Research On Ferroresonance Mechanism And Its Suppressive Measures For Neutral Point Ungrounded Systems

Ferro-resonance due to the potentialtransformer (PT) occurs as a normal fault of power systems, and data add that in the220kV and below voltage level of urban power networks have the knowledge of similar incidents happening before. Based on the nonlinear excitation characteristics of potential transformer, its saturation is considered to be blamed caused the Ferro-resonance.In the neutral ungrounded power system, in order tosupervise three phase voltages of the system, the grounded-wye connectedthree-phase potential transformer (PT) fix on the bus. When the system failures happen (such as single-phase-to-earth faults disappear), Ferro-resonance phenomenacausedby the matching of nonlinear excitation inductance of PT and network to earth capacitance will ready to happen without suitable prevention measure adopted. Once Ferro produced, saturated iron cores of PT present nonlinearity, which conduce to the great increase of excitation current. With the over currents, all circuit components are considered to be physically at risk, and PT are likely to be burned, even exploding. In consequence, finding an effective prevention measure to restrain resonance must be taken into consideration as a meter of the safe and reliable operation of power network.In the past decades, foreign and domestic experts have done many studies and experiments on Ferro-resonance and have proposed a number of prevention measures. Taken together, the prevention measures widely used at present in our country are: nonlinear resistance at broken delta of the secondary assistant windings; the neural point of PT high voltage side grounded through single-phase PT (4PT); the neural point of PT high voltage side grounded through a nonlinear resistance, etc.With the ATP-EMTP platform and Graph program helping, this paper puts its emphasis on building the formulas for the non-linear excitation characteristics of PT which can explain the mechanism of Ferro-resonance by fitting curve though the formulas. Withthe curve, PT model is fitted in ATP-EMTP, and followedbyseveral typical resonance form to discuss the regulations behind the ferromagnetic resonance.Then in the last part, the study is focused on the prevention measures. Comparing the measures to identify their own usingranges and precautions, this study has significant meaning in the control of Ferro-resonance used at present in our country.