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

A&D October 2006 

Inverter commutation failure in line-commutated converters 

caused by line voltage failures or dips and remedial 

solution based on a quenching mechanism (SIMOREG DC- 

MASTER Converter Commutation Protector) 

Dipl.-Ing. Franz Wöhrer, Siemens AG Austria, Vienna; 

Ing. Wilfried Hofmüller, Siemens AG Austria, Vienna; 

Prof. Dr. Dipl.-Ing. Felix Himmelstoss, Vienna University of Applied 

Sciences, Vienna; 

Summary 

Despite the successful progress of three-phase AC drives into all sectors of industrial drive 

technology, the DC drive – comprising a DC motor and line-commutated converter – can 

offer advantages for many types of application. One drawback of this technology, however, is 

that so-called inverter commutation failures can occur in response to line voltage failures or 

dips when the drive is operating in generator mode. The rise in current associated with this 

commutation failure can rupture fuses or destroy thyristors. Based on the example of the 

most common line-commutated converter connection, the 6-pulse three-phase bridge 

connection, the following article will explain the process of load current commutation in the 

thyristor arms and describe the scenarios in which inverter commutation can fail. 

By combining a line-commutated converter with a special quenching mechanism ("Converter 

Commutation Protector“, CCP), it is possible to prevent fuse ruptures or permanent thyristor 

damage following inverter commutation failure. Downtimes of production machinery and 

plant can be significantly reduced, so helping to revive the popularity of DC technology. The 

operating principle of the quenching mechanism (CCP), which consists of diodes, thyristors, 

capacitors, charging circuits and a voltage limiter, is described in detail below. This 

mechanism quenches the converter currents after commutation failure, but does not carry 

load current in fault-free operation. It also functions as a line-side and motor-side voltage 

limiter. 

Version vom 5.10.2006 Seite 1 von 17 Wöhrer, Hofmüller, Himmelstoss

1. Introduction 

Despite the successful progress of three-phase AC drives, comprising a frequency converter 

and three-phase motor, into all sectors of industrial drive technology, the tried and tested DC 

drive, consisting of a DC motor and line-commutated converter, still plays an important role in 

many applications and is still the drive of choice for many new installations. 

To make the right decision as to whether a DC or AC drive system is the better option for a 

particular application, it is necessary to evaluate the whole system, i.e. the motor, the supply 

power section and any other necessary additional electrical components such as line filters 

or commutating reactors. 

A decision in favor of DC drive technology is often influenced by the significantly lower power 

loss of the power electronic components, the space-saving design and the lower cost of the 

overall drive system. 

In addition to the increased maintenance requirements for DC motors as compared to AC 

motors, there is one other disadvantage which weighs against DC drive technology. This is 

the problem of so-called "inverter commutation failure", a fault which can occur under certain 

operating conditions and which causes fuses to blow and, in the worst-case scenario, 

permanent thyristor damage. 

2. The 6-pulse three-phase bridge connection 

The most common form of line-commutated converter connection, i.e. the controlled 6-pulse 

three-phase bridge connection (B6C) as shown in Figure 1 is examined below. Separately 

excited DC machines are employed for most industrial applications which use DC drives. 

Figure 1 shows the armature circuit of the DC machine as an equivalent circuit diagram 

comprising armature circuit inductance L a , armature circuit resistance R a and the induced 

source voltage U q . 

u 31 

u 12 

u 23 

V1 V3 V5 

L K 

L K 

I d 

u d α 

R a 

L a 

U q 

Controlled 6-pulse threephase 

bridge connection 

(B6C) for supplying the 

armature circuit of a 

separately excited DC 

V4 V6 V2 

machine 

a 


L K 

V1 

I d 

R a 

Armature current I d passes 

through V1 and V2. 

u 13 

u 13 → u d α 

L a 

L K 

V2 

U q 

b 

u 13 

u 12 

u 23 

V1 V3 

L K 

i K21 

L K 

i v1 

u 21 L K 

i v3 

I d 

u13 + u23 

→ udα 

2 

R a 

L a 

The commutation period 

commences when V3 is fired. 

The "commutating" line 

voltage u 21 causes shortcircuit 

current i k21 to develop U q . 

V2 

c 

V3 

I d 

R a 

After successful 

commutation, armature 

u 23 

L K 

L K 

u 23 →u 

d α 

L a 

current I d flows through V3 

and V2. 

V2 

U q 

d 

Figure 1 a Controlled 6-pulse three-phase bridge connection (B6C), b to d show the 3 

phases of commutation illustrated by the example of commutation sequence of 

thyristor V1 to thyristor V3 

The thyristor bridge, consisting of thyristors V1 to V6, is supplied by an idealized 3-phase 

voltage system (line-to-line sinusoidal voltages u 12 , u 23 , u 31 ). The inductance L k includes all 

the inductance of the supply system plus the effective inductance of line-side transformers 

and commutating reactors. The operating principle of this converter type is based on the 

well-known "line commutation" method, i.e. the thyristors must be fired in such a sequence 

that the load current I d flowing through the DC machine is always switched from one line 

phase to the next (u 12 → u 13 → u 23 → u 21 → u 31 → u 32 ). The thyristors are numbered (1 to 6) 

in the same sequence in which they are fired when a 3-phase voltage supply with clockwise 

phase sequence is applied (i.e. u 23 lags 120 degrees and u 31 lags 240 degrees behind the 

line-to-line voltage u 12 ). In steady-state operation, a "new" thyristor is fired at every 60 degree 

interval. To allow conduction with a discontinuous load current, the previously conducting 


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. 


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). 


Figure 2 Voltages and currents in a B6C thyristor bridge: 4 successful commutations close 

to the inverter stability limit can be seen on the left. On the right, a minor dip in 

the line voltage from instant ωt G causes an increase in armature current 

and commutation failure between V2 and V4. 

During the commutation period (angle of overlap u), the waveform of the converter output 

voltage u dα approximately follows the arithmetic average value of the line voltages u 13 and 

u 23 . involved in the commutation for L a >> L k . The voltage-time area (see shaded areas in 

Figure 2) which spans this average value and one of the involved line-to-line supply voltages 

during the commutation period is proportional to the load current I d and causes a change in 

voltage ("inductive direct voltage drop“, also referred to as "Dällenbach drop") from the 

idealized scenario with zero overlap time. This "inductive direct voltage drop" has exactly the 

same effect as a voltage drop across the armature circuit resistance. 

I d 


The said voltage-time area corresponds exactly to the integral of the voltage at inductance L k 

over the commutation period, or to half the value of the integral of the "commutating" line 

voltage u 21 over the commutation period and is also referred to as the "commutating voltage 

area“. As this voltage-time area is proportional to the commutating load current I d , it is clearly 

evident that the commutation period is also dependent on delay angle α, because 

commutation commences at different instantaneous values of the effective "commutating" 

line voltage u 21 as a function of delay angle α. 

The time characteristic of current i V3 in the incoming ("turning on") thyristor V3 represents a 

segment (beginning at ωt D ) from a cosine wave i k21 displaced in the y axis. Figure 2 shows 

i k21 for the commutation from V1 to V3. The displacement in the y axis depends on the 

instantaneous value of the "commutating" line voltage u 21 as commutation commences and 

thus also on the delay angle. The imaginary amplitude of the cosine-wave short-circuit 

current i k21 produced is not determined by the output direct current I d , but only by the 

inductance L k and the amplitude of the line voltage u 21 . If one were to cut a segment with 

amplitude I d out of this displaced cosine wave, the commutation periods would vary in length 

for I d values of different magnitude (overlap angle), although the commutation periods are not 

proportional to I d . 

After it has reached zero current, the outgoing thyristor V1 requires a negative voltage 

(blocking voltage) between the anode and cathode for as long as the thyristor takes to 

acquire its ability to block positive voltage. To prevent the development of an on-state 

current, this so-called "recovery time“ τ q , during which the minority charge carriers recombine 

in the barrier junction, must be allowed to elapse before the anode-cathode voltage becomes 

positive again. 

It is clear from Figure 1 c that the commutating line voltage u 21 , which is still positive at the 

end of the commutation period (from instant ωt E according to Figure 2), is applied as a 

blocking voltage at thyristor V1. The turn-off angle γ drawn in Figure 2 is the remaining angle 

until the sign of the commutating line voltage u 21 is reversed (instant ωt F or point P) and it 

must at least equal the recovery time τ q of thyristor V1 (γ min = ωτ q ). 


4. Inverter commutation failure 

One reason for commutation failure is when the current in the outgoing thyristor V1 has not 

reached zero before the sign of the commutating line voltage u 21 is reversed (instant ωt F or 

point P). In this case, the commutation current – driven by u 21 – is transferred from thyristor 

V3 back to V1. This type of failure is illustrated in Figure 2 which shows the transfer of 

commutation current from V2 to V4 (i.e. one firing cycle after the successful commutation 

from V1 to V3 described in the example). In this case, u 31 is the commutating voltage and the 

current commutates – driven by the sign reversal of u 31 – from thyristor V4 back to V2. V4 is 

turned off again and V2 conducts the full load current I d again from instant ωt J . However, 

because the original line voltage u 23 is "switched through" to the output again, the load 

current increases (similar to a short-circuit current) to an impermissibly high value due to the 

rapidly rising voltage difference between the source voltage U q of the DC machine and the 

instantaneous value of the line voltage u 23 . This process is known as "inverter commutation 

failure" (or "conduction-through“). The load current will tend towards steady-state value U q /R a 

on which an AC component is superimposed as a result of the driving line voltage. It must 

therefore be interrupted, for example, by fuses in the thyristor arms, and can cause 

permanent damage to the thyristors. The thyristor current waveforms following a 

commutation failure are also dependent on whether other thyristors have been fired after the 

failure. In this case, the turn-on of additional thyristors can cause a so-called "shoot-through 

fault", with the result that the DC machine is directly shorted via 2 thyristors and placed at 

extreme risk of damage. 

Another reason for commutation failure is when the current in the outgoing thyristor V1 has 

dropped to zero before the commutating line voltage u 21 reverses polarity, but the required 

recovery time of the thyristor as defined by γ min is not allowed to elapse. Since, in this 

instance, the thyristor has not yet regained its ability to block positive voltage when the 

commutating line voltage u 21 changes polarity, the full load current I d likewise in this case 

commutates (driven by u 21 ) from thyristor V3 back to V1 and a commutation failure occurs. 

In order to guarantee reliable commutation in the converter under normal operating 

conditions, the following must apply: α + u + γ min < 180 degrees. 

In order to prevent commutation failure, the delay angle α is limited to a maximum 

permissible value known as the "inverter stability limit“ α w (typical setting is 150 degrees). 

Inverter commutation failure can also occur despite a permissible delay angle if the voltage in 

the three-phase system briefly dips or fails altogether. For a particular current to commutate 

to the required thyristor arm, a specific voltage-time area (commutating voltage area) is 

needed at a given inductance L k . The commutation period therefore increases at a lower line 


voltage and the associated reduction in commutating voltage in the shorted circuit. If the 

current in the outgoing thyristor does not reach zero promptly, i.e. within the prescribed 

recovery time τ q , before the commutating line voltage reverses polarity, then commutation 

will fail in the current firing cycle. 

Sometimes, however, commutation failure does not occur until one of the following firing 

cycles because the reduction in average value of the converter output voltage u dα associated 

with a lower line voltage at a constant source voltage of the DC machine will result in a rapid 

rise in the load current I d , thereby further increasing the angle of overlap and causing a 

commutation failure. 

Under the conditions depicted in Figure 2, a dip in the line voltage (from instant ωt G ) by a 

mere 5 % leads to instantaneous commutation failure. The reason in this case is the rise in 

load current associated with the reduction in converter output voltage. The commutating 

voltage area (represented by shaded area from instant ωt H ) is not sufficient to allow the load 

current (which is rising sharply during commutation) to decay completely in V2. Commutation 

from V2 to V4 fails. As a result of the reversal in the commutating voltage sign, the current 

commutates from V4 back to V2 and, from instant ωt J onwards, the full, rapidly increasing 

load current I d starts to flow across V2 (and V3) again. 

If the line voltage is expected to fluctuate, the inverter stability limit α w must be lowered even 

further, resulting in a corresponding reduction in the absolute value of the maximum 

permissible source voltage U q of the DC machine for inverter operation. However, it must be 

mentioned at this point that inverter mode is not intrinsically unreliable. When linecommutated 

converters are used to supply a drive, it operates close to the inverter stability 

limit for only a small proportion of its total operating time. As soon as the delay angle drops 

below α w , the probability of failure, even following dips in the line voltage, decreases 

correspondingly. With a small enough delay angle, the converter can even recover itself. 

Nevertheless, it must be borne in mind that a converter may be required to continuously 

operate close to the inverter instability limit with some applications, e.g. brake generator. 


5. Outline description of problem and known potential remedies 

The problem of inverter commutation failure can occur in line-commutated converters 

operating in regenerative feedback mode. If the line voltage dips, for example, and the 

source voltage of the DC machine is high enough, an overcurrent, which the converter itself 

cannot quench, can develop and build. The fuses provided for the protection of the thyristors 

will inevitably trip, resulting in a prolonged outage of the converter and the DC machine it 

supplies. The time required to replace the protective fuses, which are normally fast-acting 

semiconductor fuses, can cause significant delays. An important objective in the past, 

therefore, was to build devices (of complex design in some cases) which could limit and 

interrupt this overcurrent, i.e. which could prevent inverter commutation failures or remedy 

them in a controlled manner. 

One known solution, for example, is the inclusion of high-speed, mechanical DC circuitbreakers 

in the DC circuit. However, to guarantee the reliable operation of such breakers, it 

is necessary to use highly controllable reactors to limit the current rise But reactors of this 

type are not only costly, but also bulky to install. They are difficult to rate correctly and 

require regular maintenance. 

Other methods of direct thyristor turn-off using quenching capacitors are also known, for 

example, the one which turns off only one of the two converter half-bridges using a capacitor. 

In this instance, however, the valves of the other half-bridge conduct the full motor current 

until it decays to zero which means that the method does not afford comprehensive 

protection. Another similar method uses two capacitors with a voltage limiter connected in 

parallel with each to turn off both half-bridges, but this option allows surge voltages to 

develop on the motor. Furthermore, neither of these methods protects against overvoltage 

following a line interruption on the primary side of a transformer supplying the converter 

bridge. 

6. Requirements of a quenching mechanism 

A "quenching mechanism for converters" must be capable of turning off the thyristors of a 

converter operating in regenerative feedback mode so quickly that the semiconductor fuses 

connected on the line side (to protect individual thyristors or the whole converter) are 

protected (especially in the case of inverter commutation failure) against melting or lifeshortening 

damage. Where fuses are fitted, they must not be loaded to their full pre-arcing i²t 

value. In operation without fuses, thyristors must not be loaded to their full i²t value. In 

addition, the converter must be protected against overvoltages such as those which typically 


esult from line interruptions on the primary side of a supply transformer and which often lead 

to "false turn-on" and irreparable damage to thyristors. 

7. General description of the new quenching mechanism 

By combining a line-commutated converter with a special new quenching mechanism 

("Converter Commutation Protector“, CCP) developed by SIEMENS AG Austria, it is possible 

to prevent fuse ruptures or permanent thyristor damage following inverter commutation 

failure. Downtimes of production machinery and plant can be significantly reduced, so 

helping to revive the popularity of DC technology. The operating principle of the new 

quenching mechanism ("Converter Commutation Protector“, CCP), which consists of diodes, 

thyristors, capacitors, charging circuits and a voltage limiter, is described in detail below. This 

mechanism quenches the converter currents after commutation failure, but does not carry 

load current in fault-free operation. It also functions as a line-side and motor-side voltage 

limiter. 

The quenching mechanism employs a method which affords particularly smooth turn-off of 

both half-bridges using two capacitors. A voltage limiter is connected in parallel with the 

motor and limits surge voltages at the motor end. A connection is provided between a line 

voltage bridge rectifier (comprising 6 diodes) and the voltage limiter via a further two diodes. 

This also protects against line-side overvoltages caused, for example, by a line interruption 

on the primary side of a transformer supplying the bridge. By connecting another quenching 

thyristor in series with each of the quenching capacitors, a charging circuit without 

transformer can be implemented. Furthermore, this configuration prevents a double voltage 

load on the quenching thyristors. A high-speed chopper resistor is employed as a voltage 

limiter. 

The quenching mechanism is tripped according to a process based on a software algorithm 

which evaluates several criteria. These include measurement of the available commutating 

voltage time area and analysis of the waveform of the measured load current. 

8. Operating principle and design of the quenching mechanism 

Figure 3 shows the circuit diagram for a combination of line-commutated reversible converter 

SRB (circulating-current-free anti-parallel connection of two three-phase bridge connections) 

and special quenching mechanism LOV. The LOV is connected in parallel with the threephase 

AC and DC terminals of the converter (5 power connections). 

A DC machine MOT is supplied by the converter. The gating circuit AST contains the logic 

for generating the synchronous firing pulses for the converter bridges. The direct current is 


sensed by current transformers in line phases L1 (U) and L3 (W) of the converter. The trigger 

unit ALE contains the logic required to trip the quenching mechanism. 

The quenching mechanism LOV is equipped with a quenching capacitor C1 or C2 for each 

half-bridge V11, V13, V15 or V14, V16, V12 (with reverse load current direction for V21, V23, 

V25 or V24, V26, V22). Each of these capacitors is charged at the polarity shown in the 

diagram. The two charged capacitors (C1 and C2) are connected to the converter on one 

side via diodes (V41, ..., V46) and on the other via thyristors (V31, ..., V34). 

The DC terminals (motor) are connected to capacitor C3 and its parallel voltage limiter SBG 

via thyristors (V35, ..., V38). Capacitor C3 is not essential to the operating principle, but 

allows a chopper resistor to be employed as a voltage limiter. It must be noted that a voltage 

of the same polarity is always applied to capacitor C3 (unlike quenching capacitors C1 and 

C2). 

The DC end of the bridge arrangement of diodes V41, ..., V46 is connected to the input of the 

voltage limiter SBG and C3 via diodes V47 and V48. On the one hand, these allow the 

current decay in line inductances L k while the quenching mechanism is active and, on the 

other, the transfer of transient overvoltages from the supply (such as those which occur with 

primary-side line interruptions on a supply transformer) and commutation voltage peaks from 

the converter itself when the converter bridges are operating normally (in motor and 

generator mode). 

L k includes all the inductance of the supply system, plus the effective inductance of line-side 

transformers and commutating reactors. Ls, L1 and L2 are parasitic inductance, i.e. air-core 

inductors, which are included only when required (for limiting current rise). 


Figure 3 

Circuit diagram: Combination of reversible converter SRB and quenching 

mechanism LOV Gating circuit AST generates firing pulses for SRB. Trigger unit 

ALE contains the logic required to trip LOV. Quenching capacitors C1 and C2 are 

precharged via R and switch S and quench a current in the top or bottom halfbridge 

of the converter. Voltage limiter SBG limits the line and motor voltages. 


V11 

I d 

V41 

i C1 

L K 

L S 

L 1 

V31 

+ 

C1 

- 

V39 

V35 

R a 

u 13 

C3 

- 

+ 

i 3 

L a 

V42 

i C2 

V40 

C2 

L K 

L S 

L 2 

V33 

V12 

+ 

- 

V37 

SBG 

- 

+ 

U q 

Figure 4 

Relevant circuits when a current, which was originally flowing through V11 and 

V12, is quenched. Due to the currents produced (i C1 for the one half-bridge and i C2 

for the other), the full load current I d commutates first to the quenching capacitor 

paths. The load current then recharges quenching capacitors C1 and C2 and 

commutates to capacitor C3 or the voltage limiter SBG when the voltage is high 

enough. The load current I d is then reduced to zero as a result of the difference 

between the voltage at C3 and the source voltage U q . 

9. Quenching process 

The quenching process for the polarity of motor voltage and load current I d (motor current) 

shown in Figure 3 is described below. The circuit must include quenching capacitors C1 and 

C2 as shown in Figure 3. To illustrate this quenching process more clearly, Figure 4 shows 

the circuits which are relevant for current flow in V11 and V12. 

When the quenching thyristors V31, V39, V33 and V40 are fired by trigger unit ALE, the 

currents commutate from both the upper half-bridge V11, V13, V15 to capacitor C1 and the 

lower half-bridge V14, V16, V12 to capacitor C2. All currents in the converter operating in 

regenerative feedback mode (V11, ..., V16) are quenched immediately. The motor voltage 

polarity is reversed briefly by the quenching process. 

At the same time as the quenching pulses (firing pulses for quenching thyristors) are output, 

the firing pulses for the bridge SRB are disabled. 


C1 and C2 are discharged by the load current and recharged with the reverse polarity. 

Recharging C1 and C2 with the reverse polarity increases the voltage on the DC machine. 

When the voltage level at C3 is reached, the load current commutates – at approximately the 

instant of voltage zero at C1 and C2 – to the capacitor C3 on the voltage limiter SBG across 

thyristors V35 and V37 which have just been fired. If the voltage at C3 has not reached the 

limit level of limiter SBG, it is charged up to this value by the load current. It must be noted 

here that currents flow through thyristors V32, V34, V36 and V38 only when the load current 

changes direction. 

In order to ensure that the load current driven by the armature inductance is subsequently 

reduced to zero, the voltage at C3 must be higher than the source voltage U q of the DC 

machine. The voltage limiter SBG limits the voltage at C3 to a maximum value. Voltage 

limiting can be implemented, for example, by means of a two-position controller with 

appropriate hysteresis, which switches load resistors in and out (chopper resistor). 

10. Charging circuit 

The two quenching capacitors C1 and C2 must be charged to a percentage of the peak value 

of the line-to-line system voltage. This function is performed by a charging circuit with two 

charging resistors R per capacitor, that are taken to the plus pole and minus pole of the 

bridge rectifier (V41, ..., V46), and two semiconductor switches S that are controlled by a 

two-position controller. A minimum charging voltage is required to ensure protection of the 

converter thyristor recovery time during recharging – until approximately the instant of 

polarity reversal of the voltage at C1 and C2 – following the actual quenching process. With a 

given recovery period and defined capacitance value of C1 and C2, the minimum charging 

voltage required is determined by the maximum current value to be quenched, the maximum 

anticipated source voltage U q of the DC machine and by the armature inductance. 

When quenching occurs, the semiconductor switches S of the charging circuit must be 

opened as soon as the quenching thyristors are fired in order to prevent these from 

conducting current via the charging resistors after the current has successfully decayed. 

11. Controlling the quenching mechanism and tripping algorithms 

The entire quenching mechanism can be controlled by means of an analog circuit with 

microprocessor which also provides communication with the converter. 

The quenching mechanism is tripped according to a process based on a software algorithm 

which evaluates several criteria. The time characteristics of the measured voltages and 

currents provide the basis for evaluation. A first criterion for activating the quenching 


mechanism is to measure the commutating voltage area still available after commutation and 

to compare this with the theoretical value calculated for a successful commutation. This 

voltage-time area is obtained by integration for approximately 30° of the corresponding 

commutating line voltage measured at the converter terminals, starting at the firing instant. 

The quenching mechanism is then triggered if the measured area is a certain percentage 

smaller than the theoretically available commutating voltage area. 

Another criterion for activating the quenching mechanism is that the second derivative 

obtained from the measured output current I d is calculated according to time, which is 

negative between firing instants with "normal" armature current peak values. d 2 i d /dt 2 < 0 

must therefore apply for each ("normal") current peak. If this is not the case, i.e. if the current 

peak begins to curve at the top (as shown, for example, during commutation failure in Figure 

2), it must be assumed than an overcurrent will soon occur as a result of a line voltage dip in 

which case the quenching mechanism can be activated in advance of an overcurrent event. 

The height of the armature current peak can be evaluated as the third criterion, with the crest 

value corresponding, for example, to two and a half times the rated current of the converter 

bridge. 

12. Examples of faults which can be controlled by the quenching mechanism 

Lightning strikes in high-voltage or medium-voltage installations ignite protective spark gaps 

or gas-filled surge protective devices, causing low-resistance line voltage dips lasting several 

milliseconds. Other scenarios involving low-resistance line voltage dips can be caused by 

any type of short circuit in the supply system. As explained in this article, dips in the line 

voltage can cause commutation failure. However, the resultant overcurrent is quenched by 

tripping of the quenching mechanism. In some cases, no significant overcurrent can develop 

at all thanks to early detection of the commutation failure and tripping of the quenching 

mechanism. 

Overvoltages in the supply system occur, for example, when a short circuit develops in a 

parallel circuit on the same supply, causing the relevant fuse to disconnect the faulty circuit 

from the supply. The overvoltage peaks which develop on system recovery are limited by the 

quenching mechanism at the converter terminals. 

Even in the event of a line interruption on the primary side of a supply transformer, e.g. at 

medium-voltage level, the magnetizing energy stored in the transformer can cause 

overvoltages which are limited by the quenching mechanism described above at the 

converter terminals. "False turn-on" of thyristors can thus be prevented. A line interruption on 

the primary side of a supply transformer can also cause a sharp rise in the effective line 


inductance for converter commutation, another potential cause of converter commutation 

failure (i.e. the last conductive thyristor pair remain conductive). In this instance, an 

overcurrent does not develop until other converter thyristors are fired, causing a short circuit 

on the DC machine. Thanks to the quenching mechanism, an overcurrent does not develop 

at all because the mechanism is tripped on detection of commutation failure and prevents all 

further thyristor firing. 

13. Available on the market: The "SIMOREG DC-MASTER Converter Commutation 

Protector" (CCP) 

SIEMENS AG Austria / SIMEA (Siemens Industrial Manufacturing, Engineering and 

Applications) has developed a series of quenching mechanisms of the type described in this 

article for use in combination with converters in the product range "SIMOREG DC-MASTER 

6RA70“. This series of quenching devices has been launched on the market under the 

product name "SIMOREG DC-MASTER Converter Commutation Protector“ or "SIMOREG 

DC-MASTER CCP“ for short. The current CCP product spectrum covers converters with DC 

current ratings of 300 A to 2000 A and rated voltages up to 690 V 3AC. 

14. Summary 

As described in this article, it is possible to prevent fuse ruptures or permanent thyristor 

damage following inverter commutation failure by combining a line-commutated converter 

with a special quenching mechanism. Downtimes of production machinery and plant can be 

significantly reduced, so helping to revive the popularity of DC technology. 

References 

[1] Möltgen, G.: Netzgeführte Stromrichter mit Thyristoren (Line-Commutated Converters 

with Thyristors). Siemens AG, Berlin-Munich, 3rd Edition/1974. 

[2] Zach, F.: Leistungselektronik (Power Electronics). Springer-Verlag, Vienna-New York, 

1979. 

[3] Kolar, J; Drofenik, U.: Interactive Power Electronics Seminar (iPES), Netzgeführte 

Thyristorschaltungen, Wechselrichterkippen (Line-Commutated Thyristor Circuits, 

Inverter Commutation Failure). ETH Zürich, Oct. 2004. Available at 

http://www.ipes.ethz.ch/ipes/kommutierung/kipp.html

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

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