Research on operation characteristics and control strategy of hybrid multi-feed HVDC system

Flexible DC transmission technology based on voltage source converter (VSC-HVDC) and high voltage DC transmission technology based on grid commutation converter (LCC-HVDC) have complementary characteristics, so the hybrid DC technology combined with LCC-HVDC and VSC-HVDC can make them complement each other and become the development trend of power grid technology in the future. In the existing hybrid multi-infeed DC system, conventional DC transmission (LCC-HVDC) and flexible DC transmission (VSC-HVDC) operate independently and are not coordinated with each other, but there is a blank in the research on the protection coordination relationship between VSC/LCC parallel DC hybrid system and its near area AC system. Therefore, it is necessary to theoretically analyse and study the operation characteristics, control strategy and protection coordination relationship between near-area AC systems of hybrid multi-infeed HVDC systems. Due to the limited space, this paper mainly analyses the operation characteristics and control strategy of hybrid multi-infeed HVDC system, and verifies the correctness of the theory through simulation research. In the next step, the corresponding protection is configured in the above simulation model, and the protection adaptability analysis and protection optimization scheme of hybrid multi-infeed HVDC system are obtained.


Introduction
Due to the increasing depletion of traditional primary energy and the increasing requirements of various countries for environmental protection, the demand for photovoltaic wind power, and other new energy generation is in-creasing.New energy generation has special dynamic characteristics.When AC technology and traditional DC technology are used to connect to the power grid, there are some technical defects such as poor power quality and lack of flexibility [1][2][3][4][5].With the continuous development of power electronics technology, the problems of new energy power generation access, transmission and absorption are becoming more and more prominent.The flexible DC transmission technology using voltage converter can solve this problem well [6][7][8][9][10][11], but there are many powers electronic devices in VSC-HVDC, so it has some problems such as high loss, high investment cost, complex control and so on.The HVDC transmission technology of grid commutation (LCC-HVDC) has the advantages of mature technology, large transmission capacity, narrow transmission corridor, effective elimination of DC fault current and so on.However, the inverter station using LCC as converter is prone to commutation failure, resulting in transmission power interruption, and will consume a lot of reactive power in the operation process, unable to supply power to the passive AC system and other problems.The advantages and disadvantages of VSC-HVDC and LCC-HVDC are complementary, so the hybrid DC technology that combines the two can make them complement each other and become the development trend of power grid technology in the future.
With the operation of a back-to-back networking project, a VSC/LCC DC parallel hybrid system structure is passively formed between an inverter station and a rectifier station.This is the first case of State Grid Corporation, but considering the characteristics of relatively concentrated power supply and load in our country, this system structure is also the inevitable result of the development of HVDC technology in our country.In the existing hybrid multi-feed HVDC system, conventional HVDC and flexible HVDC operate independently and are not coordinated, so there is still a gap in the research on the protection cooperation relationship between the hybrid multi-feed HVDC system formed by them and its neighboring AC system.Therefore, it is urgent to carry out the research on the operation characteristics, control strategy and protection coordination of the hybrid multi-infeed HVDC system.Due to the limited space, this paper mainly introduces the control strategy of hybrid multi-infeed DC system.By establishing the control system model of VSC-HVDC and LCC-HVDC subsystems, combined with the structure and operating parameters of the actual hybrid multi-feed DC system, the operation characteristics of each subsystem are simulated and analyzed, and the correctness of the hybrid multi-feed DC system control strategy is verified.

LCC-HVDC subsystem control system model
The HVDC control system consists of three layers: the valve control layer, the pole control layer and the main control layer, as shown in Figure 1.The first layer is called the main control layer, also known as the bipolar control layer, which obtains the DC transmission power instruction value (P set ) from the control center.After the analysis and calculation of the relevant control strategies, it is determined that the DC current instruction (I set ) is sent to the polar control layer (level 2).The polar control level determines the trigger angle instruction (α ord ) through the control operation and sends it to each valve group control unit (level 3 valve control layer).In this structure, in order to ensure that the bottom control can match and track the high-level control instructions and make the whole control system run smoothly, the three levels have different response time, and the higher the control level is, the slower the response speed is.

VSC-HVDC subsystem control system model
The control system of VSC-HVDC is the system level control, converter station level control and bottom control according to function from high to low, as shown in Figure 2. The system-level control mainly receives the active and reactive set values of the power system dispatching center, and calculates the active and reactive command values of the converter station level control.Its main function is that the calculated instruction value can maintain the active power balance and voltage stability of the hybrid multi-feed system and ensure the continuous and stable operation of the system.The active and reactive command values of the system are controlled by the level control receiving system of the converter station, and the calculated modulation ratio M and phase angle α provide reference values for the pulse trigger control of the bottom control.The bottom control receives the M and α calculated by the converter station level control, and generates the PWM trigger pulse through the corresponding modulation mode, and finally realizes the control of the switch device.

Hybrid HVDC system model
Back-to-back networking schemes form a hybrid multi-feed DC system on the rectifier side.A typical topology of a hybrid multifed HVDC system is shown in Figure 3 below.The VSC-HVDC collects the power directly to the commutation bus of the LCC-HVDC rectifier station, and the LCC-HVDC sends the power to the load center.The characteristic of this topology is that VSC-HVDC and LCC-HVDC converter stations are connected to the same power grid, the electrical distance is relatively close, and may even be connected to the same commutation bus, the strength of the power grid at the sending end is not high, the moment of inertia is low, and the LCC-HVDC commutation failure has a great impact on the power grid at the sending end.But at the same time, because of the close electrical distance between VSC-HVDC and LCC-HVDC, VSC-HVDC can provide reactive power support for LCC-HVDC and improve the operating conditions of LCC-HVDC.The simulation model of hybrid multi-feed DC system is shown in Figure 3 above, and the simulation software is PSCAD and EMTDC.The VSC-HVDC inverter station and the LCC-HVDC rectifier station are connected by a 20 km long tie line.The simulation parameters are shown in Table 1.VSC-HVDC adopts double closed-loop current vector control system, which can decouple the active power and reactive power based on dq coordinate system.These controls include phase-locked synchronization, inner loop controller, outer loop controller and trigger pulse generator.VSC-HVDC rectifier station and inverter station adopt constant DC voltage control and constant active power control respectively.LCC-HVDC rectifier station and inverter station adopt constant power control and constant DC voltage control respectively.Through these controls, the reference value of active power and DC voltage is calculated to obtain the reference value of DC current, and the command of triggering Angle is obtained through current regulation.Due to the frequent switching of filters in the LCC-HVDC system, the cost and expense of the equipment will be increased.Therefore, a reactive power coordination control strategy is proposed according to the advantages of hybrid multi-feed system.The specific method is as follows.

Research on reactive power coordinated control strategy
3.2.1.Reactive power coordination control module.The flexible DC system can realize fast and continuous adjustment of reactive power, but the adjustable capacity is small, while the filter can be adjusted with large capacity, but it can not realize fast and continuous adjustment of reactive power.The reactive power system of hybrid multi-feed system can be adjusted by the above two ways, so that the advantages of flexible DC can be fully utilized to reduce the reactive power mutation and voltage fluctuation caused by the filter switching.
The reactive power coordination control chart is shown in Figure 4.The Q EX_m in the figure is the reactive power between the grid and the conventional rectifier station.When the Q EX_m is positive, the grid injects reactive power into the conventional rectifier station, and vice versa.Q EX_ref_H is the upper limit of the dead zone of reactive power coordinated control, and Q EX_ref_L is the lower limit of the dead zone of reactive power coordinated control.Q VSC_MAX is the maximum value of reactive power of VSC inverter station and Q VSC_MIN is its minimum value.Q VSC_ref is the additional reactive power compensation value of the fixed reactive power control link in the outer loop of the VSC-HVDC inverter station.Its value is positive to indicate the reactive power generated and negative to indicate the absorption of reactive power.The measured reactive power exchanged in the AC/DC system is used to simulate the communication delay of the first order inertial link with the proportional constant G=1.0 and the time constant T=0.01s.The steps of the reactive power coordination control policy are as follows: (1) The filter does not run At this time, the main thing is to let the VSC-HVDC system to adjust the reactive power.When the value of Q EX_m exceeds the range of [Q EX_ref_H , Q EX_ref_L ], the deviation value is converted into reactive power compensation value, and the reactive power of VSC-HVDC inverter station is controlled by fixed reactive power control link in the outer loop.
(2) give priority to the reactive power of the filter.At this time, the filter of LCC-HVDC is mainly switched on and off, and the reactive power of VSC-HVDC inverter station is put into the process of switching, so that the sudden change of reactive power and voltage fluctuation can be suppressed.Considering the switching period of the filter, when the reactive power reaches Q VSC_MAX or Q VSC_MIN and the 500ms persists, and the Q EX_m exceeds [Q filter_L , Q filter_H ], a command is issued to switch the filter.
Because the reactive power of the system will inevitably fluctuate in the process of operation, in order to avoid oscillation at a certain critical point of reactive power, the dead zone of reactive power control is added in this paper, so we only need to measure the value of Q EX_m .In addition, the PI controller improves the reactive power response speed and has a filter switching criterion, which can solve the reactive power control conflict between VSC-HVDC and LCC-HVDC.

Comparative analysis of reactive power coordinated control.
In the actual operation of the LCC-HVDC converter station, the more active power is transmitted, the more reactive power will be consumed.It is generally believed that 40% to 60% of the active power is the reactive power consumed.The LCC-HVDC uses a 12-pulse converter valve, and the reactive power consumed by Q LCC is shown in Formula (1).Where I d is the DC current, μ is the commutation angle and U dr0 is the ideal no-load DC voltage.
Figures 5-6 shows the transient reactive power response process of constant voltage control and reactive power coordinated control in VSC-HVDC inverter station when LCC-HVDC transmission power changes.In the figure, I dref is the DC current setting value, U r is the commutation bus voltage on the rectifier side of LCC-HVDC, and U ref is the voltage setting value for fixed AC voltage control.For the VSC-HVDC inverter station, the AC voltage is mainly controlled, and the reactive power is compensated only when the rectifier side converter bus voltage deviates from the set value.When the reactive power coordinated control is adopted, if the reactive power exchanged by the AC / DC system exceeds a certain range, the reactive power will be compensated immediately, so that the reactive power can be compensated without voltage change, and the reactive power response speed can be improved in a short time.Transient low voltage and transient overvoltage are better suppressed.It should be noted that the communication delay between hybrid multi-feed systems is not taken into account in the above analysis, because the electrical distance of this system is close, and the communication delay is small.If the electrical distance between the conventional rectifier station and the flexible inverter station in the hybrid multi-feed system is too long, the communication delay will be long, and the response speed of reactive power control will be lower than that of AC constant voltage control.

Simulation under normal operating conditions
In the hybrid multi-feed system model in this paper.The active power of the flexible DC system is controlled by constant DC voltage, the U dcref value is set to 420kV, the reactive power control is controlled by constant active power, and the P sref value is set to 1000MW and the Q sref value is set to 0Mvar.In the flexible straight system, the reactive power control on the inverter side adopts constant power control, in which Q sref is set to 0Mvar and γ ref is set to 15 °.LCC-HVDC system, and DC current instruction value I ref =1.0 (p.u.).
The parameter curve of the hybrid multi-feed system under rated conditions is shown in Figure 7.As shown in Figure 7, under rated operating conditions, the AC / DC voltage, active power and reactive power of the hybrid multi-feed system can operate stably at the rated value.The active power received by the hybrid multi-feed system from one AC side is about 1519MW, while the active power sent to the other side is about 1000MW.The active power loss has reached more than 30%.The reason for such a large active power loss is caused by converters and DC cables.The reactive power output of both the sending end and the receiving end of the flexible system is about 0. In this process, the DC voltage at the receiving end of the flexible system is lower than that at the sending end.For example, the receiving voltage of the flexible system is about 396.3kV, while the voltage at the sending end is about 420.3kV, which is caused by the resistance consumption in the cable.As shown in Figure 8, in the hybrid multi-feed DC system shown in this paper, the system can operate stably under the rated active power, the DC voltage is stable, and the effective control of the system is realized.Like the flexible DC system, due to the active power loss of the transmission line and converter, the absorbed active power is greater than the transmitted active power.The reactive power consumed by the rectifier side of the conventional DC system is about 1700Mvar and the reactive power consumed by the inverter side is about 1580Mvar.Compared with the actual operating system, in the case of rated operating conditions, the error between the reactive power value calculated by conventional DC system simulation and the actual system is less than 2%.The simulation results show that when a single-phase fault occurs in the system, after the fault is removed, the system can return to the rated running state before the fault, and the recoverability of the model is verified.

Steady-state reactive power coordinated control strategy
In the conventional DC system, due to the frequent switching of the filter, the service life of the filter will be reduced and the loss will be increased.In order to solve this problem, according to the advantages of hybrid multi-feed DC system, this paper proposes a control strategy of reactive power mutual compensation, which uses flexible DC to compensate reactive power quickly, which reduces the frequent switching of filter and reduces the project cost.Through the comparison of overload (1.1 times load) and low load (90% load), the simulation verifies the feasibility of the proposed reactive power control strategy.Due to the limitation of space, this paper introduces the validity verification of LCC-HVDC with different transmission power.
When the LCC-HVDC system runs under overload, it will increase the reactive power consumption of its rectifier station, which requires reactive power compensation to restrain the reduction of converter bus voltage, and vice versa.
In order to better evaluate the proposed reactive power coordinated control strategy, this paper compares the two parameters: the number of filters and the error ratio ∆ (Formula (2)) of operating voltage to rated voltage.The smaller the error between the operating voltage and the rated voltage, the better the effect of the proposed control strategy.
Among them, U ac is the instantaneous voltage of the commutation bus.U ac_R is the rated voltage of the converter bus.When the standard unit value of the transmission power changes, the switch number of the filter and the change of the ∆ are shown in Table 2.As can be seen from Table 2, the reactive power control strategy proposed in this paper can indeed reduce the switching of the filter, at the same time, it can restrain the transient low voltage and high voltage, and make the voltage fluctuation more slowly.Compared with the constant AC voltage, the ∆ of reactive power coordinated control is larger when the load fluctuation range is 10% to 80%, but it is smaller under other operating conditions.Generally speaking, reactive power coordinated control is slightly better than constant AC voltage control in reactive power response and transient low voltage and transient high voltage control, and reactive power coordinated control is obviously better than constant AC voltage control in restraining the sudden change of reactive power and slowing down voltage fluctuation.

Summary
This paper briefly introduces the model and control principle of conventional DC system and flexible DC system, and takes a back-to-back networking project as the research object to simulate the hybrid multi-feed DC system.The simulation results show that the simulation model and reactive power control strategy are correct.On this basis, we will apply the suitable scheme to the protection configuration of the hybrid multi-fed DC system, and can be realized in the simulation model, so as to carry on the next step of research, and get the optimal protection scheme suitable for the hybrid multifed DC system.

Figure 1 .
Figure 1.Hierarchical structure diagram of HVDC control system.

Figure 2 .
Figure 2. Main circuit structure of VSC-HVDC transmission control system.

Figure 4 .
Figure 4. Block diagram of reactive power coordinated control.

Figure 7 .
Figure 7. Parameter curve of VSC-HVDC system under rated operating conditions.

Figure 8 .
Figure 8. Parameter curve of LCC-HVDC system under rated operating conditions.

Table 2 .
Filter switching number and ∆ change table under different power.