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| Analysis of DC Bias Voltage Formation and Voltage Transformer Melting Under Typical Distribution Faults |
| Wen Weijie1, Hu Tiewei1, Chen Xiaolong1, Wang Lei2, Li Bin1 |
1. School of Electrical Automation and Information Engineering Tianjin University Tianjin 300072 China;
2. Wuhan Branch of China Electric Power Research Institute Wuhan 300010 China |
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Abstract The melting of fuse installed at the high-voltage side of the electromagnetic potential transformers (PTs) occurs frequently in the ungrounded power distribution system with rated voltage of 10~35 kV, increasing the maintenance burden and endangering the reliability of power system. Focused on this problem, research on the mechanism of PT fuse melting is conducted, and the research shows that the deep saturation of the PT under the effect of the DC bias voltage (DCBV) is the ultimate cause of the fuse melting.
The paper presents theoretical derivations, simulation verifications, and experimental results regarding the generation conditions of DCBV and the mechanism of PT fuse melting. Firstly, the equivalent circuit of the power system without load during the whole fault process (single-phase ground fault, intermittent arcing ground fault, and two-phase ground fault) were derived. Subsequently, based on the principle of superposition, the equivalent circuit after the fault disappearance was decomposed into zero-input and zero-state circuits. By superimposing the zero-input response and zero-state response, the analytical expression of system voltage was obtained. According to the relationship between the DCBV and the net charge (Qn) of stray capacitors at the moment when the fault disappears, it is inferred Qn≠0 is the generation condition of DCBV.
Changing the discharge channels of Qn may affect characteristics of DCBV, showing different waveform and decay time. By taking stray capacitance, PT impedance mode, load, and reactive power compensation device into consideration, factors related with DCBV were discussed. Finally, simulation in PSCAD/EMTDC and down-scaled experiments were conducted for verification. The main findings and conclusions of this paper are listed as follows:
(1) In an ungrounded power system, if Qn≠0, and without discharge channels, Qn has to be redistributed on the stray capacitors, resulting in the DCBV. In the case of single-phase ground faults (intermittent arc ground faults), two-phase ground faults, three-phase ground faults, Qn may not be zero at the fault disappear moment, generating DCBV; In the case of a two-phase short circuit fault, Qn is always zero, hence there is no existence of DCBV.
(2) Factors including stray capacitance, PT impedance, load, and reactive power compensation capacity, are related with the characteristics of DCBV. A greater stray capacitance and reactive power compensation capacity could lead to a slower decay speed of DCBV. Under light load and no-load conditions, the DCBV persists for a longer duration, whereas under heavy load, the DCBV quickly diminishes. The use of different PT impedance modes can lead to variations in the form of the capacitor voltage.
(3) Large impedance (inductance and resistance) of high voltage winding of PT can result in the stray capacitor discharge circuit entering an overdamped state, leading to a decaying DC component in the zero-input response of the capacitor voltage. On the other hand, small impedance of high voltage winding of PT may trigger the switching of the PT branch between saturation and non-saturation states. The voltage on stray capacitors exhibits a waveform similar to ferro resonance. This continuous excitation current pulsing heat accumulation is the ultimate cause of the fuse melting.
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Received: 16 November 2023
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2025, Vol. 40 
