Precautions When Using Transformers
All the precautions necessary when using single-phase transformers are necessary in the case of multi-phase installations. There are one or two additional considerations occasioned by the interaction of three phases.
The principal consideration is the need to keep the three limb fluxes equal by keeping the three driving voltages equal unless the system has been specially designed otherwise. This means that the three phase currents will be unequal to the extent that the load impedances are unequal. In particular, it is difficult without taking special precautions to equalise the power in three zones - even if this were possible it would be unlikely to be achieved except at one specific element temperature - unless the furnace or load is designed with this need in mind.
This highlights the fact that a three-phase transformer cannot be treated in any way as three separate units - its economy lies in the interaction of the phases.
A further complication lies in the property of the three sine waves of the three-phase system to ‘cancel each other out’ enabling a three-phase system the ability to operate without a neutral wire.
In a similar way, a three-phase transformer is designed often with three limbs only, so that the three fluxes - which follow the voltages - also ‘cancel each other out’.
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.
These are applications, including two-zone furnaces and salt baths where, for supply reasons, energy for only two zones is taken equally (as near as possible) from the three phases of the supply. The tool which is often used for this function is the Scott-wound transformer.
The ‘Scott’ is a three-to-two arrangement where the primary is usually star and the two secondaries are usually separate, as shown in Figure 4.
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.
The one really important precaution is the avoidance of parasite voltages in the primaries which can damage thyristor equipment.
This can occur where secondaries are not, in fact, galvanically isolated. A current flowing in one can pass, wholly or partly, through the other and this can in turn generate through ‘backwards’ transformer action, an e.m.f in its appropriate primary. If the thyristor unit in this circuit is off, the e.m.f. can be large enough to cause damage. Where this problem is likely, expert advice should be sought.
In all transformer applications of thyristors, it should be remembered that the transformer is essentially a 50Hz device - it is indeed a very good 50Hz. The generation of all the harmonics will cause the core to warm up more than in ‘normal’ use and this, coupled with the need to avoid inrush current difficulties means that the core should be run at a relatively low flux density. A value of around 1.2 Tesla is usually used.