News Details

Cooling method

Large-capacity transformers commonly adopt two cooling methods: forced directed air cooling (ODAF) and forced directed water cooling (ODWF), both of which have strong cooling effects. The main working principle is to use an oil pump to pump the hot oil from the transformer tank into the oil cooler, where it is cooled and then returned to the tank to cool the windings and the tank. The oil cooler is made into a special shape that is easy to dissipate heat, using fans to blow air or circulating water as the cooling medium to remove heat from the oil. For transformers with directed cooling, certain structural measures (such as adding oil baffle paperboards and paper cylinders) are taken to make the oil flow along a specific path. With directed cooling, the cold oil from the pump outlet is sent under certain pressure into the oil ducts between coils, between pancakes, and in the core, which can improve cooling efficiency.

 When using the forced oil and air cooling method, if the power supply to the oil pump and fans is lost, the transformer cannot operate for a long time even at no load. Therefore, two independent power supplies should be selected to supply the cooler. This clause is generally clearly specified in the tender documents, and bidders can meet the requirements to ensure normal operation of the transformer. It is generally not listed as a rejection or scoring item.

  Neutral point grounding method

 There are generally three methods for grounding the neutral point of a power transformer: ungrounded, grounded through an arc suppression coil, and directly grounded. In a system with an ungrounded neutral point, when a single-phase ground fault occurs, the symmetry of the three-phase system is not destroyed, and the system can continue to operate normally, but the phase-to-ground voltage of the non-faulted phases will increase accordingly, and long-term operation is not allowed. When the system capacity is large and the lines are long, the ground arc may not self-extinguish. To prevent arc overvoltage, the method of grounding through an arc suppression coil can be adopted. When a single-phase ground fault occurs, the inductive current in the arc suppression coil can compensate for the capacitive current of the single-phase ground fault. The direct grounding method can reduce the cost of equipment insulation. The grounding method of the neutral point of the main transformer in a power plant is generally clearly stated in the special report on system integration, and strict compliance is required when placing orders.

  Short-circuit impedance

 Short-circuit impedance is an important item in the performance indicators of a transformer. When the transformer operates at full load, the magnitude of the short-circuit impedance affects the output voltage on the secondary side. A small short-circuit impedance results in a small voltage drop; when the load is short-circuited, a small short-circuit impedance leads to a large short-circuit current, and the transformer withstands a large electrodynamic force, while a large short-circuit impedance results in a small short-circuit current and a small electrodynamic force. The deviation between the measured value at the factory and the specified value is strictly required, generally assessed as not exceeding ±5%. The deviation of the short-circuit impedance can be included as a scoring item in the bid evaluation method.

  Transformer temperature rise

 Temperature rise refers to the difference between the transformer temperature and the surrounding air temperature. In terms of transformer life, the main cause of insulation aging is temperature. Since the heat inside the transformer is not evenly distributed, the temperatures at various parts of the transformer vary greatly. Therefore, it is necessary to specify the temperature rise at each part when the transformer is at rated load. This is the allowable temperature rise of the transformer. According to the insulation heat resistance class, the transformer temperature rise also needs to be corrected based on altitude. Generally, for air-cooled transformers, the correction is a decrease of 1°C for every 250m increase in altitude.

  Efficiency and losses

 Transformer losses can be divided into two categories: copper loss and iron loss. Copper loss is the loss caused by the DC resistance when current flows through the transformer windings, and it is proportional to the square of the load current, also known as load loss. Iron loss is the hysteresis and eddy current loss in the transformer core, which is approximately proportional to the square of the primary voltage for a completed transformer. Since the primary voltage is equal to the network voltage, it is also called constant loss (no-load loss). The best way to reduce load loss is to improve the insulation structure of the transformer, reducing the coil size by reducing the insulation volume, thereby reducing load loss. To reduce no-load loss, good silicon steel sheets should be used, and the core structure and manufacturing process should be improved.

 The efficiency of large transformers is above 99%. When preparing tender documents, it is generally hoped that the efficiency is as high as possible and the losses are as low as possible. However, due to the limitations of existing conductors, insulating materials, core materials, and processing technology, the efficiency and losses of transformers are close to the limit. Therefore, it is recommended that this clause use the efficiency standard specified in national standards as a rejection clause, and also include it as a scoring item, with the upper limit of the scoring weight set to highlight the importance of this parameter.

  Partial discharge level

 Partial discharge refers to an electrical discharge that bridges the insulation between conductors. It is divided into bubble-type partial discharge and partial discharge in oil. The main causes of partial discharge include sharp corners, burrs on conductors and solid insulation in high-voltage electric fields, trace bubbles in transformer oil, air gaps in solid insulation, dust on the surface of insulation, and floating potentials in high-voltage electric fields. The occurrence of partial discharge will not cause breakdown of the entire path in a short time, but it can erode the surrounding insulation and gradually spread to form a channel, causing breakdown discharge. Its harm is obvious, so the partial discharge level of the transformer should be strictly controlled. Generally, phenolic bolts, nuts, and Weidmann paperboard are used, and good-quality insulating materials are used to ensure that partial discharge remains at a low level. In the tender documents, it is recommended to clearly specify the material requirements for insulating materials, which can be used as a rejection clause to ensure that all bidders compete under the same conditions, and the partial discharge level can be used as a scoring item.

  Transformer load rejection and noise level

 When the generator rejects load, the transformer should be able to withstand 1.4 times the rated voltage for 5 seconds without abnormal phenomena. This item generally requires the bidder to provide a transformer dynamic stability calculation report, with a safety factor not less than 1.8; at the same time, a sudden short-circuit impact test report should be provided. When the transformer operates at full load under 100% forced oil circulation cooling, the noise at a distance of 2m from the transformer body should not exceed 80dB.