CN107240934A - Alternating current-direct current mixing microgrid multi-mode operation control method for coordinating and smooth-switching method - Google Patents
Alternating current-direct current mixing microgrid multi-mode operation control method for coordinating and smooth-switching method Download PDFInfo
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Abstract
A kind of alternating current-direct current mixing microgrid multi-mode operation control method for coordinating and smooth-switching method, are divided into exchanging area by alternating current-direct current mixing microgrid, hand over straight trend section area and DC area；Degree when alternating current-direct current mixes micro-grid connection operation according to DC area unbalanced power amount is divided into autonomous operation pattern and the straight operational modes of Jiao Ding；In alternating current-direct current mixing microgrid off-grid operation, select two-way AC/DC transverters based on one in many two-way AC/DC transverters, remaining is, from two-way AC/DC transverters, operational mode to be divided into straight friendship small-power operational mode surely according to the number of units difference that two-way AC/DC transverters are accessed and directly high-power operational mode is handed over surely.Secondary Control is carried out respectively by DC bus-bar voltage and ac bus frequency to alternating current-direct current mixing microgrid and trend is calmed down, alternating current-direct current mixing microgrid is realized from handing over fixed straight operational mode to directly handing over small-power operational mode to take over seamlessly surely, the high-power operational modes of Zhi Dingjiao is switched to autonomous operation mode smooth.The present invention improves the quality of power supply of system.
Description
Technical Field
The invention relates to a power coordination control method for an alternating current-direct current hybrid microgrid. In particular to an alternating current and direct current hybrid microgrid multi-mode operation coordination control method and a smooth switching method.
Background
Along with the increase of the variety and the quantity of distributed power sources and the popularization of direct current loads, the structure of a power distribution network is complex and diverse, an alternating current micro-grid cannot completely meet the increasing power supply and utilization requirements, and the research on an alternating current-direct current hybrid micro-grid is required to be actively developed for ensuring the efficient consumption and utilization of new energy and renewable energy and meeting the diversified power requirements of users. The alternating current-direct current hybrid micro-grid can effectively integrate respective advantages of the alternating current micro-grid and the direct current micro-grid to form an alternating current-direct current complementary energy supply system.
High-density distributed energy accessed in the alternating current and direct current hybrid micro-grid has strong randomness, and energy between the large power grid and an alternating current area, between the alternating current area and a direct current area, and between the direct current area and an energy storage system area flows in two directions, so that the micro-grid has various tidal current changes, and higher requirements are provided for the stable operation of the alternating current and direct current hybrid micro-grid. The problem that power cannot be instantly released in an alternating current-direct current section in the switching process of a grid-connected operation mode and an off-grid operation mode is further researched, and transient voltage and current impact is caused to a system, so that the research on the suitable alternating current-direct current hybrid micro-grid multi-mode stable operation and grid-connected and off-grid smooth switching is significant.
The existing literature is not perfect in research in this respect, and the literature Zulong and Tang Wei is published in 'VSC-based AC/DC hybrid medium voltage distribution network power-voltage coordination control' on 'China Motor engineering Commission' No. 36, No. 22 of '20 th of' 11 th month in 2016 for operating the distribution network from the aspect of optimization operationThe method is divided into a normal state and a risk state, the economic and safe operation of the power grid is realized by using a mathematical model and an optimization algorithm, and the problems of voltage fluctuation and power balance are not analyzed from the system level. The document li peng, early dawning and Zhao wave is published in 2016 (statement of Chinese Motor engineering, 5 d 1.1.36, vol. "hybrid sensitivity-based AC/DC hybrid microgrid AC/DC section voltage H)∞Robust control-based H for voltages on both sides of a section in the text∞Robust control is realized, the performance of the alternating current-direct current hybrid micro-grid for resisting alternating current disturbance and direct current disturbance is improved, a bidirectional AC/DC converter control method focusing on alternating current-direct current sections is researched, and bidirectional flow of alternating current-direct current section power is not considered. In the text of the unified control strategy for the off-grid and on-grid operation of the direct-current microgrid, which is published in the unified control strategy for the off-grid and on-grid operation of the direct-current microgrid at the 17 th volume of the 35 th volume of the report of the Chinese Motor engineering, 9.5.2015, Wangbei and Wangbei, a coordination autonomous control scheme based on voltage information of the direct-current bus is designed, the direct-current voltage is stably operated by reducing power through an unimportant load, and the modes can be switched according to a preset control scheme, so that the unified control of the on-grid and off-grid operation of the direct-current microgrid is realized, but only the logic realization process of multi-mode switching is explained in the text, and the.
Therefore, how to provide a control method capable of meeting the requirements of multi-mode stable operation of the alternating-current/direct-current hybrid microgrid and smooth switching between grid connection and grid disconnection is a problem which needs to be solved by technical personnel in the field at present.
Disclosure of Invention
The invention aims to solve the technical problem of providing a multi-mode operation coordination control method and a smooth switching method for adjusting power flow of an alternating current-direct current hybrid micro-grid according to actual operation conditions.
The technical scheme adopted by the invention is as follows: a multi-mode operation coordination control method for an alternating-current and direct-current hybrid microgrid is a multi-mode operation coordination control and smooth switching method applied to a plurality of bidirectional AC/DC converters and energy storage bidirectional DC/DC converters, and comprises 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.
In the autonomous operation mode, the amount of the power unbalance of the direct current area of the alternating current and direct current hybrid micro-grid is smaller than the charge and discharge power limit value of the 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.
The autonomous operation mode and the AC-DC operation mode are operated according to the set charge-discharge power limit valueAnd DC bus voltage deviation thresholdFreely switching the operation mode; specifically, the energy storage is achievedLimit value of charge and 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.
In the direct-current fixed AC low-power operation mode, the amount of the power unbalance of the AC area is smaller than the capacity of the 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.
The direct-current/alternating-current low-power operation mode and the direct-current/alternating-current high-power operation mode are operated according to the capacity of the main bidirectional AC/DC converterAnd AC bus frequency deviation thresholdValue ofThe 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.
A smooth switching method based on an alternating current-direct current hybrid microgrid multi-mode operation coordination control method is characterized in that secondary regulation and tide smoothing are sequentially performed on direct current bus voltage and alternating current bus frequency of an alternating current-direct current hybrid microgrid respectively, so that the alternating current-direct current hybrid microgrid can be smoothly switched from an alternating current-fixed direct operation mode to a direct current-fixed alternating current low-power operation mode, and the direct current-alternating current high-power operation mode can be smoothly switched to an autonomous operation mode.
The secondary regulation is that when the alternating current-fixed-direct current running mode runs, after the alternating current-direct current mixed micro-grid detects an off-grid signal, secondary voltage regulation is carried out on the voltage of the direct current bus through a plurality of controllers of the 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 energy storageAfter the secondary regulation is finished, the energy storage increases the charging and discharging power to keep the power balance of the direct current region, so that the power transmitted on a plurality of bidirectional AC/DC convertersThe power is reduced to zero and is restored to a standby state, at the moment, the static switch is segmented, the alternating-current and direct-current mixed micro-grid is smoothly transited to a direct-current and alternating-current low-power operation mode to operate, the stored energy is controlled by adopting the voltage of a fixed direct-current bus, the frequency and the voltage of the alternating-current bus are controlled by the plurality of bidirectional AC/DC converters, and the current flowing before the static switch is segmented is compensated in the alternating-current control link of the plurality of bidirectional AC/DC converters.
The secondary regulation is that when the direct-alternating current high-power operation mode operates, after the alternating-current and direct-current mixed micro-grid detects a grid-connected signal, secondary frequency modulation is carried out on the frequency of an alternating-current bus through a plurality of controllers of the bidirectional AC/DC converter in a detection period, the frequency is adjusted to a standard frequency value, and the 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.
According to the multi-mode operation coordination control method and the smooth switching method for the alternating current-direct current hybrid micro-grid, the power flow can be adjusted according to the actual operation condition of the alternating current-direct current hybrid micro-grid, the transient voltage and current impact in the switching process of grid connection and grid disconnection modes are reduced on the premise that the stable voltage and frequency of the system are guaranteed, and the electric energy quality of the system is improved. The method is equivalent to the arrangement of a locking interval for a plurality of bidirectional AC/DC converters by setting a direct current bus voltage deviation threshold and an alternating current bus frequency deviation threshold, thereby avoiding the frequent change of the tidal current in small disturbance, improving the efficiency of the transmission power of the bidirectional converters and reducing the power loss of an alternating current-direct current power flow sectional area. The energy storage charging and discharging power limitation is set, the problem of insufficient energy storage charging and discharging climbing rate when the micro-grid fluctuates in high power is solved, meanwhile, the energy storage is ensured to maintain a larger charge state, and sufficient residual capacity can be supplied for allocation and long-term operation in off-grid operation. The smooth switching control method provided by the invention reduces the instantaneous impact of the direct current bus voltage when the grid is disconnected and the alternating current bus frequency when the grid is connected, avoids the power sudden change caused by the switching control strategy of a plurality of bidirectional AC/DC converters, realizes the multi-mode smooth switching of the alternating current-direct current hybrid micro-grid, and improves the stability and dynamic response of the system.
Drawings
Fig. 1 is a schematic diagram of an ac/dc hybrid microgrid partition structure;
fig. 2 is an equivalent topology circuit diagram of an alternating current-direct current hybrid microgrid model;
FIG. 3a is a constant DC voltage-limited power control graph of the energy storage bidirectional DC/DC converter;
FIG. 3b is a graph of DC voltage deviation droop control for a plurality of bi-directional AC/DC converters;
FIG. 4 is a schematic diagram of a bi-directional DC/DC converter controller configuration;
FIG. 5 is a schematic diagram of a DC offset droop controller;
FIG. 6a is a graph of the constant AC frequency control of the main bi-directional AC/DC converter;
FIG. 6b is a graph of a plurality of droop control curves from the AC frequency deviation of the inverter;
FIG. 7 is a schematic diagram of an AC frequency deviation droop control architecture;
fig. 8 is a switching process of grid-connected and off-grid operation of an alternating current and direct current hybrid microgrid;
FIG. 9 is a sequence of switching from grid-connected mode two to off-grid mode three;
FIG. 10 is a transition sequence for switching from off-grid mode to on-grid mode;
fig. 11 is an equivalent circuit of an ac/dc hybrid microgrid used in the present invention;
FIG. 12 is a simulation result of a case switch from a grid-connected mode of operation to an off-grid mode of operation;
fig. 13 is a simulation result of the case switching from the off-grid operation mode to the on-grid operation mode.
Detailed Description
The following describes in detail the ac/dc hybrid microgrid multi-mode operation coordination control method and the smooth switching method according to the present invention with reference to the embodiments and the accompanying drawings.
The invention relates to a multi-mode operation coordination control method for an alternating-current and direct-current hybrid microgrid, which is a multi-mode operation coordination control and smooth switching method applied to a plurality of bidirectional AC/DC converters and energy storage bidirectional DC/DC converters, and comprises 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.
In the autonomous operation mode, the amount of the power unbalance of the direct current area of the alternating current and direct current hybrid micro-grid is smaller than the charge and discharge power limit value of the 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.
The autonomous operation mode and the AC-DC operation mode are operated according to the set charge-discharge power limit valueAnd DC bus voltage deviation thresholdFreely switching the operation 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 area is reduced and the voltage deviation of the DC bus is automatically reduced to the limit value of the charging and discharging powerAnd the DC bus voltage deviation is reduced to the voltage deviation thresholdAnd 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.
The power unbalance amount of the alternating current area is smaller than the capacity of the main bidirectional AC/DC converterOnly the main bidirectional AC/DC converter adopts fixed AC bus frequency-voltage control, and the other secondary bidirectional AThe C/DC converter is 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.
The direct-current/alternating-current low-power operation mode and the direct-current/alternating-current high-power operation mode are operated according to the capacity of the main bidirectional AC/DC converterAnd AC bus frequency deviation thresholdThe free switching operation mode specifically comprises the following steps: transmitting power exceeding capacity limit value in main bidirectional AC/DC converterWhen the frequency of the alternating current bus deviates, a plurality of slave bidirectional AC/DC converters start to operate after the frequency deviation threshold is reached, so that the power of an alternating current area is balanced and the voltage and the frequency are kept constant; when the working condition changes and the frequency deviation of the alternating current area is smaller than the deviation threshold value, the slave bidirectional AC/DC converters quit to be in a standby state, and the master bidirectional AC/DC converter automatically returns to the fixed alternating current bus frequency-voltage control operation from the constant-power operation.
The invention discloses a smooth switching method based on an alternating current-direct current hybrid microgrid multi-mode operation coordination control method.
The secondary regulation is that when the AC-DC hybrid micro-grid operates in the AC-DC operation mode, the AC-DC hybrid micro-grid detects an off-grid signalThen, the voltage of the direct current bus is regulated to a standard voltage value for the second time through the controllers of the plurality of bidirectional AC/DC converters during the checking period, and the limited value of the charging and discharging power is cancelled by the stored energyAfter 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 and is restored to a standby state, at the moment, the static switch STS is segmented, the alternating current and direct current mixed microgrid is smoothly transited to a direct current constant alternating current low-power operation mode to operate, the energy storage adopts constant direct current bus voltage control, the bidirectional AC/DC converters control the frequency and voltage of an alternating current bus, and the current flowing in front of the static switch segments is compensated in the alternating current control link of the bidirectional AC/DC converters.
The secondary regulation is that when the direct-alternating current high-power operation mode operates, after the alternating-current and direct-current mixed micro-grid detects a grid-connected signal, secondary frequency modulation is carried out on the frequency of a parallel-current bus through a plurality of controllers of the bidirectional AC/DC converter in a synchronization detection period, the frequency is adjusted to a standard frequency value, and the 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.
The following further describes the ac/dc hybrid microgrid multi-mode operation coordination control and smooth switching method of the present invention with reference to the accompanying drawings.
Fig. 1 is a structure of an ac-coupled low-voltage ac/dc hybrid microgrid system used in the present invention, which is divided into an ac region, a dc region, and an ac/dc power flow cross-sectional region. The alternating current area comprises photovoltaic power generation, wind power generation, alternating current loads and a grid-connected static switch, and the direct current area comprises direct current loads and energy storage units besides distributed power generation units. The AC/DC power flow section area is composed of a plurality of bidirectional AC/DC convertersAn AC side filter and a DC side capacitor. FIG. 2 shows an AC/DC hybrid microgrid equivalent model topology circuit u according to an embodiment of the present inventionIC,abc，iIC,abcIs the AC three-phase voltage and current u after being filtered by the AC side filterIC，iICIs the DC voltage and current after passing through the DC side capacitor uC，iCFor the direct voltage and current i output by the energy storage system after conversion by a bidirectional DC/DC converterB,abc，udcThe alternating current and direct current hybrid micro-grid is divided into grid-connected operation and off-grid operation according to the state of the static switch.
1. Grid-connected operation
The large power grid can be equivalent to an infinite power supply during grid-connected operation, the voltage and the frequency of an alternating current area and a direct current area of the alternating current and direct current mixed micro-grid are clamped by the large power grid, the power change of each unit in the direct current area breaks the energy balance relation during the original stable operation of the system, the alternating current and direct current mixed micro-grid operates in different working modes, and the system is divided into an autonomous operation mode and an alternating current and fixed direct operation mode according to different degrees of power unbalance. The autonomous operation mode is characterized in that the power of a direct current area is disturbed in real time by considering the continuous and unstable output of renewable energy sources such as photovoltaic and fans and the randomness of loads, meanwhile, the error delay and the control accuracy insufficiency of signals in each micro-controller are considered, the power and the direct current bus voltage fluctuate around a set standard value, and when the power unbalance amount of the direct current area of the alternating current-direct current mixed micro-grid is smaller than the charge-discharge power limit value of stored energyDuring the operation, the power unbalance amount can realize the power balance of the direct current area and the voltage stability of the direct current bus through energy storage absorption/release, namely, the bidirectional AC/DC converters are in a standby state according to a preset voltage threshold, and the energy storage operation is controlled by constant direct current voltage-limited power. When in the AC-DC operation mode, the method is characterized in that the large-capacity load change and the sudden change of the output power of the micro-source in the DC area are considered, so that the power unbalance amount of the DC area of the AC-DC hybrid micro-grid is increasedCharging and discharging power limit value of over-stored energyBecause the energy storage output power has the problem of climbing rate, the power fluctuation of a direct current area cannot be quickly adjusted, at the moment, the power output of the plurality of bidirectional AC/DC converters can be quickly adjusted according to the voltage change of a direct current area network, the power of the direct current area is balanced through rectification and inversion so as to stabilize the voltage of a direct current bus, namely, energy storage constant power charging and discharging operation is carried out, and the plurality of bidirectional AC/DC converters adopt direct current voltage deviation droop control.
Fig. 3a and 3b are coordination control graphs of the dc bus voltage of the present invention. Wherein,
FIG. 3a is a constant DC voltage-limited power control curve diagram of the energy storage bidirectional DC/DC converter, according to the characteristic curve, a power limiter is designed after a voltage regulation loop in grid-connected operation to limit a reference value of a current loopAnd the power limitation is cancelled in the off-grid operation, and the controller structure of the bidirectional DC/DC converter is shown in figure 4.
FIG. 3b is a DC voltage deviation droop control graph of a plurality of bidirectional AC/DC converters, FIG. 5 is a DC voltage deviation droop controller structure, △ udcThe actual voltage deviation between the direct current side capacitor bank voltage of the bidirectional AC/DC converter and the standard value of the direct current bus is obtained. SUFor the logic selection switch, the actual value of the voltage deviation is compared with the voltage deviation threshold value, and different enabling signals are selected to enable the current direct axis reference signalAnd an alternating current regulation link issued to the plurality of bidirectional AC/DC converters controls the bidirectional AC/DC converters to operate in a standby state and a direct current droop operation, and the logic selection formula is as follows:
the expressions of the direct-current voltage droop control and direct-current bus voltage deviation compensation control link are as follows:
wherein R isIC,iThe droop slope of the outer ring of the bidirectional AC/DC converter is different, is equivalent to virtual internal impedance, and is multiplied by the current of the capacitor bank to form output voltage feedforward.For voltage deviation compensation, voltage deviation caused when energy storage constant voltage control transits to bidirectional AC/DC converter droop control is reduced. The AC/DC power flow section can transmit energy in two directions, so that the voltage deviation directions are different, SdWhen the voltage is 1, the voltage at the operating point is higher than the standard value, the bidirectional AC/DC converter operates in an inversion mode, and when the voltage is in an inversion mode, the bidirectional AC/DC converter operates in a rectification mode.
2. Off-grid operation
Under the off-grid mode, the stored energy in the alternating current-direct current hybrid micro-grid is used as the only stable power supply, the power balance and the system stability of the whole micro-grid need to be born, and the normal power supply of important alternating current loads and direct current loads is ensured. The microgrid direct current region is always charged and discharged by energy storage to maintain power balance and constant direct current bus voltage, and a grid-connected access point is a direct current region balance node; the alternating current area needs to stabilize the power fluctuation of the alternating current area and control the frequency/voltage of an alternating current bus through an alternating current-direct current section, and a grid connection point on the alternating current side of the alternating current area is a balance node of the alternating current area. Due to the fact that photovoltaic power, fan power and load power have strong randomness, the power transmitted by an AC/DC power flow section area changes in real time and cannot be accurately predicted, reasonable distribution needs to be conducted according to the power grade of a bidirectional AC/DC converter, and an AC/DC hybrid micro-grid is divided into a direct-fixed AC low-power operation mode and a direct-fixed AC high-power operation mode in an off-grid mode according to the fact that the number of the connected bidirectional AC/DC converters is different. When the direct current-fixed alternating current low-power operation mode is adopted, the amount of the power unbalance of the alternating current area is smaller than that of the main powerBidirectional AC/DC converter capacityOnly the main bidirectional AC/DC converter adopts fixed AC bus frequency-voltage to control AC bus voltage and frequency, and the other auxiliary bidirectional AC/DC converters are in a standby state. When the direct current/alternating current high-power operation mode is adopted, the power unbalance amount of the alternating current area exceeds the capacity of the main bidirectional AC/DC converterThe main bidirectional AC/DC converter operates at constant power, the plurality of slave bidirectional AC/DC converters adopt frequency deviation droop control, and the active power and the reactive power are adjusted to enable the AC area to reach a power balance state.
Fig. 6a and 6b are graphs of the ac bus frequency coordination control of the present invention. Wherein,
FIG. 6a is a constant AC frequency control curve diagram of the main bidirectional AC/DC converter, wherein the frequency of the AC region is always controlled to beAt a voltage of
Fig. 6b is a graph showing a droop control of the AC frequency deviation of the plurality of slave bidirectional AC/DC converters, and fig. 7 is a schematic diagram showing a droop control structure of the AC frequency deviation of the plurality of slave bidirectional AC/DC converters.
By taking the three-phase current i on the filter on the AC sideIC,abcAnd three phase voltage uIC,abcCalculating active power and reactive power transmitted to the alternating current area, filtering the active power and the reactive power by a first-order low-pass filter, and then using the active power and the reactive power as feedback power for alternating current droop control, wherein the formula is as follows:
wherein: omegapIs the low pass filter break frequency. The parallel-connected multiple slave bidirectional AC/DC converters distribute the unbalanced power of the AC area in proportion, and the AC droop and deviation compensation expression is as follows:
wherein:respectively, the standard frequency and the standard voltage value of the AC bus, mICI and nICI is respectively different frequency droop coefficients and voltage droop coefficients of the bidirectional AC/DC converter,and respectively exchanging droop active power and reactive power reference values, and setting the values as 0 by default.And compensating the frequency deviation generated when the power of the main bidirectional AC/DC converter exceeds the rated capacity for the frequency deviation threshold value. Drooping link calculating and outputting real-time voltage reference value EICAnd a phase reference value thetaICThen, the voltage command expression is generated as follows:
the obtained result is subjected to the voltage regulator and the current regulator no-difference tracking control, and a plurality of bidirectional AC/DC converters output the same alternating current frequency and voltage in parallel.
Fig. 8 is an analysis of the switching process of the grid-connected and off-grid operation of the ac/dc hybrid microgrid, and the method illustrates the smooth switching process when different operation modes of the grid-connected and off-grid are switched.
1. Switching from AC-DC fixed operation mode to DC-AC low-power operation mode
When the AC-DC operation mode is operated, a plurality of bidirectional AC/DC converters adopt droop control of the deviation of the same direct current voltage, the voltage of a direct current bus naturally droops along a droop curve, and the difference value between the voltage of the direct current bus and the standard value of the bus is as follows:
the method comprises the steps that after a microgrid central controller detects an off-grid switching signal, the microgrid enters an inspection period, power limitation is cancelled by stored energy, and direct current bus voltage is independently controlled, because of the existence of a voltage difference value, the direct current bus voltage needs to be secondarily regulated to a standard value, secondary regulation of a traditional power system is simulated, the secondary regulation can be realized by regulating reference voltage values of droop curves of a plurality of bidirectional AC/DC converters, the regulating quantity is equal to the voltage deviation quantity, the positive and negative of the regulating quantity are related to the power transmission direction, namely, the droop curve of an inverter section is downwards moved by △ udcThe droop curve of the right rectifying section is upwards moved △ udcAnd the reference voltage expression is obtained again as follows:
the adjustment process of the ith bidirectional AC/DC converter participating in secondary voltage regulation is shown in FIG. 3b, wherein the solid line is a voltage deviation droop curve, and the bidirectional AC/DC converteriWorking at a rectification operating point A and sharing active powerPower PIC,iThe output direct current bus voltage of each bidirectional AC/DC converter is uICAt the moment, the microgrid direct-current bus falls △ udcWhen the off-grid signal is detected, the droop curve is translated upwards, namely the microgrid central controller adjusts the reference voltage toSo that the bidirectional AC/DC converter outputs a voltageOperating at the new operating point B.
After secondary voltage regulation, the voltage of the direct current bus operates at a standard value, but the bidirectional AC/DC converter i still transmits power PIC,iDirect switching control strategies can result in severe power fluctuations. At the moment, the power of the alternating current-direct current flow section area can be increased through energy storage to increase the charge-discharge power balance, so that the power of the direct current area is balanced. The change value of the output power of the energy storage in the flat recovery period is the total power transmitted by each bidirectional AC/DC converter, namely:
when the transmission power of the bidirectional AC/DC converter is reduced to zero and returns to a standby state, namely, the bidirectional AC/DC converter is transited from an operation point B to an O point, at the moment, the static switch is segmented, the microgrid is operated off the grid, the main bidirectional AC/DC converter can be directly switched to constant alternating current frequency-voltage control, the power of a large power grid balancing alternating current area during grid-connected operation is considered, namely, the current flowing through the static switch before off the grid is compensated, the current reference value is distributed to the current regulating ring of each controller according to the capacity of each bidirectional AC/DC converter, and the power fluctuation of the alternating current area is avoided. The specific adjustment process for switching from grid-connection to off-grid at different time sequences is shown in fig. 9.
2. Switching from direct-alternating-current high-power operation mode to autonomous operation mode
When the direct current/alternating current high-power operation mode is operated, the voltage of the energy storage fixed direct current bus controls a stable direct current area, the main bidirectional AC/DC converter outputs the maximum power, the plurality of slave bidirectional AC/DC converters adopt the droop control of alternating current frequency deviation, and the difference value of the real-time frequency of the alternating current bus and the standard frequency is
When the microgrid central controller detects a grid-connected switching signal, the alternating current bus needs to be directly connected with the large power grid, so that the frequency of the bus needs to be restored to a standard value through secondary frequency modulation in the detection period. Directly translating a plurality of reference values for changing the frequency from the droop curve of the bidirectional AC/DC converter, wherein the updated frequency reference expression is as follows:
the direction of the inversion section is positive when the grid-connected operation is carried out, and the positive direction is opposite to the positive direction when the grid-connected operation is carried out, so that the adjustment quantity Sd△facThe value is negative. The regulation process of the ith slave bidirectional AC/DC converter participating in secondary frequency modulation is shown in fig. 6b, the ith slave bidirectional AC/DC converter works at an inversion operation point C, and the active power transmitted to an alternating current area is PIC,iAll the frequencies obtained by droop control of the bidirectional AC/DC converter are fICThe drop amounts are △ fac. In order to enable the frequency of the alternating current bus to be equal to the standard frequency of the large power grid, the droop curve is enabled to move upwards to an operation point D by adjusting the central controller of the microgrid, and at the moment fIC＝50Hz。
After secondary frequency modulation, the static switch is closed, the balance point of the alternating current area is transferred to the grid-connected point of the alternating current-direct current hybrid micro-grid and the large power grid from the grid-connected point of the alternating current side of the bidirectional AC/DC converter, the power of the large power grid is smoothly balanced, the power of the alternating current-direct current cross-sectional area is gradually reduced to zero, namely the power of the alternating current-direct current cross-sectional area is transferred to an operating point O from an operating point D, and the compensation power total amount of the large power grid:
after the smooth switching is completed, the alternating current-direct current hybrid micro-grid operates in an autonomous operation mode, the bidirectional AC/DC converters are in a standby state, energy storage is only stable, the power balance of the direct current area is balanced, and the power threshold is reset. The specific adjustment process for switching from off-grid to grid-connection in different time sequences is shown in fig. 10.
Finally, in order to verify the effectiveness and feasibility of the provided coordination control and smooth switching method, according to the simulation verification that the alternating current-direct current hybrid microgrid equivalent circuit shown in fig. 11 is switched in different modes on an MATLAB/Simulink platform, 560V of direct current bus voltage, 10kV of alternating current bus voltage, 250kVA of rated capacity and 1MWh of energy storage capacity of three bidirectional AC/DC converters are selected, and the limit value of energy storage charging and discharging power is setIs 50kW, and a DC voltage deviation threshold value is presetIs 6V, and the frequency deviation threshold of the alternating current bus is presetIs 0.1 Hz. 2 working conditions verify that the alternating current-direct current hybrid microgrid multi-mode operation coordination control method and the smooth switching method are provided by the invention.
Working condition 1: when the grid connection of the system stably and autonomously operates for 0.5s, the direct current load is suddenly increased by 450kW, when the system operates for 1.1s, the alternating current-direct current hybrid micro-grid receives an off-grid signal, and the simulation experiment result is shown in FIG. 12.
According to simulation results, when the direct-current load suddenly increases, the direct-current bus voltage greatly drops, the three bidirectional AC/DC converters automatically share the active power imbalance of the direct-current area by adopting direct-current voltage deviation droop control, and the direct-current bus voltage is stabilized at 540V. The three bidirectional AC/DC converters have the same capacity and the same droop coefficient, and the active power transmitted after the system is stable is 150 kW. After the dynamic change of 0.2s, the alternating current-direct current hybrid micro-grid operates in an alternating-direct mode. After an off-grid signal is detected within 1.1s, the energy storage cancels power limitation, the power balance of the power in the direct current area is directly completed, namely the power transmitted by the alternating current-direct current flow section is stabilized, at the moment, the voltage of the direct current bus is restored to the standard value of 560V, and the transmission power of the three bidirectional AC/DC converters is zero. After the static switch is disconnected, the main bidirectional AC/DC converter transmits-100 kW power from the direct current area to the alternating current area, and the alternating current area needs to absorb 100kW from a large power grid due to power balance in the grid-connected operation, so that power compensation for power shortage of the alternating current area needs to be provided after the grid is disconnected, and the system is smoothly switched to the direct-current low-power operation.
Working condition 2: when the system is stably and directly operated for 2s under the low AC power, the AC load is suddenly increased by 350kW, when the system is operated for 3s, the AC/DC hybrid microgrid receives grid-connected signals, and the simulation experiment result is shown in FIG. 13.
According to simulation results, when alternating current load suddenly increases, the frequency of an alternating current bus instantly drops, after three bidirectional AC/DC converters simultaneously start frequency deviation droop control, the bidirectional AC/DC converters transmit-150 kW, the frequency is reduced to 49.7Hz, the active power of energy storage output is increased to 470kW, and after 0.2s adjustment, the system directly operates at alternating large power. And when the system detects a grid-connected signal in 3s, the frequency of the alternating current bus is restored to 50Hz after secondary frequency modulation, at the moment, the static switch is closed, the power of the alternating current area is directly balanced by the large power grid, the transmission power of the three bidirectional AC/DC converters is reduced to zero and is in a standby state, the energy storage output power is reduced to 20kW after the transition is completed, and the system is smoothly switched to an 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.
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