Large-capacity transformers generally use two types of cooling methods: forced-oriented air cooling (ODAF) and strong oil-oriented water cooling (ODWF). These two cooling methods have strong cooling effects. Its main working principle is to drive the hot oil in the transformer oil tank into the oil cooler with an oil pump, return to the oil tank after cooling, cool the windings and the oil tank. The oil cooler is made into a special shape that is easy to dissipate heat. The air in the fan or circulating water is used as the cooling medium to remove the heat from the oil. The guided cooling transformer adopts certain measures in the structure (such as oil-retaining cardboard and paper tube) to make the oil flow along a certain path. Guided cooling is adopted, and the cold oil at the pump port is sent to the oil passage between the coils, wire cakes and the oil passage of the iron core under a certain pressure, which can improve the cooling efficiency.
When strong oil air cooling is selected, when the oil pump and fan lose power supply, the transformer cannot run for a long time even with no load. Therefore, two independent power supplies should be selected for the cooler. This clause is generally specified in the bidding documents, and the bidders can meet the requirements and meet the requirements for the normal operation of the transformer. Generally, it is not listed as a veto and scoring item.
Neutral grounding method
There are generally three types of grounding methods for neutral points of power transformers: ungrounded, grounded via arc suppression coils, and directly grounded. In a neutral point ungrounded system, when single-phase grounding occurs, the symmetry of the three-phase system is not damaged, and the system can operate normally, but the voltage to ground of the non-grounded phase will increase accordingly, and long-term operation is not allowed. When the system capacity is large and the line is long, the ground arc may not extinguish by itself. In order to prevent the arc over-voltage, it can be grounded through the arc suppression coil. When single-phase grounding occurs, the inductive current in the arc suppression coil can compensate the capacitor current of the single-phase grounding. The direct neutral grounding method can reduce the cost of equipment insulation. The grounding method of the neutral point of the main transformer of the power plant is generally clearly stated in the special report of the access system, and it must be strictly implemented when ordering.
The short-circuit impedance is an important item in the performance index of the transformer. When the transformer is running at full load, the size of the short-circuit impedance has an impact on the secondary output voltage. The short-circuit impedance is small and the voltage drop is small. With large current, the transformer can withstand large electromotive force, short-circuit resistance is large, short-circuit current is small, and the electromotive force withstand by the transformer is small. The deviation between the actual measured value and the specified value at the time of shipment is strict, and it is generally assessed in accordance with not more than ± 5%. The short-circuit impedance deviation value can be included in the evaluation method as a scoring clause.
Transformer temperature rise
Temperature rise is the difference between the temperature of the transformer and the temperature of the surrounding air. In terms of transformer life, the main cause of insulation aging is temperature. Due to the uneven heat transmission inside the transformer, the temperature of various parts of the transformer is very different. Therefore, it is necessary to make provisions for the temperature rise of each part of the transformer at the rated load. This is the permissible temperature rise of the transformer. According to the insulation and heat resistance level, the temperature rise of the transformer needs to be corrected according to the altitude. Generally, air-cooled transformers are corrected by decreasing by 1 ° C for each 250m increase in altitude.
Efficiency and losses
Transformer loss can be divided into two categories: copper loss and iron loss. Copper loss is the loss of DC resistance generated when current flows through the transformer coil, which is directly proportional to the square of the load current, also called load loss. Iron loss is the hysteresis and eddy current loss in the transformer core. For the completed transformer, it is approximately the square of the primary voltage. Because the primary voltage is equal to the network voltage, it is also called constant loss (no-load loss). The best way to reduce the load loss is to improve the insulation structure of the transformer, and reduce the coil size by reducing the insulation volume, thereby reducing the load loss. To reduce no-load loss, a good silicon steel sheet should be used to improve the structure and technology of the core.
The efficiency of large transformers is above 99%. When preparing bidding documents, it is generally expected that the higher the efficiency, the better, and the lower the loss. The better, but subject to the existing wires, insulation materials, core materials and processing technology, the transformer's Efficiency and loss are close to their limits. Therefore, it is recommended that the article adopt the efficiency standard stipulated by the national standard as the veto clause, and also include the scoring clause, and set it according to the upper limit of the scoring proportion to highlight the importance of this parameter.
Partial discharge level
Partial discharge refers to an electrical discharge that is insulated between conductors. It is divided into bubble partial discharges and partial discharges in oil. The main causes of partial discharges are high voltage electric fields, sharp corners, burrs, and transformers in solid insulation. Caused by micro-bubbles in oil, air gaps in solid insulation, dust on the surface of insulators, and floating potentials in high-voltage electric fields. The occurrence of partial discharge does not cause breakdown of the entire via in a short time. However, it can erode the surrounding insulation, and gradually spread to form a channel and cause breakdown discharge. The harm is obvious. Therefore, the partial discharge level of the transformer must be strictly controlled. Generally, phenolic bolts, nuts and Weidmann's cardboard are used, with good insulation materials to ensure a low level of partial discharge. In the bidding documents, it is recommended that the material requirements for the insulation material be specified. The material requirements can be used as a veto clause to ensure that each company bids under the same conditions. The level of partial discharge can be used as the scoring clause.
Load dump and noise levels of transformers
When the generator dumps the load, the transformer should be able to withstand 1.4 times the rated voltage for 5 seconds without anomalies. This item generally requires the bidder to provide a transformer dynamic stability calculation report with a safety factor of not less than 1.8; at the same time, a sudden short-circuit impact test report should be provided. The transformer runs at full load under 100% forced oil circulation cooling mode, and the noise is not more than 80dB from the transformer body 2m.