Grid-connected Inverter Control Strategy of New Energy Microgrid

The traditional power system is composed of a synchronous generator. Because the rotor of the synchronous generator has the characteristics of the moment of inertia and damping, it can provide or absorb excess energy when the system’s frequency fluctuates. The inertia and damping of synchronous generators determine the frequency dynamic response process of the power grid, which further affects the operation, control, and protection of the whole power grid. However, because renewable energy is connected to the power grid by power electronic equipment, it does not have mechanical inertia and damping characteristics. With the increase of installed capacity of new energy, the whole power system shows low inertia characteristics. In this case, the power grid is more sensitive to interference, which may further lead to load shedding or system instability, or even system collapse. Virtual synchronous generator technology can solve this problem.


Introduction
With the exhaustion of fossil fuels and the requirement for low-carbon emission reduction, the global electric energy system is facing some changes.In recent years, due to the cost reduction of power electronic components and renewable energy technology, wind energy and solar photovoltaic power generation have been widely studied and applied.Distributed renewable energy is usually connected to the power grid by power electronic equipment.The modern power system is becoming more complicated than the traditional system based on synchronous generators because of its high permeability of new energy.From the perspective of power system operation, there are two main challenges to the high permeability of renewable energy.First of all, solar and wind power generation have randomness and fluctuation.Their output will change with the change of environment, which leads to the challenge to power system balance and resource adequacy during the peak demand period.Secondly, the new energy system is connected to the power grid through power electronic converters.The traditional synchronous generators are largely replaced by distributed power electronic equipment, which brings low inertia challenges to the power system and reduces the reliability and stability of the power grid.
Virtual synchronous generator technology can be applied to grid-connected photovoltaic systems.Through the improvement of power electronic converter control, the photovoltaic inverter has the ability of frequency support.When the photovoltaic inverter based on virtual synchronous generator technology participates in grid frequency modulation, the photovoltaic grid-connected system can provide additional energy according to the frequency change like the rotor of a synchronous generator.Therefore, the photovoltaic grid-connected system needs to be equipped with energy storage equipment or keep some active power.The former usually needs to transform the grid-connected system, and the cost of energy storage equipment is high.Still, it can not only provide or absorb the extra energy during frequency modulation but also restrain the power fluctuation of photovoltaic cells and smoothly input the power of the grid.The latter does not need additional hardware support, has simple control, and can realize large-scale transformation of photovoltaic systems.

Research status of photovoltaic grid-connected inverter
Because photovoltaic cells emit direct current and cannot be directly connected with the AC power grid, the direct current is inverted between photovoltaic cells and the power grid through power electronic equipment and then connected to the power grid.A power electronic converter is composed of power electronic devices, and the circuit model is essentially nonlinear, which will produce large harmonics in actual operation.Thus, it needs to be suppressed by hardware filters and advanced control means.
In order to reduce the injection of switching frequency harmonics of power electronic converters, Ltype, LC-type, or LCL-type high-order passive filters are used at the inverter output end of the photovoltaic grid-connected system to filter out higher switching frequency harmonics.In addition, the transformer-free inverter can be used to meet the requirements of high efficiency, small size, low weight, and low price.In this case, the modulation strategy needs to be specially designed to solve the leakage current problem.
At present, photovoltaic grid-connected inverters are connected to the power grid in two common ways, namely single-stage structure and two-stage structure, as shown in Figure 1 and Figure 2.Each grid-connected photovoltaic cell is composed of a series of parallel photovoltaic cells or photovoltaic series and then connected to the power grid through a DC-DC converter and inverter or directly through the inverter.The type of structure used in a photovoltaic grid-connected system depends on the voltage level and rated power.As shown in Figure 3, the centralized large-scale photovoltaic power station generally adopts a single-stage central inverter, and the overall system structure is relatively simple to achieve lower loss and cost.Large-scale photovoltaic cells are directly connected to the inverter, and the DC input voltage is usually as high as 1000 V or above.In this case, long-distance DC cables may lead to greater power loss and lower efficiency.In addition, because the control of the inverter realizes the functions of photovoltaic maximum power tracking and inverter at the same time, the control is difficult.Moreover, due to the influence of clouds or dust, the maximum power of different photovoltaic cells is not the same, and the single-stage grid-connected photovoltaic structure adopts a unified maximum power tracking method.Hence, the overall power generation efficiency of the system decreases.In contrast, the two-stage grid-connected photovoltaic structure is suitable for small and medium-power photovoltaic power plants.Because the output voltage of photovoltaic cells is low, the front-stage DC-DC circuit usually adopts a boost circuit and realizes the maximum power tracking function of photovoltaic cells.At the same time, the rear-stage inverter maintains the stability of the intermediate DC bus voltage and realizes power injection.

Virtual synchronization machine research status
In order to solve the problem of lack of inertia and damping characteristics of power electronic equipment, virtual synchronous generator technology came into being.Through the improvement of the control strategy, this technology enables the inverter to simulate the basic characteristics of synchronous generators.The control scheme obtains the phase and voltage amplitude reference values of the inverter output voltage by simulating the rotor swing formula and the reactive voltage droop formula.It realizes the virtual inertia, enhances the frequency stability of the system, and can realize seamless switching between the isolated island and grid-connected modes.Professor Zhong Qingchang put forward a scheme called Synchronverter, which not only has the advantages of the above methods but also further simulates the electromagnetic characteristics of synchronous machines.From the external characteristics, the inverter is closer to the synchronous generator, which can reduce the transformation of the system caused by the new energy generation grid connection.

Control principle of virtual synchronous machine
The traditional power system is composed of many synchronous generators (SG).By storing the rotational kinetic energy of SG, the inertial energy can be used to effectively alleviate or improve the influence of grid frequency change on the system.In order to meet the challenge of low inertia brought by large-scale power electronic equipment connected to the power system, the concept of virtual inertia is introduced and applied to the inverter to imitate the motion law of a synchronous motor.By improving the control algorithm, the characteristics of the inverter simulating SG are changed, and the virtual inertia is provided.This control method is called virtual synchronous machine (VSG) control.The principle of VSG droop control stems from SG's natural droop characteristics, in which voltage and frequency undergo droop load changes to adjust the exchange of active and reactive power with the power grid so as to control the frequency and amplitude of power grid voltage, respectively.
Consider VSG as an ideal controllable voltage source that is connected to an infinite power grid through a given line impedance, such as in Figure 3.The active and reactive power transmitted to the power grid is: . Simplified equivalent circuit of connection circuit between virtual synchronous machine and power grid where the sum is the active power and reactive power flowing from VSG(A) to the power grid (B), respectively, and is the voltage magnitude of VSG and power grid, corresponding to the phase difference between the two voltage vectors, line impedance, line impedance angle.
In addition, the power angle in these lines is quite small, so the sum can be assumed.Formulas (1) and ( 2) can be recalculated as: Formulas ( 3) and ( 4) show the direct relationship between power angle and active power and the direct relationship between voltage difference and reactive power.According to Formulas (3) and ( 4), through small signal analysis, the relationship between the change of active power and the change of grid frequency and the relationship between the change of reactive power and the change of voltage can be determined.The expression of this relationship is called droop control expression.
These droop relationships can be expressed by controlling the values of active power and reactive power output by the inverter to the grid and adjusting the grid frequency and voltage of the VSG grid connection point, which can be represented by the droop characteristic graph as shown in Figure 4.Among them, by changing the parameter settings of sum, that is, the slope of frequency and voltage droop characteristics.VSG will adjust its active power and reactive power reference according to its sum droop characteristics and participate in the adjustment of power grid frequency and voltage, respectively.Therefore, according to Formulas ( 5) and ( 6), the droop control loop of VSG is shown in Figure 5.Because the rotor of the synchronous generator has inertia, it can reserve certain rotational inertia energy.Inertia is an attribute of the object itself, which can be vividly understood as the resistance that hinders the change of the object's own motion state.The greater the inertia of an object is, the less it is affected by external forces.The inertia of an object is approximately proportional to its own mass.A high-quality object can resist the change of motion state more than a low-quality object.
In a synchronous generator, the moment of inertia, angular acceleration, and acceleration torque of the rotor have the following relations.


T J (7) For a given angular acceleration, a larger moment of inertia will lead to a higher acceleration torque.Since the rotor can approximate the shape of a cylinder, the formula for calculating the moment of inertia of a cylindrical object is as follows.
2   J r dm (8) The above formula shows that the greater the mass is, the greater the moment of inertia for cylinders with the same radius will be.The rotational inertia of synchronous generators in conventional power systems contributes significantly to the frequency stability of the overall power system.The rotor of SG, together with the moment of inertia of other motors (such as turbines), provides inertia to prevent frequency change and reduces the speed of frequency change.The fault of the power system or the change of system load will lead to an unbalanced power balance of the power system, which will lead to a frequency change.The inertia of the whole power system makes SG release or absorb energy.It slows down the frequency change, triggers the protection measures of other or new power generation units, and prevents them from causing excessive frequency change in the system, leading to major offgrid events and even major power outages.And the acceleration torque represents the resistance to the frequency change, which is expressed as: where  is angular acceleration, which is defined as follows:  d dt   (10) where the input torque of the prime mover makes the rotor of the synchronous generator rotate to generate the mechanical torque of electromotive force; T e is the electromagnetic torque, indicating the restraining torque of the pair;  is the angular frequency or mechanical rotor speed.The swing formula of the synchronous generator can be obtained as follows.

 
And there is the following relationship between mechanical power and electromagnetic power and corresponding torque.
In the steady state, there is no interference in the system, and the mechanical torque is equal to the electromagnetic torque.Therefore, the angular frequency of the synchronous generator remains unchanged at zero.However, due to the rotational inertia and the delay of the controller, the mechanical torque controlled by the main motor will not change immediately but only after a few seconds.Therefore, at the first moment of power interference, the frequency will change due to the difference between electromagnetic torque and mechanical torque.Depending on the size of the interference, there will be damping and continuous oscillation, which may be unstable.Obviously, for the same moment of inertia is larger, it will be smaller.Taking the frequency before power disturbance as an example, the formula is compared by integrating, the frequency can be obtained as follows: Therefore, a larger moment of inertia will lead to a lower frequency deviation.
Because the rotor of the synchronous generator has damping winding, there is damping torque in actual operation.The magnitude is related to the frequency change rate, as shown below.

 
where k TD is the damping torque coefficient.By substituting Formula (15) into the swing formula, it can be rewritten as: Since active power is a more common variable in power system research, Formula (16) can be written as: where k D is the damping coefficient.The power system swing formula is a nonlinear model caused by term.However, the frequency variation is considered low enough.Therefore, for small disturbances, the multiplication is considered to be constant.Therefore, the linear model of the swing formula is as follows.If there is a large-scale photovoltaic grid connection, its inertia will gradually decay, which will affect the stability of the power grid.In order to solve this problem, the swing formula of the virtual rotor and virtual excitation can be embedded in the control of the inverter to adjust the output power.The control inverter will work like a synchronous generator, providing the inertia and damping characteristics of traditional synchronous generators.The virtual rotor formula of the virtual synchronous machine is the same as Formula (18), and the specific control block diagram is as follows.The closed-loop transfer function of the active power control link of the virtual synchronous machine is: The transfer function is a typical second-order system from which the natural oscillation angular frequency and damping ratio can be obtained as follows: According to the above transfer function, the dynamic effect of unit step response of active power command under different moments of inertia and damping coefficient can be obtained, as shown in Figure 8 and Figure 9. Through the above analysis, as the moment of inertia gradually increases, the natural oscillation angular frequency and damping ratio of the second-order system gradually decreases.Thus, the oscillation becomes more intense, and the stable time becomes longer.With the increase of the damping coefficient, the natural oscillation angular frequency remains unchanged.In contrast, the damping ratio increases gradually, so the more stable and stable the response curve is, the shorter the time is, and the more stable the system is.

Figure 1 .Figure 2 .
Figure 1.Structure diagram of single-stage grid-connected photovoltaic power generation system

Figure 4 .Figure 5 .
Figure 4. Frequency and voltage sag characteristics rocking formula can be written as follows:

Figure 6 .Figure 7 .
Figure 6.Virtual rotor control block diagram 4. Simulation verification of the influence of moment of inertia and damping coefficient on VSG performance Figures 7 (a) and (b) show the response results of angular frequency under different moments of inertia and damping coefficient when the input power responds step by step.From Figures7 (a) and (b), it is known that for the same input power deviation step response, the greater the moment of inertia is, the slower the frequency changes and the longer the time for the frequency to reach stability.The larger the damping coefficient is, the better the suppression effect of frequency deviation is and the shorter the frequency recovery time is.

Figure 8 .
Figure 8.When D =20, the active unit of power step response corresponding to different moments of inertia.

Figure 9 .
Figure 9.When J =1 Kg•m 2 , the active unit of power step response corresponding to different damping coefficients.5.Conclusions1.Microgrids are small-scale networks that use low-power gas turbines, fuel cells, solar cells, ultracapacitors, flywheels, and batteries to provide energy.They can be connected to the larger grid or disconnected and operated independently when there are problems with the grid or power demand.2.Virtual Synchronous Machine (VSG) control technology has become the mainstream control of microgrid inverters, which has the advantage of simulating the external characteristics of synchronous generators and making grid-connected inverters have virtual inertia and damping.Moreover, under the control of VSG, grid-connected inverter can participate in the adjustment of grid voltage and frequency, which is more flexible and more widely used in microgrids.