An Enhanced PCC Harmonic Voltage Mitigation and Reactive Power Sharing in Islanded Microgrid
Minh-Duc Pham*, Hong-Hee Lee**
School of Electrical Engineering, University of Ulsan, South Korea
*minhducpham2009@gmail.com,**hhlee@ulsan.ac kr Abstract— Parallel distributed generators (DGs) in the
islanded microgrid are generally operated autonomously by means of the droop control scheme. However, the traditional droop control methods which use the P-ω and Q-E curve to share power between DGs are still concerned to improve the accuracy of reactive power sharing. Moreover, the uncontrolled harmonic power reduces the point of common coupling (PCC) voltage quality and microgrid stability. In order to solve these problems, this paper proposes an enhanced PCC harmonic control strategy and an improved reactive power sharing control scheme. Based on the low bandwidth communications, a secondary control is implemented with both central controller and local controller.
The effectiveness of the proposed control scheme is analyzed through the simulation.
Keywords— islanded microgrid, reactive power control, secondary control, PCC harmonic mitigation.
I. INTRODUCTION
Microgrid (MG) is clusters of distributed generator (DG) units which operate in coordination to solve the voltage quality and power sharing problems [1], [2].
Fig. 1 shows a typical configuration of islanded microgrid when the static switch (STS) is open. In an islanded microgrid, multiple DG converters equipped with energy storage system and renewable energy resources are responsible for forming the voltage/frequency and sharing the load power proportionally according to their power capability. For this purpose, DGs are generally operated in the voltage control mode (VCM) to maintain the constant voltage at the point of common (PCC) bus based on the droop control concept [3]. In addition, the control scheme can be easily extended to a number of DGs in microgrid without voltage and current information. However, because the droop control generally controls the fundamental component, any uncontrolled harmonic power caused by the nonlinear load can degrade the microgrid voltage quality and stability.
Therefore, it is necessary to compensate the selective harmonic simultaneously without affecting the stability of the grid. In [5], the harmonic compensation approaches are introduced based on the virtual resistance at the harmonic frequency. In this control strategy, the resistive active power filter (R-APF) is simply regarded as the virtual harmonic resistor. However, in case of long transmission line, this method is not effective to compensate harmonics due to the increased inductive feeder impedance. To solve this problem, the authors in [5] presented the negative virtual harmonic
impedance to improve the voltage quality. However, this method requires the exact value of the line impedance, which makes real applications hard.
In this paper, an enhance PCC harmonic control strategy is proposed to compensate the voltage harmonics in islanded microgrid. In addition, the improved reactive power sharing is also proposed by adaptively control virtual impedance. The proposed control is a feedback compensation method based on virtual harmonic impedance to compensate and share the harmonic power between DGs.
II. PROPOSED CONTROL STRATEGY
A. Droop control analysis
In the islanded microgrid, in order to share the power and regulate the output voltage and frequency autonomously, the droop control is usually applied:
0 G PP ,
w w= - (1)
0 Q ,
E E= -G Q (2) where w0 and E0 are the nominal values of the output frequency and voltage. P and Q are the fundamental active power and reactive power injected into the grid. Then, the output voltage reference for DG is given as following:
( )
* sin
V =E
ò
wdt (3)In (3), the references of the conventional droop controls only consist of the fundamental component. Therefore, when non-linear load is connected to the grid, the PCC voltage is distorted, and the system can be unstable with poor power quality.
Fig. 1. Typical islanded microgrid configuration.
B. Outer and Inner Loop Control
In order to control the output voltage, the following multi proportional-resonant (PR) voltage and current controllers are applied:
( )
2 21,3,5,7
2 2
Vi CV
V pV
CV o
i
K s
G s K
s s
w
w w
=
= +
+ +
å
(4)( )
I pI
G s =K (5)
whereKpVand KpIare the proportional gains,KV iis the resonant gain of the i-th PR controller.wCVis the cut-off frequency of the voltage control loop. The control diagram of the inner and outer control diagram is shown in Fig. 2.
C. The Proposed Secondary Control
To compensate the PPC harmonic voltage, the harmonic virtual impedance is proposed by considering the harmonic voltage drop caused by the line impedance, and the new Vrefis defined as follows:
* * *
_ _ ir
h
ref V VVir har V PCC h V
V = - = -V K (6)
where VVir har*_ and KVhir are the virtual harmonic impedance drop and the harmonic impedance factor, respectively.
VPCCis measured by MGCC with the harmonic extraction method in [6], and the harmonic distortion (HD) is calculated as
( ) ( ) ( ) ( )
h 2 h 2 / 1 2 1 2h d q d q
HD = V + V V + V (7)
To compensate the harmonic voltage properly, the virtual harmonic impedance factor at hthfrequency is adjusted by the external secondary loop control through PI controller:
( ) ( )
ir P_ I_
h
V vir ref h vir ref h
K =K HD -HD +K
ò
HD -HD dt, (8) where HDref is the demanded HD value to meet the PCC voltage requirement, and HDh is the current HD at hth frequency of the PCC. In this paper to keep the quality of the VPCC as good as possible, required HDh at 3rd, 5th, 7th harmonics are set to be 1.0%. These informations is sent from microgrid central controller (MGCC) to DG using a low bandwidth communication.The voltage difference between DG output voltage and PCC is obtained as follows:
C PCC
o
V V V XQ
D = - @ V . (9)
To compensate the mismatched voltage drop between DGs, the virtual impedance is properly selected by the external loop controller:
( ) ( )
Vir pQ ave i iQ ave i
X =K Q -Q +K
ò
Q -Q dt , (10) where Qaverageis average of reactive power calculated from the total number of DGs, n:1 n i i ave
Q Q
n
=
å
= . (11)Fig. 3 shows the proposed control diagram.
Fig. 3. The proposed control diagram for the VSI in DG.
Fig. 2. The inner and outer loop control diagram.
III. SIMULATION RESULTS AND ANALYSIS
In order to verify the effectiveness of the proposed control, the simulation is executed with the microgrid with 2 DGs which have same power capacity and droop parameters. The voltage of PCC is 110V, 50Hz, and the total load power 1.3kW is shared with DG1 and DG2.
Fig. 4 shows the performance of the active and reactive power with the proposed controller. Thanks to the reactive power compensation, the reactive power sharing is achieved accurately within a few periods. In order to examine the performance of the proposed harmonic compensation control method, the harmonic compensation starts at 5.5s, and the load power changes at 9.0s. As shown in Fig. 5(a), the HD3, HD5, and HD7 are reduced significantly from 3.749%, 4.41%, and 3.85%, to 1%, respectively, after a short transient time of almost 0.5s. In spite of the load variation at 9.0s, the performance of harmonic compensation is guaranteed as can be seen in Fig. 5(b). In Fig. 6, the PCC harmonic voltage quality is shown corresponding to the condition applied for Fig. 5(a).
From the simulations, the proposed harmonic compensation method shows a good performance with a fast response even though the load power changes. Also, the accurate power sharing is achieved with very small power sharing error
IV. CONCLUSIONS
This paper has mitigated the PCC voltage harmonics by compensating the virtual harmonic voltage drop at the output of DG in islanded microgrid. The virtual harmonic voltage drop is regulated by the external controller in DG, and the controller gains are adaptively tuned to ensure accurate harmonic compensation in spite of the load power variation. In addition, the reactive power sharing error is eliminated using the fundamental virtual impedance.
As a result, the improved PCC voltage quality together with the accurate power sharing are achieved. Furthermore, the whole microgrid system becomes more stable despite any load change. A series of simulation results show the effectiveness of the proposed control scheme.
Acknowledgment
This work was partly supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20174030201490) and the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2015R1D1A1A09058166)
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Fig. 4. The active and reactive power sharing with the proposed controller
Fig. 5. (a) The performance of the PCC harmonic voltage compensation with the proposed controller. (b) The dynamic response of the proposed
controller when load change.
Fig. 6. The PCC harmonic voltage quality with the proposed controller
