CN108306574A - A kind of frequency converter safety operation area computational methods - Google Patents

Abstract

The invention discloses a method for calculating the safe working area of a frequency converter. Through in-depth research on the internal structure of the frequency converter, systematic research and calculation are carried out on the rectification and inverter links inside the frequency converter and the mechanical characteristics of the external motor operation. The output voltage and current of the rectification link, the output voltage of the inverter link and the rotor current and speed of the three-phase asynchronous motor calculated after each theoretical voltage sag are compared with the parameters that the inverter and the three-phase asynchronous motor can work normally comparison, so as to obtain the safe working area in which the inverter as a whole can work normally when a voltage sag occurs; the present invention considers the three links of rectification, inverter and motor operation as a whole to calculate The safe working area of the inverter, the calculation speed is fast, the calculation accuracy is high, and it will not cause any damage to the inverter and the motor.

Description

A Calculation Method of Safe Working Area of Inverter

technical field

The invention relates to the technical field of theoretical analysis of power system power quality, in particular to a calculation method for a safe working area of a frequency converter under the condition of a voltage sag.

Background technique

With the rapid development of science and technology and economy, more and more users use sensitive loads in the power grid, which puts forward higher and higher requirements for power quality; voltage sag is the main power quality problem that affects the normal operation of electrical equipment. Different loads have different operating characteristics and are therefore not equally affected by voltage sags. For the load equipment in the power supply network, if the voltage changes or suddenly changes and causes it to fail to work normally, causing serious losses and hazards, we call such loads sensitive equipment, such as computer power supplies, adjustable speed motors, AC contactors, frequency converter, programmable logic controller, etc.

Compared with other equipment, the internal structure of the frequency converter is more complicated, and the power electronic devices such as the internal rectifier and inverter have higher requirements on power quality, and the voltage range that can be tolerated is smaller. At this stage, there are many analyzes on the sensitivity of AC contactors, computer power supplies and other equipment to voltage sags and the safe working area, and more experimental measurement methods are used, that is, using experimental equipment to simulate the occurrence of voltage sags and measuring and recording equipment. Working conditions; and the method of experimental measurement is not only inefficient, but also easily damages the equipment due to the need for actual voltage sag, which is very unsuitable for complex and sensitive frequency converters.

Therefore, in view of the prevalence and severity of the voltage sag problem and the extensive use of frequency converters in industrial production processes, it is necessary to develop a set of methods for analyzing and calculating the safe working area of frequency converters through theoretical calculations under the condition of voltage sags. Useful ways.

Contents of the invention

The purpose of the present invention is to provide a method for calculating the safe working area of the frequency converter, which can obtain the safe working area of the frequency converter through theoretical calculation, has high calculation efficiency, and will not cause any damage to the frequency converter.

The technical solution adopted by the present invention is: a method for calculating the safe working area of a frequency converter, comprising the following steps:

Step A. Set the amplitude of a voltage sag at the power supply terminal, and perform a theoretical voltage sag calculation at the power supply terminal from the rated voltage according to the set amplitude of a voltage sag, that is, subtract the primary voltage from the rated voltage The amplitude of the sag, to obtain the theoretical voltage after the voltage sag occurs at the power supply terminal;

Step B. Calculate the output voltage and current of the rectification link in the frequency converter according to the theoretical voltage after the voltage sag occurs at the power supply terminal;

Step C, calculating the output voltage of the inverter link in the frequency converter according to the output voltage and current of the rectification link calculated in step B;

Step D, calculating the rotor speed and rotor current of the three-phase asynchronous motor according to the output voltage of the inverter link calculated in step C;

Step E, according to the magnitude of a voltage sag set in step A, calculate the voltage sag in sequence at the power supply end until the voltage at the power supply terminal changes from the rated voltage to 0, and repeat step B and step 1 after each voltage sag calculation C and step D respectively calculate the output voltage and current of the rectification link, the output voltage of the inverter link, and the rotor current and speed of the three-phase asynchronous motor, and then calculate the output voltage and current of the rectification link after each voltage sag, The output voltage of the inverter link and the rotor current and speed of the three-phase asynchronous motor are compared with the parameters that the inverter and the three-phase asynchronous motor can work normally, and the safe working area that the inverter as a whole can work normally when a voltage sag occurs is obtained .

Further, in step A, the magnitude of a voltage sag set at the power supply terminal is 5% or 10% of the rated voltage.

Further, in step B, the output voltage of the rectification link is set to be V _D , the output current of the rectification link is I _D , and the calculation formula of the output voltage V _D of the rectification link is:

In the formula, V ₁ is the effective value of the line voltage, V _S is the effective value of the phase voltage, and v _AB is the line voltage;

The calculation method of the output current _ID of the rectification link is:

If the load inductance L=0 and the load resistance is R, then I _D =V _D /R, the waveform of I _D is the same as V _D , each diode in the rectifier conducts 120°, and the AC power current i _A =v _AB /R ; If the load inductance L≠0, the load resistance is R, and L is so large that in a DC voltage pulse cycle, _ID is approximately a constant value, then i _A will be an ideal rectangular wave with a pulse width of 120°, and the current amplitude Value I _D =V _D /R, R represents the resistance value of the load resistor of the rectification link.

Further in step C, calculating the output voltage of the inverter link according to the output voltage and current of the rectification link in the frequency converter calculated in step B includes the following steps:

Step C1, obtain the Fourier analysis result of the line voltage v _AB , set the zero point of the time coordinate of the line voltage v _AB at point N, and the ordinate is NY, then the Fourier analysis result of v _AB is:

Step C2, calculating the amplitude of the fundamental wave of the line voltage and the effective value of the fundamental wave of the line voltage;

The amplitude of the fundamental wave of the line voltage is:

The effective value of the fundamental wave of the line voltage is:

Take the amplitude V _1m of the fundamental wave voltage as the amplitude of the AC voltage output by the inverter link.

Further in step D, the synchronous speed of the rotor of the three-phase asynchronous motor

In the formula, _f1 is the grid frequency, and p is the number of pole pairs of the winding of the three-phase asynchronous motor;

So when the asynchronous motor rotor rotates at a constant speed n, the rotor current is expressed as

In the formula, and X _2s are respectively the electromotive force and leakage reactance of one phase of the rotor winding when the rotor of the three-phase asynchronous motor rotates; and X ₂ are respectively the electromotive force, current and leakage reactance of one phase when the rotor of the three-phase asynchronous motor does not rotate, and s is the slip rate.

The present invention studies the internal structure of the frequency converter in depth, and systematically studies the rectification and inverter links inside the frequency converter and the mechanical characteristics of the external motor operation, and calculates the rectification link after each theoretical voltage sag The output voltage and current of the inverter, the output voltage of the inverter link and the rotor current and speed of the three-phase asynchronous motor are compared with the parameters that the inverter and the three-phase asynchronous motor can work normally, so that it can be concluded that when the overall voltage sag of the inverter occurs A safe working area that can work normally; this invention considers the three links of rectification, inverter and motor operation as a whole to calculate the safe working area of the frequency converter without borrowing experimental equipment, with fast calculation speed and high calculation accuracy , and will not cause any damage to the inverter and motor.

Description of drawings

Fig. 1 is a flowchart of the present invention;

Fig. 2 is the overall structure schematic diagram of the frequency converter of the present invention;

Fig. 3 is the structural representation of the rectifier in the frequency converter of the present invention;

Fig. 4 is a structural schematic diagram of an inverter in a frequency converter according to the present invention;

Fig. 5 is an equivalent structural diagram of a three-phase asynchronous motor according to the present invention.

Detailed ways

As shown in Figure 2, Figure 3 and Figure 4, the inverter includes a rectifier and an inverter inside, and the A phase, B phase, and C phase of the three-phase AC power supply are connected to the rectifier in the inverter, and the inverter in the inverter The inverter is connected with the external three-phase asynchronous motor; D ₁ ~ D ₆ in Figure 3 are diodes, V _D and _ID are DC voltage and DC current respectively; the three-phase inverter in Figure 4 has 6 bridge arms, each The bridge arm is composed of a controllable switching device and an anti-parallel diode, T ₁ ~ T ₆ are controllable switching devices, D ₁ ~ D ₆ are diodes, P and Q are the positive and negative terminals of the DC terminal respectively.

As shown in Figure 1, the method for calculating the safe working area of the frequency converter described in the invention mainly includes the following steps:

Step A. Set the amplitude of a voltage sag at the power supply terminal. The amplitude of a voltage sag is preferably 5% or 10% of the rated voltage. Through a small voltage sag, the accuracy of the inverter safety zone can be calculated Higher; according to the set amplitude of a voltage sag, a theoretical voltage sag calculation is performed at the power supply terminal from the rated voltage, that is, the rated voltage is subtracted from the amplitude of a voltage sag to obtain the theoretical voltage sag that occurs at the power supply terminal. Voltage after a voltage sag.

Step B. Calculate the output voltage and current of the rectification link in the frequency converter according to the theoretical voltage after the voltage sag occurs at the power supply terminal.

As shown in Figure 3, a power supply cycle T _S of three-phase AC voltage is divided into six time zones, which are respectively zone I, zone II, zone III, zone IV, zone V and zone VI, at ωt=ωt ₁ ~ωt In the 60° time zone I of ₂ , the voltage of v _A is the highest, so phase A conducts electricity through D ₁ , and because v _B has the lowest voltage, phase B conducts electricity through D ₆ , so the potential at point P in zone I is v _p =v _A , and at point N Potential v _N ＝v _B ; rectified voltage V _D ＝v _PN ＝v _P -v _N ＝v _A -v _B ＝v _AB , that is, D ₁ and D ₆ conduct electricity and add the maximum positive line voltage v _AB to the load .

Similarly, in the following time zones II, III, IV, V, and VI, the line voltages are v _AC (D ₁ , D ₂ conduction), v _BC (D ₂ , D ₃ conduction), v _BA (D ₃ , D ₄ conduction), v _CA (D ₄ , D ₅ conduction) and v _CB (D ₅ , D ₆ conduction) are the largest, so in one power cycle T _S , V _D consists of 6 identical pulse waves (m= 6), each pulse width is 60°(π/3, T _S /6), and its DC average value is:

In the formula, V ₁ is the effective value of the line voltage, and V _S is the effective value of the phase voltage.

If the load inductance L=0 and the load resistance is R, then I _D =V _D /R, the waveform of I _D is the same as V _D , each diode in the rectifier conducts 120°, and the AC power current i _A =v _AB (v _AC )/R. If the load inductance L≠0, the load resistance is R, and L is so large that in a DC voltage pulse cycle (60°), _ID is approximately a constant value, then i _A will be an ideal rectangular wave with a pulse width of 120° , current amplitude _ID = V _D /R.

Step C, calculating the output voltage of the inverter link in the frequency converter according to the output voltage and current of the rectification link calculated and obtained in step B.

In the output half cycle, the inverter has the following three working modes (switching states):

Mode 1: During the period of 0≤ωt≤π/3, T ₅ , T ₆ , and T ₁ have driving signals; A and C points of the three-phase bridge are connected to the positive terminal P, point B is connected to the negative terminal Q, and R is the output of the inverter The load resistance at the terminal, the equivalent resistance R _E and the DC side current i ₁ are respectively:

v _AN =v _CN =v _D /3

v _BN ＝-i ₁ R＝-2V _D /3

Mode 2: During the period of π/3≤ωt≤2π/3, T ₆ , T ₁ , and T ₂ have driving signals; point A of the three-phase bridge is connected to the positive terminal P, B and C are connected to the negative terminal Q, and R is the inverter The load resistance at the output end, the equivalent resistance _RE and the DC side current _i2 are respectively:

Mode 3: During the period of 2π/3≤ωt≤π, T ₁ , T ₂ , and T ₃ have driving signals; points A and B of the three-phase bridge are connected to the positive terminal P, C is connected to the negative terminal Q, and R is the output terminal of the inverter. The load resistance, the equivalent resistance _RE and the DC side current _i3 are respectively:

v _AN =v _BN =i ₃ R/2=v _D /3

v _CN ＝-i ₃ R＝-2V _D /3

According to the above analysis, when the load resistance of the inverter is connected in star form, the waveforms of its phase voltages v _AN , v _BN , and v _CN are ladder waves; if the starting point of the time coordinate is taken at the starting point of the ladder wave, use Analysis, the instantaneous value of A-phase load voltage v _AN is:

The line voltage is a square wave with a width of 120° and an amplitude of V _D ; if the zero point of the time coordinate of the line voltage v _AB is taken at point N, and the ordinate is NY, then the Fourier analysis result of v _AB is:

The amplitude V _1m of the line voltage fundamental wave is:

The effective value of the line voltage fundamental wave V ₁ is:

Step D, calculating the rotor speed and rotor current of the three-phase asynchronous motor according to the output voltage of the inverter link calculated and obtained in step C.

The synchronous speed n ₁ of the three-phase asynchronous motor is:

In the formula, f ₁ is the grid frequency, and p is the number of pole pairs of the winding of the three-phase asynchronous motor.

The value of each phase voltage _U1 of the three-phase asynchronous motor is:

U ₁ ≈ E ₁ ＝4.44f ₁ N ₁ k _dp1 φ ₁ (10)

In the formula, E ₁ is the induced electromotive force of one phase winding of the stator, N ₁ is the number of series turns of each phase of the stator winding, k _N1 is the winding coefficient, and φ _{m is} the main magnetic flux.

The slip s is:

Where n is the rotational speed of the rotor of the three-phase asynchronous motor.

When the asynchronous motor rotor rotates at a constant speed n, the frequency _f2 of the electromotive force and current in the rotor winding is:

When the rotor rotates, the electromotive force E _2s of each phase of the rotor winding is:

In the formula, E ₂ corresponds to the rotor phase winding electromotive force at the stator frequency, that is, the rotor phase electromotive force when the rotor does not rotate and the main magnetic flux is still φ _m , N ₂ is the number of series turns of each phase of the stator winding, and k _N2 is the winding coefficient.

The rotor leakage reactance X _2s corresponding to the rotor current frequency f ₂ is:

X _2s =sX ₂ (14)

Therefore, when the rotor of a three-phase asynchronous motor rotates at a constant speed n, the rotor current Can be expressed as:

In the formula, X _2s are respectively the electromotive force, current and leakage reactance of one phase of the rotor winding when the rotor of the asynchronous motor rotates; X ₂ are respectively the electromotive force, current and leakage reactance of one phase of the rotor winding when the motor rotor is not rotating.

Since there is no direct connection between the stator and the rotor of the asynchronous motor, the rotor only acts on the stator through its magnetomotive force, so as long as the magnetomotive force remains unchanged, a stationary rotor can be used to replace the rotating rotor, and the physical quantities of the stator No change occurs, i.e. equivalent to the grid. Therefore, according to frequency conversion, the asynchronous motor is equivalently simplified as shown in Figure 4. In the figure, I ₁ , I ₂ ′ and I ₀ ′ are stator current, rotor current and excitation current respectively, and R ₁ , X _1σ , R ₂ , X _2σ , R _m , X _m are the equivalent impedance and excitation impedance of the stator and rotor respectively, and the rotor current I ₂ ′ can be obtained from the simplified equivalent circuit as:

The electromagnetic power P _em is:

The electromagnetic torque T _em is:

In the process of implementing frequency conversion and speed regulation to the motor through the frequency converter, it is necessary to maintain Constant, in the process of frequency conversion speed regulation, the electromagnetic torque of the motor

Maximum torque The corresponding slip is s _m , namely

therefore

Substituting formula (22) into formula (19), we get

In the formula, L ₂ ′ is the equivalent value of the leakage inductance coefficient of the rotor phase winding when the rotor is stationary, X ₂ ′=2πf ₁ L ₂ ′.

The speed drop at the maximum torque is

It can be seen from formula (23) and formula (24), in the process of frequency conversion speed regulation, if keep Constant, the maximum torque T _m is a constant, independent of frequency, and the speed drop corresponding to the maximum torque is equal, that is, the mechanical characteristics of different frequencies are parallel and the hardness is the same.

Step E, according to the amplitude of the primary voltage sag set in step A, calculate the voltage sag in sequence at the power supply end, that is, subtract the amplitude of the primary voltage sag set in step A from the previous voltage sag, and then Cycle until the voltage at the power supply terminal changes from the rated voltage to 0, and repeat step B, step C and step D after each voltage sag calculation to calculate the output voltage and current of the rectifier link, the output voltage of the inverter link, and the three-phase The rotor current and speed of the asynchronous motor, and then the output voltage and current of the rectification link, the output voltage of the inverter link, and the rotor current and speed of the three-phase asynchronous motor calculated after each voltage sag are compared with the frequency converter and the three-phase asynchronous By comparing the parameters that the motor can work normally, the safe working area of the frequency converter as a whole can be obtained when the voltage sag occurs.

When the voltage sag occurs at the power supply end, the rectification and inverter links inside the frequency converter and the normal operation of the external three-phase asynchronous motor will be affected; The safe working area of the frequency converter is the ultimate goal. Systematic research has been carried out on the internal rectification and inverter links of the frequency converter and the mechanical characteristics of the external motor. Without borrowing experimental equipment, the rectification, inverter and motor operation The three links are considered as a whole to calculate the safe working area of the inverter, the calculation speed is fast, the calculation accuracy is high, and it will not cause any damage to the inverter and the motor.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it still The technical solutions described in the foregoing embodiments can be modified, or some or all of the technical features can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims (5)

1. a kind of frequency converter safety operation area computational methods, which is characterized in that include the following steps：

Step A, the amplitude that primary voltage temporarily drops occurs for setting power end, and the amplitude temporarily dropped according to the primary voltage of setting is in power supply End proceeds by primary theoretic voltage dip from rated voltage and calculates, i.e., subtracts what primary voltage temporarily dropped using rated voltage Amplitude obtains theoretically power end and the voltage after voltage dip occurs；

Step B, the output voltage that the voltage after voltage dip calculates rectification link in frequency converter occurs according to theoretically power end And electric current；

Step C, according to step B calculate the rectification link acquired output voltage and inversion link in Current calculation frequency converter it is defeated Go out voltage；

Step D, the voltage that the inversion link output acquired is calculated according to step C calculates the rotor speed of threephase asynchronous machine and turns Electron current；

Step E, the amplitude temporarily dropped according to the primary voltage of step A settings carries out voltage dip calculating successively in power end, until Power supply terminal voltage changes to 0 from rated voltage, and step B, step C and D points of step are repeated after the calculating of voltage dip each time Not Ji Suan rectification link output voltage and electric current, the output voltage of inversion link and threephase asynchronous machine rotor current and Rotating speed, then by the output voltage of the output voltage and electric current, inversion link of the rectification link being calculated after each voltage dip And the rotor current and rotating speed of threephase asynchronous machine are carried out with the parameter that frequency converter and threephase asynchronous machine can work normally Comparison obtains the safety operation area that frequency converter can integrally be worked normally when voltage dip occurs.

2. frequency converter safety operation area as described in claim 1 computational methods, it is characterised in that：In the step A, power end The amplitude that the generation primary voltage of setting temporarily drops is the 5% or 10% of rated voltage.

3. frequency converter safety operation area as described in claim 1 computational methods, it is characterised in that：In the step B, setting is whole The output voltage for flowing link is V_D, the output current of rectification link is I_D, the output voltage V of rectification link_DCalculation formula be：

V in formula₁For line voltage virtual value, V_SFor phase voltage virtual value, v_ABFor line voltage；

The output current I of rectification link_DComputational methods be：

If load inductance L=0, load resistance R, then I_D=V_D/ R, I_DWaveform and V_DIt is identical, each diode in rectifier It is 120 ° conductive, AC power electric current i_A=v_AB/R；If load inductance L ≠ 0, load resistance R, and L is very big so that at one In DC voltage Pulse period, I_DBe approximately steady state value, then i_ATo be the ideal rectangle wave of 120 ° of pulsewidths, current amplitude I_D=V_D/ R, R indicate the resistance value of the load resistance of rectification link.

4. frequency converter safety operation area as described in claim 1 computational methods, it is characterised in that：In the step C, according to step It includes following step that rapid B, which calculates rectification link output voltage and the output voltage of Current calculation inversion link in the frequency converter acquired, Suddenly：Step C1, line voltage v is sought_ABFourier analysis as a result, setting line voltage v_ABThe zero of time coordinate takes in N points, indulges Coordinate is NY, then v_ABFourier analysis the result is that：

Step C2, line voltage fundamental voltage amplitude and line voltage fundamental wave virtual value are sought；

Line voltage fundamental voltage amplitude is：

Line voltage fundamental wave virtual value is：

Take the amplitude V of fundamental voltage_1mFor the amplitude of the alternating voltage of inversion link output.

5. frequency converter safety operation area as described in claim 1 computational methods, it is characterised in that：In the step D, three is different Walk the synchronous rotational speed of the rotor of motor

In formula, f₁For mains frequency, p is the number of pole-pairs of the winding of threephase asynchronous machine；

So when asynchronous motor rotor is with rotating speed n constant speed rotaries, rotor current is expressed as

In formula,And X_2sWhen respectively threephase asynchronous machine rotor rotates, the electromotive force and leakage reactance of one phase of rotor windings； And X₂When respectively threephase asynchronous machine rotor does not turn, electromotive force, electric current and the leakage reactance of a phase, s is revolutional slip.