the converter overcurrent protection is activated, the low-voltage circuit breaker on the input power side can effectively trip the converter, and simultaneously close the series transformer high-voltage side bypass switch.

[0078] (3) The converter capacity can be expanded by connecting multiple machines in parallel. When combined with a step-down transformer and a series transformer, it can achieve system access with multiple voltages and capacities. (4) Full-bridge AC / AC converter is used. The converter is located at low potential, which ensures reliable operation and easy maintenance. (6) The high switching frequency greatly reduces the size and cost of the isolation transformer and improves the overall power density of the device; (7) The entire device adopts centralized control, the power adjustment range of the entire system is large, and the response speed is fast, reaching millisecond level; (9) The entire system can achieve four-quadrant operation, and can adjust both the output active power and the output reactive power. It can output a capacitive compensation voltage perpendicular to the line current, and a resistive compensation voltage in phase with the line current. At the same time, it can achieve bidirectional reactive power flow, and can be used as an SVG reactive power compensation device to compensate for the input AC power supply, and output harmonic current to suppress line harmonics, achieving "one machine for multiple uses".

[0079] The protection strategy of the dynamic voltage compensation device meets the relevant standards for AC inverter access to the power grid and has, but is not limited to, the following protection functions: (1) Grid voltage abnormality protection; (2) Grid frequency anomaly protection; (3) Input overvoltage protection (4) Input overcurrent protection (5) Output overload protection; (6) Output short circuit protection; (7) DC overvoltage protection; (8) DC reverse connection protection; (9) Low voltage ride-through protection; (10) Restore grid connection protection; (11) Power recovery rate control protection (12) DC voltage control protection; Structure with high-power thyristor as core component The 10kV power electronic compensation substation with high-power thyristor voltage regulation adopts the thyristor fast shutdown function to detect the line voltage, compare it with the target voltage, determine the compensation voltage value, control the thyristor to open, and couple the voltage to be compensated into the line through multiple compensation transformers, thereby achieving the purpose of voltage stabilization. The principle diagram is as follows: The above two types of power electronic compensation substations achieve the same function, namely, voltage regulation accuracy of ±1%, response time of 20ms, and only the primary coil of the isolation transformer is connected in series in the line. The voltage regulation unit and devices are all 380V, isolated from the main line, safe and reliable.

[0080] The equipment has the function of automatically detecting parameters at the installation point and running independently. It also has the function of accepting commands from the central control system and running according to the instructions of the central control system.

[0081] Equipment access and bypass system Both methods have three-stage bypass systems, such as Figure 9 : Once bypass is required, first quickly close the low-voltage bypass switch, which can be completed within 20ms, and all power electronic systems are disconnected. At the same time, start the high-voltage automatic bypass system, complete the high-voltage bypass within 200ms, and disconnect all power electronic compensation substations. If the power electronic compensation substation needs to be repaired or moved, manually operate the maintenance isolation switch cabinet to achieve safe maintenance and removal of equipment in the power electronic compensation substation.

[0082] like Figure 10 , these operating lines do not have any power supply and will not cause any adverse effects on the system.

[0083] This device is a power electronic device with IGBT as its core component. It can simultaneously solve the problems of 10kV line power factor, three-phase imbalance, capacitive and inductive load compensation, and eliminate high-order harmonics below 19. The small capacity of 10kV SVG can only reach 1000kVar. The capacity required for our project is 200-400kVar. The capacity we have developed can reach 30-1000kVar.

[0084] Using the principle of active compensation, the 10kV voltage is reduced to 1.14kV in the form of voltage reduction. By sampling the current and voltage of the 10kV line, and through controller calculation and control of the IGBT, the 1.14kV power electronic devices generate a negative-sequence current, zero-sequence current and high-order harmonic current in the opposite direction and 8.8 times the 10kV demand value. These currents are injected into the 10kV line through the 10 / 1.14kV three-phase transformer to offset the negative-sequence current, zero-sequence current and high-order harmonic current in the 10kV line, and eliminate the negative-sequence current, zero-sequence current and high-order harmonic current in the 10kV line, thereby solving the needs of solving the three-phase imbalance of the 10kV line, eliminating high-order harmonics, and maintaining a power factor ≥ 0.95, thereby achieving the goal of improving the power quality of the line.

[0085] Key features: 1) The power factor is compensated to above 0.95, and can be compensated under capacitive and inductive loads, and can achieve timely and smooth conversion of capacitive and inductive loads, with linear and timely compensation; 2) Eliminate the system's negative-sequence current and zero-sequence current to balance the system's three phases; 3) Eliminate high-order harmonics; 4) Increase system power supply capacity by 30%; 5) Stabilize the system voltage. The voltage on the 10kV side is generally required to be lower than 12kV. (Adjustable); 6) Dynamic compensation rated output current THD≤3%; 7) Compensation reactive capacity automatically tracks grid changes; 8) Dynamic compensation response time ≤5ms; 9) Allow short-term overload capacity of 1.2 times; 10) Complete protection function; 11) Friendly human-computer interface; 12) Flexible communication interface, which can remotely monitor equipment operation and record operation data; 13) The cooling method is forced air cooling.

[0086] Equipment operation control The 10kV power electronic compensation substation instantly detects the direction of line current. When the line current flows from the load side to the grid side, it indicates that high voltage is generated on the line. Within 20ms, the device selects step-down operation mode and controls the load-side output voltage to 9.5kV. If the line current changes from the grid side to the load side, it means that the voltage is decreasing along the line. In this case, the device selects step-up operation mode within 20ms and controls the load-side output voltage to 10.5kV.

[0087] By adopting this mode and rationally arranging the equipment installation location, it is possible to simultaneously solve the problems of high line voltage when photovoltaic power generation is in progress and low line voltage when photovoltaic power generation is not in progress.

[0088] 10kV power quality compensator

[0089] Overall plan: This device utilizes voltage compensation, employing a high-speed, reliable digital processor as the primary controller, a high-power thyristor as the actuator, and a compensation transformer as the voltage regulator. The device detects the direction of line current. When current flows from the load side to the grid side, the voltage regulator's step-down function reduces the voltage by 15%. When the line current flows from the grid side to the load side, the device bypasses the circuit or operates in boost mode, also with a 15% boost capability. It features fast switching times of less than 20ms, local power factor compensation, and protection and bypass functions. The voltage regulator is available in single-phase and three-phase versions, with selectable capacities and output voltage ranges.

[0090] The voltage control high-power thyristor in this solution adopts high-quality solid-state relays, adopts optocoupler isolation, has an RC spike absorption circuit, and has fast fuses to protect the solid-state relays. The relay is in the control circuit, and there are no power electronic devices in the main circuit, ensuring that there is no power outage in the power supply circuit during the adjustment process.

[0091] Main technical specifications

[0092]

[0093] Installation Capacity less than 100kVA, single pole, installed with bracket.

[0094] Capacity greater than 150kVA, double pole, installed with bracket platform.

[0095] Comparison with other solutions: Existing methods for handling high voltage mainly focus on controlling distributed photovoltaic inverters, regulating the output voltage of substation transformers, and using voltage regulators.

[0096] Photovoltaic inverter control: The voltage increase in distributed photovoltaic lines is not caused by inverter voltage increases. Inverter voltage control cannot solve the problem of line voltage increases. The only way to reduce line voltage is to control the inverter's power generation and reduce the reverse current. However, users invest primarily to sell electricity, so controlling power generation is unacceptable to them.

[0097] Substation transformer gear adjustment: This method works well when the line is short and the reverse current is low. However, there is a significant problem: the transformer gear control is manual and cannot automatically track changes in power generation, which can cause low line voltage. This also increases the workload for substation staff and the maintenance of substation transformers.

[0098] Using a voltage regulator: The voltage regulator is an autotransformer with an on-load tap changer. Since the on-load tap changer is a mechanical switch, the transformer oil needs to be replaced after 2,000 voltage adjustments. Moreover, when adjusting the voltage, it is adjusted step by step according to the gear position and cannot quickly track the grid voltage. It generally operates in an electric voltage regulation state and relies on manual operation. Once the photovoltaic power generation stops, the voltage regulator needs to be operated quickly by humans, which cannot meet the line requirements.

[0099] Reactive power compensation equipment: At present, the reactive power compensation equipment in the line basically adopts capacitor compensation. When the line current flows to the load side, it is basically inductive and can compensate for the line reactive power. When the current flows to the grid side, it may be capacitive reactive power and the capacitor compensation cannot be put into operation.

[0100] Our solution: It adopts power electronics technology and flexible compensation technology to automatically compensate for voltage and line reactive current. By instantly detecting the direction of line current, it can select the equipment control mode, instantly control the equipment to operate in the correct mode, and respond quickly to make the line voltage qualified, including high voltage control and low voltage control, at the same time.

[0101] The equipment adopts a fully automatic maintenance-free design to minimize line maintenance.

[0102] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications are intended to fall within the scope of the claims.

Claims

1. A method for over / under voltage compensation judgment and flexible management of distributed photovoltaics based on the instantaneous line current direction, characterized in that: The voltage compensation method is adopted, that is, a high-speed and reliable digital processor is used as the main controller, a high-power thyristor is used as the executive device, and a compensation transformer is used as the voltage regulating element to detect the direction of the line current. When the current flows from the load side to the grid side, the line shows overvoltage. The voltage regulator operates in the step-down function and can reduce the voltage by 15%; when the line current flows from the grid side to the load side, the line shows undervoltage and operates in the step-up mode with a step-up capacity of 15%; the switching is fast, the time is no more than 20ms, there is on-site power factor compensation, and there are protection and bypass functions; the voltage regulator is available in single-phase and three-phase, with optional capacity and output voltage range; the distributed photovoltaic high voltage is managed by taking into account both 10kV lines and 380V lines, and different management plans are designed according to different line conditions.

2. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instant line current direction according to claim 1, characterized in that: Through the analysis of the formation mechanism of low voltage and high voltage in the line, and a large amount of research, a method of determining whether the compensation mode is step-up or step-down through complex parameter detection and calculation was found. It was found that as long as the direction of the line current is detected, it can be determined whether the voltage compensation required by the line is step-up or step-down. A control method and device for controlling the working mode of the compensation equipment after line current detection was developed; the high-power thyristor used for voltage control adopts solid-state relays, adopts optocoupler isolation, has an RC spike absorption circuit, and has a fast fuse to protect the solid-state relay. The relay is in the control circuit, and there are no power electronic devices in the main circuit, which can ensure that there is no power outage in the power supply circuit during the adjustment process.

3. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 1, characterized in that: The system monitors the effective values ​​of current and voltage in real time, uses the AMR processor to switch the thyristor at zero crossing to generate corresponding compensation voltage and current, and performs compensation in phases to stabilize the output voltage within ±3% of the set value, and realize three-phase voltage imbalance and power factor compensation.

4. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 1, characterized in that: When managing high voltage in distributed photovoltaics, it is also necessary to ensure that low voltage in the lines does not occur due to the management of high voltage.

5. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 1, characterized in that: It includes low voltage control strategies when there is no photovoltaic power generation and high voltage control strategies when there is photovoltaic power generation and current is sent up; When there is no photovoltaic power generation, low voltage control calculation: According to the circuit diagram, divide the line into several sections and calculate the resistance R, reactance X, and impedance value of each section according to different wire specifications; Calculate the current distribution along the line based on the load distribution and transformer load rate; Establish a calculation model to calculate the voltage value along the line; Based on the existing power factor value of the line, calculate the reactive power compensation required to compensate to a power factor of 0.98, using 400kvar per unit as the compensation unit and distribute it in the line; Calculate the line current distribution after reactive power compensation, and use this current to calculate the voltage distribution along the line. When the voltage is around 9.5kV, add a 10kV power electronic compensation substation to increase the voltage to 10.5kV. At this point, the layout of the low-voltage management intelligent reactive power compensator and the 10kV power electronic compensation substation has been completed; Photovoltaic power generation, when there is current transmission, high voltage control strategy: Based on the photovoltaic distribution and the maximum substation current during power transmission, the voltage distribution along the line is calculated using the formula U=E+I·Z. Based on the location and capacity of the low voltage control equipment, the 10kV power electronic compensation substation is designed to reduce the voltage to 9.5kV. Calculations are performed to determine whether the voltage of the entire line is within the acceptable range. If low-voltage equipment is used as high-voltage control equipment and cannot completely control the high voltage, a 10kV power electronic compensation substation can be installed at a suitable location based on the low-voltage control equipment to achieve the high-voltage control goal.

6. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 5, characterized in that: High voltage management strategies include: 10kV line current no reverse transmission, or partial line current reverse transmission management solutions; In this case, the 10kV line voltage does not exceed the upper limit, and only the low-voltage lines in some substations have high voltages exceeding the upper limit; This type of situation cannot be managed on the 10kV line, but must be managed on the low-voltage line where there is an over-limit transformer. By collecting low-voltage line parameters, including wire diameter, length, load distribution, total photovoltaic power generation and other data, and through calculation, a low-voltage flexible voltage management device is added where the voltage exceeds 415V. If the voltage increases by more than 10%, multiple low-voltage flexible voltage management devices are required to manage the voltage simultaneously. The location and capacity are determined based on simulation calculations.

7. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 5, characterized in that: High voltage management strategies include: 10kV line current reverse management solution: In this type of line, the 10kV line voltage exceeds the limit. If this is managed on the low-voltage line, equipment would need to be installed on each substation, which is very troublesome and requires a large investment. Instead, a 10kV flexible voltage management device and an intelligent flexible power quality management device can be installed on the 10kV line to manage the voltage. This solution can reduce the number of devices, lower equipment investment, and shorten the installation workload.

8. Due to the intermittent nature of photovoltaic power generation and the randomness of 10kV line current direction changes, to ensure line voltage compliance, the equipment must promptly adjust the control method of the 10kV flexible voltage management device when the line current direction changes, while also taking into account low line voltage management. When the current reverses, reactive power may shift from inductive to capacitive, requiring power quality compensation devices to track and compensate for reactive power of varying properties.

9. By collecting 10kV line drawings, load distribution, photovoltaic reverse transmission capacity, and line load when there is no photovoltaic power generation, a simulation model is built. Through simulation calculations, the line compensation plan is determined, taking into account both high voltage and low voltage management.

10. The method for over / under voltage compensation judgment and flexible management of distributed photovoltaic based on instantaneous line current direction according to claim 5, characterized in that: High voltage management strategies include: 10kV line high voltage management plus some low voltage line management solutions; In order to ensure that the 10kV voltage is within the range of ±7%, the voltage of some substations in the 10kV line will be around 10.5kV. When the substation near this point has photovoltaic power transmission, the low-voltage line will also have high voltage; or the substations at other points have large photovoltaic power generation capacity and large reverse current. Even if the substation voltage is low, it will cause high voltage for some users on the line. In this case, the 10kV line is first treated to ensure that the entire 10kV line operates within the qualified voltage range. At the same time, treatment is carried out on the voltage lines that need treatment to ensure that the voltage for all users is qualified.

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