Distribution transformer replacement

       

What are the decision factors in replacing an operating distribution transformer with a new one

Usually, distribution transformers are only replaced after failing.

Such failure occurs when the quality of the internal insulation system fails. This typically happens when the degree of polymerisation of the insulating paper drops below a threshold value. In such cases, the paper becomes very brittle and the breakdown voltage is reduced. A failure may be caused by a lightning strike or a fault on the line, or, in the worst case, an internal arc can occur without any external trigger.

In theory, distribution transformers don’t have an age limit. If they are constructed, operated and maintained well, the insulation paper can preserve its quality for a very long time. Some of them have been in operation for 60 years. On the other hand, newly purchased transformers can fail when circumstances are bad. So, age is a bad criterion for replacing distribution transformers that are still operational.

Four criteria

The following four criteria can trigger a replacement before failure:

1. Regulation

Most OECD countries have regulations for phasing out PCB insulating oil transformers. In the meantime, most of those transformers have been exchanged in the OECD.

2. Doubts about reliability

• To assess the reliability of a transformer, a risk analysis has to be executed. The following aspects play a role in such an analysis:
  • The required level of reliability on the concerned location in the grid. For example, supplying a hospital or an airport radar calls for a high reliability, while the presence of a back-up transformer reduces the required reliability level.
  • The state of the transformer. How can this be assessed?
    • Punctual measurements can be executed on one transformer of a series, or on one transformer on a particular key location. Useful measurement data include a Dissolved Gas Analysis (DGA) of the oil, the water content in the oil, the acidity of the oil, the content of furan derivates in the oil, and the Degree of Polymerisation (DP) at key spots in the paper insulation (see annex on transformer insulation chemistry).
    • Permanent condition monitoring - such as is done on many power transformers - is considered too expensive for distribution transformers.
    • Reliability statistics of particular series, models or manufacturers might be useful when extensive measurements are judged too expensive.
  • The short-circuit current, which depends on the location of the transformer in the network. The weaker the grid, the lower the short-circuit current. If this value is high, there is a high risk that the next short circuit will destroy the transformer.
  • The risk of lightning strikes. A new transformer has more chance of surviving a lightning strike than an older, degraded one.
• The cost of maintenance if the transformer remains in operation. This cost originates, among other things, from the addition of oxidation inhibitors in the oil, oil bath replacements, the removal of paper insulation decay products and additional condition measurements.
• When considering replacement on reliability criteria, one should also take into account what can be gained in terms of efficiency.

3. Doubts about energy efficiency

These doubts may come from the fact that the transformer is old and in the meantime more efficient types of transformer have become available and affordable, or one may discover that the transformer was not well chosen for the concerned location in the network. When such doubts arise, the following actions are required:

• A comparative calculation of the total cost of losses (no-load losses + load losses) between the two scenarios (keeping the current transformer until its end of life, or replacing it immediately with a suitable energy efficient model on the market).
  • The basic calculation is simple. Suppose the old transformer is currently 20 years old, and the estimated lifetime of both the old and the new transformer is 40 years. Suppose also that with the estimated load profile, the total losses are A kWatt for the old type and B kWatt for the new type. If transformer is exchanged only at the end of its life, the total integrated losses over the next 40 years are: (20 y x A kW + 20 y x B kW). If the transformer is replaced right away, the total integrated losses are (40 y x B kW).
  • What complicates the above calculation is that good estimates are required for the future load profile, the residual lifetime of the old transformer, and the life expectancy of the new transformer. This last figure in particular might pose problems, since no field data is available on the newest types of transformers. Moreover, to translate the figure of energy losses into a financial cost, one needs good estimates of interest rates and electricity prices.
  • In principle, one should also consider the decline in energy efficiency over time due to deteriorating insulation quality and transformer repairs.
• The cost of losses should be added to the depreciation of the purchase and installation cost to gain an idea of the Total Cost of Ownership (TCO) of both scenarios. This depreciation is not the one used in accounting, but should be made over the complete estimated lifetime of the transformer. In the replacement scenario, the depreciation of the old transformer continues during the remaining life that was estimated for this transformer.
• The premium for recycling the materials of an old transformer should be subtracted from the purchase cost of the new transformer. Although this premium will be taken into account in both scenarios, it will nevertheless influence the result. Due to this premium, the weight of the purchase cost in the TCO is reduced and thus the weight of the energy losses increased. Consequently, taking the recycling premium into account will be in favour of the replacing scenario.
• When considering a replacement for energy efficiency reasons, the decrease of maintenance costs and the gain in reliability should also be taken into account.

4. A fundamental change of load profile

Such a change has its effect on both the reliability and the energy efficiency. Consequently, both criteria 2 and 3 should be investigated.

Conclusion

In the case of a distribution transformer, it will be difficult to quantify all criteria into a reasonably exact estimate of reality. As a result, the decision to replace a transformer or not will be based on an intuitive weighing of all the above criteria by a skilled engineer, rather than the result of a TCO equation.

Nevertheless, drawing the complete TCO equation can be of help to gain insight into the issue.

Such a cost equation essentially comprises two parts.

1. The depreciation of the purchase and installation cost over the actual lifetime of the transformer. The recycling price premium is reduced from this cost.
2. The variable costs, consisting of: the no-load losses, the load losses, maintenance costs, and the risk of an unexpected failure (calculated as a cost). All of these costs increase as the transformer grows older.

The TCO is then calculated by integrating both of the above cost functions over time and adding them up.

Addendum: basic transformer insulation chemistry

The dielectrical insulation of a transformer consists of oil and paper. The quality of this insulation can deteriorate due to two phenomena: oil oxidation and paper depolymerisation.

1. Oil oxidation. This is accelerated by the presence of oxygen and moisture in the oil, and by increased temperature levels. Oil oxidation results in acidic materials that are present in the oil, and eventually in sludge. Useful measurements are water content (preventive), presence of acidic materials, and presence of sludge particles. Oxidation can be restricted by oxidation inhibitors. They only function for a limited period of time, after which a new inhibitor has to be added, or the oil bath must be replaced.
2. Paper degradation. This is accelerated by increased temperature levels and by the presence in the oil of moisture and acidic materials (formed by oil oxidation). When paper degrades, its cellulose molecular chains shorten. Furan derivates, as well as CO and CO2 gasses are released in the oil. Useful measurements are the water content of the oil (preventive), acidic materials in the oil (preventive), a Dissolved Gas Analysis (DGA) of the oil, the concentration of furan derivates in the oil, and the degree of polymerisation (DG) of the insulating paper at critical spots.