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The 6kV, 10kV and 35kV power grids in the power system generally adopt the neutral ungrounded operation mode. The low-voltage side of the main transformer in the power grid is generally connected in a triangle way, and there is no neutral point that can be grounded. When a single-phase grounding fault occurs in a neutral ungrounded system, the line voltage triangle remains symmetrical. The power system can continue to supply power to users for 1 to 2 hours, and the capacitive current is relatively small (less than 10A), which will not cause intermittent arc. Some transient grounding faults can disappear automatically, which is very effective for improving power supply reliability and reducing power outage accidents. However, with the continuous expansion of urban power grid and the continuous increase of cable outgoing lines, the capacitive current of the system to the ground increases sharply, and the capacitive current flowing through the fault point after single-phase grounding is large (more than 10A).






The arc is not easy to extinguish, it is easy to excite ferromagnetic resonance overvoltage and produce intermittent arc light grounding overvoltage, which may cause insulation damage, trip the line and expand the accident

Specifically:




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.




The air dissociation caused by the continuous arc destroys the insulation of the surrounding air and is prone to phase to phase short circuit.




Ferromagnetic resonance over-voltage may easily burn out the voltage transformer and cause damage to the arrester, which 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 reduce the capacitive current to the ground in case of single-phase grounding fault, it is necessary to install arc suppression coil and other compensation devices at the neutral point of the transformer. Therefore, it is necessary to manually establish a neutral point so that the arc suppression coil can be connected at the neutral point to reduce the grounding short-circuit breaking current and improve the reliability of the system power supply.





■ Current use at home and abroad





The grounding transformer in China usually adopts Z-type wiring (or zigzag wiring). In order to save investment and substation space, a third winding is usually added to the grounding transformer to replace the transformer used to supply power for the equipment used in the substation. According to the national standard of Reactor, the grounding mode of grounding transformer can be divided into direct grounding; It is grounded through reactor, resistance and arc suppression coil. Direct grounding has not been used in China, but some electric power research departments have begun to discuss this aspect. The grounding transformer in foreign countries usually adopts or Z-type connection, which is used for 10kV ungrounded system and constitutes the grounding protection of distribution network. When the system has a grounding fault, the grounding transformer presents high impedance to positive sequence and negative sequence current and low resistance to zero sequence current, making the grounding protection operate reliably.



■ Three phase grounding transformer

Three phase grounding transformer This type of transformer uses Z-type wiring (or zigzag wiring). The difference from ordinary transformers is that each phase coil is divided into two groups and wound on the magnetic pole of this phase in reverse. 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 transformers flows along the magnetic leakage circuit. Therefore, the zero sequence impedance of Z-type grounding transformers is very small (about 10 Ω), while that of ordinary transformers is much larger. According to the regulations, the capacity of ordinary transformer with arc suppression coil shall not exceed 20% of the transformer capacity. Z-type transformer can be equipped with arc suppression coil with capacity of 90%~100%. In addition to arc suppression coil, grounding transformer can also be equipped with secondary load, which can replace station transformer, thus saving investment cost.





■ Single phase grounding transformer

Single phase grounding transformer Single phase grounding transformer is mainly used for generator with neutral point and neutral point grounding resistance cabinet of Satons transformer to reduce the cost and volume of resistance cabinet.






■ Working characteristics






(1) Low zero sequence impedance to ensure the output of zero sequence current;






(2) High excitation impedance to reduce no-load current;






(3) Low no-load loss to save energy consumption for daily operation.






■ Wiring mode





YNyn connection





The transformer with this connection mode generally adopts three-phase three column iron core, and the neutral point on the high-voltage side can be connected with arc suppression coil to realize grounding. However, when the 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 iron core, so that a large number of zero sequence magnetic flux can only pass through the clamp, oil and oil tank body to form a closed loop, thus causing additional loss in the oil tank and clamp, resulting in local overheating, The utilization of transformer capacity is limited. The relevant operation regulations of China's power sector have made the following provisions on the working state of the neutral point connection arc suppression coil of YNyn connection transformer:






(1) The capacity of arc suppression coil shall not exceed 20% of the rated capacity of transformer;






(2) The zero sequence voltage drop generated by the zero sequence current flowing through the arc suppression coil in the transformer shall not exceed 10% of the rated phase voltage;

The connection mode of YNd connection transformer and arc suppression coil XL is characterized by that the triangular connection on the secondary side can provide a closed path of zero sequence current, so the zero sequence reactance is small. In addition, since the zero sequence magnetic potential of the primary and secondary windings on each core column is balanced, the zero sequence magnetic leakage is also small. However, when the YN connection winding is outside, the zero sequence additional loss caused in the oil tank and other components cannot be completely avoided. When it is connected with arc suppression coil, the utilization of its capacity will still be limited. The foreign test research shows that the allowable working mode of YNd connected grounding transformer is:





(1) When the secondary full load is normal, the capacity of the arc suppression coil connected at YN side shall not exceed 50% of the rated capacity of the transformer;






(2) When the secondary load is only 50% of the transformer capacity at ordinary times, the arc suppression coil capacity can be equal to the rated capacity of the transformer.






Although the secondary side of this connection can supply power to regional loads or the substation itself, its application will be greatly limited because the triangular connection is difficult to supply power to hybrid power and lighting users at the same time.


YN, open d connection is connected with arc-suppression coil XL, which is similar to YNd connection. The connection mode of open d can be connected with a resistor or reactor on the side of the open triangle to adjust the zero-sequence reactance of the transformer, and the connection of the resistor can also suppress the ferromagnetic resonance of the network. If the three-phase five column iron core is adopted, the zero sequence impedance value can also be greatly increased, and it is even possible to omit an arc suppression coil, but the structure is complex and the cost is increased. In addition, the secondary use of open triangle connection can not meet the needs of power supply to regional loads and self use power, so this method is not widely used.


ZN, yn connection transformer is connected with arc suppression coil XL, which is a common connection mode for grounding transformer. Because the zero sequence magnetic potential in the upper and lower half windings on the same iron core column of the zigzag connection method is just the same size and opposite direction, which counteracts each other, the zero sequence leakage flux is reduced to a very small value, so that its zero sequence reactance value is very small, and its capacity can be equal to the capacity of the connected arc suppression coil.






The grounding transformer widely used at home and abroad is mainly connected in this way. Since the yn junction method is adopted at the low-voltage side, it can supply local power or the self use power of the substation at the same time. The capacity of the low voltage side is usually smaller than that of the high voltage side. In most cases, the capacity of the low voltage side is within the range of 80-200kVA.






Although the rated capacity of the high-voltage side can be equal to the capacity of the connected arc suppression coil, the Z-shaped connection will have 1.15 times more turns than the Y-shaped connection, so the actual capacity of the grounding transformer should be 1.15 times of the capacity of the arc suppression coil.


■ Working principle

The working principle diagram of the grounding transformer in case of single-phase fault in the system is illustrated by the common ZNyn wiring. When the grounding transformer passes through a certain size of zero sequence current during operation, the current flowing through the two single-phase windings on the same iron core column is in opposite directions and the size is equal, so that the magnetic potential generated by the zero sequence current is just opposite to offset, so that the zero sequence impedance is also very small.






When the grounding transformer fails, the neutral point can flow through the compensation current. Due to the small zero-sequence impedance, when the zero-sequence current passes through, the impedance voltage drop generated should be as small as possible to ensure the safety of the system. Because the grounding transformer has the characteristics of low zero sequence impedance, when a single-phase grounding fault occurs in phase C, the phase C ground current I flows into the neutral point through the earth, and is divided into three equal parts to flow into the grounding transformer. Since the three-phase currents flowing into the grounding transformer are equal, the displacement of neutral point N remains unchanged, and the three-phase line voltage remains symmetrical.






However, in the manufacturing process, the turns and geometric dimensions of the upper and lower windings of the high-voltage winding cannot be completely equal, which makes it impossible for the magnetic potential generated by the zero sequence current to be exactly offset in the opposite direction, and still generates a certain zero sequence impedance, usually about 6-10 Ω. Compared with the zero sequence impedance of the star connected transformer of 600 Ω, its advantages are self-evident. In addition, the zigzag grounding transformer can also make the no-load current and no-load loss as small as possible. Compared with ordinary star connection transformer, each phase iron core of zigzag connection transformer is composed of windings of two iron core columns. According to its vector diagram, when compared with ordinary star connection transformer, it needs to be wound 1.16 times more when the voltage is the same. The amplitude of zero sequence impedance and positive sequence impedance of urban distribution network with single-phase grounding under neutral point resistance grounding mode differs greatly. When three-phase positive and negative sequence currents flow through, the magnetic potential on each iron core column of the grounding transformer is the sum of the phasors of the magnetic potential of two windings of different phases on the iron core column. The magnetic potential on the three iron core pillars is a group of three-phase equilibrium quantities with a phase difference of 120 °. The magnetic flux generated can form a loop on the three iron core pillars. The magnetic resistance of the magnetic circuit is small, the magnetic flux is large, and the induced potential is large, showing a large positive sequence and negative sequence impedance; Therefore, the grounding transformer has the characteristics of large positive and negative sequence impedance and small zero sequence impedance.






■ Main technical parameters





In order to meet the needs of arc suppression coil grounding compensation in distribution network, and also meet the needs of substation power and lighting load, Z-type connection transformer is selected, and the main parameters of grounding transformer need to be reasonably set.






(1) The primary side capacity of grounding transformer with rated capacity shall match with the capacity of 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 as 1.05-1.15 times of the capacity of the arc suppression coil. For example, the capacity of the grounding transformer equipped with one 200kVA arc suppression coil is 215kVA.






(2) Total current flowing through transformer neutral point in case of single-phase fault of neutral point compensation current:


Where:

U is the line voltage of distribution network (V); Zx is the impedance of arc suppression coil (Ω);




Zd is the primary zero sequence impedance of the grounding transformer (Ω/phase);




Zs is the system impedance (Ω);




The duration of the neutral point compensation current shall be the same as that of the arc suppression coil, which shall be 2 hours as specified.






(3) Zero sequence impedance Zero sequence impedance is an important parameter of grounding transformer, which has an important influence on relay protection to limit single-phase grounding short-circuit current and suppress overvoltage. For zigzag (Z type) and star/open delta connected grounding transformers without secondary coils, there is only one impedance, namely zero sequence impedance, so that the manufacturing department can meet the requirements of the power department.






(4) 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 with the same capacity. For the load loss, when the secondary side is in full load operation, the load loss of the primary side is less than that of the double winding transformer with the same capacity as the secondary side because of the light load of the primary side.






(5) According to the national standard, the temperature rise of grounding transformer is as follows:






1) The temperature rise under the rated continuous current shall comply with the provisions of the national standard for dry type transformers of general power transformers, but it is mainly applicable to the grounding transformers with frequent load on the secondary side;






2) When the duration of short-time load current is less than 10s (mainly when the neutral point is connected with the resistance), the temperature rise shall comply with the provisions of the national standard power transformer on the temperature rise limit under short circuit conditions;






3) The temperature rise of the grounding transformer and the arc suppression coil when operating together shall comply with the provisions on the temperature rise of the arc suppression coil: the winding temperature that continuously flows through the rated current is 80K, which is mainly applicable to the star/open delta connection grounding transformer; For the winding with the maximum flow time of rated current specified as 2h, the specified temperature is 100K.


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