Off-grid cascade emergency energy storage system and control method

Abstract

The invention provides an off-grid cascade emergency energy storage system and a control method, three-phase star connection is adopted, each phase is formed by cascading AC sides of N H-bridge power units with DC filtering links, the DC side is connected with a battery cluster, and the AC side is directly connected with a load or a power grid through a filter. The system is provided with a unified controller, an off-grid V / F control mode is automatically switched to when double power supplies of a power grid lose power, and rated voltage and frequency are established; and after the power grid is recovered, executing pre-synchronization closed-loop adjustment according to an amplitude-frequency-phase sequence, and setting a gradually broadened frequency amplitude limiting interval to realize non-impact closing of the grid-connected contactor. A positive sequence d / q axis voltage outer ring, a current inner ring and a negative sequence current closed ring are arranged in the controller, sequential operation is carried out in the same interruption period, a cross decoupling and negative sequence suppression strategy is adopted, voltage is rapidly established at a 10ms level, and unbalanced load harmonic waves are eliminated. Seamless switching from off-grid to grid-connected is achieved, and the method is suitable for important load emergency power supply.

Description

Technical Field

[0001] This invention belongs to the field of energy storage, specifically relating to an off-grid cascaded emergency energy storage system and its control method. Background Technology

[0002] Current industrial development is driving increasing energy demand, and traditional fossil fuels can no longer meet the needs of current development. The energy structure is rapidly shifting towards a low-carbon model, with renewable energy sources such as solar and wind power receiving sustained and widespread attention. However, renewable energy sources are characterized by strong intermittency and volatility, and their integration into the power grid can cause fluctuations in grid power output. Battery energy storage technology can not only store excess electrical energy but also smooth out grid fluctuations, providing emergency power supply and peak shaving / valley filling functions, and is considered one of the effective ways to solve this problem.

[0003] Currently, the most researched and widely applied battery energy storage systems involve adding a step-up transformer to the energy storage battery side and connecting it to the power grid. These systems primarily employ two-level and three-level topologies. However, due to their structural characteristics, achieving high-capacity, high-voltage outputs requires parallel connection of components, leading to complex control and significant losses. Cascaded energy storage topologies, characterized by large single-unit capacity, high output voltage levels, and fast dynamic response, have attracted widespread attention. Due to the unique properties of cascaded topologies, AC transformers are no longer needed, allowing direct connection to the power grid and overcoming the shortcomings of traditional technologies.

[0004] However, current research on cascaded energy storage systems mainly focuses on grid-connected operation, with limited research on off-grid operation and the synchronization process between off-grid and grid-connected operation. When both circuits of the grid fail, cascaded energy storage systems can serve as emergency energy storage power sources to maintain system voltage and ensure the continuity of power supply to critical loads. Furthermore, the synchronization process from off-grid to grid-connected operation after grid power is restored is also worthy of study. Therefore, there is an urgent need for an off-grid cascaded emergency energy storage system and its control method. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an off-grid cascaded emergency energy storage system and its control method. It improves upon traditional V / F control (a control method for constant AC voltage amplitude and frequency) by employing multi-vector feedforward interactive control. In the d-axis voltage outer loop, feedforward is introduced for both the positive-sequence d-axis component of the AC voltage and the positive-sequence d-axis component of the commutator current, which are not decoupled by positive or negative sequence. In the q-axis voltage outer loop, feedforward is introduced for both the positive-sequence q-axis component of the AC voltage and the positive-sequence q-axis component of the commutator current, which are not decoupled by positive or negative sequence. The positive-sequence d- and q-axis current reference command values ​​of the commutator current in the inner loop, obtained through proportional limiting calculation, are used as the control link for inductor cross-decoupling. The d-axis current command value is applied to the output value of the q-axis current inner loop via a first proportional adjustment coefficient; the q-axis current command value is then applied to the output value of the d-axis current inner loop via a second proportional adjustment coefficient. Simultaneously, the previously calculated positive-sequence d and q-axis current reference command values ​​of the undecoupled positive-sequence inner loop commutator chain are transformed into dq / αβ coordinates and αβ / dq coordinates to change the phase to the negative-sequence dq coordinate system. Then, the current inner loop control is superimposed on the original V / F (Voltage / Frequency, a control method for constant AC voltage amplitude and frequency) control to eliminate the harmonic components caused by the negative sequence.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] An off-grid cascaded emergency energy storage system includes:

[0008] The three-phase star-connected power submodule string consists of N H-bridge power units with DC filtering circuits cascaded on the AC side. The cascaded output is directly connected to the load or the power grid via an AC filter.

[0009] The DC side of the H-bridge power unit is used to connect to the battery cluster;

[0010] The system is configured with the same controller, which automatically switches to off-grid V / F control mode when the dual power supply of the grid fails, and performs pre-synchronous closed-loop regulation in amplitude-frequency-phase sequence after the grid is restored, until the grid-connected contactor closes without impact.

[0011] Furthermore, the controller incorporates a positive-sequence d-axis voltage outer loop, a positive-sequence q-axis voltage outer loop, a positive-sequence d-axis current inner loop, a positive-sequence q-axis current inner loop, and a negative-sequence current closed loop. Each loop is operated sequentially within the same interrupt cycle, and the operation results are synchronously sent to each H-bridge power unit after PWM modulation.

[0012] Furthermore, the controller sets a progressively widening frequency limiting range during the pre-synchronization phase, with an initial limit of ±0.2 Hz, which is increased by 0.1 Hz every 1 second until it reaches ±0.6 Hz, in order to shorten the phase capture time and keep the load-side frequency stable.

[0013] This invention also provides a control method for an off-grid cascaded emergency energy storage system, comprising the following steps:

[0014] Step 1: Monitor the grid voltage in real time. When the voltage of any phase is lower than 0.9 pu for 100 ms, it is determined to be a power failure.

[0015] Step 2: Immediately after power failure is detected, the grid-connected contactor is locked, and the off-grid V / F control is started at the same time to establish three-phase AC voltage with rated voltage and rated frequency as the target.

[0016] Step 3: Under V / F control, continuously sample the load current, maintain amplitude and frequency stability through positive sequence voltage-current dual-loop feedforward and cross-decoupling operation, and supply power to important loads;

[0017] Step 4: Monitor the mains voltage in real time. When the mains voltage rises back to 0.9 pu and remains there for 100 ms, it is determined that the power supply has been restored.

[0018] Step 5: Using the grid voltage as a reference, sequentially adjust the amplitude, frequency, and phase of the system output voltage in a closed loop until the amplitude difference and q-axis difference are both within the set threshold and maintained for 100 ms.

[0019] Step 6: Immediately after pre-synchronization is completed, close the grid-connected contactor to seamlessly switch the system from off-grid mode to grid-connected mode;

[0020] Step 7: After successful closing, lock out the V / F reference source, switch to grid-connected power control mode, and end the emergency procedure.

[0021] Furthermore, in step 2, the positive-sequence d-axis voltage outer loop output is first limited and then used as the d-axis current inner loop reference, and the positive-sequence q-axis voltage outer loop output is first limited and then used as the q-axis current inner loop reference. The two inner loop outputs are cross-decoupled and then synchronized to complete the rapid voltage establishment within a 10 ms interruption period.

[0022] Furthermore, in step 5, the frequency closed loop initially operates at a limit of ±0.2 Hz, and automatically increases by 0.1 Hz every 1 second until it reaches ±0.6 Hz, so that phase locking can still be completed when the grid frequency deviation is at its maximum of ±0.5 Hz.

[0023] Furthermore, in step 5, the amplitude closed loop targets the d-axis component of the grid voltage, and the phase closed loop targets the q-axis component of the grid voltage to approach zero. The two closed loops are executed sequentially within the same cycle, and the execution results directly refresh the V / F reference angle to ensure that the closing judgment is entered only after the amplitude and phase synchronization is completed.

[0024] Furthermore, the cross-decoupling operation in step 3 superimposes the d-axis current reference onto the q-axis voltage output via a first scaling factor, and superimposes the q-axis current reference onto the d-axis voltage output via a second scaling factor, in order to cancel the inter-axis coupling caused by the filter inductor.

[0025] Furthermore, it also includes negative sequence suppression, in which the positive sequence d-axis and q-axis current references are phase-shifted by coordinate transformation and sent into the negative sequence current inner loop. The output of the negative sequence loop is superimposed with the output of the positive sequence loop to eliminate negative sequence harmonics caused by unbalanced loads.

[0026] Furthermore, a 20 ms delay interlock confirmation is provided between step 6 and step 7 to ensure that the control mode is switched only after the contactor is fully closed, preventing instantaneous open-circuit impact.

[0027] Beneficial effects:

[0028] To ensure constant output voltage and frequency of the energy storage converter, the off-grid control strategy employs V / F control, a combination of an outer voltage loop and an inner current loop. The outer voltage loop provides a reference voltage, boosting it from zero to maintain a stable output AC voltage. The inner current loop accelerates the system's dynamic response and improves its anti-interference capability. Furthermore, considering that the system's load may be switching equipment such as a 12-pulse rectifier, which introduces harmonics, a dual decoupled synchronous reference coordinate system (DDSRF) method is used to separate the positive and negative sequence currents. The positive and negative sequence currents are controlled independently, and the control in the negative sequence coordinate system is used to achieve closed-loop, zero-steady-state-error control of the negative sequence current component, thereby suppressing and compensating for negative sequence current. This control method, while ensuring system control accuracy in steady state, incorporates a current command feedforward loop. When the current command changes, the feedforward loop compensates for the damping effect of the inductor on the current during dynamic changes, improving the current response speed and enabling the system to achieve rapid response. Attached Figure Description

[0029] Figure 1 Topology diagram of an off-grid cascaded emergency energy storage system;

[0030] Figure 2 This is a schematic diagram of an off-grid model of an off-grid cascaded emergency energy storage system.

[0031] Figure 3Off-grid control topology diagram for an off-grid cascaded emergency energy storage system;

[0032] Figure 4 This is a schematic diagram showing the synchronization of the energy storage converter with the power grid.

[0033] Figure 5 This is a schematic diagram of the off-grid pre-synchronization line voltage waveform;

[0034] Figure 6 This is a schematic diagram of off-grid output power;

[0035] Figure 7 This is a schematic diagram showing the pre-synchronization frequency difference (top figure) and the grid connection point frequency value (bottom figure). Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0037] like Figure 1 As shown, the off-grid cascaded emergency energy storage system of this invention is a three-phase star connection, consisting of N cascaded power submodules. Each phase, after the battery cluster passes through a DC-side filter to remove high-order harmonics and suppress some DC-side secondary pulsations, is connected to the DC side of a cascaded H-bridge converter. The AC output terminals of the H-bridge converter are connected in series to increase the voltage, which is then used to power the load or directly connect to the grid through an AC-side filter. In Figure 1, L... dc For DC-side filter inductance; C dc S1 is the DC-side filter capacitor. a1 —S4 a1 Form an H-bridge converter; U Bij U represents the battery voltage of one module in each phase. Dij The bus capacitor voltage of one module in each phase; An is the nth power submodule of phase A; u i and i i These represent the inverter's output voltage and output current, respectively, where i takes values ​​of a, b, and c; L s It is an AC-side filter inductor with an AC-side equivalent internal resistance of Rs; Z load It is the load impedance of the system; N is the neutral point of the star-connected load; E abc This represents the load voltage per phase; the subscript dq indicates the component in the dq coordinate system; the subscript ref is the reference value of the corresponding variable.

[0038] The off-grid cascaded emergency energy storage system is equipped with the same controller. When the dual power supply of the grid fails, the controller automatically switches to the off-grid V / F control mode and performs pre-synchronous closed-loop regulation in amplitude-frequency-phase sequence after the grid is restored, until the grid-connected contactor closes without impact.

[0039] Off-grid model of off-grid cascaded emergency energy storage system, such as Figure 2 As shown, it consists of the output voltage of the energy storage converter, the filter inductor, and the load. Figure 2 Middle U i L is the output voltage of each phase converter. s It is a filter inductor with an equivalent internal resistance of R. s I i E is the load current per phase. i This represents the voltage of each phase of the power grid.

[0040] according to Figure 2 The off-grid operating domain mathematical model of a cascaded energy storage system can be expressed as:

[0041] .

[0042] This invention also provides a control method for an off-grid cascaded emergency energy storage system, including:

[0043] When both power sources in the grid fail, the entire cascaded energy storage system operates off-grid. In this state, the system functions as an emergency power source, with the energy storage converter supporting the voltage to maintain system voltage and frequency stability, providing a continuous and reliable power supply to critical loads. The off-grid control topology of the off-grid cascaded emergency energy storage system is as follows: Figure 3 As shown. The instantaneous values ​​of the three-phase voltage u in the abc coordinate system are obtained through voltage sampling. a u b u c Then, the instantaneous value of the three-phase voltage U in the dq rotating coordinate system is obtained through the abc / dq coordinate transformation. d U q , reference value U dref U qref The difference between the actual value and the calculated error value is processed by a PI controller. The output value is then compared with the instantaneous three-phase current value i in the dq rotating coordinate system. d i q Adding them together yields the positive-sequence d-axis and q-axis current reference command values ​​for the inner loop commutation chain. The current reference command value I... dref I qref The difference between the actual value and the calculated error value is processed by a PI controller. The output value is then added to or subtracted from the decoupled voltage and current feedforward values ​​to obtain the output voltage value m. d m qThen, perform a dq / abc coordinate transformation on it. Then, use the previously calculated positive-sequence d- and q-axis reference values ​​I of the undecoupled inner loop commutator current. dref I qref The phase is changed to the negative sequence dq coordinate system by performing dq / αβ and αβ / dq coordinate transformations, and then the positive sequence output voltage m is obtained by superimposing the current inner loop control with the original V / F control. abc .

[0044] For a 10kV cascaded energy storage system, the off-grid to synchronization process includes:

[0045] like Figure 4 As shown, the purpose of synchronizing with the power supply is to ensure that the bus voltage and the grid voltage are synchronized in amplitude, frequency, and phase before the bus incoming circuit breaker is closed, thereby minimizing the impact when the power supply is connected to the bus. The 110kV voltage is transformed to the 10kV side according to the transformer ratio and connection group to obtain the virtual 10kV voltage U. i10abc . When U i10 If the voltage spike is greater than 0.9 pu for 100 ms, the power supply is considered to have resumed operation, and the energy storage converter begins to synchronize with the power supply; otherwise, they are not synchronized. (U...) i10 As the target value for off-grid AC voltage, the PCS output voltage is made to be the same as the power supply voltage and amplitude.

[0046] Will U i10q Δω obtained after PI control and amplitude limiting sys With reference frequency ω ref The phase θ used for off-grid V / F control is obtained by superposition and integration. Considering the energy storage converter synchronizing with the load, the output is limited to ±0.2Hz to prevent significant phase changes during synchronization. It is assumed that θ and U are initially synchronized. i10abc If the actual phase difference is at most 180 degrees (represented as pi in radians), then the time required for synchronization is approximately Considering that the actual power grid frequency deviation may be greater than ±0.2Hz, with a maximum of ±0.5Hz, the PI output limit is increased by 0.1Hz every 1s after the synchronization begins, until it reaches ±0.6Hz.

[0047] When the amplitude and phase of the energy storage converter's output voltage deviate from those of the power supply within a certain range, synchronization is considered complete, i.e., AVE(|U i10d -U d |) 10ms set1 And|U i10q | set2 If the duration is 100ms, it is considered to be completed simultaneously.

[0048] The cascaded energy storage system builds up voltage from 0 to 1 (pu). At this point, the system's off-grid load condition is as follows:​​ Figure 5 As shown, it can normally carry the load. Before the synchronization command was issued, there was a certain deviation in amplitude and phase between the system output voltage and the grid voltage, such as... Figure 6 As shown; after the 0.5s synchronization command is issued, the output phase synchronizes with the grid phase after 3 cycles, and the amplitude is also consistent, completing the pre-synchronization and meeting the conditions for closing and grid connection. Simultaneously, from... Figure 7 As can be seen, when the synchronous command is first issued, the system output frequency is raised to the limit of 50Hz, and then reduced to the same frequency as the power grid of 50.2Hz within 200ms. Among these, Figure 7 The above figure shows the pre-synchronization frequency difference. Figure 7 The following figure shows the frequency values ​​at the grid connection point.

[0049] During the off-grid pre-synchronization process, there are no obvious voltage fluctuations in the AC bus voltage waveform, enabling seamless switching.

[0050] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An off-grid cascaded emergency energy storage system, characterized in that, Comprise: a three-phase star-connected power sub-module string, each phase being formed by cascading N H-bridge power units with DC filter links, and the cascaded output end being directly connected to a load or a power grid through an AC filter; the DC side of the H-bridge power unit being used for connecting a battery cluster; the system being configured with a same controller, which automatically switches to an off-grid V / F control mode when the power grid double power supply loses power, and executes pre-synchronization closed-loop regulation in the order of amplitude-frequency-phase after the power grid recovers, until the non-impact closing of the grid contactor; V / F represents a control mode of fixing the amplitude and frequency of the AC voltage.

2. The off-grid cascaded emergency energy storage system of claim 1, wherein, the controller being internally provided with a positive sequence d-axis voltage outer loop, a positive sequence q-axis voltage outer loop, a positive sequence d-axis current inner loop, a positive sequence q-axis current inner loop and a negative sequence current closed loop, each loop being sequentially operated in the same interrupt period, and the operation results being synchronously sent to each H-bridge power unit after PWM modulation.

3. The off-grid cascaded emergency energy storage system according to claim 1 or 2, characterized in that, the controller setting a gradually widened frequency amplitude interval in the pre-synchronization stage, the initial amplitude being ±0.2 Hz, and the amplitude being widened by 0.1 Hz every 1 s until ±0.6 Hz, for shortening the phase capture time and keeping the load side frequency stable.

4. A control method of an off-grid type cascaded emergency energy storage system, characterized in that, Comprise the following steps: Step 1, real-time monitoring of the power grid voltage, when any phase voltage is lower than 0.9 pu and lasts for 100 ms, it is determined that the power is lost; Step 2, immediately after the power loss is determined, the grid contactor is locked, and the off-grid V / F control is started to establish a three-phase AC voltage with the rated voltage and the rated frequency; V / F represents a control mode of fixing the amplitude and frequency of the AC voltage; Step 3, continuously sampling the load current under the V / F control, maintaining the amplitude and frequency stability through the positive sequence voltage-current double loop feedforward and cross-decoupling operation, and supplying power to important loads; Step 4, real-time detection of the power grid voltage, when the power grid voltage rises to 0.9 pu and lasts for 100 ms, it is determined that the power supply is restored; Step 5, taking the power grid voltage as a reference, the amplitude, frequency and phase of the system output voltage are sequentially closed-loop regulated until the amplitude difference and q-axis difference are within the set threshold and remain for 100 ms; Step 6, immediately after the pre-synchronization is completed, the grid contactor is closed, and the system is seamlessly switched from the off-grid mode to the grid-connected mode; Step 7, after the closing is successful, the V / F reference source is locked, the grid-connected power control mode is switched to, and the emergency process is ended.

5. The control method according to claim 4, characterized by, In the step 2, the output of the positive sequence d-axis voltage outer loop is first limited in amplitude and then used as the d-axis current inner loop reference, the output of the positive sequence q-axis voltage outer loop is first limited in amplitude and then used as the q-axis current inner loop reference, and the outputs of the two inner loops are synchronously used after cross-decoupling, for completing the fast establishment of voltage in the 10 ms interrupt period.

6. The control method according to claim 5, characterized by In the step 5, the frequency closed loop is first limited in amplitude at ±0.2 Hz, and then automatically widened by 0.1 Hz every 1 s until ±0.6 Hz, for still being able to complete phase locking when the maximum power grid frequency deviation is ±0.5 Hz.

7. The control method according to claim 6, characterized by In step 5, the amplitude closed loop targets the grid voltage d-axis component, and the phase closed loop targets the grid voltage q-axis component to zero, and the two closed loops are sequentially executed in the same cycle, and the execution result directly refreshes the V / F reference angle, which is used to ensure that the amplitude and phase are synchronized before entering the closing judgment.

8. The control method according to claim 7, characterized by, In step 3, the cross-decoupling operation adds the d-axis current reference to the q-axis voltage output through a first proportional coefficient and adds the q-axis current reference to the d-axis voltage output through a second proportional coefficient, which is used to offset the inter-axis coupling caused by the filter inductance.

9. The control method according to claim 8, characterized by, It also includes negative sequence suppression, which sends the positive sequence d, q-axis current reference after phase shift through coordinate transformation into the negative sequence current inner loop, and the negative sequence loop output is superimposed with the positive sequence loop output, which is used to eliminate the negative sequence harmonics caused by unbalanced load.

10. The control method according to claim 9, characterized by An interlock confirmation with a delay of 20 ms is provided between step 6 and step 7, which is used to ensure that the contactor is completely closed before switching the control mode, preventing the occurrence of transient open circuit impact.

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