Mitigation of lightning surge stresses in the high voltage windings of distribution transformers by introducing an electrostatic shield

Abstract

Abstract:The Medium Voltage (MV) Electricity Distribution Networks are frequently exposed to lightning and it is the main suspected reason for distribution transformer failures. Appreciable number of transformer failures occur due to insulation failures notably at points near line ends. The short rise time of a surge prompted by a lightning impulse can cause deterioration in the insulation and ultimately lead to a dielectric breakdown. The voltage distribution along transformer High Voltage (HV) winding becomes non-linear under surge conditions due to the capacitive-inductive currents. This research focuses on the possibility of neutralizing this effect with the aid of an electrostatic shield inserted in to the HV winding.Comparison of the behaviour of the winding under surge conditions with and without electrostatic shield requires appropriate transient models suitable for simulations. The necessary calculations required for this purpose were carried out and the proposed models are presented. The simulations were carried out with PSCAD software and the results are included in this paper. The oscillatory nature due to the combined effect of capacitors and inductors is observed in the wave tail region and some of the peaks created have potentials exceeding the peak potential of the surge waveform as well.The proposed transformer model have a definite positive effect compared with the available model as the results obtained by simulations clearly reveal the reduction of the stresses on the transformer HV windings when a shield is used.

Investigation on the Resonant Oscillations in an 11 kV Distribution Transformer under Standard and Chopped Lightning Impulse Overvoltages with Different Shield Placement Configurations

“…On the other hand, the placement of the electrostatic shield depends on the location of the overvoltage surges. A study in [26] has placed the electrostatic shield at the outer layer of the HV winding due to overvoltage surges occurrence at that location. The resonant oscillations in the windings due to external transient overvoltage surges are generally different for each transformer.…”

Investigation on the Transient Voltage in a 11 kV Transformer Under Lightning Surges

“…2 and it was used as the input to the RLC equivalent circuit of the transformer model. Since, majority of the transformer winding failures occurred at the line ends [12], the lightning impulse was applied at the outermost layer. Hence, the present transient study is only focussed on HV layers.…”

“…The Cll for layer 1 to 12 of the HV winding can be calculated based on (11). The capacitance between end phase of the winding and the transformer tank, Cgt was calculated based on (12) [16], (12) where h = h + d, h is height of the winding, do is the outer diameter of the inner layer, d is the gap between two layers, t is the internal width of the tank, which is 383.15 mm. The windings are concentrically arranged around the core.…”

A Surge Absorber for the Sri Lankan Distribution Substation to Mitigate Lightning Damage

“…Oscillatory shape of the surge is created by the coupling of winding inductance and capacitances, and to understand the phenomena, different circuit models are proposed [6,7]. But analyzing with complicated models can be troublesome since it can be unvarying and takes a lot of time.…”

“…But analyzing with complicated models can be troublesome since it can be unvarying and takes a lot of time. A three phase transformer model has been used to investigate the influence of mutual coupling of LC components, with analysis conducted in both the time and frequency domains [6,7].…”