Electrical transient interaction between transformers and the power system

January 9th, 2015, Published in Articles: Energize

 

Transformers are constantly exposed to different types of transient events during their daily operation, which often imposes high stresses on their insulation structure. Field experience has shown that even when good insulation coordination studies and well-known insulation design practices are applied, a significant number of transformers suffer dielectric failures.

Such failures may be caused by previous transient events, which are not necessarily related to any system condition at the time of the failure. The analysis of the failures and their prevention requires a thorough knowledge of the transient interaction between the transformer and the power system.

In this context, another important aspect to consider is the fact that, under any new power system deregulation scenario, the necessity to integrate different entities, such as the transmission system operators, generators and distributors, requires the development of new operation procedures. These new system operation conditions, when combined with a more extensive usage of transient generating technologies and the trend of keeping the equipment longer in operation, create a new electrical environment for transformers, which increases the dielectric stress on their insulation.

Standards and service experience

From its earliest development, the improvement of power grid reliability has been a constant goal of engineers. This focus has not only been on improving the reliability of operation but more specifically the improvement of the reliability of components. A power transformer is one of the most strategic and costly piece of equipment in the power system, requiring a high level of reliability and availability. As such, transient overvoltages have long been recognised as one of the important causes of equipment failure and thus, unavailability. Figs. 1, 2 and 3 illustrate some of these failures.

Fig. 1: Damaged insulation.

Fig. 1: Damaged insulation.

Fig. 2: Insulation burning.

Fig. 2: Insulation burning.

Fig. 3: Inside view of the winding.

Fig. 3: Inside view of the winding.

Electrical network transient modelling

Proper modelling of the network is an important issue, since it will have a major effect on surges entering the transformers. The modelling of the different equipment at high frequencies is required for lines, cables, substations, circuit breakers, disconnectors, and surge arresters. The case of switching surges due to faults, lightning surges and very high frequency phenomena due disconnectors switching must be considered, with a special focus on the re-ignition phenomena in circuit breakers.

Network interaction with transformer

All transformers have internal resonances in the range of a few kHz to hundreds of kHz. Any system transient that includes frequency components in this range can potentially provoke resonant overvoltages in a transformer, even when the overvoltage impinging the transformer terminals is lower than the protective level of surge arresters. Typical situations include circuit breaker closing and opening operations, disconnector operations, and fault initiation. The actual network layout, lengths of transmission lines and cable sections are major factors which affect the impinging voltage wave shape and its dominant frequency components. Transformers experiencing frequent switching operations are more likely to fail from resonant overvoltages, due to the stochastic nature of switching operations, breakers’ location and characteristics.

Assessment of transformer voltage stress

Different methodologies to evaluate the voltage stress on transformers due to transient overvoltages exist. The conventional approach, often used by a manufacturer, involves the conversion of the transient overvoltage to a corresponding lightning impulse stress followed by a comparison with the acceptance test level. Another method considers a time-based model where the transient voltage at each position of the winding is compared to the transient of the acceptance test, thus comparing the in-coming voltage stress to the one the transformer is tested for.

The ratio, named Time Domain Severity Factor (TDSF), used in the evaluation is presented below:

cigre-eqn-1

 

(1)

 

In a third methodology, similar to the second, the transient is also compared to the acceptance test, but now in the frequency domain. All spectra of the incoming transients Fast Fourier Transform (FFT) are evaluated against the spectrum amplitude of the acceptance tests. The ratio, named Frequency Domain Severity Factor (FDSF), used in the evaluation is:

cigre_eqn-2

 

(2)

 

Impact on transformer insulation

The effects of insulation aging are presented based on the degradation of insulating oil and paper based on field-aged transformers. Even though the degradation degree is small, the breakdown voltages of insulating oil and paper do decrease due to aging. Therefore, for older transformers, or transformers which are exposed to overvoltages frequently, consideration must be given to age-related degradation of the insulation performance. Another important aspect is the effect of repetitive impulses on the decrease of insulation strength. For transformers installed in a system that has switches, which operate frequently, a decrease in the dielectric strength due to repetitive voltage stresses is to be expected. Finally, it is emphasised that a full understanding of how rapidly rising voltages affects insulation system is still a challenge and deserves future investigation.
Some work has shown an important influence of the wave shape on the voltage breakdown, and a possible decrease of the withstand due to shorter rise times. A better understanding of this effect will provide an improvement to the design of the insulation system when exposed to fast transients.

Fig. 4: Insulation structure between two windings.

Fig. 4: Insulation structure between two windings.

Fig. 5: Single 400 kV disc winding.

Fig. 5: Single 400 kV disc winding.

Recommendations

Transformer specifications should reflect the unique requirements caused by the power system, especially in critical configurations.  It is highly desirable that the manufacturer provides the utility with an appropriate high frequency model of the transformer to allow for system transient studies. The importance of close cooperation between the manufacturer and purchaser cannot be underestimated. The technical specification is the most important means of this communication and joint analysis can be carried out in the design stage and verified in the design review. Some specific evaluation (transient measurement, system studies, etc.,) should be carried out as part of transformer failure analysis when transients may be involved. Many failures are considered unknown due to a lack of this type of evaluation which may be complex and time consuming.

Conclusion

Some transformers have failed in conditions where the cause of failure could not be identified. In some cases, the evidence led to system interaction as the most probable cause. Digital simulations have shown that the voltage stresses over the transformer terminals are usually restricted to frequencies below 200 kHz. However, when these stresses are compared with the specified standardised waves, they may exceed the transformer withstand design capability.
As far as the transformer design is concerned, an improvement in reliability may be achieved by an upgrade of transformer specifications, the implementation of design review practices or even the improvement of standard dielectric tests to make them cover a wider range of system condition. Alternatively, using controlled switching or introducing pre-insertion resistors in circuit breakers, or even a resistor-capacitor snubber to reduce the stress amplitude and increase the damping at higher frequencies, may be considered.

Acknowledgement

This article was published in Electra, April 2014, and is republished here with permission.

Contact Rob Stephen, Eskom, Tel 031 563-0063, rob.stephen@eskom.co.za

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