Research And Test On Transformer Neutral Dc Current Blocking Device

With the construction and operation of many HVDC projects in China, under monopole operation mode with earth return, the impact of grounding current passed through grounding pole on the nearby transformers with earthed neutral point in AC system has been highly regarded by electronic power industry.Putting a capacitor in series between transformer neutrals, putting a resister in series between transformer neutrals, the way of DC electric potential compensation and putting a capacitor in series on AC transmission system are all measures to restrain DC. Considering that larger DC current flows through transformer neutrals and that when fault occurs larger fault current flows through transformer neutrals, transformer neutral DC current bloking device has been developed and tested by the way of putting a capacitor in series between transformer neutrals.On the base of the project to restrain the DC current flowed through transformer neutrals in LING AO and DA YA BAY nuclear power station, the principle, structure and test plan of transformer neutral DC current bloking device are studied in the paper. The main functions of the device are researched and the device is tested to meet these functions. Technology problems that has been found in experiments are analyzed, also the methods and measures that can resolve the flaw are proposed. Through monomial tests, simulative operation tests and high-current tests, the device has been improved.The paper's main contents are as follows:1) The technology and performance requirements that transformer neutral DC current bloking device should meet are proposed in the paper. It is detailedly discussed about transistor collocation, current limit inductance winding's resistance, maintained loop's design and its resistance, breaker's close time and its capability.2) The main technology indexes are studied . The way of test is confirmed and the schemes are set down. These include monomial test schemes, simulative operation test schemes and high-current test schemes.3)On the base of theoretical analysis to device's elements, the monomial tests and simulative operation tests are made to the main elements including fast bypass breaker, current limit inductance, transistor and maintained loop. The capabilities of each device's elements are mensurated, the results of test are analyzed and the parameters of each device's element are optimized.4) For simulating the fault current flowing through the device and the voltage added on the device when transformer neutral DC current bloking device was installed at transformer neutrals, high-current experiments are carried out. The waves of current flowing through the device are analyzed. To the problems occurred in the test, the unreasonable parts of the device are mended for improving its reliability.