WO2022011760A1

WO2022011760A1 - Method for starting hybrid multi-terminal direct-current power transmission system

Info

Publication number: WO2022011760A1
Application number: PCT/CN2020/107342
Authority: WO
Inventors: 李周; 魏子昂
Original assignee: 南京东博智慧能源研究院有限公司
Priority date: 2020-07-16
Filing date: 2020-08-06
Publication date: 2022-01-20
Also published as: CN112039104B; LU500824A1; LU500824B1; CN112039104A

Abstract

A method for starting a hybrid multi-terminal direct-current power transmission system, the method comprising: closing alternating-current circuits of a rectification side LCC and an inversion side LCC, and disconnecting an alternating-current circuit of an inversion side VSC; an LCC station using constant current control, the inversion side LCC using fixed extinction angle control, and the rectification side LCC charging the inversion side LCC and the inversion side VSC; the inversion side VSC being not locked during the charging process, and performing capacitor voltage equalization control or alternate touch control on an MMC at the same time; closing the alternating-current circuit of the inversion side VSC, and initializing value assignment of a controller of the inversion side VSC; raising a VSC power reference value under constant power control on an inversion side to a rated value according to a slope, raising an LCC current reference value under constant current control to a rated value according to a slope, and keeping the rising speed for the two reference values the same; and inputting, in batches, alternating-current systems connected to a VSC that uses amplitude and phase control. By means of the starting method, different starting speeds are realized, and an impact current during the normal input of a VSC controller is reduced.

Description

A start-up method of a hybrid multi-terminal direct current transmission system

technical field

The invention belongs to the technical field of electric power systems, relates to the safety and stability analysis technology of electric power systems, and in particular relates to a startup method of a hybrid multi-terminal direct current transmission system.

Background technique

Flexible direct current transmission (VSC-HVDC) has been more and more widely used due to its advantages of decoupling control of active and reactive power, supplying power to passive networks, and providing reactive power support. However, limited by the withstand voltage and current level of IGBT, VSC-HVDC is not suitable for high-voltage, large-capacity DC transmission. While traditional direct current transmission (LCC-HVDC) meets the needs of high-voltage and large-capacity transmission, it is difficult to achieve flexible control and distribution of power. Therefore, LCC-HVDC and VSC-HVDC will coexist and complement each other for a long time.

Hybrid DC transmission combines the advantages of traditional LCC-HVDC and VSC-HVDC, and can be applied to different scenarios by adopting different topologies. The series-parallel hybrid multi-terminal DC transmission system has broad application prospects because it can meet the requirements of high-voltage and large-capacity DC transmission, and the existence of parallel VSC-HVDC improves the flexibility of power distribution in the entire system. However, the complexity of the DC network topology also brings challenges to the startup strategy.

In view of the increasingly complex HVDC transmission network, it is necessary to design a reasonable start-up strategy for the HVDC transmission system, so as to realize the rapid start-up of the system and reduce the shock during the start-up process. In the current prior art, there are still the following problems: 1) when the VSC is actively started through the AC network, a current limiting resistor needs to be connected in series; 2) the startup cannot be realized when the hybrid DC system is connected to the passive network; 3) the traditional DC side is adopted During startup, the overcurrent of the bridge arm is caused by the halving of the voltage at the moment of unlocking the MMC; 4) The traditional AC startup and DC startup both need to lock the VSC, and the overcurrent will occur due to the state mismatch at the moment of unlocking.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a startup method for a hybrid multi-terminal DC power transmission system in view of the deficiencies of the background technology. The reference value is used to achieve different startup speeds, and the initialization strategy reduces the inrush current when the VSC controller is normally switched on.

The present invention adopts the following technical solutions for solving the above-mentioned technical problems:

A startup method for a hybrid multi-terminal direct current transmission system, comprising i series LCCs on the rectifier side, j series LCCs on the inverter side, and k parallel VSCs on the inverter side, where i≧1, j≧1 , k≧1; specifically includes the following steps;

Step 1, close the LCC AC lines on the rectifier side and the inverter side, and disconnect each VSC AC line on the inverter side;

Step 2, select a suitable LCC converter station for constant current control, thereby limiting the bridge arm current of the inverter side VSC during the charging process;

Step 3, the non-constant current control station in the rectifier side LCC adopts constant voltage control, the inverter side non-constant current control station adopts constant arc extinguishing angle control, and the rectifier side LCC charges the inverter side LCC and VSC;

Step 4: Do not block each VSC on the inverter side, and each VSC can arbitrarily take constant voltage control, amplitude phase control, constant power control, clear the controller integrator, and at the same time put capacitor voltage equalization control or voltage equalization control on the multi-level VSC. wheel touch control;

Step 5, when the voltage of each converter station of the system rises to the rated value or reaches the upper limit value of the voltage, close the AC line of one or more VSCs on the inverter side at the same time, and initialize the controller of each VSC on the inverter side;

Step 6: Raise the VSC power reference value that adopts or maintain constant power control on the inverter side to the rated value according to the slope, and increase the LCC current reference value that adopts constant current control to the rated value or the upper limit of safe current according to the slope. The rate of ascent remains the same;

In step 7, the AC system connected to the VSC with amplitude and phase control is put into operation in batches.

As a further preferred solution of the start-up method of a hybrid multi-terminal DC power transmission system of the present invention, in the step 2, selecting an appropriate LCC converter station to perform constant current control is as follows:

Step 2.1, select the first converter station in the i series LCCs on the rectifier side for constant current control;

Step 2.2, select the rth converter station in the i series LCCs on the rectifier side for constant current control, where 1﹤r≦i;

Step 2.3, select the s-th converter station in the j series-connected LCCs on the inverter side for constant current control, where 1≦s≦j.

As a further preferred solution of the startup method of a hybrid multi-terminal DC power transmission system of the present invention, the step 3 is specifically as follows:

Step 3.1, when the first converter station in the i series LCCs on the rectifier side is selected for constant current control and the backup constant voltage control is adopted, the reference value of the constant voltage control satisfies

Among them, U _dcR1 and U _dcRmax are the voltage reference value and voltage upper limit of the first LCC on the rectifier side, respectively, and U _dc(t) and U _max(t) are the current DC voltage and voltage upper limit of the k-th VSC on the inverter side, respectively;

The constant voltage of other LCCs on the rectifier side is the rated value, and the fixed arc extinguishing angle of the inverter side LCC is the rated value; during the charging process, the LCC voltage on the inverter side gradually rises to the rated value, and then the VSC voltage gradually rises to the rated value;

Step 3.2, when the rth converter station in the i series LCCs on the rectifier side is selected for constant current control, where 1﹤r≦i, the first one on the rectifier side adopts constant voltage control, and the voltage command increases from 0 according to the slope To the rated value, the constant voltage of other LCCs on the rectifier side is the rated value, and the constant arc extinguishing angle of the LCC on the inverter side is the rated value; during the charging process, the voltage of each converter station gradually rises;

Step 3.3, when selecting the s-th converter station in the j series LCCs on the inverter side for constant current control, where 1≦s≦j, the fixed arc-extinguishing angle of other LCCs on the inverter side is the rated value, and the rectifier side is the first One adopts constant voltage control, the voltage command increases from 0 to the rated value according to the slope, and the other LCC constant voltage on the rectifier side is the rated value; during the charging process, the voltage of each converter station gradually increases.

As a further preferred solution of the method for starting a hybrid multi-terminal DC power transmission system of the present invention, the step 5 is specifically as follows:

Step 5.1, reset the integral value of the VSC converter station that is maintained or switched to constant voltage control: for the outer loop proportional integrator, correspondingly set the initial value of the integrator to the actual electrical value;

The reset value of the integrator in the inner loop control is:

Among them, U _c and U _s are the rms voltage at the outlet of the AC side of the converter station and the rms value of the AC voltage at the common connection point, v _dreset and v _qreset are the integrator reset values in the d-axis control and the integrator reset values in the q-axis control. The integrator reset values, R _c and X _c are the equivalent resistance and commutating reactance _{of the converter station, isd} and i _sq are the dq-axis components of the AC side current of the converter station, and P is the active power injected at the PCC point.

In step 5.2, the integral value of the VSC converter station that is maintained or switched to amplitude-phase control is reset to:

Step 5.3, for a VSC converter station that maintains or switches to constant active power, the power reference value should be initialized as

Among them, I _dc is the t (1≦t≦k) charging current, and U _dc is the VSC rated DC voltage;

The integral value in the outer loop control is reset to:

Among them, v _dreset and v _qreset are the integrator reset values in the d-axis and q-axis outer loop control, respectively, P _ref and Q _ref are the active power and reactive power reference values, respectively; the integrator reset value in the inner loop control for:

where v _dreset and v _qreset are the integrator reset values in the d-axis and q-axis inner loop control, respectively, U _sd and U _sq are the dq-axis components of the AC voltage at the common connection point, and _isd and i _sq are the converter stations The dq-axis component of the AC side current.

As a further preferred solution of a method for starting a hybrid multi-terminal DC power transmission system of the present invention, in step 6, the current upper limit value satisfies:

Among them, I _dcR(r) , I _dci(s) and I _dc(t) are the current values of the rectifier side LCC, the inverter side LCC and the inverter side VSC, respectively, I _dcR(r)max , I _{dci(s) max} and I _dc(t)max are the corresponding current upper limit of each converter station, respectively.

Compared with the prior art, the present invention adopts the above technical scheme, and has the following technical effects:

1. In the present invention, the dispatching system can freely choose the LCC converter station that adopts constant current control, and can freely specify the reference value of the constant current control of the LCC to achieve different starting speeds, and the initialization strategy reduces the normal input of the VSC controller. impulse current at time;

2. The startup method of the present invention includes i series LCCs on the rectifier side, j series LCCs on the inverter side, and k parallel VSCs on the inverter side. current without the current-limiting resistors required in traditional start-up methods, for both VSC connected active and passive networks;

3. The start-up speed of the present invention is relatively fast, and different start-up speeds can be achieved through different start-up currents. This method is suitable for various single-bipolar series hybrid, parallel hybrid and series-parallel hybrid structures, and is suitable for two-, three-level and multi-level. level VSC, the proposed start-up method is general and applicable to a wide range.

Description of drawings

Fig. 1 is that Fig. 1 is the flow chart of the startup method of the present invention;

Fig. 2 is the general structure of the hybrid multi-terminal direct current transmission system to which the method of the present invention is applicable;

Figure 3 is a schematic diagram of a four-terminal hybrid DC transmission system;

Figure 4 is the simulation waveform under the starting method, in which Figure 4(a) is the DC voltage of each converter station, Figure 4(b) is the DC line current, Figure 4(c) is the LCC1 firing angle and the LCC2 arc extinguishing angle, Figure 4(d) is the active power of each converter station, Figure 4(e) is the voltage (phase A) of the upper and lower arms of MMC3 and MMC4, and Figure 4(f) is the current (phase A) of the upper and lower arms of MMC3 and MMC4. .

detailed description

Below in conjunction with accompanying drawing, the technical scheme of the present invention is described in further detail:

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a 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 those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

A start-up method of a hybrid multi-terminal DC transmission system, in which the dispatching system can freely choose the LCC converter station that adopts constant current control, and can freely specify the reference value of the constant current control of the LCC to achieve different start-up speeds, and at the same time initialize The strategy reduces the inrush current when the VSC controller is normally switched on. The startup method of the present invention includes i LCCs in series on the rectifier side, j LCCs in series on the inverter side, and k VSCs on the inverter side in parallel. , as shown in Figures 2 to 3.

As shown in Figure 1, it includes the following steps:

Close the LCC AC lines on the rectifier side and the inverter side, and disconnect the VSC AC lines on the inverter side;

Select a suitable LCC converter station for constant current control, other rectifier side LCCs adopt constant voltage control, inverter side LCCs adopt constant arc extinguishing angle control, and the inverter side LCC and VSC are charged through the rectifier side LCC;

The VSC charging process on the inverter side is not blocked, and constant voltage control, constant power control, and amplitude and phase control can be arbitrarily adopted, and at the same time, the multi-level MMC is put into capacitor voltage equalization control or wheel touch control;

When the voltage of each converter station of the system rises to the rated value, the AC line of each VSC on the inverter side is closed at the same time, and the controller of each VSC on the inverter side is initialized and assigned;

The VSC power reference value controlled by constant power on the inverter side is increased to the rated value according to the slope, and the LCC current reference value controlled by constant current is increased to the rated value according to the slope, and the rising speed of the two reference values remains the same;

The AC system connected to the VSC with amplitude and phase control will be put into operation in batches.

Further, it specifically includes the following steps:

Step 1: At the initial moment of startup, close the LCC AC lines on the rectifier side and the inverter side, and disconnect the VSC AC lines on the inverter side;

Step 2: Select a suitable LCC converter station and adopt constant current control to limit the bridge arm current of the inverter side VSC during the charging process:

1) Select the first converter station in the i series LCCs on the rectifier side for constant current control;

2) Select the r (1﹤r≦i) converter station in the i series LCCs on the rectifier side for constant current control;

3) Select the s (1≦s≦j) converter station in the j series LCCs on the inverter side for constant current control;

Step 3: The non-constant current control station in the rectifier side LCC adopts constant voltage control, the inverter side non-constant current control station adopts constant arc extinguishing angle control, and the rectifier side LCC charges the inverter side LCC and VSC:

1) When the first converter station in the i series LCCs on the rectifier side is selected for constant current control and backup constant voltage control is adopted, the reference value of constant voltage control satisfies

Among them, U _dcR1 and U _dcRmax are the voltage reference value and upper limit of the first LCC on the rectifier side, respectively, and U _dc(t) and U _max(t) are the current DC voltage and the upper limit of the voltage of the kth VSC on the inverter side, respectively.

2) When the r (1﹤r≦i) converter station in the i series LCCs on the rectifier side is selected for constant current control, the first one on the rectifier side adopts constant voltage control, and the voltage command rises from 0 to the rated value according to the slope. The constant voltage of other LCCs on the rectifier side is the rated value, and the fixed arc-extinguishing angle of the LCC on the inverter side is the rated value; during the charging process, the voltage of each converter station gradually increases;

3) When the s (1≦s≦j) converter station among the j series LCCs on the inverter side is selected for constant current control, the fixed arc extinguishing angle of the other LCCs on the inverter side is the rated value, and the first one on the rectifier side adopts the constant current control. Constant voltage control, the voltage command rises from 0 to the rated value according to the slope, and the other LCC constant voltage on the rectifier side is the rated value; during the charging process, the voltage of each converter station gradually increases;

Step 4: Each VSC on the inverter side is not blocked, and each VSC can arbitrarily adopt constant voltage control, amplitude and phase control, and constant power control, clear the controller integrator, and at the same time put the capacitor voltage equalization control or wheel touch control;

Step 5: When the voltage of each converter station of the system rises to the rated value or reaches the upper limit value of the voltage, the AC lines of the k VSCs on the inverter side are closed at the same time, and the controllers of each VSC on the inverter side are initialized and assigned:

1) The integral value of the VSC converter station maintained or switched to constant voltage control is reset to:

For the outer loop proportional integrator, set the initial value of the integrator accordingly to the actual electrical value;

The reset value of the integrator in the inner loop control is:

2) The integral value of the VSC converter station that is maintained or switched to amplitude-phase control is reset to:

3) For a VSC converter station maintained or switched to constant active power, the power reference value should be initialized as

Among them, I _dc is the t (1≦t≦k) charging current, and U _dc is the rated DC voltage of the VSC.

The integral value in the outer loop control is reset to:

where v _dreset and v _qreset are the integrator reset values in the d-axis and q-axis outer loop control, respectively. _Pref and _Qref are active power and reactive power reference values, respectively.

The reset value of the integrator in the inner loop control is:

where v _dreset and v _qreset are the integrator reset values in the d-axis and q-axis inner loop control, respectively. U _sd and U _sq are the dq-axis components of the AC voltage at the common connection point, and _isd and i _sq are the dq-axis components of the AC side current of the converter station.

Step 5: Raise the VSC power reference value that adopts or maintain constant power control on the inverter side to the rated value according to the slope, and increase the LCC current reference value that adopts constant current control to the rated value or the safe current upper limit according to the slope. The rising speed remains the same, and the current upper limit value satisfies:

I _dcR(r) , I _dci(s) and I _dc(t) are the current values of the rectifier side LCC, the inverter side LCC and the inverter side VSC, respectively, I _dcR(r)max , I _dci(s)max and I _dc(t)max is the upper limit of the corresponding current of each converter station, respectively.

Step 7: The AC system connected to the VSC with amplitude and phase control is put into operation in batches.

Taking the four-terminal hybrid direct current transmission system shown in FIG. 4 as an example, the startup method proposed by the present invention will be specifically described. When the system is in normal operation, the constant current of LCC1 on the rectifier side is 3kA, the constant arc extinguishing angle is 17° for LCC2 on the inverter side, and the constant voltage of MMC3 and MMC4 on the inverter side is 400kV and -600MW respectively. The rated voltage of the DC line is 800kV.

For the startup method proposed by the present invention:

0s<t≦0.28s: Close the AC lines of LCC1 and LCC2, disconnect the AC lines of MMC3 and MMC4, but do not block, and at the same time the MMC voltage controller and constant power controller integrator are cleared; the rectified LCC constant current is 1kA, which is The LCC and MMC on the inverter side are charged; the fixed arc extinguishing angle of the inverter LCC is 17°, and the voltage is gradually increased to the rated value; MMC3 and MMC4 have not yet established DC voltage;

0.28s<t≦1.78s: Keep the rectifier LCC constant current control and the inverter LCC constant arc extinguishing angle control unchanged, the inverter LCC voltage rises to 400kV and remains stable; the rectifier LCC starts to charge MMC3 and 4, and the MMC voltage gradually rise to the rated value;

1.78s<t≦5s: When the voltage of MMC3 and 4 rises to the rated value of 400kV, after the system voltage is established: (1) Close the MMC3 AC line, initialize the MMC3 voltage integrator, and the voltage control works normally, and the voltage reference value is 400kV; (2) Close the MMC4 AC line, initialize the reference value of the MMC4 constant power controller to -200MW, and initialize the integrator; (3) The MMC3 constant power controller ramps up the command value from -200MW to the rated power -600MW; (4) The rectifier LCC constant current controller increases from 1kA to the rated value of 3kA according to the slope;

5s≦t≦10s: System startup is complete.

The simulation waveform is shown in Fig. 4. Among them, Fig. 4(a) is the DC voltage of each converter station, Fig. 4(b) is the DC line current, Fig. 4(c) is the LCC1 firing angle and LCC2 arc extinguishing angle, and Fig. 4(d) is each converter station Active power, Figure 4(e) is the voltage (phase A) of the upper and lower bridge arms of MMC3 and MMC4, and Figure 4(f) is the current (phase A) of the upper and lower bridge arms of MMC3 and MMC4; for the proposed startup method: rectifier side The LCC constant current control limits the possible overcurrent during the MMC charging process, and the MMC initialization strategy reduces the impact when the MMC controller starts to run normally, and the startup speed is faster, which is beneficial to the safety of the entire hybrid DC transmission system. Stable operation.

The technical means disclosed in the solution of the present invention are not limited to the technical means disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be pointed out that for those skilled in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications are also regarded as the protection scope of the present invention.

It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as herein, are not to be taken in an idealized or overly formal sense. explain.

The above embodiments are only to illustrate the technical idea of the present invention, and cannot limit the protection scope of the present invention. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the present invention. Inside. The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments, and within the scope of knowledge possessed by those of ordinary skill in the art, various changes can also be made without departing from the spirit of the present invention. .

Claims

A method for starting a hybrid multi-terminal direct current power transmission system, characterized in that: it includes i series LCCs on the rectifier side, the inverter side includes j series LCCs on the inverter side, and k parallel VSCs on the inverter side, wherein i≧1 , j≧1, k≧1; specifically includes the following steps;

Step 1, close the LCC AC lines on the rectifier side and the inverter side, and disconnect each VSC AC line on the inverter side;

Step 2, select an appropriate LCC converter station for constant current control, thereby limiting the bridge arm current of the inverter side VSC during the charging process;

Step 3, the non-constant current control station in the rectifier side LCC adopts constant voltage control, the inverter side non-constant current control station adopts constant arc extinguishing angle control, and the rectifier side LCC charges the inverter side LCC and VSC;

Step 4: Do not block each VSC on the inverter side, and each VSC can arbitrarily take constant voltage control, amplitude phase control, constant power control, clear the controller integrator, and at the same time put capacitor voltage equalization control or voltage equalization control on the multi-level VSC. wheel touch control;

Step 5, when the voltage of each converter station of the system rises to the rated value or reaches the upper limit value of the voltage, close the AC line of one or more VSCs on the inverter side at the same time, and initialize the controller of each VSC on the inverter side;

Step 6: The VSC power reference value that adopts or maintains constant power control on the inverter side is increased to the rated value according to the slope, and the LCC current reference value that adopts constant current control is increased to the rated value or the upper limit of safe current according to the slope. The rate of ascent remains the same;

In step 7, the AC system connected to the VSC with amplitude and phase control is put into operation in batches.
The startup method of a hybrid multi-terminal DC power transmission system according to claim 1, wherein in the step 2, selecting an appropriate LCC converter station to perform constant current control is as follows:

Step 2.1, select the first converter station in the i series LCCs on the rectifier side for constant current control;

Step 2.2, select the rth converter station in the i series LCCs on the rectifier side for constant current control, where 1﹤r≦i;

Step 2.3, select the s-th converter station in the j series-connected LCCs on the inverter side for constant current control, where 1≦s≦j.
The method for starting a hybrid multi-terminal DC power transmission system according to claim 1, wherein the step 3 is as follows:

Step 3.1, when the first converter station in the i series LCCs on the rectifier side is selected for constant current control and the backup constant voltage control is adopted, the reference value of the constant voltage control satisfies

Among them, U dcR1 and U dcRmax are the voltage reference value and voltage upper limit of the first LCC on the rectifier side, respectively, and U dc(t) and U max(t) are the current DC voltage and voltage upper limit of the k-th VSC on the inverter side, respectively;

The constant voltage of other LCCs on the rectifier side is the rated value, and the fixed arc extinguishing angle of the inverter side LCC is the rated value; during the charging process, the LCC voltage on the inverter side gradually rises to the rated value, and then the VSC voltage gradually rises to the rated value;

Step 3.2, when the rth converter station in the i series LCCs on the rectifier side is selected for constant current control, where 1﹤r≦i, the first one on the rectifier side adopts constant voltage control, and the voltage command increases from 0 according to the slope When it reaches the rated value, the rated voltage of other LCCs on the rectifier side is rated as the rated value, and the constant arc-extinguishing angle of the LCC on the inverter side is rated as the rated value; during the charging process, the voltage of each converter station gradually increases;

Step 3.3, when selecting the s-th converter station in the j series LCCs on the inverter side for constant current control, where 1≦s≦j, the fixed arc-extinguishing angle of other LCCs on the inverter side is the rated value, and the rectifier side is the first One adopts constant voltage control, the voltage command increases from 0 to the rated value according to the slope, and the other LCC constant voltage on the rectifier side is the rated value; during the charging process, the voltage of each converter station gradually increases.
The method for starting a hybrid multi-terminal DC power transmission system according to claim 1, wherein the step 5 is as follows:

Step 5.1, reset the integral value of the VSC converter station that is maintained or switched to constant voltage control: for the outer loop proportional integrator, correspondingly set the initial value of the integrator to the actual electrical value;

The reset value of the integrator in the inner loop control is:

Among them, U c and U s are the rms voltage at the outlet of the AC side of the converter station and the rms value of the AC voltage at the common connection point, v dreset and v qreset are the integrator reset values in the d-axis control and the integrator reset values in the q-axis control. The integrator reset values, R c and X c are the equivalent resistance and commutating reactance of the converter station, isd and i sq are the dq-axis components of the AC side current of the converter station, and P is the active power injected at the PCC point.

Step 5.2, the integral value of the VSC converter station that is maintained or switched to amplitude-phase control is reset to:

Step 5.3, for a VSC converter station that maintains or switches to constant active power, the power reference value should be initialized as

Among them, I dc is the t (1≦t≦k) charging current, and U dc is the VSC rated DC voltage;

The integral value in the outer loop control is reset to:

Among them, v dreset and v qreset are the integrator reset values in the d-axis and q-axis outer loop control, respectively, P ref and Q ref are the active power and reactive power reference values, respectively; the integrator reset value in the inner loop control for:

where v dreset and v qreset are the integrator reset values in the d-axis and q-axis inner loop control, respectively, U sd and U sq are the dq-axis components of the AC voltage at the common connection point, and isd and i sq are the converter stations The dq-axis component of the AC side current.
A method for starting a hybrid multi-terminal DC power transmission system according to claim 1, wherein in step 6, the current upper limit value satisfies:

Among them, I dcR(r) , I dci(s) and I dc(t) are the current values of the rectifier side LCC, the inverter side LCC and the inverter side VSC, respectively, I dcR(r)max , I dci(s) max and I dc(t)max are the corresponding current upper limit of each converter station, respectively.