1. Change the high voltage and high current of the primary circuit to the standard value of the secondary circuit.
Usually the voltage transformer secondary winding is rated at 100V or V. The current rating of the secondary winding of the current transformer is generally 5A or 1A.
2. The low-voltage secondary system is electrically isolated from the high-voltage primary system, and the secondary side of the transformer is grounded to ensure the safety of personnel and equipment.
The purpose of grounding the secondary winding of the transformer is to reduce the ground potential of the secondary system when the primary and secondary windings break down. The smaller the grounding resistance is, the lower the low potential is, so as to ensure personal safety, so it is called protection. Ground.
The primary winding of the three-phase voltage transformer is connected to the star and then the neutral point is grounded. The purpose is to make each phase of the first and second windings react to the ground voltage of each phase of the power grid to react to the ground short circuit fault. Therefore, the grounding is called Working ground.
3. Obtain the zero-sequence current and voltage components for the relay protection device with reactive ground fault.
The zero-sequence current of the branch circuit, so the secondary winding of the three-phase current transformer is connected in parallel, so that the total output current is the sum of the three-phase currents, that is, the zero-sequence current of the primary power grid is obtained. If a three-phase of a primary circuit (such as a cable circuit) is passed through a core, a zero-sequence current is output around the secondary winding on the core.
The zero-sequence component of the grid-to-ground voltage is three relative ground voltages. See Figure 4-22(d), (e). The voltage transformer capable of grounding monitoring has two secondary windings: the first secondary winding is connected in a star shape for general measurement and protection, and provides line voltage and phase voltage. The second auxiliary winding (also referred to as the auxiliary winding) is connected three-phase end to end to form the sum of the relative triangular voltages of the open delta reaction, that is, the zero sequence component of the ground voltage.