VSC-MTDC multi-port improved coordination control strategy for acceleration stabilization under large power disturbance

Abstract

The invention discloses a VSC-MTDC multi-port improved coordination control strategy capable of accelerating and stabilizing under large power disturbance, which is suitable for the field of high-voltage direct current transmission. The strategy comprises the following steps: aiming at unbalanced power flowing into a direct-current power grid, designing an improved output reference value based on typical direct-current voltage droop control so as to realize active balance of the reference value and the unbalanced power, and realizing direct-current voltage dynamic stability and extremely surplus capacity utilization of each coordination station under large disturbance; the droop coefficient is improved, rapid crossing control of the transient process is achieved, and the transient stability is improved. According to the strategy, the advantages of typical coordination control are integrated, stable control over the DC voltage rated value and reasonable utilization of the surplus capacity of the converter station are achieved, and meanwhile the DC voltage stable recovery rate in the transient process is increased.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-voltage direct-current power transmission, and particularly relates to an improved coordinated control strategy for multi-port direct-current power grid output fluctuation and fast stability characteristics under large disturbance in the case of large-scale grid connection of new energy. BACKGROUND

[0002] Due to the randomness of new energy and low inertia, the traditional AC-DC system cannot meet the requirements of grid interconnection with large-scale new energy access. Multi-terminal flexible direct-current transmission (VSC-MTDC) has flexible operation characteristics and can realize multi-point grid connection and coordination, has strong adaptability to new energy transmission and consumption, and provides a reliable solution for large-scale grid connection of new energy bases. The current typical VSC-MTDC coordinated control strategy has a series of problems such as poor dynamic characteristics, dependence on inter-station communication, waste of surplus capacity, and voltage difference in steady state. In addition, large-capacity new energy access has higher requirements for the transient recovery speed of coordinated control.

[0003] At present, scholars at home and abroad have carried out a lot of research work on the coordinated control strategy of VSC-MTDC system. Mainly from two aspects: direct-current voltage stability coordinated control and active power output coordinated control. For the problem of direct-current voltage stability, some scholars propose to use the direct-current voltage deviation to correct the direct-current voltage and active power reference value, so as to eliminate the steady-state error of direct-current voltage. Some scholars propose an optimized droop control strategy for direct-current voltage correction, which realizes the minimum power change and optimal correction of direct-current voltage through immune clone selection algorithm, effectively coordinating the output of VSC-MTDC system and the dynamic stability of direct-current voltage. For the coordination problem of power between stations, some scholars propose an improved droop control considering power margin combined with fixed direct-current voltage control, which can realize flexible allocation of active power output of each station, reduce the risk of overload, and realize direct-current voltage regulation without difference, but it does not have high control accuracy. In addition, some scholars propose a direct-current grid power coordination strategy to track and adjust the power reference value of each station to improve the stability recovery ability of direct-current grid, but this method depends on inter-station communication and does not consider whether the grid can operate reliably when the inter-station communication is out.

[0004] Therefore, it is necessary to consider the coordination ability of the coordinated control between multiple terminals for direct-current voltage and output control, and based on this, a coordinated control strategy with high reliability and fast stability characteristics is proposed. SUMMARY

[0005] In view of the deficiencies or improvement needs of the existing VSC-MTDC coordinated control strategy, the present application is directed to the following aspects: Figure 1The new energy four-terminal VSC-MTDC grid-connected system shown, an improved coordinated control strategy for large disturbance fast recovery is proposed, and the control mode is as shown in Figure 2 Compared with the conventional coordinated control method, the active power balance strategy and the adaptive droop coefficient adjustment strategy are improved based on the DC voltage droop control.

[0006] Active power balance control. The present application is based on the typical VSC-MTDC DC voltage droop control, and the power distribution coefficient correction formula is introduced to improve and optimize the control outer loop active reference value. The active power balance strategy proposed in the present application is shown in Figure 3 The formula (1) is the output distribution and DC voltage offset function form of the standard DC voltage droop control, ΔP0 is the unbalanced power flowing into the DC grid, k j is the droop coefficient of each droop station. The power distribution coefficient is introduced by analogy to the droop coefficient to construct the power correction equation shown in formula (2), wherein P' refj is the corrected active reference value of the converter station, ΔP′ is the unbalanced power distribution of the converter station, K j is the introduced power distribution coefficient, k p，rev is the proportional coefficient, k i,rev is the integral coefficient. Through the correction link described in formula (2), when ΔP0 meets formula (3) again, the output of each active balance correction station and the DC grid voltage at this time are as formula (4).

[0007]

[0008] P r ′ efj =P refj -ΔP j ′=P refj -K j (k p,rev ΔP+k i,rev ∫ΔP) (2)

[0009]

[0010]

[0011] Droop coefficient adaptive control. The present application is based on the typical VSC-MTDC DC voltage droop control, and the adaptive droop coefficient correction formula is introduced to improve and optimize the droop coefficient. The adaptive droop coefficient control link proposed in the present application is shown in Figure 3 The calculation is based on the droop coefficient reference value of the dynamic output response of the converter station. Figure 4 The correction principle diagram of the droop coefficient in the dynamic process is shown in formula (5), wherein U dc,rev1 is the DC voltage of point M, U dc,rev2 is the DC voltage of point N, PM P is the active power of M point; P N P is the active power of N point. According to the droop control equation, there is a relationship shown in equation (5) when no correction is made. After the droop coefficient correction, it is shown in equation (6). By combining equation (5) and equation (6), and simultaneously performing the normalization design, let ΔU dcr is a constant, and ΔP dc,rev increases with the power offset value |P refj -P j | increases, so that ΔP dc,rev satisfies ΔP dc,rev =a|P refj -P j |, wherein a is a constant. The basic form of the droop coefficient correction function shown in equation (7) can be obtained, and the dynamic correction of the droop coefficient is realized.

[0012]

[0013]

[0014]

[0015] The application provides a coordinated control strategy of a VSC-MTDC system with large-capacity new energy access. Active power balance control adjusts the droop control power reference value dynamically, distributes and absorbs the unbalanced power according to the distribution coefficient, and realizes dynamic stability control before and after the DC voltage large disturbance. The droop coefficient adaptive control dynamically changes the actual value of the droop coefficient, improves the dynamic recovery rate during large disturbance, and enables the system to reach a stable state more quickly. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Fig. 1 is a schematic diagram of a four-terminal VSC=MTDC system topology with new energy grid connection.

[0017] Figure 2 Fig. 3 is a basic control principle diagram of the four-terminal VSC=MTDC system.

[0018] Figure 3 Fig. 5 is a schematic diagram of the coordinated control for fast recovery of large disturbance.

[0019] Figure 4 Fig. 7 is a dynamic correction principle diagram of the droop coefficient.

[0020] Figure 5 Fig. 9 is a DC voltage simulation comparison diagram of the droop coefficient correction control effect.

[0021] Figure 6 Fig. 11 is a control capability simulation diagram of the coordinated control strategy. Typical master-slave control and DC voltage droop control are respectively constructed, and compared with the control strategy of the application. Figure 6(a) is the VSC1 side active and DC voltage waveform chart, Figure 6 (b) is the VSC2 side active and DC voltage waveform chart, Figure 6 (c) is the VSC3 side active and DC voltage waveform chart, Figure 6 (d) is the VSC4 side active and DC voltage waveform chart. DETAILED DESCRIPTION

[0022] The application is further illustrated below in combination with the drawings and embodiments.

[0023] Figure 1 The figure shows a new energy grid-connected four-terminal VSC=MTDC system topology diagram. The flexible DC converter station VSC1 accesses the wind-solar hybrid new energy unit, the station VSC3 accesses the wind farm unit, and the station VSC2 and VSC4 access the traditional AC power grid.

[0024] Figure 2 It is a four-terminal VSC=MTDC system basic control principle diagram. Among them, the rectifier side of the new energy sending terminal is a constant U / f control. The collected DC voltage and current are transformed to obtain dq axis components, and the preset voltage reference value u sref and the frequency reference value f ref are controlled, and the PI link is obtained to obtain the rectifier side inner loop current reference value i sdref and i sqref input current inner loop. The active inverter side adopts DC voltage droop control, and the DC voltage U dcj and the active power P j reference value and the actual sampling value are subtracted, and then the droop coefficient k j is passed through PI control to obtain the corresponding inner loop current reference value i sdref and i sqref .

[0025] The rectifier side dq transformation phase angle is obtained by a constant f ref , while the active inverter side depends on the phase-locked loop for tracking. The basic pull equation of the phase-locked loop is shown in equation (8). Where f ref is the rated frequency of the receiving terminal AC power grid, usually 50Hz, and the voltage q component u sqref is 0 to realize the tracking of the grid connection point voltage. The input u sq is the measured q-axis voltage component, k p and k i are the proportional coefficient and integral coefficient of the PI control link.

[0026]

[0027] There is also a current inner loop decoupling control link between the outer loop control and the actual modulation link. The current reference value i sdrefandi sqref To achieve the control objective, the dq-axis control components are individually controlled through decoupling compensation of the dq-axis components. The obtained converter station outlet voltage reference value is then sent to the modulation loop for converter station switching control to achieve the control objective. The inner loop control function is shown in equation (9).

[0028]

[0029] Figure 3 This is a schematic diagram of coordinated control for rapid recovery from large disturbances. Typical master-slave control offers strong DC voltage stability, but suffers from poor dynamic characteristics, slow stabilization speed, reliance on inter-station communication for station pole switching commands, and the master station bearing all unbalanced power, resulting in wasted surplus capacity at slave stations. The DC voltage droop control mode significantly improves the utilization rate of surplus capacity at each station, but its drawback lies in the inherent error of droop control and low power allocation accuracy between stations. Based on the characteristics of these two control modes, this invention proposes an improved coordinated control mechanism, such as... Figure 3 As shown. Equation (1) is a typical DC voltage droop control formula for unbalanced power distribution and DC voltage deviation, where ΔP0 is the unbalanced power flowing into the DC grid, and k j Let P' be the droop coefficient for each droop station. Equation (2) is the active power correction equation, used to correct the output reference value, P′ refj The corrected active power reference value for the converter station is given by ΔP′, where ΔP′ represents the unbalanced power distribution at the converter station, and K is the power source. j For the introduced power allocation factor, k p，rev k is the proportionality coefficient. i,rev For the integral coefficient. After the correction process described in equation (2), when ΔP0 satisfies equation (3) again, the output of each active balancing correction station and the DC grid voltage at this time are as shown in equation (4).

[0030] Figure 4 This is a schematic diagram illustrating the dynamic correction principle of the droop coefficient. The invention is based on typical VSC=MTDC DC voltage droop control, and introduces an adaptive droop coefficient correction mechanism to improve and optimize the droop coefficient. The adaptive droop coefficient control mechanism proposed in this invention is as follows: Figure 3 As shown, the reference value of the droop coefficient is calculated based on the dynamic output response of the converter station. Figure 4 This is a diagram illustrating the correction principle for the droop coefficient during dynamic processes, where U... dc,rev1 U is the DC voltage at point M. dc,rev2 Let P be the DC voltage at point N. M The active power output is at point M; P N Let N be the active power output. From the droop control equation, we know that under normal conditions without correction, the relationship is as shown in equation (5). After droop coefficient correction, it is as shown in equation (6). Combining equations (5) and (6), and simultaneously performing standardized design, let ΔU... dcr The constant is given, and ΔP is known.dc,rev With the power offset value |P refj -P j | increase, so that ΔP dc,rev satisfies ΔP dc,rev = a|P refj -P j |, where a is a constant. The basic form of the droop coefficient correction function shown in equation (7) is obtained, and the dynamic correction of the droop coefficient is realized.

[0031] The specific embodiments of the present application will be described in detail in combination with the drawings and examples, but the present application is not limited by the described examples. The steps are as follows:

[0032] 1) In the PSCAD electromagnetic simulation software, the simulation model of the new energy grid-connected four-terminal VSC=MTDC system is built as shown in Figure 1 .

[0033] 2) Test in N-1 scenario, VSC1 at 3s station is out of operation due to fault, respectively build master-slave control, DC voltage droop control and the improved coordinated control of the present application, simulation comparison verifies the control effect of the improved coordinated control.

[0034] It can be seen from Figure 5 and Figure 6 that compared with the typical master-slave control and DC voltage droop control, the improved coordinated control strategy proposed in the present application combines the advantages of the two, realizes the stable control of the DC voltage rated value and the reasonable use of the surplus power of the converter station, and also improves the recovery rate of the DC voltage stability in the transient process.

Claims

1. A VSC-MTDC multi-port improved coordinated control strategy under power large disturbance acceleration stability, characterized in that, For the unbalanced power flowing into the DC grid, based on the typical DC voltage droop control, the improved output reference value is designed to realize the active balance of the reference value and the unbalanced power, and to realize the dynamic stability of the DC voltage under large disturbance and the utilization of the surplus capacity of each coordinated station. The droop coefficient is improved to realize the fast crossing control of the transient process and improve the transient stability capability.

2. According to the requirements of claim 1, the improved output reference value strategy includes an active power balance correction method.

3. According to the requirements of claim 1, the improved droop coefficient strategy includes a droop coefficient adaptive correction method.

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