CN111371368B - Method for preventing power supply voltage drop of cascade four-quadrant frequency converter - Google Patents
Method for preventing power supply voltage drop of cascade four-quadrant frequency converter Download PDFInfo
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- CN111371368B CN111371368B CN202010196540.7A CN202010196540A CN111371368B CN 111371368 B CN111371368 B CN 111371368B CN 202010196540 A CN202010196540 A CN 202010196540A CN 111371368 B CN111371368 B CN 111371368B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P23/00—Arrangements or methods for the control of AC motors characterised by a control method other than vector control
- H02P23/06—Controlling the motor in four quadrants
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P27/00—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage
- H02P27/04—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage
- H02P27/06—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage using dc to ac converters or inverters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P2201/00—Indexing scheme relating to controlling arrangements characterised by the converter used
- H02P2201/01—AC-AC converter stage controlled to provide a defined AC voltage
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Abstract
The invention discloses a method for preventing a cascade four-quadrant frequency converter from power supply voltage drop, which comprises the following specific steps of: monitoring the power supply voltage in real time, judging the power supply voltage drop, and calculating the power supply voltage drop depth; distinguishing a system power generation running state and an electric running state; when the power supply voltage drops, correspondingly executing power supply voltage drop control under the power generation working condition or power supply voltage drop control under the electric working condition according to whether the system operates in the power generation state or the electric state, and adopting a corresponding control method according to severe drop and non-severe drop of the power supply voltage; when the power supply voltage is normal, normal operation control is executed. The cascade frequency converter power supply voltage drop resistant device is suitable for the cascade four-quadrant frequency converter power supply voltage drop resistance, is compatible with the cascade frequency converter power supply voltage drop resistance only operating under the power generation working condition or only operating under the electric working condition, provides reference and reference significance for the power supply voltage drop resistant function of the common frequency converter, and is strong in universality.
Description
Technical Field
The invention relates to the technical field of power electronics and frequency converter control, in particular to a method for preventing power supply voltage drop of a cascade four-quadrant frequency converter.
Background
The main current technology of the cascade type frequency converter adopts a multi-level scheme of cascading a plurality of power conversion units, has the technical characteristics of high voltage and large capacity, is important equipment for controlling, regulating and soft starting a high-power motor in industry or military, and can drive the motor to operate in an electric or power generation state, wherein the cascade type four-quadrant frequency converter can realize bidirectional power flow and can simultaneously meet the driving requirements of electric and power generation operation of the motor. With the increase of the input force of the country in the aspects of energy conservation and emission reduction, the cascade frequency converter is more and more widely applied to high-energy-consumption industries such as electric power, metallurgy, petrochemical industry, water supply, cement, mining and the like. Typical applications in China today include: military tests (wind tunnel test beds), gas pressure transmission stations (such as western gas and east gas transmission), chemical industry (such as LNG and light hydrocarbon recovery) and large thermal power plants (boiler water supply systems) are increasingly critical in application occasions, and are particularly applied to power equipment of national major projects, and once the equipment is abnormally shut down, huge loss and great influence can be caused.
The power supply of the cascade frequency converter is generally a public power grid or a bus in a factory, and the abnormal drop of the power supply voltage of the cascade frequency converter can be caused by factors such as power grid faults, bus switching, starting of other high-power equipment on the same power grid or bus, and the abnormal shutdown of a unit can be usually caused if the cascade frequency converter does not have the function of resisting the power supply voltage drop. Meanwhile, the high-performance power supply voltage drop resisting function of the connected frequency converter is developed by considering different depths and durations of power supply voltage drops, reducing factors such as disturbance impact on loads and the like. The method can be used for reducing the huge loss and the great influence on load disturbance impact or abnormal shutdown caused by the drop of the power supply voltage of a unit system particularly applied to national important projects.
Aiming at the problems, a method for preventing the power supply voltage drop of the cascade four-quadrant frequency converter is provided according to the characteristics of the cascade four-quadrant frequency converter and by combining the engineering technical experience of preventing the power supply voltage drop of the current universal frequency converter.
Disclosure of Invention
In order to solve the technical problems in the background art, the invention provides a method for preventing power supply voltage drop of a cascade four-quadrant frequency converter, which can meet the requirements of power supply voltage drop control under the power generation working condition and the electric working condition, is continuous and not out of control in the control process, reduces the impact and disturbance in the control switching process and the recovery process, and improves the success rate and the reliability of resisting the power supply voltage drop.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for preventing power supply voltage drop of a cascade four-quadrant frequency converter comprises the following steps:
monitoring power supply voltage in real time, judging power supply voltage drop, and calculating power supply voltage drop depth;
distinguishing a power generation running state and an electric running state of the system;
step three, when the power supply voltage drops, if the system runs in a power generation state, executing power supply voltage drop control under the power generation working condition;
1) and corresponding to the non-severe drop of the power supply voltage, executing the non-severe drop control of the power supply voltage under the power generation working condition:
the motor side frequency converter executes normal power generation control, maintains the current motor rotating speed and load power, and keeps the rotating speed and the power stable;
the power supply side frequency converter executes derating operation according to the power supply voltage dropping depth, and if the reactive compensation requirement exists, the power supply side frequency converter provides capacitive reactive power according to the voltage dropping depth to support power supply voltage recovery, each VLU on the direct current side of the cascade type four-quadrant frequency converter automatically puts in when the direct current bus voltage of each VLU exceeds the limit value, redundant power on the direct current side of the cascade type four-quadrant frequency converter is absorbed, and the safety of the cascade type four-quadrant frequency converter is protected;
2) and (3) executing power supply voltage severe drop control under the power generation working condition corresponding to the power supply voltage severe drop:
the motor side frequency converter executes normal power generation control, maintains the current motor rotating speed and load power, and keeps the rotating speed and the power stable;
the power supply side frequency converter executes zero power output, each VLU on the direct current side of the cascade four-quadrant frequency converter is automatically put in when the direct current bus voltage of each VLU exceeds a limit value, the power fed back to the direct current side of the cascade four-quadrant frequency converter by the motor side frequency converter is absorbed, and the safety of the cascade four-quadrant frequency converter equipment is protected;
step four, when the power supply voltage drops, if the system runs in an electric state, executing power supply voltage drop control under an electric working condition;
1) and (3) corresponding to the non-severe drop of the power supply voltage, executing the non-severe drop control of the power supply voltage under the electric working condition:
the motor side frequency converter executes derating operation, and the executing derating degree of the motor side frequency converter depends on the power control capability of the power supply side frequency converter under the current power supply voltage drop depth;
the power supply side frequency converter executes derating operation according to the power supply voltage drop depth, provides power required by the motor side frequency converter, stabilizes each direct current bus voltage, and provides capacitive reactive power according to the voltage drop depth if reactive power compensation is required, so as to support power supply voltage recovery;
2) corresponding to the serious drop of the power supply voltage, executing the serious drop control of the power supply voltage under an electric working condition, executing zero-power input by a power supply side frequency converter, executing zero active output by a motor side frequency converter, not blocking a driving pulse, namely not out of control and not stopping, simultaneously keeping the excitation operation of a motor, normally calculating the flux linkage and the rotating speed of the motor, and executing a kinetic energy feedback function by the motor side frequency converter when the serious drop time of the power supply voltage lasts longer, maintaining the voltage of each direct current bus and realizing the capability of resisting the serious drop of the power supply voltage for a longer time;
and step five, executing normal operation control when the power supply voltage is normal.
Further, the first step specifically includes:
the method comprises the steps of collecting and monitoring power supply voltage in real time, judging that voltage power supply drop occurs if the actual power supply voltage is lower than a set power supply voltage limit value, otherwise judging that the power supply voltage is normal, calculating the voltage drop depth according to the current power supply voltage and rated power supply voltage when the power supply voltage drops, and dividing the power supply voltage drop into power supply voltage severe drop and power supply voltage non-severe drop according to the power supply voltage drop depth;
when the power supply voltage drops, the voltage drop depth is calculated according to the current power supply voltage and the rated power supply voltage, and the following results can be obtained:
D≥DLim (2)
wherein, UNFor mains voltage rating, UACurrent value of the supply voltage, D is the supply voltage drop depth, DLimThe method is a power supply voltage falling depth judgment limit value, D is more than or equal to 0 and less than or equal to 1, and D is 1 and represents the current power supply voltage UAFalling to 0, when D is more than or equal to DLimAnd judging that the power supply voltage drops seriously.
Further, the second step is specifically as follows: according to the current operation mode of the system, the system is distinguished to operate in a power generation state or an electric state, wherein the power generation state refers to the operation state that power flows from the motor to the power supply through the cascade type four-quadrant frequency converter, and the electric state refers to the operation state that power flows from the power supply to the motor through the cascade type four-quadrant frequency converter.
Further, each VLU on the dc side of the cascaded four-quadrant frequency converter described in the third step specifically includes:
the cascade four-quadrant frequency converter is composed of a plurality of power conversion units, each power conversion unit is provided with a VLU, the VLU is automatically activated when the voltage of a corresponding direct current bus exceeds a limit value and absorbs redundant energy at a direct current side, and the activated VLU is automatically locked when the voltage of the corresponding direct current bus is lower than a set limit value.
Further, the executing the kinetic energy feedback function in the fourth step specifically includes:
converting the rotating mechanical kinetic energy of the motor and the load thereof into electric energy and feeding the electric energy back to the direct current bus side of each power conversion unit to maintain the voltage of each direct current bus above a set limit value;
the method comprises the steps of calculating the minimum value of all direct-current bus voltages of the cascade four-quadrant frequency converter in real time, carrying out closed-loop control on the minimum value and the direct-current bus voltage maintaining target value, and using the output value of the closed-loop control as the fine adjustment quantity of the motor torque or the rotating speed to realize the conversion of controllable rotary mechanical kinetic energy to electric energy.
Further, the normal operation control is executed in step five, when the power supply voltage is normal, that is, when the power supply voltage is not in the period of falling of the power supply voltage, the normal power generation control is executed in the power generation mode, and the normal motoring control is executed in the motoring mode.
Compared with the prior art, the invention has the beneficial effects that:
1. the invention provides a method for preventing power supply voltage drop of a cascade four-quadrant frequency converter, which can meet different power supply voltage drop depths, including complete drop of zero power supply voltage and power supply voltage drop control under a power generation working condition, can maintain the current motor rotating speed and load power, keep the rotating speed and power stable, cannot generate disturbance on process operation, can control power supply voltage drop under an electric working condition, and can resist power supply voltage drop for a longer time by utilizing a kinetic energy feedback function.
2. No matter in a power generation working condition or an electric working condition, in the process of controlling the power supply voltage drop, the driving pulse is not blocked, the excitation of the motor is kept to continuously operate, the rotating speed and the flux linkage of the motor are normally calculated, the non-stop and non-runaway are realized, the impact and disturbance of the control switching process and the recovery process are reduced, and the success rate and the reliability of resisting the power supply voltage drop are greatly improved.
3. The cascade frequency converter power supply voltage drop resistant device is suitable for the cascade four-quadrant frequency converter power supply voltage drop resistance, is compatible with the cascade frequency converter power supply voltage drop resistance only operating under the power generation working condition or only operating under the electric working condition, provides reference and reference significance for the power supply voltage drop resistant function of the common frequency converter, and is strong in universality.
Drawings
FIG. 1 is a schematic diagram of the method steps of the invention for preventing power supply voltage drop of a cascade four-quadrant frequency converter;
FIG. 2 is a schematic diagram of the power conversion unit configuration VLU of the present invention;
fig. 3 is a block diagram of the kinetic energy feedback control applicable to the cascade type frequency converter according to the present invention.
Detailed Description
The following detailed description of the present invention will be made with reference to the accompanying drawings.
As shown in fig. 1, a method for preventing a power supply voltage drop of a cascaded four-quadrant converter includes the following steps:
monitoring power supply voltage in real time, judging power supply voltage drop, and calculating power supply voltage drop depth;
distinguishing a power generation running state and an electric running state of the system;
step three, when the power supply voltage drops, if the system runs in a power generation state, executing power supply voltage drop control under the power generation working condition;
step four, when the power supply voltage drops, if the system runs in an electric state, executing power supply voltage drop control under an electric working condition;
and step five, executing normal operation control when the power supply voltage is normal.
The processing related to the power supply voltage in the first step specifically includes:
the method comprises the steps of collecting and monitoring power supply voltage in real time, judging whether the actual power supply voltage is lower than a set power supply voltage limit value or not, judging whether the power supply voltage is normal or not, calculating the voltage drop depth according to the current power supply voltage and rated power supply voltage when the power supply voltage drops, and dividing the power supply voltage drop into power supply voltage severe drop and power supply voltage non-severe drop according to the power supply voltage drop depth.
For example, the power supply may be a public power grid or a bus in a plant, and when a power supply voltage drop occurs, the voltage drop depth is calculated according to the current power supply voltage and the rated power supply voltage, so that:
D≥DLim (2)
wherein, UNFor mains voltage rating, UAThe current value of the supply voltage, D is the supply voltage drop depth,DLimthe method is a power supply voltage falling depth judgment limit value, D is more than or equal to 0 and less than or equal to 1, and D is 1 and represents the current power supply voltage UAFalling to 0, when D is more than or equal to DLimAnd judging that the power supply voltage drops seriously.
And step two, distinguishing the system running state, namely distinguishing whether the system runs in a power generation state or an electric state according to the current running mode of the system, wherein the power generation state refers to the running state that power flows from the motor to the power supply through the cascade type four-quadrant frequency converter, and the electric state refers to the running state that power flows from the power supply to the motor through the cascade type four-quadrant frequency converter.
Step three, executing power supply voltage drop control under the power generation working condition, specifically comprising:
corresponding to the non-severe drop of the power supply Voltage, performing the non-severe drop control of the power supply Voltage under the power generation working condition, performing normal power generation control by the motor side frequency converter, maintaining the current motor rotating speed and load power, keeping the rotating speed and power stable, performing derating operation by the power supply side frequency converter according to the power supply Voltage drop depth, providing capacitive reactive power by the power supply side frequency converter according to the Voltage drop depth if a reactive power compensation requirement exists, supporting the power supply Voltage recovery, and automatically putting each VLU (Voltage Limiting Unit) at the direct current side of the cascade four-quadrant frequency converter into use when the direct current bus Voltage of each VLU exceeds the quadrant limit value, absorbing the redundant power at the direct current side of the cascade four-quadrant frequency converter and protecting the equipment of the cascade four-quadrant frequency converter from self safety;
the method comprises the steps that corresponding to the serious drop of power supply Voltage, the serious drop control of the power supply Voltage under the power generation working condition is executed, a motor side frequency converter executes normal power generation control, the current motor rotating speed and load power are maintained, the rotating speed and power are kept stable, the power supply side frequency converter executes zero-power output, each VLU (Voltage Limiting Unit) on the direct current side of the cascade type four-quadrant frequency converter is automatically put in when the direct current bus Voltage of each VLU exceeds a limit value, the power fed back to the direct current side of the cascade type four-quadrant frequency converter by the motor side frequency converter is absorbed, and the safety of the cascade type four-quadrant frequency converter equipment is protected.
Illustratively, when the non-severe drop of the power supply voltage is corresponded, the power supply side frequency converter executes derating operation according to the drop depth of the power supply voltage, and calculates derating active current according to the drop depth of the power supply voltage and the operation power of the motor side frequency converter, so as to obtain:
IP_g≤IP_g_Lim (4)
wherein, PmThe power of a motor side frequency converter when the power supply voltage drops, k is a power conversion coefficient, IP_gFor the target value of active current during derating operation of the frequency converter on the power supply side, IP_g_LimThe upper limit value of the active current of the power supply side frequency converter.
If reactive compensation is needed, the active current and the reactive current of the power supply side frequency converter meet the following limiting conditions:
wherein, IQ_gIs a reactive current target value of the power supply side frequency converter, Ig_LimThe upper limit value of the current carrying capacity of the power supply side frequency converter.
Step four, executing power supply voltage drop control under the electric working condition, specifically comprising:
corresponding to the non-severe drop of the power supply voltage, performing the non-severe drop control of the power supply voltage under the electric working condition, performing derating operation on a motor side frequency converter, wherein the derating degree of the motor side frequency converter depends on the power control capability of the power supply side frequency converter under the current power supply voltage drop depth, performing derating operation on the power supply side frequency converter according to the power supply voltage drop depth, providing power required by the motor side frequency converter, stabilizing each direct-current bus voltage, and providing capacitive reactive power according to the voltage drop depth if a reactive power compensation requirement exists, and supporting the recovery of the power supply voltage;
corresponding to the serious drop of the power supply voltage, executing the serious drop control of the power supply voltage under the electric working condition, executing the zero-power input by the power supply side frequency converter, executing the zero-power output by the motor side frequency converter, not blocking the driving pulse, namely, not out of control and not stopping, simultaneously keeping the excitation operation of the motor, normally calculating the flux linkage and the rotating speed of the motor, and executing the kinetic energy feedback function by the motor side frequency converter when the serious drop time of the power supply voltage lasts for a long time to maintain the voltage of each direct current bus, thereby realizing the capability of resisting the serious drop of the power supply voltage for a longer time.
And step five, executing normal operation control, wherein when the power supply voltage is normal, namely when the power supply voltage is not in a power supply voltage drop period, executing normal power generation control in a power generation mode, and executing normal electric control in an electric mode.
In an embodiment of the present invention, each VLU (Voltage Limiting Unit) on the dc side of the cascaded four-quadrant converter specifically includes:
the cascade four-quadrant frequency converter is composed of a plurality of power conversion units, each power conversion unit is provided with a VLU, the VLU is automatically activated when the voltage of a corresponding direct current bus exceeds a limit value and absorbs redundant energy at a direct current side, and the activated VLU is automatically locked when the voltage of the corresponding direct current bus is lower than a set limit value.
VLUs are configured corresponding to each of the power conversion units, and fig. 2 shows the power conversion units and the VLUs, which includes a power source side power conversion unit 201, a VLU202, a motor side power conversion unit 203, a dc support capacitor 204, all the power source side power conversion units 201 forming a power source side frequency converter, all the motor side power conversion units 203 forming a motor side frequency converter, in this example, the VLU202 is formed by a fully controlled power device and a brake resistor, the fully controlled power device is turned on when the VLU202 is activated, the dc side is connected to the brake resistor, the fully controlled power device is turned off when the VLU202 is locked, and the dc side is disconnected from the brake resistor.
In one embodiment of the present invention, the performing the kinetic energy feedback function specifically includes:
converting the rotating mechanical kinetic energy of the motor and the load thereof into electric energy and feeding the electric energy back to the direct current bus side of each power conversion unit to maintain the voltage of each direct current bus above a set limit value;
FIG. 3 shows a kinetic energy feedback control block diagram suitable for a cascade type frequency converter, which calculates each DC bus voltage (U) of the cascade type four-quadrant frequency converter in real time through an operation selectordc_1，Udc_2，…Udc_n) Minimum value U ofdc_minAnd maintains the target value U with the DC bus voltagedc_refAnd performing closed-loop control, wherein the output value of the closed-loop control is used as the fine adjustment quantity of the motor torque delta T or the rotating speed delta v, so that the conversion of controllable rotary mechanical kinetic energy to electric energy is realized.
The above embodiments are implemented on the premise of the technical solution of the present invention, and detailed embodiments and specific operation procedures are given, but the scope of the present invention is not limited to the above embodiments. The methods used in the above examples are conventional methods unless otherwise specified.
Claims (5)
1. A method for preventing power supply voltage drop of a cascade four-quadrant frequency converter is characterized by comprising the following steps:
monitoring power supply voltage in real time, judging power supply voltage drop, and calculating power supply voltage drop depth;
distinguishing a power generation running state and an electric running state of the system;
step three, when the power supply voltage drops, if the system runs in a power generation state, executing power supply voltage drop control under the power generation working condition;
1) and corresponding to the non-severe drop of the power supply voltage, executing the non-severe drop control of the power supply voltage under the power generation working condition:
the motor side frequency converter executes normal power generation control, maintains the current motor rotating speed and load power, and keeps the rotating speed and the power stable;
the power supply side frequency converter executes derating operation according to the power supply voltage dropping depth, and if the reactive compensation requirement exists, the power supply side frequency converter provides capacitive reactive power according to the voltage dropping depth to support power supply voltage recovery, each VLU on the direct current side of the cascade type four-quadrant frequency converter automatically puts in when the direct current bus voltage of each VLU exceeds the limit value, redundant power on the direct current side of the cascade type four-quadrant frequency converter is absorbed, and the safety of the cascade type four-quadrant frequency converter is protected;
2) and (3) executing power supply voltage severe drop control under the power generation working condition corresponding to the power supply voltage severe drop:
the motor side frequency converter executes normal power generation control, maintains the current motor rotating speed and load power, and keeps the rotating speed and the power stable;
the power supply side frequency converter executes zero power output, each VLU on the direct current side of the cascade four-quadrant frequency converter is automatically put in when the direct current bus voltage of each VLU exceeds a limit value, the power fed back to the direct current side of the cascade four-quadrant frequency converter by the motor side frequency converter is absorbed, and the safety of the cascade four-quadrant frequency converter equipment is protected;
step four, when the power supply voltage drops, if the system runs in an electric state, executing power supply voltage drop control under an electric working condition;
1) and (3) corresponding to the non-severe drop of the power supply voltage, executing the non-severe drop control of the power supply voltage under the electric working condition:
the motor side frequency converter executes derating operation, and the executing derating degree of the motor side frequency converter depends on the power control capability of the power supply side frequency converter under the current power supply voltage drop depth;
the power supply side frequency converter executes derating operation according to the power supply voltage drop depth, provides power required by the motor side frequency converter, stabilizes each direct current bus voltage, and provides capacitive reactive power according to the voltage drop depth if reactive power compensation is required, so as to support power supply voltage recovery;
2) and (3) executing power supply voltage severe drop control under an electric working condition corresponding to the power supply voltage severe drop:
the power supply side frequency converter performs zero-power input, the motor side frequency converter performs zero active output, but does not block driving pulses, namely, the motor is out of control and does not stop, the motor is kept in excitation operation, the flux linkage and the rotating speed of the motor are normally calculated, and when the serious drop time of the power supply voltage lasts for a long time, the motor side frequency converter performs a kinetic energy feedback function to maintain the voltage of each direct-current bus and realize the longer-time capability of resisting the serious drop of the power supply voltage;
step five, executing normal operation control when the power supply voltage is normal;
the first step specifically comprises:
the method comprises the steps of collecting and monitoring power supply voltage in real time, judging that voltage power supply drop occurs if the actual power supply voltage is lower than a set power supply voltage limit value, otherwise judging that the power supply voltage is normal, calculating the voltage drop depth according to the current power supply voltage and rated power supply voltage when the power supply voltage drops, and dividing the power supply voltage drop into power supply voltage severe drop and power supply voltage non-severe drop according to the power supply voltage drop depth;
when the power supply voltage drops, the voltage drop depth is calculated according to the current power supply voltage and the rated power supply voltage, and the following results can be obtained:
wherein, UNFor mains voltage rating, UACurrent value of the supply voltage, D is the supply voltage drop depth, DLimThe method is a power supply voltage falling depth judgment limit value, D is more than or equal to 0 and less than or equal to 1, and D is 1 and represents the current power supply voltage UAFalling to 0, when D is more than or equal to DLimAnd judging that the power supply voltage drops seriously.
2. The method for preventing the power supply voltage drop of the cascade four-quadrant frequency converter according to claim 1, wherein the second step specifically comprises: according to the current operation mode of the system, the system is distinguished to operate in a power generation state or an electric state, wherein the power generation state refers to the operation state that power flows from the motor to the power supply through the cascade type four-quadrant frequency converter, and the electric state refers to the operation state that power flows from the power supply to the motor through the cascade type four-quadrant frequency converter.
3. The method according to claim 1, wherein the VLUs on the dc side of the cascaded four-quadrant frequency converter in the third step specifically include:
the cascade four-quadrant frequency converter is composed of a plurality of power conversion units, each power conversion unit is provided with a VLU, the VLU is automatically activated when the voltage of a corresponding direct current bus exceeds a limit value and absorbs redundant energy at a direct current side, and the activated VLU is automatically locked when the voltage of the corresponding direct current bus is lower than a set limit value.
4. The method according to claim 1, wherein the performing the kinetic energy feedback function in step four includes:
converting the rotating mechanical kinetic energy of the motor and the load thereof into electric energy and feeding the electric energy back to the direct current bus side of each power conversion unit to maintain the voltage of each direct current bus above a set limit value;
the method comprises the steps of calculating the minimum value of all direct-current bus voltages of the cascade four-quadrant frequency converter in real time, carrying out closed-loop control on the minimum value and the direct-current bus voltage maintaining target value, and using the output value of the closed-loop control as the fine adjustment quantity of the motor torque or the rotating speed to realize the conversion of controllable rotary mechanical kinetic energy to electric energy.
5. The method according to claim 1, wherein the step five performs normal operation control, wherein when the power supply voltage is normal, i.e. not in the period of power supply voltage drop, the normal power generation control is performed in the power generation mode, and the normal motoring control is performed in the motoring mode.
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