Front-End Power Converter Topologies for Plug-In Electric Vehicles

INTRODUCTION

Numbers of electric vehicles (EVs) and plug-in hybrid EVs (PHEVs) have increased in recent years and expected to ascend soon. To reduce fuel import rate and pollution. the governments are promoting the EVs into the automobile market with subsidies and tax exclusions. However, still, the EVs do not have the substantial market place in the automobile sectors due to constraints in driving range, charging time, expensive and massive battery requirement, and deficient charging station (CS) infrastructures. Further, nation-wide infrastructure development remains a huge task, even in developed countries. The United Kingdom aims to increase the charging infrastructure through building charging ports in newly designed homes, office buildings, and parking-adjacent street lights. The EVs/PHEVs are having different charging options, such as (i) the level 1/level 2 onboard charger and (ii) off- board chargers. Structure of an EV with the onboard charger is shown in Figure 5.1. Charging several vehicles may become an extra burden to the electricity grid during peak demand schedule. In various literature, the issues related to power demand due to uncontrolled EV/PHEV charging systems are addressed [1]. Also, the vehicle- to-home (V2H) capability of EV/PHEV has been used for power backup systems, but the problems of uncontrolled charging were not addressed. Additionally, power backup systems market in India is increasing, with the main focus on replacing diesel engine generator systems (DEGs) with uninterruptible power supply (UPS) systems. To replace the DEGs, the UPS shall have the capabilities, such as high energy storage and better power quality (PQ) delivery. By using EV/PHEV battery packs for UPS systems, the following advantages can be utilized:

  • • Battery packs used in EV/PHEVs have a balance between power and energy densities. In EVs, the energy density and power density are considered for driving range and acceleration, respectively. Similarly, in UPS applications, the energy density and power density are considered for the long backup duration and delivering high PQ in dynamic load behavior conditions, respectively.
  • • UPS system, with same battery pack, can be used as household battery swapping station.
  • • Demand level in local distribution transformer (DT) can be reduced by switching the UPS to backup mode, in the case of critical low voltages.
  • • Charge power (PEV) optimization/smart charging can be done through bidirectional DC-DC converter (BDC).

A typical V2H UPS system is developed in MATLAB®/Simulink® to demonstrate the functionalities mentioned above. Vehicle-to-grid (V2G) concepts have also emerged to compensate for the peak demand in the distribution system. The UPS application, as mentioned earlier with off-board chargers and the onboard chargers with V2G capability needs efficient, reliable, and controllable power electronic converters. For both the claims, a combination of front-end AC-DC converter which connects the EV charger to the electricity grid and a BDC converter w'ith battery energy management capability is the essential power electronic converters. The front-end converter converts the input voltage into pulsating DC with power factor correction (PFC), and the DC-DC stage converts the pulsating DC to regulated DC voltage for storage application. In this chapter, in Section 5.2 and 5.3, various power converters topologies for the AC-DC converters and the BDC converters are carefully examined with their operations and advantages/disadvantages. In Section 5.4, a typical V2H UPS system is configured, and Section 5.5 gives the simulation results with discussions.

AC-DC PFC CONVERTER TOPOLOGIES

The sequences of switching pulses, used in power electronics converters, draw non-sinusoidal current from the grid and generate harmonic currents. This nonlinear load effect reduces the degree of utilization of the power from the grid. Thus, PFC is an essential part of the UPS system to obtain unity power factor, and the AC-DC converter with PFC plays a vital role to transfer the electrical power from grid to vehicle and vice versa. For power quality improvement, various circuit topologies have been derived. Front-end AC-DC converter is classified into two types: (1) Two-stage approach and (2) single-stage approach. The single-stage approach is more fit for the low power application and fits for lead-acid battery storage system due to the low ripple in the output current. Two-stage systems are fit for high-power application and suitable for lithium-ion battery. A single-stage AC-DC PFC converter is illustrated in Figure 5.2 [2]. This section reviews the existing front-end AC-DC converter topologies for PHEVs with its general operations, advantages, and disadvantages. The selection considerations for the suitable PFC converters are given in Table 5.1.

 
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