Modern Trends of Electric Drives In Electric Traction System SUBMITTED BY:- SURINDER KUMAR B.TECH (E.E.) 100170593752
Introduction• Traction motors can use both ac/dc currents with suitable control electronics. They are operating in high voltage current.• They can widely used for vehicles, elevators, locomotives etc.• Traction motor refers to an electric motor providing the primary rotational torque of a machine.• Traction motors are used in electrically powers rail vehicles such as electric multiple units and electric locomotives.
Requirements of Traction System• High starting pulling effort in order to have rapid acceleration.• Equipment capable of withstanding large temporary overloads for short periods.• The locomotive should be self contained and able to run on any route.• Capability of withstanding voltage fluctuations and temporary interruption of supply.• Parallel running usually more than one motor (two or four motors) should be possible.
Electronic Speed Control Method for DC Motor• Compared to the electric and electro-mechanical systems of speed control, the electronic methods have Higher accuracy Greater reliability Quick response Higher efficiency
Recent Trends in Electrical Traction SystemMulti-Stage Converters:• The multi-stage converter has a chain of 4 converters per phase (4- Stage Converter), with a “Main converter” that manages more than 80% of the total power.• Three “Slave converters” that take the rest of the power (less than 20%).• It can generate almost perfect current & voltage waveforms, because it is modulated by amplitude instead of pulse-width modulation.
Continued…..• It can generate near sinusoidal voltages with only fundamental frequency switching.• It have almost no electromagnetic interference of common mode voltages.• They are suitable for large volt-ampere rated motor drives & high voltages and it is lower frequency devices.
Multistage Converter • S2 and S1’ are switched on the output is –V dc. • Either pair S1 and S2 or S1’ and S2’ are on the output is zero. • S1 and S2’ are on the output is +V dc.
4 Level Multi Stage Converter Main components of Multistage Converter
2-level converter• The two-level converter is the simplest type of three-phase voltage-source converter.• It is a six pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes.• The voltage at the AC output of each phase is switched between two discrete voltage levels.• Corresponding to the electrical potentials of the positive and negative DC terminals.• When the upper of the two valves in a phase is turned on, the AC output terminal is connected to the positive DC terminal.• Resulting in an output voltage of +½ Ud with respect to the midpoint potential of the converter.• When the lower valve in a phase is turned on, the AC output terminal is connected to the negative DC terminal, resulting in an output voltage of -½ Ud.
Three Level Converter• To improve the poor harmonic performance of the two-level converter, some systems have been built with three level converters.• Three-level converters can only two discrete voltage levels at the AC terminal of each phase: +½ Ud, 0 and -½ Ud.• A common type of three-level converter is the diode-clamped (or neutral-point- clamped) converter, where each phase contains four IGBT valves.• Each rated at half of the DC line to line voltage, along with two clamping diode valves.• The DC capacitor is split into two series-connected branches, with the clamping diode.• To obtain a positive output voltage (+½ Ud) the top two IGBT valves are turned on.• To obtain a negative output voltage (-½ Ud) the bottom two IGBT valves are turned on• To obtain zero output voltage the middle two IGBT valves are turned on
Modular Multi-Level Converter• The MMC differs from other types of converters.• In that the current flows continuously in all six valves of the converter throughout the mains-frequency cycle.• As a result, concepts such as “on-state” and “off-state” have no meaning in the MMC.• The direct current splits equally into the three phases.• The alternating current splits equally into the upper and lower valve of each phase.• The current in each valve is therefore related to the direct current Id and alternating current Iac as follows:
Advantages• It can generate perfect current & voltages waveforms.• The power losses into the machine due to harmonic current are also eliminates.• Reducing power losses at minimum value.• Power rating of semiconductor is very small.• Multi stage converter working at low switching frequency.• Operation is fast & quick response.
Applications• Active power filters.• Sinusoidal current rectifiers.• Machine drives.• Power factor compensators.• Back to back frequency link system.• Traction drive system.