A Survey of Techniques Used to Control Microgrid Generation and Storage during Island Operation

Transcription

1 A Surey o Techniques Used to Control Microgrid Generation and Storage during Island Operation Michael Angelo Pedrasa Department o Electrical and Electronics Engineering, Uniersity o the Philippines Quezon City, Philippines m.pedrasa@student.unsw.edu.au ABSTRACT A microgrid is a cluster o interconnected distributed generators, loads and intermediate storage units that cooperate with each other to be collectiely treated by the grid as a controllable load or generator. Power quality eents and pre-set conditions will make the microgrid disconnect rom the main grid and operate as a separate island. This paper enumerates and discusses the issues concerning the island operation o microgrids. The microsources and storage deice should co-operate with each other to maintain the integrity o the islanded microgrid. Six co-operation approaches proposed by arious authors are presented. The strategies are then compared based on their applicability to dierent control requirements. 1. INTRODUCTION A microgrid is a cluster o interconnected distributed generators, loads and intermediate energy storage units that co-operate with each to be collectiely treated by the grid as a controllable load or generator [1]. It is connected to the grid at only one point, the point o common coupling or PCC. The main objectie o its conception is to acilitate the high penetration o distributed generators without causing power quality problems to the distribution network. Another important objectie is to proide high quality and reliable energy supply to sensitie loads. The components that constitute the microgrid may be physically close to each other or distributed geographically. Figure 1 depicts a typical microgrid. The microsources are the primary energy sources within the microgrid. They may be rotating generators or distributed energy (DE) sources interaced by power electronic inerters. The installed DE may be biomass, uel cells, geothermal, solar, wind, steam or gas turbines and reciprocation internal combustion engines. The oerall eiciency may be improed by using combined heat and power sources (CHP). The connected loads may be critical or non-critical. Critical loads require reliable source o energy and demand stringent power quality. These loads usually own the microsources because they require a continuous supply o energy. Non-critical loads may be shed during Ted Spooner School o Electrical Engineering and Telecommunications, Uniersity o New South Wales Sydney, Australia e.spooner@unsw.edu.au emergency situations and when required as set by the microgrid operating policies. The intermediate energy storage deice is an inerterinteraced battery bank, supercapacitors or lywheel. The storage deice in the microgrid is analogous to the spinning resere o large generators in the conentional grid. They ensure the balance between energy generation and consumption especially during abrupt changes in load or generation. The microgrid in Figure 1 has a dedicated storage deice. Another method o integrating energy storage to the microgrid is to install battery banks in the dc links o the inerters o the microsources [2]. Grid MGCC Intermediate storage MV/LV Transormer Non-critical loads Figure 1: An example microgrid The microgrid can operate in grid-connected mode or in island mode. In grid-connected mode, the microgrid either draws or supplies power to the main grid, depending on the generation and load mix and implemented market policies. The microgrid can separate rom the main grid wheneer a power quality eent in the main grid occurs [3]. The microgrid components are controlled using a decentralized decision making process in order to balance demand and supply coming rom the microsources and the grid. The degree o decentralization may ary rom a ully decentralized approach or using hierarchical control (Hatziargyriou, Dimeas et al. 2005). The latter makes use o a central controller (microgrid central controller or MGCC) that controls the actions o all components o the microgrid (Hatziargyriou, Jenkins et al. 2006). The MGCC optimizes operation by maximizing the alue o the microgrid. It is also capable o implementing demand side management. G PV FC

2 2. ISLAND OPERATION OF MICROGRIDS When a microgrid is grid-connected, it behaes as a controllable load or source. It should not actiely regulate the oltage at the PCC [6]. Furthermore, the harmonics and dc current it injects to the grid should be below the required leels. During this mode o operation, the primary unction o the microgrid is to satisy all o its load requirements and contractual obligations with the grid. The microgrid should disconnect when an abnormal condition occurs in the grid. It shits to island mode o operation, and the microgrid is aced with the ollowing issues: 1. Voltage and requency management The oltage and requency are established by the grid when the microgrid is connected. When the microgrid islands, one or more primary or intermediate energy sources should orm the grid by establishing its oltage and requency, otherwise, the microgrid will collapse. Both oltage and requency should be regulated within acceptable limits. I the requency has dropped to excessiely low leels, loads may be shed to hasten its recoery towards the nominal alue [7]. 2. Balance between supply and demand I the microgrid is exporting or importing power to the grid beore disconnection, then secondary control actions should be implemented to balance generation and consumption in island mode. I the connected load exceeds the aailable generation, demand side management should be implemented. Also, there should be enough energy storage capacity to ensure initial balance ater an abrupt change in load or generation. 3. Power quality The microgrid should maintain an acceptable power quality while in island operation. There should be an adequate supply o reactie energy to mitigate oltage sags. The energy storage deice should be capable o reacting quickly to requency and oltage deiations and injecting or absorbing large amounts o real or reactie power. Finally, the microgrid should be able to supply the harmonics required by nonlinear loads. 4. Microsource issues A major dierence between the primary energy sources in the grid and microsources connected to the microgrid is that the latter has no inertia [1]. The microgrid does not hae the spinning reseres that are inherently present in the conentional grid. Most microsources (e.g. turbines and uel cells) hae slow response or ramp-time when implementing secondary oltage and requency control. The intermediate storage units and microsources with built-in battery banks are thereore expected to delier the beneits that should be deried rom spinning reseres. The power electronics interace enabled these deices to respond quickly to abrupt command signals and changes in the power low leels. The types, ownership and combination o connected microsources also complicate the control o a microgrid during island operation. Microsources relying on renewable energy like wind and solar are operated at their maximum outputs to maximize production, and require orecasting methods to predict their output. CHP sources are only operated when their heat productions are required. Some microsources may be required to meet the energy demands o speciic loads beore they should proide or other loads. 4. Communication among microgrid components The aailability o communication inrastructure between the microgrid components is another aspect considered when choosing the control approach on an islanded microgrid. The microgrid should hae a plugand-play architecture so that the microsources will rely on locally aailable inormation to control their generated power [1]. I communication between components is required (e.g. MGCC sending setpoints to microsources or negotiation between agents controlling the microsources), the delay within the communication network should not present problems. The microsources and storage deice should co-operate with each other to maintain the integrity o the islanded microgrid. This paper enumerates and discusses six dierent co-operation approaches proposed by seeral authors. The approaches dier on how the microsources and storage deice will co-operate when the microgrid is islanded. The strengths and weaknesses o each approach will be examined. Perceied implementation issues will also be discussed. Finally, the scenarios where these approaches are suited will be suggested. 3. CO-OPERATION OF MICROSOURCES AND STORAGE DEVICES The irst part o this section describes how the output power o the microsources is controlled. The second part enumerates the island mode co-operation techniques o the microsources POWER OUTPUT CONTROL A microsource may be controlled so that its real and reactie power output is constant. This type o control is called PQ control [8]. The microsource behaes like a oltage-controlled current source. The terminal oltage is measured and broken down to its direct and quadrature components. The direct and quadrature components o the output current are computed rom the oltage components and required real and reactie power output. Figure 2 shows how the output current is controlled. The droop control o microsources proposed in [9] emulates the operation o a rotating generator in a conentional grid. The output requency and oltage o the inerters ollow the droop characteristics as shown in Figure 3. The microsources will change their output by ΔP (or ΔQ) when the requency (or oltage) changes by Δ (or Δ) rom the nominal alues 0 (or 0 ).

3 Figure 2: PQ control o microsources with two microsources and one storage deice that is importing power (P grid ) rom the grid. The output power o the storage deice and two microsources are zero, P 1, and P 2, and the requency is 0. When the microgrid islands, the microsources and storage deice will proide the power originally imported rom the grid. The indiidual droop characteristics will determine the new requency, new, as shown in Figure 4(a). The storage deice and microsources will output additional powers ΔP 0, ΔP 1 and ΔP 2, where ΔP 0 + ΔP 1 + ΔP 2 = P grid (1) 0 new Storage Microsource 1 Microsource 2 P 0 P 1 P 2 0 P 1 P 2 (a) (a) 0 new Storage Microsource 1 Microsource 2 Droops, V VSI Q 0 Q 1 Q 2 P, Q P and Q computation (b) Figure 3: (a) Droop characteristics. (b) Inerter with droop characteristics CO-OPERATION OF MICROSOURCES Six co-operation methods will be presented. Fie o which apply to microgrids with dedicated energy storage while one applies to microgrids which microsources hae installed storage capacity. In the irst our approaches, all microsources and storage deice are in PQ control when the microgrid is gridconnected. The output power setpoints are determined by the MGCC or by the owners. The real power output o the storage deice is zero. PV and wind microsources are operated at maximum output leels. For purpose o identiication in this paper, the subsection heading will be used as name o the described co-operation technique PURE DROOP CONTROL V, I Microgrid In the approach presented by Georgakis, etal in [10], the storage deice and all microsources capable o regulating their power outputs switch to droop control when the microgrid islands. These deices will control the oltage and requency o the microgrid. The resulting oltage and requency are determined by their droop characteristics. As example, consider a microgrid 0 Q 1 Q 2 (b) Figure 4: Pure droop control, (a) requency control and (b) oltage control The new microgrid oltage is determined by the indiidual Q- cures as shown in Figure 4(b) INVERTER MODES CONTROL The co-operation approach proposed by Pecas Lopes, etal in [7, 8 & 11] is called microgrid operation based on inerters control modes. In their approach, the microsources remain in PQ control when the microgrid islands and the storage deice shits to droop control. A PI controller in the microsources compute the real power setpoint (P* in Figure 2) so that the requency will return to the pre-island alues. This action orces the microsources to produce the right amount o power so the storage deice will not contribute to the oerall generation. The oltage is controlled using droop control as shown in Figure 4(b). VQ droop PI Q* P* PQ-controlled inerter Microgrid Figure 5: Microsource controller or inerter modes control.

4 This operation is called single master operation because only one inerter (the one associated with the storage deice) regulates the requency and oltage in the microgrid. I battery banks are installed in the dc bus o the microsource inerters, these microsources can also shit to droop control and participate in the regulation o the requency and oltage. This is now called multimaster operation [8]. 0 Q -Q max Q max PRIMARY ENERGY SOURCE CONTROL Another approach proposed by Peças Lopes etal in [11] is called microgrid operation regarding primary energy source control. When the microgrid islands, the storage deice behaes like a synchronous generator: it perorms secondary control actions to restore the oltage and requency to pre-island alues. The secondary actions inole the shiting o the droop characteristics. Figure 6: Primary energy source control Figure 6 shows how requency and oltage are regulated. Assume that the storage deice is generating no real power and Q 0 reactie power, and the microgrid is importing real (P grid ) and reactie (Q add ) rom the grid (point A). When the microgrid islands, the storage deice should produce the real and reactie powers imported rom the grid, so the requency and oltage will drop to isl and isl (point B). The requency and oltage are restored to the pre-island alues ( 0 and 0 ) by shiting the droop cures o the microsources to the right (point C) REVERSE DROOP CONTROL A A 0 Q 0 P grid 0 isl In the approach presented by Laaksonen, etal in [12], all microsources remain in PQ control and the storage deice shits to droop control when the microgrid islands. The droop control o the storage inerter, howeer, regulates the microgrid oltage by controlling the output real power, and the requency is regulated by controlling the output reactie power. They proposed this droop relationship because o the resistie nature o LV distribution lines. In this paper, this co-operation approach will be called reerse droop control. The droop characteristics are shown in Figure 7. C B C B Q add 0 isl 0 -P max P max Figure 7: Reerse droops control AUTONOMOUS CONTROL The approach proposed by Lasseter and Piagi in [2] is called autonomous control o microsources. They emphasized that the microgrid should ollow a peer-topeer and plug-and-play model. They aoided the installation o a single point o ailure like MGCC and dedicated storage units, so the microsources hae integrated storage (battery bank in the dc bus o the inerter). The microsources are controlled using unit power control coniguration, eeder low control coniguration or combination o both. In unit power low coniguration, the microsources assume droop control in both grid-tied and island modes o operation. The output power o a microsource is constant when the microgrid is grid-tied because the grid sets the requency. When the microgrid islands, the microgrid requency is determined by the droop cures as described in Figure 3(a). In eeder low coniguration, a microsource regulates the real power lowing through a particular eeder in the microgrid. It controls the amount o power it is generating to regulate the amount o power lowing through the particular eeder. Figure 8 shows the relationship between the power lowing through a eeder, F, and requency,. The microgrid requency is determined by the F- cures o the microsources when the microgrid islands. 0 F F 0 microsource Figure 8: Feeder low coniguration MULTI-AGENT BASED PQ CONTROL The approach proposed by Oyarzabal, etal in [13] commands all microsources to remain in PQ control while the storage deice shits to droop control. The storage deice will control the oltage and requency o the microgrid. The output power setpoints o the microsources are computed by the MGCC and sent to P P F Rest o the microgrid

5 the microsources eery 30 seconds. The storage deice acts as a load ollowing unit: it generates or absorbs power to ensure energy balance at all times. Intelligent agents are used to manage the microgrid, and a control agent in the MGCC takes charge o the scheduling o the operation o the microsources. 4. COMPARISON OF CO-OPERATION STRATEGIES The undamental requirement o a microgrid in island operation is the presence o an inerter or rotating machine that will establish oltage and requency and keep these alues within acceptable limits. The slope o the droop cures, and the range o power exchange between generators and loads determine the oltage and requency extremes in the pure droop, reerse droop and autonomous control. Furthermore, the requency and magnitude o requency and oltage luctuations depend on the requency and magnitude o load luctuations. Frequent and large load swings will cause requent and signiicant oer- and under-oltages and operation beyond requency limits. These control approaches may be adopted i the connected loads can tolerate the luctuations (e.g. constant impedance loads and loads with power electronics ront end). On the other hand, the inerters mode, primary energy source and multiagent based PQ control guarantee that the oltage and requency would not deiate much rom the nominal alues, resulting in high quality o power. The approaches are applicable to microgrids that drie power quality-sensitie loads. The serices required rom the storage deice determine the storage capacity required by the microgrid. Small capacity is required i it only proides quick reaction to imbalances between supply and consumption during abrupt changes in generation or load. The inerter modes and multi-agent based co-operation schemes may used or microgrids with small storage capacity because the storage deice only unctions as a ast power absorbing or generating unit but does not output power or extended periods. In contrast, large storage capacity should be installed i the storage deice will participate in the oerall power generation o the microgrid. The pure droop, primary energy source or reerse droops cooperation approaches demand large storage capacity because the storage deice directly participates in requency control. It is required to output or absorb real power or prolonged periods o time. The microgrid or storage deice owner should hae means o recoering the installation costs o the storage deice. As suggested in [14], the storage deice can participate in the market operation o the microgrid the latter is grid-connected. All co-operation strategies except the multi-agent based PQ control will not require a communication inrastructure between microsources, storage deices and controllers during island operation i the inerters are capable o island detection. The storage deices and microsources with island detection capability can automatically transer rom PQ control to droop control and ice ersa. Howeer, i the microsources rely on the central controller in determining the state o the microgrid, a ast communication line should be installed between the controller and the microsources. The multi-agent based PQ control needs a communication inrastructure like a LAN because the generation setpoints are periodically sent by the central controller. The success o multi-agent based control depends on the reliability o communication exchange between the agents. Communication between the central controller and loads are required i load shedding is implemented especially when the connected load exceeds the generation capacity or the requency dropped to excessiely low leels immediately ater disconnection. Black starting will also require communication among the microgrid components, howeer, the black start procedure may be executed by the operators using a telephone. The choice o co-operation strategy should take into account the types o installed microsources. I intermittent (solar or wind) or CHP microsources are present, there should be at least one microsource that would act as a load ollowing unit. A load ollowing unit is required because o the stochastic nature o the intermittent sources and CHP sources operate only when heating is required. The load ollowing microsource simply adjusts its output power to balance the instantaneous generation and consumption. I the installed storage capacity is small, the inerter modes and multi-agent based PQ control are best choices because the other microsources are orced to assume the load ollowing unction, and there is no risk o depleting or oercharging the storage deice. I the installed storage capacity is large, pure droop control is the next best option because the load ollowing responsibility is shared by the storage deice and microsources. Reerse droop control may be also chosen i the other microsources will also assume reerse droop characteristics. The primary energy source control is the worst choice because the storage deice is solely responsible in load ollowing. I autonomous control is chosen, the eeder low coniguration should be used on the eeder where the intermittent source is connected. The microsource associated with the eeder low coniguration acts as the load ollowing unit. The ownership o the microgrid components also aects the choice o co-operation approach. I the microsources hae seeral owners, they tend to operate the microsources in a way that will maximize their indiidual proits. All approaches may be used i the microgrid is normally importing power rom the grid. When the microgrid islands, the microsources will remain operating at their maximum leels and some loads are shed to achiee balance between generation and consumption. Howeer, i the microgrid is normally selling power to the grid, the shortage o consumers would orce the microsources to decrease generation leels. They must compete on who will supply energy to the connected loads. The multi-agent based PQ control is best suited or this situation. Autonomous control may also be used or this case, but the consumers and microsource owners should pre-arrange through contracts the amount o energy that will be exchanged.

6 As mentioned earlier, the storage deice is a critical component o an islanded microgrid because it establishes the oltage and requency. Thereore, its owner must be compensated or proiding this important serice. This dilemma is aoided i the entire microgrid has only one owner. All co-operation approaches may be used i the microgrid has only one owner. Haing one owner implies that the microsources do not hae to compete and the objectie o the microgrid controller is to maximize the alue o the entire microgrid, and not the alue o indiidual components. 5. CONCLUSIONS This paper presented six co-operation approaches or the microsources and storage deice when the microgrid is in island operation. All approaches addressed the issues concerning island operation o microgrids. Factors like power quality and stability, installed storage capacity, requirement o a communication inrastructure, types o installed microsources and microgrid ownership aect the choice o co-operation strategy. REFERENCES [10] D. Georgakis, etal, Operation o a Prototype Microgrid System Based on Microsources Equipped with Fast-acting Power Electronics Interaces. 35th Annual IEEE Power Electronics Specialists Conerence, 2004, ol. 4, pp [11] J.A. Peças Lopes, etal, Control Strategies or MicroGrids Emergency Operation. International Conerence on Future Power Systems, No [12] H. Laaksonen, P. Saari and R. Komulainen, Voltage and Frequency Control o Inerter Based Weak LV Network Microgrid, International Conerence on Future Power Systems, No [13] J. Oyarzabal, etal, "Agent-based Micro Grid Management System," International Conerence on Future Power Systems, Noember [14] F. Neiuwenhout, etal, "Electricity Storage or Distributed Generation in the Built Enironment," International Conerence on Future Power Systems, No [1] Robert Lasseter, "Microgrids," IEEE Power Engineering Society Winter Meeting, 2001, ol. 1, pp [2] Paolo Piagi and Robert Lasseter, "Autonomous Control o Microgrids," IEEE PES Meeting, [3] Robert Lasseter, etal, "White Paper on Integration o Distributed Energy Resources: The CERTS MicroGrid Concept," Caliornia Energy Commission, [4] Nikos Hatziargyriou, etal, Management o Microgrids in Market Enironment, International Conerence on Future Power Systems, [5] Nikos Hatziargyriou etal, Microgrids - Large Scale Integration o Microgeneration to Low Voltage Grids, Session CIGRE International Council o Large Electric Systems, Paris, France. [6] IEEE , IEEE Standard or Interconnecting Distributed Resources with Electric Power Systems [7] J. Pecas Lopes, etal, Deining Control Strategies or MicroGrids Islanded Operation, IEEE Transactions on Power Systems, ol. 21, issue 2, pp [8] J. Pecas Lopes, etal, "Microgrids Black Start and Islanded Operation," 15th Power Systems Computation Conerence, Liège, Belgium, [9] Alred Engler and Nikos Soultanis, "Droop Control in LV-Grids," 2005 International Conerence on Future Power Systems, 2005