Sometimes in transformer step-down installations, a breakdown discharge may occur between the low and high voltage windings, as well as a significant increase in the potential difference on the low voltage windings. In connection with such cases, it became necessary to use protective devices, such as punch fuses. Now almost all step-down transformer substations use these protective devices.
Breakdown fuse
In the event of emergency situations in transformers between high and low voltage windings, breakdown and a significant increase in voltage on the transformer plates occur, which can damage all connected equipment. This phenomenon is called transient voltage, with it the voltage from the high side goes to the low side, destroying its insulation, since the low side may not be designed for high voltages. To avoid this, special equipment is used - a breakdown fuse.
There are several options for connecting low side windings. When connecting the low side windings to the star, the breakdown fuse of the transformer is connected to the neutral and then to the ground. When connecting the low side windings into a triangle, a fuse is connected to one end of the winding and then to ground.
What the fuse consists of
The breakdown fuse consists of two metal electrodes separated by a mica plate. The dimensions of the plate vary depending on the power and voltage of the windings of the low side of the transformer. Special holes are made in the plates for the passage of the discharge. For what it is necessary - we will explain below.
One of the fuse electrodes is connected to the neutral, or to one of the phases of the transformer, if there is no neutral. The use of these fuses greatly simplifies the monitoring and maintenance of transformer substations.
Operating principle
When the transition voltage occurs in the transformers, the voltage increases on the low side windings. In this case, a breakdown of the spark occurs, discharges pass through the holes in the mica plate between the electrodes of the breakdown fuse, thereby switching between them and the increased voltage goes through grounding. As mentioned above, the dimensions and thickness of the mica plate itself, as well as the holes in it, depend on the rated operating voltage of the high side of the transformer.
Such fuses are used when the voltage on the high side is higher than 3000 V, but if the voltage is lower than 3000 V, then they simply use blank grounding, or fuses according to a special order of the customer-consumer.
Specifications
Punch fuses with a rated operating voltage of 400 to 690 V are currently manufactured and used (in rare cases, fuses for normal operation voltage of 230 V are made on special orders), the breakdown voltage limits vary from 300 to 1000 V. The discharge gap between the electrodes varies from 0 , 08 to 0.3 mm, depending on the transition voltage.
The breakdown fuse can withstand a ground current of up to 200 A for 30 minutes. In this case, welding of working electrodes during breakdown often occurs. During the porcelain insulation test, 2000 V voltage is applied to the ends of the fuse electrodes for 1 minute. Normal insulation resistance must not be lower than 4 ohms. After passing the tests, a marking with an operating voltage is applied to the bottom of the porcelain case. All current-carrying parts of the fuse are nickel-plated, and the joints, fasteners are coated with zinc.
During installation, this protective device must be installed strictly symmetrically to the vertical axis. When installing transformers outdoors from above, the fuses are covered with a special cover to protect them from dust and moisture. Fuses are a one-off means of protection, that is, if a breakdown occurs through the mica plate, it should subsequently be replaced with a new one, especially if during the check of the breakdown fuses it was revealed that the electrodes were welded together.
Application
When calculating the power supply of any section of energy consumption, many special protection devices for electrical installations must be introduced in order to avoid their failure. As described above, one of these devices is a breakdown fuse. It is used to protect low voltage windings in a transformer installation with a voltage on the high side of 3000 V.
The main advantage of this type of fuse is its ease of manufacture, low cost, as well as ease of maintenance. Sometimes, according to the technical requirements of the customer’s conditions, installation organizations use analogs of breakdown fuses.