Research On Characteristics And Design Of Three-phase Power Electronics Load

In recent years, with numbers of power supply equipment widely applying in various fields of industry application and people's living, the requirements on the performance of power supply test equipments are also increasing. Traditional test equipments are mainly built-up with passive components, such as resistance, inductance, capacitance, etc. There are many disadvantages exist in it, mainly is large energy consumption, bulky, inflexible on characteristics simulating. This paper presents a three-phase Power Electronics Load (PEL), which adopts back-to-back PWM converter as the main circle, and it could be applied in the three-phase power supply test experiment to replace the traditional loads. This PEL consists with Simulating Converter (SC) and Grid Connected Converter (GCC). SC must accurately and flexible simulate several of load characteristics, such as balance resistance, emotional resistance, capacitive resistance, unbalance, nonlinear, dynamic and so on. GCC must firstly quickly and efficiently feedback the energy absorbed by SC, meanwhile it would be much better if GCC could compensate the harmonics current caused by the Devices Be Tested (DBT).The basic mathematical model of three-phase voltage source rectifier has been present. Based on it, the principle and control method on static coordinate flame was analyzed in detail, especially of the harmonic characteristics of DC link voltage and its affect on the system.The accuracy and dynamic response of SC's current loop are the main characteristics of PEL. When the parameters of main circle have been designed, the choice and design of the current loop control method are particularly important. This paper compared all kinds of control method, including PID controller, hysteresis control, one cycle control, etc. Then the resonant controller was chosen as the basic controller method of SC's current loop, because it could achieve zero steady error when tracking the periodic sinusoid command, meanwhile it performs strong anti-interference on the periodic sinusoid disturbance. Using root locus to analyze the system, it is obviously unstable when traditional resonant controller was adopted. Then the pole-zero cancellation method and a lead compensator were applied to improve the traditional resonant, which lead to a good performance in continue domain. Implementing this improved resonant controller in the discrete system, it is also unstable when the one-step-delay caused by DSP system was taken into consideration. Therefore, a state observer was adopted to counteract the effect of one-step-delay control, and a repetitive compensator was used to reduce the periodic errors of this observer.In the actual experiment system, DBT produces abundant harmonics, and the harmonics enter into the grid directly if GCC doesn't compensate them. In order to develop a green PEL, this paper has researched the method to reduce the harmonics caused by DBT. Rapid and accurate detecting technique is the primary task of harmonic compensation. Therefore, a variable step-size adaptive algorithm was adopted to detect the harmonics of DBT input current. The traditional variable step-size adaptive algorithm only takes the amplitude of active and reactive current as its weight, which leads to a large proportion harmonics in the steady state error of the algorithm, and the detecting accuracy was seriously reduced. Adding each amplitudes of active and reactive harmonics current lower than 25th could solve this problem, and this method was implement by this paper, which guarantees the accuracy of the algorithm. After the harmonics of DBT input current had been detected, the reference of GCC's current loop became non-linear current, and it was difficult to tracking this non-linear reference current. Based onαβcoordinate system, a current control strategy with repetitive controller and PI controller in series connection was proposed PI controller was designed follow the traditional way to enhance the system's bandwidth, and the repetitive controller was designed to reduce the periodic errors and anti-interference the disturbance.All the theoretical analysis and the effect of control methods were verified by simulation based on MATLAB A 10kVA prototype of three-phase PEL and an inverter as DBT were successfully developed, the results of tests and experiments testify the effective and performance of this prototype.