Black start method and system based on bipolar LCC-HVDC

Abstract

The invention relates to the technical field of power system black start, and discloses a black start method and system based on a bipolar LCC-HVDC, and the method comprises the steps: entering a DC system black start stage, starting a bipolar LCC-HVDC system through an ice melting simulation mode, and the bipolar LCC-HVDC system comprises two heat exchanger groups at a sending end and two heat exchanger groups at a receiving end; and entering a load recovery stage, improving the output power of the direct current system, accessing a load, and transitioning to a normal operation state of two-stage forward transmission to complete black start of the receiving end power grid. Based on a bipolar reverse transmission mode and a special control mode of a converter, the bipolar LCC-HVDC system has voltage source characteristics at the initial stage of black start, so that the bipolar LCC-HVDC system can provide voltage and frequency support for a passive power grid at a receiving end; a black start strategy of the bipolar LCC-HVDC system based on AC / DC system characteristics when a receiving end is in a power failure state; the invention discloses a strategy for supporting load recovery of a bipolar LCC-HVDC system based on alternating current and direct current system characteristics.

Description

Technical Field

[0001] The present invention relates to the technical field of black start of electric power system, and in particular to a black start method and system based on bipolar LCC-HVDC. Background Art

[0002] In the context of low-carbon energy transformation, my country's power system presents the characteristics of high proportion of new energy and high power electronics. At the same time, the development of ultra-high voltage AC / DC hybrid and the rapid advancement of the power market have brought about major changes in the scale, structure and operation mode of the system. In this "new type" power system, the uncertainty of new energy output, the coupling effect of AC / DC, the complexity of power electronic equipment control, and the complexity and variability of operation mode bring great challenges to the system operation and control, making the system have the potential risk of major power outages. Therefore, black start recovery research is a major demand for the security defense of the new type of power system, and it has important practical significance for ensuring energy security.

[0003] The key to black start is the start of the power source. Traditional black start power sources mainly include gas units and hydropower units. This type of starting power source has typical advantages such as large installed capacity, fast start speed, and high start smoothness. In the new power system environment, these traditional units will not all be shut down. Most of them will be converted into peak-shaving and frequency-regulating units to improve the regulation and control capabilities of the new power grid and transform them into black start recovery power sources. At the same time, in the case of a system collapse and system-wide power outage due to various factors in the new power system, other forms of substations or power sources may also be used as black start power sources, including: fast switching back (FCB) units, high-voltage direct current transmission (HVDC) converter stations, new energy power plants, and temporary emergency power sources. These power sources themselves have self-starting characteristics, or can be used as black start power sources under certain conditions, thereby accelerating the overall recovery process of the power grid.

[0004] A power outage at the receiving end of HVDC will not affect the operation of the sending end grid. Therefore, during the grid restoration process, if the sending end grid is normal, the HVDC converter station can be used as the power source for the receiving end grid under certain conditions. HVDC converter stations are mainly divided into two types: thyristor (semi-controlled thyristor) converter station and IGBT (fully controlled thyristor) converter station. The DC transmission system composed of IGBT converter stations is also called flexible DC transmission (VSC-HVDC). It can realize independent control of active power and reactive power transmission, thereby providing voltage and frequency support for the receiving end grid at the same time. Research has proved that it has strong black start capability.

[0005] Currently, it is difficult to use thyristor converter stations as black start power sources. The phase-commutated converter type high-voltage direct current transmission system (LCC-HVDC) with thyristor converter stations as the core uses semi-controlled devices, which has a large reactive power demand and is difficult to control. It exhibits current source characteristics in the receiving power grid and is difficult to provide voltage and frequency support. Studies have shown that LCC-HVDC can participate in and accelerate the recovery process of weak AC systems, but it is difficult to achieve a passive network black start with it as the only power source. Summary of the invention

[0006] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0007] In view of the above existing problems, the present invention is proposed. Therefore, the present invention provides a black start method based on bipolar LCC-HVDC to solve the problem.

[0008] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0009] In a first aspect, the present invention provides a black start method based on a bipolar LCC-HVDC, comprising: entering a DC system black start phase, wherein the DC system black start phase comprises: starting a bipolar LCC-HVDC system in an ice melting simulation mode, wherein the bipolar LCC-HVDC system comprises two heat exchanger groups at a sending end and two heat exchanger groups at a receiving end, wherein no power is output and no load is connected in the DC system black start phase;

[0010] Entering the load recovery stage, the load recovery stage includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operation state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

[0011] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, the bipolar LCC-HVDC system comprises:

[0012] The bipolar LCC-HVDC system comprises a first converter, a second converter, a third converter and a fourth converter, each converter group comprising a twelve-pulse converter and a matching converter transformer;

[0013] The first converter and the second converter are connected via a DC line L1, and the third converter and the fourth converter are connected via a DC line L2;

[0014] The first converter and the third converter are both located at the sending end, and the AC busbar at the sending end is connected to the external power grid;

[0015] The second converter and the fourth converter are both located at the receiving end, and the AC busbar at the receiving end is connected to the power outage system, the reactive power compensation device and the load.

[0016] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, the DC system black start stage includes:

[0017] In the initial state, the AC filter and capacitor of the receiving end AC bus are connected to the grid, the receiving end AC bus is in a no-load state, the first converter is started as a rectifier, the third converter is started as an inverter, the DC line L1 operates in an open circuit voltage mode, and a DC voltage U d1 ;

[0018] When the DC voltage U d1 Reach the starting reference voltage value U dlulk When , the second converter is started as an inverter, and the first converter is switched to constant current control;

[0019] When the AC voltage of the AC busbar at the receiving end and the DC current of the DC line L1 are respectively greater than the voltage setting value and the current setting value of the fourth converter unlocking, the fourth converter is started as a rectifier, the DC line L2 is turned on and the current is drawn back from the receiving end to the sending end;

[0020] When the DC current of the DC line L1 is equal to the DC current of the DC line L2, the positive current and the negative reverse current are equal, reaching the simulated ice melting operation state, and completing the startup of the bipolar LCC-HVDC system.

[0021] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, the load recovery stage includes:

[0022] The receiving end AC busbar is gradually loaded until the negative pole reverse current is reduced to zero, and the DC system has voltage source characteristics;

[0023] The DC system enters a bipolar forward power supply mode, the receiving-end AC busbar is put into reactive compensation equipment and load, and transitions to a normal operating state of bipolar forward transmission, completing a black start of the receiving-end power grid.

[0024] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, the stepwise loading of the receiving-end AC bus comprises:

[0025] The receiving end AC busbar is gradually loaded, and the total active load input is less than the positive pole transmission power, and the inverter station arc extinction angle meets the following constraints, which can be expressed as:

[0026]

[0027] Among them, R load The equivalent resistance representing the total active load input, U d1 represents the DC voltage of DC line L1, γ min represents the critical shutdown angle, β represents the inverter station advance trigger angle, k represents the converter transformer ratio, ωL C Indicates the commutation reactance of the converter;

[0028] Assuming that the instantaneous DC voltage remains unchanged, the critical commutation voltage U is obtained. acmin , and the minimum AC voltage is greater than the critical commutation voltage U acmin , respectively expressed as:

[0029]

[0030]

[0031] Among them, R C represents the commutation impedance, I d represents the DC current, k represents the transformer ratio, U' L It represents the average AC voltage of the receiving end AC busbar at the moment before the load is put into operation, U L It represents the average value of the AC voltage of the receiving-end AC bus at the moment when the load is put into operation. R0 represents the total equivalent resistance of all loads and reactive compensation devices connected to the AC bus at the receiving end. X0 represents the total equivalent reactance of all loads and reactive compensation devices connected to the AC bus at the receiving end. R represents the equivalent resistance of the single active load input, and X represents the equivalent reactance of the single active load input.

[0032] When the total active load power input by the receiving-end AC bus is equal to the positive pole transmission power, the negative pole reverse transmission power will drop to zero, the DC line L2 will be locked, and the DC system will be switched from the simulated ice melting mode to the unipolar power transmission mode.

[0033] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, wherein: the DC system enters the bipolar forward power supply mode including:

[0034] The third converter is operated as a rectifier and a constant current control is adopted, the fourth converter is operated as an inverter and a constant trigger frequency control is adopted, and the current of the DC line L2 is transmitted in the forward direction;

[0035] By increasing the current setting value of the third converter, the DC current of the DC line L2 increases, and the AC voltage U ac Carry out detection to determine the input of load so that the AC voltage U ac Maintain within the voltage reference value of the AC bus at the receiving end;

[0036] According to the active power balance and the voltage reference value of the AC bus at the receiving end, the required total active load input amount and the single active load input amount are obtained;

[0037] In order to meet the constraint condition of the arc extinction angle of the inverter station, a second constraint condition is set, which is expressed as:

[0038]

[0039] Among them, Q C and Q L Respectively represent the reactive power of the reactive compensation device and the reactive power of the load, U d2 I dL1 tanθ、U d4 I dL2 tanθ represents the reactive power consumed by the second converter and the fourth converter, θ min Indicates the minimum power factor angle;

[0040] The reactive power of the reactive power compensation device should meet the following requirements:

[0041] Q C >2(U d2 I dL1 +U d4 I dL2 )tanθ min +Q L

[0042] When the forward power transmitted by the two poles of the DC system is equal, the black start of the receiving-end power grid is completed and the normal power transmission state to the receiving-end AC bus is entered.

[0043] As a preferred solution of the black start method based on bipolar LCC-HVDC of the present invention, wherein: the DC voltage U d1 The relationship between the trigger angle is expressed as:

[0044]

[0045] Wherein, E0 represents the effective value of the AC voltage at the sending end, R and C represent the equivalent resistance and equivalent capacitance respectively when the DC line L1 is open, C0 represents the buffer capacitance of the converter valve, and α represents the trigger angle of the converter.

[0046] In a second aspect, the present invention provides a black start system based on a bipolar LCC-HVDC, comprising:

[0047] A black start module is used to enter a DC system black start phase, wherein the DC system black start phase includes: starting a bipolar LCC-HVDC system in an ice melting simulation mode, wherein the bipolar LCC-HVDC system includes two heat exchanger groups at a sending end and two heat exchanger groups at a receiving end, wherein there is no power output and no load connection in the DC system black start phase;

[0048] The recovery module is used to enter the load recovery phase, which includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operation state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

[0049] In a third aspect, the present invention provides an electronic device, comprising:

[0050] Memory and processor;

[0051] The memory is used to store computer executable instructions, and the processor is used to execute the computer executable instructions. When the computer executable instructions are executed by the processor, the steps of the black start method based on bipolar LCC-HVDC are implemented.

[0052] In a fourth aspect, the present invention provides a computer-readable storage medium storing computer-executable instructions, which, when executed by a processor, implement the steps of the black start method based on bipolar LCC-HVDC.

[0053] Compared with the prior art, the present invention has the following beneficial effects: based on the bipolar reverse transmission mode and the special control mode of the converter, the present invention enables the bipolar LCC-HVDC system to have voltage source characteristics at the initial stage of black start, so that it can provide voltage and frequency support for the passive power grid at the receiving end; based on the characteristics of the AC and DC system, the black start strategy of the bipolar LCC-HVDC system when the receiving end is in a power outage state; based on the characteristics of the AC and DC system, the strategy of supporting load recovery of the bipolar LCC-HVDC system; including the effectiveness of constant impedance load, constant power load and motor load recovery. The present invention can maintain consistency in theoretical analysis and simulation experiments, and correspondingly proves the effectiveness and feasibility of the method. The comprehensive consideration of factors is relatively complete and the engineering implementation is convenient, and correspondingly has good engineering promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0055] Figure 1 It is a schematic diagram of the overall process of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0056] Figure 2 A schematic diagram of a bipolar LCC-HVDC system of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0057] Figure 3 A schematic diagram of power transmission of a DC system during a black start process of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0058] Figure 4 This is a schematic diagram of the process of the DC system black start phase in step S101 of the black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0059] Figure 5 A schematic diagram of a six-pulse converter power outage state of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0060] Figure 6 A schematic diagram of the initial power-on state of a six-pulse converter according to a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0061] Figure 7 A schematic diagram of initial commutation of a six-pulse converter according to a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0062] Figure 8 A schematic diagram of the initial commutation end state of a six-pulse converter according to a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0063] Fig. 9 A schematic diagram of constant AC voltage control of a converter 4 according to a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0064] Fig.10 It is a schematic diagram of the overall process of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0065] Fig.11 This is a schematic diagram of the load recovery phase process in step S102 of the black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0066] Fig.12 A schematic diagram of simulation results of a DC system black start phase of a black start method based on a bipolar LCC-HVDC according to an embodiment of the present invention;

[0067] Fig.13 It is a schematic diagram of simulation results of constant impedance load recovery in the reverse power reduction sub-phase of the load recovery phase of the black start method based on bipolar LCC-HVDC according to an embodiment of the present invention;

[0068] Fig.14 It is a schematic diagram of simulation results of constant impedance, constant power and motor model composite load recovery in two stages of black start of a black start method based on bipolar LCC-HVDC according to an embodiment of the present invention. DETAILED DESCRIPTION

[0069] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0070] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0071] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0072] The present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0073] At the same time, in the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0074] In the present invention, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0075] Example 1

[0076] Reference Figure 1-Figure 10 , is an embodiment of the present invention, and provides a black start method based on bipolar LCC-HVDC, such as Figure 1 As shown, including:

[0077] S101, entering the DC system black start stage, the DC system black start stage includes: starting the bipolar LCC-HVDC system through the simulated ice melting mode, the bipolar LCC-HVDC system includes two heat exchanger groups at the sending end and two heat exchanger groups at the receiving end, wherein there is no power output and load connection in the DC system black start stage;

[0078] S102, entering the load recovery stage, which includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operating state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

[0079] Preferably, the bipolar LCC-HVDC system comprises:

[0080] The bipolar LCC-HVDC system includes a first converter, a second converter, a third converter and a fourth converter, and each converter group includes a twelve-pulse converter and a matching converter transformer;

[0081] The first converter and the second converter are connected via a DC line L1, and the third converter and the fourth converter are connected via a DC line L2;

[0082] The first converter and the third converter are both located at the sending end, and the AC busbar at the sending end is connected to the external power grid;

[0083] The second converter and the fourth converter are both located at the receiving end, and the AC busbar at the receiving end is connected to the power outage system, the reactive power compensation device and the load.

[0084] Specifically, in this embodiment, a bipolar LCC-HVDC system is used as a black start power supply, and its structure is as follows: Figure 2 As shown, the bipolar LCC-HVDC system includes four converter groups, namely the first converter, the second converter, the third converter and the fourth converter, each converter group includes a twelve-pulse converter and a matching converter transformer, the first converter and the second converter are connected through a DC line L1, the third converter and the fourth converter are connected through a DC line L2, the sending-end AC bus S1 side is a normally operating external power grid, the first converter and the third converter are located on this side, the receiving-end AC bus S2 side is connected to the power outage system, that is, it is in a passive power outage state, the second converter and the fourth converter are located on this side, and the receiving-end AC bus is connected to sufficient reactive power compensation devices and multiple loads, and the DC line L1 and the DC line L2 are bipolar DC lines.

[0085] In a preferred embodiment, step S101, the DC system black start phase includes:

[0086] Step 1: In the initial state, the AC filter and capacitor of the receiving end AC bus are connected to the grid. The receiving end AC bus is in a no-load state. The first converter is started as a rectifier, the third converter is started as an inverter, and the DC line L1 operates in the open circuit voltage mode to establish a DC voltage U d1 ;

[0087] Step 2: When the DC voltage U on the DC line L1 d1 Reach the starting reference voltage value U dlulk When , the second converter is started as an inverter, and the first converter is switched to constant current control;

[0088] Step 3: When the AC voltage of the AC busbar at the receiving end and the DC current of the DC line L1 are respectively greater than the voltage setting value and the current setting value of the fourth converter unlocking, the fourth converter is started as a rectifier, the DC line L2 is turned on and the current is drawn back from the receiving end to the sending end;

[0089] Step 4: When the DC current of the DC line L1 is equal to the DC current of the DC line L2, the positive current and the negative reverse current are equal, reaching the simulated ice melting operation state (i.e., bipolar reverse transmission mode), and completing the startup of the bipolar LCC-HVDC system.

[0090] Preferably, in step 1, the DC voltage U d1 The relationship between the trigger angle is expressed as:

[0091]

[0092] Wherein, E0 represents the effective value of the AC voltage at the sending end, R and C represent the equivalent resistance and equivalent capacitance respectively when the DC line L1 is open, C0 represents the buffer capacitance of the converter valve, and α represents the trigger angle of the converter.

[0093] Exemplarily, in step 2, the starting condition of the second converter is expressed as:

[0094] U d1 >U dlulk

[0095] Among them, U dlulk As the starting reference voltage value, the value is 0.5pu;

[0096] The second converter adopts fixed trigger pulse control based on reference frequency. The reference frequency signal is obtained from the sending end AC bus and transmitted to the receiving end AC bus through the signal line between the sending and receiving ends, thereby ensuring that the voltage and current frequency of the passive network at the receiving end is a stable 50Hz at the initial stage of black start. After the second converter is unlocked, the capacitive network at the receiving end is used to provide commutation reactive current to achieve successful commutation and obtain AC voltage.

[0097] Figures 5 to 8 The initial commutation process of the six-pulse converter in the passive network is shown, where D1 to D6 are six thyristors, and the thyristor filled in red indicates that the thyristor is in the on state. Figures 5 to 8 The four stages of initial commutation are given in sequence as follows:

[0098] like Figure 5 As shown, the receiving-end network is initially in a passive power-off state, all thyristors of the converter are in the cut-off state, and there is no residual charge in the receiving-end equivalent capacitor;

[0099] like Figure 6 As shown in Figure 2, assuming that thyristors D3 and D4 are turned on for the first time, the conduction path is as follows: Figure 6 As shown in , this process will charge the equivalent capacitance of phase A and phase B;

[0100] like Figure 7 After the initial conduction, the phase change from D3 to D5 will be carried out. At this time, the conduction path and the polarity of each phase capacitor are as follows: Figure 7 As shown in , the B phase charging capacitor will provide the commutation voltage required for the first commutation, and also provide the negative polarity required for the thyristor D3 to turn off;

[0101] like Figure 8 This is the state after the initial commutation is completed.

[0102] After the second converter is turned on, the DC current of the DC line L1 charges the receiving-end AC bus and capacitor through the inverter. Since the equivalent impedance value is very small, the DC voltage of the DC line L1 drops to a very low level. At this time, the constant current setting value of the first converter is increased from the minimum setting value of 0.1pu to increase the current injected into the receiving-end power grid by the DC line L1, further charge the filter and capacitor group on the receiving-end AC bus, and increase the receiving-end bus AC voltage.

[0103] Exemplarily, in step 3, the AC voltage of the receiving-end AC bus and the DC current of the DC line L1 are respectively greater than the voltage setting value and the current setting value of the fourth converter unlocking, which is expressed as:

[0104]

[0105] Among them, U aculk ,I dlulk They represent the voltage setting value and current setting value of the fourth converter unlocking, which are equal to 0.5pu and 0.1pu respectively. ac Indicates the AC voltage of the receiving end AC bus, I d1 Indicates the DC current of the DC line L1.

[0106] The third converter uses constant voltage control as an inverter, and the fourth converter uses a special constant AC voltage control as a rectifier, so that when the AC voltage at the receiving end is lower than the set value, its trigger angle is reduced to reduce the reverse power, and vice versa, the trigger angle is increased to increase the reverse power to ensure the stable AC voltage of the receiving end network. The control block diagram of the fourth converter is as follows: Fig. 9 shown.

[0107] In a preferred embodiment, step S102 is as follows Fig.10 As shown, the load recovery phase includes:

[0108] Step 1: The receiving end AC busbar is gradually loaded until the negative reverse current is reduced to zero, and the DC system has voltage source characteristics;

[0109] Step 2: The DC system enters the bipolar forward power supply mode, the receiving-end AC busbar is put into reactive power compensation equipment and load, and transitions to the normal operating state of two-stage forward transmission, completing the black start of the receiving-end power grid.

[0110] In a preferred embodiment, in step 1 of S102, gradually adding load to the receiving-end AC bus includes:

[0111] The receiving end AC busbar is gradually loaded with loads, and the total active load input is less than the positive pole transmission power, which can be expressed as:

[0112]

[0113] The input of reactive load will lead to a decrease in the arc extinction angle of the inverter station. It is necessary to ensure that no commutation failure occurs during this process. Therefore, the arc extinction angle of the inverter station satisfies the following constraints, which can be expressed as:

[0114]

[0115] Among them, R load The equivalent resistance representing the total active load input, U d1 represents the DC voltage of DC line L1, γ min represents the critical shutdown angle, β represents the inverter station advance trigger angle, k represents the converter transformer ratio, ωL C Represents the converter commutation reactance.

[0116] Since the arc extinction angle cannot be directly controlled, it needs to be indirectly controlled through DC current and AC voltage. Assuming that the instantaneous DC voltage remains unchanged, the critical commutation voltage U acmin , expressed as:

[0117]

[0118] At the moment of load input, due to the hysteresis of the DC system controller, the DC system behaves as a current source, which will cause the instantaneous AC voltage drop at the receiving end. Therefore, the minimum AC voltage should be greater than the critical commutation voltage U acmin , expressed as:

[0119]

[0120] Among them, R C represents the commutation impedance, I d represents the DC current, k represents the transformer ratio, U' L It represents the average AC voltage of the receiving end AC busbar at the moment before the load is put into operation, U L It represents the average value of the AC voltage of the receiving-end AC bus at the moment when the load is put into operation. R0 represents the total equivalent resistance of all loads and reactive compensation devices connected to the AC bus at the receiving end. X0 represents the total equivalent reactance of all loads and reactive compensation devices connected to the AC bus at the receiving end. R represents the equivalent resistance of the single active load input, and X represents the equivalent reactance of the single active load input.

[0121] Among them, R and X must satisfy:

[0122]

[0123] Under the premise that the total active load input meets the condition that the total active load input is less than the positive pole transmission power, and the single active load input meets the R and X conditions, as the receiving-end AC bus gradually puts the load in, the third converter and the fourth converter are respectively controlled according to the fixed DC voltage control and the fixed AC bus voltage control, so that the reverse current of the DC line L2 is automatically reduced, and the AC voltage of the receiving-end AC bus is maintained stable.

[0124] When the total active load power input by the receiving-end AC bus is equal to the positive pole transmission power, the negative pole reverse transmission power will drop to zero, the DC line L2 will be locked, and the DC system will be switched from the simulated ice melting mode to the unipolar power transmission mode.

[0125] In a preferred embodiment, in step 2 of S102, the DC system enters the bipolar forward power supply mode including:

[0126] The third converter is operated as a rectifier and adopts constant current control, the fourth converter is operated as an inverter and adopts constant trigger frequency control, and the current of the DC line L2 is transmitted in the forward direction.

[0127] By increasing the current setting value of the third converter, the DC current of the DC line L2 increases, and the AC voltage U ac Carry out detection to determine the input of load so that the AC voltage U ac Maintain within the voltage reference value of the receiving AC bus, expressed as:

[0128] U ac >1.05U b

[0129] Among them, U b Indicates the voltage reference value of the AC bus at the receiving end;

[0130] According to the active power balance and the voltage reference value of the AC bus at the receiving end, the required total active load input amount and the single active load input amount are obtained;

[0131] Specifically, the load input time can be determined according to the above formula. In order to maintain the AC bus voltage at the receiving end equal to the set value, that is, the reference voltage, the total active load input required is obtained according to the active power balance, which is expressed as:

[0132]

[0133] Where, θ represents the power factor angle of the receiving end AC bus, U d2 I dL1 Indicates the active power transmitted by DC line L1, U d4 I dL2 Indicates the active power transmitted by the DC line L2.

[0134] Then the single input amount of active load is obtained, which is expressed as:

[0135]

[0136] At the same time, it is necessary to ensure that the receiving-end converter station will not fail to commutate during this process, that is, to ensure that the inverter station arc extinction angle meets the set constraints. Correspondingly, the receiving-end AC system is required to have a slightly leading power factor. Therefore, in order to meet the constraints of the inverter station arc extinction angle, the second constraint is set, which is expressed as:

[0137]

[0138] That can be expressed as:

[0139]

[0140] Among them, Q C and Q L Respectively represent the reactive power of the reactive compensation device and the reactive power of the load, U d2 I dL1 tanθ、U d4 I dL2 tanθ represents the reactive power consumed by the second converter and the fourth converter, θ min Indicates the minimum power factor angle;

[0141] The reactive power of the reactive power compensation device should meet the following requirements:

[0142] Q C >2(U d2 I dL1 +U d4 I dL2 )tanθ min +Q L

[0143] When the forward power transmitted by the two poles of the DC system is equal, the black start of the receiving-end power grid is completed and the normal power transmission state to the receiving-end AC bus is entered.

[0144] Specifically, the transmission power from step S101 to step S102 changes as follows: Figure 3 shown.

[0145] It should be noted that based on the bipolar reverse transmission mode and the special control mode of the converter, the bipolar LCC-HVDC system has voltage source characteristics at the initial stage of black start, so that it can provide voltage and frequency support for the passive power grid at the receiving end; (2) the black start strategy of the bipolar LCC-HVDC system based on the characteristics of the AC and DC system when the receiving end is in a power outage state; (3) the strategy of the bipolar LCC-HVDC system supporting load recovery based on the characteristics of the AC and DC system; (4) including the effectiveness of constant impedance load, constant power load and motor load recovery. In summary, the method of this application can be consistent in theoretical analysis and simulation experiments, which correspondingly proves the effectiveness and feasibility of the method of this application; at the same time, the comprehensive consideration of this application is relatively complete and the engineering implementation is convenient, and it has a good engineering promotion value.

[0146] The above is a schematic scheme of a black start method based on bipolar LCC-HVDC in this embodiment. It should be noted that the technical scheme of the black start system based on bipolar LCC-HVDC and the technical scheme of the black start method based on bipolar LCC-HVDC are of the same concept, and the details of the technical scheme of the black start system based on bipolar LCC-HVDC in this embodiment that are not described in detail can all be referred to the description of the technical scheme of the black start method based on bipolar LCC-HVDC.

[0147] The black start system based on bipolar LCC-HVDC in this embodiment includes:

[0148] A black start module is used to enter the black start phase of the DC system. The DC system black start phase includes: starting the bipolar LCC-HVDC system through an ice melting simulation mode. The bipolar LCC-HVDC system includes two heat exchanger groups at the sending end and two heat exchanger groups at the receiving end. There is no power output and load connection in the DC system black start phase;

[0149] The recovery module is used to enter the load recovery phase, which includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operating state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

[0150] This embodiment further provides an electronic device, which is applicable to a black start based on a bipolar LCC-HVDC, including:

[0151] Memory and processor; the memory is used to store computer executable instructions, and the processor is used to execute computer executable instructions to implement the black start method based on bipolar LCC-HVDC as proposed in the above embodiment.

[0152] This embodiment further provides a storage medium on which a computer program is stored. When the program is executed by a processor, the black start method based on bipolar LCC-HVDC as proposed in the above embodiment is implemented.

[0153] The storage medium proposed in this embodiment and the black start method based on bipolar LCC-HVDC proposed in the above embodiment belong to the same inventive concept. The technical details not fully described in this embodiment can be referred to the above embodiment, and this embodiment has the same beneficial effects as the above embodiment.

[0154] Through the above description of the implementation methods, the technicians in the relevant field can clearly understand that the present invention can be implemented by means of software and necessary general hardware, and of course can also be implemented by hardware, but in many cases the former is a better implementation method. Based on such an understanding, the technical solution of the present invention is essentially or the part that contributes to the prior art can be embodied in the form of a software product, and the computer software product can be stored in a computer-readable storage medium, such as a computer floppy disk, read-only memory (ReadOnly, Memory, ROM), random access memory (RandomAccess Memory, RAM), flash memory (FLASH), hard disk or optical disk, etc., including a number of instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform the methods of various embodiments of the present invention.

[0155] Example 2

[0156] Reference Figure 11-Figure 14 , which is an embodiment of the present invention, provides a black start method based on bipolar LCC-HVDC. In order to verify its beneficial effects, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0157] In this embodiment, a simulation model of a bipolar LCC-HVDC system including its sending-end and receiving-end passive networks is constructed. The bipolar LCC-HVDC system simulation model constructed based on the CIGRE standard LCC-HVDC model includes 4 converter groups, each of which includes a twelve-pulse converter and a matching converter transformer. The sending-end AC side is a normally operating external power grid, and the receiving-end AC bus is connected to the power outage system, that is, it is in a passive power outage state, and is connected to sufficient reactive compensation devices and various loads. The DC line L1 and the DC line L2 are bipolar DC lines.

[0158] Secondly, a special control system for the bipolar LCC-HVDC system converter is constructed, including a constant frequency control system and a constant AC voltage control system of the receiving-end inverter. Then, simulation experiments are carried out according to the processes and strategies of step S101 and step S102, and the simulation results are analyzed to verify the effectiveness of the strategy of this method.

[0159] Specifically, the receiving end load simulation model includes:

[0160] (1) Constant impedance load simulation model, which is invested according to the strategy during the simulation process to verify the correctness of the above theoretical analysis and the effectiveness of the strategy.

[0161] Fig.11 This is the first stage of black start, i.e., the DC system black start stage. For specific simulation results, it lasts from the 0th second to the 2nd second from the start of black start.

[0162] a. At 0.05s, the two converters on the S1 side are unlocked, with the first converter acting as a rectifier and the third converter acting as an inverter, and a DC voltage begins to be established on the DC line L1.

[0163] b. At 0.11s, the second converter is unlocked as an inverter and controlled by a fixed trigger pulse. The converter station completes the initial commutation. The DC current of the DC line L1 increases according to the constant current instruction of the first converter. At the same time, the current is injected into the AC grid on the S2 side through the DC line L1 to charge the capacitor and the filter, so that the AC voltage on the S2 side gradually increases. During this process, the DC voltage of the DC line L1 is kept at a low level.

[0164] c. At about 0.3s, the fourth converter is unlocked as a rectifier, and the minimum current and constant AC voltage are used for control. The DC voltage of the DC line L2 quickly rises to the set value. During this period, the arc extinction angle of the second converter has been operating at a relatively high state (close to 90°). This is due to the capacitive AC network on the S2 side. At about 0.65s, the AC voltage on the S2 side rises to about 0.95pu, and the arc extinction angle of converter 2 begins to decrease, causing the DC voltage of the DC line L1 and the reverse DC current of the DC line L2 to begin to rise, and the active power transmitted by the two lines increases.

[0165] d. The bipolar reverse transmission is balanced at about 1.6s, and the arc extinction angle of converter 2 drops to about 30°. So far, the DC system is stably operating in the simulated ice melting mode, and the AC voltage on the S2 side is stable at 1.0pu, completing the initial system black start.

[0166] Fig.12 This is the reverse power reduction sub-stage of the second stage of black start, i.e., the load recovery stage. For specific simulation results, it lasts from the 2nd second to the 3.5th second from the start of black start.

[0167] a. In this stage, the load is gradually connected according to the constraints of S31. As the reactive power consumption of converter 4 decreases, the inverter arc extinction angle of converter 2 increases to a certain extent.

[0168] b. Under the regulation of the control system, the reverse transmission power of the DC line L2 gradually decreases to 0, so that the output active power continues to increase. The maximum active output is about 400MW, the reactive output is 100MVar, and the inverter extinction angle reaches up to 38°.

[0169] Fig.13 This is the positive power increase sub-stage of the second stage of black start, i.e., the load recovery stage. For specific simulation results, it lasts from 3.5s to 5s from the start of black start.

[0170] a. Re-unlock converter 3 and converter 4 from 3.5s to enable forward transmission of L2 line power. Converter 3 as a rectifier adopts a gradually increasing constant current control, and converter 4 adopts a constant pulse frequency control. As the L2 current increases, the load and reactive power compensation device are put into operation according to the constraint of S22.

[0171] b. The final bipolar DC voltage is 0.8pu, the DC current is 0.6pu, the total active power output is about 1000MW, the reactive power output is 120Mvar, and the load impedance angle is about 6.8°.

[0172] In particular, the increase in load inductive reactance will lead to a decrease in system capacitance, which will lead to a decrease in the arc extinction angle of the DC inverter station and cause a commutation failure. For this simulation, the minimum value of the inverter arc extinction angle of the second and fourth converters is 10.2°. Although there is no commutation failure, the commutation margin is very small at this time, and the ability to resist impact is weak. This simulation can verify that this LCC-HVDC converter station requires a load impedance angle less than 6.8°, otherwise a commutation failure will occur.

[0173] (2) Constant impedance, constant power and motor multi-load simulation models, which are invested according to the strategy during the simulation process to verify the effectiveness and applicability of the strategy.

[0174] As a stable and reliable large-capacity power source, the recovery of thermal power plants will quickly improve the power supply capacity of the region, thereby accelerating the system recovery process. The plant loads of thermal power plants are mainly high-power coal mills and blower equipment, which can be modeled as induction motors. This experiment uses four 710KW and one 4MW induction motor models with rated power, and adds some constant impedance and constant power loads to simulate the plant loads of large thermal power plants.

[0175] The startup process finally completes the two stages of black start. Fig.14The simulation results of the two stages of black start are shown in Figure 2. During the reverse power reduction stage (from 2s to 3.5s), the load of the thermal power plant is restored, the minimum inverter arc extinction angle is 16°, no commutation failure occurs, and the final output active power and reactive power are 1050MW and 150Mvar respectively.

[0176] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A black start method based on bipolar LCC-HVDC, characterized in that: include: Entering the DC system black start stage, the DC system black start stage includes: starting the bipolar LCC-HVDC system through the simulated ice melting mode, the bipolar LCC-HVDC system includes two heat exchanger groups at the sending end and two heat exchanger groups at the receiving end, wherein there is no power output and load connection in the DC system black start stage; Entering the load recovery stage, the load recovery stage includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operation state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

2. The black start method based on bipolar LCC-HVDC according to claim 1, characterized in that: The bipolar LCC-HVDC system comprises: The bipolar LCC-HVDC system comprises a first converter, a second converter, a third converter and a fourth converter, each converter group comprising a twelve-pulse converter and a matching converter transformer; The first converter and the second converter are connected via a DC line L1, and the third converter and the fourth converter are connected via a DC line L2; The first converter and the third converter are both located at the sending end, and the AC busbar at the sending end is connected to the external power grid; The second converter and the fourth converter are both located at the receiving end, and the AC busbar at the receiving end is connected to the power outage system, the reactive power compensation device and the load.

3. The black start method based on bipolar LCC-HVDC according to claim 1 or 2, characterized in that: The DC system black start phase includes: In the initial state, the AC filter and capacitor of the receiving end AC bus are connected to the grid, the receiving end AC bus is in a no-load state, the first converter is started as a rectifier, the third converter is started as an inverter, the DC line L1 operates in an open circuit voltage mode, and a DC voltage U d1 ; When the DC voltage U d1 Reach the starting reference voltage value U dlulk When , the second converter is started as an inverter, and the first converter is switched to constant current control; When the AC voltage of the AC busbar at the receiving end and the DC current of the DC line L1 are respectively greater than the voltage setting value and the current setting value of the fourth converter unlocking, the fourth converter is started as a rectifier, the DC line L2 is turned on and the current is drawn back from the receiving end to the sending end; When the DC current of the DC line L1 is equal to the DC current of the DC line L2, the positive current and the negative reverse current are equal, reaching the simulated ice melting operation state, and completing the startup of the bipolar LCC-HVDC system.

4. The black start method based on bipolar LCC-HVDC according to claim 3, characterized in that: The load recovery phase includes: The receiving end AC busbar is gradually loaded until the negative pole reverse current is reduced to zero, and the DC system has voltage source characteristics; The DC system enters a bipolar forward power supply mode, the receiving-end AC busbar is put into reactive compensation equipment and load, and transitions to a normal operating state of bipolar forward transmission, completing a black start of the receiving-end power grid.

5. The black start method based on bipolar LCC-HVDC according to claim 4, characterized in that: The receiving-end AC busbar gradually puts load on the receiving end including: The receiving end AC busbar is gradually loaded, and the total active load input is less than the positive pole transmission power, and the inverter station arc extinction angle meets the following constraints, which can be expressed as: Among them, R load The equivalent resistance representing the total active load input, U d1 represents the DC voltage of DC line L1, γ min represents the critical shutdown angle, β represents the inverter station advance trigger angle, k represents the converter transformer ratio, ωL C Indicates the commutation reactance of the converter; Assuming that the instantaneous DC voltage remains unchanged, the critical commutation voltage U is obtained. acmin , and the minimum AC voltage is greater than the critical commutation voltage U acmin , respectively expressed as: Among them, R C represents the commutation impedance, I d represents the DC current, k represents the transformer ratio, U' L It represents the average AC voltage of the receiving end AC busbar at the moment before the load is put into operation, U L It represents the average value of the AC voltage of the receiving-end AC bus at the moment when the load is put into operation. R0 represents the total equivalent resistance of all loads and reactive compensation devices connected to the AC bus at the receiving end. X0 represents the total equivalent reactance of all loads and reactive compensation devices connected to the AC bus at the receiving end. R represents the equivalent resistance of the single active load input, and X represents the equivalent reactance of the single active load input. When the total active load power input by the receiving-end AC bus is equal to the positive pole transmission power, the negative pole reverse transmission power will drop to zero, the DC line L2 will be locked, and the DC system will be switched from the simulated ice melting mode to the unipolar power transmission mode.

6. The black start method based on bipolar LCC-HVDC according to claim 5, characterized in that: The DC system enters the bipolar forward power supply mode including: The third converter is operated as a rectifier and a constant current control is adopted, the fourth converter is operated as an inverter and a constant trigger frequency control is adopted, and the current of the DC line L2 is transmitted in the forward direction; By increasing the current setting value of the third converter, the DC current of the DC line L2 increases, and the AC voltage U ac Carry out detection to determine the input of load so that the AC voltage U ac Maintain within the voltage reference value of the AC bus at the receiving end; According to the active power balance and the voltage reference value of the AC bus at the receiving end, the required total active load input amount and the single active load input amount are obtained; In order to meet the constraint condition of the arc extinction angle of the inverter station, a second constraint condition is set, which is expressed as: Among them, Q C and Q L Respectively represent the reactive power of the reactive compensation device and the reactive power of the load, U d2 I dL1 tanθ、U d4 I dL2 tanθ represents the reactive power consumed by the second converter and the fourth converter, θ min Indicates the minimum power factor angle; The reactive power of the reactive power compensation device should meet the following requirements: Q C >2(U d2 I dL1 +U d4 I dL2 )tanθ min +Q L When the forward power transmitted by the two poles of the DC system is equal, the black start of the receiving-end power grid is completed and the normal power transmission state to the receiving-end AC bus is entered.

7. The black start method based on bipolar LCC-HVDC according to claim 3, characterized in that: The DC voltage U d1 The relationship between the trigger angle is expressed as: Wherein, E0 represents the effective value of the AC voltage at the sending end, R and C represent the equivalent resistance and equivalent capacitance respectively when the DC line L1 is open, C0 represents the buffer capacitance of the converter valve, and α represents the trigger angle of the converter.

8. A black start system based on bipolar LCC-HVDC, characterized in that: include, A black start module is used to enter a DC system black start phase, wherein the DC system black start phase includes: starting a bipolar LCC-HVDC system in an ice melting simulation mode, wherein the bipolar LCC-HVDC system includes two heat exchanger groups at a sending end and two heat exchanger groups at a receiving end, wherein there is no power output and no load connection in the DC system black start phase; The recovery module is used to enter the load recovery phase, which includes: increasing the output power of the DC system, connecting the load, transitioning to the normal operation state of the two-stage forward transmission, and completing the black start of the receiving-end power grid.

9. An electronic device, characterized in that: include: Memory and processor; The memory is used to store computer executable instructions, and the processor is used to execute the computer executable instructions. When the computer executable instructions are executed by the processor, the steps of the black start method based on bipolar LCC-HVDC according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium, characterized in that: It stores computer executable instructions, which, when executed by a processor, implement the steps of the black start method based on bipolar LCC-HVDC as claimed in any one of claims 1 to 7.

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