CN108494007A - Virtual synchronous generator control method based on direct Power Control when unbalanced source voltage - Google Patents

Abstract

Virtual synchronous generator control method based on direct Power Control when a kind of unbalanced source voltage obtains network voltage and grid-connected current positive-sequence component, inverter output instantaneous power and mean instantaneous power first with ROGI；Using mean instantaneous power as feedback quantity, the reference modulation signal under α β static coordinates is obtained using traditional virtual Strategy For Synchronization Control；Direct Power Control ring is established using vector scale integral resonance controller, using output instantaneous power as feedback quantity, by the way that different power command values is respectively set, obtains correcting modulated signal using direct Power Control；By reference modulation signal and modulated signal superposition is corrected, final modulated signal is obtained, is sent into SVPWM modulation modules, realizes that output current is sinusoidal and balance, output power are constant without two independent control targets of fluctuation respectively.Present invention preserves the original control characteristics of VSG, are not necessarily to phaselocked loop, improve system dynamic response capability, be easy to Project Realization.

Description

Virtual synchronous generator control based on direct power control when grid voltage is unbalanced method

technical field

The invention belongs to the technical field of distributed power generation, and relates to a virtual synchronous generator control method when the grid voltage is unbalanced.

Background technique

The development of renewable new energy and the improvement of the utilization efficiency of renewable energy are the inevitable ways to solve human energy problems. Renewable energy needs to be connected to the grid through power electronic converters, with the goal of injecting maximum power into the grid. When the installed capacity of distributed generation is relatively low, the original traditional synchronous generators in the system can provide sufficient support for the power system, and the access of distributed generation units will not pose a threat to the stability of the power system. Since the distributed generation units interfaced with power electronic converters do not have the inertia and damping of synchronous generators, with the large-scale access of distributed generation units, the installed ratio of traditional synchronous generators gradually decreases, resulting in The rotating reserve capacity and the moment of inertia are relatively reduced. At this time, the power system is easily affected by power fluctuations and system failures, resulting in system instability. As the penetration rate of distributed renewable energy continues to increase, distributed generation units will pose a greater threat to the stability of the power system. The virtual synchronization technology realizes the synchronous machine of the power electronic converter, so that the distributed power supply can have the internal mechanism and external characteristics of the synchronous machine, so that it can participate in the regulation of the power grid autonomously like the conventional synchronous machine.

The distributed generation unit is installed at the end of the power grid, far away from the main power network. The power grid is fragile. At the same time, the power grid environment is relatively complex, with high harmonic content of the power grid voltage and large grid impedance, which is prone to grid voltage imbalance faults. These non-ideal grid environments will directly affect the power quality and stable operation of the virtual synchronous generator, and also have a negative impact on the safe and stable operation of the grid. Therefore, studying the operation mechanism and optimal control strategy of virtual synchronous generators in non-ideal grid environments has important theoretical and practical significance for improving the viability of virtual synchronous generators, improving the quality of output power, and expanding the application of virtual synchronous generators. .

Contents of the invention

The purpose of the present invention is to realize two independent control objectives of grid-connected virtual synchronous generator output power without fluctuation, output current three-phase balance and sinusoidal when the grid voltage is unbalanced, and propose a two-phase static coordinates based on direct power control Virtual Synchronous Generator Control Method.

The present invention is achieved through the following technical solutions.

A virtual synchronous generator control method based on direct power control when the grid voltage is unbalanced according to the present invention is characterized in that it comprises the following steps:

(1) Use the Reduced Order Generalized Integrator (Reduced Order Generalized Integrator, ROGI) to separate the grid voltage and the positive sequence component of the grid-connected current; calculate the instantaneous power and average instantaneous power;

(2) Substituting the average instantaneous power into the traditional virtual synchronous control strategy (Virtual Synchronous Generator, VSG) to obtain the reference modulation signal;

(3) Establish a direct power control loop. By setting the power command value when the grid voltage is unbalanced, the corrected modulation signal when the output power has no fluctuation, the output current is sinusoidal, and the three-phase balance is obtained through the direct power control loop. ;

(4) Superimpose the reference modulation signal and the correction modulation signal, and send them to the Space Vector Pulse Width Modulation (SVPWM) module to obtain the control signal of the switching device.

Further, the use of ROGI to separate the grid voltage and grid-connected current positive sequence component described in step (1) The method of calculating instantaneous power and average instantaneous power is:

(1-1) The voltage and current positive sequence components are obtained by using discretized ROGI, and the expression of discretized ROGI is shown in formula (1).

In the formula, ω is the angular frequency, T _s is the sampling period, u _α ⁺ is the positive sequence component of the voltage α axis after ROGI processing, u _β ⁺ is the positive sequence component of the voltage β axis after ROGI processing, e _α ⁺ is the positive sequence component of the voltage α axis sent to ROGI, e _β ⁺ is the positive sequence component of the voltage β axis sent to ROGI, the subscripts α and β represent the α and β components under the two-phase stationary αβ coordinates, and the superscript + represents positive sequence component, n is the nth sample.

(1-2) Calculate the instantaneous power P _g , Q _g , and the average instantaneous power P ₀ , Q ₀ , and the calculation formulas are shown in formulas (2) and (3).

In the formula, e _α and e _β are the α and β axis components of the grid voltage in the two-phase stationary αβ coordinate system, i _α and i _β are the α and β axis components of the output current in the two-phase stationary αβ coordinate system, and the superscript + indicates a positive sequence component.

Further, the method for obtaining the reference modulation signal by substituting the average instantaneous power into the traditional VSG control described in step (2) is:

(2-1) Substitute the active power command P _ref and the average instantaneous power P ₀ into formula (4) to obtain the voltage phase.

In the formula, J is the virtual moment of inertia, D _p is the damping coefficient, ω is the angular frequency, ω _n is the rated angular frequency, and θ is the phase angle.

(2-2) Substitute the reactive power command Q _ref and the average instantaneous power Q ₀ into formula (5) to obtain the voltage amplitude.

U＝K∫(Q _ref -Q ₀ )dt (5)

In the formula, K is the integral coefficient, and U is the effective value of the output voltage.

(2-3) Take the output of the active link as the frequency and phase of the modulating wave, and the output of the reactive link as the amplitude of the modulating wave, and synthesize the reference three-phase modulating voltages u _am , _ubm , u _cm according to formula (6), and pass through Clark ( clark) transformation to obtain the reference modulation voltage u _α , u _β in the two-phase stationary coordinates, and the clark transformation formula is shown in formula (7).

Further, the establishment of the direct power control loop described in step (3), by setting the power command value when the grid voltage is unbalanced, two different control objectives of stable output power, sinusoidal output current and three-phase balance are obtained through the direct power control loop The method of correcting the modulation signal when the grid voltage is unbalanced by using direct power control is as follows:

(3-1) The direct power control loop is established by using a vector proportional integral (Vector Proportional Integral, VPI) resonant controller. The expression of the VPI controller is shown in formula (8).

In the formula, K _p and K _i are the control parameters of double-frequency VPI controller, ω _c is the bandwidth of VPI control, and ω _n is the rated angular frequency.

(3-2) In order to ensure that the output power is stable when the grid voltage is unbalanced, set the power command value of direct power control P _com =0, Q _com =0, and obtain the output results u′ _{α_com} , u′ of the output power stable control target _{beta_com} .

(3-3) In order to ensure that the output current is sinusoidal and three-phase balanced when the grid voltage is unbalanced, reset the direct power control power command values P _com and Q _com to obtain the output result u′ of the control target when the current is sinusoidal and three-phase balanced _{α_com} , u′ _{β_com} , to achieve sinusoidal current and three-phase balance, set the power command value according to formula (9).

In the formula, P _g and Q _g are the output instantaneous power, P ₀ and Q ₀ are the output average instantaneous power.

(3-4) Convert the output signal of the direct power control into a modified modulation signal through the conversion matrix M, the conversion equation is shown in formula (10), and M is shown in formula (11).

In the formula, u _{α_com} and u _{β_com} are the modified modulation signals in the αβ coordinate system, u′ _{α_com} and u′ _{β_com} are the direct power control output in the αβ coordinate system respectively, and e _α and e _β are the two-phase static αβ coordinate system The α and β axis components of the lower grid voltage, i _α and i _β are the α and β axis components of the output current in the two-phase static αβ coordinate system.

Further, as described in step (4), the reference modulation signal and the correction modulation signal are superimposed, and sent to the SVPWM modulation module to obtain the control signal of the switching device.

Features and beneficial effects of the present invention:

(1) When the grid voltage is unbalanced, the traditional virtual synchronous control technology is used to obtain the reference modulation signal; the direct power control loop is established, and the power command value of the direct power control is set when the grid voltage is unbalanced, and obtained through the direct power control loop The modified modulation signal when the output power is stable, the output current is three-phase balanced, and the sinusoidal two different control targets are achieved; the reference modulation signal and the modified modulation signal are superimposed to obtain the final modulation signal.

(2) When the grid voltage is unbalanced, the improved virtual synchronous generator control method based on direct power control does not change the VSG control structure, retains the original control characteristics of the VSG, and is implemented in a two-phase stationary coordinate system without a phase-locked loop , improve the dynamic response capability of the system, and be easy to implement in engineering.

Description of drawings

Figure 1 is the overall block diagram of the VSG control inverter.

Accompanying drawing 2 system control flowchart.

Accompanying drawing 3 is VSG control block diagram.

Accompanying drawing 4 is a direct power control block diagram.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and working principles.

As shown in Figure 1, in the figure, U _dc is the DC side voltage, R _s , L _s and C are the internal resistance of the filter inductor, filter inductor and filter capacitor respectively; _ia , _ib and _ic are the output current of the inverter ; u _a , u _b and u _c are the output three-phase voltages of the inverter side; e _a , e _b and e _c are the three-phase grid voltages; L _g is the line inductance; P ^* and Q ^* are active power and reactive power respectively Power given value; P _g , Q _g are measured values of instantaneous active and reactive power output by the inverter, and P ₀ and Q ₀ are measured values of average instantaneous active and reactive power output by the inverter. The present invention is a virtual synchronous generator control technology based on direct power control when the grid voltage is unbalanced. When the grid voltage is unbalanced, the voltage amplitude and phase are obtained through the VSG control algorithm from the given power value and the average instantaneous power. The base modulation signal is synthesized; the direct power control loop is designed, and when the grid voltage is unbalanced, different correction modulation signals are obtained by setting the direct power control power command value; the reference modulation signal and the correction modulation signal are superimposed to obtain the final modulation signal, Send it to the SVPWM link to get the control signal of the switching device, and realize the control goal by controlling the switching device on and off.

As shown in Figure 2, the control method of the embodiment of the present invention includes the following steps:

1. Sampling and obtaining the inverter side output three-phase current i _abc and three-phase grid voltage e _abc ,

2. Discretize the ROGI function, and the transfer function of the ROGI continuous domain is:

In the formula, k is used to adjust the separation speed.

According to formula (1), the continuous domain transfer function relationship between u _αβ ⁺ , u _αβ ^- and u _αβ is obtained as:

In the formula, the subscript αβ indicates the component in the two-phase static αβ coordinate, and the superscript + indicates the positive sequence component.

The formula (2) is simplified by using the orthogonal relationship of the αβ axis, taking the positive sequence component as an example:

In the formula, e _αβ ⁺ is the input component of positive sequence ROGI, and u _αβ ⁺ is the output component of ROGI.

Multiplying the left and right sides of equation (3) by s-jω, we can get:

Tustin bilinear transformation is used for discretization processing, Tustin transformation is shown in formula (5), and T _s is the sampling period.

The discretized ROGI expression is shown in formula (6).

In the formula, ω is the angular frequency, T _s is the sampling period, u _α ⁺ is the positive sequence component of the voltage α axis after ROGI processing, u _β ⁺ is the positive sequence component of the voltage β axis after ROGI processing, e _α ⁺ is the positive sequence component of the voltage α axis sent to ROGI, e _β ⁺ is the positive sequence component of the voltage β axis sent to ROGI, the subscripts α and β represent the α and β components under the two-phase stationary αβ coordinates, and the superscript + represents positive sequence component, n is the nth sample.

3. Calculate the instantaneous power P _g , Q _g and the average instantaneous power P _g , Q _g .

4. Substitute the active power command P _ref and the average instantaneous power P ₀ into the active power-frequency calculation link (7) to obtain the virtual phase.

In the formula, J is the virtual moment of inertia, D _p is the damping coefficient, ω is the angular frequency, and ω _n is the rated angular frequency.

Substitute the active power command Q _ref and the average instantaneous power Q ₀ into the reactive power-voltage calculation link (8) to obtain the voltage amplitude.

U＝K∫(Q _ref -Q ₀ )dt (8)

In the formula, K is the integral coefficient, and U is the effective value of the output voltage.

4. Synthesize the reference modulation voltages u _am , u _bm and u _cm , and perform clark transformation to obtain the reference modulation voltages u _α and u _β in the two-phase stationary coordinates.

5. Establish direct power control. By setting different power command values, different control objectives can be achieved when the grid voltage is unbalanced. Specifically: _In order to achieve stable output power without fluctuation, _the power command value is set to 0, and the power command value is set according to formula (9) to realize sinusoidal current and three-phase balance. The block diagram of direct power control is shown in Fig. 4.

In the formula, P _g and Q _g are the output instantaneous power, P ₀ and Q ₀ are the output average instantaneous power.

The mathematical model of the grid-connected inverter in the αβ coordinate system is established from Figure 1:

In the formula, the subscripts α and β represent the components under the two-phase static coordinates; L=L _g +L _s , L _s is the internal resistance of the filter inductor, and L _g is the line inductance; e _α and e _β are the two-phase static αβ The α and β axis components of the grid voltage in the coordinate system, i _α and i _β are the α and β axis components of the output current in the two-phase stationary αβ coordinate system. When the grid voltage is unbalanced, the grid voltage can be expressed in the two-phase stationary coordinate system as:

In the formula, the superscript + indicates the positive sequence component, and the superscript - indicates the negative sequence component.

According to formula (10), we can get:

According to formula (11), we can get:

In the formula, ω _n is the rated angular frequency of the power grid.

The output instantaneous power is:

Deriving formula (14), the power derivative can be obtained as

Substituting formula (12) (13) into (15) can get the expression of output voltage under unbalanced power grid:

The equation of direct power control under unbalanced grid can be obtained as:

In the formula, M is the conversion matrix, and the output signal of the direct power control is converted into a modified modulation signal through the conversion matrix M, P _com and Q _com are active and reactive power command values respectively, and G _VPI is the expression of the VPI controller.

6. Superimpose the reference modulation signal and the correction modulation signal, and send them to the SVPWM modulation link to obtain the switching device control signal.

Claims (4)

1. a virtual synchronous generator control method based on direct power control when a grid voltage is unbalanced, is characterized in that, comprises the steps: (1) Use the reduced-order generalized integrator to separate the grid voltage and the positive sequence component of the grid-connected current; calculate the instantaneous power and average instantaneous power; (2) Substituting the average instantaneous power into the traditional virtual synchronous control strategy to obtain the reference modulation signal; (3) Establish a direct power control loop. By setting the power command value when the grid voltage is unbalanced, the corrected modulation signal when the output power has no fluctuation, the output current is sinusoidal, and the three-phase balance is obtained through the direct power control loop. ; (4) The reference modulation signal and the correction modulation signal are superimposed and sent to the space vector pulse width modulation module to obtain the control signal of the switching device. 2. The virtual synchronous generator control method based on direct power control when a kind of grid voltage is unbalanced according to claim 1, it is characterized in that the step (1) described utilizes the reduced-order generalized integrator to separate grid voltage and grid-connected Current positive sequence component, the method of calculating instantaneous power and average instantaneous power is: (1-1) Use the reduced-order generalized integrator after discretization to obtain the positive sequence components of voltage and current. The expression of the reduced-order generalized integrator after discretization is shown in formula (1): In the formula, ω is the angular frequency, T _s is the sampling period, u _α ⁺ is the positive sequence component of the voltage α axis obtained after processing by the reduced-order generalized integrator, and u _β ⁺ is the voltage β after processing by the reduced-order generalized integrator axis positive sequence component, e _α ⁺ is the positive sequence component of the voltage α axis sent to the reduced-order generalized integrator, e _β ⁺ is the positive sequence component of the voltage β axis sent to the reduced-order generalized integrator, and the subscripts α and β represent two The α and β components under the phase-stationary αβ coordinates, the superscript + indicates the positive sequence component, and n is the nth sampling; (1-2) Calculate the instantaneous power P _g , Q _g , the average instantaneous power P ₀ , Q ₀ , and the calculation formulas are shown in formulas (2) and (3): In the formula, e _α and e _β are the α and β axis components of the grid voltage in the two-phase stationary αβ coordinate system, i _α and i _β are the α and β axis components of the output current in the two-phase stationary αβ coordinate system, and the superscript + indicates a positive sequence component. 3. The virtual synchronous generator control method based on direct power control when a kind of grid voltage is unbalanced according to claim 1, it is characterized in that the average instantaneous power is substituted into the traditional virtual synchronous control strategy control in step (2) to obtain The method of reference modulation signal is: (2-1) Substituting the active power command P _ref and the average instantaneous power P ₀ into formula (4) to obtain the voltage phase: In the formula, J is the virtual moment of inertia, D _p is the damping coefficient, ω is the angular frequency, ω _n is the rated angular frequency, θ is the phase angle; (2-2) Substitute the reactive power command Q _ref and the average instantaneous power Q ₀ into formula (5) to obtain the voltage amplitude: U＝K∫(Q _ref -Q ₀ )dt (5) In the formula, K is the integral coefficient, and U is the effective value of the output voltage; (2-3) Take the output of the active link as the frequency and phase of the modulating wave, and the output of the reactive link as the amplitude of the modulating wave, and synthesize the reference three-phase modulating voltages u _am , _ubm , u _cm according to formula (6), and undergo Clarke transformation The reference modulation voltages u _α and u _β in the two-phase stationary coordinates are obtained, and the Clark transformation formula is shown in formula (7): 4. The virtual synchronous generator control method based on direct power control when a kind of grid voltage is unbalanced according to claim 1, it is characterized in that the described establishment direct power control loop of step (3), by setting grid voltage The power command value at balance is obtained through the direct power control loop to obtain the corrected modulation signal when the output power is stable, the output current is sinusoidal and the three-phase balance is two different control objectives, and the method of correcting the modulated signal when the grid voltage is unbalanced is obtained by using direct power control for: (3-1) The vector proportional integral resonant controller is used to establish a direct power control loop, and the expression of the vector proportional integral controller is shown in formula (8): In the formula, K _p and K _i are the control parameters of double frequency vector proportional integral controller, ω _c is the bandwidth of vector proportional integral control, and ω _n is the rated angular frequency; (3-2) In order to ensure that the output power is stable when the grid voltage is unbalanced, set the power command value of direct power control P _com =0, Q _com =0, and obtain the output results u′ _{α_com} , u′ of the output power stable control target _{beta_com} ; (3-3) In order to ensure that the output current is sinusoidal and three-phase balanced when the grid voltage is unbalanced, reset the direct power control power command values P _com and Q _com to obtain the output result u′ of the control target when the current is sinusoidal and three-phase balanced _{α_com} , u′ _{β_com} , to achieve sinusoidal current and three-phase balance, set the power command value according to formula (9): In the formula, P _g and Q _g are output instantaneous power, P ₀ and Q ₀ are output average instantaneous power; (3-4) Convert the output signal of the direct power control into a modified modulation signal through the conversion matrix M, the conversion equation is shown in formula (10), and M is shown in formula (11): In the formula, u _{α_com} and u _{β_com} are the modified modulation signals in the αβ coordinate system, u′ _{α_com} and u′ _{β_com} are the direct power control output in the αβ coordinate system respectively, and e _α and e _β are the two-phase static αβ coordinate system The α and β axis components of the lower grid voltage, i _α and i _β are the α and β axis components of the output current in the two-phase static αβ coordinate system.