Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in
Analysis of Bidirectional Converter for Power Sharing From Vehicle to Grid 1 2
Sasikumar S1, Krishnamoorthi K2, Soniya N1
Research Scholar, Department of Electrical Engineering, Sona college of Technology, Associate Professor, Department of Electrical Engineering, Sona college of Technology, 3 PG Scholar, Department of Electrical Engineering, Sona college of Technology,
Abstract- According to the growth of electric vehicle industry, the demand of new battery energy storage systems for vehicle- to-grid has been increased rapidly to improve electric power utilization. Projected BESS penetration levels across utility customer are a promising clue that BESS may dominate the market in the near future. In this paper, a novel BESS system with V2G function for on-board charger which composed of a bidirectional DC/DC converter using voltage compensation configuration is proposed and the mode of charging/discharging is verified by simulation. Keywords: Bidirectional DC-DC converter, NBDC, NBDC with Coupled Inductor and IBDC. .
converter. According to the voltage compensation and its power sharing, the size of proposed on-board BESS can be reduced. [n this paper, the power sharing is analysed and the mode of charging/discharging is verified by simulation. From the simulation results, the validity of proposed system can be confirmed.
1. Introduction
2. Novel Topology of the Battery Energy Storage System
According to the growth of electric vehicle (EV) industry, the demand of new battery energy storage systems (BESS) for vehicle-to-grid (V2G) has been increased rapidly to improve electric power utilization [1]. Sharing power assets between battery and power system is the focus of BESS that can create new economic value as shown Fig 1. An example of a generated energy according to load conditions is shown in Fig. 1 [2-3]. As shown in Fig. 1, the peak-load condition could be occurred in daytime from 10 a.m. to 12 p.m. or from 4 p.m. to 6 p.m. when is swamped with electric power demand in factories or buildings. Because of the quantity of demand power depends on the power consumption of loads, the utility grid has to secure a supply of electric energy which is larger than the required power consumption under peak load-condition. It means that the waste of electric power could be heavy at the other time than peak-load period. Projected BESS penetration levels across utility customer are a promising clue that BESS may dominate the market in the near future. The on-board BESS is configured as Fig 2. As known, the BESS with V2G function needs a bidirectional dc-dc converter. This paper presents a novel BESS system with V2G function for on-board charger. To reduce the size of entire on-board BESS, a voltage compensation technique is applied to the conventional full-bridge based bidirectional DC-DC
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Fig.1. Illustrate schematic of vehicle to grid system including bidirectional converter
The proposed dc-dc converter system uses the series compensating method [4-5], although the efficiency of a dc- dc converter is slightly low, the system efficiency can be dramatically high, because the compensated power by the converter is small compared to the system power. 100% of the renewable system power can be handled by 30% of the dc-dc convertor capacity. The novel bidirectional topology which has battery charging mode and discharging mode. Both modes were used the phaseshift technique. In charging mode, primary side uses phase-shift technique, battery side is not operate switch. In discharging mode, battery side uses phaseshift technique, primary side does not operate switch. The bidirectional dc-dc converter has to control the input/output current of the battery. Fig. 1 shows a top-level architecture of an intelligent vehicle-to-grid (V2G) charging system for electrical vehicles. The system consists of the front-end converter, a power converter with high frequency transformer, bidirectional PWM converter, and intelligent digital controller. The front-end converter is regulated by receiving renewable PV and wind sources, and it transfers the power energy to the power converter that is integrated to a new multi-input DC-DC converter. The converter consists of two power stages, input-side and output-side stages. The inputside stage is designed using a full-bride topology Page 122
Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in with four main switches. It is possible to allow the operation of the converter with a predictive control. The converter can achieve soft switching for the bottom-side switches by using a leakage inductance in the transformer [5]. The bi-directional converter offers power conversion between both the dc link voltage at the EV battery pack and the commercial AC line. The converter usually transfers the ac source to the dc source at the dc link for charging the EV battery pack. Reversely, the DC energy source flows back to the grid from the batteries in EV through PWM converter as a three-phase inverter. For a predictive controller, the load sharing is considered based on the characteristics of renewable sources [6]. The controller works for effective power flow and communication networks. The controller also operates based on the charging profile that provides the charging curve data to inform both the user and the grid regarding the vehicle’s energy needs, and to control the vehicle charging based on feedback provided by the grid operator. When the grid uses renewable energy like wind and/or solar power, the predictive controller should provide the command to discharge the vehicle for the power grid.
3. Operation and Modeling of the Bidirectional Converter For Battery Charger To connect a high voltage DC-link with a battery of low voltage a DC- DC bi-directional converter with high conversion ratio is necessary. Due to the possibility of the bidirectional power flow, charging and discharging of the battery is possible. The structure of the bidirectional Buck and Boost converter is depicted in Fig. 2.
Fig. 2. The proposed of Buck and Boost bidirectional converter. The converter can operate under the buck or boost control. It is defined as follows; when the energy flows from the renewable energy source to the battery (charging mode), the converter operates under the buck control. The buck switch is controlled and the boost switch is in off state. When the energy flows from the battery to the grid (discharging mode), the converter operates under the boost control so the boost switch is controlled and the buck switch goes to off state. The buck and boost control modes change according to the detected grid and battery voltages. Depending on the switch states, the bidirectional Buck and Boost converter has two different modes; if all the circuit elements are
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assumed ideal, the averaged state equations of the converter, which illustrate the CCM (Continuous Current Mode) operation of the converter, can be written as follows [7];
4. The Topology and Operating Principle Voltage-source PWM converter could achieve sinusoidal current in grid side, operate under the unit power factor, and transform power bilaterally. In the low-to-medium power occasion, a well-known single-phase full-bridge VSC is usually used, as shown in Fig. 1. Asymmetrical half-bridge (AHB) resonant DC/DC converter as shown in Fig. 2 is a basic form of LLC resonant converter [14], which has good characteristics, such as zero-switching (ZVS) for primary-side switches, zero-current switching (ZCS) for secondary-side rectifiers, low switch voltage stress, and small circulating current. Hence, the AHB resonant DC/DC converter [15]-[17]can operate at a high switching frequency with lesser switch loss. The resonant inductor Lr, the magnetizing inductor Lm of the high-frequency transformer, and the resonant capacitor Cr constitute the resonant tank. Because there are three dynamic components in circuit, the converter can output different dc voltage under different working frequency. By changing the working frequency of the converter, it can be obtained what output voltage you need within a certain range. It is very suitable for the on-board charger of EVs, whose nominal dc voltages are different. However, for the V2G system, the most basic requirement of the converter is transforming power bilaterally, which is impossible for traditional AHB resonant DC/DC converter because of the diodes in the secondary-side rectifier. So, if one would have a LLC resonant converter used in V2G system, some necessary improvement is must to be done. Fig. 3 shows the proposed topology of bidirectional power converter for V2G system. The topology has two-stage: AC/DC converter (frontstage) and DC/DC Converter (post-stage). The frontstage is a single-phase full-bridge of voltage source PWM converter, which makes the voltage of DC bus steadily at a constant value. The post-stage is a novel half-bridge LLC resonant (HB LLC) converter which is called symmetrical half-bridge LLC (SHB LLC) resonant converter.
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in Fig. 3. The proposed topology of bidirectional power converter for V2G The SHB LLC converter means (post-stage) that the circuit topologies on both sides of the highfrequency transformer are exactly the same. Each side of the SHB LLC converters could work as a typical HB LLC resonant converter. When the converter on one side of the high-frequency transformer works in the high-frequency inverter mode, the one on the other side will work in the high-frequency rectifier mode. The SHB LLC resonant converter is composed of a switching network, a resonant network and a rectifier network in series. V11, V12 and V21, V22 constitute the two side switching network, respectively. It is used to get symmetrical square wave signal with complementary with primary switch pulse (on 50% and off 50%) which is given to the resonant tank as an input. One resonant tank is consisted by the resonant inductor Lr1, the magnetizing inductor Lm of the highfrequency transformer, the resonant capacitor C11 and C12. The other is consisted by the resonant inductor Lr2, the magnetizing inductor L’m of the highfrequency transformer, the resonant capacitor C21 and C22, similarly. The uncontrolled full-bridge rectifiers in the two side are composed of the diodes (VD 11, VD12, VD15, VD16) and (VD17, VD18, VD21, VD22). Depending on the direction of power transmission, the proposed converter has two modes: the forward mode and the reverse mode, as shown in Fig. 4
Fig. 4. Two modes of the SHB LLC resonant converter The SHB LLC resonant converter could realize ZVS/ZCS operational mode in the whole load range. It reduces the switching loss effectively and slows down the transient over-voltage and overcurrent of the switches. This topology can solve the problem which is accepted that it is difficult for the lagging arm to achieve soft switching using the traditional ZVS bridge phase-shift PWM converter or the bridge ZVS PWM converter. Compared with the single resonant capacitor topology, using split resonant capacitor, the ripple and root mean square (RMS) of the input current through resonant capacitors are both smaller. The split resonant capacitor receives only half RMS current of the
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single resonant capacitor and the capacitance of the split resonant capacitor is also only half of the single resonant capacitor. On the resonance side, the clamp diodes circuit can be used as the over-voltage protection of the resonant inductor in the resonant network, while the symmetrical clamp diodes automatically convert to a rectifier arm of the single-phase full-bridge rectifier and separate the unused resonant inductor on the output side from the main circuit. It avoids large internal impedance voltage drop in the output loop. Similarly, the clamp diodes can also be used as overvoltage protection of the resonant capacitors in the resonant network, while the symmetrical clamp diodes on the output side. It can effectively suppress the LC resonance phenomena that may occur in the output-rectifier circuit. Therefore the diode clamp circuit of the resonant capacitors has a complex function of resonant voltage clamping protection and inhibiting resonance in the rectifier circuit.
5. Control Theory and Design The Fig. 5 shows the control diagram of bidirectional power converter proposed above. The control method of front-stage (VSC converter) adopts double-closed-loop control based on dq transformation with outer voltage loop and inner current loop. It could realize the unit power factor and low harmonic in grid side, whether VSC work in the rectifier mode or inverter mode. The control method of post-stage (SHB LLC resonant converter) has two parts: and reverse control module. The two modules share a VCO (voltage-controlled oscillator) circuit and a driving signal producer circuit. When the bidirectional power converter works in the forward control mode, the switch S connects to the forward control module circuit. Hence the Voltage and current of the EV battery are under control in order to adapt different voltage level and capacity of the EV batteries. And when the bidirectional power converter works in the reverse control mode, the switch S connects to the reverse control module circuit. It can control the current of the DC bus for the VSC at a constant value. Then the VSC inverter the power flow to the grid side. ZVS can be ensured in the primary side switches by keeping the current through these switches negative on the instant they are turned ON. The primary current should be able to charge and discharge the output capacitors of the primary side switches during the dead-time. The magnitude of this current depends on the magnetizing inductance and the duration of the deadtime. So, the ZVS in the primary side depends on the magnetizing inductance, the switch output capacitance and the dead-time duration. The operation of this converter during dead-time is similar to the operation of LLC resonant converter during dead-time. So, the magnetizing inductance Page 124
Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in can be designed using the same expression as the full-bridge LLC resonant converter [26]. Magnetizing inductance cannot be too low. As, it would make the magnetizing current very high, resulting in huge conduction losses, increased apparent power requirements for switches and increased peak voltage requirement for the primary side capacitor. Large magnetizing inductance will result in a small magnetizing current, but it limits the voltage gain of the converter. So, magnetizing inductance cannot be too large. For longer dead-time, the magnetizing inductance can be made large to reduce the magnetizing current, but it will result in large primary RMS current as no energy is transferred during dead-time. All these factors should be kept in mind while designing magnetizing inductance.
Fig.5.Battery power and bidirectional input power.
6. Simulation Results Now we can achieve optimum input to minimizing quadratic performance index. Simulation has been carried out in the MATLAB/ Simulink environment to verify the good performance of the proposed method. The performances obtained with the NMPC are outlined in Figs. 4 and 5 for the Boost mode operation at the desired output reference voltage Vref = 750V. The robustness to disturbances is illustrated in the case of a load disturbance (at the time instant equal to 3 second R drops to 50% of the nominal value). For Buck mode operation at output voltage reference Vref = 100V, output voltage waveforms are illustrated for input voltage and load current step changes in Figs. 6 and 7. The results obtained show that the nonlinear model performance than the one achieved with the conventional linear controllers and that it guarantees a stable operation under ill conditions. Now let’s compare the performance of NMPC with some conventional methods like current program control and feedback linearization control. Test of the charging current parameters shown in Fig.8. It shows that the current flows are able to follow the different references. Boost converter testing is also done by observing the voltage DC link as in Fig.11. The picture shows that the voltage is maintained at 600 Volts. It shows that the bidirectional converter as a voltage regulator was able to regulate the flow of power so that the voltage becomes stable.
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Fig.6.Battery power and bidirectional output power. Fig.5.The process of change in boost mode charge into trickle charge.(a) Battery current,(b) Voltage DC link,(c) PV power,(d) The battery voltage
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in Fig.5. presents changes in input and output bidirectional power converter. The picture shows the charging process. It also shows the power that flows to the bidirectional converter is impaired due to losses in the circuit. Table III shows us that the bidirectional converter efficiency is 90.04 present. Fig. 6 presents the simulation results of the power Required exceeds the PV power. Fig. (a) Present battery voltage continues to rise due to the charging process. Fig. 6 (a) presents the current flowing curve of the battery. When the battery voltage has exceeded 267 volts, the battery current change of mode boost charge into trickle charge. Fig. 6 (b) presents the DC link voltage. DC link voltage momentarily increased when there is a change charging mode. However, as the operation of the controller in the PV power down, the voltage stabilizes at a value of 620 Volts. PV power reduction can be observed in Fig.6. (c). The controller responds excess PV power by reducing the power output of PV. Thus the DC link voltage can still be maintained despite the PV power exceeds the power charging.
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