CN105280331A - Transformer iron core demagnetization method - Google Patents

Abstract

The invention discloses a transformer iron core demagnetization method and especially provides a method for demagnetizing the iron core of a transformer after the DC resistance test. According to the invention, the voltage is applied onto the primary winding of a transformer by a DC power supply for three times, wherein the voltage-applying amplitude is the same each time and the polarity thereof is alternately changed. The voltage-applying duration is determined based on the acquired current waveform of the primary side of the transformer. During the first two times, the current applied onto the primary winding of the transformer reaches a forward peak value and a reverse peak value respectively. During the third time, the current applied onto the primary winding of the transformer occupies only half of the duration from the forward peak value to 63.2% of the reverse peak value. Through the above three voltage application processes, the iron core of the transformer is demagnetized. Based on the above method, the demagnetization duration is greatly shortened and the demagnetization effect is ensured at the same time. Meanwhile, the demagnetization equipment is small in power and good in practicality.

Description

A kind of transformer core demagnetizing method

Technical field

The invention belongs to transformer residual technique field, be specifically related to a kind of transformer core demagnetizing method.

Background technology

According to " power equipment preventive trial code " requirement, high-power transformer, after maintenance or after running 1 to 3 year, all will carry out DC Resistance Test of Transformer, and the magnetic field produced due to direct current will make transformer core leave a large amount of remanent magnetism.When transformer during no-load closing, affect by iron core remanent magnetism and breaker closing phase angle, the saturated of iron core may be caused, cause transformer to produce very large magnetizing inrush current in the side of voltage jump, cause differential protection misoperation to trip, make the failure time and again that puts into operation of transformer; The magnetizing inrush current that numerical value is very large also can cause transformer and circuit breaker because of electric power excessive and impaired; Bring out transformer that other transformer station contiguous etc. running and produce " sympathetic inrush " and mistrip, cause large-area power-cuts; A large amount of harmonic waves wherein also can cause serious pollution to the electrical network quality of power supply.Therefore should demagnetize to iron core after transformer directly hinders test.

Remanent magnetism produces exists hysteresis characteristic mainly due to transformer fe core material.Transformer main magnetic circuit generally adopts the silicon steel sheet that magnetic property is higher and core loss is less to make.When being placed on remagnetization in alternating magnetic field, rising magnetization curve does not overlap with decline magnetization curve, and during decline, the change of magnetic flux density B always lags behind the change of magnetic field intensity H, and when H drops to zero, B does not drop to zero but drops to a certain numerical value B _r, this phenomenon is called magnetic hysteresis.B _rbe called residual magnetic flux density, be called for short remanent magnetism.If can ensure that the scene temperature simultaneously on-the-spot electromagnetic noise of the Curie point that exceedes iron core of can not suddenling change is very little, be not enough to the change affecting magnetic field, just can think remanent magnetism time to time change hardly.

The transformer core demagnetizing method of current domestic employing is generally apply the magnetic field H that is greater than remanence magnetisation to transformer, then constantly change magnetic direction and reduce magnetic field intensity gradually to null value (by completing to the curtage of transformer applying alternation and decay), the B-H magnetic hysteresis loop of generation is progressively decremented in the process of zero in additional magnetic field amplitude, reduce hysteresis ring, magnetic flux is less gradually, finally close to zero.But this method demagnetization required time is long, and easily in the process changing magnetic direction and size, cause hysteresis ring to offset, cause degaussing failure, sometimes even can increase transformer remanent magnetism.

Summary of the invention

In order to solve above-mentioned prior art Problems existing, the invention provides a kind of transformer core demagnetizing method.The method only need can complete iron core demagnetization by DC power supply to Transformer Winding three pressurizations, substantially reduce the demagnetization time, and demagnetizer power is little, has practicality while ensure that demagnetization effectiveness.

In order to achieve the above object, the present invention adopts following technical scheme:

A kind of transformer core demagnetizing method, comprises the steps:

Step 1: by DC power supply to the pressurization of transformer primary side winding, signal pickup assembly is sampled to transformer voltage current waveform by instrument transformer, when primary current no longer increases, controller record saturation current peak value i _satand by the pressurization of switching circuit upset transformer primary side winding voltage polarity, start timing;

Step 2: when primary current oppositely and reach-63.2%i _sattime, stop timing, record duration is Δ t; Continue to be forced into primary current and reach-i _satand no longer increase, controller again by switching circuit upset transformer primary side winding voltage polarity, when this pressurization duration reaches Δ t/2, cuts out DC power supply, completes demagnetization.

Compared to the prior art comparatively, the present invention possesses following advantage:

1, the required time that demagnetizes is short.

2. can demagnetize without the need to predicting the initial remanent magnetism of iron core, and demagnetization effectiveness is better.

Accompanying drawing explanation

Fig. 1 is demagnetization process flow diagram.

Fig. 2 is demagnetizer figure.

Fig. 3 is demagnetization equivalent circuit diagram.

Fig. 4 is single-phase transformer demagnetization simulation model.

Fig. 5 is demagnetization DC voltage waveform figure.

Fig. 6 is transformer core flux waveforms figure.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in detail:

As depicted in figs. 1 and 2, the present invention discloses a kind of transformer core demagnetizing method, comprises the steps:

Step 1: by DC power supply to the pressurization of transformer primary side winding, signal pickup assembly is sampled to transformer voltage current waveform by instrument transformer, when primary current no longer increases, controller record saturation current peak value i _satand by the pressurization of switching circuit upset transformer primary side winding voltage polarity, start timing;

Step 2: when primary current oppositely and reach-63.2%i _sattime, stop timing, record duration is Δ t; Continue to be forced into primary current and reach-i _satand no longer increase, controller again by switching circuit upset transformer primary side winding voltage polarity, when this pressurization duration reaches Δ t/2, cuts out DC power supply, completes demagnetization.

Demagnetization equivalent electric circuit below in conjunction with Fig. 3 is described the present invention's principle of demagnetizing:

First time, when pressurizeing to transformer primary side winding, iron core magnetic flux constantly increased, and when primary current no longer increases, iron core is saturated, reaches maximum magnetic flux can obtain former limit winding induced voltage is

$U_L=L\frac{di}{dt}=0---\left(1\right)$

Primary current is

$i_{sat}=\frac{U}{R}---\left(2\right)$

Wherein L is transformer core equivalent inductance, and U is direct voltage amplitude, and R is DCR of Transformer, and i is former limit winding current.

Now overturn former limit winding applied voltage polarity, to the pressurization of transformer second time.Can obtain according to the analysis of DC circuit threefactor method:

$U_L=-{\mathrm{Ue}}^{-\frac{t}{\mathrm{\τ}}}---\left(3\right)$

$i=-\frac{U}{R}+\frac{U}{R}{e}^{-\frac{t}{\mathrm{\τ}}}---\left(4\right)$

Wherein according to the law of electromagnetic induction,

From formula (6), to the additional duration of transformer be direct voltage U can make transformer core magnetic flux by be kept to zero.

As t=τ,

i＝-63.2％I _sat(7)

Therefore record second time pressure process in primary current from forward peak value i _satto-63.2%i _satrequired time Δ t; Continue to be forced into primary current simultaneously and reach-i _satand controller is pressurizeed to transformer third time by switching circuit upset transformer primary side winding voltage polarity again after no longer increasing; This Δ t/2 duration that need pressurize can make transformer core magnetic flux by transit to zero, powered-down completes demagnetization afterwards.

Embodiment

For a single-phase transformer demagnetization, build simulation model as shown in Figure 4, in figure: U represents DC power supply, R indication transformer D.C. resistance, n indication transformer no-load voltage ratio, L ₁indication transformer leakage inductance, L ₂indication transformer equivalence magnetizing inductance, P indication transformer high-pressure side, S indication transformer low-pressure side; Transformer rated capacity is 403MVA, rated voltage is 199.18kV/162.64kV, high-pressure side rated flux is 634V*s, DCR of Transformer is 0.08 Ω, demagnetizes from high voltage side of transformer, and demagnetization direct voltage amplitude is 8V, initial remanent magnetism is-500V*s, demagnetize by the inventive method, Fig. 5 is demagnetization DC voltage waveform figure, Fig. 6 is transformer core flux waveforms.Visible through about 600s can complete iron core demagnetization, demagnetization after iron core remanent magnetism be 6V*s, demagnetization effectiveness is good.

Below by reference to the accompanying drawings the specific embodiment of the present invention is described; but these explanations can not be understood to limit scope of the present invention; protection scope of the present invention is limited by the claims of enclosing, and any change on the claims in the present invention basis is all protection scope of the present invention.

Claims (1)

1. a transformer core demagnetizing method, is characterized in that: comprise the steps:

Step 1: by DC power supply to the pressurization of transformer primary side winding, signal pickup assembly is sampled to transformer voltage current waveform by instrument transformer, when primary current no longer increases, controller record saturation current peak value i _satand by the pressurization of switching circuit upset transformer primary side winding voltage polarity, start timing;

Step 2: when primary current oppositely and reach-63.2%i _sattime, stop timing, record duration is Δ t; Continue to be forced into primary current and reach-i _satand no longer increase, controller again by switching circuit upset transformer primary side winding voltage polarity, when this pressurization duration reaches Δ t/2, cuts out DC power supply, completes demagnetization.