According to the filling medium, the grounding transformer can be divided into oil type and dry type. According to the number of phases, the grounding transformer can be divided into three-phase grounding transformer and single-phase grounding transformer.
The function of the grounding transformer is to draw out the neutral point to add arc suppression coil or resistance when the system is △-type connection or Y-type connection. The difference with ordinary transformers is that each phase coil is divided into two groups and wound on the magnetic column of that phase. The advantage of this connection is that the zero-sequence magnetic flux can flow along the magnetic column, while the zero-sequence magnetic flux of the ordinary transformer is along the magnetic column. The leakage magnetic circuit flows, 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 ordinary transformers with arc suppression coils are used, their capacity shall not exceed 20% of the transformer capacity. The Z-type transformer can be equipped with a 90%-100% capacity arc suppression coil. In addition to the arc suppression coil, this kind of transformer earthing types can also be equipped with a secondary load, which can replace the transformer used, thereby saving investment costs.
Single-phase grounding transformer is mainly used for neutral grounding resistance cabinets of generators and transformers with neutral points to reduce the cost and volume of resistance cabinets.
1. Intermittent extinguishment and reignition of single-phase grounding arcs will produce arc grounding overvoltages, the amplitude of which can reach 4U (U is the normal phase voltage peak) or higher, and the duration is long, which will insulate electrical equipment Cause great harm, cause breakdown in weak insulation; cause heavy losses.
2. The dissociation of the air caused by the continuous arc destroys the insulation of the surrounding air and is prone to short-circuit between phases.
3. Ferroresonance overvoltage is generated, which may easily burn out the voltage transformer and cause damage to the arrester or even explosion of the arrester. These consequences will seriously threaten the insulation of power grid equipment and endanger the safe operation of the power grid.
In order to prevent the occurrence of the above-mentioned accidents and provide enough zero-sequence current and zero-sequence voltage for the system to make the grounding protection work reliably, it is necessary to artificially establish a neutral point to connect the grounding resistance at the neutral point. Grounding transformer (referred to as grounding transformer) is produced under such circumstances. The grounding change is to artificially create a neutral grounding resistance, and its grounding resistance is generally very small (generally required to be less than 5 ohms). In addition, the grounding transformer has electromagnetic characteristics, and has high impedance to the positive sequence and negative sequence currents, and only a small excitation current flows through the windings. Since the winding directions of the two windings on each iron core column are opposite, the two windings on the concentric column flow with equal zero-sequence current and present low impedance, and the voltage drop of the zero-sequence current on the windings is very small. That is, when a ground fault occurs in the system, positive sequence, negative sequence and zero sequence currents will flow in the windings. The windings present high impedance to the positive sequence and negative sequence currents. For the zero sequence currents, due to the The two windings are connected in series with opposite polarity, and the induced electromotive force is equal in magnitude and opposite in direction, which exactly cancels each other out, so it presents a low impedance.
Since many grounding transformers supplied by distribution transformer manufacturers only provide a small resistance for neutral 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 the no-load state. However, when the power grid fails, the fault current will only pass in a short period of time. When a single-phase grounding fault occurs in the power grid with a neutral point grounded by a small resistance, the high-sensitivity zero-sequence protection judges and cuts off the faulty line in a short time. The grounding transformer only works during the period from the ground fault to the removal of the faulty line by the zero-sequence protection action of the faulted line, and the neutral grounding resistance and the grounding transformer will pass through the zero-sequence current.
The grounding transformer is to artificially create a neutral point to connect the grounding resistance. When a ground fault occurs in the system, it has high impedance to the positive sequence and negative sequence currents, and low impedance to the zero sequence currents, so that the grounding protection can operate reliably.