Researches On Electronic Voltage Transformers Based On Detecting Capacitor Current And Its Related Technologies

Electronic Voltage Transformer (EVT), which is one of the key equipments needed fordigital substation and smart grid, is highlights and frontiers at home and abroad in voltagetransformer research. This paper mainly studies a new type of EVT, and in-depth does ancomprehensive research to its related technologies.Firstly, the paper summarizes the problems of traditional electromagnetic transformersand present situation of studies about EVT, and then in-depth studies the principle andstructure of EVT based on detecting capacitor current, which is a new method of passive EVT,discusses its advantages, proposes three related technologies. The primary side equivalentcircuit is established considering equivalent inductance of TA and equivalent resistance ofhigh voltage capacitor, error calculation formula is dirived considering frequency as theindependent variable, and then frequency characteristic of EVT is analyzed. The paperclassifies stray capacitance in the light of electrode composition, derives the formula of straycapacitance and points out that stray capacitance affects the transformers’ steady-stateperformance by changing capacitance, the influence can be eliminated in the commissioningphase. Starting from the structure of high voltage all film oil capacitor, the paper studies themechanism how temperature can affect output voltage. When temperture changes, thecapacitance will alter and causes capacitor current vary, then leads to the change of secondoutput voltage. the paper also proposes two methods, analog circuits method and digitalcircuits method, for temperture compensation. The analog circuit method is simple, and hassmall compensation error; the digital circuit method has high precision, but its’ structure ismore complex than the former. The laboratory simulation testing platform is established, andsteady-state accuracy experiment, temperature experiment and transient experiment is done inaccordance with national standards. The problems encountered in experments is analysed, andcorresponding solutions is proposed.EVT based on detecting current in capacitor has the strongpoints such as simple structure,smart sensing principle and reliably. It also has high anti-interference ability and non-leaddrop because it uses cable to transmiss current signal between switching station and controlroom. It doesn’t need power supply in switching station as the processing circuit places in thecontrol room, so the new type of EVT is passive and of great practical value and prospects.