Failure sometimes even leads to big fires, which draws significant attention from the media, especially when sensitive industries are involved. It is not difficult to track such events. Last April, in the US media quite a few of such events could be spotted. In one case, the reporter covering the event quoted local officials by saying that “transformer explosion with fire is not very unusual”. In the same week, a transformer explosion in Berkeley cut-off power to 43.000.
In many cases, a rigid testing scheme can avoid such catastrophic events. Apart from the usual routine (mainly high-voltage) tests, more and more attention is drawn to prove that a transformer can survive a short circuit.
During a short circuit, the passage of short-circuit current through the transformer generates excessive electro-dynamical forces, increasing with the size and complexity of the transformer. These forces are mainly absorbed by the mechanical (clamping) design system. Therefore, proper design of the mechanical structure of transformer windings is a key issue. A recent (2015) extensive survey of about 1000 major transformer failures by CIGRE showed that:
- Windings are the most common failure location,
- mechanical failures account for over 20% of all failures of substation transformers,
- external short-circuit is the second largest known failure cause (after ageing).
Verification of short-circuit withstand capability is often thought to be demonstrated by calculation methods. The calculation process, commonly called "design review", consists of a comparison of calculated stresses in the transformer under review to those estimated from positively tested similarly designed winding structures. By keeping the forces below the tested levels or below known yield levels, a certain impression of short-circuit withstand capability might be obtained. But material, manufacturing or other quality imperfections cannot be accounted for, nor failure modes that are an (in)direct result of mechanical damage.
Testing is the reliable alternative and the request for short-circuit tests (especially for power transformers > 25 MVA) is rising over the years. At KEMA Laboratories, now around 20 - 30 large power transformers per year are tested for short-circuit withstand capability, up to a voltage class of 800 kV. Such tests require the maximum power from laboratories because the actual system power must be available. Preferably, such tests must be performed in three-phase circuits, because a realistic flux linkage of the three phases must be represented. When there is insufficient power available for three-phase tests, so-called “1.5 phase” tests are recommended, in which the full current is flowing through one winding, and half of the current through each of the other two. This guarantees at least an equivalent current magnitude in all three phases at the critical moment of maximum asymmetrical current.
Test results from KEMA Laboratories confirm that short-circuit withstand capability is not automatically guaranteed: in the 297 large power transformer test-series (25 – 440 MVA) in the period 1996 – 2015, in 67 cases transformers (23%) failed to pass the test already in the first stage of the test procedure. Around half of the transformers involved were re-tested after reinforcement in the factory. Mostly the failure was an increase of reactance beyond the standardized limits but also other, more direct observations were made frequently such as severe oil spraying, rupture of bushing porcelain, internal flash-over etc.
Mostly, the reason for increase of reactance is deformation of the winding structure. The short-circuit radial forces tend to compress the inner windings and to expand the outer ones, whereas axial (and combined) forces can dislocate or twist windings. Application of short-circuit current is the first part of the test procedure. After this critical high-power test, visual inspection at the manufacturer's site has to be carried out, followed by routine tests. If this is passed, the transformer is installed in service, since a short-circuit test is not a destructive test.
Cost considerations and a limited capacity of test laboratories means that only a fraction of all transformers can be short-circuit tested. Thus, a risk assessment must be made. Transformers having a design, vulnerable to electro-dynamical forces, such as split-winding types, may be candidates for a test. Also, transformers at critical locations, such as step-up transformers at power plants are regularly short-circuit tested. Finally, transformers having to operate under conditions of high fault incidence, such as traction transformers, are frequently tested.
With many years of testing experience, one can observe that manufacturers, forced by major utilities to have their products short-circuit tested, ultimately take their lessons from short-circuit testing by reducing their failure rate in tests year by year. In China, an increasing transformer field failure rate due to short-circuit was brought to a halt by requiring short-circuit testing and opening of a dedicated power laboratory.