Method And Experiment Research For Distribution Transformer Negative Sequence Current

With wide use of the power electronic devices and dynamic nonlinear loads (suchas electric arc furnace, electrified railway, electric welder and so on) in the powersystem, it resulted in distribution transformers working in three-phase unbalance inlong-term and serious harmonic pollution, at the same time superior negative sequencecurrent and harmonic current emerged on the power grid, which would give rise tomalfunction when the negative sequence component protecting the starting components,and it threaten seriously the safe operation of the power system. Aimed at reducing theharm of negative sequence current, the author analyzed all kinds of impact factors, thenresearched and developed a kind of distribution transformer negative sequence currentdynamic inhibition system, the system could restrained effectively negative sequencecurrent that was caused by the three-phase unbalanced current, decreased influence ofthe negative sequence current that was caused by the harmonic current, as well asincreased the power factor, decreased wastage of the distribution transformer, andimproved safe operation of the power system. The main researches as follows:①The author analyzed generation and impact factors of the negative sequencecurrent, and came up with a method about distribution transformer negative sequencecurrent dynamic inhibition, and built simulation experiment model of that method withthe system stable operation condition which that method is used, and the simulationresults proved effectively both three-phase imbalance current and harmonic current haveimpact on the negative sequence current and the validity and feasibility of the method ofrestraining negative sequence current.②In combination with the practical engineering environment of distributiontransformer, the author researched a set of distribution transformer negative sequencecurrent dynamic inhibition system, which is divided into two parts of three phaseunbalance current governance and harmonic current filter according to the function.And whole capacitor compensation mode is used to design three-phase unbalancecurrent governance part, including the design of electric parameter acquisition unit,central control unit and capacitor cabinet framework components and grade group ofcompensation capacitor and so on. Combining with the existing APF device andconsidering the reactive power coupling phenomenon, the author design the harmoniccurrent part to ensure the good collaborative operation of the whole system. ③To ensure the safe stable operation of the system and the remote statemonitoring of distribution transformer, the author write the client-side and server-sidesoftware program of this system. And the client-side software program including dataextraction, identification subroutine, rapid calculation of reactive power regulation,capacitor bank switching controlling subroutine and General Packet RadioService(GPRS) wireless communication subroutine and so on; meanwhile, takingadvantage of VC++2010and MySQL5.0developed the system matching datamonitoring software of server-side software program, it realized online monitoring ofdistribution transformer and experiment system, it benefits for staff.④Based on the distribution transformer negative sequence current dynamicinhibition method and system, the author had experimented in both laboratory and onthe scene. On the condition of the lab, the computational results indicated the systemwithout harmonic current could suppressed effectively negative sequence current thatcaused by three-phase imbalance current; meanwhile, on the condition of scene, thecomputational results showed the system with harmonic current could suppressedeffectively negative sequence current that caused by three-phase current and impacts ofnegative sequence current that caused by harmonic current, it also improved its powerfactor and reduced additional losses, both of the two experiments reached the intendedtarget.