Key Laboratory of Control of Power Transmission and Conversio n, Ministry of ... controller, the inverter-based microgrid system is able to switch between ...
1
Modeling, Control and Testing of a Voltage-Source-Inverter-Based Microgrid Guojie Li, Zhijian Jin
Liping Su Key Laboratory of Control of Power Transmission and Conversio n, Ministry of Education Shanghai Jiaotong University Shanghai, China
controllable loads that can operate either interconnected or isolated from the main distribution grid as a controlled entity.
Abstract— Microgrids are low-voltage distribution networks comprising various distributed generators (DGs), storage devices, and controllable loads that can operate either interconnected or isolated from the main distribution grid as a controlled entity. This paper concentrates on the modeling and control of a voltage-source-inverter-based microgrid. Considering the dispersion of DGs and loads, the controllers are designed using different control strategies respectively corresponding to the types of the DGs and ways of combining of storage devices. The Power and Voltage Controller coupled with the microsources provides fast response to disturbances and load changes without relying on communications. PQ control can realize the designated control of real and reactive power of DGs according to actual operation situation. V/f control can realize power sharing between different DGs when the load power changes, and provide frequency support when in the islanded mode. With the implementation of the unified controller, the inverter-based microgrid system is able to switch between islanding and grid-connected modes without disrupting connected critical loads. The two control strategies have been verified in simulation using MATLAB/SIMULINK by switching between interconnected and islanded modes.
Many forms of DGs such as fuel-cells, photo-voltaic and micro-turbines are interfaced to the network through power electronic converters. These interface devices make the sources more flexible in their operation and control compared to the conventional electrical machines. However, due to their negligible physical inertia they also make the system potentially susceptible to oscillations resulting from network disturbances [1] [2]. So Microgrid control is critical, which needs to ensure that: new microsources can be added to the system without modification of existing equipment, the Microgrid can connect to or isolate itself from the grid in a rapid and seamless fashion, reactive and active power can be independently controlled, voltage sag and system imbalances can be corrected, and that the Microgrid can meet the grid’s load dynamics requirements [2]. In this paper, a systematic approach to modeling an inverter-based microgrid is presented. Considering the dispersion of the DGs and loads, the controllers are designed using different control strategies including V/f control and PQ control respectively corresponding to the types of the DGs and ways of combining of storage devices. The performance of the proposed controller has been tested extensively in simulation using MATLAB/SIMULINK and verified the accuracy of the two control strategies.
Keywords-distributed generation (DG); mircogrid; V/f control; PQ control; inverter
I. INTRODUCTION The penetration of distributed generation at medium and low voltages (MV and LV), both in utility networks and downstream of meters, is increasing worldwide. The presence of generation close to demand can reduce carbon emissions, increase the power quality and reliability (PQR) of electricity delivered to sensitive loads, potentially reduce the need for traditional system expansion and so on. But controlling a potentially huge number of DGs creates a daunting new challenge for operating and controlling the network safely and efficiently [1]. This challenge can be partially addressed by microgrids. The Consortium for Electric Reliability Technology Solutions (CERTS) Microgrid concept assumes an aggregation of loads and microsources operating as a single system providing both power and heat. Microgrids are low-voltage distribution networks comprising various distributed generators (DGs), storage devices, and
II. MICROGRID MODELING A multi-bus microgrid configuration system considered in this paper is shown in Fig. 1, where two paralleled DG systems 1 and 2 are employed. Each DG system is comprised of a dc source, a pulse-width modulation (PWM) voltage source inverter (VSI) and LC filters used to filter out the high frequency harmonics. Sensitive loads 1 and 2 access bus 2 and bus 3 respectively, while variable load 3 access bus1. Then the microgrid which consists of two DGs and three loads connects to the distribution network through switches and a transformer. A typical characteristic of a microgrid is that it can be operated either in grid connected or islanded mode. Under normal mode of operation, the microgrid is connected to the
This work was supported by the Science Project from the Science and Technology Commission of Shanghai Municipality, China.
978-1-4577-0365-2/11/$26.00 ©2011 IEEE
Key Laboratory of Control of Power Transmission and Conversion, Ministry of Education Shanghai Jiaotong University Shanghai,China
724
2
utility system at the point of common coupling (PCC). In this mode, the two DG systems both adopt PQ control to provide local power and voltage support for critical loads 1 and 2. This configuration reduces the burden of generation and delivery of power directly from the utility grid and enhances the immunity of critical loads to system disturbances in the utility grid. When disturbances such as faults occur, the microgrid turns to the islanded mode [3]. In this mode, the two DGs are the sole power sources left to regulate the load voltage and supply uninterrupted power to critical loads. At this time, DG2 is the master DG and DG1 is the affiliated DG. The control of DG2 turns to V/f control to provide voltage and frequency reference. After islanding, the reconnection of the microgrid is achieved autonomously after the tripping event is no longer present.
iL is the output current of inverter, iLd and iLq are currents of d axis and q axis of iL by dq transformation, respectively. Assuming that the output active power and reactive power of inverter are Pref and Qref , respectively, and due to
unq 0
(1)
Then
iLdref iLqref
Pref und Qref und
(2)
The above two equations represent the P&Q Control module in Fig. 2 and show that there are an external power control and inner current control [6]. The tracking of the reference active power Pref and reactive power Qref is to track the reference current iLref . P is determined by
iLd and Q is determined by iLq .
Thus, control of P and Q is decoupled.
Fig.1 Microgrid modeling
III. CONTROLLER DESIGN This section presents the two basic control strategies respectively corresponding to the type of the DG. The DGs such as photovoltaic generation (PV) and wind turbines use PQ control strategy due to its output power influenced by the weather [4]. And the DGs such as microturbines and fuel cells, whose output power is easy to be controlled, so both PQ and V/f control strategies are suitable for them. It should be noted that the proposed controller illustrated in a simplified two-DG system in this study can be expanded to be used in a more complex microgrid with more DG systems.
Fig.2 PQ control schematic
A. PQ Control Fig. 2 is the PQ control schematic for the three-phase grid-interfacing inverter.
B. V/f Control Fig. 3 is the V/f control schematic; Fig. 4 is the power control module; Fig. 5 is the two feedback loop control of voltage and current.
If the DG needs inverter to connect to the conventional distribution system and the capacity of power and energy storage device is enough, each feeder in Fig. 1 can be equivalent to the part above the dashed line of Fig. 2 [5].
In Fig. 4, the P and Q calculation blocks that calculate the values of active and reactive power will use the knowledge of instantaneous values of d axis and q axis voltages and currents. The equations used are:
725
3
P und id unq iq Q unq id und iq
(3)
The droop control block includes “Q verse V Droop” and “P verse f Droop”. They can be expressed in the following equations [7]:
Pn P f f n m V V Q 0 n
Fig.4 Power control schematic
(4)
The parameters m and n can be obtained by the followings:
Pmax Pn m f f n min n Qmax V0 Vmin
(5)
Fig.5 Voltage and current control loop
In Fig. 5, the external voltage control loop uses PI controller to stabilize the system and the inner current control loop uses proportion controller to improve the system dynamic response. As shown, outer capacitor voltage feedback compensator wC f u nref is used to force the capacitor voltages to track their
The reference frequency and amplitude of the output voltage can be obtained by droop control, and then the symmetric three-phase reference voltage uref can be obtained by f and V through voltage synthesis controller, then udref and uqref are
sinusoidal reference waveforms stiffly with an acceptable low output total harmonic distortion (THD) [4].
obtained by dq transformation as the inputs of Fig. 5 [8].
IV. SIMULATION VERIFICATION OF MODEL In this paper, the whole system control method is called master-slave operation, which means that one DG acts as the master and the others as affiliates. When in grid-connected mode all DGs adopt PQ control and when in islanded mode the master DG turns to V/f control to provide voltage and frequency reference to the other DGs. In this paper, DG2 is the master DG and DG1is the affiliated one. The performance of the proposed controller is tested in MATLAB/SIMULINK using the system parameters shown in Table I. TABLE I
SYSTEM PARAMETERS
Parameters DC voltage
Fig.3 V/f control schematic
DG1
Values
Inverter filter resistance
Rf
0.01
Inverter filter inductance
Lf
0.6 103 H
Inverter filter capacitance C f
1.5 103 F
Inverter switching frequency f s
Pn
8 103 Hz 8kW
Qn
0kW
Reference active power Reference reactive power
726
800V
vdc
4 DG2
Same as DG1, except
fn PQ Controller
380V
v0 k kvp
0.00125 0.5
kvi
20
1/m
k kvp
1105 3 104 1 10
kvi
2000
1/n V/f Controller
R1n 0.347 / km
The line of 10kV
X 1n 0.234 / km R2 n 0.641 / km X 2 n 0.101 / km
The line of 380V Load 1
R1 5, X1 0.628
Load 2
R2 5, X 2 0.628
Load 3
Transformer
transition from grid-connected to islanding mode at t=0.5s is shown in Figs. 6~9, respectively. When disconnected from distribution network, the output active power of DG2 increases, which means the master DG need to output more active power for power balance. At the same time, the output active power of DG1 is unchanged because of PQ control. Fig. 7shows that the output reactive power of DG2 decreases whose value is equivalent to the one distribution network output to the microgrid before disconnection. And the output reactive power of DG1 is zero because its power factor is 1. The voltage of BUS2 increases because the output reactive power of DG1 decreases. Fig.9 shows that the frequency decreases within permitted range when disconnected from distribution network and it verifies that the V/f control can provide stable frequency support.
Pn 20kW 50Hz
R3 10, X 3 1.57 Rating capacity 300kVA Rating voltage 0.4kV /10kV 4% Zk*
Fig.6 The output active power of DG1 and DG2
4.26kW
pk Distribution network
10kV
The action set is shown in Table II. Before 0.5s, the system is in grid-connected mode and the two DGs both adopt PQ control. DG2 turns to V/f control when the microgrid transits from grid-connected to islanding mode at t=0.5s. TABLE II ACTION SET Time 0s~0.5s 0.5s 0.5s~2s 1s 1.5s 2s
Action DG2 — PQ K open DG2 — V/f K3 open K3 close K close DG2 — PQ
Fig.7 The output reactive power of DG1 and DG2
Status In grid-connected
In islanded Shedding load 3 Adding load 3 In grid-connected
A. Transition from Grid-Connected to Islanding Mode Response of the output active and reactive power of DG1 and DG2, the BUS2voltage, and the system frequency during
Fig.8 The voltage of BUS2
727
5
Fig.9 The frequency of system Fig.12 The voltage of BUS2
B. In Islanding Mode, Switching Load 3 Response of the output active and reactive power of DG1 and DG2, the BUS2voltage, and the system frequency when shedding load 3 at t=1.0s and adding load 3 at t=1.5s in islanding mode is shown in Figs. 10~13, respectively. Figs. 10 and 11show that in islanding mode, the change of power by switching load 3 is tracked by the master DG2.The output active and reactive power of DG2 decrease when shedding load 3 and increase when adding load 3. At the same time, the output active and reactive power of DG1 keep unchanged during switching load 3. The results show that V/f control can track load interruption perfectly and PQ control can provide stable output power. Fig.12 shows that the voltage of sensitive load1is stable. Fig.13 shows that the change of frequency reflects the change of active power of DG2 and by V/f control it can be maintained within permitted range.
Fig.13 The frequency of system
C. Transition from Islanding to Grid-Connected Mode Response of the output active and reactive power of DG1 and DG2, the BUS2 voltage, and the system frequency during transition from islanding mode to grid-connected at t=2s is shown in Figs. 14~17, respectively. The output active power of DG1 remains unchanged and that of DG2 decreases when microgrid reconnects to distribution network that outputs active power to microgrid. The output reactive power of DG1 remains zero and that of DG2 also increases to zero because of turning to PQ control. The BUS2 voltage has been affected a little bit due to the decrease of reactive power from DG2. Fig.17 shows that the system frequency has been affected during operation mode change but the variation is quite small within permitted range.
Fig.10 The output active power of DG1 and DG2
Fig.14 The output active power of DG1 and DG2
Fig.11 The output reactive power of DG1 and DG2
728
6
grid-connected and islanded modes, and during operation mode transitions. VI. REFERENCES [1] [2] [3]
[4]
Fig.15 The output reactive power of DG1 and DG2
[5]
[6]
[7] [8]
Fig.16 The voltage of BUS2
Fig.17 The frequency of system
V. CONCLUSIONS In this paper, a three-bus microgrid is presented, which includes microsources, inverters and loads. The control of inverters is critical because the control flexibility allows the microgrid to present itself to the bulk power system as a single controlled unit, have plug-and-play simplicity for each microsource, and meet the customers’ local needs. This paper presents two control strategies for the microgrid, PQ control and V/f control respectively. The PQ control is suitable for the DGs whose output power is adjustable and the V/f control is suitable for the DGs whose output power is stable. The PQ control makes the DGs output the reference power according to the actual situation and the V/f control makes the DGs share of power between the DG systems when the microgrid islands from the utility and provide frequency support when the microgrid is in islanded mode. The simulation by MATLAB/SIMULINK confirmed the effectiveness and robustness of the two control strategies in
729
N. Hatziargyriou, H. Asano, R. Iravani and C. Marnay, “Microgrids,” IEEE power & energy magi, July/august 2007, 1540-7977. R. Lassetter, “Control Methods for MicroGrids: The CERTS Microgrid Concept,” CERT Rep., Apr.2002 N. M. Abdel-Rahim and J. E. Quiche, “Analysis and design of a multiple feedback loop control strategy for single-phase voltage-source UPS inverters,” IEEE Trans. Power Electron, vol. PE-11, pp. 532-541, July 1996. Esmaili, Xul, Nicholsn D K, “A new control method of permanent magnet generator for maximum power tracking in wind turbine application,” Proceedings of Power Engineering Society General Meeting, vol. 3, Jun 12-16, 2005, San Francisco, CA, USA: 2090-2095 M. N. Marwari and A. Keyhani, “Control of distributed generation system-Part I: voltage and current control,” IEEE Trans. Power Electric, vol. 19, no. 6, pp. 1541-1550, Nov. 2004. M. Radanovich, T. Green, and H. Mansur, “A survey of control methods for parallel three-phase inverters connection,” Proc. Inst. Elect. Eng., no. 475, pp. 472-477, Sep.2000. C. S. Wang, “Synthetical Control and Analysis of Microgrid,” Automation of Electric Power Systems, vol. 32, no. 7, Apr. 10, 2008. W. J. Yang, “Simulation and Research of Grid Connected Photovoltaic Generation and Microgrid Operation Control,” M. D. thesis, Sichuan, Southwest Jiao Tong University, 2010(in Chinese).
