Offshore wind power direct current sending-out system structure based on S7 bridge arm current source converter and control method of offshore wind power direct current sending-out system structure

Abstract

The invention provides an offshore wind power direct current sending-out system structure based on an S7 bridge arm current source converter and a control method of the offshore wind power direct current sending-out system structure. The topological characteristic provided by the invention is that an S7 bridge arm is connected in parallel with the direct current side of the 6-pulse CSC, and the switch of the S7 bridge arm and the switch of each bridge arm of the 6-pulse CSC are distributed, so that multi-active and multi-reactive independent control is realized. The active quantity of the land CSC is controlled by constant direct current voltage, and the reactive quantity of the land CSC is controlled by reactive power. When the collected submarine cable has a fault, the S7CSC reduces the reference value of the AC bus and reduces the current fed by the CSC to a fault point, thereby preventing fault transient overcurrent and realizing fault ride-through. The technical scheme provided by the invention has the advantages that: S7CSC has two control degrees of freedom, can realize independent control of active components and reactive components, stabilizes the offshore AC voltage and frequency, can adapt to the fluctuation of wind power output, effectively inhibits the increase of fault current at the offshore AC collection fault, and realizes fault ride-through.

Description

Technical Field

[0001] The present invention relates to the technical field of HVDC transmission and power electronics, and particularly relates to a structure and a control method of a DC transmission system for offshore wind power based on an S7 bridge arm current source converter. Background Art

[0002] With the gradual green transformation of China's power system, vigorously developing offshore wind power has become a key link. With the expansion of wind turbines and the optimization research of more technical solutions, offshore wind power is developing towards the direction of large-capacity in the far sea. Taking the Rudong offshore wind power project as an example, realizing the efficient grid connection of the wind farm through a modular multilevel converter (MMC) is the mainstream offshore wind power transmission solution in the world. However, although this solution has advantages such as low loss and large transmission distance, the MMC is composed of power sub-modules, and the energy storage capacitors therein occupy a large amount of space. With the continuous increase in capacity, the volume and weight of the offshore converter gradually increase, which is particularly prominent in the construction of offshore platforms, resulting in a significant increase in the cost of the converter station.

[0003] In recent years, the research on the current source converter (CSC) based on IGCT has gradually become a research hotspot at home and abroad. The active commutation type current source converter is composed of reverse-blocking IGCTs and has the advantages of small volume, light weight, and flexible control, and is a more promising offshore wind power transmission solution in the future. Some scholars have proposed a CSC and LCC hybrid power transmission scheme, using CSC on the offshore side and LCC on the onshore side and proposing corresponding control methods, which greatly reduces the floor area of the offshore platform. However, this scheme has large switching losses and relatively severe DC voltage fluctuations. The reason is that pulse width modulation (PWM) is adopted. Therefore, a modulation method with a low switching frequency needs to be studied. Some scholars have proposed to adopt a fundamental frequency modulation (FFM) method. By adjusting the firing angle, similar to LCC, each device is turned on and off once within a switching cycle, and the switching frequency is greatly reduced. However, the disadvantage is that there is only one control variable, the firing angle. Therefore, it is necessary to cooperate with the fan to control to establish the offshore AC voltage. Some scholars have proposed an offshore wind power grid connection system based on CSC (AC-DC hybrid system) and designed corresponding control methods for steady-state conditions. Summary of the Invention

[0004] In view of the problems existing in the application of FFM-CSC in offshore wind power, the present invention proposes a structure of an offshore wind power DC transmission system based on an S7 bridge arm current source converter, and proposes a corresponding control method. It is characterized in that an S7 bridge arm is integrated on the basis of the original CSC topology. The three-phase bridge arms of the CSC still adopt fundamental frequency modulation. At the same time, the switching control of the S7 bridge arm is combined with the fundamental frequency modulation, so as to have two control degrees of freedom and realize the stable control of the offshore AC voltage and frequency. The proposed low-voltage ride-through method for the AC collection system fault can effectively suppress the growth of the fault current at the offshore AC collection fault and realize fault ride-through.

[0005] Each bridge arm of the S7CSC is cascaded by a plurality of fully controlled devices that can withstand reverse voltage (such as the combination of reverse-conducting IGCT / IGBT in series with a diode, reverse-blocking IGCT / IGBT), and adopts fundamental wave frequency modulation, reducing the switching frequency of the IGCT to the power grid fundamental frequency, significantly reducing the switching loss. In the FFM-CSC, each bridge arm conducts only once in a switching cycle, conducts only one phase of the upper bridge arm or the lower bridge arm at any moment, and actively turns off after the conduction angle of each bridge arm is 120°.

[0006] The overall structure of the offshore wind power transmission system proposed by the present invention is as follows: The AC electric energy output by the offshore wind turbine generator is stepped up by a 35kV / 220kV offshore step-up transformer and then collected to the AC bus of the offshore converter platform through a short-distance submarine cable. The offshore S7CSC rectifies and transmits the bus electric energy and sends it to the onshore converter station through a long-distance DC submarine cable. Finally, it is sent into the onshore power grid by the receiving-end CSC. The 12-pulse S7CSC is formed by connecting two 6-pulse S7CSC valve groups through a star-delta transformer. An LC filter bank is connected to the AC side outlet of each S7CSC valve group, where the filter inductor is L0 and the filter capacitor is C, and a smoothing reactor L is equipped on the DC side. dc Different from the ordinary CSC valve group, a new bridge arm branch is connected in parallel on the valve side of the DC link of the S7CSC as a pre-switching branch, that is, S7.

[0007] The control method of the offshore wind power DC transmission system is as follows.

[0008] The S7CSC has two control degrees of freedom, namely the trigger angle α of the 6-bridge arm CSC r and the duty cycle D of the S7 bridge arm r . By adjusting these two control parameters, the converter can show more flexible and stable characteristics under different working conditions.

[0009] For an offshore converter station, considering that the stability of the offshore AC voltage amplitude and frequency depends on the real-time balance between the instantaneous power generated by the wind power system and the instantaneous power absorbed by the offshore CSC, the calculation formulas for the AC side voltage and current of the offshore converter are shown in Equation (1):

[0010]

[0011] Among them, is the phasor of the 12-pulse CSC equivalent valve outlet current, and L eq = k T 2 L / 2 is the equivalent filter inductance, and C eq = 2C / k T 2 is the equivalent filter capacitor.

[0012] The triggering angle α r and the S7 duty cycle D r of the S7CSC can be calculated by back-calculation, and the calculation formulas are as follows:

[0013]

[0014] Therefore, the offshore AC voltage frequency control method can control the on-off of the converter by back-calculating the triggering angle α r and the S7 duty cycle D r , and finally realize the stable control of the offshore AC voltage amplitude and frequency.

[0015] For the control method of the onshore CSC, in order to achieve stable power transmission and minimize the voltage fluctuation in the valve as much as possible, the DC voltage in the CSC-HVDC should be stabilized. At the same time, in order to achieve the unity power factor operation of the onshore AC power grid, the active power of the onshore CSC adopts constant DC voltage control, and the reactive power adopts reactive power control.

[0016] In order to solve the problem that when a fault occurs in the gathering submarine cable, since the V / F control method adopted by the rectifier side converter station will maintain the AC port voltage stable by increasing the port voltage, resulting in a sharp increase in the fault current, the AC bus reference value can be reduced during the fault, thereby reducing the current fed by the CSC to the fault point. The present invention proposes a corresponding step-down control method to suppress the fault current by actively stepping down, thereby preventing the fault transient overcurrent and realizing fault ride-through.

[0017] The beneficial effect of the present invention is that the offshore wind power transmission system adopting the S7CSC has the triggering angle α r and the duty cycle D rWith two degrees of control freedom, it can independently control the AC voltage and frequency at sea, is suitable for application in the scenario of transmitting offshore wind power, and can also achieve low-voltage fault ride-through to realize stable control of the offshore wind power DC transmission system. Description of the Drawings

[0018] Figure 1 is the topology structure diagram of the offshore wind power DC transmission system based on the S7 bridge-arm current source converter provided by the present invention;

[0019] Figure 2 is the modulation method diagram of the S7 bridge-arm current source converter provided by the present invention

[0020] Figure 3 is the AC voltage and frequency control method diagram of the offshore converter station provided by the present invention

[0021] Figure 4 is the control method diagram of the onshore converter station provided by the present invention. Detailed Embodiments

[0022] The preferred embodiments will be described in detail below with reference to the accompanying drawings. It should be emphasized that the following description is merely exemplary and not intended to limit the scope of the present invention and its applications.

[0023] Figure 1 is the topology structure diagram of the offshore wind power DC transmission system based on the S7 bridge-arm current source converter. As Figure 1 shown, the AC electric energy output by the offshore wind turbine is collected to the AC bus of the offshore converter platform through a short-distance submarine cable after passing through a 35kV / 220kV offshore step-up transformer. The offshore S7CSC rectifies and transmits the bus electric energy and sends it to the onshore converter station through a long-distance DC submarine cable. Finally, it is sent into the onshore power grid by the receiving-end CSC. The 12-pulse S7CSC is formed by connecting two 6-pulse S7CSC valve groups through a star-delta transformer. An LC filter bank is connected to the AC side outlet of each S7CSC valve group, where the filter inductor is L0 and the filter capacitor is C, and a smoothing reactor L is equipped on the DC side dc . Different from the ordinary CSC valve group, a new bridge-arm branch is connected in parallel on the valve side of the DC link of the S7CSC as a pre-switching branch, that is, S7.

[0024] The control method of the wind turbine is as follows: The machine-side converter of the wind turbine adopts zero d-axis current control. At the same time, in order to realize the efficient utilization of wind energy, the maximum power tracking method is adopted. The grid-side converter of the fan adopts a constant DC voltage control method to keep the DC voltage stable. In addition, the GSC also adopts a reactive power control method, and usually sets the reactive power control command value to 0.

[0025] There is often a DC-chopper installed inside a wind turbine. When the output power of the wind turbine is blocked and the DC voltage inside its back-to-back converter increases, the DC-chopper is put into operation for energy consumption. The energy-consuming resistor in the DC-chopper inside the wind turbine is usually designed to consume the rated power of the wind turbine, in order to limit the DC voltage U of the wind turbine converter dcWT <1.1 pu.

[0026] Figure 2 It is the modulation method of the S7 bridge arm current source converter and the a-phase AC current diagram. As Figure 2 shown, when S1 is normally turned on, a trigger signal for S1 is delayed by a D r angle. At this time, S1 is in the off state. In order not to let the DC current be discontinuous, a trigger signal for S7 is given at this time to make it conduct for a D r angle, as Figure 2 (b) and Figure 2 (c) shown. Since S1 is in the off state at this time, the current does not pass through S6, but directly passes through S7 to realize the current circulation. Subsequently, S7 is turned off and S1 is turned on, and the current returns to the normal conduction situation. At the moment when S2 is normally turned on, the above process is repeated, so that the conduction moment of S2 is delayed by a D r angle, and S7 continues to conduct for a D r angle, as Figure 2 (b) and Figure 2 (d) shown. Therefore, within the entire switching cycle, S7 conducts a duty cycle angle at the normal conduction moments of S1 to S6, while S1 to S6 are all delayed by a duty cycle angle to conduct. Figure 2 (e) shows the a-phase AC current under this modulation method.

[0027] The detailed control methods of each converter are as follows:

[0028] (1) For the offshore converter station, considering that the stability of the offshore AC voltage amplitude and frequency depends on the real-time balance of the instantaneous power generated by the wind power system and the instantaneous power absorbed by the offshore CSC, the control method mainly consists of three parts. For the offshore voltage controller part, the reference values u rd * and u rq * of the dq-axis components of the AC voltage are respectively compared with the measured values u rd and u rq , and the reference values i rd * and i rq * of the dq-axis components of the grid-side current are obtained through a PI controller. After obtaining i rd * and i rq , through the inner-loop current control, the dq components i rvd * and i rvq of the reference value of the equivalent valve outlet current can be obtained.*, and then inversely calculate through Equation (2) to calculate the firing angle α r and the S7 duty angle D r , so as to control the on-off of the converter, and finally realize the stable control of the amplitude and frequency of the offshore AC voltage.

[0029] (2) For the onshore converter station, in order to achieve stable power transmission and minimize the voltage fluctuation in the valve as much as possible, the DC voltage in the CSC-HVDC should be stabilized. At the same time, in order to achieve the operation of the onshore AC power grid with a unity power factor, the active power of the onshore CSC adopts constant DC voltage control, and the reactive power adopts reactive power control.

[0030] The method for the offshore collection system to ride through low voltage faults is as follows:

[0031] After a fault occurs in the offshore collection system, since the CSC using V / F control has the characteristics of a voltage source, it will keep the AC port voltage stable by increasing the line voltage of the AC bus. Therefore, the current fluctuation caused by a ±10% voltage fluctuation is used as the switching criterion for the low voltage ride-through method.

[0032] Detect the local three-phase AC current after the fault occurs. When the AC current of a certain phase exceeds 1.32Iset, start the step-down control method. When the rectifier side reduces the reference value of the AC voltage to 0.4 pu to suppress the growth of the overcurrent, and continue to detect the current at the same time. When the current continues to increase to 2Iset, it means that the step-down method cannot suppress the overcurrent and the fault is serious. Therefore, lock the converter station. If the current drops to 1.32Iset, exit the step-down method, otherwise keep running.

[0033] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those of ordinary skill in the art can still modify the specific implementation manners of the present invention or make equivalent replacements with reference to the above embodiments. Any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention pending approval.

Claims

1. A topological structure suitable for an offshore wind power DC transmission system, characterized in that: The converter is an active commutation current source converter S7CSC based on the S7 bridge arm. The offshore S7CSC rectifies and sends the bus power to the onshore converter station through a long-distance DC submarine cable. Finally, it is sent to the onshore power grid by the receiving CSC.

2. The system topology according to claim 1, characterized in that: The 12-pulse S7CSC is composed of two 6-pulse S7CSC valve groups connected through a star-delta transformer, and a new bridge arm branch is connected in parallel on the DC link valve side as a pre-switch branch, called the 7th bridge arm (S7).

3. The system topology according to claim 1, characterized in that: The AC outlet of each S7CSC valve group is connected to an LC filter group, where the filter inductor is L0, the filter capacitor is C, and a smoothing reactor L is installed on the DC side. dc .

4. The system topology according to claim 1, characterized in that: Each bridge arm of the S7CSC is cascaded by multiple fully-controlled devices that can withstand reverse voltage (such as reverse-conducting IGCT / IGBT series diode combination, reverse-resistance IGCT / IGBT).

5. The system topology according to claim 1, characterized in that: The S7CSC uses fundamental frequency modulation (FFM), the modulation frequency is the AC system frequency, and has two control degrees of freedom, namely the trigger angle α of the 6-arm CSC r And the duty angle D of the S7 bridge arm r .

6. A steady-state control method for an offshore wind power DC transmission system based on S7CSC, characterized in that: The firing angle α of S7CSC is obtained by inverse calculation of the outlet current of 12-pulse CSC equivalent valve. r and S7 duty angle D r The on and off of the converter is then controlled, ultimately achieving stable control of the offshore AC voltage amplitude and frequency. The active power of the onshore converter station is controlled by a constant DC voltage, and the reactive power is controlled by reactive power.

7. A fault ride-through method for an offshore wind power DC transmission system based on S7CSC, characterized in that: When a fault occurs in the gathering submarine cable, the S7CSC will lower the AC bus reference value, thereby reducing the current fed by the CSC to the fault point, suppressing the fault current, thereby preventing transient overcurrent and achieving fault ride-through.

Images