Copyright © 2021 Inderscience Enterprises Ltd.
Multi-function control of small-scaled grid-connected PV systems
Esmaeil Zangeneh Bighash and Seyed Mohammad Sadeghzadeh*
Faculty of Electrical Engineering, Shahed University,
Tehran, Iran
Email: [email protected] Email: [email protected]
*Corresponding author
Esmaeil Ebrahimzadeh and Frede Blaabjerg
Department of Energy Technology, Aalborg University, Denmark Email: [email protected] Email: [email protected]
Abstract: The installation capacity of the grid-connected small-scaled PV systems in distribution grids is remarkable. Therefore, the capacity of these systems can play an important role in distribution grids. By applying a proper control, these systems can, in addition to their energy source, be used as a harmonic and voltage compensators at the point of common coupling (PCC).
This paper presents a multi-functions control scheme for small-scaled PV systems. In the presented approach, the control system, in addition to delivering the energy of the panels to the grid, will put the solar system in the harmonic and voltage compensation process at the PCC. Both the voltage and harmonic compensation strategies are implemented by on-line analysing the PCC voltage.
The structure in the proposed approach is a single-phase transformerless HERIC inverter with the LCL filter at the output. Finally, the results will be evaluated by MATLAB simulator and verified by experimental prototype.
Keywords: resistive-active power filter; R-APF; active power filter;
APF; dynamic voltage restore; DVR; static synchronous compensator;
STATCOM; photovoltaic; PV; uninterruptible power supply; UPS; low-voltage ride-through; LVRT.
Reference to this paper should be made as follows: Bighash, E.Z., Sadeghzadeh, S.M., Ebrahimzadeh, E. and Blaabjerg, F. (2021) ‘Multi-function control of small-scaled grid-connected PV systems’, Int. J. Power Electronics, Vol. 14, No. 2, pp.239–256.
Biographical notes: Esmaeil Zangeneh Bighash received his MSc in Electrical Engineering from the University of Tehran, Tehran, Iran, where he has also been a Lecturer for undergraduate lab courses. In 2018, he was graduated in PhD from the Faculty of Engineering, Shahed University, Tehran, Iran.
He has been a Visiting Scholar at the Aalborg University, Aalborg, Denmark, in 2016. His research interests include modelling, design, and control of
power-electronic converters in different applications like renewable energy systems, and his main current project is focusing on power quality of grid connected PV inverters.
Seyed Mohammad Sadeghzadeh received his ‘Doctorat de l’INPG’ in Electrical Networks from the Electrical Engineering Laboratory of the Institut National Polytechnique de Grenoble, France. He also holds a BSc in 1990, MSc in 1992 and PhD in 1997 in Electrical Engineering from the Sharif University of Technology, Tehran Iran. He has been a faculty member of the Electric Power Engineering Department of Shahed University in Tehran, Iran since 1998 where he has founded the Shahed University PV Solar Research Laboratory. He has been a member of Technology Executive Committee (TEC) of United Nation Framwork on Climate Change (UNFCCC) in Bonn, Germany representing Asian and Pacific countries in 2013–2015.
Esmaeil Ebrahimzadeh received his MSc in Electrical Engineering from the University of Tehran, Tehran, Iran, where he has also been a Lecturer for undergraduate lab courses. Since 2015, he was employed as a PhD Fellow at the Department of Energy Technology, Aalborg University, Aalborg, Denmark.
He has been a Visiting R&D Engineer at the Vestas Wind Systems A/S, Aarhus, Denmark, in 2017. His research interests include modelling, design, and control of power-electronic converters in different applications like renewable energy systems, and his main current project is focusing on power quality and stability analysis in large wind power plants. He is an IEEE student member and received the best paper awards at IEEE PEDG 2016 and IEEE PES GM 2017.
Frede Blaabjerg’s current research interests include power electronics and its applications such as in wind turbines, PV systems, reliability, harmonics and adjustable speed drives. He received 18 IEEE Prize Paper Awards, the IEEE PELS Distinguished Service Award in 2009, the EPE-PEMC Council Award in 2010, the IEEE William E. Newell Power Electronics Award 2014 and the VillumKann Rasmussen Research Award 2014. He was nominated in 2014, 2015 and 2016 by Thomson Reuters to be between the 250 Most Cited Researchers in Engineering in the World. In 2017, he became Honoris Causa at University Politehnica Timisoara (UPT), Romania.
1 Introduction
Nowadays, voltage and harmonic compensations in distribution grids are going to get more and more attention. For the voltage compensation, some interesting devices like DVR and STATCOM have been proposed and used. The DVR tries to compensate the voltage sag by injecting a series voltage to the grid (Nielsen et al., 2004; Somayajula and Crow, 2015). STATCOM is connected adjacent loads in parallel with the grid and helps the voltage stability by supporting the reactive power of some loads (Aziz et al., 2013;
Ota et al., 2015). Also for the harmonic compensation, APF and R-APF are more attractive. Like STATCOM, the APF and R-APF are connected adjacent the nonlinear loads in parallel with the grid. The APF compensator senses the current of the nonlinear loads and exactly supports the harmonic currents of these loads. In this case, the harmonic currents do not cross the communication lines and the power loss decrease (Salimi et al., 2017). APF needs to directly have current information of the nonlinear
loads and since the nonlinear loads are dispersed overall the distribution grid, it is difficult to access the currents of these loads. Therefore, harmonic compensation for these loads by the APF would be complicated and costly. In contrast to the APF, the R-APF does not need any current information of the nonlinear loads. In fact, these filters act like a virtual harmonic resistance and improve the grid voltage harmonics by sensing only the PCC voltage (Sun et al., 2013; Bai et al., 2018). However, due to the relationship between the electrical length of the feeder and wave-length of any frequency component, one R-APF is typically not practical at the end of a feeder and should be dispersed overall the feeder (Wada et al., 2002). Hence, using R-APF for distribution grid can be costly to.
Recently, installation capacity of the PV systems are remarkably increasing [European Photovoltaic Industry Association (EPIA), 2018]. Usually, these systems are connected to the grid in two capacity kinds, large PV for power plant and small-scaled PV. In this case, small-scaled PV systems are widespread in the distribution grid (Global Market Outlook for Solar Power 2016–2020). Hence, these systems can be an attractive option for the harmonic compensation in distribution grid. Therefore, by a proper control scheme, the small-scaled PV systems can act like the R-APF and STATCOM for both grid voltage and harmonic compensations and no additional costs are required when the PV systems are in distribution grids.
Different linear and nonlinear control methods have been presented for grid-connected small-scaled PV systems. Hysteretic controllers (Stefanutti and Mattavelli, 2006), proportional-resonant (PR) controller (Castilla et al., 2009), voltage oriented controller (VOC) (Monfared and Golestan, 2012), sliding mode controller (Hao et al., 2013), predictive controllers (Bighash et al., 2018), artificial intelligence controllers (Li et al., 2014), direct quadrature (DQ) rotating frame controllers (Mehrasa et al., 2016), and power-based (PQ) controllers (Mehrasa et al., 2019).
Among these control methods, PR controller which is based on an internal-modern theory, is widely used in the grid-connected voltage-source converters, such as DVR system UPS, PV inverters and active power filter APF.
In the PR controllers, the selective harmonic compensation is realised by cascading several proportional resonant blocks tuned to resonant at the selected low-order harmonic frequencies (Guo and Liu, 2010). Therefore, due to needing individual harmonic compensation, this paper presents a multi-functions controller based on cascade PR controller for single-phase grid-connected PV inverters.
In the presented approach, the control system, in addition to delivering the energy of the panels to the grid, will put the solar system in the harmonic and voltage compensation process at the PCC. Three control functions have been embedded. The first control function is for delivering the regular power from the PV panels to the PCC. The second control function is responsible for the PCC voltage compensation. In this case, when a voltage drop occurs at the PCC, the control system activates the LVRT operation for the PV inverter. Here, the PV inverter helps to recover the PCC voltage by injecting a proper reactive power to the PCC like a power plant. The last control function, in addition to deliver the power from the panels to the PCC, makes the PV inverter acts like an R-APF in order to improve the PCC voltage harmonics. In this case, the control system detects some specific harmonic components of the PCC voltage and improves these harmonics by using a virtual harmonic resistance compensator. Finally, a 1 kW transformerless HERIC inverter is used to connect the PV system to the grid.
This paper is organised as follows: first the structure of single-phase grid connected for PV systems is explained. Then, the control mode functions including normal function in Section 3, LVRT capability in Section 4, and PCC voltage harmonic compensation for the PV inverters are presented in Section 5, in Section 6, the current controller is illustrated. Simulation results are shown in Section 7 and Section 8. Finally, in Section 9, the conclusion part is discussed.
2 Single-phase grid-connected transformerless inverter
One of the most important factors for the PV system is its efficiency. Also according to German DIN VDE 0126-1-1 standard (DIN_VDE Normo, 2008), the injected common-mode current (leakage current) by the grid-connected PV system should be limited. This current may be limited by using a line transformer at the inverter output. However, the efficiency of the inverter is reduced. In order to address this challenge, transformerless inverters have been presented (Shen et al., 2012; Yu et al., 2011) and they are preferred. In this field, the HERIC inverter has a high efficiency with a low common-mode current (Zhang et al., 2014). Also using, an LCL filter at the output leads to high-frequency harmonic attenuation and operating in both stand alone and grid connected mode for a single-phase inverter (Liserre et al., 2005; Mohamed, 2011).
Therefore, in this paper a transformerless HERIC inverter with an LCL filter at the output is used to connect the PV system to the grid. Figure 1 shows the hardware structure and control system of the proposed scheme.
Figure 1 The proposed control scheme of two stage single-phase PV inverter (see online version for colours)
Sag Detection
Li Lg
C LCL Filter S1
S3 S2
S4 S6
HERIC Inverter AC Bypass PV Array
Vg+ - GridZg Full-Bridge
Sag Signal
Fundamental Current Reference
Calculation (5) Signal
Conditioning
S2 S3 S4 S5 S6 S1
PWM
DC DC
Upstream Control MPPT
Power Profile (Power References)
Sag Signal + -
+
- DC/DC Converter
Multi-functions controller Gate
Pulse
Harmonic Processing (SDFT) and
current reference calculation
PCC
+ -
+ -
+ -
+ + -
Gate Pulses
S5
Signal Conditioning
Signal Conditioning
3 PV system in normal operation
Most of single phase PV inverters are based on two stage energy conversion. The first stage includes a DC/DC converter which extract the maximum power point of the panels by MPPT control algorithms. The next stage is a grid-connected inverter which is responsible for delivering the DC link power to the grid.
Almost, the PV systems work only in normal operation. In this operation, PV system works with unity power factor to deliver the power PV panels to the grid. Therefore, in this control mode, the active power reference is equal to the injected power to the DC link by DC/DC converter (P* = PMPPT) and Q* = 0.
4 LVRT capability of PV inverter
Recently, due to some standard rules in some countries, the future generation of the PV inverters have been forced to support LVRT capability. In this regards, the PV inverter should inject reactive power during the fault time in order to recover the grid voltage.
Figure 2 shows the determined standard for LVRT strategy in some countries. Also Figure 3 shows the reactive current requirement in order to recovery of the PCC voltage by the grid connected inverters.
According to Figure 3, due to amount of the grid voltage, a certain reactive power reference is calculated for the inverter. In this case, different reactive power control strategies have been presented in Yang et al. (2014b). Among the proposed strategies, the constant peak current strategy is selected in this paper for LVRT operation.
Figure 2 LVRT requirements forced in some countries (Yang et al., 2014b, 2014a) for PV inverters (see online version for colours)
Figure 3 Reactive current injection requirements in E.ON grid code (see online version for colours)
Source: Neumann and Erlich (2012)
4.1 Constant peak current strategy
This strategy gives a guaranty to the inverter against the overcurrent and it will keep the inverter current around the nominal current. In this case, the active and reactive current references for the inverter are determined as:
(
1)
q pcc peak
I =k −v I (1)
2 2
d peak q
I = I −I (2)
2 2
peak N d q
I =I = I +I (3)
where vPCC is the PCC voltage in pu. IN and Ipeak are the inverter nominal current and peak current, Id and Iq are the active and reactive power currents for the inverter. Finally, due to Figure 3, k ≥ 2. The power reference for the inverter is calculated by:
* 1
2 d d
P = V I (4)
* 1
2 d q
Q = V I (5)
where P* is the active power reference and Q* is the reactive power reference. In LVRT operation, in order to keep safe the inverter against to overcurrent, the DC/DC converter should exit from the MPPT algorithm and decrease the injected power in order to keep the DC link in balance mode. In this case, the inverter delivers the active power P* determined from equation (4), and the DC/DC converter delivers this power, P*, to the DC link to keep it constant.
5 Improving PCC voltage harmonic components
In this case, the inverter acts like a virtual harmonic resistance by absorbing the harmonic components of the PCC voltage. Here the reference current determined from the controller has two parts. The first part is the fundamental current reference for delivering P* and Q* to the grid and the second part is the harmonic current reference in order to compensate the PCC harmonics.
* * *
1
g g gh
i =i +i (6)
where i*g1 is the fundamental reference calculated from equation (7) and i*gh is the harmonic current reference obtained from equation (14).
[ ]
**1 2 2 1_ 1_ *
1_ 1_
2
g PCC PCC
PCC PCC
i V V P
V V Q
=
+ α β
α β
(7) where VPCC1_α1 and VPCC1_β are the fundamental of orthogonal components for the PCC
voltage, which are calculated by a sliding discrete Fourier transform (SDFT) algorithm.
The harmonic current reference can be calculated as:
* PCCh
gh h
i V
= R (8)
where VPCCh is the harmonic components of the PCC voltage and Rh is the virtual harmonic resistance, which is automatically tuned for each harmonic component as will be described in the next sections.
5.1 Calculation of the PCC voltage harmonics (VPCCh)
The harmonic components of the PCC voltage are detected by SDFT. SDFT can individually detect each harmonic component of the PCC voltage. Figure 4 shows the block diagram of the SDFT algorithm.
Figure 4 SDFT algorithm for the kth harmonic
+ –
Re[Xk(n)]
Non-recursive part Sinθk
Resonator
Recursive part Comb filter
Z–1
Z–1 2Cosθk
+ – +
Z–N – + x(n)
Im[Xk(n)]
Cosθk
Source: Sumathi and Janakiraman (2008)
In Figure 4, Re[Xk(n)] is the real part and Im[Xk(n)] is the imaginary part for kth harmonic.
In this paper, some specific harmonics of the PCC voltage like 3rd, 5th, and 7th harmonics are considered for compensation.
In order to calculate the PCC voltage harmonic components, a dq-domain transform is needed. The SDFT algorithm detects the orthogonal signals in the αβ-domain and then αβ-domain is transformed into the dq-domain by park transform as:
_ _
_ _
cos sin
sin cos
PCCh d PCCh
PCC q PCCh
V ωh ωh V
V ωh ωh V
=−
α
β (9)
where VPCCh_α and VPCCh_β are the real and imaginary part of the PCC voltage (VPCCh) made by SDFT algorithm, also VPCCh_d and VPCCh_q are the dq-domain of the grid voltage.
Due to the nature of the dq-domain, VPCCh_q is zero and VPCCh_d is equal to the amplitude of the PCC voltage. Therefore, VPCCh_d is considered for calculating the harmonic components. Finally, the PCC voltage harmonics for each component is calculated as:
3_
3
1_
5 5 _ 1_
7 _ 7
1_
PCC d PCC d PCC d PCC d PCC d gPCC d
h V V h V
V h V
V
=
=
=
(10)
5.2 The harmonic current reference calculation
In this case, in order to protect the inverter against overcurrent, the proposed controller will dedicate a determined part from the inverter current capacity for the harmonic compensation. It means, the remaining current capacity between the fundamental current and maximum rating is reserved for the harmonic compensation as:
2 2
maxh max_inv max 1g
i = i −i (11)
where imaxh is the current capacity and the inverter can dedicate it for compensating the harmonics, imax_inv is the maximum current capacity of the inverter, which can be obtained from equation (12) and also imaxg1 is the peak of the fundamental grid current reference determined from equation (7).
Figure 5 Harmonic compensation scheme in a simple phase
SDFT transformdq Harmonic
calculations
Update the harmonic current
references
max_
max inv 2
PCC
i S
= V (12)
where VPCCmax is the peak of the PCC voltage and S is the complex power of the inverter.
In the proposed approach, imaxh should be dedicated for compensating each harmonic component. In order to share Imaxh among the harmonic components, an index H is defined as equation (13).
3 5 7
H=h +h +h (13)
Finally, due to index H and imaxh, igh* for compensating each harmonic is calculated by equation (14).
( ) ( ) ( )
*3 max 3
*5 max 5
*7 max 7
3 sin
5 sin
7 sin
g h
g h
g h
i h i ωt
H
i h i ωt
H
i h i ωt
H
= × ×
= × ×
= × ×
(14)
where ω3, ω5, and ω7 are the frequencies of each harmonic detected by the SDFT.
Finally, a block diagram for determining the harmonic current references is shown in Figure 5.
6 Current controller
The current controller in this paper is a cascaded PR controller. After determining the current references, equation (14), these references are compared with the measured grid current ig as equation (19), and consequently the error errh will be controlled by PR controllers. In this case, each current reference, for each harmonic component, is controlled by a separated PR controller as equations (15)–(18). Finally, the inverter voltage reference Vinv* is calculated by equation (20).
The PR transfer function and inverter voltage references for each harmonic component can be shown as:
1 2 1 2
1
2 2
i c
p p
c
k ωs
G k
s ωs ω
= +
+ + (15)
3 2 3 2
3
2 2
i c
p p
c
k ωs
G k
s ωs ω
= +
+ + (16)
5 2 5 2
5
2 2
i c
p p
c
k ωs
G k
s ωs ω
= +
+ + (17)
7 2 7 2
7
2 2
i c
p p
c
k ωs
G k
s ωs ω
= +
+ + (18)
( ) ( )
( ) ( )
* *
1 1 3 3
* *
5 5 7 7
g g g g
g g g g
err i i err i i
err i i err i i
= − = −
= − = − (19)
* 1 1 3 3 5 5 7 7
inv p p p p
V =err G +err G +err G +err G (20)
where kp and kih are the proportional and integrator gains at the order h. ωc is the cut-off frequency and ωh is the angular frequency at the fundamental and selected harmonic frequency.
7 Simulation results
Simulations have been conducted in Matlbab/Simulink using PLECS blocks. The system parameters are listed in Table 1. Three case studies are considered in the simulations.
In the first case study, the PV system works in normal operation with unity power factor.
The second case study is conducted when a voltage sag occurs at the PCC voltage.
Finally, in the last case, some harmonics are added to the PCC voltage and the inverter is controlled as a harmonic compensator to improve the PCC voltage harmonics.
Table 1 System parameters
Grid frequency ω = 2π × 50 rad/s
Grid voltage Vg = 230 V
Inverter rating S = 1 kVA
DC-bus voltage Vdc = 400 V
Switching frequency Fsw = 10 kHz
LCL-filter capacitance C = 2.35 µF
Inverter side inductance Li = 3.6 mH
Grid side inductance Lg = 708 µH
7.1 Normal operation
In order to keep the DC link voltage balanced in the nominal operation, the injected power to the PCC should be equal to the injected power to the DC link, P* = PMPPT. Therefore, assuming the injected power to the DC link is 1 kW and the inverter works in unity power factor, the active and reactive power references are P* = 1 kW and Q* = 0.
Figure 6 shows the results for this case. As it is seen in Figure 6, the inverter works in unity power factor and delivers 1 kW active power to the grid.
Also, in order to evaluate the performance of the controller, the active power reference is suddenly decreased from 1 kW to 500 W. Here, the controller should immediately reduce the injected power to the PCC from 1 kW to 500 W. Figure 7 shows the results of this case. Somehow, Figure 7(a) shows the grid voltage and injected current to the grid. Also Figure 7(b) shows the injected active and reactive powers to the grid. As it is shown in Figure 7(a), when the active reference decrease from 1 kW to 500 W the injected current decreased in order to deliver the power references to the grid.
Figure 6 Normal operation of the PV inverter, (a) PCC voltage and grid current (b) injected active and reactive powers to the grid (see online version for colours)
0.1 0.15 0.2 0.25
Time [s]
ig [7 A/div]
Vpcc [200 V/div]
0.1 0.15 0.2 0.25
-500 0 500 1000 1500
Time [s]
Active power [W]
Reactive power [Var]
(a) (b)
Figure 7 Normal operation of the PV inverter with a step in active power, (a) PCC voltage and grid current (b) injected active and reactive powers to the grid (see online version for colours)
0.2 0.25 0.3 0.35 0.4
Time [s]
ig [7 A/div]
Vpcc [200 V/div]
0.1 0.2 0.3 0.4 0.5
0 500 1000 1500
Time [s]
Active power [W]
Reactive power [Var]
(a) (b)
Figure 8 LVRT operation of the PV inverter, (a) PCC voltage and grid current (b) injected active and reactive powers to the grid (see online version for colours)
0.1 0.2 0.3
Time [s]
ig [10 A/div ] Vpcc [200 V/div]
0.1 0.2 0.3 0.4 0.5
0 500 1000 1500
Time [s]
Active power [W]
Reactive power [Var]
(a) (b)
7.2 LVRT operation
In this case study, during the inverter works in normal operation, a voltage drop occurs in the PCC. Therefore, the control system switches to the LVRT operation and injects a proper reactive power in order to recover the PCC voltage. In this case, a voltage drop occurs at 0.2 s and the PCC voltage drops to 0.6 pu. The results of this case study are shown in Figure 8, where the controller immediately switches to LVRT operation and due to the LVRT scheme, the controller injects a proper active and reactive powers.
7.3 Harmonic compensation
In this case study, there are some harmonic components in the PCC voltage which are the, 3rd, 5th, and 7th harmonic components. Therefore, by detecting the harmonic components in the PCC voltage, the proposed control system activates the harmonic compensator in order to decrease the PCC voltage harmonics.
Figure 9 shows the PCC voltage and the injected current before and after the harmonic compensator activation. In this case, before the harmonic compensation strategy, the inverter injects the fundamental current in order to deliver the power from the panels to the grid. After activating the compensator, the controller engages the extra capacity of the inverter to improve the PCC voltage harmonics.
Figure 9 Harmonic compensation strategy of the PV inverter, (a) PCC voltage and grid current, (b) the THD of PCC voltage without the harmonic compensation, (c) THD of the PCC voltage with the harmonic compensation (see online version for colours)
0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22
Time [s]
ig [7 A/div ] Vpcc [200 V/div]
Before compensation After compensation
0 200 400 600 800 1000
0 2 4 6
8 Fundamental (50Hz) = 325.2 , THD= 8.88%
Vpcc Mag (% of Fundamental)
Frequency (Hz)
Before compensation
(a) (b)
0 200 400 600 800 1000
0 2 4 6 8
Frequency (Hz) Fundamental (50Hz) = 324.4 , THD= 2.44%
Vpcc Mag (% of Fundamental)
After compensation
(c)
As it is shown in Figure 9(a), before the compensation, the injected current has only the first harmonic component and when the compensator is activated in 0.14 s, the injected current has other components in order to compensate the PCC voltage harmonics. Figure 9(b) and Figure 9(c) show the FFT for the PCC voltage harmonics before and after the harmonic compensation. After compensation, as it is shown, the harmonic components of the PCC voltage decreased.
Finally, in order to evaluate the ability of the proposed control in all control functions, in addition to activation of the harmonic compensator, the active and reactive power references change from 800 W to 600 W and from 0 VAR to 450 VAR at 0.14 s.
The results of this case are shown in Figure 10. In this case, as it is shown in Figure 10, considered active and reactive powers have been properly injected to the grid and the harmonics of the PCC voltage have been decreased.
Figure 10 Harmonic compensation strategy and reactive power injection of the PV inverter, (a) PCC voltage and grid current (b) injected active and reactive power strategies (see online version for colours)
0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22
Time [s]
ig [7 A/div]
Vpcc [200 V/div]
0.1 0.15 0.2 0.25
0 150 300 450 600 800
Time [s]
Active power [W]
Reactive power [Var]
(a) (b)
8 Experimental results
Experimental results have been conducted in a LAB with a 1 kW Danfoss inverter controlled by dSPACE 1007 and connected to the grid via a grid simulator, model Chroma 61845. The experimental parameters are considered like simulation tests.
Figure 11 shows the configuration setup. The experimental results have been shown in Figure 12, Figure 13 and Figure 14 for two conditions, LVRT operation and harmonic compensation.
Figure 12 shows the experimental results for LVRT operation of single phase grid connected inverter.
As it is shown in Figure 12(a), due to the constant peak current strategy, the amplitude of the injected current to the grid is constant during LVRT. The injected active and reactive powers have been shown in Figure 12(b). As it mentioned, in order to protect the inverter against the overcurrent, the injected active current is reduced which is done by constant peak current strategy.
Figure 11 LAB prototype of the grid connected inverter (see online version for colours)
Figure 12 Reactive power injection during harmonic compensation process, (a) grid voltage and current (b) injected active and reactive powers (see online version for colours)
Vg 100 V/div ig 5 A/div Time 40 ms ig
Vpcc
(a)
Time 40 ms
P
Q
450 Var
1000 W
(b)
Figure 13 Without harmonic compensation process, (a) grid voltage and current (b) harmonic order of the PCC voltage (see online version for colours)
Vg 100 V/div ig 5 A/div Time 20 ms Vpcc
ig
(a)
(b)
The experimental results for harmonic compensation process have been shown in Figure 13 and Figure 14. The first operation is without compensation and the inverter works in normal mode. In the second operation, the harmonic compensator is active.
In this case, in addition to deliver the energy of the panels, the inverter injects the harmonic current references to the PCC.
As it is shown in Figure 13(a), when the harmonic compensator isn’t work, the inverter only deliver the energy of the panels by injecting the fundamental harmonic component. Therefore, the harmonic components of the PCC voltage have not reduced.
Figure 13(b) shows the harmonic components of this operation.
On the other hand, Figure 14 shows the results for the PCC voltage. In this case, as it is shown in Figure 14(a), the inverter in addition to the fundamental current injects 3rd, 5th, and 7th harmonic components to the grid. Finally, reduced harmonic components for the PCC voltage is shown in Figure 14(b).
Figure 14 With harmonic compensation process, (a) grid voltage and current (b) harmonic order of the PCC voltage (see online version for colours)
Vg 100 V/div ig 5 A/div Time 20 ms Vpcc
ig
(a)
(b)
9 Conclusions
This paper presented a multi-functions control method for small-scaled grid-connected PV systems. The proposed scheme includes a cascaded PR controller, SDFT harmonic calculator, harmonic compensator and the LVRT operator. In the proposed approach, the PCC voltage condition is analysed real-time and by SDFT algorithm the harmonic components are extracted. In this case, if the PCC voltage is in normal condition, the controller works in normal operation else due to existing harmonics or voltage drop in PCC voltage, harmonic compensator or LVRT operator are activated.
Simulations with a couple of studies have been done in order to evaluate the proposed approach. The results show that the PV systems, in addition to deliver the energy of the panels to the grid, can participate in the voltage drop and harmonic compensation in distribution grids. In the proposed control method, the PV system is a versatile device in distribution grid. Therefore, the future of the PV systems can be smarter and work like a custom power device in order to improve the power quality.
References
Aziz, T., Hossain, M.J., Saha, T.K. and Mithulananthan, N. (2013) ‘Var planning with tuning of STATCOM in a DG integrated industrial system’, IEEE Trans. Power Del., Vol. 28, No. 2, pp.875–885.
Bai, H., Wang, X. and Blaabjerg, F. (2018) ‘A grid-voltage-sensorless resistive active power filter with series LC-filter’, IEEE Trans. Power Electron., No. 99, pp.1–1.
Bighash, E.Z., Sadeghzadeh, S.M., Ebrahimzadeh, E. and Blaabjerg, F. (2018) ‘High quality model predictive control for single phase grid-connected photovoltaic inverter’, Electr. Power Syst.
Res., Vol. 158, pp.115–125.
Castilla, M., Miret, J., Matas, J., Garcia de Vicuna, L. and Guerrero, J.M. (2009) ‘Control design guidelines for single-phase grid-connected photovoltaic inverters with damped resonant harmonic compensators’, IEEE Trans. Ind. Electron., Vol. 56, No. 11, pp.4492–4501.
DIN_VDE Normo (2008) VDE 0126-1-1-2006 Automatic Disconnection Device between a Generator and the Public Low-Voltage Grid.
European Photovoltaic Industry Association (EPIA) (2018) Solar Photovoltaics-competing in the Energy Sector [online] http://www.epia.org.
Global Market Outlook for Solar Power 2016–2020.
Guo, S. and Liu, D. (2010) ‘Proportional-resonant based high-performance control strategy for voltage-quality in dynamic voltage restorer system’, The 2nd International Symposium on Power Electronics for Distributed Generation Systems, pp.721–726.
Hao, X., Yang, X., Liu, T., Huang, L. and Chen, W. (2013) ‘A sliding-mode controller with multiresonant sliding surface for single-phase grid-connected VSI with an LCL filter’, IEEE Trans. Power Electron., Vol. 28, No. 5, pp.2259–2268.
Li, S., Fairbank, M., Johnson, C., Wunsch, D.C., Alonso, E. and Proano, J.L. (2014) ‘Artificial neural networks for control of a grid-connected rectifier/inverter under disturbance, dynamic and power converter switching conditions’, IEEE Trans. Neural Netw. Learn. Syst., Vol. 25, No. 4, pp.738–750.
Liserre, M., Blaabjerg, F. and Hansen, S. (2005) ‘Design and control of an LCL filter-based three-phase active rectifier’, IEEE Trans. Ind. Appl., Vol. 41, No. 5, pp.1281–1291.
Mehrasa, M., Pouresmaeil, E., Sepehr, A., Pournazarian, B., Marzband, M. and Catalao, J. (2019)
‘Control technique for the operation of grid-tied converters with high penetration of renewable energy resources’, Electr. Power Syst. Res., Vol. 166, pp.18–28.
Mehrasa, M., Rezanejhad, M., Pouresmaeil, E., Catalao, J. and Zabihi, S. (2016) ‘Analysis and control of single-phase converters for integration of small-scaled renewable energy sources into the power grid’, Proc. of IEEE PEDSTC, pp.384–389.
Mohamed, Y.A-R.I. (2011) ‘Suppression of low- and high-frequency instabilities and grid-induced disturbances in distributed generation inverters’, IEEE Trans. Power Electron., Vol. 26, No. 12, pp.3790–3803.
Monfared, M. and Golestan, S. (2012) ‘Control strategies for single-phase grid integration of small-scale renewable energy sources: a reviews’, Renewable and Sustainable Energy Reviews, Vol. 16, pp.4982–4992.
Neumann, T. and Erlich, I. (2012) ‘Modelling and control of photovoltaic inverter systems with respect to German grid code requirements’, Proc. of 2012 IEEE Power and Energy Society General Meeting, San Diego, CA, pp.1–8.
Nielsen, J.G., Newman, M., Nielsen, H. and Blaabjerg, F. (2004) ‘Control and testing of a dynamic voltage restorer (DVR) at medium voltage level’, IEEE Trans. Power Electron., Vol. 10.
Ota, J.I.Y., Shibano, Y. and Akagi, H. (2015) ‘A phase-shifted PWM D- STATCOM using a modular multilevel cascade converter (SSBC) – part II: zero-voltage-ride-through capability’, IEEE Trans. Ind. Appl., Vol. 51, No. 1, pp.289–296.
Salimi, M., Soltani, J. and Zakipour, A. (2017) ‘Experimental design of the adaptive backstepping control technique for single-phase shunt active power filters’, IET Power Electron., Vol. 10, No. 8, pp.911–918.
Shen, J.M., Jou, H.L. and Wu, J.C. (2012) ‘Novel transformerless grid-connected power converter with negative grounding for photovoltaic generation system’, IEEE Trans. Power Electron., Vol. 27, No. 4, pp.1818–1829.
Somayajula, D. and Crow, M.L. (2015) ‘An integrated dynamic voltage restorer-ultracapacitor design for improving power quality of the distribution grid’, IEEE Trans. Sustain. Energy, Vol. 6, No. 2, pp.616–624.
Stefanutti, W. and Mattavelli, P. (2006) ‘Fully digital hysteresis modulation with switching-time prediction’, IEEE Trans. Ind. Appl., Vol. 42, No. 3, pp.763–769.
Sumathi, P. and Janakiraman, P.A. (2008) ‘Integrated phase-locking scheme for SDFT based harmonic analysis of periodic signals’, IEEE Trans. Circuits Syst. II, Exp. Briefs, Vol. 55, No. 1, pp.51–55.
Sun, X., Zeng, J. and Chen, Z. (2013) ‘Site selection strategy of single frequency tuned R-APF for background harmonic voltage damping in power systems’, IEEE Trans. Power Electron., Vol. 28, No. 1, pp.135–143.
Wada, K., Fujita, H. and Akagi, H. (2002) ‘Considerations of a shunt active filter based on voltage detection for installation on a long distribution feeder’, IEEE Trans. Ind. Appl., Vol. 38, No. 4, pp.1123–1130.
Yang, Y., Blaabjerg, F. and Wang, H. (2014a) ‘Low-voltage ride-through of single-phase transformerless photovoltaic inverters’, IEEE Trans. Ind. Appl., Vol. 50, No. 3, pp.1942–1952.
Yang, Y., Wang, H. and Blaabjerg, F. (2014b) ‘Reactive power injection strategies for single-phase photovoltaic systems considering grid requirements’, IEEE Trans. Ind. Appl., Vol. 50, No. 6, pp.4065–4076.
Yu, W., Lai, J.S.J., Qian, H. and Hutchens, C. (2011) ‘High-efficiency MOSFET inverter with H6-type configuration for photovoltaic nonisolated ac-module applications’, IEEE Trans.
Power Electron., Vol. 26, No. 4, pp.1253–1260.
Zhang, L., Sun, K., Xing, Y. and Xing, M. (2014) ‘H6 transformerless full-bridge PV grid-tied inverters’, IEEE Trans. Power Electron., Vol. 29, No. 3, pp.1229–1238.
