CN104333930A - Selecting method for transformer low frequency heating frequency based on square wave modulation - Google Patents

Abstract

The invention discloses a selecting method for transformer low frequency heating frequency based on square wave modulation, applied to the low frequency heating device for power transformer. While low frequency heating for the transformer based on square wave modulation, the frequency of the output current of AC-AC variable frequency device can be calculated according to the formula: f=78.54I2R2/UN, wherein I2 is the short circuit side current of the heated transformer, R2 is the DC resistance of the winding at the short circuit side of the heated transformer, UN is the nominal voltage of the heated transformer. The theoretical calculation formula used for calculating low frequency heating frequency minimum value can offer the optimal executing solution for low frequency heating the large-scale power transformer and realize the maximization economic benefit for low frequency heating field.

Description

Based on the system of selection of the transformer low frequency heating frequency of square-wave frequency modulation

Technical field

The present invention relates to transformer test technical field, specifically a kind of system of selection of the transformer low frequency heating frequency based on square-wave frequency modulation.

Background technology

Along with progress and the development of project of transmitting and converting electricity, under some operating mode such as large-scale power transformer is assembled at the scene, disassembling overhaul, completion put into operation, need to carry out heat drying.

A kind of low frequency heating means are disclosed in Chinese invention patent (application number 201410101817.8) " a kind of large-scale power transformer low frequency heater and implementation method ".This method realizes based on short-circuit method, and by transformer side short circuit in winding, opposite side winding applies the electric current of low frequency, and the low frequency Short-Circuit Current on Transformer utilizing short circuit end winding to flow through heats.Its physical circuit as shown in Figure 1, comprise phase shifting transformer 2 and AC-AC converter plant 3, the input of described phase shifting transformer 2 connects three-phase alternating-current supply 1, the output of phase shifting transformer 2 connects the input of described AC-AC converter plant 3, the output of AC-AC converter plant 3 is connected with by a side ports of heating transformer 4, described by the opposite side port short circuit of heating transformer 4; Described AC-AC converter plant 3 comprises two three-phase commutation bridges be connected in antiparallel.

This patent, on the basis of said apparatus, utilizes low-frequency current to heat, and compared with heating with power frequency, when exporting equal heating current, the capacity of required heater can significantly reduce; And low frequency heating is without the need to carrying out reactive power compensation.Low-frequency ac power is as transformer field heating power supply, and its input is powered by 380V station electricity consumption; Being flow through low frequency short circuit current in heating transformer winding generates heat, and then heating transformer dielectric, reaches the effect that tide is dry.Further, change output voltage and the frequency of low-frequency ac power, the heating power of Transformer Winding can be regulated; By measuring transformer winding temperature and transformer top-oil temperature, can Real-Time Monitoring by the temperature rise of heating transformer.This low frequency heating efficiency of heating surface based on short-circuit method realization is high, device volume is little, to shorten transformer installation period, improve transformer field heat drying effect, improve transformer insulation state, ensure that transformer safety is run significant, can great economic benefit be brought.

But in low frequency heating process, how to select heating frequency to be the key issue needing emphasis to solve.Frequency is lower, and the efficiency of heating surface is higher, and meanwhile, the magnetically saturated risk of transformer core is larger; And frequency also can not be too high, if heating frequency is close to power frequency 50Hz, then low frequency heating compare power frequency heating efficient, time short advantage will not exist.

Summary of the invention

The invention provides a kind of system of selection of the transformer low frequency heating frequency based on square-wave frequency modulation, it is in conjunction with the characteristic of power supply square-wave frequency modulation, can provide a kind of computational methods of best heating frequency.

Technical scheme of the present invention is:

A kind of system of selection of the transformer low frequency heating frequency based on square-wave frequency modulation, it is applied in power transformer low frequency heater, described power transformer low frequency heater comprises phase shifting transformer and AC-AC converter plant, the input of described phase shifting transformer connects three-phase alternating-current supply, the output of phase shifting transformer connects the input of described AC-AC converter plant, the output of AC-AC converter plant is connected with by a side ports of heating transformer, described by the opposite side port short circuit of heating transformer; Described AC-AC converter plant comprises two three-phase commutation bridges be connected in antiparallel, and when the transformer low frequency heating based on square-wave frequency modulation, the output current frequency of AC-AC converter plant calculates according to following formula:

f＝78.54I ₂R ₂/U _N(1)

Wherein, I ₂for by heating transformer short circuit end electric current, R ₂for by heating transformer short circuit end winding D.C. resistance, U _nfor by heating transformer rated voltage.

The present invention is when the large-scale power transformer low frequency heating based on short-circuit method, in conjunction with the characteristic of power supply square-wave frequency modulation, a kind of computational methods of best heating frequency are provided, the calculating of supply frequency in the transformer low frequency heat run based on square-wave frequency modulation can be directly applied to.

Transformer magnetic potential balance equation when the present invention heats according to low frequency, the ingenious transformer magnetic core under rated condition that utilizes, close to this design feature saturated, obtains transformer through deriving and utilizes square-wave frequency modulation power supply to carry out the minimizing computing formula of low frequency heating process medium frequency.The minimizing theoretical foundation of determination low frequency heating frequency of the present invention is that magnetic core there will not be saturation condition when carrying out low frequency heating.From theory analysis, low frequency heating process medium frequency is lower, then the efficiency of heating surface is higher, and economic benefit is better.Therefore, the theoretical calculation formula of calculating low frequency heating frequency minimum value provided by the invention, for the heating of Transformer on Site low frequency provides optimum embodiment, achieves the maximization of low frequency heating economic benefit.

Accompanying drawing explanation

Fig. 1 is the structural representation of existing short-circuit method transformer low frequency heater.

In figure: 1-three-phase alternating-current supply, 2-phase shifting transformer, 3-AC-AC converter plant, 4-by heating transformer.

Embodiment

Below in conjunction with embodiment, the present invention is further described:

When transformer based on square-wave frequency modulation carries out low frequency heating, the square wave after transformer primary side winding input modulation, vice-side winding short circuit, by the short circuit current flow through in winding, realizes the heating to transformer.The circuit equation of short circuit end is as follows:

In formula (2), u ₂t () is the induced potential of transformer secondary (short circuit end), i ₂t electric current that () is flow through for short circuit end winding, R ₂, L ₂for resistance and the inductance of short circuit end winding, N ₂for short circuit end coil turn, for magnetic flux.

Can to be derived magnetic flux by formula (2) expression formula:

When low frequency heats, frequency is extremely low, di ₂t ()/dt is very little, L ₂also very little, therefore formula (3) can be reduced to:

The close B of magnetic be magnetic flux divided by core section, expression formula is:

$B\left(t\right)=-\frac{R_2}{{\mathrm{SN}}_2}\underset{0}{\overset{t}{\∫}}{i}_2\left(t\right)\mathrm{dt}+B\left(0\right)---\left(5\right)$

In formula (5), S is that core section amasss.

During square wave heating, current i ₂t () is square wave, if square wave maximum is I ₂, heating power supply output current frequency (i.e. the output current frequency of AC-AC converter plant 3) is f, then formula (5) can be changed into:

$B\left(t\right)=-\frac{R_2}{{\mathrm{SN}}_2}\·\frac{I_2}{2f}+B\left(0\right)---\left(6\right)$

In the limiting case, B (t)=B _max, B (0)=-B _max, now frequency f is minimum, has:

$f_{\min }=-\frac{I_2{R}_2}{4B_{\max }{\mathrm{SN}}_2}---\left(7\right)$

Transformer is when normally working, and the magnetic flux in iron core is:

When without remanent magnetism, u (t)=U is had under rated operation _ncos (wt), w are angular frequency, w=2 π f, wherein f=50Hz.The maximum of magnetic flux can be calculated according to formula (8), and then obtain the close maximum of magnetic:

Formula (9) is substituted in formula (7) and can obtain:

$f_{\min }=-\frac{I_2{R}_2}{4B_{\max }{\mathrm{SN}}_2}=\frac{I_2{R}_2}{4\frac{U_N}{{\mathrm{SN}}_2\·100\mathrm{\π}}{\mathrm{SN}}_2}=78.54I_2{R}_2/U_N---\left(10\right)$

Formula (10) is the computational methods of optimum frequency when low frequency provided by the invention heats.

Such as, certain ± 800kV converter transformer carry out square-wave frequency modulation low frequency heating time, the parameter of this transformer is: U _n=171.9kV, I ₂=1054A, R ₂=0.0549 Ω, can calculate according to formula (10), and best low frequency heating frequency is: f _min=0.03Hz.By the frequency of this 0.03Hz, the heating means mentioned with background technology and device, the change of current of a 800kV is become and carries out low frequency heating, only need heat 4 days by Circulation and Hot Oil Spray, after the basis of Circulation and Hot Oil Spray increases low frequency heating, foreshorten to 12 hours heating time, significantly shorten the time needed for on-the-spot transformer installation and operation, create huge economic benefit and social benefit.

The invention has the beneficial effects as follows, when the large-scale power transformer low frequency heating based on short-circuit method, in conjunction with the characteristic of power supply square-wave frequency modulation, provide a kind of computational methods of best heating frequency.The present invention can directly apply to the calculating of supply frequency in the transformer low frequency heat run based on square-wave frequency modulation.

The above; be only the specific embodiment of the present invention, but protection scope of the present invention is not limited thereto, anyly belongs to those skilled in the art in the technical scope that the present invention discloses; the change that can expect easily or replacement, all should be encompassed within protection scope of the present invention.Therefore, protection scope of the present invention should be as the criterion with the protection range of claim.

Claims (1)

1. the system of selection based on the transformer low frequency heating frequency of square-wave frequency modulation, it is applied in power transformer low frequency heater, described power transformer low frequency heater comprises phase shifting transformer (2) and AC-AC converter plant (3), the input of described phase shifting transformer (2) connects three-phase alternating-current supply (1), the output of phase shifting transformer (2) connects the input of described AC-AC converter plant (3), the output of AC-AC converter plant (3) is connected with by a side ports of heating transformer (4), described by the opposite side port short circuit of heating transformer (4), described AC-AC converter plant (3) comprises two three-phase commutation bridges be connected in antiparallel, it is characterized in that: when the transformer low frequency heating based on square-wave frequency modulation, the output current frequency of AC-AC converter plant (3) calculates according to following formula:

f＝78.54I ₂R ₂/U _N(1)

Wherein, I ₂for by heating transformer (4) short circuit end electric current, R ₂for by heating transformer (4) short circuit end winding D.C. resistance, U _nfor by heating transformer (4) rated voltage.