Las penas privativas de libertad de larga duración

The current Source Converter (CSC) technology is widely used in industry for medium voltage drives. Figure 2.1 shows a schematic of the CSC drive topology. The CSC

drive system comprises of; input filter capacitorsC_gon the main ac input, a Current

Source Rectifier (CSR), a dc link inductorL_dc (energy storage element), an output

Current Source Inverter (CSI) and output filter capacitorsC_m which connect to the

machine. Current source based drive topologies using both PWM based inverters and Line Commutated Inverters (LCI) are in wide use in industry today. The PWM based CSC are mainly used in power levels up to 10MW whereas LCI based inverters tend to be employed for higher power ratings up to 100MW. Whole wafer power electronic switching devices such as Thyristors, Gate Turn Off Thristors (GTO), Gate Commutated Thyristors (GCT) are used with switching frequencies ranging from machine fundamental frequency for LCI topologies up to 500Hz for the PWM

2.2 Current Source and Voltage Source Converters ~ M g C m C dc L dc L CSR CSI

Fig. 2.1 Current Source Converter Topology

topologies. The restriction in switching frequency is due to the high thermal resistance of the Thyristor based semiconductor (GCTs) devices which prevent efficient heat transfer from the devices to the heatsink. Additionally, the restriction on switching frequency helps to minimise the converter switching losses and increase its efficiency. Current source converter topologies for both dc and ac connected drive systems are

widely reported in literature [[59–61], [62], [63], [64], [65]] et-al and will not be

discussed in detail here, only the key advantages and limitations of this topology will be highlighted. The main desirable characteristics of the CSC based drive topologies are:

• Output Voltage Waveform: Typical output voltage waveforms from the CSI

are suitable for insulation ratings of standard machines and transformers. The

output voltage waveforms are near sinusoidal and do not contain high dv_dt which

pose increased stress on machine insulation and increased common mode noise.

Owing to the very low dv_dt there is no restriction on cable length to the machine

as is typical for VSC based systems where voltage reflections along cables can

result in voltage doubling effects due to the high dv_dt converter output voltages.

• Short Circuit Protection: In case of short circuits on converter output or shoot

through faults, the rate of rise of output current is limited by the dc link inductor which allows sufficient reaction time for the CSR controller to rapidly quench the fault current by producing a negative dc link voltage to rapidly force the fault current to zero.

2.2 Current Source and Voltage Source Converters

• Four Quadrant Operation: This topology has inherent full four quadrant

operation, where the power reversal is conveniently achieved by reversing the dc link voltage polarity with no additional hardware requirement.

• Low device switching frequencies: Owing to the low switching frequencies,

CSC topology is attractive for very high power applications up 100MW where the voltage source inverters normally cannot compete in terms of the cost and energy efficiency of the system.

The main limitations of the CSC topology are:

• Harmonics and Torque Pulsations: A consequence of the inherently low

device switching frequency is the presence of harmonics on both the input and machine side that can be detrimental to system operation if not carefully evaluated and accounted for at the design stages. The detrimental effects of these harmonics include; increased harmonic losses on the machine as well as increased low frequency machine output torque pulsations which can excite mechanical resonant modes leading to increased stresses on the mechanical drive system. For cost and footprint reasons, the dc link inductor size is often not big enough to provide perfect decoupling between the network and machine side converters. This results in both integer and non integer harmonics being fed back to the ac supply. Often passive filters are necessary to absorb these unwanted harmonics or multi-winding transformers are employed to cancel these unwanted harmonics adding cost to the overall drive solution.

• Limited Dynamic Performance: Owing to the use of a dc link reactor which

limits the rate of rise of dc link current, the dynamic performance of this drive topology is limited. As a consequence this topology is not used for applications that require very high dynamic performance.

• LC Resonant Modes: In this topology, resonant modes exist between the in-

put and output filter capacitances and the respective ac network and machine inductances. The filter capacitors have to be sized to ensure that the LC resonant frequencies are lower than the lowest harmonics produced by the converter,

2.2 Current Source and Voltage Source Converters

which often results in increased filter size. The increase in filter size can how- ever be mitigated by employing active damping control schemes which aim to synthesise artificial damping resistance via the control scheme to effectively damp these resonant modes. This however comes at the expense of control modulation margin which has to be reserved for this active damping algorithm.