The paper presents some problems related to the designing, realization and testing of a PWM inverter implemented in the IGBT technology. The PWM inver... Inverters - Pulse width modulation - Voltage control - Traction motors - Choppers - Acceleration - Insulated gate bipolar transistors - electric vehicles - induction motor drives - insulated gate bipolar transistors - power bipolar transistors - power filters - PWM invertors - regenerative braking - traction motor drives - modern urban transportation system - PWM inverter - IGBT technology - asynchronous motor - electric transportation system - driving system - voltage waveforms - intermediate circuit - DC current - intermediate circuit filter - AC motor drives - inverters - industrial electronics
SPEEDAM 2008 International Symposium on Power Electronics, Electrical Drives, Automation and Motion
Modern Urban Transportation System based on a PWM Inverter P.M. Nicolae*, and D.G. Stanescu** * Faculty of Electrical Engineering, University of Craiova, Bvd. Decebal, no.107, Craiova, (Romania) ** Faculty of Electrical Engineering, University of Craiova, Bvd. Decebal, no.107, Craiova, (Romania) Abstract - The paper presents some problems related to the designing, realization and testing of a PWM inverter implemented in the IGBT technology. The PWM inverter is used for the realization of a driving system with asynchronous motor for electric transportation system. Firstly, the structure of the proposed driving system is described. The links between the control block and the driving system for running and braking are considered. The structure and the manufacturing of the PWM inverter are presented. The intermediate circuit filter is discussed in terms of parameters and designing stage. Some tests concerning the behavior of the entire system are presented. Finally, the registration of the voltage waveforms from the intermediate circuit of d.c. current for cases “forward” and “backward” – for two different situations, are discussed.
II. DRIVING SYSTEM STRUCTURE
Index Terms-- AC motor drives, inverters, industrial electronics
Fig.1. Driving system structure
I. INTRODUCTION The developing of public transportation systems in cities is a practice in many European countries. Its purpose is to reduce the transportation jams and intense pollution. Within this strategy, a significant role is played by the urban transportation systems that use electric energy in electro-mechanic drive systems. An example in this sense consists of the urban transportation systems by trolley-bus, where one can notice a trend in substituting the d.c. drive by a.c. drive. The main advantage of trolley-buses over the buses with Diesel propulsion consists in the reduced gases emissions. The recent progress of power device technology, particularly related to the introduction of IGBT modules in power inverters, favoured the reduction of energy losses and the improvement of performances in terms of converter weight and switching frequency. These performances enhancements have been possible due to the increase of modules current and voltage ranges and to the reduction of their switching times . The trolleybuses driving is presently achieved using the classic solution (through d.c. motors and resistances for velocity control), whose reliability is very poor and provides a low energetic efficiency. The driving alternative that uses choppers, with performances superior to those from the classic solution (d.c. motor and velocity control through resistances), due to the d.c. motors utilization, does not entirely eliminates the potential sources of fault (collector, brushes, contactors) .
The driving system structure presented in Fig. 1 is as follows: 1- network 2- line filter 3- block with diodes and thyristors 4- d.c. link 5- induction motor 6- electronic control block The electronic control block is used to perform: - the interface with the power electric circuits; - the interface with the vehicle (running conditions); - the prescription of electric quantities; - the control and regulation; - the determination of parameters on the vehicle; - the acquisition of numerical signals and generation of numerical commands; - diagnosis and protection; - display; - lighting and acoustic signalization. The links between the control block and power side were supplied through connectors placed so as to provide a maximum access for handling. The control equipment provides pulses to the PWM inverter so as to assure continuous control to two quantities: stator current and rotor frequency.
This work was supported by Romanian National Authorities for Research, The Program ”Partnerships in Priorities Domains”, the VII-th Modul, and The Romanian National Program AMTRANS.
Control electronic block Fig. 2. Relation between the control block and the power part of the driving
Figure 2 presents the links between the control block and the driving system . The entire system requires an information on the velocity, so that the microcontroller should be able to perform a real time computation of the asynchronous motor control frequency as described by :
f N r f 2 'f 2
where f1 is stator supply frequency; fN - frequency proportional to the vehicle velocity; r - “+” for running; “-“ for braking; f2 - frequency of slip; ǻf2 - frequency correction. After the determination of the starting period, when the motor terminals are submitted to the entire line voltage, one performs an automated flow weakening, so that the vehicle might reach full speed . In the brake regime, the control block controls the current through motor and the voltage in the intermediate circuit. With respect to their values the braking can be either electrical-recovering (when the braking energy is transferred in the supplying network), or electrical rheostatic (when there are no consumers on the line and the braking energy is dissipated along the braking resistances through the braking chopper).
- maximum phase current (as RMS value): 410 A; - switching frequency: 2000 Hz; - technology: IGBT; - construction: modular; - cooling: forced; - operating temperature: (-25…40) oC. Traction application basically requires a temperature range of -250C to + 400C. The high maximum ambient temperature limits the available temperature range and dissipatable power for a maximum junction temperature of 12500C. A high potential for power increase is therefore the rise of the maximum junction temperature. The minimum ambient temperature not only influences the choice of materials, but also the available blocking voltage; this voltage decreases for low temperatures as it is shown in the next figure: Vdc [V] 1 700 1 600 1 500
Fig. 3. Blocking voltage vs. temperature for 1700 [V] IGBT
III. DESIGNING AND REALIZATION OF TRACTION PWM INVERTER The inverter was designed considering the following rated data: - supplying voltage: 750 V d.c.; - maximum apparently power: 300 kVA; - maximum active power: 200 kW;
Along with the inverting bridge and the braking chopper, the traction inverter also contains the control unit, the voltage transducers, the smoothing coils and the filtering capacitors. The 6 transistors from the inverter’s three-phase bridge are connected to the 6 PWM outputs of the inverter’s microprocessor.
150 50 IA[YR]
The traction PWM inverter is presented in Fig. 4. The inverter phase output voltage is written as (using the Fourier series):
voltage admitted by the transistor due to the current injected by motor in the intermediate circuit of d.c. current . Figure 6 depicts a solution used for the reduction of this exceeding voltage, consisting in the connection of a resistance in the intermediate circuit through a braking chopper. The processor has an active output PWM called BRAKE, dedicated to the control of the transistor from the braking circuit. The braking chopper is provided with a voltage transducer that triggers the value of the voltage from the intermediate circuit and a current transducer that monitorizes the value of the braking current. The chopper transistor (of IGBT type) is controlled with a constant frequency. The signal has an adjustable filling factor, controlled by microcontroller. Braking chopper has the following components: - braking resistance RF; - transistor realised in IGBT technology; - “snubber” protection circuit, realised with D2, C2, R2; - diode D1 for the introduction of the braking resistance in the corresponding protection circuit; - current smoothing inductivity L1;
v(t ) V0 ¦ 2 VQ sin (QZt MQ )
where Vȣ is the RMS value of the phase voltage for ȣ harmonic, and ĳȣ is the phase angle for the same harmonic (V0 is the continuous component of voltage distorted). The inverter phase output current can be written as a function of the phase voltages and its harmonics :
2 VQ sin (QZt MQ arctg
QX 1 R
UAREF UBREF Fig. 5. Waveforms of currents and voltages at inverter output
Fig. 4. Traction inverter made in IGBT technology
V0 n i (t ) ¦ R Q1
300 450 600 100 150 200 Acquisition time Acquisition Time (ms) IB[YR]
R 2 (QX 1 ) 2
In the following one presents the waveforms of voltages and currents corresponding to the phases A and B from the inverter output operating with the load (fig. 4). The total RMS phase voltage is equal to 230 V and the total RMS value of the phase current is equal to 228.3 A . The output active powers (single phase (P1) and threephase (P) active powers) was calculated based on the next relations:
PWM1 PWM6 PWM3 PWM2 PWM5
P1 V0 I 0 ¦ VQ IQ co sMQ
P1 P2 P3
The graph emphasizes the phase difference equal to 2ʌ/3 between currents and voltages, as well as the phase difference between the current and voltage on the same phase . The currents waveform shape is almost sinusoidal, providing the motor with a stable behavior and the developing of a good torque over the entire frequency range. In some cases, during the braking regime, the d.c. voltage from the inverter intermediate circuit can grow up to dangerous values that can exceed the maxim value of
Brake resistor Brake chopper
Fig. 6. PWM outputs used for three-phase inverter control
voltage Vout1 to the 2 Vdc
load power factor.
For capacitor selection it is necessary to find the point of maximum VSI input ripple current . VSI input ripple RMS current is obtained as follows:
2 I inh
D2 Intermediate circuit filter
Fig. 7. Chopper electric schematic for resistive braking
A special attention was given to the intermediate circuit filter. This filter (Ci, Rd , Ki, and Ri) reduces up to acceptable limits the supplying network pulsations for the running regime and takes the electric motor instantaneous energy in braking regime. It also provides a sufficient number of pulsations whose value does not influence the normal operation of the regulators from the control schematic. The intermediate circuit electric schematic that contains this filter is depicted by Fig. 7. The contactor Ki for loading through resistor from the intermediate circuit is opened at the control voltage apparition and is automatted connected at the brake press or at voltage decrease under the value Ui. It is d.c. link capacitors that account for a major part of the volume, weight and also cost of an inverter. Therefore, reduction of d.c. link size is a very important integration factor . Generally, in such applications the d.c. link capacitor deals with voltage fluctuations due to the source internal impedance and ripple current associated with inverter switching. It is also important to take into account voltage transients due to stray inductance and device switching. Significant reduction of voltage spikes is achieved mostly due to low inductive busbar design which dominates the overall stray inductance. Consequently, the d.c. link design should be based on the inverter input ripple current handling taking into account high ambient temperatures. VSI input rms current value depends on VSI modulation index, output fundamental current and load 2 power factor. The squared d.c. link current ripple ki factor
Converter topologies used in traction are basically determinated by the necessary d.c.-link voltage and the available blocking voltage of the semiconductors. Traction is marked by a wide range of supply voltages. For inner cities light rail 750 V d.c. is the dominating voltage. The tolerance of the supply voltage is usually high, in d.c. grids full power is usually required down to (-30%) and the maximum voltage can reach up to (+25%). A series of tests were made for the motor senses “forward” and “backward”. The waveforms corresponding to the voltage from the intermediate circuit are presented in Figure 8. This figure depicts the recorded waveform for the voltage from the intermediate circuit of d.c. current in the case „forward”, whose value is equal to 750 V. The small deviations of the voltage from the intermediate circuit proves a filter correct sizing. 1000
600 400 200 100 150 200 Acquisition Time (s)
1000 800 600 400 200 50
100 150 200 Acquisition Time (s)
Fig. 9. The voltage from the intermediate circuit of d.c. current in the case „backward”, for increased acceleration
component magnitude of the inverter line to neutral output
Fig. 8. Voltage from the intermediate circuit of d.c. current in the case „forward”.
Figure 9 depicts the recorded waveform for the voltage from the intermediate circuit of d.c. current in the case „backward”, for increased acceleration.
Through the realization of this PWM inverter one intended to obtain a high technology equipment for the trolley-buses driving in order to extend the environment friendly urban transportation systems. The employed technology and tests were in agreement with the operational conditions of the electric driving system that is to be done. The inverter was made considering the following standards: EN 50207; EN 50121-3-1; EN 50124-1; EN 50153; EN 50155; EN 50163; EN 61373; EN 60529.
The range of variations recorded by the voltage from the intermediate circuit varies is closed to that from fig. 8, fact that proves an inverter good behavior. Figure 10 depicts the recorded waveform for the voltage from the intermediate circuit of d.c. current in the case „backward”, for reduced acceleration. 1000 Voltage (V)
800 600 400
100 150 200 Acquisition Time (s)
Fig. 10. The voltage from the intermediate circuit of d.c. current in the case „backward”, for reduced acceleration 
For the reduced acceleration, the maximum magnitude over the angular velocity control and the angular velocity oscillations are almost zero.
V. CONCLUSIONS The recent achievements of power device technology, particularly related to the introduction of IGBT modules in power inverters, provide the reduction of energy losses and the improvement of performances in terms of converter weight and switching frequency. The present solution has a superior energetic efficiency: - the electric braking might be regenerative until stop and the energy dissipation over the braking resistances is no longer present; - the asynchronous motor has identical characteristics for both regimes: operating as motor and during braking; - the asynchronous motor exhibits insignificant losses and no thermal problems; - the recovered braking energy can be used by auxiliary services when no other consumers are present on the line. An increased reliability of this solution for the entire system is obtained because: - the inverter is not provided with mechanical contactors, fuses, moving parts; - the electric braking does not require the reversing of current flow, so no contactors are required. The waveforms corresponding to the voltage from the intermediate circuit are presented in the case „forward”, whose value is equal to 750 V. Also, the voltage waveforms from the intermediate circuit of d.c. current in the case „backward” - for increased acceleration and for reduced acceleration are discussed. As a conclusion, one can mention that for the „backward” sense in the electrical driving case one must use small prescribed accelerations and reduced speeds.
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