March 26-29
Control of Power Converters for Microgrids I. VECHIU1, A. LLARIA1,2, O. CUREA1 and H. CAMBLONG1,2 (1) ESTIA-Recherche, Technopôle Izarbel, 64 210 Bidart, France Tel: + 33 (0) 559 438 474 Fax: + 33 (0) 559 438 405 (2) UPV/EHU, Basque Country, SPAIN E-mail:
[email protected] URL: http://www.estia.fr
Copyright © 2009 MC2D & MITI
Abstract: Nowadays, Distributed Generation and Microgrids (MGs) are becoming an important research line because of their peculiar characteristics. MGs are composed of small power sources which can be renewable, placed near customer sites. Moreover, they have the inherent property of islanding: the disconnection of either the MG from the main grid or a part of a MG from the rest of the MG. In this paper, the behaviour of a particular MG during grid connected and islanding operation is investigated. The studded MG is based on different energy sources: a wind turbine, a PV array, a backup diesel generator and a storage system. The renewable sources and the storage system are connected on DC-bus which is interconnected with a main grid trough a Voltage Source Inverter (VSI). The attention focuses on the control technique of the VSI during grid connected and islanding operation of the MG. The evolution of the AC signals on the point of common coupling between the MG and the main grid as well as the DC signals on the DC-bus on which are connected renewable energy sources of the MG has been investigated on simulation using a MATLAB/Simulink model. Keywords: Microgrid, Distributed Generation, Islanding, VSI control. 1. Introduction In recent years, the development of Distributed Generation Systems (DGS), and especially Microgrids, has been increased considerably. A DGS is composed by electrical sources placed near local loads and it becomes more interesting when those sources are of different kinds [1, 2], such as wind turbines, photovoltaic (PV) panels, fuel cells, etc. In that case, the DGS can be considered a Hybrid Energy System (HES) and it is very suitable to support the electrical network in rural areas and remote sites. A MG is an HES with an inherent characteristic: the islanding capability. Islanding is the disconnection of the MG from the main grid without interruption of the energy generation
for the loads connected to the islanded part. It is also possible to have an islanding of only one part of the MG instead of all the MG. Moreover, islanding can be either planned or unplanned. The former corresponds to disconnections for maintenance operations, but the latter is due to faults or disturbances described in the standard IEEE 1547 [3] which occur in the grid or in a feeder of the MG. The habitual sources which exist in a MG are usually low power micro-sources (MS), normally under 100-200kW, with power electronics interfaces. The main advantages of MS are their high reliability, low cost and in addition, they have a low level of greenhouse gazes production.
PCC D
Wind Turbine
Static Switch Circuit Braker
C
Power Flow Controller
Battery
Energy Manager B
Electrical Load Diesel Generator
A PV Panel
Fig. 1. General MG architecture
The simplest architecture of a MG consists of a radial connection of several feeders with a set of associated loads and MS [4]. Fig. 1 shows a model of MG with four feeders, named A, B, C and D. In general, the maximum voltage in a feeder is around 480V. As it can be seen, each feeder has a circuit breaker and a power flow controller commanded by the energy manager. The radial system is connected to the distribution system through the point of common coupling (PCC). The static switch is a separation device used to island all the MG from the main grid, while the circuit breaker is used to island a feeder from the rest of the MG. An important characteristic of energy resources such as wind energy or PV is their unpredictability; they fluctuate independently from demand. Thus, to have a correct level of reliability when using them, it is necessary to use energy storage systems which ensure an uninterruptible power supply. The control and flexibility needed by the MG is achieved by means of power electronics interfaces. MGs can be considered as inverter dominated electrical networks because those power converters are employed as interface with the main grid. When the MG is connected to the main grid, inverters use the signal of main grid as reference to obtain an AC signal with the correct frequency and voltage. But, in islanding, the reference of the main grid is lost, so inverters must find new references to continue the generation of good power quality. So some control techniques are necessary to allow the good operation of inverters with MG in islanding. This paper outlines a first research work in order to investigate the behaviour of a particular MG. The aim of this paper is not to study the process of islanding but two different
control strategies allowing grid connected and islanding operation of the MG. The paper is organized as follows: Section 2 describes the architecture of the investigated MG, Section 3 the control techniques for the VSI used as power interface between the MG and the main grid, while Section 4 presents simulation results. Finally, Section 5 summarizes the main points of this paper. 2. Investigated microgrid Many different architectures are presented in the literature. A first example is the Microgrid Central Controller (MGCC) which was firstly developed in the EU Microgrids Project [5]. In these MGs, the Central Controller, which is installed on the side of the medium voltage/low voltage (MV/LV) substation connected to low voltage line (LV) is the most important element. Its function is to carry out a high level management of the MG operation by means of technical and economic functions. Another microgrid architecture example is CERTS Microgrid (CM) [2]. This concept proposes the unlimited installation of distributed energy equipments in the electrical grid. CM paradigm permits a smoothly islanding, provides electricity to relatively small installations (under 2 MW of peak) and without complex and expensive control systems. In other words, none of the devices is essential by itself for the MG operation, giving a high level of reliability. The architecture of the microgrid investigated in this paper is presented in Fig. 2. This architecture consists of a DC-bus and an ACbus with their associated loads and microsources. The investigated MG is based on a
Main Grid
DG
Electrical Loads
Control Signal
AC bus
Master
AC DC
DC bus DC
DC
DC
DC
DC
AC
Wind Turbine
PV Panel
Fig. 2. Investigated MG architecture
wind turbine, a PV array, a storage system, a diesel generator (DG) and a VSI. This converter is the power interface between the MG and a main grid.
3. Control techniques for inverters In islanding operation, there are two general inverter coordination strategies to manage the MG [6]: Single Master Operation or Multi Master Operation. The MG presented in Fig. 2 uses the Single Master Operation during islanding.
A MG is a weak grid where the effects of renewable resources or load variations are more intense than in interconnected systems. For a proper analysis and evaluation of the behaviour of the MG a complete model of the investigated system has been developed.
During the grid connected operation, the VSI (Fig.2) uses the main grid electrical signals as reference. However, in islanding, the inverters lose that reference and new references must be fund. Consequently, the VSI must be able to Renewable Energy Sources VDC
vDC v*DC +
PI
vd
i*d
+ id
i*q
+ -
-
PI ωLf ωLf PI
* + v Sd
+ + +
v*Sα
v*Sq
vq
v*Sa αβ
dq
v*Sβ
αβ
v*Sb
v*a v*b v*c
db
*
abc
v
Sb
da
da db dc
dc
θ
iq id
αβ
iα
abc
ic ib
iq
iβ
dq
αβ
ia
θ θ
Lf
αβ
vd αβ
vq
dq
vα
abc
vβ
ic ib ia
vC vB vA
αβ L O A D S
iGD D G iL
iG Main Grid
Fig. 3. Control strategy for the VSI during grid connected operation
operate in both configurations as grid connected inverter and stand-alone inverter.
and the second is synchronously rotating dq coordinate system. This transformation, combined with PI controllers allows the steadystate error to be eliminated. The voltage equations in dq synchronous reference are as follows:
When the MG is in islanding, it is necessary to start the pertinent control mechanisms. The objective is to obtain a frequency deviation equal to zero and to maintain the amplitude voltage value in the islanded MG. To assure this, the inverters must find new voltage and frequency references, maintaining a good power quality. In this case, the MG can be considered as an inverter dominated system, because its frequency is controlled by the power electronics.
did + ωL f iq dt diq 0 = vSq − R f iq − L f − ωL f id dt
vd = vSd − R f id − L f
(1) (2)
As can be seen in the Fig. 3, the q-axis current is set by the q-axis voltage controller to control the reactive power flow while the reference of d-axis current is set by the DC-bus voltage controller to control the active power flow between the AC-bus and the DC-bus. The terms -ωLfid, ωLfiq are inserted to decouple dq axes dynamics.
A Converter control during grid connected operation Fig. 3 shows the inverter control scheme in case of grid connected operation. To control the active power through the inverter, the voltage DC-bus VSI side is measured and compared to DC-bus reference. The DC-bus voltage error feeds a PI controller having a current reference as output. In this configuration, the DG is not in operation and the inverter is line-commuted. Its control is performed in dq synchronous frame rotating [7] at the fundamental frequency based on current loop controller (Fig. 3). A characteristic feature for this voltage and current controller is processing of signals in two coordinate systems. The first is stationary αβ
B Converter control during islanding operation In this second case, the DC-bus voltage VDC is set at the reference voltage by controlling the energy flow from the batteries to the DC-bus as shown in Fig. 4. The DC/DC converter operates as a voltage stabiliser. The DC-bus voltage error feeds a PI-controller having the duty ratio db as output. When energy from the renewable side is insufficient to supply sudden increments in load demand, the DC bus voltage drops below the Renewable Energy Sources Icb
Ib Vbat
=
=
db
v *d
+ -
v*q + -
PI
vd
i*d
+ id
PI
i*q + iq
-
PI ωLf ωLf PI
-
PI V*DC
* + v Sd
v*Sa
v*Sα αβ
dq
+ * + + v Sq
αβ
vq
θ
id
αβ
v*Sβ iα
v*Sb
*
*
da
*
v av b vc
db
*
abc
abc
v
Sb
VDC
+
da db dc
dc
ic ib
iq
ω
dq
∫
αβ
ia
Lf
θ
vd vq
iβ
αβ dq
vα
abc
vβ
ic ib ia
vC vB vA
αβ L O A D S
Fig. 4. Control strategy for the VSI during islanding operation
D G iL
iGD
reference value. Energy is pumped to the DC bus from the batteries in order to maintain the voltage at the set reference value. When there is excess energy, the batteries are recharged. In this configuration, the inverter is self-commuted and sets the frequency and the amplitude of the voltage. The complete control structure for the inverter used during islanding operation is shown in Fig. 4. It consists of an inner current loop controller and an outer voltage loop controller [8], in rotating frame. The reference of d-axis current is set by the d-axis voltage controller to control the active power flow and the q-axis current is set by the q-axis voltage controller to regulate the reactive power flow. The output signals from PI controllers after the inverse dq/αβ transformation are used to generate the duty ratio signals. 4. Simulation results Simulations have been run with the system shown in Fig. 2 to investigate its behaviour during grid connected operation and islanding operation. For both cases the three phase bi-
directional converter presented in Table I.
il [A]
the
parameters
Table I PARAMETERS
Input/Output
Filter Components
VALUE Input Voltage (VDC)
700 [V]
Output Voltage (va, vb, vc)
230 [Vrms], 50 [Hz]
Rated Power (total)
70 [KW]
DC capacitor
2200 [µF]
Filter inductance (Lf) 3 [mH] Filter resistance (Rf) 0.1 [Ω]
A first simulation has been conducted with the investigated MG connected to the main grid. Fig. 5a,b illustrates simulation results of the currents at the point of common coupling (load current il, inverter current iin and main grid current iG), the AC output voltage amplitude Vl and the DC-bus voltage VDC during a step increase of a resistive load from 16 to 40 kW. During this simulation, there are no wind or irradiation variations and the DG is not in operation. As can be seen in Fig. 5a, when the load
100
400 350 Vl [V]
0
-100 2.48
has
2.49
2.5
2.51
2.52
2.53
2.54
2.55
Vl * Vl
300 250 200 2.3
-50 2.48
2.5
2.6
2.7
2.8
2.9
3
720
2.49
2.5
2.51
2.52
2.53
2.54
2.55
50
iG [A]
2.4
0
0
700 690
-50 2.48
VDC * VDC
710 VDC [V]
iin [A]
50
2.49
2.5
2.51
2.52
2.53
2.54
680 2.3
2.55
2.4
2.5
Time [s]
2.6 2.7 Time [s]
2.8
2.9
3
(a) (b) Fig 5: Grid connected operation. (a) AC current at the point of common coupling, (b) AC-bus output voltage amplitude and DC-bus voltage. 400
V l [V]
il [A]
100
0
-100 2.48
2.49
2.5
2.51
2.52
2.53
2.54
200 2.3
2.55
VDC [V]
iin [A]
0
2.49
2.5
2.51
2.52
2.53
2.54
Ib [A]
iDG [A]
2.5
2.6
2.7
2.8
2.9
3
0
2.49
2.5
2.51 2.52 Time [s]
2.53
2.54
2.55
700
V DC
699
V *DC
698 2.3 200
2.55
100
-100 2.48
2.4
701
50
-50 2.48
Vl * Vl
300
2.4
2.5
2.6
2.7
2.8
2.9
3
2.4
2.5
2.6
2.7
2.8
2.9
3
100 0 -100 2.3
Time [s]
(a) (b) Fig. 6: Islanding operation. (a) AC current at the point of common coupling, (b) AC-bus output voltage amplitude, DC-bus voltage and the current through the storage system.
increases, the main grid ensures the needed power. At the moment of load increasing, AC output voltage and DC-bus voltage drop can be noticed (Fig. 5b). The DC-bus and the AC-bus have also been analysed during islanding operation and the results are illustrated in Fig. 6a,b. For these simulations, the main grid and the MG are disconnected and the VSI operate in stand-alone. It fixes the frequency and the amplitude of the MG. The DG operates as a backup system at rated power in parallel with the VSI and there are no wind or irradiation variations. The Fig. 6a shows the evolution of the currents at the point of common coupling. As can be seen, before the increasing of the load, there is an excess energy on the AC-bus which is used to charge the storage system. In this case, the power flow trough the VSI from the AC-bus to the DC-bus. At the moment t=2.5s, the load needs more energy and the power flows trough the VSI from the DC-bus to the AC-bus. The Fig. 6b shows the evolution of the AC-bus and the DC-bus voltage as well as the storage system current during islanding operation. 5. Conclusion A particular MG architecture has been modelled in order to analyse its behaviour during grid connected and islanding operation. The model is based on a wind turbine, a PV panels array, a backup DG and a VSI used for the interconnection with the main grid. The VSI plays an important role because it acts as interface and fixes the AC voltage amplitude and frequency of the signal into the MG. In islanding, the reference given by the grid is lost and a different control technique is required by the VSI to create new references. In this particular case, simulations results show that using a VSI and classical control strategies, it is possible to have a good power quality on the MG during grid connexion and islanding operation. The present work is a first step in the MG behaviour investigation field. Islanding process tests should be done in simulations in order to better predict the physical system performances.
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“The CERTS Microgrid Concept,” White paper for Transmission Reliability Program, Office of Power Technologies, U.S. Department of Energy, 2002. [3] “IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems. Standard IEEE 1547-2003,” 2003. [4] H. Ibrahim, A. Ilinca and J. Perron, “Energy Storage Systems – Characteristics and Comparisons,” Elsevier Renewable & Sustainable Energy Reviews, Vol. 12, pp. 1221-1250, 2008. [5] B. Kroposki, R. Lasseter, T. Ise, S. Morozumi, S. Papathanassiou and N. Hatziargyriou, “Making Microgrids Work,” IEEE Power & Energy Magazine, Vol. 6, pp. 40-53, 2008. [6] J. A. Peças Lopes, C. L. Moreira and A. G. Madureira, “Defining Control Strategies for Analysing Microgrids Islanded Operation,” in Proc. IEEE Russia Power Tech, pp. 1-7, 2005. [7] M. P. Kazmierkowski, R. Krishnan, F. Blaabjerg, “Control in Power Electronics Selected Problems”, Elsevier Science, 2002. [8] I. Vechiu, H. Camblong, Member, IEEE, O. Curea, B. Dakyo, G. Tapia “Performance Analysis of Four-Leg VSC under Unbalanced Load Conditions for Hybrid Power System Application” PELINCEC Conference, Warsaw, 2005.
