Abstract: This article introduces the insulation level and representation method of transformers, as well as the coordination of transformer neutral point lightning arresters.
Keywords: Transformer insulation neutral point lightning arrester
The insulation level of a transformer, also known as insulation strength, is the level that matches the protection level and other insulation parts, i.e. the withstand voltage value, determined by the highest voltage Um of the equipment.
The highest voltage Um of the equipment is the effective value of the highest phase to phase voltage of the winding for a transformer. From the perspective of insulation, Um is the highest effective voltage of the system to which the winding can be connected. Therefore, Um is the standard value that can be greater than or equal to the rated voltage of the winding.
The insulation of a winding with the same ground frequency withstand voltage at all output terminals is called full insulation; The insulation level of the grounding or neutral point of a winding is lower than that of the wire end, which is called graded insulation.
The rated withstand voltage of the winding is marked with the following letter codes:
LI - Lightning impulse withstand voltage
SI - Operating impulse withstand voltage
AC - Power frequency withstand voltage
The insulation level of transformers is represented by listing the withstand voltage values in the order of high voltage, medium voltage, and low voltage windings (with the impulse level first), separated by diagonal lines. The insulation level of the neutral point with graded insulation is listed with a horizontal line after its insulation level at the end of the line.
For example: LI850AC360- LI400AC200/LI480AC200- LI250AC95/LI75AC35.
The meaning is: a 220KV three side graded insulation main transformer, with the first being the high-voltage side lead terminal, neutral point, medium voltage side lead terminal, neutral point, and low-voltage side.
For the selection of lightning arrester protection:
Generally speaking, the selection of busbar side lightning arresters is relatively easy, and there are generally no problems with choosing according to the voltage level produced and used by the manufacturer. However, the selection of neutral points is quite difficult. A few years ago, Guangdong Power Grid Company specifically issued a document pointing out that there were many mismatched lightning arresters in different places and corrected them. A brief explanation will be given on the selection of neutral point lightning arrester for the 110kV neutral point protection method of the main transformer.
Single use lightning arrester protection method
A Y1W-73/200 lightning arrester can be used for a neutral point with an insulation level of 60kV. Its DC 1mA voltage of 103kV is equivalent to the 73kV power frequency peak, and the neutral point can withstand a short-term power frequency overvoltage of 1 times the phase voltage; The residual voltage of 1kA is 200kV, and the lightning withstand voltage level can be calculated as U-withstand=1.1 × (1.1U residual+15) kV. The current residual voltage is 200kV, so the equipment insulation of 258kV can meet the requirements. A lightning arrester with a residual voltage of 225kV can also meet insulation coordination requirements for insulation with a lightning tolerance of 300kV.
The neutral point with 44kV insulation level can be equipped with Y1W-60/144 type lightning arrester. Its DC 1mA voltage of 86kV is equivalent to the peak power frequency of 60kV. When single-phase grounded, the highest voltage of the 110kV neutral point is about 43.8kV, which can withstand general single-phase grounding overvoltage. However, it is difficult to withstand short-term power frequency overvoltage of 1 times the phase voltage. Choosing Y1W-73/200 type lightning arrester is much safer for the lightning arrester, but the insulation protection margin of the main transformer is reduced. Generally, reducing the protection margin of the main transformer should not be used to protect lightning arresters.
The selection of a neutral point lightning arrester with 35kV insulation level is not a problem with residual voltage, but the possibility of damage from power frequency overvoltage increases. Therefore, it should be considered that the increase in neutral point potential during single-phase grounding of the system will not damage the lightning arrester. The voltage is about 43.8kV. Therefore, the DC 1mA voltage of the lightning arrester should be above 60KV (43.8KV × √ 2 ≈ 60KV), and Y1W-48/109 type lightning arrester can be used. The lightning arrester cannot withstand a short-term power frequency overvoltage of 1 times the phase voltage.