control discharge, reflecting a very good coverage of energy demand. Keywords- microgrid, critical consumer, backup source, supercapacitor.
Hybrid Backup Power Source Behaviour in a Microgrid Elena Dănilă, Daniel Sticea, Gheorghe Livint, Dorin Dumitru Lucache Department of Energy Utilisation, Electrical Drives and Industrial Automation Faculty of Electrical Engineering, Technical University Gheorghe Asachi of Iasi Iasi, Romania Abstract— By their nature, microgrids are dependent on the continuity of power supply and require backup sources with the highest degree of efficiency and availability. In addition, most users connected to a low voltage network have individual loads or groups of receivers that need quality energy supply (higher than those offered by the public grid) and they also require backup power source. The availability, redundancy, resiliency and effectiveness of a hybrid backup source (with batteries and supercapacitors) were analyzed in this paper by physical modeling a critical consumer. Experimental stand, scaled 1:9.5, is centered on supercapacitors’ storage system, which supplies critical consumer in parallel with two batteries. Load profile obtained both with supplying through the converter and without control discharge, reflecting a very good coverage of energy demand. Keywords- microgrid, critical consumer, backup source, supercapacitor.
I.
INTRODUCTION
A microgrid is seen as an integrated energy system consisting of distributed energy resources and multiple electrical loads operating as single, autonomous grid either in parallel or “islanded” [1] from the existing power grid. In the vision of Siemens [2], this discrete system ads to the main one the following values: efficiency (lower energy intensity and distribution system loss), reliability (near 100% uptime for critical loads), security (enable cyber and physical security), quality (stable power to meet exacting consumer energy requirements), sustainability (expand generation to renewables and cleaner fuel sources). A modern microgrid includes renewable distributed generation, energy storage units, and load management subsystems [3]. In grid connected mode, distributed generators and storage units within the microgrid synchronize the frequency and magnitude of the voltage [4] at the own terminals to the grid voltage and optimize the energy supply. In the islanded mode, the storage units retrieve the role of supplying the loads according to the energy management sub-system [5]. Since in case of an upstream fault, the IEEE 1547 standard [6] compels network operators to insulate consumers from the distribution system or through special devices automatically connect them to the nearest distributed generation center, it is necessary for the dispatcher to control all local loads. However, the performance expectations of microgrids significantly exceed the capacities of generation systems, and mostly the
physical characteristics (such as the number and size of conventional resources). For increased reliability (of embedded microgrids) and to ensure continuity of electricity to consumers is better to create the possibility of rapid transition from a microgrid to another (from one AC to DC one and vice versa). Such a possibility requires additionally a consumer’s universality (the ability to work both AC and DC). Globally, the capacity of microgrids grew strongly from 2011, reaching 1.1 GW [7]. Experts estimate an annual growth rate for installed power of 13.8% by 2014 and 17% in the period 2014 to 2022, projected to reach a value of over 15GW in 2022 [8]. In economic terms, the potential of this market is currently over $5 billion and is projected to increase until 2022: for an annual rate of 17%, the capital market will be $ 27 billion [9]. Besides microgrids, charging infrastructure and energy storage systems (ESS) gain a very good market share. Starting from small bases, the ESS fills all the gaps in renewable markets, reaching, from roughly $100 million in global revenue in 2011, over $700 million in 2013 [10]. II.
CRITICAL CONSUMER’S LOAD PROFILE ANALYSIS
Basically, all consumers connected to a low voltage network have individual loads or groups of receivers that require quality and reliability of energy supply, higher than those offered by the public distribution grid. Often these consumers’ strict requirements can be easily met using an auxiliary power source (backup source). To understand the behavior of the hybrid backup power source, first was considered as place of consumption “Computer Programming” laboratory, appreciated as critical consumer as it should be continuously supplied during the classes. The receivers within this microgrid (place of consumption) are: 35 workstations (LCD monitor, central processing unit, keyboard and mouse) Pentium IV model, 12 luminaires with linear fluorescent lamps and one air conditioning unit. The PC’s were used by students in the usual pace of work, the reference period being two weeks – 10 effective operating days, 4 hours per day. On each computer was installed an energy tracking software, that records the workstation’s power consumption, as long as it is turned on during laboratory classes. The load profile from figure 1 renders the power consumption and the load profile, on elements, revealed on time-pattern, for a 2 hours application class (100 minutes of continuous usage).
In case of power line disturbances, as outages, voltage fluctuations, surges, under-voltage, over-voltage, voltage transients, voltage harmonics or voltage bursts [11], a backup
source must cover the need of load of each receiver within the place of consumption.
Figure 1. a) Power consumption during a class, give on each type of student activity: launching application, solving tasks, saving data work, compilation, resuming work, oral discussion (idle mode) and exiting the applications; b) consumers’ load profile.
III.
EXPERIMENTAL MICROGRID AND TEST RESULTS
Hybrid backup power source used in the experimental analysis consists of two VRLA batteries (12V/7Ah) and a module of 5 series supercapacitors (2.5 V/400F) PowerStor XV Series. The storage unit must provide energy to the critical consumer according to load profile in Figure 1. Since the phase voltage in the place of consumption is 230V and the source voltage is 24V, this process is 1:9.5 scale physical modeled. Parallel connection of hybrid source (batteries and supercapacitors) with the load provides high energy density and series connection provides the voltage across the load. As the aim is to supply energy to the consumer so as to cover the load profile, the hybrid storage system configuration is parallel (figure 2). The consumer is modeled by a resistance that will "require" to the source the total current from load profile (figure 1.b), keeping time variation. Coupling resistance in circuit is made through a static switch – IGBT - with reverse command from a microcontroller. DC / DC converter is bidirectional buck-boost type, with role of controlling charging/ discharging currents and of balancing
supercapacitors’ voltage with bus voltage (24V). For protecting Rload resistance, was added a discharging diode (caster) in antiparallel. Current acquisition is done by three current transducers LEM type (Hall effect sensor), which send signals to the microcontroller development board with preprogrammed dsPIC30F6010A. The same microcontroller transmits a PWM command signal to the static switch with integrated driver that connects / disconnects the load in circuit. After the supercapacitors and the batteries have been completely charged, respectively at 24.4V and 13.5V, the resistance Rload is connected in circuit and is supplied following the operating scenario: -
from the beginning there is a power line disturbance and the load is supplied 5 minutes from the backup source;
-
from minute 5 to 10, the main electrical grid is remediated and the load is supplied from the public network;
-
from minute 10 to 32 there appears again a failure and the backup source returns into service;
-
up to 52 minute mark, the load is supplied again from the public network, then - until the end of the considered time range – there appears another failure
in the public distribution network and the storage unit takes over the energy providing role (figure 3).
Figure 2. Electrical scheme of experimental stand, modelling the place of consumption.
Figure 3. Profile of total load current consumption in case of three interruptions in the the main electrical grid.
Analyzing the profile in Figure 3 is found that the two energy sources (batteries and supercapacitors module), with the total provided energy, compensate very well the load demand when Ipublic network = 0, helping to improve security in the supply of electricity to receivers at “Computer Programming” laboratory. The sampling was set in the data acquisition program to 500ms, so the time base of the profile of the above figure is multiplied by 0.106 to achieve correspondence with the time base of the laboratory’s consumption profile from Figure 1.b. The same sampling time rate is applied to all following load profiles. In case when the energy supply disturbance appears and the storage units are not completely charged, can be observed on the profile from Figure 4 a short dynamic regime at batteries (orange characteristic), occurred when Ipublic network = 0. This derives from the fact that the voltages on the supercapacitors module and on the VRLAs did not have time to balance.
The observed response of storage devices from Figure 3 and Figure 4 shows the batteries’ inertia in transient processes. When the current demanded by load (red characteristic) decreases in value, the supercapacitors are charging from the current that batteries - due to inertia - still provide. As the voltage measured on supercapacitors module (Uinit) is 13.5V, results that it can provide a maximum energy of: Emax =
1 1 C 1 2 2 Cechiv ⋅ U init = ⋅ ⋅ U init = ⋅ 80 ⋅ 182.25 = 7290Wh = 121.5W min 2 2 5 2
having a depth of discharge of: DOD = [1 −
U min 0,856 ] ⋅ 100 = [1 − ] ⋅ 100 = 93,7% U max 13.5
The battery system can provide a maximum energy of: E bat = (∑ U bat ) ⋅ I bat ⋅ 1h = 24 ⋅ 7 ⋅ 1 = 168Wh = 2,84W min
The behavior of the proposed hybrid storage system is identical, regardless of the load curve’s shape (namely the energy demand) of the consumer it serves. In Figure 5 is
Figure 4.
presented, for validating the previous observation, a power supply chart for a random consumer according to its specific load curve (arbitrarily generated from the control program).
Profile of total load current consumption and of provided current in case of one main grid interruption, with partially charged backup sources
Figure 5. Current supply profile for a random consumer, in case of three main grid interruptions, with fully charged backup sources
The DC-DC buck-boost converter ensures a proper and efficient charging control and monitors the supercapacitor module in the whole battery voltage range. Its charging current is limited to 15A to avoid overloading. The output current control ensures a good current regulation over the forward voltage spread characteristics in any case of operation. If the converter is removed form the scheme of experimental stand and Ipublic network = 0, the supercapacitors will discharge directly on the resistance Rload and the supplying characteristics will look as in Figure 6. It results from the chart profile the supercapacitors module provides all the energy in the first 4.2 minutes of hybrid power source’s operation, after that the battery takes over the load supply and supercapacitors discharge. When the load value decreases (84 minutes) and the battery must provide less current to supply the consumer, supercapacitors are recharging from the battery. When the load demanded by the consumer increases (at 91 minute), supercapacitors begin to provide
current they also. When the load becomes 0 (100 minutes after the reference) batteries’ and module’s voltages tend to equilibrate. Supercapacitors are in the third discharging cycle much slower, which takes about 11 minutes. It highlights from the current waveform (the blue one) the two defining characteristics of supercapacitor: very high power density and a low time constant. The designed hybrid storage system comprises units that serve two different types of needs: a primary source with high energy (the batteries) and an auxiliary one with lower energy capacity that solves transient demands for higher power levels (the supercapacitors), handled by the primary source. This means that when there is no high power demand, the high energy source recharges the high power source, as the voltage rises. To accomplish the energy demand from the consumer they serve, these two units have to operate in parallel. The
hybrid backup power source proves to have a high availability, regardless of when the fault occurs in main energy supply. To add redundancy to the critical consumer’s grid, the hybrid backup power source can be multiplied, to obtain a distributed-redundant power system which provides a
significant improvement in maintainability, fault tolerance and which is simpler and less expensive than parallel-redundant or isolated ones. The goal of distributed redundancy is to bring power system redundancy to every piece of load equipment, as close as possible to the input terminals [12].
Figure 6. Profile of total load (red line) current supply by batteries (green line) and supercapacitors (blue line), without the DC-DC buck-boost converter
IV.
REFERENCES
CONCLUSIONS
The reliability of a microgrid is given by its each element’s reliability, and poor operation or malfunction of the smallest element can compromise the entire microgrid. Therefore, ensuring resilience and availability is vital in operating, especially when it comes to supply critical consumers. In parallel, due to continuous changes in energy production filed and in energy utilization landscape, the energy storage systems are rapidly gaining ground. Physical modeling of a certain consumer’s electrical microgrid through an experimental stand on 1:9.5 aimed to describe the behavior of a hybrid energy backup source in very different operating modes: fully or partially charged, with one or more main electricity grid failure, supplying the load through or without a converter. There is revealed that batteries and supercapacitors are operating complementary as they are able to optimally fulfill the consumer’s diverse needs. The maximum available energy from the supercapacitors module is 42.7 times higher than the maximum batteries’ energy, but its time constant is low. In stationary applications this is an advantage because the supercapacitor is designed to be recharged fast, several times during a hybrid backup supply operation if needed.
[1]
R. H. Lasseter, Smart distribution: Coupled microgrids, Proceedings of the IEEE, 2011, 99(6), pp. 1074-1082. [2] R. L. Dohn, The bussiness case of microgrids – White paper: The new face of energy modernization, Siemems AG 2011. [3] F. Katiraei, M. R. Iravani, Power management strategies for a microgrid with multiple distributed generation units, Power Systems, IEEE Transactions on 21.4, 2006, pp. 1821-1831. [4] I. J.Balaguer, Q. Lei, S. Yang, U. Supatti, F.Z. Peng, Control for gridconnected and intentional islanding operations of distributed power generation, IEEE Transactions on Industrial Electronics, 2011, 58(1), pp. 147-157. [5] M. H. Cintuglu, O. A. Mohammed, Simulation of digitalized power system using PMU and intelligent control, In Industry Applications Society Annual Meeting, 2013 IEEE (pp. 1-8). [6] http://grouper.ieee.org/groups/scc21/1547/1547_index.html [7] Global Status Report, Renewables 2013, REN21. [8] Oil Sands Co-Generation and Connection Survey, Desiderata Energy consulting Report 2014, prepared for OSCA Power and Co-Generatiobn Task Group. [9] U.S. Energy Information Administration, Annual Energy Outlook 2013 Early Release Overview. [10] Advanced Energy Now 2014 Market Report, Global and U.S. Markets by Revenue 2011-2013 and Key Trends in Advanced Energy Growth. [11] Uninterruptible Power Supply (UPS) Glossary. [12] High-Availability Power Systems, Part II: Redundancy Options, Liebert White Paper 2003.
