CN111711196A - A control method for seamless switching of operating modes of AC-DC hybrid distribution network - Google Patents

Abstract

The invention proposes a seamless switching control method for the operation mode of an AC/DC hybrid distribution network. The involved AC/DC hybrid distribution network includes two AC sub-networks in order to ensure the reliability of power supply, and both are connected to the public power grid through converters. , the AC and DC sub-grids are connected through two AC/DC converters, and the energy storage system is connected to the DC side. The control method includes: in the normal working mode, two AC/DC converters provide bus voltage support and power sharing for the DC sub-network, the energy storage system only controls the energy storage SOC, and adopts a constant power control mode; In the fault mode, a bidirectional AC/DC converter and the energy storage system provide bus voltage support and power sharing for the DC sub-grid; in the dual fault mode, the energy storage system provides the DC bus voltage support. The AC-DC hybrid distribution network operating mode seamless switching control method proposed by the invention can realize voltage stability, power adaptive sharing and energy storage SOC regulation of the DC sub-network in normal and fault modes.

Description

A control method for seamless switching of operating modes of AC-DC hybrid distribution network

technical field

The invention relates to a control method for seamless switching of operation modes of an AC/DC hybrid distribution network, which belongs to the cross field of power system and control system design.

Background technique

With the increasing demand for energy, more and more attention has been paid to distributed generation of new energy at home and abroad. The new energy is connected to the public grid in the form of micro-grid and connected to the grid, which is the most effective way to give full play to the distributed power generation of new energy. Since most of the electric energy generated by new energy power generation is DC, and the DC load has also increased in recent years, DC microgrids have developed rapidly, and AC-DC hybrid distribution networks with both AC and DC characteristics have also become a research topic. hot spot. Compared with the traditional AC distribution network, the AC/DC hybrid distribution network has the advantages of DC power on the basis of the AC. The AC/DC hybrid distribution network can directly access the DC load and distributed power, and the distributed power can The DC bus directly powers the DC load, reducing the need for an AC/DC converter.

The AC/DC hybrid distribution network consists of an AC sub-network and a DC sub-network. The AC/DC sub-networks realize power exchange through AC/DC converters, including energy storage systems, distributed power sources, DC loads and AC loads. At present, the research of AC/DC hybrid distribution network mainly includes topology, control strategy, stable operation, fault protection and optimal scheduling, etc. Among them, the coordinated control and mode switching of multiple converters are the key to the stable operation of the distribution network. In the prior art about mode seamless switching control, it mainly focuses on the switching between the grid-connected and islanded operation modes of the microgrid to ensure the rapidity of switching between the grid-connected and isolated modes of the microgrid. There are few studies on the seamless switching control of distribution network modes.

SUMMARY OF THE INVENTION

The present invention aims to propose a seamless switching control method for the operation mode of an AC/DC hybrid distribution network, which is used to solve the problem of coordinated control between multiple converters and realize the switching control of multiple operation modes of the hybrid distribution network.

In order to achieve the above object, the concrete technical scheme of the present invention is as follows:

An AC/DC hybrid distribution network operation mode seamless switching control method, the hybrid distribution network includes two AC sub-networks and one DC sub-network, the two AC sub-networks are connected to the DC sub-network through an AC/DC converter respectively. At the same time, the two AC sub-networks are connected to the public power grid through AC/AC; wherein, the hybrid distribution network is connected with an AC load on the AC side, and is connected with an energy storage system and a DC load on the DC side. In the system, the energy storage unit is connected to the DC sub-grid through the energy storage converter. The method includes the following steps:

(1) Using two AC/DC converters and energy storage converters as three controlled voltage sources, the central controller issues control commands ΔV _x (x =1, 2, 3), where 1 represents the first AC/DC converter, 2 represents the second AC/DC converter, and 3 represents the energy storage converter; to achieve voltage stability and power in different operating modes Adaptive sharing and energy storage SOC regulation include the following three modes:

(a) In normal operation mode, the dual AC/DC converters are sufficient to meet the power demand of the DC load, and the central controller issues a command ΔV to adjust the dual AC/DC converters to provide voltage support and power sharing for the DC bus. In operation mode, the energy storage converter only controls the energy storage SOC, and adopts constant power control;

(b) In the single-circuit fault mode, the AC/DC converter of the faulty circuit is removed, and the central controller issues a command ΔV to adjust the energy storage converter and the non-faulty AC/DC converter to provide voltage support and Power sharing.

(c) In the dual-circuit fault mode, the dual-circuit AC/DC converters are cut off, the DC load power demand is provided by the energy storage converter, and the central controller issues a command ΔV to adjust the energy storage converter to provide the DC bus. Voltage support and power requirements.

(2) The three controlled voltage outer loop controls all adopt consistent DC voltage droop control. According to the rated voltage of the DC bus and the actual output current of the three controlled voltage sources, and introduce the command ΔV issued by the central controller, the The reference voltages for the three controlled voltage sources are calculated.

(3) The present invention obtains the DC output reference voltages of the three controlled voltage sources according to the calculation in (2), and uses PI regulation in the controlled voltage sources to realize the tracking of the output reference voltages, thereby realizing the hybrid distribution network in the Voltage support, power adaptive sharing, and energy storage SOC regulation in normal operation and failure modes.

Further, the control command ΔV _x (x=1,2) issued by the central controller to the dual AC/DC converter in the normal working mode is obtained by calculating in the following manner:

ΔV _x =(K _P1 +K _I1 /s)(v ^* -v _dcbus )+(K _P2 +K _I2 /s)(i _average -i _dc,x ),x=1,2 (1)

Among them, x=1, 2, respectively represent the serial numbers of the corresponding two AC/DC converters, K _P1 , K _P2 correspond to the proportional coefficients of the PI regulators of voltage and current, respectively, K _I1 , K _I2 correspond to The integral coefficient of the PI regulator for voltage and current, 1/s represents the integral link of PI regulation, v ^* is the rated voltage of the DC bus, v _dcbus is the actual voltage of the DC bus, i _dc,x is the AC/DC converter Output current, i _average is the reference current of the AC/DC converter that participates in DC power sharing. When the voltage is the same, the power sharing can be achieved by controlling the current ratio.

The energy storage converter only controls the energy storage SOC and adopts constant power control. The command ΔV ₃ issued by the central controller can be calculated and obtained in the following way:

Among them, i _dc,3,ref is the charging and discharging reference current of the constant power control of the energy storage system, and i _dc,3,ref >0, i _dc,3 is the actual charging and discharging current of the energy storage system, i _dc,3 >0 Indicates that the energy storage system is discharged, and i _dc,3 < 0 indicates that the energy storage system is charged.

Further, the control command ΔV _x (x=1 or 2, 3) issued by the central controller to the energy storage converter and the non-faulty AC/DC converter in the single-circuit fault mode can be calculated by the following method: get:

ΔV _x =(K _P1 +K _I1 /s)(v ^* -v _dcbus )+(K _P2 +K _I2 /s)(i _average -i _dc,x ), x=1 or 2,3 (3) where , where i _dc,x , where x=1 or 2, is the actual output current of the non-faulty AC/DC converter, and i _dc,3 is the actual output current of the energy storage converter.

Further, the control command ΔV ₃ issued by the central controller to the energy storage converter in the dual fault mode can be calculated by the following method:

ΔV ₃ =(K _P1 +K _I1 /s)(v ^* -v _dcbus ) (4)

Further, in the step 2, the outer loop control of the three controlled voltage sources is based on the DC voltage droop control, and the voltage deviation is introduced to realize the dual AC/DC converter and storage according to ΔV. The DC reference voltage v _dc,x,ref of the energy converter is as follows:

v _dc,x,ref =v ^* -ki _dc,x +ΔV _x ,x=1,2,3 (5)

Where v _dc,x,ref is the output reference voltage of the controlled voltage source, k is the droop coefficient, i _dc,x is the output current of the controlled voltage source, ΔV _x is the command issued by the central controller.

Further, the hybrid power distribution network is also connected with a photovoltaic power generation system on the DC side, and the photovoltaic power generation system operates at the maximum power point.

Compared with the prior art, the present invention has the advantage of realizing the coordinated control among the multiple converters of the multi-sub-network AC/DC hybrid distribution network, and the proposed seamless switching control method is based on the DC voltage droop control, and in normal It is not necessary to change the underlying control of the controlled voltage source before switching between the operating mode and the fault mode. By adjusting the commands issued by the central controller, the stability of the DC bus can be ensured, adaptive power sharing among multiple converters, and energy storage SOC control. It can ensure the seamless switching control of the operation mode of the hybrid distribution network.

Description of drawings

1 is a topology diagram of an AC/DC hybrid distribution network involved in the present invention;

Figure 2 shows the normal working mode of the hybrid distribution network;

Figure 3 is a schematic diagram of the inner loop control of the AC/DC converter;

Figure 4 is a schematic diagram of the inner loop control of the energy storage converter.

Detailed ways

The invention proposes a seamless switching control method for the operation mode of an AC-DC hybrid distribution network. This method introduces a voltage deviation on the basis of the DC voltage droop control. The voltage deviation is an instruction issued by a central controller. The controller realizes the coordinated control of the controlled voltage source through the issued command, so as to realize the voltage support of the DC bus, the self-adaptive power sharing and the energy storage SOC control. In the normal working mode, the power demand of the DC load is met by two AC/DC converters, and the energy storage adopts constant power control to realize the energy storage SOC control; The faulty AC/DC converter realizes the DC bus voltage support and power sharing; in the dual fault mode, the DC load power demand is met by the energy storage converter, and the DC bus voltage is also supported by the energy storage converter.

Below in conjunction with accompanying drawing, the present invention is described in further detail:

Considering that the photovoltaic modules in the DC sub-network are controlled by the maximum power point tracking, and the output power is assumed to be constant, it has no effect on the mode seamless switching control method proposed by the present invention. The normal working mode is shown in Figure 2. There are three controlled voltage sources, two AC/DC converters and an energy storage system in the DC sub-network. The energy storage system is controlled by the energy storage converter. They are respectively marked with serial numbers, wherein 1 represents the first AC/DC converter, 2 represents the second AC/DC converter, and 3 represents the energy storage converter;

In the normal working mode, the circuit breakers S1, S2, S3, and S4 in Fig. 2 are all closed, the dual AC/DC converter is sufficient to meet the power demand of the DC load, and the central controller issues commands ΔV ₁ and ΔV ₂ Adjust the dual AC/DC converter to provide voltage support and power sharing for the DC bus, and the control commands ΔV ₁ and ΔV ₂ can be calculated by formula (1); in this operating mode, the energy storage system only controls the energy storage SOC, using For constant power control, the control command ΔV ₃ issued by the central controller can be calculated by formula (2). When the circuit breaker S1 or S2 in Fig. 2 is accidentally disconnected, the hybrid distribution network works in the single-circuit fault mode. Taking the disconnection of S1 as an example to illustrate: when it is detected that S1 is disconnected unexpectedly, the central controller actively disconnects S3, And adjust the control command ΔV ₃ issued to the energy storage converter, so that the energy storage system provides voltage support for the DC bus and shares the DC load power demand with the AC/DC converter 2, and the energy storage commutation in the single fault mode The control command ΔV ₃ of the AC/DC converter 2 can be calculated by formula (3), and the control command ΔV ₂ of the AC/DC converter 2 is the same as that in the normal working mode. When both S1 and S2 are accidentally disconnected in Figure 2, the hybrid distribution network works in a dual-circuit fault mode: when it is detected that S1 and S2 are disconnected, the central controller actively disconnects S3 and S4, and adjusts the distribution to the energy storage. The control command ΔV ₃ of the converter enables the energy storage system to provide voltage support for the DC bus and fully bear the power demand of the DC load. The control command ΔV ₃ of the energy storage converter in the dual fault mode can be calculated by formula (4) Calculated.

In the hybrid distribution network shown in Figure 2, there are three controlled voltage sources on the DC side: dual AC/DC converters and energy storage converters. The outer loop control of the three controlled voltage sources is based on DC voltage droop control, and introduces a voltage deviation ΔV _x , which is used as a control command for mode switching and is given by the central controller according to the operation mode of the hybrid distribution network. , according to which the reference voltage controlled by the inner loop of the controlled voltage source can be calculated respectively, and the specific implementation is shown in formula (5).

The two AC/DC converters use the same control. The inner loop control schematic diagram is shown in Figure 3. It mainly includes the voltage inner loop, the current inner loop and the pulse generator module. x=1 means the first AC/DC converter. converter, x=2 represents the second AC/DC converter. First, the collected three-phase voltage v _x,abc and three-phase current i _x,abc of the AC side are used to calculate the active power P _x and reactive power Q _x , and perform Park transformation on i _x,abc , as shown in formula (6) As shown, the d-axis component i _x,d , the q-axis component i _x,q and the 0-axis component i _x,0 of the current are obtained by calculation.

In order to obtain the reference AC voltage v ^* _x,abc of the reference pulse generator, it is necessary to obtain the reference voltage components v ^* _x,d , v ^* _x,q and v ^* _x,0 in the dq0 coordinate system. In order to obtain v ^* _x,d , subtract the actual output DC voltage v _dc,x from the DC reference voltage v _dc,x,ref obtained by the outer loop control, and then obtain the inner loop current reference value i _x,dref through PI adjustment, using i _x,d is subtracted from i _x,dref , and it can be adjusted by PI; in order to obtain v ^* _x,q , since the DC side does not consider the reactive power, the calculated AC side reactive power Q _x is subtracted from 0, After PI adjustment, the inner loop reference current i _x,qref is obtained, and i _x,qref is subtracted from i _x,q , and it can be adjusted by PI; in order to obtain v ^* _x,0 , the calculated 0-axis component i ₀ is subtracted Go to the 0 axis reference value 0, and then adjust by PI. Converting the obtained v ^* _x,d , v ^* _x,q and v ^* _x,0 to v ^* _x,abc needs to undergo inverse Park transformation, as shown in equation (7).

v ^* _{x, abc} can generate 6 pulse signals by comparing with the triangular carrier through the pulse generator module, which are used to control the opening and closing of the 6 IGBT switches of the AC/DC converter, so as to realize the AC/DC commutation The actual output DC voltage of the controller tracks the given DC reference voltage v _dc,x,ref by the outer loop control.

The control principle of the inner loop of the energy storage converter is shown in Figure 4, which also includes a voltage inner loop, a current inner loop and a pulse generator. Subtract the v _dc,3,ref given by the outer loop control with the measured actual output voltage v _dc,3 of the energy storage converter, and obtain the current inner loop reference value i _dc,3,ref through PI adjustment, and then The i _dc,3,ref is subtracted from the actual output current i _dc,3 of the energy storage converter, and after PI adjustment, the pulse signal of the switching tube of the energy storage converter can be obtained after inputting to the pulse generator. Using the obtained switching tube pulse signal to control the switching tube of the energy storage converter on and off, the actual output of the energy storage converter can track the given reference voltage v _dc,3,ref by the outer loop control.

The central controller adjusts and issues the control command ΔV _x (x=1,2,3) according to the actual operation mode of the hybrid distribution network. The outer loop control of the dual AC/DC converter and the energy storage converter adopts DC droop control. The voltage deviation is introduced to realize the DC reference voltage v _dc,x,ref of the dual AC/DC converter and the energy storage converter according to ΔV _x , and the inner loop control realizes the given reference voltage v for the outer loop control The tracking of _dc,x,ref can realize the DC bus voltage support, power adaptive sharing and energy storage SOC control under the normal working mode and fault mode of the hybrid distribution network.

Claims (6)

1. A control method for seamless switching of operating modes of an AC/DC hybrid distribution network, wherein the hybrid distribution network includes two AC sub-networks and one DC sub-network, and the two AC sub-networks are connected to each other through an AC/DC converter respectively. The DC sub-network is connected, and at the same time, the two AC sub-networks are connected to the public power grid via AC/AC; wherein, the hybrid distribution network is connected with an AC load on the AC side, and is connected with an energy storage system and a DC load on the DC side. In the energy storage system, the energy storage unit is connected to the DC sub-grid through the energy storage converter. It is characterized in that, the method comprises the following steps: (1) Two AC/DC converters and energy storage converters are used as three controlled voltage sources, and the central controller issues control commands ΔV to the three controlled voltage sources by monitoring the operating status of the power grid to achieve different Voltage stabilization, power adaptive sharing, and energy storage SOC regulation in operating mode include the following three modes: (a) When both lines of the AC sub-network are connected to the grid normally, the hybrid distribution network operates in the normal working mode, and the two AC/DC converters operate normally. Assuming that it is sufficient to meet the power demand of the DC load, the central controller will Send command ΔV to control the two-way AC/DC converter to provide DC bus voltage support, and at the same time correct the power sharing ratio, the DC energy storage converter only needs to control the energy storage SOC, and adopts constant power control; (b) When one fault occurs in the two-way grid connection of the AC sub-network, the hybrid distribution network operates in a single-way fault mode, the central controller removes the AC/DC converter corresponding to the fault, and the DC voltage changes from energy storage to energy storage. The converter is supported by another non-faulty AC/DC converter, and the central controller issues a command ΔV to regulate the energy storage converter and the non-faulty AC/DC converter to participate in DC bus voltage support and adaptive power sharing ; (c) When the two-way grid connection of the AC sub-network fails, the hybrid distribution network operates in the two-way fault mode, the central controller cuts off the two-way AC/DC converter, and the DC voltage is only converted by the energy storage. The central controller sends ΔV to the energy storage converter to support the DC voltage and provide the power required by the DC load. (2) The outer loop control of the three controlled voltage sources is based on the DC voltage droop control, and introduces the voltage deviation to realize the DC voltage of the dual AC/DC converter and the energy storage converter according to ΔV. reference voltage. The inner loop control is the reference voltage tracking control based on the PI regulator, which realizes the seamless switching control of the operation mode of the hybrid distribution network. 2 . The control method for seamless switching of operating modes of an AC/DC hybrid distribution network according to claim 1 , wherein the control command issued by the central controller to the dual-circuit AC/DC converter under the normal operating mode is 2 . ΔV _x (x=1,2) is calculated as follows: ΔV _x =(K _P1 +K _I1 /s)(v ^* -v _dcbus )+(K _P2 +K _I2 /s)(i _average -i _dc,x ),x=1,2 (1) Among them, x=1, 2, respectively represent the serial numbers of the corresponding two AC/DC converters, K _P1 , K _P2 correspond to the proportional coefficients of the PI regulators of voltage and current, respectively, K _I1 , K _I2 correspond to The integral coefficient of the PI regulator for voltage and current, 1/s represents the integral link of PI regulation, v ^* is the rated voltage of the DC bus, v _dcbus is the actual voltage of the DC bus, i _dc,x is the AC/DC converter Output current, i _average is the reference current of the AC/DC converter that participates in DC power sharing. When the voltage is the same, the power sharing can be achieved by controlling the current ratio. The energy storage converter only controls the energy storage SOC and adopts constant power control. The command ΔV ₃ issued by the central controller can be calculated and obtained in the following way:

Among them, i _dc,3,ref is the charging and discharging reference current of the constant power control of the energy storage system, and i _dc,3,ref >0, i _dc,3 is the actual charging and discharging current of the energy storage system, i _dc,3 >0 Indicates that the energy storage system is discharged, and i _dc,3 < 0 indicates that the energy storage system is charged. 3 . The control method for seamless switching of operating modes of an AC/DC hybrid distribution network according to claim 1 , wherein in the single-circuit fault mode, the central controller issues the control method to the energy storage converter and the non-faulty AC/DC converter. 4 . The control command ΔV _x (x=1 or 2,3) of the DC converter can be obtained by the following method: ΔV _x =(K _P1 +K _I1 /s)(v ^* -v _dcbus )+(K _P2 +K _I2 /s)(i _average -i _dc,x ), x=1 or 2,3 (3) Wherein, i _dc,x , where x=1 or 2, is the actual output current of the non-faulty AC/DC converter, and i _dc,3 is the actual output current of the energy storage converter. 4 . The control method for seamless switching of operating modes of an AC-DC hybrid distribution network according to claim 1 , wherein the control command ΔV ₃ issued by the central controller to the energy storage converter in the dual-circuit fault mode It can be calculated by the following method: ΔV ₃ =(K _P1 +K _I1 /s)(v ^* -v _dcbus ) (4) 5 . The control method for seamless switching of operating modes of an AC/DC hybrid distribution network according to claim 1 , wherein, in the step 2, the outer loops of the three controlled voltage sources are controlled in a DC voltage droop control mode. 6 . On this basis, the voltage deviation is introduced to achieve the DC reference voltage v _dc,x,ref of the dual AC/DC converter and the energy storage converter according to ΔV, as follows: v _dc,x,ref =v ^* -ki _dc,x +ΔV _x ,x=1,2,3 (5) Where v _dc,x,ref is the output reference voltage of the controlled voltage source, k is the droop coefficient, i _dc,x is the output current of the controlled voltage source, ΔV _x is the command issued by the central controller. 6 . The control method for seamless switching of operating modes of an AC/DC hybrid distribution network according to claim 1 , wherein the hybrid distribution network is further connected with a photovoltaic power generation system on the DC side, and the photovoltaic power generation system is configured with a maximum Power point operation.

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