Implementation of high performance V/F control in medium voltage inverter

introduction

The control speed regulation system is the core of the medium voltage inverter device. The constant voltage-frequency ratio (V/f) control of the speed open loop is the simplest control method, which is suitable for general AC speed regulation occasions without high dynamic performance requirements, such as fans, water pumps, etc.

The general frequency converters that have been maturely applied at present include vector control and direct torque control, but V/f control is still retained. Under V/f control, the frequency converter has torque control, no "tripping", improved mechanical characteristics, excavator characteristics, etc. Introducing the V/f control concept into the unit series medium voltage frequency converter can make the medium voltage frequency converter have higher performance.

2 Control Principle

The control method based on the constant rotor flux (i.e., constant E/f1 control) uses flux compensator, slip compensator and current limit controller to realize the torque control function. This fully reflects the basic idea of ​​high-performance V/f control. However, it is difficult to achieve constant rotor flux control, so constant Eg/f1 control is implemented, and current limit control is used to achieve excavator characteristics to prevent "tripping"; slip compensation control improves the hardness of mechanical characteristics to achieve the minimum speed error under speed open-loop control: IR drop compensation keeps the air gap flux constant at all times, thereby realizing high-performance V/f control without "tripping", and its principle block diagram is shown in Figure 1.

3. Stator resistance (IR drop) compensation

The starting point of constant voltage-frequency ratio control is to keep the air gap flux φm unchanged, make full use of the motor core, and give full play to the motor's ability to generate torque. According to the principle of motor science, when adjusting the speed below the base frequency, in order to keep the air gap flux φm unchanged, it is sufficient to control Eg/f1 to a constant value, but Eg cannot be directly controlled. Generally, the stator voltage U1 is used to replace Eg to form V/f control. When the frequency is high, the stator resistance voltage drop can be ignored due to the high voltage applied to the stator; but at low frequencies, the influence of the stator resistance cannot be ignored, and the constant voltage-frequency ratio control cannot effectively keep the flux unchanged. The maximum output torque of the speed control system will drop to a very small level, limiting the system's load capacity, or even failing to carry load. At this time, stator voltage drop compensation (IR compensation) can be used to appropriately increase the stator voltage and enhance the load capacity.

Stator resistance compensation is based on keeping the stator flux amplitude unchanged. Since the stator leakage inductance L only accounts for 2% to 5% of the stator total inductance, the stator leakage inductance can be ignored in engineering applications, that is, L=0, so it is approximately considered that the stator flux is equal to the air gap flux. On this basis, the vector compensation IR drop method is adopted. Figure 2 is the steady-state equivalent circuit of the asynchronous motor.

According to the phasor diagram of the steady-state equivalent circuit of the asynchronous motor in Figure 2, it can be obtained:

Define the amplitude of Eg at rated frequency frated as Urated. When the output frequency is f, the amplitude of Eg is required to be Urated(f/frated). Substituting this value into formula (1), we get:

In the formula, the stator resistance R1, I, cosφ and I1, sinφ are unknown quantities. Once these quantities are calculated, U1 can be calculated in vector form.

As shown in Figure 2, since compensating for the stator resistance voltage drop requires increasing the voltage, and increasing the voltage will further increase the stator resistance voltage drop, this forms a positive feedback. To ensure the stability of the system, U1 calculated by equation (2) can be divided into two parts, one for the basic V/f component and the other for compensating for the stator resistance voltage drop. The latter is added to the basic V/f component after passing through the first-order inertia link (suppressing the change speed of this part), thereby obtaining the voltage output U1. All calculations are calculated using the normalized value after the rated value, which greatly reduces the amount of calculation.

4 Slip compensation

Asynchronous motors must have a certain slip s to generate electromagnetic torque. When the motor speed is high, such as the rated speed, s is about 3%, and its influence can be ignored. When the frequency converter is running at variable frequency, in order to generate the same electromagnetic torque, s is inversely proportional to the synchronous frequency. As the synchronous frequency decreases, s becomes larger and larger. When the synchronous frequency is low to a certain extent, the motor may stop rotating, that is, the slip s seriously affects the accuracy of the motor speed regulation at low speed.

From the principle of motor science, it is known that when the electromagnetic torque (TL) of the asynchronous motor is less than the maximum torque (Tm), the mechanical characteristics of the asynchronous motor at different synchronous speeds are approximately a set of parallel lines, that is, to produce the same electromagnetic torque, the speed drop is basically the same at different synchronous frequencies, which is the starting point of slip compensation. When the synchronous frequency is f0, the speed drop is △f to output the torque of T0. In order to ensure the motor speed f0, the synchronous frequency must be increased to f0+△f.

The purpose of slip compensation is to improve the hardness of the motor's mechanical characteristics and accurately perform slip compensation, which requires knowing the functional relationship between slip and electromagnetic torque. Under the constant Er/f1 control mode, the motor's mechanical characteristics are straight lines, so the slip is linearly related to the electromagnetic torque, that is, under the condition of keeping the rotor flux amplitude unchanged, the electromagnetic torque is proportional to the slip frequency. Under constant U1/f1 control and constant Eg/f1 control, the electromagnetic torque is nonlinearly related to the slip frequency, but when the electromagnetic torque (TL) is much smaller than the maximum torque (Tm), the electromagnetic torque is approximately linearly related to the slip frequency, except that the approximate linear segment of the constant Eg/f1 control is longer, and because the maximum torque (Tm) of the constant Eg/f1 control at each synchronous frequency is unchanged, this approximate relationship will not change with the change of the synchronous frequency. Therefore, when compensating for the stator resistance drop (IR drop) and keeping Eg/f1 constant, the same linear compensation method as that under the constant Er/f1 control mode is usually used.

5 Current Limit Control

The purpose of current limiting control is to enable the motor to generate a certain maximum torque, and the inverter will not trip regardless of the load (even if it is locked), that is, to achieve the excavator characteristics. Since the medium-voltage inverter referred to here is 18 units connected in series, using carrier horizontal phase-shift PWM modulation, the output power of each unit of the 18-unit medium-voltage inverter is evenly distributed. Assuming the input power of each unit is Pm, the total power is:

Where Ed is the power unit inverter input voltage; Id is the power unit inverter input current; ωr is the motor rotor rotation angular frequency.

If Ed is constant under the action of the large capacitor on the DC side, the current Id will be proportional to Tf1. As the load increases, the torque increases, and Id can always be limited below a certain level by appropriately reducing f1. In order to ensure the realization of the current limiting function, the output voltage must be appropriately adjusted. For this purpose, a special PI regulator is designed to control the maximum current limit and realize the excavator characteristics.

Figure 3 is a partial block diagram of the maximum current limit regulation, the maximum current limit PI regulator. With the maximum allowable current (set by parameter P106) as the given value, the current effective value (per unit value) output by the "stator current 3/2 conversion and decomposition" module is used as feedback, and two identical adjustment quantities △f (given by variable KK0118, frequency adjustment quantity) and △u (given by variable KK0119, voltage adjustment quantity) are output. △u plays an auxiliary role and is adjusted by the proportional coefficient P121. It should be noted that although the current feedback value here is an effective value (per unit value), it is calculated in real time through the instantaneous values ​​of the A and C phase currents (current vector amplitude), so it can meet the dynamic requirements of current regulation; the PI regulator is aimed at limiting the maximum current, and the current can only be adjusted from large (exceeding the maximum allowable value) to small, but not from small to large. Therefore, the regulator output must be limited (less than zero). According to the needs of the inverter current limit, the designed PI regulator is different from the ordinary PI regulator in two aspects: output adjustment quantity and output limit.

For convenience, the designed digital PI regulator adopts an incremental algorithm. In order to verify the effect of the maximum current limiting regulator, a starting current limiting experiment is carried out. In this experiment, the proportional coefficient P121 of the output KK0119 of the voltage regulation quantity is 1.0. P100 is the integral gain of the PI regulator; P113 is the proportional gain of the PI regulator; P106 is the maximum current effective value allowed (per unit value); P107 is the connector for selecting the maximum current analog quantity; KK0102 is the actual current effective value of the feedback (per unit value, before digital filtering); KK0103 is the actual current effective value of the feedback (per unit value, after digital filtering); P101 is the actual current effective value filter time constant; KK0118 is the frequency regulation output.

Figure 4 shows the experimental waveforms of limiting the starting current, where the experimental parameters of Figures 4a and 4b are: P100 = 50 ms, P113 = 0.05, P106 = 13%, P101 = 1 000 ms, and the control feedback amount is KK0103. Figure 4a shows the waveforms of KK0108 (1 wave) and KK0103 (2 waves), and Figure 4b shows the waveforms of KK0118 (9 waves) and KK0102 (10 waves); the experimental parameters of Figure 4c are: P100 = 50 ms, P113 = 0.10, P106 = 13%. The control feedback amount is KK0102, and Figure 4c shows the waveforms of KK0118 (7 waves) and KK0102 (8 waves).

As can be seen from Figures 4a and 4b, the effective value of the filtered current is used as feedback. Due to the delay of the feedback signal, the control quantity output lags. Although the regulator controls the filtered current KK0103 (Figure 4a) well, the actual current value KK0102 is not well controlled at this time (Figure 4b). The current dynamic peak reaches 7 V, while the actual maximum allowable current value is 1.3 V, which cannot achieve "tripping". In order to limit the dynamic current, Figure 4c increases the proportional gain and uses KK0102 without delay as feedback. At this time, the maximum dynamic current is limited to about 2 V, which better achieves the purpose of current limiting. Therefore, more appropriate adjustment of parameters can improve the regulation effect of the regulator.

6 Conclusion

In summary, the parameters of the stator resistance (IR drop) compensation, slip compensator and maximum current limit controller are adjusted to achieve a better adjustment effect of the regulator; at the same time, since the given value P106 for current limit is set to limit overload, its value is smaller than the overcurrent value of "tripping". Therefore, the designed regulator can play a role in limiting the maximum current, thereby realizing the excavator characteristics.