Available online at www.sciencedirect.com
Procedia Engineering 16 (2011) 192 – 197
International Workshop on Automobile, Power and Energy Engineering
Study on protection Scheme for Micro-grid with Mobile Energy Storage Units Zhao Xiao-xiao,Xia Ming-chao, He Xuan-hu, Zhou Yuan School of Electrical Engineering, Beijing Jiaotong University, ,100044,Beijiing China
Abstract A method of dividing complex micro-grid into minor micro-grids and finally to the minimal micro-grid which consists of one AC bus or one DC bus is presented according to classification principles. The protection scheme of minimal micro-grid is discussed and then applied to the whole micro-grid. By analyzing parallel operation mode and islanding operation mode of micro-grid, the protection scheme of each mode is given respectively.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Automobile, Power and Energy Engineering Keywords:Smart Grid; micro-grid; Electric Vehicle; charging station; V2G; DG
1. Introduction Building charging stations of electric vehicles is an important way to promote energy conservation and low-carbon cost economy, and is a pilot project of construction of Strong Smart Grid. The energy storage unit in an electric vehicle is mainly car batteries, so the electric vehicle can be called mobile energy storage unit. Smart Grid calls for self-healing, interaction, compatibility, exchange of information etc, and compatibility contains the access of renewable energy which needs the support of energy storage. Electric vehicle, acting as the mobile energy storage unit, can interact with grid. With the mature technology and the rapid increase in the number of electric vehicles and charging stations, V2G (Vehicle to Grid) has a bright future. As a major part of V2G, mobile distributed energy storage units, which can realize user participation and power interactivity, become the trend of developing future grid. Research on the charger with high efficiency, high power factor, low harmonic content and two-way power flow, as well as the intelligent charging stations that are efficient, safe, convenient, and compatible are becoming focused gradually.
* Corresponding author. Tel.: +86-15210576893 E-mail address:
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1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.08.1071
Zhao Xiao-xiao et al. / Procedia Engineering 16 (2011) 192 – 197
Combining electric vehicle charging stations and micro-grid technology, energy storage technology, and photovoltaic technology together, micro-grid system with charging stations presents significant social and economic benefits, which can not only fully improve resource utilization, but also improve the characteristics of grid loads. Electric energy from the distributed generators (DGS) should be given priority to use when charging, and only if the energy * Corresponding author. Tel:+86-15210576893 from DGS is insufficient for charging electric vehicles and energy storage batteries can distribution network supply the electric energy. During the low power consumption period, charging station micro-grid can charge the electric vehicles and energy storage batteries to fill up the power consumption valley. During the high power consumption period, charging station micro-grid can offer distribution network some electric energy support for a short time, meaning that distribution generator, electric vehicles and energy storage batteries can transmit electric energy to distribution network to clip the power consumption peak. There are two ways to charge the electric vehicles, one is to change the storage batteries, which usually charged by DC chargers, the other one is to charge the electric vehicles directly. If the vehicles need to be fast-charged, AC chargers can be adopted, otherwise, DC chargers are is preferred. This paper discussed that the topology of complex micro-grid can be divided into several levels, and studied the protection schemes of minimal micro-grid of each level, analyzed the faults on DGS, chargers, buses and lines of minimal micro-gird and the protection schemes of each kind of fault are presented. 2. Topology Structure Modeling of Micro-grid Due to the operation of micro-grid contains parallel operation mode and islanding operation mode, the protection schemes need to fit both modes. The structure of complex micro-grid [1] [2] [3] contains charge station units, distributed generator units are shown in Fig.1. It can be divided into several levels when research on control and protection of complex micro-grid. As Fig.1, complex micro-grid consists of minor micro-grids 1 and 2 which can be regarded as loads or bi-directional sources and internal structure as shown in Fig.2 and Fig.3.
Fig.1 Complex micro-grid topology structure According to this method, complex micro-grid can be divided into several parts until minimal microgrid, which has only one switch connect with senior level network and only one AC or DC bus. Researches on the protection scheme of micro-grid can start from the minimal micro-grid gradually, each
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level can be regarded as a minimal micro-grid and its upper level can be seen as the main grid, while its lower level can be regarded as loads or bi-directional source. Higher level is usually with complex structure. Types of loads and sources, power transfer directions, and sizes of power are usually the main concern factors of entire protection scheme. 3.Minimum Micro-grid topology and Protection Scheme
PCC K1
Control center
QF6
K2
QF9 K3 AC/DC
DC bi-directional charger
AC/DC bi-directional
DC charger
AC charger
Grid converter
DG
Storage batteries
Secondary load
Important load
3.1 AC bus minimal micro-grid Fig.2 AC bus minimal micro-grid topology structure AC bus minimal micro-grid as shown in fig.2 is suitable for the situation of mass AC loads and AC sources in micro-grid. DC devices are connected with AC bus through bidirectional converter under this circumstance. • Line protection In Parallel operation mode, when fault occurs on the line that connecting AC bus minimal micro-grid with senior level, the short-circuit current of fault point comes from senior and minimal micro-grid.
(a)
(b) Fig.3 AC bus minimal micro-grid line topology structure
As shown in fig. 3(a), fault occurs at K1 point on the line between circuit breaker and PCC, the fault current that flow through the circuit breaker QF6 is Ik1. The value of Ik1 related to the short capacity of QF6 toward micro-grid. Fault occurs at K2 point on the line between circuit breaker and transformer, as shown in fig. 3(b), the fault current that flow through the circuit breaker QF6 is Ik2, and its value related to the short capacity of QF6 toward PCC, and usually higher than Ik1. Since micro-grid contains distributed generators, when fault occurs on each side of QF6, the fault current would flow through QF6. Its value differs as the different places the fault occurs which can be judged by the direction of short circuit power. So the main protection for line fault is adaptive protection which can choose the setting values according to power direction. This adaptive mode extends from sub-
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micro-grid to senior level network. In addition, when fault occurs on K2 point, the differential protection of bus can be used to isolate the fault. • Bus protection The main protection of bus in micro-grid adopts complete current differential protection, the protection of senior circuit breaker acts as backup protection. Because the protection of bus is related to every element connected to bus, the protection should be coordinate with relays and automatic devices of each element when design wiring of protection of bus. The protection of AC bus has not only backup protection but also circuit breaker failure protection [4], which is to avoid the rejecting act of circuit breaker that cause largescale fault. • Charger protection During the charging process, when a fault occurs on a single charger, If DC charger have been in overcurrent or over-voltage charging state, the internal protection of charger acts as main protection, and sends the fault information to control center at the same time. After a setting time delay, the control center sends the order to disconnect DC charger line. The main protection of charger lines is instantaneous trip current protection, the protection of QF6 acts as backup protection. AC chargers use similar protection scheme. • DG internal faults are protected by themselves. DG external faults aWhen fault occurs on the line which DG connects, the fault current contains fault current I1of bus and I2 of DG. In general, the value of I1 is higher. The situation is similar to 2.1.1. The main protection for DG line fault is adaptive protection which can choose the action values according to power direction, the protection of QF6 act as backup protection. bWhen fault occurs on the line which DG connects and the line is switch off and DG disconnect with micro-grid. Reclosing of DG to bus needs synchronization check to assure DG is synchronous with other sources of micro-grid. • Protection deploy of islanding operation Islanding operation means that when fault occurs in senior level grid, minimal micro-grid is disconnected with senior level grid, DGS power the whole micro-grid normally on the condition that the micro-grid system is secure. In this way, micro-grid transform into self-supplying mode. Islanding operation contains intentional Islanding operation and non-intentional Islanding operation [6] (a) Intentional islanding operation is active when there is no fault occurs in senior level micro-grid, but something wrong with the power quality. In order to assure the power quality of important load in micro-grid, micro-grid runs on islanding operation by disconnecting with PCC. Before parallel operation transform to islanding operation, it enacts reasonable islanding operation zone, which is able to assure reliable operation. Intentional islanding operation protection scheme: since the circuit breaker is disconnected, the protection can be skipped. The main protection on bus adopts complete current differential protection. The action current value of differential relay is reset according to the principle that the value should be higher than the highest unbalance current of micro-grid external faults and higher than the break transformer secondary circuit current. When faults occur on other lines of micro-grid, since the lack of short capacity which comes from senior level micro-grid, the value should be reset according to the actual short capacity during the islanding operation. The protection scheme of parallel operation is the same as islanding operation, but the protection value needs to be reset based on the short capacity on islanding operation of minimal micro-grid. (b) Non-intentional island operation is active when fault occurs in senior level grid, and the PCC has to cut off to disconnect the minimal and senior level of micro-grid. It may not keep the reliability of voltage and frequency under this situation and finally leads to the collapse of voltage and frequency. The uncertainty and contingency of non-intentional islanding operation may lead to the failure in control of voltage and frequency. On the one hand, if voltage and frequency exceed permit range, devices may be damaged, and protections may mis-operation [6].When source capacity is less than load in micro-grid, unimportant loads can be cut off to transfer non-intentional islanding operation mode into intentional islanding operation and keep the micro-grid secure. 3.2 DC bus minimal micro-grid
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Fig.4 DC bus minimal micro-grid topology structure DC bus minimal micro-grid is shown as fig.4. This structure is suitable for mass DC loads and DC sources in micro-grid (Solar energy for instance). AC devices are connected with DC bus through bidirectional converter under this situation. • Line fault In Parallel operation mode, when fault occurs on the line that connecting DC bus minimal micro-grid with senior level, the short-circuit current of fault point comes from senior and minimal micro-grid.
(a)
(b) Fig.5 DC bus minimal micro-grid line topology structure
As shown in fig. 5(a), fault occurs at K1 point on the line between circuit breaker and PCC, the fault current that flow through the circuit breaker QF6 is Ik1. Fault occurs at K2 point on the line between circuit breaker and transformer, as shown in fig. 5(b), the fault current that flow through the circuit breaker QF6 is Ik2. The situation is similar to 2.1.1. In DC system current direction can be used directly, so the main protection of line fault is adaptive protection which can choose the action values according to current direction. Bidirectional converter fault can be protected by its own internal protection. • Bus fault The main protection of faults of DC bus can also use current differential protection. Over-current protection of circuit breaker on every line connected with bus act as backup protection. Similarly, the protection of DC bus has not only backup protection, but also circuit breaker failure protection. • Charger and the lines faults When faults occur in a single charger, if DC charger occur over-current or over-voltage charging, internal protection of charger acts as main protection, and sends the fault information to control center at the same time. After a setting time delay, the control center sends the order to disconnect DC charger line. When faults on lines between charger and DC bus, instantaneous trip current protection is main protection, current velocity of variation di/dt and current variation ΔI can be used. The protection of QF6 acts as backup protection. Other chargers use similar methods to enact protection scheme. • DG fault DG internal faults are protected by themselves. The protection scheme of DG external connection lines fault is same to the charger lines protection, but the protection values are different. Setting of protection values must take account of the capacity of DG and the distance of line. • The island protection scheme of DC bus in minimal micro-grid
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The protection schemes are same when the minimal micro-grid runs in parallel operation mode and islanding operation mode, but the load current and short-circuit current will change remarkably [12][13][14]. So the protection settings of circuit breakers that installed on the lines which DG and chargers connect with bus, need to be set according to the short-circuit capacity. 4. Conclusion Complex micro-grid can be subdivided into several levels of minor micro-grids until the minimal micro-grids. AC bus or DC bus minimal micro-grid can be regarded as load or bi-directional source when setting the protection scheme of entire micro-grid. The complex micro-grid protection scheme can be like the protection scheme of minimal micro-grid.
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