www.ietdl.org Published in IET Generation, Transmission & Distribution Received on 1st December 2012 Revised on 31st July 2013 Accepted on 15th August 2013 doi: 10.1049/iet-gtd.2012.0687
ISSN 1751-8687
Energy storage application in low-voltage microgrids for energy management and power quality improvement Irena Wasiak, Ryszard Pawelek, Rozmyslaw Mienski Institute of Electrical Power Engineering, Lodz University of Technology, 18/22 Stefanowskiego Street, 90-924 Lodz, Poland E-mail:
[email protected]
Abstract: The study deals with the application of energy storage connected to the low-voltage microgrid by coupling inverter for simultaneous energy management and ancillary services that include the compensation of power quality disturbances. The usefulness of storage equipment as a solution to various problems that accompany microgrid development is discussed. Then, the idea is presented to join different tasks the storage can perform in one application. Control algorithm is presented for storage inverter, which enables storage unit to be charged or discharged according to assumed schedule and to contribute to power quality improvement through the compensation of reactive power, current harmonics and unbalance. Effectiveness of compensation is examined on the simulation model of test microgrid. Moreover, the results of tests performed in real test microgrid configured in the Laboratory of Distributed Generation at the Lodz University of Technology are presented in the study.
1
Introduction
Together with the growing use of distributed energy sources (DESs), conventional low-voltage (LV) distribution networks change their structure from passive to active. An active network in which the processes of energy generation, distribution and use are executed in a controllable way forms an electrical power microgrid [1]. The microgrid integrates energy sources, including renewables, storage systems, controllable and uncontrollable loads. Energy sources and storage equipment are connected directly or through power electronic converters. The microgrid can operate autonomously, but usually it operates in connection with the supplying grid. The transition of passive networks to active microgrids involves unique challenges for the utilities and energy consumers. The key issue here is new architecture and properly designed management and control systems. On the other hand the integration of DESs in the power system causes varies technical problems to be solved, relating to maintaining the required power quality (PQ), stability and continuity of supply. The highest amount of distributed generation that can be integrated in a specific network without PQ limits being violated is called ‘hosting capacity’ [2]. In many practical cases, the network hosting capacity limits the wide penetration of DESs, unless some measures are applied that will facilitate the integration process and assure the required quality of supply. Such measures include energy storage equipment. In conventional LV networks, energy storage devices have been used mainly by end-users for peak shaving or as protection against short supply interruptions. With the IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
advent of microgrids and development of storage technology the role of this equipment has been continuously growing. Storage systems have becoming the subject of interest also for utilities and energy producers or prosumers [Prosumer is defined here as end-user exploiting energy sources (simultaneous consumer and producer).] as they can help to increase the efficiency of supply and maintain the required PQ in the complex microgrid environment [1]. There are many technologies that make energy storage possible [3–5]. Storage systems designated for specific tasks differ with power capacity and energy charging/discharging rate (amount of stored energy). According to the energy discharge rate the devices can be classified into three categories [6]: (1) systems that supply energy for more than a few minutes to several hours for energy management, including microgrid island operation; (2) systems that supply energy within minutes to address power shortages (uninterruptible power supply); (3) systems that supply energy within seconds or less to address the compensation of PQ events. From among the different technologies that are available at present, battery storages have been used more often in LV networks, as their technology is commercially mature and economically favourable [6]. The devices are connected to the grid through coupling inverters. Their basic task is transmitting active power from or into the grid, thus they store or release energy, according to the established 463
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www.ietdl.org schedule. The idea presented in this paper is to widen the functionality of the storage inverter applied basically for energy management, and involve it simultaneously in additional functions aimed at PQ improvement. These functions, called ancillary services, may include voltage control, Var support and the compensation of disturbances, such as harmonics or unbalance. The approach in which different tasks are joined in one system makes the storage application more effective. Some results of investigations undertaken by the authors in the field of storage utilisation were presented in their earlier works. In [7, 8], simulation results illustrating the application of battery storage for load levelling and voltage stabilisation were reported. Paper [9] showed the use of storage for joint energy management and harmonic compensation. This paper is an extension of the previous works and demonstrates the ability of energy storage to be operated for energy management and simultaneous compensation of load reactive power, harmonics and unbalance. The presented results come from simulation and laboratory tests carried out in a test microgrid configured in the Laboratory of Distributed Generation at the Lodz University of Technology [10]. The paper is organised as follows. In Section 2, the role of energy storage equipment in microgrids development is discussed. It is followed by Section 3 presenting study microgrid and the principle of storage control to perform the assumed tasks. Section 4 deals with microgrid modelling and simulation with using PSCAD/EMTDC program. Laboratory tests are presented in Section 5. The paper is finished with conclusions.
2 Role of energy storages in microgrids development 2.1
Integration of renewable energy sources
Much work has been carried out on the integration of renewable energy sources (RESs), such as wind and solar systems, with the supplying network. Daily power generation of such sources is stochastic and does not correspond to a typical demand curve. Usually, the highest generation is during off-peak hours. The application of a storage system enables one to store the surplus energy generated during low-load periods and release it during high-load periods. In this way local generation decreases peak power demand and reduce power flows in the grid [11, 12]. The storage system plays the role of a power and energy buffer and makes dispatching power generated by RES possible. From the market point of view it means that energy is stored at times of low-energy prices (low load) and injected to the grid at times of high prices. The benefit for the source owner can be in saving energy delivered from the grid to cover his demand. As regards big wind farms supplying energy to the grid, it also means economical profits. The technical and economical impact of this application is analysed in [11, 13]. Another purpose of the application is mitigation of power output fluctuations, thus improving PQ in the network [11, 14–16]. This function is important when there is a high level of RES penetration. Storages can lead to the increasing use of renewables, which in consequence contributes to increase in grid efficiency. 464 & The Institution of Engineering and Technology 2014
2.2
Load levelling
In electrical power grid, the consumer’s demand changes according to the daily load curve. As power generation must match power consumption, this requires the grid to have proper capacity to handle demand fluctuation, regardless of how it runs. Traditionally, electricity networks are dimensioned on peak power. Investigations showed that the energy transported through lines as a percentage of the total energy capacity needed for the peak demand is no more than 68% [17]. Using energy storage at the consumer’s site can provide load levelling, reducing the stress on the network for peak loads. Thus, the power transmitted from the network is kept at lower level, which allows one to dimension the network for a lower value of the demand power. 2.3
Peak shaving
When a distinction is made in tariffs between peak and off-peak hours, energy delivered during peak hours is more expensive. In such a situation end-users may be interested in reducing power demand from the supplying grid in peak hours, which is possible using energy storage. The storage is charged during off-peak hours when energy is cheaper and it is discharged during peak-load periods supplying customer loads. For industrial users, who are charged not only according to energy consumption but also according to the highest power demand, the reduction of peak power demand means decreasing demand charges. This application is profitable, in particular, when the ratio of peak to low demand is high and there is a large difference between peak and off-peak energy charge. 2.4
PQ improvement
Many existing LV networks, especially the majority of rural networks, are not suited to connection of considerable number of DESs. An obsolete construction and relatively small cross-sections of lines cause that the voltage rise in network nodes after the connection of a source may exceed PQ limits. In some cases, especially during off-peak hours, the line load carrying capacity may also be exceeded. Some DESs impact PQ in a similar way as disturbing loads. Single-phase sources, typical in LV applications, contribute to increasing the network unbalance. Disturbances such as harmonics may be introduced by devices connected through power electronic converters. Renewable energy sources, such as photovoltaic (PV) or wind ones, may also cause voltage fluctuations as the result of primary energy variability. Thus, one can expect that if DESs are installed in networks with disturbing loads problems with PQ will heighten. Power electronic compensators, such as active power filters and distribution static synchronous compensators have been common measures for mitigating PQ disturbances in distribution networks. Similar functionalities can be obtained for energy storages that are connected to the grid through coupling inverters. Specific tasks performed by the storage depend on inverter control mode. In current control mode the inverter must inject currents to the network, such that undesirable current components introduced to the network, for example, harmonic, reactive and negative sequence are cancelled and the network current becomes a fundamental harmonic, positive sequence or in phase with voltage. This kind of application may be the interest of IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
www.ietdl.org Table 1 Possible applications of energy storage in LV microgrids Area of applications utility network
end-users: customers, prosumers
Storage task
Purpose of application
load levelling
† reducing peak loads † better utilisation of network transmission capacity † reducing power and energy losses † connecting new customers with no necessity of investment in network reinforcement
voltage regulation and disturbance compensation storing energy in low-load periods and supplying customer loads in peak-load periods supplying loads during dips and short interruptions RESs integration
PQ improvement
disturbance compensation
reducing energy consumption in peak hours; reducing energy costs lowering costs of losses because of dips and interruptions † storing the surplus energy generated during low-load periods and releasing it during high-load periods † compensation of voltage fluctuations
PQ improvement
end-users mainly. In voltage control mode, the control of power generating by the storage system and voltage at its terminals is achieved by regulating the phase and amplitude of inverter output voltage with respect to the supplying voltage at the point of connection. This kind of application is more typical for utility network. Different possible applications of storage devices in LV microgrids are summarised in Table 1.
3 3.1
Study microgrid
Fig. 1 Diagram of the microgrid under study
single-phase thyristor bridges, with different loads on dc side. Energy storage is composed of single cells, EC type, designed for cyclic operation. Storage inverter has been constructed specially for laboratory purposes, with the aim to test different control algorithms. The circuit of inverter DC side has been designed in such a way that to limit the flow of fast varying current components through the battery circuit. The resistance R and reactance X in Fig. 1 represent the supplying feeder. The short-circuit power at the primary side of the transformer is 5 MVA. 3.2 Energy storage control for energy management and PQ improvement As described in Section 2, the typical applications of storages in electrical power networks are aimed at energy
Microgrid configuration
The idea proposed by the authors to join different tasks of the storage in one system was examined by means of simulation and laboratory tests. The study microgrid is shown in Fig. 1. The microgrid operates in connection with the LV network and consists of the following equipment: † distribution transformer of 70 kVA, † stationary PV panels of total rated power 6.5 kWp, † three-phase controllable load of 30 kW maximum power, † lead-acid battery energy storage of 8 kW with three-phase four-wire pulse-width modulated (PWM) inverter. PV panels are connected by means of three single-phase inverters. The load is composed of linear and unbalanced three-phase RL element of variable power and three IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
Fig. 2 Block diagram of the storage inverter control 465
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www.ietdl.org management. Consequently, charging and discharging pattern for the storage is defined for active power only. In this section, a control algorithm is presented in which additional functionalities are added to the storage, which allow it to be applied not only for energy management but for the simultaneous compensation of PQ disturbances as well. The algorithm requires two independent control loops, one dedicated to active power control and the other one dedicated to disturbance compensation. The storage inverter operates in current control mode. The reference currents are the sum of the following components i = i P + i com
(1)
T is the phase currents, where i = ia ib ic T the phase current active i P = iaP ibP icP is T components, i com = ia com ib com ic com is the phase current additional compensating components. The active components of phase current are calculated in open loop on the basis of required active power value and measured voltage signal. The icom component vector depends on disturbances being compensated. In general, it can include
harmonic and unbalance mitigation, respectively T i Q = ia Q ib Q ic Q , i h = ia h T i un = ia un ib un ic un
ib h
ic h
T
The active currents are determined dependently on the value of active power transmitted by the storage and defined by energy management algorithm; it is the power the storage system is charged or discharged with. For the calculation of compensating components, the theory of instantaneous power was used [18, 19]. First, current and voltage signals are transformed from phase (abc) coordinates to αβ0 coordinates. For the voltage the transformation is described by the following equations ⎡
(2)
⎤ 1 1 1 √ √ √ ⎡ ⎤ ⎢ 2 ⎡ ⎤ 2 2 ⎥ ⎢ ⎥ ua u0 ⎢ ⎥ 2 ⎣ ua ⎦ = ⎢ 1 − 1 − 1 ⎥⎣ ub ⎦ ⎢ ⎥ 3 ⎢ ub √2 √2 ⎥ uc ⎣ 3 3⎦ − 0 2 2
where iQ is the component responsible for reactive power compensation, ih and iun are the ones responsible for
Equations for current transformation are analogous.
i com = i Q + i h + i un
,
(3)
Fig. 3 Simulation results of active and reactive power consumption a Active and b Reactive power balance in study microgrid Indices in symbols stand for: g – grid, t – prosumer’s total power, s – storage, pv – PV source, l – load 466 & The Institution of Engineering and Technology 2014
IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
www.ietdl.org In three-phase systems real instantaneous power is defined as follows p = ua ia + ub ib + u0 i0
Taking into account (6) and (7), iα and iβ current components are derived
(4)
whereas imaginary instantaneous power q = u a ib − u b ia
ub ua
1 ua = 2 ua + u2b ub
−ub ua
pav + posc qav + qosc
(8)
(5)
For a three-wire system, (4) and (5) can be written as ua p = −ub q
ia ib
ia ib
(6)
When harmonic and unbalance components occur in phase currents varying components appear in both real and imaginary instantaneous power course p = pav + posc q = qav + qosc
Equation (8) is the base for determination of the reference current compensating components. The compensation idea consists in identification and calculation of the oscillatory components of instantaneous powers responsible for PQ disturbances and – optionally – the reactive power to be compensated. The value of reactive power may be equal to load power or may have the value dependent on required power factor at the point of connection to the power grid. Assuming the full compensation of reactive power, the reference current components iα ref and iβref are calculated from the formula
(7)
where pav, qav are the average values of real and imaginary power, and posc, qosc are the varying components of the power.
ia ref ib ref
1 = 2 uag + u2bg
u ag
−ubg
u bg
u ag
posc com
qav com + qosc com (9)
Fig. 4 Current waveforms in selected time period – results of simulation a Resultant current of load and PV source b Storage current c Current in the supplying feeder IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
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Fig. 5 Voltage and current waveforms in phase a iat – resultant current of load and PV source, ig – current in the supplying feeder
Fig. 6 Operation of storage under discharging mode and simultaneous compensation of reactive power, unbalance and harmonics a Voltage in a (L1) phase (the rms value is equal to 230 V) and three phase currents (in A) of compensated devices (top) and the supplying feeder (bottom) b Harmonic spectrum of currents of compensated devices (top) and the supplying feeder (bottom) 468 & The Institution of Engineering and Technology 2014
IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
www.ietdl.org where index ‘g’ refers to the grid and index ‘com’ refers to the compensated devices. Finally, the control algorithm is derived using iα ref, iβ ref with additional zero sequence current i0, after transformation from αβ0 coordinates to abc coordinates ⎡
⎤ 1 √ 1 0 ⎢ 2 ⎥⎡ ⎡ ⎤ ⎤ ⎢ i0 √ ⎥ ia ref ⎢ ⎥ 3 ⎥⎢ 1 2⎢ 1 ⎢ ⎥ ⎥ ⎣ ib ref ⎦ = ⎦ ⎢ √ − ⎥⎣ i 2 ⎥ a ref 2 3⎢ 2 √ ⎥ ib ref ⎢ ic ref ⎣ 1 3⎦ 1 √ − − 2 2 2 ⎡ ⎤ 1 √ 1 0 ⎢ 2 ⎥⎡ ia com + ib com + ic com ⎤ ⎢ √ ⎥ √ ⎢ ⎢ ⎥ 3 ⎥ 1 2⎢ 1 3 ⎥ ⎥ = ⎢ √ − ⎥⎢ ⎣ ⎦ i a ref 2 ⎥ 2 3⎢ 2 √ ⎥ ⎢ ⎣ 1 ib ref 3⎦ 1 √ − − 2 2 2 (10) Block diagram of the inverter control is presented in Fig. 2. In the ‘p, q calculation’ block, on the basis of the current and voltage signals transformed to αβ0 coordinates, real and imaginary instantaneous powers are determined. The oscillatory component of the real power is extracted using low-reject type filter described by second-order transfer function with characteristic frequency equal to 5 Hz. The phase currents active component and additional compensating component are used to form the reference current for the inverter. Hysteresis control was applied, in which switching input signal to the inverter was generated when the measured inverter currents exceeded the bandwidth of reference currents.
4
Modelling and simulation
4.1
Model description
The considered microgrid was modelled in PSCAD/EMTDC environment. In developed model, the supplying network is represented by an equivalent three-phase voltage source. PV
sources are modelled as current sources in which current values are determined dependently on stochastically varying power generated by the source. Load model consists of three single phase thyristor bridges with resistances connected to dc side, values of which are adapted independently to obtain the preset values of active and reactive powers consumed by the load. Energy storage is modelled by means of constant dc voltage source and three-phase six-pulse PWM inverter connecting it to the network. The inverter consists of fully controllable switches which are turned on and off through a gate drive circuit. Operating in current control mode, the inverter generates currents corresponding to the reference currents which are calculated using the appropriate PSCAD modules. Active power generated or consumed by the storage is controllable; in the simulation, it was set according to the assumed profile. Reactive power was set to zero. Each device is equipped with a measuring instrument that enables the measurement of phase currents, voltages, active and reactive powers. It was assumed as a rule that power delivered to the node is negative, and received from the node is positive. 4.2
Simulation results
Simulation was aimed to show the storage performance during active power consumption/generation and simultaneous compensation of current harmonics, unbalance and reactive power of fundamental frequency. It was assumed that PV source generated active power Ppv = 3.6 kW reducing prosumer’s active power demand from the supplying grid. Reactive power of the source Qpv = 0, so the prosumer’s reactive power consumption resulted from load demand. Simulations were performed for different load conditions. In the 2 s and 5 s of the simulation, load active power consumption Pl was reduced from 5.8 to 4.1 kW and to 2.9 kW, respectively (Fig. 3a). At the same time the reactive power Ql decreased from 4.3 to 2.7 kVar and to 1.7 kVar, respectively (Fig. 3b). The change in reactive power caused the appropriate reaction of the storage and reduction of generated reactive power Qs in the same extent. In the 3 s of simulation, the storage active power Ps was changed from 3.9 to − 4.0 kW, and in 6 s from − 4.0 to − 2.1 kW.
Table 2 Experimental results obtained in the test microgrid for the storage performing the tasks of energy management and PQ improvement Storage in discharging phase ( −2.85 kW)
Measured quantity
Pa Pb Pc ∑P Qa Qb Qc ∑Q Ihh,a Ihh,b Ihh,c THDUa THDUb THDUc k2i k2U
kW kW kW kW kvar kvar kvar kvar A A A % % % % %
Storage in charging phase (3.05 kW)
Prosumer
Supplying feeder
Prosumer
Supplying feeder
0.73 −0.52 −0.52 −0.31 0.64 0.70 0.15 1.49 2.59 2.44 2.48 4.19 4.07 3.99 112.46 1.49
−1.02 −1.08 −1.10 −3.20 −0.07 0.03 −0.02 −0.07 1.76 1.51 1.60 4.01 3.79 3.86 3.61 1.42
0.69 −0.59 −0.59 −0.49 0.60 0.68 0.13 1.41 2.48 2.42 2.41 3.66 3.64 3.78 119.83 1.60
0.83 0.87 0.85 2.55 0.02 0.03 0.04 0.10 1.52 1.30 1.30 3.47 3.41 3.40 3.60 1.56
IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
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www.ietdl.org Both active and reactive power balance is shown in Fig. 3. It is clearly visible that reactive power compensation is accomplished in both phase of the storage operation, that is, during energy consumption as well as generation. The effect of harmonic and unbalance compensation is visible in grid currents, shown in Figs. 4 and 5.
5
Laboratory tests
The study microgrid from Fig. 1 was configured in the Laboratory of Distributed Generation at the Lodz University of Technology [10]. Tests were performed to validate the
inverter control algorithm and to show the effectiveness of disturbance compensation under the bidirectional power flow operation of energy storage. The sources of disturbances being compensated were three-phase rectifier, three-phase asymmetric inductance and three single-phase PV panels, including one with a defective solar tracker. The values of active and reactive power, currents and voltages as well as PQ indices were measured using the laboratory monitoring system developed in Dasylab environment. Figures below show some of the obtained results. The first presented case (Fig. 6) concerns the storage in discharging phase, generating the active power to the grid.
Fig. 7 Operation of storage under charging mode and simultaneous compensation of reactive power, unbalance and harmonics a Voltage in a (L1) phase (the rms value is equal to 230 V) and three phase currents (in A) of compensated devices (top) and the supplying feeder (bottom) b Harmonic spectrum of currents of compensated devices (top) and the supplying feeder (bottom) 470 & The Institution of Engineering and Technology 2014
IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
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Fig. 8 Active power balance in the test microgrid during experiment
Load consumed the power of Pl = 4.58 kW and Ql = 1.68 kVar, PV panels generated the active power of Ppv = − 4.89 kW and the storage active power was set on Ps = − 2.85 kW. The option of reactive power compensation was active, so the storage inverter generated the reactive power of Qs = − 1.49 kVar. As the result, the total power of Pg = − 3.20 kW and Qg = − 0.07 kVar was introduced to the supplying grid. The values of measured powers as well as PQ indices are put together in Table 2. It can be noted that as an effect of compensation the power factor at the point of prosumer’s terminals changed from cosj = 0.2 (without storage) to practically cosj = 1.0 (with storage in operation for Ps = − 2.85 kW). The power factor improvement is also significant for storage active power set on 0, in such a case cosj = 0.98. It is visible in Fig. 6 that current harmonics of lower rows were mitigated; however, because of inverter switching some other harmonics of switching frequency appeared. It should be noted that the applied inverter switching frequency was not very high, thus the hysteresis bandwidth and resulting accuracy of reference current tracking was not as good as it could be in commercially manufactured inverters. Current total harmonic distortion (THD) Ihh, measured in amperes, decreased of about 1 A, what resulted in reducing voltage THD factor. The balancing effect of phase current is significant, and the unbalance factor k2i was reduced from 112.46 to 3.61%. The unbalance factor is defined as the ratio of negative sequence current fundamental harmonic to positive sequence current fundamental harmonic. The next case (Fig. 7) refers to the charging phase of the storage. The resultant power of load and PV source was Pt = − 0.49 kW and Qt = 1.41 kVar, which gave the value of power factor cosj = 0.33. With the storage in operation, the power factor was increased to cosj = 0.999 (for the storage power Ps = 3.05 kW). The option of reactive power compensation for the storage power Ps = 0 results in cosj = 0.98. Similar to the previous case the reduction of lower current harmonics is visible. As the result of unbalance compensation current unbalance factor k2i was reduced from 119.83 to 3.60% (Table 2). Next test was carried out to present the power balance and the stable operation of the storage in the longer period of time. The results are shown in Fig. 8. The experiments proved that the storage inverter can perform the compensation tasks in both charging and discharging mode, IET Gener. Transm. Distrib., 2014, Vol. 8, Iss. 3, pp. 463–472 doi: 10.1049/iet-gtd.2012.0687
independently on active power value. The contribution for PQ improvement is clear. It should be noted that for consumers utilising RESs to cover local load demand the power factor varies in a wide range, dependently on the active power production, and may have very low values. In such a situation the presented storage application seems to be particularly good solution, that makes it possible to control active power exchange with the supplying grid and gives Var support to keep constant power factor.
6
Conclusions
Energy storage equipment plays an important role in facilitating the process of DESs integration to electrical power networks. With an appropriate control it can provide solutions for various technical problems that accompanies DESs integration, thus enhance the network hosting capacity. The idea presented in the paper of using energy storage, connected to the grid through coupling inverter, for simultaneous energy management and PQ improvement gives additional benefits for utilities and end-users. Maintaining the high level of supply quality is one of the tasks for the network operator. High quality of power is required by customers and contributes to the improvement of energy efficiency. Conventionally, additional ‘custom power’ equipment has been applied for the mitigation of electromagnetic disturbances and PQ improvement. As shown in the paper, energy storage equipment can accomplish tasks similar to those performed by dedicated power electronic compensators. The capability of storage equipment to provide energy management and ancillary services at the same time has been proved by the results of simulation and laboratory tests. This kind of storage applications seems to be a promising option for future microgrids. An important issue in practical applications is how the operation of storage affects its life. The authors assume that the battery is rated on its primary duty of energy management, depending on a specific application (Table 1). Thus, charging and discharging pattern is imposed by the executed management strategy. It would be appropriate if additional functionalities of storage inverter did not additionally reduce the battery life. To this end, the storage inverter should be adequately designed, for example, equipped with additional capacitor on DC side, in order to 471
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References
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