An Advanced Current Control Compensation Scheme to Improve the Microgrid Power Quality without
Using Dedicated Compensation Devices
A. Naderipoura, A. A. Mohd Zinb and M.H.
Habibuddinc
Faculty of Electrical Engineering, Universiti Teknologi Malaysia (UTM), 81310, Skudai, Malaysia
anaderipour@fkegraduate.utm.my, b*abdullah@fke.utm.my and cmhafiz@fke.utm.my
Josep M. Guerrero
Institute of Energy Technology, Aalborg University, Aalborg East DK-9220, Denmark
joz@et.aau.dk
Abstract— For grid-connected inverters (GCI), switching harmonics can be effectively attenuated through some methods. This paper present an advanced current control method for GCI of distributed generators (DGs), which responsible for the DG to controlling the power injection to the grid and the cancellation of the harmonics in Microgrid (MG) and between MG and Power Common Coupling (PCC). The current harmonics in the grid and MG are compensated without using dedicated compensation devices, such as active power filters (APFs), by the proposed current controller which is contain an advanced synchronous reference frame (ASRF) controllers. The merged control methods are proposed for the interface inverter to perform the comprehensive activity of compensating for the harmonics, such as a correction of the system imbalance and the removal of the harmonics. The simulation results are presented to demonstrate the effective performance of the proposed method.
Keywords— harmonic current, distributed generation (DG), grid- connected inverter, microgrid
I. Introduction
The increasing concern about the environmental pollution and fossil energy shortage has given a high impetus to the use of Distributed Power Generation Systems (DPGSs), such as solar energy, fuel cells, and micro-turbine. The output of solar cells, fuel cells and micro-turbine are dc voltage. Therefore, the grid-connected inverters play an important part in the distributed generation systems. A Microgeid (MG) is a local grid composed of Distributed Generators (DGs), energy storage systems and loads and can operate in grid-connected and island modes [1]. Because distortion sources can signify a high proportion of the total loads in small-scale systems the problem with power quality is a specific concern in MGs [2], [3]. Active Power Filter (APF) has been proved as a flexible solution for compensating the harmonic distortion caused by various nonlinear loads in power distribution power systems. In 1976, Gyugyi and Strycula [4] presented a family of shunt and series active power filters and established the concept of an APF consisting of a Pulse Width Modulation (PWM) inverter using a power transistor. Hybrid compensation has the advantages of both passive and APF for improving power quality problems
[5], but it is not cost-effective. Traditionally, the interface converters used in MGs have behaved as current sources when they are connected to the main grid. The interface converter controller must be able to cope with unbalanced utility grid currents and current harmonics, which are within the range given by the waveform quality requirements of the local loads and MGs. The primary goal of a power-electronic interface converter is to control the power injection. However, compensation for the power quality problem, such as current harmonics, can be achieved through appropriate control strategies. Consequently, the control of DGs must be improved to meet the requirements when connected to the grid. Blaabjerg et al in [6] has been analysed difficulties in the control of grid- connected inverters include low-harmonics current control, passive filters, dc-link voltage control and phase-locked loop (PLL).
The methods in these studies ([7] and [8]) have been proposed to compensate for current harmonics in grid- connected MGs. The other study [8] for the cascaded current and voltage control strategy has been proposed for the interface converter in MGs. M. Hamzeh et al. [9] proposed a control strategy, including a multi proportional resonant controller (MPRC) with adjustable resonance frequency and a harmonic impedance controller (HIC). Additionally, one author presented a control strategy for a multi-bus MV MG under unbalanced load conditions. A few controllers, namely, PI controllers implemented in the frame (also called the synchronous reference frame), the resonant controller, the PI controller implemented in the frame, and the DB predictive controller, were proposed in the literature [10]. In another study [11], the proposed methods were designed for the compensation of current imbalance at the DG terminal, while the power quality at the Point of Common Coupling (PCC) is usually the main concern due to sensitive loads that may be connected. The application of the active power filters as efficient interface for power quality improvement in distribution networks is gaining more attention with the advances in power electronics technology. However, the high cost of investment, poor performance under severe unbalanced and nonlinear load conditions are main challenges associated with active power
filters. Hence, it is important to propose the improved control schemes to enhance the power quality of the power system.
In this study, a new inverter hybrid control method for harmonic compensation is presented. The proposed control strategy consists of adjustable synchronous reference frame (ASRF) methods. The ASRF is proposed to control power injection to the grid, and used for harmonic current compensation. The focus of the present paper is the current quality at PCC, namely, the reduction of THD at the PCC and MG. Current harmonics and imbalance compensation in PCC and MGs as a new feature of the hybrid control methods are the main contributions of the present study. Furthermore, simulation studies are presented, discussed and analyzed.
II. Proposed Control Method
To enhance grid and microgrid current quality, an advanced current control method for the GCI is presented. The proposed method contains two units: the active and reactive power control unit and the harmonic current compensation unit. Fig. 1 presents a schematic block diagram of the proposed control strategy for its grid-connected inverter. This block diagram is applicable for main and remaining harmonic compensation, supplying reactive power for distortion sources and the correction of the system unbalance.
Fig. 1. Studied system configuration with nonlinear loads and dispersed generation sources.
III. Active and reactive power control unit
In the grid-connected power control mode, essentially all the available power that can be achieved from the Microturbine (MT) and the Photovoltaic (PV) is delivered to the grid.
Furthermore, compensation of the reactive power is possible. A block diagram of the control arrangement for the grid- connected control mode is shown in Figure 2.
Eq. (1)
&
Eq. (2) PLL
ia, ib
vab, vbc
Id
dq Iq
Vd
Vq
Vd
Vq
Iq*
Id*
Id*
Iq*
P* Q*
ϴ
SPWM abc
dq abc
dq
abc S1-S6
ωL 6
ωL
Fig. 2. Block diagram of active and reactive power control.
A few controllers, namely, the PI controllers, are implemented in the frame (also called the Synchronous Reference Frame (SRF)) to adjust the grid currents in the dq-synchronous frame.
The well-known SRF method can be used for control of the grid-connected inverter, and it also can be written as in [7].
This method uses a reference frame transformation module, abc to dq. The dq transformation can be used to convert the three phase currents injected by the inverter into three constant DC components defined as the direct, quadrature and zero components: Id, Iq and I0, respectively. In general, three phase voltages and currents are transformed into coordinates by the Park transformation, as shown by the matrix [L]:
> @ > @
0 0
ud uA id iA
uq L uB and iq L iB
u i
u C i C
ª º ª º ª º ª º
« » « » « » « »
« » « » « » « »
« » « » ¬ ¼ « » « » ¬ ¼
¬ ¼ ¬ ¼
(1)
> @
2 2
sin sin sin
3 3
2 2 2
cos cos cos
3 3 3
1 1 1
2 2 2
L
S S
D D D
S S
D D D
ª º
« »
« »
« »
« »
« »
« »
¬ ¼
(2)
The phase angle of the voltage and current signals are set as a reference current, which achieves the SRF while I*q=0. Refer to Figure 1 for the current controller that is under study [12].
The Sinusoidal Pulse Width Modulation (SPWM) voltage frame is guaranteed. The voltage reference and design PLL synchronize the inverter with the grid. Hence, I*q and I*d as the reference currents in the dq transform are re-calculated as:
The reference currents in the dq-axes, I*d and I*q, can be drawn from the following relations:
0 * *
Vq P
P V Id dV Iq q
o
Id Vd (3)0 * *
Vq Q
Q V Id qV Iq d
o
Iq Vd (4) where Vd and Vq are the grid voltages in the dq transform.Furthermore, the inverter is able to deliver P* and Q*, which are the reference active and reactive power, respectively.
The simplified active and reactive powers are calculated as:
P V Id d
Q V Iqd (5)
Equation (5) shows that, while the system voltage is constant (Vd), the currents of dq control the active and reactive powers.
The real power injection from GCI is controlled by the reference signal of I*d, whereas the reactive power is set to zero (I*d=0). The reference current I*d is extracted from dynamic analysis of the DC-Link capacitor. A constant DC voltage across the capacitor shows that the DG matches the power. The equation is shown as:
2 2( )
d VDC Pin Pout
dt C (6)
The DC-DC converter controls the power input (Pin) to the capacitor, to generate the maximum DG output power. To keep the inverter output voltage constant, the inverter controls the capacitor output power (Pout). The reference current is extracted from the difference between Pin and Pout using the Professional integral (PI) Controller.
* 1 ( ( ) ( ) )
Id Kp Pin Pout KI Pin Pout dt
Vd ³ (7)
From these parameters, the command voltages V*d and V*q
for the inverter gates SPWM can be acquired using:
* ( * ) ( * ) )
* ( * ) ( * ) )
Vd Kp Id Id KI Id I dtd L If q Vd
Vq Kp qI Iq KI Iq I dtq L If d Vq Z
Z
³
³ (8)
The command voltages are directed to the inverter for SPWM modulation. The control strategy applied to the interface converter usually includes two cascaded loops. An external voltage loop controls the DC-link voltage, and a fast internal current loop regulates the grid current. The DC-link voltage in this structure is controlled by the essential output power, which is the reference for the active current controller.
Typically, the dq control methods are associated with PI controllers because they have satisfactory behaviour when regulating DC variables [13].
the current reference iref and is also used to maintain the GCI synchronism with the grid.
IV. Harmonic current compensation control unit
In the control unit for the compensation of harmonic currents, a three-phase stationary rotating frame is converted to a synchronous rotating frame via Park′s transformation. The three-phase load currents are transformed in dq0 co-ordinates by
> @
L> @
0
iLd iLA iLq L iLB
iL iLC
ª º ª º
« » « »
« » « »
« » « ¬ » ¼
¬ ¼
(9)
Therefore, by averaging
i
Ld andi
Lq in domain>
0 2 S@
results in components
i
Ld andi
Lq, that is1 2
2 0
1 2
2 0
iLd iLdd t iLq iLqd t
S S Z
S Z
S
³
³
(10)
where
sin 2
2 3
3 2
sin 3
iLAsin t iLB t iLd
iLC t
Z Z S
Z S
ª º
« »
« »
« »
¬ ¼
(11)
cos cos 2
2 3
3 2
cos 3
iLA t iLB t iLq
iLC t
Z Z S
Z S
ª § ¨ · ¸ º
« © ¹ »
« »
§ ·
« ¨ ¸ »
« »
¬ © ¹ ¼
(12)
2 2
( )
1 3 1 3
i i
aA id t and bA i tq (13)
The three-phase distorted currents of the three-phase circuit can be represented as follows:
( )1sin ωt ( )1cos ωt
2π 2π
( ) ( )
sin 1 (ωt 3) 1 cos (ωt 3)
2π 2π
( )1sin (ωt 3) 1 cos (ωt 3)
i i
aA bA
iLSA i i
iLS iLSB ab bb
iLSC aci bci
ª º
« »
ª º « »
« » « »
« » « »
« »
¬ ¼ « »
« »
¬ ¼
(14)
Equation (13) gives the relationship between the dc component of
i
Ld,i
Lq and the coefficients ofi
Ls, the compensating objective of the proposed control method (PCM). Equation (14) is the compensating objective of the PCM. In practice, it is importance to find out the optimum compensating objective of PCM. The compensating currents provided by PCM is:iPCM iLiLS (15)
Substituting (13) in (14) gives
i
Lsand substitutingi
Lsin (15) gives iPCM with iLknown. Here,i
Lsand iPCM are calculated in abc co-ordinates.The three-phase load currents are transformed indq0co- ordinates as follows
> @
0
ud uA
uq L uB
u uC
ª º ª º
« » « »
« » « »
« » « » ¬ ¼
¬ ¼
(16)
Similarly, the averages of
u
d andu
q are calculated, and the coefficients ofu
A are2 2
( )
1 3 1 3
u u
aA ud t and bA u tq (17)
Hence, the following equations can be obtained
sin sin( 2 )
2 3
3 2
sin( )
3 uA t uB t vd
uC t
Z Z S
Z S
§ ·
¨ ¸
¨ ¸
¨ ¸
© ¹
(18)
cos cos( 2 )
2 3
3 2
cos( )
3 uA t uB t vq
uC t
Z Z S
Z S
§ ·
¨ ¸
¨ ¸
¨ ¸
© ¹
(19)1( )
0 3
v vAvBvC (20)
The control variables then become dc values; consequently, filtering and controlling can be easily achieved.
V. SIMULATION RESULTS
In a basic MG architecture (Figure 3), the electrical system is assumed to be radial with several feeders and a collection of loads. The DGs, such as the MT, and PV, are connected to the system through interface converters.
Grid
PCC
ZS Zt
MG Bus
Case Study I
DGs
&
NLLs
(a)
PCC ZS Zt
MG Bus
Case Study II
MT PV P
Proposed Control Method
P Grid
(b)
Fig. 3. (a) Studied system configuration: (a) nonlinear loads and dispersed generation sources without proposed control method; (b) Grid-connected inverter with proposed control method.
The parameters of load/DGs and control method parameters can be found in Tables 1and 2 respectively.
Table I. N-Load/DG Parameters and Conditions for the System
Load/DGs Parameters Values
Microturbine
Inverter switching frequency 4 kHz
Inverter resistance 4 Ω
Inverter capacitance 5 μF
DC-link voltage 545 V
Photovoltaic Inverter resistance 0.2 mΩ
DC-link voltage 675
Rating of nonlinear load 1 RL 30 kW, 10 kVAr 13A Rating of nonlinear load 2 Resistor 0.3 Ω 24A
Table II. control parameters
Controller Parameter value
Vdc (V ) 688
Proportional gain (K p) 0.4
Integral gain (Ki) 10
Fundamental Frequency (H z) 50 Vabc (V ) and Vabc DG( V ) 220
Iabc (A) and Iabc DG ( A) 260
This MG includes three DGs, such as the MT, the FC and PV which are connected to the grid by the GCI. The proposed control methods are applied to the MT and PV; however, the FC is connected to the grid by the ordinary interface converter without the control strategy.
In this system, the voltage is assumed sinusoidal.
To demonstrate the effectiveness of the proposed control method, the system in Figure 3 was simulated in MATLAB/Simulink. In the simulation, two case study are taken into account.
Case study I: Without any compensation.
Case study II: With the absence of compensation devices and using just propose control method.
A. Case study I
In this case study, the resulting system waveforms are shown in Figure 4 without any compensation devices. DG sources and nonlinear loads make the system current non-linear and unbalanced.
0.1 0.11 0.12 0.13 0.14 0.15
-200 0 200
Grid (A)
T ime (s) 180.4(A) Ia
(a)
0.36 0.37 0.38 0.39 0.4 0.41
-50 0 50
Photovoltaic (A)
T ime (s) 46.76(A) Ia
0.2 0.21 0.22 0.23 0.24 0.25 0.26
-40 -20 0 20 40
Micro-turbine (A)
T ime (s) 27.37 (A) Ia
(c)
Fig. 4. Grid and DG units current waveforms without any compensation: (a) Grid currents; (b) PV currents; (c) MT currents.
B. Case study II
This case study has an improved power quality with the absence of compensation devices such as PF, LCL and APF in the MG. The compensated system currents are explained in this subsection. Figures 5 (a), (b) and (c) show the effective compensation values of the harmonic current for the DG unit and the system, respectively. This case study reveals that there has been a proposed control method which can compensate for the current system (PCC) and DGs with the absence of power compensation devices.
0.06 0.07 0.08 0.09 0.1 0.11 -500
0 500
Grid (A)
T ime (s)
761.1 (A) Ia
(a)
0.39 0.4 0.41 0.42 0.43 0.44 -200
0 200
Photovoltaic (A)
T ime (s)
Ia 244.7 (A)
(b)
0.21 0.22 0.23 0.24 0.25 0.26 -500
0 500
Micro-turbine (A)
T ime (s)
443.7 (A) Ia
(c)
Fig. 5. Grid and DG unit current waveforms with propose control method; (a) system currents; (b) Photovoltaic; (c) Micro-turbine.
When all of the loads and DGs are connected, the THD current without any compensation was 13.43%. As shown in Figure 5(a), THD is reduced to 2.27% in the proposed control method. The simulation results of Figures 4 (b) and (c) shows the performance of the proposed control method to compensate the distorted waveform of Figures 5 (b) and (c). Consequently, it is capable of meeting the IEEE 519-1992 recommended harmonic standard limits. The Current and THD value of study system in case study II (after propose control method) is given in Tables 3.
Table III. Current and THD Results Before
Compensation
After Propose Control Method Current (A) THD % Current (A) THD % Grid 180.4 13.43 761.1 2.27
PV 46.76 8.66 244.7 1.38
MT 27.37 38.23 443.7 2.44
VI. Conclusions
This study proposes a new control strategy for harmonic current compensation for DG interface converters in a MG and between MG and PCC. The proposed control method consists of the ASRF control methods. When nonlinear, unbalanced loads and DGs are connected to the grid, the proposed strategy significantly and simultaneously improves the THD of the grid- connected inverter of DGs and the grid current.
In the proposed method, the harmonic currents of the nonlinear loads and DGs in the MG and PCC are completely compensated. The proposed method is responsible for controlling the power injection to the grid, is responsible for compensating for the harmonic current due to the unbalanced load. The simulation results verify the feasibility and effectiveness of the newly designed control method for a grid- connected converter in a MG.
Acknowledgment
The authors sincerely would like to express their appreciation to the Universiti Teknologi Malaysia (UTM) for supporting this work through GUP Grant (Vote No : 15H65) and Ministry of Higher Education (MOHE) for providing funds to carry out the research reported in this paper. Furthermore, I would also like to thank Aalborg University, Aalborg East, Denmark, for their cooperation.
References
[1] D. G. Photovoltaics and E. Storage, “IEEE Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems,” 2011.
[2] A. Naderipour, A. A. M. Zin, and M. H. Bin Habibuddin, “IMPROVED POWER-FLOW CONTROL SCHEME FOR A GRID-CONNECTED MICROGRID,” Electron. WORLD, vol. 122, no. 1963, pp. 42–45, 2016.
[3] A. A. M. Zin, A. Naderipour, M. H. Habibuddin, and J. M. Guerrero,
“Harmonic currents compensator GCI at the microgrid,” Electron. Lett., 2016.
[4] L. Gyugyi and E. C. Strycula, “Active ac power filters,” in Proc.
IEEE/IAS Annu. Meeting, 1976, vol. 19, pp. 529–535.
[5] Z. Chen, F. Blaabjerg, and J. K. Pedersen, “Hybrid compensation arrangement in dispersed generation systems,” Power Deliv. IEEE Trans., vol. 20, no. 2, pp. 1719–1727, 2005.
[6] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V Timbus, “Overview of control and grid synchronization for distributed power generation systems,” Ind. Electron. IEEE Trans., vol. 53, no. 5, pp. 1398–1409, 2006.
[7] Q.-N. Trinh and H.-H. Lee, “An Enhanced Grid Current Compensator for Grid-Connected Distributed Generation Under Nonlinear Loads and Grid Voltage Distortions,” 2014.
[8] Q.-C. Zhong and T. Hornik, “Cascaded current–voltage control to improve the power quality for a grid-connected inverter with a local load,” Ind. Electron. IEEE Trans., vol. 60, no. 4, pp. 1344–1355, 2013.
[9] M. Hamzeh, H. Karimi, and H. Mokhtari, “Harmonic and Negative- Sequence Current Control in an Islanded Multi-Bus MV Microgrid,”
Smart Grid, IEEE Trans., vol. 5, no. 1, pp. 167–176, 2014.
[10] A. Timbus, M. Liserre, R. Teodorescu, P. Rodriguez, and F. Blaabjerg,
“Evaluation of current controllers for distributed power generation systems,” Power Electron. IEEE Trans., vol. 24, no. 3, pp. 654–664, 2009.
[11] N. B. Lai and K.-H. Kim, “An Improved Current Control Strategy for a Grid-Connected Inverter under Distorted Grid Conditions,” Energies, vol. 9, no. 3, p. 190, 2016.
[12] H.-L. Jou, W.-J. Chiang, and J.-C. Wu, “A simplified control method for the grid-connected inverter with the function of islanding detection,”
Power Electron. IEEE Trans., vol. 23, no. 6, pp. 2775–2783, 2008.
[13] E. Twining and D. G. Holmes, “Grid current regulation of a three-phase voltage source inverter with an LCL input filter,” Power Electron. IEEE Trans., vol. 18, no. 3, pp. 888–895, 2003.
[14] L. N. Arruda and S. M. Silva, “PLL structures for utility connected systems,” in Industry Applications Conference, 2001. Thirty-Sixth IAS Annual Meeting. Conference Record of the 2001 IEEE, 2001, vol. 4, pp.
2655–2660.
