CN111682560B - Method for suppressing electromechanical oscillation of power grid based on fast power support of photovoltaic power generation system - Google Patents

Abstract

The invention belongs to the field of power electronics, and specifically discloses a method for suppressing electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, including: a grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, and a synchronous machine speed acquisition module , a speed deviation judgment module, a speed droop control module, a photovoltaic fast power modulation module and a photovoltaic steady-state power generation module. The invention divides the working process of the photovoltaic power generation system to suppress the electromechanical oscillation into four typical working conditions, and sets the conditions for switching between different working conditions and the corresponding power output commands according to the deviation and change rate of the rotor speed of the synchronous machine in different oscillation periods. It is a simple, effective and practical method with good application value.

Description

Method for suppressing electromechanical oscillation of power grid based on fast power support of photovoltaic power generation system

technical field

The invention relates to the field of power electronics, in particular to a method for suppressing electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, which is suitable for quickly suppressing the rotor speed oscillation phenomenon of a synchronous machine caused by power imbalance in a power system.

Background technique

The power system is often subjected to various power disturbances, which can easily cause rotor speed oscillation phenomenon involving electromechanical energy conversion (called electromechanical oscillation) in synchronous generators, which is not conducive to the stable operation of the power system. With the large-scale connection of new energy power generation devices, mainly photovoltaic power generation systems, to the grid, the proportion of traditional synchronous generators has gradually decreased, reducing the inherent damping capacity and inertia level of the power system. In addition, photovoltaic grid-connected power generation systems are generally considered to have low inertia and weak damping, which further weakens the damping capacity and inertia level of power systems containing new energy power generation devices, making it difficult to effectively suppress the electromechanical oscillation of the power grid.

The essence of the electromechanical oscillation phenomenon in the power grid is the imbalance of active power in the power system, which causes the oscillation phenomenon of acceleration or deceleration in the rotor speed of the synchronous generator alternately. Therefore, adding an effective power control strategy to the photovoltaic power generation system to compensate for the power deficit or absorb the surplus power in the system can quickly calm the electromechanical oscillation phenomenon of the synchronous machine rotor, which is of great significance to the stable operation of the power system.

Existing grid electromechanical oscillation suppression strategies based on photovoltaic power generation systems mainly include droop control and inertia control. Usually, the active power output of the photovoltaic power generation system is regulated according to the speed deviation and speed change rate of the synchronous machine rotor; During the period of electromechanical oscillation phenomenon, the photovoltaic power generation system works as a static var compensator to adjust the grid voltage to indirectly suppress the unbalanced power of the grid. However, none of the existing control strategies can maximize the potential of photovoltaic power generation systems to suppress electromechanical oscillations in the power grid.

Contents of the invention

The present invention aims at the shortcomings of the existing photovoltaic power generation system control strategy to suppress the electromechanical oscillation of the power grid, and provides a method for suppressing the electromechanical oscillation of the power grid based on the rapid power support technology of the photovoltaic power generation system, which can effectively suppress the unbalanced power of the power system , to quickly calm the electromechanical oscillation phenomenon of the power grid.

The present invention is realized through the following technical solutions:

Based on the method of rapid power support of the photovoltaic power generation system to suppress the electromechanical oscillation of the power grid, firstly, the operating range of the DC voltage in the photovoltaic power generation system is set between the voltage corresponding to the maximum power output point and the open circuit voltage, and half of the frequency modulation reserve capacity is reserved. That is to say, the generating power of the photovoltaic power generation system is set to half of the maximum output power. The invention combines the advantages of fast power support and droop control strategies, divides the working process of photovoltaic power generation system to suppress electromechanical oscillation into four typical working conditions, and sets different The condition of working mode switching and the corresponding power output command is a simple, effective and practical method, and has good application value. The technology disclosed in the invention has a simple implementation process, can maximize the development and utilization of the photovoltaic power generation system to suppress the electromechanical oscillation of the power grid, can quickly calm the rotor speed oscillation phenomenon caused by electromechanical oscillation, and improve the frequency stability of the power system.

A method for suppressing electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, comprising: a grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, a synchronous machine speed acquisition module, a speed deviation judgment module, and a speed droop control module , photovoltaic fast power modulation module and photovoltaic steady-state power generation module.

The grid voltage and current acquisition module acquires three-phase signals of grid voltage and current;

The synchronous rotating coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current DC components under the rotating coordinate system;

The voltage phase acquisition module collects the phase signal of the grid voltage in real time, and outputs it to the synchronous rotation coordinate transformation module;

The synchronous machine rotational speed acquisition module acquires the real-time rotational speed signal of the rotor in the synchronous generator;

The speed deviation judging module judges the detected synchronous generator rotor speed deviation and its rate of change as switching conditions for operating conditions of the photovoltaic power generation system;

The speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judgment instruction obtained by the speed deviation judgment module;

The photovoltaic fast power modulation module makes the output power of the photovoltaic power generation system operate in the fast power mode according to the judgment command obtained by the speed deviation judgment module, that is, the photovoltaic power generation system only has two working conditions of output maximum power and minimum power 0;

The photovoltaic steady-state power generation module makes the photovoltaic power generation system operate in a steady-state power generation state according to the judgment instruction obtained by the speed deviation judgment module, and no longer outputs controllable active power to suppress the electromechanical oscillation of the power grid.

(1) Under the action of droop control strategy (proportional controller) or inertial control strategy (differential controller), the active current I _d output to the synchronous machine grid can be described as:

I _d = K _p (ω ₀ -ω) = K _p Δω

I _d = sK _d (ω ₀ -ω) = sK _d Δω

Among them, ω is the actual speed of the rotor of the synchronous generator, ω ₀ is the synchronous speed of the rotor, K _p and K _d are the gains of the proportional controller and the differential controller in the speed control loop, respectively, and s is the differential operator.

(2) If the output power _PPV injected by the photovoltaic power generation system into the grid is taken into account, the damping coefficient T _D and inertia coefficient T _J of the power system can be described as:

Among them, D and H are the inherent damping coefficient and inertia coefficient of the system, and K _e is the control coefficient of the photovoltaic power generation system, which characterizes the influence of the photovoltaic power generation system on the dynamic characteristics of the power grid under the action of the control strategy.

Obviously, the larger the controller gain K _p is, the larger the damping coefficient T _D of the system is, the larger the damping torque provided by the photovoltaic power generation system to the grid can effectively suppress the speed deviation Δω; similarly, the larger K _{d is} , the greater the inertia of the system The larger the coefficient _TJ , the greater the inertia support provided to the grid, which can improve the ability of the photovoltaic power generation system to suppress the speed change rate dω/dt. Obviously, adjusting the controller gain can equivalently change the process of rotor speed oscillation.

(3) Taking the drooping control strategy of the photovoltaic power generation system as an example to illustrate, through steps (1) and (2), it can be seen that increasing K _p can increase the active current I _d of the photovoltaic power generation system, that is, increase the photovoltaic output power _PPV output, thereby enhancing the damping torque effect of the synchronous machine rotor. However, when K _p increases to the limit, the photovoltaic inverter will continue to output alternately based on the maximum photovoltaic output power P _mppt or the minimum power 0 according to the positive and negative polarity of the speed deviation in the whole period. During this period, the ability of the photovoltaic power generation system to suppress the electromechanical oscillation of the power grid will reach its limit. analyse as below:

Among them, the rotor motion equation of synchronous machine:

Among them, P _m is the mechanical power of the prime mover, P _e is the electromagnetic power of the synchronous machine, assuming that the grid load suddenly decreases by ΔP _m , and ΔP _e is the change in electromagnetic power P _e when the photovoltaic output power changes.

When the photovoltaic power generation system operates in a linear inertia control strategy, that is, the output power of the photovoltaic power generation system is no longer constant output maximum power P _mppt in a local period, the control gain K _d of the differential controller should meet the following setting conditions:

Considering that the rate of change of rotor speed reaches the maximum at the initial moment of electromechanical oscillation of the power grid, and the photovoltaic power generation system has not yet output controllable active power to participate in suppression, the rate of change of rotor speed at this time is:

It can be seen that the gain K _d of the differential controller should satisfy:

Substituting it into the rotor motion equation of the synchronous machine can be obtained:

At this time, the rate of change of the rotor speed of the synchronous machine is:

When the photovoltaic power generation system operates in the fast power support mode, the change rate of the rotor speed of the synchronous machine is:

Comparing with the speed change rate of synchronous machine as standard, we can get:

(4) From the comparison results of the speed change rate in step (3), it can be seen that the fast power support mode has a stronger ability to suppress dω/dt than the linear inertia control; similarly, the fast power support mode has obvious suppression ability to Δω Stronger than linear damping control. Its physical mechanism is as follows:

The root cause of electromechanical oscillation is: disturbance power causes temporary power imbalance in the power grid, power surplus or shortage will prompt the rotor to accelerate or decelerate, and the more serious the power imbalance, the greater the energy of acceleration and deceleration, and the more serious the oscillation phenomenon. That is, the greater the dω/dt and Δω indicators, the greater the probability of triggering the action of the relay protection device. The fast power support mode can make full use of the power resources of the photovoltaic power generation system, so that it can quickly emit or absorb energy at its maximum capacity, so as to effectively compensate for the power shortage of the system or absorb the excess energy of the system, weakening the dω/dt, Δω is too large acceleration and deceleration energy.

(5) Table 1 shows the switching thresholds and corresponding power output commands of the four operating conditions corresponding to the photovoltaic power generation system control strategy of the present invention.

Among them: ω _th1 is the rotor speed deviation threshold of the synchronous machine for the photovoltaic power generation system to implement the droop control strategy, ω _th2 and R _B are the speed deviation and speed change rate thresholds for the photovoltaic power generation system to implement the fast power support strategy, respectively.

Table 1 Switching commands and corresponding current output commands in different working conditions

When dω/dt, Δω and other indicators are within the safe range, that is, ω ₀ -ω _th1 <ω<ω ₀ +ω _th1 and |dω/dt| _≤RB , it indicates that the system does not generate electromechanical oscillation or the oscillation process is completely over , the photovoltaic power generation system should operate in the steady-state power generation mode. At this time, _PPV = P ₀ , where _PPV represents the photovoltaic output power, and P ₀ represents the photovoltaic steady-state output power. This corresponds to working condition I in Table 1. If the rotational speed index satisfies ω _th1 <|Δω|≤ω _th2 and |dω/dt| _≤RB , then the photovoltaic power generation system can switch to the rotational speed drooping mode, corresponding to working condition IV. According to the rotation speed deviation, the output power of the photovoltaic at this time is: 1.5V _d K _p Δω+P ₀ , where V _d is the DC component of the grid voltage on the d-axis under the action of the synchronous rotating coordinate transformation module. Through the photovoltaic droop control strategy, the necessary damping torque can be provided to the grid, forcing the electromechanical oscillation to be further weakened, so as to improve the frequency quality of the grid.

(6) If the rotational speed evaluation indicators dω/dt and Δω meet the thresholds set by working condition II, that is, Δω>ω _th2 and |dω/dt|>R _B , it indicates that the power system is seriously short of power at this time, and the photovoltaic power generation system The active power output should be increased in time to fill the power shortage in the power system. Its current command I _d ^* should immediately increase to the forward current limit, that is, the current I _mppt corresponding to the maximum photovoltaic output power, and continuously provide the maximum forward power support to the grid. At this time, P _PV = P _mppt , so as to significantly suppress Δω, dω/dt, corresponding to working condition II. Conversely, if Δω<-ω _th2 or |dω/dt|>R _B , it indicates that the power system is seriously surplus, then the photovoltaic power generation system should immediately minimize the output of active power, and its current command I _d ^* should respond immediately Decrease to 0 to reduce the photovoltaic output power PP _PV to 0, and provide reverse fast power support to the grid to reduce the power surplus, corresponding to working condition III. Compared with the prior art, the present invention has the following advantages:

Compared with the droop control and inertia control strategies of the existing conventional photovoltaic power generation system, the electromechanical oscillation suppression strategy proposed in the present invention does not require complicated controller parameter setting process, and only needs to set the switching thresholds of the four working conditions according to the grid operation standard. It can maximize the use of the photovoltaic power generation system's ability to suppress electromechanical oscillations in the power grid, and quickly calm the rotor oscillations caused by power disturbances without adding other oscillation suppression equipment. It has good industrial application value and low cost.

Through the above technical scheme, when the rotor speed oscillation is caused by the power disturbance of the synchronous machine grid, this method can quickly calm the electromechanical oscillation phenomenon of the grid without real-time adjustment of the parameters of the controller in the droop and inertia control strategy, which is a simple, effective and practical method. Photovoltaic power generation system control strategy.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. Ordinary technicians can also obtain other drawings based on these drawings without paying creative work.

Figure 1 is a nonlinear operation diagram of the output power of the photovoltaic power generation system;

Fig. 2 is a working principle diagram of the photovoltaic power generation system of the present invention suppressing the electromechanical oscillation of the power grid;

Fig. 3 is the working principle diagram of grid voltage and current acquisition module and synchronous machine speed acquisition module of the present invention;

Fig. 4 is the control schematic diagram of the DC output current of the photovoltaic power generation system of the present invention;

Fig. 5 is the comparison result of the output power of the photovoltaic power generation system under different proportional controller gains of the present invention;

Fig. 6 is the comparison result of the output power of the photovoltaic power generation system under different differential controller gains of the present invention;

Fig. 7 is a schematic diagram of four operating conditions of the photovoltaic power generation system of the present invention;

Fig. 8 is a schematic diagram of the power output of four operating conditions of the photovoltaic power generation system of the present invention;

Fig. 9 is a flow chart of the control strategy proposed by the present invention based on the fast power support technology;

Fig. 10 is a comparison of the speed suppression effect of the strategy proposed in the present invention and the conventional control;

Figure 11 is a comparison of the suppression effect of the speed change rate between the proposed strategy of the present invention and the conventional control strategy;

Fig. 12 is a comparison result of the output current of the power generation system between the proposed strategy of the present invention and the conventional control strategy.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

Example

A method for suppressing electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, comprising: a grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, a synchronous machine speed acquisition module, a speed deviation judgment module, and a speed droop control module , photovoltaic fast power modulation module and photovoltaic steady-state power generation module.

The grid voltage and current acquisition module acquires three-phase signals of grid voltage and current;

The synchronous rotating coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current DC components under the rotating coordinate system;

The voltage phase acquisition module collects the phase signal of the grid voltage in real time, and outputs it to the synchronous rotation coordinate transformation module;

The synchronous machine rotational speed acquisition module acquires the real-time rotational speed signal of the rotor in the synchronous generator;

The speed deviation judging module judges the detected synchronous generator rotor speed deviation and its rate of change as switching conditions for operating conditions of the photovoltaic power generation system;

The speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judgment instruction obtained by the speed deviation judgment module;

The photovoltaic fast power modulation module makes the output power of the photovoltaic power generation system operate in the fast power mode according to the judgment command obtained by the speed deviation judgment module, that is, the photovoltaic power generation system only has two working conditions of output maximum power and minimum power 0;

The photovoltaic steady-state power generation module makes the photovoltaic power generation system operate in a steady-state power generation state according to the judgment instruction obtained by the speed deviation judgment module, and no longer outputs controllable active power to suppress the electromechanical oscillation of the power grid.

(1) Under the action of droop control strategy (proportional controller) or inertial control strategy (differential controller), the active current I _d output to the synchronous machine grid can be described as:

I _d = K _p (ω ₀ -ω) = K _p Δω

I _d = sK _d (ω ₀ -ω) = sK _d Δω

Among them, ω is the actual speed of the rotor of the synchronous generator, ω ₀ is the synchronous speed of the rotor, K _p and K _d are the gains of the proportional controller and the differential controller in the speed control loop, respectively, and s is the differential operator.

(2) If the output power _PPV injected by the photovoltaic power generation system into the grid is taken into account, the damping coefficient T _D and inertia coefficient T _J of the power system can be described as:

Among them, D and H are the inherent damping coefficient and inertia coefficient of the system, and K _e is the control coefficient of the photovoltaic power generation system, which characterizes the influence of the photovoltaic power generation system on the dynamic characteristics of the power grid under the action of the control strategy.

Obviously, the larger the controller gain K _p is, the larger the damping coefficient T _D of the system is, the larger the damping torque provided by the photovoltaic power generation system to the grid can effectively suppress the speed deviation Δω; similarly, the larger K _{d is} , the greater the inertia of the system The larger the coefficient _TJ , the greater the inertia support provided to the grid, which can improve the ability of the photovoltaic power generation system to suppress the speed change rate dω/dt. Obviously, adjusting the controller gain can equivalently change the process of rotor speed oscillation.

(3) Taking the drooping control strategy of the photovoltaic power generation system as an example to illustrate, through steps (1) and (2), it can be seen that increasing K _p can increase the active current I _d of the photovoltaic power generation system, that is, increase the photovoltaic output power _PPV output, thereby enhancing the damping torque effect of the synchronous machine rotor. However, when K _p increases to the limit, the photovoltaic inverter will continue to output alternately based on the maximum photovoltaic output power P _mppt or the minimum power 0 according to the positive and negative polarity of the speed deviation in the whole period. During this period, the ability of the photovoltaic power generation system to suppress the electromechanical oscillation of the power grid will reach its limit. analyse as below:

Among them, the rotor motion equation of synchronous machine:

Among them, P _m is the mechanical power of the prime mover, P _e is the electromagnetic power of the synchronous machine, assuming that the grid load suddenly decreases by ΔP _m , and ΔP _e is the change in electromagnetic power P _e when the photovoltaic output power changes.

When the photovoltaic power generation system operates in a linear inertia control strategy, that is, the output power of the photovoltaic power generation system is no longer constant output maximum power P _mppt in a local period, the control gain K _d of the differential controller should meet the following setting conditions:

Considering that the rate of change of rotor speed reaches the maximum at the initial moment of electromechanical oscillation of the power grid, and the photovoltaic power generation system has not yet output controllable active power to participate in suppression, the rate of change of rotor speed at this time is:

It can be seen that the gain K _d of the differential controller should satisfy:

Substituting it into the rotor motion equation of the synchronous machine can be obtained:

At this time, the rate of change of the rotor speed of the synchronous machine is:

When the photovoltaic power generation system operates in the fast power support mode, the change rate of the rotor speed of the synchronous machine is:

Comparing with the speed change rate of synchronous machine as standard, we can get:

(4) From the comparison results of the speed change rate in step (3), it can be seen that the fast power support mode has a stronger ability to suppress dω/dt than the linear inertia control; similarly, the fast power support mode has obvious suppression ability to Δω Stronger than linear damping control. Its physical mechanism is as follows:

The root cause of electromechanical oscillation is: disturbance power causes temporary power imbalance in the power grid, power surplus or shortage will prompt the rotor to accelerate or decelerate, and the more serious the power imbalance, the greater the energy of acceleration and deceleration, and the more serious the oscillation phenomenon. That is, the greater the dω/dt and Δω indicators, the greater the probability of triggering the action of the relay protection device. The fast power support mode can make full use of the power resources of the photovoltaic power generation system, so that it can quickly emit or absorb energy at its maximum capacity, so as to effectively compensate for the power shortage of the system or absorb the excess energy of the system, weakening the dω/dt, Δω is too large acceleration and deceleration energy.

(5) The switching thresholds of the four operating conditions corresponding to the photovoltaic power generation system control strategy of the present invention and the corresponding power output instructions are as follows:

Among them: ω _th1 is the rotor speed deviation threshold of the synchronous machine for the photovoltaic power generation system to implement the droop control strategy, ω _th2 and R _B are the speed deviation and speed change rate thresholds for the photovoltaic power generation system to implement the fast power support strategy, respectively.

When dω/dt, Δω and other indicators are within the safe range, that is, ω ₀ -ω _th1 <ω<ω ₀ +ω _th1 and |dω/dt| _≤RB , it indicates that the system does not generate electromechanical oscillation or the oscillation process is completely over , the photovoltaic power generation system should operate in the steady-state power generation mode. At this time, _PPV = P ₀ , where _PPV represents the photovoltaic output power, and P ₀ represents the photovoltaic steady-state output power. This corresponds to working condition I in Table 1. If the rotational speed index satisfies ω _th1 <|Δω|≤ω _th2 and |dω/dt| _≤RB , then the photovoltaic power generation system can switch to the rotational speed drooping mode, corresponding to working condition IV. According to the rotation speed deviation, the output power of the photovoltaic at this time is: 1.5V _d K _p Δω+P ₀ , where V _d is the DC component of the grid voltage on the d-axis under the action of the synchronous rotating coordinate transformation module. Through the photovoltaic droop control strategy, the necessary damping torque can be provided to the grid, forcing the electromechanical oscillation to be further weakened, so as to improve the frequency quality of the grid.

(6) If the rotational speed evaluation indicators dω/dt and Δω meet the thresholds set by working condition II, that is, Δω>ω _th2 and |dω/dt|>R _B , it indicates that the power system is seriously short of power at this time, and the photovoltaic power generation system The active power output should be increased in time to fill the power shortage in the power system. Its current command I _d ^* should immediately increase to the forward current limit, that is, the current I _mppt corresponding to the maximum photovoltaic output power, and continuously provide the maximum forward power support to the grid. At this time, P _PV = P _mppt , so as to significantly suppress Δω, dω/dt, corresponding to working condition II. Conversely, if Δω<-ω _th2 or |dω/dt|>R _B , it indicates that the power system is seriously surplus, then the photovoltaic power generation system should immediately minimize the output of active power, and its current command I _d ^* should respond immediately Decrease to 0 to reduce the photovoltaic output power PP _PV to 0, and provide reverse fast power support to the grid to reduce the power surplus, corresponding to working condition III. Figure 1 is the nonlinear operation diagram of the output power of the photovoltaic power generation system. First, the nonlinear power output curve of the photovoltaic cell is explained, where the abscissa is the DC side capacitor voltage value U _dc , where U _dc is the minimum inverse of the photovoltaic inverter Variable voltage, U _mmpt is the DC side voltage corresponding to the photovoltaic maximum power output point, and U _oc is the open circuit voltage of the photovoltaic cell. The ordinate is the output power _PPV of the photovoltaic cell. It can be seen from Figure 1 that if the photovoltaic output power is limited to [U _mmpt , U _oc ], the output power P _PV output presents monotonicity, and the photovoltaic power output range can also be realized as [0, P _mmpt ], that is, global regulation, which is beneficial to Design a negative feedback control system.

Figure 2 shows the schematic diagram of the photovoltaic power generation system connected to the synchronous machine grid. By adding a conventional control strategy to the grid-connected inverter, the steady-state power generation function of the photovoltaic power generation system and the electromechanical oscillation suppression function of the grid can be realized.

Figure 3 shows the schematic diagram of the synchronous rotating coordinate transformation module and the voltage phase acquisition module, which can convert the collected three-phase voltage V _abc and current signal I _abc of the grid-connected point into the voltage and current DC components in the rotating coordinate system, namely V _d , V _q , I _d , I _q . Then, the phase angle θ of the three-phase grid voltage can be captured through the voltage phase acquisition module, and input to the rotating coordinate transformation module to complete the coordinate transformation function. Where f is the actual frequency of the grid. Figure 4 is the control schematic diagram of the photovoltaic grid-connected inverter, in which I _d ^* , I _q ^* , U _d ^* , U _q ^* are the reference signals of the DC current and voltage components respectively, and U _abc ^* is the reference signal of the three-phase voltage Signal, by controlling the reference signal of the DC component of the current, the active power output of the photovoltaic inverter can be controlled.

Figure 5 shows the change law of the active power output of the photovoltaic power generation system when the gain K _p of the proportional controller is increased after the droop control strategy is added to the photovoltaic inverter. It can be seen from Figure 5 that when the gain K _p increases to a certain level, the photovoltaic power generation system will continue to output at its maximum power P _mppt within a certain period of time, that is, nonlinear droop control. It can be developed and utilized to the greatest extent, and is beneficial to quickly suppress the speed deviation of the rotor of the synchronous machine.

Figure 6 shows the change law of the active power output of the photovoltaic power generation system when the gain K _d of the differential controller is increased after the inertia control strategy is added to the photovoltaic inverter. It can be seen from Figure 6 that when the gain K _d increases to a certain extent, the photovoltaic power generation system will continue to output at its maximum power P _mppt within a certain period of time, that is, nonlinear inertia control. The net provides inertial support, which is conducive to quickly suppressing the rate of change of the rotor speed.

Figure 7 is a schematic diagram of the grid electromechanical oscillation suppression strategy based on fast power support technology proposed by the present invention. Through the judgment results of the speed deviation judgment module, different inverter active current output conditions are selected to maximize development and utilization The damping and inertia capability of the photovoltaic power generation system to the electromechanical oscillation of the grid.

FIG. 8 is a power schematic diagram of the oscillation suppression strategy proposed in the present invention. At the initial stage of oscillation, the phenomenon of rotor speed oscillation is the most serious. At this time, the photovoltaic active power output switches back and forth between 0 and P _mppt to suppress the unbalanced power to the greatest extent. In the later stage of the oscillation, when the rotor speed index reaches the set range, the photovoltaic power generation system switches to the droop control strategy, which linearly adjusts the photovoltaic active power output according to the speed deviation, further suppresses the rotor speed deviation, and improves the stability of the power system.

Fig. 9 is a flow chart of the strategy proposed by the present invention, specifically the criteria for judging and selecting the four working conditions.

Fig. 10 is a comparison effect diagram of the suppression effect of the photovoltaic power generation system on the electromechanical oscillation of the power grid under the strategy proposed by the present invention and the conventional droop control, nonlinear droop control and no power control strategies. It can be seen from Figure 10 that the suppression strategy proposed in the present invention has the most obvious suppression effect on the oscillation phenomenon of the rotor speed, and can quickly calm down the electromechanical oscillation phenomenon of the power grid.

FIG. 11 is a waveform of the change law of the absolute value of the rotor speed change rate dω/dt described in FIG. 10 . Figure 11 shows that the suppression effect of the proposed strategy on the rotor speed change rate is also significantly stronger than that of the conventional control strategy, which can effectively avoid triggering related frequency protection devices and is conducive to improving the frequency stability of the power grid.

Fig. 12 is a waveform diagram of the three-phase current I _abc output by the photovoltaic inverter when the photovoltaic power generation system in Figs. 10 and 11 operates under the proposed strategy. It can be seen from Figure 12 that in the early stage of electromechanical oscillation of the power grid, the photovoltaic power output switches between 0 and P _mppt , and in the later stage of oscillation, the current output shows a linearization law.

Claims (1)

1. A method for suppressing electromechanical oscillations of a power grid based on rapid power support of a photovoltaic power generation system, comprising: the system comprises a power grid voltage and current acquisition module, a synchronous rotation coordinate conversion module, a voltage phase acquisition module, a synchronous machine rotating speed acquisition module, a rotating speed deviation judgment module, a rotating speed sagging control module, a photovoltaic rapid power modulation module and a photovoltaic steady-state power generation module;

(1) The power grid voltage and current acquisition module acquires three-phase signals of power grid voltage and current;

(2) The synchronous rotation coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current direct current components under a rotation coordinate system;

(3) The voltage phase acquisition module acquires phase signals of the power grid voltage in real time and outputs the phase signals to the synchronous rotation coordinate transformation module;

(4) The synchronous machine rotating speed acquisition module acquires a real-time rotating speed signal of a rotor in the synchronous generator;

(5) The rotating speed deviation judging module judges the detected rotating speed deviation and the change rate of the rotor of the synchronous generator, and the rotating speed deviation and the change rate of the rotor of the synchronous generator are used as switching conditions of the operation working conditions of the photovoltaic power generation system;

(6) The rotating speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judging instruction obtained by the rotating speed deviation judging module;

(7) The photovoltaic rapid power modulation module enables the output power of the photovoltaic power generation system to operate in a rapid power mode according to the judging instruction obtained by the rotating speed deviation judging module, namely, the photovoltaic power generation system only has two working conditions of outputting maximum power and minimum power of 0;

(8) The photovoltaic steady-state power generation module enables the photovoltaic power generation system to run in a steady-state power generation state according to the judging instruction obtained by the rotating speed deviation judging module, and controllable active power is not output any more to inhibit electromechanical oscillation of the power grid;

the switching threshold values of the 4 operation conditions corresponding to the control strategy of the photovoltaic power generation system and the corresponding power output instruction are specifically as follows: wherein:ω _th1 a synchronous machine rotor rotational speed deviation threshold value of a sagging control strategy module is executed for the photovoltaic power generation system,ω _th2 and (3) withR _B The method comprises the steps that a threshold value of a rotational speed deviation and a threshold value of a rotational speed change rate of a rapid power support strategy are respectively executed for a photovoltaic power generation system;

ωin order to synchronize the actual rotational speed of the generator rotor,ω ₀ d is the synchronous rotation speed of the rotorω/dtSuppressing the rate of change of rotational speed for photovoltaic power generation systems，ΔωIn order to be a deviation of the rotational speed,K _p is the gain of the proportional controller,P _mppt is the maximum output power of the photovoltaic;

(a) When dω/dt、ΔωWhen the index is within the safe interval, i.e.ω ₀ -ω _th1 <ω<ω ₀ +ω _th1 And |dω/dt|≤R _B Indicating that the system does not generate electromechanical oscillation or the oscillation process is completely finished, the photovoltaic power generation system should operate in a steady-state generation power mode at the momentP _PV =P ₀ WhereinP _PV The output power of the photovoltaic is represented by,P ₀ representing the photovoltaic steady-state output power, which corresponds to the working condition I;

(b) If the rotation speed index meetsω _th1 <|Δω|≤ω _th2 And |dω/dt|≤R _B At the moment, the photovoltaic power generation system is switched to a rotation speed sagging mode, and the rotation speed sagging mode corresponds to a working condition IV; the output power of the photovoltaic is obtained according to the rotation speed deviation: 1.5V _d K _p Δω+P ₀ Wherein V is _d The method comprises the steps that a voltage direct current component of the power grid voltage on a d axis under the action of a synchronous rotation coordinate transformation module is used;

(c) If deltaω>ω _th2 And |dω/dt|>R _B Indicating that the power of the power system is seriously absent at the moment, the photovoltaic power generation system increases the active power output in time so as to fill the absent power in the power system; its current commandI _d ^* Immediately increasing to the forward current limit, i.e. the current corresponding to the maximum output power of the photovoltaicI _mppt Continuously providing positive maximum power support to the grid, at this timeP _PV =P _mppt To significantly inhibit deltaω、dω/dtCorresponding to a working condition II;

(d) If deltaω<-ω _th2 Or |dω/dt|>R _B Indicating that the power of the power system is seriously excessive, the photovoltaic power generation system immediately reduces the output of active power to the maximum extent,its current commandI _d ^* Should be immediately reversely reduced to 0 so as to lead the output power of the photovoltaicP _PV Decreasing 0 provides reverse rapid power support to the grid to reduce power surplus, corresponding to condition iii.