A&D October 2006 Inverter commutation failure in line ... - Siemens

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thyristor must be fired simultaneously, a process which has given rise to the term "double pulses". Each thyristor conducts the load current for 2 successive firing cycles (twice within a maximum of 60 degrees in steady state). The average value of the rectified output voltage u dα can be adjusted by the so-called delay or gating angle α. The delay angle α is measured from the so-called "natural commutation instant“ N. This is the point at which current commutation would commence in a new line phase in a diode bridge (intersection between u 12 and u 13 , u 13 and u 23 , u 23 and u 21 , u 21 and u 31 , u 31 and u 32 , u 32 and u 12 ). One segment at a time (with interval that can be varied by delay angle α) from one of the line-to-line supply voltages (u 13 , u 23 , u 21 , u 31 , u 32 , u 12 ) is "switched through" continuously to the DC machine via a pair of thyristors (V1+V2, V2+V3, V3+V4, V4+V5, V5+V6, V6+V1). Due to the inductance L k in the circuit, however, the load current I d cannot transfer instantaneously from the outgoing ("turning off") thyristor arm to the incoming ("turning on") thyristor arm. With a continuous load current I d , the "commutation period" or "overlap" commences in each case when the "new" (incoming) thyristor is fired, while the load current I d from one terminal of the DC machine is split between two line phases via the outgoing and incoming thyristors. The other terminal of the DC machine is connected to the remaining supply connection via the thyristor fired in the previous cycle, which is now in the second conduction phase and conducting the full load current I d . While the 6-pulse three-phase bridge connection can supply only a positive load current I d , it is capable of generating positive or negative average values of the rectified output voltage u dα through variation of delay angle α. Depending on whether the DC machine is operating as a motor or generator, the converter bridge is said to be operating in "rectifier mode" or "inverter mode". A single three-phase bridge connection is therefore suitable for a drive with only positive torque in different directions of rotation. A typical application for this type of operation in the 1st and 4th speed/torque quadrant would be the hoisting or lowering of a hoist load. If the DC machine is also required to operate with negative torques, then a double converter (also referred to as "reversible converter") must be used. The most common design of reversible converter is based on a circulating-current-free anti-parallel connection of two three-phase bridge connections. Version vom 5.10.2006 Seite 4 von 17 Wöhrer, Hofmüller, Himmelstoss
3. Commutation of current in thyristor arms To explain the process of inverter commutation failure, commutation of the load current from V1 to V3 as illustrated in Figure 1 b to d is examined in more detail in the following example. The voltage and current waveforms associated with this type of commutation in regenerative feedback mode are shown in Figure 2. In the initial state (as shown in Figure 1 b), the full load current I d flows across thyristors V1 and V2 and, as a result, the line voltage u 13 is "switched through" to the output. At instant ωt D as shown in Figure 2, i.e. delayed by angle α in relation to the natural commutation instant N, the "new" (incoming) thyristor V3 is fired for the purpose of switching through the line-to-line supply voltage u 23 in the correct phase sequence to the output (thyristor V2 is fired simultaneously). Due to the inductance L k in the circuit, the current cannot transfer instantaneously from V1 to V3. The period of "commutation" or "overlap" commences while the load current I d is split between V1 (i V1 ) and V3 (i V3 ) (interval between ωt D and ωt E as shown in Figure 2). If the inductance in the load circuit (armature inductance L a of DC machine including an armature current smoothing reactor if used) is very much higher than the effective commutation inductance L k , then the load current I d can be regarded as an approximately constant quantity over the commutation period (the armature circuit inductance L a acts like a power source). Figure 1 c shows that the line-to-line supply voltage u 21 (i.e. the difference between the lineto-line supply voltages u 13 and u 23 involved in the commutation) is shorted across V1 and V3 during the commutation period. Inductance 2*L k is effective in the shorted circuit and the "commutating" line voltage u 21 causes short-circuit current i k21 to develop. This is overlaid on the direct current I d in the outgoing thyristor V1 in such a way that the sum of both gradually decreases. Once it has reached zero, it ceases to flow as a result of the blocking effect of thyristor V1. At the same time, the current i k21 across V3 has increased from zero to I d , at which point the commutation period ends. On completion of commutation, the line voltage u 23 is the output voltage and I d flows (as shown in Figure 1 d) through thyristors V3 and V2. V2 is now in the second conduction phase and is conducting load current I d until the commutation is (or should be) transferred from V2 to V4 when V4 is next fired (V3 is fired simultaneously). Version vom 5.10.2006 Seite 5 von 17 Wöhrer, Hofmüller, Himmelstoss
Page 1 and 2: A&D October 2006 Inverter commutati
Page 3: L K V1 I d R a Armature current I d
Page 7 and 8: The said voltage-time area correspo
Page 9 and 10: voltage and the associated reductio
Page 11 and 12: esult from line interruptions on th
Page 13 and 14: Figure 3 Circuit diagram: Combinati
Page 15 and 16: C1 and C2 are discharged by the loa
Page 17: inductance for converter commutatio

A&D October 2006 Inverter commutation failure in line ... - Siemens

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