CN103368170A - Converter and control method of multi-end soft direct current power transmission system - Google Patents

Abstract

The present invention _relates to a converter control method of a multi-terminal flexible direct current transmission system and the converter and flexible direct current transmission system adopting the control method. When u _{dc_ref_low} is between, the converter operates in constant power control mode; when the DC bus voltage is outside the preset voltage upper limit u _{dc_ref_high} and lower limit u _{dc_ref_low} , the converter operates in DC bus voltage control mode , when the bus voltage is higher than the set upper limit, the converter adjusts the DC bus voltage to stabilize at the upper limit, and when the bus voltage is lower than the lower limit, the converter adjusts the DC bus voltage to stabilize at the lower limit. With this method, the coordinated control of DC voltage and power flow can be carried out simultaneously, which is suitable for large-scale multi-drop distributed multi-terminal flexible DC system engineering.

Description

A converter of a multi-terminal flexible direct current transmission system and its control method

technical field

The invention relates to a converter control method of a multi-terminal flexible direct current transmission system, a converter using the control method and a flexible direct current transmission system using the converter.

Background technique

Flexible DC transmission technology is based on power electronic devices that can be shut down and voltage source converters (VSCs). Most of them are BtB topological structures and use underground cables and submarine cables as transmission media to realize power transmission. Among them, the voltage source converter (VSC) is the basic unit of the transmission system. At present, the topological structures used in practical engineering include two-level, three-level and modular multi-level topological structures.

A multi-terminal DC transmission (MTDC) system refers to a DC transmission system that includes multiple rectifier stations or multiple inverter stations. Its notable feature is that it can realize multi-power supply and multi-drop power receiving, and provide a more flexible and fast power transmission method. MTDC power transmission system is mainly used in: transmission of electric energy from multiple energy bases to multiple load centers in the distance; large cities or industrial centers that cannot use overhead line corridors; middle branches of direct current transmission lines connected to loads or power sources; several isolated AC The DC transmission line is used between the systems to realize the asynchronous connection of the power grid, etc. With the further development of high-power power electronic fully-controlled switching device technology, the research of new control strategies, the gradual reduction of DC transmission costs, and the improvement of power quality requirements, based on conventional current source converters (Current Source Converted CSC) and voltage The hybrid MTDC power transmission technology of Voltage Source Converter VSC, the MTDC power transmission technology based on FACTS, and the new MTDC technology based on VSC will be developed rapidly, which will greatly improve the operational reliability and practicability of the MTDC power transmission system. Expand the scope of application of the MTDC transmission system, provide more new interconnection modes for the power grid, provide new ideas for multi-point power receiving of DC power supply in large cities, provide new methods for other forms of new energy to connect to the power grid, and provide high-quality electric energy storage The establishment of a new way.

Different from the traditional CSC-MTDC system, the VSC-MTDC system does not change the direction of the DC voltage when the power flow is reversed, but reverses the direction of the DC current, and does not require mechanical operation, which is faster and more reliable. It is a control method with high reliability and flexibility, so VSC-MTDC is a suitable scheme for constructing parallel multi-terminal DC system.

The VSC-MTDC system has more technical advantages than the two-end system in terms of operation flexibility and reliability, but its operation control is relatively complicated. Just as the current control is the basic control method in the current source direct current transmission (CSC-HVDC) system, the DC voltage control is the basic control in the VSC-HVDC system. In the VSC-MTDC system, the converter station controlled by sampling DC voltage is equivalent to an active power balance node, which plays the role of power balance and stable DC voltage, and its control performance and reliability determine the operating characteristics and reliability of the entire system. sex.

VSC-MTDC DC voltage and power flow coordination control strategy has the following types: (1) master-slave voltage control method, which is characterized by simple control and the strongest controllability, but requires high-speed communication to coordinate system commands; (2) voltage The advantage of the descending method is that it does not require the adjustment value coordination of the upper controller, has low requirements for mutual communication, and has certain scalability. The disadvantage is that the configuration of the slope is complicated, and the power flow control is affected by the main wiring of the system and the parameters of the DC cable. The biggest defect is Active power flow control cannot be accurately performed; (3) DC voltage coordination control method based on voltage deviation, which is characterized by independent adjustment without communication, flexible configuration, and strong scalability, but in terms of both DC voltage coordination and power flow optimization control , limited by its own control structure, its control performance is limited. Among them, the second-order voltage deviation method automatically cuts off the power flow control when performing voltage deviation control, so voltage coordination and power flow control cannot be carried out at the same time, and the third-order voltage deviation method also has this problem , while the control capability of each stage is limited.

Contents of the invention

The object of the present invention is to provide a converter control method for a multi-terminal flexible direct current transmission system, a converter using the control method and a flexible direct current transmission system using the converter, to solve the problems in the existing VSC-MTDC system The problem that DC voltage and power flow coordination control cannot be carried out simultaneously.

To achieve the above object, the solution of the present invention includes: a converter control method of a multi-terminal flexible direct current transmission system, a converter applying the control method, and a flexible direct current transmission system applying the converter.

A converter control method for a multi-terminal flexible direct current transmission system: when the DC bus voltage is between a preset voltage upper limit value u _{dc_ref_high} and a lower limit value u _{dc_ref_low} , the converter operates in a constant power control mode; when When the DC bus voltage is outside the preset voltage upper limit value u _{dc_ref_high} and lower limit value u _{dc_ref_low} , the converter operates in the DC bus voltage control mode. When the bus voltage is higher than the set upper limit value, the converter regulates The DC bus voltage is stable at the upper limit, and when the bus voltage is lower than the lower limit, the converter adjusts the DC bus voltage to be stable at the lower limit.

The converter in this control method adopts the direct current control method based on the rotating coordinate system, and its outer loop control based on the rotating coordinate system includes a power controller for generating the active current command _idref , and the power controller includes an active power flow controller and DC voltage controller, when the converter operates in the constant power control mode, the output of the active power flow controller is the input of the active current command i _dref ; when the converter operates in the DC bus voltage control mode, the output of the DC voltage controller It is the input of active current instruction _idref .

The DC voltage controller in the control method includes a voltage lower limit controller and a voltage upper limit controller.

The converter using this control method: when the DC bus voltage is between the preset voltage upper limit value u _{dc_ref_high} and the lower limit value u _{dc_ref_low} , the converter operates in the constant power control mode; when the DC bus voltage is in the preset When the set voltage upper limit value u _{dc_ref_high} and lower limit value u _{dc_ref_low} are outside, the converter operates in the DC bus voltage control mode. When the bus voltage is higher than the set upper limit value, the converter adjusts the DC bus voltage to stabilize at Upper limit value, when the bus voltage is lower than the lower limit value, the converter adjusts the DC bus voltage to stabilize at the lower limit value.

The converter adopts the direct current control method based on the rotating coordinate system, and its outer loop control based on the rotating coordinate system includes a power controller for generating the active current command _idref , and the power controller includes an active power flow controller and a DC voltage controller When the converter operates in the constant power control mode, the output of the active power flow controller is the input of the active current command i _dref ; when the converter operates in the DC bus voltage control mode, the output of the DC voltage controller is the active current command Input for i _dref .

The DC voltage controller of the converter includes a voltage low limit controller and a voltage high limit controller.

A multi-terminal flexible direct current transmission system using this control method: each converter in the direct current transmission system sets different voltage upper limit values u _{dc_ref_high} and lower limit values u _{dc_ref_low} according to the importance of loads.

The beneficial effects of the present invention are: adopting the control method and the DC power transmission system of the converter, the converter can automatically switch its own operating mode according to the operating state of the DC system: when the DC bus voltage at one or more terminals deviates, the Switch the operation mode from the constant power control mode to the DC voltage control mode, stabilize the DC bus voltage within the set upper and lower limits, and prevent accidents from expanding; set different voltage upper and lower limits according to the importance of the load to avoid accidents in the DC system In case of failure, priority is given to ensuring the power supply of key loads. The DC operating point of the multi-terminal system adopting this method is fixed, the control structure is simple, the system configuration is flexible, and the scalability is strong, which is suitable for large-scale multi-drop distributed multi-terminal flexible DC system engineering.

Description of drawings

Figure 1 is a schematic diagram of the structure of the multi-terminal flexible direct current transmission;

Figure 2a is the equivalent circuit of a modular multilevel flexible direct current transmission converter;

Fig. 2b is a control block diagram of a modular multilevel flexible direct current transmission converter;

Fig. 3a is a control block diagram of a power controller with a DC bus voltage high and low voltage limit control function;

Figure 3b is an external control characteristic diagram of a power controller with a DC bus voltage high and low voltage limit control function;

Fig. 4 is a structural diagram of a simulation system of a modular multilevel direct current transmission converter;

Figure 5a is the AC voltage waveform of the multi-terminal system AC voltage zero-voltage drop simulation test of the modular multi-level flexible DC transmission converter;

Figure 5b is the DC bus voltage waveform in the simulation test of the AC voltage zero voltage drop of the multi-terminal system of the modular multi-level flexible DC transmission converter;

Figure 5c is the DC current waveform of the AC voltage zero voltage drop simulation test of the multi-terminal system of the modular multi-level flexible DC transmission converter;

Figure 6a is the DC voltage waveform of the multi-terminal system power operation simulation test of the modular multi-level flexible DC transmission converter;

Figure 6b is the DC current waveform of the multi-terminal system power operation simulation test of the modular multi-level flexible DC transmission converter.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

The structure diagram of the multi-terminal flexible DC power transmission system is shown in Figure 1: the positive direction of power flowing from the AC side to the DC side is taken as the positive direction, S1, S3, and S4 are power supply sources, and Load2 and Load5 are power loads. Figure 2a is the equivalent circuit of the converter of the flexible direct current transmission system, where P _s and Q _s are the active and reactive power of the AC side, and I _a1 , I _b1 , I _c1 , U _sa , U _sb , and U _sc are the AC side The current and voltage of the three phases on the side, U _ca , U _cb , and U _cc are the AC voltages consumed by the converter impedance, and P _c and I _dc are the power and current input by the converter to the DC grid. Flowing to the DC side is the positive direction, and U _dc is the DC side terminal voltage of the converter, and the specific working method will not be repeated here. Figure 2b is a control block diagram of a flexible direct current transmission converter. The constant power control method involved in this control block diagram, the principle and measures of the direct current voltage control (constant voltage control) method itself, and the labels in the figure, etc. belong to the prior art, and will not be discussed here. For details, refer to the paper "Control Strategy and Simulation Research of Flexible DC Transmission Based on dq0 Synchronous Coordinates" (Power System Protection and Detection, Volume 37, Issue 22). Add descriptions for each symbol.

The specific control method of the converter of the multi-terminal flexible direct current transmission is as follows:

The upper limit value u _{dc_ref_high} and the lower limit value u _{dc_ref_low} of the converter and the rectification and inversion trade-off function are set. During normal operation, the bus voltage is within the range of the set voltage upper limit value u _{dc_ref_high} and lower limit value u _{dc_ref_low} , after the calculation of the trade-off function, the converter is controlled to operate in the constant power control mode; when the DC bus voltage is in the preset When the voltage upper limit value u _{dc_ref_high} and the lower limit value u _{dc_ref_low} are outside, after the calculation of the trade-off function, the converter is switched to operate in the DC bus voltage control mode. When the bus voltage exceeds the upper limit value, the control bus voltage is stabilized at the upper limit value, when the bus voltage is lower than the lower limit, the control bus voltage is stable at the lower limit. When the converter is running in the constant power control mode, the active power will be maintained according to its power command value P _order , at this time the output of the active power flow controller is used as the input of the active current command _idref in Figure 2b; when the converter is running In the DC bus voltage control mode, it will switch from the original control by its power command value P _order to the control by the DC bus voltage value U _dc , the output of the DC voltage controller is used as the input of the active current command _idref in Figure 2b, through which In this way, the converter can smoothly switch between the two working modes, which solves the original problem that a converter cannot work in both the constant power control mode and the stable DC bus voltage mode. The control block diagram of the power controller that completes the mode switching operation is shown in Figure 3a. Figure 3a is a coordinated controller structure based on the principle of voltage deviation. The power controller includes a DC voltage control part and a related power flow control part. Among them, the voltage high and low limit controllers are all PI controllers, which are used to adjust the active power when the DC voltage exceeds the deviation range to control the DC voltage. The active power flow controller is a PI controller based on the steady-state inverse model. This controller is used for DC voltage Fast closed-loop control of power flow is carried out within the range of high and low limits; since the outputs of the three controllers are added together as active current commands through the adder, conditions are created for the competition between the voltage controller and the power flow controller, and the trade-off function is Adjust the output of the power flow controller based on the output results of the DC voltage high and low limit controller, and then perform coordinated control of the DC voltage and active power flow according to the pre-configured parameters. The trade-off function can be different according to the coordination requirements of the system's DC voltage and active power flow Configuration, the simplest configuration is a constant, which is combined with the output of the voltage controller and compared with the power flow controller, combined with the actual DC bus voltage of the current converter to output active power, based on the DC voltage stability and power flow The optimal allocation principle finally determines the output active current command. The input of the coordination controller has 5 variables, which are the voltage high limit command Udc_ref_High, the voltage low limit command Udc_ref_Low, the active power command P_ref, the DC bus voltage Udc and the actual active power P of the AC side of the converter, and the output is the converter The active current command Id_ref; the PI parameters of the voltage high and low limit controller and the active power flow controller are configured by the system transfer function based on the dynamic characteristics; the output limit of the voltage low limit controller is the rectification Active power limit, where the lower limit Pvdc_Low_lower is set to zero, and the configuration of the upper limit Pvdc_Low_upper directly determines the rectification active capability of this terminal participating in voltage coordination control, which is actually configured in combination with engineering requirements; the output limit of the voltage high limit controller is changed for this terminal. The active power limit of the inverter participating in the coordinated control of inverters, where the upper limit Pvdc_High_upper is set to zero, and the configuration of the lower limit Pvdc_High_lower directly determines the active power of the inverter participating in voltage coordinated control at this end, and is actually configured according to engineering requirements; the configuration of the trade-off function There are many forms, but there are generally three schemes: the first scheme is that the DC voltage high and low limit controllers are fully prioritized. At this time, when the DC voltage exceeds the high limit, the active power flow will be changed to meet the maximum stability of the DC voltage. , the actual active power flow and the command are no longer equal; this scheme is suitable for the AC power grid end with strong emergency power flow support; the second scheme is active power flow priority, and at this time the converter at this end performs the initial stage when the DC voltage exceeds the upper and lower limits. A certain degree of DC voltage control, but because the trade-off function is configured to give priority to active power flow, the active power flow is still guaranteed to be the closed-loop command value in the steady state. This scheme can perform DC voltage control at the initial stage of dynamics, and still ensure power flow transmission after stabilization. This scheme is suitable for the AC grid end with certain transient emergency power flow support; the third scheme is the limited coordination mode of active power flow. The transmission demand of the power flow at this end will be considered. In this way, the AC power flow change due to the coordination effect is between the first and second schemes. This scheme is suitable for the AC grid end where the converter must ensure a certain degree of power flow transmission. ; and the last thing is to ensure that the converter at this end operates under the allowable capacity, where Id_upper is the limit value of the maximum rectified active current of the converter at this end, Id_lower is the limit value of the maximum inverter active current of the converter at this end, and the active current The limit value is generally set to the maximum continuous operating load current allowed by the converter.

The external characteristics of the active power controller with the DC bus voltage high and low voltage limit control function meet the requirements of Figure 3b:

During the control process, when the bus voltage exceeds the set upper and lower limits, the power controller works in the DC voltage control mode, and the voltage is constant at the corresponding set upper and lower limits; when the bus voltage is within the upper and lower limits When it is inside, switch to constant power control mode, and the power is constant at this time.

Select the high and low voltage limits of the DC bus voltage at each end. The limit values at each end can be the same, or they can be selected as different values. Different selections of the limit values will cause different power flow distribution priorities. In the following simulation example, different priorities are set for different converters corresponding to loads—that is, different upper and lower limit values.

The effectiveness of the DC voltage high and low voltage limit control method is verified below through simulation.

According to the above control strategy, the simulation model of the multi-terminal system of the modular multi-level flexible DC transmission converter is built in MATLAB/SIMULINK software. The model is connected to the system by the common point of the DC side of five converters, and the wiring diagram is shown in Figure 4. Among them, the rated power of 1# converter is 350MW, the rated power of 2# converter is 100MW, the rated power of 3# converter is 100MW, the rated power of 4# converter is 200MW, and the rated power of 5# converter The rated power is 100MW, and the rated voltage of the DC bus is 400kV. The 1# converter is in the constant DC bus voltage operation mode, and the 2-5# converters are in the constant power operation mode. 2#, 3#, 4#, and 5# are all power controllers with DC bus voltage high and low voltage limit control function, of which 4# low voltage limit is 370kV, and other low voltage limits are 380kV.

Simulation 1: 1# converter is in constant DC bus voltage mode, the bus voltage command is 400kV; 2# converter draws 50MW from the DC side, 3# converter draws 100MW from the DC side, 4# converter The power drawn from the DC side is 100MW, and the 5# converter draws 100MW from the DC side. At 0.6S, a three-phase zero voltage drop fault occurred on the grid side of the 1# converter. Figure 5a is the AC voltage waveform of the 1# converter, Figure 5b is the DC bus voltage waveform, and Figure 5c is the DC bus current waveform of the 1-5# converter (the flow from the AC side of the converter to the DC side is the positive direction). From the simulation waveform, it can be seen that after 0.6S fault, the 1# converter exits the operation, and its DC current drops to 0A, and the 2#, 3#, 5# converters switch from the previous constant power mode to the DC bus voltage control state , and stabilize the bus voltage at the lower limit of 380kV, and the 4# converter still maintains 100MW power operation.

Simulation 2: 1# converter is in constant DC bus voltage mode, and the bus voltage command is 400kV; 0.2S2# starts to draw power from the DC side of 100MW, 3# converter draws power from the DC side to 50MW, and 5# draws power from the DC side Power 100MW, 4# starts to draw power 150MW from the DC side at 0.5S, 4# turns to send power to the DC side at 0.7S; 1.0S2# sends power to DC 70MW, 3# sends power to DC 70MW, 5# starts Send power 70MW to DC, 1.3S4# send power 200MW to DC, 1.7S4# turn to draw power 20MW from DC side. Figures 6a and 6b are the simulation waveforms of this experiment. It can be seen from the waveform that due to the limited power transmission of 1# at 0.5S, the sum of the power pumped by the 4 converters 2-5# exceeds the transmission capacity of the 1# converter. At this time, the 1# converter station is converted to rectification Direction maximum power output operation, DC bus voltage drops, 2#, 3#, 5# switch to DC bus voltage control at 0.5S, stabilize the bus voltage at its lower limit of 380kV, when 0.7S, 2-5# from DC The sum of side pumping power is converted to 50MW. At this time, the power of 1# converter can meet the operation requirements of the other four converters. 1# converter controls the bus voltage at 400kV again, 2#, 3#, 5# Switch to the original constant power control, and its transmission power returns to the power command value. 1.0S2#, 3#, 5# turn to output power to the DC side, at this time 4# also outputs power to the DC side, the total output power of the 4 converters to the DC side is 310MW, the system is running normally, at 1.3S , the output power of 4# to the DC side is increased to 200MW and increased by 100WM. At this time, the total power delivered by the 4 converters to the DC side is 410MW, which exceeds the transmission capacity of the 1# converter. At this time, the 1# converter The inverter turns to the maximum power output operation in the inverter direction, the DC bus voltage rises, 2#, 3#, 5# 1.3S cuts in to the DC bus voltage control, and stabilizes the bus voltage at its upper limit of 400kV. When 1.7S, 2 - The sum of the power sent by 5# to the DC side is converted to 190MW. At this time, the power of the 1# converter can meet the operating requirements of the other four converters. The 1# converter controls the bus voltage at 400kV again. 2#, 3# and 5# switch to the original constant power control, and their transmission power returns to the power command value.

The above simulations verify the effectiveness of this control method.

Claims (7)

1. the converter control method of a multiterminal flexible DC power transmission system is characterized in that, when DC bus-bar voltage is in predefined upper voltage limit value u _{Dc_ref_high}And lower limit u _{Dc_ref_low}Between the time, converter runs on decides power control mode; When DC bus-bar voltage is in predefined upper voltage limit value u _{Dc_ref_high}And lower limit u _{Dc_ref_low}In the time of in addition, converter runs on the DC bus-bar voltage control model, and when busbar voltage was higher than the capping value, converter was regulated DC bus-bar voltage and is stable at higher limit, when busbar voltage was lower than lower limit, converter was regulated DC bus-bar voltage and is stable at lower limit.

2. the converter control method of a kind of multiterminal flexible DC power transmission according to claim 1 system, it is characterized in that, converter adopts the Direct Current Control method based on rotating coordinate system, and its outer shroud control based on rotating coordinate system comprises for generation of active current instruction i _DrefPower controller, power controller comprises meritorious flow controller and dc voltage controller, decides power control mode when converter runs on, the power flow control device is output as active current instruction i _DrefInput; When converter runs on the DC bus-bar voltage control model, dc voltage controller is output as active current instruction i _DrefInput.

3. the converter control method of a kind of multiterminal flexible DC power transmission according to claim 2 system is characterized in that described dc voltage controller comprises voltage lower bound controller and the high limit of voltage controller.

4. a converter that adopts the described control method of claim 1 is characterized in that, when DC bus-bar voltage is in predefined upper voltage limit value u _{Dc_ref_high}And lower limit u _{Dc_ref_low}Between the time, converter runs on decides power control mode; When DC bus-bar voltage is in predefined upper voltage limit value u _{Dc_ref_high}And lower limit u _{Dc_ref_low}In the time of in addition, converter runs on the DC bus-bar voltage control model, and when busbar voltage was higher than the capping value, converter was regulated DC bus-bar voltage and is stable at higher limit, when busbar voltage was lower than lower limit, converter was regulated DC bus-bar voltage and is stable at lower limit.

5. converter according to claim 4 is characterized in that, converter adopts the Direct Current Control method based on rotating coordinate system, and its outer shroud control based on rotating coordinate system comprises for generation of active current instruction i _DrefPower controller, power controller comprises meritorious flow controller and dc voltage controller, decides power control mode when converter runs on, the power flow control device is output as active current instruction i _DrefInput; When converter runs on the DC bus-bar voltage control model, dc voltage controller is output as active current instruction i _DrefInput.

6. converter according to claim 5 is characterized in that, described dc voltage controller comprises voltage lower bound controller and the high limit of voltage controller.

7. a multiterminal flexible DC power transmission system that adopts control method claimed in claim 1 is characterized in that, each converter is according to the different upper voltage limit value u of different set of load importance in this DC transmission system _{Dc_ref_high}And lower limit u _{Dc_ref_low}

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