A Method for Remote Transmission of LCC-MMC Three-Terminal Hybrid Flexible Renewable Energy

Abstract

The present invention belongs to the field of power systems with high proportions of wind power and photovoltaic power access, cross-region and long-distance external transmission and consumption, and specifically relates to an LCC-MMC three-terminal hybrid flexible renewable energy remote external transmission method. The method includes the following steps: Step 1: Based on the actual operation of the LCC-MMC Yunnan-Guangdong DC project, new energy is connected to the sending-end system, and the receiving-end system is a converter station, and a model is built; Step 2: The rectifier side of the sending-end system controls the DC current, and the circuit is adjusted to the normal value; Step 3: The inner-loop current control of the inverter side of the receiving-end system; Step 4: The outer-loop current control of the inverter side of the receiving-end system; achieving the purpose of reducing the probability of continuous commutation failure in the multi-infeed DC system and reducing the manufacturing cost.

Description

Technical Field

[0001] The invention belongs to the field of power systems with high proportions of wind power and photovoltaic power access, and cross-regional and long-distance external transmission and consumption, and particularly relates to an LCC-MMC three-terminal hybrid flexible renewable energy remote external transmission method. Background Art

[0002] Gansu is located at the throat of the new Silk Road economic belt in China, the hub of the new energy base group, and the energy transmission corridor, and has unique advantages in building a high-proportion new energy base characterized by "UHV grid + smart grid + new energy". Recently, the National Energy Administration has approved Gansu to build a national new energy comprehensive demonstration area. As of the end of December 2018, the proportion of new energy in the province's installed capacity in the whole network reached 40.68% , and new energy has become the largest main power source in Gansu. Considering the installed capacity of hydropower, the proportion of new energy in the installed capacity of the whole network reached 58.6%.

[0003] Renewable energy power generation such as wind power and photovoltaic power has the characteristics of intermittency, randomness, and poor schedulability. After large-scale access, it will have a greater impact on the external transmission characteristics of the power system. For example: frequency, voltage, transmission power, etc. In addition, since the initial stage of wind power development in China, there have been requirements for large-scale, highly concentrated development and long-distance, high-voltage transmission, showing significant differences from the foreign wind power development model. The resulting grid technical and economic problems are particularly prominent and more complex. Most of the remote wind power bases are facing bottlenecks in power transmission, and the problem of power rationing is very serious. According to the St.Clair curve, when the transmission distance exceeds 700km, AC power transmission without series compensation cannot reach its natural power, resulting in a decrease in transmission capacity. Therefore, even for a 1000kV UHV AC transmission line without a series compensation device, its transmission capacity will not exceed its natural power. To solve the above problems, higher DC transmission methods are generally considered, such as the Jiuquan-Huainan DC transmission project, etc. However, for conventional DC transmission, the problem of multiple DC feed-ins seriously affects safety and stability. Therefore, the hybrid flexible DC transmission technology LCC-MMC-HVDC is proposed to enhance grid stability and improve transmission capacity. When using the LCC-MMC hybrid flexible DC transmission technology, there is no commutation failure problem in the receiving-end system. Therefore, it will not cause commutation failures in multiple converter stations at the same time, and can solve the problem of multiple DC feed-ins. At the same time, the MMC can provide dynamic reactive power support to the receiving-end system to ensure the voltage stability of the receiving-end system. Summary of the Invention

[0004] For the power grid in the Hexi region, when a large amount of new energy is transmitted over a long distance across regions, the hybrid flexible DC technology LCC-MMC is used to achieve long-distance and large-capacity power transmission. Combining the respective advantages of LCC-HVDC and MMC-HVDC, the hybrid DC transmission system with LCC on the rectifier side and MMC on the inverter side can eliminate the commutation failure problem existing on the inverter side of traditional DC transmission. The present invention provides an LCC-MMC three-terminal hybrid flexible renewable energy long-distance transmission method, aiming to reduce the probability of consecutive commutation failures occurring in a multi-infeed DC system and lower the manufacturing cost.

[0005] The technical solution adopted by the present invention is as follows:

[0006] An LCC-MMC three-terminal hybrid flexible renewable energy long-distance transmission method includes the following steps:

[0007] S1. Based on the actual operating LCC-MMC Yunnan-Guangdong DC project, new energy is connected to the sending end system, and the receiving end system is a converter station, and a model is built;

[0008] S2. The rectifier side of the sending end system controls the DC current to be constant and adjusts the circuit to the normal value;

[0009] S3. The inner-loop current control of the receiving end system;

[0010] S4. The outer-loop current control of the receiving end system.

[0011] In the above S1, 50% new energy is connected to the sending end system.

[0012] In the above S2, the DC current control of the rectifier side of the sending end system satisfies

[0013]

[0014] where: I d is the DC line current, V dor , V doi are the no-load voltages on the valve sides of the converter transformers on the rectifier side and the inverter side respectively, R dr is the DC line resistance, R cr , R ci are the equivalent commutation resistances of the rectifier and the inverter respectively, α is the delay angle of the rectifier, and γ is the turn-off angle of the inverter.

[0015] In the above S3, the inner-loop current control is divided into the input variable control by the positive-sequence current controller, the input variable control by the negative-sequence current controller, the dynamic control of the positive-sequence dq-axis current component, and the dynamic control of the negative-sequence dq-axis current component.

[0016] The input variable of the positive-sequence current controller satisfies

[0017]

[0018]

[0019] The input variable values of the negative sequence current controller satisfy that,

[0020]

[0021]

[0022] The dynamic expressions of the positive sequence dq-axis current components are

[0023]

[0024] The dynamic expressions of the negative sequence dq-axis current components are,

[0025]

[0026] In the formula, and are state variables; is the disturbance component; is the input variable; is the voltage coupling compensation term; is the AC grid voltage feedforward term.

[0027] In the said S4, the reference value of the negative sequence current for the outer loop current control of the receiving-end system is set to zero.

[0028] When the reference value of the said negative sequence current is set to zero, the reference values of the positive sequence dq-axis current are solved according to the reference values of the active and reactive powers as,

[0029]

[0030]

[0031] In the formula: P * is the reference value of the active power, Q * is the reference value of the reactive power, are the reference currents of the dq-axis respectively

[0032] In the said S4, the expression of the reference value of the positive sequence current for the outer loop current control of the receiving-end system is

[0033]

[0034] In the formula: u dc is the actual voltage of the system, is the reference voltage, k p5 is the proportionality coefficient

[0035] The beneficial effects of the present invention are as follows: For the power grid in the Hexi region, when a large amount of new energy is transmitted over a long distance across regions, the hybrid flexible DC technology LCC-MMC is used to achieve long-distance and large-capacity power transmission. Combining the respective advantages of LCC-HVDC and MMC-HVDC, the hybrid DC transmission system with LCC on the rectifier side and MMC on the inverter side can eliminate the commutation failure problem existing on the inverter side of traditional DC transmission, reduce the probability of consecutive commutation failures in a multi-infeed DC system, and lower the manufacturing cost.

[0036] The three-terminal structure can realize the multi-region transmission of new energy, solve the single "point-to-point" transmission mode of traditional DC transmission, and the receiving-end system adopts the NLM modulation method, which improves the power quality of the receiving-end system. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is the flow chart of the present invention;

[0038] Figure 2 is the three-terminal hybrid flexible DC transmission topology;

[0039] Figure 3 is the model for transmitting the bundled power sources of wind power, photovoltaic power, and thermal power using the three-terminal hybrid flexible transmission technology LCC-MMC;

[0040] Figure 4 is the topological structure diagram of the MMC sub-module (SM);

[0041] Figure 5 is the working principle diagram of the MMC;

[0042] Figure 6 is the block diagram of the positive-sequence and negative-sequence inner-loop current controllers;

[0043] Figure 7 is the block diagram of the outer-loop current control;

[0044] Figure 8 is the working state diagram of the MMC. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments:

[0046] Embodiment 1

[0047] The three-terminal hybrid flexible DC transmission topology is as Figure 2 shown,

[0048] The mathematical model of the rectifier side is shown in Equation (1),

[0049]

[0050] where, Uad is the effective value of the no-load line voltage on the converter side, U r is the DC voltage of the rectifier station; I r is the DC output current of the converter; P ad is the DC output power; Q ad is the reactive power required by the converter; X r is the reactance per phase; α is the firing angle; μ is the commutation overlap angle.

[0051] The mathematical model of the inverter side is shown in the following equations (2) and (3).

[0052]

[0053]

[0054] Among them, P i , Q i are the active power injected into the MMC by the AC system, respectively. P dci is the DC output power of the converter, I di is the DC output current of the MMC, i pk , i circk are the arm current of phase k and the circulating current k respectively, taking a certain phase in the three-phase AC system; i k is the instantaneous value of the current of phase k on the AC side of the converter.

[0055] As Figure 4 shown is the topological structure of a sub-module (SM). T1 and T2 represent IGBTs, D1 and D2 represent anti-parallel diodes, and C0 represents the DC-side capacitor of the sub-module; u c is the voltage of the capacitor, u sm is the voltage across the sub-module, i sm is the current flowing into the sub-module, and the reference directions of each physical quantity are shown in the figure. From Figure 6 it can be seen that each sub-module has a connection port for series connection to the main circuit topology, and the MMC supports the voltage of the DC bus through the DC-side capacitor voltages of each sub-module. Analysis shows that the sub-module has three working states, as shown in Figure 8 . According to the on-state and current direction of the IGBTs in the upper and lower arms of the sub-module, it can be divided into 6 working modes. When both T1 and T2 are given turn-off signals, it is in working state 1. There are two working modes, namely mode 1 and mode 4, depending on which of the anti-parallel diodes D1 and D2 conducts. The current charges the capacitor through D1; corresponding to mode 4, D2 conducts and the current bypasses the capacitor through D2. This working state is called the abnormal working state and is used to charge the sub-module capacitor during MMC startup or to bypass the sub-module capacitor during a fault.

[0056] When T1 is turned on for communication while T2 is turned off by the signal, it is called operating state 2. At this time, T2 is in the off state due to the turn-off signal, and D2 is also in the off state because it bears the reverse voltage. There are also two operating modes in operating state 2, namely mode 2 and mode 5, depending on the flowing direction of the sub-module current. Corresponding to mode 2, D1 is in the conducting state at this time, while T1 bears the reverse voltage and remains in the off state despite the applied turn-on signal, and the current charges the capacitor through D1. Corresponding to mode 5, T1 is in the conducting state at this time, while D1 bears the reverse voltage and is in the off state, and the current discharges the capacitor through T1. When the sub-module is in operating state 2, the DC-side capacitor is always connected to the main circuit (charging or discharging), and the output voltage of the sub-module is the capacitor voltage u c When T1 is turned off by the signal while T2 is turned on by the signal, it is called operating state 3. At this time, T1 is in the off state due to the turn-off signal, and D1 is also in the off state because it bears the reverse voltage. There are also two operating modes in operating state 3, namely mode 3 and mode 6, depending on the flowing direction of the sub-module current. Corresponding to mode 3, [element] is in the conducting state at this time, while [element] bears the reverse voltage, and the current bypasses the capacitor through [element]. Corresponding to mode 6, [element] is in the conducting state at this time, while [element] bears the reverse voltage and remains in the off state despite the applied turn-on signal, and the current discharges the capacitor through [element] to bypass the capacitor. When the sub-module is in operating state 3, the output voltage of the sub-module is zero for the capacitor voltage, that is, the sub-module is bypassed from the main circuit.

[0057] Based on the actual project background, a hybrid three-terminal AC system with a voltage of 500 kV and a frequency of 50 Hz is to be built. Converter station 1 is a bipolar 12-pulse LCC converter station, and converter stations B and C are both bipolar MMC 21-level converter stations. The voltage level of the DC line is plus or minus 800 kV. Converter station B adopts constant DC voltage and constant AC voltage control, and converter station C adopts constant active power and constant AC voltage control. The transmission power is 8000 MW for converter station A, -5000 MW for converter station B, and -3000 MW for converter station C. After the fault is removed, the MMC will be blocked. If the blocking is to be modified, the blocking logic needs to be modified.

[0058] Step 2: Constant current control on the rectifier side

[0059] Constant DC current control means that due to reasons such as faults, the DC current on the DC line changes, and the current is quickly adjusted to the normal value. The DC current value satisfies the following relationship in formula (4),

[0060]

[0061] where, I d is the DC current of the line, V dor 、V doiare the valve-side no-load voltages of the converter transformers on the rectifier side and the inverter side, respectively, and R dr is the DC line resistance, R cr , R ci are the equivalent commutation resistances of the rectifier and the inverter, respectively. α is the delay angle of the rectifier, and γ is the turn-off angle of the inverter.

[0062] Step 3: Inner-loop current control method of MMC

[0063] and are state variables, is the disturbance component is the input variable. It can be seen that there is coupling between the d-axis and the q-axis. Introduce the voltage coupling compensation term and the AC grid voltage feedforward term When using proportional-integral (PI) control, the input variable values of the positive-sequence current controller satisfy the relationship in Equation (5).

[0064]

[0065] Similarly, the input variable values of the negative-sequence current controller can also be obtained to satisfy the relationship in Equation (6).

[0066]

[0067] Substituting Equation (5) into Equation (7) gives the dynamic expression (8) of the positive-sequence dq-axis current components.

[0068]

[0069]

[0070] Substituting Equation (6) into Equation (7) gives the dynamic expression (9) of the negative-sequence dq-axis current components.

[0071]

[0072] Step 4: Outer-loop current control method of MMC

[0073] The function of the inner-loop current controller is to make and track their reference values, while the outer-loop controller calculates the inner-loop current reference values according to the reference values of active power, reactive power, and DC voltage, etc. To suppress negative-sequence current and prevent overcurrent of power electronic devices, the reference value of the negative-sequence current can be set to zero.

[0074]

[0075] When the negative sequence current is zero, the positive sequence dq-axis current reference values are solved according to the active and reactive power reference values as follows,

[0076]

[0077]

[0078] When constant DC voltage control is adopted, the positive sequence d-axis current reference value can be obtained according to the DC voltage reference value,

[0079]

[0080] This application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowchart and / or block diagram, and the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the specified functions in one process Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0081] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implements the specified functions in one process Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0082] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process, and thus the instructions executed on the computer or other programmable device provide steps for implementing the specified functions in one process Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific implementation manners of the present invention or make equivalent substitutions, and any modification or equivalent substitution that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A method for remotely transmitting flexible renewable energy of LCC-MMC three-terminal hybrid, characterized in that, It includes the following steps: S1. Based on the background of the LCC-MMC DC project, new energy is connected to the sending-end system, and the receiving-end system is a converter station, and a model is built; S2. The rectifier side of the sending-end system controls the DC current to a fixed value, and adjusts the circuit to the normal value; S3. The inner-loop current control of the inverter side of the receiving-end system; Among them, in the said S3, the inner-loop current control is divided into the input variable control of the positive-sequence current controller, the input variable control of the negative-sequence current controller, the dynamic control of the positive-sequence dq-axis current component, and the dynamic control of the negative-sequence dq-axis current component; The input variables of the positive-sequence current controller satisfy The input variables of the negative-sequence current controller take values that satisfy The dynamic expression of the positive-sequence dq-axis current component is The dynamic expression of the negative-sequence dq-axis current component is wherein, and are state variables; is a disturbance component; is an input variable; is a voltage coupling compensation term; is an AC grid voltage feedforward term; S4. The outer-loop current control of the inverter side of the receiving-end system; In the said S4, the expression of the positive-sequence current reference value of the outer-loop current control of the receiving-end system is Where: u dc is the actual voltage of the system, is the reference voltage, and k p5 is the proportionality coefficient.

2. A method for remotely transmitting a three-terminal hybrid flexible renewable energy of LCC-MMC according to claim 1, characterized in that: In the said S1, 50% new energy is connected to the sending-end system.

3. A method for remotely transmitting a three-terminal hybrid flexible renewable energy of LCC-MMC according to claim 1, characterized in that: In the said S2, the DC current fixed value of the rectifier side of the sending-end system satisfies Where: I d is the DC current of the line, V dor , V doi are the no-load voltages on the valve sides of the converter transformers on the rectifier side and the inverter side respectively, R dr is the DC line resistance, R cr , R ci are the equivalent commutation resistances of the rectifier and the inverter respectively. α is the delay angle of the rectifier and γ is the turn-off angle of the inverter.

4. A method for remotely transmitting a three-terminal hybrid flexible renewable energy of LCC-MMC according to claim 1, characterized in that: In the said S4, the reference value of the negative-sequence current of the outer-loop current control of the receiving-end system is set to zero.

5. A method for remotely transmitting a three-terminal hybrid flexible renewable energy of LCC-MMC according to claim 4, characterized in that: When the reference value of the negative-sequence current is set to zero, the reference values of the positive-sequence dq-axis current are solved respectively according to the reference values of the active and reactive powers Where: P * is the reference value of active power, Q * is the reference value of reactive power, are the reference currents of dq axes respectively.

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