1. Introduction
The development of wind turbines, photovoltaic cells and other renewable energy sources has increased the importance of implementing optimal control, operation and grid-connection of distributed generation (DG) units [
1,
2,
3]. In general, as a result of sudden load fluctuation or motor overload in a traditional power system, the rotor of a traditional synchronous generator (SG) releases its kinetic energy gradually to make up for the power shortage and restrain the system frequency from changing rapidly [
4]. However, distributed energy sources are connected to the power grid on a large scale through grid-connected inverter, and the DG units making up a distributed generation units lack mechanical inertia. As a result, when a power system based on these sources is disturbed, the power electronic inverter reacts rapidly, which results in a sudden change of all performance parameters that subsequently affects the stability of the power grid operations.
To deal with the instability in power grid operations in a system consisting of distributed energy sources, the concept of a virtual synchronous generator (VSG) was first proposed in [
5]. By introducing the rotor swing equation, the virtual inertia and primary frequency regulation instructions were conceived to simulate the external characteristics of the SG using this concept. The VSG can increase the inertia of the inverter and improve the system output response [
6,
7,
8,
9]. Although the VSG is equivalent to the traditional synchronous generator in terms of output characteristics, it is still difficult to maintain system stability under high transient conditions.
As a common control method of a voltage source in the islanded mode, the VSG control is similar to droop control: when the load changes, it causes a permanent frequency offset [
10]. Therefore, when the system is connected to the grid, a phase-locked loop (PLL) is always needed to obtain the frequency and the initial angle of the power grid [
11]. In [
12], a seamless transfer control based on the PLL was proposed. In this control method, the phase and magnitude of the load voltage were matched to the grid voltage, and there was no distortion of the load voltage and current when transferring from islanded mode to grid-connected mode. As the PLL measurement accuracy is often dependent on a parameter design, a synchronization method based on virtual impedance was proposed in [
13]. This method did not require a PLL. When the virtual current decreased to zero, there was no power exchange with the power grid and the output voltage was indirectly consistent with the power grid voltage state. However, it still took nearly one second to synchronize the two voltages. All the methods mentioned previously require a pre-synchronization process. As a result, when the output voltage is inconsistent with the grid voltage, the system is likely to become unstable.
To further improve the grid frequency response, virtual inertia and damping can be used for parameter design. In [
14], the inertia response of a synchronous motor was simulated by the energy storage system, and the frequency and power changes were reduced based on an optimal parameter selection algorithm. In [
15], the damping effect of the variable inertia scheme was investigated using transient energy analysis, and the influence on the stability of adjacent motors was discussed. In [
16], the effects of virtual inertia and damping were analyzed comprehensively to obtain fast and stable load disturbance frequency in grid-connected mode.
When dealing with high-order nonlinear systems, backstepping control (BSC) method can be applied. In this method, we can simplify the system in order to obtain low-order subsystems, and then intermediate virtual control variables and Lyapunov functions are selected in turn. The design of the control law of the whole system is completed recursively based on the Lyapunov stability principle [
17,
18]. However, with the increase of system order, the controller design becomes more complex as the analytic form of derivative of the virtual control quantity has to be calculated [
19]. The use of command filter can effectively reduce the computational complexity in the BSC method and enhance the anti-interference ability of the system [
20]. The traditional backstepping control needs accurate information of the controlled system model, and an adaptive method is widely used to deal with parameter uncertainty in the system modeling. In [
21], a feedback control method combining the backstepping control and fuzzy controllers was adopted, which could approximate nonlinear functions and achieve good tracking. In [
22], an artificial neural network was added to the backstepping control to control the induction motor drive system, which could efficiently estimate uncertain parameters online.
In this paper, the modeling errors of a VSG system are estimated using an adaptive method. These modeling errors include external bounded disturbances, virtual inertial and damping parameter variation disturbances. A projection operator is further introduced. This method can estimate uncertain parameters online and ensure the boundedness of these value. Several control strategies are compared in
Table 1.
Accordingly, the novel contributions of this paper are as follows:
A nonlinear controller is proposed and it adds a supplementary signal to the VSG system to guarantee the stability of the system in the islanded, grid-connected and transition modes.
The command filter is used to eliminate the differential expansion in the backstepping controller, and simplify the derivation process and controller design.
In order to improve the frequency response of VSG in the transition process, this paper focuses on the adaptive adjustment of virtual inertia and damping.
In view of actual model error and external disturbances, a projection operator is introduced. The use of the operator always keeps the estimated value of the parameter in the preset range, which ensures system convergence.
The rest of this paper is organized as follows. In
Section 2, a mathematical model of the VSG system is introduced, followed by the adaptive adjustment of virtual inertia and damping of VSG. In
Section 3, a state space model is obtained according to the structure of the controller. In
Section 4, the adaptive command filter backstepping (ACB) control is designed for the VSG system and the stability of the controller is proved using the Lyapunov stability theorem. In
Section 5, simulation results and comparisons with traditional control methods are presented to demonstrate the effectiveness of the proposed controller.
4. Controller Design
In view of the above model, the controller is designed by using the backstepping method. The commond filter is introduced to avoid the repeated derivation of the virtual controller, and the projection operator is used to keep the estimated values of parameters
and
within the preset range to ensure the convergence of the system. Define the error
where
is the given reference instruction,
and
are the output of the command filter. The structure of command filter is shown in
Figure 5.
In the design of backstepping controller, there is the problem of calculating expansion, and considering the input saturation in practice, the derivative of its virtual control variables can be obtained by integration using the commond filter [
20,
26]. The state equation of the command filter is given as
where
u is the input
of the command filter,
and
are amplitude limiting and rate limiting,
and
are the damping and bandwidth of the command filter. The derivative of the error can be obtained from Equation (
24) to Equation (
26)
Step 1. To stabilize Equation (
28), Lyapunov function was selected as
Take the derivative of Equation (
31) and substitute Equation (
28) into it
where
is a constant greater than 0. Select the virtual control variable
Replace
with
in Equation (
32), and
is obtained as
In order to compensate the filtering error of the command filter, the error variable is redefined as
where
is the error compensation signal of the command filter, and it is selected as
Step 2. To stabilize Equation (
29), Lyapunov function was selected as
Then we get the derivative of Equation (
37)
Take the derivative of Equation (
35), and using Equations (
28), (
33) and (
36), it can be calculated as
Then
is simplified to
where
is a constant greater than 0. Select the virtual control variable
Replace
with
in Equation (
40), and
is obtained as
When external interference occurs, model parameters (including parameters of
J and
D in the transition process) will fluctuate. In addition, it is assumed that the change of voltage is much faster than the change of current, which may lead to certain errors. Therefore, adaptive estimated values
and
are used to replace
and
, and Equation (
41) can be rewritten as
In order to compensate the filtering error of the command filter, the error variable is redefined
where
is the error compensation signal of the command filter, and it is selected as
Step 3. To stabilize Equation (
30), Lyapunov function was selected as
where
,
is the parameter estimation error,
and
are the adaptive gains. Take the derivative of Equation (
44), and using Equations (
29), (
43) and (
45), it can be calculated as
Then
is simplified to
where
is a constant greater than 0. Then the controller is designed as
The parameter adaptive law can be chosen as
where
is the projection operator,
is an estimate of
,
is the estimated error. Thus the adaptive law can be designed as
where
is a constant greater than 0,
x is the determined adaptive function, and the discontinuous projection operator is defined as
In [
27], it indicates that
has the following property for any
x:
According to the property of the projection operator, the estimated value of the parameter is within the preset range, and can be obtained
Substitute Equations (
52), (
53) and
u into Equation (
48) to obtain
Assuming that
, Equation (
54) can be rewritten as
According to [
28],
is a bounded function of time
t. If
, that is to say
, Equation (
55) can be rewritten as
The above formula indicates that
,
,
,
and
are uniformly bounded [
29], and the filtering error compensation signals
and
are bounded, so the tracking errors
and
are also bounded.
Figure 6 shows the block diagram of ACB controller for VSG system.
5. Simulation Results
This section verifies the effectiveness of the designed controller through simulation analysis. The main parameters of the electrical simulation model built based on Matlab/Simulink are shown in
Table 2. According to the Lyapunov stability theory, the appropriate
,
and
can be selected to achieve the rapid stability of the system. For a better control effect, the parameters in this paper are selected as
,
, and
.
5.1. VSG’s Connection and Disconnection to the Grid
In order to investigate effectiveness of the proposed controller, the system with a load of 13 kW is simulated to observe the performance parameters of VSG’s connection and disconnection to the grid. The moment of inertia and damping factor are selected as , , respectively.
Figure 7 shows the variation of frequency during the transition process and
Figure 8 exhibits the variation of active power under ACB control strategy. According to the simulation results, at first, the VSG system operates in islanded mode. In this case, the VSG system provides 13 kW active power to supply essential local loads, and the frequency decreases by about 0.074 Hz. Since there are no fault conditions in the grid, it is necessary to connect the system to the grid at
0.3 s. Due to the re-establishment of the power angle relationship, the active power has a certain vibration and the settling time is less than 0.1 s. By using the ACB control method, the maximum oscillation range is
kW. As
Figure 9 shows, the maximum oscillation range for the traditional backstepping method is
kW. It is evident that after the addition of the command filter and projection adaptive algorithm, the oscillation is reduced. After stabilization, about 3 kW active power is provided by the power grid, and the system frequency tends to be consistent with the power grid. At
0.6 s, due to a fault in the grid, the system is disconnected from the grid. No system reconfiguration changes are required during the transition from grid-connected to islanded mode. The VSG system output power increases immediately, and the frequency is reduced. It is obvious that during the transition process, the active power increases with no overshoot, and due to the existence of virtual inertia and damping, the system frequency changes smoothly. As can be seen from
Figure 10 and
Figure 11, using ACB control strategy, the voltage waveform basically remains unchanged except for the part that decreases due to reactive power, while the current waveform shows slight fluctuations when it is connected to the grid.
Figure 12 shows the variation of current under VSG control strategy without nonlinear controller. When connected directly to the grid, the current waveform is seriously unstable, and with the increase of the inconsistency between the output voltage and the grid voltage, the impulse current will be further increased, which must be avoided in normal operation.
The above results mainly verify that the designed controller can achieve smooth switching in the transition process, especially when connected to the power grid, the system can achieve improved stability without the need for pre-synchronous operation.
5.2. Load Power Disturbance and Power Distribution
Two systems run in parallel. System 1: kW, , , and the local load is 7 kW; System 2: kW, , , and the local load is 12 kW. At s, the common load changes from 10 kW to 3 kW. The simulation results are as follows.
Figure 13 exhibits the active power distribution of parallel systems.
Figure 14 shows frequency variation. As can be observed from the simulation results, before the load is removed, the active power of system 1 is 11.4 kW, and that of system 2 is 17 kW. The total load power is higher than the preset powers and the frequency offset occurs. After load variation, active power stabilizes at 8.8 kW and 13.2 kW, respectively. Since the total load power is lower than the preset powers, both system frequency increase and about 0.4 s later, the whole system becomes stable and the frequency of both subsystems will tend to be the same value (50.03 Hz). It can be noted that the generated active power of each system (11.4:17 kW and 8.8:13.2 kW) is approximate to the damping factor (
), and the
Figure 14 also shows the overshoot of frequency of system 2 is less then that of system 1. It can be indicated that a larger moment of inertia
J can increase the inertia of the system, that is, the system frequency can be changed smoothly by setting
J reasonably.
5.3. Frequency Improvement during Transition
The influence of moment of inertia and damping factor on frequency in the transition process is considered below. Increasing the moment of inertia
J at the initial stage of frequency fluctuation can reduce the frequency offset peak and the initial rate of change. When the frequency tends to be stable from the offset peak in grid-connected mode, increasing the damping factor
D can reduce the adjustment time of frequency stability.
Figure 15 shows the frequency response under different moment of inertia values. With the increase of
J, the frequency change rate decreases, and no overstepping occurs. However, it still takes a long time to reach the rated value. The main reason is that the active power after the grid connection does not reach the rated value, i.e., there is a power shortage. In
Figure 16, the moment of inertia remains unchanged and the damping factor increases instead. Initially, the damping factor is selected as
, and changes to
when the system is connected to the power grid. It can be observed that the frequency reaches the rated value more quickly with the increased damping factor, which proves that the damping factor can be increased appropriately to reduce the frequency adjustment time and quickly stabilize the system.
Since the damping factor will affect the steady-state deviation of the frequency, it will not change in islanded mode.
Figure 17 shows frequency response under different moment of inertia when disconnected from the grid, where
,
, and
is the adaptive moment of inertia. Accordingly, the steady-state moment of inertia
, the regulating factor
, and the threshold value
are selected. As can be seen in
Figure 18, with the rapid decrease of frequency, the adaptive
becomes larger to suppress the rapid change of frequency and avoid overshoot. When the frequency tends to the stable value, the rate of frequency change decreases, and a smaller
can further reduce the frequency adjustment time.