When oil samples are analysed on site, the water content is usually the first indicator to be asked for, because it is easy to measure and gives a clear figure. But what often decides whether a transformer can still be operated safely is the breakdown voltage. The water content is a single variable, the breakdown voltage is a combined result, and the two are not linked by a one-to-one conversion.
What the breakdown voltage measures
The breakdown voltage measures the voltage strength needed to break down the oil sample under a specified electrode gap and a specified waveform. It does not reflect any single contaminant but the overall dielectric withstand capability of the oil, which is the combined result of water content, particles, dissolved gas, acid value and oxidation products. Where the water content of a batch is within limits but the breakdown voltage is low, the problem may lie with particles, with ageing products or with the sampling stage, rather than with the water content alone.
There is one precondition that is easily overlooked: the breakdown voltage is the result for the sample under standard test conditions, and the figure varies with the test method, the electrode gap and the waveform. Taking a figure on site and comparing it directly with the figure in a laboratory report seldom holds up, because the comparison is only valid when the methods are the same.
How the three types of interference act
- Water: in the emulsified state it has the strongest effect on the breakdown voltage; the effect of a small amount of dissolved water is milder, which is also why different measurement methods give different results.
- Particles: conductive metal swarf in particular forms a continuous conductive path in the electric field, which brings the breakdown voltage down sharply, and the extent of the drop is related to the number of particles and the size distribution.
- Gas and ageing products: dissolved gas and polar oxidation products change the dielectric properties of the oil, and they are the most direct signal that the oil has aged.
| Interference factor | Mechanism | Effect on the breakdown voltage |
|---|---|---|
| Emulsified water | The water phase forms a conductive path in the electric field | A clear decrease |
| Conductive particles | The particles align in the field and bridge the electrodes | A sharp decrease, with wide fluctuation |
| Dissolved gas | Reduces the dielectric strength of the oil | A gradual decrease |
| Oxidation products | Polar molecules adsorb onto the charged surfaces | A gradual decrease |
The trap in the sampling stage
The breakdown voltage is extremely sensitive to the condition of the sample. The sample container must be clean and dry, with a sealed cap, so that moisture and particles from the air cannot fall in; the sampling line should be flushed and drained of residual liquid first, otherwise residues and moisture on the pipe wall pull the reading down directly; the sample should be tested as soon as possible and should not stay in contact with the air for long. After treatment in the ZYD-100 the breakdown voltage of the oil can reach ≥65 kV (GB/T 507). This figure is obtained after a complete purification process, and it is hard to achieve consistently by fitting a new element on site.
A breakdown voltage within limits does not mean the oil is new. It only shows that the dielectric withstand capability is within limits; the water content, the cleanliness, the colour value and the acid value still have to be confirmed separately.