CN108365772A - A kind of current transformer current inner loop optimum gain determines method - Google Patents

A kind of current transformer current inner loop optimum gain determines method Download PDF

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Publication number
CN108365772A
CN108365772A CN201810200441.4A CN201810200441A CN108365772A CN 108365772 A CN108365772 A CN 108365772A CN 201810200441 A CN201810200441 A CN 201810200441A CN 108365772 A CN108365772 A CN 108365772A
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loop
filter
gain
current
closed
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赵志宏
刘钊
季振东
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Nanjing University of Science and Technology
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Nanjing University of Science and Technology
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Priority to CN201810200441.4A priority Critical patent/CN108365772A/en
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/539Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency
    • H02M7/5395Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency by pulse-width modulation
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00Details of apparatus for conversion
    • H02M1/0003Details of control, feedback or regulation circuits
    • H02M1/0012Control circuits using digital or numerical techniques
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00Details of apparatus for conversion
    • H02M1/0003Details of control, feedback or regulation circuits
    • H02M1/0038Circuits or arrangements for suppressing, e.g. by masking incorrect turn-on or turn-off signals, e.g. due to current spikes in current mode control

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Electrical Variables (AREA)

Abstract

The invention discloses a kind of current transformer current inner loop optimum gains to determine method.This method is:The mathematical model of controlled device is established in discrete domain first;Then by defining closed-loop pole to unit circle apart from minimum value function, scanning monitor gain simultaneously solves the position that each controller gain corresponds to closed-loop pole, obtains closed-loop pole to the relation curve of unit circle minimum range;Finally under the premise of system is stablized, closed-loop pole is calculated to the maximum of unit circle minimum range relation curve, the controller gain corresponding to the maximum, as electric current loop optimum gain.The present invention provides a kind of easy intuitive determining method for the current inner loop gain of current transformer double-closed-loop control, under the premise of realizing optimum gain, optimize system damping, it is suppressed that the resonance of output LC filters provides theoretical foundation for the parameter designing of outer voltage.

Description

Method for determining optimal gain of current inner loop of converter
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a method for determining the optimal gain of a current inner loop of a converter.
Background
The converter can provide reliable power supply for local load, and typical application occasions include an energy storage converter (PCS), an Uninterruptible Power Supply (UPS), independent distributed power generation, a microgrid, a shore power supply and the like which operate independently. In these applications, the converter is required to be able to provide a high quality supply voltage with constant frequency and amplitude, balanced three phases, and low THD to the load, while the output current is determined by the load. Because of the variety of types of loads, such as three-phase balanced loads, three-phase unbalanced loads, and single-phase loads, as well as rectifying nonlinear loads, the waveforms and frequency spectrums of the currents are very complicated. The converter is required to be capable of providing three-phase stable voltage no matter what kind of load is connected, and the converter is equivalent to a controllable voltage source for the load, so that the quality of the output voltage of the converter is the key of the whole control system.
Because the output of the Voltage Source Converter (VSC) is a pulse width modulation Voltage, an LC filter is generally used for a high-power Converter to filter the high-frequency component of the output Voltage, and the design of the LC filter is to meet the requirements of cut-off frequency (filtering capability), no-load reactive current, and full-load Voltage drop (dc bus output capability). The LC filter has an inherent series resonance point, and the VSC outlet pulse width modulated voltage has very abundant frequency components, so voltage harmonic components easily cause the LC filter to resonate, resulting in the converter overcurrent and deteriorating the output voltage quality, and therefore this problem should be avoided when designing the control system.
The double closed loop control adopts a structure of a voltage outer loop and a current inner loop, virtual damping is provided for an output LC filter through proportional gain of the current inner loop, frequency response of the current inner loop is improved on the premise of not increasing loss, resonance is restrained, the problem of current impact in the moment of load input can be avoided due to introduction of the current loop, the risk of overcurrent of a converter is reduced, in addition, compensation of unbalanced and nonlinear loads can be realized by introducing negative sequence and adjustment of each subharmonic component into the voltage outer loop, and therefore high output voltage quality can be obtained. However, the current loop function is not clearly located, although the current loop has been widely recognized to improve the system damping function, there is still a wrong idea that the current loop bandwidth needs to be increased to achieve the adjustment without the static error, and the research on how to set the current loop gain to achieve the optimization of the system damping is not deep enough, which is also the key of the dual-loop control parameter design, and is directly related to the system stability and the parameter design of the external voltage loop.
So far, most of analysis and controller design related to the voltage source output converter is carried out in a continuous domain, one-beat time delay of digital control is not considered, or a first-order Pade function approximation is used, which is effective in the case of high switching frequency (such as 10kHz and above) because the analysis error is small. However, in order to meet the requirement of temperature rise and heat dissipation, the switching frequency of the high-power converter is generally limited to about 3kHz to 4kHz, so that the accuracy and reliability of analyzing and designing the voltage source output converter in a continuous domain cannot be met.
Disclosure of Invention
The invention aims to provide a method for determining the optimal damping gain of a current inner loop, which can enable double closed loop control to achieve the aim of optimizing system damping, inhibit the resonance of an output LC filter to the maximum extent and provide a basis for the parameter design of a voltage outer loop.
The technical solution for realizing the purpose of the invention is as follows: a method for determining the optimal gain of a current inner loop of a current transformer comprises the following steps:
step 1, establishing a discrete model of a controlled object;
step 2, defining a minimum function min | D of the distance from the closed loop pole to the unit circlez|;
Step 3, gain K of scanning controllerITo find the corresponding closed loop pole pclThe position of (a);
step 4, obtaining each pclCorresponding to min | DzTaking value of |, obtaining KIAnd min | Dz| relation curve fs
Step 5, on the premise of system stability, calculating a curve fsThe gain of the controller corresponding to the maximum value is obtained and used as the optimal gain of the current loop
Further, step (ii)Step 1, establishing a discrete model G of a controlled objectiL(z), specifically as follows:
wherein,for outputting the resonance angular frequency of the LC filter, L is the filter reactance value of the LC filter, C is the filter capacitance value of the LC filter, and TsIs the sampling period.
Further, the distance minimum function min | D described in step 2zThe method specifically comprises the following steps:
wherein theta is an included angle between a unit circle vector in a z plane and a real axis, and the value range is theta epsilon [0,2 pi ].
Further, the scan controller gain K in step 3ITo find the corresponding closed loop pole pclThe positions of (a) are as follows:
calculating the closed loop transfer function needed by the closed loop poleComprises the following steps:
in the formula,is a current loop open loop transfer function;
wherein,for outputting the resonance angular frequency of the LC filter, L is the filter reactance value of the LC filter, C is the filter capacitance value of the LC filter, and TsIs the sampling period.
Compared with the prior art, the invention has the following remarkable advantages: (1) the frequency characteristic of the controlled object is improved, and the resonance and the peak value of the output LC filter are restrained to the maximum extent; (2) the stability margin of the system is improved, the damping characteristic of the system is improved, and the determination of an external voltage ring is facilitated.
Drawings
Fig. 1 is a schematic flow chart of a method for determining an optimal gain of a current inner loop of a current transformer according to the present invention.
Fig. 2 is a schematic diagram of a main circuit topology of a voltage source output converter.
Fig. 3 is a block diagram of a dual closed-loop control.
Fig. 4 is a current inner loop control block diagram.
FIG. 5 shows the current inner loop closed loop pole with the controller gain KISchematic diagram of the variation trace of (2).
Fig. 6 is a schematic diagram of the local enlargement of the root track branch 1 and the definition of the distance function min | D (z) |.
FIG. 7 shows the controller gain KIGraph of the relationship with min | D (z) |.
Detailed Description
The invention will be further described with reference to the accompanying drawings.
The method for determining the optimal gain of the current inner loop of the converter directly considers and includes digital control one-beat time delay by taking digital control as a starting point, and establishes a mathematical model of a controlled object in a discrete domain.By defining a minimum function min D (z) of the distance from the closed loop pole to the unit circle, the gain K of the scan controllerIAnd solving the position of the closed loop pole corresponding to the gain of each controller to obtain a relation curve of the minimum distance from the closed loop pole to the unit circle. On the premise of system stability, the maximum value of the relation curve from the closed loop pole to the minimum distance of the unit circle is obtained, and the controller gain corresponding to the maximum value is the optimal gain of the current loop
With reference to fig. 1, the method for determining the optimal gain of the current inner loop of the current transformer of the present invention includes the following steps:
step 1, establishing a discrete model of a controlled object, specifically as follows:
(1.1) establishing a mathematical model of the controlled object in a discrete domain according to the characteristic that the modulation wave keeps unchanged in each switching period;
(1.2) starting from digital control, directly including digital control of one-beat delay, and proportional gain KISuppressing resonance of an output LC filter to obtain a current loop closed loop transfer function;
step 2, defining a minimum function min | D of the distance from the closed loop pole to the unit circlez|;
Step 3, gain K of scanning controllerITo find the corresponding closed loop pole pclThe position of (a);
step 4, obtaining each pclCorresponding to min | DzTaking value of |, obtaining KIAnd min | Dz| relation curve fs
Step 5, under the premise of system stability, curve fsObtaining the controller gain corresponding to the maximum value as the optimal gain of the current loop
Further, the discrete model G of the controlled objectiL(z) is:
wherein,for outputting the resonance angular frequency of the LC filter, L is the filter reactance value of the LC filter, C is the filter capacitance value of the LC filter, and TsIs the sampling period.
Further, the distance function min | Dz| is defined as:
wherein theta is an included angle between a unit circle vector in a z plane and a real axis, and the value range is theta epsilon [0,2 pi ].
Further, the closed loop transfer function required by calculating the closed loop pole is calculatedComprises the following steps:
whereinIs a current loop open loop transfer function.
The invention is described in further detail below with reference to the figures and the embodiments.
Example 1
The main circuit topology of the voltage source output converter is shown in fig. 2, wherein the direct current side is different according to different application occasions, for a two-stage PCS, the direct current side is provided by a DC/DC and an energy storage medium, for photovoltaic power generation, the direct current side is obtained by boosting through a front-stage Boost, and for a shore power supply, the direct current side is obtained by rectifying a grid diode or performing PWM rectification. For high-power converter outputs, LC filters are generally connected, where RLThe output end of the LC filter can be directly connected with a load for the loss equivalent resistance of the reactor and the power switch device, and for the occasions needing isolation (such as a shore power supply and the like), the output end of the LC filter needs to be connected with the load through an isolation transformer, and at the moment, the primary side of the transformer adopts a triangular connection method and the secondary side of the transformer adopts a star connection method, so that a neutral line is provided for the load.
As shown in fig. 2, the adjusting process of the double closed-loop control is as follows, firstly, the line voltage of the capacitor end is detected and compared with the reference voltage, the error signal is sent to the voltage controller, the output of the voltage controller is used as the reference signal of the current inner loop, wherein the current loop can adopt either the inductive current or the capacitive current for feedback, the current loop adopts the proportional controller, the output of the current controller is the modulation wave, and the SVPWM modulation is adopted to improve the utilization rate of the dc bus voltage.
The double closed-loop digital control flow of the voltage outer loop and the current inner loop of the converter is shown in FIG. 3, and each voltage/current component is given in the form of a vector of a static coordinate system, such asWherein C isV(z) is a voltage controller, CI(z) is a current controller. Neglecting the equivalent resistance of the filter reactor, i.e. R, during the analysisLThe output LC filter is in a zero damping state, i.e. worst case, 0. Considering the load current as a disturbance component, without considering the influence of the load impedance Z, it can be derived from fig. 3:
further obtaining:
whereinTo output the resonant angular frequency of the LC filter, since the inverter PWM output has a zero-order keeper characteristic, the transfer function in the discrete domain is further obtained as follows:
combining fig. 3 and equation (3), a current inner loop control block diagram can be obtained, as shown in fig. 4, where the current loop open loop transfer function is as follows:
as shown in fig. 5, with the controller gain KIIncreasing current loop root trajectory curve, visible with gain KIThe closed loop poles at branch 1 and branch 2 move first towards the unit circle, then gradually approach and finally lie outside the unit circle. Further, as can be seen from the enlarged view of the part of the track branch 1 in FIG. 6, whenThe minimum value min | D (z) | of the distance between the closed-loop pole and the unit circle is maximum, and the corresponding damping characteristic and the system stability are the best. When inThe pole of the time-closed loop is on the unit circle, and the system is in critical stabilitySteady state, therefore, in order to keep the system stable, it is required
The minimum closed loop pole to unit circle distance is defined as follows:
where theta is equal to 0,2 pi](5)
Set up KIGradually increasing from zero and finding out the corresponding closed-loop poleThen get the corresponding min | D (z) | value from equation (5), and finally get KIThe relationship with min | D (z) | is shown in FIG. 7. Depending on whether the current loop is stable, K is adjustedIIs divided into a stable region and an unstable region whenThe closed loop system is stable whenThe closed loop pole is outside the unit circle and the system is unstable. On the premise of system stability whenPole of time closed loopSince the minimum distance min | D (z) | from the unit circle has a maximum value of 0.1312, the minimum distance min | D (z) | corresponds to the maximum valueThe controller gain is optimized for the current loop.
In summary, the invention provides a simple and intuitive design method for the current inner loop gain of the converter double closed loop control, realizes the purpose of optimizing the system damping under the action of the optimal gain, can inhibit the resonance of the output LC filter to the maximum extent, and provides a basis for the parameter design of the voltage outer loop.

Claims (4)

1. A method for determining the optimal gain of a current inner loop of a current transformer is characterized by comprising the following steps:
step 1, establishing a discrete model of a controlled object;
step 2, defining a minimum function min | D of the distance from the closed loop pole to the unit circlez|;
Step 3, gain K of scanning controllerITo find the corresponding closed loop pole pclThe position of (a);
step 4, obtaining each pclCorresponding to min | DzTaking value of |, obtaining KIAnd min | Dz| relation curve fs
Step 5, on the premise of system stability, calculating a curve fsThe gain of the controller corresponding to the maximum value is obtained and used as the optimal gain of the current loop
2. The method for determining the optimal gain of the current transformer inner loop as claimed in claim 1, wherein the step 1 of establishing the discrete model G of the controlled objectiL(z), specifically as follows:
wherein,for outputting the resonance angular frequency of the LC filter, L is the filter reactance value of the LC filter, C is the filter capacitance value of the LC filter, and TsIs the sampling period.
3. The method for determining optimal gain of current transformer inner loop as claimed in claim 1, wherein the distance minimum function min | D in step 2zThe method specifically comprises the following steps:
wherein theta is an included angle between a unit circle vector in a z plane and a real axis, and the value range is theta epsilon [0,2 pi ].
4. The method of claim 1, wherein the scan controller gain K of step 3 is determined by determining the optimal gain of the current transformer inner loopITo find the corresponding closed loop pole pclThe positions of (a) are as follows:
calculating the closed loop transfer function needed by the closed loop poleComprises the following steps:
in the formula,is a current loop open loop transfer function;
wherein,for outputting the resonance angular frequency of the LC filter, L is the filter reactance value of the LC filter, C is the filter capacitance value of the LC filter, and TsIs the sampling period.
CN201810200441.4A 2018-03-12 2018-03-12 A kind of current transformer current inner loop optimum gain determines method Pending CN108365772A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110086179A (en) * 2019-04-19 2019-08-02 南京理工大学 A kind of LC type converter control method based on low resonant frequency

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