Parallel inverter control for power sharing in
islanded operation of AC micro-grid
Guo-Jie Li
1, Suleman Haider
2 1,2Department of Electrical Engineering/Shanghai Jiaotong, University/China
Abstract
The key issues related to Micro-grids (MG) have concentration to massive struggle to expand their penetration in power systems especially when system works parallel in the islanded mode. The distributed generators (DGs) in the islanding operation of the micro-grid are commonly coupled via inverters to an AC distributed system.The aim of this study is to investigate the feasibility of power sharing among parallel distributed generators (DGs). In this paper modeling and control of parallel inverters are presented. The frequency and voltage variation caused by connecting two inverters in parallel is an issue in droop control strategy. To address these issues, the DG units are interfaced with two parallel inverters in an islanded mode. The enhanced droop control implemented for frequency restoration and voltage control among the distributed generators with predefined droop characteristics. The PQ control is investigated for controlling output real and reactive power of the distributed generator by assigning their references. The study proposes islanded operation in the AC Micro-grid without communication link under enhanced droop control. The inverter has been implemented using the MATLAB/SIMULINK environment. The results show validity and power sharing performance of the system while maintaining stable operation when the DGs work in the islanding mode.1
Keywords: Micro-grid, islanded mode, enhanced droop control, parallel inverter control.
I. INTRODUCTION
The key issues related to Micro-grids (MG) have concentration to massive struggle to expand their penetration in power systems especially when system works parallel in the islanded mode [1]. The distributed generators (DGs) in the islanding operation of the micro-grid are commonly coupled via inverters to an AC distributed system. Various kinds of techniques have been introduced for controlling the inverter parallel operation or in particularly for the power sharing under combining different controlled schemes [2]. The concepts of micro-grids contain operation, control structure, power and voltage regulation and energy management [3]. The basic structure based on one or several renewable energy sources (RES), types of load and energy backup systems integrated together [4]. Meanwhile, the power level of the utility grid is greater than that of the micro grid. The actual performance is judged when the micro grid works in the islanded mode [5]. The micro-grid control and operation as well as switching among the operation modes are the challenges and these challenges to be solved by using the micro-grid competently. A MG is modeled by various energy generators which are linked to the point of common coupling (PCC) that feeds the required loads. For increasing the reliability and decreasing the transmission losses the whole system should be intelligent to work under grid tie or an islanded mode [6].
In grid tie mode the micro-grid supplies energy to the grid or desired load, charging backup, etc. In this scenario, the MG works as current source inverter. Hence, the inverter acts as voltage supporter and the distributed power handled through real power reference which is linked to the generated energy. However, the power flow structure becomes a sensitive situation caused by the fluctuation of grid [7]. Moreover, in the islanded mode, a decentralized method is commonly used and has benefit to work as wireless control operation. It is useful specially, when a number of distributed generators are placed far from each other and have no chance of communication connection among them [8]. However, droop control has numerous limitations and challenges, such as frequency and voltage dropped by
1
changing load. In case of inverter failure, all operation continuously works under normal conditions without affecting the other inverters. The key purpose of this control methodology is to control of whole system in which the DGs are responsible for power delivery [9].
This paper assumes that, at first, the both DGs work as the inverter interface. The losses of inverters and harmonics are negligible; it has showed two different control strategies for the islanded AC micro-grid. The P-f/Q-E droop controller delivers the control real and reactive power and provides frequency and voltage regulation. A decentralized droop control method is proposed with Frequency Restoration Scheme (FRS) to restore the frequency and provide exact real and reactive power. Moreover, the PQ control goal is to adjust the power tracking and to achieve fast dynamic response.
The following is organized as: section 2 presents combined PQ control and droop control. Section 3 presents the Frequency Restoration Scheme (FRS) and voltage control. Section 4 shows the simulation results and Section 5 is conclusion.
II. COMBINEDPQCONTROLANDDROOPCONTROL
Fig.1 shows the parallel inverters PQ control and droop control strategies, in which the enhanced droop control system takes decision according to the situation. The micro-grid under islanding operation where, one inverter maintains the voltage stable and the other one can run as instructed demand operation mode. It can be controlled to track the maximum set- point of power. When the DGs work together and neglecting the losses in power, the actual
power delivered to the load is with the following rulePtotal=P1+P2.
Fig.1.Parallel structure of micro-grid A. PQ Control
In PQ control mode, the inverter is used to deliver the given real and reactive power. The controller consists of current and power control loop. The operation of PQ control based on DQ reference frame defines the components of d-axis and q-axis of ac side current.
In Fig.2 the reference frame d-axis is in phase with the grid voltage and it is responsible for controlling the real power (P). The q-axis is responsible to control the reactive power (Q) of the inverter. Under the DQ coordinate system the inverter output, real and reactive power can be described as shown in Fig.3.
Fig.3:Power controller schematic
In Fig.3, the main objective of controlling active and reactive power is to control the current or we can say that, at AC side inverter active and reactive power are controlled through tracking the current reference.
P v g i g v g i g
r e f d d q q (1)
-Q v g i g v g i g
r e f q d d q (2)
P r e f i
d r e f v
d
,
-Q r e f i
q r e f v
d
(3)
Similarly, the output of current controller is
-
-k d I
v k i i L i v
d r e f d P s d r e f d q d
(4)
-
k q Iv k i i L i v
q r e f q p s q r e f q d q
(5)
B. Droop Control
Droop control is a well-known method for controlling micro-grid in islanded mode. The distributed generated unit is attached to a common bus with the transmission line as shown in Fig.4 where the apparent power deliverable to the bus from the distributed generator (DG) is equal to:
*
in v in v in v in v in v
S P jQ V I (6)
Where Pinv and Qinv are the deliverable active and reactive power andIi n v* is the current that is passing through
transmission line from inverter to bus, which is represented as:
* V 0
in v E I Z (7) 2 in v
E V E V V
S V
Z Z Z
(8)
Here, E is the inverter voltage and V is the common bus voltage. So is the phase angle of inverter side voltage,Z is
Fig.4: Equivalent circuit of MG
If X >>Rfor the transmission line, therefore the inverter output power is:
s in
E V P
X
(9)
( c o s )
V
Q E V
X
(10)
So, the above presents the voltage angle can be controlled by regulating the real power while the inverter voltage is controlled by the reactive power.
Fig.5: Schematic of droop control scheme
The real power stream and power angle controls through frequency. By adjusting P and Q independently, the
amplitude and frequency of the grid voltage are regulated while, it acts like the synchronous actions in power systems. The frequency of the inverter is dropped when the load real power rises and the inverter voltage amplitude is dropped when the load reactive power rises [10]. These conclusions determine from the frequency and voltage droop regulation relationship through real and reactive power:
( c a l )
m P P
(11)
( c a l )
E En Q Q (12)
III. FREQUENCYRESTORATIONANDVOLTAGECONTROL
In islanded operation, voltage and frequency are dependent on load. For the combined PQ control and droop control methodology in parallel inverters, droop control provides better load sharing. However, rustles having large frequency and voltage deviation at the connecting point. To overcome this issue, Frequency Restoration Scheme and
voltage control are needed. The frequency is verified by droop gain (m) and deviation between calculated real power
(Qcal) and reactive power set-point Q*. So, (11) shows that the frequency will be deviated by changing the load. To
improve the frequency, the Frequency Restoration Scheme (FRS) can be applied to restore the frequency to its
minimal value*
. To realize this scheme is added in (11) as.
( c a l )
m P P
(13)
For the steady-state operation m P( c a l P) is reformed as.
( )
d
K d t
(14)
Here is the frequency error and the constant K controls the overall system frequency restoration. The frequency
restoration scheme is based on Laplace transform of (13), (14) and it schematically represents in Fig.5. The constant
K shows the difference of vertical moving of the inverter. For reducing this variation in the DGs, the constant K
should be reduced. But this action does not eliminate the error completely. The Frequency Restoration Scheme (FRS) will be started after smoothing the dynamic response [11]. Reactive power sharing is realized with the Q-E droop control and it is affected by voltage dropped and loads. So (16) becomes:
( )
c a l
E En Q Q (15)
The voltage amplitude dropped relationship obtained by:
P c c
E E V
(16)
(16) represents the relation between the DGs’ reactive power output and the voltage magnitude difference. The
varying rang is limited and small forVP c cand assume that the value of K is a constant slope. Because of critical load,
the difference in supplied power and load turns over at PCC, the reactive power provided by both DGs is equal to
the load side reactive power soEis reformed as
( P c c)
E K E V
(17)
The voltage dropped occurs at the connecting point because generated reactive power of DG1 is higher than that of
DG2. In the droop control strategy, the change in load reserved by distributed generators in a prearranged way and
decentralized wireless control of parallel inverters is to attain with the operational system frequency as a communication link within the micro-grid.
IV. SIMULATIONRESULTS
To verify the efficiency of control strategy, the study presents the micro-grid working criteria in islanding operation under combined droop and PQ control, which are simulated in MATLAB environment. The parallel inverters are
connected with R-L load, where the rated frequency is 50Hz. And P1ref=10kW, P2ref=8kW, Q1ref=10kVar, Q2ref
=5kVar. At the PCC point the output reactive power can be regulated through DG2 inverter. Fig.6 represents the MG
with PQ controlled and droop controlled inverter. At the initial point for power regulation performance, where
micro-grid DG1 inverter supplies the required real and reactive power to the load under the droop control strategy as
shown in Fig.5. During the timespan t=0 to t=0.3s the power consumption of load (L1) rises to its rated value under
droop control (PL1=8kW, QL1=5kvar). At t=0.3s DG2 under PQ control connect to the micro-grid. The response of
load voltage and current at PCC is shown in Fig.6 (a). It can be noted that at t=0.3s when DGs connect with each other a spike appears caused by unexpected variation in reactive power. After connecting, both GDs supply power to the desired load.
The output power of PQ controlled inverter shown in Figs. 6(c) and 6(d) where DG measured power accurately
tracks with respect to its reference power command and having fast dynamic response. The DG2 injects constantly
observed that the frequency at PCC increases at the point of synchronization and at the same time disturbance occur in load side power.
The waveform of MG frequency at PCC is shown in Figs. 6(e) and 6(f) with and without Frequency Restoration Scheme (FRS). At connecting point t=0.3s the frequency will increase at the point of synchronization. After synchronization both DGs work together to supply power to the load. The frequency dropped happens because of the load demand. After connecting within 0.2s, the frequency will be going to restore to a stable position according to the Frequency Restoration Scheme (FRS) and droop characteristics. The frequency deviation happens because of
combined control strategy and droop characteristic. When the load (L2) and (L3) are connected to the Micro-grid
(PL2=2kW, QL2=1kVar), (PL3=2kw, QL3=1kVar) at t=0.5s, t=0.7s the frequency decrease caused by the load
demand. Therefore, the real and the reactive power reallocate through droop control between two inverters. The DG1
power increases to supply the demandpower according to the supervisory steeper droop control. During the load variation, the frequency is in a stable level. The delivered power with combined control strategy is shown in Figs. 6(g) and 6(h). It can be seen at the PCC the system voltage is reliable. In this all scenario, it is observed that the measured PQ controller output power tracks the reference power accurately and the performance of droop controller is effective in terms of power sharing and have a fast response when load changing occurs.
a) Output voltage and current waveform at PCC b) close up waveform of current and voltage
Connecting
Qmeasured Q
set
Connecting
measured P
Pset
0.1
c) Output active power of PQ control inverter d) output reactive power of PQ control inverter
Time (seconds) Time (seconds)
F
r
e
q
u
e
n
c
y
F
r
e
q
u
e
n
c
y
1
Connecting
Connecting
g) Active power response at PCC h) Reactive power response at PCC Fig.6: Micro-grid performance with combined PQ control and droop controlled inverter
CONCLUSION
In this paper, the enhanced droop control with frequency restoration and voltage regulation function and PQ control strategies for the controlling of parallel DGs in the islanding mode of AC micro-grid has been investigated to achieve the flexible power regulation for power delivery. The PQ controller performance is effective to track the active and reactive power sharing and droop control is for frequency and voltage control in the islanded mode. The main advantage of this droop control strategy is to work without any communication between the parallel DGs. The supervisory droop control implemented for power reallocation with predefined droop characteristics and for frequency regulation using the Frequency Restoration Scheme (FRS) among the distributed generators. Furthermore, the droop controller provides stable voltage. The tracking error between the set -point power and measured power of PQ control based inverter has been investigated. The DG voltage can quickly respond to the needed voltage demand. These criteria of control strategy make this system quickly responding to the load change. The results achieved from MATLAB/SIMULINK verify stability of the load voltage and frequency.
REFERENCES
[1] S. Parhizi, H. Lotfi, A. Khodaei, S. Bahramirad, “State of the art in research on micro-grids: A review”, IEEE
Access, no.3, 2015, pp.890-925.
[2] J.A.P. Lopes, C.L. Moreira, A.G. Madureira, “Defining control strategies for micro-grids islanded
operation”, IEEE Trans. on power systems, vol.21, no.2,2006, pp.916-924.
[3] M Najeeb, “An overview on microgrid control strategies”, International Journal of Advanced Technology and
Engineering Exploration, vol. 4, no.5, 2015, pp.93-98.
[4] R.H. Lasseter, “Micro-grids” ,Power Engineering Society Winter Meeting, 2002, vol.1, 2002, pp.305-308.
[5] P. Basak, A. K. Saha, S. Chowdhury, S.P. Chowdhury, “Micro-grid: Control techniques and
modeling”, Universities Power Engineering Conference, 2010, pp.1-5.
[6] J. Huang, C. Jiang, and R. Xu, “A review on distributed energy resources and Microgrid”, Renewable and
Sustainable Energy Reviews, vol.12, no.8, 2008, pp.2472-2483.
[7] B.Kroposki, R. Lasseter, T.Ise,S.Morozumi, “Making microgrids work”, Power& Energy Magazine
IEEE,vol.6, no. 3, 2008, pp.40-53.
[8] M. Ramezani, S. Li, and Y. Sun, “Combining droop and direct current vector control for control of parallel
inverters in micro-grid”, IET Renewable Power Generation, vol.11, no.1, 2017, pp.107-114.
[9] M.S. Mahmoud, S. A. Hussain, and M.A. Abido, “Modeling and control of micro-grid: An overview”, Journal
of the Franklin Institute, vol.351, no.5, 2014, pp. 2822-2859.
[10] H. Han, X. Hou, J. Yang, J. Wu, M. Su,et al, “Review of Power Sharing Control Strategies for Islanding
Operation of AC Micro-grids” ,IEEE Trans. on Smart Grid, vol.7, no.1, 2015, pp.200-215.
[11] M. Kosari and S. H. Hosseinian, “Decentralized Reactive Power Sharing and Frequency Restoration in
