Three-phase four-bridge-arm flexible interconnection device and switching control method

Abstract

The invention relates to a three-phase four-bridge-arm flexible interconnection device and a switching control method, the three-phase four-bridge-arm flexible interconnection device in a back-to-back design is adopted, a main converter station adopts UdcQ control, a slave converter station adopts transient damping VSG control, a power difference value is obtained through a power instruction value and a feedback value of the slave converter station, and the three-phase four-bridge-arm flexible interconnection device and the switching control method are used for switching the three-phase four-bridge-arm flexible interconnection device. An engine power angle is obtained through a rotor mechanical equation, a function relation between the power angle and inverter output is obtained based on a linearization model, a VSG active loop closed-loop transfer function is obtained, a VSG signal model is established, and output power is controlled through an input power instruction and an input system frequency of the VSG signal model. When the operation mode of the flexible interconnection device is switched, smooth transition of voltage and current on the direct current side can be achieved, and inertia and damping support can be provided for a power grid during steady-state operation. Compared with the prior art, the method has the advantages of high stability, high reliability and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of power system applications, and in particular to a three-phase four-bridge arm flexible interconnection device and a switching control method. Background Art

[0002] In recent years, with the rapid development of renewable energy and the growth of electricity demand, flexible interconnection devices have gradually attracted attention. As one of the important topologies, the three-phase four-leg inverter is widely used because it can achieve higher flexibility and efficiency.

[0003] Compared with the traditional three-phase three-bridge-arm structure, the three-phase four-bridge-arm inverter improves the control capability of unbalanced load and the stability of the system by adding a bridge arm. This design can effectively use the surplus current and enhance the regulation capability of the DC side voltage, thereby achieving smooth switching control in grid operation.

[0004] In actual operation, how to achieve smooth switching control on the DC side is an important issue. Especially when a fault occurs or the load changes, the traditional control strategy often cannot respond quickly, resulting in voltage fluctuations and damage to the equipment. Therefore, it is particularly important to develop new control strategies to improve the dynamic response capability and steady-state performance of the system.

[0005] In recent years, a variety of control strategies have been proposed to address this problem, such as methods based on the combination of deadbeat control and repetitive control to improve control accuracy and robustness. These methods can effectively improve the performance of the system under unbalanced loads and ensure the safe and stable operation of power electronic equipment by stabilizing the DC bus voltage when the power grid changes.

[0006] A major feature of the flexible interconnection device is the flexibility of its operation mode. When a fault occurs in the power system connected to the flexible interconnection device, it can maintain the normal operation of the power grid in the non-fault area by coordinating the operating status of each port and flexibly switching the working mode, and quickly restore the power supply of the key load in the fault area, providing a guarantee for the stable operation of the power system. However, when the flexible interconnection device needs to switch modes due to a line fault, it often has a certain communication delay. During the delay period, the working state of the flexible interconnection device has changed with the line fault, and its control strategy cannot be switched in time because the instructions of the energy management system have not yet been received. Therefore, the stable operation of the power system cannot be guaranteed during the delay period; at the same time, after the communication system delay, the switching of the control strategy of the flexible interconnection device may also cause a certain power shock to the power line. Therefore, the smooth switching of the operating mode of the flexible interconnection device is crucial to the stable operation of the power system, and the research on the smooth switching strategy of the operating mode of the flexible interconnection device is imminent. Summary of the invention

[0007] The purpose of the present invention is to provide a three-phase four-bridge arm flexible interconnection device and a switching control method in order to overcome the defects of the above-mentioned prior art.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] A three-phase four-bridge-arm flexible interconnection device, the flexible interconnection device is composed of two three-phase four-bridge-arm converters connected back to back, the three-phase four-bridge-arm converter is a two-level converter, each bridge arm of the three-phase four-bridge-arm converter is composed of two switches, the bridge arms are respectively connected to the A phase, B phase, C phase and N phase of the corresponding three-phase power grid, and the two three-phase four-bridge-arm converters are respectively used as a master converter station and a slave converter station.

[0010] A second aspect of the present invention is a switching control method for a three-phase four-bridge-arm flexible interconnection device. Based on the above three-phase four-bridge-arm flexible interconnection device, the switching control method comprises the following steps:

[0011] The main converter station adopts U dc Q control strategy, using transient damping VSG control strategy from the converter station;

[0012] The transient damping VSG control strategy includes: based on the line active power formula of the traditional grid-connected inverter, the power difference is obtained from the power command value and feedback value of the converter station, the engine power angle is obtained through the rotor mechanical equation, and the functional relationship between the power angle and the inverter output is obtained based on the linearization model. The VSG active loop closed-loop transfer function is obtained, and a VSG signal model is established. The output power is controlled by the input power command and input system frequency of the VSG signal model to realize transient damping VSG control.

[0013] Furthermore, the transient damping VSG control includes a power loop, a voltage loop and a current loop. The output of the power loop is given by the voltage loop, and the output of the voltage loop is given by the current loop. When the DC voltage is stable, the power loop given value is within the upper and lower limits of the voltage loop amplitude. When the DC voltage exceeds the upper and lower limits of the voltage loop amplitude, the DC voltage is maintained stable through the voltage loop.

[0014] Furthermore, the calculation expression of the closed-loop transfer function of the VSG active loop is:

[0015]

[0016] Among them, P ref and P represent the power base and output power of VSG respectively; J is the virtual moment of inertia; D is the damping coefficient; D p is the virtual damping coefficient of VSG; E is the amplitude of the virtual internal potential of VSG; U g is the voltage amplitude at the common coupling point of the power grid; X fis the line impedance; ω0 is the rated angular frequency of VSG.

[0017] Furthermore, the calculation expression of the split form of the VSG signal model is:

[0018] K=(EU g ) / X f

[0019]

[0020] In the formula, ΔP e =P / P ref is the ratio of active power to active reference, ΔP1 is the part related only to the active reference change rate, and ΔP2 is the part related only to the frequency change rate; ΔP ref is the active reference change rate, and Δw is the frequency change rate.

[0021] Furthermore, in the split form of the VSG signal model, the calculation expression of the steady-state error of ΔP2 is:

[0022]

[0023] Where E is the steady-state error.

[0024] Furthermore, the control switching method includes a single-end power supply mode: when one of the main converter station power grid or the slave converter station power grid fails, power is supplied through the AC power grid at the other end. At this time, the flexible interconnection device adopts U dc Q control.

[0025] Furthermore, the control switching method includes a double-end power supply mode: jointly supplying power to the DC side grid and the respective AC side loads, at which time one end of the two-end grid is U dc Q control controls the DC bus voltage; one end is PQ control to adjust the power.

[0026] Furthermore, the control switching method includes a double-end isolation mode: when the two power grids are not connected, both ends use U dc Q control.

[0027] Furthermore, the control switching method includes a reactive power compensation mode: when the DC grid transmits reactive power to the AC grids on both sides for reactive power compensation, the main converter station is U dc Q control controls the DC bus voltage; the slave converter station is used for transient damping VSG control to compensate for the reactive power of the power grid.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1) The present invention adopts U dcQ is used as the control mode of the master converter station. The slave converter station adopts the virtual synchronization strategy based on transient damping. When the grid dispatching system fails to adjust the slave converter station in time or the adjustment value exceeds the allowable range, the DC bus voltage is limited to the preset range, so that the slave converter station works at the upper / lower limit of the DC voltage U dc Q control mode is used to maintain the DC bus voltage within the range consistent with grid operation and improve system stability.

[0030] 2) The present invention supports switching control under various working conditions, and can realize smooth switching of the DC side of the flexible interconnection device during transient operation. At the same time, it can provide inertia and damping support to the power grid during stable operation, thereby improving the stability of power grid operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is an overall structural diagram of a three-phase four-bridge-arm flexible interconnection device of the system of the present invention;

[0032] Figure 2 It is the main converter station U of the system of the present invention. dc Q control mode;

[0033] Figure 3 It is a virtual synchronization strategy control mode based on transient damping of the slave circulation station of the system of the present invention;

[0034] Figure 4 It is a comparison of the DC bus voltage waveform before and after the control strategy of the system of the present invention is improved;

[0035] Figure 5 It is a comparison of the DC side current waveform before and after the control strategy of the system of the present invention is improved;

[0036] Figure 6 It is the active waveform of the flexible interconnection device when the active power of the system of the present invention steps;

[0037] Figure 7 It is the active waveform of the flexible interconnection device when the grid frequency of the system of the present invention decreases. DETAILED DESCRIPTION

[0038] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0039] Example 1

[0040] The present invention is a three-phase four-bridge arm flexible interconnection device, such as Figure 1As shown, the flexible interconnection device is composed of two three-phase four-bridge-arm converters connected back to back. The three-phase four-bridge-arm converter is a two-level converter. Each bridge arm of the three-phase four-bridge-arm converter is composed of two switches. The bridge arms are respectively connected to the corresponding A phase, B phase, C phase and N phase of the three-phase power grid. The two three-phase four-bridge-arm converters serve as the main converter station and the slave converter station respectively.

[0041] Based on the three-phase four-bridge-arm flexible interconnection device of the present invention, the following switching control method is also proposed:

[0042] like Figure 2 As shown, the main converter station of the system of the present invention adopts U dc Q control method is used to stabilize the voltage of the DC bus.

[0043] Introduce VSG control into the traditional control strategy, and analyze the small signal model of VSG. The small signal model of VSG simplifies the control model of VSG into a transfer function through the principle of automatic control. After that, it is not necessary to establish the real control model for analyzing the VSG system, but only to analyze its small signal model. Of course, adding the small signal model of VSG to the traditional control link can also make the control link have VSG characteristics. The idea of ​​VSG small signal modeling is: first, △P is obtained from the power command value and the feedback value, and then the generator power angle δ is obtained through the rotor mechanical equation. Then, based on the linearization model, a relationship between the power angle and the inverter output is obtained, and they are approximately simplified into a proportional function, so that there is a linear relationship between δ and Pe. After simplification step by step, the small signal model of VSG can be obtained. The specific calculation process is as follows:

[0044] The line active power formula of the traditional grid-connected inverter is as follows:

[0045]

[0046] Among them, E0 is the inverter electromotive force amplitude, U g is the voltage amplitude at the common coupling point PCC of the power grid, Z is the line impedance, α is the impedance angle of the filter circuit, it is generally believed that α=π / 2, the generator power angle δ0, so the above formula is simplified to:

[0047]

[0048] Since α≈π / 2, X>>R, so we can assume that Z=X f , further simplify the above formula:

[0049]

[0050] Among them, X f is the line inductive reactance.

[0051] Because δ approaches 0 when the synchronous generator operates normally, sinδ≈δ, and we have:

[0052]

[0053] From the simplified control of VSG active loop, the closed-loop transfer function of VSG active loop can be obtained as follows:

[0054]

[0055] Among them, P ref and P represent the power base and output power of VSG respectively; J is the virtual moment of inertia; D is the damping coefficient; D p is the virtual damping coefficient of VSG; E is the amplitude of the virtual internal potential of VSG; Ug is the voltage amplitude at the common coupling point of the power grid; ω0 is the rated angular frequency of VSG.

[0056] make D p Replace with k f +D, then:

[0057]

[0058] Formula (6) is the small signal model of VSG.

[0059] An in-depth analysis of the VSG small signal model shows that its output power is determined by two input channels, namely the power command and the system frequency. Therefore, the small signal model of the VSG shown in equation (6) can be decomposed as shown in equation (7):

[0060]

[0061] By analyzing ΔP2 in the above formula, the steady-state error of ΔP2 is obtained as shown in formula (8).

[0062]

[0063] When the moment of inertia is fixed, as D increases, the system damping increases, and the larger the moment of inertia, the smaller the system damping. At the same time, the damping coefficient is not large and cannot eliminate system oscillations. Therefore, the damping coefficient should be larger. However, from the analysis of the steady-state error shown in formula (8), it can be found that if the damping coefficient D is large, it will inevitably bring a certain steady-state error to the system and disrupt the primary frequency modulation plan.

[0064] The control strategy of the present invention includes a power loop, a voltage loop and a current loop. The output of the power loop is given by the voltage loop, and the output of the voltage loop is given by the current loop. Figure 3As shown in the figure, when the DC voltage is stable, the power loop set value is within the upper and lower limits of the voltage loop amplitude, so the power loop works to distribute power. When the main converter station is not working, the DC voltage is unstable. When the DC voltage exceeds the upper and lower limits of the voltage loop amplitude, the power loop does not work, and the voltage loop maintains the stability of the DC voltage.

[0065] The main converter station adopts U dc Q control, the purpose is to control the DC bus voltage at a constant value, to provide guarantee for the slave power control. The slave converter station adopts the VSG control based on transient damping proposed by the present invention, which is composed of a virtual synchronous machine transient damping small signal model, a voltage loop with voltage limiting, and a current loop. The part of the strategy excluding the VSG loop can be regarded as a U with voltage limiting. dc Q control, the voltage upper and lower limits are usually set according to the allowable fluctuation range of the line voltage, and the adjustable power range is determined by the allowable fluctuation range of the voltage. The wider the voltage upper and lower limits are set, the larger the adjustable power range is. Usually, the voltage upper limit is set to 105% of the rated value of the DC bus voltage, and the lower limit is set to 95% of the rated value of the DC bus voltage.

[0066] When the flexible interconnection device using traditional master-slave control switches the operation mode under fault conditions, the DC side will be subjected to a large impact current, damaging the power line electrical equipment. In addition, the operation mode switching is performed by the power grid communication system in a timely manner, which will also cause certain pressure on the power grid communication system. In order to solve this problem and improve the control effect of the control strategy of the flexible interconnection device, a transient damping virtual synchronization control strategy based on formula (7) is proposed in the present invention. The control strategy is as follows: Figure 3 As shown in the figure, this strategy can provide stable support for the DC bus voltage when the operating mode of the flexible interconnected device is switched. At the same time, there is no need to frequently switch the control strategy like traditional control, so as to achieve stable control of the flexible interconnected device in each operating mode, reduce the pressure on the power grid communication system, and reduce the dependence on the EMS energy management system. At the same time, when the flexible interconnected device is working normally, the control strategy has the dynamic characteristics of VSG, which can provide inertia and damping support to the power grid, which is conducive to the stable operation of the power grid.

[0067] The system active power reference value is set to 10kW, the DC bus voltage reference value is set to 700V, the upper and lower limits of the DC bus voltage fluctuation are set to 735V and 665V (±5%), and the flexible interconnection device is set to operate in the double-end power supply mode at the beginning of the simulation. At 2s, the DC bus fails and the flexible interconnection device switches to the double-end isolation operation mode. The control group is set to the traditional control strategy: at 0-2s, the main converter station U dc Q control is controlled by the slave converter station PQ, and switches to the main converter station U after 2s. dc Q controls the output of the converter station U dcQ control. Considering the time required for system communication, the communication delay is uniformly set to 100ms, and the DC bus voltage is obtained as follows Figure 4 As shown, the DC side current is Figure 5 shown.

[0068] observe Figure 4 and 5 It can be found that when the flexible interconnection device using the traditional control strategy needs to switch the control strategy due to system failure, there will be large voltage fluctuations and impact currents, which will damage the electrical equipment on the power line. However, the flexible interconnection device using the virtual synchronous control strategy based on transient damping can stabilize the DC bus voltage at the set lower limit (or upper limit) of the voltage allowed to fluctuate at the moment of disconnection on the DC side, and the voltage transition process is relatively smooth. Although the DC bus voltage does not operate at the rated value, it also operates within the allowable fluctuation range, which can ensure that the normal operation of the load and the electrical equipment on the line is not affected; and the flexible interconnection device using the strategy proposed by the present invention does not switch the control strategy, so the DC side current does not generate impact current and can smoothly transition to a new steady state. It is verified that the proposed strategy can achieve smooth switching of the DC side of the flexible interconnection device, and can maintain stable operation of the power grid during the switching process, thereby improving the stability of the power grid.

[0069] In order to verify the superiority of the strategy proposed in this invention in power regulation, it is assumed that the flexible interconnection device is out of operation from the power grid of the converter station, and carries an AC load of 10kW from the converter station. The active power reference value is set to 10kW. At 5s, the active power command is adjusted to 12kW, and the active power output waveform of the flexible interconnection device is obtained as follows: Figure 6 shown.

[0070] Depend on Figure 6 It can be seen that the strategy proposed in the present invention can not only effectively follow the power command, but also has the characteristics of inertia damping. The active transition process is smooth. Although there is a small amount of overshoot, it is within the allowable fluctuation range and can quickly return to stability. It can be seen that the strategy proposed in the present invention can enable the flexible interconnection device to provide inertia and damping support for the connected power grid.

[0071] In order to verify the superiority of the strategy proposed in this invention in supporting the stability of the power grid, the flexible interconnection device is assumed to operate in the double-end power supply mode, the active reference value is set to 10kW, and at 5s, the power grid frequency from the converter station drops by 0.1Hz, and the active power output waveform of the flexible interconnection device is obtained as follows: Figure 7 Observation Figure 7 It can be seen that when the power grid frequency from the converter station drops, the flexible interconnection device responds immediately and increases power to make up for the active power shortage. It can be seen that the strategy proposed in the present invention also has the primary frequency regulation characteristics of VSG and can provide stable support for the power grid.

[0072] The present invention also includes four control modes under different working conditions, namely single-end power supply mode, double-end power supply mode, double-end isolation mode and reactive power compensation mode.

[0073] The single-ended power supply mode means that one end of the power grid is out of operation due to a fault or maintenance, and the DC side load is powered by the other end of the power grid. At this time, the flexible interconnection device must work in U dc Q-mode.

[0074] The power grid dispatching system fails to monitor in time or cannot adjust the slave power reference value in time due to loss of communication, or the adjusted power reference value exceeds the power adjustment range allowed by the upper and lower limits of the DC voltage, resulting in the slave output power being unable to track the reference value and the flexible interconnection device losing the ability to distribute power. Even in this case, the virtual synchronous control based on transient damping can limit the DC bus voltage to the preset upper and lower limits, so that the slave converter station can work at the upper / lower limit of the DC voltage U dc Q control mode is used to maintain the DC bus voltage within the range that is consistent with grid operation.

[0075] The double-end power supply mode means that the two ends of the power grid are interconnected through a flexible interconnection device to jointly supply power to the DC side power grid and their respective AC side loads. At this time, the flexible interconnection device plays a role in power allocation for the two ends of the power grid. At this time, one end of the two ends of the power grid is U dc Q control controls the DC bus voltage; one end is PQ control to adjust the power.

[0076] The double-end isolation mode means that the DC side of the flexible interconnection device connecting the two ends of the power grid is disconnected, and the two ends of the power grid are each a system, independently supplying power to their own loads. dc Q control.

[0077] The reactive power compensation mode refers to the DC power grid connected by the flexible interconnection device transmitting reactive power to the AC power grids on both sides for reactive power compensation.

[0078] When switching from double-ended power supply to double-ended isolation operation or single-ended power supply, the traditional control strategy needs to switch the converter station control strategy from PQ control to U dc Q control, which may generate a large impact current due to the switching of the operating mode. In this case, the strategy proposed by the present invention does not need to change the control mode. The transient damping VSG control will stabilize the DC bus voltage within the range allowed by normal operation due to the upper and lower limits of the fixed DC voltage when the operating mode changes. When the control strategy is switched from double-end isolation operation or single-end power supply to double-end power supply, the traditional control strategy needs to change the converter station control strategy from U dcQ control switches to PQ control, and the strategy proposed in the present invention does not need to change the control strategy in this case, and will become a transient damping VSG control mode because the DC voltage upper limit / fixed DC voltage lower limit control does not work.

[0079] In summary, the virtual synchronous control strategy based on transient damping proposed in the present invention can make the voltage and current transition smoothly when the flexible interconnected device switches on the DC side due to transient events, and can realize smooth switching of the operation mode of the flexible interconnected device without switching the control strategy. At the same time, when the flexible interconnected device is in steady-state operation, it provides inertia and damping support for the power grid, so that the flexible interconnected device can realize flexible switching of the distribution network while providing strong support for the stable operation of the power grid. The virtual synchronous control strategy based on transient damping proposed in the present invention is effective and has certain advantages in the stable operation of the power grid.

[0080] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A three-phase four-bridge arm flexible interconnection device, characterized in that: The flexible interconnection device includes two three-phase four-bridge-arm converters connected back to back, the three-phase four-bridge-arm converter is a two-level converter, each bridge arm of the three-phase four-bridge-arm converter includes two switches, the bridge arms are respectively connected to the A phase, B phase, C phase and N phase of the corresponding three-phase power grid, and the two three-phase four-bridge-arm converters serve as the main converter station and the slave converter station respectively.

2. A switching control method for a three-phase four-bridge arm flexible interconnection device, characterized in that: Based on the three-phase four-bridge-arm flexible interconnection device according to claim 1, the switching control method comprises the following steps: The main converter station adopts U dc Q control strategy, the slave converter station adopts transient damping VSG control strategy; The transient damping VSG control strategy includes: based on the line active power formula of the traditional grid-connected inverter, obtaining the power difference from the power command value and feedback value of the converter station, obtaining the engine power angle through the rotor mechanical equation, obtaining the functional relationship between the power angle and the inverter output based on the linearization model, obtaining the VSG active loop closed-loop transfer function, and establishing a VSG signal model, controlling the output power through the input power command and input system frequency of the VSG signal model, and realizing transient damping VSG control.

3. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2 is characterized in that: The transient damping VSG control includes a power loop, a voltage loop and a current loop. The output of the power loop is the given value of the voltage loop, and the output of the voltage loop is the given value of the current loop. When the DC voltage is stable, the power loop given value is within the upper and lower limits of the voltage loop amplitude. When the DC voltage exceeds the upper and lower limits of the voltage loop amplitude, the DC voltage is maintained stable through the voltage loop.

4. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2 is characterized in that: The calculation expression of the VSG active loop closed-loop transfer function is: Among them, P ref and P represent the power base and output power of VSG respectively; J is the virtual moment of inertia; D is the damping coefficient; D p is the virtual damping coefficient of VSG; E is the amplitude of the virtual internal potential of VSG; U g is the voltage amplitude at the common coupling point of the power grid; X f is the line impedance; ω0 is the rated angular frequency of VSG.

5. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 4 is characterized in that: The calculation expression of the split form of the VSG signal model is: In the formula, ΔP e =P / P ref is the ratio of active power to active reference, ΔP1 is the part related only to the active reference change rate, and ΔP2 is the part related only to the frequency change rate; ΔP ref is the active reference change rate, and Δw is the frequency change rate.

6. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 5, characterized in that: In the split form of the VSG signal model, the calculation expression of the steady-state error of ΔP2 is: Where E is the steady-state error.

7. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2, characterized in that: The control switching method includes a single-end power supply mode: when one of the main converter station power grid or the slave converter station power grid fails, power is supplied through the AC power grid at the other end. At this time, the flexible interconnection device adopts U dc Q control.

8. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2 is characterized in that: The control switching method includes a double-end power supply mode: jointly supplying power to the DC side grid and the respective AC side loads, at which time one end of the two-end grid is U dc Q control controls the DC bus voltage; one end is PQ control to adjust the power.

9. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2, characterized in that: The control switching method includes a double-end isolation mode: when the two power grids are not connected, both ends use U dc Q control.

10. The switching control method of a three-phase four-bridge-arm flexible interconnection device according to claim 2, characterized in that: The control switching method includes a reactive power compensation mode: when the DC power grid transmits reactive power to the AC power grids on both sides for reactive power compensation, the main converter station is U dc Q control controls the DC bus voltage; the slave converter station is used for transient damping VSG control to compensate for the reactive power of the power grid.

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