Thyristors and Transformers

It can be advantageous and cost effective to mount a thyristor unit in the primary circuit of a transformer. The following article describes precautions to be taken.

Transformers are frequently used in electrical energy applications for one of three purposes:
i. To achieve galvanic isolation
ii. To achieve a reduction of the supply voltage at the load
iii. To achieve an increase in the supply voltage at the load

The use of thyristors with transformers needs some care as the ‘inrush’ current on connecting the supply to the transformer can easily exceed the maximum current rating of the thyristor unit and cause failure of the semiconductor fuse. The inrush current is of the order of 20 times the normal running current, as a
rule of thumb.

Consider first the single-phase case:
Where thyristors are connected between transformer and load, the thyristor unit characters are determined by the load only and no attention needs to be paid to the transformer. In many cases, though, thyristors are connected in the transformer primary - i.e. between the supply and the transformer - for reasons of economy; generally one pays more for current than for voltage capacity. In these cases, it is important to observe some straightforward precautions.

If the B-H curve of Figure 1 is considered, it is a matter of chance where on the curve, thus the magnetic
state of the transformer, the equipment is left on disconnection. Even if, as is usual with thyristors, the state is left.
corresponding to H=0, there will be residual magnetism in the transformer core. The problem is what whilst the circuit is switched off as current falls to zero, it is switched on as voltage rises from zero. Unity
power is rare!
If no precautions are taken then, one time in two, switching on will drive the transformer into saturation in the same direction, allowing excess current to flow. In practice, the fuse will fail about one time in four.
On switching on, if the rms current is increased carefully from zero, the inrush problem will be overcome. This technique is referred to as ‘soft start’ and in most cases is necessary over only a few cycles. Where
current limiting also is required on starting, this same ‘soft start’ is usually satisfactory for both requirements. Once running, the magnetic state of the transformer can be easily inferred, so the same soft-starting precaution is not always necessary every time the load is re-energised, having been ‘off’ for a period - as is common in temperature control applications. Even so, the problem of ‘off with current, on with voltage zeros’ remains. To cope with this, a technique of burst firing with delayed first cycle firing has been developed.

Usually a delay of half a half-cycle (as shown in figure 2)  is satisfactory for most applications, though the enhancement to be able to adjust the delay start from zero to 90° as a commissioning feature - to cope with individual Q factors - is almost always beneficial. 
Thus inrush currents can be minimised whilst the benefits of burst firing control are realised.
Designers are still required to exercise considerable caution when working with loads which are other than purely resistive, however, as experience of control using burst firing techniques in the primary of transformers is still limited. Further, where the technique is used with confidence with elements which run at relatively low temperature, there may be difficulty - either mechanical or thermal - in the case of high-temperature elements. The guidance of the element manufacturer should always be sought when in doubt.
The well-established ‘phase angle’ method of control is now thought to be suitable for all applications.
If in difficulty, the designer would be well advised to seek expert assistance from manufacturers with considerable applications experience.

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