High Power 3-level Power Conversion System for Renewable Energy Nano-grid_Yang Jiao

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Hi everyone, this is Yang Jiao in this video nugget, I will introduce a high power high frequency 3-level neutral point clamped power conversion system for renewable energy and nano grid applications First, let me briefly review the system structure This is a 2-stage power conversion system it contains a DC/AC converter and a DC/DC chopper The AC side is connected to 480V or 600 V Utility grid and the DC side can be connected to 400-800V DC bus The AC and DC side is decoupled by the 1200V DC link The whole system has 200kVA power rating with 20kHz switching frequency It has bidirectional power flow with 4-quadrant operation capability This power conversion system can serve as grid interface for renewable energy or energy storage system It can also work as power supply for varies loads or drive motor in industrial or transportation applications The whole system has modularized structure and over 98.5% efficiency with high power and high frequency The modularized phase leg building block uses 3 layers of laminated bus bar as 3-level rail to reduce the parasitic But the parasitics in switch module and inter connection still results in two different switching loops The short loop contains 2 modules and 1 connection while the long loop contains 4 modules and 3 connections The 4-quadrant switching loop for DNPC varies with power factor To avoid the long switching loop with larger parasitic conventional ANPC phase leg can be used with only short switching loop To further improve the switching performance I proposed a new switching mode for ANPC The 4-quadrant switching loops use paralleled long loop and short loop to reduce the commutation current This proposed method has reduced switching stress and loss The maximum switching stress for each device in the phase leg is measured for different modulation scheme The proposed method has lower voltage and current stress compared with DNPC and conventional ANPC Moreover, the switching stress is distributed more evenly among all the devices The phase leg loss distribution is also compared for the conventional and proposed ANPC modulation The result shows the proposed method has more evenly distributed loss for each switch Finally, the total system loss for the converter is compared for conventional and proposed ANPC modulation The proposed method has largely reduced conduction loss because of the paralleled neutral current path As a result the efficiency for the whole system is up to 98.5% This is the modularized phase leg building block It uses 1200V/400A single IGBT module The gate drivers are customized for switching performance optimization They are interfaced with the controller by fiber optics and an interface board Here is the 3-level NPC power conversion system hardware The left-hand side of the cabinet contains the DC/AC inverter and the right hand side is the DC/DC chopper The modularized phase leg building blocks are mounted on the top and the filter and auxiliary circuit sit on the bottom The control boards are installed on the back of the cabinet The DC/AC power stage contains 3 phase leg building blocks and an optimized and customized LCL filter The passive damping circuit for filter has very low loss The common mode chokes are placed at both inverter side and grid side There are also pre-charge circuit, sensor board and fuses as auxiliary circuit The layout of the DC/DC power stage is similar to that of the DC/AC part The integrated coupled inductor for the DC output is a very novel design to reduce the volume of passive component The common mode choke at DC side suppress leakage current and avoid the large transformer There are also pre-charge circuit to establish DC link voltage from DC side The whole power conversion system is controlled by a modularized digital controller system This is the control card with DSP and AD chip This part is the modularized signal conditioning circuit . There are some flexibly configured plug in board for signal input, output and display This is the Ethernet card for human machine interface This is the modularized gate driver interface board for DC/AC and DC/DC Here shows some experimental waveforms of the power conversion system First shown here is the test result for the DC/AC part The converter is running at close loop rectifier mode The output current and DC link voltage is controlled The green colored waveform is the line to line voltage and the other three sinusoidal waveforms are the 3 phase grid current waveform This waveform shows the test result of the DC/DC part. It is running at interleaved buck mode The phase leg output voltage is in green color. It is a three-level output voltage These are the two phase inductor current in the coupled inductor They contain both common mode and differential mode component The yellow waveform is the total output current waveform This screen shows the human machine interface which is used to control the system and display the system variables The start command can be send to the 2 stages separately The running status like PLL state, contactor state are indicated The key variables like AC grid voltage and current, DC link voltage, active and reactive power DC side voltage and current are also reflected by the human machine interface So that’s all for my video nugget. Thank you for watching.

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