CN110635499B - Smooth switching method of energy storage converter based on subsection control of grid-connected and off-grid transient process - Google Patents

Abstract

The invention discloses an energy storage converter smooth switching method based on grid-connected and off-grid transient process sectional control. The invention combines the characteristics of the change-over switch, processes the grid-connected and off-grid conversion transient process in a segmented manner, can further improve the reliability and smoothness of the grid-connected and off-grid switching process of the system, and adapts to the complexity of the actual working condition.

Description

Smooth switching method of energy storage converter based on subsection control of grid-connected and off-grid transient process

technical field

The invention relates to a method for smooth switching of energy storage converters, in particular to a method for smooth switching of energy storage converters based on subsection control of grid-connected and off-grid transient processes.

Background technique

With the gradual deepening of the penetration rate of new energy distributed power generation, the problem of power rationing is serious. At the same time, in order to improve the stability of power consumption by key loads on the user side and the continuity of power consumption when the power grid fails, the importance of the microgrid system is gradually emerging. The main components of the microgrid system include energy storage devices, distributed power generation units, intelligent switch cabinets, low-voltage power grids, and local loads. Among them, the intelligent switchgear is the key equipment connecting the local microgrid and the large power grid. Normally, in order to realize the on-off grid switching function of the energy storage converter in the micro-grid system, the smart switchgear can generally be equipped with a semiconductor type switch, such as a static switch, or a mechanical switch, such as a contactor. The contactor is a mechanical switch, which has a long operating time. For the working conditions with low requirements, small voltage drop or allowable recovery time of 100 milliseconds, a cost-effective contactor can be selected; while the static switch belongs to semiconductor power. device, with its fast operating time, is suitable for reliable power supply of critical loads.

The on-grid and off-grid control strategies of energy storage converters in micro-grid systems are mostly based on special system topologies, such as matching various micro-sources or AC-DC hybrid micro-grid systems, and various optimized control algorithms, such as improving Droop control, fuzzy sliding mode control, PQ/VF, virtual synchronous motor, etc., and hybrid energy storage systems, such as lithium batteries, supercapacitors, and flywheels as energy storage media, are all improving energy storage and conversion. It plays a better role in the smoothness of the switching process between the switch and the off-grid switch. However, when the voltage or frequency of the grid changes and triggers the on-off grid switching condition, most of the existing technologies do not fully consider the on-off grid switching when switching on-grid and off-grid algorithms and controlling the switch in the smart switchgear. In practical applications, the voltage and current waveforms of the switching process may be seriously distorted, especially in severe working conditions, and even the switching fails.

Although the existing technology proposes many optimization and improvement methods for the topology of the microgrid system and the control algorithm for grid connection and disconnection, none of them analyze the transient process of grid connection and disconnection in detail, and seek to further improve the smoothness of switching from the transient process. Under normal circumstances, contactors or static switches are used in intelligent switch cabinets, and the closed and open states of the two and the corresponding feedback signals are actually not synchronized, and there is a certain time difference. In addition, when switching between grid-connected operation and off-grid operation as mentioned in the prior art, factors such as the switching time and phase locking of the two algorithms determine the complexity of the transient process of grid-connected and off-grid switching. If the transient process of on-off and off-grid switching is not considered, especially when two different control algorithms are used before and after switching on and off-grid, it will not only cause large fluctuations on the DC side, but also reduce the storage capacity of the switching process on the AC side. The output voltage and current waveform of the energy converter is distorted, which triggers a shutdown fault. For example, when the SOC is low, the power grid is charged at full power and the power grid is faulty when connected to the full load. After switching from grid-connected operation to off-grid operation, the energy storage is quickly restored without waiting for the contactor in the smart switch to be disconnected. The output voltage of the converter, at this time, the probability of triggering battery undervoltage is high, so that the battery BMS will trigger a three-level alarm and shut down. In addition, the output voltage of the AC side energy storage The rapid increase, accompanied by the rapid increase of the output current of the energy storage converter, has a high risk of triggering hardware over-current shutdown.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for smooth switching of energy storage converters based on subsection control of the grid-connected and off-grid transient process, which takes into account the grid-connected and off-grid transient process to make the entire switching process smoother.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A method for smooth switching of energy storage converters based on subsection control of on-off grid transient process, characterized in that it comprises the following steps:

Step 1: Grid-connected to off-grid switching, the initial state of the energy storage converter is grid-connected operation, working in the current source mode;

1.1 Grid failure detection delay period: The grid fails at time A1; after the fault inspection delay time, the grid fault is judged at time B1, and at the same time a contactor disconnection command is issued to switch the grid-connected algorithm to the off-grid algorithm;

1.2 The grid-connected to off-grid algorithm is switched to the actual contactor disconnection period: the contactor is actually cut off at time C1;

1.3 The period from the actual disconnection of the contactor to the disconnection state of the contactor feedback: the corresponding feedback signal of the contactor is detected at time D1;

1.4 Contactor feedback off state period: the output voltage target value of the energy storage converter restores the off-grid operating voltage command to the rated value at a certain speed;

Step 2: Off-grid to grid-connected switching, the initial state of the energy storage converter is off-grid operation, working in voltage source mode;

2.1 The power grid returns to normal and begins phase-locking to phase-locking completion period: the power grid recovers from a fault at time A2, and automatically starts phase-locking based on the power grid; after the phase-locking delay time, phase-locking is successful at time B2, and the contactor is closed at the same time instructions, but still maintain the off-grid algorithm;

2.2 The period from sending the contactor closing command to the contactor feedback closed state signal: at C2, the contactor feedback signal shows that it is closed, but it is actually still in the disconnected state, and the off-grid algorithm is still maintained during this period;

2.3 The period from the start of the contactor feedback closed state to the actual closing of the contactor: after a delay for a period of time, the contactor is actually closed at time D2, which cannot be directly detected at time D2;

2.4 The actual closing stage of the contactor: A certain delay is taken on the basis of time C2 to ensure that the off-grid algorithm is switched to the grid-connected algorithm after the contactor is actually closed.

Further, the above 1.1 is specifically that the power grid fails at time A1, and after the fault inspection delay time, it is judged that the power grid is faulty at time B1, and at the same time, a contactor disconnection command is issued, and the grid-connected algorithm is switched to the off-grid algorithm, that is, switched to voltage Source mode, at this time, the output voltage target value of the energy storage converter should be consistent with the grid side, and the initial phase should be consistent, and the off-grid control should be performed at the rated frequency to avoid the obvious voltage difference between the voltage sources on both sides of the contactor. Grid-connected current impact.

Further, the above-mentioned 1.2 specifically refers to the actual disconnection of the contactor at time C1, which cannot be directly detected. Therefore, before the time C1, the output voltage of the energy storage converter cannot start to recover, so as to prevent the output voltage of the energy storage converter from colliding with the faulty power grid. A large current surge caused by a rapidly increasing voltage difference between voltages.

Further, in the above 1.3, the detected disconnection feedback signal at the moment of contactor D1 lags behind the actual disconnection action of contactor C1 at the moment.

Further, the above 1.4 specifically uses the feedback signal at time D1 as the recovery flag of the output voltage command of the energy storage converter, and the output voltage target value of the energy storage converter recovers the off-grid operating voltage command to the rated value at a certain speed.

Further, the above-mentioned 2.1 is specifically that the power grid recovers from a fault at time A2, and automatically starts phase-locking based on the power grid; after the phase-locking delay time, phase-locking is successful at time B2, and at the same time a contactor closing command is issued, but the disconnection is still maintained. At this time, the output voltage target value of the energy storage converter is consistent with the grid side, avoiding the obvious voltage difference between the voltage sources on both sides of the contactor, which will cause grid-connected current impact when the contactor is closed, and ensure that the energy storage converter realizes Smooth switching.

Further, the above 2.4 is specifically to take a certain delay on the basis of time C2 based on the measured data, and ensure that the off-grid algorithm is switched to the grid-connected algorithm after the contactor is actually closed, and the energy storage converter works at the current In the source mode, the active power and reactive power when off-grid are taken as the initial state, and the active power and reactive power of the system can be restored to the grid-connected operating power command at a certain speed from the current value.

Compared with the prior art, the present invention has the following advantages and effects:

1. The present invention combines the characteristics of the switch itself to process the transient process of on-grid and off-grid switching in segments, which can further improve the reliability and smoothness of the system's on-grid and off-grid switching process, and adapt to the complexity of actual working conditions;

2. The present invention does not change the components of the original micro-grid system, and does not affect the use of the original related technical advantages. For different micro-grid system topologies, single-phase or three-phase energy storage converters, different closed-loop control algorithms, and different switching The switch is universal;

3. The present invention is especially applicable to the situation where two different algorithms are used before and after switching, and for the use of a unified algorithm, such as droop control and virtual synchronous motor algorithm, the characteristics of the switch itself can also be used as the basis for key logic judgments.

Description of drawings

Fig. 1 is a schematic diagram of the method for smooth switching of energy storage converters based on subsection control of grid-connected and off-grid transient processes according to the present invention.

Detailed ways

The present invention will be described in further detail below through examples, and the following examples are explanations of the present invention and the present invention is not limited to the following examples.

As shown in Fig. 1, a method for smooth switching of energy storage converters based on subsection control of grid-connected and off-grid transient processes in the present invention includes the following steps:

Step 1: Grid-connected to off-grid switching, the initial state of the energy storage converter is grid-connected operation, working in the current source mode.

1.1 Grid fault detection delay period (time A1 to B1): grid failure occurs at time A1; after the fault inspection delay time, the grid fault is judged at time B1, and the contactor disconnection command is issued at the same time, and the grid-connected algorithm is switched to the off-grid algorithm , that is to switch to the voltage source mode, but at this time the output voltage target value of the energy storage converter should be consistent with the grid side, and the initial phase should be consistent, and the off-grid control should be performed at the rated frequency to avoid the voltage source on both sides of the contactor There is an obvious voltage difference that causes grid-connected current impact.

1.2 The grid-connected to off-grid algorithm switches to the actual contactor disconnection period (time B1 to C1): the contactor is actually cut off at time C1, although from this moment in the off-grid control loop, the output voltage target value of the energy storage converter can be A certain speed restores the off-grid operating voltage command to the rated value, but this moment cannot be directly detected. If the output voltage of the energy storage converter starts to recover before time C1, a large current impact will be caused due to the rapidly increasing voltage difference between the output voltage of the energy storage converter and the faulty grid voltage.

1.3 The period from the actual disconnection of the contactor to the disconnection state of the contactor feedback (time C1 to D1): at the time D1, the corresponding feedback signal of the contactor can be detected. It can be seen from the figure that the contactor disconnection feedback signal (D1 moment) lags behind the actual contactor disconnection action (C1 moment).

1.4 Contactor feedback disconnection state period (after time D1): the output voltage target value of the energy storage converter returns to the rated value at a certain speed; the feedback signal at time D1 is used as the output of the energy storage converter Voltage command recovery flag, the output voltage target value of the energy storage converter restores the off-grid operation voltage command to the rated value at a certain speed.

Step 2: Off-grid to grid-connected switching, the initial state of the energy storage converter is off-grid operation, working in voltage source mode;

2.1 The power grid returns to normal and starts phase-locking to phase-locking completion period (time A2 to B2): the power grid recovers from a fault at time A2, and automatically starts phase-locking based on the power grid; after the phase-locking delay time, phase-locking succeeds at time B2 , at the same time issue the contactor closing command, but still maintain the off-grid algorithm, at this time the output voltage target value of the energy storage converter should be consistent with the grid side, to avoid the obvious voltage difference between the voltage sources on both sides of the contactor when the contactor is closed The grid-connected current impact is caused, which is the key to ensure the smooth switching of the energy storage converter.

2.2 The period from sending the contactor closing command to the contactor feedback closed state signal (time B2 to C2): Although the contactor feedback signal at time C2 shows that it is closed, it is actually still in the open state. The off-grid algorithm in 2.1 is still maintained at this stage.

2.3 The period from the start of the contactor feedback closed state to the actual closing of the contactor (time C2 to D2): After a delay for a period of time, the contactor is actually closed at time D2. The moment D2 is the best moment for the off-grid algorithm to switch to the grid-connected algorithm. The moment D2 cannot be detected directly, but the delay from C2 to D2 should be considered in the program. Otherwise, switching the algorithm at this time and trying to restore the active power and reactive power to the corresponding power command value will destroy the power balance required to maintain the normal operation of the load at this time, causing a sudden change in the output voltage of the energy storage converter, also accompanied by Larger current impact, resulting in system switching failure.

2.4 The actual closing stage of the contactor (after time D2): According to the measured data, a certain delay is taken on the basis of time C2, and it is ensured that after the contactor is actually closed, the off-grid algorithm is switched to the grid-connected algorithm. The inverter works in the current source mode, and the active power and reactive power when off-grid are taken as the initial state. The active power and reactive power of the system can be restored from the current value to the grid-connected operating power command at a certain speed, and can be issued later according to the background The power command is adjusted to ensure the stability of the system switching transient process.

The times shown in the example of Figure 1 are typical values for contactor AF1250-30-11. The static switch is a semiconductor power device with a fast action speed of about microseconds, while the cycle of the control algorithm interrupt service program is about 100 microseconds. When the method of the present invention actually controls the static switch, the collocation with its feedback signal is simpler than that of the contactor, and the method of the present invention can be simplified as follows:

In the process of grid-connected to off-grid, at time B1, after the grid fault is detected, the static switch can be disconnected and switched to the off-grid algorithm. When the feedback signal shows that it is disconnected, the off-grid operating voltage command is restored to the rated value. Before time B1, it is treated in the same way as the contactor scheme.

In the process of off-grid transfer to grid-connection, the phase-locking is successful at time B2, and after the static switch closing command is issued, when the feedback signal shows that it is closed, the off-grid algorithm is switched to the grid-connection algorithm, and the power can be changed from the current value at a certain speed. Restore the grid-connected operation power command to the rated value. Before time B2, it is treated in the same way as the contactor scheme.

The present invention does not change the components of the original micro-grid system and does not affect the advantages of the original related technologies, and combines the characteristics of the switch itself to process the transient process of the grid-connected and off-grid conversion in segments, which can further improve the parallelism of the system. Off-grid switching process, so this strategy is universal for different microgrid system topologies, single-phase or three-phase energy storage converters, different closed-loop control algorithms, and different switching switches. It is especially suitable for the situation where two different algorithms are used before and after switching, and for the use of a unified algorithm, such as droop control and virtual synchronous motor algorithm, the characteristics of the switch itself can also be used as the basis for key logic judgments.

The above content described in this specification is only an illustration of the present invention. Those skilled in the technical field to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, as long as they do not deviate from the content of the present invention specification or exceed the scope defined in the claims, all should Belong to the protection scope of the present invention.

Claims (7)

1. A method for smooth switching of energy storage converters based on segmented control of the transient process of grid-connected and off-grid, characterized in that it comprises the following steps: Step 1: Grid-connected to off-grid switching, the initial state of the energy storage converter is grid-connected operation, working in the current source mode; 1.1 Grid failure detection delay period: The grid fails at time A1; after the fault inspection delay time, the grid fault is judged at time B1, and at the same time a contactor disconnection command is issued to switch the grid-connected algorithm to the off-grid algorithm; 1.2 The grid-connected to off-grid algorithm is switched to the actual contactor disconnection period: the contactor is actually cut off at time C1; 1.3 The period from the actual disconnection of the contactor to the disconnection state of the contactor feedback: the corresponding feedback signal of the contactor is detected at time D1; 1.4 Contactor feedback off state period: the output voltage target value of the energy storage converter restores the off-grid operating voltage command to the rated value at a certain speed; Step 2: Off-grid to grid-connected switching, the initial state of the energy storage converter is off-grid operation, working in voltage source mode; 2.1 The power grid returns to normal and starts phase-locking to phase-locking completion period: the power grid recovers from a fault at time A2, and automatically starts phase-locking based on the power grid; after the phase-locking delay time, phase-locking is successful at time B2, and the contactor is closed at the same time instructions, but still maintain the off-grid algorithm; 2.2 The period from sending the contactor closing command to the contactor feedback closed state signal: at C2, the contactor feedback signal shows that it is closed, but it is actually still in the disconnected state, and the off-grid algorithm is still maintained during this period; 2.3 The period from the start of the contactor feedback closed state to the actual closing of the contactor: after a delay for a period of time, the contactor is actually closed at time D2, which cannot be directly detected at time D2; 2.4 The actual closing stage of the contactor: A certain delay is taken on the basis of time C2 to ensure that the off-grid algorithm is switched to the grid-connected algorithm after the contactor is actually closed. 2. The energy storage converter smooth switching method based on segmented control of grid-connected and off-grid transient processes according to claim 1, characterized in that: said 1.1 is specifically that the power grid fails at time A1, and after the fault inspection delay time , judge the power grid fault at time B1, and at the same time issue a command to disconnect the contactor, and switch the grid-connected algorithm to the off-grid algorithm, that is, switch to the voltage source mode. At this time, the output voltage target value of the energy storage converter should be consistent with the grid side , and ensure that the initial phase is consistent, and the off-grid control is performed at the rated frequency, so as to avoid the obvious voltage difference between the voltage sources on both sides of the contactor and cause the grid-connected current impact. 3. According to claim 1, the energy storage converter smooth switching method based on segmental control of on-grid and off-grid transient process, is characterized in that: said 1.2 is specifically the actual removal of the contactor at C1 time, and the C1 time cannot be directly Therefore, before the time C1, the output voltage of the energy storage converter cannot start to recover, so as to avoid the large current impact caused by the rapidly increasing voltage difference between the output voltage of the energy storage converter and the voltage of the faulty grid. 4. The energy storage converter smooth switching method based on subsection control of grid-connected and off-grid transient processes according to claim 1, characterized in that: in the above 1.3, the detected disconnection feedback signal at the moment of contactor D1 The actual disconnection action at the moment of contactor C1 is hysteresis. 5. The energy storage converter smooth switching method based on segmental control of on-grid and off-grid transient process according to claim 1, characterized in that: said 1.4 specifically uses the feedback signal at time D1 as the energy storage converter Output voltage command recovery flag, the output voltage target value of the energy storage converter recovers the off-grid operation voltage command to the rated value at a certain speed. 6. The energy storage converter smooth switching method based on subsection control of grid-connected and off-grid transient processes according to claim 1, characterized in that: said 2.1 is specifically that the grid recovers from a fault at time A2, and automatically starts to use the grid Phase-locking is performed as the benchmark; after the phase-locking delay time, the phase-locking is successful at time B2, and the contactor closing command is issued at the same time, but the off-grid algorithm is still maintained. At this time, the output voltage target value of the energy storage converter is consistent with the grid side, Avoid the obvious voltage difference between the voltage sources on both sides of the contactor, which will cause grid-connected current impact when the contactor is closed, and ensure the smooth switching of the energy storage converter. 7. According to claim 1, the method for smooth switching of energy storage converters based on subsection control of grid-connected and off-grid transient processes is characterized in that: said 2.4 is based on actual measured data, and a certain delay is taken on the basis of time C2 , and ensure that after the contactor is actually closed, the off-grid algorithm is switched to the grid-connected algorithm. At this time, the energy storage converter works in the current source mode, and the active power and reactive power when off-grid are taken as the initial state. The active power of the system The reactive power can be restored to the grid-connected operation power command at a certain speed from the current value.

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