ITEC Asia-Pacific 2014 1569951217
The Voltage Coordinated Control for Vehicle to DC Microgrid Xue Zhang, Wei Pei, Yingying Sun, Wei Deng, Hui Qu Institute of Electrical Engineering, Chinese Academy of Sciences, Haidian District, Beijing 100190, China
[email protected] Abstract: With the quickly development of electric vehicle and dc microgrid, the vehicle to dc microgrid technology has obtained widely attentions. The battery energy storage units are replaced by the dynamic energy storage units of electric vehicle, and the dc voltage coordinated control of vehicle to dc microgrid is one of the most important problems. In order to deal with the problems, an effective control method is proposed in the paper. The dc voltage coordinated control methods are analyzed under grid-connected operation, converter current limit and islanding operation conditions. The control method of electric vehicle can be switched to achieve short support for dc bus voltage. Finally, the simulation results verify the feasibility and effectiveness of proposed control method. Keyword: Electric vehicle; coordinated control; dc bus voltage; dc microgrid; grid-connected converter
I.
provide ancillary services for grid, and also provide ancillary services for microgrid. The paper mainly concerned with vehicle to microgrid. The dc microgrid is better choice compare with ac microgrid due to the advantages of less number of energy conversion, high efficiency, low cost, simple control structure without frequency and reactive power concern, and the electric vehicle is more adapted to access into dc microgrid according to its internal structure. In order to ensure the reliability of operation for dc microgrid, a mass of battery energy storage system usually be accessed into the system. It has the properties of high cost and large volume, and lead to high price for establishing the microgrid. The electric vehicles have the feature of mobile energy storage units, and then the static battery energy storage units can be replaced, and further reduce cost and obtain batter economic. The battery energy storage system replaced electric vehicles in the dc microgrid, and the electric vehicle should have the similar function with battery to provide ancillary services for grid, and enhance the stability and reliability of dc microgrid. The dc bus voltage is an important indicator of the power balance, and similar to its role in the frequency of the AC network. Therefore, the control of dc bus voltage is to ensure safe and reliable operation of the dc microgrid important condition. The paper mainly concern the dc voltage coordinated control for electric vehicle and dc microgrid. In order to solve the problem, a plug-in hybrid electric vehicle (PHEV) energy storage systems and control strategies based on power electrics are proposed, the structure and control strategy of super capacitors, fuel cells and batteries in series and parallel are analyzed, the performance of multiple bidirectional DC/DC converter is compared [12]. An energy management strategy based on hierarchical fuzzy control is proposed on the basis of the structure of photovoltaic generation and PHEV system, and the fuzzy control divided three layers depend on power supply situation, and the fuzzy rule with two variables are selected, and improve accurate and effective operation of the system [13]. The control method proposed for smart building with PHEV, and the four layers multi-agent system structure is applied, the indoor temperature, indoor light and air quality is effective controlled by particle swarm optimization algorithm, the support and optimization of system are analyzed emphatically under islanding operation conditions
INTRODUCTION
With the shortage of fossil fuels and serious environment problems, the development of renewable energy resource and electric vehicle technology has become important initiative for energy-saving and emission reduction [1], [2]. However, the large-scale electric vehicles access to grid will have adverse impact on the power grid, especially the during peak load, the difference of the load peak and off-peak will further increase, and then lead to the grid instability. In addition, the random charging of electric vehicle also has adverse impact on the grid [3]. To solve the problems, the electric vehicle technologies mainly contain the following respects: the electric vehicle charging station [4], [5], scheduling for charging and discharging of electric vehicles [6], [7], energy management [8]-[11], vehicle to grid (V2G) [12]-[14] etc.
Fig. 1.
The current research fields for electric vehicle.
In the above research field, the V2G technology has aroused the widely attentions. The core concept of V2G is that a large number of electric vehicles are applied to achieve energy buffer between the grid and renewable energy resource. The power of electric vehicle can be injected into grid when the load power is larger. The power of grid can be stored into the battery of the electric vehicle when the load power is smaller [15]. The electric vehicle can
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The battery can send the power into grid when the required load power over than grid. The vehicle to grid can be achieved through grid-connected operation mode. The electric vehicle connected with grid by GCC.
[14]. There is less research on the dc voltage coordinated control of vehicle to dc microgrid according to the above research results. In order to deal with the problem, an effective dc voltage coordinated control method is proposed. The control methods of grid-connected converter and electric vehicle energy storage system are given under grid-connected operation, converter current-limit and islanding operation conditions. The control method of electric vehicle energy storage system can be flexible switched to provide short-term support for dc bus voltage. Finally, the simulation results verify the feasibility and effectiveness of proposed control methods. II.
Fig. 3.
THE STRUCTURE AND MODEL of ELECTRIC
C. The Charging and Discharging Characteristic
VEHICLE
of Battery The battery of electric vehicle mainly include Lead-acid, Nickel metal hydride, Nickel-Cadmium, Lithium-ion and sodium-sulfur ect. The performance of Lithium-ion battery is best of all by comparing specific energy, specific power, charging technology and life for use and so on [17]. The electric vehicle energy storage system consists of Lithium ion battery and bi-directional DC/DC converter. The charging and discharging model of Lithium-ion battery can be expressed as Q Q Edisc = E0 − K ⋅ ⋅ i* − K ⋅ ⋅ it + A ⋅ e − B ⋅it (1) Q − it Q − it
A. The Structure of DC Microgrid The structure of dc microgrid can be shown in Fig.2. The dc microgrid includes grid-connected converter (GCC), wind generation system, PV generation system, battery energy system (BES), electric vehicle (EV) and local load. The function of GCC is to keep dc voltage constant. The interface of battery and electric vehicle apply bi-directional DC/DC converter, and the purpose of them are achieve bi-directional flow of energy. The two-stage grid-connected structure is used for PV generation system, and the direct-driven permanent magnet synchronous generator (PMSG) is applied for wind generation system.
Fig. 2.
The simply structure of electric vehicle to DC microgrid.
Ec = E0 − K ⋅
Q Q ⋅ i* − K ⋅ ⋅ it + A ⋅ e − B ⋅it Q − it 0.1Q + it
(2)
Where Edisc is discharging current, Ec is charging current, E0 is voltage constant, K is polarization constant, Q is maximum battery capacity, it is extracted capacity, i* is Low frequency current dynamics, A is exponential voltage and B is exponential capacity. The state of charge (SOC) of Lithium-ion battery can be expressed as 1 t SOC = 1 − ∫ i ( t ) dt (3) Q 0 In order to imitate the Lithium-ion battery characteristic, the battery energy system with controlled voltage source, the function of controlled voltage source is to imitate constant dc bus voltage. The amplitude of discharging of charging current is set at 10A, the initial SOC of battery is 80%, and the simulation results can be shown in Fig.4.
The structure of DC-Microgrid.
B. The Simplify Model of Electric Vehicle To simplify the analysis, the maximum power point tracking (MPPT) control is applied for the PV and wind generation all the times, and then they are neglected due to do not participate dc voltage coordinated control. The static battery energy storage system is replaced by electric vehicle, and the simplify structure can be shown in Fig.3. As shown in Fig.3, the electric vehicle consists of battery, bi-directional DC/DC converter, motor and inverter. The battery and bi-directional DC/DC converter is power supply of electric vehicle, and could achieve bi-directional energy flow during charging and discharging. The battery can store the surplus power when the grid power exceeds load power.
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reference of electric vehicle can be gained through energy management system (EMS).
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B. GCC Current Limit Operation The GCC current-limit operation is aroused by two modes. One is required load power over than the maximum power of GCC; another is the output power of distributed generation units exceed the maximum power of GCC. The above conditions would lead to dc bus voltage generally increasing and decreasing, and further make the overall system collapse.
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Fig. 6.
The simulation results of Lithium ion battery charge and discharge
The current reference of battery is -10A under initial conditions, and then the electric vehicle working at charging mode, the charging power is 1kW, and the SOC of battery is generally increasing. At 0.5s, the current reference of battery is switched to 10A, and then the electric vehicle working at discharging mode, the SOC of battery is generally decreasing. III.
THE DC VOLTAGE COORDINATED CONTROL
C. The Islanding Operation The PCC voltage sags under power grid fault occurs. The GCC stopped because the drive pulses are blocked by the fault detection modules under short circuit fault conditions. The constant current charging control method is switched to dc voltage droop control method for electric vehicle to ensure the ripple of dc bus voltage within fluctuation allows. The control structure of electric vehicle can be shown in Fig.7.
As shown in Fig.3, the electric vehicle can be described by battery and bi-directional DC/DC converter during stop work conditions, the structure is similar to battery energy storage system. Therefore the electric vehicle should participate in dc voltage coordinated control, and provide ancillary services for dc microgrid. The control methods of battery and bi-directional DC/DC converter are analyzed under grid-connected, converter current-limit and islanding operation mode.
* Iαβ
* uαβ
Fig. 7.
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The control structure of islanding mode.
IV.
(b) Fig. 5.
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The selection principle of electric vehicles is decided by SOC of battery, the electric vehicles with SOC>80% will participate in the control of dc bus voltage to provide short-term support for dc microgrid. The droop control method is switched to stop mode when islanding operation continuously time over than γ minutes.
(a) I b*
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A. The Grid-Connected Operation The control strategy of grid-connected operation mode can be shown in Fig.5. * dc
The control structures of GCC limit current operation.
As mentioned before, the PV and wind generation system are neglected in the paper, and then only the first case is considered. In order to ensure the dc bus voltage stable under converter current-limit condition, the electric vehicle load shedding depend on the SOC of battery when Δ|Udc|>ε, the less SOC of battery, the high priority of electric vehicles. The Fig.6 shows the control structure of electric vehicles, the charging and discharging current is switched to zero when it receive load shedding signal, and then the electric vehicle stopped.
characteristic. (a) Battery power. (b) Battery current. (a) State of charge.
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The control structures of GCC and EV. (a) GCC. (b) EV.
SIMULATION RESULTS
The matlab/simulink simulation software is applied to verify the feasibility and effectiveness of proposed control method. The dc bus voltage is set at 400V, grid phase voltage peak is set at 160V. The GCC connected with LCL filter to reduce high frequency harmonic, which is aroused by SPWM modulation. The maximum ripple degree of dc
The voltage and current double close loop control is applied to achieve zero steady-state error regulation for grid-connected converter in the two phase stationary frame [18]. The constant current charging control method is applied for electric vehicle under grid-connected operation conditions, and the charging and discharging current
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voltage is set at 20V. In order to simplify analysis, there are only two electric vehicles with rated power 3kW Lithium-ion battery in the dc microgrid, the rated capacity of electric vehicles is set at 96V/12Ah. As mentioned before, the distributed generation units are neglected for dc microgrid. The effectiveness of grid-connected operation is not analyzed because the previous researches have verified the grid-connected operation mode. Therefore the following were converter current-limit and islanding operation to be researched.
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The simulation results of converter limit-current operation mode.
SOC (%)
(a) Grid voltage. (b) Grid current. (c) Dc-bus voltage. (d) Battery current. (e) EV1 SOC. (f) EV2 SOC.
B. The Islanding Operation The Fig.9 shows the simulation results of grid fault conditions. The EV1 and EV2 are accessed into dc microgrid under initial conditions, and constant current charging with 10A, the GCC transfer power to electric vehicle and keep dc voltage constant. At 0.5s, the PCC voltage sags due to three-phase short circuit fault. The grid-connect converter is disconnected with grid to avoid damage. The control method of EV1 with SOC>80% is switched to dc voltage droop control mode for prevent dc bus voltage reduction, and the droop coefficient is selected as 1.6, and the dc voltage is regulated at 395V, within the law. At 1s, the grid fault is cleared, the GCC again control dc bus voltage, and then the control mode of EV1 is switched to constant current charging mode, the SOC of battery is generally increasing.
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A. GCC Current-Limit Operation The Fig.8 gives the simulation results of converter current-limit operation mode. The EV1 access to dc microgrid and charging with 20A constant current under initial condition, and then the dc voltage is controlled by GCC. At 0.5s, the EV2 access to dc microgrid, and the output current of GCC is clamped in 12.5A because the required load power exceed the maximum power of GCC. As shown in fig.8 (c), the EV2 is unloaded when Δ|Udc|>ε, the dc voltage is controlled again through GCC. At 0.9s, the SOC of EV1 reach in 95%, the charging current reference of EV1 is switched to zero to avoid overcharging, and then the EV2 access to dc microgrid with constant current charging control strategy.
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The simulation results of islanding operation mode. (a) Grid
voltage. (b) Grid current. (c) DC-bus voltage. (d) Battery current. (e) EV1 SOC. (f) EV2 SOC.
VI.
CONCLUSION
The paper proposes a dc voltage coordinated control method for vehicle to dc microgrid. The control methods of GCC and electric vehicle energy storage system are analyzed under grid-connected operation, converter current-limit and islanding operation conditions. The constant current charge control method of electric vehicle is switched to the droop control to provide short support for dc bus voltage in islanding operation conditions. Finally, the simulation results verify the feasibility and effectiveness of proposed control methods. ACKNOWLEDGMENT The authors would like to thank the National Natural Science Foundation of China (51277170, 51377152) and Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2-EW-328) for their support. REFERENCE
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