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Energy Procedia
Energy Procedia 00 (2011)12000–000 Energy Procedia (2011) 1008 – 1014
www.elsevier.com/locate/procedia
ICSGCE 2011: 27–30 September 2011, Chengdu, China
Design and Realization of Microgrid Composing of Photovoltaic and Energy Storage System Junliang Wang*, Xiaomeng Li, Haijing Yang, Shengli Kong Henan Electric Power Research Institute, Zhengzhou, China
Abstract Considering the photovoltaic power has the characteristic of stochastic waving, the microgrid composed of batteries storage energy and photovoltaic cells is adopted. A control system of three layers structure is designed, which are local layer, concentrating layer and center layer. The master-slave control mode is used for microgrid operation. The scheme has been used in the actual project; the whole system is operated well in terms of stability, reliability and economy. It provides an example of the photovoltaic cells used for electric power generation project. © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.
Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC) Keywords: microgrid; photovoltaic; battery storage energy; delamination control
1. Introduction Recently as the general energy being scarcer, developing clean energy becomes the research focus [1][2]. The solar energy has clean and sustainable use characteristics, so it is prevalent that using photovoltaic electricity. But its emitting power is stochastic, large power converting rates cause poor electric quality, and distributed electric energy brings change of conventional power grid structure, these questions challenge the operation control and management of electric grid. Paper [3] designed an overall power management strategy for the proposed system to manage power flows among the different energy sources and the storage unit in the system. Paper [4] designed a hybrid solar system with a multilayered building envelope. Recently the microgrid with photovoltaic power, storage energy and electric loads form is a valid mode connecting power system. In the microgrid energy storage system connects photovoltaic power by AC bus bar, the energy storage system can compensate photovoltaic power fluctuation, especially when the microgrid deviates from online. It also can reduce the frequency waving. The tracking control mode is used in energy storage system. When microgrid switches from online to
* Corresponding author. Tel.: +86-371-67905496. E-mail address:
[email protected].
1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC). Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2011.10.132
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offline state, it can discharge rapidly and enlarge power supply rate, so the power waving can be reduced. In this paper according to minimum disturbance principle, a new microgrid operating control strategy is designed and applied in the project. 2. Characteristic of Photovoltaic and Energy Storage System 2.1. The working principle of photovoltaic and its characteristic Photovoltaic cell has photovoltaic effect, after the crystal silicon cell absorbs the solar light, pairs of positive charges and negative electrons will come into being in the PN section, when the circuit is closed, there will be current in it, at the same time the solar energy change into electric energy directly. A photovoltaic generating electricity system is composed of photovoltaic cell and controller and inverter, its equivalent circuit is shown in Fig. 1 [5][6][7]. IL: photovoltaic current; Id: diode current; Rsh: PN section equivalent resistance; Rs: photovoltaic cell inner resistance. Known from Kirchhoff’s Current Law: I = I L − I d − I sh ⎡ ⎛ Q(V + IRs ) ⎞ ⎤ V + I ⋅ Rs (1) = I L − I d ⎢ exp ⎜ ⎟ − 1⎥ − R AkT ⎝ ⎠ ⎣ ⎦ sh where V and I are output voltage and current of photovoltaic cell respectively. Ish is leaking current of photovoltaic cell. Id is saturation current of the diode. Q is Coulomb constant(1.6×10-19C).A is PN section constant. k is Boltzmann constant (1.38×10-23J/K). T is absolute temperature of photovoltaic cell. IL is short circuit current of photovoltaic cell, it depends on light intension and temperature. Id just depends on temperature.
Fig. 1. Photovoltaic cell equivalent circuit.
Fig. 2. Photovoltaic P-V characteristic.
The solar energy photovoltaic cell is nonlinear electric resource, its output characteristic is impacted by light intension and environment temperature and magnitude of loads. In certain conditions of light intension and environment temperature, the photovoltaic cell works as different output voltage, but when the voltage reaches some numerical value, its output power attains maximum, this point is the tiptop of power-voltage curve, namely maximum power point, shown in Fig. 2. If we improve the whole efficiency of photovoltaic system, the working point must be adjusted in real time, and operating at the point nearly, the process is named maximum power point tracking (MPPT) [8]. 2.2. The storage battery characteristic and structure of energy storage system The storage battery stores energy as chemical energy form, it may work in charge or discharge state, and expediently adjust the microgrid operating characteristic [9][10]. Seen from electrical characteristic, the storage battery is a voltage resource with inner exiguity resistance, shown in Fig. 3. [11].
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Fig. 3. The storage battery equivalent circuit.
Fig. 4. The storage battery storing energy system.
As load resistance RL is equal to battery inner resistance RE, the output power of battery is maximum,
( L). its value is The storage battery storing energy system is composed of battery pile (BP), power control system (PCS), battery management system (BMS) and main control system(MCS), shown in Fig. 4. BP is connected to 380 voltage bus bar through PCS after direct current inverting to alternating current. PCS controls charge or discharge state of BP, BMS manages and harmonizes the charge or discharge process of BP. MCS collects information from PCS and BMS respectively, and harmonizes PCS and BP, it achieves the control function of energy storage system charging or discharging, and sends information to controlling center synchronously. P = E 2 4R
3. Operating Control of Microgrid System The photovoltaic system’s dynamic and stable characteristics are different from traditional generators, its maximum power can’t be controlled, moreover the single phase load will cause an unbalance of threephase power, and therefore operating control of microgrid system is different from traditional electric power system [12][13]. In this paper the control structure of three layers is designed, they are local layer and concentrating layer and center layer. The master slave control mode is used for microgrid operating. Local layer collects digital and stimulant information from various units including photovoltaic system, storage energy system, loads, breakers, and completes collection, conversion, disposing and transmitting. The concentrating layer achieves microgrid energy management and its online and offline controlling. The center layer accomplishes microgrid economical operating according to income maximizing principle. The center controller is master controlling unit. The upper layer manages distributed power resources and various loads in nether layer, nether layer contacts upper layer by communications, shown in Fig. 5. In working center controller adjusts the electric resource’s operating point and decides loads on or off according to photovoltaic output power and changes of loads demand in microgrid. The dynamic balance of power is completed by electric resource controller in bottom layer. When microgrid is offline the storage energy system in normal working restrains the stochastic waving of photovoltaic power and disturbance of loads. It provides voltage resource for photovoltaic system while microgrid is offline. When the power of photovoltaic system is abundant the storage energy system works in charge state or in discharge state when power is deficient. Seen from the alternating current bus bar, the storage energy system may work in charge or discharge state, it can absorb or utter power, and has a large regulation range, so it can reduce the power striking on AC bus bar, and benefit microgrid operating stability.
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Fig. 5. The microgrid control structure
When microgrid is operating online, the frequency is controlled by electric system, photovoltaic system and storage energy system are operating in active power and reactive power closed loop control mode (P/Q mode), it just sends out active power and reactive power, and doesn’t attend to adjust system frequency [14], P/Q mode is shown in Fig. 6. When microgrid is operating in offline state, the main control unit in control system holds the frequency and voltage of system, it adopts the voltage and frequency closed loop control mode (V/f mode), and maintains the system stable operation, V/f mode is shown in Fig. 7.
Fig. 6. P/Q control model
Fig. 7. V/f control model
When outer electric power system operates in well state, the microgrid is merged into it by synchronizing device, here microgrid operates employing control mode of united optimizing power, it makes microgrid be the highest efficiency. The storage energy system output or input power tracks the changes of loads, in order to meet the power grid dispatching department’s demand of power waving. The controller commands the online breaker to be off while the fault occurs in outer electric power system, the microgrid goes to be offline, here storage energy system establishes voltage on bus bar, then photovoltaic system is started rapidly, and loads go to be on one by one according to optimized sequence. The storage energy equipments decides its charge or discharge state according to warp between photovoltaic output
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power and total loads on bus bar, then light energy is utilized maximally. The microgrid system control flow is shown in Fig. 8.
Fig. 8. The microgrid control flow.
4. The Scheme Applied In Project The scheme designed in this paper has been applied in “The gold sun” project. In the project photovoltaic system and battery storage system are merged into electric power system according to distributing method, they are connected to two buses that voltage is 380V respectively by inverter. Two PCS are equipped in the project, its capacity is 100kW per group. The total power of photovoltaic system is 380kWp, it is divided into two groups and its power is 190kWp per group. The microgrid is merged into electric power system through 380V voltage, the main circuit is shown in Fig. 9. The RS485 is applied to build communication net, it connects photovoltaic inverter and storage battery controller and low voltage measuring and controlling units to the Ethernet, and joints microgrid controller to industry control computer, it completes microgrid communication net, and contacts dispatching department of electric power system through special light fiber. In project the storage energy element is ferric and lithium phosphate battery, the surveillance and control system of storage energy is equipped with the battery management system, it fulfill function include charge and discharge and temperature protections and so on, the system can complete its safe operating and choice of control strategy and run mode.
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Fig. 9. The microgrid main circuit of photovoltaic united to storage energy.
The online or offline controller applies voltage harmonics detecting method to judge whether the microgrid is “isolated island” or not [15]. When the controller opens the online breaker, and at the same time sends the information to PCS and microgrid control system, the loads controller cuts off unimportant loads. After PCS receives the offline message, it works into V/f control mode. The microgrid control system starts online to offline control strategy immediately while it receives offline message [16]. When microgrid is online, the storage energy inverter works in current resource control mode, it gives off power based on the microgrid’s instructions. However when it is offline, the inverter works in voltage resource control mode, it gives off power in V/f mode. As the main control unit, it receives the online or offline signal and completes to switch running mode rapidly. When microgrid is offline, the collecting controller reins the whole microgrid to run in stable state, the photovoltaic system works in MPPT mode, it regulates the storage energy power to meet the loads demand, thus the PV power is used sufficiently and the important loads power supplied continuously is assured. However, when microgrid is online, it realizes the uniform dispatching of PV and storage energy and loads, and makes the distributed power net be more flexible, in urgent things it may be regarded a spare electric resource that can realize the optimized running for distributed grid. 5. Conclusion The microgrid system united by photovoltaic and storage energy utilizes the characteristic that storage battery has charge and discharge functions and its power may be regulated rapidly, this may restrain the power waving because of photovoltaic power being random, the solar energy is used adequately. They two unite to form the microgrid, it may operate in online or offline mode, besides it has a characteristic that can restrain the striking and disturbance of loads. The project practice shows that the whole microgrid system works is stable, reliable and economical; it provides a paradigm for photovoltaic generating electric power application. References [1]
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