The grounding transformer is divided into ZNyn wiring (Z type) or YNd wiring according to the wiring method, and its neutral point can be connected to the arc suppression coil or the grounding resistance grounding. Nowadays, Z-type (zigzag type) grounding transformers are mostly used for grounding through arc suppression coils or small resistances.
1. Z-type grounding transformer
There are two types of Z-type grounding transformers: oil-immersed and dry-type insulation. Among them, the resin casting type is a kind of dry-type insulation. Its structure is the same as that of ordinary three-phase core power transformers, except that the windings on each phase core are divided into The upper and lower parts are two parts with equal number of turns, and then the end of each phase winding is connected in series with the end of the other phase winding.
Z-type grounding transformers can also be equipped with low-voltage windings, connected to star-shaped neutral point grounding, etc., and used as station transformers.
The advantages of the Z-type transformer zigzag connection method: ① In the case of a single-phase short circuit, the ground current is roughly evenly distributed in the three-phase windings, and the magnetic potentials of the two windings on each column are opposite, so there is no damping effect, and the current can flow smoothly from the neutral point to the line;
②There is no third harmonic component in the phase voltage of the winding, because the three single-phase transformers connected in a zigzag way have the characteristics that the vectors of the third harmonic are in the same direction and the same size. The electromotive force is close to a sine wave.
The zero-sequence currents in the two half windings on the same iron core column of the Z-type grounding transformer are in opposite directions, so the zero-sequence reactance is very small, and there is no choke effect on the zero-sequence current.
The principle of reducing the zero-sequence impedance is: in each phase of the three-phase iron core of the grounding transformer, there are two windings with the same number of turns, which are respectively connected to different phase voltages.
When the three-phase positive and negative sequence voltages are added to the line end of the grounding transformer, the magnetomotive force generated on each iron core column of the grounding transformer is the phasor sum of the two-phase winding magnetomotive force.
The synthetic magnetomotive force on a single iron core column differs by 120°, which is a set of three-phase balance quantities. The single-phase magnetomotive force can form a magnetic path with each other on the three iron core columns, the magnetic resistance is small, the magnetic flux is large, the induced electromotive force is large, and the excitation impedance is large.
When the zero-sequence voltage is applied to the three-phase line end of the grounding transformer, the magnetomotive force generated in the two windings on each iron core column is equal in magnitude and opposite in direction, and the resultant magnetomotive force is zero, and the three-phase iron core column is There is no zero sequence magnetomotive force.
The zero-sequence magnetomotive force can only form a closed loop through the shell and the surrounding medium, the magnetoresistance is large, and the zero-sequence magnetomotive force is small, so the zero-sequence impedance is also small.
In order to meet the needs of arc suppression coil grounding compensation in the distribution network, and also meet the needs of power and lighting loads in substations, the main parameters of the grounding transformer need to be reasonably set when selecting a transformer connected by Z-type wiring.
1. Rated capacity
The capacity and needs of the primary side of the grounding transformer are matched with the capacity of the arc suppression coil. According to the capacity specification of the existing arc suppression coil, it is recommended to set the capacity of the grounding transformer to 1.05-1.15 times the capacity of the arc suppression coil.
2. Zero sequence impedance
The zero-sequence impedance is an important parameter of the grounding transformer, which has an important influence on the relay protection limiting single-phase grounding short-circuit current and suppressing overvoltage.
For zigzag (Z-type) and star/open delta connected grounded transformers without secondary coils, there is only one impedance, the zero-sequence impedance, so that the manufacturing department can meet the requirements of the power department.
Loss is an important performance parameter of grounding transformer. For grounding transformer with secondary coil, its no-load loss can be the same as that of double-winding transformer of the same capacity.
For the load loss, when the secondary side is running at full load, due to the light load on the primary side, the load loss is smaller than the load loss of a dual-winding transformer with the same capacity as the secondary side.