Pre-synchronization control method and system for grid-following type energy storage converter based on phase angle compensation

Abstract

The invention discloses a pre-synchronization control method and system for a grid-following type energy storage converter based on phase angle compensation, and the method comprises the steps: controlling the voltage amplitude and frequency of an energy storage converter side to be consistent with those of a power grid side through a control module when the energy storage converter is off-grid; measuring the phase angles of the energy storage variable current and the two sides of the power grid through a measurement module; generating a phase angle compensation amount based on the phase angles on the two sides through a calculation module, synthesizing a three-phase voltage reference value based on the amplitude and frequency of the voltage on the power grid side and the phase angle compensation amount, and performing coordinate transformation on the three-phase voltage reference value to obtain a voltage loop reference value; through a compensation module, based on a voltage loop reference value, a grid-connected point phase angle is adjusted through double-closed-loop control and current-limiting protection; and monitoring electrical values of the converter side and the power grid side through a monitoring module so as to judge and switch grid-connected conditions. According to the invention, long-time waiting under a small frequency difference is avoided, the frequency is kept relatively stable, and frequency hopping and voltage / current impact risks caused by large frequency difference adjustment are eliminated.

Description

Technical Field

[0001] This invention belongs to the field of power system grid connection and off-grid control technology, specifically relating to a pre-synchronization control method and system for grid-connected energy storage converters based on phase angle compensation. Background Technology

[0002] A microgrid is a small-scale power generation and distribution system consisting of distributed power sources (such as photovoltaic and wind power), energy storage devices, loads, monitoring and protection facilities, and a control system. It possesses self-control and autonomous management capabilities. Its core feature is that it can operate in parallel with the external power grid to achieve bidirectional energy exchange; it can also switch to off-grid operation (islanding mode) in the event of a grid failure to ensure continuous power supply to critical loads.

[0003] The energy storage converter is the core hub for the smooth switching between grid-connected and off-grid operation of a microgrid. Through real-time dynamic switching control modes, it uses PQ control to regulate active and reactive power during grid-connected operation, providing power to the load along with the main grid. When a grid line fault occurs, the connection between the grid and the microgrid is quickly disconnected, and the microgrid transitions from grid-connected to off-grid (i.e., islanded) operation. During off-grid operation, the energy storage converter switches to VF control to provide voltage and frequency support to the microgrid, maintaining stable islanded operation. After fault clearance, pre-synchronization control of the energy storage converter is required during reconnection. If strict synchronization of voltage amplitude, frequency, and phase between the microgrid and the grid is not achieved (i.e., synchronization conditions are not met), the switching transient will trigger overvoltage and overcurrent surges, seriously threatening equipment safety and system stability.

[0004] Currently, the mainstream approach for switching energy storage converters from off-grid to grid-connected mode is to use VF control to achieve pre-synchronization of voltage amplitude and frequency, and to adjust the phase angle by using a fixed frequency difference between the energy storage converter's grid connection point and the grid to meet synchronization requirements. However, this approach has significant limitations: when the frequency difference between the grid connection point and the grid is too small, the phase synchronization process takes too long; while when the frequency difference is large, it can cause voltage and current surges at the moment of grid connection, threatening system safety. Therefore, it is necessary to study a fast and reliable pre-synchronization control method. Summary of the Invention

[0005] To address the shortcomings of existing technologies and improve the efficiency and reliability of microgrid grid connection, this invention adopts the following technical solution:

[0006] The pre-synchronization control method for grid-connected energy storage converters based on phase angle compensation includes the following steps:

[0007] Step S1: When the energy storage converter is off-grid, control the voltage amplitude and frequency on the energy storage converter side to be consistent with those on the grid side;

[0008] Step S2: Measure the phase angles between the energy storage converter and both sides of the power grid;

[0009] Step S3: Generate phase angle compensation amount based on the phase angles on both sides;

[0010] Step S4: Based on the voltage amplitude and frequency on the grid side, and the phase angle compensation amount, synthesize the three-phase voltage reference value, and then perform coordinate transformation on the three-phase voltage reference value to obtain the voltage loop reference value;

[0011] Step S5: Based on the voltage loop reference value, adjust the phase angle of the grid connection point through dual closed-loop control;

[0012] Step S6: Monitor the electrical values ​​on the energy storage converter side and the grid side to determine grid connection conditions and switch modes.

[0013] Furthermore, in step S1, the control mode of the energy storage converter is switched according to the grid-connected or off-grid status of the grid-connected switch of the energy storage converter. When connected to the grid, the energy storage converter adopts PQ control mode to control the output of the required active current and reactive current. When off-grid, the energy storage converter adopts VF control to control the voltage amplitude and frequency on the energy storage converter side to be consistent with the grid side.

[0014] Further, in step S2, pre-synchronization begins based on the pre-synchronization enable signal, and the phase angles on both sides of the grid-connected switch of the energy storage converter are measured in real time via a phase-locked loop; in step S3, the phase angle difference between the two sides is sent to the integral controller to obtain the phase angle compensation amount Δθ.

[0015]

[0016] Where, θ b θ represents the phase angle on the energy storage converter side. g Let represent the phase angle on the grid side, s represent the Laplace operator, and k represent the phase angle on the grid side. i This represents the integration parameter.

[0017] Further, in step S1, a voltage reference value V is obtained based on the consistent voltage amplitude and frequency. ref With frequency reference value f ref In step S4, the synthesized three-phase voltage reference values ​​are denoted as v. aref v bref v cref The calculation formula is as follows:

[0018]

[0019] Where Δθ represents the phase angle compensation amount.

[0020] Further, in step S4, the three-phase voltage reference value is transformed by abc / dq to obtain the voltage loop reference value u. dref u qrefThe transformation formula is as follows:

[0021]

[0022] Where, θ p This indicates the phase angle used in the transformation.

[0023] Furthermore, in step S4, the phase angle θ used for coordinate transformation p The calculation method is as follows:

[0024]

[0025] Where t represents time.

[0026] Further, in step S5, the voltage outer loop reference value is input to the proportional-integral PI controller to obtain the current reference value, and then the current reference value is input to the current inner loop controller to obtain the voltage signal. After dq / abc conversion and SPWM, a drive pulse signal is generated to control the energy storage inverter and adjust the phase angle of the grid connection point.

[0027] Furthermore, in step S5, the current reference value is limited to suppress the risk of transient overcurrent.

[0028] Furthermore, in step S6, the amplitude difference ΔU, phase angle difference Δθ, and frequency difference Δf of the voltages on both sides of the grid-connected switch of the energy storage converter are monitored in real time to see if they meet the grid connection conditions (ΔU<10%, Δθ<10°, Δf<0.2Hz). If they do, the grid-connected switch is closed, the energy storage converter control mode PQ control is switched, a pre-synchronization end signal is issued, and the pre-synchronization ends.

[0029] The pre-synchronization control system for grid-connected energy storage converter based on phase angle compensation includes a control module, a measurement module, a calculation module, a compensation module, and a monitoring module. It performs pre-synchronization control according to the pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation.

[0030] The control module controls the voltage amplitude and frequency on the energy storage converter side to be consistent with the grid side when the energy storage converter is off-grid.

[0031] The measurement module measures the phase angle between the energy storage converter and both sides of the power grid.

[0032] The calculation module generates a phase angle compensation amount based on the phase angles on both sides, and synthesizes a three-phase voltage reference value based on the voltage amplitude and frequency consistent between the energy storage converter side and the grid side, as well as the phase angle compensation amount. Then, the three-phase voltage reference value is transformed to obtain a voltage loop reference value.

[0033] The compensation module, based on the voltage loop reference value, adjusts the phase angle at the grid connection point through dual closed-loop control;

[0034] The monitoring module monitors the electrical values ​​on the energy storage converter side and the grid side, so as to determine and switch grid connection conditions through the control module.

[0035] The advantages and beneficial effects of this invention are as follows:

[0036] This invention utilizes VF control to achieve voltage amplitude and frequency synchronization. A phase-locked loop (PLL) then acquires phase information between the energy storage converter side and the grid side (on both sides of the grid-connected switch). The phase difference is fed back to the voltage reference value generation stage via an integral controller to generate a phase angle compensation. Through dual closed-loop control, the voltage phase angle on the converter side is adjusted, achieving rapid phase synchronization. This directly avoids the inherent contradictions of fixed frequency difference regulation. On the one hand, phase closed-loop control accelerates the synchronization process, avoiding long waiting times under small frequency differences; on the other hand, it maintains relatively stable frequency, eliminating the risks of frequency jumps and voltage / current surges caused by large frequency difference regulation. Simultaneously, the use of an integral controller solves the problem of excessive initial compensation in traditional PI control, leading to voltage instability at the initial stage of phase angle pre-synchronization. Furthermore, current limiting control is adopted to reduce transient overcurrent caused by control mode switching. Attached Figure Description

[0037] Figure 1 This is a flowchart of the method in an embodiment of the present invention.

[0038] Figure 2 This is a schematic diagram of the energy storage grid connection model in an embodiment of the present invention.

[0039] Figure 3 This is a block diagram of the grid-connected pre-synchronization control of the energy storage system in an embodiment of the present invention.

[0040] Figure 4 This is a schematic diagram of the dual closed-loop control strategy in an embodiment of the present invention.

[0041] Figure 5 This is a schematic diagram illustrating the calculation of phase angle compensation in an embodiment of the present invention.

[0042] Figure 6 This is a simulation result diagram of the phase A voltage on the energy storage converter side and the phase A voltage on the grid side in an embodiment of the present invention.

[0043] Figure 7 This is a simulation result diagram of the voltage phase angle (blue) on the energy storage converter side and the voltage phase angle (red) on the grid side in an embodiment of the present invention.

[0044] Figure 8 This is a simulation result diagram of the voltage frequency on the energy storage converter side and the voltage frequency on the grid side in an embodiment of the present invention.

[0045] Figure 9 This is a simulation result diagram of the active power output of energy storage in an embodiment of the present invention.

[0046] Figure 10 This is a simulation result diagram of the energy storage output current in an embodiment of the present invention.

[0047] Figure 11 This is a schematic diagram of the system structure in an embodiment of the present invention. Detailed Implementation

[0048] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0049] When the microgrid is connected to the grid, the energy storage converter adopts PQ control, which regulates the active / reactive output through a dual closed loop of power outer loop and current inner loop; when off-grid, it switches to VF control, which maintains voltage and frequency support through a dual closed loop of voltage outer loop and current inner loop.

[0050] The core strategy for most current energy storage converters to achieve smooth off-grid to grid-connected switching is to pre-synchronize voltage amplitude and frequency based on VF control, while adjusting the phase angle through a fixed frequency difference (Δf) to meet the grid-connected synchronization requirements. However, this approach has inherent contradictions: when the frequency difference between the energy storage converter's grid connection point and the grid is small, phase angle synchronization relies on natural accumulation, leading to a significant extension of the pre-synchronization time (up to several seconds), affecting the system response speed; if the frequency difference is too large, the phase angle adjustment rate will be too fast, causing voltage / current surges, which are prone to occur during grid connection and threaten system safety. A fixed frequency difference Δf cannot simultaneously achieve both adjustment speed and transient stability. Furthermore, at the moment of switching from off-grid to grid-connected, the control mode switching will cause a jump in the current reference value of the inner current loop, generating transient overcurrent.

[0051] To address the above problems, this invention proposes a pre-synchronization control method for grid-connected energy storage converters based on phase angle compensation, such as... Figure 1 As shown, the specific steps include the following:

[0052] Step S1: Dynamically switch the control mode of the energy storage converter;

[0053] The main controller detects the open / closed state of the grid-connected switch and switches the control mode of the energy storage converter according to the state of the grid-connected switch. When the grid-connected switch is closed, the energy storage converter adopts PQ control to control the output of the required active and reactive currents; when the grid-connected switch is open, the energy storage converter adopts VF control to ensure that the voltage amplitude and frequency are consistent with the grid side. The voltage reference value and frequency reference value are denoted as V0 and V1, respectively. ref f ref .

[0054] Step S2: Measure the phase angles on both sides of the grid-connected switch;

[0055] After the fault is cleared, the main control sends a pre-synchronization enable signal, and pre-synchronization begins. The phase angles on both sides of the grid-connected switch are measured in real time via a phase-locked loop (PLL), and the phase angle on the energy storage converter side is denoted as θ. b The phase angle on the power grid side is denoted as θ. g .

[0056] Step S3: Generate phase angle compensation Δθ;

[0057] The difference between the phase angle on the energy storage converter side and the phase angle on the grid side is sent to the integral controller to obtain the phase angle compensation amount Δθ. The calculation formula is as follows:

[0058]

[0059] In the formula, s represents the Laplace operator, k i This represents the integration parameter.

[0060] Furthermore, the integrator freezes its calculations before receiving the pre-synchronization enable signal; real-time compensation is performed after reception to avoid voltage oscillations caused by initial overshoot.

[0061] Step S4: Based on the voltage reference value V ref Frequency reference value f ref And the phase angle compensation amount Δθ, generate the voltage loop reference value u dref u qref ;

[0062] The combined three-phase voltage reference values ​​are denoted as v. aref v bref v cref The calculation formula is as follows:

[0063]

[0064] Three-phase voltage reference value v aref v bref v cref u is obtained through abc / dq transformation dref u qref The transformation formula is as follows:

[0065]

[0066] Among them, the phase angle θ used for transformation p The calculation method is as follows:

[0067]

[0068] In the formula, t represents time.

[0069] Step S5: Based on the voltage loop reference value, perform dual closed-loop control and current limiting protection by adjusting the phase angle of the grid connection point;

[0070] with u dref u qref As input, the current reference value i is obtained through a PI controller. dref i qref Then it is input to the current inner loop controller to obtain the modulated voltage signal u. sd u sq The drive pulse signal is generated by dq / abc conversion and SPWM to control the energy storage inverter and adjust the phase angle at the grid connection point.

[0071] Furthermore, the current reference value is limited to suppress the risk of transient overcurrent.

[0072] Step S6: Monitor both sides of the grid connection switch to determine grid connection conditions and perform smooth switching;

[0073] The amplitude difference ΔU, phase angle difference Δθ, and frequency difference Δf of the voltage on both sides of the grid-connected switch are monitored in real time. When the grid connection conditions are met (ΔU<10%, Δθ<10°, Δf<0.2Hz), the switch is closed, the main controller issues a command to switch the energy storage converter control mode PQ control, and issues a pre-synchronization end signal to end the pre-synchronization.

[0074] To illustrate the application examples of this invention, a simulation platform based on PSCAD / EMTDC is built as follows: Figure 2 The energy storage grid connection model shown has a grid connection pre-synchronization control as follows: Figure 3 As shown, the dual closed-loop control strategy is as follows: Figure 4 As shown, the phase angle compensation is calculated as follows: Figure 5 As shown in Table 1, the main parameters of the simulation model are as follows.

[0075] Table 1 Simulation parameters of the energy storage grid-connected model

[0076]

[0077] right Figure 2 The energy storage grid-connected model shown is simulated. Initially, the energy storage is operating in grid-connected mode, using PQ control, with an output of 4MW of active power. A fault occurs at 0.5s (three-phase ground fault, fault duration 150ms). At 0.63s, the grid-connected switch opens (switch opening time 100ms, protection action 30ms). The main controller sends a control mode switching signal, and the energy storage converter switches to VF control. At 1.5s, the fault is cleared, and the main controller sends a pre-synchronization signal, initiating pre-synchronization. At 1.78s, the grid connection conditions are met (ΔU<5%, Δθ<10°, Δf<0.2Hz), the grid-connected switch closes, and the main controller sends a control mode switching signal and a pre-synchronization end signal. The energy storage converter switches to PQ control, and pre-synchronization ends.

[0078] Depend on Figure 6It can be seen that before the 1.5s pre-synchronization start, the voltage on the energy storage converter side and the voltage on the grid side have the same amplitude, but there is a significant phase difference. At the start of the 1.5s pre-synchronization, the output voltage on the energy storage side (blue line) gradually coincides with the voltage on the grid side (red line), and remains consistent after 1.78s. Figure 7 As shown, this verifies the feasibility of the phase difference integral compensation mechanism.

[0079] Depend on Figure 8 It can be seen that before 1.5s, the voltage and frequency of the VF-controlled energy storage converter strictly track the grid. At 1.5s, pre-synchronization begins, and the voltage and frequency of the energy storage side are dynamically adjusted according to the compensation algorithm (fluctuation range 49.8–50.2Hz). The phase angle difference Δθ converges rapidly with the frequency gradient. At 1.78s, the grid connection condition for the phase angle is met, and the frequency experiences a small jump, gradually recovering to a steady state of 50Hz (the recovery speed is related to the PLL parameters). The frequency-limited adjustment strategy effectively suppresses the risk of loss of synchronization. This demonstrates the stability of the pre-synchronization method used in this invention.

[0080] Depend on Figure 9 It can be seen that when the energy storage is reconnected to the grid, the output active power of the energy storage rises from 1MW to 4MW without overshoot, with no power oscillation throughout, demonstrating the smoothness of the PQ control mode switching. Figure 10 It can be seen that the energy storage output current did not overshoot when reconnected to the grid, which proves the effect of the current inner loop limiting strategy used in this invention on suppressing the inrush current.

[0081] In summary, by adopting the pre-synchronization control method proposed in this invention, rapid and stable grid connection of grid-type energy storage can be achieved when a line fault occurs.

[0082] like Figure 11 As shown, a pre-synchronization control system for a grid-connected energy storage converter based on phase angle compensation includes a control module, a measurement module, a calculation module, a compensation module, and a monitoring module. Pre-synchronization control is performed according to the pre-synchronization control method for a grid-connected energy storage converter based on phase angle compensation.

[0083] The control module controls the voltage amplitude and frequency on the energy storage converter side to be consistent with the grid side when the energy storage converter is off-grid.

[0084] The measurement module measures the phase angle between the energy storage converter and both sides of the power grid.

[0085] The calculation module generates phase angle compensation based on the phase angles on both sides, and synthesizes three-phase voltage reference values ​​based on the voltage amplitude and frequency consistent between the energy storage converter side and the grid side, as well as the phase angle compensation. Then, the three-phase voltage reference values ​​are transformed to obtain voltage loop reference values.

[0086] The compensation module, based on the voltage loop reference value, adjusts the phase angle at the grid connection point through dual closed-loop control.

[0087] The monitoring module monitors both the energy storage converter side and the grid side, and uses the control module to determine and switch grid connection conditions.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A pre-synchronization control method for grid-connected energy storage converters based on phase angle compensation, characterized in that... Includes the following steps: Step S1: When the energy storage converter is off-grid, control the voltage amplitude and frequency on the energy storage converter side to be consistent with those on the grid side; Step S2: Measure the phase angles between the energy storage converter and both sides of the power grid; Step S3: Generate phase angle compensation amount based on the phase angles on both sides; Step S4: Based on the voltage amplitude and frequency on the grid side, and the phase angle compensation amount, synthesize the three-phase voltage reference value, and then transform the three-phase voltage reference value to obtain the voltage outer loop reference value; Step S5: Based on the aforementioned voltage outer loop reference value, perform dual closed-loop control by adjusting the phase angle at the grid connection point; Step S6: Monitor the electrical values ​​on the energy storage converter side and the grid side to determine grid connection conditions and switch modes.

2. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 1, characterized in that: In step S1, the control mode of the energy storage converter is switched according to the open or closed state of the grid-connected switch of the energy storage converter. When connected to the grid, the energy storage converter adopts the PQ control mode to control the required active current and reactive current output. When disconnected from the grid, the energy storage converter adopts VF control to control the voltage amplitude and frequency of the micro energy storage converter side to be consistent with the grid side.

3. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 2, characterized in that: In step S2, pre-synchronization begins based on the pre-synchronization enable signal, and the phase angles on both sides of the grid-connected switch of the energy storage converter are measured in real time via a phase-locked loop; in step S3, the phase angle difference between the two sides is sent to the integral controller to obtain the phase angle compensation amount Δθ. Where, θ b θ represents the phase angle on the energy storage converter side. g Let represent the phase angle on the grid side, s represent the Laplace operator, and k represent the phase angle on the grid side. i This represents the integration parameter.

4. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 1, characterized in that: In step S1, a voltage reference value V is obtained based on the voltage amplitude and frequency consistent with those on the grid side. ref With frequency reference value f ref In step S4, the synthesized three-phase voltage reference values ​​are denoted as v. aref v bref v cref The calculation formula is as follows: Where Δθ represents the phase angle compensation amount.

5. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 4, characterized in that: In step S4, the three-phase voltage reference value is transformed to obtain the voltage loop reference value u. dref u qref The transformation formula is as follows: Where, θ p This indicates the phase angle used in the transformation.

6. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 5, characterized in that: In step S4, the phase angle θ used for transformation is... p The calculation method is as follows: Where t represents time.

7. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 1, characterized in that: In step S5, the voltage loop reference value is input to the proportional-integral controller to obtain the current reference value, and then the current reference value is input to the inner current loop controller to obtain the modulated voltage signal. After coordinate transformation and sinusoidal pulse width modulation, a drive pulse signal is generated to control the energy storage inverter and adjust the phase angle of the grid connection point.

8. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 7, characterized in that: In step S5, the current reference value is limited.

9. The pre-synchronization control method for grid-connected energy storage converter based on phase angle compensation according to claim 1, characterized in that: In step S6, the amplitude difference, phase angle difference, and frequency difference of the voltage on both sides of the grid-connected switch of the energy storage converter are monitored in real time to see if they meet the grid connection conditions. If they do, the grid-connected switch is closed, the control mode of the energy storage converter is switched, and the pre-synchronization ends.

10. A pre-synchronization control system for a grid-connected energy storage converter based on phase angle compensation, comprising a control module, a measurement module, a calculation module, a compensation module, and a monitoring module, characterized in that: Pre-synchronization control is performed using the pre-synchronization control method for grid-connected energy storage converters based on phase angle compensation as described in any one of claims 1 to 9; The control module controls the voltage amplitude and frequency on the energy storage converter side to be consistent with the grid side when the energy storage converter is off-grid. The measurement module measures the phase angle between the energy storage converter and both sides of the power grid. The calculation module generates a phase angle compensation amount based on the phase angles on both sides, and synthesizes a three-phase voltage reference value based on the voltage amplitude and frequency on the grid side and the phase angle compensation amount. Then, it performs coordinate transformation on the three-phase voltage reference value to obtain the voltage loop reference value. The compensation module performs dual closed-loop control based on the voltage loop reference value by adjusting the phase angle of the grid connection point; The monitoring module monitors the energy storage converter side and the grid side, so as to determine and switch grid connection conditions through the control module.

Images