CN105071405B - Micro-grid system with unbalanced nonlinear loads and Power balance control method - Google Patents

Abstract

The invention discloses a kind of micro-grid system with unbalanced nonlinear loads and Power balance control method, system includes multiple DG units in parallel and the line impedance being connected with each DG unit respectively, line impedance is connected on micro-capacitance sensor bus by PCC points, the three-phase equilibrium resistive load of load unit, asymmetric linear load and diode rectification nonlinear load access micro-capacitance sensor bus by PCC points, it is also associated with micro-capacitance sensor bus for measuring PCC point voltage fundamental positive sequences, the measurement module of negative sequence component and harmonic component, micro-capacitance sensor bus passes sequentially through static switch and the transformer connection main power networks of 10kV.The present invention realizes the idle harmonic power equalization of micro-capacitance sensor using the selective virtual impedance based on virtual fundamental positive sequence, negative sequence impedance and virtual variable harmonic impedance, harmonic wave and unbalance voltage compensating controller can realize dividing equally for imbalance power and harmonic power, solve micro-capacitance sensor harmonic wave and voltage imbalance question.

Description

Microgrid system with asymmetric nonlinear load and power balance control method

technical field

The invention belongs to the technical field of new energy distributed power generation and micro-grid control in power systems, and relates to a micro-grid system with asymmetric nonlinear loads and a power balance control method, in particular to an island with asymmetric nonlinear loads Unbalanced reactive power and harmonic power balance control method for microgrid.

Background technique

The widespread application of new energy distributed generation (Distributed Generation, DG) systems such as photovoltaic power generation and wind power generation has made the microgrid system based on distributed power sources, energy storage devices, loads and control devices an important part of the smart grid. Different from the traditional distribution network, the microgrid system can work in both grid-connected and islanded operation modes. In the island mode, the microgrid system needs to realize power sharing between DG units. However, the power system distribution network often has a large number of unbalanced and nonlinear loads, which cause power quality problems and affect the operational stability of the microgrid system. For low-voltage micro-grid systems, GB/T 14549-1993 and GB/T 15543-2008 national standards stipulate the total harmonic distortion (Total Harmonic Distortion, THD) of the grid system public connection point (Point of Common Coupling, PCC) under normal conditions and three-phase voltage unbalance are not more than 5% and 2% respectively. Therefore, measures need to be taken to enable the microgrid system to operate reliably under unbalanced and nonlinear loads, ensure the overall performance of the system, and maintain the unbalanced reactive and harmonic power balance of the microgrid.

Active power filters are usually used in power systems to ensure the power quality of distribution networks. Series active power filters inject negative sequence and harmonic voltages into distribution lines through coupling transformers to compensate for grid voltage imbalance and harmonic components. However, for microgrids based on distributed generation systems, an additional active power filter on each DG unit will significantly increase the cost. The distributed power sources and loads in the island microgrid system are connected by voltage source inverters, so the grid voltage imbalance and harmonic components can be compensated by controlling the inverters. The Chinese patent with the authorized announcement number CN103296700B proposes to inject an appropriate proportion of harmonic and reactive current into the microgrid, thereby realizing the effective control of the harmonic and reactive current of the microgrid, but this method will cause distortion of the output voltage and does not consider the microgrid The PCC point voltage unbalance fault often occurs in the power grid system, and the calculation process is complicated, and the compensation effect is poor. The Chinese patent with the authorized announcement number CN103236702B improves the steady-state and dynamic sharing characteristics of reactive power by dynamically changing the droop coefficient and virtual impedance in real time. However, for microgrid systems with nonlinear loads, the virtual impedance at this time And the method of dynamic variable coefficient and transient variable coefficient cannot realize the equal sharing of reactive power. The Chinese patent with application publication number CN104578884A proposes a voltage unbalance control method for multi-inverter parallel connection of low-voltage microgrid, which uses robust droop control, virtual negative impedance and current deadbeat control to realize unbalanced operation of microgrid islands , but this control method is only for the case of linear asymmetric loads, it does not consider the balance of unbalanced reactive power and it is difficult to implement. Mehdi Savaghebi's article entitled "Autonomous voltage unbalance compensation in an islanded droop-controlled microgrid" published in IEEE Transactionson Industrial Electronics proposed an adaptive microgrid negative sequence compensation control method to maintain the voltage balance of the busbar at the PCC point, but the The method also only considers the case of a single linear asymmetric load, and is not suitable for the actual asymmetric nonlinear microgrid system. The Chinese patent with the authorized announcement number CN103368191B realizes the unbalanced compensation of the microgrid voltage by introducing a negative sequence reactive power Q ^- -G unbalanced droop control loop, but this method is only limited to asymmetrical linear loads. The microgrid system of the load is not applicable, and the unbalanced situation of the line impedance of the DG unit is not considered.

To sum up, the existing microgrid control technology mainly studies the single case of linear symmetric load or nonlinear load, and there is no report on the case of asymmetric nonlinear load and inconsistent line impedance. Therefore, it is necessary to study a control method that simultaneously realizes unbalanced reactive power and harmonic power balance, which can be widely used in microgrid systems with asymmetric nonlinear loads.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a method to realize reactive power and harmonic power balance of microgrid by using selective virtual impedance based on virtual fundamental wave positive sequence, negative sequence impedance and virtual variable harmonic impedance , the harmonic and unbalanced voltage compensation controller can realize the equal sharing of unbalanced power and harmonic power, solve the problem of microgrid harmonic and voltage unbalanced microgrid system with asymmetric nonlinear load and power balance control method .

The purpose of the present invention is achieved through the following technical solutions: a novel AC microgrid system, including a plurality of parallel DG units and line impedances, load units and static switches connected to each DG unit respectively, and the line impedance is passed through the PCC The load unit includes three-phase balanced resistive load, asymmetric linear load and diode rectified nonlinear load, and the three-phase balanced resistive load, asymmetric linear load and diode rectified nonlinear load all pass through the PCC point Connected to the microgrid bus, the microgrid bus is also connected with a measurement module for measuring the fundamental positive sequence, negative sequence components and harmonic components of the PCC point voltage. The microgrid bus is connected to the 10kV main grid through static switches and transformers in turn.

Further, the DG unit is composed of a renewable energy source, a three-phase full-bridge inverter, an LCL filter and a local controller of the DG unit, and the renewable energy source is connected to the three-phase full-bridge inverter and the LCL filter in sequence, The LCL filter is connected to the line impedance, and the local controller of the DG unit is connected to the measurement module through a Low Bandwidth Communication (LBC) cable, and the positive sequence, negative sequence components and harmonics of the fundamental voltage of the PCC point detected by the measurement module are The components are transmitted to the local controller of the DG unit, and the output of the local controller of the DG unit is connected to the three-phase full-bridge inverter.

A novel AC microgrid system power balance control method of the present invention comprises the following steps:

S1. Real-time detection of the three-phase full-bridge inverter side current i _Labc , three-phase output voltage v _oabc and three-phase output current i _oabc in the DG unit of the microgrid system, and convert the detected data into αβ coordinates through Clark transformation Inverter side current i _Lαβ , output voltage v _oαβ and output current i _{oαβ under} the axis;

S2. Accurately extract the fundamental wave of the output current i _oαβ through the second order generalized integral (Second Order Generalized Integrator, SOGI) and delayed signal cancellation (Delayed Signal Cancellation, DSC) module of the inverter output current i _oαβ obtained by Clark transformation Positive sequence component Fundamental negative sequence component and harmonic components with harmonic orders 5, 7, 11 and 13 with

S3, the positive sequence component of the fundamental wave of the output current obtained by S2 Fundamental negative sequence component and harmonic components with harmonic orders 5, 7, 11 and 13 with Combined with the inverter output voltage v _oαβ , the instantaneous power is calculated, and the fundamental positive-sequence active power P ⁺ and the fundamental positive-sequence reactive power Q ⁺ , fundamental harmonic power D and fundamental negative sequence unbalanced reactive power Q _N ;

S4. The fundamental positive-sequence active power P ⁺ and fundamental positive-sequence reactive power Q ⁺ obtained in S3 are controlled by the fundamental positive-sequence active-frequency (P ⁺ -ω) droop control and the positive-sequence reactive-voltage amplitude (Q ⁺ -E) Droop control, constructing the amplitude E and frequency ω component of the inverter reference voltage signal _vref,αβ , the expression of which is as follows:

Among them, ω ^* and E ^* represent rated frequency and rated voltage amplitude, P ^{+ *} and Q ^{+ *} represent the reference value of positive sequence fundamental active power and reactive power, k _p and k _q represent fundamental positive sequence active power- Frequency (P ⁺ -ω) and positive sequence reactive power-voltage amplitude (Q ⁺ -E) droop coefficient, when the microgrid is operating in an island, the values of P ^+* and Q ^+* are usually set to 0;

S5. According to the structure diagram of the harmonic and voltage unbalance compensation controller shown in FIG. 4, the output voltage v _oabc of the three-phase inverter is transformed into the voltage component under the dq coordinate axis through Park transformation, and the voltage component extracted by the PLL is compared with The local output voltage angular frequency ω _lf , each harmonic order and the low-pass filter cooperate to extract the negative sequence of the voltage fundamental wave of the three-phase inverter output voltage v _oabc and the 5th, 7th, 11th and 13th harmonic components with Construct two constant values K _N and K _H to multiply the fundamental negative-sequence unbalanced power Q _N and the fundamental harmonic Component and the sum of each harmonic component After multiplying and adding the obtained results, the compensation coefficient K _UHCR of harmonic and unbalanced voltage equalization compensation control can be obtained:

Among them, K _N and K _H are the compensation setting values of voltage unbalance and harmonic components, and harmonic and unbalanced voltage equalization compensation controllers can effectively compensate harmonics and unbalanced power; it is worth noting that K The values of _N and K _H need to ensure the stability of the microgrid system while compensating for voltage unbalance and harmonic components.

S6, the positive sequence of the constructed output current fundamental wave Fundamental negative sequence and harmonic components with harmonic orders 5, 7, 11 and 13 with Interact with the selective virtual impedance based on the virtual fundamental positive sequence impedance, the virtual fundamental negative sequence impedance and the virtual variable harmonic impedance respectively to obtain the selective virtual Impedance drop:

Selective virtual impedance drop:

Virtual fundamental wave positive sequence impedance voltage drop:

Virtual fundamental wave negative sequence impedance voltage drop:

Virtual variable harmonic impedance drop:

where, ω _f represents the fundamental angular frequency, with Denote the virtual fundamental positive sequence resistance and inductance, with Represents the virtual fundamental negative sequence resistance and inductance, R _v,h and L _v,h represent the virtual resistance and inductance of the h-order harmonic component, h=5, 7, 11, 13;

S7. Detect the PCC point voltage in real time, and extract the positive sequence of the fundamental wave of the PCC point voltage under the dq axis negative sequence and main harmonic components with The fundamental positive and negative sequences and harmonic components of the PCC point voltage are transmitted to the harmonic distortion rate calculation unit through the LBC line, and the harmonic distortion indices HD _v5± , HD _v7± , HD _v11± and HD _v13 of each voltage are calculated _± , and voltage harmonic distortion index reference value with After comparison, through the saturation limiter and proportional integral controller (Proportional Integral, PI), and then with the PCC point voltage fundamental wave positive sequence negative sequence and main harmonic components with Multiply to construct the PCC point voltage harmonic compensation value with And send it to PCC point voltage harmonic compensation controller through LBC;

S8. Extract the positive sequence of the fundamental wave of the inverter output current i _oαβ on the α axis Fundamental negative sequence And the harmonic components i _oα,5± , i _oα,7± , i _oα,11± and i _oα,13± mainly including the 5th, 7th, 11th and 13th harmonics, output each through the harmonic distortion calculation unit Each harmonic current distortion index HD _I,5± , HD _I,7± , HD _I,11± and HD _I,13± of the sub-harmonic current, compare it with the maximum value of each voltage harmonic distortion index Compare the obtained value with the PCC point voltage harmonic compensation value Multiply with the compensation coefficient CG _h± of the hth voltage harmonic, and the output value is then equal to the total power occupied by the rated power S _0,i of the DG unit Multiply the proportional coefficient and construct the harmonic compensation value of each voltage of the DG unit under the dq axis Among them, n is the number of DG units, and the coordinate transformation will be converted to Among them, θ is the phase extracted by the PCC point voltage through the PLL, and finally the compensation value of each voltage harmonic is accumulated to obtain the final PCC point voltage harmonic compensation value

S9. The inverter output voltage v _oαβ , the selective virtual impedance voltage drop v _vαβ , the reference voltage signal v _ref,αβ , the harmonic and unbalanced voltage equalization compensation coefficient K _UHCR and the PCC point voltage harmonic compensation value Add and subtract to get the inverter output voltage reference signal

The above voltage reference signal After the voltage and current double closed-loop control and space vector pulse width modulation (Space Vector Pulse Width Modulation, SVPWM) composed of the voltage controller and the current controller, the trigger signal required by the inverter H-bridge IGBT is constructed.

Further, the specific implementation method of the step S1 is as follows:

in,

Further, the specific implementation method of step S2 is as follows: firstly, the fundamental wave and harmonic components of the output current are obtained through the SOGI algorithm, and then the DSC obtains the positive and negative values of the fundamental frequency of the output current after T/4 delay for the fundamental current component. ordinal component Construct the negative sequence component of the 5th harmonic output current after T/20 delay for the 5th harmonic current Construct the positive sequence component of the 7th harmonic output current after T/28 delay for the 7th harmonic current Construct the negative sequence component of the 11th harmonic output current after T/44 delay for the 11th harmonic current The positive sequence component of the 13th harmonic output current is constructed by T/52 delay for the 13th harmonic current T is the fundamental frequency period.

Further, the specific calculation method of fundamental positive sequence active power P ⁺ , fundamental positive sequence reactive power Q ⁺ , fundamental harmonic power D and fundamental negative sequence unbalanced power Q _N in step S3 is as follows:

Among them, ω _LPF is the cut-off frequency of the low-pass filter, and v ^* represents the instantaneous value of the rated DG phase voltage.

Further, the current controller G _i (s) in the step S9 is a proportional (k _c ) current controller, and the voltage controller G _v (s) is a voltage controller with multi-resonant proportional resonance, the expression for:

Among them, k _pv represents the proportional gain of the voltage controller, k _rv represents the resonance gain of the voltage controller on the fundamental wave, k _hv represents the resonance gain of the voltage controller’s hth harmonic, and ω _c is the shear frequency of the voltage controller , ω _f is the fundamental angular frequency.

The beneficial effects of the present invention are:

1. The present invention proposes a new control method for unbalanced reactive power and harmonic power balance of the microgrid to realize the power sharing of multiple converters, using the virtual fundamental positive sequence, negative sequence impedance and virtual variable The selective virtual impedance of the variable harmonic impedance is used to realize the reactive and harmonic power balance of the microgrid. The harmonic and unbalanced voltage compensation controller can realize the equal sharing of unbalanced power and harmonic power. When the microgrid system has asymmetric nonlinear loads, the microgrid harmonics and voltage imbalance problems;

2. In the case of an isolated island microgrid with asymmetric nonlinear loads, even if the line impedance of each DG is inconsistent, the control method proposed in the present invention can also realize the equal sharing of reactive power and harmonic power among multiple parallel DG units;

3. The voltage harmonic compensation controller for the PCC point of the microgrid proposed by the present invention can compensate the main voltage harmonic components of the PCC point, control the voltage harmonics of each node of the entire microgrid island system, and effectively improve the microgrid. Improve the power quality of the grid and enhance the power sharing capability among multiple parallel converters in the microgrid;

4. The present invention can realize the balanced control of unbalanced reactive power and harmonic power only through the comprehensive control strategy of the microgrid, improve the power quality of the microgrid, reduce the investment cost of the microgrid, and improve the complex load of the island microgrid system. Stability and reliability of operation under working conditions.

Description of drawings

Fig. 1 is a topological structure diagram of the AC microgrid system of the present invention;

Fig. 2 is the circuit structure and control schematic diagram of a single DG unit of the microgrid system of the present invention;

Fig. 3 is a structure diagram of inverter voltage and current double closed-loop control with selective virtual impedance and based on DSC and SOGI current fundamental wave positive sequence, negative sequence component and harmonic component extraction modules;

Fig. 4 is a structural diagram of a harmonic and voltage unbalance compensation controller;

Fig. 5 is a schematic structural diagram of the PCC point voltage harmonic compensation controller;

Figure 6 is a structural diagram of the microgrid simulation model;

Figure 7 is a simulation waveform diagram of the output voltage and current of DG1 and DG2 using the traditional droop control strategy in the microgrid system under unbalanced linear load;

Fig. 8 is a simulation waveform diagram of DG1 and DG2 output voltage and current of DG1 and DG2 using the control strategy proposed by the present invention under unbalanced linear load;

Figure 9 is a simulation waveform diagram of the output voltage and current of DG1 and DG2 using the traditional droop control strategy in the microgrid system under unbalanced nonlinear load;

Fig. 10 is a simulation waveform diagram of DG1 and DG2 output voltage and current of the microgrid system using the control strategy proposed by the present invention under unbalanced nonlinear load;

Fig. 11 is a simulation waveform diagram of active power and reactive power of the microgrid system respectively using the traditional droop control strategy and the control strategy proposed by the present invention.

detailed description

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figure 1, a new AC microgrid system includes multiple parallel DG units and line impedances, load units and static switches connected to each DG unit respectively. The line impedance is connected to the microgrid bus through the PCC point , the load unit includes three-phase balanced resistive load, asymmetric linear load and diode rectified nonlinear load. The grid bus is also connected with a measurement module for measuring the fundamental positive sequence, negative sequence and harmonic components of the PCC point voltage. The microgrid bus is connected to the 10kV main grid through static switches and transformers in turn.

Further, the DG unit is composed of a renewable energy source, a three-phase full-bridge inverter, an LCL filter and a local controller of the DG unit, and the renewable energy source is connected to the three-phase full-bridge inverter and the LCL filter in sequence, The LCL filter is connected to the line impedance, and the local controller of the DG unit is connected to the measurement module through a Low Bandwidth Communication (LBC) cable, and the positive sequence and negative voltage fundamental waves of the PCC point (common connection point) detected by the measurement module are The sequence component and harmonic component are transmitted to the local controller of the DG unit, and the output of the local controller of the DG unit is connected to the three-phase full-bridge inverter.

As shown in Figure 2, a novel AC microgrid system power balance control method of the present invention includes the following steps:

S1. Real-time detection of the three-phase full-bridge inverter side current i _Labc , three-phase output voltage v _oabc and three-phase output current i _oabc in the DG unit of the microgrid system, and convert the detected data into αβ coordinates through Clark transformation Inverter side current i _Lαβ , output voltage v _oαβ and output current i _{oαβ under} the axis;

S2. Accurately extract the fundamental wave of the output current i _oαβ through the second order generalized integral (Second Order Generalized Integrator, SOGI) and delayed signal cancellation (Delayed Signal Cancellation, DSC) module of the inverter output current i _oαβ obtained by Clark transformation Positive sequence component Fundamental negative sequence component and harmonic components with harmonic orders 5, 7, 11 and 13 with

S3, the positive sequence component of the fundamental wave of the output current obtained by S2 Fundamental negative sequence component and harmonic components with harmonic orders 5, 7, 11 and 13 with Combined with the inverter output voltage v _oαβ , the instantaneous power is calculated, and the fundamental positive-sequence active power P ⁺ and the fundamental positive-sequence reactive power Q ⁺ , fundamental harmonic power D and fundamental negative sequence unbalanced reactive power Q _N ;

S4. The fundamental positive-sequence active power P ⁺ and fundamental positive-sequence reactive power Q ⁺ obtained in S3 are controlled by the fundamental positive-sequence active-frequency (P ⁺ -ω) droop control and the positive-sequence reactive-voltage amplitude (Q ⁺ -E) Droop control, constructing the amplitude E and frequency ω component of the inverter reference voltage signal _vref,αβ , the expression of which is as follows:

Among them, ω ^* and E ^* represent rated frequency and rated voltage amplitude, P ^{+ *} and Q ^{+ *} represent the reference value of positive sequence fundamental active power and reactive power, k _p and k _q represent fundamental positive sequence active power- Frequency (P ⁺ -ω) and positive sequence reactive power-voltage amplitude (Q ⁺ -E) droop coefficient, when the microgrid is operating in an island, the values of P ^+* and Q ^+* are usually set to 0;

S5. Convert the output voltage v _oabc of the three-phase inverter into a voltage component under the dq coordinate axis through Park transformation, and compare it with the angular frequency ω _lf of the local output voltage extracted through the PLL, the order of each harmonic and the low-pass filter Cooperate to extract the voltage fundamental wave negative sequence of the three-phase inverter output voltage v _oabc and the 5th, 7th, 11th and 13th harmonic components with Construct two constant values K _N and K _H to multiply the fundamental negative-sequence unbalanced power Q _N and the fundamental harmonic Component and the sum of each harmonic component After multiplying and adding the obtained results, the compensation coefficient K _UHCR of harmonic and unbalanced voltage equalization compensation control can be obtained:

Among them, K _N and K _H are the compensation setting values of voltage unbalance and harmonic components, and harmonic and unbalanced voltage equalization compensation controllers can effectively compensate harmonics and unbalanced power; it is worth noting that K The values of _N and K _H need to ensure the stability of the microgrid system while compensating for voltage unbalance and harmonic components.

S6, the positive sequence of the constructed output current fundamental wave Fundamental negative sequence and harmonic components with harmonic orders 5, 7, 11 and 13 with Interact with the selective virtual impedance based on the virtual fundamental positive sequence impedance, the virtual fundamental negative sequence impedance and the virtual variable harmonic impedance respectively to obtain the selective virtual Impedance voltage drop; the voltage and current double closed-loop control block diagram shown in Figure 3 describes in detail the structure of the positive sequence, negative sequence components and harmonic components of the fundamental wave of the output current and the selective virtual impedance. As shown in Figure 3, by the virtual fundamental wave positive sequence impedance voltage drop Virtual fundamental wave negative sequence impedance voltage drop The expression of the selective virtual impedance voltage drop v _vαβ composed of virtual variable harmonic impedance voltage drop v _vαβ,h is:

Selective virtual impedance drop:

Virtual fundamental wave positive sequence impedance voltage drop:

Virtual fundamental wave negative sequence impedance voltage drop:

Virtual variable harmonic impedance drop:

where, ω _f represents the fundamental angular frequency, with Denote the virtual fundamental positive sequence resistance and inductance, with Represents the virtual fundamental negative sequence resistance and inductance, R _v,h and L _v,h represent the virtual resistance and inductance of the h-order harmonic component, h=5, 7, 11, 13;

S7. Aiming at the presence of a large number of harmonic components in the PCC point voltage of the microgrid under nonlinear load conditions, the present invention adopts the control method shown in FIG. 2 and FIG. 5 to realize harmonic compensation of the PCC point voltage. As shown in Figure 2, the PCC point voltage is detected in real time, and the fundamental positive sequence of the PCC point voltage under the dq axis is extracted negative sequence and main harmonic components with The fundamental positive and negative sequences and harmonic components of the PCC point voltage are transmitted to the harmonic distortion rate calculation unit through the LBC (Low Bandwidth Communication) line, and the harmonic distortion indices HD _v5± , HD _v7± , HD of each voltage are calculated _v11± and HD _v13± , with voltage harmonic distortion index reference value with After comparison, through the saturation limiter and proportional integral controller (ProportionalIntegral, PI), and then with the positive sequence of the PCC point voltage fundamental wave negative sequence and main harmonic components with Multiplied together, the PCC point voltage harmonic compensation value shown in Figure 2 is constructed with And sent to the PCC point voltage harmonic compensation controller shown in Figure 5 through the LBC;

S8. Extract the positive sequence of the fundamental wave of the inverter output current i _oαβ on the α axis Fundamental negative sequence And the harmonic components i _oα,5± , i _oα,7± , i _oα,11± and i _oα,13± mainly including the 5th, 7th, 11th and 13th harmonics, output each through the harmonic distortion calculation unit Each harmonic current distortion index HD _I,5± , HD _I,7± , HD _I,11± and HD _I,13± of the sub-harmonic current, compare it with the maximum value of each voltage harmonic distortion index Compare the obtained value with the PCC point voltage harmonic compensation value Multiply with the compensation coefficient CG _h± of the hth voltage harmonic, and the output value is then equal to the total power occupied by the rated power S _0,i of the DG unit Multiply the proportional coefficient and construct the harmonic compensation value of each voltage of the DG unit under the dq axis Among them, n is the number of DG units, and the coordinate transformation will be converted to Among them, θ is the phase extracted by the PCC point voltage through the PLL (Phase Locked Loop), and finally the compensation value of each voltage harmonic is accumulated to obtain the final PCC point voltage harmonic compensation value As shown in Figure 5, the PCC point voltage harmonic compensation value is:

S9. The inverter output voltage v _oαβ , the selective virtual impedance voltage drop v _vαβ , the reference voltage signal v _ref,αβ , the harmonic and unbalanced voltage equalization compensation coefficient K _UHCR and the PCC point voltage harmonic compensation value Add and subtract to get the inverter output voltage reference signal

The above voltage reference signal After the double closed-loop control of voltage and current composed of voltage controller and current controller and space vector pulse width modulation (Space Vector Pulse Width Modulation, SVPWM), the trigger signal required by the inverter H-bridge IGBT is constructed, as shown in Figure 3 It shows that there is a delay G _d (s)=1/(1+1.5T _s s) in the process of SVPWM processing signal transmission and calculation in the microgrid system, and T _s is the sampling period.

Further, the specific implementation method of the step S1 is as follows:

in,

Further, the specific implementation method of step S2 is as follows: firstly, the fundamental wave and harmonic components of the output current are obtained through the SOGI algorithm, and then the DSC obtains the positive and negative values of the fundamental frequency of the output current after T/4 delay for the fundamental current component. ordinal component Construct the negative sequence component of the 5th harmonic output current after T/20 delay for the 5th harmonic current Construct the positive sequence component of the 7th harmonic output current after T/28 delay for the 7th harmonic current Construct the negative sequence component of the 11th harmonic output current after T/44 delay for the 11th harmonic current The positive sequence component of the 13th harmonic output current is constructed by T/52 delay for the 13th harmonic current T is the fundamental frequency period.

Further, the specific calculation method of fundamental positive sequence active power P ⁺ , fundamental positive sequence reactive power Q ⁺ , fundamental harmonic power D and fundamental negative sequence unbalanced reactive power Q _N in step S3 for:

Among them, ω _LPF is the cut-off frequency of the low-pass filter, and v ^* represents the instantaneous value of the rated DG phase voltage.

Further, the current controller G _i (s) in the step S9 is a proportional (k _c ) current controller, and the voltage controller G _v (s) is a voltage controller with multi-resonant proportional resonance, the expression for:

Among them, k _pv represents the proportional gain of the voltage controller, k _rv represents the resonance gain of the voltage controller on the fundamental wave, k _hv represents the resonance gain of the voltage controller’s hth harmonic, and ω _c is the shear frequency of the voltage controller , ω _f is the fundamental angular frequency.

In the microgrid system analyzed above, the implementation of unbalanced reactive power and harmonic power balance control for one DG unit in the microgrid system is applicable to the case of multiple DG units connected in parallel.

In order to verify the feasibility of the method proposed in the present invention, as shown in Figure 6, a microgrid simulation platform containing two DG units under nonlinear load and asymmetric linear load conditions was built on Matlab/Simulink. The diode rectified nonlinear load and linear asymmetric load are connected to the PCC through switch 1 and switch 2; the IGBT switching frequency of the inverter H bridge is 10kHz, the DC side voltage of the two inverters is 650V, and the two DG The inverter side inductance L ₁ and L ₂ of the unit LCL filter are both 1.8mH, the filter capacitors C ₁ and C ₂ are both 25μF, the filter output inductance L _o1 and L _o2 are both 1.8mH; the line impedance 1 The inductance L _f1 is 3mH, the resistance R _f1 is 0.2Ω, the inductance L _f2 of the line impedance 2 is 1mH, the resistance R _f2 is 0.2Ω; the asymmetrical load R _UL is 230Ω, the nonlinear load inductance L _NL of the three-phase diode rectifier bridge is 84μF, resistor R _NL is 460Ω, and capacitor C _NL is 235μF.

In Fig. 6, switch 1 is open and switch 2 is closed. Under the condition of unconnected asymmetrical load of phase C, the three-phase output voltage and current simulation waveforms of DG1 and DG2 before and after compensation are shown in Fig. 7 and Fig. 8, respectively. , where v _o1abc and v _o2abc represent the three-phase output voltages of DG1 and DG2 respectively, and i _o1abc and i _o2abc represent the three-phase output currents of DG1 and DG2 respectively. The simulation waveform when unbalanced voltage compensation and selective virtual impedance are not used is shown in Figure 7, the output currents of phase a and b of DG1 should be i _o1a and i _o1b much smaller than the output currents of phase a and b of DG2 i _o2a and i _o2b , and the amplitude of the c-phase output current i _o1c of DG1 is relatively large, there are relatively large circulating currents and unbalanced currents between DG1 and DG2, and the unbalanced power is not equally divided; using the unbalanced voltage compensation and selection of the present invention The simulation waveform after the permanent virtual impedance is shown in Figure 8. At this time, the output currents of DG1 and DG2 are consistent, and the circulating current is effectively suppressed, realizing the unbalanced reactive power balance of the microgrid system.

In the case of asymmetrical linear loads and nonlinear loads acting at the same time, that is, switch 1 and switch 2 are closed at the same time, the simulation waveforms of the three-phase output voltage and current of DG1 and DG2 before and after compensation are shown in Figure 9 and Figure 10, respectively . It can be seen from Figure 9 that when harmonic and unbalanced voltage compensation and selective virtual impedance are not added, since the line impedance of DG1 is small, it distributes more load current, and there is an imbalance of unbalanced reactive power and harmonic power. sub-problem; when the selective virtual impedance based on virtual fundamental wave positive sequence, negative sequence impedance and virtual variable harmonic impedance and harmonic unbalanced voltage compensation controller are applied to the microgrid system, the three-phase output voltage and current simulation As shown in the waveform diagram 10, the output current waveforms of DG1 and DG2 are consistent, and the circulating current is effectively suppressed, indicating that the control method proposed in the present invention can effectively solve the power sharing problem of the microgrid under nonlinear and asymmetric loads, and realize the microgrid system Unbalanced reactive power and harmonic power balance control. In addition, the harmonics of the DG1 and DG2 three-phase output voltage waveforms shown in Figure 9 and Figure 10 are effectively suppressed under the action of the PCC point harmonic compensator and the multi-resonance controller.

Fig. 11 shows the simulated waveforms of active power and reactive power of the microgrid system respectively using the traditional droop control strategy and the control strategy proposed by the present invention. It can be seen from the figure that when the traditional droop control strategy is used in 0~3s, the reactive power Q1 of _DG1 is about 100Var, and the reactive power _Q2 of DG2 is about -40Var, and the total reactive power of the microgrid is not evenly divided; At t=3s, after switching the control strategy proposed by the present invention, Q ₁ =Q ₂ ≈30Var, and the total reactive power of the system is balancedly controlled, further verifying the effectiveness and feasibility of the method proposed by the present invention.

The foregoing is only a specific embodiment of the present invention, and those skilled in the art will understand that within the technical scope disclosed in the present invention, various modifications, replacements and changes can be made to the present invention, so the present invention should not be limited by the above-mentioned Instances are limited.

Those skilled in the art will appreciate that the embodiments described here are to help readers understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical revelations disclosed in the present invention without departing from the essence of the present invention, and these modifications and combinations are still within the protection scope of the present invention.

Claims (5)

1. The method is characterized by comprising a plurality of DG units connected in parallel, and a line impedance, a load unit and a static switch which are respectively connected with each DG unit, wherein the line impedance is connected to a microgrid bus through a PCC (point of common coupling), the load unit comprises a three-phase balanced resistive load, an asymmetric linear load and a diode rectification nonlinear load, the three-phase balanced resistive load, the asymmetric linear load and the diode rectification nonlinear load are all connected to the microgrid bus through the PCC, the microgrid bus is also connected with a measurement module for measuring fundamental wave positive sequence, negative sequence components and harmonic components of the voltage of the PCC, and the microgrid bus is connected with a 10kV main power grid through the static switch and a transformer in sequence;

the DG unit is composed of a renewable energy source, a three-phase full-bridge inverter, an LCL filter and a DG unit local controller, wherein the renewable energy source is sequentially connected with the three-phase full-bridge inverter and the LCL filter, the LCL filter is connected with a line impedance, the DG unit local controller is connected with a measurement module through a low-bandwidth communication cable LBC, a voltage fundamental wave positive sequence component, a negative sequence component and a harmonic component of a PCC point detected by the measurement module are transmitted to the DG unit local controller, and the output of the DG unit local controller is connected with the three-phase full-bridge inverter;

the method comprises the following steps:

s1, detecting three-phase full-bridge inverter side current i in microgrid system DG unit in real time_LabcThree-phase output voltage v_oabcAnd three-phase output current i_oabcAnd converting the detected data into an inverter side current i in αβ coordinate axis by Clark conversion_LαβOutput voltage v_oαβAnd an output current i_oαβ；

S2 inverter output current i obtained by Clark conversion_oαβOutput current i is accurately extracted through second-order generalized integral and delay signal cancellation module_oαβFundamental positive sequence component ofFundamental negative sequence componentAnd harmonic components of harmonic orders 5, 7, 11 and 13And

s3, obtaining the output current fundamental wave positive sequence component obtained in S2Fundamental negative sequence componentAnd harmonic components of harmonic orders 5, 7, 11 and 13Andand the inverter output voltage v_oαβCombining, calculating instantaneous power, and obtaining fundamental positive sequence active power P output by the filter through a low-pass filter⁺Fundamental wave positive sequence reactive power Q⁺Fundamental harmonic power D and fundamental negative sequence unbalanced reactive power Q_N；

S4, obtaining the fundamental wave positive sequence active power P from S3⁺Sum fundamental positive sequence reactive power Q⁺Constructing a reference voltage signal v of the inverter by fundamental positive sequence active-frequency droop control and positive sequence reactive-voltage amplitude droop control_ref,αβThe amplitude E and frequency ω components of (a) are expressed as follows:

$\left\{\begin{array}{c}\mathrm{\ω}={\mathrm{\ω}}^{*}-k_p(P^{+}-P^{+*})\\ E=E^{*}-k_q(Q^{+}-Q^{+*})\end{array}\right.$

wherein, ω is^*And E^*Representing nominal frequency and nominal voltage amplitude, P^+*And Q^+*Reference value, k, representing the positive sequence fundamental active and reactive power_pAnd k_qRepresenting fundamental positive sequence active-frequency and positive sequence reactive-voltage amplitude droop coefficients;

s5, outputting voltage v by the three-phase inverter_oabcConverting the voltage component into voltage component under dq coordinate axis through Park conversion, and extracting local output voltage angular frequency omega through PLL_lfExtracting the output voltage v of the three-phase inverter by matching the harmonic frequency and the low-pass filter_oabcVoltage fundamental negative sequence ofAnd 5, 7, 11 and 13 th harmonic componentsAndconstruct two constant values K_NAnd K_HPower Q unbalanced with fundamental negative sequence_NAnd the fundamental harmonic power D are respectively multiplied, and the obtained values are respectively multiplied with the fundamental negative sequence of the output voltageSum of the components and the sub-harmonic componentsMultiplying, and adding the obtained results to obtain the compensation coefficient K for harmonic and unbalanced voltage balance compensation control_UHCR：

$K_{UHCR}=K_N\·Q_N\·v_{o\mathrm{\α}\mathrm{\β},f}^{-}+K_H\·D\·\underset{h=5,7,11,13}{\Σ}{v}_{o\mathrm{\α}\mathrm{\β},h}^{\±}$

Wherein, K_NAnd K_HCompensation set values for voltage imbalance and harmonic components;

s6, positive sequence of output current fundamental wave constructedNegative sequence of fundamental waveAnd harmonic components of harmonic orders 5, 7, 11 and 13Andand respectively interacting with selective virtual impedances based on virtual fundamental positive sequence impedance, virtual fundamental negative sequence impedance and virtual variable harmonic impedance to obtain selective virtual impedance voltage drop for realizing reactive power and harmonic power balance of the microgrid:

selective virtual impedance drop:

virtual fundamental positive sequence impedance voltage drop:

virtual fundamental negative sequence impedance voltage drop:

virtual variable harmonic impedance drop:

wherein, ω is_fWhich represents the angular frequency of the fundamental wave,andrepresenting a virtual fundamental positive sequence resistance and inductance,andrepresenting virtual fundamental negative sequence resistance and inductance, R_v,hAnd L_v,hRepresents h harmonic component virtual resistance and inductance, h is 5, 7, 11, 13;

s7, detecting the PCC point voltage in real time, and extracting the fundamental wave positive sequence of the PCC point voltage under the dq axisNegative sequenceAnd major harmonic componentsAndthe fundamental positive and negative sequence and harmonic components of the PCC point voltage are transmitted to a harmonic distortion rate calculation unit through an LBC line, and harmonic distortion indexes HD of all sub-voltages are calculated_v5±、HD_v7±、HD_v11±And HD_v13±And voltage harmonic distortion index reference valueAndcompared with the PCC, the PCC is in positive sequence with the fundamental wave of the PCC point voltageNegative sequenceAnd major harmonic componentsAndmultiplying to construct the harmonic compensation value of the PCC point voltageAndand transmitting the voltage harmonic compensation signal to a PCC point voltage harmonic compensation controller through the LBC;

s8, extracting inverter output current i_oαβFundamental positive sequence at α axesNegative sequence of fundamental waveAnd a harmonic component i mainly containing harmonics of 5, 7, 11 and 13 th order_oα,5±、i_oα,7±、i_oα,11±And i_oα,13±The harmonic distortion factor calculation unit outputs the harmonic current distortion index HD of the harmonic current_I,5±、HD_I,7±、HD_I,11±And HD_I,13±It is compared with the maximum value of harmonic distortion index of each voltageComparing the obtained value with the PCC point voltage harmonic compensation valueAnd h-order voltage harmonic compensation coefficient CG_h±Multiplying the output value by the rated power S of the DG unit_0,iTotal power occupiedMultiplying the proportional coefficients and constructing the harmonic compensation value of each sub-voltage of the DG unit under the dq axisWherein n is the number of DG units, byThe coordinate transformation willIs converted intoTheta is the phase extracted by the PCC point voltage through PLL, and the final harmonic compensation value of the PCC point voltage can be obtained by accumulating the compensation values of the voltage harmonics of each time

$C_{\mathrm{\α}\mathrm{\β}}^{*}=\underset{h=5,7,11,13}{\mathrm{\Σ}}\{({\mathrm{HD}}_{I,h\±}^{\max }-{\mathrm{HD}}_{I,h\±})\·D_{dq}^{h\±}\·{\mathrm{CG}}_{h\±}\·\frac{S_{0,i}}{\underset{j}{\overset{n}{\mathrm{\Σ}}}{S}_{0,j}}\·\left[\begin{array}{cc}\cos \mathrm{\θ}& -\sin \mathrm{\θ}\\ \sin \mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\};$

S9, outputting the voltage v by the inverter_oαβSelective virtual impedance voltage drop v_vαβReference voltage signal v_ref,αβHarmonic and unbalanced voltage balance compensation coefficient K_UHCRAnd PCC point voltage harmonic compensation valueAdding or subtracting to obtain the reference signal of the output voltage of the inverter

$v_{o\mathrm{\α}\mathrm{\β}}^{*}=C_{\mathrm{\α}\mathrm{\β}}^{*}-K_{UHCR}+v_{ref,\mathrm{\α}\mathrm{\β}}-v_{v\mathrm{\α}\mathrm{\β}}-v_{o\mathrm{\α}\mathrm{\β}}$

Reference the voltage to a signalAnd a trigger signal required by the H-bridge IGBT of the inverter is constructed after voltage and current double closed-loop control and space vector pulse width modulation which are composed of a voltage controller and a current controller.

2. The microgrid system power balancing control method with an asymmetric nonlinear load as claimed in claim 1, wherein the step S1 is implemented by the following method:

$i_{Labc}=\sqrt{\frac{2}{3}}\left[\begin{array}{cc}1& 0\\ -\frac{1}{2}& \frac{\sqrt{3}}{2}\\ -\frac{1}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\·i_{L\mathrm{\α}\mathrm{\β}},v_{oabc}=\sqrt{\frac{2}{3}}\left[\begin{array}{cc}1& 0\\ -\frac{1}{2}& \frac{\sqrt{3}}{2}\\ -\frac{1}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\·v_{o\mathrm{\α}\mathrm{\β}},i_{oabc}=\sqrt{\frac{2}{3}}\left[\begin{array}{cc}1& 0\\ -\frac{1}{2}& \frac{\sqrt{3}}{2}\\ -\frac{1}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\·i_{o\mathrm{\α}\mathrm{\β}}$

wherein,

3. the microgrid system power balancing control method with an asymmetric nonlinear load as claimed in claim 2, wherein the step S2 is implemented by the following method: firstly, obtaining output current fundamental wave and each subharmonic component through an SOGI algorithm, then, obtaining positive and negative sequence components of output current fundamental wave frequency through DSC (differential scanning calorimetry) on the fundamental wave current component through T/4 time delayConstructing the negative sequence component of the 5 th harmonic output current by T/20 time delay on the 5 th harmonic currentConstructing the positive sequence component of the 7 th harmonic output current by T/28 time delay on the 7 th harmonic currentConstructing negative sequence component of 11 th harmonic output current by T/44 time delay on 11 th harmonic currentConstructing the positive sequence component of the 13 th harmonic output current by the T/52 time delay of the 13 th harmonic currentT is the fundamental frequency period.

4. The microgrid system power balancing control method with asymmetric nonlinear loads as claimed in claim 3, characterized in that the fundamental positive sequence active power P in step S3⁺Fundamental wave positive sequence reactive power Q⁺Harmonic of fundamental wavePower D and fundamental negative sequence unbalanced power Q_NThe specific calculation method comprises the following steps:

$P^{+}=\frac{{\mathrm{\ω}}_{LPF}}{S+{\mathrm{\ω}}_{LPF}}(v_{o\mathrm{\α}}{i}_{o\mathrm{\α},f}^{+}+v_{o\mathrm{\β}}{i}_{o\mathrm{\β},f}^{+})$

$Q^{+}=\frac{{\mathrm{\ω}}_{LPF}}{s+{\mathrm{\ω}}_{LPF}}(v_{o\mathrm{\β}}{i}_{o\mathrm{\α},f}^{+}-v_{o\mathrm{\α}}{i}_{o\mathrm{\β},f}^{+})$

$D=v^{*}\sqrt{{\left(i_{o\mathrm{\α},5}^{-}\right)}^2+{\left(i_{o\mathrm{\β},5}^{-}\right)}^2+{\left(i_{o\mathrm{\α},7}^{+}\right)}^2+{\left(i_{o\mathrm{\β},7}^{+}\right)}^2+{\left(i_{o\mathrm{\α},11}^{-}\right)}^2+{\left(i_{o\mathrm{\β},11}^{-}\right)}^2+{\left(i_{o\mathrm{\α},13}^{+}\right)}^2+{\left(i_{o\mathrm{\β},13}^{+}\right)}^2}$

$Q_N=v^{*}\sqrt{{\left(i_{o\mathrm{\α},f}^{-}\right)}^2+{\left(i_{o\mathrm{\β},f}^{-}\right)}^2}$

wherein, ω is_LPFIs the cut-off frequency, v, of the low-pass filter^*Representing the nominal DG phase voltage instantaneous value.

5. The microgrid system power balancing control method with asymmetric nonlinear loads as claimed in claim 3, wherein the current controller G in the step S9_i(s) is a proportional current controller, a voltage controller G_v(s) is a voltage controller containing multi-resonance ratio resonance, and the expression is as follows:

$G_v\left(s\right)=k_{pv}+\frac{2k_{rv}{\mathrm{\ω}}_{c}s}{s^2+2{\mathrm{\ω}}_{c}s+{{\mathrm{\ω}}_f}^2}+\underset{h=5,7,11,13}{\Σ}\frac{2k_{hv}{\mathrm{\ω}}_{c}s}{s^2+2{\mathrm{\ω}}_{c}s+{\left({\mathrm{\ω}}_{f}h\right)}^2}$

wherein k is_pvIndicating the proportional gain, k, of the voltage controller_rvRepresenting the resonant gain, k, of the voltage controller on the fundamental wave_hvRepresenting the resonant gain, omega, of the h harmonic of the voltage controller_cIs the shear frequency, omega, of a voltage controller_fIs the fundamental angular frequency.