The function and principle of voltage transformer equipped with primary harmonic eliminator
Before understanding the function of the voltage transformer to eliminate the neutral point harmonic, we may wish to discuss the neutral point operation mode of the power system. In the three-phase AC power system, as a power supply generator and transformer neutral point, there are three ways of operation: one is the neutral point of the power supply is not grounded; One is that the neutral point of the power supply is grounded through the arc suppression coil; One is the direct grounding of the neutral point of the power supply. The first two are called neutral non-effective grounding, or a small current grounding system, and the latter is called neutral direct grounding, or high current grounding.
1 The power system is grounded at neutral point through arc suppression coil, and arc suppression grounding has been taken into account in this system (as described in article 2 above). In the voltage transformer of the system, the Yo connection does not consider installing a primary harmonic eliminator.
2 We generally refer to the PT cabinet with a harmonic reducer, which refers to the nonlinear resistor installed in the 6-35kV electromagnetic voltage transformer (referred to as voltage transformer) once wound between the neutral point of the Yo junction line and the ground, which plays the role of damping and current limiting. In 6-35kV power generation and substation, we often encounter that the neutral point of the power grid is not grounded, and the electromagnetic voltage change primary winding of the Yo connection on the parent line becomes the only metal channel for the neutral point ungrounded power grid to the ground, and the charging and discharging path of the power grid relative to the ground capacitance must pass through the voltage change primary winding. This slow change process saturates the depth of the transformer core. When the grounding of the power network disappears, the inrush current of several amps will appear in the transformer primary winding, and the voltage transformer 0.5A high voltage fuse will be blown. Even if this inrush current has not reached the fusing value of the fuse, it still exceeds the rated current of the voltage transformer, and the voltage transformer operating in the overcurrent state for a long time will be burned, and then cause other accidents. If a primary attenuator is selected, this phenomenon will not occur. When the single-phase grounding capacitance current is less than a certain value, it will not produce a larger inrush current in the voltage change primary winding, and has no effect on the voltage change and high voltage fuse, from the perspective of economy and product cost, it can not be installed. If requested by the customer, the installation of a detuning device on the primary side of the voltage transformer will add a layer of protection to the operation of the equipment.
3 Mention the voltage change and install a harmonic reducer, do not mistake as long as the PT cabinet is installed, because in the 2PT cabinet, the voltage transformer is V-V wiring, mainly used for measurement, measurement, insulation monitoring, there is no problem of neutral point grounding (there can be no charging and discharging path of the grid relative capacitance), there is no need to install a harmonic reducer.
4 In some engineering designs, the user has access to a certain size of capacitors on the bus side according to the actual situation of the on-site power grid, so that the ratio of the capacitive impedance (Xc) and inductive impedance (XL) of the line is less than 0.01, which can avoid resonance. In this distribution system, the neutral point of the voltage transformer does not need to install a harmonic eliminator.