CN114552675B - Grid-connected inverter transient stability control method and device based on virtual synchronous machine - Google Patents
Grid-connected inverter transient stability control method and device based on virtual synchronous machine Download PDFInfo
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Abstract
The invention relates to a grid-connected inverter transient stability control method and device based on a virtual synchronous machine, wherein an active power control loop, a reactive power control loop and the virtual synchronous machine are used for controlling the grid-connected inverter, when the active power control loop is used for realizing control, the reactive power deviation delta Q and the frequency deviation delta omega are multiplied by a multiplier, then compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop. The method can effectively improve the transient stability of the networking grid-connected inverter and is easy to realize.
Description
Technical Field
The invention relates to a grid-connected inverter transient stability control method and device based on a virtual synchronous machine.
Background
Voltage source inverters (voltage source inverter, VSI) are widely used as grid interfaces in renewable energy systems, distributed generation systems, and electrified transportation systems to convert direct current to alternating current. However, VSI may operate under severe three-phase voltage drops due to short-circuit faults or the like.
Currently, research on grid-tied VSI transient stability during voltage dips is mainly focused on more-grid-type VSIs and networking-type VSIs employing vector control. Because of the small short-circuit ratio (SCR) of the weak grid, phase-locked loops in grid-like VSIs tend to lose stability during grid voltage dips. The networking type VSI does not contain a phase-locked loop, is equivalent to a controlled voltage source, and has several networking type VSI control methods using different active power control schemes, including droop control, power synchronous control, virtual oscillator control and virtual synchronous generator (virtual synchronous generator, VSG) control. Among these methods, VSG control is the simplest and most widely used case, where VSI is controlled to simulate the dynamics of a synchronous generator.
The implementation flexibility of the VSG controller enables a corresponding auxiliary control method to be designed on the premise of knowing a transient instability mechanism so as to enhance transient stability. In the prior art, a method for improving transient stability by adding a low-pass filter to a reactive power control loop is proposed, but the method increases the order of a control system, so that system analysis is more complex. In the prior art, a self-adaptive inertia method is also provided, which can reduce frequency deviation during power grid faults so as to improve transient stability. However, this method relies on a differentiating element to detect the direction of the frequency change, which is complex to implement. In the prior art, a method for switching VSG modes is also provided, and the method can change the VSG modes when a power grid fails, and can realize fault ride-through even without a balance working point, so that the transient stability is improved. However, this method requires detection of frequency and active power changes, and is also dependent on differential components, which is relatively complex to implement in practice. In the prior art, a method is also provided, and a transient damper is designed based on the difference between the grid frequency estimated by the phase-locked loop and the angular frequency in the active power control loop of the VSG. However, this approach relies on a phase locked loop to acquire the frequency of the grid, which may lose stability during voltage dips.
Disclosure of Invention
The invention aims to provide a grid-connected inverter transient stability control method and device based on a virtual synchronous machine, which can effectively improve the transient stability of a grid-connected inverter and are easy to realize.
Based on the same inventive concept, the invention has two independent technical schemes:
1. the utility model provides a grid-connected inverter transient stability control method based on virtual synchronous machine, is controlled grid-connected inverter through active power control ring, reactive power control ring and virtual synchronous machine, when active power control ring realizes controlling, multiplies reactive power deviation DeltaQ and frequency deviation Deltaω through a multiplier, obtains compensation power DeltaP v through a proportion link again, and finally will compensate power DeltaP v negative feedback to active power control ring front end.
Further, the compensation power DeltaP v is obtained by the following formula,
ΔPv＝Kv(Q-Qref)·Δω
Wherein Δω is a frequency deviation, K v is a scaling factor of a scaling link, Q is reactive power, and Q ref is a reactive power reference value.
Further, the control model formula of the active power control loop is as follows,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
Further, the proportionality coefficient K v is obtained by the following method,
Step 2.1: setting a reference value of a proportion coefficient K v;
Step 2.2: calculating to obtain a current virtual power angle delta based on a proportionality coefficient K v;
Step 2.3: judging whether the current virtual power angle delta p is larger than delta m,δm or not, wherein the virtual power angle corresponding to the unstable balance point of the system when K v =0; if yes, entering a step 4, otherwise entering a step 5;
Step 2.4: k v＝Kv -Deltak, wherein Deltak is the iteration step length of K v, and returning to the step 2;
Step 2.5: the current scaling factor K v is taken as the output value.
Further, in step 2.2, the virtual power angle delta is calculated by the following formula,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, ω g is the current grid voltage frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference value, K v is the scaling factor of the scaling link, Q is the reactive power, and Q ref is the reactive power reference value.
Further, in step 2.3, δ m has a value of pi.
Further, in step 2.4, the iteration step Δk has a value of 0.1p.u.
Further, the method comprises the following steps:
Step 1: the method comprises the steps of collecting three-phase current I f output by a grid-connected inverter, three-phase current instantaneous value I pcc and three-phase voltage instantaneous value U pcc of a grid-connected point through a sensor, and calculating three-phase instantaneous active power P and reactive power Q;
Step 2: the active power P calculates a power grid voltage phase angle reference value theta ref through an active power control loop;
Step 3: the reactive power Q calculates a reference voltage amplitude U ref through a reactive power control loop;
Step 4: i f、Ipcc、Upcc、θref and U ref are input into a virtual synchronous machine, and the virtual synchronous machine outputs a PMW signal through pulse width modulation to control the grid-connected inverter.
2. The control device for improving the transient stability of the networking grid-connected inverter comprises an active power control loop, a reactive power control loop and a virtual synchronous machine, wherein the virtual synchronous machine is used for controlling the grid-connected inverter by outputting a PMW signal through pulse width modulation based on a grid voltage phase angle reference value theta ref and a reference voltage amplitude U ref output by the active power control loop and the reactive power control loop, and the active power control loop is provided with a control branch circuit which comprises a proportion module and a multiplier; the control branch performs the following control,
When the active power control loop realizes control, the reactive power deviation delta Q and the frequency deviation delta omega are multiplied by a multiplier, compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop.
Further, the control model formula of the active power control loop is as follows,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
The invention has the beneficial effects that:
When VSI is unstable during transient, the difference Δq between the grid-side reactive power Q and the reactive power reference value Q ref in the reactive power controller is greater than 0, i.e., Q-Q ref =Δq >0; while the angular frequency deviation in the active power controller is always greater than 0, i.e. Δω >0, while the angular frequency deviation Δω is equal to 0 at steady state. According to the invention, by utilizing the characteristics that the existing virtual synchronous machine control type VSI is constant in delta Q >0 when in transient instability and delta omega=0 when in steady state, an auxiliary control branch is added on the basis of the existing control method when the active power control loop is used for realizing control, reactive power deviation delta Q and frequency deviation delta omega are multiplied by a multiplier, compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop, so that the stability of the virtual synchronous machine control type VSI is improved, and the operation is simple and easy to realize.
The compensation power deltap v of the present invention is obtained by the following formula,
ΔPv＝Kv(Q-Qref)·Δω
Wherein Δω is a frequency deviation, K v is a scaling factor of a scaling link, Q is reactive power, and Q ref is a reactive power reference value.
The control model formula of the active power control loop is as follows,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
The scaling factor K v is obtained by the following method,
Step 2.1: setting a reference value of a proportion coefficient K v;
Step 2.2: calculating to obtain a current virtual power angle delta based on a proportionality coefficient K v;
Step 2.3: judging whether the current virtual power angle delta p is larger than delta m,δm or not, wherein the virtual power angle corresponding to the unstable balance point of the system when K v =0; if yes, entering a step 4, otherwise entering a step 5;
Step 2.4: k v＝Kv -Deltak, wherein Deltak is the iteration step length of K v, and returning to the step 2;
Step 2.5: the current scaling factor K v is taken as the output value.
In step 2.2, the virtual power angle delta is calculated and obtained by the following formula,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, ω g is the current grid voltage frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference value, K v is the scaling factor of the scaling link, Q is the reactive power, and Q ref is the reactive power reference value.
Delta m takes pi as the value.
The iteration step Δk takes a value of 0.1p.u.
According to the method and the specific parameters, the proportionality coefficient K v is obtained, so that the calculation accuracy and the calculation efficiency of the feedback power delta P are further ensured, and the transient stability of the networking grid-connected inverter is further effectively improved.
Drawings
FIG. 1 is a control schematic diagram of a prior art control method of the present invention;
FIG. 2 is a control schematic of the control method of the present invention;
FIG. 3 is a flow chart of the calculation of the scaling factor K v according to the present invention;
fig. 4 is a simulated waveform diagram when the grid voltage drops when K v =0;
Fig. 5 is a simulated waveform diagram of K v =2.6p.u., when the grid voltage drops.
Detailed Description
The present invention will be described in detail below with reference to the embodiments shown in the drawings, but it should be understood that the embodiments are not limited to the present invention, and functional, method, or structural equivalents and alternatives according to the embodiments are within the scope of protection of the present invention by those skilled in the art.
Embodiment one:
grid-connected inverter transient stability control method based on virtual synchronous machine
As shown in fig. 1, the grid-connected inverter is controlled by an active power control loop, a reactive power control loop and a current-voltage control loop, which is the prior art.
As shown in fig. 2, the present invention adds an auxiliary control branch based on the existing control method. When the active power control loop realizes control, the reactive power deviation delta Q and the frequency deviation delta omega are multiplied by a multiplier, compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop.
The compensation power deltap v is obtained by the following formula,
ΔPv＝Kv(Q-Qref)·Δω (1)
Wherein Δω is a frequency deviation, K v is a scaling factor of a scaling link, Q is reactive power, and Q ref is a reactive power reference value. Δp v represents the transient compensation power. The frequency deviation Δω=0 at steady state, and therefore Δp v =0 at steady state, the auxiliary control leg in the present invention does not affect the steady state characteristics of the VSI.
The control model formula of the active power control loop is as follows,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
Defining a virtual power angle δ=θ ref-θg, and assuming that the grid voltage frequency is maintained at the reference frequency ω 0, equation (3) can be rewritten as follows:
From the reactive power controller:
Uref＝U0+Kq(Qref-Q) (5)
K q is the Q-V droop coefficient, U 0 is the reference peak value of the grid phase voltage, and Q ref is the grid reactive power reference value. The above analysis is performed in a power control loop, and in order to build a large signal model that can be used for transient stability analysis, the relationship of the actual circuit to the power controller must also be considered. Generally, the bandwidth of the voltage-current inner loop is higher than the bandwidth of the outer loop power controller, which means that the inner loop has a faster response speed. According to time scale decoupling, when establishing mathematical model analysis, the inner loop control can be considered to accurately track the voltage reference U ref and the phase reference θ ref, simplifying VSI into a controlled voltage source that satisfies V pcc＝Vref, with transient stability primarily determined by the outer loop power controller. Thus, from fig. 2, P and Q on the grid side can be deduced:
Where U g represents the grid voltage, Z g represents the grid-side line impedance magnitude, and j g represents the grid-side line impedance angle. Substituting (7) into (5) to solve U ref,
Wherein the method comprises the steps ofWith longer formula (8), the intermediate variable is denoted by Z t, i.e
Therefore, a large signal model of the control method proposed by the present invention can be obtained as shown in the formula (9).
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, ω g is the current grid voltage frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference value, K v is the scaling factor of the scaling link, Q is the reactive power, and Q ref is the reactive power reference value.
As shown in fig. 3, the scaling factor K v is obtained by,
Step 2.1: setting a reference value of a proportion coefficient K v; the reference value of K v is 1U 0/Pmax,Pmax, which is the rated capacity of VSI.
Step 2.2: calculating to obtain a current virtual power angle delta based on a proportionality coefficient K v;
In step 2.2, the virtual power angle delta is obtained through the calculation of the formula (9),
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, ω g is the current grid voltage frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference value, K v is the scaling factor of the scaling link, Q is the reactive power, and Q ref is the reactive power reference value.
Step 2.3: judging whether the current virtual power angle delta p is larger than delta m,δm or not, wherein the virtual power angle corresponding to the unstable balance point of the system when K v =0; if yes, go to step 4, otherwise go to step 5. In this embodiment, δ m takes pi, because the virtual power angle in transient instability must exceed pi when the integration time is long enough.
Step 2.4: k v＝Kv -Deltak, wherein Deltak is the iteration step length of K v, and returning to the step 2; in this embodiment, the iteration step Δk has a value of 0.1p.u., so that the influence of the inner loop on the K v error can be reduced, and the iteration calculation can be accelerated.
Step 2.5: the current scaling factor K v is taken as the output value.
The control method of the invention comprises the following steps:
Step 1: the method comprises the steps of collecting three-phase current I f output by a grid-connected inverter, three-phase current instantaneous value I pcc and three-phase voltage instantaneous value U pcc of a grid-connected point through a sensor, and calculating three-phase instantaneous active power P and reactive power Q;
Step 2: the active power P calculates a power grid voltage phase angle reference value theta ref through an active power control loop;
Step 3: the reactive power Q calculates a reference voltage amplitude U ref through a reactive power control loop;
Step 4: i f、Ipcc、Upcc、θref and U ref are input into a virtual synchronous machine, and the virtual synchronous machine outputs a PMW signal through pulse width modulation to control a grid-connected inverter (VSI).
In order to evaluate the transient performance of the virtual synchronous machine control type VSI control method provided by the invention, a simulation model shown in figure 2 is built by utilizing Matlab/Simulink for verification. Setting a power grid reference frequency to be omega 0 =50 Hz, an active power reference value P ref =40 kW, a VSI rated capacity to be P max =40 kW, a reactive power reference value Q ref＝0,J＝Dp＝10Pref/ω0.,Dq =0.0001V/W, a power grid line voltage to be 380V, a line impedance Z g = 3.288 Ω and an impedance angle to beThe VSI DC side voltage source is 1.2kV. Fig. 4 is a simulated waveform diagram of the grid three-phase voltage suddenly dropping from 1p.u. to 0.56p.u. when K v =0 (i.e. VSI controlled by the existing virtual synchronous machine), where the simulated waveforms are, from top to bottom, the grid three-phase voltage V gabc, the grid three-phase current I gabc, the grid side active power P, the frequency deviation Δω, and the virtual power angle δ, respectively. As can be seen from fig. 4, at time t=0 the grid voltage drops to 0.56p.u., the system transient is unstable and low frequency oscillations can be observed in the waveforms of I gabc, P, Δω and δ. Fig. 5 is a simulated waveform diagram when the grid three-phase voltage suddenly drops from 1p.u. to 0.56p.u. at time t=0 when K v =2.6p.u. In fig. 5, it can be seen that the system can still maintain stable operation when the grid voltage drops three phases; when reaching steady state, the output power p=40 kW, and it can be seen that the control method proposed by the present invention does not change the steady state output power of the VSI. Therefore, according to the virtual synchronous machine control type VSI control method provided by the invention, the critical K v is obtained by combining the parameter design scheme of K v, and the transient stability of the virtual synchronous machine control type grid-connected inverter can be greatly enhanced as long as the selected K v is larger than the critical K v value.
Embodiment two:
control device for improving transient stability of networking grid-connected inverter
As shown in fig. 2, the system comprises an active power control loop, a reactive power control loop and a virtual synchronous machine, wherein the virtual synchronous machine is used for controlling a grid-connected inverter by pulse width modulation based on a grid voltage phase angle reference value theta ref and a reference voltage amplitude U ref output by the active power control loop and the reactive power control loop, and the active power control loop is provided with a control branch circuit which comprises a proportion module and a multiplier; the control branch performs the following control,
When the active power control loop realizes control, the reactive power deviation delta Q and the frequency deviation delta omega are multiplied by a multiplier, compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop.
The control model formula of the active power control loop is as follows,
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
The above list of detailed descriptions is only specific to practical embodiments of the present invention, and they are not intended to limit the scope of the present invention, and all equivalent embodiments or modifications that do not depart from the spirit of the present invention should be included in the scope of the present invention.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (7)
1. The utility model provides a grid-connected inverter transient stability control method based on virtual synchro machine, is through active power control ring, reactive power control ring and virtual synchro machine to grid-connected inverter control, its characterized in that: when the active power control loop realizes control, multiplying reactive power deviation delta Q and frequency deviation delta omega by a multiplier, obtaining compensation power delta P v through a proportional link, and finally negatively feeding back the compensation power delta P v to the front end of the active power control loop;
the compensation power deltap v is obtained by the following formula,
；
Wherein delta omega is frequency deviation, K v is a proportionality coefficient of a proportionality link, Q is reactive power, and Q ref is a reactive power reference value;
The control model formula of the active power control loop is as follows,
；
；
Wherein D p is a damping coefficient, J is a virtual inertia, deltaω is a frequency deviation, omega 0 is a grid voltage reference frequency, deltaP v is compensation power, P is grid active power, P ref is a grid active power reference value, and theta ref is a grid voltage phase angle reference value and also is an output value of an active power control loop;
The scaling factor K v is obtained by the following method,
Step 2.1: setting a reference value of a proportion coefficient K v;
Step 2.2: calculating and obtaining current virtual power angle based on proportionality coefficient K v ；
Step 2.3: judging the current virtual power angleWhether or not it is greater than，The virtual power angle corresponding to the unstable balance point of the system when K v =0; if yes, entering a step 4, otherwise entering a step 5;
Step 2.4: k v= Kv -Deltak, wherein Deltak is the iteration step length of K v, and returning to the step 2;
Step 2.5: the current scaling factor K v is taken as the output value.
2. The virtual synchronous machine-based grid-connected inverter transient stability control method according to claim 1, wherein the method comprises the following steps of: in step 2.2, the virtual power angle is calculated by the following formula,
；
Wherein D p is a damping coefficient, J is a virtual inertia, deltaω is a frequency deviation, omega 0 is a grid voltage reference frequency, omega g is a current grid voltage frequency, P is a grid active power, P ref is a grid active power reference value, K v is a scaling factor of a scaling link, Q is reactive power, and Q ref is a reactive power reference value.
3. The virtual synchronous machine-based grid-connected inverter transient stability control method according to claim 1, wherein the method comprises the following steps of: in step 2.3, δ m takes on pi.
4. The virtual synchronous machine-based grid-connected inverter transient stability control method according to claim 1, wherein the method comprises the following steps of: in step 2.4, the iteration step Δk has a value of 0.1 p.u.
5. The virtual synchronous machine-based grid-connected inverter transient stability control method according to any one of claims 1 to 4, comprising the steps of:
Step 1: the method comprises the steps of collecting three-phase current I f output by a grid-connected inverter, three-phase current instantaneous value I pcc and three-phase voltage instantaneous value U pcc of a grid-connected point through a sensor, and calculating three-phase instantaneous active power P and reactive power Q;
Step 2: the active power P calculates a power grid voltage phase angle reference value theta ref through an active power control loop;
Step 3: the reactive power Q calculates a reference voltage amplitude U ref through a reactive power control loop;
Step 4: i f、Ipcc、Upcc、θref and U ref are input into a virtual synchronous machine, and the virtual synchronous machine outputs a PMW signal through pulse width modulation to control the grid-connected inverter.
6. The control device for improving transient stability of the networking grid-connected inverter for realizing the method of any one of claims 1 to 5 comprises an active power control loop, a reactive power control loop and a virtual synchronous machine, wherein the virtual synchronous machine is used for controlling the grid-connected inverter by pulse width modulation based on a grid voltage phase angle reference value theta ref and a reference voltage amplitude U ref output by the active power control loop and the reactive power control loop, and is characterized in that the active power control loop is provided with a control branch circuit, and the control branch circuit comprises a proportion module and a multiplier; the control branch performs the following control,
When the active power control loop realizes control, the reactive power deviation delta Q and the frequency deviation delta omega are multiplied by a multiplier, compensation power delta P v is obtained through a proportional link, and finally the compensation power delta P v is negatively fed back to the front end of the active power control loop.
7. The apparatus according to claim 6, wherein: the control model formula of the active power control loop is as follows,
；
；
Where D p is the damping coefficient, J is the virtual inertia, Δω is the frequency deviation, ω 0 is the grid voltage reference frequency, Δp v is the compensation power, P is the grid active power, P ref is the grid active power reference, θ ref is the grid voltage phase angle reference, and also is the output value of the active power control loop.
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