Asymmetric Voltage Active Support Method Based on the Remaining Reactive Power Capacity of Microgrid Clusters

Abstract

The present invention belongs to the technical field of microgrid operation control, and specifically relates to an asymmetric voltage active support method based on the remaining reactive power capacity of a microgrid cluster, including the calculation of the reactive power support amount for the remaining capacity of the microgrid cluster and the active voltage control considering asymmetric faults; the calculation of the reactive power support amount for the remaining capacity of the microgrid cluster adaptively calculates the respective reactive power-voltage droop coefficients according to the respective remaining reactive power capacities of the microgrids in the cluster, and obtains the reactive power support amount in this voltage situation through the positive-sequence voltage at the grid connection point of the microgrid cluster; the active voltage control considering asymmetric faults calculates the positive- and negative-sequence current reference values corresponding to the reactive power support amount according to different control objectives, and realizes the rapid output of the reactive power support amount through dual current-loop control. This method gives full play to the power support ability of the microgrid cluster, improves the operation economy, enhances the system voltage stability, and improves the consumption capacity of new energy at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of microgrid operation control, and particularly relates to an asymmetric voltage active support method based on the remaining reactive power capacity of a microgrid group. Background Art

[0002] Most of the distributed power sources in a microgrid are connected through grid-connected inverters. As the interface of the distributed power source, the control method of the grid-connected inverter determines the power quality of the system. Traditional grid-connected inverters adopt active power control methods, where the active power is the maximum output power of distributed generation, and the reactive power output satisfies the power factor at the grid connection point to provide power support for the system. When a grid fault occurs, in order to actively support the grid voltage under the fault, experts and scholars at home and abroad have carried out relevant research and proposed cooperative control strategies based on distributed power sources or microgrids to support the voltage. However, a single microgrid can provide a limited power support capacity and has weak power supply reliability. When a distributed power source disconnects from the grid or an asymmetric grid fault occurs, it will cause the grid voltage to drop. Therefore, the power supply capacity of a single microgrid is limited, so the support effect on the grid voltage is not obvious and it is impossible to raise the voltage to achieve effective support.

[0003] A microgrid group is composed of multiple microgrids in the power grid. When the microgrid group performs reactive voltage support, it is necessary to consider the support capabilities of different microgrids, and the reactive power support amount of each microgrid needs to be proportional to its support capability. However, voltage deviations inevitably exist at each grid connection point of the microgrid group. Therefore, additional control links are required to improve the microgrid power distribution accuracy. Traditional adjustment methods mainly include centralized methods and decentralized methods. The centralized method uses a centralized controller to collect information of the microgrid group, perform calculation and processing, and then issue reactive power support instructions. Although this method is simple to implement, it has a heavy communication network burden and is overly dependent on the centralized controller, with low reliability and poor real-time performance. The decentralized method uses local information for adjustment. Since it does not require a communication network, it has a low cost and good real-time performance. However, due to the locality of local information, it cannot achieve precise power distribution.

[0004] The invention patent with the application number 201310007059.9 discloses a method for configuring the capacity of a multi - element composite energy storage for a micro - grid group. Its main scheme is as follows: Calculate the power generation and load data of wind power generation and photovoltaic power generation in a sub - micro - grid within any period of time; Calculate the output power of the wind turbines in the sub - micro - grid; Determine the output power of photovoltaic power generation in the sub - micro - grid; Calculate the power imbalance between power generation and load in the sub - micro - grid; Calculate the cumulative imbalance energy of the sub - micro - grid from time tj when it starts to operate off - grid until T1 - T3; Calculate the minimum discharge energy required for energy storage backup when the sub - micro - grid continues to operate stably from time tj+T1 - T3 to time T3 under a fault condition; Calculate the minimum discharge energy that needs to be provided by the energy storage; Calculate the minimum charging energy absorbed by the energy storage; Calculate the minimum capacity that needs to be provided by the energy storage; Calculate the minimum rated capacity of the energy - type energy storage in the sub - micro - grid; Calculate the maximum power demand of the load in each sub - micro - grid; Calculate the rated power of the power - type energy storage in the sub - micro - grid; According to the energy storage capacity configuration of each sub - micro - grid, further configure the main energy storage capacity of the micro - grid group. This configuration method is general in estimation and has a single energy storage type, and is not instructive for practical applications.

[0005] The invention patent with the publication number CN112134306B discloses a control method for improving the high - frequency stability of a three - phase voltage - supporting grid - connected inverter. The method steps include: Detect the inductor current iLx and the grid current igx, perform coordinate transformation, and obtain the feedback signal iLmf and the feed - forward signal igmf through the inductor current feedback function Gi and the grid current feed - forward function Gf; Detect the capacitor voltage vox and perform coordinate transformation to obtain the voltage signal vom; The power synchronization controller calculates the output voltage controller command vom_ref according to the active power P, reactive power Q and their commands; According to vom and vom_ref, calculate the error signal e1m; The controller performs a closed - loop process on e1m to obtain the command iLmf_ref of the inductor current feedback signal iLmf and the error signal; Use e2m to generate the PWM control signal of the inverter bridge switching tubes.

[0006] From the above - disclosed prior art, it can be seen that no one has noticed or paid attention to the grid - connection support problem when distributed power sources are disconnected from the grid or the grid has an asymmetric fault. However, at this time, it will cause a drop in the grid voltage. Because the power supply capacity of a single micro - grid is limited, the support effect on the grid voltage is not obvious, and it is impossible to raise the voltage to achieve effective support. Therefore, in order to fully exploit the active support ability of the micro - grid group, it is of great significance to propose an active support method for asymmetric voltage considering the remaining reactive power capacity of the micro - grid group. Summary of the Invention

[0007] The object of the present invention is to provide an active asymmetric voltage support method based on the remaining reactive power capacity of a microgrid cluster for the problems existing in the prior art. This method realizes voltage support under grid asymmetric faults through calculating the reactive power support amount considering the remaining capacity of the microgrid cluster and active voltage control considering asymmetric faults, ensures that each sub-microgrid provides power support according to the proportion of its own remaining capacity, fully exerts the power support ability of the microgrid cluster, improves the operation economy, enhances the system voltage stability, and improves the accommodation capacity of new energy at the same time.

[0008] The technical solution of the present invention is as follows:

[0009] An active asymmetric voltage support method based on the remaining reactive power capacity of a microgrid cluster includes calculating the reactive power support amount of the remaining capacity of the microgrid cluster and active voltage control considering asymmetric faults;

[0010] The calculation of the reactive power support amount of the remaining capacity of the microgrid cluster adaptively calculates its own reactive-voltage droop coefficient according to the respective remaining reactive power capacities of the microgrids in the cluster, and obtains the reactive power support amount in this voltage situation through the positive-sequence voltage at the grid connection point of the microgrid cluster;

[0011] The active voltage control considering asymmetric faults calculates the positive and negative sequence current reference values corresponding to the reactive power support amount according to different control objectives, and realizes the rapid output of the reactive power support amount through dual current loop control.

[0012] Specifically, the calculation of the reactive power support amount of the remaining capacity of the microgrid cluster includes the following steps:

[0013] S1: According to the real-time operation data of each microgrid in the microgrid cluster, calculate the remaining reactive power capacity of the microgrids at different positions and capacities, and determine the ability of the microgrids to participate in active voltage support:

[0014]

[0015] where S i,j represents the capacity of the jth converter in the ith microgrid, P i,j,t represents the active power output by the jth converter in the ith microgrid, n is the total number of microgrids in the microgrid cluster, M is the number of converters inside the microgrid, and Q si,t is the remaining reactive power capacity of the ith microgrid at time t;

[0016] S2: Each microgrid calculates its own adaptive reactive-voltage droop coefficient according to the real-time reactive power dispatch instruction:

[0017]

[0018]

[0019] Wherein, is the reactive power dispatch command value of the i-th microgrid at time t, is the per-unit value of the reactive power dispatch command based on the remaining capacity, R i,t is the reactive power-voltage droop coefficient of the i-th microgrid at time t, R t is the global droop coefficient of the microgrid group, which is the same for each microgrid, V i is the lower limit of the normal voltage range, V low_TW is the minimum voltage value for the microgrid to go off-grid;

[0020] S3: Adopt the reactive power-voltage droop control method, and based on the positive-sequence voltage value of the connection point of each microgrid, calculate the reactive power support amount of each microgrid according to the following formula:

[0021]

[0022] Wherein, V i,t is the real-time voltage of the connection point of the microgrid, is the reactive power support amount of the i-th microgrid at time t;

[0023] S4: Each microgrid further distributes the reactive power support amount based on the capacity of the internal converter, and then according to the proportion of the remaining reactive power capacity of the converter as the reactive power output command of each converter, so that the reactive power output by each converter during reactive power-voltage support is proportional to its own remaining reactive power capacity.

[0024] Specifically, the active voltage control considering asymmetric faults includes the following steps:

[0025] S1: Calculate the positive and negative sequence current reference values corresponding to the reactive power support amount according to the asymmetric component of the connection point voltage after the asymmetric fault, as follows:

[0026]

[0027] Wherein, P o is the active power output of the converter, are the dq-axis components of the positive and negative sequence voltages of the connection point respectively, k is the control mode flag bit;

[0028] S2: Set the control target. When k = 0, the reference value of the negative sequence current component is zero, and both the output active power and reactive power fluctuate, realizing control mode one; when k = 1, the output active power has no fluctuation, and the reactive power fluctuates, realizing control mode two; when k = -1, the output reactive power has no fluctuation, and the active power fluctuates, realizing control mode three;

[0029] S3: Use the calculated positive and negative sequence current reference values as the inputs of the dual current loop controller. The controller generates a driving signal to enable the converter to quickly track the reactive power support amount and provide an asymmetric current component, thereby completing the support of the positive sequence voltage and the suppression of the negative sequence voltage under faults.

[0030] Specifically, the dual current loop controller realizes the fast tracking of positive and negative sequence current commands. The dual current controllers decouple and control the positive sequence current and the negative sequence current respectively, and operate independently of each other. Finally, an asymmetric control quantity is synthesized at the converter end to output positive and negative sequence reactive current components, realizing the active support control of the grid-connected point voltage.

[0031] Vigorously develop new energy and build a new power system with new energy as the main body. The access of some distributed power sources forms a microgrid. A single microgrid can provide limited power supply capacity and has weak power supply reliability. When a distributed power source is disconnected from the grid or an asymmetric fault occurs in the grid, it will cause the grid voltage to drop. Due to the limited power supply capacity of a single microgrid, the support effect on the grid voltage is not obvious, and it is impossible to raise the voltage to achieve active support.

[0032] With the increase in the number of microgrids, geographically adjacent microgrids are interconnected to form a microgrid cluster system. Through the energy scheduling between microgrids and between microgrids and the large grid, the penetration rate of distributed power sources and the overall power supply reliability are further improved. In order to make the overall economy and stability of the interconnected microgrids better, it is necessary to reasonably coordinate and configure the energy storage system capacity of multiple microgrids. If the capacity configuration is too small, it may be difficult to achieve the expected flexibility and economic benefits; if the capacity configuration is too large, the investment cost will increase.

[0033] Currently, there are studies on the energy storage capacity configuration of single microgrids in the existing technologies. There is a literature introducing a wavelet-based energy storage capacity configuration algorithm and a real-time coordination control strategy for wind power generation in microgrids to reasonably configure the hybrid energy storage capacity to suppress fluctuations and smooth the output. In addition, there are also studies considering renewable energy generation, hybrid energy storage methods, and typical loads in the island operation mode of microgrids, establishing a mathematical model with the minimum daily cost of the energy storage system as the single objective function, and using the particle swarm optimization algorithm with quantum behavior to configure the capacity of the hybrid energy storage. In addition, a two-layer optimization mathematical model with an inner layer of the battery life model and an outer layer of the optimal model for the economic operation of the system is proposed, and a two-stage model iteration solution is adopted by combining the network adaptive direct search algorithm and the improved particle swarm algorithm. However, the methods in the existing technologies currently are difficult to actively support the grid voltage under asymmetric faults in microgrids.

[0034] In order to achieve the active support of the grid voltage under asymmetric faults, experts and scholars at home and abroad have conducted relevant research and proposed some control strategies to achieve voltage support. Some use the quasi-proportional resonance control strategy to inject current into the grid to raise the grid voltage, but do not consider the suppression of power fluctuations. Others use the unbalanced control strategy of quasi-proportional resonance control, which can effectively suppress the active power fluctuations and reduce the current distortion rate, but do not consider the situation of reactive power fluctuations. There are also voltage control strategies using multi-agent systems, which establish agent control models to achieve voltage support for the grid, but only consider the control and scheduling of a single microgrid. In addition, in order to achieve zero-voltage crossing of photovoltaic grid-connected inverters, a phase-advance compensation link for the grid voltage feed-forward component is introduced, but the control strategy is relatively complex in operation and may cause delays. To sum up, there is little research on the active voltage support for microgrid clusters under grid faults. However, microgrid clusters have a larger power supply capacity and stronger power supply reliability, and further improve the penetration rate of distributed power sources, which can effectively achieve voltage support during grid faults.

[0035] The beneficial effects of the present invention are as follows: Compared with the prior art, the present invention can provide voltage support during faults by microgrid clusters with power support capabilities, thereby raising the voltage at the new energy grid connection point and reducing the probability of new energy disconnection from the grid. This active voltage support method ensures that each sub-microgrid provides power support according to the proportion of its remaining capacity, fully exerts the power support capabilities of the microgrid cluster, and improves the economic efficiency of operation. The dual-current-loop controller can suppress the second harmonic of the DC voltage by controlling the second pulsating component of the instantaneous active power on the grid side to zero, and achieve the maximum power factor operation on the grid side; the dual-current-loop controller can make the output current at the grid connection point sinusoidal and symmetric by controlling the negative-sequence current to zero; the dual-current-loop controller can make the second pulsating component of the reactive power zero, and although the output current at the grid connection point is unbalanced, its sinusoidality is good. For the control of the current inner loop, the positive-sequence current and the negative-sequence current are independently controlled in the positive- and negative-sequence rotating coordinate systems respectively. Since both sets of regulators control the DC quantity, there is no need to increase the control gain, reducing the unstable factors of the system, and the static-error-free control of the positive- and negative-sequence currents can also be achieved. Brief Description of the Drawings

[0036] Figure 1 It is the principle block diagram of the method flow of the present invention.

[0037] Figure 2 It is the block diagram of the positive- and negative-sequence dual-current-loop controller. Detailed Embodiments

[0038] The technical solutions of the present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0039] The present invention provides an asymmetric voltage active support method based on the remaining reactive power capacity of a microgrid group, which is applicable to the safe and stable operation of a high-proportion new energy microgrid group. The flowchart of the asymmetric voltage active support method based on the remaining reactive power capacity of the microgrid group proposed by the present invention is as Figure 1 shown. This method mainly includes two parts: calculating the reactive power support amount considering the remaining capacity of the microgrid group and actively controlling the voltage considering asymmetric faults.

[0040] Embodiment 1

[0041] In this embodiment, the calculation of the reactive power support amount of the remaining capacity of the microgrid group adaptively calculates the reactive power-voltage droop coefficient according to the respective remaining reactive power capacities of the microgrids within the group, and obtains the reactive power support amount in this voltage situation through the positive sequence voltage at the grid connection point of the microgrid group;

[0042] The active voltage control considering asymmetric faults calculates the positive and negative sequence current reference values corresponding to the reactive power support amount according to different control objectives, and realizes the rapid output of the reactive power support amount through double current loop control.

[0043] The calculation of the reactive power support amount of the remaining capacity of the microgrid group includes the following steps:

[0044] S1: According to the real-time operation data of each microgrid in the microgrid group, calculate the remaining reactive power capacity of the microgrids at different positions and capacities, and determine the ability of the microgrids to participate in active voltage support:

[0045]

[0046] In the formula, S i,j represents the capacity of the jth converter in the ith microgrid, P i,j,t represents the active power output by the jth converter in the ith microgrid, n is the total number of microgrids in the microgrid group, M is the number of converters inside the microgrid, and Q si,t is the remaining reactive power capacity of the ith microgrid at time t;

[0047] S2: Each microgrid calculates the adaptive reactive power-voltage droop coefficient according to the real-time reactive power dispatch instruction:

[0048]

[0049]

[0050] In the formula, is the reactive power dispatch instruction value of the ith microgrid at time t, is the per-unit value of the reactive power dispatch instruction based on the remaining capacity, R i,t is the reactive power-voltage droop coefficient of the ith microgrid at time t, R tis the global droop coefficient of the microgrid cluster, which is the same for each microgrid. V i is the lower limit of the normal voltage range, V low_TW is the minimum voltage value for the microgrid to go off-grid;

[0051] S3: Adopt the reactive power-voltage droop control method. Based on the positive-sequence voltage value at the grid connection point of each microgrid, calculate the reactive power support amount of each microgrid according to the following formula:

[0052]

[0053] In the formula, V i,t is the real-time voltage at the grid connection point of the microgrid, is the reactive power support amount of the i-th microgrid at time t;

[0054] S4: Each microgrid further distributes the reactive power support amount based on the capacity of the internal converter and according to the proportion of the remaining reactive power capacity of the converter as the reactive power output command of each converter, so that the reactive power output by each converter during reactive power-voltage support is proportional to its own remaining reactive power capacity.

[0055] Embodiment 2

[0056] Under an asymmetric fault, due to the existence of negative-sequence voltage, the original single-current-loop control strategy cannot be applied. Therefore, it is necessary to separate the positive and negative sequences of voltage and current and control them separately using a double-current loop.

[0057] The active voltage control considering asymmetric faults includes the following steps:

[0058] S1: Calculate the positive and negative sequence current reference values corresponding to the reactive power support amount according to the asymmetric component of the grid connection point voltage after the asymmetric fault, as follows:

[0059]

[0060] In the formula, P o is the active power output of the converter, are the dq-axis components of the positive and negative sequence voltages at the grid connection point respectively, k is the control mode flag bit;

[0061] S2: Set the control target. When k = 0, the reference value of the negative-sequence current component is zero, and both the output active power and reactive power fluctuate, realizing Control Mode 1; when k = 1, the output active power has no fluctuation, and the reactive power fluctuates, realizing Control Mode 2; when k = -1, the output reactive power has no fluctuation, and the active power fluctuates, realizing Control Mode 3;

[0062] S3: Use the calculated positive and negative sequence current reference values as the inputs of the dual-current-loop controller. The controller generates a driving signal to enable the converter to quickly track the reactive power support amount and be able to provide an asymmetric current component, thereby completing the support of the positive sequence voltage and the suppression of the negative sequence voltage under faults.

[0063] In this embodiment, the dual-current-loop controller realizes the fast tracking of the positive and negative sequence current commands. The dual-current controllers respectively perform decoupling control on the positive sequence current and the negative sequence current, and operate independently of each other. Finally, an asymmetric control quantity is synthesized at the converter end, thereby outputting the positive and negative sequence reactive current components to realize the active support control of the grid-connected point voltage. Its dual-loop control structure is as Figure 2 shown. In the figure, are respectively the dq-axis components of the positive and negative sequence reference currents output by the grid-connected converter; are respectively the dq-axis components of the actual positive and negative sequence currents output by the grid-connected converter; k is the mode control flag, PI is the proportional-integral controller, and L is the reactance of the output filter of the grid-connected converter; P o is the active output of the converter, and Q DG is the reactive power support amount allocated by the microgrid to the converter.

[0064] Finally, it should be noted that: The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the specific implementation manners of the present invention or perform equivalent replacements on some technical features; without departing from the spirit of the technical solutions of the present invention, they should all be covered within the scope of the technical solutions claimed by the present invention.

Claims

1. An active support method for asymmetric voltage based on the remaining reactive power capacity of a microgrid cluster, characterized in that It includes the calculation of the reactive power support amount for the remaining capacity of the microgrid cluster and the active voltage control considering asymmetric faults; The calculation of the reactive power support amount for the remaining capacity of the microgrid cluster adaptively calculates the respective reactive-voltage droop coefficients according to the remaining reactive power capacities of the microgrids within the cluster, and obtains the reactive power support amount in this voltage situation through the positive-sequence voltage at the connection point of the microgrid cluster; The active voltage control considering asymmetric faults calculates the positive- and negative-sequence current reference values corresponding to the reactive power support amount according to different control objectives, and realizes the rapid output of the reactive power support amount through double-current-loop control; The calculation of the reactive power support amount for the remaining capacity of the microgrid cluster includes the following steps: S1: According to the real-time operation data of each microgrid in the microgrid cluster, calculate the remaining reactive power capacity of the microgrids at different positions and capacities, and determine the ability of the microgrids to participate in active voltage support; Where, S i,j represents the capacity of the j-th converter in the i-th microgrid, P i,j,t represents the active power output by the j-th converter in the i-th microgrid, n is the total number of microgrids in the microgrid cluster, M is the number of converters within the microgrid, Q si,t is the remaining reactive power capacity of the i-th microgrid at time t; S2: Each microgrid calculates the adaptive reactive-voltage droop coefficient according to the real-time reactive power dispatch instruction; wherein, is the reactive power scheduling command value of the i-th microgrid at time t, is the per-unit value of the reactive power scheduling command based on the remaining capacity, R i,t is the reactive power-voltage droop coefficient of the i-th microgrid at time t, R t is the global droop coefficient of the microgrid group, which is the same for each microgrid, V i is the lower limit of the normal voltage range, V low_TW is the minimum voltage value for the microgrid to go off-grid; S3: Adopt the reactive-voltage droop control method, and based on the positive-sequence voltage value at the connection point of each microgrid, calculate the reactive power support amount of each microgrid according to the following formula: Where, V i,t is the real-time voltage at the grid connection point of the microgrid, is the reactive power support of the i-th microgrid at time t; S4: Each microgrid further distributes the reactive power support amount based on the capacity of the internal converter and according to the proportion of the remaining reactive power capacity of the converter. As the reactive power output command of each converter, the reactive power output by each converter during reactive power-voltage support is proportional to its own remaining reactive power capacity.

2. The asymmetric voltage active support method based on the remaining reactive power capacity of the microgrid group according to claim 1, wherein The active voltage control considering asymmetric faults includes the following steps: S1: Calculate the positive- and negative-sequence current reference values corresponding to the reactive power support amount according to the asymmetric component of the voltage at the connection point after the asymmetric fault, as follows: Wherein, P o is the active output of the converter, are respectively the dq-axis components of the positive and negative sequence voltages at the point of common coupling, k is the control mode flag bit; S2: Set the control objective. When k = 0, the reference value of the negative-sequence current component of the current is zero, and both the output active power and reactive power fluctuate, realizing control mode one; when k = 1, the output active power has no fluctuation, and the reactive power fluctuates, realizing control mode two; when k = -1, the output reactive power has no fluctuation, and the active power fluctuates, realizing control mode three; S3: Use the calculated positive- and negative-sequence current reference values as the inputs of the double-current-loop controller. The controller generates a drive signal, enabling the converter to quickly track the reactive power support amount and be able to provide asymmetric current components, thereby completing the support of the positive-sequence voltage and the suppression of the negative-sequence voltage under the fault.

3. The asymmetric voltage active support method based on the remaining reactive power capacity of the microgrid group according to claim 2, characterized in that The double-current-loop controller realizes the rapid tracking of the positive- and negative-sequence current commands. The double-current controller decouples and controls the positive-sequence current and the negative-sequence current respectively, and operates independently of each other. Finally, an asymmetric control quantity is synthesized at the converter end, thereby outputting the positive-sequence and negative-sequence reactive current components to realize the active support control of the voltage at the connection point.

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