In order to utilize the energy from the renewable energy sources, power conversion system is necessary, in which the voltage source inverter (VSI) is usually the last stage for injecting power to the grid. It is an economical solution to add the function of power quality conditioning to the grid-connected VSI in the low-voltage distribution system. Two multifunctional VSIs are studied in this paper, that is, inductive-coupling VSI and capacitive-coupling VSI, which are named after the fundamental frequency impedance of their coupling branch. The operation voltages of the two VSIs are compared when they are used for renewable energy integration and power quality conditioning simultaneously. The operation voltage of the capacitive-coupling VSI can be set much lower than that of the inductive-coupling VSI when reactive power is for compensating inductive loads. Since a large portion of the loads in the distribution system are inductive, the capacitive-coupling VSI is further studied. The design and control method of the multifunctional capacitive-coupling VSI are proposed in this paper. Simulation and experimental results are provided to show its validity.
Microgrids are emerging as a consequence of rapidly growing distributed power generation systems and energy storage systems [
Originally, the VSI for RES integration only transfers active power to the grid [
The control and implementation of the multifunctional VSI for renewable energy integration and power quality conditioning have been discussed in previous work [
Another group of grid-connected VSI, which is named capacitive-coupling VSI, has been used as power quality conditioners mainly for compensating reactive power and harmonics [
The two multifunctional VSIs are first compared in Section
In this section, the fundamental frequency power flow control capability of the two VSIs is first analyzed. The system configurations of the two grid-connected VSIs for renewable energy integration and power quality conditioning are shown in Figure
Comparisons for fundamental frequency power flow control.
Inductive-coupling VSI | Capacitive-coupling VSI |
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System configuration of the grid-connected VSI.
If the grid-side voltage and coupling impedance are fixed, the power flow varies in terms of the operation voltage of the inverter, as expressed in [
In (
In (
The 3-dimensional plot of the relationship between the operation voltage and the power to be transferred is shown in Figure
Operation voltage varies in terms of power flow.
The top views of the Figure
Top view of Figure
In order to reduce the operation voltage, the VSI is selected in terms of the reactive power required at the PCC. For example, only positive reactive power is required for improving the power factor at the PCC, when loads are inductive. Under this circumstance, the capacitive-coupling VSI is able to transfer active power and improve the power factor simultaneously with a lower operation voltage, as illustrated in Figure
As mentioned in previous part, more nonlinear loads are connected to the distribution system. The harmonic suppression capability is also necessary in many applications. The VSIs can reduce harmonics flowing from loads to the grid by injecting harmonic currents to the PCC. However, harmonic compensation increases the overall VSI operation voltage. When harmonic currents are injected together with the fundamental frequency current, the inverter output voltage is calculated as follows. Equation (
It is obvious that the impedance of the coupling branch at
Comparisons with harmonic suppression capability.
Inductive-coupling VSI | Capacitive-coupling VSI |
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See Figure |
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Coupling impedance varies in terms of harmonic order.
The coupling impedance of the capacitive-coupling VSI is the summation of the coupling inductor and capacitor. It is assumed that the impedance of capacitance
The impedance of the LC branch at
When more than one harmonic are compensated, the value of
The expression for determining the harmonic compensation voltage can be obtained, as shown in
The value of
If only harmonic at a specific frequency, that is,
Three cases are shown in Figure
Based on above analyses, the capacitive-coupling VSI also shows advantage when harmonic suppression capability is considered. The capacitive-coupling VSI is a good alternative to serve as grid-connected VSI with active power transfer, reactive power compensation, and harmonic suppression capabilities for the low-voltage distribution system.
In this section, the comprehensive design procedure of the multifunctional capacitor-coupling VSI is proposed. The system configuration is shown in Figure
With reference to (
Based on previous discussions and analysis, the detail procedure for the capacitive-coupling VSI design for minimum operation voltage under both fundamental and harmonic model is provided as follows. Select the coupling impedance according to
Calculate the coupling capacitance Calculate the coupling inductance according to
Determine the dc-link operation voltage according to
The dc-link voltage of the inverter is selected to satisfy the peak value of the inverter output voltage. The coeffcient
Figure
Control block diagram of the capacitive-coupling VSI.
The instantaneous reactive power theory (IRP) is used to calculate the load power [
Block diagram of the software PLL.
The grid voltage is then divided by its peak value
The output of the PLL block and the peak voltage
The reference currents are sent to the PWM unit; the output currents of the capacitive-coupling VSI are controlled to follow the reference, so that the capacitive-coupling VSI transfer active, reactive power to the grid as well as to compensate harmonics.
Simulation models are built by using PSCAD/EMTDC. Both inductive-coupling VSI and capacitive-coupling VSI are used to achieve renewable energy integration and power quality conditioning. The system configuration is given in Figure
System parameters in the simulation.
System setting | |
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Grid voltage |
220 V |
Source inductor |
0.1 mH |
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Inductive-coupling VSI | |
Coupling Inductor ( |
8 mH |
dc-link voltage ( |
370 V |
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Capacitive-coupling VSI | |
Coupling capacitor (Cc) | 130 uF |
Coupling inductor ( |
3.5 mH |
dc-link voltage ( |
160 V |
Simulation models.
The linear loads are connected to the grid and the VSI is plugged in at 0.1 s. The two multifunctional VSIs are controlled to inject active power and reactive power to the PCC. The simulated grid voltage, dc voltage, source current, and load currents are shown in Figure
Simulation results without harmonics compensation.
RMS | Active power | Power factor | Current THD | |
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Linear load current | 10.32 A | 1525 W | 0.66 | 0.1% |
Grid side (inductive-coupling VSI) | 5.51 A | 1062 W | 1.00 | 2.81% |
Grid side (capacitive-coupling VSI) | 5.59 A | 1058 W | 1.00 | 2.74% |
Simulation results without harmonic compensation: (a) inductive-coupling VSI and (b) capacitive-coupling VSI.
The nonlinear load is plugged in at 0.4 s. The simulation results are shown in Figure
Simulation results with harmonics compensation.
RMS | Active power | Power factor | Current THD | |
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Nonlinear load current | 13.5 A | 2162 W | 0.74 | 15.3% |
Grid side (inductive-coupling VSI) | 7.76 A | 1707 W | 1.00 | 2.75% |
Grid side (capacitive-coupling VSI) | 7.73 A | 1695 W | 1.00 | 2.00% |
Simulation results with harmonic compensation: (a) inductive-coupling VSI and (b) capacitive-coupling VSI.
A small capacity prototype of the capacitive-coupling VSI is built and the system configuration is as given in Figure
Experimental results of capacitive-coupling VSI.
Current RMS | Active power | Power factor | Current THD | |
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Linear load current | 2.57 A | 90 W | 0.64 | — |
Source current | 1.44 A | 72 W | 0.98 | 5.91% |
Nonlinear load current | 2.86 A | 99 W | 0.63 | 14.2% |
Source current | 1.50 A | 79 W | 0.98 | 6.61% |
Experimental results of capacitive-coupling VSI (a) without harmonics and (b) with harmonic compensation.
In this paper, two multifunctional VSIs for renewable energy integration and power quality conditioning are studied and compared. When the capacitive-coupling VSI provides reactive power for the inductive loads, its operation voltage is much lower than that of an inductive-coupling VSI. As a result, the system initial cost and operation losses are greatly reduced. The design and control system of the capacitive-coupling VSI are presented. The simulation and experimental results are provided to show the validity of the capacitive-coupling VSI in active power transfer, reactive power compensation, and harmonics suppression.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to acknowledge the Science and Technology Development Fund, Macao SAR Government with the project (072/2012/A3), and University of Macau Research Committee with the project (MYRG135(Y2-L2)-FST11-DNY) for the financial support.