Design of DC-DC Converter for LV Battery Charging for Renewable Energy Applications

Pakistan is facing power shortages in the access of at least 4000 MW, and this shortage increases as the demand for the power grows. The power shortage is because of higher power demand, and because of transmission losses in the distribution lines as the power generated is delivered to far off areas

Project Title

Design of DC-DC Converter for LV Battery Charging for Renewable Energy Applications

Project Area of Specialization

Electrical/Electronic Engineering

Project Summary

Pakistan is facing power shortages in the access of at least 4000 MW, and this shortage increases as the demand for the power grows. The power shortage is because of higher power demand, and because of transmission losses in the distribution lines as the power generated is delivered to far off areas using these lines. One possible solution to the power shortage problem is to take advantage of solar power. Solar power is free, and is a good alternative owing to its pollution-free nature, one-time installation and low maintenance cost. Solar power is particularly suitable for Pakistan as it gets increased exposure to the sun throughout the year.

Most of the recently merged areas (formerly known as Federally Administered Tribal Areas) and some areas in northern Khyber Pakhtunkhwa are away from the National Grid. Connecting these areas to the National Grid will not be easy as it requires the transmission lines have to be laid first which require a huge capital cost. If small Photovoltaic (PV) power generation systems are established in the areas needing electric power, there will no need for the transmission lines too, thus reducing the cost and the losses in the system.

The purpose of the work is to propose a PV system with greatly improved power conversion efficiency and a cloud-based control system.

The proposed PV system consists of the following three sub-modules,

a) High-Voltage (HV) High-Power (HP) power converters to generate 220V AC voltage from the DC power extracted from the PV panels

b) Embedded system to control the power converter as well as extract maximum power available from the PV panels

c) Cloud-based system for remote telemetry and control of the PV System

The HV HP power converters consist of the following three power converter,

i) DC-DC power converter with Maximum Power Point Tracking (MPPT)

ii) DC-DC LV Battery Charger

iii) DC-AC Inverter

The scope of this project is to design a DC-DC LV battery charging system for the PV system. This converter is capable of charging an LV battery (12V, 24V and 48V) of a 350V DC voltage. In this project, Infineon C7 Mosfets based full-bridge phase-shift converter operating at a switching frequency of 50kHz is used. To reduce the Electromagnetic Interference (EMI) noise, a Zero-Voltage Switching (ZVS) techniques will be used.

Project Objectives

The power converters in the PV system take the DC power from the PV panels and converts it into AC power. There are three power converters in the PV system. The first one is responsible for extracting maximum available DC power from the PV panels, and converts it into HV DC voltage. The second power converter is responsible for charging the LV batteries. The last power converter is responsible for converting the HV DC into AC power. All these three power converters need embedded systems to generate PWM signals and to implement the controllers to regulate the output voltage and power.

The scope of this project is to design a DC-DC LV battery charger for the PV system. It is represented by a light green box in the following figure,

The main objectives of this project are to,

i) Efficiently charge the LV battery of a DC voltage

ii) Be able to regulate the output current for a range of battery voltages (12V, 24V, 48V)

iii) Be able to regulate the output current even if there is variation in the input voltage because of shading on the PV panels.

iv) Measure the output voltage and current, input voltage and current and output power for control and telemetry purposes.

v) The peak-to-peak ripple in the output current is as small as possible.

vi) Be able to turn off the system as fast as possible in case of a critical fault.

Project Implementation Method

The hardware for the proposed DC-DC LV battery charger consists of,

a) The power-stage made up of single-phase bridge circuit on both sides of the transformer.

b) Sensors for measurement of input and output DC voltages and currents.

c) Embedded system implemented on Texas Instruments (TI) Delphino Microcontroller (TMS320F28379D) Launchpad.

In this project, the power-stage for the DC-DC LV battery charger will be designed. With the Pulse Width Modulation (PWM) and Phase-Shift control techniques, the power converter will be able to charge the LV batteries at the set current. It can also deliver the power from the LV batteries to the DC-AC inverter.

Project Implementation Stages:

Simulations:

The HV HP power-stages and the embedded system for the power converters are developed in parallel. These power converters are first simulated in PSIM software to validate the design. The controllers are designed for the power converters to regulate the output current and voltage at any battery voltage. The design of the controllers is also validated in simulations.

The simulations are used to obtain the dummy data for the Firmware development. The simulation results give the upper and lower limit for the PWM frequencies, duty-cycles and phase-shifts. It also provides the waveforms of the sensed data.

Schematics and PCB Design:

The simulation results give all the information about all the components in the power-stage. Based on the simulation results, components are selected. The schematics for the power-stage is designed in Altium. After carefully designing the schematics, the Printed Circuit Board (PCB) is designed for the power-stage. Since this power-stage has to process very high voltage and power, special care has to be taken. The creepage and clearance of the traces are checked.

Once both the schematics and PCB designs are validated, PCB design files are sent for fabrication.

Low-Voltage (LV) and Low-Power Testing of the PCB Boards:

Once the PCBs are fabricated, the components are soldered on the board. The LV circuits like the power-supplies, gate-drives and sensors are tested first. Once all the LV circuits are tested at all corner cases, the boards are ready for low-power testing. The output load is kept at a minimum, and the input voltage is increased in small steps. After these LV and low-power tests, the boards are ready for high-power testing.

High-Power Full Load Testing:

After validating the power-stage design at low-power, the boards will be tested at high power now. The output power is increased in small steps up to full load. The sensors are tested at full load. With these tests, the power-stage design is validated.

Benefits of the Project

The benefits offered by the designed PV system are,

i) It is indigenously developed.

ii) It has higher end-to-end power efficiency as compared to the PV systems available in the market

iii) It is compact, has a longer life and lower cost as compared to the PV systems available in the market

iv) It is ‘smart’ in the sense that after detecting the shading on the panels, it can adjust the output charging power

v) With special system-level control strategies, the PV system can support temporary overload conditions during the daytime by getting the power from the LV batteries. This feature is currently not available in the commercially available PV systems.

vi) It has a Cloud-based telemetry and control system. With this feature, the PV system can be monitored and controlled from anywhere in the world.

vii) With this project, a state-of-the-art research facility has been established where the students conduct research in Renewable Energy Systems.

Technical Details of Final Deliverable

The end product of this project is a 1kW prototype PV system for residential applications. The designed system will have three power converters capable extracting maximum power from the PV panels, charging LV Batteries, and providing AC power.

The target end-to-end power efficiency is more than 75%. This system will be able to send telemetry data to the Cloud (via GSM and Wifi modules) and to User (via Bluetooth module). It will be able to receive configuration setting from Cloud (via GSM and Wifi modules) and from User (via Bluetooth module).

Final Deliverable of the Project

HW/SW integrated system

Core Industry

Energy

Other Industries

Core Technology

Cloud Infrastructure

Other Technologies

Internet of Things (IoT)

Sustainable Development Goals

Affordable and Clean Energy, Climate Action

Required Resources

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