6kW V2H Power Converter Using Isolated CLLC DAB Converter

Abstract

Recently, as interest in eco-friendliness grows, the supply of hybrid electric vehicles and pure electric vehicles (EVs) for improving fuel efficiency of automobiles is rapidly expanding. The average daily energy consumption of electric vehicles is less than 20 [%] of the total ESS capacity of the vehicle, and research on additional functions using the ESS of the vehicle is urgently needed to expand the supply of electric vehicles. V2H(Vehicle to Home), like V2G(Vehicle to Grid), includes the concept of cooperating with system stabilization using ESS of electric vehicles. In addition, it includes various operations that can realize home welfare, such as uninterrupted power supply in case of power outage at home, and power supply for home DC devices. Therefore, in order to expand the supply of eco-friendly electric vehicles, it is urgently required to develop a V2H system with various functions that can realize home welfare. In this paper, we propose a V2H system with a CLLC resonant converter and a non-isolated step-up converter that can solve different impedance and resonant frequencies depending on the power transfer direction. The proposed V2H system is 6 [kVA] applicable to 150-450 [V], the voltage range that can use the ESS voltage for electric vehicles, and is designed with a capacity that can handle instantaneous electricity use at home. In addition, in order to verify the feasibility, an experiment by Psim simulation and prototype production was performed.

1. Introduction

Recently, the supply of hybrid electric vehicles and pure electric vehicles (EVs) is rapidly expanding due to changes in the concept of eco-friendliness and improvement of fuel efficiency of automobiles[1]. The average daily energy consumption of electric vehicles is within 20 [%] of the total Energy Storage System(ESS) capacity of the vehicle. Therefore, in order to expand the supply of electric vehicles, it is urgently necessary to study additional functions using the ESS of automobiles. Currently, a representative additional function using ESS of electric vehicles is V2G (Vehicle To Grid)[2][3], which corresponds to the hourly rate system. The term was first used by AC Propulsion, Inc. It is a technology that connects the battery stored in the electric vehicle to the power system by using it like an ESS. In general, the V2G system is performed through several stages depending on the system conditions and the ESS conditions of the electric vehicle. A representative operating system is largely composed of a Distribution System Operator (DSO), an Aggregator, a Bidirectional Charger, and an EV. Each component is the same as the state such as the frequency, load amount and voltage of the grid, the number of electric vehicles that can be charged and discharged, and the state of charge (SoC) of the electric vehicle battery. Based on this various information, the ESS of the electric vehicle performs the charging/discharging operation. These technologies are being established as an industrial model that can continue sustainable growth by reducing carbon dioxide emissions while being eco friendly and economical. In the future, by utilizing V2G, it will not only reduce peak power, but also create various power auxiliary services and added value such as power system frequency adjustment. Currently, the supply of electric vehicles is concentrated on passenger vehicles. Most electric vehicles are owned by households, so in order to expand the supply of electric vehicles, it is urgently necessary to preemptively improve the home electricity rate system. In developed countries, even at home, an hourly rate system that differentiates electricity rates during the daytime when electricity consumption is high and at night and morning hours when electricity consumption is low is being implemented. In Korea, an hourly rate system that can equalize electricity consumption is being operated on a trial basis. At this point in time when electric vehicles are concentrated on passenger vehicles and hourly rate system is not implemented, electricity management changed from V2G concept to V2H (Vehicle To Home) concept is urgently required. V2H, like V2G, includes the concept of cooperating with system stabilization using ESS of electric vehicles. It includes various operations that can realize home welfare, such as uninterrupted power supply and power supply for home DC appliances in case of power outage at home. Therefore, in order to expand the supply of eco-friendly electric vehicles, the development of a V2H system with various functions that can realize home welfare is urgently required[4]-[6]. In this paper, we propose a V2H system with a CLLC resonant converter and a non-isolated step-up converter that can solve different impedances and resonant frequencies depending on the power transfer direction. The proposed V2H system is 6 [kVA] applicable to 150-450 [V], the voltage range that can use the ESS voltage for electric vehicles, and is designed with a capacity that can handle instantaneous electricity use at home. In addition, in order to verify the feasibility, an experiment by Psim simulation and prototype production was performed[7]-[10].

2. Power Coverter for V2H

Globally, the electric vehicle market is rapidly increasing due to the change of the paradigm for eco-friendliness. As shown in Fig. 1, the electric vehicle market is expected to reach $4790 billion in 2025.

Fig. 1 Electric vehicle market change

Currently, most households owning electric vehicles have built and operated a slow charging station as shown in Fig. 2. Such a charging system simply performs a charging function due to a unidirectional power conversion device. In addition, the electric vehicle ESS is equipped with a capacity to cope with long-distance driving of several hundred kilometers. However, the average daily mileage during city driving operation is low, so the ESS usage rate is very low. Therefore, securing economic feasibility to improve ESS utilization is very important.

Fig. 2 Application example of V2H system

The main purpose of using the batteries stored in electric vehicles as ESS to connect them to the power system is a technology for grid safety through averaging power demand. As shown in Fig. 3, the electric vehicle ESS is charged during times of low power demand and discharged when the power demand is high to equalize the power demand of the entire system.

Fig. 3 The role of V2G

For various operations using the ESS of electric vehicles, it is essential to build a two-way charging/discharging system that breaks away from the existing one-way charging system. DC/DC converter and DC/AC inverter technology are very important to build a bidirectional charging/discharging system[11]-[12].

2.1 CLLC isolated DAB converter

Fig. 4 shows an isolated resonant bidirectional converter. The CLLC converter is a power transfer converter using the resonance of input/output impedance. There is no problem of voltage drop due to the reactor of the transformer, and it has the advantage of analyzing the characteristics by the fundamental wave component of the switching frequency[13]. Since the resonance frequency voltage applied to the primary and secondary sides of the transformer is a square wave and the current is a sine wave, the characteristics of the entire system can be analyzed by analyzing the fundamental voltage. In Fig. 4, the output voltage of the primary-side full-bridge converter appears as a square wave. Expanding this as a Fourier series gives the following equation.

Fig. 4 Isolated resonant bidirectional DAB converter

\(v _ { S } ( t ) = \frac { 4 V _ { i } } { \pi } \sum _ { n = 1.3 .5 \ldots . } ^ { \infty } \frac { 1 } { n } \operatorname { sin } ( 2 \pi n f _ { s } t )\)       (1)

In the CLLC DAB converter, the fundamental wave component of the voltage determines the characteristics before output transmission, so the fundamental wave component of Equation (1) is the same as Equation (2).

\(v _ { S I } ( t ) = \frac { 4 } { \pi } V _ { i } \operatorname { sin } ( 2 \pi f _ { s } t )\)       (2)

The RMS value of the fundamental component of Equation (2) is as follows.

\(V _ { S 1 } = \frac { 2 \sqrt { 2 } } { \pi } V _ { i }\)       (3)

Also, since the output voltage of the secondary side full-bridge converter is a square wave having a phase difference ϕ with the primary side, using Equation (1), it is developed as a Fourier series as the following equation.

\(v _ { B } ( t ) = \frac { 4 V _ { o } } { \pi } \sum _ { n = 1,3.5 \ldots } ^ { \infty } \frac { 1 } { n } \operatorname { sin } ( 2 \pi n f _ { s } t - \phi )\)       (4)

In Equation (4), the fundamental wave component is expressed as the following equation.

\(v _ { B 1 } ( t ) = \frac { 4 } { \pi } V _ { o } \operatorname { sin } ( 2 \pi f _ { s } t - \phi )\)       (5)

The RMS value of the fundamental component of Equation (5) is as follows.

\(V _ { B 1 } = \frac { 2 \sqrt { 2 } } { \pi } V _ { a }\)       (6)

If the transformer primary current flows as in Equation (7), the average value of the current can be expressed as Equation (8).

\(i _ { S 1 } ( t ) = \sqrt { 2 } I _ { S 1 } \operatorname { sin } ( 2 \pi f _ { s } t - \phi )\)       (7)

\(I _ { o } = \frac { 2 } { T _ { s } } \int _ { 0 } ^ { \frac { T _ { s } } { 2 } } | i _ { S I } ( t ) | d t = \frac { 2 \sqrt { 2 } } { \pi } I _ { S I }\)       (8)

From Equations (6) and (8), the equivalent resistance to the load resistance (Ro) is defined as the following equation.

\(R _ { e } = \frac { V _ { B 1 } } { I _ { O } } = \frac { 8 } { \pi ^ { 2 } } R _ { o }\)       (9)

Fig. 5 is the equivalent circuit of a resonant bidirectional DAB converter.

Fig. 5 Equivalent circuit of resonant bidirectional DAB converter

2.2 V2G system configuration

The ESS voltage for electric vehicles tends to be proportional to the vehicle class, and the voltage varies from 150 [V] to 450 [V] for small and medium-sized passenger vehicles. Therefore, a suitable V2H system design is required. In addition, the V2H system should be able to supply power to the home in case of a power outage, and considering that the contract power of the household is 3 [kW], a capacity of at least this is required. In particular, in households owning mid-size electric vehicles, the quality of life has improved and the power consumption is rapidly increasing. Considering this situation, a V2H system of 6 [kW] or more is required. Fig. 6 shows the 6 [kW] class V2H system configuration and operation algorithm proposed in this paper. Fig. 6(a), which is a diagram of the V2H system, is largely composed of a non-isolated bidirectional step-up converter, an isolated CLLC converter, and a single-phase full-bridge inverter. The isolated CLLC converter part acts as a high-efficiency DC/DC transformer for isolation, and a non-isolated bidirectional step-up converter controls the DC-Link voltage of the inverter. The single-phase full-bridge inverter controls the output AC voltage in independent operation mode, and controls the incoming power to the grid in the case of grid connected type. As shown in Fig. 6(b), the V2H operation algorithm, the bi-directional step-up converter controls the output voltage of the CLLC converter according to the vehicle ESS voltage through the control of the ratio. When the ESS voltage is 360 [V] or higher, the operation of the step-up/down converter is stopped, and the DC-Link voltage of the inverter is determined by the ESS voltage. The maximum current of the non-isolated bidirectional converter is 20 [A], and when the ESS voltage is 150 [V], it is possible to output 6 [kW] from 3 [kW] to 300 [V] or more. The inverter has a rated output voltage of 220 [V] and a maximum output current of 27 [A]. Even when the ESS voltage is 150 [V], it is possible to output up to 6 [kVA] under the condition that the active power is 3 [kW] or less.

Fig. 6 V2H system block diagram and operation algorithm

3. Simulation and Experiment

3.1 Simulation results

Fig. 7 is a simulation circuit diagram to analyze the characteristics of the V2H power conversion system when applied to non-isolated converters, isolated CLLC converters, and independent and linked inverters. A DLL control block using Visual C++ was used to implement the operation sequence and control algorithm.

Fig. 7 Simulation circuit diagram for V2H power conversion system

Fig. 8 shows the simulation results. In order to analyze the 6 [kW] bidirectional power transfer characteristics, Fig. 8(a) shows the DAB primary voltage, DAB primary current, transformer excitation current, DAB primary average current, DAB primary and secondary DC voltage, and power waveforms. is indicating As shown in the figure, it can be seen that the magnitude of the DAB primary side current decreases when the power direction is changed, and the phase is reversed, and the transformer excitation current is about 5 [%] of the rated current. In addition, it can be seen that the DAB works well as a voltage source DC transformer whose power direction is instantaneously changed by the difference between the primary and secondary DC voltages. Fig. 8(b) shows the waveforms of DAB 1st and 2nd side voltage, DAB 1st and 2nd side current, Gate signal, transformer excitation current, and DAB 1st and 2nd capacitor voltage waveforms. As shown in the figure, the resonance of the CLLC converter is good in synchronization with the gate signal, and the peak capacitor voltage is about 11 [V]. In order to analyze the characteristics of the grid-connected inverter, Fig. 8(c) shows the waveforms of DAB primary side current, grid voltage, grid current, DC_link voltage, and inverter output voltage. As shown in the figure, it can be seen that the grid current and the grid voltage maintain the same phase and become a good sine wave, and it is confirmed that the DC_link voltage has a voltage ripple characteristic of 120 [Hz].

Fig. 8 Simulation results

Fig. 9 is a picture of the V2H power conversion system configuration, Fig. 9(a) is a V2H power conversion system, and Ti's DSP28335 is used as the controller, and the setpoint is performed by Modbus communication, 485 communication. Fig. 9(b) is a picture of the V2H power conversion system configuration for the experiment. It consists of an oscilloscope for instantaneous waveform measurement, a power meter for power and efficiency measurement, a laptop computer that is a high-level controller for remote control, and a graphic LCD for status monitoring.

Fig. 9 V2H power conversion system configuration

Fig. 10 is a waveform analysis of 3 [kW] steady-state independent operation characteristics. As shown in Fig. 10(a), the resonance waveform of the CLLC converter operates in synchronization with the current waveform of the linked inverter, and Fig. 10(b) is an enlarged waveform of the Point_A part of

Fig. 10 3 [kW] steady-state independent operation characteristics

Fig. 10(a). could confirm that zero-current soft-switching was achieved with good resonance.

Fig. 11 shows the starting and stopping characteristics when operating in 3[kW] standalone inverter mode. As shown in Fig. 11(a), the inverter output voltage shows soft starting characteristics, and after the output voltage is stabilized, power is supplied to the load by the output MC. Fig. 11(b) is an enlarged waveform of the initial region where MC is turned on and power is supplied, showing good characteristics without current overshooter of the CLLC converter. Fig. 11(c) is a waveform during stop, and it was confirmed that the system stopped after the steady-state current decreased softly.

Fig. 11 3 [kW] independent starting and stopping characteristics

Fig. 12 shows the steady-state characteristics when operating in 0 [kW] and 3 [kW] linked inverter mode. Fig. 12(a) is a waveform when 0 [kW] relay is connected, and it can be seen that the connection current and the CLLC resonance current are controlled close to zero. Fig. 12(b) shows the waveform when 3 [kW] grid is connected, and it can be seen that the ESS current has a ripple of 120 [Hz]. Fig. 13 shows the system efficiency characteristics including the non-isolated converter, the DAB CLLC converter, and the linked inverter. Fig. 13 (a) shows the system efficiency of 93.9 [%] in the case of connecting 3 [kW] at the input voltage of 180 [V], Fig. 13(b) shows that the system efficiency was excellent at 95.85 [%] when the input voltage was 400 [V] and 6.6 [kW] was connected.

Fig. 12 3 [kW] system connection characteristics

Fig. 13 System Efficiency Characteristics

4. Conclusion

As the spread of eco-friendly electric vehicles expands, there is an urgent need for research on the additional use of ESS for electric vehicles. Electricity use using ESS in electric passenger vehicle can be broadly classified into self-power use and electric power transaction using grid connection. In order to implement these functions, it is a single-phase inverter system that can be standalone or linked.Recently, research on a V2H system, which is a power converter having such a function, is being actively conducted in Japan and Europe.In Korea, too, there is an urgent need to develop a V2H system with various functions in preparation for the implementation of the time zone rate system for home electricity.

In this paper, a CLLC resonant converter that is an insulated bidirectional converter capable of soft switching for reducing switching loss, a half-bridge converter that is a non-isolated bidirectional converter for ESS power control, and a V2H system applied with a single-phase inverter were proposed. In addition, as a result of simulation and experiment, the following conclusions were obtained.

• Since the CLLC resonant converter using a number of turns ratio of 1:1 operates smoothly at the same resonant frequency regardless of the power transmission direction, soft switching is confirmed in all load areas.
• The proposed V2H system input operating voltage is 150-450 [V], and it has been confirmed that it can be safely applied to most passenger car ESSs as it has built-in soft-start and soft-stop functions.
• The proposed V2H system efficiency including CLLC converter for non-isolated converter DAB and linked inverter is 3 [kW] at input voltage 180 [V], output efficiency is 93.9 [%], and 6.6 [kW] at input voltage 400 [V] The output efficiency was 95.85 [%], which was excellent.