High-impedance Fault Detection and Location Methods for Three-core Submarine Cables Based on Voltage Distribution Features

Remarks on “The resistance of faults in submarine cables”

Summary form only given as follows. This paper presents a new approach to the detection of high impedance fault (HIF). The proposed approach (or detector which implements this approach) has four distinct features. First, the detector input signal is the three-phase unbalanced current (or feeder 3I/sub 0/), rather than the conventional three individual phase-currents. Secondly, the detector is designed by monitoring the energy variance for the second, fourth and sixth harmonics of 3I/sub 0/ individually, and by requiring the increment of randomness of all these harmonics to indicate an HIF. Thirdly, a self-tuning scheme based on the statistical confidence is applied to the automatic setting of threshold level for the energy variances monitored. Fourthly, effective counters are designed to detect both persistent and intermittent arcings. The detector has been tested on 51 staged faults and 8 unfaulted feeders. The test results show that the detector can have 100% fault detection rate so long as the arcing current at fault location is not less than 5 A. As to the dependability tests, no false alarm (or tripping) has happened for all of the unfaulted feeders under test. These feeders either supply special (including arc furnace) load, or have leakage current (referring to the intermittent zero-sequence load due to the feeder location along coast), or are under the normal capacitor or load switchings. The test results and the investigation for the high sensitivity and dependability and for the limitation of proposed fault detection method are presented.

A novel fault location scheme using feeder terminal units (FTUs) for high-impedance grounding faults in resonant grounding systems (RGSs) is presented. The parallel resonance for a high-impedance fault (HIF) in an RGS with inverter-interfaced distributed generators (IIDGs) is analyzed. The process exhibits unique distribution features for transient zero-sequence components involving underdamped and overdamped states. We conclude that the transient zero-sequence admittances (TZSAs) from FTUs upstream of the fault point have the maximum value, while the TZSAs obtained from FTUs downstream of the fault point are small for HIFs. Therefore, the proposed location scheme uses FTUs to calculate the maximum values of the TZSA. By comparing with action criteria, the distribution automation (DA) master station utilizes the above results of each FTU to locate the faulty feeder section. The accuracy of the fault characteristics and the proposed method are validated by applying a real-time digital simulator (RTDS) and an FTU-based monitoring system. Additionally, the proposed scheme demonstrates an excellent and reliable performance, considering various faulty feeder sections, fault resistances, arcing faults, fault inception angles, imbalances and the presence of IIDGs and noise.

Layout of offshore wind farm (OSWF) plays a major role in capital cost of its installation. The cost of OSWF depends on the cost of electrical collector system which includes, submarine cables, number of wind turbines, offshore platforms etc. The optimal design of OSWF will give better economical outcome. This paper explains the methodology for optimal design of OSWF electrical collector system and cost model of collector system. The methodology is based on the combination of Ant Colony optimization and Travelling Salesman problem. The objective is to minimize the length of submarine cable connected between wind turbines in wind farm. It will reduce the cost of cable and cable power loss. With the help of North Hoyle OSWF connected with 30 wind turbines, the optimization approach designs optimal OSWF based on minimum length of cable for radial and ring topologies.

Most offshore wind farms use a large capacity offshore collector substation in order to stably transfer generated power. Many bundles of wind turbines are connected to offshore platforms by using submarine cables that contain bigger capacitance than the overhead one. This means that the line impedance has a resonant frequency, and it will be changed depending on the connection of offshore wind turbines. When the additional offshore wind turbines are linked to an existing offshore substation, the resonant phenomenon may be caused by submarine cables due to the fact that the inverter system of a wind turbine can make harmonics through the switching operation. From this perspective, this paper analyses the resonant frequency of a transmission line based on a large-scale offshore wind farm. In order to verify this analysis, the models of a 75 MW wind farm will be simulated by using the PSCAD/EMTDC program. The simulation results presented the response of a wind farm injecting resonant frequency in accordance with the composition of offshore submarine cables.

In the offshore wind power transmission system(WPTS) through the AC submarine cable(SC), as the scale of the offshore wind farm(WF) gradually increases, the transmission distance of the AC SC continues to increase. Due to the large charging power of the SC, there are many problems in the actual offshore WF connection project, such as excessive reactive power injection at the point of common connection(PCC) of offshore WFs, over voltage and over current carrying capacity along the SC, and over voltage on the high voltage side of the offshore substation. In response to the above problems, this paper establishes a mathematical model for the WPTS by the 220kV AC SCs, and analyses the WPTS based on the principle of layered division of reactive power compensation. Under the condition of voltage and current limitation along the cable, the offshore high voltage reactor(HVR) value of compensation is determined. Onshore high voltage reactor and SVG are installed on land to make the reactive power exchange between the WPTS and the land AC network zero. When the SVG capacity is minimum, onshore HVR value connected to the system is given. Combined with a specific project in China, the recommended reactive power configuration plan is calculated.

Offshore windfarms is an emerging technology in the wind energy conversion system. For onshore wind farms high voltage overhead lines are used for transmitting power from the wind farm to the grid. However this option is not possible in the case of an offshore windfarm as large part of the distance to connection point must be covered by submarine cables. The application of high voltage dc (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage (EHV) ac transmission due to a variety of reasons like efficiency, economics etc. In the case of HVDC transmission a converter is connected between the wind farm and the utility grid. Even though CSC's and VSC's are widely used for the realization of large HVDC systems, as an alternative approach Bridge of Bridge converter (BoBC) is introduced recently. In this work the performance of the VSC and BoBC are compared and analyzed. The simulation is done using PSCAD/EMTDC software. Compared with the onshore wind farms, the offshore wind farms have access to significantly better wind energy resources and hence offer larger energy generating capability. Therefore offshore wind farms are gaining importance and the BoBC proves to be better than the VSC for similar applications.

In order to study switching transients in an offshore wind farm (OWF) collector system, we employ modeling methods of the main components in OWFs, including vacuum circuit breakers (VCBs), submarine cables, and wind turbine transformers (WTTs). In particular, a high frequency (HF) VCB model that reflects the prestrike characteristics of VCBs was developed. Moreover, a simplified experimental system of an OWF electric collection system was set up to verify the developed models, and a typical OWF medium voltage (MV) cable collection system was built in PSCAD/EMTDC based on the developed models. Finally, we investigated the influences of both the initial closing phase angle of VCBs and typical system operation scenarios on the amplitude and steepness of transient overvoltages (TOVs) at the high-voltage side of WTTs.

High voltage direct current systems through submarine cables for offshore wind farms: A life-cycle cost analysis with voltage source converters for bulk power transmission

Submarine cables are critical assets that play an indispensable role in interconnected power systems, particularly in cross-sea energy transmission and offshore wind turbines. Their importance is further accentuated in exigent situations such as natural disasters and war, which emphasize the need for secure, reliable and uninterrupted energy supply. Furthermore, the investment and operating costs of submarine cables are relatively higher than other power system equipment, making it essential to operate them under the rated operating conditions to prevent possible faults and ensure power system stability. As thermal stress can lead to damage of cable insulation, it is an essential parameter to consider. Overloading increases thermal stress, resulting in rapid aging of the cable insulation and a shorter cable lifetime. Therefore, it is imperative to determine the maximum conductor temperatures and current carrying capacities of submarine cables under varying loading rates and ambient conditions using thermal analysis. In this study, thermal analyses are carried out for a three-phase, 220 kV HVAC, XLPE insulated submarine cable under different loading conditions. The findings demonstrate that the maximum temperature, current carrying capacity, and total losses of the cable are significantly impacted by loading rate, phase imbalance, and seawater temperature.

Life cycle assessment of the offshore wind farm alpha ventus

The high voltage alternating current transmission with submarine cables is the common scheme to integrate offshore wind farms close to the land. Due to the capacitive effect of AC cables, the association of the long AC cables and the onshore grid can cause resonances, which may amplify grid background harmonics. This paper focuses on the mitigation of background harmonic amplification caused by offshore wind farm integration. First, the background harmonic amplification mechanism in the offshore wind farm integration system is analysed qualitatively, and the mitigation approaches are given in principle. Then, taking a practical project as a study case, the harmonic modelling methods of the offshore grid and the onshore grid are elaborated, including the harmonic models of wind turbines, transformers, cables, the high voltage and the extra‐high voltage transmission grid, and the low voltage distribution network. According to the established harmonic models, the cause of background harmonic amplification is analysed. On this basis, two mitigation methods of installing the series blocking filter and the shunt filter are compared and the recommended mitigation scheme is proposed. Finally, the feasibility of the proposed scheme is verified by calculation results under different grid operating conditions.

A new high-impedance fault (HIF) detection method using time-frequency analysis for feature extraction is proposed. A pattern classifier is trained whose feature set consists of current waveform energy and normalized joint time-frequency moments. The proposed method shows high efficacy in all of the detection criteria defined in this paper. The method is verified using real-world data, acquired from HIF tests on three different materials (concrete, grass, and tree branch) and under two different conditions (wet and dry). Several nonfault events, which often confuse HIF detection systems, were simulated, such as capacitor switching, transformer inrush current, nonlinear loads, and power-electronics sources. A new set of criteria for fault detection is proposed. Using these criteria, the proposed method is evaluated and its performance is compared with the existing methods. These criteria are accuracy, dependability, security, safety, sensibility, cost, objectivity, completeness, and speed. The proposed method is compared with the existing methods, and it is shown to be more reliable and efficient than its existing counterparts. The effect of choice of the pattern classifier on method efficacy is also investigated.

Simulation and calculation of maximum transmission power for offshore wind plants accounting for the Electro-Magnetic transient process

Microgrids (MGs) offers grid-connected (GC) and islanded (ID) operational modes. However, these dynamic modes of operation pose different microgrid protection challenges. This paper presents a new protection method for MGs based on a discrete one-dimensional recursive Median filter (1-DRMF). In the first step, the 1-DRMF is applied on a measured current signal on every single phase individually for targeted feature extraction. Then, the median filter deviation (MFD) and the selected harmonic distortion (SHD) are computed from the current signals of all phases independently. In the second step, the upsurges in the MFD and the SHD of all phases are cross-checked with the pre-established threshold value of 0.3 to identify and categorize fault incidents. Finally, the directional properties of three-phase (3-p) reactive energy are employed in order to pinpoint the faulty line section (LS). Many simulations were executed on MATLAB/Simulink to validate the sustainable performance of the established method. Results prove that the scheme can detect, classify, and locate the solid and high impedance faults (HIF) in the GC as well as the ID modes under radial and meshed scenarios.