Background
The high-power frequency converter is widely applied to industrial production, such as a scraper conveyor in the mine industry, a fracturing pump in the oil and gas industry, a pipeline pump, a rolling mill in the metallurgy industry, a traction system in the rail transit industry and the like. The power grade of the high-power frequency converter is generally not lower than megawatt level, and most of the core power electronic switching devices are medium-voltage IGBTs or IGCTs. In the state of the art of power electronic switching devices, it is necessary to limit the switching frequency thereof to avoid the failure of the device due to excessive switching loss. Therefore, the switching rate of high power frequency converters is typically below 1kHz, and in some applications even below 200 Hz.
In a conventional Pulse Width Modulation (PWM) strategy, taking typical sine wave PWM as an example, a modulated wave is usually kept unchanged in one carrier period or half of the carrier period, and the modulated wave is compared with the carrier to obtain a switching control signal of a power electronic device. Under the low carrier frequency ratio working condition of the high-power frequency converter, the PWM strategy has the condition of error modulation, namely when a carrier passes through an adjacent period of a modulation wave to update a step, extra switching action is generated, so that the problems of higher switching loss and narrow pulse are caused, and the equipment safety is influenced. Moreover, the lower the carrier frequency ratio, the more prominent the problem.
To solve the problem of pulse width modulation with low carrier frequency ratio, a specific harmonic cancellation (SHE) PWM strategy is currently generally adopted. The basic principle of SHE-PWM is to set notches at specific positions of a voltage waveform, properly control the waveform of the inverter pulse width modulation voltage through multiple commutations of the inverter in each half period, and convert the square wave voltage output by the inverter into an equivalent sine wave through a pulse width averaging method so as to eliminate certain specific times of harmonics. The modulation strategy is essentially an off-line modulation algorithm, and the switching angle of the off-line modulation algorithm needs to be calculated off-line in advance and stored in a digital controller for use in operation control. The off-line calculation mode determines that the dynamic characteristic is poor, and the modulation precision under the working conditions of sudden load torque and the like is difficult to meet the requirement.
In summary, for a high-power frequency converter, especially under the condition of relatively low carrier frequency, the existing pulse width modulation strategy has certain disadvantages.
Disclosure of Invention
The invention provides a pulse width modulation strategy of a high-power frequency converter, which solves the problem of realizing high-precision online real-time modulation in low carrier frequency ratio, and adopts the technical scheme as follows:
a pulse width modulation strategy of a high-power frequency converter comprises the following steps:
s1: calculating angular velocity of current control period
And modulation angle
And storing the angular velocity of the last control period
And three-phase voltage modulation prediction value
、
、
;
S2: predicting the angular velocity of the next control cycle: assuming that the acceleration is constant in the control period, according to the current angular velocity
And angular velocity of the last cycle
Predicting and calculating the angular speed of the next period in a forward difference mode
;
S3: predicting the modulation angle of the next control period: according to the control period
And angular velocity
Calculating the predicted value of modulation angle increment of the next control period
;
S4: according to the current modulation angle
Respectively calculating three-phase sinusoidal voltage modulation values
、
、
(ii) a Combining predicted modulation angle increments
Calculating the predicted value of three-phase sinusoidal voltage modulation
、
、
;
S5: calculating zero sequence injection voltage
Respectively superposed on the three-phase sinusoidal voltage modulation predicted values to obtain the three-phase voltage modulation predicted values
、
、
;
S6: and comparing a slope between two points with a carrier wave to obtain a control instruction of the corresponding power electronic switching device in the current period by taking the voltage modulation predicted value of each phase calculated in the previous period as the starting point of the modulation wave in the current period and the voltage modulation predicted value of the next period as the end point of the modulation wave.
Further, in step S2, the angular velocity
The formula of (1) is:
。
further, in step S3, the modulation angle increment predicted value of the next control period
The formula is as follows:
wherein k is more than or equal to 0 and less than or equal to 1 and is an adjusting coefficient.
Further, in step S4, the three-phase sinusoidal voltage modulation value
、
、
The method comprises the following steps:
wherein,
is a modulation factor;
the three-phase sinusoidal voltage modulation prediction value
、
、
The method comprises the following steps:
further, in step S5, the three-phase voltage modulation prediction value
、
、
The method comprises the following steps:
further, in step S6, the oblique lines are expressed as follows:
the a-phase modulation wave starting point is denoted as a1 (0,
) The endpoint is designated as a2 (Ts,
) (ii) a The B-phase modulation wave starts from B1 (0,
) The endpoint is designated as B2 (Ts,
) (ii) a The C-phase modulation wave starting point is denoted as C1 (0,
) The endpoint is designated as C2 (Ts,
) Then the slash is expressed as:
further, in step S6, after the oblique line is compared with the carrier, a control command of the power electronic switching device corresponding to the current cycle is obtained, where the control command includes the following steps:
s8: and judging whether the current control period is the first half carrier period or the second half carrier period of the switching period.
S9: if the step S8 judges that the carrier wave period is the first half carrier wave period, calling the carrier wave straight line of the first half carrier wave period
(ii) a If judging that the carrier period is the second half, jumping to step S11;
s10: comparing the modulation wave shown by the formulas in the step S7 and the step S9 with the carrier wave, and respectively calculating a comparison value and an action moment of the A, B, C three-phase power electronic switching device in the first half carrier wave period; when the modulation wave is higher than the carrier wave, the corresponding upper tube device is switched on, and the lower tube is switched off; and vice versa;
s11: calling the carrier line of the next half of the switching period
;
S12: comparing the modulation wave shown by the formulas in the step S7 and the step S11 with the carrier wave, and respectively calculating a comparison value and an action moment of the A, B, C three-phase power electronic switching device in the second half of the carrier wave period; when the modulation wave is higher than the carrier wave, the corresponding upper tube device is switched on, and the lower tube is switched off; and vice versa;
s13: switching control of the three-phase power electronic switching device is performed in accordance with the calculation results of steps S10 and S12.
In step S10, the operation time of the A, B, C three-phase power electronic switching device in the first half carrier cycle is:
in step S12, the operation time of the A, B, C three-phase power electronic switching device in the second half carrier cycle is:
and the pulse width modulation strategy of the high-power frequency converter obtains a modulation predicted value of the next control period according to the prediction calculation of the angular velocity and the modulation angle, is connected with the current modulation value to obtain a modulation wave, and is compared with a carrier wave to realize pulse width modulation. The method eliminates the modulation wave step of the conventional regular sampling, thereby solving the problem of error modulation under low carrier frequency ratio. Because the modulation wave is closer to the actual waveform, the wave-sending control precision is improved, and the output harmonic wave is reduced.
Detailed Description
The invention provides a pulse width modulation strategy of a high-power frequency converter, which is combined with a pulse width modulation strategy flow chart shown in figure 5 to concretely introduce the implementation steps of the invention:
s1: calculating angular velocity of current control period
And modulation angle
And storing the angular velocity of the last control period
And three-phase voltage modulation prediction value
、
、
;
S2: predicting the angular velocity of the next control cycle: assuming constant acceleration in the control period, based on the current angleSpeed of rotation
And angular velocity of the last cycle
Predicting and calculating the angular speed of the next period in a forward difference mode
Namely:
;
s3: predicting the modulation angle of the next control period: according to the control period
And angular velocity
Calculating the predicted value of modulation angle increment of the next control period
Namely:
wherein k is more than or equal to 0 and less than or equal to 1 and is an adjusting coefficient.
S4: according to the current modulation angle
Respectively calculating three-phase sinusoidal voltage modulation values
、
、
(ii) a Namely:
wherein,
is the modulation factor.
S5: at three-phase sinusoidal voltage modulation value
、
、
On the basis, the predicted modulation angle increment is combined
Calculating the predicted value of three-phase sinusoidal voltage modulation
、
、
Namely:
s6: calculating zero sequence injection voltage according to application requirements
Respectively superposed on the three-phase sinusoidal voltage modulation predicted values to obtain the three-phase voltage modulation predicted values
、
、
Namely:
s7: and taking the voltage modulation predicted value of each phase calculated in the previous period as the modulation wave starting point of the period, taking the voltage modulation predicted value of the next period as the modulation wave end point, obtaining a straight line according to the two points, and then calculating the intersection point of the straight line and the carrier wave.
Here, the a-phase modulation wave start point is represented as a1 (0,
) The endpoint is designated as a2 (Ts,
) (ii) a The B-phase modulation wave starts from B1 (0,
) The endpoint is designated as B2 (Ts,
) (ii) a The C-phase modulation wave starting point is denoted as C1 (0,
) The endpoint is designated as C2 (Ts,
) Then the slash is expressed as:
s8: and judging whether the current control period is the first half carrier period or the second half carrier period of the switching period.
S9: if the step S8 judges that the carrier wave period is the first half carrier wave period, calling the carrier wave straight line of the first half carrier wave period
(ii) a If judging that the carrier period is the second half, jumping to step S11;
s10: comparing the modulation wave with the carrier wave shown in the formulas of step S7 and step S9, and respectively calculating a comparison value and an action time of the A, B, C three-phase power electronic switching device in the first half of the carrier wave period, namely:
when the modulation wave is higher than the carrier wave, the corresponding upper tube device is switched on, and the lower tube is switched off; and vice versa.
S11: calling the carrier line of the next half of the switching period
;
S12: comparing the modulation wave with the carrier wave shown in the formulas of step S7 and step S11, and respectively calculating a comparison value and an action time of the A, B, C three-phase power electronic switching device in the second half of the carrier wave period, namely:
when the modulation wave is higher than the carrier wave, the corresponding upper tube device is switched on, and the lower tube is switched off; and vice versa.
S13: switching control of the three-phase power electronic switching device is performed in accordance with the calculation results of steps S10 and S12.
Compared with fig. 1 and fig. 3, and compared with fig. 2 and fig. 4, it can be seen that the pulse width modulation strategy of the high-power frequency converter provided by the invention can obtain a modulation predicted value of the next control period according to the prediction calculation of the angular velocity and the modulation angle, and connect with the current modulation value to obtain a modulation wave, which is compared with the carrier wave to implement pulse width modulation. The method eliminates the modulation wave step of the conventional regular sampling, thereby solving the problem of error modulation under low carrier frequency ratio. Because the modulation wave is closer to the actual waveform, the wave-sending control precision is improved, and the output harmonic wave is reduced.