Reactive power coordinated control method of LCC converter, filter and SVG on the AC side

Abstract

The present invention relates to a reactive coordination control method for an AC-side LCC converter, a filter and an SVG, comprising: in a steady-state condition without a fault, the SVG controls the first section reactive power of the converter, the passive filter, the SVG and the AC system to be Qaim1; as the DC power increases or decreases, when the SVG output reaches the limit value and loses the reactive power control capability, the DC converter controls the reactive power of the first section to be Qaim2 by adjusting the cut-off angle of the inverter side of the converter, Qaim2>Qaim1; as the DC power increases or decreases, when the cut-off angle of the DC converter reaches the limit value and loses the reactive power control capability, the filter controls the reactive power of the first section to be between Qmin and Qmax by switching on or off. The present invention combines the reactive output characteristics of the SVG, the filter and the DC converter, controls them together, coordinates them with each other, keeps them unified in both transient and steady states, and reduces various mutations.

Description

Technical Field

[0001] The present invention relates to a reactive coordination control method for an LCC converter (grid commutation converter), a filter and an SVG (static var generator) on the AC side of a DC project, and relates to the technical field of reactive compensation. Background Art

[0002] LCC (grid-commutated direct current) transmission technology is widely used in China. Its strong transmission capacity, good stability, and high reliability make it important for long-distance energy transmission. However, LCC DC projects have some inherent drawbacks in reactive power compensation on the AC side, mainly including:

[0003] 1) When the LCC converter is in operation, it will inevitably consume a large amount of capacitive reactive power, which is roughly half of the active power transmitted by the converter. This inevitably requires the installation of a large amount of reactive power compensation equipment, which limits the operational flexibility of the DC project.

[0004] 2) Traditional LCC converters use passive filters as reactive power compensation devices. These filters have a fixed reactive capacity and cannot be continuously adjusted. They must be switched on and off in groups using circuit breakers. This slow switching speed leads to step-like fluctuations in voltage and reactive power. To limit these fluctuations, the filter capacity must be limited, which in turn results in a large number of filters and a large footprint. Furthermore, due to the very slow switching speed of the circuit breakers, the filters cannot be switched on or off in time when a system fault occurs, making it impossible to support DC system recovery. In the event of a DC system lockout, this inability to switch off the filters can even cause significant overvoltages, threatening the safe operation of the DC system.

[0005] 3) The LCC converter itself can perform a certain amount of reactive power absorption regulation by adjusting the DC inverter-side converter's turn-off angle. However, the LCC converter's inverter-side γ angle adjustment is limited. The minimum value of the γ angle is limited by commutation failures and generally does not exceed 15° in projects. Excessively large γ angles lead to excessive thyristor losses. In the event of an AC system fault, the converter must adjust the trigger and turn-off angles to control the DC system power. Therefore, the converter's reactive power regulation function cannot be used during the fault. Summary of the Invention

[0006] The present invention aims to address at least one of the technical problems existing in the prior art. To address this issue, the present invention provides a method for coordinated reactive power control of an AC-side LCC converter, filter, and SVG. This method can comprehensively utilize the capabilities of these three elements to optimize the reactive power compensation capability of the LCC DC project, reduce interference with the AC system, and improve DC project performance.

[0007] To achieve the above-mentioned object, the present invention adopts the following technical solution: a method for coordinated reactive power control of an LCC converter, filter, and SVG on the AC side of a DC project provided by the present invention, comprising:

[0008] Under steady-state conditions without any faults, the reactive power of the first section of the SVG-controlled converter station and AC system is Qaim1, which includes the DC converter, SVG, and passive filter.

[0009] As DC power increases or decreases, when the SVG output reaches its limit and loses its reactive power control capability, the DC converter adjusts the inverter-side shutdown angle (γ angle) to control the reactive power of the first section between the converter station and the AC system to Qaim2. The capacitive reactive power output from the DC converter station to the AC is positive, with Qaim2 > Qaim1. This parameter value is intended to keep the γ angle at a minimum when the SVG has the ability to regulate.

[0010] As the DC power increases or decreases, when the DC converter's turn-off γ angle reaches the limit value and loses its reactive power control capability, the filter controls the first-section reactive power of the converter station and the AC system to be between Qmin and Qmax by switching on or off.

[0011] Furthermore, as the DC power decreases, the conditions satisfied by the various parameters of reactive power control are:

[0012] Qmax-Qacf <Qaim1;

[0013] To control reactive power fluctuations within Qlimit, the following are required:

[0014] Qmax-Qaim1 <Qlimit,Qacf-(Q2-Q1)<Qlimit,Qacf-(Q4-Q3)<Qlimit;

[0015] Where Qacf is the capacitive reactive power of the filter, Qlimit is the reactive power fluctuation limit of the AC system, Q1~Q2 are the reactive power capabilities of the SVG, and Q3~Q4 are the reactive power variation range when the DC converter adjusts the γ angle.

[0016] Furthermore, as the DC power increases, the conditions that the various parameters of reactive power control meet are:

[0017] Qmin+Qacf>Qaim2;

[0018] To control reactive power fluctuations within Qlimit, the following are required:

[0019] Qaim2-Qmin <Qlimit, Qacf-(Q2-Q1)<Qlimit, Qacf-(Q4-Q3)<Qlimit;

[0020] Among them, Qacf is the capacitive reactive power of the filter, Qlimit is the reactive power fluctuation limit of the AC system, Q1~Q2 are the reactive power capacity of the SVG, and Q3~Q4 are the reactive power variation range when the DC converter adjusts the γ angle.

[0021] Furthermore, when the shutdown γ angle of the DC converter remains unchanged, the SVG is used to adjust the reactive power, controlling the reactive power exchanged between the converter station and the first section of the AC system to Qaim1 within the control capability, and controlling the reactive power fluctuation caused by the filter switching to be within Qlimit; when the SVG is exited, the DC converter is used to adjust the reactive power, controlling the reactive power exchanged between the converter station and the first section of the AC system to Qaim2 within the control capability, and controlling the reactive power fluctuation caused by the filter switching to be within Qlimit.

[0022] Furthermore, Qaim1 and Qaim2 are set fixed values ​​or set reactive power variation ranges. When the reactive power of the first section is at Qaim1, the SVG no longer changes its output and maintains the reactive power output of the previous moment. When the reactive power of the first section is at Qaim2, the shutdown angle of the DC converter remains at the value of the previous moment, thereby avoiding continuous operation of the SVG and / or DC converter.

[0023] Furthermore, when a fault occurs, the converter, filter, and SVG enter a transient reactive power coordinated control process, including:

[0024] When the DC system AC side bus voltage is greater than Umax, or the DC system AC side bus voltage is less than Umin, the system is considered to have a fault and enter a transient state, and the SVG enters voltage control mode;

[0025] Set the SVG voltage control value Uaim so that the SVG quickly generates reactive power to help the system recover from the fault and restore the voltage to normal as soon as possible.

[0026] After the fault is restored, the SVG control mode exits the voltage control mode and returns to the reactive power control mode.

[0027] Furthermore, the voltage control value Uaim of the SVG is generally set to (Umax+Umin) / 2.

[0028] Furthermore, after the fault is restored, the SVG switches from the voltage control mode to the reactive power control mode. After the reactive power of the SVG is removed, it should not re-enter the transient operating state.

[0029] Furthermore, when the voltage on the AC side of the DC system is less than Uup and greater than Udown for a duration of T, the transient process is considered to have ended. Here, Uup = Umax - △U, and Udown = Umin + △U. △U is the maximum jump value of the AC voltage caused by the output change of the SVG. If the reactive power regulation capability of the SVG is Q1~Q2, then △Q = Q1-Q2, and the corresponding voltage in the AC system is △U. The time T should be greater than the maximum fault recovery time of the AC system.

[0030] Furthermore, when the SVG enters voltage control mode, the SVG's reactive output is no longer controllable. The filter and DC converter are no longer controlled based on the reactive power of the first section, which includes the SVG's reactive power. Instead, the reactive power of the second section, which only includes the filter and DC converter, is used as the control variable. The control process includes: when the SVG enters voltage control mode, the filter controls the reactive power of the second section to [Qmin - Q2, Qmax - Q1], and the DC converter's turn-off γ angle controls the reactive power of the second section to Qaim2 - Q1. This maintains consistency with steady-state operating conditions and prevents fluctuations when entering and exiting transient operating conditions. Q1 and Q2 represent the reactive power of the SVG.

[0031] The present invention adopts the above technical solution, which has the following characteristics:

[0032] 1. This invention utilizes the continuous reactive power regulation capability of SVG and converters to reduce reactive power and voltage fluctuations caused by filter switching, and reduces reactive power exchange between the DC and AC systems. This also increases the capacity of filters installed in the LCC DC system, reduces the number of filters installed, and reduces the land area of ​​the DC project.

[0033] 2. This invention utilizes the SVG's ability to quickly generate reactive power in a fault state. When a fault occurs on the AC side of a DC system, the SVG quickly generates reactive power to control voltage, helping to recover from the fault and improving the DC system's fault recovery speed.

[0034] 3. The present invention combines the reactive output characteristics of SVG, filter and DC converter to form a complete reactive power compensation system for converter station. The system is jointly controlled and coordinated with each other to maintain uniformity in both transient and steady states, thus reducing various sudden changes.

[0035] In summary, the present invention incorporates an SVG (Insulated Gate Bipolar Transistor) device (IGBT) capable of high-speed and continuous reactive power regulation into a hybrid commutation field in LCC DC projects. Leveraging the high-speed control capabilities of the SVG, this technology overcomes the limitations, inflexibility, and discontinuity of reactive power compensation equipment in LCC converters, reduces disturbances to the AC system, and supports reactive power in transient conditions. This approach can be widely applied in grid-commutated DC transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. Throughout the drawings, the same reference numerals are used to denote the same components. In the drawings:

[0037] Figure 1 Schematic diagram of the structure of a hybrid grid-commutated converter according to an embodiment of the present invention;

[0038] Figure 2 Schematic diagram of a hybrid grid-commutated converter in a "2-3" operating condition according to an embodiment of the present invention;

[0039] Figure 3 Schematic diagram of a hybrid grid-commutated converter in a "3-4" operating condition according to an embodiment of the present invention;

[0040] Figure 4 A schematic diagram of controlling power rise and fall using different trigger angles according to an embodiment of the present invention;

[0041] Figure 5 FIG. 1 is a schematic diagram of controlling power rise and fall by using different DC voltages according to an embodiment of the present invention.

[0042] Figure 6 This is a schematic diagram of the reactive coordinated control principle of an embodiment of the present invention. DETAILED DESCRIPTION

[0043] It should be understood that the terms used herein are for the purpose of describing specific example embodiments only and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "one", "an" and "said" as used herein may also be meant to include plural forms. The terms "comprise", "include", "contain" and "have" are inclusive and therefore specify the presence of stated features, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, steps, operations, elements, parts, and / or combinations thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring them to be performed in the specific order described or illustrated, unless the order of execution is clearly indicated. It should also be understood that additional or alternative steps may be used.

[0044] Although the terms first, second, third, etc. can be used in the text to describe multiple elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms can only be used to distinguish an element, component, region, layer or section from another region, layer or section. Unless the context clearly indicates otherwise, terms such as "first", "second" and other numerical terms do not imply order or sequence when used in the text. Therefore, the first element, component, region, layer or section discussed below can be referred to as the second element, component, region, layer or section without departing from the teaching of the example embodiments.

[0045] For ease of description, spatially relative terms may be used herein to describe the relationship of one element or feature relative to another element or feature as shown in the figures, such as "inside," "outside," "inner side," "outer side," "lower," "upper," etc. Such spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures.

[0046] To optimize the inherent defects of reactive power compensation in LCC DC projects, and by deeply integrating the ability of the switchable device converter to quickly output reactive power, the present invention proposes a method for coordinated steady-state reactive power control of the DC project's AC-side converter, filter, and SVG. This method includes utilizing the continuous reactive power regulation capability of the SVG or DC converter to reduce reactive power fluctuations caused by filter switching and reactive power exchange between the converter station and the AC system. Specifically, by simultaneously controlling the reactive power exchanged between the SVG, filter, and DC converter and the AC system, different control targets are set based on the different control capabilities and control speeds of the three reactive power compensation devices, and the control parameters are coordinated with each other, so that the reactive power fluctuation range exchanged between the converter station and the AC system is minimized and the capacity of the passive filter is maximized. In addition, the present invention also proposes a transient reactive power control method for the AC-side converter, filter, and SVG of a DC project. The method includes: utilizing the rapid reactive power output of the SVG to switch from reactive power control to voltage control in the event of an AC system fault; setting appropriate control thresholds to enable the SVG to output power as quickly as possible in the event of a fault; and resuming steady-state control from the transient state as quickly as possible after the fault is recovered. Transient thresholds are set for the DC converter and filter to ensure that their control remains consistent with steady-state control when the SVG's reactive power output is uncontrollable.

[0047] Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0048] like Figure 1 As shown, the method for coordinated reactive power control of the converter, filter, and SVG on the AC side of a DC project provided in this embodiment is based on the reactive power compensation on the AC side of a traditional LCC converter station. In addition to the DC converter and filter, an SVG is added. The three jointly control the reactive power exchanged between the converter station and the AC system, i.e., the reactive power of section 1.

[0049] In this embodiment, the SVG includes an IGBT converter and a reactor. By fully controlling the continuous voltage generated by the IGBT converter, it can quickly generate any reactive power within a range, such as Q1-Q2. Generally, the inductive and capacitive reactive power capacities of an SVG are the same. However, in this embodiment, because reactive power may need to be reserved for fault output during transient processes, the steady-state reactive power output of the SVG can be set to a limit range of Q1-Q2. The SVG generates and adjusts reactive power extremely quickly, with a time constant at the μs level. Since the SVG's reactive power generation capability is continuous, the SVG can control the reactive power exchanged between the converter station and the AC system to a fixed value within its capability. The reactive power characteristics generated by the SVG are as follows: Figure 2 As shown in Figure 1, when an AC system fails, the SVG can quickly provide reactive power to support the system.

[0050] In this embodiment, the filter is a reactive power compensation device in a traditional LCC converter station, mainly composed of high-voltage capacitors. Its reactive power Qacf is also mainly provided by high-voltage capacitors. It is fixed under a fixed steady-state voltage and is switched on and off by a circuit breaker. Therefore, the time constant of the filter to adjust the reactive power is in the order of hundreds of milliseconds. The converter station is generally equipped with multiple sets of filters. By continuously switching on and off, the reactive power can be increased in a step-by-step manner, and the reactive power exchanged between the converter station and the AC system can be controlled within a range of Qmin~Qmax, where Qmax-Qmin must be greater than Qacf to avoid cutting off after being put into operation. The reactive power characteristics generated by the filter are as follows: Figure 3 As shown in the figure, since the filter switching time is too long, the filter cannot be switched on in time when a system fault occurs, so the filter has no ability to support transient conditions.

[0051] In this embodiment, the reactive power generated by the rectifier and inverter sides of the DC converter is shown in equations (1) and (2). It can be seen that the reactive power of the converter is affected by the trigger angle α on the rectifier side and the turn-off angle γ on the inverter side. Since the trigger angle α is used to control the DC current, only the turn-off angle γ can be used to control the reactive power, and the change of the γ angle on the inverter side will synchronously affect the α angle on the rectifier side. Therefore, the γ angle can be used to adjust the reactive power absorbed by the rectifier and inverter side converters without affecting the DC power. The smaller the γ angle, the less inductive reactive power absorbed by the DC system, which is equivalent to the greater the capacitive reactive power emitted. The adjustment time constant of the γ angle is in the millisecond level. However, the minimum value of the converter's γ angle is limited by commutation failure. Generally, DC projects will not exceed 15°. The maximum value is limited by the loss of the thyristor converter valve. In order to reduce the loss, this value cannot be too large for a long time, especially under high power. When the AC system fails, the DC system needs to first ensure the transmission power, so it loses the reactive power regulation capability.

[0052] Furthermore, the reactive power consumed by the rectifier (six-pulse converter) can be calculated using the following formula (for the inverter, α should be replaced by γ):

[0053] (1)

[0054] (2)

[0055] (3)

[0056] in, represents the reactive power consumed by the converter, is the DC current, The ideal no-load DC voltage for each 6-pulsation valve group, is the valve side AC line voltage, is the relative inductor voltage drop, is the commutation angle, is the trigger angle.

[0057] Example 1: To reduce reactive power fluctuations caused by filter switching, assuming the goal is to limit reactive power fluctuations in the AC system to within Qlimit, the reactive power capacity of the SVG is Q1-Q2, and the reactive power change when the DC converter adjusts the γ angle is Q3-Q4. Taking into account the adjustment ranges and time constants of the various reactive power regulation devices mentioned above, this example provides a method for coordinated steady-state reactive power control of the AC-side converter, filter, and SVG in a DC project, including:

[0058] S1. Under steady-state conditions (no fault conditions), the DC converter, filter, and SVG are used to control the reactive power of Section 1 of the converter station and AC system. The SVG with the shortest reactive output time constant and continuous reactive power regulation capability controls the reactive power of Section 1 to a certain intermediate value Qaim1.

[0059] S2. As DC power increases or decreases, the SVG's output reaches its limit and loses its reactive power control capability. The DC converter, which has the second shortest reactive power output time constant and continuous reactive power regulation capability, controls the reactive power of section 1 to a different reactive power value, Qaim2, by adjusting the inverter-side turn-off angle (γ angle), where Qaim2 > Qaim1.

[0060] S3: As DC power increases or decreases, the DC converter's γ angle reaches its limit, and the DC converter loses its reactive power control capability. The filter with the longest reactive power output time constant and discrete reactive power regulation capability then switches on or off to control section 1's reactive power within a reactive power range of Qmin to Qmax.

[0061] It should be noted that the capacitive reactive power flowing from the DC system into the AC system at section 1 is considered positive. Under the steady-state condition without faults and in the condition of power decline, the SVG with the shortest reactive power output time constant and continuous reactive power regulation ability controls the reactive power at section 1 to an intermediate value Qaim1. Since Qaim1 < Qaim2, the γ angle of the DC converter is at its minimum value at this time. As the DC power decreases, the inductive reactive power absorbed by the DC converter also decreases, the capacitive reactive power flowing to section 1 increases, and the output of the SVG gradually reaches the maximum inductive reactive power of its equipment, and finally loses the reactive power control ability due to equipment capacity limitations. After that, the capacitive reactive power at section 1 continues to increase until it reaches the control target value Qaim2 of the γ angle. At this time, the DC converter with the second shortest reactive power output time constant and continuous reactive power regulation ability reduces the capacitive reactive power of the DC converter by increasing the γ angle, so as to control the section reactive power to remain at Qaim2. As the DC power decreases further, the inductive reactive power absorbed by the DC converter is even lower, the capacitive reactive power at section 1 is greater, the γ angle of the DC converter reaches the maximum value, and the DC converter loses the reactive power control ability. The reactive power at section 1 reaches the control limit Qmax of the filter. Then, the filter with the longest reactive power output time constant and discrete reactive power regulation ability reduces the capacitive reactive power at section 1 by being disconnected. At this time, if the reactive power is lower than Qaim1, the SVG withdraws from the maximum inductive reactive power and attempts to control the reactive power at section 1 to Qaim1, and the γ angle of the DC converter shrinks from the maximum value and attempts to control the reactive power at section 1 to Qaim2. Since the control speed of the SVG is much higher than that of the DC converter, the reactive power at section 1 will remain at Qaim1, and at the same time, the γ angle will continue to shrink, "replacing" the inductive reactive power until the reactive power regulation ability of the SVG is exhausted. Finally, the magnitude of the section reactive power depends on the reactive power regulation ability of the SVG and the DC converter. If the section reactive power is higher than Qaim1 after the filter is disconnected, this design is meaningless. Therefore, Qmax - Qacf < Qaim1, and since the reactive power fluctuation needs to be controlled within Qlimit, so Qmax - Qaim1 < Qlimit, Qacf - (Q2 - Q1) < Qlimit, Qacf - (Q4 - Q3) < Qlimit, as specifically shown in Figure 4 shown.

[0062] Further, under the condition of increasing DC power, first, the SVG with the shortest reactive power output time constant and continuous reactive power regulation ability controls the reactive power of section 1 at an intermediate value Qaim1. Since Qaim1 < Qaim2, the γ angle of the DC converter is at the minimum value at this time. As the DC power rises, the inductive reactive power absorbed by the DC converter also increases, the capacitive reactive power flowing to section 1 decreases, and the output of the SVG gradually reaches the maximum capacitive reactive power of its equipment, and finally loses the reactive power control ability due to equipment capacity limitations. After that, the capacitive reactive power of section 1 continues to decrease and reaches the control limit value Qmin of the filter. Then, the filter with the longest reactive power output time constant and discrete reactive power regulation ability increases the capacitive reactive power of section 1 by being put into operation. At this time, if the section reactive power is higher than Qaim2, the SVG withdraws from the maximum capacitive reactive power and attempts to control the reactive power of section 1 to Qaim1, and the γ angle of the DC converter increases from the minimum value and attempts to control the reactive power of section 1 to Qaim2. Since the control speed of the SVG is much higher than that of the DC converter, the reactive power of section 1 will remain at Qaim1 and the γ angle will not change. Finally, the magnitude of the section reactive power depends on the reactive power regulation ability of the SVG. If the section reactive power is lower than Qaim2 after the filter is put into operation, the design is meaningless. Therefore, Qmin + Qacf > Qaim2, and since the reactive power fluctuation needs to be controlled within Qlimit, so Qaim2 - Qmin < Qlimit, Qacf - (Q2 - Q1) < Qlimit, Qacf - (Q4 - Q3) < Qlimit, as specifically shown in Figure 5 shown.

[0063] As can be seen from the above description, keeping Qaim1 < Qaim2 is to make the SVG continuously adjust the reactive power output to make the reactive power of section 1 approach Qaim1 when the SVG has the ability to regulate reactive power, and at the same time the DC converter continuously adjusts the γ angle to make the reactive power of section 1 approach Qaim2. Since the reactive power regulation speed of the SVG is faster, the reactive power of section 1 always remains at Qaim1. In this process, the γ angle continuously decreases, generating capacitive reactive power, while the SVG continuously reduces the capacitive reactive power, which is equivalent to the SVG "replacing" the reactive power of the DC converter until the γ angle reaches the minimum value. The purpose of this design is to keep the γ angle of the DC converter at the minimum when the SVG has the ability to regulate reactive power and reduce the loss of the DC converter. This reactive power coordination control strategy is still applicable when the SVG fails and exits or the γ angle of the DC converter must remain unchanged, and all control parameters remain the same. When the SVG exits, the DC converter adjusts the reactive power and controls the reactive power exchanged with the AC system section within Qaim2 within its control ability, and controls the reactive power fluctuation of the filter switching within Qlimit; when the γ angle of the DC converter remains unchanged, the SVG adjusts the reactive power and controls the reactive power exchanged with the AC system section within Qaim1 within its control ability, and controls the reactive power fluctuation of the filter switching within Qlimit.

[0064] Furthermore, in this control strategy, Qaim1 and Qaim2 can be fixed values ​​or have a smaller reactive power variation range. When the reactive power of section 1 is within this small range, the SVG output is no longer changed, maintaining the reactive power output and the inverter gamma angle at the previous moment. This prevents the SVG and inverter gamma angle from constantly changing.

[0065] In summary, the reactive power coordination control method of the present invention fully combines the high-speed control flexibility of turnable power electronic devices, the controllability of thyristor converters, and the stability of passive devices. It is a reactive power coordination method that properly utilizes the characteristics of different reactive compensation devices and has both flexibility and low losses. It controls the reactive power exchanged between the converter station and the AC system within a smaller range, while reducing the fluctuation of filter switching, increasing the filter capacity, reducing the number of filter groups, and reducing interference with the AC system. It has great application prospects in DC systems connected to weak AC systems, greatly reducing the space occupied by converter stations, and improving the adaptability of DC systems.

[0066] Example 2: Figure 6 As shown, this embodiment also discloses a method for coordinated transient reactive power control of a converter, a filter, and an SVG, which utilizes the capabilities of the SVG to support system recovery when an AC side fault occurs. The specific process includes:

[0067] S1: Use voltage as a sign of entering transient operating conditions. When the AC voltage is greater than Umax or lower than Umin, it is considered that the system has entered a transient operating condition and a fault has occurred in the AC system. At this time, SVG switches to voltage control.

[0068] The fast reactive power output of S2 and SVG helps the system recover from faults, reduces overvoltage, supports low voltage, and restores the voltage to normal as soon as possible.

[0069] S3. After the fault is restored, the SVG should exit the transient state as soon as possible and return to the steady state to allow reactive power compensation to proceed normally.

[0070] In a preferred embodiment of the present invention, to achieve this goal, Umax and Umin are voltage limits that the AC side will not enter during normal operation, and any such limits indicate a fault condition. These threshold voltages have a zero probability of occurrence during normal operation. The SVG voltage control value is set to (Umax + Umin) / 2, ensuring that once the voltage enters the transient range, the SVG output helps the system recover to a steady state.

[0071] In a preferred embodiment of the present invention, to quickly exit the transient state, Uup and Udown are set. When the system voltage returns to within the range between Uup and Udown for a duration of T, the system is considered to have returned to normal, and the SVG switches from voltage control to reactive power control, resuming the steady-state control strategy. The design of Uup and Udown must be considered in conjunction with Umax and Umin. Uup is calculated based on Umax, taking into account the SVG's reactive power regulation capability and a certain margin. Udown is calculated based on Umin, taking into account the SVG's reactive power regulation capability and a certain margin. The setting of T must take into account the AC system's maximum fault recovery time.

[0072] Specifically, if the SVG's reactive power regulation capability is Q1-Q2, and the voltage corresponding to △U is △Q (Q1-Q2), then Uup = Umax-△U, Udown = Umin+△U, and T is the maximum fault recovery time specified by the AC system dispatcher. This ensures that the AC system has indeed returned to a steady state after a fault and recovers quickly. It also ensures that after returning to steady-state operation, the SVG switches from voltage control to reactive power control. After the SVG's reactive power capability is removed, the AC voltage jumps by a maximum of △U and does not re-enter transient conditions.

[0073] In a preferred embodiment of the present invention, when the SVG enters a transient state, the reactive output of the SVG is no longer controllable. Therefore, the control of the filter and DC converter is no longer based on the reactive power of Section 1, which includes the reactive power of the SVG, but instead is based on the reactive power of Section 2, which includes only the filter and DC converter.

[0074] In the steady-state operating condition of Example 1, when the filter is put into operation, the reactive power of Section 1 is Qmin, and the capacitive reactive power of SVG has reached the maximum value Q2. Therefore, the reactive power of Section 2 is Qmin-Q2. When the filter is cut off, the reactive power of Section 1 is Qmax, the capacitive reactive power of SVG has reached the minimum value Q1, and the reactive power of Section 2 is Qmax-Q1. Therefore, the filter should control the reactive power of Section 2 to [Qmin-Q2, Qmax-Q1].

[0075] In Example 1, when the reactive power of the DC converter controlling Section 1 is Qaim2, since Qaim2 > Qaim1, the SVG's reactive output reaches the capacitive minimum value, Q1. Therefore, the reactive power of Section 2 is controlled at Qaim2 - Q1. Therefore, when the SVG enters transient operation, the reactive power of the filter controlling Section 2 is [Qmin - Q2, Qmax - Q1], while the reactive power of the DC converter controlling Section 2 is Qaim2 - Q1. This maintains consistency with steady-state operation, preventing fluctuations when entering and exiting transient conditions.

[0076] The following describes a specific case of the AC side of the back-to-back ±100kV DC project in the Fujian-Guangdong interconnection as an example to illustrate the reactive coordinated control method of the converter, filter and SVG on the AC side of the DC project of the present invention.

[0077] The Fujian-Guangdong interconnection project has two ±100kV, 1000MW converter units with a total power of 2000MW. Nine filters, each 140MVA, are deployed on the Guangdong side. They are equipped with SVGs, and their reactive power regulation capabilities are Q1-Q2, which are [-120 MVA, 120 MVA]. The rated γ angle of the DC converter is 17°, with a maximum γ angle of 53° at low power and 25° at high power. According to Equation (1), the reactive power regulation capability of the DC converter increases with increasing DC power. At low power, the reactive power regulation range of the DC converter, Q3-Q4, is [67-274 MVar], and at high power, the reactive power regulation range, Q3-Q4, is [1098-1398 MVar]. The fluctuation Qlimit of the AC system filter input is limited to less than 70MVA. The control target Qaim1 of section 1 of the SVG is designed to be 0MVar, the control target Qaim2 of section 1 of the DC converter is designed to be [+13~+15MVar], and the reactive power range Qmin~Qmax of the filter control section 1 is designed to be [-54 MVA, +54 MVar].

[0078] Assuming the converter maintains a constant γ angle and the SVG controls reactive power, before the filter is applied, the reactive power of Section 1 reaches -54 MVA, while the SVG's reactive power is +120 MVA. After the filter is applied, the reactive power of Section 1 reaches -54 + 140 = 86 MVA. The SVG's reactive power then decreases by 86 MVA, to 34 MVA, controlling the reactive power of Section 1 to 0 MVA. The reactive power change in Section 1 is 54 MVA, which is within the Qlimit limit. Before the filter is removed, the reactive power of Section 1 reaches +54 MVA, while the SVG's reactive power is -120 MVA. After the filter is removed, the reactive power of Section 1 reaches +54 - 140 = -86 MVA. The SVG's reactive power increases by 86 MVA, to -34 MVA, controlling the reactive power of Section 1 to 0 MVA, and the reactive power change in Section 1 is now 54 MVA.

[0079] Assume that the SVG fails and stops operating, with the converter maintaining the γ angle unchanged. Under low DC power, before the filter is activated, the reactive power of Section 2 reaches -54 MVA. At this time, the γ angle is at its minimum value of 17°, absorbing 67 MVA of reactive power. After the filter is activated, the reactive power of Section 2 reaches -54 + 140 = 86 MVA. At this time, the γ angle increases to 33°, and the DC converter absorbs an additional 71 MVA of reactive power, controlling the reactive power of Section 2 to 15 MVA. The reactive power of Section 2 now changes to 69 MVA, which is within the Qlimit limit. Before the filter is removed, the reactive power of Section 2 reaches +54MVA. At this time, the γ angle is at its maximum value of 53°, absorbing 274MVA of reactive power. After the filter is removed, the reactive power of Section 2 reaches +54-140MVA=-86MVA. At this time, the γ angle becomes smaller than 40°, and the DC converter absorbs 99MVA less reactive power, controlling the reactive power of Section 2 at 13MVA. At this time, the reactive power of Section 2 changes to 41MVA.

[0080] Since the AC side voltage range during normal operation of a DC project is 500kV to 550kV, the DC system's protection will activate if this voltage range is exceeded. However, the AC system can continue to operate even if the voltage reaches any value within this range, even with a small probability. Therefore, Umax is set to 550kV and Umin to 500kV. During transient conditions, the control target is an AC system voltage of 525kV. The SVG's reactive power range is [-120MVA to 120MVA]. For this AC system, 240MVA can cause a maximum voltage fluctuation of 9kV. Therefore, Uup is set to 540kV and Udown to 510kV. After the SVG enters the transient process, the reactive power of the filter control section 2 is [-54-120, 54-(-120)], that is, [-174MVA, 174MVA], and the reactive power of the DC converter control section 2 is Qaim2-Q1=[133MVA~135MVA].

[0081] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In the description of this specification, the reference terms "a preferred embodiment", "further", "specifically", "in the present embodiment", etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of this specification. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

[0082] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (apparatus), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0083] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0084] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for coordinated control of reactive power of LCC converter, filter and SVG on the AC side, characterized in that include: Under steady-state conditions without any fault, the reactive power of the first section of the SVG-controlled converter station and the AC system is Qaim1, and the first section includes the DC converter, SVG and passive filter; As the DC power increases or decreases, when the SVG output reaches the limit value and loses the reactive power control capability, the DC converter controls the reactive power of the first section of the converter station and the AC system to Qaim2 by adjusting the shutdown γ angle of the inverter side of the converter, and the capacitive reactive power output from the DC converter station to the AC is positive, where Qaim2>Qaim1; As the DC power increases or decreases, when the DC converter's turn-off γ angle reaches the limit value and loses its reactive power control capability, the filter controls the first section reactive power of the converter station and the AC system to be between Qmin and Qmax by switching on or off, where: As the DC power decreases, the conditions satisfied by the various parameters of reactive power control are: Qmax-Qacf <Qaim1; Since the reactive power fluctuation needs to be controlled within Qlimit, therefore: Qmax-Qaim1 <Qlimit,Qacf-(Q2-Q1)< Qlimit,Qacf-(Q4-Q3)<Qlimit; In the formula, Qacf is the capacitive reactive power of the filter, Qlimit is the reactive power fluctuation limit value of the AC system, Q1~Q2 is the reactive power capacity of the SVG, and Q3~Q4 is the reactive power variation range when the DC converter adjusts the γ angle; As the DC power increases, the conditions that various parameters of reactive power control meet are: Qmin+Qacf>Qaim2; Since the reactive power fluctuation needs to be controlled within Qlimit, therefore: Qaim2-Qmin <Qlimit, Qacf-(Q2-Q1)< Qlimit, Qacf-(Q4-Q3)<Qlimit; Among them, Qacf is the capacitive reactive power of the filter, Qlimit is the reactive power fluctuation limit value of the AC system, Q1~Q2 is the reactive power capacity of the SVG, and Q3~Q4 is the reactive power variation range when the DC converter adjusts the turn-off γ angle.

2. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 1, characterized in that: When the shutdown γ angle of the DC converter remains unchanged, the SVG is used to adjust the reactive power, and the reactive power exchanged between the converter station and the first section of the AC system is controlled to be within the control capability at Qaim1, and the reactive power fluctuation of the filter switching is controlled within Qlimit; when the SVG exits, the DC converter is used to adjust the reactive power, and the reactive power exchanged between the converter station and the first section of the AC system is controlled to be within the control capability at Qaim2, and the reactive power fluctuation of the filter switching is controlled within Qlimit.

3. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 1, characterized in that: Qaim1 and Qaim2 are set fixed values ​​or set reactive power variation ranges. When the reactive power of the first section is at Qaim1, the SVG no longer changes its output and maintains the reactive power output of the previous moment. When the reactive power of the first section is at Qaim2, the shutdown γ angle of the DC converter maintains the value of the previous moment to avoid continuous operation of the SVG and / or the DC converter.

4. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 1, characterized in that: When a fault occurs, the converter, filter and SVG enter the transient reactive power coordinated control process, including: When the bus voltage on the AC side of the DC system is greater than Umax, or the bus voltage on the AC side of the DC system is less than Umin, it is considered that the system has failed and entered a transient state, and the SVG enters the voltage control mode; Set the voltage control value Uaim of the SVG so that the SVG can quickly generate reactive power to help the system recover from the fault and restore the voltage to normal as soon as possible. After the fault is restored, the SVG control mode exits the voltage control mode and returns to the reactive power control mode.

5. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 4, characterized in that: The voltage control value Uaim of SVG is set to (Umax+Umin) / 2.

6. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 5, characterized in that: After the fault is restored, the SVG enters the reactive power control mode from the voltage control mode. After the reactive power of the SVG is removed, it should not re-enter the transient operating state.

7. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 6, characterized in that: When the voltage on the AC side of the DC system is less than Uup and greater than Udown and lasts for T, the transient process is considered to have ended, where Uup=Umax-△U, Udown=Umin+△U, and △U is the maximum jump value of the AC voltage caused by the change in SVG output.

8. The method for coordinated control of reactive power of AC-side LCC converter, filter and SVG according to claim 4, characterized in that: When SVG enters the voltage control mode, the reactive output of SVG is no longer controllable. The control of the filter and the DC converter can no longer use the reactive power of the first section including the reactive power of SVG as the control quantity, but use the reactive power of the second section including only the filter and the DC converter as the control quantity. The control process includes: when SVG enters the voltage control mode, the filter controls the reactive power of the second section to be [Qmin-Q2, Qmax-Q1], and the DC converter turns off the γ angle to control the reactive power of the second section to be Qaim2-Q1. At this time, it can be consistent with the steady-state operating condition and will not cause fluctuations when entering and exiting the transient operating condition. Among them, Q1~Q2 are the reactive capacity of SVG.

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