Abstracts:Increased monitoring of distribution networks and power system assets present utilities with new opportunities to predict and forestall system failures. Although automated pattern recognition methodologies have given other industries significant advantage, power system operators face additional challenges before these can be realized. The effort of apportioning ground truth to fault data creates a knowledge bottleneck that can make utilizing automatic classification techniques impossible. Surrogate approaches using operational process outputs such as maintenance tickets as labels can be challenging owing to the causal ambiguity of these written records. To approach a solution, this paper demonstrates utilizing natural language processing techniques to disambiguate the free text in maintenance tickets for onward use in supervised learning of fault prediction and classification techniques. A demonstration of this approach on an established power quality fault data set is provided for illustration.

Abstracts:This letter proposes an analytical approach to estimate local and the network wide influence of a static compensator (STATCOM), configured for voltage unbalance (VU) mitigation. In addition, it provides guidelines for determining the most appropriate location for active mitigation. The proposed generalised formulation is able to demonstrate the VU mitigation provided by a STATCOM regardless of the control strategy used or the network conditions.

Abstracts:Conservation Voltage Reduction (CVR) is a well-known technique to reduce energy consumption by reducing the supplied voltage within statutory limits. Historically, the efficacy of CVR schemes has been quantified using the CVR factor (CVRf) which relates, in a given period, voltage reduction to energy savings. This metric has proved useful in the past, where fixed voltage reductions were applied. However, this letter shows from a theoretical perspective and with a realistic case study that when used for active CVR schemes (voltages actively controlled throughout the day), the CVRf can be smaller than that for fixed voltage reductions despite the larger energy savings. This could be misleading for decision makers considering active CVR schemes as an energy efficiency measure. Consequently, it is recommended that comparisons among CVR schemes are carried out in terms of actual energy reductions.

Abstracts:This paper presents a new fault location algorithm for radial distribution networks employing synchronized distributed voltage traveling wave (TW) observers. A robust and accurate fault location algorithm significantly improves the distribution networks reliability and reduces the risk of bush fires and electrocution resulting from sustained undetected faults. The medium voltage distribution networks include numerous junctions and many shunt and series connected devices, such as capacitor banks, transformers and cables, which makes fault location far more complicated. This paper investigates the effect of power system components on the propagation of traveling waves and proposes a method for a fault location in heavily branched radial distribution feeders. Results show that parasitic shunt capacitances in transformers have a significant impact on traveling time of incident waves to the location of the TW observers and compensation for this effect will improve the accuracy of fault location.

Abstracts:DC short-circuit current calculation for the modular multilevel converter based multi-terminal direct current (MMC-MTDC) grid is critical and fundamental in equipment selection and protection setting. The existing MTDC grid short-circuit current calculation methods suffer from low efficiency, poor adaptability, and inflexibility. To address these issues, this paper proposes an efficient and general companion circuit-based method to estimate the short-circuit currents of asymmetric bipolar MTDC grids. First, an MMC is equivalent to an RLC series circuit in parallel with a time-dependent controlled current source. The equivalent model considers not only the discharge currents of the sub-module capacitors but also the ac in-feed current determined by MMC controllers. Then, the equivalent model for a bipolar MTDC grid with the dedicated metallic return is established by combing the equivalent MMC model with the lumped model of the dc transmission lines. Next, the companion circuit method is employed to efficiently and readily solve the equivalent model of the dc grid to obtain post-fault branch currents and node voltages. Both theoretical analyses and intensive tests on the CIGRE benchmark MTDC grid and Zhangbei bipolar MTDC grid verify the improved accuracy, efficiency, adaptability, and flexibility of the presented method.

Abstracts:This article discusses the mechanisms enabling spatial resolution in fault location methods based on full transient signals, as opposed to those only using their early-time portion. This idea is found in recent travelling-wave methods (TWM) and those based on electromagnetic time reversal (EMTR). Their spatial resolution is discussed in terms of the sensitivity of a system resonances to change in the fault position and their coherence bandwidth. It is proven that using the entire transient signal it is possible to bypass the Fourier transform uncertainty principle, which limits the spatial resolution of time-domain reflectometry and standard early-time TWM. Super-resolved fault location is shown to be possible only for resonating systems, enabling high spatial resolution without relying on wide-band data. A detailed theoretical analysis for laterals and numerical results for networks and a three-phase line show that significant differences can be observed for the spatial resolution associated to each resonance, most often resulting in a loss of spatial resolution. The interaction between separate resonant structures, such as laterals in networks and coupled conductors in three-phase lines are shown to be the main cause of resolution loss.

Abstracts:Large fault direct current with fast rising speed has always been a significant threat to high voltage direct current systems. Fault current limiters (FCLs) are one of the most promising solutions. However, the biased permanent magnets (PM)s in traditional FCLs have large magnetic reluctance that causes FCLs to have small inductance changing ratios when a fault occurs. In this study, a novel three-limb topology of a saturated iron core-type fault current limiter (TSFCL) is proposed and a central PM bias limb is inserted to force flux flow through this limb. Therefore, the inductance changing ratio is greatly enhanced and the steep rising trend of fault current will be restrained effectively. The operating principles and characteristics of the TSFCL are analyzed to verify the deductions. Moreover, different lengths and cross-sectional areas of the inserted PM are tested and optimization study is carried out. The results show that the clipping inductance has a 55% increase compared with a traditional two-limb model. Moreover, compared with the existing smoothing reactor and traditional two-limb model, proposed topology can decrease the fault current by 45% and 16% within 5 ms, respectively. Finally, a typical HVDC system was built and the effectiveness of proposed TSFCL was verified.

Abstracts:The offshore ac side impedance model of the modular multilevel converter (MMC) based high voltage direct current (HVdc) system is essential for analyzing the interaction stability between MMC-HVdc and the offshore wind power plants. This paper develops the offshore ac side impedance model of an MMC-HVdc system for wind power integration taking the effects of offshore station, dc cable and onshore station into consideration. The dc impedance model of the onshore station is first derived which includes the effects of dual closed loop dc bus voltage control and circulating current control. Then, the dc impedance of the onshore station and dc cable impedance are used to derive the ac side impedance of the offshore station under open loop control. In addition, the influence of the dual closed loop based ac bus voltage and frequency (VF) control on the offshore ac impedance is analytically derived. As a result, the proposed model can be used to reveal the coupling between offshore offshore ac system and dc system as well as to investigate the influence of the dc system and VF controller in the offshore ac impedance of the MMC-HVdc system. Furthermore, the proposed model overcomes the deficiencies in harmonic resonance analysis of MMC-HVdc based offshore wind power integration system. The results of the simulation in PSCAD/EMTDC validate the proposed models and analyses.

Abstracts:With the capability of limiting the increasing short-circuit current of power grid, high voltage built-in high-impedance transformer (HVBHT) has been widely used in recent years. However, due to its special structure, the magnetizing inrush with high magnitude and slow decay is likely to occur during no-load energization, which may lead to the mal-operation of zero-sequence overcurrent protection of the upstream transmission line. This phenomenon has become a serious problem in the actual production in China. To solve this problem, a modified time difference method is proposed based on the substation area information, so as to accurately identify the magnetizing inrush, internal fault as well as current transformer saturation. Subsequently, a countermeasure on preventing line zero-sequence overcurrent protection from mal-operation due to HVBHT energization is proposed. Simulation results based on PSCAD indicate that the proposed protection scheme can cope with various kinds of energization and fault situations effectively. Thus, the correct operation of zero-sequence overcurrent protection can be ensured. Compared with other existing identification methods of magnetizing inrush, the proposed method shows great superiority.