J.F.G. Cobben

Basis-Adaptive Sparse Polynomial Chaos Expansion for Probabilistic Power Flow

This paper introduces the basis-adaptive sparse polynomial chaos (BASPC) expansion to perform the probabilistic power flow (PPF) analysis in power systems. The proposed method takes advantage of three state-of-the-art uncertainty quantification methodologies reasonably: the hyperbolic scheme to truncate the infinite polynomial chaos (PC) series; the least angle regression (LARS) technique to select the optimal degree of each univariate PC series; and the Copula to deal with nonlinear correlations among random input variables. Consequently, the proposed method brings appealing features to PPF, including the ability to handle the large-scale uncertainty sources; to tackle the nonlinear correlation among the random inputs; to analytically calculate representative statistics of the desired outputs; and to dramatically alleviate the computational burden as of traditional methods. The accuracy and efficiency of the proposed method are verified through either quantitative indicators or graphical results of PPF on both the IEEE European Low Voltage Test Feeder and the IEEE 123 Node Test Feeder, in the presence of more than 100 correlated uncertain input variables.

Current distribution in LV networks during 1-phase MV short-circuit

This paper describes the consequences of a fault in a medium voltage network on the grounding systems at the LV-side. To study the current distribution and to verify the models, we deliberately introduced one phase to ground faults in the 10 kV floating MV network. The selected site was the end of a 2 km long buried MV cable feeding a small group of houses; the soil there had a high resistivity. The fault current was 127 A. The current distribution and the relevant voltages were measured at several positions in MV and LV net. We compare the results for a TT and a quasi-TN earthing system in the houses, also taking the telecom cable into account. The distribution of the fault current over the MV cable, measured at another MV station at 2.2 km distance, are in agreement with calculations, based on a model which includes the contact of the lead shield of the MV cable with the ground. Current and voltage data are compared with calculations. Eindhoven University of Technology and the Dutch power distribution company NUON, carry out a joint research project on global earthing. In this project we treat different faults and various interference sources e.g. lightning and switching events together.

Distributed energy resources for a zero-energy neighborhood

Zero energy buildings are on the increasing trend. They are perceived as appropriate technology to reducing CO2 emissions, improving energy efficiency and alleviating energy poverty. The main goal is that a grid-connected building produces enough energy on site to equal or exceed its annual energy requirement while using the grid as a buffer. Many municipalities see this concept as a prospective solution for developing future neighborhoods and thereby aim to develop a neighborhood with net zero energy concept. This paper proposes passive designs measures and distributed power generations required in designing such a neighborhood.

Scenario analysis of generic feeders to assess the adequacy of residential LV-grids in the coming decades

The energy transition will bring large changes to residential load profiles. In order to assess whether the current low voltage (LV)-grids are capable of handling these changes, future scenario analysis must be used to assess the grid adequacy. To this end, an approach is proposed, incorporating scenario-based household load modelling, LV-feeder clustering and probabilistic load flow calculations. All combinations of independent scenarios on the drivers of change in the residential load have been assessed with probabilistic load flow calculations . Generic feeders are chosen, which are representative for the LV-grid of a Dutch distribution network operator. The scenario assessment shows the chances of overloading and/or exceeding of the voltage limits for the LV-feeders over different time horizons and scenarios. The approach can also be used to assess the main scenario drivers and grid parameters responsible for changes in the future adequacy of the LV-grid.

Coordinated control to mitigate over voltage and under voltage in LV networks

Increasing penetration of distributed renewable energy resources (DRES) and smart loads into the LV network lead to new power quality challenges. Important power quality challenges are overvoltage and undervoltage. A number of solutions are already developed to mitigate these voltage variations. In this research, On Load Tap Changer (OLTC)-based control technique is used to mitigate overvoltage and undervoltage. A specific voltage variation case is assumed in this research, in which, both overvoltage and undervoltage occur simultaneously. In this paper, a solution to mitigate the aforementioned voltage problems is proposed. Importantly, the proposed control is capable of mitigating both overvoltage and undervoltage simultaneously. The solution includes coordination of OLTC and the photo voltaic systems. Furthermore, the proposed control is verified using a co-simulation platform.

Evaluation and updating of harmonic voltage limits

Characteristics of the current emission for new developed devices are changing resulting in lower low order harmonics and an increase in high order harmonics. For the low voltage network this results in an increase in high order harmonic voltages, violating more often the limits for the harmonic voltages at the 15th and 21th order harmonic. To improve this situation harmonic current could be reduced, network impedance could be decreased or the harmonic voltage limits could be increased. Since there are no complaints regarding these higher harmonic voltages, this paper presents an alternative approach to update these harmonic voltage limits.

A LV network overvoltage mitigation strategy based on epsilon-decomposition

To increase the penetration of distributed renewable energy resources (DRES) in the existing LV network, new control solutions need to be developed. In this paper an algorithm for overvoltage mitigation in LV networks with DRES is proposed. Complex communication infrastructure for monitoring and controlling an extensive part of the LV network is not economically attractive for distribution system operators (DSO). Thus, in this study, a large LV network is decomposed into a number of smaller sub-networks using the epsilon-decomposition algorithm. As a result, applying the proposed control strategy requires communication only within the individual sub-networks. The sensitivity matrix-based method is used to mitigate the overvoltage problem by partially curtailing the output power of DRES. The proposed control strategy can mitigate the overvoltage also in case loss of communication occurs in few DRES units. Finally, it is demonstrated that the proposed approach to decomposing the network can be performed offline using system data that is readily available to the DSO, and consequently it can be easily applied in real LV networks.

Offshore wind farm harmonic resonance analysis — Part I: Converter harmonic model

Offshore wind power technology has matured significantly and now directly competes with conventional and onshore wind power generation. Thanks to continuous technological developments and significant cost reduction, offshore wind power is closing the gap with the onshore wind power about the levelized cost of electricity; however the technology challenges in the offshore environment still poses threats to the long-term operational reliability and profitability of offshore wind farm projects. This article focuses on one such threat: harmonic resonance in the offshore grid and presents an analytical solution to identify and mitigate it by applying advanced harmonic modelling of an offshore wind turbine. The analytical derivation of the frequency dependent positive and negative sequence impedance of a converter harmonic model of a 8MW generic wind turbine is presented. It concludes by comparing the frequency domain converter harmonic model of a generic 8MW offshore wind turbine with its time domain counterpart as well as the ideal current source model (conventional solution) under the distorted grid condition. The results obtained confirm that the converter harmonic model demonstrates superior accuracy when compared to the ideal current source model (conventional solution) for low order harmonics resonance interaction assessment. To conclude, the discrepancies in the results obtained from the time domain and the frequency domain models respectively deserves further investigation. Particularly the frequency dependent impedance as derived from the average model of the associated power electronics should be examined in detail as its validity depends strongly on the phenomena one is interested in (typically the average model provides sufficient accuracy when the frequency of interest under investigation is ten times smaller than the switching frequency).

Mitigation strategies to improve the performance of AC contactor against voltage dips

The performance of a process against voltage dips depends on the severity of voltage dips and the robustness of process equipment to voltage dips. This paper discusses about the performance of an AC contactor, one of the voltage sensitive equipment, against various types of voltage dips without and with mitigation techniques. First, lab tests are conducted to find the voltage-tolerance curves of the contactor without mitigation. Then, the tests are repeated with the AC-coil mitigated by Coil-Lock and phase-phase supply; and the tolerance curves of the device without and with mitigation are compared. To get better insight into the performance of the contactor, the severity of various types of voltage dips obtained from field measurements are compared with the voltage-tolerance curves of the contactor in both situations.

Applicability of true voltage unbalance approximation formula for unbalance monitoring in LV networks with single-phase distributed generation

In the hierarchy of power transmission and distribution systems, the three-phase LV distribution networks are most susceptible to voltage unbalance (VU). The main causes are large presence of randomly distributed single-phase loads and, following the latest trends, the increasing presence of single-phase distributed generators. Most widely accepted VU calculation is based on percentile ratio of negative and positive sequence voltage (voltage unbalance factor, VUF). Obtaining sequence voltages is a complex domain calculation and requires simultaneous sampling of three-phase voltages and angles. This is why the existing VU monitoring and mitigation solutions are dominantly three-phase. Without an additional three-phase aggregation device, there is an inherent gap in VU monitoring for single-phase loads and generators. In this paper, the data concentrators for a growing PV micro-inverter niche are identified as an infrastructure that could be exploited to somewhat close this gap. Due to potential technical limitations of PV data concentrators, a non-complex VUF approximation formula is tested as a “light” calculation alternative, by comparing it against conventional VUF. The comparison results are obtained from Monte Carlo load flow simulation for an unbalanced LV network.