Parallel Operation of Thyristors
The need to connect thyristors in parallel arises when the current or over current to be handled by the apparatus or equipment exceeds the rating of a single thyristor. Thyristors can be connected directly in parallel with each other if they have identical forward V-I characteristics. This is rarely the case unless very special selection of the thyristors is made to ensure good current sharing during normal load and under overload and faulty conditions.
The shape of the forward voltage of thyristors makes it difficult for them to achieve good sharing without assistance, as even quite a small voltage drop difference can result in a wide difference of load current, as shown in Figure 31 shows the sharing of current between two parallel connected thyristors of same type.
Figure 31 (b) illustrates that for the same voltage drop VT, SCRT2 shares a rated current I2, whereas SCR T1 carries a current I1 much less than the rated current I2. Total rated current of the parallel unit is (I1 + I2) instead of 2I2. This unequal distribution of current in parallel connected thyristors leads to a thermal-runaway problem. For example, thyristor T2 in Figure 31 carries more current than the thyristor T1. Due to large forward, current, its internal power dissipation will be more, thereby raising its junction temperature and reducing the dynamic resistance. This, in turn, will increase the current shared by SCR T2 and the process becomes repetitive. The cumulative increase in current results in permanent damage to the SCR T2, followed by the burning of SCR T1. Therefore, all SCRs must operate at the same temperature when used in parallel connection. This can be done by a common heat-sink.
The unequal distribution of current in a parallel unit is also caused by the inductive effect of current carrying conductors. When SCRs are arranged unsymmetrically, the one which have more inductance due to more flux linkages, less current flows through it. This unequal current distribution
can be avoided by mounting the SCRs symmetrically on the heat-sink.
The difference in the turn-on characteristics can also influence the operation of thyristors in parallel. Differences in finger-voltages (the minimum forward anode voltage at which an SCR can be successfully turned-on with a trigger signal of sufficient magnitude) may prohibit the SCR with highest turn-on voltages to trigger. This is explained in Figure 32. If SCRT1 is directly connected in parallel with SCRT2 having identical characteristics, SCRT2 will never turn-on in application requiring zero voltage triggering. When SCRT1 is switched ON, the anode voltage of SCRT2 would be that of the on-state voltage of SCRT1 and consequently, it will never equal or exceed the minimum required anode voltage to trigger SCR T2 even if the width of the trigger pulse is greater than the delaytime of the SCR T2. Hence, it is very essential that in direct paralleling of SCRs, the forward characteristic of each and every SCR must be properly matched. Matched thyristors are generally available for direct parallel operation.
Methods For Ensuring Proper Current Sharing: In d.c. circuits, a series resistor R may be added to each arm of the parallel arrangement to improve the sharing of current. To force steady state current sharing, the resistors are connected as shown in Figure 33. In other words, due to the connection of series resistor, the difference in the value of dynamic resistance RT of thyristors is compensated.
If the two thyristors are of identical rating, then the two external series resistors R1 and R2 are chosen such that the total voltage drops are equal. If two SCRs of different forward current ratings IT1 and IT2 are to be operated in parallel, then the same resistance R can be used for both units to ensure proper current sharing by the SCRs. Obviously, such a addition of resistance affects overall efficiency and regulation.
Current sharing with reactors is a more efficient method than with resistors. In an a.c. circuit, the current distribution can be made more uniform by the magnetic coupling of the parallel paths as shown in Figure 34. Figure 34 shows a 1:1 ratio reactor in bucking connection for two SCRs in parallel operation.
If currents IT1 and IT2 are equal, then the voltage drop in the reactor will be zero because of the mutual cancellation of flux linkages in the coils. If the current through SCR T1 tends to increase above the current through SCR T2 (i.e., IT1 > IT2), a counter EMF will be induced proportional to the unbalanced current and tends to reduce the current through SCRT1. At the same time, a boosting voltage is induced in series with SCR T2, increasing the current flow through the SCR T2. The most important magnetic requirements of such a reactor are high saturation and low residual flux densities in order to provide as great a change in total flux each cycle as possible.