Aiming at the resonance problem in the process of grid connection of LCL filter microgrid inverter, a multi-resonance LCL harmonic suppression strategy is proposed. On the basis of analyzing the principle and establishing the mathematical model in detail, the realization process of the multi-resonance constant power compound control strategy is studied emphatically. Through the simulation, the validity of the control strategy is verified, The results show that the scheme stabilizes the output power and reduces the total harmonic distortion of the grid-connected inverter to 0.12%, and the corresponding phase current distortion rate drops to 0.02%.The suppression effect is obvious, it is an effective harmonic suppression method.

With the depletion of traditional energy sources, the new energy power generation system with microgrid as the carrier has been developed rapidly because of its flexible, decentralized, small, close to users and the use of clean energy. Due to the energy structure of the micro grid mainly clean energy such as wind power, photovoltaic power generation, the distributed energy will generally need electricity to the grid through power electronic converter device to realize grid connected. Therefore, a lot of power electronic devices access to power grid harmonics, caused the converter power factor lower and parallel resonant circuit or series resonance, decrease active reactive power measurement accuracy, reduce the quality of power supply a series of problems, give the user the safe and security, economic operation of power system brings great harm. so harmonic suppression is very important.

Current research of harmonic suppression methods, mainly has: the harmonic suppression method based on active filter [

Figure _{ref} Q_{ref}_{abc}, i_{abc}_{d}, v_{q}_{qref}^{dref}_{qref}_{cd}, i_{cq}, i_{cdref}, i_{cqref}

The figure includes PQ control module, current loop multi-resonance control module, and PWM modulation module etc. Micro-network inverter grid output voltage _{abc}_{oabc}_{α}, v_{β} and i_{α}, i_{β}, will be sent to the PQ controller; PQ controller according the active and reactive setpoint _{ref}_{ref}_{αref} andi_{βref} and then compare with the current detection value of the αβ components i_{α} and i_{β}. And after the ratio multi-resonant regulator G_{2}(S) in the current loop control module, the reference values i_{cαref}, i_{cβref} of the capacitive current are obtained, then compared with the capacitance current detection value αβ component i_{cα} and i_{cβ}. And then adjusted by the proportional regulator G_1 (S), the control PWM circuit drives the inverter, so that the inverter output active power and reactive power constant. In order to meet the requirements of system stability, the current loop control module in Figure

Principle block diagram of multi resonance LCL harmonic suppression strategy

_{1} is the inductance, _{1} is its internal resistance and the equivalent resistance between the upper and lower legs of each phase, _{2} is the internal resistance of _{2}. In the case of three-phase grid voltage symmetry, the mathematical model is as follows:

Where _{k}_{k}_{k}

_{1α}, _{1β}, _{2α}, _{2β} are the α and β components of the input and output currents in the αβ coordinate system; _{cα}, i_{cβ}, u_{cα}, u_{cβ}_{α}_{β}

LCL-type filter in the better suppression of high-frequency harmonics at the same time, because of its own structure for a third-order system, easy to produce resonance, the frequency near the narrow band and too high gain, will lead to the system and the load Parameter changes are very sensitive, affecting the stability of the system to bring a series of impact and harm to the grid. In order to reduce its sensitivity and high gain characteristics, to achieve the AC signal without static tracking, this paper on the basis of the use of active damping introduced into the capacitor current loop regulation to suppress high frequency interference, and the external loop current using proportional resonance control, Constructs a transfer function that performs AC compensation on the reference input signal. So that in a specific bandwidth in the same frequency response characteristics, to meet the system stability requirements, so that the output at the resonant frequency at high gain, the other frequency segment attenuation. Thus reducing the resonance, improve the stability of the system and control accuracy.

The control block diagram is showas follows:

Block diagram of current loop control

As shown in Figure _{2}(S) selected proportional resonance regulator, G_{1}(S) selected proportional regulator. After the current is transformed by the coordinate, the voltage and current v_{α}, v_{β} and i_{α}, i_{β} in the two stationary coordinates are obtained and sent to the PQ calculation module to obtain the reference current i_{αref}, i_{βref}, and then compared with i_{α} and i_{β} obtained the deviation by the proportionmulti-resonant regulator G_{1}(S) in the current loop control module, get the capacitor current reference value i_{cαref} and i_{cβref}, And it is compared with the capacitance current detection value αβ component i_{cα} and i_{cβ}, after adjusting the proportional regulator G_{2}(S), then control PWM circuit drives the inverter, so that the inverter output active power and reactive power constant.

Since the PR regulator is equivalent to the PI modulator in the stationary coordinate system under the αβ coordinate, the PR regulator can also be used to design the PI regulator parameter. Figure _{cref}, i_{cref}

The corresponding characteristic equation is:

According to the Rouse stability criterion, the system stability condition is calculated as:

According to the stability criterion (formula 9) to set the parameters, so that when the grid to reach a stable state.

A single PR regulator generates an infinite gain at a specific frequency. In order to ensure its stability and easy to achieve, using the approximate structure; its transfer function is as follows:The transfer function is as follows:

Where K_{p} is the scale factor, w_{c} is the frequency adjustment coefficient, K_{h}is the resonance coefficient, and w_{h} is the resonant frequency.

In order to achieve the 5,7,11 harmonic current compensation need to re-connect three resonant controller.

The transfer function of the current inner loop multi-resonance controller is:

The minimum value of the resonant frequency in the LCL grid-connected inverter is:

Then the minimum value of K is:

In order to ensure the stability of the control system, the cutoff frequency should be chosen to be less than so the K_{P} in the multi-resonance PR control can be approximated by the cutoff frequency f_{c}:

According to the scope of K and the specific control requirements, through the control system open-loop baud diagram for parameter adjustment.

Based on the detailed analysis of the principle of multi-resonance constant power control, the simulation results of the mathematical model are as follows

DC side voltage control after active, no power waveform

As shown above, the active and reactive power of the output after LCL multi-resonant constant power control is constant, which ensures the stable operation of the system.

By LCL multi-resonant constant power control before and after the voltage waveform

From Fig.

By LCL multi-resonant constant power control before and after the current waveform

From the above figure can be found by the LCL multi-resonant constant power control after the current waveform has improved.

In order to analyze the filtering effect by LCL multiresolution constant power control, the voltage distortion rate, the total voltage distortion rate and the current distortion rate of the respective voltage waveforms before and after the control are summarized as shown in Table

LCL MULTI-RESONANT CONSTANT POWER GRID-CONNECTED CONTROL BEFORE AND AFTER THE VOLTAGE HARMONIC CONTENT

Voltage distortion rate | current distortion rate | |
---|---|---|

76.03% | 15.23% | |

0.12% | 0.02% |

LCL MULTI-RESONANT CONSTANT POWER CONTROL BEFORE AND AFTER THE VOLTAGE AND CURRENT DISTORTION

5 | 7 | 11 | |

2.86% | 0.93% | 0.98% | |

0.08% | 0.04% | 0.01% |

From Table

Table

In this paper, a harmonic suppression strategy for micro-grid inverter combined with LCL constant power control and multi-resonant PI control is proposed for the resonant problem of LCL filter microgrid inverters. It is found that the scheme stabilizes the output power LCL and reduces the total harmonic distortion rate of the grid inverter to 0.12% and the corresponding phase current distortion rate is reduced to 0.02%. The harmonic suppression effect is obvious.