Wojciech Tomasz Wiechowski

Field Test and Simulation of a 400-kV Cross-Bonded Cable System

This paper discusses cable modeling for long high-voltage ac underground cables. In investigating the possibility of using long cables instead of overhead lines, the simulation results must be trustworthy. Therefore, model validation is of great importance. This paper gives a benchmark case for measurements on a 400-kV cable system with cross-bonded sheaths. This paper describes in detail the modeling procedure for the cable system and compares simulation results with the transient field test results. It is shown that although the main characteristics of the waveforms are well reproduced in the initial transient, there are significant deviations between the simulation and measurement results. An analysis indicates that the main cause for the deviation is inadequate representation of the current distribution on conductors since the modeling approach does not take proximity effects into account. The measurement results can be received by contacting the first author of this paper.

Selected studies on offshore wind farm cable connections - challenges and experience of the Danish TSO

The Danish power system is characterised by a high share of wind power production. Most of the wind power has so far been distributed onshore and connected to the lower voltage levels. The recent tendency is to group it in large offshore wind farms. The wind farms are often connected directly to the high voltage network via long AC cable lines compensated by shunt reactors. These cable connections impose some additional challenges that should be considered in the planning stage. This paper presents the recent experience of the Danish TSO with long cable connections of wind farms and shows the results of selected simulation studies made of the planned Horns Rev B wind farm connection.

A New Control Structure for Hybrid Power Filter to Reduce the Inverter Power Rating

This paper describes a control structure of a hybrid power filter, which can be used in current harmonic mitigation. The selected hybrid filter topology consists of an active power inverter connected between the capacitor and inductor of a shunt passive filter. In this paper the inverter power rating is seen as the main factor of interest, because it determines the overall cost of the installation. The aim of the proposed control is to reduce the amount of power losses in the inverter by using a current loop that controls the fundamental reactive power. The principles of operation as well as design considerations of the presented harmonic mitigation solution are presented. Several comparisons are given to describe the advantages and limitations of the proposed topology. Experimental results confirm the theoretical analysis on a laboratory setup with a 7 kVA power inverter

Double layered sheath in accurate HV XLPE cable modeling

This paper discusses modelling of high voltage AC underground cables. For long cables, when crossbonding points are present, not only the coaxial mode of propagation is excited during transient phenomena, but also the intersheath mode. This causes inaccurate simulation results for high frequency studies of crossbonded cables. For the intersheath mode, the correct physical representation of the cables sheath as well as proximity affect play a large role and will ensure correct calculations of the series impedance matrix and therefore a correct simulation for the actual cable. This paper gives a new, more correct method for modelling the actual physical layout of the sheath. It is shown by comparison to field measurements how the new method of simulating the cables sheath results in simulations with less deviation from field test results.

Identification of problems when using long high voltage AC cable in transmission system II: Resonance & harmonic resonance

Within European countries, Denmark was been at the forefront of replacing the transmission lines with cables. The project was supplying the power to the greater Copenhagen area in early 1990s. Recently, the Danish TSO Energinet.dk has initiated a research project for analyzing the HV underground cable in transmission system. The objective of this paper and the companion paper is to address the most important problems expected in transmission system with relatively larger share of long HV underground cables. The end goal will be a guideline to special solutions and precautions to avoid dangerous over voltage problems and also resonance problems in a transmission network with future increased share of cables. Two major categories of problems are switching transient and resonance problems. In each category of the possible problems, first some theoretical background is provided and then the problem and countermeasures are discussed. In this paper most important resonance problems are addressed and discussed. Three main categories of resonance problems are: near resonance, harmonic resonance and ferroresonance.

Methods to Minimize Zero-Missing Phenomenon

With the increasing use of high-voltage ac cables at transmission levels, phenomena, such as current zero missing, start to appear more often in transmission systems. Zero-missing phenomenon can occur when energizing cable lines with shunt reactors. This could considerably delay the opening of the circuit breaker (CB), leaving the system unprotected and vulnerable to failures. Methods to prevent zero-missing phenomenon are still being studied and compared in order to identify effective countermeasures. This paper contributes to these efforts by presenting several countermeasures that can be applied to reduce the hazards of zero-missing phenomenon. The authors discovered that this phenomenon can be eliminated, merely by using an extra CB or a preinsertion resistor.

Overvoltage Protection of Large Power Transformers—A Real-Life Study Case

This paper demonstrates the results from a detailed study of the overvoltage protection of a particular 400/150-kV 400-MVA power transformer. The work presented here is based on a real-life power system substation design and data and initiated by Danish TSO Energinet.dk as a consequence of serious transformer overvoltage damage. A simulation model for the entire system consisting of overhead line, transformer, surge arrester, and earth grid has been created in PSCAD/EMTDC. The main focus has been put on the earth grid, which has been submodeled in detail in MATLAB using an electromagnetic transient approach based on the thin-wire program made by J. H. Richmond for NASA in 1974. The earth grid model is verified with excellent agreement compared to already published results. The overvoltage performance of the particular case is analyzed, and it shows that the transformers LIWL have probably been exceeded. It is clearly illustrated that the transient performance of the earth grid plays an important role in the overall overvoltage protection system design.

Derivation of Theoretical Formulas of the Frequency Component Contained in the Overvoltage Related to Long EHV Cables

Recent studies of long extremely high-voltage (EHV) cables show the importance of performing temporary overvoltage analyses. As the switching of EHV cables can trigger temporary overvoltages, it is important to find the dominant frequency component contained in the switching overvoltages of these cables. Since there are no theoretical formulas to find the dominant frequency, it is generally found by means of time domain simulations or frequency scans. The derivation of theoretical formulas has been desired as the formulas would be useful in verifying the results of time-domain simulations or frequency scans. In addition, the formulas could eliminate the necessity of building simulation models of some network components. In order to address this need, this paper derives theoretical formulas to find the dominant frequency in long cable energization overvoltages. The accuracy and practical usefulness of the formulas are confirmed by comparing them with the results of time-domain simulations.

Use of a pre-insertion resistor to minimize zero-missing phenomenon and switching overvoltages

With the increasing use of High-Voltage Cables, which have different electric characteristics from Overhead Lines, phenomenon like current zero-missing start to appear more often on the transmission systems. Methods to prevent zero-missing phenomenon are still being studied and compared to see which countermeasure works the best. Technically the best way to avoid zero-missing phenomenon produces very high switching overvoltages, making the operator to choose to either avoid the zero-missing phenomenon or to minimize the switching transients. This paper presents a method of determining an optimal value of the resistance of the pre-insertion resistor that results in minimizing both the zero-missing phenomenon and switching overvoltages simultaneously.

Harmonic domain modeling of a distribution system using the DIgSILENT PowerFactory software

The first part of this paper presents the comparison between two models of distribution system created in computer simulation software PowerFactory (PF). Model A is an existing simplified equivalent model of the distribution system used by transmission system operator (TSO) Eltra for balanced load-flow calculation and stability studies. Model B is accurate model of the distribution system created on the basis of the detailed data of the investigated network and is used as a reference. The harmonic impedance of the two models is compared. In the second part of the paper, the sensitivity of the harmonic impedance to basic system parameters (load active power, motor load fraction and reactive power of PFC capacitors) is analyzed