A Bidirectional DC–DC Converter With Overlapping ...

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A Bidirectional DC–DC Converter With Overlapping Input and Output Voltage Ranges and Vehicle to Grid Energy Transfer Capability Article · March 2012 DOI: 10.1109/IEVC.2012.6183258

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3 authors: Mehnaz Akhter Khan

I. Husain

North Carolina State University

North Carolina State University

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Available from: Mehnaz Akhter Khan Retrieved on: 29 April 2016

A Bi-directional DC-DC Converter with Overlapping Input and Output Voltage Ranges and Vehicle to Grid Energy Transfer Capability Mehnaz Akhter Khan Student Member IEEE ECE, North Carolina State University Raleigh, NC,USA

Iqbal Husain Fellow IEEE ECE, North Carolina State University Raleigh, NC,USA

Abstract — A bi-directional buck-boost DC-DC converter is presented for vehicle charging as well as vehicle to grid (V2G) energy transfer. The chosen cascaded buck-boost topology allows overlapping input and output voltage ranges and a higher intermediate DC bus voltage where the electric drivetrain traction inverter can be connected. The intermediate DC-link capacitor voltage is varied to improve the transient performance of the converter. The reference voltage of this capacitor is set based on the input and output voltage levels, and the power demand. Experimental results have shown that with its modularity, control flexibility and transient performance the converter is a viable candidate for vehicle-to-grid energy transfer.

T

I.

INTRODUCTION

he increased penetration of renewable energy sources into the power grid will help ease our dependence on fossil fuel based energy sources. The main problem associated with the alternative energy sources such as solar energy, wind energy, and oceanic energy is their variability in energy production, which could be alleviated through interconnected energy storage systems. The electric vehicles penetrating into the marketplace can be used as an on demand energy storage. Electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV) are important for zero emissions on the roadway and subsequent environmental concerns compared to the combustion engine vehicles. Therefore, if they are used as storage devices, then it would be helpful to stabilize the grid. The electricity demand is high during the day and low at night. If energy is stored in the batteries of the electric vehicles during the night time and is withdrawn during the peak time, then the EVs can support the grid during the peak demand period. The installed power capacity needs to be high to meet the peak demand. A large number of vehicles connected to the grid at a time with their large capacity energy storage will support the grid to alleviate the load leveling problems. To enable power transfer in both directions between the grid and an electric vehicle, a DC-

Yilmaz Sozer Member IEEE ECE, University of Akron Akron, Ohio, USA

DC converter is needed with bidirectional power flow capability. The bi-directional DC-DC converter for PHEV and EV should be designed in such a way that the vehicle can contribute in the power compensation, voltage regulation and peak shaving [1]. The converter topology used in [2-3] has a low maximum overall efficiency, and the efficiency drops significantly for large voltage transfer ratios [4]. Another topology uses reversible rectifiers on each side of a contactless inductive power transfer system to control the amount and the direction of power flow [5]. Interleaving technique in the converters has been used to reduce filtering requirement, and improve the dynamic response and power management, and a multilevel converter has been used to decrease the voltage stress on the transistors and to reduce the need for large inductors [6]. In the research presented in this paper, a cascaded buckboost DC-DC converter has been developed to transfer power between the vehicle batteries and the grid. The developed non-isolated DC-DC converter provides simple structure, high reliability, high efficiency, and low parts count compared to several other converters in the same category [7-9]. The topology is not suited where electrical isolation is needed which would require different topologies to be used [10]. The developed converter allows the input and output voltage ranges to overlap, and a capacitor link in the middle is used in an intermediate stage. An alternative topology uses an inductor in the intermediate stage [4], but the developed topology provides modular design and allows individual modules to be interleaved on either the input or the output side; this is a significant benefit that allows component size reduction and integration of multiple vehicles into the system. II.

VEHICLE-TO-GRID POWER EXCHANGE

The bidirectional DC-DC converter is the part of the vehicle high power electrical system to make connection between the battery and the DC bus of the grid interface; the inverter is assumed to be part of the grid interface

DC BUS

+ Vehicle 1

Vehicle 2

C

Inverter

GRID

S1

IL1

S3

IL2

C1 L1

L2

Battery

S4

S2

Stage1

C2

DC BUS

infrastructure which will be in the charging station of a parking lot. A number of vehicles with a DC output are envisioned to be connected to the DC bus of the charging station as shown in Fig.1. If the battery of the vehicle is undercharged and needs energy, it will then draw it from the grid; conversely, if the battery has reserve capacity more than it needs for the commute and the power demand on the grid is high, then it will deliver power to the grid. The power exchange between the grid and the vehicle is required for a large number of different types of vehicles with different input and output voltage ranges. A bi-directional DC-DC converter that can operate in both buck and boost modes in either direction is necessary to achieve the objective [4]. The developed converter has the capability to allow a wide range and overlapping of input and output voltage ranges.

Inverter

GRID

Stage2

Fig.2: Proposed DC-DC converter.

The power is transferred between the input and output DC stages utilizing the DC-link capacitor as an intermediate storage unit. The converter has two stages, each one of which can act either as a buck converter or a boost converter. For example, during the power flow from the battery to the DC bus, the stage 1 will act as a boost converter and stage 2 will act as a buck converter. The DC-link capacitor voltage is maintained at a set level by the converter controller. Conversely, when power is to flow from the DC bus to the battery, stage 2 operates in the boost mode and stage 1 operates in the buck mode. The controller operation flow chart is shown in Fig. 3. Start

Battery Voltage= ADC1 Intermediate stage Voltage=ADC2 Load Voltage=ADC3

Vehicle n

Stage 1 Inductor Current= ADC4 Stage 2 Inductor Current= ADC5

Fig.1: Interface of vehicles and grid.

III.

According to Battery Voltage and Load Voltage Set the intermediate stage reference Voltage

CASCADED DC-DC CONVERTER TOPOLOGY

The DC-DC converter plays a vital role for power exchange between vehicle and grid allowing power transfer in either direction according to the need. A boost converter is often used in the electric drivetrain of an electric/hybrid vehicle to boost the battery voltage so that the DC-link voltage supplying the traction inverter operates at a higher voltage. However, the charging station DC bus voltage range may need to be at a lower voltage. The battery voltage also varies around a nominal voltage. In these systems, the battery voltage range will overlap with the charging station DC bus voltage range. The developed converter topology that is capable of operating in both buck and boost modes in either direction is shown in Fig. 2. The topology is suitable for the interfacing the high voltage battery with the high voltage DC-bus where an electrical isolation is not required. The cascaded buck-boost converter has an intermediate stage to store energy at a higher voltage and allows the overlap between battery voltage and DC bus voltage in the whole operating range. The high efficiency operation is achieved in the topology through the switching of only one switch in a bridge, and reducing the voltage stress on the switches through the bridge configuration.

Y

N

Battery Voltage>nominal voltage and SOC

N

Intermediate Stage Voltage< Its reference Voltage

Intermediate Stage Voltage> Its reference Voltage

Y

Y

Load Voltage>Its reference voltage

N Stage 2 Buck (PWM3)

N

Load Voltage