| As one of the harmonic suppression devices often used in power systems,active power fiLters have the characteristics of fast response speed,reliable stabi Lity,and excellent harmonic suppression effect.Previous APF systems wanted to achieve the purpose of harmonic suppression by correcting all the harmonics of the distortion current,but because the harmonic current in the power system is mainly a form of specific sub-harmonics,such as: 5,7,11,13,17,19,etc.Active power fi Lters can effectively suppress or even eliminate high-order harmonics in power systems.However,in actual power systems,higher-order harmonics account for less of a certain component,and specific harmonics in power systems are mainly based on Low-order harmonics are dominant.Therefore,what has the most research value in the power system is how to suppress the low-order harmonics in the power system.The main research direction of this paper is how the APF control system based on the diode-clamped three-level circuit structure compensates for the specific harmonics,so as to achieve the purpose of harmonic suppression and reduce the power system losses.The first step: first discuss the operation and control principle of the parallel APF control system,and establish a mathematical model of the parallel APF system in the xyz and dq coordinate systems according to the working principle of the parallel APF.Then realize the voltage regulation control of the dc side voltage through the active and reactive decoupling algorithm.At the same time,this time it also researches how to solve the problem of the unbalanced capacitor voltage on the dc side of the three-level diode clamp converter.It mainly focuses on the research of the inductive auxi Liary voltage balancing circuit.The circuit solves the problem of unbalanced midpoint capacitor voltage on the dc side of the three-level diode clamp converter.The second step: According to the dq0 full harmonic detection method,an improved detection method for a specific harmonic is obtained.This detection method extracts n-th harmonic from the load-side distortion current as the reference value of the harmonic current for harmonic compensation control.Make a difference with the n-th harmonic compensation current to obtain the difference between the two currents.The PI controller then adjusts the PI current to obtain the command current of the nth harmonic compensation control.Then,the command current is converted into the command voltage of the modulation signal by calculating the relationship between the voltage and current vectors in the dq coordinate system.The PWM output of the harmonic compensation control is obtained from the n-th SPWM modulation process.Simi Larly,if other specific harmonic compensation modules are processed in parallel,APF control can be used to correct multiple specific harmonics.Step 3: construct an APF low-power verification system based on a three-level diode clamp circuit structure,and design a set of synchronous coordinate transformation,harmonic detection and compensation control,phase lock loop and Source and reactive power coupling control and other related modules in one fully digital control system.The APF low-power verification system has been established to verify whether the suppression of specific harmonics can be achieved in practice.Finally,through experiments,the results confirmed that the APF control algorithm based on the three-level diode clamping circuit structure can indeed be used.compensates for specific harmonics.By compensating the 5th,7th,11th,13th,17th,19th,23rd,25th,30th,and 31st order harmonics in the grid-side current,it is verified that the THD value of the grid-side current can be reduced from 20.3% before compensation After it is reduced to 0.9% after compensation,the THD value of the grid-side current is reduced by nearly 20%,which effectively suppresses harmonics,reduces the loss of the power system,and improves the efficiency of the power system. |
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