Thyristors and Transformers

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.

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.

Unfortunately, this cancelling is true only at the fundamental frequency of the supply (50Hz in the UK). Most transformers are controlled by thyristors in the ‘phase angle’ mode, which means that the current in the windings consists of a range of odd harmonics; dominant amongst which is the third harmonic, (setting aside the fundamental). Reference to Figure 3 will demonstrate that, far from ‘cancelling out’, the third harmonics reinforce each other. This property can have interesting effects on a transformer core, resulting from overheating, since the flux will try to ‘close’ via any metal in the vicinity – frequently the steel case.

There are two easy ways to avoid difficulty. The first is to wind the primary (usually, though the secondary is equally suitable) in ‘delta’ so that the three fluxes will add in a ring round the core, with a resultant of zero. The alternative is to use a five-limb transformer – the outer limbs forming the return paths. These are not the only way to avoid a problem, nor is it guaranteed that they are always satisfactory. However, they are the most usual and fail only in very unusual cases.

There are many installations which have been running without problems for years which do not employ a delta. For whatever reason, whether it is because the transformer is lightly loaded or the third harmonic content is low at the normal working point, or a combination of other factors, these systems emphasise that the rules concentrating the third harmonic are only guidelines.

All the rules and guidelines applying to 3-phase transformers apply also to ‘Scott’. The notable exception is the delta-primary which is not practical. For this reason, Scott transformers are often wound with five limbs when used with thyristors. Additionally, the unusual flux patterns often necessitates a slower ‘soft start’ than with the  ‘conventional’ transformer, as because of the harmonic content of load currents, the line currents will not be equal for all phase angle conditions and the power will not necessarily be equal in the two load circuits.