Low-voltage fault ride-through coordination control method for short-distance two-terminal low-voltage DC power transmission system

Abstract

The application discloses a low-voltage fault ride-through coordination control method of a close-range low-voltage DC power transmission system, a two-stage circuit at a power transmission end transmits active power from an AC network side to a DC power transmission line side, 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 voltages between the two-stage circuits, and a DAB converter works in a phase-shift working mode to stabilize a DC power transmission line bus voltage, so that low-voltage fault ride-through coordination control at the power transmission end side is realized; a two-stage circuit at a power receiving end absorbs active power from the DC power transmission line side and transmits reactive power to the AC network side, the DAB converter works in the phase-shift 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 a PWM inversion state to perform low-voltage fault ride-through coordination control at the power receiving end side. The method realizes the compensation of different amplitudes of three-phase grid voltages respectively.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a low-voltage fault ride-through coordination control method of a near-distance two-end low-voltage DC power transmission system. BACKGROUND

[0002] In recent years, with the increase of the types of power consumption of users in the distribution network, the power demand of different distribution areas presents a diversification trend. The distance between adjacent distribution areas is generally short, and there are generally large load demand in some distribution areas and small load demand in some distribution areas. At present, for the case of large load demand in the distribution area, only measures such as transformer capacity expansion or introduction of new energy generation can be taken to meet the increasing demand for power and power reliability. These solutions often come with high construction costs, and at the same time, the existing distribution network is difficult to effectively optimize the load demand of each area and improve the utilization rate of distribution transformers, and the market demand potential is large. With the rapid development of power electronics technology, low-voltage DC power transmission is one of the best ways to solve the above problems, that is, a low-voltage DC power transmission system is installed near the transformer of each adjacent distribution area, and the excess energy of the lightly loaded area transformer can be transmitted to the adjacent distribution area with overload load demand through the low-voltage DC bus, thereby optimizing the load demand between distribution areas and improving the utilization rate of area transformers. However, when a transient fault such as single-phase grounding, two-phase short-circuit, three-phase grounding fault occurs in a distribution area, the two-end low-voltage DC power transmission system needs to be coordinated and controlled to effectively ride through the low-voltage fault.

[0003] At present, low-voltage DC power transmission systems for the above purposes are not common. In order to adapt to the frequent load imbalance of low-voltage distribution systems, the low-voltage DC power transmission system needs to adopt a four-bridge converter structure, and a high-frequency transformer is used for electrical isolation, so the low-voltage DC power 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 two-end low-voltage DC power transmission system is interconnected through a DC bus. When a transient fault such as single-phase grounding, two-phase short-circuit, three-phase grounding fault occurs in one end of the power grid, the low-voltage DC power transmission system at the fault end is mainly responsible for the support control of reactive power, and the low-voltage DC power transmission system at the normal end is mainly responsible for the limit 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 of a single new energy generation grid-connected converter for low-voltage ride-through is to output active current and reactive current symmetrically according to the three-phase fundamental positive sequence voltage drop depth during the voltage drop of the power grid, and does not distinguish which fault it is, and cannot achieve separate compensation of different amplitudes of three-phase grid voltage, resulting in overcompensation of some phases and undercompensation of some phases. SUMMARY

[0005] The application aims to provide a low-voltage fault ride-through coordination control method for a short-distance two-terminal low-voltage DC power transmission system, and solve the problem that the existing control method cannot realize the separate compensation of different amplitudes of three-phase power grid voltage, resulting in over-compensation of a certain phase and under-compensation of a certain phase.

[0006] The technical solution of the application is as follows: the low-voltage fault ride-through coordination control method for a short-distance two-terminal low-voltage DC power transmission system, a two-stage circuit located at a power transmission end transmits active power from an AC grid side to a DC power 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-shift working mode to stabilize the DC power transmission line bus voltage, so as to realize low-voltage fault ride-through coordination control at the power transmission end side; the two-stage circuit located at a power receiving end absorbs the active power transmitted by the power transmission end from the DC power transmission line side and transmits reactive power to the AC grid side of the power receiving end, the DAB converter works in the phase-shift 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 a PWM inversion state to perform low-voltage fault ride-through coordination control at the power receiving end side.

[0007] The application also has the characteristics that,

[0008] The specific process of the low-voltage fault ride-through coordination control at the power transmission end side is as follows:

[0009] Step 1: parameter initialization of the low-voltage DC power transmission system at the power transmission end;

[0010] Step 2: setting the positive and negative bus voltage reference values between the two-stage circuits in normal working state U pn_ref_T , the DC power transmission line bus voltage reference value U dc_ref , and the upper threshold value of the DC power transmission line bus voltage when a low-voltage fault occurs at the power receiving end U dc_max ;

[0011] Step 3: collecting the three-phase voltages of the AC grid at the power transmission end u a_T , u b_T , u c_T , the three-phase filter inductor currents of the T-type three-level grid-connected converter with the fourth bridge arm i a_T , i b_T , i c_T , the positive and negative bus voltages between the two-stage circuits u pn_T , and the positive and negative bus output currents between the two-stage circuits ipn_T , DC line bus voltage u dc ;

[0012] Step 4, when the DC line bus voltage u dc is less than the upper threshold value U dc_max , the two-end low-voltage DC power transmission system is in a normal working state, and the process jumps to Step 5; when the DC line bus voltage u dc is greater than or equal to the upper threshold value U dc_max , the receiving-end low-voltage DC power transmission system is in a low-voltage fault ride-through state, and the process jumps to Step 7;

[0013] Step 5, the T-type three-level grid-connected converter with the fourth bridge arm is in a normal working state, a proportional-resonant control is used to adjust the three-phase filter inductor current i a_T , i b_T , i c_T , as a current inner loop control, a PI control is used to adjust the positive and negative bus voltages between the two-stage circuits U pn , as a voltage outer loop control, and PWM signals of the four bridge arms are generated, and the active power in the normal working state is calculated according to formula (1) P T1 ;

[0014] (1)

[0015] Step 6, the DAB converter is in a normal working state, a PI control is used to adjust the DC line bus voltage u dc , as a voltage outer loop control, and a PI control is used to adjust the DC line bus current i dc , as a current inner loop control, a phase-shift angle D0 is generated, an extended phase-shift angle D1 is calculated through an extended phase-shift algorithm, PWM signals of the eight switching tubes are generated, and the process jumps to Step 2;

[0016] Step 7, at this time, it is indicated that a low-voltage fault occurs at the receiving end, and the active power imbalance of the two-end low-voltage DC power transmission system causes the DC line bus voltage u dc to rapidly rise, and the positive and negative bus voltage reference values between the two-stage circuits of the DC voltage outer loop of the T-type three-level grid-connected converter with the fourth bridge arm at the sending end U pn_ref_TThe lowest DC voltage value set to meet the PWM rectification condition is denoted as Vdcmin U pn_ref_fault At this time, the double-loop control strategy of the DC voltage outer loop and the filter inductor current inner loop in step 5 is still adopted, and the positive and negative bus output voltages between the two-stage circuits are u pn_T reduced to 0.2 to limit the active power output of the power transmission end, and the active power under the stable condition of the low-voltage DC power transmission system after the fault occurs and at both ends is calculated according to formula (1) P fault

[0017] Step 8, the voltage transfer ratio of the DAB converter of the power transmission end is controlled 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 U dc_ref is equal to the input positive and negative bus voltages between the two-stage circuits U pn , the control method of the DC voltage outer loop and the current inner loop in step 6 is still adopted, and the DC transmission line bus voltage of the power transmission end is controlled by extending the phase-shifted operation mode;

[0018] Step 9, the instantaneous active power of the power transmission end is calculated according to formula (1) P T2 When the absolute value of the difference between P T2 and P fault is less than a threshold value ε1, and the absolute value of the difference between P T2 and P T1 is greater than a threshold value ε2, it is indicated that the low-voltage fault of the power receiving end still exists, and the process jumps to step 7;

[0019] Step 10, when the absolute value of the difference between P T2 and P fault is not less than a threshold value ε2, and the absolute value of the difference between P T2 and P T1 is not greater than a threshold value ε1, it is indicated that the low-voltage fault of the power receiving end disappears, and the control mode returns to normal, and the process jumps to step 2.

[0020] The specific process of the low-voltage fault ride-through coordination control on the power receiving end side is as follows:

[0021] Step 1, the low-voltage DC power transmission system parameters of the power transmission end are initialized;

[0022] Step 2, the three-phase AC grid voltage of the power receiving end is acquired​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 circuit u pn_R ;

[0023] Step 3, control the voltage transfer ratio of the DAB converter at the power receiving end to be 1, so as to give the positive and negative bus voltage reference value of the two-stage circuit U pn_ref_R equal to the DC transmission line bus voltage u dc , the DAB converter adopts an extended phase-shift control method with voltage outer loop and current inner loop to generate PWM signals of 8 switching tubes;

[0024] Step 4, calculate the root mean square value of each phase voltage at the grid connection point in real time U a_RMS 、 U b_RMS 、 U c_RMS ;

[0025] Step 5, calculate the voltage drop depth of each phase grid voltage according to the grid voltage M a 、 M b 、 M c ;

[0026] Step 6, according to the national standard, the dynamic reactive current output by the inverter I q should track the voltage change at the grid connection point in real time, and formula (4) is obtained:

[0027] (4)

[0028] wherein K1 is the proportion value of the dynamic reactive current output by the inverter to the voltage change, 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 fourth bridge arm at the power receiving end;

[0029] introduce the power factor angle To limit the output current during low voltage fault ride through, we have:

[0030] (5)

[0031] (6)

[0032] where, I q is the dynamic reactive current, I d is the dynamic active current;

[0033] Substitute equation (5) into equation (4), we have i ( i The expression of phase power factor angle (7) is as follows:

[0034] (7)

[0035] According to the depth of each phase drop M i To get the phase of the output current reference value of the T-type three-level grid-connected converter with the fourth bridge arm at the power receiving end, i The phase of the output current reference value is equal to i the phase power factor angle;

[0036] Step 7, calculate the three-phase filter inductance current reference value of the T-type three-level grid-connected converter with the fourth bridge arm at the power receiving end I a_ref_R , I b_ref_R、 I c_ref_R The calculation process is as follows:

[0037] To derive the current reference value, there are two requirements in principle, one is that the sum of the three-phase modulation currents is 0, to reduce the double frequency fluctuation of the positive and negative bus voltage, the second is to increase the reference current of the drop phase to the amplitude, to raise the drop phase voltage;

[0038] Take the a-phase ground fault as an example, b-phase and c-phase are the same, the a-phase voltage drop depth M ≥0.7, take the zero sequence current as 0 as the principle, the reference amplitude of the a-phase current is the rated current amplitude, the calculation formula (8) is:

[0039] (8)

[0040] (9)

[0041] Each phase current is decomposed into d-axis and q-axis components, to get the calculation formula (10) of the d-axis direction and the calculation formula (11) of the q-axis direction:

[0042] (10)

[0043] (11)

[0044] wherein, I mi is the three-phase current amplitude, i =a,b,c;

[0045] By the above formula, the amplitude of the other normal two-phase current is obtained;

[0046] (12)

[0047] (13)

[0048] At the same time, the following needs to be considered and ;

[0049] According to the above principle, M when the zero sequence current is constant 0, but M when the normal phase current cannot meet the rated current less than 1.1 times, the current command is distributed as the normal phase current amplitude being 1.1 times of the rated current and the other phase current amplitude being 0;

[0050] Thus, we get i ( i =a,b,c) phase inductance current reference value as the modulation wave parameter for control during fault;

[0051] Step 8, real-time detection of grid connection point three-phase voltage, select the maximum root mean square value of the three-phase voltage U RMS_MAX , coordinate control of two-stage circuit positive and negative bus voltage reference value U pn_ref_R as the modulation wave parameter for control during fault;

[0052] Step 9, when the receiving end of the power transmission system detects a sudden rise of the DC transmission line bus voltage u dc , indicating that the receiving end has a low voltage fault, then execute steps 7-10 in the low voltage fault ride-through coordination control process of the sending end side, reduce the DC transmission line bus voltage, and timely limit the active power output of the sending end.

[0053] The root mean square value of the voltage of each phase of the grid connection point U a_RMS , U b_RMS , U c_RMSThe expression of is:

[0054] (2)

[0055] In the formula, N is the total sampling point number in half of the power frequency cycle of the alternating current grid voltage.

[0056] The phase grid voltage drop depth M a , M b , M c The expression of is:

[0057] (3)

[0058] In the formula, U N is the rated effective value of the grid phase voltage.

[0059] i The expression of the phase inductance current reference value is:

[0060] (14)

[0061] In the formula, is the grid angular frequency.

[0062] The expression of the two-stage circuit positive and negative bus voltage reference value U pn_ref_R is:

[0063] (15).

[0064] The beneficial effects of the present application are:

[0065] (1) The coordinated control method of the present application, when the power receiving end grid energy is accumulated on the transmission line bus capacitor, causes the transmission line DC voltage to rise, and the power sending end senses the transmission line voltage rise, identifies the fault between the stations under the condition of no interconnection line, and timely limits the active power output, thereby solving the power imbalance problem when the voltage drops;

[0066] (2) The coordinated control method of the present application completely separates the three-phase grid voltage, solves the three-phase low voltage fault ride-through problem with the idea of single-phase low voltage fault ride-through, simplifies the complexity of low voltage fault ride-through control, and selects the proportional resonant control output current of the power receiving end converter, which is more rapid in response than proportional integral control, more conducive to ride-through low voltage fault, and simultaneously realizes the separate compensation of three-phase grid voltages with different amplitudes;

[0067] (3) The asymmetric fault type is fuzzed in the coordination control method, and the fault types such as single-phase grounding, two-phase short circuit and three-phase grounding are no longer distinguished, and the single-phase low-voltage fault is simply regarded as a single-phase low-voltage fault, so that the fault identification process is simplified. BRIEF DESCRIPTION OF DRAWINGS

[0068] Figure 1 is a block diagram of a short-distance two-end low-voltage DC power transmission system to which the method of the present application is applied;

[0069] Figure 2 is a two-stage circuit diagram of a low-voltage DC power transmission system in the method of the present application;

[0070] Figure 3 is a phase i voltage and current control block diagram of a T-type three-level converter at a power transmission end in the method of the present application;

[0071] Figure 4 is a control block diagram of a DAB converter at a power transmission end in the method of the present application;

[0072] Figure 5 is a phase i voltage and current control block diagram of a T-type three-level converter at a power receiving end in the method of the present application;

[0073] Figure 6 is a control block diagram of a DAB converter at a power receiving end in the method of the present application;

[0074] Figure 7 is a low-voltage fault ride-through control flowchart of a power transmission end in the method of the present application;

[0075] Figure 8 is a low-voltage fault ride-through control flowchart of a power receiving end in the method of the present application. DETAILED DESCRIPTION

[0076] The present application will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0077] Embodiment 1

[0078] The short-distance two-end low-voltage DC power transmission system applied in the low-voltage fault ride-through coordination control method of the short-distance two-end low-voltage DC power transmission system of the present application is shown in Figure 1 The power of the two-end low-voltage DC power transmission system can flow in both directions, so it can be a power receiving end and a power transmission end, the circuit structure of the two-end low-voltage DC power transmission system is completely the same, and is composed of AC-DC converter and DC-DC converter two-stage circuits, the two-end low-voltage DC power transmission system is connected through a DC bus of a transmission line, the AC-DC converter and the DC-DC converter two-stage circuits are connected through positive and negative buses, and the AC-DC converter is connected with an AC power grid.

[0079] Specifically, the two-stage circuit of each low-voltage DC power transmission system is shown in Figure 2As 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 and the T-type three-level grid-connected converter with the fourth bridge arm are connected through positive and negative busbars.

[0080] The DAB converter consists of 4 switching tubes S 1. S 2. S 3. S 4 constitute the H bridge of the secondary side of the transformer, 4 switching 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 and input filter capacitor C in The two ends are connected in parallel, and the positive and negative bus voltages between the two-stage circuits U pn and output filter capacitors C in The two ends of the switch are connected in parallel; S 1's collector and switch tube S 2, and is connected to the collector of the two-stage circuit, and is connected to the positive and negative bus voltages U pn The positive pole of the switch tube is connected; S 3's emitter and switch tube S 4's 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; S 5 collector and switch tube S 6, and is also 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 S 8 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 to the energy storage inductor at the same time 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 is connected to the collector, and is also connected to the other end of the secondary side of the transformer; the switch tubeS 5's emitter and switch tube S 7's collector is connected, and the same name end of the transformer primary side is connected; switch tube S 6's emitter and switch tube S 8's collector is connected, and the opposite name end of the transformer primary side is connected.

[0081] The T-type three-level grid-connected converter with the fourth bridge arm comprises a positive DC bus capacitor C1, one end of the positive DC bus capacitor C1 is connected with the positive bus voltage between the two-stage circuit U pn The positive electrode of the positive DC bus capacitor C1 is connected with the positive bus voltage between the two-stage circuit U pn The other end of the positive DC bus capacitor C1 is connected with one end of a negative DC bus capacitor C2, the other end of the negative DC bus capacitor C2 is connected with the negative bus voltage between the two-stage circuit S x1 、 S x2 、 S x3 、 S x4 ( x =a, b, c) constitute three-phase bridge arms of the three-level converter, two switch tubes S n1 、 S n4 constitute the fourth bridge arm of the converter, and the grid neutral point is connected with the midpoint of the fourth bridge arm through a filter inductor; switch tube S x1 's emitter and switch tube S x4 's collector is connected, and switch tube S x1 's collector is connected with the collector of switch tube S n1 's collector, and is connected with the positive bus voltage between the two-stage circuit U pn The positive electrode of the positive DC bus capacitor C1 is connected with the positive bus voltage between the two-stage circuit S x2 's emitter and switch tube S x3 's emitter is connected, and switch S x2 's collector is connected with the neutral point O; switch tube S x3 's collector and switch tube S x1 's emitter is connected, and the collector of switch tube S x4 is connected; switch tubeS n1 emitter of the transistor and the switch tube S n4 collector of the transistor and the switch tube S n4 emitter of the transistor and the positive and negative bus voltage between the two-stage circuit U pn negative of the transistor and the switch tube S n4 emitter of the transistor and the switch tube S x4 emitter of the transistor and the positive and negative bus voltage between the two-stage circuit U pn negative of the transistor and the switch tube S a3 collector of the transistor and one end of the filter inductor L ai the other end of the filter inductor L ai one end of the filter capacitor and the power grid C a the other end of the filter capacitor C a one end of the filter capacitor C b the other end of the filter capacitor C b one end of the filter inductor and the power grid L bi the other end of the filter inductor L bi collector of the switch tube S b3 collector of the switch tube S c3 collector of the transistor and one end of the filter inductor L ci the other end of the filter inductor L ci one end of the filter capacitor and the power grid C c the other end of the filter capacitor C c one end of the filter capacitor C b the other end of the filter capacitor

[0082] Embodiment 2

[0083] The low-voltage fault ride-through coordination control method of the short-distance two-end low-voltage DC power transmission system is characterized in that: the two-stage circuit at the power transmission end transmits active power from the AC grid side to the DC power transmission line side, the T-type three-level grid-connected converter with the fourth bridge arm operates in the PWM rectification state to stabilize the positive and negative bus voltages between the two-stage circuits, and the DAB converter operates in the phase-shift working mode to stabilize the DC power transmission line bus voltage, so as to realize the low-voltage fault ride-through coordination control at the power transmission end side; the two-stage circuit at the power receiving end absorbs the active power transmitted by the power transmission end from the DC power transmission line side and transmits reactive power to the AC grid side at the power receiving end, the DAB converter operates in the phase-shift 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 operates in the PWM inversion state to perform the low-voltage fault ride-through coordination control at the power receiving end side.

[0084] The specific process of the low-voltage fault ride-through coordination control at the power transmission end side is as follows, as shown in Figure 7

[0085] Step 1, parameter initialization of the low-voltage DC power transmission system at the power transmission end;

[0086] Step 2, the reference values of the positive and negative bus voltages between the two-stage circuits in normal working condition are given U pn_ref_T , the reference value of the DC power transmission line bus voltage is given U dc_ref , and the upper threshold value of the DC power transmission line bus voltage when the low-voltage fault occurs at the power receiving end is given U dc_max .

[0087] Step 3, the three-phase voltages of the AC grid at the power transmission end are collected u a_T , u b_T , u c_T , the three-phase filter inductor currents of the T-type three-level grid-connected converter with the fourth bridge arm are collected i a_T , i b_T , i c_T , the positive and negative bus voltages between the two-stage circuits are collected u pn_T , the positive and negative bus output currents between the two-stage circuits are collected i pn_T , and the DC power transmission line bus voltage is collected u dc .

[0088] Step 4, when the DC power transmission line bus voltage u dc is less than the upper threshold value U ​dc_max When the low-voltage DC transmission systems at both ends are in normal working condition, jump to step 5; when the DC transmission line bus voltage u dc Greater than or equal to the upper threshold U dc_max When , the receiving-end low-voltage DC transmission system is in the low-voltage fault ride-through state, jump to step 7;

[0089] 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 resonant control is used to adjust the three-phase filter inductor current. i a_T 、 i b_T 、 i c_T To adjust the positive and negative bus voltages between the two-stage circuits, PI control is used as the current inner loop control. U pn Adjust as the voltage outer loop control to generate PWM signals of the four bridge arms, and calculate the active power under normal working conditions according to formula (1): P T1 ;

[0090] (1)

[0091] Step 6, such as Figure 4 As shown in the figure, 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 the current as the inner loop control to generate the phase shift angle D0. Calculate the extended phase shift angle D1 through the extended phase shift algorithm to generate the PWM signals of the 8 switching tubes, and jump to step 2.

[0092] 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 DC transmission line bus voltage to u dc Rapidly rising, the positive and negative bus voltage reference value of the two-stage circuit of 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 U N_peak (U N_peak The peak value of the AC grid line voltage at the sending end is measured (denoted as Vgrid,peak), and the double-loop control strategy of the DC voltage outer loop and the filter inductor current inner loop in step 5 is still adopted to control the output voltage of the positive and negative bus lines between the two-stage circuit u pn_T to 0.2 to limit the active power output at the sending end, and the active power at the sending end under the stable condition of the low-voltage DC power transmission system after the fault occurs and the active power at the receiving end is calculated according to formula (1) P fault ;

[0093] In step 8, the voltage transfer ratio of the DAB converter at the sending end is controlled to be 1, that is, the input DC transmission line bus voltage is equal to the output DC transmission line bus voltage, and the DC transmission line bus voltage reference value of the voltage outer loop of the DAB converter U dc_ref is equal to the input voltage of the positive and negative bus lines between the two-stage circuit U pn The control method of the DC voltage outer loop and the current inner loop in step 6 is still adopted to control the DC transmission line bus voltage at the sending end by extending the phase-shifted operation mode, so that the voltage transfer ratio of the input DC transmission line bus voltage and the output DC transmission line bus voltage is maintained at 1.

[0094] In step 9, the instantaneous active power at the sending end is calculated according to formula (1) P T2 When the absolute value of the difference between Vgrid,peak and Vgrid is less than a threshold value ε1, and the absolute value of the difference between Pactive and Pactive,0 is greater than a threshold value ε2, it indicates that the low-voltage fault at the receiving end still exists, and the process jumps to step 7. P T2 and P fault When the absolute value of the difference between Vgrid,peak and Vgrid is less than a threshold value ε1, and the absolute value of the difference between Pactive and Pactive,0 is greater than a threshold value ε2, it indicates that the low-voltage fault at the receiving end still exists, and the process jumps to step 7. P T2 and P T1 When the absolute value of the difference between Vgrid,peak and Vgrid is less than a threshold value ε1, and the absolute value of the difference between Pactive and Pactive,0 is greater than a threshold value ε2, it indicates that the low-voltage fault at the receiving end still exists, and the process jumps to step 7.

[0095] In step 10, when the absolute value of the difference between Vgrid,peak and Vgrid is not less than a threshold value ε2, and the absolute value of the difference between Pactive and Pactive,0 is not greater than a threshold value ε1, it indicates that the low-voltage fault at the receiving end disappears, the active power at the receiving end increases, and the control mode returns to normal, that is, the control is performed according to the control mode before the fault occurs, and the process jumps to step 2. P T2 and P fault In step 10, when the absolute value of the difference between Vgrid,peak and Vgrid is not less than a threshold value ε2, and the absolute value of the difference between Pactive and Pactive,0 is not greater than a threshold value ε1, it indicates that the low-voltage fault at the receiving end disappears, the active power at the receiving end increases, and the control mode returns to normal, that is, the control is performed according to the control mode before the fault occurs, and the process jumps to step 2. P T2 and P T1 In step 10, when the absolute value of the difference between Vgrid,peak and Vgrid is not less than a threshold value ε2, and the absolute value of the difference between Pactive and Pactive,0 is not greater than a threshold value ε1, it indicates that the low-voltage fault at the receiving end disappears, the active power at the receiving end increases, and the control mode returns to normal, that is, the control is performed according to the control mode before the fault occurs, and the process jumps to step 2.

[0096] The specific process of the low-voltage fault ride-through coordination control at the receiving end side is as follows, for example Figure 8As shown:

[0097] Step 1: Initialize the parameters of the low voltage direct current transmission system at the power transmission end;

[0098] Step 2, such as Figure 5 As shown, the three-phase voltage of the AC 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 ;

[0099] Step 3: Control the voltage transfer ratio of the DAB converter at the receiving end to be 1, and then give the positive and negative bus voltage reference values ​​of the two-stage circuit 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 to keep the voltage transfer ratio between the input DC transmission line bus voltage and the output DC transmission line bus voltage at 1, generating PWM signals for 8 switching tubes;

[0100] 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:

[0101] (2)

[0102] Where, N is the total number of sampling points in half a power frequency cycle of the AC grid voltage;

[0103] 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:

[0104] (3)

[0105] wherein, U N is the grid phase voltage rated effective value, i.e. in our country U N =220V;

[0106] Step 6, according to the national standard, the dynamic reactive current output by the inverter I q should track the grid point voltage change in real time, and formula (4) is obtained:

[0107] (4)

[0108] wherein, K1 is the dynamic reactive current output by the inverter and the voltage change proportion value, 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 a fourth bridge arm at the power receiving end;

[0109] The power factor angle is introduced to limit the output current during low voltage fault ride through, and then formula (5) is obtained:

[0110] (5)

[0111] (6)

[0112] wherein, I q is the dynamic reactive current, I d is the dynamic active current.

[0113] Substitute formula (5) into formula (4) to obtain i ( i The expression (7) of the phase power factor angle of each phase is as follows:

[0114] (7)

[0115] According to the falling depth of each phase M i , the phase of the output current reference value of each phase of the T-type three-level grid-connected converter with a fourth bridge arm at the power receiving end is obtained, i the phase of the output current reference value of each phase is equal to i the phase power factor angle;

[0116] Step 7, calculate the three-phase filter inductance current reference value of the T-type three-level grid-connected converter with a fourth bridge arm at the power receiving end I a_ref_R , Ib_ref_R、 I c_ref_R , the calculation process is as follows:

[0117] In principle, there are two requirements for deriving the current reference value. First, the sum of the three-phase modulated current is 0 to reduce the double frequency fluctuation of the positive and negative bus voltages. Second, the reference current of the droop phase is increased to the amplitude to raise the droop phase voltage.

[0118] Taking phase a ground fault as an example, the same applies to phases b and c. 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:

[0119] (8)

[0120] (9)

[0121] 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:

[0122] (10)

[0123] (11)

[0124] Where, I mi is the three-phase current amplitude, i =a,b,c;

[0125] The amplitudes of other normal two-phase currents are obtained through the above formula.

[0126] (12)

[0127] (13)

[0128] Also need to consider as well as

[0129] 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. Therefore, the zero-sequence current can be appropriately released. 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.

[0130] From this we get i ( i=a,b,c) phase inductor current reference values ​​are as follows:

[0131] (14)

[0132] Where, is the grid angular frequency, which is 314pi;

[0133] 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 controlled during a fault.

[0134] 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 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:

[0135] (15)

[0136] Two-stage circuit positive and negative bus voltage reference values U pn_ref_R It 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.

[0137] Step 9: When the receiving end of the 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 has occurred at the receiving end, steps 7 to 10 of the coordinated control process for low voltage fault ride-through at the transmitting end are executed to reduce the bus voltage of the DC transmission line and limit the active power output at the transmitting end in a timely manner.

[0138] The PI control method in the present invention consists of a proportional controller (P) and an integral controller (I). The proportional controller provides a fast response, while the integral controller eliminates static errors in the system and improves stability. By adjusting the proportional coefficient and the integral time constant, system performance can be optimized to achieve optimal control results. Extended phase-shifting algorithms, PI control, and proportional resonant control are all conventional techniques in the art and will not be elaborated upon in detail in this invention.

[0139] Example 3

[0140] The coordinated control of low voltage fault ride-through at the receiving end is divided into two stages, taking phase A drop as an example:

[0141] Shallow voltage drop stage 1: When the voltage drop depth of phase a at the grid connection point is MLess 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. In order 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.

[0142] 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 are used as the reference values. 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 and the DC bus voltage reference value to reduce the fluctuation of the DC bus voltage.

Claims

1. A low-voltage fault ride-through coordination control method for a short-distance high-voltage direct current transmission system with low-voltage at both ends, characterized in that, The two-stage circuit at the power sending end transmits active power from the AC network 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-shifted operation mode to stabilize the DC transmission line bus voltage, thereby realizing the coordinated control of low-voltage fault ride-through at the power sending end side; the two-stage circuit at the power receiving end absorbs the active power transmitted by the power sending end from the DC transmission line side and transmits reactive power to the AC network side at the power receiving end, the DAB converter works in the phase-shifted operation 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 the coordinated control of low-voltage fault ride-through at the power receiving end side; The process of the coordinated control of low-voltage fault ride-through at the power sending end side is as follows: Step 1, initialization of the low-voltage DC transmission system parameters at the power sending end; Step 2, given the normal working two-stage circuit between the positive and negative bus voltage reference value U pn_ref_T , DC transmission line bus voltage reference value U dc_ref , when the low voltage fault occurs in the power receiving end, the upper threshold of DC transmission line bus voltage 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 , three-phase filter inductance 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 DC transmission line bus voltage u dc is less than the upper threshold value U dc_max , both ends of the low-voltage DC transmission system are in a normal working state, and jump to Step 5; when the DC transmission line bus voltage u dc is greater than or equal to the upper threshold value U dc_max , the receiving end low-voltage DC transmission system is in a low-voltage fault ride-through state, and jump to Step 7; Step 5, the T-type three-level grid-connected converter with the fourth bridge arm is in normal operation, the proportional-resonant control is used to regulate the three-phase filter inductor current i a_T 、 i b_T 、 i c_T , as the current inner loop control, the PI control is used to regulate the positive and negative bus voltage between the two-stage circuits U pn , as the voltage outer loop control, the PWM signals of the four bridge arms are generated, and the active power in the normal operation is calculated according to formula (1) P T1 ; （1） Step 6, the DAB converter is in normal working state, the PI control is used to the DC transmission line bus voltage u dc Adjustment is made, as voltage outer loop control, the PI control is used to the DC transmission line bus current i dc Adjustment is made, as current inner loop control, the phase shift angle D0 is generated, the extended phase shift angle D1 is calculated through the extended phase shift algorithm, the PWM signals of 8 switch tubes are generated, and the step 2 is jumped to. Step 7, at this time, the low voltage fault of the receiving end is indicated, the active power imbalance of the two-end HVDC system leads to the bus voltage of the DC transmission line u dc Rapidly rises, the positive and negative bus voltage reference value between the two-stage circuit of the DC voltage outer ring of the T-type three-level grid-connected converter with the fourth bridge arm of the sending end U pn_ref_T The lowest DC voltage value satisfying the PWM rectification condition is set, which is expressed as U pn_ref_fault At this time, the double-loop control strategy of the DC voltage outer ring and the filter inductor current inner ring of step 5 is still adopted, and the positive and negative bus output voltage between the two-stage circuit u pn_T is reduced to 0.2 to limit the active power output of the sending end, and the active power under the stable condition of the two-end HVDC system after the fault occurs is calculated according to formula (1) P fault ; Step 8, the voltage transfer ratio of the DAB converter of the power sending end is controlled 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 U dc_ref is equal to the input two-stage inter-circuit positive and negative bus voltage U pn , still using the control method of the DC voltage outer loop and the current inner loop in step 6, the DC transmission line bus voltage of the power sending end is controlled through the extended 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 the absolute value of the difference between P fault and is not less than a threshold value ε2, and P T2 the absolute value of the difference between P T1 and is not greater than a threshold value ε1, it indicates that the low-voltage fault of the powered end disappears, the control mode returns to normal, and the process jumps to Step 2.

2. The low-voltage fault ride-through coordination control method of a close-range two-end low-voltage DC power transmission system according to claim 1, characterized in that, The process of the coordinated control of low-voltage fault ride-through at the power receiving end side is as follows: Step 1, initialization of the low-voltage DC transmission system parameters at the power sending end; Step 2, collect the AC grid three-phase voltage of the power receiving end u a_R 、 u b_R 、 u c_R , three-phase filter inductance 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 voltages between two-stage circuits u pn_R ; Step 3, the voltage transfer ratio of the DAB converter of the power receiving end is controlled 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 DC transmission line bus voltage u dc The DAB converter adopts an extended phase shift control method with a voltage outer loop and a current inner loop to generate PWM signals of 8 switch tubes. Step 4, calculate the RMS value of each phase voltage of the point of common coupling in real time U a_RMS 、 U b_RMS 、 U c_RMS ; Step 5, calculating the phase grid voltage drop depth according to 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 I q The grid point voltage variation should be tracked in real time, and formula (4) is obtained （4） In the formula, K1 is the proportion value of the inverter output dynamic reactive current and voltage change, K1 should be in the range of 1.5-2.5, K1=2, I N The rated output current value of the T-type three-level grid-connected converter with the fourth bridge arm at the power receiving end; Introducing the power factor angle To limit the output current during low voltage fault ride through, we have: （5） （6） wherein I q is the dynamic reactive current, I d is the dynamic active current; Substituting equation (5) into equation (4), we obtain i ( i The expression (7) of the phase angle of the phase (a, b, c) power factor is as follows: （7） According to the falling depth of each phase M i to obtain the phase of the per-phase output current reference value of the T-type three-level grid-connected converter with the fourth bridge arm at the power receiving end, i the phase of the per-phase output current reference value is equal to i the phase power factor angle; Step 7, calculate the three-phase filter inductor current reference value of the T-type three-level grid-connected converter with the fourth bridge arm at the power receiving end I a_ref_R 、 I b_ref_R、 I c_ref_R The calculation process is as follows: The current reference value is derived, and there are two requirements in principle, one is that the sum of the three-phase modulation currents is 0, to reduce the double-frequency fluctuation of the positive and negative bus voltages, and the other is that the reference current of the drop phase is increased in amplitude, to raise the drop phase voltage; Take the a-phase grounding fault as an example, the b-phase and c-phase are the same, the a-phase voltage drop depth M When ≥0.7, take the zero sequence current as 0 as a 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, to obtain the calculation formula (10) in the d-axis direction and the calculation formula (11) in the q-axis direction: （10） （11） wherein I mi is the three-phase current amplitude, i =a, b, c; Through the above formula, the amplitudes of the other normal two-phase currents are obtained; （12） （13） At the same time, it is necessary to consider and ; According to the above principles, M ≥ 0.7, the zero sequence current can be constant 0, but M < 0.7, the normal phase current cannot meet the requirement of less than 1.1 times of the rated current, the current instruction is distributed as one phase current amplitude of 1.1 times of the rated current and the other phase current amplitude of 0 in the normal phase. Thus obtained i ( i = a, b, c) phase inductance current reference value as the fault control modulation wave parameters; Step 8, real-time detection of grid point three-phase voltage, selecting the maximum root mean square value of three-phase voltage U RMS_MAX , coordinating control of two-stage circuit positive and negative bus voltage reference value U pn_ref_R , as the modulation wave parameter of fault control; Step 9, when the receiving end of the power transmission system detects a step-up of the DC transmission line bus voltage u dc of the receiving end, indicating that the receiving end has a low voltage fault, steps 7-10 in the low voltage fault ride-through coordination control process of the sending end are executed, the DC transmission line bus voltage is reduced, and the active power output of the sending end is timely limited.

3. The low-voltage fault ride-through coordination control method of a close-range two-terminal HVDC power transmission system according to claim 2, characterized by, RMS value of each phase voltage of the point of common coupling U a_RMS , U b_RMS , U c_RMS The expression is: （2） In the formula, N is the total number of sampling points in half of the power frequency cycle of the AC network voltage.

4. The low-voltage fault ride-through coordination control method of a close-range two-end low-voltage DC power transmission system according to claim 3, characterized by, Phase grid voltage dip depth M a , M b , M c The expression is: （3） In the formula, U N Us is the nominal effective value of the grid phase voltage.

5. The low-voltage fault ride-through coordination control method of a close-range two-end low-voltage DC power transmission system according to claim 4, characterized by, i The expression of phase inductance current reference value is: （14） In the formula, is the grid angular frequency.

6. The low-voltage fault ride-through coordination control method of a close-range two-end low-voltage DC power transmission system according to claim 5, 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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