Objeciones a la Resocialización a través de la cárcel

The Voltage Source Converter (VSC) technology is widely used in industry for low and medium voltage drives. Figure 2.2 shows a schematic of the VSC drive topology.

The VSC drive system comprises of an input PWM filterPW M_Filter and PWM ripple

current limiting reactorL_pwm on the main ac input, an input rectifierV SRwhich can

be in the form of a diode rectifier or Thyristor rectifier or Active Front-End rectifier

(AFE), a dc link capacitorC_dc (energy storage element) that provides low-impedance

voltage source characteristics at the inverter’s input, an output PWM inverterV SI

which connects to the machine. Very fast switching power electronic devices such as

~ Filter PWM dc C VSR _VSI M pwm L

Fig. 2.2 Voltage Source Converter Topology

MOSFETs and IGBTs are employed owing to the high PWM switching frequencies of several kilo-Hertz often used in VSC drive systems. Often passive ac line filters are required to absorb the high frequency PWM harmonics generated by the AFE rectifier. The size of the ac filter is often dictated by the PWM frequency employed, higher PWM frequencies result in smaller ac filter footprint. Often, a tradeoff is required on the choice of switching frequency between minimizing input filter size and converter switching losses. On the machine side, the machine reactance is in general sufficient

2.2 Current Source and Voltage Source Converters

to effectively filter the output inverter PWM voltage and yield very good sinusoidal motor current waveform. As a result, output sinewave filters are rarely used with VSC fed machines and very low torque pulsations can be achieved if the PWM switching frequency is sufficiently high, 1-5kHz being typical for high power drives.

Owing to the use of a dc link capacitor, the capacitors provide the instantaneous current to the inverter required in high dynamic systems, and therefore enables high bandwidth current control of the VSC topology. As a consequence, this topology is widely used in applications that require high dynamic performance. Unlike CSC fed drives, the converter output voltage is independent of machine load conditions and is dependent on the dc link voltage and PWM modulation depth.

A key drawback of the VSC topology is the highdv_dt of the synthesised PWM voltage

waveforms which requires higher insulation systems on machines and transformers

connected to the VSC drive system. Another undesirable consequence of the high dv_dt

is the increased common mode currents which can cause system Electro-Magnetic Interference (EMI) issues if not adequately addressed.

In comparison to VSC drive topology, the CSC drive generally features simple converter structure, motor friendly waveforms, inherent four-quadrant operation capa- bility and reliable fuseless short-circuit protection. However, the cost, size, and weight of electrolytic capacitors employed in VSC tend to be significantly lower than DC link inductors employed in comparably rated voltage and current source configurations. Furthermore, new generations of gate-controlled power switches that lack reverse voltage blocking capability tend to be more naturally suited to voltage source inverter requirements. As a result, current source inverters are generally reserved for special applications such as high power drives which can benefit from the Thyristor type whole wafer power electronics devices that have high current handling capabilities.

In all dc systems, the generator ac output is converted to dc by power electronic converters. Even in classical dc generators, the role of the power electronic converter is done by passive brushed mechanical Commutators/converters. For electrical power generating machines whose output ac is rectified to feed a dc bus as is typical in dc systems, the nature of the dc fault current depends on the converter power electronics topology and characteristics of the generating electrical machine. Having alluded to the need for converter fault current limiting capability in dc systems, the IGBT based

2.2 Current Source and Voltage Source Converters

VSC type topologies widely used in industry have a major drawback when used in ac/dc conversion stages of the generators. The PWM VSC freewheel diodes rectify the generator output and cannot be switched off, therefore the fault current magnitude is initially defined by the generator impedances and the subsequent duration of the fault is defined by de-magnetisation characteristics of the generator. On the other hand, a CSC topology on the generator output employing power electronic devices with current turn off capability can interrupt the fault current. From the fault current limiting perspective, CSC topology is an attractive choice and it lends itself well to generators ac/dc conversion stages for dc systems. Similar CSC systems have been used in the Thyristor based HVDC transmission systems. The desirable features of CSC topologies have not been fully appreciated and exploited in dc power generation and delivery systems. Further research in CSC based generator/converter topologies can potentially yield significant benefits in terms of alleviating or better still eliminating the stringent operational duty requirements on protection switchgear in dc systems.

The topologies presented in this work are all based on the CSC drive topology. It will be shown that the proposed topologies presented in this work addresses some of the drawbacks of CSC topologies presented above, such as harmonics & torque pulsations, LC resonant modes and limited dynamic performance. Each of the machine topologies considered in this work consists of a plurality of coils in the machine stator winding slots that form a plurality of stator/armature phases and a rotor which comprises of a number of magnetic poles. The machine topologies are essentially DC machines turned inside out with the traditional mechanical commutators replaced by actively controlled electronic commutators. A schematic diagram of a current source fed conventional mechanically commutated dc machine is depicted in in figure 2.3. The armature circuit employ mechanical brushes for current commutation. Unlike conventional brush-less DC machines, this topology features: a high number of stator phases (12, 15, 24 phases and even higher), and is current source fed with electronic commutator power devices that switch at the very low machine fundamental frequencies rather than the high Pulse Width Modulated frequencies of most conventional drives.

2.3 Brush Commutated DC Machines