Design and Implementation of a Variable-Frequency Drive Using a Multi-Level Topology

Transcription

1 University of Manitoba Department of Electrical & Computer Engineering ECE 4600 Group Design Project Project Proposal Design and Implementation of a Variable-Frequency Drive Using a Multi-Level Topology by Group 03 Kale Ewasiuk Curtis Shumski Edwin Ifionu Matt Szyda Academic Supervisor(s) Shaahin Filizadeh Industry Supervisors Steven Howell Manitoba Hydro Date of Submission September 26, 2014 Copyright 2014 Kale Ewasiuk, Edwin Ifionu, Curtis Shumski, Matt Szyda

2 CONTENTS Contents 1 Introduction Project Details Implementation Topology Description Simulation Component Values Selection User Interface and Control System Switches and Gate Driver Circuitry Sub-Module Capacitor Voltage and Arm Current Measurement System Future Provisions Specifications Division of Labour GANTT Chart Budget Conclusions References

3 1 Introduction 1 Introduction The development of fully controlled power electronic converters has led to improvements in variable-frequency drive (VFD) performance and efficiency. Voltage-source converters (VSC) are a popular type of VFD that use power electronic switches and capacitive energy to emulate an alternating output voltage from a dc-link. Common VSC topologies use pulse-width modulation (PWM) with two output levels to achieve an alternating voltage with a desired magnitude and fundamental frequency. The concept of voltage-source conversion can be extended to produce a staircase waveform with multiple output voltage levels. The purpose of this project is to design and implement a small scale VFD using a multi-level VSC topology. Multi-level converters have shown several advantages compared to conventional two-level VSCs. Benefits such as improved output waveform quality, reduced switching frequency, and lower levels of produced interference have been demonstrated[1]. The converter topology to be designed is the modular multi-level converter (MMC) [2, 3]. The MMC will be used for its scalability and simplicity of topology compared to the other multi-level converters. The MMC allows for increasing output levels by adding identical sub-modules (SM)[3] which can reduce switch and capacitor voltage ratings and improve harmonic performance. This project will contribute to the practical application of the MMC as it is a relatively new topology that has grown in popularity over the past few years. 2 Project Details This section gives specific details of how the project will be implemented, as well as the system components needed to meet project objectives

4 2 Project Details 2.1 Implementation A basic version of the converter will first be implemented by testing a single SM. Sub-modules will then be cascaded and their individual control will be tested. Half-bridge SMs will be used because they allow for simpler control schemes and fewer components compared to full-bridge SMs. The control system will be simultaneously developed as the circuit complexity and functionality increases. Real-Time Digital Simulator (RTDS) will be used to simulate firing-pulses to allow for independent development of the control system if required. The MMC and control system will be integrated and used to drive an induction motor with open-loop controls upon successful test results. The performance of the converter will be tested under various load conditions which can be set by a dynamometer. The system overview is shown in Figure 1. Modular Multilevel Inverter + V dc Sub-module capacitor voltages and arm currents.... C dc C dc Valve controllers (VC) 2... Gate drivers, deadtime generators, isolators Half-Bridge Sub-Modules (SM) Reference wave controller (RC) Frequency and voltage control and display UV LV L arm L arm i ac Motor speed and voltage Speed and voltage controller (MC) Induction Motor 175 W 120 V RMS + v ac - M Speed display and load torque control Speed and voltage control and display 3-phases and closed-loop speed control if time permits Fig. 1: MMC circuit and control system overview - 3 -

5 2 Project Details 2.2 Topology Description In steady state, the converter operates by switching SMs in the upper and lower valve (denoted UV and LV on Figure 1) such that the potential v ac in between them is alternating as described in equation 1. v ac = v l v u 2 L arm 2 di ac dt (1) If there are N SM capacitors C inserted in the converter at all times, then their nominal voltages should be the same at V dc /N. As current is drawn from the converter, the voltage in each SM capacitor changes. The arm inductors L arm are used to limit unwanted harmonics in current deriving from the voltage imbalance between the dc-link voltage V dc and the sum of the upper and lower valve voltages. Balanced capacitor voltages will result in minimal deviation from a staircase waveform and reduced harmonic content in the arm currents [4]. 2.3 Simulation A simulation case will be developed as the project progresses to ensure that the design choices made are met. Power systems CAD (PSCAD/EMTDC) will be used to simulate a high-level representation of the circuit topology and control system with the goal of accurately reflecting the designed system. A key part of the converter simulation is to aid in component selection decisions. 2.4 Component Values Selection The components in the MMC circuit to be selected are the sub-module switches and capacitors, dc-link capacitors, and arm inductors. The voltage ratings of the components depend on the dc-link voltage and the number of active SMs in the converter. The dc-link capacitors will be selected to allow for an acceptable sag in voltage. The SM capacitor and arm inductor will be chosen based on equations developed in references [5] and [6] respectively. The selected component values will - 4 -

6 2 Project Details be finalized depending on their performance through simulation and availability for purchase. 2.5 User Interface and Control System An interactive control system will be implemented for the design of the overall system. The user is allowed two inputs via a 4x4 keypad; the ac-voltage magnitude and frequency. This information is sent to a reference waveform controller (RC) in the form of a modulation index and reference frequency. The reference controller calculates and passes a time-varying reference wave with the integer number of SMs to be inserted in the upper and lower arms to their respective valve controllers (VC). The VCs will perform a sort and balance routine that is used to determine the correct number of SMs to be inserted based on their respective voltage levels and direction of current. The VCs will then be responsible for sending the firing signals to the respective gate drivers. 2.6 Switches and Gate Driver Circuitry The gate driver circuit is the interface between the control system and the high power switches of the converter. MOSFETs have been chosen over IGBTs for the design due to their efficiency at lower power levels[7]. A gate driver circuit with a floating channel will be used to alleviate changing MOSFET source voltages and allow for bootstrap operation. Optocouplers will be used between the controller and the other gate driver circuitry to provide electrical isolation from the high voltage gate driver circuit. A dead-time generator will be used to delay the firing pulse of each SM switch to avoid short circuiting during finite switching time. 2.7 Sub-Module Capacitor Voltage and Arm Current Measurement System The voltage across the capacitor in each sub-module has to be measured continuously to determine the switching routine. The voltage will be measured with a voltage divider or DC/DC converter to produce a scaled voltage with a maximum output of 5V (logic level). The valve - 5 -

7 3 Specifications controller will serially acquire the measured voltage through an analog multiplexer. The current through each arm will be measured with a hall effect sensor with a digital output directly connected to the valve controller. 2.8 Future Provisions The system design will allow for additional features such as scaling up to a three-phase converter with a closed-loop speed control system. The controls will include functionality that allows for user input of speed; the main controller determines the appropriate magnitude and frequency of the output ac voltage that will achieve the desired motor speed to a certain accuracy (that is not specified at this time). 3 Specifications The designed converter is required to meet the specified distortion level under the nominal conditions listed in Table 1. The properties and ratings of the motor to be driven is listed in Table

8 3 Specifications Table 1: Project specifications Paramater DC-link voltage Value or Range 350 V Converter requirements Nominal power output Nominal voltage range Nominal frequency range 100 W V RMS Hz Total distortion for harmonic orders < 15 th 3% Number of sub-modules per arm 3 (minimum) Table 2: Motor properties Motor properties Model Lab-Volt Type Power rating Voltage rating Operating frequency Maximum speed Single-phase capacitor start 175 W 120 V RMS 1-60 Hz 1715 rpm - 7 -

9 4 Division of Labour 4 Division of Labour The tasks for this project have been divided into four sections: simulation, circuit, control, and user interface. Each section has several milestones that are associated with it. Table 3 shows how the milestones have been divided between the team along with an estimated completion date. Table 3: Division of labour Simulation Task P.I.C. E.D.O.C Development of basic simulation model with capacitor voltage balancing K & M 09/28/2014 Development of advanced simulation model with converter controls E & K 01/19/2014 Circuit Selection and implementation of passive components, switches and gate driver circuit Implemention of a working cascaded sub-modules prototype with batteries as capacitors Design and implementation of arm current and capacitor voltage measurement system Physical building, testing, and troubleshooting of the single-phase converter Controls K & M 12/10/2014 K & M 11/23/2014 E & K 12/22/2014 ALL 01/18/2015 Design of control system C & E 02/11/2014 Implemention of reference controller with variable voltage and frequency C & E 11/16/2014 Implemention and interfacing valve controller with cicruit measurement system E & M 12/22/2014 Interfacing of control system with gate-drivers C & M 01/11/2015 User Interface Implementation of converter voltage and frequency control and display C 12/22/2014 Design Expansions Design and implementation of three-phase converter ALL 08/02/2015 Design and implementation of motor speed closed-loop control system ALL 02/15/2015 NOTE: C = Curtis, E = Edwin, K = Kale, M = Matt P.I.C = Person(s) in Charge, E.D.O.C = Expected Date of Completion - 8 -

10 - 9-5 GANTT Chart The GANTT chart shown in Figure 2 divides the project milestones into more detailed sections with an estimated work timeline. Fig. 2: GANTT Chart for converter design project. G03 Proposal GANTT Chart

11 6 Budget 6 Budget The proposed budget for the project is shown below in Table 4. The predicted costs are based off of preliminary research, and both cost and parts are subject to change. The approved budget for the project is $400.00, and any additional costs will be provided by team members, the project supervisor Dr. Shaahin Filizadeh, or further contributions from the Department of Electrical and Computer Engineering

12 6 Budget Table 4: Estimated budget Half-bridge switches (includes diodes and mosfet) IRFI4019HG-117P Digikey 10 $4.72 $47.20 Gate drivers IR2110STRPBF Digikey 10 $4.30 $43.00 Gate driver capacitors N/A U. of Manitoba 20 $0.00 $0.00 DC/DC converter for gate supply VIBLSD1-S5-S15-SIP Digikey 10 $5.78 $57.80 DC/DC converter for logic supply VIBLSD1-S5-S5-SIP Digikey 10 $5.78 $57.80 Optocoupler H11L2SM Digikey 10 $1.34 $13.40 Dead-time generators (3 phase) IDXP630PI Digikey 4 $5.33 $21.32 Controls components Reference controller PIC16F877A Digikey 1 $7.10 $7.10 Valve and speed controllers PIC16F87 Digikey 2 $3.50 $7.00 PICkit-3 in-ciruit debugger PG Digikey 2 $57.40 $ pin connector header GRPB061VWVN-RC Digikey 2 $0.75 $1.50 Measurement components Analog multiplexer ADG5408BRUZ Digikey 2 $7.28 $14.56 Resistors N/A U. of Manitoba 20 User interface LCD screen (for speed display) Item P/N Supplier QTY Cost Sub- Total DC-link capacitors WBR A Digikey 2 $12.70 $25.40 Sub-Module capacitors UHE2A221MHD Digikey 10 $1.04 $10.40 Arm inductors C-49U Digikey 2 $38.36 $76.72 NHD-0216XZ-FSW- GBW Digikey 1 $13.17 $13.17 Keypad Digikey 1 $6.99 $6.99 Equipment Single-phase induction motor Lab-Volt U. of Manitoba 1 $0.00 $0.00 Dynamonmeter Lab-Volt U. of Manitoba 1 $0.00 $0.00 Other Contingency $50.00 $50.00 Machine shop time 8 hrs Subtotal $ Taxes $73.86 TOTAL $

13 7 Conclusions 7 Conclusions This proposal discussed the basic theory and the design of an MMC for the application of a VFD. The proposed design incorporates user input which allows the product to be used for various applications that require different voltages and frequencies. The proposed design allows for expansion to a three-phase MMC with advanced closed-loop controls

14 REFERENCES References [1] L. Tolbert, F. Z. Peng, and T. Habetler, Multilevel converters for large electric drives, Industry Applications, IEEE Transactions on, vol. 35, no. 1, pp , Jan [2] A. Lesnicar and R. Marquardt, An Innovative Modular Multilevel Converter Topology Suitable for a Wide Power Range, in Power Tech Conference Proceedings, 2003 IEEE Bologna, vol. 3, June 2003, pp. 6 pp. Vol.3. [3] R. Marquardt, in Power Electronics Conference (IPEC), 2010 International, title=modular Multilevel Converter: An Universal Concept for HVDC-Networks and Extended DC-Bus- Applications, June 2010, pp [4] D. Siemaszko, A. Antonopoulos, K. Ilves, M. Vasiladiotis, L. Angquist, and H.-P. Nee, Evaluation of control and modulation methods for modular multilevel converters, in Power Electronics Conference (IPEC), 2010 International, June 2010, pp [5] K. Ilves, S. Norrga, L. Harnefors, and H.-P. Nee, On energy storage requirements in modular multilevel converters, Power Electronics, IEEE Transactions on, vol. 29, no. 1, pp , Jan [6] Q. Tu, Z. Xu, H. Huang, and J. Zhang, Parameter design principle of the arm inductor in modular multilevel converter based hvdc, in Power System Technology (POWERCON), 2010 International Conference on, Oct 2010, pp [7] A. Dubhashi and B. Pelly, A comparison of igbts and power mosfets for variable frequency motor drives, in Applied Power Electronics Conference and Exposition, APEC 89. Conference Proceedings 1989., Fourth Annual IEEE, Mar 1989, pp