An Islanded Microgrid Frequency and Voltage Control Method

Abstract

The present invention discloses an islanded microgrid frequency and voltage control method, which includes the following steps: Step 1, model the renewable energy and electric vehicle systems of the microgrid; Step 2, use a virtual synchronous generator to imitate the behavior of a synchronous generator and list the droop control equation; Step 3, obtain the discrete state equation of the virtual synchronous generator; Step 4, express the cost function in model predictive control according to the discrete equation; Step 5, in order to obtain better dynamic responses for frequency and voltage, adjust the active power and reactive power of the virtual synchronous generator. The present invention avoids the complex parameter adjustment process in traditional voltage and current double-loop control, predicts the change amount of the reference values of the active power and reactive power of the VSG, enhances the dynamic characteristics of the VSG, uses energy storage and electric vehicles to provide inertial support and power compensation, can make the generator output power more stable, and the proposed controller has a simple structure and can realize the comprehensive adjustment of damping and inertia.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability control, and in particular to a frequency and voltage control method for an islanded microgrid. Background Art

[0002] Microgrids use distributed energy sources to provide energy support for the power grid and have been widely used as controllable local energy grids. Many distributed energy sources, such as photovoltaic, wind energy, fuel cells, microturbines, and storage units, are connected to the AC bus of the microgrid through voltage source converters (VSCs). As renewable energy sources such as solar and wind energy increasingly penetrate into the power system, especially the distribution system, serious stability problems will occur, and thus the flexibility and reliability of the system will be affected. Renewable energy sources are connected using power electronic devices, which have different characteristics from synchronous generators, especially the lack of inertia. Therefore, the interface of such power electronics-based inverters will affect the global stability and inertia of the power grid and will cause problems of system frequency and voltage fluctuations. The simple current control methods used in these inverters do not naturally have inertia like traditional synchronous generators. There are secondary and tertiary control methods to maintain grid stability by keeping the voltage and frequency constant, but this method requires high communication between inverters, which will reduce the reliability of the system. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a frequency and voltage control method for an islanded microgrid, which uses the advantages of a virtual synchronous generator in providing inertia, damping oscillation, and voltage regulation to improve the power quality of the system.

[0004] To solve the above technical problem, the present invention provides a frequency and voltage control method for an islanded microgrid, including the following steps:

[0005] Step 1: Model the renewable energy and electric vehicle systems of the microgrid;

[0006] Step 2: Use a virtual synchronous generator to imitate the behavior of a synchronous generator and list the droop control equation;

[0007] Step 3: Obtain the discrete state equation of the virtual synchronous generator;

[0008] Step 4: Express the cost function in model predictive control according to the discrete equation;

[0009] Step 5: Adjust the active power and reactive power of the virtual synchronous generator to obtain better dynamic responses for frequency and voltage.

[0010] Preferably, in Step 1, modeling the renewable energy and electric vehicle systems of the microgrid specifically includes the following steps:

[0011] Step 11: Calculate the output power P of the photovoltaic power generation system PV :

[0012]

[0013] In the formula, ψ, and S are the irradiance, conversion efficiency and effective area of the solar cell array, and T A is the ambient temperature;

[0014] Step 12: Calculate the output power P of the wind power generation system wind :

[0015]

[0016] In the formula, C p is the capture efficiency, λ is the speed ratio, β is the pitch angle of the wind turbine, η is the efficiency, and ρ a is the air density, and V is the wind speed;

[0017] Step 13: Calculate the power change ΔP of the electric vehicle EV :

[0018]

[0019] In the formula, K EV and T EV are the battery gain and time constant of the electric vehicle respectively, and Δf is the frequency change.

[0020] Preferably, in Step 2, a virtual synchronous generator is used to imitate the behavior of a synchronous generator, and the specific steps for listing the droop control equation are as follows:

[0021] Step 21: The active-power - frequency expression in the droop control method is:

[0022] P = P ref + m(ω ref - ω g )

[0023] In the formula, m is the frequency droop coefficient, P ref and ω ref are the reference values of the active power and the VSG angular frequency respectively, and ω g is the measured angular frequency;

[0024] Step 22: The reactive-power - voltage expression in the droop control method is:

[0025] U = U ref + n(Q ref - Q e )

[0026] where n is the reactive power sag coefficient, Q ref and U ref are the reference values of reactive power and terminal voltage respectively, and Q e are the measured reactive powers respectively.

[0027] Preferably, in step 3, obtaining the discrete state equation of the virtual synchronous generator specifically includes the following steps:

[0028] Step 31: The inertia equation expression of the virtual synchronous generator is:

[0029]

[0030] where J and D are the moment of inertia and damping coefficient respectively, and P m is the mechanical power, and P e is the electromagnetic power, ω m and ω0 are the mechanical angular frequency and rated angular frequency respectively;

[0031] Step 32: The state space model of the virtual synchronous generator is:

[0032]

[0033] where ω = ω m - ω0. The fluctuation of the renewable output power will cause the power imbalance between the supply and demand sides, thus causing further frequency fluctuations. For VSG, the frequency fluctuation is small, approximately ω m ≈ ω0, and the output power P e can be regarded as a perturbation, and P m can be regarded as a controllable input variable;

[0034] Step 33: The discrete state equation of the virtual synchronous generator is:

[0035]

[0036] where T S is the sampling time.

[0037] Preferably, in step 4, expressing the cost function in the model predictive control according to the discrete equation specifically includes the following steps:

[0038] Step 41: The changes in frequency, mechanical and electrical can be written as:

[0039]

[0040] Step 42: The cost function considers the frequency deviation ω and rated power change P of the virtual synchronous generator m , and is expressed as

[0041]

[0042] Wherein, α and β are the weight coefficients of frequency and power changes respectively, and Δω(k) and ΔP m (k) are the angular velocity error and active power error at the k-th moment respectively.

[0043] Preferably, in step 5, in order to obtain better dynamic responses for frequency and voltage, adjusting the active power and reactive power of the virtual synchronous generator specifically includes the following steps:

[0044] Step 51, the change in the active power reference of the virtual synchronous generator is:

[0045] ΔP VSG (k) = ΔP m (k);

[0046] Step 52, the change in the reactive power reference of the virtual synchronous generator is:

[0047]

[0048] Step 53, the power reference expression of the virtual synchronous generator is:

[0049]

[0050] The beneficial effects of the present invention are as follows: In an islanded microgrid with high penetration, the present invention provides a method for suppressing frequency and voltage fluctuations of a virtual synchronous generator based on model predictive control, avoiding the complex parameter adjustment process in traditional voltage and current double-loop control, predicting the changes in the reference values of the active power and reactive power of the VSG, enhancing the dynamic characteristics of the VSG, and using energy storage and electric vehicles to provide inertial support and power compensation, which can make the output power of the generator more stable. The proposed controller has a simple structure and can realize the comprehensive adjustment of damping and inertia. Description of the Drawings

[0051] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0052] Figure 2 It is a schematic diagram of the control structure of the virtual synchronous generator of the present invention.

[0053] Figure 3(a) is a waveform diagram of the frequency when the load changes in the present invention.

[0054] Figure 3(b) is a waveform diagram of the frequency change rate when the load changes in the present invention.

[0055] Figure 4(a) is a waveform diagram of the grid change when the load is connected in the present invention.

[0056] Figure 4(b) is the waveform diagram of the power grid change when the load of the present invention is disconnected.

[0057] Figure 5(a) is the waveform diagram of the output power of the synchronous generator when the load of the present invention changes.

[0058] Figure 5(b) is the waveform diagram of the output power of the energy storage and electric vehicle when the load of the present invention changes. Detailed implementation manners

[0059] Figure 1 is the schematic diagram of the system structure of the present invention. The main energy sources are diesel generators and renewable energy sources, including a wind farm and a solar photovoltaic system; the electric vehicle and the energy storage system can both supply energy and absorb energy; finally, the household load is used as the electricity consumer. When the microgrid operates in island mode, frequency and voltage fluctuations will occur due to the lack of inertia support, so a virtual synchronous generator is needed to provide inertia and damping support.

[0060] Figure 2 is the control block diagram of the virtual synchronous generator using model predictive control, which avoids the complex parameter adjustment process in the traditional voltage and current double-loop control, corrects the power reference value of the virtual synchronous generator, reduces the changes in the grid frequency and voltage, and provides inertia support through the energy storage and electric vehicle.

[0061] The calculation steps of the output power of the renewable energy and electric vehicle in the microgrid are as follows:

[0062] Step 1, calculate the output power P of the photovoltaic power generation system PV :

[0063]

[0064] In the formula, ψ, and S are the irradiance, conversion efficiency and effective area of the solar cell array; T A is the ambient temperature.

[0065] Step 2, calculate the output power P of the wind power generation system wind :

[0066]

[0067] In the formula, C p is the capture efficiency; λ is the speed ratio; β is the pitch angle of the wind turbine; η is the efficiency; ρ a is the air density; V is the wind speed.

[0068] Step 3, calculate the power change ΔP of the electric vehicle EV :

[0069]

[0070] In the formula, K EV and T EV are the electric vehicle battery gain and time constant respectively; Δf is the frequency change.

[0071] The steps required for the model predictive control virtual synchronous generator (MPC-VSG) are as follows:

[0072] Step 1, the active-power - frequency expression in the droop control method is:

[0073]

[0074] In the formula, m and n are the frequency and reactive power droop coefficients respectively; P ref , Q ref , U ref , ω ref are the reference values of active power, reactive power, PCC terminal voltage, and VSG angular frequency respectively; ω g and Q e are the measured angular frequency and reactive power respectively.

[0075] Step 2, the inertia equation expression of the virtual synchronous generator is:

[0076]

[0077] In the formula, J and D are the moment of inertia and damping coefficient respectively; P m is the mechanical power, P e is the electromagnetic power; ω m and ω0 are the mechanical angular frequency and rated angular frequency respectively.

[0078] Step 3, the state space model of the virtual synchronous generator is:

[0079]

[0080] In the formula, ω = ω m - ω0, the fluctuation of the renewable output power will cause the power imbalance between the supply and demand sides, thus causing further frequency fluctuations. For VSG, the frequency fluctuation is small, approximately ω m ≈ ω0. The output power P e can be regarded as a perturbation, and P m can be regarded as a controllable input variable.

[0081] Step 4, the discrete state equation of the virtual synchronous generator is:

[0082]

[0083] In the formula, T S is the sampling time.

[0084] Step 5, the variations of frequency, mechanics, and electricity can be written as:

[0085]

[0086] Step 6, the cost function takes into account the frequency deviation ω and the rated power variation P of the virtual synchronous generator m , expressed as

[0087]

[0088] where α and β are the weight coefficients of frequency and power variations respectively; Δω(k) and ΔP m (k) are the angular velocity error and the active power error at time k respectively.

[0089] The steps for regulating the power reference of the virtual synchronous generator are as follows:

[0090] Step 1, the change in the active power reference of the virtual synchronous generator is:

[0091] ΔP VSG (k) = ΔP m (k)

[0092] Step 2, the change in the reactive power reference of the virtual synchronous generator is:

[0093]

[0094] Step 3, the expression of the power reference of the virtual synchronous generator is:

[0095]

[0096] The strategy for suppressing frequency and voltage fluctuations of the virtual synchronous generator using model predictive control (MPC-VSG) proposed in the present invention will be further described below with reference to FIGS. 3 to 5 as examples:

[0097] To compare the frequency and voltage fluctuations under load changes, a 5 kW load is connected at 3 s and disconnected at 7 s. FIGS. 3(a) and 3(b) show the comparison diagrams of the frequency and the rate of change of frequency under the traditional method and the proposed method under load changes. From FIG. 3(a), it can be seen that the maximum frequency change under the traditional method is about 0.7 HZ, and the frequency change under the proposed method is about 0.2 HZ, with the frequency change reduced by about 71%. The frequency fluctuation is reduced, significantly improving the power quality and enhancing the system stability. Along with the change of frequency, the rate of change of frequency is also worthy of attention. As shown in FIG. 3(b). Under the traditional method, the maximum rate of change of frequency is about 2.5 Hz / s, and under the MPC-VSG method, the rate of change of frequency is about 0.2 HZ / s, which is less than 0.6 Hz / s, meeting the requirements of the International Organization for Standardization.

[0098] Figures 4(a) and 4(b) are the curve graphs of the AC side voltage changes when the load is connected and disconnected. Figure 4(a) is the comparison graph of the voltage fluctuations under two control methods when the load is connected. The load is connected at 3 s. The voltage decreases by 5.6 V under the traditional method, and the voltage decreases by 4.5 V under the MPC-VSG method. Figure 4(b) is the comparison graph of the voltage fluctuations when the load is disconnected. The load is disconnected at 7 s. The voltage increases by 5.2 V under the traditional method, and the voltage increases by 3.8 V under the MPC-VSG method. The proposed method can reduce the voltage fluctuations by 19.6% and 26.9% respectively. The voltage fluctuations are effectively suppressed, and the voltage dynamic performance is improved.

[0099] Figures 5(a) and 5(b) are the curve graphs of the power outputs of the synchronous generator, energy storage, and electric vehicle under the traditional method and the MPC-VSG method. When the load is connected, the output power of the synchronous generator without energy storage and electric vehicle compensation immediately increases by 5 kW, and the change range is large. MPC-VSG uses energy storage and electric vehicles for compensation, which can provide additional power to make the rising amplitude of the synchronous generator smoother. When the load is disconnected, the output power of the synchronous generator without energy storage and electric vehicle compensation drops rapidly. MPC-VSG uses energy storage and electric vehicles to absorb the excess power to slow down the speed of the synchronous generator output power drop.

Claims

1. An islanded microgrid frequency and voltage control method, characterized in that It includes the following steps: Step 1: Model the renewable energy and electric vehicle system of Weaver Network; specifically, it includes the following steps: Step 11. Calculate the output power P of the photovoltaic power generation system PV : where ψ, and S are the irradiance, conversion efficiency, and effective area of the solar cell array, and T A is the ambient temperature; Step 12: Calculate the output power P of the wind power generation system wind : where C p is the capture efficiency, λ is the speed ratio, β is the pitch angle of the wind turbine, η is the efficiency, and ρ a is the air density, and V is the wind speed; Step 13, calculate the power change ΔP of the electric vehicle EV : where K EV and T EV are the electric vehicle battery gain and time constant, respectively, and Δf is the frequency change; Step 2: Use a virtual synchronous generator to imitate the behavior of a synchronous generator and list the droop control equations; specifically, it includes the following steps: Step 21: The active-power - frequency expression in the droop control method is: P = P ref +m(ω ref -ω g ) where m is the frequency sag coefficient, P ref and ω ref are the reference values of the active power and the VSG angular frequency respectively, and ω g is the measured angular frequency; Step 22: The reactive-power - voltage expression in the droop control method is: U = U ref + n(Q ref - Q e ) where n is the reactive power sag coefficient, Q ref and U ref are the reference values of reactive power and terminal voltage respectively, and Q e are the measured reactive powers respectively; Step 3: Obtain the discrete state equation of the virtual synchronous generator; specifically, it includes the following steps: Step 31: The inertia equation expression of the virtual synchronous generator is: where J and D are the moment of inertia and damping coefficient respectively, P m is the mechanical power, and P e is the electromagnetic power, ω m and ω0 are the mechanical angular frequency and the rated angular frequency respectively; Step 32: The state space model of the virtual synchronous generator is: where ω = ω m - ω0, the fluctuation of the renewable output power will cause the power imbalance between the supply and demand sides, thus causing further frequency fluctuations. For VSG, the frequency fluctuation is small, approximately ω m ≈ ω0, the output power P e can be regarded as a perturbation, P m can be regarded as a controllable input variable; Step 33: The discrete state equation of the virtual synchronous generator is: In the formula, T S is the sampling time; Step 4: Express the cost function in model predictive control according to the discrete equation; specifically, it includes the following steps: Step 41: The changes in frequency, mechanical and electric power are written as: Step 42: The cost function takes into account the frequency deviation ω and the rated power change P of the virtual synchronous generator m , which is expressed as where α and β are the weight coefficients of frequency and power variations respectively, and Δω(k) and ΔP m (k) are the angular velocity error and active power error at the k-th moment respectively; Step 5: Adjust the active power and reactive power of the virtual synchronous generator to obtain better dynamic responses for frequency and voltage.

2. The frequency and voltage control method for the islanded microgrid as claimed in claim 1, wherein In Step 5, to obtain better dynamic responses for frequency and voltage, adjusting the active power and reactive power of the virtual synchronous generator specifically includes the following steps: Step 51: The change in the active power reference of the virtual synchronous generator is: ΔP VSG y(k) = ΔP m (k); Step 52: The change in the reactive power reference of the virtual synchronous generator is: Step 53: The power reference expression of the virtual synchronous generator is:

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