Voltage synchronization control method for off-grid to grid-connected

Abstract

The off-grid to grid-connected voltage synchronization control method disclosed in the present invention restores the grid voltage and frequency of an off-grid power grid according to a preset time, executes a phase synchronization program based on the sampled grid voltage, phase-locks the grid voltage, and ensures that the q-axis component is equal to 0, achieving phase synchronization; uses the effective value of the grid voltage as the reference for the voltage effective value loop of the inverter; controls low-order harmonic synchronization based on the formula: inverter voltage reference = voltage effective value loop + grid voltage instantaneous sampling value - grid voltage low-pass filtered value; controls high-order harmonic synchronization based on the modulation target voltage = inverter current loop output + grid voltage feedforward; and outputs a drive signal for the IGBT based on PWM modulation to synchronize the amplitude and immediately switch to grid-connected operation. During off-grid to grid-connected transition, a control method is implemented to ensure that the high and low harmonics of the off-grid voltage are consistent with those of the grid, thereby ensuring zero-current switching during off-grid to grid-connected transition and avoiding inrush current.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic system operation control, and in particular to a method for voltage synchronization control in off-grid to grid-connected switching. Background Art

[0002] At present, during the operation of photovoltaic systems, when switching from off-grid to grid-connected, the harmonic current is relatively large. When the grid-connected relay is not powered off, the off-grid voltage harmonics cannot be consistent with the grid voltage, and only the fundamental wave can be consistent, resulting in a large impact on the grid. When new energy power generation is connected to the grid, the power quality of the traditional grid is greatly impacted, which is not conducive to the control of power quality. This large power impact will have a certain degree of harm to the voltage and frequency of the traditional grid, causing voltage instability, affecting users' electricity consumption, and in more serious cases, damaging the grid-connected equipment itself. Summary of the Invention

[0003] The purpose of the present invention is to solve the deficiencies of the above-mentioned technologies and to provide a method for voltage synchronization control in off-grid to grid-connected power supply. The specific method is as follows.

[0004] The off-grid to grid-connected voltage synchronization control method designed by the present invention is characterized by comprising the following steps:

[0005] Step S1: Restore the grid voltage and frequency of the off-grid power grid according to a preset time. If the recovery time is not reached and the grid voltage and frequency are abnormal, restart the recovery; if the recovery time is reached and the grid voltage and frequency are normal, proceed to step S2;

[0006] Step S2: Sampling the grid voltage during off-grid operation and executing a phase synchronization procedure based on the sampled grid voltage to phase-lock the grid voltage and set the q-axis component to 0, thereby achieving phase synchronization; if the phase synchronization is unsuccessful, the phase synchronization procedure is repeated until the phase synchronization is successful; if the phase synchronization is successful, the process proceeds to step S3;

[0007] Step S3, effective value synchronization, using the grid voltage effective value as the inverter voltage effective value loop reference, and proceeding to step S4;

[0008] Step S4, low-order harmonic synchronization, according to the formula of inverter voltage reference = voltage RMS value loop + grid voltage instantaneous sampling value - grid voltage low-pass filter value, controls the low-order harmonics of the off-grid voltage to be synchronized with the low-order harmonics of the grid voltage, and proceeds to step S5;

[0009] Step S5, high-order harmonic synchronization, according to the modulation target voltage = inverter current loop output + grid voltage feedforward, controls the high-order harmonics of the off-grid voltage to be synchronized with the high-order harmonics of the grid voltage, and proceeds to step S6;

[0010] Step S6, outputting a driving signal for driving the IGBT according to PWM modulation to synchronize the amplitude; if the amplitude synchronization is successful, immediately switching from off-grid operation to grid-connected operation; if the amplitude synchronization fails.

[0011] According to the above-mentioned off-grid to grid-connected voltage synchronization control method, the execution method of the phase synchronization program is as follows:

[0012] Step S21, performing closed-loop regulation based on the sampled grid voltage so that the q-axis component of the grid voltage is 0, and outputting a phase-locked frequency fo, which is used as the current off-grid operating frequency;

[0013] In step S22, the frequency fo is integrated to obtain the phase-locked angle PLL_Angle, and the phase-locked angle PLL_Angle performs a dq transformation on the grid voltage to obtain the q-axis component of the grid voltage. If the grid voltage is successfully phase-locked, that is, the q-axis component is equal to 0, then the phase synchronization is successful. If the grid voltage is not successfully phase-locked, that is, the q-axis component is not equal to 0, then the phase synchronization fails, and steps S21 and S22 are repeated.

[0014] The off-grid to grid-connected voltage synchronization control method described in the present invention utilizes amplitude synchronization to enable the voltage loop in off-grid operation to output low-frequency harmonics contained in the grid, i.e., a value smaller than the loop bandwidth of the voltage loop. Higher-order harmonics rely on the feedforward of the current loop, i.e., using the instantaneous value of the grid voltage for feedforward. This ensures that the off-grid output voltage of the inverter has the same frequency, phase, amplitude, and harmonics as the grid voltage, avoiding inrush current during off-grid to grid-connected conversion and simultaneously ensuring that the balanced power distribution of multiple off-grid parallel units is not affected.

[0015] In addition, when switching from off-grid to grid-connected, a control method is implemented to ensure that the high and low harmonics of the off-grid voltage are consistent with those of the grid, thereby ensuring zero-current switching from off-grid to grid-connected. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the control flow chart when switching to grid connection during off-grid operation;

[0017] Figure 2 It is a phase synchronization control flow chart;

[0018] Figure 3 Amplitude synchronization control flow chart. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention are within the scope of protection of the present invention.

[0020] Example:

[0021] like Figure 1-Figure 3 As shown, the off-grid to grid-connected voltage synchronization control method described in this embodiment is characterized by comprising the following steps:

[0022] Step S1: Restore the grid voltage and frequency of the off-grid power grid according to a preset time. If the recovery time is not reached and the grid voltage and frequency are abnormal, restart the recovery; if the recovery time is reached and the grid voltage and frequency are normal, proceed to step S2;

[0023] Step S2: Sampling the grid voltage during off-grid operation and executing a phase synchronization procedure based on the sampled grid voltage to phase-lock the grid voltage and set the q-axis component to 0, thereby achieving phase synchronization; if the phase synchronization is unsuccessful, the phase synchronization procedure is repeated until the phase synchronization is successful; if the phase synchronization is successful, the process proceeds to step S3;

[0024] Step S3, effective value synchronization, using the grid voltage effective value as the inverter voltage effective value loop reference, and proceeding to step S4;

[0025] Step S4, low-order harmonic synchronization, according to the formula of inverter voltage reference = voltage effective value loop + grid voltage instantaneous sampling value - grid voltage low-pass filter value, controls the synchronization of low-order harmonics of off-grid voltage and low-order harmonics of grid voltage, and enters step S5; wherein, the low-order harmonics of the grid are extracted and superimposed on the reference value of the voltage instantaneous value loop to participate in the control. The loop implementation method has a slow response and can only track low-order harmonics, thereby achieving synchronization of low-order harmonics of off-grid voltage and low-order harmonics of grid voltage.

[0026] Step S5, high-order harmonic synchronization, based on the modulation target voltage = inverter current loop output + grid voltage feedforward, controls the high-order harmonics of the off-grid voltage to be synchronized with the high-order harmonics of the grid voltage, and proceeds to step S6; wherein, the instantaneous value of the grid voltage minus the filtered value of the grid voltage is obtained, and the harmonic components are used as voltage feedforward and current loop output to act on PWM together, with a faster response, and finally an inverter voltage synchronized with the grid voltage is obtained.

[0027] Step S6 modulates the output drive signal of the IGBT according to PWM to synchronize the amplitudes. If amplitude synchronization is successful, the system immediately switches from off-grid operation to grid-connected operation. If amplitude synchronization fails, PWM is a common method for achieving strong power control using weak signals in power electronics control. The IGBT is the execution unit and acts as an amplification unit. Here, when amplitude asynchrony is detected, feedback closed-loop control is used to adjust the PWM. After execution by the IGBT, the inverter output voltage is adjusted, gradually approaching the target voltage, and ultimately controlling the steady-state error to 0, thereby achieving amplitude synchronization.

[0028] According to the above-mentioned off-grid to grid-connected voltage synchronization control method, the execution method of the phase synchronization program is as follows:

[0029] Step S21, performing closed-loop regulation based on the sampled grid voltage so that the q-axis component of the grid voltage is 0, and outputting a phase-locked frequency fo, which is used as the current off-grid operating frequency;

[0030] In step S22, the frequency fo is integrated to obtain the phase-locked angle PLL_Angle, and the phase-locked angle PLL_Angle performs a dq transformation on the grid voltage to obtain the q-axis component of the grid voltage. If the grid voltage is successfully phase-locked, that is, the q-axis component is equal to 0, then the phase synchronization is successful. If the grid voltage is not successfully phase-locked, that is, the q-axis component is not equal to 0, then the phase synchronization fails, and steps S21 and S22 are repeated.

[0031] The off-grid to grid-connected voltage synchronization control method described in the present invention utilizes amplitude synchronization to enable the voltage loop in off-grid operation to output low-frequency harmonics contained in the grid, i.e., a value smaller than the loop bandwidth of the voltage loop. Higher-order harmonics rely on the feedforward of the current loop, i.e., using the instantaneous value of the grid voltage for feedforward. This ensures that the off-grid output voltage of the inverter has the same frequency, phase, amplitude, and harmonics as the grid voltage, avoiding inrush current during off-grid to grid-connected conversion and simultaneously ensuring that the balanced power distribution of multiple off-grid parallel units is not affected.

[0032] In addition, when switching from off-grid to grid-connected, a control method is implemented to ensure that the high and low harmonics of the off-grid voltage are consistent with those of the grid, thereby ensuring zero-current switching from off-grid to grid-connected.

[0033] The present invention is not limited to the above-mentioned optimal implementation mode. Anyone can derive other forms of products under the inspiration of the present invention. However, no matter what changes are made in the shape or structure, any technical solution that is the same or similar to that of the present application falls within the scope of protection of the present invention.

Claims

1. A method for controlling voltage synchronization from off-grid to grid-connected, characterized in that: The steps are as follows: Step S1: Restore the grid voltage and frequency of the off-grid power grid according to the preset time. If the recovery time is not reached and the grid voltage and frequency are abnormal, restart the recovery; if the recovery time is reached and the grid voltage and frequency are normal, proceed to step S2; Step S2, sampling the grid voltage in off-grid operation, and executing a phase synchronization procedure based on the sampled grid voltage to phase-lock the grid voltage, and the q-axis component is equal to 0, thereby achieving phase synchronization; If the phase synchronization is not successful, the phase synchronization procedure is repeated until the phase synchronization is successful; If the phase synchronization is successful, proceed to step S3; Step S3, effective value synchronization, using the grid voltage effective value as the inverter voltage effective value loop reference, and proceeding to step S4; Step S4, low-order harmonic synchronization, according to the formula of inverter voltage reference = voltage RMS loop output + grid voltage instantaneous sampling value - grid voltage low-pass filter value, controls the synchronization of low-order harmonics of off-grid voltage with low-order harmonics of grid voltage, and proceeds to step S5; Step S5, high-order harmonic synchronization, according to the modulation target voltage = inverter current loop output + grid voltage feedforward, control the high-order harmonics of the off-grid voltage to be synchronized with the high-order harmonics of the grid voltage, and then enter step S6; Step S6, outputting a driving signal for driving the IGBT according to PWM modulation to synchronize the amplitudes; If the amplitude synchronization is successful, the system will immediately switch from off-grid operation to grid-connected operation; If the amplitude synchronization fails, the PWM is adjusted through feedback closed-loop control. After execution by the IGBT, the inverter output voltage is adjusted and gradually approaches the target voltage. Finally, the steady-state error is controlled to 0, thereby achieving amplitude synchronization.

2. The off-grid to grid-connected voltage synchronization control method according to claim 1, characterized in that: The phase synchronization procedure is performed as follows: Step S21, performing closed-loop regulation based on the sampled grid voltage so that the q-axis component of the grid voltage is 0, and outputting a phase-locked frequency fo, which is used as the current off-grid operating frequency; In step S22, the frequency fo is integrated to obtain the phase-locked angle PLL_Angle, and the phase-locked angle PLL_Angle performs a dq transformation on the grid voltage to obtain the q-axis component of the grid voltage. If the grid voltage is successfully phase-locked, that is, the q-axis component is equal to 0, then the phase synchronization is successful. If the grid voltage is not successfully phase-locked, that is, the q-axis component is not equal to 0, then the phase synchronization fails, and steps S21 and S22 are repeated.

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