The photovoltaic box transformer is an electrical device that uses the principle of electromagnetic induction to transform the low-value AC voltage output by the photovoltaic inverter into a higher-level AC voltage (see Figure 1). For centralized photovoltaic power plants, it is not suitable to be directly integrated into the grid. Because low-voltage grid-connection leads to too many photovoltaic grid-connected points, it is not conducive to electric energy metering and grid flow stability; at the same time, low-voltage grid-connection leads to excessive current, which is not conducive to the stable operation of switchgear.
According to the different cooling methods of transformers, it can generally be divided into two categories: dry-type transformers and oil-immersed transformers. Dry-type transformers are generally insulated by resin and are cooled by natural air, and large-capacity is cooled by fans; while oil-type transformers are insulated by insulating oil, and the heat generated by the coil is transferred to the radiator of the transformer by the circulation of insulating oil inside the transformer for heat dissipation. . From the perspective of power station design, according to the different site environment and the requirements of the electrical design code for construction design, oil-immersed transformers are generally used in areas with high dust and dust, and the use of dry-type transformers may cause more dust to accumulate and increase maintenance workload. . The following is an introduction to the specific equipment structure of different types of transformers:
(1) Dry-type transformer
Dry-type transformers are mainly composed of an iron core composed of silicon steel sheets and a coil cast by epoxy resin. An insulating tube is placed between the high and low voltage coils to increase electrical insulation, and the coil is supported and constrained by spacer blocks. Fasteners for overlapping parts All have loose performance (see Figure 2). Dry-type transformers have the advantages of strong short-circuit resistance, small maintenance workload, high operating efficiency, small size, and low noise.
Dry-type transformer cooling methods are divided into natural air cooling and forced air cooling (AF). With natural air cooling, the transformer can operate continuously for a long time under the rated capacity. When forced air cooling, the output capacity of the transformer can be increased by 50%. It is suitable for intermittent overload operation or emergency accident overload operation; due to the large increase in load loss and impedance voltage during overload, it is in a non-economic operation state, so it should not be kept in continuous overload operation for a long time.
(2) Oil-immersed transformer
Oil-immersed transformers are mainly composed of iron cores, windings, fuel tanks, oil pillows, insulating sleeves, tap changers and gas relays, etc. (see Figure 3). In a large-capacity transformer, the heat generated by the core loss can be The insulating oil is fully taken away during circulation to achieve a good cooling effect, and cooling oil channels are often provided in the iron core.
The body (winding and iron core) of the oil-immersed transformer is installed in an oil tank filled with transformer oil, which is welded with steel plates. The oil tank of medium and small transformers is composed of a tank and a tank cover. The body of the transformer is placed in the tank shell, and the body of the transformer can be lifted out for maintenance by opening the cover. In order to strengthen the insulation and cooling conditions, the transformer core and windings are immersed in an oil tank filled with transformer oil.
The performance characteristics of oil-immersed transformers mainly include:
(1) The low-voltage windings of oil-immersed transformers generally adopt a cylindrical structure with copper foil around the shaft, except for the small-capacity copper wires; The magnet is small, the elevator has high strength, and the ability to withstand short circuits is strong.
(2) The iron core and the winding are respectively fastened. The height of the device, the low-voltage lead and other fastening parts are all equipped with self-locking lock nuts, and the non-suspending structure is adopted, which can withstand the shock of transportation.
(3) The coil and iron core are vacuum dried, and the transformer oil is vacuum filtered and oiled to minimize the moisture inside the transformer.
(4) The oil tank adopts corrugated sheet and has a breathing function to compensate for the volume change of the oil caused by temperature changes, so there is no oil conservator, which reduces the height of the transformer.
(5) Because the corrugated sheet replaces the oil conservator, the transformer oil is isolated from the outside world, which effectively prevents the entry of oxygen and moisture and leads to a decrease in insulation performance.
(6) Oil-immersed transformers do not need to change oil during normal operation, which greatly reduces the maintenance cost of the transformer and prolongs the service life of the transformer.