Grid to Vehicle and to Grid Energy Transfer using Bidirectional AC – DC Converter

Grid to Vehicle and to Grid Energy Transfer using Bidirectional AC – DC Converter

A. Anandan* K. Balaji* V. Udayakumar* G. Rajasekaran* P. Jamuna+

*PG scholars in Department of Electrical & Electronics Engineering

+Assistant Professor in Department of Electrical & Electronics Engineering Sri Manakaula Vinayagar Engineering College

Puducherry-107, INDIA

AbstractThis paper proposes a bidirectional ac/dc converter for vehicle to grid power transfer. The proposed converter consist of single-phase current-source rectifier (SCSR) and an auxiliary switching network (ASN). This offers bidirectional power flow between battery and grid in buck or boost mode with wider voltage range, The topology structure and operating principles of the proposed converter are analyzed in detail. An indirect control algorithm is used to realize the charging and discharging of the battery. This paper main objective is to present the principles of charging and discharging with buck- boost modes and to verify the efficiency of the proposed converter the circuit will be simulated using MATLAB simulink.

Keywords Bidirectional AC/DC converter,.Single-phase current-source rectifier (SCSR), Auxiliary switching network (ASN).

1. INTRODUCTION

   Now a day with the increasing cost of petroleum and growing acceptance of global climate change due to co2 emission by vehicles as a critical environmental problem, policy makers, engineers and business leaders are searching for alternative energy solutions. Re-electrification of automobile transportation and enhancement of the existing power system grid is one such solution. The electric power system is a complex and critical infrastructural system, yet it lacks energy storage capacity, so electricity must be simultaneously produced and consumed[1]

   For this energy storage capability the Developing plug-in hybrid electric vehicles (PHEV) or electric vehicles (EV) is an effective way to solve the increasingly serious energy crisis and global warming issues because of their reduced fuel usage and greenhouse emissions[2] [3]. In the longer term, there may be potential for smart-grid technology to enable EVs to be used as distributed storage devices, feeding electricity stored in their batteries back into the system when needed (vehicle-to-grid, or V2G, supply). This can help to reduce electricity system costs by providing a cost-effective means of providing regulation services, spinning reserves and peak-shaving capacity.

   In a V2G system, the battery charger plays a critical role, because the charging time, the quality of the power and

   the battery life are linked to it [3], [4]. A battery charger with a high power density, low cost, high efficiency, and flexibility, has been the main focus of current research in both industrial and academic communities.

   Nowadays, a two-stage topology that voltage-source rectifier (VSR) cascades a dc-dc converter is widely applied

   in EV/PHEV [5]-[6]. This type of charger offers a two-way electrical energy flow and communication. In addition, the quality of the input current is improved by the PWM technique.

   According to the duality between the VSR and the current-source rectifier (CSR), the CSR is suitable for a battery charger. However, there are relatively few studies on their application to the battery area. Actually the conventional CSR used as a charger was reported early [7]-[8]. A charger based on the CSR and H-bridge inverter was reported, where the H-bridge inverter works as an inverting link [9]. It supports bidirectional energy flow and has fewer switches when compared with AC/DC matrix converters. However, undesired redundant switches still exits and that may restrict the scope of its application. Therefore, a simplified topology which can perform multiple functions was studied [10]. For reducing the switch power losses and EMI noise, a bidirectional swiss rectifier [11], [12] and a bidirectional 3rd harmonic injection active filter rectifier [13] were proposed. Their common advantage is that the rectifier works as the fundamental frequency. All the CSCs mentioned above are three phase chargers. When a low power charger is required, for example, when charging at home or in an office, a single phase charger is usually preferred [4]. Various topologies and schemes based on a diode rectifier or a voltage-source rectifier have been reported for single phase chargers [4], [14]-[15]. However, few studies have focused on single phase current-source chargers. In [16], a novel inductor voltage control method was proposed for a single phase CSC to achieve any input power factor. This papers main objective is to present the principles of charging and discharging with buck-boost modes and to verify the feasibility of the proposed topology, modulation scheme and control algorithm.

   This paper is organized as follows: Section I, II, III introduces the topology, the operation modes and the averaged switching modeling of the proposed converter. Section IV shows simulation and experimental results to validate the capabilities of the proposed converter. Finally, the main points of this paper are summarized in Section V.

2. PROPOSED DC DC CONVERTER

   1. Circuit Configuration and Detail

      The topology of the proposed single-phase current- source bidirectional converter is shown in Fig. 1. It consists of an LC input filter, a conventional SCSR and an ASN. For the reverse voltage blocking capability, each IGBT in the SCSR is connected with a diode in series. Under normal operation switches S1 and S3 are complementary; the same restriction is applied to S2 and S4. The ASN includes two IGBTs, two diodes, an inductor Ldc and a capacitor Co, which is the key part of the system. Its main function is to change the direction of the power flow and converter voltage.

      Fig. 1. Circuit diagram of Bidirectional AC-DC Converter.

      Its front-end is a conventional CSR and its rear-end is an ASN composed of two IGBTs and two diodes. The role of the front-end SCSR is to convert the supply ac voltage into an intermediate dc link voltage and to realize PFC. The ASN is designed to control the direction of the power flow (charging or discharging). In addition, it works in a PWM manner to help in boosting the output voltage, which greatly expands its application range. Compared with the bidirectional buck-boost converter mentioned in [32], the proposed topology has fewer components, a wider output voltage range and better quality in the input current.

   2. Modes of Operation

   The proposed converter operation can be divided into four operating states. According to the characteristics of the CSR, the dc current idc cannot be reversed. However, by controlling the ASN, the direction of the current flowing through the battery can be changed to achieve charging and discharging functionalities. Fig. 2 to 5 depicts the four operating modes of the ASN, where the CSR is represented by a controlled voltage source druc. dr is the difference between the steady state duty ratios of switches S1 and S2. The red lines illustrate the current paths. Under the charging operation, ug and ub are the input and output voltages, respectively, and the operating modes alternate between mode I and mode II.

   Mode I: In this mode, switches S5 and S6 are in the off state, diodes D5 and D6 are forward-biased and the battery absorbs energy.

   Fig. 2:Battery charging

   Mode II: In this mode switch S5 is in the on state and a corresponding diode D6 is forward-biased, which disconnects the battery from the circuit.

   Fig. 3:Freewheeling action

   Under the discharing operation, ub and ug are the input and output voltages, respectively, and the operating modes alternate between mode and mode .

   Mode III: In this mode, switches S5 and S6 are in the on state, diodes D5 and D6 are reverse-biased and the battery supplies energy.

   where, ig is the gird current, uc is the terminal voltage of capacitor C, ub is the battery voltage, idc is the dc current flowing through inductor Ldc, dr=d1-d2, db=1-d5-d6. di is the duty ratio of switch Si (i{1,26}).

   Assume that ug=Usin(t), and that U and represent the grid voltage magnitude and the angle frequency, respectively. In order to obtain a sine grid current with a unity power factor, according to (2), the following equation should be satisfied in the steady state:

   ig dridc I sin(t)

   (5)

   Fig. 4: Battery discharging

   Mode IV: In this mode , plays the role of freewheeling. The ASN works as a buck circuit and the system is still a buck-

   where, I is the amplitude of the input current. If I is greater than zero, the system works in the charging status; else the system works in the discharging status. For satisfying the volt-second balance of the inductor Ldc, according to equation (3), equation (6) can be obtained (ignoring the effect of the input filter) as follows:

   boost converter.

   drug druc dbub

   (6)

   where, -1dr, db1. It is clear that the value of the battery voltage can be larger or smaller than that of the grid voltage. As a result, the charging/discharging operation under buck/boost mode can be realized. According to (5) and (6), there is one degree of freedom available. Therefore there are lots of methods to accomplish the control task mentioned above in theory. In this context, idc is controlled to be a constant and dr is controlled to vary with a sine shape, as shown in (7).

   i *

   dr c.sgn( I ).sin(t). dc i

   (7)

   dc

   Fig. 5: Freewheeling action

3. MODELING OF THE PROPOSED CONVERTER

   Assume that the system works in continuous mode. According to Fig 1 , the state space average switching model for the proposed converter is formulated as follows:

   where c is the modulation index, and sgn() is the sign function. idc* is the reference dc current. (idc* /idc) idc idc is an extra compensation term to improve the dynamic responses of the duty cycle to prevent distortion of the grid current. In particular, when the grid voltage ug is near the zero-crossings under the charging operation, the power losses of the circuit and switches are more than the grid can supply.

   L dig

   dt

   duc

   ug

   • uc

     (1)

     From (5) and (7), I, c and idc satisfy the restriction |I|=c·idc. Taking the power losses into account, a minimal idc is expected. Therefore, according to (6) and (7), c can be given

     c dt ig dridc

     (2)

     by:

     c min ub ,1

     (8)

     L didc d u d u

     (3)

     u

     dt r c b b

     C dub

     0 dt

     dbidc

   • ib

     (4)

     where, min() is the minimum function. According to (6), db can be obtained as follows:

     d c.U.sin2 (t)i * (i u )

     (9)

     b dc dc b

     1. Control Algorithm

        A control diagram for realizing the charging/discharging is shown in Fig. 4. To improve the S1

        reliability and reduce the cost, only the grid voltage, the

        battery terminal voltage and the current through the inductance Ldc are sampled. To regulate the current through S2 the inductor, a control scheme including feed-forward control

        and PI feedback control is presented. The feed-forward S3 control compensates the effects of the grid voltage directly,

        which increases the dynamic response of the current control. S4 The PI control will help to stabilizing the system and handle

        some unknown disturbance. A simple open-loop control is

        used for modifying dr to obtain a sine grid current and to S5 realize PFC.

        S6

        uL

        Charging Discharging

        -dr/2 -dr/2

        dr db

        -db/2 -db/2

        ug ug ug ug

        0 0 0

        . ug- ub ug- ub

        idc

        Ts Ts

        Fig.7 Switching pattern arrangements under different modesug>0,ug<ub

        TABLE I

        PARAMETERS USED FOR SIMULATON

        Fig.6 Control diagram of charging, and discharging.

     2. Switching Pattern Arrangements

   The performance of the converter depends on both the modulation strategy and the switching pattern arrangement. In this paper, reducing the switching power losses and output current ripple are the main consideration when arranging the switching pattern. Therefore, a symmetrical switching pattern is usually applied [20]. Both the front-end switching pattern and the rear-end switching pattern need to be arranged. In addition, coordination control between the front-end switching pattern and the rear-end switching pattern is required.

   In this paper, the in-phase carrier PWM modulator is adapted under the charging operation, and the interleaved carrier PWM modulator is adapted under the discharging operation. which illustrates the specific operation of each switch in each switching period. An example is given in Fig.7 to show the changes of the voltage across inductor L and the dc current idc. Here the voltage drops across the IGBTs and the diodes are neglected. From Fig.6 the switching patterns are bilaterally symmetrical, and twice commutations occur in each switching period.

   parameters	value
   Amplitude of input voltage U	230V
   Input filter inductor L	0.1H
   Input filter capacitor C	10uF
   Output filter inductor Ldc	400µH
   Output filter capacitor C0	470µF
   Switching frequency fs	20Hz
   The reference current idc*	8A
   Value of the battery voltage ub	120V

   parameters	value
   Amplitude of input voltage U	230V
   Input filter inductor L	0.1H
   Input filter capacitor C	10uF
   Output filter inductor Ldc	400µH
   Output filter capacitor C0	470µF
   Switching frequency fs	20Hz
   The reference current idc*	8A
   Value of the battery voltage ub	120V

4. SIMULATION RESULT

   To validate the proposed topology and method, simulations based on the Matlab/Simulink environment are implemented in this section. The models of the semiconductor devices and battery are from the SimPowerSystems/Power Electronics library.

   A schematic diagram of the single-phase current source bidirectional converter in the simulation is shown in Fig. 1. The parameters used in the simulation are listed in Table I. The loads are 40Ah capacity lead-acid battery with nominal voltage of 120V. The simulation results in Fig. 8 show the performance of the proposed converter. From the top, the grid voltage ug, the grid current ig, the output voltage udc, the dc

   current idc , the battery voltage ub and the battery current ib are sequentially displayed.

   Grid voltage

   (A)

   It can be clearly seen that in the charging mode, the grid currents are in phase with the grid voltages, while in the discharging mode, the input currents and the grid voltages are out of phase by 1800. An interesting phenomenon can be found in the fact that the inductor current is distorted near the grid voltage zero-crossing region in all of the charging modes, even though it is designed to be controlled as a constant value.

   In addition, in th discharging mode, as the battery is a constant voltage source, it can overcome the forward voltage and keep the inductor current constant. As a result, the inductor current is controlled well without distortion. In conclusion, the input current is in phase with the input voltage, and it is close to a unity power factor, and the input current is sinusoidal in all modes.

   Battery voltage

   (B)

   Fig.8.Simulation results: (A) Buck charging.

   (B) Boost discharging.

5. CONCLUSION

A bidirectional AC/DC converter for V2G application has been designed simulated mathematical models for the Buck and Boost modes of AC/DC converter are derived. An average model of AC-DC converter is obtained using MATLAB simulink. It is observed that current change its direction quickly as the mode of operation changes with out any overshoot of the peak values hence the converter voltage and current are stable in its operation on both direction.

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