Amir Hayati Soloot

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

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.

The effect of winding design on transformer frequency response with application on offshore wind farm energization

The application of transformers in offshore wind farms requires more consideration in the sense of reliability and protection than onshore ones due to the higher cost of accessibility and repair. Switching of offshore wind farms and lightning strikes are the main two transient phenomena which can lead to harmful damages to Wind Turbine Transformer (WTT) as well as other equipment. Thus, the frequency response of WTTs should be analyzed especially for 10 kHz<;f<;10 MHz. To obtain the necessary protection methods for WTTs, the measured frequency response of a special 500 kVA transformer with layer, disc and pancake windings are assessed and compared in this paper. The results show that all winding designs have a resonance frequency around 800 kHz with disc windings showing the lowest transferred overvoltages. The layer winding, also has a resonance at 1.6 MHz with an even higher transferred overvoltages. The disc winding have pronounced resonance frequencies also around 100 kHz and this results in higher internal overvoltages.

Frequency domain investigation of switching transients in Offshore Wind Farms

Offshore Wind Farm (OWF) as one of the renewable energy alternatives has received much focus in recent years. The installation of wind turbines, cable interconnections and power transformers in offshore areas requires more attention to maintenance and repair. Wind Turbine Transformers (WTTs) are subjected to high stresses due to switching transients in offshore systems.

Two dimensional simulation of Post Arc phenomenon in Vacuum Interrupter with Axial Magnetic Field contacts

The Post Arc (PA) phase of Vacuum Interrupters (VIs) and the effects of dominant physical parameters of vacuum arc investigated in this paper. 2-D movement of PA plasma in the presence of Transient Recovery Voltage (TRV) is simulated in a proper Finite Element Method (FEM).

Internal resonant overvoltage in wind turbine transformers- sensitivity analysis of measurement techniques

The installation of medium voltage transformers in wind turbines motivates the wind turbine manufacture, operators and researchers to review the challenges of insulation failure owing to overvoltages. Among these overvoltages, internal resonant overvoltages are most crucial and destructive. One of the protective measures, which decrease the potential of internal resonances, is the selection of appropriate winding type, i.e. disc, layer, and pancake. In this paper, the internal resonances of the three windings are compared and the least venerable one, pancake, is introduced. The focus will be on the assessment of measurement techniques and the sensitivity analysis of the obtained results on them. The motivation was to obtain high quality measurements in the range of 10 kHz-10 MHz so that the modeling and calculation results can be compared with. Measurement results show that the RG58A/U coaxial cables have loading effect due to stray capacitance and influence the voltage measurements. It is recommended to apply oscilloscope papers with x10 attenuation. Measurement results for input impedance are sensitive on both voltage and current probes. The selection of current transformer CM-100-6L-IR50 provides quite better results compared to current probe Tektronix P6021. It is also found out that the installation of foil around transformer presents more realistic results for both terminal and internal resonances.

Upon The Impact of Power System and Vacuum Circuit Breaker Parameters on Transient Recovery Voltage

The aim of this paper is to investigate how Transient Recovery Voltage (TRV) characteristics are varied due to network and Vacuum Circuit Breaker (VCB) parameters. The role of RC damping branch to mitigate TRV is observed for Short Circuit (SC) and inductive load interruption. For this, a numerical method is taken to solve network and VCB equations simultaneously just after Current Zero (CZ). To evaluate numerical method, an analytical method base on step response of second order system is considered. The sensitivity of TRV amplitude to RC branch values is analytically discussed. As an internal VCB parameter, Post Arc Current (PAC) is considered and the effect of PAC on TRV after SC interruption is surveyed. Furthermore, the TRV computation method applied in this paper is evaluated with Alternative Transient Program (ATP) simulations.

Stability influence of renewable energy systems: Connection to DC nanogrids

Stability analysis of power systems dominated by power electronics devices is a challenge for the future. This paper focuses on the analysis, based on impedance representation of a nanogrid and a renewable energy system. The renewable energy system can be composed by a photovoltaic energy conversion module or a renewable energy source with a permanent magnet generator. Therefore, the common coupling point stability has been studied by the small signal Nyquist test. The real time simulation and hardware in a loop test of the nanogrid where the renewable source is going to be connected was carried-out with an OPAL-RT and a network analyzer. The impedance of the renewable energy system was obtained by the application of a current perturbation to the output DC side, as well as the nanogrid impedance. The obtained results show that the coupling of both system has a stable Nyquist test.

The peculiarities of Transient Recovery Voltage in presence of Post Arc Current in Vacuum Circuit Breakers

The post arc phase is a critical part in the current interruption process of Vacuum Circuit Breakers (VCB). During this step, the dielectric strength of VCB has to be recovered. In order to improve the performance of VCB, investigation of Post Arc Current (PAC) in the presence of Transient Recovery Voltage (TRV) is presented in this paper.