CN113346531A - Active grid-connected and off-grid switching method for cascaded energy storage system - Google Patents
Active grid-connected and off-grid switching method for cascaded energy storage system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/40—Synchronising a generator for connection to a network or to another generator
- H02J3/42—Synchronising a generator for connection to a network or to another generator with automatic parallel connection when synchronisation is achieved
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J9/00—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting
- H02J9/04—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source
- H02J9/06—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems
Abstract
The active grid-connected and off-grid switching method of the cascade energy storage system is characterized in that: when the cascade energy storage system is switched from a grid-connected state to an off-grid state, the cascade energy storage system enters an off-grid voltage control loop by taking the voltage amplitude, the phase and the angular frequency of the current alternating current power grid as initial values; when the cascade energy storage system is switched from the off-grid state to the grid-connected state, the amplitude and the phase of the voltage output by the cascade energy storage system in the off-grid state are adjusted until the voltage and the amplitude are synchronous, and the cascade energy storage system is switched to the grid-connected state. The grid-connected and off-grid switching method solves the problem that local key equipment cannot normally work due to short-term (such as 100ms) power failure caused by the fact that a locking device is adopted for grid connection and off-grid connection in the prior art, and also solves the technical problems that when droop control and virtual synchronous machines are adopted for grid connection and off-grid connection in the prior art, the droop coefficient and the virtual synchronous machine rotational inertia are difficult to determine, and the system is easy to cause instability.
Description
Technical Field
The invention relates to a grid-connected and grid-disconnected switching method, in particular to an active grid-connected and grid-disconnected switching method of a cascade energy storage system.
Background
With the increasing installation of distributed renewable energy sources and energy storage devices, structural changes are brought to modern power distribution systems. The output voltage grade of a general photovoltaic power generation or energy storage device is low, and the general photovoltaic power generation or energy storage device needs to be connected to a high-voltage grade power grid through a step-up transformer. The high-voltage cascade energy storage system is a novel energy storage converter. The converter achieves higher AC side output voltage through the cascade topology of the H bridge, can be directly connected with 10kV and 35kV high-voltage-class power grids, and omits a step-up transformer, thereby improving the efficiency of the converter.
The micro-grid based on the cascaded energy storage system also faces the problem of grid-to-grid switching. In the application field of large-scale energy storage power stations, in order to ensure successful grid-connected and grid-disconnected switching, a mode of outputting by a locking device and restarting equipment after finishing actions of a grid-connected and grid-disconnected switch is generally adopted. Since the response time of the common grid-connected and off-grid switch is about 100ms, a short power failure can be caused in the period, and the local key load cannot work normally.
The existing seamless grid-connected and off-grid switching method mostly adopts droop control and a virtual synchronous machine, but the droop control and the virtual synchronous machine control can bring the change of the amplitude and the frequency of output voltage, and the droop coefficient and the rotational inertia of the virtual synchronous machine are difficult to determine, so that the system is easy to be unstable.
Disclosure of Invention
In order to overcome the defects of the technical problems, the invention provides an active grid-connected and off-grid switching method of a cascade energy storage system.
The invention relates to an active grid-connected and off-grid switching method of a cascade energy storage system.A power supply end of a local load is connected to the output end of the cascade energy storage system at the rear side of a grid-connected and off-grid switch; the method is characterized in that: when the cascade energy storage system is switched from a grid-connected state to an off-grid state, the cascade energy storage system enters an off-grid voltage control loop by taking the voltage amplitude, the phase and the angular frequency of the current alternating current power grid as initial values; when the cascade energy storage system is switched from the off-grid state to the on-grid state, the amplitude and the phase of the voltage output by the cascade energy storage system in the off-grid state are adjusted until the voltage and the amplitude output by the cascade energy storage system are synchronous with the voltage and the amplitude of the current alternating current power grid, and the cascade energy storage system is switched to the on-grid state.
According to the active grid-connected and grid-disconnected switching method of the cascade energy storage system, the cascade energy storage system is switched from a grid-connected state to an off-grid state through the following steps:
a) collecting the grid information and calculating a voltage reference value, and collecting the voltage amplitude U of the AC grid when receiving a command for switching from a grid-connected state to an off-grid stateg1Phase thetag1And angular frequency ω;
b) calculating an off-grid voltage reference value, and calculating an off-grid output voltage reference value of the cascade energy storage system through a formula (1):
in the formula, thetapcs_refFor off-grid voltage-loop phase reference, uAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
c) closed-loop voltage regulation, which is to independently control the three-phase voltage output by the cascade energy storage system to be off-grid, and the closed-loop voltage regulation is carried out through a formula (2):
uA_m＝uAref+kp·(uAref-uApcs)
uB_m＝uBref+kp·(uBref-uBpcs) (2)
uC_m＝uCref+kp·(uCref-uCpcs)
in the formula uA_m、uB_mAnd uB_mRespectively outputting modulation signals of A-phase, B-phase and C-phase voltages, u-phase and C-phase voltages for the cascade type energy storage systemApcs、uBpcsAnd uCpcsThe instantaneous values of the A-phase alternating-current voltage, the B-phase alternating-current voltage and the C-phase alternating-current voltage output by the cascade energy storage system are respectively; k is a radical ofpAdjusting the coefficient for voltage closed loop;
entering an off-grid voltage control loop;
d) and off-grid switching, namely disconnecting a grid-connected and off-grid switch of the cascade energy storage system after entering an off-grid voltage control loop, and finishing the active switching of the cascade energy storage system from grid connection to off-grid.
The active grid-connected and grid-disconnected switching method of the cascade energy storage system is characterized in that the cascade energy storage system is switched from the grid-disconnected state to the grid-connected state through the following steps:
1) collecting the grid and energy storage system information, and collecting the voltage amplitude U of the alternating current grid when receiving a command of switching from an off-grid state to a grid-connected stategAnd phase thetagAnd the voltage amplitude U output by the cascade energy storage system under the off-grid statepcsAnd phase thetapcs；
2) Calculating an off-grid voltage reference value, and calculating the off-grid voltage reference value of the cascade energy storage system through a formula (3):
wherein, the sigma is a small value which is set manually and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u. ofAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
performing off-grid voltage loop control on the off-grid voltage reference value obtained according to the formula (3);
3) grid-connected control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of a formula (4) or not by acquiring the voltage amplitude and the phase of the cascade energy storage system in real time:
in the formula eta1And η2A smaller value is set artificially and used for indicating that the error of the two variables meets the requirement;
and when the voltage amplitude and the phase of the cascade energy storage system meet the formula (4), closing a grid-connected and off-grid switch of the cascade energy storage system, and entering a grid-connected power control loop to finish the active switching from off-grid to grid-connected of the cascade energy storage system.
The invention has the beneficial effects that: according to the active grid-connected and grid-disconnected switching method of the cascade energy storage system, when the cascade energy storage system is switched from a grid-connected state to an off-grid state, reference values of an off-grid voltage loop phase and three-phase voltage are calculated by collecting the voltage amplitude, the phase and the angular frequency of a power grid as initial values, and then the output voltage of the cascade energy storage system to be off-grid is controlled through closed-loop voltage regulation, so that the voltage amplitude is ensured not to fluctuate before and after the cascade energy storage system is off-grid, and uninterrupted power supply of load equipment can be realized. When the cascade energy storage system is switched from an off-grid state to a grid-connected state, firstly, acquiring the voltage amplitude and phase of an alternating current power grid and the voltage amplitude and phase of the cascade energy storage system, then establishing an off-grid voltage reference value according to the phase difference between the power grid and the energy storage system and the amplitude of the energy storage system, and switching the energy storage system to the grid-connected state when the phase difference and the voltage amplitude difference between the energy storage system and the power grid are smaller than a constraint condition, thereby realizing the flexible switching between the two states of the energy storage system between the grid-connected state and the off-grid state, ensuring the reliable power supply of a local load, solving the problem that the local key equipment cannot normally work due to the short (such as 100ms) power failure caused by the adoption of a locking device and the off-grid, and also solving the problems that the droop coefficient and the rotational inertia of a virtual synchronizer are difficult to determine when the adoption of droop control and the adoption of the existing virtual synchronizer and the off-grid connection and connection of the virtual synchronizer, Easily cause the technical problem of unstable system.
Drawings
FIG. 1 is a schematic diagram of a system of a cascaded energy storage system according to the present invention;
FIG. 2 is a flowchart of an active grid-connected and off-grid switching method of the cascaded energy storage system according to the present invention;
fig. 3 is a graph showing the ac voltage, ac current, load current and grid-connected/off switch action of the energy storage system when the active grid-connected/off switching method of the cascaded energy storage system of the present invention is used for grid-connected/off operation.
In the figure: the system comprises an alternating current power grid 1, a cascade energy storage system 2, a local load 3, a power unit 4 and an off-grid switch 5.
Detailed Description
The invention is further described with reference to the following figures and examples.
As shown in fig. 1, a schematic diagram of a system principle of the cascade energy storage system of the present invention is provided, the cascade energy storage system 2 is connected to an ac power grid 1 through a grid-connected and off-grid switch 5, a three-phase circuit of the cascade energy storage system is composed of a plurality of power units 4 cascaded together, each power unit 4 is composed of a plurality of controllable power devices, and a battery pack for storing electric energy is arranged in each power unit 4. The local load 3 is connected to the output of the cascaded energy storage system 2.
The cascade energy storage system can store the electric energy on the alternating current power grid 1 in the battery pack through rectification, and can invert the electric energy stored in the battery pack into alternating current through inversion to be transmitted to the power grid. Thus, if the voltage on the alternating current power grid 1 is lower than the preset value, the alternating current power grid 1 is in a wave trough state, and the electric energy stored in the battery pack is released and transmitted to the power grid; if the voltage of the alternating current power grid 1 is higher than the preset voltage, indicating that the alternating current power grid 1 is in a wave crest state, storing abundant electric energy on the power grid into a battery pack; it can be seen that it has the effect of peak clipping and valley filling.
When the alternating current power grid 1 to which the cascade energy storage system 2 is connected needs to be maintained or overhauled, the cascade energy storage system 2 needs to be switched from a grid-connected state to an off-grid state, and after maintenance or overhaul is finished, the cascade energy storage system 2 needs to be switched from the off-grid state to the grid-connected state. For the existing method of implementing grid connection and grid disconnection by using a locking device, a short (e.g. 100ms) power failure may be caused, and thus, the critical equipment in the local load 3 may not work normally. For the existing method adopting droop control and virtual synchronous machine grid-connection and off-grid, the determination of the droop coefficient and the virtual synchronous machine moment of inertia is difficult, and the instability of the system is easily caused. The active grid-connected and off-grid switching method of the cascade energy storage system can better solve the technical problem.
As shown in fig. 2, a flowchart of the active grid-connected and off-grid switching method of the cascaded energy storage system of the present invention is shown,
the cascade energy storage system is switched from a grid-connected state to an off-grid state through the following steps:
a) collecting the grid information and calculating a voltage reference value, and collecting the voltage amplitude U of the AC grid when receiving a command for switching from a grid-connected state to an off-grid stateg1Phase thetag1And angular frequency ω;
b) calculating an off-grid voltage reference value, and calculating an off-grid output voltage reference value of the cascade energy storage system through a formula (1):
in the formula, thetapcs_refFor off-grid voltage-loop phase reference, uAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
c) closed-loop voltage regulation, which is to independently control the three-phase voltage output by the cascade energy storage system to be off-grid, and the closed-loop voltage regulation is carried out through a formula (2):
uA_m＝uAref+kp·(uAref-uApcs)
uB_m＝uBref+kp·(uBref-uBpcs) (2)
uC_m＝uCref+kp·(uCref-uCpcs)
in the formula uA_m、uB_mAnd uB_mRespectively outputting modulation signals of A-phase, B-phase and C-phase voltages, u-phase and C-phase voltages for the cascade type energy storage systemApcs、uBpcsAnd uCpcsThe instantaneous values of the A-phase alternating-current voltage, the B-phase alternating-current voltage and the C-phase alternating-current voltage output by the cascade energy storage system are respectively; k is a radical ofpAdjusting the coefficient for voltage closed loop;
entering an off-grid voltage control loop;
d) and off-grid switching, namely disconnecting a grid-connected and off-grid switch of the cascade energy storage system after entering an off-grid voltage control loop, and finishing the active switching of the cascade energy storage system from grid connection to off-grid.
The cascaded energy storage system is switched from an off-grid state to a grid-connected state through the following steps:
1) collecting the grid and energy storage system information, and collecting the voltage amplitude U of the alternating current grid when receiving a command of switching from an off-grid state to a grid-connected stategAnd phase thetagAnd the voltage amplitude U output by the cascade energy storage system under the off-grid statepcsAnd phase thetapcs；
2) Calculating an off-grid voltage reference value, and calculating the off-grid voltage reference value of the cascade energy storage system through a formula (3):
wherein, the sigma is a small value which is set manually and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u. ofAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
performing off-grid voltage loop control on the off-grid voltage reference value obtained according to the formula (3);
3) grid-connected control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of a formula (4) or not by acquiring the voltage amplitude and the phase of the cascade energy storage system in real time:
in the formula eta1And η2A smaller value is set artificially and used for indicating that the error of the two variables meets the requirement;
and when the voltage amplitude and the phase of the cascade energy storage system meet the formula (4), closing a grid-connected and off-grid switch of the cascade energy storage system, and entering a grid-connected power control loop to finish the active switching from off-grid to grid-connected of the cascade energy storage system.
As shown in fig. 3, a graph of the ac voltage, the ac current, the load current and the grid-connected/disconnected switch action of the energy storage system when the active grid-connected/disconnected switching method of the cascaded energy storage system of the present invention is used for grid-connected/disconnected operation is shown.
The active grid-connected and grid-disconnected switching method of the cascade energy storage system is applied to the cascade energy storage system shown in fig. 1 for grid-connected and grid-disconnected operation, and a parameter curve shown in fig. 3 is obtained, so that the whole energy storage system is in a grid-connected operation state before 0.2s, and a cascade energy storage system grid-disconnected command is issued at the moment of 0.2 s. And when the off-grid operation of the cascaded energy storage system is stable, issuing a grid connection command of the cascaded energy storage system at the moment of 0.4 s.
Fig. 3(a) shows the ac terminal voltage of the cascaded energy storage system and the local load ac terminal voltage, and it can be seen that no matter the grid-connected state is switched to the off-grid state or the off-grid state is switched to the grid-connected state, the ac terminal voltage does not change significantly before and after the switching, and the seamless switching between the grid-connected state and the off-grid state is realized. Fig. 3(b) shows the current at the ac end of the cascaded energy storage system, which shows that the current response is fast before and after switching, and the current is provided to the local load in time. Fig. 3(c) shows the local load ac terminal current, and it can be seen that the local load current has almost no change before and after switching, which proves that the grid-connected and off-grid switching does not affect the reliable power supply of the local load. Fig. 3(d) shows an action state of a grid-connected and grid-disconnected switch, a grid-disconnected command is issued at 0.2s, the cascaded energy storage system enters an off-grid voltage control loop according to the step a), the step b) and the step c), and then the grid-connected and grid-disconnected switch is disconnected to complete switching from grid connection to an off-grid state; therefore, the off-grid command and the on-off-grid switch have a certain time delay. And issuing a grid-connected command at 0.4s, starting to adjust the amplitude and the phase of the alternating current output voltage by the cascade energy storage system according to the step 1) and the step 2), closing a grid-connected switch when the constraint condition requirement of the formula (4) is met, and then entering a grid-connected control loop to complete the switching from the off-grid state to the grid-connected state.
Therefore, the active grid-connection and grid-disconnection switching method suitable for the cascade energy storage system can realize seamless and flexible switching of the cascade energy storage converter from grid connection to grid disconnection and from grid disconnection to grid connection, and ensures reliable power supply of local critical loads.
Claims (3)
1. An output end of a cascade energy storage system (2) is connected to an alternating current power grid (1) through a grid-connected and off-grid switch (5), and a power supply end of a local load is connected to an output end of the cascade energy storage system on the rear side of the grid-connected and off-grid switch; the method is characterized in that: when the cascade energy storage system is switched from a grid-connected state to an off-grid state, the cascade energy storage system enters an off-grid voltage control loop by taking the voltage amplitude, the phase and the angular frequency of the current alternating current power grid as initial values; when the cascade energy storage system is switched from the off-grid state to the on-grid state, the amplitude and the phase of the voltage output by the cascade energy storage system in the off-grid state are adjusted until the voltage and the amplitude output by the cascade energy storage system are synchronous with the voltage and the amplitude of the current alternating current power grid, and the cascade energy storage system is switched to the on-grid state.
2. The active grid-connected and grid-disconnected switching method of the cascaded energy storage system according to claim 1, wherein the cascaded energy storage system is switched from a grid-connected state to an off-grid state by the following steps:
a) collecting the grid information and calculating a voltage reference value, and collecting the voltage amplitude U of the AC grid when receiving a command for switching from a grid-connected state to an off-grid stateg1Phase thetag1And angular frequency ω;
b) calculating an off-grid voltage reference value, and calculating an off-grid output voltage reference value of the cascade energy storage system through a formula (1):
in the formula, thetapcs_refFor off-grid voltage-loop phase reference, uAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
c) closed-loop voltage regulation, which is to independently control the three-phase voltage output by the cascade energy storage system to be off-grid, and the closed-loop voltage regulation is carried out through a formula (2):
in the formula uA_m、uB_mAnd uB_mRespectively outputting modulation signals of A-phase, B-phase and C-phase voltages, u-phase and C-phase voltages for the cascade type energy storage systemApcs、uBpcsAnd uCpcsThe instantaneous values of the A-phase alternating-current voltage, the B-phase alternating-current voltage and the C-phase alternating-current voltage output by the cascade energy storage system are respectively; k is a radical ofpAdjusting the coefficient for voltage closed loop;
entering an off-grid voltage control loop;
d) and off-grid switching, namely disconnecting a grid-connected and off-grid switch of the cascade energy storage system after entering an off-grid voltage control loop, and finishing the active switching of the cascade energy storage system from grid connection to off-grid.
3. The active grid-connected and grid-disconnected switching method of the cascaded energy storage system according to claim 1 or 2, wherein the cascaded energy storage system is switched from the grid-disconnected state to the grid-connected state by the following steps:
1) collecting the grid and energy storage system information, and collecting the voltage amplitude U of the alternating current grid when receiving a command of switching from an off-grid state to a grid-connected stategAnd phase thetagAnd the voltage amplitude U output by the cascade energy storage system under the off-grid statepcsAnd phase thetapcs；
2) Calculating an off-grid voltage reference value, and calculating the off-grid voltage reference value of the cascade energy storage system through a formula (3):
wherein, the sigma is a small value which is set manually and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u. ofAref、uBrefAnd uCrefReference values of an off-grid voltage ring A phase, a B phase and a C phase are respectively set;
performing off-grid voltage loop control on the off-grid voltage reference value obtained according to the formula (3);
3) grid-connected control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of a formula (4) or not by acquiring the voltage amplitude and the phase of the cascade energy storage system in real time:
in the formula eta1And η2A smaller value is set artificially and used for indicating that the error of the two variables meets the requirement;
and when the voltage amplitude and the phase of the cascade energy storage system meet the formula (4), closing a grid-connected and off-grid switch of the cascade energy storage system, and entering a grid-connected power control loop to finish the active switching from off-grid to grid-connected of the cascade energy storage system.
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