What is grounding and earthing transformer ?
A grounding transformer is also called earthing transformer. Which is a type of auxiliary transformer used for generating a earthing fault current (when the fault occur) to neutral for relay protection purpose in three-phase electric power systems. It is a grounding / earthing path to either an ungrounded wye or a delta-connected system.
Zig-zag Grounding / Earthing transformer
Grounding / Earthing transformer usually incorporate a single winding transformer with a zig -zag winding configuration. Some of is wye-delta winding transformer.
Neutral Grounding / Earthing transformer
It is very common on generators in power plants and wind farms. Neutral grounding transformers are applied on high-voltage (sub-transmission) systems, such as at 33 kV system circuit not have a grounding.
Principles and function of Grounding / Earthing transformer
With the need of urban construction and development and the increase of power supply load, many places 132 / 11kV terminal substations are being built in urban areas. The voltage 132kV incoming line is adopted on the primary side. With the majority of the 11kV outgoing in urban areas are overhead cable outgoing, and the single-phase capacitive current in the 11kV distribution network will increase sharply.
According to the "Overvoltage Protection and Insulation Cooperation of AC Electrical Devices" of the former Ministry of Electric Power Industry, when the single-phase ground fault capacitor current of 3-66KV system exceeds 10A, the arc elimination coil grounding mode should be adopted. In the general 132 / 11kV substation, the transformer low voltage side is △ wiring, and the system low voltage side has no neutral point to lead out the transformer. Therefore, the 11kV grounding transformer setting should be considered in the substation design.
The principle of Grounding / Earthing Transformer
For the triangular connected distribution system, to cause the neutral point of the system, it must be connected to the ground transformer. There are two kinds of grounding transformer: Z-type grounding transformers (ZN, ZN, yn) and star / triangular connection transformer (YN, d). Now, the multiple Z-type grounding transformer, its neutral point can be connected to the arc elimination coil.
The Z-type grounding transformer is the same as the ordinary three-phase core power transformer in structure, but the winding on each phase iron core is divided into two parts of the equal number of upper and lower turns, which are connected in twists and turns. Different wiring modes are divided into ZN, yn1 and ZN and yn11.
The zero order current direction in the winding of the same column of the Z-type grounding transformer is opposite, so the zero order reactance is very small, and there is no choke effect on the zero order current. When the neutral point of the Z-type grounding transformer is connected to the arc suppression coil, the compensation current of the arc suppression coil can flow freely, so the Z-type transformer is widely used as the grounding transformer.
Z-type grounding transformer, can also be equipped with low-voltage winding, connected into a star neutral point grounding (yn) and other ways, used as the transformer used.
The Z-type grounding transformer has oil immersion type and dry type insulation two kinds, among which the resin pouring type is a kind of dry type insulation.
Scope of application: Suitable for oil-immersed Z-type grounding transformer with a capacity of 220 kVA and a voltage of 33 kV and below.
For the 33KV and 66KV distribution network
The transformer winding usually adopts the Y connection method, with a neutral point leading out, so there is no need to use the grounding transformer.
For the 6KV and 11KV distribution network
The transformer winding usually adopts the △ connection method, with no neutral point to lead out, which requires the grounding transformer to lead out to the neutral point.
The function of the grounding transformer is to draw out the neutral point to connect the arc elimination coil when the system is △ wiring or Y wiring neutral point is not drawn.
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The grounding transformer adopts Z-type wiring (or winding wiring),
That is, each phase coil is wound around two magnetic columns, and the zero order magnetic flux generated by the two-phase winding counteracts each other, so the zero order impedance of the Z-type grounding transformer is very small (generally less than 10 Ω), the no-load loss is low, and the transformer capacity can be used by more than 90%.
The ordinary transformer zero-order impedance is much larger, the arc elimination coil capacity generally should not exceed 20% of the transformer capacity, so it can be seen that the Z-type wiring transformer as a grounding transformer is a better choice.
Generally, when the system imbalance voltage is large, the three-phase winding of Z transformer made into balance type can meet the measurement needs. When the system unbalanced voltage is small (e. g., the full cable network), the neutral point of the Z-type transformer should make an unbalanced voltage of 30 V to 70 V to meet the measurement needs.
Grounding transformer can take arc elimination coil, grounding transformer can also take secondary load, instead of the station transformer. With the secondary load, the primary capacity of the grounding transformer shall be the sum of the arc-reducing coil capacity and the secondary load capacity.
1-phase earthing 3-phase without secondary winding 3-phase with secondary winding
Function of Grounding / Earthing transformer
The grounding transformer is specially designed for the arc suppression coil. Generally, the arc suppression coil is installed on the triangle side of the transformer in the small current grounding system to compensate the grounding capacitive current when the grid is single-phase grounded. However, there is no neutral point on the triangle side of the transformer, and the grounding transformer provides artificial neutral point for the installation of arc suppression coil.
In the power system, 6kV, 11kV and 33kV power grids generally adopt the operation mode of ungrounded neutral point. In the power grid, the distribution voltage side of the main transformer is generally delta connected, and there is no neutral point for the grounding resistance.
When a single-phase grounding fault occurs in a neutral ungrounded system, the line voltage triangle remains symmetrical, which has little impact on users' continuous work, and when the capacitive current is relatively small (less than 10A), some transient grounding faults can disappear automatically, which is very effective for improving power supply reliability and reducing power outage accidents. Because of its simple operation mode and low investment, this operation mode has been adopted in the initial stage of China's power grid and has played a good role.
However, with the growing growth and development of the electric power industry, this simple way is no longer able to meet the current demand. With the increase of cable circuits in urban power grids, the capacitive current is increasing (more than 10A). At this time, the grounding arc cannot be reliably extinguished, which will lead to the following consequences.
1. Intermittent extinction and reignition of single-phase grounding arc will produce arc grounding over-voltage with amplitude up to 4U (U is the peak value of normal phase voltage) or higher and long duration, which will cause great harm to the insulation of electrical equipment and cause breakdown at weak insulation; Cause heavy losses.
2. Due to the air dissociation caused by the continuous arc, the insulation of the surrounding air is damaged, and the interphase short circuit is easy to occur;
3. Ferromagnetic resonance over-voltage is generated, which is easy to burn out the voltage transformer and cause damage to the arrester, and may even cause the arrester to explode;
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 accidents, provide sufficient zero sequence current and zero sequence voltage for the system, and make the grounding protection act reliably, it is necessary to manually establish a neutral point, so as to connect the grounding resistance at the neutral point. In order to solve this problem. The grounding transformer (referred to as grounding transformer for short) is produced under such circumstances. The grounding transformer is a man-made neutral grounding resistance, which is generally very small (generally less than 5 Ω).
In addition, the grounding transformer has electromagnetic characteristics and high impedance to positive and negative sequence currents. Only a small excitation current flows through the winding. Since the winding directions of the two windings on each iron core column are opposite, the zero sequence current flowing through the two windings on the concentric column is of low impedance, and the voltage drop of the zero sequence current on the winding is very small.
When the system has ground fault, positive sequence, negative sequence and zero sequence currents will flow through the windings. The winding presents high impedance for positive and negative sequence currents, while for zero sequence currents, because two windings in the same phase are connected in reverse polarity in series, the induced electromotive force is equal in size and opposite in direction, just offset each other, so it presents low impedance.
In the working state of grounding transformer, many grounding transformers only provide small neutral grounding resistance without load. Therefore, many grounding transformers are non secondary. When the grounding transformer operates normally in the power grid, the grounding transformer is equivalent to the no-load state.
However, in case of power grid fault, when single-phase grounding fault occurs in the power grid through the fault current only in a short time, and the neutral point is grounded through a small resistance, the highly sensitive zero sequence protection judges and removes the fault line for a short time, and the grounding transformer only functions during the period from the grounding fault to the time when the fault line is removed by the zero sequence protection action of the fault line, Only the neutral point grounding resistance and grounding transformer can pass the zero sequence circuit with IR=(U is the system phase voltage, R1 is the neutral point grounding resistance, R2 is the additional resistance for grounding fault bribery). According to the above analysis, the operating characteristics of the grounding transformer are:; No load for a long time, overload for a short time.
In short, the grounding transformer is artificially made a neutral point to connect the grounding resistance. When ground fault occurs in the system, the positive sequence negative sequence current shows high impedance, and the zero sequence current shows low impedance, which makes the ground protection act reliably.
The function of the grounding transformer is to lead out the neutral point for adding arc suppression coil when the system is delta type wiring or Y-type wiring neutral point cannot be led out. The transformer uses Z-type wiring (or zigzag wiring). The difference between the transformer and ordinary transformer is that each phase coil is wound on two magnetic poles respectively. The advantage of this connection is that the zero sequence magnetic flux can flow along the magnetic pole, while the zero sequence magnetic flux of ordinary transformer flows along the magnetic leakage circuit, Therefore, the zero sequence impedance of Z-type grounding transformer is very small (about 10 Ω), while that of ordinary transformer is much larger.
Therefore, the regulations stipulate that when ordinary transformers are used with arc suppression coils, their capacity shall not exceed 20% of the transformer capacity, while Z-type transformers can be used with arc suppression coils of 90%~100% capacity. In addition to the arc suppression coils, grounding transformers can also be used with secondary loads, which can replace the transformers used, thus saving investment costs.
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