CN113346531B - Active parallel-to-off-grid switching method for cascade energy storage system - Google Patents
Active parallel-to-off-grid switching method for cascade energy storage system Download PDFInfo
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- CN113346531B CN113346531B CN202110695296.3A CN202110695296A CN113346531B CN 113346531 B CN113346531 B CN 113346531B CN 202110695296 A CN202110695296 A CN 202110695296A CN 113346531 B CN113346531 B CN 113346531B
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- 238000007665 sagging Methods 0.000 abstract description 3
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Classifications
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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
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Abstract
The invention relates to an active parallel off-network switching method of a cascade energy storage system, which is characterized by comprising the following steps of: 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 off-grid state of the cascade energy storage system are adjusted until the voltage and the amplitude are synchronous, and the voltage and the amplitude are switched to the grid-connected state. The network connection and disconnection switching method solves the problem that the local key equipment cannot work normally caused by a short (such as 100 ms) power failure due to the fact that a locking device is adopted and disconnected, and also solves the technical problems that the determination of a sagging coefficient and the moment of inertia of a virtual synchronous machine is difficult and unstable system is easy to cause when the sagging control and the virtual synchronous machine are adopted and disconnected.
Description
Technical Field
The invention relates to a parallel-to-off network switching method, in particular to an active parallel-to-off network 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 introduced to modern power distribution systems. The output voltage level of a general photovoltaic power generation or energy storage device is low, and a high-voltage level power grid is connected through a step-up transformer. The high-voltage cascade energy storage system is a novel energy storage converter. The converter achieves higher alternating current side output voltage through cascade topology of the H bridge, can be directly connected with 10kV and 35kV high-voltage grade power grids, omits a step-up transformer, and therefore improves the efficiency of the converter.
Micro-grids mainly comprising cascade energy storage systems also face the problem of off-grid switching. In the application field of large-scale energy storage power stations at present, in order to ensure successful off-grid switching, a mode of restarting equipment after waiting for the off-grid switching action to be completed is generally adopted for outputting by a locking device. Because the response time of a common off-grid switch is about 100ms, a short power failure is caused during the period, and the local key load cannot work normally.
Most of the existing seamless off-grid switching methods adopt droop control and virtual synchronous machines, but the droop control and the virtual synchronous machine control can bring about changes of output voltage amplitude and frequency, and the droop coefficient and the moment of inertia of the virtual synchronous machine are difficult to determine, so that instability of a system is easy to cause.
Disclosure of Invention
In order to overcome the defects of the technical problems, the invention provides an active off-grid switching method of a cascade energy storage system.
According to the active off-grid switching method of the cascade energy storage system, the output end of the cascade energy storage system is connected to an alternating current network through the off-grid switch, and the power supply end of a local load is connected to the output end of the cascade energy storage system at the rear side of the 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 grid-connected state, the amplitude and the phase of the voltage output by the off-grid state of the cascade energy storage system 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 voltage and the amplitude are switched to the grid-connected state.
According to the active off-grid switching method of the cascade energy storage system, the cascade energy storage system is switched from the grid-connected state to the off-grid state by the following steps:
a)collecting power grid information and calculating a voltage reference value, and collecting voltage amplitude U of an alternating current power grid when a command for switching from a grid-connected state to a grid-disconnected state is received g1 Phase θ g1 And an 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 θ pcs_ref For off-grid voltage loop phase reference, u Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring;
c) Closed-loop voltage regulation, which is to independently control three-phase voltages output by the cascade energy storage system to be off-grid respectively, and is carried out through a formula (2):
u A_m ＝u Aref +k p ·(u Aref -u Apcs )
u B_m ＝u Bref +k p ·(u Bref -u Bpcs ) (2)
u C_m ＝u Cref +k p ·(u Cref -u Cpcs )
wherein u is A_m 、u B_m And u B_m The modulating signals of A phase, B phase and C phase voltages are respectively output by the cascade energy storage system, u Apcs 、u Bpcs And u Cpcs The instantaneous values of the phase A, phase B and phase C alternating current voltage are output by the cascade energy storage system respectively; k (k) p The voltage closed-loop adjustment coefficient is used;
entering an off-grid voltage control loop;
d) And switching off the grid, and after entering an off-grid voltage control loop, disconnecting a grid-connected switch of the cascade energy storage system to finish the active switching of the cascade energy storage system from grid connection to off-grid.
According to the active off-grid switching method of the cascade energy storage system, the cascade energy storage system is switched from the off-grid state to the grid-connected state by the following steps:
1) Collecting information of a power grid and an energy storage system, and collecting voltage amplitude U of an alternating current power grid when a command of switching from an off-grid state to a grid-connected state is received g And phase theta g And voltage amplitude U output by cascade energy storage system in off-grid state pcs And phase theta pcs ；
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, sigma is a smaller numerical value which is set by people and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u (u) Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring;
performing off-grid voltage loop control according to the off-grid voltage reference value obtained in the formula (3);
3) Grid connection control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of the formula (4) or not by collecting the voltage amplitude and the phase of the cascade energy storage system in real time:
wherein eta is 1 And eta 2 A smaller numerical value set by people is used for representing that two variable errors meet the requirements;
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 of the cascade energy storage system from off-grid to grid-connection.
The beneficial effects of the invention are as follows: according to the active off-grid switching method of the cascade energy storage system, when the cascade energy storage system is switched from the grid-connected state to the off-grid state, the voltage amplitude, the phase and the angular frequency of the power grid are collected to serve as initial values, the reference values of the off-grid voltage loop phase and the three-phase voltage are calculated, the output voltage of the cascade energy storage system to be off-grid is controlled through closed-loop voltage regulation, the fact that the voltage amplitude does not fluctuate before and after off-grid is guaranteed, and uninterrupted power supply to load equipment can be achieved. When the cascade energy storage system is switched from the off-grid state to the grid-connected state, the voltage amplitude and the phase of the alternating current power grid and the voltage amplitude and the phase of the cascade energy storage system are firstly collected, then an off-grid voltage reference value is established according to the phase difference between the power grid and the energy storage system and the amplitude of the energy storage system, when the difference between the phase difference between the energy storage system and the power grid and the voltage amplitude is smaller than constraint conditions, the energy storage system is switched to the grid-connected state, the flexible switching of the energy storage system between the grid-connected state and the off-grid state is realized, the reliable power supply of the local load is ensured, the problem that the local key equipment cannot work normally due to short power failure (such as 100 ms) caused by the adoption of a locking device in the prior art is solved, and the technical problems that the determination of the sagging coefficient and the moment of inertia of a virtual synchronous machine is difficult and the system instability is easy to cause when the existing system is in the off-grid connection are solved.
Drawings
FIG. 1 is a schematic diagram of a cascade energy storage system according to the present invention;
FIG. 2 is a flow chart of an active and off-grid switching method of the cascade energy storage system of the present invention;
fig. 3 is a graph of ac voltage, ac current, load current and off-grid switching action of the energy storage system when the active off-grid switching method of the cascade energy storage system of the present invention is used for off-grid 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 will be further described with reference to the drawings and examples.
As shown in fig. 1, a schematic system principle diagram of a cascade energy storage system in the present invention is provided, wherein the cascade energy storage system 2 is connected to an ac power grid 1 through a parallel off-grid switch 5, a three-phase circuit of the cascade energy storage system is composed of a plurality of power units 4 that are cascaded together, each power unit 4 is composed of a controllable power device, and a battery pack for storing electric energy is provided in each power unit 4. The local load 3 is connected to the output of the cascade energy storage system 2.
The cascade energy storage system can store electric energy on the alternating current power grid 1 in the battery pack through rectification, and can also 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 ac power grid 1 is lower than the preset value, indicating that the ac power grid 1 is in a trough state, releasing the electric energy stored in the battery pack to the power grid; if the voltage of the alternating current network 1 is higher than the preset value, indicating that the alternating current network 1 is in a peak state, storing the surplus electric energy on the network into a battery pack; it can be seen that it has peak clipping and valley filling effects.
When maintenance or overhaul is required for the ac power grid 1 to which the cascade energy storage system 2 is connected, the cascade energy storage system 2 needs to be switched from the grid-connected state to the off-grid state, and after the maintenance or overhaul is completed, 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 and off-line by using the locking device, a short (e.g. 100 ms) power failure is caused, so that key equipment in the local load 3 cannot work normally. For the existing method of adopting droop control and a virtual synchronous machine for off-grid, the determination of the droop coefficient and the moment of inertia of the virtual synchronous machine is difficult, and the instability of the system is easy to cause. The active off-grid switching method of the cascade energy storage system can better solve the technical problem.
As shown in fig. 2, a flow chart of the active and off-grid switching method of the cascade energy storage system of the present invention is provided,
switching the cascade energy storage system from the grid-connected state to the off-grid state is achieved by:
a) Collecting power grid information and calculating voltage reference value, when receivingWhen a command for switching from a grid-connected state to an off-grid state is received, the voltage amplitude U of the alternating current power grid is acquired g1 Phase θ g1 And an 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 θ pcs_ref For off-grid voltage loop phase reference, u Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring;
c) Closed-loop voltage regulation, which is to independently control three-phase voltages output by the cascade energy storage system to be off-grid respectively, and is carried out through a formula (2):
u A_m ＝u Aref +k p ·(u Aref -u Apcs )
u B_m ＝u Bref +k p ·(u Bref -u Bpcs ) (2)
u C_m ＝u Cref +k p ·(u Cref -u Cpcs )
wherein u is A_m 、u B_m And u B_m The modulating signals of A phase, B phase and C phase voltages are respectively output by the cascade energy storage system, u Apcs 、u Bpcs And u Cpcs The instantaneous values of the phase A, phase B and phase C alternating current voltage are output by the cascade energy storage system respectively; k (k) p The voltage closed-loop adjustment coefficient is used;
entering an off-grid voltage control loop;
d) And switching off the grid, and after entering an off-grid voltage control loop, disconnecting a grid-connected switch of the cascade energy storage system to finish the active switching of the cascade energy storage system from grid connection to off-grid.
Switching the cascade energy storage system from the off-grid state to the grid-connected state is achieved by:
1) Collecting power grid and storageThe system information can be obtained, and when a command of switching from the off-grid state to the grid-connected state is received, the voltage amplitude U of the alternating current power grid is collected g And phase theta g And voltage amplitude U output by cascade energy storage system in off-grid state pcs And phase theta pcs ；
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, sigma is a smaller numerical value which is set by people and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u (u) Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring;
performing off-grid voltage loop control according to the off-grid voltage reference value obtained in the formula (3);
3) Grid connection control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of the formula (4) or not by collecting the voltage amplitude and the phase of the cascade energy storage system in real time:
wherein eta is 1 And eta 2 A smaller numerical value set by people is used for representing that two variable errors meet the requirements;
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 of the cascade energy storage system from off-grid to grid-connection.
As shown in fig. 3, a graph of ac voltage, ac current, load current and off-grid switching action of the energy storage system is shown when the active off-grid switching method of the cascade energy storage system is used for off-grid operation.
The active and off-grid switching method of the cascade energy storage system is applied to the cascade energy storage system shown in fig. 1 to perform the on-grid and off-grid 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 an off-grid command of the cascade energy storage system is issued at 0.2 s. And when the off-grid operation of the cascade energy storage system is stable, issuing a grid-connected command of the cascade energy storage system at 0.4 s.
Fig. 3 (a) shows the ac terminal voltage of the cascade energy storage system and the local load ac terminal voltage, so that no matter the ac terminal voltage is switched from the grid-connected state to the off-grid state or from the off-grid state to the grid-connected state, no obvious change is generated between the ac terminal voltage and the off-grid state before and after the switching, and seamless switching between the off-grid state and the off-grid state is realized. Fig. 3 (b) shows the ac end current of the cascade energy storage system, and it can be seen that the current response before and after switching is rapid, and the current is provided for the local load in time. Fig. 3 (c) shows the local load ac end current, and it can be seen that there is almost no change in the local load current before and after switching, which proves that the off-grid switching does not affect the reliable power supply of the local load. FIG. 3 (d) is a state of the off-grid switch, an off-grid command is issued at 0.2s, the cascade energy storage system enters an off-grid voltage control loop according to the steps a), b) and c), and then the off-grid switch is disconnected and turned off, so that the grid connection to the off-grid state is completed; therefore, the off-network command and the off-network switch action have a certain delay. And (3) 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 finish switching from grid-connected to grid-connected state.
Therefore, the active parallel off-grid switching method suitable for the cascade energy storage system can realize seamless and flexible switching from grid connection to off-grid and from off-grid to grid connection of the cascade energy storage converter, and ensure reliable power supply of local key loads.
Claims (1)
1. The active grid-connected and off-grid switching method of the cascade energy storage system comprises the steps that an output end of the 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 at 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 grid-connected state, the amplitude and the phase of the voltage output by the off-grid state of the cascade energy storage system 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 voltage and the amplitude are switched to the grid-connected state;
switching the cascade energy storage system from the grid-connected state to the off-grid state is achieved by: a) Collecting power grid information and calculating a voltage reference value, and collecting voltage amplitude U of an alternating current power grid when a command for switching from a grid-connected state to a grid-disconnected state is received g1 Phase θ g1 And an 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 θ pcs_ref For off-grid voltage loop phase reference, u Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring; c) Closed-loop voltage regulation, which is to independently control three-phase voltages output by the cascade energy storage system to be off-grid respectively, and is carried out through a formula (2):
wherein u is A_m 、u B_m And u B_m The modulating signals of A phase, B phase and C phase voltages are respectively output by the cascade energy storage system, u Apcs 、u Bpcs And u Cpcs A phase A, a phase B and a phase C alternating current voltage respectively output by the cascade energy storage systemInstantaneous values; k (k) p The voltage closed-loop adjustment coefficient is used; entering an off-grid voltage control loop; d) Switching off the grid, disconnecting a grid-connected switch of the cascade energy storage system after entering the grid-connected voltage control loop, and completing active switching of the cascade energy storage system from grid connection to grid disconnection;
switching the cascade energy storage system from the off-grid state to the grid-connected state is achieved by: 1) Collecting information of a power grid and an energy storage system, and collecting voltage amplitude U of an alternating current power grid when a command of switching from an off-grid state to a grid-connected state is received g And phase theta g And voltage amplitude U output by cascade energy storage system in off-grid state pcs And phase theta pcs The method comprises the steps of carrying out a first treatment on the surface of the 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, sigma is a smaller numerical value which is set by people and is larger than 0, and is used for controlling the change of the output frequency of the off-grid voltage loop; u (u) Aref 、u Bref And u Cref Respectively the reference values of the A phase, the B phase and the C phase of the off-grid voltage ring; performing off-grid voltage loop control according to the off-grid voltage reference value obtained in the formula (3); 3) Grid connection control, namely judging whether the voltage amplitude and the phase of the cascade energy storage system meet the constraint condition of the formula (4) or not by collecting the voltage amplitude and the phase of the cascade energy storage system in real time:
wherein eta is 1 And eta 2 A smaller numerical value set by people is used for representing that two variable errors meet the requirements; 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 of the cascade energy storage system from off-grid to grid-connection.
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