CN107240934A - Alternating current-direct current mixing microgrid multi-mode operation control method for coordinating and smooth-switching method - Google Patents

Abstract

An AC-DC hybrid microgrid multi-mode operation coordination control method and a smooth switching method, which divides the AC-DC hybrid microgrid into an AC area, an AC-DC power flow section area, and a DC area; The degree of power unbalance is divided into autonomous operation mode and AC fixed-direction operation mode; when the AC-DC hybrid microgrid is running off-grid, one of the multiple bidirectional AC/DC converters is selected as the main bidirectional AC/DC converter. Converters, and the rest are secondary bidirectional AC/DC converters. According to the number of bidirectional AC/DC converters connected, the operating modes are divided into direct-stationary low-power operation mode and direct-stationary high-power operation mode. By performing secondary adjustment and power flow smoothing on the DC bus voltage and AC bus frequency of the AC-DC hybrid microgrid, the AC-DC hybrid microgrid can be smoothly switched from the AC-regulated DC operation mode to the DC-regulated AC low-power operation mode, making the DC Smoothly switch from fixed-delivery high-power operation mode to autonomous operation mode. The invention improves the power quality of the system.

Description

Coordinated control method and smooth switching method for multi-mode operation of AC-DC hybrid microgrid

technical field

The invention relates to a power coordination control method of an AC-DC hybrid microgrid. In particular, it relates to a multi-mode operation coordination control method and a smooth switching method of an AC-DC hybrid microgrid.

Background technique

With the increase in the types and quantities of distributed power sources, the popularity of DC loads, and the complexity and variety of distribution network structures, it is difficult for AC micro-grids to fully meet the growing demand for power supply and consumption. In order to ensure the efficiency of new energy and renewable energy It is necessary to actively carry out AC-DC hybrid micro-grid research to accommodate and utilize and meet the diversified power needs of users. The AC-DC hybrid microgrid can effectively integrate the respective advantages of the AC microgrid and the DC microgrid to form an AC-DC complementary energy supply system.

The high-density distributed energy connected to the AC-DC hybrid microgrid has strong randomness, and the two-way flow of energy between the large power grid and the AC area, the AC area and the DC area, and the DC area and the energy storage system area will inevitably cause changes in the power flow of the microgrid. Diversification puts forward higher requirements for the stable operation of the AC-DC hybrid microgrid. Further research on the switching process of grid-connected operation and off-grid operation mode, there is a problem that the power in the AC-DC power flow section cannot be released instantaneously, which will cause transient voltage and current impact on the system, so research on suitable AC-DC hybrid microgrid multi-mode stable operation and grid connection , Off-grid smooth switching is of great significance.

The existing literature research in this area is not perfect. The literature Zhang Lu and Tang Wei published "VSC-based AC-DC hybrid medium-voltage power distribution" on November 20, 2016 in the "Proceedings of the Chinese Society for Electrical Engineering", Volume 36, Issue 22. Grid Power-Voltage Coordinated Control" divides distribution network operation into normal and risk states from the optimization operation level, uses mathematical models and optimization algorithms to realize economical and safe operation of the power grid, and does not analyze voltage fluctuations and power levels from the system level. balance problem. Literature Li Peng, Yu Xiaomeng, and Zhao Bo published "H _∞ Robust Control of AC-DC Cross-section Voltage in AC-DC Hybrid Microgrid Based on Mixed Sensitivity" published in "Proceedings of the Chinese Society for Electrical Engineering", Volume 36, Issue 1, January 5, 2016 "In this paper, a H _∞ robust control based on mixed sensitivity is proposed for the voltage on both sides of the section, which improves the performance of the AC-DC hybrid microgrid against AC and DC disturbances. The research focuses on the bidirectional AC/DC converter control of the AC-DC section. The method does not consider the two-way flow of power in the AC and DC power flow sections. Literature Wang Panbao and Wang Wei published "Unified Control Strategy for DC Microgrid Off-grid and Grid-connected Operation" on September 5, 2015 in "Proceedings of the Chinese Society for Electrical Engineering" Volume 35, Issue 17. The coordination and autonomous control scheme of information can stabilize the DC voltage by reducing the power of non-important loads, and can switch between various modes according to the predetermined control scheme, realizing the unified control of DC micro-grid and off-grid operation, but the article only explains the multi-mode The logic implementation process of switching does not consider the smooth transition during the switching process of on-grid and off-grid modes.

Therefore, how to provide a control method capable of satisfying the multi-mode stable operation of the AC/DC hybrid microgrid and the smooth switching between grid-connected and off-grid is an urgent problem to be solved by those skilled in the art.

Contents of the invention

The technical problem to be solved by the present invention is to provide a multi-mode operation coordination control method and a smooth switching method in which the AC/DC hybrid microgrid adjusts the power flow according to the actual operation situation.

The technical solution adopted in the present invention is: an AC-DC hybrid microgrid multi-mode operation coordination control method, which is applied to the multi-mode operation coordination of multiple bidirectional AC/DC converters and energy storage bidirectional DC/DC converters A method for controlling and smooth switching, the method comprising: dividing the AC-DC hybrid microgrid into an AC area, an AC-DC power flow section area, and a DC area; It is the autonomous operation mode and the AC-constant-direction operation mode; when the AC-DC hybrid microgrid is running off-grid, one of the multiple bidirectional AC/DC converters is selected as the main bidirectional AC/DC converter, and the rest are From the bidirectional AC/DC converter, the operation mode is divided into direct-stationary low-power operation mode and direct-stationary high-power operation mode according to the number of bidirectional AC/DC converters connected.

In the autonomous operation mode, the power unbalance in the DC region of the AC-DC hybrid microgrid is less than the limit value of the charging and discharging power of the energy storage The energy storage bidirectional DC/DC converter adopts a constant DC bus voltage-limited power control method to maintain a constant DC bus voltage, and multiple bidirectional AC/DC converters are in a standby state; The power imbalance in the DC area of the microgrid is greater than or equal to the limit value of the charging and discharging power of the energy storage Energy storage to charge and discharge power limit Constant power operation, multiple bidirectional AC/DC converters use DC voltage deviation droop control to keep the DC bus voltage of the AC-DC hybrid microgrid constant.

The autonomous operation mode and the AC/DC operation mode are based on the set charging and discharging power limit value and DC bus voltage deviation threshold Freely switch operation modes; specifically, when the energy storage reaches the limit value of charging and discharging power When the DC bus voltage deviation occurs, when the preset voltage deviation threshold is reached After that, multiple bidirectional AC/DC converters are started to operate to stabilize the DC bus voltage; when the power imbalance in the DC area is reduced to the charge and discharge power limit within, and the DC bus voltage deviation decreases to the voltage deviation threshold Within the period, the energy storage automatically returns from constant power operation to constant DC bus voltage-limited power control operation, and multiple bidirectional AC/DC converters exit and are in standby mode.

In the direct AC low power operation mode, the power imbalance in the AC area is less than the capacity of the main bidirectional AC/DC converter Only the main bidirectional AC/DC converter adopts constant AC bus frequency-voltage control, and the rest of the secondary bidirectional AC/DC converters are in standby mode; in the direct fixed AC high-power operation mode, the power unbalance in the AC area exceeds the main bidirectional AC/DC converter. DC converter capacity The master bidirectional AC/DC converter operates at constant power, and multiple slave bidirectional AC/DC converters adopt frequency deviation droop control.

The described DC-AC low-power operation mode and DC-AC high-power operation mode are based on the capacity of the main bidirectional AC/DC converter and AC bus frequency deviation threshold Free switching operation mode, specifically: when the main bidirectional AC/DC converter transmits power exceeding the capacity , the AC bus frequency deviates, and the deviation reaches the frequency deviation threshold Afterwards, multiple slave bidirectional AC/DC converters start running to balance the power in the AC area and keep the voltage and frequency constant; when the working conditions change, the frequency deviation in the AC area is less than the deviation threshold At this time, multiple secondary bidirectional AC/DC converters exit the standby state, and the main bidirectional AC/DC converter automatically returns from constant power operation to constant AC bus frequency-voltage control operation.

A smooth switching method based on the multi-mode operation coordination control method of the AC-DC hybrid microgrid, which is to make the DC bus voltage and AC bus frequency of the AC-DC hybrid microgrid secondary adjustment and power flow smoothing respectively, so that the AC-DC hybrid microgrid The network realizes smooth switching from the direct-connection direct operation mode to the direct-connection low-power operation mode, so that the direct-direction high-power operation mode is smoothly switched to the autonomous operation mode.

The secondary adjustment is to perform secondary adjustment on the DC bus voltage through the controllers of multiple bidirectional AC/DC converters during the inspection period after the AC/DC hybrid micro-grid detects the off-grid signal when the AC-DC operation mode is running. Secondary voltage regulation, adjusted to the voltage standard value, and the energy storage cancels the charge and discharge power limit value After completing the secondary adjustment, the energy storage increases the charging and discharging power to maintain the power balance in the DC area, so that the power transmitted on multiple bidirectional AC/DC converters is reduced to zero, and the state is restored to the standby state. At this time, the static switch is segmented , the AC-DC hybrid micro-grid smoothly transitions to the direct fixed AC low power operation mode, the energy storage adopts constant DC bus voltage control, multiple bidirectional AC/DC converters control the frequency and voltage of the AC bus, and In the AC current control link of the DC converter, the current flowing before the static switch section is compensated.

The secondary regulation is that when the AC/DC hybrid micro-grid detects the grid-connected signal when the direct AC/DC high-power operation mode is running, multiple controllers from the bidirectional AC/DC converters check the AC bus frequency at the same time. Carry out secondary frequency modulation, adjust to the frequency standard value, and close the static switch; after the AC-DC hybrid micro-grid is connected to the grid, the large power grid will balance the power in the AC area, reducing the power transmitted by multiple bidirectional AC/DC converters to Zero, return to the standby state, the energy storage charging and discharging power is reduced and only the power balance in the DC area is maintained. After the charging and discharging limit value is reset, the AC-DC hybrid microgrid completes the smooth switching from the direct AC high-power operation mode to the autonomous operation mode.

The multi-mode operation coordination control method and smooth switching method of the AC-DC hybrid microgrid of the present invention can adjust the power flow according to the actual operation of the AC-DC hybrid microgrid, and reduce the grid-connected and The transient voltage and current impact during off-grid mode switching improves the power quality of the system. The present invention sets the DC bus voltage deviation threshold and the AC bus frequency deviation threshold, which is equivalent to setting a blocking interval for multiple bidirectional AC/DC converters, avoiding frequent changes in the power flow during small disturbances, and improving the transmission power efficiency of the bidirectional converters , reducing the power loss in the cross-section area of the AC-DC flow. Setting the energy storage charge and discharge power limit eliminates the problem of insufficient energy storage charge and discharge ramp rate when the micro-grid fluctuates greatly, and at the same time ensures that the energy storage maintains a large state of charge, which is conducive to sufficient remaining capacity for off-grid operation. Scaling and long-running. The smooth switching control method proposed by the present invention reduces the instantaneous impact of DC bus voltage when off-grid and AC bus frequency when grid-connected, avoids power mutation caused by multiple bidirectional AC/DC converters switching control strategies, and realizes The multi-mode smooth switching of the AC-DC hybrid microgrid improves the stability and dynamic response of the system.

Description of drawings

Figure 1 is a schematic diagram of the partition structure of the AC-DC hybrid microgrid;

Figure 2 is the equivalent topology circuit diagram of the AC-DC hybrid microgrid model;

Fig. 3a is a constant DC voltage-limited power control curve diagram of an energy storage bidirectional DC/DC converter;

Fig. 3b is a curve diagram of DC voltage deviation droop control of multiple bidirectional AC/DC converters;

Fig. 4 is a structural schematic diagram of a bidirectional DC/DC converter controller;

Fig. 5 is a structural schematic diagram of a DC voltage deviation drooping controller;

Fig. 6a is a constant AC frequency control curve diagram of the main bidirectional AC/DC converter;

Figure 6b is a droop control curve diagram of AC frequency deviation of multiple slave converters;

Fig. 7 is a schematic diagram of the AC frequency deviation droop control structure;

Figure 8 is the switching process of AC-DC hybrid microgrid grid-connected and off-grid operation;

Figure 9 is the timing process of switching from grid-connected mode 2 to off-grid mode 3;

Fig. 10 is the sequence process of switching from off-grid mode 4 to grid-connected mode 1;

Figure 11 is the equivalent circuit of the AC/DC hybrid microgrid used in the case of the present invention;

Figure 12 is the simulation result of the case switching from the grid-connected operation mode to the off-grid operation mode;

Figure 13 is the simulation result of the case switching from off-grid operation mode to grid-connected operation mode.

detailed description

The method for coordinating and controlling the multi-mode operation of the AC/DC hybrid microgrid and the smooth switching method of the present invention will be described in detail below with reference to the embodiments and the accompanying drawings.

The multi-mode operation coordination control method of the AC-DC hybrid microgrid of the present invention is a method for multi-mode operation coordination control and smooth switching applied to multiple bidirectional AC/DC converters and energy storage bidirectional DC/DC converters. The method described includes: the AC-DC hybrid microgrid is divided into the AC area, the AC-DC power flow section area and the DC area; when the AC-DC hybrid microgrid is connected to the grid, it is divided into the autonomous operation mode and the AC-fixed DC area according to the degree of power imbalance in the DC area. Operation mode; when the AC/DC hybrid microgrid is running off-grid, one of the multiple bidirectional AC/DC converters is selected as the master bidirectional AC/DC converter, and the rest are slave bidirectional AC/DC converters According to the number of bidirectional AC/DC converters connected, the operation mode can be divided into direct-stationary low-power operation mode and direct-stationary high-power operation mode.

In the autonomous operation mode, the power unbalance in the DC region of the AC-DC hybrid microgrid is less than the limit value of the charging and discharging power of the energy storage The energy storage bidirectional DC/DC converter adopts a constant DC bus voltage-limited power control method to maintain a constant DC bus voltage, and multiple bidirectional AC/DC converters are in a standby state; The power imbalance in the DC area of the microgrid is greater than or equal to the limit value of the charging and discharging power of the energy storage Energy storage to charge and discharge power limit Constant power operation, multiple bidirectional AC/DC converters use DC voltage deviation droop control to keep the DC bus voltage of the AC-DC hybrid microgrid constant.

The autonomous operation mode and the AC/DC operation mode are based on the set charging and discharging power limit value and DC bus voltage deviation threshold Freely switch operation modes; specifically, when the energy storage reaches the limit value of charging and discharging power When the DC bus voltage deviation occurs, when the preset voltage deviation threshold is reached Finally, multiple bidirectional AC/DC converters are started to stabilize the DC bus voltage; when the power imbalance in the DC area decreases and the DC bus voltage deviation automatically decreases to the limit value of charging and discharging power , and the DC bus voltage deviation decreases to the voltage deviation threshold Within the time period, the energy storage will automatically return from constant power operation to constant DC bus voltage-limited power control operation, and multiple bidirectional AC/DC converters will exit and be in standby mode.

The power imbalance in the AC area is less than the capacity of the main bidirectional AC/DC converter Only the main bidirectional AC/DC converter adopts constant AC bus frequency-voltage control, and the rest of the secondary bidirectional AC/DC converters are in standby mode; in the direct fixed AC high-power operation mode, the power unbalance in the AC area exceeds the main bidirectional AC/DC converter. DC converter capacity The master bidirectional AC/DC converter operates at constant power, and multiple slave bidirectional AC/DC converters adopt frequency deviation droop control.

The described DC-AC low-power operation mode and DC-AC high-power operation mode are based on the capacity of the main bidirectional AC/DC converter and AC bus frequency deviation threshold Free switching operation mode, specifically: when the transmission power of the main bidirectional AC/DC converter exceeds the capacity limit value When the AC bus frequency deviates, when the frequency deviation threshold is reached, multiple slave bidirectional AC/DC converters start running to balance the power in the AC area and keep the voltage and frequency constant; when the working conditions change, the frequency deviation in the AC area is less than the deviation threshold At this time, multiple secondary bidirectional AC/DC converters exit the standby state, and the main bidirectional AC/DC converter automatically returns from constant power operation to constant AC bus frequency-voltage control operation.

The smooth switching method based on the multi-mode operation coordination control method of the AC-DC hybrid micro-grid of the present invention is to make the AC-DC hybrid micro-grid through the secondary adjustment and power flow smoothing of the DC bus voltage and the AC bus frequency of the AC-DC hybrid micro-grid. The network realizes smooth switching from the direct-connection direct operation mode to the direct-connection low-power operation mode, so that the direct-direction high-power operation mode is smoothly switched to the autonomous operation mode.

The secondary adjustment is to perform secondary adjustment on the DC bus voltage through the controllers of multiple bidirectional AC/DC converters during the inspection period after the AC/DC hybrid micro-grid detects the off-grid signal when the AC-DC operation mode is running. The secondary voltage regulation is adjusted to the voltage standard value, and the energy storage cancels the charge and discharge power limit value After the secondary regulation is completed, the energy storage increases the charge and discharge power to maintain the power balance in the DC area, so that the power transmitted by multiple bidirectional AC/DC converters is reduced to zero and returns to the standby state. At this time, the static switch STS section, the AC-DC hybrid micro-grid smoothly transitions to direct fixed AC low-power operation mode, energy storage is controlled by constant DC bus voltage, multiple bidirectional AC/DC converters control the frequency and voltage of the AC bus, and multiple bidirectional AC In the AC current control link of the /DC converter, the current flowing before the static switch section is compensated.

The secondary regulation is that when the AC/DC hybrid micro-grid detects the grid-connected signal when the direct AC/DC high-power operation mode is running, the controllers of multiple slave bidirectional AC/DC converters check the parallel current bus in the same period. The frequency is adjusted to the frequency standard value twice, and the static switch is closed; after the AC-DC hybrid micro-grid is connected to the grid, the large power grid balances the power in the AC area, so that the power transmitted by multiple bidirectional AC/DC converters is reduced. To zero, return to the standby state, the energy storage charging and discharging power is reduced and only the power balance in the DC area is maintained. After the charging and discharging limit value is reset, the AC-DC hybrid microgrid completes the smooth switching from the direct AC high-power operation mode to the autonomous operation mode.

The method for coordinated control and smooth switching of the multi-mode operation of the AC/DC hybrid microgrid of the present invention will be further described below in conjunction with the accompanying drawings.

Figure 1 shows the structure of the AC-coupled low-voltage AC-DC hybrid microgrid system used in the present invention, which is divided into three parts: the AC area, the DC area, and the AC-DC power flow section area. The AC area includes photovoltaic power generation, wind power generation, AC loads and grid-connected static switches, and the DC area includes DC loads and energy storage units in addition to distributed power generation units. The AC-DC power flow section area is composed of multiple bidirectional AC/DC converters, AC-side filters and DC-side capacitors. Fig. 2 is an example of the AC/DC hybrid microgrid equivalent model topology circuit of the present invention, u _{IC, abc} , i _{IC, abc} are the AC three-phase voltage and current filtered by the AC side filter, u _IC , i _IC are the The DC voltage and current after the DC side capacitor, u _C , i _C are the DC voltage and current output by the energy storage system after being transformed by the bidirectional DC/DC converter, i _{B, abc} , u _dc are the AC bus current and DC Bus voltage, the AC-DC hybrid microgrid is divided into grid-connected operation and off-grid operation according to the state of the static switch.

1. Grid-connected operation

During grid-connected operation, the large power grid can be equivalent to an infinite power supply. The voltage and frequency in the AC area of the AC-DC hybrid microgrid are clamped by the large grid. The power changes of each unit in the DC area will break the original energy balance relationship of the system during stable operation, making the The AC-DC hybrid microgrid operates in different working modes, and the system is divided into autonomous operation mode and AC-fixed-direction operation mode according to the degree of power imbalance. The autonomous operation mode is to consider the continuous unstable output of renewable energy sources such as photovoltaics and wind turbines and the randomness of loads to cause real-time disturbances in the power in the DC region, and at the same time consider the error delay and control accuracy of the signals in each micro-source controller Insufficient, the power and DC bus voltage fluctuate around the set standard value, when the power imbalance in the DC area of the AC-DC hybrid microgrid is less than the charge and discharge power limit value of the energy storage , the power unbalance can be absorbed/released by energy storage to realize the power balance of the DC region itself and the stability of the DC bus voltage, that is, multiple bidirectional AC/DC converters are in the standby state according to the preset voltage threshold, and the energy storage operation adopts Constant DC voltage-limited power control. The AC-DC operation mode is characterized by considering the large-capacity load change and the sudden change in the output power of the micro-source in the DC area, so that the power imbalance in the DC area of the AC-DC hybrid micro-grid exceeds the limit value of the charging and discharging power of the energy storage Due to the ramp rate problem of the energy storage output power, it is impossible to quickly adjust the power fluctuation in the DC area. At this time, multiple bidirectional AC/DC converters can quickly adjust the power output according to the voltage change of the DC area network, and balance the DC through rectification and inverter. District power to stabilize the DC bus voltage, that is, energy storage constant power charging and discharging operation, multiple bidirectional AC/DC converters adopt DC voltage deviation droop control.

Fig. 3a and Fig. 3b are curve diagrams of coordinated control of DC bus voltage in the present invention. in,

Figure 3a is a constant DC voltage-limited power control curve of an energy storage bidirectional DC/DC converter. According to the characteristic curve, a power limiter is designed after the voltage regulation loop to limit the reference value of the current loop during grid-connected operation. The power limit is canceled during off-grid operation, and the controller structure of the bidirectional DC/DC converter is shown in Figure 4.

Figure 3b is the DC voltage deviation droop control curve diagram of multiple bidirectional AC/DC converters, and Figure 5 is the structure of the _DC voltage deviation droop controller; Actual voltage deviation between standard values. S _U is a logic selection switch, which compares the actual value of the voltage deviation with the threshold value of the voltage deviation, and selects different enabling signals to convert the current direct axis reference signal to The AC current regulation link issued to multiple bidirectional AC/DC converters controls the bidirectional AC/DC commutation operation in the standby state and DC drooping operation. The logic selection formula is:

The expressions of DC voltage droop control and DC bus voltage deviation compensation control link are:

Among them, R _IC,i is the drooping slope of the outer loop of different bidirectional AC/DC converters, which is equivalent to the virtual internal impedance, which is multiplied by the current of the capacitor bank to form the output voltage feedforward. For voltage deviation compensation, the voltage deviation caused by the transition from energy storage constant voltage control to bidirectional AC/DC converter droop control is reduced. Since the AC-DC power flow section can transmit energy bidirectionally, the voltage deviation direction is different. When S _d is 1, it means that the voltage at the operating point is higher than the standard value, and the bidirectional AC/DC converter is running in reverse, and vice versa, it means rectifying operation.

2. Off-grid operation

In the off-grid mode, the energy storage in the AC-DC hybrid microgrid is the only stable power source, which needs to undertake the power balance and system stability of the entire microgrid, and ensure the normal power supply of important AC loads and DC loads. The DC area of the microgrid is always charged and discharged by energy storage to maintain power balance and DC bus voltage constant, and its grid-connected access point is the balance node of the DC area; in the AC area, it is necessary to stabilize the power fluctuation in the AC area and control the frequency of the AC bus through the AC-DC power flow section /voltage, and the grid-connected point on the AC side is the balance node in the AC area. Due to the strong randomness of photovoltaic, wind turbine, and load power, the power transmitted in the AC-DC power flow section area changes in real time and cannot be accurately predicted. It needs to be reasonably allocated according to the power level of the bidirectional AC/DC converter. The number of connected devices is different. In the off-grid mode, the AC-DC hybrid micro-grid is divided into direct-stationary AC low-power operation mode and direct-stationary AC high-power operation mode. In the direct AC low power operation mode, the power unbalance in the AC area is smaller than the capacity of the main bidirectional AC/DC converter Only the master bidirectional AC/DC converter adopts constant AC bus frequency-voltage to control the voltage and frequency of the AC bus, and the rest of the slave bidirectional AC/DC converters are in a standby state. In the direct AC high-power operation mode, the power imbalance in the AC area exceeds the capacity of the main bidirectional AC/DC converter The main bidirectional AC/DC converter operates at constant power, and multiple secondary bidirectional AC/DC bidirectional converters adopt frequency deviation droop control to make the AC area reach a power balance state by adjusting active power and reactive power.

Fig. 6a and Fig. 6b are curve diagrams of the AC bus frequency coordinated control in the present invention. in,

Figure 6a is the main bidirectional AC/DC converter fixed AC frequency control curve, and the frequency in the AC area is always controlled as Voltage is

Fig. 6b is a curve diagram of AC frequency deviation droop control of multiple slave bidirectional AC/DC converters, and Fig. 7 is a schematic structural diagram of AC frequency deviation droop control of multiple slave bidirectional AC/DC converters.

Take the three-phase current i _IC,abc and the three-phase voltage u _IC,abc on the AC side filter, calculate the active power and reactive power sent to the AC area, and filter it through the first-order low-pass filter as the AC droop control Feedback power, the formula is:

Where: ω _p is the corner frequency of the low-pass filter. Multiple bidirectional AC/DC converters connected in parallel distribute the unbalanced power in the AC area proportionally, and the expressions for AC droop and deviation compensation are:

in: are the standard frequency and voltage of the AC bus, m _IC ,i and n _IC ,i are the frequency droop coefficient and voltage droop coefficient of different slave bidirectional AC/DC converters respectively, AC droop active power and reactive power reference values respectively, the default setting is 0. is the frequency deviation threshold, and compensates the frequency deviation generated when the power of the main bidirectional AC/DC converter exceeds the rated capacity. After the drooping link calculates and outputs the real-time voltage reference value E _IC and the phase reference value θ _IC , the expression of the generated voltage command is:

The obtained result is controlled by the voltage regulator and the current regulator without difference tracking, and multiple bidirectional AC/DC converters output the same AC frequency and voltage in parallel.

Fig. 8 is an analysis of the switching process between grid-connected and off-grid operation of the AC-DC hybrid microgrid, and the method illustrates the smooth switching process when switching between different operation modes of grid-connected and off-grid.

1. Switch from AC-DC operation mode to DC-DC low-power operation mode

When running in AC/DC mode, multiple bidirectional AC/DC converters adopt the same DC voltage deviation droop control, and the DC bus voltage naturally droops along the droop curve, and the difference from the bus standard value is:

When the micro-grid central controller detects the off-grid switching signal, the micro-grid enters the synchronization check, the energy storage cancels the power limit, and independently controls the DC bus voltage. Due to the existence of the voltage difference, the DC bus voltage needs to be adjusted to the standard value by secondary voltage regulation. Simulating the secondary adjustment of the traditional power system can be realized by adjusting the reference voltage value of the droop curve of multiple bidirectional AC/DC converters. The sag curve of variable section moves down △u _dc , the sag curve of right rectification section moves up △u _dc , and the reference voltage expression is obtained again:

The adjustment process of the _i -th bidirectional AC/DC converter participating in the secondary voltage regulation is shown in Fig. 3b. The solid line is the drooping curve of the voltage deviation. The active power P _IC,i , the output DC bus voltage of each bidirectional AC/DC converter is u _IC , at this time the DC bus of the microgrid drops △u _dc , and after the off-grid signal is detected, the drooping curve is shifted upward, that is, the microgrid The grid central controller adjusts the reference voltage to Makes the bidirectional AC/DC converter output voltage Work on the new operating point B.

After secondary voltage regulation, the DC bus voltage runs at the standard value, but the bidirectional AC/DC converter i still transmits power P _IC,i , and direct switching of the control strategy will lead to severe power fluctuations. At this time, the power in the AC-DC section area can be stabilized by increasing the charging and discharging power through energy storage, thereby balancing the power in the DC area. The output power change value of the energy storage during the recovery period is the total power transmitted by each bidirectional AC/DC converter, namely:

After the transmission power of the bidirectional AC/DC converter is reduced to zero and returns to the standby state, that is, the transition from operating point B to point O, at this time, the static switch is segmented, the microgrid runs off-grid, and the main bidirectional AC/DC converter It can be directly switched to constant AC frequency-voltage control, and consider the power in the balanced AC area of the large power grid during grid-connected operation, that is, to compensate the current flowing through the static switch before off-grid, according to the capacity of each bidirectional AC/DC converter Distributed to the current reference value in the current regulation loop of each controller to avoid power fluctuations in the AC area. The specific adjustment process of switching from grid-connected to off-grid at different timings is shown in Figure 9.

2. Switch from high-power operation mode to autonomous operation mode

When running in the direct fixed AC high power operation mode, the energy storage constant DC bus voltage controls the stable DC area, the main bidirectional AC/DC converter outputs at the maximum power, and multiple secondary bidirectional AC/DC converters adopt AC frequency deviation droop control, and the AC The difference between the real-time frequency of the bus and the standard frequency is

When the micro-grid central controller detects the grid-connected switching signal, since the AC bus needs to be directly connected to the large power grid, a second frequency adjustment is required to restore the bus frequency to the standard value during the inspection period. Direct translation of multiple sets from the droop curve of the bidirectional AC/DC converter to change the reference value of the frequency, the updated frequency reference expression is:

The direction of the inverter section is positive during off-grid operation, which is opposite to the positive direction during grid-connected operation, so the adjustment value S _d △f _ac is negative. The adjustment process of the i-th slave bidirectional AC/DC converter participating in the secondary frequency modulation is shown in Figure 6b. The i-th slave bi-directional AC/DC converter works at the inverter operation point C, and the active power transmitted to the AC The power is P _IC,i , the frequency obtained from the droop control of the bidirectional AC/DC converter is f _IC , and the droop amount is △f _ac . In order to make the frequency of the AC bus equal to the standard frequency of the large power grid, the droop curve is shifted up to the operating point D by adjusting the central controller of the microgrid, at this time f _IC =50Hz.

After the secondary frequency modulation, the static switch is closed, and the balance point of the AC area is transferred from the grid-connected point of the AC side of multiple bidirectional AC/DC converters to the grid-connected point of the AC-DC hybrid micro-grid and the large grid, and the large grid smoothly balances the power of the AC area, The power in the AC-DC power flow section area is gradually reduced to zero, that is, the transition from the operating point D to the operating point O. The total compensation power of the large power grid during the switching process is:

After the smooth switching is completed, the AC-DC hybrid microgrid operates in autonomous operation mode, and multiple bidirectional AC/DC converters are in standby mode. The energy storage only stabilizes the power balance in the DC area and resets the power threshold. The specific adjustment process of switching from off-grid to grid-connected in different timings is shown in Figure 10.

Finally, in order to verify the effectiveness and feasibility of the proposed coordinated control and smooth switching method, according to the AC-DC hybrid microgrid equivalent circuit shown in Figure 11, the simulation verification of different mode switching is carried out on the MATLAB/Simulink platform, and the DC bus voltage is selected The AC bus voltage is 560V, the AC bus voltage is 10kV, the rated capacity of the three bidirectional AC/DC converters is 250kVA, the energy storage capacity is 1MWh, and the limit value of energy storage charging and discharging power is set 50kW, preset DC voltage deviation threshold 6V, preset AC bus frequency deviation threshold 0.1Hz. The two working conditions verify the multi-mode operation coordination control method and smooth switching method of the AC-DC hybrid microgrid proposed by the present invention.

Working condition 1: When the system is grid-connected and runs stably and autonomously until 0.5s, the DC load suddenly increases by 450kW. When the operation reaches 1.1s, the AC-DC hybrid microgrid receives the off-grid signal. The simulation experiment results are shown in Figure 12.

From the simulation results, it can be seen that when the DC load suddenly increases, the DC bus voltage drops greatly, and the three bidirectional AC/DC converters adopt DC voltage deviation droop control to automatically share the active power imbalance in the DC area and convert the DC The bus voltage is stable at 540V. Since the three bidirectional AC/DC converters have the same capacity and the same droop coefficient settings, the active power transmitted after the system is stable is 150kW. After a dynamic change of 0.2s, the AC-DC hybrid microgrid operates in the AC-DC mode. After the off-grid signal is detected in 1.1s, the energy storage cancels the power limit and directly completes the power balance of the power in the DC area, that is, the power transmitted in the AC-DC section is stabilized. At this time, the DC bus voltage returns to the standard value of 560V, and the three bidirectional AC /DC converter transmission power is zero. After the static switch is disconnected, the main bidirectional AC/DC converter transmits -100kW power from the DC area to the AC area. Since the power balance in the AC area needs to absorb 100kW from the large grid during grid-connected operation, the energy storage needs to provide The power compensates for the power shortage in the AC area. At this time, the system smoothly switches to direct AC low-power operation.

Working condition 2: When the system runs off-grid, stable, direct AC and low power for 2s, the AC load suddenly increases by 350kW, and when it runs for 3s, the AC-DC hybrid microgrid receives a grid-connected signal. The simulation experiment results are shown in Figure 13.

It can be seen from the simulation results that when the AC load suddenly increases, the frequency of the AC bus drops instantly. After the three bidirectional AC/DC converters simultaneously start the frequency deviation droop control, the bidirectional AC/DC converters all transmit -150kW, And the frequency is reduced to 49.7Hz, and the output active power of the energy storage is increased to 470kW. After 0.2s adjustment, the system can directly operate at high power. At 3s, the system detects the grid-connected signal, and the AC bus frequency returns to 50Hz after secondary frequency modulation. At this time, the static switch is closed, and the large power grid directly balances the power in the AC area, and the transmission power of the three bidirectional AC/DC converters is reduced to Zero is in the standby state. After the transition is completed, the output power of the energy storage is reduced to 20kW, and the system smoothly switches to the autonomous operation mode.

Claims (8)

1. A multi-mode operation coordination control method for an alternating-current/direct-current hybrid microgrid is characterized by being applied to multi-mode operation coordination control and smooth switching of a plurality of bidirectional AC/DC converters and energy storage bidirectional DC/DC converters, and comprising the following steps: the alternating current-direct current hybrid micro-grid is divided into an alternating current area, an alternating current-direct current power flow section area and a direct current area; when the alternating current-direct current hybrid micro-grid is connected to the power grid, the alternating current-direct current hybrid micro-grid is divided into an autonomous operation mode and an alternating current-fixed direct operation mode according to the degree of the power unbalance of a direct current area; when the alternating-current and direct-current hybrid micro-grid is operated in an off-grid mode, one of the bidirectional AC/DC converters is selected as a main bidirectional AC/DC converter, the rest bidirectional AC/DC converters are selected as slave bidirectional AC/DC converters, and the operation modes are divided into a direct-alternating low-power operation mode and a direct-alternating high-power operation mode according to the difference of the number of the connected bidirectional AC/DC converters.

2. The method for multi-mode operation coordination control and smooth switching of the alternating current-direct current hybrid microgrid according to claim 1, characterized in that in the autonomous operation mode, the amount of direct current region power unbalance of the alternating current-direct current hybrid microgrid is smaller than a charge-discharge power limit value of stored energyThe energy storage bidirectional DC/DC converter maintains the voltage of the direct current bus to be constant by adopting a control mode of fixing the voltage of the direct current bus and limiting power, and the plurality of bidirectional AC/DC converters are in a standby state; when the alternating current-fixed direct current running mode is adopted, the amount of the power unbalance of the direct current area of the alternating current-direct current hybrid micro-grid is greater than or equal to the charge-discharge power limit value of the stored energyEnergy storage to limit the charging/discharging powerAnd (4) operating at constant power, and adopting direct-current voltage deviation droop control to keep the direct-current bus voltage of the alternating-current and direct-current mixed micro-grid constant by the aid of the bidirectional AC/DC converters.

3. The method for multi-mode operation coordination control and smooth switching of the alternating current-direct current hybrid microgrid according to claim 2, characterized in that the autonomous operation mode and the alternating current-fixed direct operation mode are performed according to set charge-discharge power limit valuesAnd DC bus voltage deviation thresholdFree switching operationA mode; specifically, the stored energy reaches the limit value of charge-discharge powerWhen the voltage deviation of the direct current bus is reached, the voltage deviation reaches a preset voltage deviation threshold valueThen, starting the bidirectional AC/DC converters to stabilize the voltage of the direct current bus; when the power unbalance amount of the DC region is reduced to the limit value of the charging and discharging powerIn addition, the voltage deviation of the DC bus is reduced to a voltage deviation threshold valueAnd if the energy is stored, the operation is automatically returned to the constant direct current bus voltage-limited power control operation from the constant power operation, and the plurality of bidirectional AC/DC converters exit and are in a standby state.

4. The method for multi-mode operation coordination control and smooth switching of the alternating current-direct current hybrid microgrid according to claim 1, characterized in that in a direct-current low-power operation mode, the amount of power unbalance of an alternating current area is smaller than the capacity of a main bidirectional AC/DC converterOnly the main bidirectional AC/DC converter adopts fixed AC bus frequency-voltage control, and the other auxiliary bidirectional AC/DC converters are in a standby state; in a DC-AC high power mode of operation, the amount of AC zone power imbalance exceeds the capacity of the main bidirectional AC/DC converterThe main bidirectional AC/DC converter operates at constant power, and the plurality of slave bidirectional AC/DC converters adopt frequency deviation droop control.

5. The method for multi-mode operation coordination control and smooth switching of the alternating current-direct current hybrid microgrid according to claim 4, characterized in that the direct-alternating current low-power operation mode and the direct-alternating current high-power operation mode are performed according to the capacity of a main bidirectional AC/DC converterAnd AC bus frequency deviation thresholdThe free switching operation mode specifically comprises the following steps: over capacity in main bidirectional AC/DC converter transmission powerWhen the frequency of the alternating current bus is deviated, the deviation reaches a frequency deviation threshold valueThen, starting a plurality of slave bidirectional AC/DC converters to operate so as to balance the power of the alternating current area and maintain the voltage and the frequency constant; when the working condition changes, the frequency deviation of the alternating current area is smaller than the deviation threshold valueWhen the master bidirectional AC/DC converter is in the standby state, the slave bidirectional AC/DC converter automatically returns to the constant alternating current bus frequency-voltage control operation from the constant power operation.

6. The method for smoothly switching the multi-mode operation coordination control method of the alternating-current/direct-current hybrid microgrid based on claim 1 is characterized in that the alternating-current/direct-current hybrid microgrid realizes smooth switching from an alternating-current/direct-current running mode to a direct-current/low-power running mode and smooth switching from the direct-current/high-power running mode to an autonomous running mode by respectively and sequentially carrying out secondary regulation and tidal current smoothing on the direct-current bus voltage and the alternating-current bus frequency of the alternating-current/direct-current hybrid microgrid.

7. The smooth switching method based on the AC/DC hybrid microgrid multi-mode operation coordination control method according to claim 6, characterized in that the secondary regulation is that when the AC/DC hybrid microgrid operates in an AC/DC operation mode, after the AC/DC hybrid microgrid detects an off-grid signal, secondary voltage regulation is performed on the DC bus voltage through a plurality of controllers of bidirectional AC/DC converters in a synchronization period, the voltage is regulated to a standard voltage value, and the limited value of the charging and discharging power is cancelled in the energy storageAfter secondary adjustment is completed, the energy storage increases the charging and discharging power to keep the power balance of the direct current area, the power transmitted on the bidirectional AC/DC converters is reduced to zero, the standby state is recovered, the static switch is segmented at the moment, the alternating current and direct current mixed micro-grid is smoothly transited to the direct alternating current and alternating current low-power operation mode to operate, the energy storage adopts the voltage control of the direct current bus, the frequency and the voltage of the alternating current bus are controlled by the bidirectional AC/DC converters, and the current flowing through the static switch before the segmentation is compensated in the alternating current control link of the bidirectional AC/DC converters.

8. The smooth switching method of the multi-mode operation coordination control method for the alternating current/direct current hybrid microgrid according to claim 6, characterized in that the secondary regulation is that when the alternating current/direct current hybrid microgrid operates in a direct alternating current high-power operation mode, after the alternating current/direct current hybrid microgrid detects a grid-connected signal, the frequency of an alternating current bus is secondarily modulated by a plurality of controllers of a bidirectional AC/DC converter in a detection period, the frequency is adjusted to a standard frequency value, and a static switch is closed; after the alternating current and direct current hybrid micro-grid is connected to the grid, the large power grid balances the power of an alternating current area, the power transmitted on the bidirectional AC/DC converters is reduced to zero, the standby state is recovered, the energy storage charging and discharging power is reduced, the power balance of the direct current area is only kept, and the alternating current and direct current hybrid micro-grid finishes smooth switching from a direct alternating current high-power operation mode to an autonomous operation mode after the charging and discharging limit value is reset.