Suppliers of energy feedback devices for frequency converters remind you that currently, simple energy consumption braking is widely used in AC frequency conversion speed control systems, which have disadvantages such as wasting electrical energy, severe resistance heating, and poor fast braking performance. When asynchronous motors frequently brake, using feedback braking is a very effective energy-saving method and avoids damage to the environment and equipment during braking. Satisfactory results have been achieved in industries such as electric locomotives and oil extraction. With the continuous emergence of new power electronic devices, increasing cost-effectiveness, and people's awareness of energy conservation and consumption reduction, there is a wide range of application prospects.
The energy feedback braking device is particularly suitable for situations where the motor power is large, such as greater than or equal to 100kw, the equipment's moment of inertia gd2 is large, and it belongs to the repeated short-term continuous working system. The deceleration reduction from high speed to low speed is large, the braking time is short, and strong braking is required. In order to improve the energy-saving effect and reduce the energy loss during the braking process, it is also necessary to recover the deceleration energy and feedback it to the power grid to achieve energy-saving effect.
Feedback braking principle
In the variable frequency speed regulation system, the deceleration and stopping of the motor are achieved by gradually reducing the frequency. At the moment when the frequency decreases, the synchronous speed of the motor decreases accordingly. However, due to mechanical inertia, the rotor speed of the motor remains unchanged, and its speed change has a certain time lag. At this time, the actual speed will be greater than the given speed, resulting in a situation where the back electromotive force e of the motor is higher than the DC terminal voltage u of the frequency converter, that is, e>u. At this point, the electric motor becomes a generator, which not only does not require power supply from the grid, but can also send electricity to the grid. This not only has a good braking effect, but also converts kinetic energy into electrical energy, which can be sent to the grid to recover energy, killing two birds with one stone. Of course, there must be an energy feedback device unit for automatic control in order to achieve it. In addition, the energy feedback circuit should also include AC and DC reactors, resistance capacitance absorbers, electronic switches, etc.
As is well known, the bridge rectifier circuit of general frequency converters is three-phase uncontrollable, so it is impossible to achieve bidirectional energy transfer between the DC circuit and the power supply. The effective solution to this problem is to use active inverter technology, and the rectifier part adopts reversible rectifier, also known as grid side converter. By controlling the grid side inverter, the regenerated electrical energy is inverted into AC power with the same frequency, phase, and frequency as the grid, and fed back to the grid to achieve braking. Previously, active inverter units mainly used thyristor circuits, which can only safely perform feedback operation under stable grid voltage that is not prone to faults (grid voltage fluctuations not exceeding 10%). This type of circuit can only safely perform feedback operation of the inverter under stable grid voltage that is not prone to faults (with grid voltage fluctuations not exceeding 10%). Because during power generation braking operation, if the grid voltage braking time is greater than 2ms, commutation failure may occur and components may be damaged. In addition, during deep control, this method has low power factor, high harmonic content, and overlapping commutation, which will cause distortion of the power grid voltage waveform. Simultaneously controlling complexity and high cost. With the practical application of fully controlled devices, people have developed chopper controlled reversible converters using PWM control. In this way, the structure of the grid side inverter is completely the same as that of the inverter, both using PWM control.
From the above analysis, it can be seen that to truly achieve energy feedback braking of the inverter, the key is to control the grid side inverter. The following text focuses on the control algorithm of the grid side inverter using fully controlled devices and PWM control method.
control algorithm
The control algorithm for grid side inverters usually adopts vector control algorithm, where vdc, v * dc, and △ vdc represent the measured value, given value, and control error of the DC bus voltage, respectively; id、i*d、 Δ id represents the measured value, given value, and control error of the d-axis of the grid side inverter; iq、i*q、 Δ iq represents the measured value, given value, and control error of the q-axis current of the grid side converter; Δ v * d, v * d, and v * q respectively represent the d-axis output voltage deviation setpoint, d-axis output voltage setpoint, and q-axis output voltage setpoint of the grid side inverter; EABC, V * ABC, and IABC respectively represent the instantaneous given values of grid potential, grid side converter output voltage, and three-phase instantaneous values of output current; e. φ represents the amplitude and phase of the grid potential, respectively.
The vector control algorithm calculates the difference between the measured DC bus voltage and the given value, and obtains the given value of the d-axis current through a PI regulator; Then, based on the measured phase of the grid voltage, the measured output current of the grid side inverter is synchronously coordinate transformed to obtain the measured values of d-axis current and q-axis current. After pi adjustment, the d-axis value is added to the amplitude of the grid voltage to obtain the given values of d-axis voltage and q-axis voltage. After synchronous coordinate inverse transformation, the output is obtained.
The advantage of this algorithm is high control accuracy and good dynamic response; The disadvantage is that there are many coordinate transformations in the control algorithm, and the algorithm is complex, requiring high computational power from the control processor.
It adopts a current tracking PWM rectifier composition. This simplified algorithm directly multiplies the d-axis current setpoint with the three-phase sine reference value obtained from the measured grid voltage phase lookup table to obtain the setpoint of the three-phase output current, and then performs simple pi adjustment to obtain the setpoint of the three-phase output voltage and output it. Due to the omission of coordinate transformation calculations in this algorithm, the computational power requirements for the control processor are relatively low. On the other hand, due to the characteristics of the PI regulator itself, there is a certain steady-state error in its control of AC flow, so the power factor of this algorithm is lower than that of the standard vector control algorithm. During dynamic processes, the fluctuation of DC bus voltage is relatively large, and the probability of DC bus voltage and other faults occurring during rapid dynamic processes is relatively high.
Feedback braking characteristics
Strictly speaking, the grid side inverter cannot be simply referred to as a "rectifier" because it can function as both a rectifier and an inverter. Due to the use of self turn off devices, the magnitude and phase of the AC current can be controlled through appropriate PWM mode, making the input current approach a sine wave and ensuring that the power factor of the system always approaches 1. When the regenerative power returned from the inverter by the motor deceleration braking increases the DC voltage, the phase of the AC input current can be reversed from the phase of the power supply voltage to achieve regenerative operation, and the regenerative power can be fed back to the AC power grid, while the system can still maintain the DC voltage at the given value. In this case, the grid side inverter operates in an active inverter state. This makes it easy to achieve bidirectional power flow and has a fast dynamic response speed. At the same time, this topology structure enables the system to fully control the exchange of reactive and active power between the AC and DC sides, with an efficiency of up to 97% and significant economic benefits. The heat loss is 1% of the energy consumption braking, and it does not pollute the power grid. The power factor is about 1, which is environmentally friendly. Therefore, feedback braking can be widely used for energy-saving operation in energy feedback braking scenarios of PWM AC transmission, especially in situations where frequent braking is required. The power of the electric motor is also high, and the energy-saving effect is significant. Depending on the operating conditions, the average energy-saving effect is about 20%. The only drawback of implementing feedback control is the complex structure of the control system.
In summary, it can be seen that the energy feedback system device has far superior advantages over energy consumption braking and DC braking. By using feedback braking to feed back regenerated electricity to the grid, it can achieve the effect of reducing energy consumption and saving electricity costs. Therefore, in the current situation of power shortages caused by rapid economic development in various parts of China, promoting and applying feedback brakes has important energy-saving significance.