Analysis And Suppression Measures Of Transformer DC Magnetic Bias Caused By HVDC Transmission

In recent years, DC magnetic bias has attached more and more attention to for itmay cause great harm to the transformer and the system. Actually, there are twocauses for the DC magnetic of the transformer: firstly, when the HVDC system is inthe mono-pole ground return operation mode, the regurgitant DC under the earth willflow into the transformer winding of the transformer substation through the neutralpoint of ground connection; Secondly, the interaction between the dynamic changesof solar plasma wind and the geomagnetic field will cause geomagnetic storm, thechange of geomagnetic field produces electric potential gradient on the earth surfaceand then produces low-frequency induced current in the transformer winding.Because of its very low frequency, it can be considered approximately as DC.This thesis makes detailed theoretical analysis and experimental research aimingat the effect of DC magnetic bias on the transformer and the restraining measures,deriving the expressions of the total flux of the transformer after the DC magneticbias. Work out the expression of field current by the simplified circuit diagram of thehybrid system of AC and DC and work out the direct factors influencing the fieldcurrent by the expressions. Elicit the expressions of in-out voltage and current by theequivalent circuit of transformer and use PSCAD/EMTDC software to simulate theinput and output voltage and current waveforms joining different sizes of DC.By comparing the core constructions of three-phase transformer, three-phasethree-column type transformer and three-phase five-column type transformer,conclusion can be drawn that three-phase transformer is easier to be influenced by theDC magnetic bias. A totally new concept—DC ratio coefficient is defined, comparingthe incidence of bias on transformers of different structures more intuitively. Underthe condition of different DC ratio coefficients, use simulation software ofPSCAD/EMTDC to simulate the magnetizing current waveforms and Ф-I curves ofthe two transformers and verify the feature of poor support on DC bias of three-phase transformer.This thesis comes up with a new circuit topological structure of neutral linecapacitance which is DC blocking and controllable bridge open and cutoff to restrainthe DC magnetic bias. Compared with the traditional capacitance DC blocking circuit,this method makes use of DC inductance in the bridge circuit to limit the risingchange rate of short-circuit current in the initial stage of fault and has no need of theresponse of control system, instantly automatically putting into use, avoidingtransient current impact to damage capacitor. This restraining method can also ensurethe transformer neutral point of effectively grounded, making the control system hasenough response time to switch working mode. It gives equivalent structurerestraining device circuit in detail, works out the analysis results of voltage andcurrent under the condition of system failure and gives evidence of the circuitparameter such as capacitor through analysis result.Finally, use simulation software of PSCAD/EMTDC to verify that thesuppression device can restrain the transformer DC magnetic bias completelyeffectively and make contrastive analysis of the effect on restraining the transientimpulse current before and after adding the isolating DC device when system faultoccurs. Low-power DC blocking experimental facility has been made, simulating theinhibitory effect on the fault current peak value under the circumstance of systemshort circuit. The experimental result coincides with the simulation, proving theeffectiveness and practicability of this restraining method.