Ⅰ Classification
The grounding transformer artificially creates a neutral point grounding resistance to artificially ground the center point. With a grounding transformer, the electrostatic charges induced by overhead lines due to charged clouds and rain will be conducted to the ground, thus avoiding possible damage to lines and equipment.
Since many grounding transformers only provide a small resistance to the neutral point grounding and do not need to carry a load, many grounding transformers are non-secondary. When the power grid is operating normally, the grounding transformer is equivalent to a no-load state. When a single-phase ground fault occurs in the power grid, the highly sensitive zero-sequence protection determines and cuts off the faulty line in a short time. The protective action takes effect during the period of time when the faulty line is removed. Therefore, the operating characteristics of the grounding transformer are: no load for a long time and overload for a short time.
With a grounding transformer, the electrostatic charges induced by overhead lines due to charged clouds and rain will be conducted to the ground, thus avoiding possible damage to lines and equipment.
⒈Three-phase grounding transformer
This type of transformer adopts Z-shaped wiring (or zigzag wiring). The difference from ordinary transformers is that each phase coil is divided into two groups and wound on the magnetic column of the phase in opposite directions. The advantage of this connection is that the zero-sequence magnetic flux can flow along the The magnetic column flows, and the zero-sequence magnetic flux of the ordinary transformer flows along the magnetic leakage path, so the zero-sequence impedance of the Z-type grounding transformer is very small (about 10Ω), while the ordinary transformer is much larger. According to regulations, when using an ordinary transformer with an arc suppression coil, its capacity shall not exceed 20% of the transformer capacity. The Z-type transformer can be equipped with an arc suppression coil of 90% to 100% capacity. In addition to the arc suppression coil, the grounding transformer can also be equipped with a secondary load, which can replace the station transformer, thereby saving investment costs.
⒉Single phase grounding transformer
Single-phase grounding transformers are mainly used in neutral point grounding resistance cabinets of generators and Satons transformers with neutral points to reduce the cost and volume of the resistance cabinets.
Ⅱ Function characteristic
⒈Low zero-sequence impedance to ensure the output of zero-sequence current;
⒉High excitation impedance to reduce no-load current;
⒊Low no-load loss to save energy consumption in daily operation
Ⅲ Wiring method
YNyn connection
Transformers with this connection method generally use a three-phase three-column iron core, and the neutral point on the high-voltage side can be connected to an arc suppression coil to achieve grounding. However, when a single-phase grounded zero-sequence current flows through the high-voltage side winding, the generated zero-sequence magnetic potential cannot be balanced by the secondary magnetic potential, and the zero-sequence magnetic flux in the same direction cannot form a loop in the three-column core. , so that a large amount of zero-sequence magnetic flux can only pass through the clamp, oil and tank body to form a closed loop, causing additional losses in the tank and clamp, resulting in local overheating and limiting the utilization of the transformer capacity.
