CN204044236U - A kind of leakage current detection circuit - Google Patents

Abstract

The utility model relates to a kind of leakage current detection circuit, it is characterized in that, comprise the transformer adopting high magnetic conductivity ferrite toroidal core, transformer has at least four windings; One end of excitation winding one connects the unit that magnetizes, and the other end connects degaussing unit on the one hand, another aspect tandem tap one and current sampling unit successively; One end of excitation winding two connects the unit that magnetizes, and the other end connects degaussing unit on the one hand, another aspect tandem tap two and current sampling unit successively; Calibration current generating unit forms loop via switch three and correcting current winding; Leakage current I _kthrough winding inflow transformer; Switch one and switch two alternate conduction under main control unit controls, main control unit also connects current sampling unit.The utility model can meet the requirement of detection bandwidth in hole diameter enlargement situation.

Description

A kind of leakage current detection circuit

Technical field

The utility model is applied to new energy grid-connected power technical field, is converted into the circuit of alternating voltage especially for a kind of DC voltage.

Background technology

Solar energy grid-connected photovoltaic system is effective one of important means utilizing new forms of energy, in distributed generation system, also occupy very consequence.Along with national governments are to the attention of utilization of new energy resources and encouragement, this generation mode is more and more general in actual applications.

Can photovoltaic DC-to-AC converter be the important component part of solar energy grid-connected photovoltaic system, converted to and electrical network amplitude, the on all four alternating current of phase place by the direct current that solar panel produces.In order to improve system effectiveness, increasing photovoltaic combining inverter adopts non-isolated design, such electrical network and photovoltaic array exist and are directly electrically connected, and there is virtual stray capacitance between photovoltaic array and ground, because which form the common mode resonance loop be made up of stray capacitance, filter element and electric network impedance.The common mode voltage that stray capacitance changes will produce corresponding common mode current (i.e. leakage current) in this common mode resonance loop.

Common mode current in photovoltaic parallel in system can bring a lot of problem and harm, as caused the distortion of grid-connected current, producing electromagnetic interference (EMI) etc. to other equipment, more seriously can form significant threat to personal safety.Therefore some countries have carried out relevant regulations to the requirement of leakage current, VDE 0126-1-1 standard as Germany specifies, when leakage current is more than 300mA, photovoltaic parallel in system must disconnect with electrical network in 0.3s, and therefore safe and reliable leakage current test technology has become the obstacle must crossed in photovoltaic grid-connected inverting system.

In order to avoid common mode current works the mischief to the person over the ground, generally take two kinds of solutions:

One is increase (power frequency or high frequency) isolating transformer.Although connect the effect that transformer can realize Voltage Cortrol and electrical isolation, also there is the shortcoming and defect that some are intrinsic, as Industrial Frequency Transformer exists, volume is large, Heavy Weight, cost are high and installation, transport difficult etc.; And although high-frequency transformer volume is little, structure is simple, and because adopting multilevel hierarchy to make Systematical control complicated, conversion efficiency is low.

Two is increase leakage current test unit (circuit), and the leakage current produced during Real-Time Monitoring invertor operation, quits work immediately once exceed safe threshold inverter, thus cuts off leakage current.This detecting unit its be a current sensor in essence, due to leakage current numerical value very little (mA rank), thus to transducer sensitivity require higher, all there are corresponding commercial product in U.S. LEM and German VAC company, but it is expensive; Also there are many inverter producers to adopt based on the scheme from concussion Fluxgate Technique, generally need to use cobalt base amorphous magnet ring.In practical application, general 2 ~ 4 strands are simultaneously through magnet ring, thus its aperture can not be too little, but be inversely proportional to from concussion frequency and magnet ring volume, which limits the detection bandwidth (relevant laws and regulations require that leakage current test unit bandwidth is at least 2kHz) of sensor.Magnet ring thickness can be reduced to address this problem, but due to material behavior, cobalt base amorphous being difficult to cuts, magnet ring model suitable is in the market fewer, its aperture is at below 15mm, in can not meeting, the application of high-power inverter, and this circuit exports as analog quantity, and rear end needs increase sample circuit and take inverter main control chip (as DSP) resource to carry out data processing.

Summary of the invention

The purpose of this utility model is to provide a kind of new leakage current test scheme, can adapt to the inverter of various power.

In order to achieve the above object, the technical solution of the utility model there is provided a kind of leakage current detection circuit, it is characterized in that, comprise the transformer adopting high magnetic conductivity ferrite toroidal core, transformer has at least four windings, be respectively excitation winding one, excitation winding two, correcting current winding and winding, winding is the conductor through high magnetic conductivity ferrite toroidal core;

One end of excitation winding one connects the unit that magnetizes, and the other end connects degaussing unit on the one hand, another aspect tandem tap one and current sampling unit successively;

One end of excitation winding two connects the unit that magnetizes, and the other end connects degaussing unit on the one hand, another aspect tandem tap two and current sampling unit successively;

Calibration current generating unit forms loop via switch three and correcting current winding;

Leakage current I _kthrough winding inflow transformer;

Switch one and switch two alternate conduction under main control unit controls, main control unit also connects current sampling unit.

Preferably, described calibration current generating unit or employing current source, or adopt voltage source and the resistance of series connection.

Preferably, described main control unit or employing microcontroller, or adopt drive circuit unit and pulse width signal processing unit, drive described switch one and described switch two by this drive circuit unit, meanwhile, electric current is detected by this drive circuit unit by described current sampling unit; Pulse width signal processing unit connects drive circuit unit.

Preferably, described pulse width signal processing unit or be comparator circuit, or be the high-speed pulse counting of series connection and D/A converting circuit.

Preferably, the unit that magnetizes described in is voltage source V _cC.

Preferably, described degaussing unit or be voltage source V _dD, or be the electric capacity C of ground connection ₁.

Preferably, described current sampling unit is the resistance of ground connection.

For photovoltaic DC-to-AC converter, the utility model proposes a kind of new leakage current test scheme, the program adopts ferrite bead to substitute cobalt base amorphous magnet ring.Enrich due to ferrite size and be convenient to cutting, the requirement of detection bandwidth can be met in hole diameter enlargement situation.This programme adopts MCU as measurement, control module simultaneously, can carry out aftertreatment easily to data, as calculated effective value, sliding average etc., directly can cut off invertor operation by MCU if desired.

Accompanying drawing explanation

Fig. 1 is the leakage current detection circuit schematic diagram in embodiment 1;

Fig. 2 is magnetization current curve map;

Fig. 3 is the magnetization current curve map that magnetic core exists DC magnetic bias postpone;

Fig. 4 is the leakage current detection circuit schematic diagram in embodiment 2;

Fig. 5 is the leakage current detection circuit schematic diagram in embodiment 3;

Fig. 6 is typical peak pulse duration comparatively circuit diagram;

Fig. 7 is leakage current linearity test curve.

Embodiment

For making the utility model become apparent, hereby with preferred embodiment, and accompanying drawing is coordinated to be described in detail below.

Embodiment 1

Disclosed in the present embodiment, a kind of new leakage current test scheme, comprises a transformer T1, a microcontroller, and one is detected resistance Rs, three K switch ₁, K ₂and K ₃, two diode D ₁and D ₂, a current source, two voltage source V _cCand V _dD, its annexation as shown in Figure 1.

Wherein transformer T1 adopts high magnetic conductivity ferrite magnetic core, comprises 4 winding: W ₁, W ₂for excitation winding, for generation of alternating voltage, make the two-way magnetization of magnetic core; W ₃for correcting current winding, for calibrating MCU two-way drive singal Pulse_A, Pulse_B pulsewidth difference and the proportionate relationship detecting electric current; W ₄for the conductor through toroidal core, flow through W ₄electric current be the required leakage current I measured _k.MCU Main Function is to provide two-way drive singal Pulse_A and Pulse_B and calculates its pulsewidth difference, the magnetization current of sample streams on resistance Rs, and relevant data processing.

If this main circuit utilizes the positive and negative difference to magnetization characteristic of magnetic material to judge whether leakage current (being also magnetization current) exceedes threshold values, and its principle of work is as follows:

Composition graphs 2, K switch ₁and K ₂alternate conduction, in first half cycle T _sin/2, K ₁during conducting, direct supply V _cCto winding W ₁magnetize, the electric current flowing through Rs rises, once reach threshold values I _pk, K ₁turn off, voltage source V _dDby diode to winding W ₁degaussing, Rs electric current drops to 0; In later half cycle T _sk in/2 ₂successively turn-on and turn-off, principle as before.

1) when not having electric current to pass (namely there is no leakage current) in magnet ring, magnetic core does not have magnetic bias, it is positive and negative to magnetization curve full symmetric, winding magnetization current symmetrical shape, therefore ON time (being also the pulse width of drive singal Pulse_A and the Pulse_B) T of two switches _on1and T _on2identical, as shown in Figure 2.

2) if there is leakage current to pass in magnet ring, because its frequency is very low relative to switches switching frequency, relative to high-frequency excitation winding W ₁and W ₂, there is a DC magnetic in magnetic core and be biased, make magnetic core positive and negative to magnetization curve generation difference.As shown in phantom in Figure 3, at W ₁on the direction of the winding current, magnetic core more easily magnetizes, and the magnetization current rate of rise accelerates, and at W ₂the more difficult magnetization of magnetic core in coil direction, the electric current rate of rise is slack-off, thus causes two switch conduction times T _on1' and T _on2' producing difference, the size of its difference is closely related with the leakage current through magnet ring.

3) by correcting current winding W ₃the relation between leakage current and ON time difference can be determined.First Closing Switch K before invertor operation ₃, current known source I ₀flow through winding W ₃, at this moment K switch ₁and K ₂oN time produce certain difference, be designated as Δ T ₀.

4) calibration steps disconnects K after completing ₃, start the leakage current detected in side circuit.

Following analyzed pulse width discrepancy delta T=T _on1-T _on2with leaked hunting electric current I _kbetween relation:

A) leakage current I is supposed _kdirection and W ₁winding current unanimously, then its magnetic field intensity H produced in magnetic core ₀for:

$H_0=\frac{I_k}{l_e}---\left(1\right);$

L in formula (1) _efor the length of magnetic path of magnetic core.

B) according to classical electromagnetics formula with winding W can be calculated ₁and W ₂magnetization current rises to I by 0 _pktime the required time be respectively:

$T_{\mathrm{on}1}=\frac{{\mathrm{NA}}_e}{V_m}{\∫}_0^{H_{\mathrm{pk}}}u(H+H_0)\mathrm{dH}=\frac{{\mathrm{NA}}_e}{V_m}{\∫}_{H_{\mathrm{pk}}}^{H_{\mathrm{pk}}+H_0}u\left(H\right)\mathrm{dH}---\left(2\right);$

$T_{\mathrm{on}2}=\frac{{\mathrm{NA}}_e}{V_m}{\∫}_0^{H_{\mathrm{pk}}}u(H-H_0)\mathrm{dH}=\frac{{\mathrm{NA}}_e}{V_m}{\∫}_{H_{\mathrm{pk}}}^{H_{\mathrm{pk}}-H_0}u\left(H\right)\mathrm{dH}---\left(3\right);$

In formula (2) and formula (3), A _efor core cross section amasss, N is winding W ₁and W ₂the number of turn, V _mfor being applied to the voltage on winding during magnetization, the V in corresponding diagram 1 _cC, u (H) is ferritic magnetic permeability under magnetic field intensity H, H _pkfor magnetization current reaches I _pkcorresponding magnetic field intensity:

$H_{\mathrm{pk}}=\frac{N\·i_{\mathrm{pk}}}{l_e}---\left(4\right).$

C) formula (3) deducts formula (2):

$\mathrm{\ΔT}=\frac{{\mathrm{NA}}_e}{V_m}({\∫}_{-H_0}^{H_0}u\left(H\right)\mathrm{dH}-{\∫}_{H_{\mathrm{pk}}-H_0}^{H_{\mathrm{pk}}+H_0}u\left(H\right)\mathrm{dH})---\left(5\right).$

D) as magnetization current convergence I _pk(namely magnetic field intensity H levels off to H _pk) time magnetic core entry altitude saturated, u (H) → 0, therefore formula (5) the right Section 2 can be ignored, so have:

$\mathrm{\ΔT}=\frac{{\mathrm{NA}}_e}{V_m}\·{\∫}_{-H_0}^{H_0}u\left(H\right)\mathrm{dH}=\frac{{2\mathrm{NA}}_e}{V_m}\·{\∫}_0^{H_0}u\left(H\right)\mathrm{dH}---\left(6\right).$

E) due to leakage current i _kvery little, can think that magnetic core is in the range of linearity, u (H) → u (0)=μ ₀μ _i, formula (6) can be rewritten as follows:

$\mathrm{\ΔT}=\frac{{2\mathrm{NA}}_e}{V_m}\·{\mathrm{\μ}}_0{\mathrm{\μ}}_i\·H_0=\frac{2{\mathrm{NA}}_e{\mathrm{\μ}}_0{\mathrm{\μ}}_i}{V_m{l}_e}\·I_k---\left(7\right);$

In formula (7), μ ₀for absolute permeability of vacuum, μ _ifor the initial relative permeability of magnetic core.Can see, under certain conditions pulse width difference Δ T and leakage current I _kbetween there is linear relationship.

F) by aforementioned currents calibration winding current known I ₀corresponding pulse width difference Δ T ₀, namely have:

${\mathrm{\ΔT}}_0=\frac{{2\mathrm{NA}}_e}{V_m}\·{\mathrm{\μ}}_0{\mathrm{\μ}}_i\·H_0=\frac{{2\mathrm{NA}}_e{\mathrm{\μ}}_0{\mathrm{\μ}}_i}{V_m{l}_e}\·I_0---\left(8\right).$

G) easily obtain according to formula (7) and formula (8):

Above computing method and step all realize by MCU.

Embodiment 2

As shown in Figure 4, the difference of the present embodiment and embodiment 1 is: voltage source V _dDby electric capacity C ₁substitute, current source voltage source V _cCrealize with known resistance Rc.Other structure and working principle are with embodiment 1.

Embodiment 3

As shown in Figure 5, the difference of the present embodiment and embodiment 1 is: the function of MCU can with a drive circuit unit U ₁with pulse width signal processing unit U ₂realize.U ₁effect be to provide two-way drive singal and gather voltage on RS, once exceed setting threshold values turn off K at once ₁or K ₂; U ₂measure the pulsewidth of drive singal and convert thereof into analog output, it can be comparator circuit as shown in Figure 6 and low-pass filter circuit, or high-speed pulse counting and D/A converting circuit.The advantage of this scheme is with low cost, and MCU crystal oscillator frequency can be avoided the restriction of sampling bandwidth simultaneously.Other structure and working principle are with embodiment 1.

In above-described embodiment, the test result of each circuit as shown in Figure 7, and experiment shows that above-mentioned testing circuit all has the good linearity.

Claims (7)

1. a leakage current detection circuit, is characterized in that, comprise the transformer (T1) adopting high magnetic conductivity ferrite toroidal core, transformer (T1) has at least four windings, is respectively excitation winding one (W ₁), excitation winding two (W ₂), correcting current winding (W ₃) and winding (W ₄), winding (W ₄) be conductor through high magnetic conductivity ferrite toroidal core;

Excitation winding one (W ₁) one end connect and to magnetize unit, the other end connects degaussing unit on the one hand, on the other hand tandem tap one (K successively ₁) and current sampling unit;

Excitation winding two (W ₂) one end connect and to magnetize unit, the other end connects degaussing unit on the one hand, on the other hand tandem tap two (K successively ₂) and current sampling unit;

Calibration current generating unit is via switch three (K ₃) and correcting current winding (W ₃) form loop;

Leakage current I _kthrough winding (W ₄) inflow transformer (T1);

Switch one (K ₁) and switch two (K ₂) alternate conduction under main control unit controls, main control unit also connects current sampling unit.

2. a kind of leakage current detection circuit as claimed in claim 1, is characterized in that, described calibration current generating unit or employing current source, or adopts voltage source and the resistance of series connection.

3. a kind of leakage current detection circuit as claimed in claim 1, is characterized in that, described main control unit or employing microcontroller, or adopts drive circuit unit (U ₁) and pulse width signal processing unit (U ₂), by this drive circuit unit (U ₁) drive described switch one (K ₁) and described switch two (K ₂), meanwhile, by this drive circuit unit (U ₁) detect electric current by described current sampling unit; Pulse width signal processing unit (U ₂) connect drive circuit unit (U ₁).

4. a kind of leakage current detection circuit as claimed in claim 3, is characterized in that, described pulse width signal processing unit (U ₂) or be comparator circuit, or be high-speed pulse counting and the D/A converting circuit of series connection.

5. a kind of leakage current detection circuit as claimed in claim 1, is characterized in that, described in the unit that magnetizes be voltage source V _cC.

6. a kind of leakage current detection circuit as claimed in claim 1, is characterized in that, described degaussing unit or be voltage source V _dD, or be the electric capacity C of ground connection ₁.

7. a kind of leakage current detection circuit as claimed in claim 1, is characterized in that, described current sampling unit is the resistance (R of ground connection _s).