Influence of the winding design of wind turbine transformers for resonant overvoltage vulnerability

Abstract

Switching transients and earth faults may lead to resonant overvoltages at wind turbine transformer terminals as well as inside High Voltage (HV) and Low Voltage (LV) windings. The winding design in a transformer could strongly influence the internal voltage distribution as function of frequency, and this has a great impact on risk of insulation failure. In this paper, resonant overvoltages in three winding designs; layer, disc and pancake, are investigated and analyzed for the application in offshore wind farm. To achieve this, a prototype 500 kVA transformer with the three winding types on the HV side and taps for voltage measurements is designed and produced. The measurements show that a HV winding of layer type gives the highest transferred voltage to the LV terminal and that this happens at 1.6 MHz which could be excited for close-up earth faults. Although disc winding seems the least vulnerable when measuring the transferred voltage to the LV terminal, it has higher potential of internal resonances in the HV winding compared to the two other types both voltage-to-ground and voltage drops. Pancake and layer windings have less vulnerability in the case of internal resonances. Pancake windings have modular design characteristic. This advantage eases the repair for offshore wind applications.

Accelerated insulation aging due to fast, repetitive voltages: A review identifying challenges and future research needs

“…Although the challenges mentioned above still have not been addressed when using WBG-based conversion systems, the similar situations concerning transformer resonant overvoltages caused by cable-transformer interaction have been reported for conventional power systems where (short length) cabletransformer (motor) interaction can cause very high overvoltages. Switching transients, especially for wind applications subjected to more frequent switching operations and earth fault are two transient phenomena that can lead to resonant overvoltages at the LV terminal of a transformer as well as inside HV and LV windings [105][106][107][108][109][110][111]. The increasing number of transformer dielectric failures led to the CIGRE Working Group A2/C4.39 initiation for the computational assessment of impinging overvoltages on transformer terminals and internal stresses.…”

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

“…where is the width of the layer, is the distance between two layers (length of the average electric flux line between 2 conductors), de is the external layer diameter, di is the internal layer diameter and r is the curvature radius of the winding. 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.…”

“…However, it should be noted that the voltage responses of windings to the input voltage impulses also depend on behaviour of the impulse waveforms [9]. For example, the magnitude of oscillations in voltage distributions depend on the wave front time of the applied impulse [10,11].…”

Effect of shield placement for transient voltage mitigation due to switching surges in a 33/11 kV transformer windings

“…Switching operations in the substations during making and breaking of power lines connected to transformers could induce high oscillatory and non-linear surges [3]. Furthermore, the operation of circuit breakers can generate multiple, Very Fast Transient Surges (VFTS) in transmission lines and transformers [4]. Without mitigation, the switching transients originating from the circuit-breaker operation could also lead to the insulation breakdown in the windings and affect the transformers reliability [5].…”