Low-voltage fault ride-through coordination control method for close-range two-end low-voltage direct-current power transmission system

Abstract

The invention discloses a low-voltage fault ride-through coordination control method for a close-range two-end low-voltage direct-current power transmission system, and the method comprises the steps: transmitting active power from an alternating-current network side to a direct-current power transmission line side through a two-stage circuit at a power transmission end; a T-type three-level grid-connected converter with a fourth bridge arm works in a PWM rectification state to stabilize positive and negative bus voltage between two-stage circuits, and a DAB converter works in a phase-shift working mode to stabilize bus voltage of a direct-current transmission line, so that low-voltage fault ride-through coordination control on a power transmission end side is realized; the two-stage circuit located at the power receiving end absorbs active power from the direct-current power transmission line side and transmits reactive power to the alternating-current network side, and the DAB converter works in a phase-shifting working mode to stabilize positive and negative bus voltage between the two-stage circuit. And the T-type three-level grid-connected converter with the fourth bridge arm works in a PWM inversion state to carry out power receiving end side low-voltage fault ride-through coordination control. The method provided by the invention realizes respective compensation of different amplitudes of three-phase power grid voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a low-voltage fault ride-through coordinated control method for a short-distance two-terminal low-voltage direct current transmission system. Background Art

[0002] In recent years, with the increase in the types of electricity consumption of distribution network users, the electricity demand of different distribution stations has shown a diversified trend. Adjacent distribution stations are generally close to each other, and it is common that some distribution stations have large load electricity demand and some have small load demand. At present, for the situation where the load electricity demand of distribution stations is large, measures such as transformer capacity increase or introduction of new energy generation can only be taken to meet the growing demand for electricity and power reliability. These solutions are often accompanied by expensive construction costs. At the same time, the existing distribution network is difficult to effectively optimize the load demand of each station and improve the utilization rate of distribution transformers. The market demand potential is huge. With the rapid development of power electronics technology, low-voltage direct current transmission is one of the best ways to solve the above problems, that is, to install a low-voltage direct current transmission system near the transformer of each adjacent distribution station. With the help of low-voltage direct current bus, the excess energy of the light-loaded station transformer can be transmitted to the adjacent distribution station with overloaded load electricity demand, thereby optimizing the load demand between distribution stations and improving the utilization rate of transformers in the station area. However, when a transient fault such as single-phase grounding, two-phase short circuit, or three-phase grounding fault occurs in a distribution station area, the interconnected low-voltage DC transmission systems at both ends need to be coordinated and controlled to effectively cross the low-voltage fault.

[0003] At present, low-voltage DC transmission systems for the above purposes are not common. In order to adapt to the frequent load imbalance in low-voltage distribution systems, low-voltage DC transmission systems need to adopt a four-arm converter structure and use high-frequency transformers for electrical isolation. Therefore, the low-voltage DC transmission system adopts a two-stage circuit topology, including an AC-DC grid-connected converter and a DC-DC converter with a high-frequency isolation transformer. The low-voltage DC transmission systems at both ends are interconnected through a DC bus. When a single-phase grounding, two-phase short circuit, three-phase grounding fault or other transient faults occur in the power grid at one end, the low-voltage DC transmission system at the fault end is mainly responsible for the support control of reactive power, and the low-voltage DC transmission system at the normal end is mainly responsible for the limiting control of active power. The coordinated control of the two can ensure the safe and stable operation of the system.

[0004] The current control method for single renewable energy power generation grid-connected converters to cope with low voltage ride through is that during the grid voltage drop, the three-phase converter symmetrically outputs active current and reactive current according to the depth of the three-phase fundamental positive sequence voltage drop. It does not distinguish between the types of faults and cannot achieve separate compensation for different amplitudes of the three-phase grid voltage, resulting in overcompensation of some phases and undercompensation of others. Summary of the invention

[0005] The purpose of the present invention is to provide a low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage DC transmission system, so as to solve the problem that the existing control method cannot achieve separate compensation for different amplitudes of the three-phase power grid voltage, resulting in over-compensation of some phases and under-compensation of some phases.

[0006] The technical scheme adopted by the present invention is a low-voltage fault ride-through coordinated control method for a short-distance two-end low-voltage DC transmission system, wherein a two-stage circuit located at the power transmission end transmits active power from the AC network side to the DC transmission line side, a T-type three-level grid-connected converter with a fourth bridge arm works in a PWM rectification state to stabilize the positive and negative bus voltages between the two-stage circuits, and a DAB converter works in a phase-shifting working mode to stabilize the bus voltage of the DC transmission line, thereby realizing low-voltage fault ride-through coordinated control on the power transmission end side; a two-stage circuit located at the power receiving end absorbs the active power transmitted by the power transmission end from the DC transmission line side and transmits reactive power to the AC network side of the power receiving end, the DAB converter works in a phase-shifting working mode to stabilize the positive and negative bus voltages between the two-stage circuits, and a T-type three-level grid-connected converter with a fourth bridge arm works in a PWM inversion state to perform low-voltage fault ride-through coordinated control on the power receiving end side.

[0007] The present invention is also characterized in that: The specific process of coordinated control of low voltage fault ride-through at the transmission end is as follows: Step 1, initializing the parameters of the low voltage DC transmission system at the power transmission end; Step 2: Give the positive and negative bus voltage reference values ​​between the two-stage circuits during normal operation U pn_ref_T , DC transmission line bus voltage reference value U dc_ref , The upper limit threshold of the bus voltage of the DC transmission line when a low voltage fault occurs at the receiving end U dc_max ; Step 3: Collect the three-phase voltage of the AC power grid at the power transmission end u a_T , u b_T , u c_T , the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm i a_T , i b_T , i c_T , positive and negative bus voltage between two-stage circuits u pn_T , positive and negative bus output current between two-stage circuits i pn_T , DC transmission line bus voltage u dc ; Step 4: When the bus voltage of the DC transmission line u dc Less than the upper threshold U dc_max When the low-voltage DC transmission systems at both ends are in normal working condition, jump to step 5; when the bus voltage of the DC transmission line u dc Greater than or equal to the upper threshold U dc_max When the low-voltage DC transmission system at the receiving end is in a low-voltage fault ride-through state, jump to step 7; Step 5: The T-type three-level grid-connected converter with the fourth bridge arm is in normal working state, and the three-phase filter inductor current is controlled by proportional resonance control. i a_T , i b_T , i c_T As the current inner loop control, PI control is used to adjust the positive and negative bus voltages between the two-stage circuits. U pn Adjustment is performed as the voltage outer loop control to generate PWM signals for the four bridge arms. The active power under normal working conditions is calculated according to formula (1): P T1 ; (1) Step 6: The DAB converter is in normal working state, and the bus voltage of the DC transmission line is controlled by PI control. u dc As the voltage outer loop control, PI control is used to adjust the bus current of the DC transmission line i dc Adjust as the current inner loop control, generate the phase shift angle D0, calculate the extended phase shift angle D1 through the extended phase shift algorithm, generate the PWM signal of 8 switch tubes, and jump to step 2; Step 7: This indicates that a low voltage fault occurs at the receiving end, and the active power of the low-voltage DC transmission system at both ends is unbalanced, causing the bus voltage of the DC transmission line to u dc Rapidly rising, the positive and negative bus voltage reference value of the two-stage circuit between the DC voltage outer loop of the T-type three-level grid-connected converter with the fourth bridge arm at the power transmission end U pn_ref_T Set to the minimum DC voltage value that meets the PWM rectification conditions, expressed as U pn_ref_fault At this time, the dual-loop control strategy of the DC voltage outer loop and the filter inductor current inner loop in step 5 is still used to adjust the positive and negative bus output voltages between the two-stage circuits. u pn_TReduced to 0.2 to limit the active power output at the transmission end, the active power under stable conditions of the low-voltage DC transmission system at both ends after the fault occurs is calculated according to formula (1): P fault ; Step 8, control the voltage transfer ratio of the DAB converter at the power transmission end to be 1, that is, the input DC transmission line bus voltage is equal to the output DC transmission line bus voltage. At this time, the DC transmission line bus voltage reference value of the voltage outer loop of the DAB converter is U dc_ref Equal to the input positive and negative bus voltage between the two-stage circuit U pn , still using the control method of the DC voltage outer loop and the current inner loop in step 6, and controlling the bus voltage of the DC transmission line at the power transmission end by extending the phase shift working mode; Step 9: Calculate the instantaneous active power at the transmission end according to formula (1): P T2 ,when P T2 and P fault The absolute value of the difference is less than the threshold ε1, and P T2 and P T1 When the absolute value of the difference is greater than the threshold ε2, it indicates that the low voltage fault at the receiving end still exists, and jump to step 7; Step 10, when P T2 and P fault The absolute value of the difference is not less than the threshold ε2, and P T2 and P T1 When the absolute value of the difference is not greater than the threshold ε1, it indicates that the low voltage fault at the receiving end disappears, the control mode returns to normal, and jumps to step 2.

[0008] The specific process of coordinated control of low voltage fault ride-through at the receiving end is as follows: Step 1, initializing the parameters of the low voltage direct current transmission system at the power transmission end; Step 2: Collect the three-phase voltage of the AC grid at the receiving end u a_R , u b_R , u c_R , Three-phase filter inductor current of T-type three-level grid-connected converter with fourth bridge arm i a_R , i b_R , i c_R, DC transmission line bus voltage u dc , positive and negative bus voltage between two-stage circuits u pn_R ; Step 3: Control the voltage transfer ratio of the DAB converter at the receiving end to be 1, so the positive and negative bus voltage reference values ​​of the two-stage circuit are given. U pn_ref_R Equal to the bus voltage of the DC transmission line u dc ,DAB converter adopts the extended phase shift control method of voltage outer loop and current inner loop to generate PWM signals of 8 switch tubes; Step 4: Calculate the RMS value of each phase voltage at the grid connection point in real time U a_RMS , U b_RMS , U c_RMS ; Step 5: Calculate the voltage drop depth of each phase of the grid based on the grid voltage M a , M b , M c ; Step 6: According to the national standard, the dynamic reactive current output by the inverter is required to be I q The voltage change of the grid connection point should be tracked in real time, and equation (4) is obtained: (4) Where, K1 is the ratio of the dynamic reactive current and voltage change of the inverter output. The value range of K1 should be 1.5~2.5, K1=2, I N is the rated output current value of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end; Introducing the power factor angle To limit the output current during low voltage fault ride-through, we can get: (5) (6) In the formula, I q is the dynamic reactive current, I d is the dynamic active current; Substituting equation (5) into equation (4), we get i ( i =a,b,c) The expression (7) for the phase power factor angle is as follows: (7) According to the drop depth of each phase M i To obtain the phase of the output current reference value of each phase of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end, i The phase of the phase output current reference value is equal to i Phase power factor angle; Step 7, calculate the reference value of the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end I a_ref_R , I b_ref_R、 I c_ref_R , the calculation process is as follows: In principle, there are two requirements for deriving the current reference value. First, the sum of the three-phase modulation current is 0 to reduce the double frequency fluctuation of the positive and negative bus voltages. Second, the reference current of the drop phase is increased to the amplitude to raise the drop phase voltage. Taking phase a ground fault as an example, the voltage drop depth of phase a is M When ≥0.7, the zero-sequence current is taken as 0 as the principle, the reference amplitude of the a-phase current is the rated current amplitude, and the calculation formula (8) is: (8) (9) Each phase current is decomposed into d-axis and q-axis components, and the calculation formula (10) in the d-axis direction and the calculation formula (11) in the q-axis direction are obtained: (10) (11) In the formula, I mi is the three-phase current amplitude, i =a,b,c; Through the above formula, the amplitudes of other normal two-phase currents are obtained; (12) (13) Also need to consider as well as ; According to the above principles, M When ≥0.7, the zero-sequence current can be constant to 0, but M When the normal phase current cannot meet the rated current less than 1.1 times when <0.7, the current command is distributed so that the current amplitude of one phase in the normal phase is 1.1 times the rated current and the current amplitude of the other phase is 0; From this we get i ( i=a, b, c) phase inductance current reference value, as the modulation wave parameter for fault control; Step 8: Real-time detection of the three-phase voltage at the grid connection point, and selection of the maximum RMS voltage value among the three phases U RMS_MAX , coordinate control of the positive and negative bus voltage reference values ​​of the two-stage circuit U pn_ref_R , as the modulation wave parameter for fault control; Step 9: When the receiving end of the power transmission system detects the bus voltage of the DC transmission line u dc If a sudden rise in voltage indicates that a low voltage fault occurs at the receiving end, steps 7 to 10 in the coordinated control process of low voltage fault ride-through at the transmission end are executed to reduce the bus voltage of the DC transmission line and limit the active power output at the transmission end in a timely manner.

[0009] The RMS value of each phase voltage at the grid connection point U a_RMS , U b_RMS , U c_RMS The expression is: (2) In the formula, N It is the total number of sampling points in half power frequency cycle of AC grid voltage.

[0010] Voltage drop depth of each phase grid M a , M b , M c The expression is: (3) In the formula, U N It is the rated effective value of the grid phase voltage.

[0011] i The expression of the phase inductor current reference value is: (14) In the formula, is the grid angular frequency.

[0012] Two-stage circuit positive and negative bus voltage reference value U pn_ref_R The expression is: (15).

[0013] The beneficial effects of the present invention are: (1) The coordinated control method of the present invention causes the DC voltage of the transmission line to rise when the energy of the power grid at the receiving end accumulates on the bus capacitor of the transmission line. The power transmitting end senses the rise in the voltage of the transmission line, identifies the fault between the substations without interconnection, and limits the active power output in time, thereby solving the power imbalance problem when the voltage drops. (2) The coordinated control method of the present invention completely separates the three-phase grid voltage and solves the three-phase low voltage fault ride-through problem with the idea of ​​single-phase low voltage fault ride-through, thus simplifying the complexity of low voltage fault ride-through control. At the same time, the receiving-end converter selects quasi-proportional resonance to control the output current. Compared with proportional integral control, the overall control of the three-phase current responds more quickly and is more conducive to riding through low voltage faults. At the same time, the different amplitudes of the three-phase grid voltage are compensated separately. (3) The coordinated control method of the present invention fuzzifies the asymmetric fault type and no longer distinguishes between single-phase grounding, two-phase short circuit, three-phase grounding and other fault types. It simply regards them as single-phase low voltage faults, thus simplifying the fault identification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a block diagram of a short-distance two-terminal low-voltage direct current transmission system to which the method of the present invention is applied; Figure 2 It is a two-stage circuit diagram of a low voltage direct current transmission system in the method of the present invention; Figure 3 It is a block diagram of the voltage and current control of the i-phase of the T-type three-level converter at the power transmission end in the method of the present invention; Figure 4 is a control block diagram of a DAB converter at the power transmission end in the method of the present invention; Figure 5 It is a block diagram of the voltage and current control of the i-phase of the T-type three-level converter at the power receiving end in the method of the present invention; Figure 6 is a control block diagram of a DAB converter at a power receiving end in the method of the present invention; Figure 7 It is a flow chart of low voltage fault ride-through control at the power transmission end in the method of the present invention; Figure 8 It is a control flow chart of low voltage fault ride-through at the power receiving end in the method of the present invention. DETAILED DESCRIPTION

[0015] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] Example 1 The low voltage fault ride-through coordinated control method for a short-distance low voltage direct current transmission system at both ends of the present invention is applied to a short-distance low voltage direct current transmission system at both ends of the present invention. Figure 1As shown, the power of the low-voltage DC transmission system at both ends can flow in both directions, so they can be the receiving end and the transmitting end of each other. The circuit structure of the low-voltage DC transmission system at both ends is exactly the same, and both are composed of two-stage circuits of AC-DC converter and DC-DC converter. The low-voltage DC transmission system at both ends is connected through the DC bus of the transmission line, the two-stage circuits of AC-DC converter and DC-DC converter are connected through the positive and negative bus, and the AC-DC converter is connected to the AC power grid.

[0017] Specifically, each low voltage DC transmission system has two-stage circuits, such as Figure 2 As shown, the DC bus side of the transmission line is connected to a dual active bridge DC-DC converter (DAB converter for short), and the AC grid side is connected to an AC-DC converter. The AC-DC converter specifically adopts a T-type three-level grid-connected converter with a fourth bridge arm. The DAB converter is connected to the T-type three-level grid-connected converter with the fourth bridge arm through positive and negative bus bars.

[0018] The DAB converter consists of 4 switching tubes S 1. S 2. S 3. S 4 form the H bridge of the secondary side of the transformer, 4 switch tubes S 5. S 6. S 7. S 8 constitutes the H bridge of the primary side of the transformer, the bus voltage of the DC transmission line U dc With input filter capacitor C in The two ends are connected in parallel, and the positive and negative bus voltages between the two-stage circuit U pn With output filter capacitor C in The two ends of the switch tube are connected in parallel; S 1 collector and switch tube S 2, and at the same time connected to the positive and negative bus voltages between the two-stage circuit U pn The positive pole of the switch tube is connected; S 3 emitter and switch tube S 4 is connected to the emitter, and at the same time to the positive and negative bus voltages between the two-stage circuit U pn The negative pole of the switch tube is connected; S 5 collector and switch tube S 6, and at the same time connected to the DC transmission line bus voltage U dc The positive pole of the switch tube is connected; S 7's emitter and switch tube S8 is connected to the emitter of the DC transmission line bus voltage U dc The negative pole of the switch tube is connected; S 1's emitter and switch tube S 3's collector is connected, and the energy storage inductor is also connected L One end of 1, energy storage inductor L The other end of 1 is connected to one end of the secondary side of the transformer; the switch tube S 2 emitter and switch tube S 4, and at the same time connected to the other end of the secondary side of the transformer; the switch tube S 5 emitter and switch tube S 7, and at the same time connected to the same end of the primary side of the transformer; the switch tube S 6 emitter and switch tube S The collector of 8 is connected to each other, and the opposite end of the primary side of the transformer is connected at the same time.

[0019] The T-type three-level grid-connected converter with a fourth bridge arm includes a positive DC bus capacitor C1, one end of the positive DC bus capacitor C1 and a positive and negative bus voltage between the two-stage circuit. U pn The positive electrode of the positive DC bus capacitor C1 is connected to one end of the negative DC bus capacitor C2, and the other end of the negative DC bus capacitor C2 is connected to the positive and negative bus voltage between the two-stage circuit. U pn The negative electrode of the DC bus capacitor C2 is connected to the positive DC bus capacitor C1, and the midpoint between the negative DC bus capacitor C2 and the positive DC bus capacitor C1 is the neutral point O; S x1 , S x2 , S x3 , S x4 ( x =a, b, c) form the three-phase bridge arm of the three-level converter, and the two switching tubes S n1 , S n4 The fourth bridge arm of the converter is formed, and the neutral point of the grid is connected to the midpoint of the fourth bridge arm through a filter inductor; the switch tube S x1 The emitter and switch tube S x4 The collector of the switch tube is connected to S x1 The collector of the fourth bridge arm switch tube is connected S n1 The collector of the two-stage circuit is connected to the positive and negative bus voltages. U pnThe positive pole of the switch tube is connected; S x2 The emitter and switch tube S x3 The emitter of the switch is connected to S x2 The collector of the switch is connected to the neutral point O; S x3 The collector and switch tube S x1 The emitter of the switch tube is connected S x4 The collector of the switch tube S n1 The emitter and switch tube S n4 The collector of the switch tube is connected to S n4 The positive and negative bus voltage between the transmitter and the two-stage circuit U pn The negative pole of the switch tube is connected; S n4 The emitter and switch tube S x4 The emitter is connected to the positive and negative bus voltages between the two-stage circuits. U pn The negative pole of the switch tube is connected to S a3 The collector of the filter inductor L ai One end of the filter inductor is connected to L ai The other end of the filter capacitor C a One end of the grid connection, filter capacitor C a The other end of the filter capacitor C b One end of the filter capacitor C b The other end of the filter inductor L bi One end, grid connection, filter inductor L bi The other end of the switch tube S b3 The collector of the switch tube S c3 The collector of the filter inductor L ci One end of the filter inductor is connected to L ci The other end of the filter capacitor C c One end of the grid connection, filter capacitorC c The other end of the filter capacitor C b one end of the .

[0020] Example 2 The invention discloses a low voltage fault ride-through coordinated control method for a short-distance two-end low voltage direct current transmission system. A two-stage circuit located at a power transmission end transmits active power from an AC network side to a DC transmission line side. A T-type three-level grid-connected converter with a fourth bridge arm works in a PWM rectification state to stabilize the positive and negative bus voltages between the two-stage circuits. A DAB converter works in a phase-shifting working mode to stabilize the bus voltage of the DC transmission line, thereby realizing low voltage fault ride-through coordinated control at a power transmission end side. A two-stage circuit located at a power receiving end absorbs active power transmitted from the power transmission end from the DC transmission line side and transmits reactive power to the AC network side of the power receiving end. The DAB converter works in a phase-shifting working mode to stabilize the positive and negative bus voltages between the two-stage circuits. A T-type three-level grid-connected converter with a fourth bridge arm works in a PWM inversion state to perform low voltage fault ride-through coordinated control at a power receiving end side.

[0021] The specific process of coordinated control of low voltage fault ride-through at the transmission end is as follows: Figure 7 As shown: Step 1, initializing the parameters of the low voltage DC transmission system at the power transmission end; Step 2: Give the positive and negative bus voltage reference values ​​between the two-stage circuits during normal operation U pn_ref_T , DC transmission line bus voltage reference value U dc_ref , The upper limit threshold of the bus voltage of the DC transmission line when a low voltage fault occurs at the receiving end U dc_max ; Step 3: Collect the three-phase voltage of the AC power grid at the power transmission end u a_T , u b_T , u c_T , the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm i a_T , i b_T , i c_T , positive and negative bus voltage between two-stage circuits u pn_T , positive and negative bus output current between two-stage circuits i pn_T , DC transmission line bus voltage u dc ; Step 4: When the bus voltage of the DC transmission line u dc Less than the upper threshold U dc_max When the low-voltage DC transmission systems at both ends are in normal working condition, jump to step 5; when the bus voltage of the DC transmission line u dc Greater than or equal to the upper threshold U dc_max When the low-voltage DC transmission system at the receiving end is in a low-voltage fault ride-through state, jump to step 7; Step 5, such as Figure 3 As shown in the figure, the T-type three-level grid-connected converter with the fourth bridge arm is in normal working state, and the proportional resonance control is used to control the three-phase filter inductor current. i a_T , i b_T , i c_T As the current inner loop control, PI control is used to adjust the positive and negative bus voltages between the two-stage circuits. U pn Adjustment is performed as the voltage outer loop control to generate PWM signals for the four bridge arms. The active power under normal working conditions is calculated according to formula (1): P T1 ; (1) Step 6, such as Figure 4 As shown in the figure, the DAB converter is in normal working state, and the PI control is used to control the bus voltage of the DC transmission line. u dc As the voltage outer loop control, PI control is used to adjust the bus current of the DC transmission line i dc Adjust as the current inner loop control, generate the phase shift angle D0, calculate the extended phase shift angle D1 through the extended phase shift algorithm, generate the PWM signal of 8 switch tubes, and jump to step 2; Step 7: This indicates that a low voltage fault occurs at the receiving end, and the active power of the low-voltage DC transmission system at both ends is unbalanced, causing the bus voltage of the DC transmission line to u dc Rapidly rising, the positive and negative bus voltage reference value of the two-stage circuit between the DC voltage outer loop of the T-type three-level grid-connected converter with the fourth bridge arm at the power transmission end U pn_ref_T Set to the minimum DC voltage value that meets the PWM rectification conditions, expressed as U pn_ref_fault ,For example U pn_ref_fault =0.2 UN_peak ( U N_peak Refers to the measured peak value of the AC power grid line voltage at the power transmission end). At this time, the dual-loop control strategy of the DC voltage outer loop and the filter inductor current inner loop in step 5 is still used to control the positive and negative bus output voltages between the two-stage circuits. u pn_T Reduced to 0.2 to limit the active power output at the transmission end, the active power under stable conditions of the low-voltage DC transmission system at both ends after the fault occurs is calculated according to formula (1): P fault ; Step 8, control the voltage transfer ratio of the DAB converter at the power transmission end to be 1, that is, the input DC transmission line bus voltage is equal to the output DC transmission line bus voltage. At this time, the DC transmission line bus voltage reference value of the voltage outer loop of the DAB converter is U dc_ref Equal to the input positive and negative bus voltage between the two-stage circuit U pn , still using the control method of the DC voltage outer loop and the current inner loop in step 6, controlling the bus voltage of the DC transmission line at the power transmission end by extending the phase shift working mode, so that the voltage transfer ratio between the input bus voltage of the DC transmission line and the output bus voltage of the DC transmission line is maintained at 1; Step 9: Calculate the instantaneous active power at the transmission end according to formula (1): P T2 ,when P T2 and P fault The absolute value of the difference is less than the threshold ε1, and P T2 and P T1 When the absolute value of the difference is greater than the threshold ε2, it indicates that the low voltage fault at the receiving end still exists, and jump to step 7; Step 10, when P T2 and P fault The absolute value of the difference is not less than the threshold ε2, and P T2 and P T1 When the absolute value of the difference is not greater than the threshold ε1, it indicates that the low voltage fault at the receiving end disappears, the receiving end outputs active power normally, resulting in an increase in the active power transmitted by the transmitting end, and the control mode returns to normal, that is, control is performed according to the control mode before the fault occurs, and jump to step 2.

[0022] The specific process of coordinated control of low voltage fault ride-through at the receiving end is as follows: Figure 8 As shown: Step 1, initializing the parameters of the low voltage direct current transmission system at the power transmission end; Step 2, such as Figure 5 As shown, the three-phase voltage of the AC power grid at the receiving end is collected u a_R , u b_R , u c_R , Three-phase filter inductor current of T-type three-level grid-connected converter with fourth bridge arm i a_R , i b_R , i c_R , DC transmission line bus voltage u dc , positive and negative bus voltage between two-stage circuits u pn_R ; Step 3: Control the voltage transfer ratio of the DAB converter at the receiving end to be 1, so the positive and negative bus voltage reference values ​​of the two-stage circuit are given. U pn_ref_R Equal to the bus voltage of the DC transmission line u dc ,The DAB converter adopts an extended phase-shift control method with a voltage outer loop and a current inner loop, so that the voltage transfer ratio between the input DC transmission line bus voltage and the output DC transmission line bus voltage is kept at 1, and PWM signals of 8 switch tubes are generated; Step 4: Calculate the RMS value of each phase voltage at the grid connection point in real time U a_RMS , U b_RMS , U c_RMS , the expression is as follows: (2) In the formula, N is the total number of sampling points in half a power frequency cycle of the AC grid voltage; Step 5: Calculate the voltage drop depth of each phase of the grid based on the grid voltage M a , M b , M c , the expression is as follows: (3) In the formula, U N is the rated effective value of the grid phase voltage, that is, U N =220V; Step 6: According to the national standard, the dynamic reactive current output by the inverter is required to be I q The voltage change of the grid connection point should be tracked in real time, and equation (4) is obtained: (4) Where, K1 is the ratio of the dynamic reactive current and voltage change of the inverter output. The value range of K1 should be 1.5~2.5, K1=2, I N is the rated output current value of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end; Introducing the power factor angle To limit the output current during low voltage fault ride-through, we can get: (5) (6) In the formula, I q is the dynamic reactive current, I d is the dynamic active current.

[0023] Substituting equation (5) into equation (4), we get i ( i =a,b,c) The expression (7) for the phase power factor angle is as follows: (7) According to the drop depth of each phase M i To obtain the phase of the output current reference value of each phase of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end, i The phase of the phase output current reference value is equal to i Phase power factor angle; Step 7, calculate the reference value of the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end I a_ref_R , I b_ref_R、 I c_ref_R , the calculation process is as follows: In principle, there are two requirements for deriving the current reference value. First, the sum of the three-phase modulation current is 0 to reduce the double frequency fluctuation of the positive and negative bus voltages. Second, the reference current of the drop phase is increased to the amplitude to raise the drop phase voltage. Taking phase a ground fault as an example, the voltage drop depth of phase a is M When ≥0.7, the zero-sequence current is taken as 0 as the principle, the reference amplitude of the a-phase current is the rated current amplitude, and the calculation formula (8) is: (8) (9) Each phase current is decomposed into d-axis and q-axis components, and the calculation formula (10) in the d-axis direction and the calculation formula (11) in the q-axis direction are obtained: (10) (11) In the formula, I mi is the three-phase current amplitude, i =a,b,c; Through the above formula, the amplitudes of other normal two-phase currents are obtained.

[0024] (12) (13) Also need to consider as well as

[0025] According to the above principles, M When ≥0.7, the zero-sequence current can be constant to 0, but M When the value is less than 0.7, the normal phase current cannot meet the rated current of less than 1.1 times, so the zero-sequence current can be appropriately released, and the current command is distributed so that the current amplitude of one phase in the normal phase is 1.1 times the rated current, and the current amplitude of the other phase is 0, so as to maintain the stability of the circuit; From this we get i ( i =a,b,c) phase inductor current reference values ​​are as follows: (14) In the formula, is the grid angular frequency, which is 314pi; The inductor current reference value is the target reference value in the control, which is the parameter of the modulation wave and is the modulation wave parameter for control during a fault.

[0026] Step 8, such as Figure 6 As shown, the three-phase voltage of the grid point is detected in real time, and the maximum RMS voltage value among the three phases is selected. U RMS_MAX , coordinate control of the positive and negative bus voltage reference values ​​of the two-stage circuit U pn_ref_R , the calculation formula is as follows: (15) Two-stage circuit positive and negative bus voltage reference value U pn_ref_RIt is the target reference value in DAB fault control. In order to reduce the bus voltage of the DC transmission line, the positive and negative bus voltage reference values ​​of the two-stage circuit are the modulation wave parameters controlled during faults. Step 9: When the receiving end of the power transmission system detects the bus voltage of the DC transmission line u dc If a sudden rise in voltage indicates that a low voltage fault occurs at the receiving end, steps 7 to 10 in the coordinated control process of low voltage fault ride-through at the transmission end are executed to reduce the bus voltage of the DC transmission line and limit the active power output at the transmission end in a timely manner.

[0027] In the method of the present invention, the PI control is composed of a proportional controller (P) and an integral controller (I), wherein the proportional controller is used for rapid response, and the integral controller is used for eliminating system static errors and improving stability. By adjusting the proportional coefficient and the integral time constant, the performance of the system can be optimized to achieve the best control effect. The extended phase shift algorithm, PI control and proportional resonance control are all conventional technologies in the art, and the present invention will not elaborate on them in detail.

[0028] Example 3 The coordinated control of low voltage fault ride-through at the receiving end side, taking phase A drop as an example, divides the low voltage ride-through process of the circuit into two stages: Shallow voltage drop stage 1: When the voltage drop depth of phase a at the grid connection point is M Less than 0.9 and greater than or equal to 0.5, it enters the shallow voltage drop stage. According to the power factor angle calculation formula (7), the phase of the modulation wave of the a-phase current is calculated, and the amplitude of the a-phase output current is equal to the rated current amplitude to support the grid voltage. Since only the a-phase voltage drops, the b and c-phase voltages are normal, and there is no need to consider reactive support. Based on the principle of zero-sequence current being 0, the amplitude of the b and c-phase currents is reduced, thereby achieving smooth and safe fault crossing. To prevent power backflow, the positive and negative bus voltage reference values ​​of the two-stage circuit are U pn_ref_R As the target reference value, that is, the positive and negative bus voltage reference values ​​of the two-stage circuit are the modulation wave parameters controlled during a fault, thereby reducing the bus voltage of the DC transmission line to reduce the fluctuation of the DC bus voltage caused by the grid voltage fault.

[0029] Voltage drop depth stage 2: When the voltage drop depth of phase a at the grid connection point is M Less than 0.5, it enters the voltage deep drop stage, the phase of the modulation wave of the a-phase current is constant at π / 2, the active power output is 0, only reactive power is injected, the output current amplitude is equal to the rated current amplitude, and the active power transmission can be appropriately reduced when the other two-phase voltages are normal; the positive and negative bus voltage reference values ​​of the two-stage circuit U pn_ref_RAs the target reference value, that is, the positive and negative bus voltage reference values ​​of the two-stage circuit are the modulation wave parameters controlled during a fault, thereby reducing the bus voltage of the DC transmission line and the DC bus voltage reference value to reduce the fluctuation of the DC bus voltage.

Claims

1. A coordinated control method for low voltage fault ride-through of a short-distance two-terminal low voltage direct current transmission system, characterized in that: The two-stage circuit at the transmission end transmits active power from the AC grid side to the DC transmission line side, the T-type three-level grid-connected converter with the fourth bridge arm works in the PWM rectification state to stabilize the positive and negative bus voltages between the two-stage circuits, and the DAB converter works in the phase-shifting working mode to stabilize the bus voltage of the DC transmission line, thereby realizing coordinated control of low-voltage fault ride-through on the transmission end side; the two-stage circuit at the receiving end absorbs the active power transmitted by the transmission end from the DC transmission line side and transmits reactive power to the AC grid side of the receiving end, the DAB converter works in the phase-shifting working mode to stabilize the positive and negative bus voltages between the two-stage circuits, and the T-type three-level grid-connected converter with the fourth bridge arm works in the PWM inversion state to perform coordinated control of low-voltage fault ride-through on the receiving end side.

2. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 1 is characterized in that: The specific process of coordinated control of low voltage fault ride-through at the transmission end is as follows: Step 1, initializing the parameters of the low voltage DC transmission system at the power transmission end; Step 2: Give the positive and negative bus voltage reference values ​​between the two-stage circuits during normal operation U pn_ref_T , DC transmission line bus voltage reference value U dc_ref , The upper limit threshold of the bus voltage of the DC transmission line when a low voltage fault occurs at the receiving end U dc_max ; Step 3: Collect the three-phase voltage of the AC power grid at the power transmission end u a_T , u b_T , u c_T , the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm i a_T , i b_T , i c_T , positive and negative bus voltage between two-stage circuits u pn_T , positive and negative bus output current between two-stage circuits i pn_T , DC transmission line bus voltage u dc ; Step 4: When the bus voltage of the DC transmission line u dc Less than the upper threshold U dc_max When the low-voltage DC transmission systems at both ends are in normal working condition, jump to step 5; when the bus voltage of the DC transmission line u dc Greater than or equal to the upper threshold U dc_max When the low-voltage DC transmission system at the receiving end is in a low-voltage fault ride-through state, jump to step 7; Step 5: The T-type three-level grid-connected converter with the fourth bridge arm is in normal working state, and the three-phase filter inductor current is controlled by proportional resonance control. i a_T , i b_T , i c_T As the current inner loop control, PI control is used to adjust the positive and negative bus voltages between the two-stage circuits. U pn Adjustment is performed as the voltage outer loop control to generate PWM signals for the four bridge arms. The active power under normal working conditions is calculated according to formula (1): P T1 ; （1） Step 6: The DAB converter is in normal working state, and the bus voltage of the DC transmission line is controlled by PI control. u dc As the voltage outer loop control, PI control is used to adjust the bus current of the DC transmission line i dc Adjust as the current inner loop control, generate the phase shift angle D0, calculate the extended phase shift angle D1 through the extended phase shift algorithm, generate the PWM signal of 8 switch tubes, and jump to step 2; Step 7: This indicates that a low voltage fault occurs at the receiving end, and the active power of the low-voltage DC transmission system at both ends is unbalanced, causing the bus voltage of the DC transmission line to u dc Rapidly rising, the positive and negative bus voltage reference value of the two-stage circuit between the DC voltage outer loop of the T-type three-level grid-connected converter with the fourth bridge arm at the power transmission end U pn_ref_T Set to the minimum DC voltage value that meets the PWM rectification conditions, expressed as U pn_ref_fault At this time, the dual-loop control strategy of the DC voltage outer loop and the filter inductor current inner loop in step 5 is still used to adjust the positive and negative bus output voltages between the two-stage circuits. u pn_T Reduced to 0.2 to limit the active power output at the transmission end, the active power under stable conditions of the low-voltage DC transmission system at both ends after the fault occurs is calculated according to formula (1): P fault ; Step 8, control the voltage transfer ratio of the DAB converter at the power transmission end to be 1, that is, the input DC transmission line bus voltage is equal to the output DC transmission line bus voltage. At this time, the DC transmission line bus voltage reference value of the voltage outer loop of the DAB converter is U dc_ref Equal to the input positive and negative bus voltage between the two-stage circuit U pn , still using the control method of the DC voltage outer loop and the current inner loop in step 6, and controlling the bus voltage of the DC transmission line at the power transmission end by extending the phase shift working mode; Step 9: Calculate the instantaneous active power at the transmission end according to formula (1): P T2 ,when P T2 and P fault The absolute value of the difference is less than the threshold ε1, and P T2 and P T1 When the absolute value of the difference is greater than the threshold ε2, it indicates that the low voltage fault at the receiving end still exists, and jump to step 7; Step 10, when P T2 and P fault The absolute value of the difference is not less than the threshold ε2, and P T2 and P T1 When the absolute value of the difference is not greater than the threshold ε1, it indicates that the low voltage fault at the receiving end disappears, the control mode returns to normal, and jumps to step 2.

3. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 2 is characterized in that: The specific process of coordinated control of low voltage fault ride-through at the receiving end is as follows: Step 1, initializing the parameters of the low voltage direct current transmission system at the power transmission end; Step 2: Collect the three-phase voltage of the AC grid at the receiving end u a_R , u b_R , u c_R , Three-phase filter inductor current of T-type three-level grid-connected converter with fourth bridge arm i a_R , i b_R , i c_R , DC transmission line bus voltage u dc , positive and negative bus voltage between two-stage circuits u pn_R ; Step 3: Control the voltage transfer ratio of the DAB converter at the receiving end to be 1, so the positive and negative bus voltage reference values ​​of the two-stage circuit are given. U pn_ref_R Equal to the bus voltage of the DC transmission line u dc ,DAB converter adopts the extended phase shift control method of voltage outer loop and current inner loop to generate PWM signals of 8 switch tubes; Step 4: Calculate the RMS value of each phase voltage at the grid connection point in real time U a_RMS , U b_RMS , U c_RMS ; Step 5: Calculate the voltage drop depth of each phase of the grid based on the grid voltage M a , M b , M c ; Step 6: According to the national standard, the dynamic reactive current output by the inverter is required to be I q The voltage change of the grid connection point should be tracked in real time, and equation (4) is obtained: （4） Where, K1 is the ratio of the dynamic reactive current and voltage change of the inverter output. The value range of K1 should be 1.5~2.5, K1=2, I N is the rated output current value of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end; Introducing the power factor angle To limit the output current during low voltage fault ride-through, we can get: （5） （6） In the formula, I q is the dynamic reactive current, I d is the dynamic active current; Substituting equation (5) into equation (4), we get i ( i =a,b,c) The expression (7) for the phase power factor angle is as follows: （7） According to the drop depth of each phase M i To obtain the phase of the output current reference value of each phase of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end, i The phase of the phase output current reference value is equal to i Phase power factor angle; Step 7, calculate the reference value of the three-phase filter inductor current of the T-type three-level grid-connected converter with the fourth bridge arm at the receiving end I a_ref_R , I b_ref_R、 I c_ref_R , the calculation process is as follows: In principle, there are two requirements for deriving the current reference value. First, the sum of the three-phase modulation current is 0 to reduce the double frequency fluctuation of the positive and negative bus voltages. Second, the reference current of the drop phase is increased to the amplitude to raise the drop phase voltage. Taking phase a ground fault as an example, the voltage drop depth of phase a is M When ≥0.7, the zero-sequence current is taken as 0 as the principle, the reference amplitude of the a-phase current is the rated current amplitude, and the calculation formula (8) is: （8） （9） Each phase current is decomposed into d-axis and q-axis components, and the calculation formula (10) in the d-axis direction and the calculation formula (11) in the q-axis direction are obtained: （10） （11） In the formula, I mi is the three-phase current amplitude, i =a,b,c; Through the above formula, the amplitudes of other normal two-phase currents are obtained; （12） （13） Also need to consider as well as ; According to the above principles, M When ≥0.7, the zero-sequence current can be constant to 0, but M When the normal phase current cannot meet the rated current less than 1.1 times when <0.7, the current command is distributed so that the current amplitude of one phase in the normal phase is 1.1 times the rated current and the current amplitude of the other phase is 0; From this we get i ( i =a, b, c) phase inductance current reference value, as the modulation wave parameter for fault control; Step 8: Real-time detection of the three-phase voltage at the grid connection point, and selection of the maximum RMS voltage value among the three phases U RMS_MAX , coordinate control of the positive and negative bus voltage reference values ​​of the two-stage circuit U pn_ref_R , as the modulation wave parameter for fault control; Step 9: When the receiving end of the power transmission system detects the bus voltage of the DC transmission line u dc If a sudden rise in voltage indicates that a low voltage fault occurs at the receiving end, steps 7 to 10 in the coordinated control process of low voltage fault ride-through at the transmission end are executed to reduce the bus voltage of the DC transmission line and limit the active power output at the transmission end in a timely manner.

4. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 3 is characterized in that: The RMS value of each phase voltage at the grid connection point U a_RMS , U b_RMS , U c_RMS The expression is: （2） In the formula, N It is the total number of sampling points in half power frequency cycle of AC grid voltage.

5. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 4 is characterized in that: Voltage drop depth of each phase grid M a , M b , M c The expression is: （3） In the formula, U N It is the rated effective value of the grid phase voltage.

6. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 5 is characterized in that: i The expression of the phase inductor current reference value is: （14） In the formula, is the grid angular frequency.

7. The low voltage fault ride-through coordinated control method for a short-distance two-terminal low voltage direct current transmission system according to claim 6 is characterized in that: Two-stage circuit positive and negative bus voltage reference value U pn_ref_R The expression is: （15）。

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