Applicable Frequency Range of Current Source Converters

APPLICABLE FREQUENCY RANGE OF CURRENT SOURCE CONVERTERS K. Moll and D. Schrooer Brown, Boveri et Cie., Mannheim, Germany

describe and analyse the operating range o£ overlapping commutation. It ia demonstrated, that the general operating of the current source converter with blocking diodea is preaerved.

ABSTRACT

A three-phase current source converter with blocking diode a feeding an asynchronous induction machine has been analysed. The analysis illustrates the three different modes of operation of this converter, especially the operation in the full overlap mode of commutation. The results were found by a computer program.

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From many publications the existence of two current source converter types is well knowns the first one needing an additional thyristor bridge for commutation, the second one operating with the commutation between the phases by blocking diodes. The commutation mechanism of both types is described by the same mathematical procedure. In the arrangement with the additional thyristor bridge there is far aore requirement on the elements. As only the converter with the commutation between the phases has a region with full overlapping commutation it distinguishes from the other one. This paper wants to

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~ .., Operation in the no overlap mode of the current source converter

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Fig.2 Commutation in the no-overlap mode OPERATION IN THE NO- OVERLAP MODE

Fig.l ahows the entire converter power circuit, Fig.2 every single section of the usual commutation sequence.The three steps it consista of are explained by the commutation troa phase 1 to phase 2 of the induction machine as follows.

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Step Is Thyristor-commutation. When the thyriator T3 turns on, TI is biased in the reverse direction by the voltage of the upper capacitor group. By the oscillating circuit with the inductance 2·LX and the capacitance C'=1.5·Cx, the thyristor TI is turned off. The whole de current Id flows to T3 (Fig.2.1). The duration of this atep is generally of little importance.

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Step 2: Linear capacitor- charging.While T3 and the diode DI are conducting (Fig.2.2), a constant-current discharging of the commutation capacitors Cl, C2 and C3

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Fig. I Current source converter power circuit 231

Klaus Moll and Dierk Schroder

232

takes place. There ia no effect on the induction machine. Aa aoon as the capacitor-voltage reachea the machine-voltage e12' the linear charging comea to an end. Step 3s Diode-commutation. At the end of step 2 there is an oacillating circuit with C'-1.5.Cx (capacitance) &ad L'-2·L G (inductaace) (Fi«.2.3). The osoillation laats until the load curreat haa coapletel;r ohaaged froa phaae 1 to phaae 2 (Fig.2.4).

ia reached whea ia the a. .e coaautation group (p or n) the ead of the precediag diode-commutation coincide a with the beginning of the follOWing linear capacitor-charging. The coamutation-capacitora are contiauall;r coaductiag, that aeana the tiaea of conatant capacitor-voltage (+UC, -UC' 0) are reduced to only a point. Vbe. exceeding the critical frequency fp' the range of the full overlapping coaautationa atarte, which ia analysed in the following chapter. The ditfe~nt frequency ranges are to be seen in Fig. }. T-f

Daring the deacribed operatioa, the following eQuationa are to be coasidered. Certain conditions must not be neglected (Ref.3). f

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the commutation generally proceeds ia two atepa, the thyristor-groupa uncouplea from the diode-groups. This means that there are commutations in the upper diodegroup and the lower thyriator-group and reversed, at the aame tiae. In the frequency range

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the diode-co-.utatioR (atep 3) of the group p orer1apa the thyriator-co-.tatioa (atep 1) and the liaear charging (atep 2) of the lower «roup a. Becauae the aubaequeat coaducting diode during atep 1 and atep 2 ia atill biaaed in the reverae directioa, there ia no mutual iafluence of the upper and lower group in ateady-atate operatioa. Aa overlapping of coamutatioaa haa no effect on the working readiaeaa of the coaverter. The frequency range of the usual commutations region I a

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current aource converter

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Fig.} The three different frequency ranges of the current sources converter Region I a Operating in the no-ov8l'" overlap aode Region IIa Operating in the partial-overlap mode Region IIIsOperating in the fulloverlap aode OPERATION IN THE FULL-OVERLAP MODE While the converter is operating in the full-overlap aode, the triple time of commutation (3Tc) would be longer than the period of the output frequency, if the coaautation sequenoe were undisturbed (a8 desoribed in Fig.2). If it should go on working, the sequence will have to be changed in the way that the equation f -

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is valid in the whole frequency range. The consequence is an additional occuring of not yet exploited current circuita. The analysis of these arrangements can be accomplished be different wa;ra. The wa;r chosen uses a simulation program on a oomputer, to prevent ti.e-consuming laborator;r experiments. The applied simulation program .erves the calculation of

Applicable frequency range of current source converters

ateadyanddynamic states in electrical circuits with semiconductors. Using this kind of programs the special conditions of the convertertechnique must be taken into account, as for example the great dirferences in time constants by continually switching actions.

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The value of the adopted frequency of the induction machine is 230 Hz (about 6 % more than f p ). The results of the simulation are to be seen in Fig. 4 and Fig.5. The essential conclusion is the readiness for operation of the current source converter in the frequency range above r~. A more detailed investigation shows that an additional charging takes place as suspected, as every diode conducts more than once a period. Operating in the no-overlap and partial-overlap mode a diode usually conducts only one time in a period. At the instant to (Fig.5) a diode commutation from Dl to D~ (thyristor T~ is conducting) is taking place in the upper group p, while there is a linear capacitor charging in the lower group n, diode D2 and thyristor T4 conducting. The diodes D4, D5 and D6 are biased in the reverse direction. The voltage of D6 being reduced to zero at the instant tl !g-

Behaviour of the current source converter at 230 HI:

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Fig.4 Operating in the full-overlap model capacitor-voltages and -currents or the current source converter at 2~0 Hz.

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Fig.6 Additional circuit during the commutation in the full-overlap mode (2~0 Hz)

Behaviour of the current source converter at 230 Hz

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Fig.5 Operating in the full-overlap model diode-voltages, diode-currents, aachine-voltages and machine-curreats at 2~0 H'Z. !he circuit, ShOWD in Fig.l, is calculated at the oomputer. The following parameters have been employed for the analyaisl Lo - 0,6 aB, Lx - 12 ~H, OK - 110 ~F, o 1 12 - ~80 V, Id - ~50 A, • - 50 From this operation point followsl TO -

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the diode starts working again in the same period with an extra conduction. Now the four diodes Dl, D~, D6 and D2 are conducting. To the circuit, as shown in Fig.2.3, consisting of C', L' and the diodes Dl and D~ a further mesh containing D6, is added (Fig.6). The supplementary current in D6 causes an additional charging of the commutation-capacitors and a reduction of the currents in phase 1 and 3 of the induction machine. !here is the advantage to the converter that an elevation of the frequency above f p is possible and that the general working readiness is preserved. The disadvantage is the reduct~on of the torquebuilding currents, while the de current remains constant. This fact prevents the region of full-overlap commutation from

Klaus Moll and Dierk Schroder

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being exploited infinitely, yet there is no risk tor the converter in this range as perhaps overvoltages. The described commutation operation comes to an end at the frequency

i.e. TC2 duration of the maohine commutation (from phase to pha.e). The general conditions are maintained furtheron. An immense increasing of frequenoy causes a conduction of more than four diodes at the same time (Fig.7), however that is no danger for the converter either. But in this range the converter is only controllable with difficulties, as there are instabilities in the change-over of motor- and generator-operating. If the converter changes from one region to the other, the phase-angle of the induction machine will change discontinuously its value.

the following requirements on the semiconductors appears thyristors u Th - Uc (8) diodes u Di - UC+u 12 Fig.5 shows that the additional charging causes a charge-shifting of the commutation-capacitoxsC4, C5 and c6 (lower group) at the instant tlsCl is loaded in the same, C5 conducting 1/3 of the de current Id is loaded in the reverse direction, C6 conducts 2/3 of Id and is also loaded in the reverse direction. By the description of the usual commutation mechanism is known, that during the commutation from thyristor T2 to T4 the capacitor-voltage -UC changes its sign. As the additional current runs reverse, the capacitor-voltage is decreasing, while the exploitation of the range with fulloverlapping commutation is increasing. The general requirement on the thyristor stays with UC. For the requirements on the diodes other conditions must be considered. In the frequency range below f p (Fig.3) the diode voltage consists of the capacitor-voltage of one group (p or n) and the machine voltage. With the beginning of full-overlapping commutations the diode voltage is set by the voltage of both capacitor-groups (p and n). The voltage of the diode D4, for example, results from the difference of the capacitor-voltages of C4 and Cl. The same bearing on the diode D5 have the voltages of C2 and C5.

REFERElICES

Behaviour of the current source converter at high frequency (300 Hz)

(1) Kenneth P. Phillipa, Current-Source Converter for AC Motor Drives, IEEE Tra-sactioas on Indust!:f Applications IA-8 110.6, 679 1972) 77 4226E

Fig.7 The current source oonverter at very high frequencys machinecurrents and diode-currents at 300 Bz. REQUIREMENTS ON THE ELEMENTS

In the range of the usual commutations (no-overlap and partial-overlap mode)

(2) B.Kazuno, Co-.utation or a ThreePhase Thyristor Bridge with Commutatioa Capacitors a-d Series Diode~ Electrical Engineering in Japan 90 110.5, 91 (1970) (3) Dierk Schroder, Selbatgeftihrter Stromrichter mit Phasenfolgeloschung und eingepragtea Stroa, Elektrotechnische Zeitschrift ~ 86 B.ll, 520 (1975)