The conditions and related factors for transformer economically practical.

Generally believed that when the annual cost on load loss equals to no-load loss, the total cost is minimum. It will be the most economical.

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Transformer transfer rate is an important factor, which is the ratio of outgoing power (P2) and incoming power (P1).

If it reached 1 at run time, the efficiency would be improved.

How to make load balance at run time, is an important factor. The parallel operation can help a lot for that.
2 transformer maintenance

2.1 The benefit of maintenance

According to the experience from management and operation departments, in the first 10 years of running, especially the first 3 years, the failure rate was highest.

According to the U.S. experience,transformer oil purification for 765kv EHV transformer, the failure rate is high at initial period. Many of them happened on first few days, or few hours, or even a few minutes. So paying more attention on maintain during early time is important.

Although it might cost on maintenance and parts replacement, it can avoid serious consequence later.

It has been proved that good maintenance can extend the life of the transformer.

2.2 Tactics of transformer maintenance

There are three main methods: predictive maintenance, preventive maintenance, state maintenance.

Predictive maintenance is using automotive devices monitoring , along with  state analysis. For example, the infrared detection, visual inspection, the dissolved gas analysis , and index of transformer oil analysis.

Preventive maintenance is periodic maintenance. It includes planned maintenance, for example, regular testing, energized testing and on-line monitoring. It is planned after state analysis.
State maintenance is directed for a certain problem of transformer. It requires detailed analysis, inspection, and then repairing or parts replacement.

3 transformer on-site detection

3.1 Methods of on-site detection

Currently methods includes visual inspection, by portable devices (such as detectors, sensors, etc.), adsorbent and detection deformation of transformer winding.

Visual inspection:
1) Check the sound of transformer;

2) Check the bus connection and earthing from transformer casing;
3) Check the ceramic tube whether clean, damage, cracks and discharge burning etc.;

4) Check that the leakage of transformer, the status of oil color, and oil quantity;
5) Check the gas relay whether normal operation, and any action;
6) Check the intact of explosion tube diaphragm, is there cracks and residual oil;

7) Check the color of silica gel dehumidifiers;
8) Check the cooling device, usually, oil pressure should be 1 to 1.5 atm higher than water pressure. Water cooler should not have leakage.

3.2 On-site partial discharge test for larger transformer

On-site discharge test is to ensure re-operation security after transformer repair.

It’s an effective method to evaluation the insulation of transformer after repair. It can save cost on transport, and can find fault or failure on time if there is a problem.

3.3 Mensuration by polarization and  non-polarized current
In most cases, the winding displacement will not immediately lead to transformer failure. However, it can bring on insulation breakdown under over-voltage stress.

3.4 Field detection devices

3.4.1 Hydrogen testing device

Years of experience shows that no matter what type of fault and where it location, dissolved gas in oil will contain hydrogen.
As well know, at 500 degree Celsius high temperature transformer oil will produce hydrogen. The quantity of hydrogen depends on the failure type and point temperature.
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