CN104155507A - Leakage current detection circuit and method - Google Patents

Abstract

For a photovoltaic inverter, the invention provides a new leakage current detection scheme including a leakage current detection circuit and a leakage current detection method with the adoption of the circuit. According to the scheme provided by the invention, a cobalt-based amorphous magnetic bead is replaced by a ferrite bead. Since the ferrite has rich sizes and is convenient to cut, the requirement of bandwidth detection can be satisfied in the condition of increasing the aperture. At the same time, an MCU is used as measurement and control units, the post-processing of data is convenient, such as the calculation of an effective value and a moving average value, and the operation of an inverter can be cut directly through the MCU when necessary.

Description

A kind of leakage current detection circuit and method

Technical field

The present invention is applied to new energy grid-connected power technical field, is converted into especially circuit and the detection method of alternating voltage for a kind of DC voltage.

Background technology

Solar energy grid-connected photovoltaic system is one of important means of effectively utilizing new forms of energy, also occupies very consequence in distributed generation system.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.

Photovoltaic DC-to-AC converter is the important component part of solar energy grid-connected photovoltaic system, and the direct current that solar panel can be produced converts to and electrical network amplitude, the on all four alternating current of phase place.In order to improve system effectiveness, increasing photovoltaic combining inverter adopts non-isolation design, there is directly electrical connection in electrical network and photovoltaic array like this, and between photovoltaic array and ground, there is virtual stray capacitance, therefore formed the common mode resonance loop being formed by stray capacitance, filter element and electric network impedance.The common mode voltage changing in stray capacitance will produce corresponding common mode current (being leakage current) in this common mode resonance loop.

Common mode current in photovoltaic parallel in system can bring a lot of problems and harm, as cause grid-connected current distortion, other equipment are produced to electromagnetic interference (EMI) etc., more seriously can form significant threat to personal safety.Therefore some countries have carried out relevant regulations to the requirement of leakage current, as German VDE 0126-1-1 standard regulation, when leakage current surpasses 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 that must cross in photovoltaic grid-connected inverting system.

For fear of common mode current over the ground, the person is worked the mischief, generally takes two kinds of solutions:

The one, increase (power frequency or high frequency) isolating transformer.Although connect the effect that transformer can be realized voltage adjustment and electrical isolation, also there are some intrinsic shortcoming and defect, as Industrial Frequency Transformer exists that volume is large, Heavy Weight, cost is high and install, transportation difficulty etc.; Although and high-frequency transformer volume is little, simple in structure, complicated because adopting multilevel hierarchy that system is controlled, conversion efficiency is low.

The 2nd, increase leakage current test unit (circuit), the leakage current producing during Real-Time Monitoring invertor operation, quits work immediately once surpass safe threshold values inverter, thereby cuts off leakage current.It is a current sensor in essence for this detecting unit, and due to leakage current numerical value very little (mA rank), so transducer sensitivity is required higher, all there are corresponding commercial product in U.S. LEM and German VAC company, but it is expensive; Also there is many inverters producer to adopt the scheme based on certainly shaking Fluxgate Technique, generally need to use cobalt base amorphous magnet ring.In practical application, general 2～4 strands are simultaneously through magnet ring, thereby its aperture can not be too little, but certainly shake frequency with magnet ring volume is inversely proportional to, and this has just limited the detection bandwidth (relevant laws and regulations requirement leakage current test unit bandwidth is at least 2kHz) of sensor.In order to address this problem, can reduce magnet ring thickness, but due to material behavior, cobalt base amorphous being difficult to cuts, suitable magnet ring model is fewer in the market, its aperture is below 15mm, in can not meeting, the application of high-power inverter, and this circuit is output as analog quantity, rear end need to increase sample circuit and take inverter main control chip (as DSP) resource carries out data processing.

Summary of the invention

The object of this invention 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, a technical scheme of the present invention has been to provide a kind of leakage current detection circuit, it is characterized in that, comprise the transformer that adopts 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, if there is leakage current I _kproduce, this leakage current I _kthrough winding, in high magnetic conductivity ferrite toroidal core, pass; Calibration current generating unit forms loop via switch three and correcting current winding;

Also comprise main control unit, the unit that magnetizes, degaussing unit, switch one, switch two and current sampling unit, switch one and switch two be alternate conduction under main control unit is controlled, during switch one conducting, magnetize unit to one charging of excitation winding, main control unit detects the electric current that is played increase by initial current value by current sampling unit, this current value increases to after setting threshold, and switch one turn-offs and switch two conductings, is recorded the ON time T of switch one by main control unit ₁, switch one closes has no progeny, and degaussing unit gives excitation winding one degaussing, and the electric current on current sampling unit drops to initial current value; During switch two conducting, magnetize unit to one charging of excitation winding, and main control unit detects the electric current that is played increase by initial current value by current sampling unit, and this current value increases to after setting threshold, switch two turn-offs and switch one conducting, is recorded the ON time T of switch two by main control unit ₂;

Before invertor operation, actuating switch three, provides calibration current I by calibration current generating unit ₀, record main control unit and record the poor Δ T that switch one and main control unit record the ON time of switch two ₀; After invertor operation, cut-off switch three, real time record main control unit records the poor Δ T that switch one and main control unit record the ON time of switch two, foundation real-Time Monitoring has leakage current I _k.

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

Preferably, described main control unit or employing microcontroller, or adopt drive circuit unit and pulse width signal processing unit, by this drive circuit unit, drive described switch one and described switch two, meanwhile, by this drive circuit unit, by described current sampling unit, detect electric current; Pulse width signal processing unit records described ON time T ₁and described ON time T ₂.

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

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 capacitor C of ground connection ₁.

A creepage detection method that adopts above-mentioned leakage current detection circuit, is characterized in that, comprises the steps:

The first step, before invertor operation, main control unit work, switch one and switch two remain alternate conduction under main control unit is controlled, the process of alternate conduction is:

Step 1.1, switch one conducting under main control unit is controlled;

When step 1.2, switch one conducting, magnetize unit to one charging of excitation winding, main control unit detects the electric current that is played increase by initial current value by current sampling unit, this current value increases to after setting threshold, switch one turn-offs and switch two conductings, is recorded the ON time T of switch one by main control unit ₁, switch one closes has no progeny, and degaussing unit gives excitation winding one degaussing, and the electric current on current sampling unit drops to initial current value;

When step 1.3, switch two conducting, magnetize unit to one charging of excitation winding, main control unit detects the electric current that is played increase by initial current value by current sampling unit, this current value increases to after setting threshold, switch two turn-offs and switch one conducting, is recorded the ON time T of switch two by main control unit ₂, return to step 1.2;

In above-mentioned steps 1.1 to the process of step 1.3, ON time T ₁with ON time T ₂equate;

Second step, by switch three conductings, calibration current generating unit via correcting current winding to transformer input calibration current I ₀, ON time T ₁with ON time T ₂at calibration current I ₀impact under produce difference, be designated as Δ T ₀;

The 3rd step, the rear invertor operation of stopcock three, if produce leakage current I on winding _k, ON time T ₁with ON time T ₂at leakage current I _kimpact under produce difference, be designated as Δ T, foundation obtain in real time leakage current I _kconcrete numerical value.

For photovoltaic DC-to-AC converter, the present invention proposes a kind of new leakage current test scheme, and this scheme adopts ferrite bead to substitute cobalt base amorphous magnet ring.Because ferrite size is abundant and be convenient to cutting, can in hole diameter enlargement situation, meet the requirement of detection bandwidth.This programme adopts MCU as measurement, control module simultaneously, can to data, carry out aftertreatment easily, as calculated effective value, sliding average etc., can directly by MCU, cut off invertor operation 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 Pulse width comparision circuit diagram;

Fig. 7 is leakage current linearity test curve.

Embodiment

For the present invention is become apparent, hereby with preferred embodiment, and coordinate accompanying drawing to be described in detail below.

Embodiment 1

The disclosed a kind of new leakage current test scheme of the present embodiment, comprises a transformer T1, a microcontroller, and one is detected resistance R s, 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 signal Pulse_A, Pulse_B pulsewidth difference and the proportionate relationship that detects electric current; W ₄for the conductor through toroidal core, W flows through ₄electric current be the leakage current I of required measurement _k.MCU Main Function is to provide two-way and drives signal Pulse_A and Pulse_B and calculate its pulsewidth difference, the magnetization current of sample streams on resistance R s, 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) surpasses threshold values, and its principle of work is as follows:

In conjunction with Fig. 2, K switch ₁and K ₂alternate conduction, in first half cycle T _sin/2, K ₁during conducting, direct supply V _cCto winding W ₁magnetize, flow through the Current rise of Rs, 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 _s/ 2 interior K ₂priority turn-on and turn-off, principle as before.

1) while not having electric current to pass (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, and winding magnetization current shape is symmetrical, therefore ON time (also driving the pulse width of signal 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 with respect to switching over frequency, with respect to high-frequency excitation winding W ₁and W ₂, in magnetic core, there is a DC magnetic biasing, 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 the more difficult magnetization of magnetic core in W2 coil direction, Current rise slope is slack-off, thereby causes two switch conduction time 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 ₃can determine the relation between leakage current and ON time difference.First Closing Switch K before invertor operation ₃, current known source I ₀winding W flows through ₃, K switch at this moment ₁and K ₂oN time produce certain difference, be designated as Δ T ₀.

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

Below analyze pulse width difference Δ T=T _on1-T _on2with leaked hunting electric current I _kbetween relation:

A) hypothesis leakage current I _kdirection and W ₁winding current unanimously, its magnetic field intensity H producing in magnetic core ₀for:

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

L in formula (1) _ethe length of magnetic path for magnetic core.

B) according to classical electromagnetics formula with can calculate winding W ₁and W ₂magnetization current rises to I by 0 _pkin time, is respectively the needed time:

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

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

In formula (2) and formula (3), A _efor magnetic core sectional area, N is winding W ₁and W ₂the number of turn, V _mduring for magnetization, be applied to the voltage on winding, 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(be that magnetic field intensity H levels off to H _pk) time magnetic core entry altitude saturated, u (H) → 0, thus second, formula (5) the right can ignore, 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 _iinitial relative permeability for 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, calibrate winding current known I ₀corresponding pulse width difference Δ T ₀, 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) according to formula (7) and formula (8), easily obtain:

Above computing method and step all can realize by MCU.

Embodiment 2

As shown in Figure 4, the difference of the present embodiment and embodiment 1 is: voltage source V _dDcan pass through capacitor C ₁substitute current source voltage source V _cCrc realizes with known resistance.Other structures and principle of work 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 be with a drive circuit unit U ₁with pulse width signal processing unit U ₂realize.U ₁effect be to provide two-way and drive signal and gather the upper voltage of RS, once surpass, set threshold values and turn-off at once K ₁or K ₂; U ₂to driving the pulsewidth of signal to measure and convert thereof into analog output, it can be comparator circuit and low-pass filter circuit as shown in Figure 6, or high-speed pulse counting and D/A converting circuit.The advantage of this scheme is with low cost, can avoid the restriction of MCU crystal oscillator frequency to sampling bandwidth simultaneously.Other structures and principle of work are with embodiment 1.

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

Claims (7)

1. a leakage current detection circuit, is characterized in that, comprises the transformer (T1) that adopts high magnetic conductivity ferrite toroidal core, and 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 the conductor through high magnetic conductivity ferrite toroidal core, if there is leakage current I _kproduce, this leakage current I _kthrough winding (W ₄) in high magnetic conductivity ferrite toroidal core, pass; Calibration current generating unit is via switch three (K ₃) and correcting current winding (W ₃) formation loop;

Also comprise main control unit, the unit that magnetizes, degaussing unit, switch one (K ₁), switch two (K ₂) and current sampling unit, switch one (K ₁) and switch two (K ₂) alternate conduction under main control unit is controlled, switch one (K ₁) during conducting, the unit that magnetizes is to excitation winding one (W ₁) charging, main control unit detects the electric current that is played increase by initial current value by current sampling unit, and this current value increases to after setting threshold, switch one (K ₁) turn-off and switch two (K ₂) conducting, by main control unit, record switch one (K ₁) ON time T ₁, switch one (K ₁) close and have no progeny, degaussing unit gives excitation winding one (W ₁) degaussing, the electric current on current sampling unit drops to initial current value; Switch two (K ₂) during conducting, the unit that magnetizes is to excitation winding one (W ₁) charging, main control unit detects the electric current that is played increase by initial current value by current sampling unit, and this current value increases to after setting threshold, switch two (K ₂) turn-off and switch one (K ₁) conducting, by main control unit, record switch two (K ₂) ON time T ₂;

Before invertor operation, actuating switch three (K ₃), by calibration current generating unit, provide calibration current I ₀, record main control unit and record switch one (K ₁) record switch two (K with main control unit ₂) the poor Δ T of ON time ₀; After invertor operation, cut-off switch three (K ₃), real time record main control unit records switch one (K ₁) record switch two (K with main control unit ₂) the poor Δ T of ON time, foundation real-Time Monitoring has leakage current I _k.

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 adopt voltage source and the resistance of connecting.

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 adopt 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) by described current sampling unit, detect electric current; Pulse width signal processing unit (U ₂) record described ON time T ₁and described ON time T ₂.

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 high-speed pulse counting and D/A converting circuit for connecting.

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 capacitor C of ground connection ₁.

7. adopt a creepage detection method for leakage current detection circuit as claimed in claim 1, it is characterized in that, comprise the steps:

The first step, before invertor operation, main control unit work, switch one (K ₁) and switch two (K ₂) under controlling, main control unit remains alternate conduction, the process of alternate conduction is:

Step 1.1, switch one (K ₁) conducting under main control unit is controlled;

Step 1.2, switch one (K ₁) during conducting, the unit that magnetizes is to excitation winding one (W ₁) charging, main control unit detects the electric current that is played increase by initial current value by current sampling unit, and this current value increases to after setting threshold, switch one (K ₁) turn-off and switch two (K ₂) conducting, by main control unit, record switch one (K ₁) ON time T ₁, switch one (K ₁) close and have no progeny, degaussing unit gives excitation winding one (W ₁) degaussing, the electric current on current sampling unit drops to initial current value;

Step 1.3, switch two (K ₂) during conducting, the unit that magnetizes is to excitation winding one (W ₁) charging, main control unit detects the electric current that is played increase by initial current value by current sampling unit, and this current value increases to after setting threshold, switch two (K ₂) turn-off and switch one (K ₁) conducting, by main control unit, record switch two (K ₂) ON time T ₂, return to step 1.2;

In above-mentioned steps 1.1 to the process of step 1.3, ON time T ₁with ON time T ₂equate;

Second step, by switch three (K ₃) conducting, calibration current generating unit is via correcting current winding (W ₃) to transformer (T1) input calibration current I ₀, ON time T ₁with ON time T ₂at calibration current I ₀impact under produce difference, be designated as Δ T ₀;

The 3rd step, stopcock three (K ₃) rear invertor operation, if at winding (W ₄) the upper leakage current I that produces _k, ON time T ₁with ON time T ₂at leakage current I _kimpact under produce difference, be designated as Δ T, foundation obtain in real time leakage current I _kconcrete numerical value.