Power distribution transformer with three-phase unbalanced loads or seasonal overloads

From the investigation results, it can be seen that there are a large number of unbalanced three-phase loads in power distribution transformer, especially in rural areas, where most of the electricity loads are single-phase loads and the load varies greatly.

Therefore, many power distribution transformers have three-phase load imbalances, which result in the asymmetric operation of three phases, producing zero-sequence current. On the one hand, this increases the losses of the transformer, and on the other hand, it reduces the effective capacity of the transformer.

Both of these situations will cause the transformer to overheat, the insulation oil to age, and the insulation level of the winding to decrease, ultimately leading to the damage of the transformer. 

The following measures can be taken:

• Investigate the load situation of the power distribution transformer, including the load for 24 hours a day and the load for four seasons in a year, and adjust the three-phase load as much as possible to make it close to symmetrical operation;

• Adjust the peak and valley time of electricity use to reduce overload; at the same time, increase the capacity of the transformer in a timely manner to avoid long-term overloading.

Poor grounding of power distribution transformer

The lightning protection work for power distribution transformer after being struck by lightning is generally done, but there are still two problems:

1. The grounding of the lightning arrester is poor;

2. Attention is only paid to installing lightning arresters on the high-voltage side, while neglecting the need to install lightning arresters on the low-voltage side (especially in areas with multiple lightning strikes). If the grounding of the lightning arrester is poor, when overvoltage occurs, the lightning arrester cannot discharge the current well, leading to insulation damage to the transformer.

If no lightning arrester is installed on the low-voltage side, when the lightning arrester on the high-voltage side discharges a large amount of lightning current to the ground, a voltage drop will occur at the grounding position, which will act on the neutral point of the low-voltage side winding at the same time as passing through the wave impedance of the low-voltage line. The following measures can be taken:

• Find out the bad grounding of the lightning arrester and reorganize it as required. It is important to connect the grounding wire of the lightning arrester directly to the outer shell of the transformer and the neutral point of the low-voltage side, and then use a common grounding device. The grounding resistance should not exceed 4Ω;

• For areas with multiple lightning strikes, install an additional set of low-voltage lightning arresters on the low-voltage side.

Oil seepage and leakage of power distribution transformer

There are also many instances where transformer oil leaks in the power distribution transformer. As the leakage reduces the amount of oil in the transformer, lowers the oil level, and allows air and moisture to enter, it accelerates the oxidation of the oil, making it deteriorate. As a result, the viscosity of the oil increases, the flow rate decreases, and the transformer's heat dissipation is affected, resulting in a high temperature rise, further accelerating the deterioration of the oil.

At the same time, the acidity of the deteriorated oil increases, causing the insulation resistance of the winding to decrease, and even damaging the insulation. If this continues, it will inevitably lead to the damage of the transformer.

The following measures can be taken:

• Find out where the oil is leaking and handle it properly;

• Check whether the transformer oil has deteriorated or not and perform a simple analysis of the oil. If the transformer oil gradually changes from pale yellow to orange or brown, and the viscosity of the oil is higher, it means that the transformer oil has deteriorated and must be purified or replaced;

• If the transformer oil has not deteriorated, check whether the oil level is too low. If so, add oil to the level indicated on the oil storage tank of the power distribution transformer;

• Check the insulation resistance of the winding.