Research And Design Of Three-phase Unbalanced Compensation Technology For Distribution Transformer

In order to meet the requirement of power supply for large amount of single-phase loads,the three-phase four-wire system is widely adopted in the urban and rural distribution network,and the distribution transformers are mostly Y/yn0 wiring.Due to the unbalanced distribution of load in each phase,three-phase unbalanced operation inevitably occurs on the distribution transformer side.The distribution loss increasing and the efficiency decreasing in this operation mode and the neutral point voltage offset also probably occur,which reduces the power quality.The development and application of three-phase unbalance compensation technology of distribution transformers possess a great practical engineering significance.In this paper,based on the unbalanced outputting current caused by unbalanced three-phase load of distribution transformer,a three-phase unbalance compensator and control strategy for distribution transformer is researched and designed,which enable the distribution transformer to output three-phase current balancedly even under the unbalanced loads.Considering the need of the zero-sequence current compensating for distribution transformer technology in the three-phase four-wire power supply mode,the application performance and characteristics of four kinds of compensation circuits with three-phase four-wire converts had been compared and analyzed in this paper.And a three-phase unbalance compensator scheme take the split-capacitor three-phase four-wire convert as main circuit is determined,also the mathematical model of the compensation system had been established.In order to accurately separate the negative sequence and zero sequence current from the three-phase unbalanced load current and calculate the instruction compensation currents,a delay algorithm based on symmetric component method and two-phase stationary coordinate system is discussed in this paper.And since the characteristics of the symmetric component method,which large amount of delays and computation in achieving current separation,a 1/4-cycle delay algorithm is used to obtain the command compensation current.A unbalanced current compensation control strategy based on quasi-PR controller is researched and designed.The parameter configuration of output filter and the design process of quasi-PR controller are discussed detailly.And the simulation shows that the three-phase unbalance compensation circuit of the distribution transformer realizes the effective tracking of the instruction compensation current under the three-phase unbalanced load and the distribution can output the three-phase balanced current.The design and manufacture of a three-phase unbalance compensation system simulation experiment platform for distribution transformers has been completed in this paper.And the compensation system take the split-capacitor three-phase four-wire converters as the main circuit and DSP28335 as the control core.Compensation control experiments and analysis had been carried out under three kinds of load unbalance.The experimental results show that the three-phase unbalance compensation strategy designed in this paper has achieved the control objective which output current balancing of distribution transformer when the three-phase unbalance loads occuring.