What is a harmonic elimination device and computer harmonic elimination?
A harmonic elimination: also called harmonic elimination damper, is actually a nonlinear harmonic elimination resistance R0, concatenated in voltage transformer PT primary side between neutral and ground.
When the normal operation of power grid, harmonic elimination on voltage (500 v, R0 has high resistance (
Up to several hundred thousand euro)
, damping effect, make the resonance in the initial stage is not easy to development;
When the single-phase grounding grid, harmonic elimination on high voltage (
10 kv power grid, the voltage of about 1.
, a low R0 (
Tens of thousands euro)
And can meet the insulation of the PT open delta voltage not less than 80 v supervisory requirements, at the same time can still damping resonance.
Prevent ferroresonance, way is to change the system parameters and damage of resonance conditions;
In the ferromagnetic resonance happens, the development of the effective damping resonance, eliminate the hazards.
When the arc grounding grid, R0 can still maintain a certain amount of resistance, can restrict the transformer inrush current, the running effect is very obvious.
A harmonic elimination devices eliminate PT saturated resonance and limitation flow two functions, but in the practical application has the following limitations: 1, neutral point is half insulating structure 2, restrict the PT resonance occurs with other 3 PT is invalid, single-phase grounding in power grid, the PT the measured value of the zero sequence voltage U0 have error, the U0 requiring higher accuracy of amplitude and Angle is not suitable to use 4, unit's own heat capacity is limited, even if choose product of heat capacity is relatively large, the duration longer intermittent grounding overvoltage inspired, device is damaged.
PT three-phase excitation inductance saturation degree is different, composition of capacitance and system oscillation extremely easily, cause ferroresonance, cause of voltage rise, flows through the winding current, voltage transformer blown fuse, even the lightning arrestor explosion, transformer burned, a serious threat to the safe operation of power grid.
Computer harmonic elimination device, also known as secondary harmonic elimination device, installed in the PT on the open delta winding of normal operation or when the single-phase grounding device is not action, once in the case of ferroresonance judgment, then make the positive and negative in open delta on both ends of the two parallel alternating thyristor zero trigger conduction, restrictions and damping ferroresonance.
3, add the connection analysis of harmonic elimination PT: in the ferromagnetic harmonic prevention measures, the neutral point with harmonic elimination PT way less restricted place, all can use in the neutral point grounding system and equipment requirements is not high, the selection of capacity, reliable performance and can not influence the normal operation.
Harmonic elimination, thyristor cut-off by oneself, to repeat the action.
From the perspective of harmonic elimination, the harmonic elimination device resistance, the greater the share of the higher the voltage, current transformer core is not easy to saturated, can effectively prevent ferroresonance.
But resistance is too large, open delta voltage transformer output voltage is reduced, affect the sensitivity of relay protection device, since not neutral point grounding directly, each phase winding voltage is not relative to the voltage, push the meaning of voltage transformer neutral point grounding.
During starting harmonic elimination device, thyristor conduction, all show low resistance state, about a few m Ω - resistance
Dozens of m Ω, such a small resistance to high frequency, the fundamental frequency and damping frequency division three types of resonance, and effective for the power grid.
The major drawback of the device, it is difficult to distinguish between fundamental wave resonance and single phase grounding, the criterion of the main difference is that the discretion of the zero sequence voltage U0, usually fundamental frequency resonance as U0 acuity 150 v, single phase grounding as 30 v or less Uo < 145 v.
To prevent maloperation when single-phase grounding device make PT long time overload and burned, had to fundamental frequency resonance to make sure that the criterion of voltage is higher, so, in the case of displacement of the power frequency voltage is not very high, the device can't action, may make the excitation characteristic of some poor iron core saturation PT easily blown fuse.
And the device is in the grid over the ground capacitance is large, to prevent the intermittent grounding or disappear instantly, transformer transient saturation caused by low frequency saturation current generated by the blown fuse, is powerless.
In addition, in the role of duration longer intermittent overvoltage, multiple excitation, action for a long time, make through the significant increase in the current of high voltage winding, thermal energy accumulation effect, still can burn out PT, operation experience has proved.
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