IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
2993
Intelligent Connection Agent for Three-Phase Grid-Connected Microgrids Joan Rocabert, Member, IEEE, Gustavo M. S. Azevedo, Student Member, IEEE, Alvaro Luna, Member, IEEE, Josep M. Guerrero, Senior Member, IEEE, Jose Ignacio Candela, Member, IEEE, and Pedro Rodr´ıguez, Senior Member, IEEE
Abstract—The high penetration of distributed generation power plants, based on renewable energy sources (RESs), is boosting the connection of power converters to the electrical network. This generation concept would permit to form local networks, microgrids, when the main grid falls due to any kind of contingency in the network. However, the connection and disconnection of these local networks may give rise to undesired transient overcurrents that should be avoided. In order to solve this drawback, this paper presents a method oriented to carry out a stable intentional disconnection/reconnection of local grids from the main electrical network under grid-fault conditions. This control method has been implemented in a grid-connected power converter that acts as an intelligent connection agent (ICA) and adapts its operation mode according to its connection state. The proposed control also manages the operation of a controlled switch, which is responsible of disconnecting/reconnecting the microgrid from the mains. In this paper, the behavior of the ICA under transient conditions will be discussed, and finally, its simulated and experimental performance will be shown. Index Terms—αβ control, dq-control, distributed power generation, grid-connection and island operation, intentional islanding, microgrid, voltage source converters.
I. INTRODUCTION HE INTEGRATION of small front-end power converters connected to low-voltage (LV) networks has experienced a fast development in the last years, mainly due to the massive penetration of renewable energy sources (RES). This feature is giving rise to a transformation of the current electrical network, where distributed generation systems (DG) are becoming more important. Among RES, the most outstanding technologies, in terms of installed power, are those based on wind and photovoltaic power plants, which are connected to the grid by means of power converters. These generation systems are likely to participate in microgrids applications. In addition, the increasing expansion of energy storage systems, configured as DG grid-tie
T
Manuscript received July 5, 2010; revised October 7, 2010; accepted January 1, 2011. Date of current version October 12, 2011. This work was supported by the Spanish Ministry of Science and Innovation under Project ENE2008-06841C02-01/ALT and TRA2009-0103. Recommended for publication by Associate Editor Dehong Xu. J. Rocabert, A. Luna, J. M. Guerrero, J. I. Candela, P. Rodriguez are with the Department of Electrical Engineering, Technical University Of Catalonia, 08222 Barcelona, Spain (e-mail:
[email protected]). G. M.S. Azevedo is with the Department of Electrical Engineering, Federal University of Pernambuco, 50740-510 Recife-PE, Brazil. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2011.2116126
through power converters, also facilitates the formation of islanded grids from the main network [1], [2]. However, nowadays the creation of islanded grids is not permitted by most of the transmission system operators (TSOs). Moreover, antiislanding techniques should be used to disconnect distributed generators in front of any grid fault [3]–[7], to prevent any unplanned island grid. According to the IEEE Standard 1547.2-2008 [8], the main ancillary services for distributed power generation systems can be summarized as scheduling, system control and dispatch services, reactive power supply and voltage- and frequency-control regulation, energy imbalance service, and operation of spinning reserve. However, considering the installation of energy storage system, these ancillary services could be expanded to increase the operational reserve capability, frequency regulation, peak shaving, intentional islanding capability, uninterruptible power system (UPS) functionalities, and management of daily wind/solar cycles. Among these new functionalities, the islanding capabilities are of special interest, as it increases the reliability of the electrical network while improving the continuity of the electricity supply in case of contingencies. Therefore, it is foreseen that the mandatory disconnection of DG systems under such conditions will gradually disappear, as the control techniques for the management of DG networks improve [9]. The intentional island operation mode is an old concept that, for instance, constitutes the basis of UPSs, responsible of feeding critical loads under grid-fault conditions, [10], [11], and the subsequent resynchronization when the network is operative again. However, in microgrids the UPS model cannot be copied, as the same communication tools and control functionalities cannot be applied [12]–[14], as in such systems the different generators and loads are not concentrated in few meters, but may be separated by kilometers. Moreover, microgrids are not conceived to work as a closed system; hence, it should be relatively easy to expand it, by means of integrating either more generation systems or loads. Different solutions for the connection and disconnection of microgrids from the main network have been already presented by several authors. Some works have even dealed with the inclusion of grid supporting functionalities and voltage/frequency regulation, as the one introduced in [15]. In all these works, it has been stated the great importance of implementing a fast and accurate grid voltage synchronization algorithm [16], [17]. In microgrid applications, the synchronization of an isolated network is usually accomplished by detecting the phase difference between the main grid and the microgrid. In [18], an
0885-8993/$26.00 © 2011 IEEE
2994
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
application based on using two synchronization phase-locked loops (PLL) for synchronizing the main network and the microgrid is presented. In this case, one PLL is devoted to detect the microgrid phase voltage, while the other one detects the main grid phase. When the mains is restored its phase angle is detected, meanwhile the microgrid is still working as an island. After synchronizing with the phase of the network the voltage in the microgrid starts shifting its phase value. The phase angle difference between both voltages is minimized by means of a PI controller. This application can be further simplified, as is implemented in [19] and [20], using the detected quadrature grid voltage from the PLL to synchronize with the microgrid. In [21] and [22], the performance of the network synchronization when using a single PLL, for detecting the phase and the amplitude error in a dq based reference frame is proposed. In [23], an improved method, based on the implementation of a synchronization system based on a frequency-locked loop (FLL) working in a stationary reference frame for detecting the phase angle of the voltage was presented. The aim of this paper is to propose a new algorithm for the formation of electric islands in three-phase systems, with the presence of loads and DG sources, when the main grid is under fault conditions. In this study, a control algorithm based on the adaptation of a dual second-order generalized integrator based on an FLL (DSOGI-FLL) [24] is proposed for controlling the disconnection, resynchronization, and reconnection of the microgrid from the grid. Taking advantage of the frequency detection, performed by the FLL, the synchronization and the current and voltage control loops are implemented in the αβ stationary reference frame. Thus, the system is less sensitive to transient phase jumps and the controllers are able to deal with balanced and unbalanced grid conditions. The proposed algorithm has been implemented in an intelligent connection agent (ICA), consisting on a voltage source inverter (VSI) and a controlled switch, SI . In the ICA, the VSI control establishes the connection state of this switch, determining when the microgrid should be connected to the main grid or, equivalently, when SI should be closed or opened. Under normal grid conditions, the VSI works as a grid observer but, when a grid fault is detected, the ICA disconnects the microgrid from the main grid. In this mode, the VSI provides a nominal voltage reference to the DG sources connected to the microgrid ac bus, acting thus as a grid-former converter. In the following, the structure of the ICA proposed in this paper will be described and its implementation under different operability conditions will be shown. The principle of the synchronization technique based on DSOGI-FLL and the main control implemented in the ICA to work in grid connection and in island mode are presented in Sections III and IV, respectively. Finally, the simulation and the experimental results obtained when disconnecting and reconnecting the microgrid will be presented and discussed in Sections V and VI. II. DESCRIPTION OF THE STUDY CASE A simplified layout of the structure of the microgrid and its connection to the network is depicted in Fig. 1. As is shown
Fig. 1.
ICA involved in the microgrid environment.
in the figure, the microgrid is modeled as an aggregated current source inverter (CSI), that emulates the behavior of a gridfeeding power plant, while a load has been included in order to consider the energy consumed by the microgrid. Both elements are connected to the main grid, through the ICA, that consists on an active switch, SI , and a grid-connected VSI. The state of this switch, as well as the operation mode of the VSI at the ICA depends upon the operability of the mains. As detailed in Fig. 1, the VSI is fed by an external dc source that emulates the behavior of a generation power plant or an energy storage system. Considering this study case, the different operating modes of the microgrid will be briefly described in the following. A. Grid-Connected Mode In this mode, the main objective of the ICA is to track the grid voltage, waiting for grid disturbances that may require any action (as a disconnection from the mains). Under these conditions, the main network feeds all loads connected into the microgrid and provides the additional power required for balancing the power between loads and DG sources. An example of this application is the so-called grid-supporting inverters, which are designed to support the voltage and/or the frequency in either standalone or grid-connected conditions, as shown in [25]. B. Island Mode Under grid-fault conditions, the VSI changes to work in island mode and the switch SI , in Fig. 1, gets opened. Under such conditions, the microgrid VSI starts acting as a grid-forming converter fed by an external dc source. Considering that the microgrid can be composed by different sources connected in parallel with the inverter, the VSI supplies, or stores, the necessary amount of energy to maintain the microgrid energy balance and to keep the frequency and the voltage within their nominal limits. By means of higher order power control loops [26] or droop control methodologies [27], the microgrid distributes the generation between the different sources connected to the island grid. In this point, it is worth to point out that during the disconnection process, the frequency and the amplitude of the reference voltage will be settled to their nominal values. In addition, the phase angle will be the one detected previously to the turn off of the main switch.
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
Fig. 2.
Block diagram of the SRF-PLL.
C. Transient Operating Mode When a grid voltage disturbance is detected by the ICA, the main switch, SI , is turned off and the microgrid is disconnected from the grid. This transition should be performed as fast as possible, in order to ensure that any load or source connected to the microgrid would experience any significant distortion, thus maintaining the power flow. Likewise, another transient occurs when the microgrid should be reconnected to the network, changing from the islanded toward the grid-connected mode. In this case, in order to allow a transient-free reconnection process, the voltage across the SI should be almost zero at the closing time. Therefore, before reclosing SI the microgrid voltage should be synchronized with the network’s voltage. Once the microgrid has been reconnected to the network, the main switch can be turned on and the VSI changes its operation to the grid-connected mode again. III. SYNCHRONIZATION SYSTEM One of the most important parts to be solved before connecting any source to the grid is to perform an accurate synchronization with the network voltage to avoid overcurrents. Most of the grid-tie systems use a synchronization loop based on a PLL, which is mainly devoted to obtain the phase angle (θ ) of the grid voltage [28]. Many grid synchronization applications for three-phase systems are based on the implementation of synchronous reference frame PLLs (SRF-PLL). The main structure of a SRF-PLL is shown in Fig. 2. As is depicted in Fig. 2, in this kind of PLLs the three-phase grid voltage is transformed using the Clarke and Park transformation into a stationary reference frame [29]. The quadrature component of the voltage resulting from this synchronous transformation, namely, vq , is conducted to zero using a PI controller. The output of the PI controller provides the estimated value of the rotating frequency of the SRF-PLL. The integration of this frequency gives rise to the phase angle of the SRF (θ). When the quadrature component, vq , is equal to zero, θ matches the phase angle of the input voltage vector, hence the PLL is synchronized with the positive-sequence component (PSC) of the grid. Although the SRF-PLL presents a proper performance under balanced voltages, its performance is highly deficient under unbalanced and distorted grid conditions. Moreover, its performance is very sensitive to sudden changes in the phase angle, what consequently makes it less reliable when synchronizing power converters with the grid. In order to overcome these problems, an advanced synchronization system, namely, DSOGI-FLL, has been used in this paper. The DSOGI-FLL was
2995
presented in [24]–[28], and it has been proven to be able to estimate the PSC of a three-phase voltage under generic grid conditions. Moreover, it is also very suitable for the voltage reference generation when the microgrid is operating in island mode. The main blocks of the DSOGI-FLL structure have been presented in Fig. 3. As is depicted in the figure, the input voltage is decomposed into vα and vβ , and each one passes through a second-order general integrator (SOGI), that behaves as a bandpass filter at ω , to obtain the filtered output signals, vα and vβ , oscillating in phase with the input at ω . On the other hand, the signals qvα and qvβ , are the in-quadrature components of vα and vβ , respectively [30]. Considering these components, vα , vβ , qvα , and qvβ , the PSC of the input signal, v+ α β , can be extracted from the following linear combination: vα 1 1 −q + vα β = (1) vβ 2 q 1 where q represents the 90◦ phase-shift operator obtained from the SOGI. The frequency error generated by the αβ-component, εf , compared with the reference error value ε∗f = 0, will be the input of the FLL. The gain of the FLL is imposed by Γ, which determines the dynamics of the synchronization. The FLL output will be the estimated frequency, ω , which is later feedback to the SOGI blocks. In the particular case of working in island mode, a normalization block is added to the output of the PSC, as shown in Fig. 4. In this block, the reference of the positive sequence voltage to be provided by the converter to the other elements linked to the microgrid is calculated. As depicted in Fig. 4, the sinusoidal inputs provided by the PSC of the DSOGI-FLL are scaled in order to obtain an output voltage, ||vα+∗,β ||, with the nominal amplitude, VNOM . In this way, variations of the voltage magnitude during transient periods are avoided, and hence, the effects of the transition between grid-connected and island modes are minimized for the elements connected to the microgrid. IV. CONTROL SYSTEM In this paper, the different control loops have been designed to work in the αβ stationary reference frame using proportional– resonant (PR) controllers [31]. The control layout for the VSI is shown in Fig. 5. From the figure, it can be noticed how the inner control loop is devoted to control the current injection of the VSI, while the outer loop regulates the VSI output voltage. The main objective of these control loops is to achieve a fast transient response and a proper operability in both grid-connected and island modes. The characteristic transfer function of PR controller in αβ frame is shown in (2) [32] β (s) = kp + GαPR
kR s s2 + ω 2
(2)
where kP is the proportional gain, kR the resonant gain, and ω the frequency detected and applied to the controller, which is given by the DSOGI-FLL.
2996
Fig. 3.
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
Dual SOGI-FLL block diagram.
Fig. 4. PSC extraction and gain normalization from the DSOGI-FLL output signals.
A. Grid-Connected Mode When the microgrid is operating in grid-connected mode, the voltage regulation is disabled by the synchronization algorithm, putting GRID= 0, as detailed in Fig. 5. Hence, the current reference i∗α ,β , is given by the power calculator, and the VSI operates as a grid-supporting converter, injecting a desired value of active and reactive power, P ∗ and Q∗ In this mode, the system controls the VSI as a grid follower and the ICA system injects zero amperes into the grid. In grid-tied mode, the grid provides the correct balance between the load consumption and the DG generation. Thus, in this operation mode, the different renewable DG connected to the microgrid can be controlled by a local MPPT control and/or by a higher level control layer, giving active and reactive power references to optimize the operation point [33], [34]. B. Islanded Mode The voltage loop will operate only when the network falls, or when microgrid is intentionally disconnected from the main network. Under these conditions, the microgrid is operating in island mode, hence the voltage control loop is enabled, putting
GRID= 1. As a result, the ICA starts operating as a gridforming converter and gives the required current, i∗α ,β , to obtain the sinusoidal reference voltage, vα∗ ,β , imposing thus the microgrid voltage and frequency, as depicted in Fig. 5. In addition, a feed-forward of the grid voltage is added in the current-control loop, in order to improve the microgrid’s voltage control. The voltage reference, vα+∗,β , at a fixed frequency ω ff (=2πf) is generated by the synchronization system DSOGI-FLL, the positive-sequence calculation block (PSC) and the normalization function. These blocks are depicted in Figs. 3 and 4. The PSC, based on the equations shown in (3), extracts the positive sequence from the reference signal, obtained in the DSOGIFLL. The normalization block is used to normalize the amplitude of the positive reference voltage. Thus, vα+∗,β will be in phase with the fundamental positive sequence voltage before the fault, and it will remain oscillating at its nominal frequency, ω ∗ , at nominal quadratic mean value ||vα∗ ,β ||. This frequency is used in PR controllers belonging to the voltage and the current control loops, Fig. 5 shows a blocks diagram describing this structure. V. DISCONNECTION AND CONNECTION TRANSIENTS A. Disconnection Process Working in grid-connected mode, but in a nondetected and unplanned island situation, can be extremely dangerous due to security reasons, as workers can be exposed to energized lines during maintenance works, but also because in this faulty situation, inverters connected to the microgrid may energize the lines and feed up to the distribution transformer. This could give rise to dangerous situations when the main grid is restored and the transformer windings register two different amplitudes or phases in the voltage. Therefore, antiislanding detection systems are required nowadays for DG systems. In the context of this paper, passive islanding detection systems have been
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
Fig. 5.
Voltage- and current-control loop in stationary reference frame.
Fig. 6.
Synchronization control loop for to intentional island connection control.
implemented to begin the process of intentional disconnection and the formation of energy islands. The passive detection methods collected in the IEEE Std. 929-2000 [35] present different monitoring techniques to avoid the islanding of DG, according maximum and minimum voltage and frequency values [36]. The DSOGI-FLL have been used to extract both the frequency and the fundamental grid voltage component under faulty-grid conditions, thus permitting to determine whether an over/under voltage (OUV) or an over/under frequency (OUF) has occurred or not. If the module of the fundamental positive sequence, extracted from the DSOGI-FLL, changes suddenly, the OUV or the OUF condition
2997
are detected, as shown in [18] and [35]. When a fault is detected, the main switch disconnects the island from the main grid and the synchronization system begins to generate the microgrid reference voltage, at constant-speed phase angle θ = ω NOM t, where ω NOM = 2πf, without any delay or phase step referred to the previous conditions before the fault occurrence. In this paper, the proposed system uses two DSOGI-FLL, as shown in Fig. 6. The first one is used to monitor the grid voltage characteristics (magnitude and frequency) giving the OUV and OUF signals. The second one operates as a synchronization system, when the microgrid is in grid-connected mode, or as a simple oscillator, when the grid is not available. It should
2998
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
be noted that these DSOGI-FLL have the same input voltage. However, when a fault is detected, c1 becomes zero and the synchronization SOGI parameter, kSOGI , as well as the FLL gain, Γ, become zero as well. In these conditions, the input voltage of this the DSOGI-FLL does not have any effect. Moreover, an analysis of Fig. 3 revels that, in this situation, the frequency ω smoothly converges to the nominal value ω ff and the DSOGI keeps oscillating at the frequency ω with constant amplitude. The microgrid connection state is mainly determined by two conditions. The first one will be referenced as c1 , which determines when the voltage and frequency are within the ranges allowed by the standards, as detailed in the following expression: 1, if 1.1 > vGRID (p.u.) > 0.85 (3) OUV = 0, if other 1, if 1.05 >ωGRID(p.u.) > 0.95 (4) OUF = 0, if other c1 = OUV · OUF.
(5)
Before the fault occurrence, the system operates in steady state, synchronized with the grid; therefore, the DSOGI error signals can be considered almost zero. Thus, the condition c2, can be defined as 1, if εV α β (p.u.) < 0.05 (6) c2 = 0, if other εv = ε2 + ε2 (7) αβ v (α ) v (β )
Fig. 7.
Resynchronization and reconnection process flowchart.
by the DSOGI-FLL itself, vVSI .
where εv α = vα − vα ε v β = vβ −
v∗ = c1 · vVSI + c1 · vGRID .
(9)
vβ
SI = c1 · c2.
(8)
The second condition checks that both grid and microgrid voltages oscillate at the same frequency and phase. This phase difference is quantified by the quadratic mean voltage error, εv , in αβ coordinates (6). This value can be obtained from the DSOGI considering the difference between the real grid voltage, v, and the estimated one, v . When the fault occurs, the system detects either a sag in the quadratic mean grid voltage value or a frequency deviation. Then, the ICA control opens the main switch SI , disconnecting the local network, and the proportional gain, that modifies the voltage error in the synchronization loop, is set to zero, kSOGI = 0. Therefore, the DSOGI-FLL operates as an oscillator at a constant frequency, whose phase matches to that of the grid just before the fault. In this moment, the positive-sequence voltage obtained from the synchronization loop, vα+∗,β , is provided as a reference to the voltage loop. The reference voltage, v∗ , used to synchronize the system is given by (9). When the system operates in grid-connected mode or during the resynchronization transient from standalone mode, the synchronization voltage will be equal to the main grid voltage, vGRID . On the contrary, when the microgrid works in standalone, the synchronizing voltage will be the voltage formed
In addition, the dynamics of the synchronization process is modified depending on the microgrid-connection state. When the system is grid-connected, it needs a fast dynamic behavior to inject currents in phase with the voltage. During intentional island mode, the dynamic is canceled since it imposes a zero value to the constants determining the dynamics of the system. In resynchronization mode (islanded from the main grid), a slow dynamics is imposed to adjust the phase difference between the main voltage and the microgrid voltage. The gain values included in the synchronization loop, kSOGI and Γ, will be also modified as function of the microgrid-connection state suggested as follows:
1 kSOGI = KSOGI · c1 · c2 + c2 · 100
c2 + c2 · 1 and Γ = Γ · c1 · . 100
(10)
In this study, the SOGI √ gain, KSOGI , during grid-connected mode has been set to 2, while the FLL gain is set to 0.411. On the other hand, when the system works isolated from the grid, both values will be zero, i.e., kSOGI = Γ = 0.
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
Fig. 8.
2999
Electrical scheme of the simulation and experimental setup.
B. Resynchronization Process The resynchronization process starts when the main grid voltage is restored. This condition is achieved when c1 = 1, from the previous island state, and the reconnection process begins, passing through different transient states. It should be noted that, at this moment, c2 is still zero because the DSOGI output is not yet in phase with the grid voltage. Thus, both gains have a low value given by (10), that results in kSOGI = K/100 and Γ = Γ/100. These low values permit having a smooth response when there is a phase jump. Equations (9) and (10) are represented in Fig. 6 as a truth table in function of the conditions c1 and c2. As shown in the figure, low-pass filters (LPFs) are also added to smooth the gain changes. In a first step, once the main voltage is recovered and stabilized, the grid frequency could be operating in a nonnominal frequency value, which should be detected. To perform this action, an additional DSOGI-FLL system is implemented, that provides the new nominal feed-forward frequency, ω ff , to be applied in the main DSOGI-FLL loop, given as the new frequency reference to the islanded grid. This module is enabled by the variable EN_FLL2, and it is active after the fault has been removed. Once the new frequency detection is stable, the module is disabled. The blocks of this system are presented in Fig. 6. Once the new nominal frequency, ω ff , has been measured, a slow resynchronization process with the grid voltage is required. The settling time of the synchronization will be a function of the phase difference between the grid and the microgrid voltage and the parameters kSOGI , and Γ. The DSOGI constants during the resynchronization stage become a hundred of times smaller than their nominal value. Once the resynchronization is done and after a security time in which both voltages must remain stable (a reasonable time of 200 ms has been established), the grid switch SI is closed.
In the island mode, the inverter works as a grid-forming inverter; therefore, while being controlled in this way, it cannot be connected to the grid. To solve this issue, before reclosing the switch, the VSI changes to as a grid-feeding control. Once its operation mode is changed, the main switch is instantaneously closed. This transient state has to be very fast, lasting only some microseconds, because it represents an unstable transient-state sensitive to load changes. Immediately after the ICA switch recloses, the VSI keeps supplying the same current levels for feeding the local load. In a few microseconds time, typically two or three voltage cycles, the amplitude of the current controller references is smoothly reduced to zero, decreasing the current injected by the VSI. Meanwhile, the grid current increases along this process, in order to cover the power demanded by the loads. This smooth transition is introduced to prevent the ocurrence of overcurrents, which could produce significant voltage dips in weak networks. Just after that the integrators in the resonant current control loop are reset and the setpoint of the current is set to the desired value. This finalizes the reconnection transient and the ICA is controlled again as a grid follower, waiting for a new disconnection process. Fig. 7 details the flowchart of the entire reconnection process.
VI. SIMULATION AND EXPERIMENTAL RESULTS The performance of the intelligent connection agent during the disconnection and reconnection processes will be tested by means of simulations and experiments in a scaled setup. In this study case, the implemented system reproduces the requirements of a general plant with passive loads and grid-feeding converters connected to a bus of a local network. The objective of the simulation and experiments is to evaluate the performance of the overall control, transient voltages, and currents between two operation modes.
3000
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
Fig. 9. Simulation of the disconnection and the reconnection of a microgrid controlled by the proposed ICA. (a) Overall view of the full simulation. The grid fault is produced in t = 3.134 s and is restored in t = 5.909 s. (b) Detailed view of the disconnection process. (c) Instant in which the grid fault is removed, but a phase difference between the grid and the microgrid exists. (d) Evolution of the synchronization process. (e) Ending of the resynchronization process, the main switch is closed in t = 7.50 s, but the load is still supplied by the VSI (now controlled as a CSI). (f) Smooth energy transfer between the VSI and the grid.
Simulations have been carried out in cosimulation between PSIM and MATLAB–Simulink software. The experimental results where obtained in a scaled setup where the proposed control was implemented in a dSPACE 1103 platform with a sample time of TS = 100 μs. In this system, the switching frequency, fS , is set to 10 kHz. The ICA has been build using an automated contactor controlled from the dSPACE by the c1·c2 condition, and a grid-tie VSI. In Fig. 8, the configuration of the three-phase ICA-VSI,
with its main switch, used in both simulation and experiments is shown. In this case, a 2.2 kVA three-phase inverter controlled through dSPACE is used. The dc voltage of the VSI is set to vdc = 350 V and the voltage at the ac side is equal to 110 Vrm s at 50 Hz. The connection of the converter to the network has been done through an ac filter, that behaves as a lowpass LC-filter, where Lf = 2.46 mH and Cf = 10 μF. In turn, the local load consist on a set of three wye-connected 54-Ω resistors.
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
3001
Fig. 10. Experimental performance of the ICA during a disconnnection and a reconnection process. The ICA switch is turned off at t = 3.134 s and turned on at t = 7.600 s. (a) Grid Power required, upper signal, and power supplied at the load, lower signal. (b) Evolution of the mean quadratic error in the DSOGI-FLL and in the SOGI-FLL, upper signal, and condition states of c1, c2 and c1·c2. (c) Sensed and reference voltage in both side of the switch, upper, and difference between them. (d) Same variables represented in the ICA switch on. (e) and (f), Measured and reference current given by the inverter.
A. Case I: Microgrid Composed by Loads Fig. 9 shows the simulation results during both steady-state and transient conditions of the disconnection and the reconnection processes. In this test, the system starts operating in a grid-connected mode when the load is fully fed by the grid, while the VSI tracks only the microgrid voltage. Then, at t = 3.134 s, an ideal and symmetrical 50% voltage sag occurs at the point of common coupling (PCC). In Fig. 9(b), the disconnection transient, where the microgrid starts working in island mode, is detailed. Along this transient,
it is important to note how the microgrid voltage as well as the current load is not significantly distorted. In fact, in the load just a reduced voltage drop is seen, due to the fast dynamics of the VSI current controller, which after the disconnection detection operates as a grid former converter. At t = 5.909 s, the voltage grid is restored again, but with a 60 ◦ phase-angle difference between the main grid and microgrid voltage signals is registered; therefore, the reconnection cannot be performed yet. When the main grid is restored and stabilized, the previously explained slow resynchronization stage starts. Fig. 9(c) and (d) shows this process. At t = 7.533 s, both phase angles are nearly
3002
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
Fig. 11. Experimental results. The scale is set to 100 V/division in voltage variables, and 5 A/division for current in all the graphs. In graph (a) time scale is set to 1 s/division, while in the rest of them, it is set to 20 ms/division. (a) Overall view. The grid fault is produced in t = 3.134 s and is restored in t = 5.909 s. (b) Detailed view of the disconnection process. (c) Instant in which the grid fault is removed. (d) Evolution of the synchronization process. (e) When the resynchronization process is finished, the main switch is closed in t = 7.50 s, but the load is still supplied by the VSI (now controlled as a CSI). (f) Energy transfer between the VSI and the grid.
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
equal and the physical reconnection is done. Fig. 9(e) shows the reconnection transient, while the deenergization in the ICA-VSI state is depicted in Fig. 9(f). Additional waveforms extracted from the simulation are added to complete the description of the disconnection and grid-reconnection processes during the same transient time. In Fig. 10(a), the power injected to the load is shown. In this figure, it can be noticed that in any microgrid mode, the load is perfectly supplied. In function of its operation mode, the load is fed by the main grid or by the VSI, as is represented in the upper graph. The grid state is indicated by the c1·c2 variable, depicted in the lower part of Fig. 10(b). In the upper diagram, depicted in dark (into the zoom), the quadrature error used to detect the frequency deviation in the nominal frequency value, ω ff 2π50, is represented previously to the resynchronization stage. When this error εα β Δ ω , is near to zero, the frequency detected by SOGI-FLL is considered to be as the new frequency to be feedback into the resynchronization loop. Then, the slow resynchronization process starts with the voltage error in α–β components being εα β DSOGI (represented with a light line in Fig. 10(b)). In Fig. 10(c), the error in the voltage reference during the intentional islanding grid is represented, while in Fig. 10(d), the performance of this error during the reconnection step is depicted. Finally, the current-control loop performance is detailed in Fig. 10(e) and (f). In these figures, the disconnection and reconnection errors are shown, where only the switching noise is appreciated. The low error value, in both figures, shows the proper performance of the voltage and current controllers in island mode. Experimental results, during disconnection and reconnection, for the microgrid phase voltage are captured in Fig. 11. The performance of these experimental waveforms corroborates the same results obtained in simulation. The scale is set to 100 V/division in voltage variables, and 5 A/division for current in all the graphs. In Fig. 11(a), the full process can be seen, with the disconnection and the reconnection processes included. For the sake of precision, the time scale is set to 1 s/division, while in the rest of plots, it is set to 20 ms/division. The grid fault is produced in t = 3.134 s and is restored in t = 5.909 s. A detailed view of the island formation is represented in Fig. 11(b). The resynchronization initial steps are shown in Fig. 11, where the grid fault clearance is captured, although a phase difference remains. In this moment, the synchronization process, whose evolution can be seen in Fig. 11(d), starts. Finally, Fig. 11(e) presents the closing process of the ICA main switch when the resynchronization process is finished. It can be noticed from the figure that the main switch is closed in t = 7.50 s, but the load is still supplied by the VSI. Finally, Fig. 11(f) shows the ICAVSI deenergization transition. It is worth to state that during the reconnection transient, no voltage distortions are observed in the oscilloscope. B. Case II: Microgrid Composed by Loads and Sources The simulation of the full system with the addition of a generation source in grid-feeding mode has been also performed. All the test conditions are the same as in the previous simulations;
3003
Fig. 12. Simulation view of disconnection and reconnection process, with presence of current source into the microgrid.
Fig. 13. Simulation view of power balance in disconnection and reconnection processes.
however, in this case, a power load of PR = 3.7 kW has been connected. The current and voltage involved in this sequence are shown in Fig. 12, while the power balance is depicted in Fig. 13. The CSI is programmed with a step profile between t1 and t6 with a nominal power of PCSI = 2 kW. Between t0 and t2 , the system is grid-connected and the load is fully fed by the grid. In t1 , the CSI is turned on; therefore, the grid only supplies the power difference between the load and generation. In t2 , a fault is produced in the main grid; therefore, the SI is turned off and the microgrid operates in island mode. In this moment, t2 , the VSI changes its operation mode as a grid-forming inverter and supplies the required power to maintain the voltage to its nominal value. It should be noted that in this transition the power consumed by the load and the generation is not affected by any significant perturbations. In t3 = 0.6 s, the main grid voltage is restored and detected; therefore, the resynchronization process starts. This process ends up at t4 , where the resynchronization is complete. As described in Section V, the VSI forming part of the ICA switches its operation mode to a current source, as a previous step to closing of the switch SI . Between t4 and t5 , the microgrid operates again in grid-connected mode, while the
3004
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 10, OCTOBER 2011
ICA inverter operates as a grid-feeding converter, taking the reference values from the last island current reference values. In t5 , a smooth cancelation of this reference current starts, while the grid increases the current and power feeding the load. Then, a secure reset of the main ICA inverter can be done, operating again as a network follower. Finally, at t6 , the output reference power into the CSI returns to zero. VII. CONCLUSION In this paper, it has been shown how the ICA manages the disconnection and reconnection of a microgrid when the main network is under fault conditions. Moreover, the ICA has been actively controlled and programmed to work depending on the connection state of the microgrid. As discussed in this paper, the proposed system has two main advantages. The first one lies on the fact that it does not require any communication system to make the different ICA agents work properly, which consequently permits to carry out an easy expansion of the microgrid. The second one is the achievement of a flexible grid-forming algorithm, which is not limited to the proposed microgrid configuration. Hence, the ICA can be applied to any microgrid topology, with grid-feeding inverters or any other grid-connected power converters. In this paper, a DSOGI-FLL has been used for synchronization purposes giving rise to good results. In addition, the FLL block of this synchronization structure has been used for providing the ω reference of the resonant controllers implemented in this study. In this study, it has been demonstrated how the proposed controller is able to follow the voltage grid in amplitude, frequency, and phase in grid-connected mode, while, in islanded mode, the ICA permits providing the proper voltage to feed the loads avoiding overcurrents during the transient. The controller and the synchronization algorithm implemented in this case ensure a smooth and safe reconnection of the microgrid from the main grid when the fault is cleared. The different simulation and experimental results have shown the correct operation of the proposed ICA in a scaled laboratory setup. REFERENCES [1] N. Hatziargyriou, H. Asano, R. Iravani, and C. Marnay, “Microgrids,” IEEE Power Energy Mag., vol. 5, no. 4, pp. 78–94, Jul./Aug. 2007. [2] F. Katiraei, R. Iravani, N. Hatziargyriou, and A. Dimeas, “Microgrids management,” IEEE Power Energy Mag., vol. 6, no. 3, pp. 54–65, May/Jun. 2008. [3] H. Karimi, A. Yazdani, and R. Iravani, “Negative-sequence current injection for fast islanding detection of a distributed resource unit,” IEEE Trans. Power Electron., vol. 23, no. 1, pp. 298–307, Jan. 2008. [4] M. Liserre, A. Pigazo, A. Dell’Aquila, and V. M. Moreno, “An antiislanding method for single-phase inverters based on a grid voltage sensorless control,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1418–1426, Oct. 2006. [5] M. Ciobotaru, V. G. Agelidis, R. Teodorescu, and F. Blaabjerg, “Accurate and less-disturbing active antiislanding method based on PLL for gridconnected converters,” IEEE Trans. Power Electron., vol. 25, no. 6, pp. 1576–1584, Jun. 2010. [6] I. Erlich, W. Winter, and A. Dittrich, “Advanced grid requirements for the integration of wind turbines into the German transmission system,” in Proc. IEEE Power Eng. Soc. Gen. Meeting, 2006, pp. 7–15. [7] C. Feltes, S. Engelhardt, J. Kretschmann, J. Fortmann, F. Koch, and I. Erlich, “Comparison of the grid support capability of DFIG-based wind
[8] [9] [10]
[11] [12] [13] [14]
[15]
[16]
[17]
[18] [19] [20] [21] [22]
[23]
[24]
[25]
[26]
[27]
[28]
farms and conventional power plants with synchronous generators,” in Proc. IEEE Power Energy Soc. Gen. Meeting, 2009, pp. 1–7. IEEE Application Guide for IEEE Std 1547, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Std 1547.2-2008, 2008. H. J. Laaksonen, “Protection principles for future microgrids,” IEEE Trans. Power Electron., vol. 25, no. 2, pp. 2910–2918, 2010. J. M. Guerrero, J. C. Vasquez, J. Matas, M. Castilla, and L. G. de Vicuna, “Control strategy for flexible microgrid based on parallel line-interactive UPS systems,” IEEE Trans. Ind. Electron., vol. 56, no. 3, pp. 726–736, Mar. 2009. Z. Yao, L. Xiao, and Y. Yan, “Seamless transfer of single-phase grid-interactive inverters between grid-connected and stand-alone modes,” IEEE Trans. Power Electron., vol. 25, no. 6, pp. 1597–1603, Jun. 2009. J. M. Guerrero, L. Hang, and J. Uceda, “Control of distributed uninterruptible power supply systems,” IEEE Trans. Ind. Electron., vol. 55, no. 8, pp. 2845–2859, Aug. 2008. J. M. Guerrero, L. G. De Vicuna, and J. Uceda, “Uninterruptible power supply systems provide protection,” IEEE Ind. Electron. Mag., vol. 1, no. 1, pp. 28–38, Spring 2007. C. Chien-Liang, W. Yubin, L. Jih-Sheng, L. Yuang-Shung, and D. Martin, “Design of parallel inverters for smooth mode transfer microgrid applications,” IEEE Trans. Power Electron., vol. 25, no. 1, pp. 6–15, Jan. 2010. K. De Brabandere, B. Bolsens, J. Van Den Keybus, A. Woyte, J. Driesen, and R. Belmans, “A voltage and frequency droop control method for parallel inverters,” IEEE Trans. Power Electron., vol. 22, no. 4, pp. 1107– 1115, Jul. 2007. P. Rodriguez, A. V. Timbus, R. Teodorescu, M. Liserre, and F. Blaabjerg, “Flexible active power control of distributed power generation systems during grid faults,” IEEE Trans. Ind. Electron., vol. 54, no. 5, pp. 2583– 2592, Oct. 2007. A. V. Timbus, P. Rodriguez, R. Teodorescu, M. Liserre, and F. Blaabjerg, “Control strategies for distributed power generation systems operating on faulty grid,” in Proc. IEEE Int. Symp. Ind. Electron., vol. 2, 2006, pp. 1601–1607. T. Thacker, F. Wang, and D. Boroyevich, “Islanding control of a distributed generation unit’s power conversion system to the electric utility grid,” in Proc. IEEE 36th Power Electron. Spec. Conf., 2005, pp. 210–216. H. Kim, T. Yu, and S. Choi, “Indirect current control algorithm for utility interactive inverters in distributed generation systems,” IEEE Trans. Power Electron., vol. 23, no. 3, pp. 1342–1347, May 2008. J. M. Guerrero, N. Berbel, J. Matas, J. L. Sosa, and L. G. de Vicuna, “Control of line-interactive UPS connected in parallel forming a microgrid,” in Proc. IEEE Int. Symp. Ind. Electron., 2007. Y. Li, D. M. Vilathgamuwa, and P. C. Loh, “Design, analysis, and realtime testing of a controller for multibus microgrid system,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1195–1204, Sep. 2004. R. Teodorescu and F. Blaabjerg, “Flexible control of small wind turbines with grid failure detection operating in stand-alone and grid-connected mode,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1323–1332, Sep. 2004. J. C. Vasquez, J. M. Guerrero, A. Luna, P. Rodriguez, and R. Teodorescu, “Adaptive droop control applied to voltage-source inverters operating in grid-connected and islanded modes,” IEEE Trans. Ind. Electron., vol. 56, no. 10, pp. 4088–4096, Oct. 2009. P. Rodriguez, A. Luna, I. Candela, R. Rosas, R. Teodorescu, and F. Blaabjerg, “Multi-resonant frequency-locked loop for grid synchronization of power converters under distorted grid conditions,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 127–138, 2010. D. Georgakis, S. Papathanassiou, N. Hatziargyriou, A. Engler, and C. Hardt, “Operation of a prototype microgrid system based on micro-sources quipped with fast-acting power electronics interfaces,” in Proc. IEEE 35th Ann. Power Electron. Spec. Conf., 2004, pp. 2521–2526. E. Barklund, N. Pogaku, M. Prodanovic, C. Hernandez-Aramburo, and T. C. Green, “Energy management in autonomous microgrid using stability-constrained droop control of inverters,” IEEE Trans. Power Electron., vol. 23, no. 5, pp. 2346–2352, Sep. 2008. L. Yun Wei and K. Ching-Nan, “An accurate power control strategy for power-electronics-interfaced distributed generation units operating in a low-voltage multibus microgrid,” IEEE Trans. Power Electron., vol. 24, no. 12, pp. 2977–2988, Dec. 2009. A. Timbus, R. Teodorescu, F. Blaabjerg, and M. Liserre, “Synchronization methods for three phase distributed power generation systems: An overview and evaluation,” in Proc. IEEE 36th Power Electron. Spec. Conf., 2005, pp. 2474–2481.
ROCABERT et al.: INTELLIGENT CONNECTION AGENT FOR THREE-PHASE GRID-CONNECTED MICROGRIDS
3005
[29] S.-K. Chung, “A phase tracking system for three phase utility interface inverters,” IEEE Trans. Power Electron., vol. 15, no. 3, pp. 431–438, May 2000. [30] P. Rodriguez, A. Luna, M. Ciobotaru, R. Teodorescu, and F. Blaabjerg, “Advanced grid synchronization system for power converters under unbalanced and distorted operating conditions,” in Proc. IEEE 32nd Ann. Conf. IEEE Ind. Electron., 2006, pp. 5173–5178. [31] D. G. Holmes, T. A. Lipo, B. P. McGrath, and W. Y. Kong, “Optimized design of stationary frame three phase AC current regulators,” IEEE Trans. Power Electron., vol. 24, no. 11, pp. 2417–2426, Nov. 2009. [32] A. Timbus, M. Liserre, R. Teodorescu, P. Rodriguez, and F. Blaabjerg, “Evaluation of current controllers for distributed power generation systems,” IEEE Trans. Power Electron., vol. 24, no. 3, pp. 654–664, Mar. 2009. [33] K. De Brabandere, K. Vanthournout, J. Driesen, G. Deconinck, and R. Belmans, “Control of microgrids,” in Proc. IEEE Power Eng. Soc. Gen. Meeting, 2007, pp. 1–7. [34] N. Pogaku, M. Prodanovic, and T. C. Green, “Modeling, analysis and testing of autonomous operation of an inverter-based microgrid,” IEEE Trans. Power Electron., vol. 22, no. 2, pp. 613–625, Mar. 2007. [35] IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems, IEEE Std 929-2000, 2000. [36] F. De Mango, M. Liserre, A. D. Aquila, and A. Pigazo, “Overview of antiislanding algorithms for PV systems. Part I: Passive methods,” in Proc. 12th Int. Power Electron. Motion Control Conf., 2006, pp. 1878–1883.
Josep M. Guerrero (S’01–M’04–SM’08) received the B.S. degree in telecommunications engineering, the M.S. degree in electronics engineering, and the Ph.D. degree in power electronics from the Technical University of Catalonia, Barcelona, Spain, in 1997, 2000, and 2003, respectively. He is currently an Associate Professor with the Department of Automatic Control Systems and Computer Engineering, Technical University of Catalonia, Barcelona, where he currently teaches courses on digital-signal processing, FPGAs, microprocessors, and renewable energy. Since 2004, he has been responsible for the Renewable Energy Laboratory, Escola Industrial de Barcelona. He has been a Visiting Professor at Zhejiang University and Aalborg University. His current research interests include power-electronics converters for distributed generation and distributed energy storage systems, control and management of microgrids and islanded minigrids, and photovoltaic and wind power plants control. Dr. Guerrero is an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS and the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS. He has been a Guest Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS SPECIAL ISSUE OF POWER ELECTRICS FOR WIND ENERGY CONVERSION, and the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS SPECIAL SECTIONS: Uninterruptible Power Supplies (UPS) systems, Renewable Energy Systems, Distributed Generation and Microgrids, and Industrial Applications and Implementation Issues of the Kalman Filter. He Chairs the Renewable Energy Systems Technical Committee of IEEE Industrial Electronics (IES). He is an elected IEEE IES Adcom member.
Joan Rocabert (S’08–M’11) was born in Barcelona, Spain. He received the M.Sc. degree in electrical engineering and the Ph.D. degree in electrical engineering from the Technical University of Catalonia, Barcelona, Spain, in 2003 and 2010, respectively, on the topic of PV microgrids control. From 2004 to 2008, he was a Research Assistant in the Department of Electronic Engineering, Technical University of Catalonia, where since 2008 he has been a Researcher and an Assistant Professor with the Department of Electrical Engineering. His current research interests include power electronics applied to photovoltaic and wind energy systems, particularly their application into microgrids.
Ignacio Candela (S’99–M’04) received the B.S., M.S., and Ph.D. degrees from the Technical University of Catalonia (UPC), Barcelona, Spain, in 1987, 2000, and 2009, respectively, all in industrial engineering. He was an Assistant Professor in 1991 and has been an Associate Professor since 1993 with UPC. He has authored or coauthored more than 30 published technical papers and has been involved in several industrial projects and educational programs in the fields of power quality conditioning and motor drives. His current research interests include power conditioning, integration of distributed energy systems, and control of power converters and motor drives. Dr. Candela is a member of the IEEE Power Electronics Society, the IEEE Industrial Electronics Society, and the IEEE Industry Application Society.
Gustavo M. S. Azevedo (S’08) was born in Belo Jardim, Brazil, in 1981. He received the B.S. and M.S. degrees in electrical engineering, in 2005 and 2007, respectively, from the Federal University of Pernambuco, Recife-PE, Brazil, where he is currently working toward the Ph.D. degree. From 2008 to 2009, he was a Visiting Scholar at the Polytechnical University of Catalunya, Barcelona, Spain. His current research interests include integration of renewable energy systems, microgrids, and power quality.
Alvaro Luna (S’07) received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from the Technical University of Catalonia (UPC), Barcelona, Spain, in 2001, 2005, and 2009, respectively. In 2005, he joined the faculty of UPC, where he is currently an Assistant Professor with the Department of Electrical Engineering. His current research interests include wind turbines control, integration of distributed generation, and power conditioning. Dr. Luna is a member of the IEEE Industrial Electronics Society and the IEEE Industrial Applications Society.
Pedro Rodr´ıguez (S’99–M’04–SM’10) received the B.S. degree in electrical engineering from the University of Granada, Granada, Spain, in 1989, and the M.S. and Ph.D. degrees in electrical engineering from the Technical University of Catalonia (UPC), Barcelona, Spain, in 1994 and 2004, respectively. In 1990, he joined the faculty of UPC as an Assistant Professor, and he became an Associate Professor in 1993. He was a Researcher with the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg, in 2005, and with the Institute of Energy Technology, Aalborg University, Aalborg, Denmark, in 2006. He is currently the Head of the Research Group on Renewable Electrical Energy Systems, Department of Electrical Engineering, UPC. He is a coauthor of about 100 papers in technical journals and conference proceedings. He is the holder of five patents. His current research interests include integration of distributed energy systems, power conditioning, and control of power converters. Dr. Rodr´ıguez is a member of the IEEE Power Electronics, the IEEE Industrial Electronics (IES), the IEEE Industry Application Societies, and the IEEE IES Technical Committee on Renewable Energy Systems. He has coorganized special sessions in several IEEE conferences on power electronics applied to renewable energies. He is an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS and the Committee Chair of the IEEE IES Gold and Student Activities.
