Hierarchical control of intelligent microgrids

JUAN C VASQUEZ, JOSEP M GUERRERO, JAUME MIRET, MIGUEL CASTILLA, and LUIS GARCI´A DE VICUN˜A © COMSTOCK & DIGITAL VISION Integration of Distributed Energy Resources into the Smart Grid W orldwide, electrical grids are expected to become smarter in the near future In this sense, there is an increasing interest in intelligent and flexible microgrids, i.e., able to operate in island or in grid-connected modes Black start operation, frequency 1932-4529/10/$26.00&2010IEEE and voltage stability, active and reactive power flow control, active power filter capabilities, and storage energy management are the functionalities expected for these small grids This way, the energy can be generated and stored near the consumption points, thus increasing the Digital Object Identifier 10.1109/MIE.2010.938720 DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 23 reliability and reducing the losses produced by the large power lines In this article, the main concepts related to the configuration, control, and energy management of intelligent microgrids are reviewed Microgrids as a Key Point to Integrate Distributed Generation into the Grid Intelligent microgrids are required to integrate distributed generation (DG), distributed storage (DS), and dispersed loads into the future smart grid This will be a key point to cope with new functionalities, as well as to integrate renewable energy resources into the grid Those small grids should be able to generate and store energy near to the consumption points This avoids large distribution lines coming from big power plants located far away from the consumption areas The impact of these distribution lines could result in low efficiency due to the high conduction losses, voltage collapse caused by reactive power instabilities, low reliability due to single point failures and contingencies, among other problems The main idea is to connect these microgrids to the main grid or interconnect them through tie lines forming microgrid clusters A microgrid can be defined as a part of the grid consisting of prime energy movers, power electronics converters, distributed energy storage systems, and local loads Microgrids should be able to operate autonomously but also interact with the main grid The seamless transfer from grid-connected mode to islanded mode is also a desirable feature These tie lines will act as interchange energy channels to balance the energy required by each microgrid, thus the power flow of these lines will be further reduced Moreover, microgrids represent a new paradigm of lowvoltage distribution systems, since the generation is not only based on small generation machines but also on small prime movers, such as photovoltaic (PV) arrays, small wind turbines (WTs), or fuel cells, that requires for power electronics interfaces such as ac–ac or dc–ac inverters Those power electronics equipments act very fast, which has full control of the transient response However, in contrast with the generation machines, power electronics not have inherent inertia that ensures the stability of the system and the steady-state synchronization of each unit With the objective to achieve this performance, virtual inertias are often implemented through control loops known as the droop method This method consists on reducing the frequency and the amplitude of the inverter output voltage proportionally to the active and reactive powers Thus, microgrids will be able to keep active and reactive power balance, as well as to avoid voltage collapses Further, microgrids should have additional performances such us low-voltage WT PV Panel System UPS Inverters PCC Utility Grid Static Transfer Switch (IBS) Distributed Loads Common ac Bus Microgrid FIGURE – Typical structure of a flexible microgrid based on renewable energy resources 24 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2010 ride-through, active power filtering and uninterruptible power supply (UPS) capabilities, black start and islanding operation, synchronization with the main grid, fully and independent active and reactive power flow control, and energy management Figure shows a microgrid based on small wind generators, PV sources, energy storage systems, and distributed loads The microgrid is connected to the point of common coupling (PCC) of the main grid through the intelligent bypass switch (IBS) The overall system consists of a number of DG and DS systems that requires for power electronics inverters It is worth saying that the microgrid can have several elements working like current-source inverters (CSIs) and other working like voltage-source inverters (VSIs) 1) CSI units are normally used for PV or WT systems that require maximum power point tracker (MPPT) algorithms However, these systems can also work as VSI, operating outside the maximum power point if necessary 2) VSI units are used for storage energy systems to support the voltage and frequency of the microgrid in island mode Nevertheless, it is necessary to add proper control loops when several units are connected in parallel Operation Modes of a Microgrid Grid-Connected Mode of Operation The microgrid energy management must be performed by considering the energy storage systems and the control of the energy flows in both operation modes, i.e., with and without connection to the public grid In this sense, the microgrid must be capable of exporting/importing energy from/to the main grid to control the active and reactive power flows and to supervise the energy storage [1], [2] In the grid-connected mode, system dynamics is fixed to a large extent by the utility grid because of the small size of the DG units Another problem is the slow response at the control signals when a change of the output power occurs The absence of synchronous machines connected to the low-voltage power grid requires for virtual inertias implemented within the control loops of the power electronic interfaces Further, the power balancing during the transient must be provided by power storage devices, such as batteries, supercapacitors, or flywheels After a blackout, the microgrid should start correctly imposing itself the frequency and amplitude conditions as well as connecting progressively loads and DG units following a hierarchical order (black start operation) Similarly in this operation mode, all the DG units must supply a specified power, e.g., to minimize the power importing from the grid (peak shaving), whose requirements depend on the global energy management system In addition, each DG unit can be controlled through voltage regulation for active and reactive power generation using a communication bus Typically, depending on the custom desire, when the microgrid is in grid-connected mode, the main grids, together with the local DG units, send all the power to the loads result, DS units will support all active power unbalances by injecting or absorbing active power proportionally to the frequency deviation To operate isolated from the main grid, the IBS will be open, disconnecting the microgrid from the main grid [3] Therefore, when the microgrid is in islanded operation mode, the DG units that feed the system are responsible for nominal voltage and frequency stability when power is shared by the generation units It is also important to avoid overloading the inverters and to ensure that load changes are controlled in a proper form Some control techniques are based on communication links as a master–slave scheme, which can be adopted in systems where neighboring DG units are connected through a common bus However, a communication link through a low-bandwidth system can be more economic, more reliable, and finally, attractive Equally, in autonomous mode, the microgrid must satisfy the following issues: n Voltage and frequency management: The system acts like a voltage source, controlling power flow through voltage and frequency control loops adjusted and regulated as reference within acceptable limits n Supply and demand balancing: In grid-connected mode, the frequency of the DG units is fixed by the grid Changing the setting frequency, new active power set points that will change the power angle between the main grid and the microgrid can be obtained n Power quality: The power quality can be established in two levels The first is reactive power compensation and harmonic current sharing inside the microgrid, and the second level is the reactive power and harmonic compensation at the PCC; thus, the microgrid can support the power quality of the main grid Also, when the microgrid is operating in islanded mode, all the DG units are constant power sources, injecting the desired power toward the utility grid Transition Between Grid-Connected and Islanded Mode As previously commented, IBS is continuously supervising both the utility grid and the microgrid status (see Figure 2) When a power supply shutdown occurs, or a fault in the main grid has been detected by the IBS, the microgrid must be disconnected and the restoration process must be reduced as much as possible to ensure a high reliability level In such a case, this switch can readjust the power reference at nominal values, although it is not strictly necessary In addition to this, if maximum permissible deviation is not exceeded (typically, 2% for frequency and 5% amplitude), the voltage amplitude and frequency can be measured inside the microgrid, and operation points (P à and Qà ) avoid the frequency deviation and amplitude of the droop method When the microgrid is in islanded mode operation, and IBS detects main grid fault-free stability, synchronization among voltage, amplitude, phase, and frequency must be realized for connecting operation The restoration procedure aimed at the plant restart, system frequency synchronization, and power generation of the main grid During this stage, some details must be considered, such as the reactive power balance, commutation of the transient ∗ E=V ∗ voltages, balancing power genω=ω eration, starting sequence, and Islanding coordination of DG units Islanded Mode of Operation The microgrid can be disconnected from the grid in the following two scenarios: n Preplanned islanded operation: If any events in the main grid are presented, such as long-time voltage dips or general faults, among others, islanded operation must be started n Nonplanned islanded operation: If there is a blackout due to a disconnection of the main grid, the microgrid should be able to detect this fact by using Bypass Off proper algorithms P = P ∗; Q = Q ∗ Import/Export In islanded mode, the sysP/Q Grid tem dynamic is depicted by its Connected Operation own DG units, which normally regulates frequency and ampliE = Vg tude voltage of the microgrid ω = ωg Bypass On Also, a small deviation from the nominal frequency and ampli- FIGURE – Operation modes and transfers of the flexible tude could be noticed As a microgrid and IBS grid status supervisory Hierarchical Control of Microgrids Functionally, the microgrid, in a similar way as the main grid, can operate by using the DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 25 following three main hierarchical control levels (see Figure 3): Tertiary Control B n Primary control is the droop W Secondary Control control used to share load Primary Control between converters n Secondary control is responsible for removing any FIGURE – Hierarchical operation modes of the flexible steady-state error introduced microgrid by the droop control respectively, and m and n coefficients n Tertiary control concerning more define the corresponding slopes P à global responsibilities decides the import or export of energy for the and Qà are the active and reactive microgrid power references, which are commonly These three levels are described set to zero when we connect UPS units in detail below in parallel autonomously, forming the energetic island (see the control diagram in Figure 5) However, if we want Primary Control: to share power with constant power P=Q Droop Control sources, the utility grid is necessary to Each inverter will have an external fix both active and reactive power sourpower loop based on droop control ces to be drawn from the unit This [4]–[6], also called autonomous or droop method increases the system decentralized control, whose purpose performance because it is allowing the is to share active and reactive power autonomous operation among the modamong DG units and to improve the ules This way, the amplitude and system performance and stability, frequency output voltage can be influadjusting at the same time both the enced by the P=Q sharing through a frequency and the magnitude of the outself-regulation mechanism that uses put voltage The droop control scheme both the active and reactive local can be expressed as (see Figure 4) power from each unit [7] x ¼ xà À mðP À P Ã Þ ð1Þ To obtain good power sharing, à à the frequency and amplitude output E ¼ E À nðQ À Q Þ; ð2Þ voltage must be fine-tuned in the control loop, with the aim of comwhere xà and E à are the frequency pensating the active and reactive and the amplitude of the output voltage, power imbalance [8], [9] This concept is derived from the classic high-power system theory, in which generator ω = ω – m(P – P ∗) frequency decreases when the grid ω utility power is increased [10], [11] In transmission systems, the grid impedance is mainly inductive; this is the reason why it is used to adopt P À x and Q À E slopes Hence, the inverter can inject desired active and reactive power to the main grid, regulating the output voltage and responding to linear load changes However, when using power electronics converters and low-voltage microgrids, the impedance is too far away to be inductive The multiloop droop control scheme shown in Figure is composed of an external loop whose function is to regulate the output voltage, whereas the inner loop supervises the inductor current [12], [13] or the capacitor current [14], [15] of the output filter to reach a fast dynamic response This control diagram provides a high viability in parameters design and a low total harmonic distortion, but it requires both complex analysis and a parameter synchronization algorithm Similarly, another relevant aspect to provide proper output impedance is the virtual output impedance loop Virtual Impedance Loop The output impedance of the closedloop inverter affects the power sharing accuracy and determines the droop control strategy Furthermore, the proper design of this output impedance can reduce the impact of the line impedance unbalance To program a stable output impedance, the output voltage reference proportionally to the output current can be ω∗ P∗ (a) Q − P Io − n + E = E ∗ – n(Q – Q ∗) + E∗ Q∗ Transformations and Power Calculation E E∗ Q∗ (b) Q FIGURE – P À x and Q À E grid scheme using Pà and Qà as set points Vo E Vo∗ P − m + P∗ FIGURE – Droop control using P=Q 26 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2010 φ Vo∗ = E sin(ω ⋅ t – φ) dropped This fast control loop is able to fix the output impedance of the inverter by subtracting a processed portion of the output current to the voltage reference of the inverter, which is obtained from the voltage reference of the inner control loops as shown in Figure Moreover, hot-swap operation, i.e., the connection of more UPS’s modules without causing large current disturbances, can be achieved by using a soft-start virtual impedance by programming a high output impedance when the UPS is connected to the microgrid and then reduce it slowly to a proper value As a control inner loop, inverters must be programmed to act as generators by including virtual inertias by means of the droop method It specifically adjusts the frequency or amplitude output voltage as a function of the desired active and reactive power Thus, active and reactive power can be shared equally among the inverters For reliability and to ensure local stability, voltage regulation is needed Without this supervision control, most of the DG units can present reactive power and operation voltage oscillations To avoid this fact, high circulating currents among the sources must be eliminated through the voltage control in such a way that reactive power generation of the DG unit be more capacitive, reducing the voltage set point value In other words, while Q is a high inductive value, the voltage reference value will be increased as shown in Figure Secondary Control: Frequency and Voltage Restoration and Synchronization To restore the microgrid voltage to nominal values, supervisor system must send the corresponding signals using low-bandwidth communication Also, this control can be used for microgrid synchronization to the main grid before performing the interconnection, transiting from islanded to grid-connected mode The power distribution through the control stage is based on a static relationship between x À P and E À Q, and it is implemented as a droop scheme Likewise, CSI units are normally used for PV or WT systems that require maximum power point tracker algorithms v + Voltage Loop − PWM + UPS Inverter Current Loop i Zo (s) Virtual Impedance Loop Q Q∗ Voltage E Vo∗Reference E sin (ωt ) ω P Droop Control P and Q Calculation P FIGURE – Multiloop control droop strategy with the virtual output impedance approach frequency and voltage restoration to their nominal values must be adjusted when a load change is realized Originally, frequency deviation from the nominal measured frequency grid brings to an integrator implementation For some parallel sources, this displacement cannot be produced equally because of measured errors In addition, if the power sources are connected in islanded mode through the main grid at different times, the load behavior cannot be completely ensured because all the initial conditions (historical) from the integrators are different Hence, it is necessary that an external secondary control be able to measure the frequency and amplitude deviations and send the necessary E ΔE E = E ∗ – nQ Capacitive Load −Qnom E∗ Inductive Load Q Qnom FIGURE – Droop characteristic when supplying capacitive or inductive loads references to push up the droop characteristics of each DG unit (see Figure 8) Tertiary Control: P=Q Import and Export In the third hierarchical control loop, the adjustment of the inverter’s references connected to the microgrid, and even of the generator’s MPPTs, is performed, so that the energy flows are optimized The set points of the microgrid inverters can be adjusted to control the power flow in global (the microgrid imports/exports energy) or local terms (hierarchy of spending energy) Normally, power flow depends on economic issues Economic data must be processed and used to make decisions in the microgrid Each controller must respond autonomously to the system changes without requiring load data, the IBS, or other sources Thus, the secondary control uses P and Q injected from the grid to control it (see Figure 9) For instance, we can adjust P-reference as a positive or negative value to absorb or inject P to the grid and fix Q-reference to zero to achieve unity power factor The controller will send the frequency and amplitude references to the secondary DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 27 ω = ω∗ – m (P ∗ – P) + δω E = E∗ – n (Q ∗ – Q) + δV Frequency Restoration Level ω ref δω Gwr(s) Droop Control and Sine Generator ωo Vref Current Control Loop δV Gvr(s) P Inner Loops Voltage Control Loop Q io Driver and PWM Generator ν Vo Virtual Impedance Loop Voltage Restoration Level Secondary Control Low-Bandwidth Communications P/Q Calculation Outer Loops Primary Control FIGURE – Primary and secondary control based on hierarchical management strategy Conclusions control, saturating them with the maximum and minimum allowed values inside the microgrid By using this control level, extra functionalities can be obtained, such as islanding detection or voltage harmonic reduction of the grid by harmonic injection Consequently, the microgrid can be fully controlled by using the multilevel hierarchical approach, which conjugates distributed and decentralized control The implementation will be related to the communication infrastructure and the future smart-grid codes Main ac Grid This article gives an overview about the hierarchical control of intelligent microgrids Also, it was shown that a number of interconnected DG and DS units can perform a flexible microgrid, showing the different operating modes of a microgrid applying the concept of multilevel control loops conceived as a control hierarchical strategy This article has shown that droop-controlled microgrids can operate in both grid-connected and islanded mode as a flexible, grid-interactive microgrid IBS Microgrid P, Q P/Q Calculation P Q – ∗ + Q P∗ + δφ Emax Gq P Emin ωmax – ωmin RMS PLL Gp Tertiary Control + – + δφ φref + Gse vref Gsw ωref Secondary Control + Synchronization Loop FIGURE – Block diagram of the tertiary control and the synchronization control loop 28 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2010 The following improvements to the conventional droop method are required to integrate microgrids to the main grid [4], [5], [14], [16], [17]: n improvement of not only the transient response of the DG and DS units but also of the microgrid n virtual impedance: harmonic power sharing and hot-swapping of DG and DS units n adaptive droop control laws to increase the interactivity of the system The following are the hierarchical controls required for an ac microgrid: n Primary control based on the droop method allows the connection of different ac sources acting like synchronous machines n Secondary control avoids the amplitude and frequency deviation produced by the primary control Only low-bandwidth communications are needed to perform this control level A synchronization loop can be added in this level to transfer from islanding to gridconnected modes n Tertiary control allows import/ export active and reactive power to the grid, estimates the grid impedance, nonplanned islanding detection, and harmonic current injection to compensate for voltage harmonics in the PCC Additional features are also required to the flexible microgrids: n voltage ride-through and power quality in the PCC n black start operation n grid impedance estimation and islanding detection n storage energy management and control These new features will allow microgrids more intelligence and flexibility to integrate DG and DS resources into the future smart grid This concept will be an impulse for the integration of clean energy resources, allowing a more sustainable electrical grid system in global terms Biographies Juan C Vasquez received his B.S degree in electronics engineering from the Universidad Autonoma de Manizales, Colombia, and his Ph.D degree in automatics, robotics, and vision from the Technical University of Catalonia, Barcelona, Spain, in 2004 and 2009, respectively He has been an assistant professor teaching courses on digital circuits, servo systems, and flexible manufacturing systems His research interests include modeling, simulation, and management applied to the DG in microgrids Josep M Guerrero (josep.m guerrero@upc.edu) received his B.S degree in telecommunications engineering, his M.S degree in electronics engineering, and his Ph.D degree in power electronics from the Technical University of Catalonia, Barcelona, Spain, in 1997, 2000, and 2003, respectively He is 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, control theory, microprocessors, and renewable energy Since 2004, he has been responsible for the Renewable Energy Laboratory, Escola Industrial de Barcelona He is the editor-in-chief of International Journal of Integrated Energy Systems His research interests include PVs, wind energy conversion, UPSs, storage energy systems, and microgrids He is a Senior Member of the IEEE Jaume Miret received his B.S degree in telecommunications and his M.S and Ph.D degrees in electronics from the Technical University of Catalonia, Barcelona, Spain, in 1992, 1999, and 2005, respectively Since 1993, he has been an assistant professor with the Department of Electronic Engineering, Technical University of Catalonia, Vilanova i la Geltru´, Spain, where he teaches courses on digital design and circuit theory His research interests include dc–ac converters, active power filters, and digital control He is a Member of the IEEE Miguel Castilla received his B.S., M.S., and Ph.D degrees in telecommunication engineering from the Technical University of Catalonia, Barcelona, Spain, in 1988, 1995, and 1998, respectively Since 2002, he has been an associate professor with the Department of Electronic Engineering, Technical University of Catalonia, Vilanova i la Geltru´, Spain, where he teaches courses on analog circuits and power electronics His research interests include power electronics, nonlinear control, and renewable energy systems Luis Garcı´a de Vicun ˜ a received his Ingeniero de Telecomunicacio´n and Dr.Ing degrees from the Technical University of Catalonia, Barcelona, Spain, in 1980 and 1990, respectively, and his Dr.Sci degree from the  Paul Sabatier, Toulouse, Universite France, in 1992 He is currently an associate professor with the Department of Electronic Engineering, Technical University of Catalonia, Vilanova i la Geltru´, Spain, where he teaches courses on power electronics His research interests include power electronics modeling, simulation and control, active power filtering, and high-power-factor ac/dc conversion References [1] P Piagi and R H Lasseter, ‘‘Autonomous control of microgrids,’’ in Proc IEEE Power Engineering Society General Meeting (PES), June 2006, p [2] N Pogaku, M Prodanovic, and T C Green, ‘‘Modeling, analysis and testing of 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Applications (EPE’07), 2007, pp 1–9 DECEMBER 2010 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 29 ... main hierarchical control levels (see Figure 3): Tertiary Control B n Primary control is the droop W Secondary Control control used to share load Primary Control between converters n Secondary control. .. the configuration, control, and energy management of intelligent microgrids are reviewed Microgrids as a Key Point to Integrate Distributed Generation into the Grid Intelligent microgrids are required... the hierarchical control of intelligent microgrids Also, it was shown that a number of interconnected DG and DS units can perform a flexible microgrid, showing the different operating modes of