A harmonic elimination device of voltage transformer protection
The damage of ferromagnetic resonance voltage transformer and the measures of ferroresonance over current and/or high voltage may cause damage of voltage transformer, especially low frequency resonance, the corresponding excitation voltage transformer impedance caused greatly reduced depth of iron core saturation, field current increases sharply, as high as dozens of times to one hundred times bigger than the rating, which seriously damage voltage transformer.
Damage the voltage transformer is usually have two situations: (1) the primary winding of voltage transformer burn out, this is caused by persistent ferroresonance.
Usually occur in the no-load busbar closing, the phase bus to ground capacitance is lesser, have three times the frequency of high frequency resonance overvoltage.
(2) high frequent fusing of fuse.
Is caused by ultra-low frequency ferromagnetic resonance.
Ultra-low frequency ferro-resonance in single-phase earth fault disappear instantly, the grid of capacitance and voltage transformer excitation inductance of a transient electromagnetic oscillation.
This kind of situation in no-load busbar closing larger bus to ground capacitance of the power grid.
Especially in lacunar arc grounding fault, the more serious.
Using voltage transformer to limit or eliminate ferroresonance measures and analysis of the method of eliminating ferroresonance has a lot of kinds, the following will introduce respectively of voltage transformer secondary and take measures, to limit or eliminate resonant overvoltage.
In the open delta winding of voltage transformer open end with a harmonic elimination device before a harmonic elimination device generally use the light bulb, model is 220 v, 500 w bulb.
Its role is at the beginning of the resonance occurs, the light bulb of the resistance is small, and when single phase static grounding resistance larger to avoid voltage transformer overload, and it is a commonly used method of simple and economic.
The more a harmonic elimination device resistance small, the more you can inhibit the occurrence of resonance, but if the value is too small, the corresponding power is too big, not only difficult to choose the appropriate resistance materials, at the same time, the capacity of PT will not be able to meet the requirements, and make the open delta voltage on both ends is too low, so that influence the correct action of the relay protection.
Therefore, using this method has its own disadvantages: 1)
When the non-metallic sex ground fault occurs, the neutral displacement voltage is very small, loop without damping effect, produced by the resonance cannot eliminate.
Takes place when the single-phase grounding fault, the requirements to open delta voltage transformer winding has a large enough capacity and prevent winding burn.
When intermittent arc grounding fault occurs, the resistance of access, will make through the voltage transformer is a significant increase in the current of winding and increases the likelihood of voltage transformer loss.
At the same time also should pay attention to each open delta voltage transformer in the system all meet resistance when works a harmonic elimination device can also be used in open delta voltage transformer winding open end with harmonic elimination device of a microcomputer.
Electronic type microcomputer a harmonic elimination device is transient when resonance happens through high impedance circuit, has the good effect of harmonic elimination.
But the low frequency oscillation.
Therefore, only suitable for small capacitance grid.
In the primary side voltage transformer neutral point via high impedance grounding.
In three-phase voltage transformer or three single phase voltage transformer neutral point on the primary side and the indirect one single phase zero sequence of TV.
Commonly known as' 4 TV, belong to a harmonic elimination.
A harmonic elimination device features: 1)
Using zero sequence TV secondary winding compensation function, eliminates the quadratic phase three times of the influence of harmonic voltage measurement.
Three-phase TV open delta circuit is short, therefore, the zero sequence TV for measuring the zero sequence voltage, three-phase TV positive sequence voltage measurement.
Will open delta connect closed triangles to eliminate three harmonics, which can eliminate ferroresonance.
Only single phase of zero sequence circuit TV a inductance, and three-phase excitation inductance, TV under excitation of overvoltage, impedance values with the grid earth capacitance are equal, the ferroresonance does not have the necessary and sufficient conditions, which have the effect of inhibition or eliminate ferroresonance.
It to restrain ferroresonance and ultra-low frequency resonance has good effect, suitable for big capacitance of power network.
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