DSP BASED CONTROL OF MULTISYSTEM POWER CONVERTER FOR PASSENGERS' COACHES Nenad Težak, KONČAR - Electrical Engineering Institute Fallerovo šetalište 22, HR-10000 Zagreb, Croatia
[email protected] Abstract. The paper deals with the DSP based control of multisystem power converters for passengers' coaches that circulate throughout Europe's railway network, and are designed to provide power supply on several standardised UIC voltages. It also contains description of implemented adaptive control structure needed in order to meet harsh operating conditions on-board rolling stock, as well as demanding technical requirements on power-quality characteristics of the converter in total. Adaptive control algorithm, with respect to detected power supply system at converter's input, reconfigures its power and control structure, selects matching control algorithm and takes care of proper, diagnostics supported, self-calibration and measuring range selection of A/D sections, during start-up sequence. Keywords. Adaptive control, power conditioning, power supplies, DSP, traction.
1. INTRODUCTION In order to accomplish versatile power supply for passengers' coaches that circulate throughout the Europe's railway network, a multisystem power converter (Fig. 1) is designed in such a way that it operates on all four standard UIC voltages (3000VDC, 1500VDC, 1500VAC 50Hz and 1000VAC 16 2/3 Hz). All equipment installed on-board rolling stock should be designed in order to meet harsh operating conditions regarding shock and vibrations resistance, extended temperature range operation (from –40 to +85 °C) etc. Additionally, power supplies for consumers in modern passenger coaches should comply to very demanding EMC related technical requirements as well as to power-quality related characteristics of the converter in total. Since the technical requirements on converter's performance differ significantly for stated AC and DC power supply systems, adaptive control structure was implemented to match all stated demands.
Multisystem static power converter type VIS50-1 has modular design as illustrated on its simplified schematic diagram (Fig. 2), which indicates basic structure of converter's power stage, as well as its inputs and outputs.
Fig. 2. Simplified schematic diagram of multisystem static power converter type VIS50-1 Adaptive control algorithm, with respect to detected power supply system at converter's input, reconfigures accordingly its power and control structure, selects matching control algorithm and takes care of proper, diagnostics supported, self-calibration and measuring range selection for data acquisition (including hardware section for signal conditioning for A/D sections and software part of A/D drivers, incorporated within application program), during start-up sequence.
Fig. 1. Multisystem power converter type VIS50-1, installed on-board passenger coach of Croatian railways (HŽ)
More detailed description of the power converter's topology, its basic operating modes and some hardware specific issues are discussed in [1].
2. CONTROL SYSTEM Since the technical requirements on control structure differ a lot on each of the previously stated UIC voltages, adaptive control strategy is considered necessary. Furthermore, digital implementation of necessary control algorithms is rather time-consuming, setting higher demands on the computing power. Thus, a DSP based controller, embedded in dedicated double euro-card module type DSP1 (Fig. 3) was employed for hardware implementation of the control system of the multisystem static power converter VIS50-1.
their final implementation in hardware and/or software and commissioning of the prototype. When presence of high-voltage is detected at the input to the power stage of converter, a dedicated circuitry determines the type of power-system converter is connected to. After confirmation that valid power supply system is detected, master controller sets the corresponding configuration of power stage of the converter, and subsequently instructs DSP based controller to set its configuration accordingly, including the adjustment of signal conditioning and measuring sections as well as selection of appropriate control structure, so that commissioning sequence can start. During the startup procedure auto calibration of signal conditioning and measuring sections on all channels is performed, in order to cancel the unwanted offset that could result in intolerable side effects during operation of power-converter. The control structure basically differs for AC and DC power supply systems according to the different set of requirements for these two types of power supply systems. When supplied from the AC network, total power factor should be maintained higher then 0.95 over a wide range of operating load. Therefore, a strong correlation between input voltage and current is needed, even for the square input voltage!
Fig. 3. Dedicated DSP based controller type DSP1 for hardware implementation of the control system for the multisystem static power converter All necessary control algorithms in this project were developed using specialized simulation software package for power electronics – Simplorer (Ansoft, for more details please see http://www.ansoft.com/products/em/simplorer/), which simplifies significantly and speeds-up analysis and design of such complex power converters together with corresponding control structure. Using different simulation models (Fig. 5), with respect to their complexity, all characteristic operating modes were explored in detail, proofing selected control methods before
Fig. 4. Waveforms of input current and input voltage of the VIS50-1 converter when supplied with square input voltage
Fig. 5. Simulation model of multisystem static power converter with corresponding control structures for AC (upper window) or DC (lower window) power supply system connected at converter's input, developed by specialized simulation software for power electronics and control – Simplorer (Ansoft)
Strong correlation between input voltage and current (Fig. 4) is achieved by deriving a current reference value directly from filtered waveform of the input voltage, together with phase-lag compensation caused by data acquisition of feedback signals. Additionally, proper operation on AC supply network requires minimal DC component of input current to the power converter, in steady state operation. For example, when converter is powered from 1500VAC 50Hz network, nominal input current is about 35 ARMS, while allowed DC level of steady state input current is about 40 mARMS! In order to achieve this goal regular auto-calibration of analog inputs during the start-up sequence was employed. In this way offsets of A/D channels are minimized, while at the same time improving system's reliability by indicating eventual malfunction of signal conditioning and measuring sections. It turned out that auto-calibration of signal conditioning and measuring sections on all channels was crucial in attaining extremely low DC components of input currents to power converter supplied from AC railway network, even at the full scale of extended temperature range (from –40° to +85°C). In order to improve dynamic characteristics of the system at sudden changes of input voltage while having approximately constant load attached to converter's outputs, feed-forward compensation with respect to the inverted mean value of the input voltage was introduced.
However, when attached to DC network total power factor leaves the focus of interest and is immediately replaced by input impedance, i.e. highest possible resistance to injected higher harmonics. This feature is very important because insufficient resistance to injected higher harmonics could accidentally trigger the security and signaling devices along the railway tracks [2]. In order to improve overall dynamic behavior of the system, including its resistance to network disturbances (e.g. network load and/or voltage variations) during normal operation, additional feed-forward signals were introduced in all AC- and DC-control structures. 3. HARDWARE & SOFTWARE IMPLEMENTATION The DSP based controller was designed around the Texas TMS320F240 processor, and its hardware and software. Its operating system, firmware for PLD's as well as the application program and all necessary development tools were completely developed within Končar-IET company. The DSP based controller is equipped with versatile peripherals (digital inputs/outputs, analog inputs/outputs, PWM outputs etc.) according to the block diagram (Fig 6.). One of the key features that enabled fulfillment of numerous demands on EMC related and power-quality
TMS320F240 DSP
Fig. 6. Block diagram of the DSP based controller type DSP1 of the multisystem static power converter type VIS50-1
characteristics of the multi-system static power converter is 9-channel PWM unit for phase-shifted control of input boost converters and synchronous control of high-frequency inverters. In order to improve resolution utilization (to achieve higher precision of measurements) on all signal measurements, adjustment of measuring range was done on all channels, according to the detected type of power supply system. To keep the processor's load at more or less similar level, switching between control structures is implemented by conditional execution of corresponding sections of DSP's application program, while preserving capability of individual tuning controller's parameters for each of the supported power supply systems at converter's input. From diagnostic point of view, DSP controller with its built-in multi-channel recorder, whose trigger can be configured to start recording of internal variables manually/event-driven/ automatically, with/without pretrigger, significantly enhances troubleshooting capabilities of the system during commissioning or exploitation period.
4. CONCLUSION Employing dedicated DSP based controller, in order to implement the control algorithm for smooth operation of multisystem power converter, provided (together with inhouse developed operating system and block oriented graphical programming tools for application program development) the flexible, high-performance hardware/software platform, for development of control structure to meet numerous demands on the operation of the system as a whole, supporting its ability of self-adaptation to various operating conditions, extending its diagnostic features (through built-in multi-channel recorder with pretrigger capabilities, that eases troubleshooting) and increasing its overall reliability. Smooth operation of more than 30 units of power converters onboard passenger coaches in Croatian railways, prove that the implemented adaptive control structure was appropriate for this application. 5. REFERENCES [1] Magzan, A.; Rajković, B. & Ungarov J.: Research into possibilities to increase input impedance of voltage and frequency converters fed from DC network. Proceedings of 10th EDPE conference in Dubrovnik (Croatia), 1998, pp. 86-91 [2] UIC-Kodex, 550 VE. (1997): Elektrische Energieversor-gungseinrichtungen für Wagen der Reisezugwagen-bauart, 10. Ausgabe vom 01.01.97. Bureau Central de UIC, Paris.
