Research On The Key Techniques Of Operating Intelligent Substation For Electronic Transformers

The construction of intelligent substation under the "strong smart grid" of our country require higher intelligent and reliable stability of electric power equipment.Electronic transformer(ET)is of the intelligent substation process an important layer transmission equipment.Its digital output and network wiring make the power system to run safer,more environmentally and friendly,more conducive to primary electrical appliances and entire transmission and distribution system of intelligent.As the continuous development of science and technology and electric power technology workers to make unremitting efforts,active electronic transformer has made a practical progress.Tracking through the performance testing program of electronic transformers in the intelligent substation operation process still exist low sensor accuracy and anti-interference ability and so on.The main reason is that the performance test of ET with off-line calibration,which can not effectively simulate the electronic transformer and network operating conditions.Therefore,the electronic transformer long-term performance testing program was launched in 2015,and the test object is 220kV active electronic transformer.According to the test results,there exists some issues in the long-term stable operation and measurement information accuracy(sensing characteristics).It mainly includes digital synchronous issue of electronic transformer output,high accuracy on-line inspection issue and the stability of power systems when the ET accessed to the grid.In this paper,we analyzed above based on the experiments.Theoretically,the influence of digital integration,signal transmission delay compensation and clock compensation mechanism on digital synchronization was studied systematically.Then we analyzed the influence of frequency fluctuation,harmonics and interharmonicson for online calibration of electronic transformer and discussed influences of data pack loss rate of power transformer on operation of system stability after connection of electronic transformer.Firstly,the server model of merge unit is established on the operation principle of distributed sampling value control block,which solved the problem of data communication flexibility in the transmission of sampled value message of electronic transformer in intelligent substation.The design idea of general purpose SMV MMS server is proposed.The communication protocol between distributed systems is mapped to Ethernet link layer and IED object model of generic SMV MMS server is constructed,which carried out the real-time communication for electronic transformer in intelligent substation.The intelligent substation process layer equipment with data communication is achieved based on the IEC61850 communication protocol process bus.Secondly,the time synchronization system of common station was deployed on the whole intelligent substation with dual configuration for the main clock.Beidou second generation system was used for intelligent substation,while access to IEEE1588 network clock source.In data transmission of intelligent substation,the forward and reverse link delay the data queue delay of communication channel due to incomplete symmetry of communication path are different,and the QIA compensation mechanism was utilized.We used phase compensation and resampling decimation filter to calculate the output synchronous sampling pulse based on the second Lagrangian interpolation due to data transmission delay for the sampled value data generated by acquisition unit.Thirdly,the FPGA chip is used to design the merge unit,and we used the smpled measurement values(SMV)chip to realize the interface communication in practical research of high voltage electronic transformers.In order to solve large transient error between measured current and ECT output signal under long running time of acquisition system,we proposed an analog-to-digital conversion method combining VFC and pulse counter.To solve the high frequency disturbance signal generated by the sampling circuit in the disconnector operation state,the electromagnetic transient simulation is carried out to set up the isolation switch division test model.According to the characteristics of overvoltage overcurrent waveforms of high frequency signal,we also contect the suppression measures of combining TVS tube with RC low pass filter to improve the electromagnetic interference immunity of electronic transformer,which ensured that the electronic transformer output accuracy to meet the 0.2S level requirements.Fourth,when electronic transformer is running online,there are many influencing factors in complex environment of intelligent substation,such as harmonic superposition and frequency change.To get rid of the influence of frequency fluctuation,spectrum leakage and high harmonics and interharmonics on signal output,an improved algorithm of data preprocessing and main spectrum correction for traditional DFT algorithm is advanced to improve the online learning of electronic transformers.Finally,on the analysis of the operational stability of the electronic transformer,the electronic transformer based on data communication is accessed to the smart grid,and the networked control system(NCS)model is established.The line power signal of power system is measured by the electronic transformer.Considering the packet loss rate in the communication network,the networked control model makes an accurate estimation of the signal after packet loss,and then it is flexible and quick to compensate by the actuator.The power system network is connected with the generator through digital interface,and the running characteristics of the system are characterized by the rotor curve of generator rotor angle.The control model maintains the operational stability and damping margin for power system in different working conditions,and finds an effective method to calculate the network minimum requirement in data transmission quality.