Conference Agenda

Installing cables in the power system results in a shift of resonances to lower frequencies, and possibly introduces problems associated with near fundamental frequency resonance overvoltages. These overvoltages were found to prevail especially during switching events in weak systems. This paper has studied a utility case by using PSCAD software as well as the sequence network method. The industry practice demonstrates that several mitigation methods exist to deal with resonance problems. These can be classified into (i) installing direct transfer trip functions for contingencies, (ii) suppressing the phenomenon (e.g. specifying surge arresters), (iii) adding interlocking between the associated circuit breakers to prevent mis-operations or (iv) avoiding the problem (e.g. using alternative system connections). Some of the mitigation methods may not be recommended according to the Safety-by-Design initiative of the utility. The surge arrester (ii) itself cannot provide full protection and should be used together with other mitigation measures. To balance the risk against cost and ensure the cables can be operated safely, this paper recommends (iv) of planning an alternative system connection to avoid the residual risk of overvoltages.

The paper investigates overvoltage propagation in a 400kV network within the National Grid Electricity Transmission system in the UK. The studies were triggered by a replacement project for two parallel 400 kV cables and the installation of an additional one. The network in the vicinity of the cables has historical issues of flashover and circuit breaker failures. Electromagnetic transient studies were performed to simulate energisation and fault inception and clearance for the three new cables to examine compliance against overvoltage standards and limits. Studies were performed using the DIgSILENT analysis tool. Studies show that due to resonance in a specific frequency range, switching overvoltage propagated and amplified at remote nodes to a level close to the equipment's rated switching voltage. As mitigation, surge arresters with characteristics different from the National Grid recommended surge arresters were proposed and found suitable to bring the overvoltage values within the design limits for temporary overvoltage.

Inan ungrounded electrical system, successive reignitions and extinctions of a single-phase to ground fault is known to potentially generate a voltage escalation phenomenon.This is the so-called arcing ground fault whose theory has been thoroughly described in literature, even though very few practical occurrences have been experienced in existing electrical systems due to the specific and rare conditions that cause its appearance. This paper studies the risk of appearance of voltage escalation inthe 10 kV auxiliary system of an industrial plant. It describes the physical principles at the origin of voltage escalation and presents a study conducted using an EMT-like program. The results show that, thanks to the presence of a permanent insulation monitor (PIM) connected to the system through voltage transformers, the successive reignitions and extinctions of single-phase faults cannot cause severe overvoltages or voltage escalation. The paper ends with some general conclusions.

This paper studies the voltages developed on a wind turbine (WT) and a medium-voltage distribution line (MVDL) connected to a wind farm subjected to lightning strikes and located on FD (FD) soils. The ground potential rise (GPR), voltages at the blade tip and on phase conductors of the MVDL are calculated using the full-wave electromagnetic software XGSLab® employing the rigorous Partial Element Equivalent Circuit (PEEC) method. The wind farm comprises four wind turbines with interconnected grounding systems using cables buried in resistivity soils of 1,000 and 5,000 Ω.m. The voltages are computed for the first positive impulse (FPI) of 100-kA 10/350 μs and for the subsequent negative impulse (SNI) of 50-kA, 1/200μs. Results have shown that voltage peaks increase notably as the soil resistivity increases. When the WTs are assumed, oscillations in the GPR waveforms for the SNI occur due to the multiple reflections between the blade and the turbine’s base. However, the voltages for the FPI present smooth time-domain responses. Furthermore, the overvoltages developed at the MVDL are significantly dependent on the soil resistivity and lightning current waveform.

In half-wavelength (HWL) transmission lines, severe temporary overvoltages are usually observed during the occurrence of line-to-line faults in specific critical locations, and it is unrealistic to ignore the impact of the corona phenomenon. To represent the corona effect, an accurate Suliciu corona model was adopted and implemented in PSCAD software, and a new technique based on the genetic algorithm (GA) was proposed to fit the model parameters. Furthermore, the paper evaluated the performance of the overvoltage mitigation method based on the installation of two spark gaps (SGs) at strategic points to detune the resonance condition. As shown, the SGs turn the HWL line
overvoltage levels similar to those observed in conventional AC lines, and the corona modelling further reduces the severity of the overvoltages.

In high-voltage industrial electrical power systems it is common to ground the neutral of the generators by high resistance to control the ground-fault currents. Grounding resistors are sized so that transient overvoltages remain at controlled amplitudes, and, for this, it is considered as a basic criterion to set the resistive component of the ground-fault current greater than or equal to the total capacitive current. Although this criterion is widespread in the literature, when the resistor is connected to the secondary of a grounding transformer this criterion can be different. Normally, the works found in the literature does not consider the transformer parameters, however, for large industrial power systems which have a high capacitive current, the transformer parameters need to be considered, hence a new criterion to keep the overvoltages at safe levels must be investigated. Thus, in this work, it is proposed a transient model to study intermittent-ground faults in a typical industrial power system. Furthermore, many simulations are performed to investigate which parameters affect the overvoltages when the grounding transformer parameters are considered.