HV Cable Systems Research Articles

Comprehensive Properties of Grafted Polypropylene Insulation Materials for AC/DC Distribution Power Cables

Polypropylene (PP) exhibits excellent insulation properties, high thermo-stability, and recyclable nature, thus expected to be the next-generation insulation material for HV cable application. Chemical grafting modification is an effective technology to improve the electrical properties of polypropylene. In this paper, we develop and report a new-type grafted PP insulation material by water-phase grafting technology. The comprehensive properties including electrical, thermal, and mechanical of it are tested and compared with traditional cable insulation material—crosslinked polyethylene (XLPE). The results show that the grafted PP holds superior thermal properties and enough mechanical flexibility. The electrical properties are of significant advantages, including resistivity enhanced by nearly two degrees of magnitudes, AC/DC breakdown strength raised by over 20%, and obviously suppressed space charge accumulation. These results indicate that grafted PP is very suitable for application in HV cable systems, either AC or DC. This research lays a foundation for the research and development of the next-generation recyclable polypropylene cables at higher voltage levels.

An Online Data-Efficient HV Cable Fault Localization Approach Via an AI-Enabled Recognition of Modal Wavefront in Sheath Current

Precise permanent fault localization is an important task for fast power restoration in underground HV cable systems. Among online fault localization methods, the fault localization accuracy strongly relies on the type and volume of measurement, and there is always a tradeoff between localization accuracy and measurement cost. To balance the tradeoff, a data-efficient HV cable fault localization framework is proposed in this paper. First, the fault characteristics of sheath currents are analyzed in modal mode, and compared with the conventional core conductor measurements. It is discerned that the sum of three phases sheath currents has similar characteristics, which can be measured by fewer sensors in a lower rating as the replacement of the conventional core conductor measurements. Second, the challenges of recognizing the wavefront arrival in the sheath are presented, and a Convolution Neural Network is introduced for the localization purpose. The proposed approach can realize high localization accuracy with low-cost measurement, and keep consistent performance under various scenarios through limited training datasets. A case study has been carried out using PSCAD/EMTDC platform to validate the effectiveness and feasibility of the proposed approach, followed by a discussion on the results.

Identification of defects in HV cable sheath based on equivalent impedance spectrum characteristic coding

In recent years, on-line monitoring and condition-based maintenance of HV cable systems have been extensively investigated to prevent cable fault. By analyzing cable sheath current, sheath defects can be identified. A new method of cable sheath defect identification is proposed in this study on the basis of the coding of the equivalent sheath impedance spectrum characteristics. In the proposed method, the sheath defects are identified by calculating the equivalent sheath impedance spectrum on the basis of the detected currents of the on-line sheath monitoring system. The equivalent circuit model of cross-bonded (CB) HV cable for sheath defect identification is constructed. The three-phase circuit model is decoupled to solve each phase independently in accordance with the CB scheme of the cable sheath. The equations of solving the equivalent sheath impedance are deduced. The criteria of sheath defect identification are proposed using the coding of characteristic parameters of the equivalent sheath impedance spectrum. Finally, a simulation model of the HV cable is built in the Simulink software to simulate 12 types of sheath defects. The simulation results have proven the correctness and efficiency of the proposed method.

Countermeasures of Zero-Missing Phenomenon in (E)HV Cable Systems

Zero-missing is a phenomenon in shunt-compensated cable systems, in which the current through the line breaker does not cross the zero point for several cycles. This paper deals with a thorough investigation on countermeasures of the zero-missing phenomenon in transmission systems and determines the requirements, benefits, and risks of applying each method. The effectiveness of countermeasures is studied on a simulated cable project with different cable lengths in an actual grid model of the Dutch 380 kV transmission system. Results are analyzed based on three criteria related to the IEC standards and the Dutch grid code. In addition, the switching sequence of circuit breakers is specified to maximize the effectiveness of the countermeasures. A statistical switching analysis is performed for the insulation coordination study since the application of some countermeasures increases the probability of high transient switching overvoltages. Moreover, the closing variation threshold of circuit breakers is calculated as a function of the circuit impedance and the shunt compensation degree.

On-line monitoring of relative dielectric losses in cross-bonded cables using sheath currents

Cross-bonded three-phase single-core Cross-linked Polyethylene (XLPE) cables are widely used in power transmission and distribution systems. A great deal has been published on on-line monitoring of HV cable systems to prevent failure. Relative Dielectric Losses (DL) monitoring reveals the general condition of the cable insulation. Generally, DL measurement relies on measuring operating voltage and insulation leakage current. The main challenge to on-line monitoring of DL in Cross-bonded High voltage (CB HV) cables is the interconnectedness of metal sheath. Measured sheath currents are a combination of the leakage currents of each section and the imbalanced circulating current. Differentiating the leakage current of each interconnected section from the measured data is complex. The second challenge is that co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. As a result, any measured value contains currents from the two sheaths connecting at a joint. The third challenge is that it is hard to obtain the reference voltage synchronously and with high accuracy. Consequently, this paper investigates separation of leakage current components and analysis of DL based on leakage currents without reference voltage. A circuit model is proposed to describe the leakage current separation method for CB HV cables to allow on-line monitoring of DL, and the criterion required for assessing relative change of DL among different phases or sections of the cable for insulation deterioration are presented. Operational measurements from a cable system are analyzed and discussed. Results show that there is no significant deterioration in this monitored major section of a CB HV cable system.

On-line monitoring and analysis of the dielectric loss in cross-bonded HV cable system

Dielectric loss has long been recognized as one of the most important indicators of cable insulation health. Although huge efforts have been made in the past to measure the dielectric loss of cable circuits, there has been no report of any successful on-line techniques for the purpose. This paper proposes a new on-line insulation dielectric loss monitoring method, based on separation of currents collected from the co-axial cables connecting the cable sheathes and the cable link boxes. The principle and theoretical foundation of the method are demonstrated. MATLAB simulations, based on the real cable parameters in a 110kV cable tunnel in China, indicate the error of the proposed method is less than 1×10−3%. The criteria for determining the relative dielectric loss in cable segments, based on the leakage current separation results, are demonstrated at the end. Initial results and analysis of implementation of the proposed method in the real world 110kV cable tunnel are presented.

The practical effect of the enlargement law on the electrothermal life model for power-cable lines

Insulation coordination is essential for designing and testing HV cable systems properly. In transient conditions, i.e., in the presence of transient voltage surges, insulation coordination is based on the individuation of a certain maximum acceptable risk of failure under the switching and lightning overvoltages that the insulation may encounter in service, and translates into a maximum number of expected failures per year per length of line that the cable line has to match. Given the design of the cable in steady-state conditions, the number of expected failures per year and per length of cable line can be reduced by a proper design of the shield wires of overhead lines, of the grounding systems of line towers and so on [1], and by selecting and installing properly voltage-surge arresters when needed.

From Slim Design, to Strategic Sourcing of Supplier, to Optimised Installation and Commissioning of HV Cable System in CLP Power HK Limited

Driven by ever-changing business environment, technological innovations and changes of supply / demand markets are very common nowadays. These may create either risks or opportunities to power business operators. CLP Power Hong Kong Limited (CLPP), a power utility in Hong Kong, has therefore developed a systematic approach to ensure high reliability o f the High Vo ltage (HV) cable system wh ile driving the cost down. This paper shares CLPP's approach in three key parts, namely prudent design procedures, strategic sourcing of suppliers and installation management. Riding on the technology advancement of cable industry, HV s limCross-linked Polyethylene(XLPE) cable system is well proven learnt fro m the sound experience in Extra High Voltage (EHV) cable industry. The reduced cable size is beneficial for cable transportation, ergonomics and environ mental protection. The Co mpany's 2-level Strategic Sourcing Model helps source price co mpetit ive yet quality suppliers. W ith in-depth investigation in suppliers' long-term sustainability, the suppliers selected through this model created significant saving to the Co mpany without sacrificing system reliability. Together with the optimised installation and co mmissioning practices, the approach has delivered significant contributions to CLPP, such as maintaining high reliability, saving cost, securing cable material supply, and achieving high safety, health, environ ment and quality (SHEQ) performance.

Upgrading of existing 400 kV FF HV cable link by redesigning and implementing forced cooling apparatuses in Vienna

30 years sleeping of 100 km cooling pipes was long enough to launch a process of evaluating and construction of a high sophisticated cooling installation of 24 km FF HV cable systems. The old cable crew remembered of easily upgrading from 600 to 1000 MVA discharging cold water into pipes. Deeper investigations showed what had to be changed of old constructions. The relationship between water temperature and pressure in the tubes for calculating their further lifetime is essential. The awareness of PRYSMIAN the old calculation would kill the cables in the middle of the span without an additional cooling station. Costs of the cooling: €14 Mill. instead of €95 Mill. for new cable systems. The evidence in reliability of FF HV cables made it worth.

A new approach to partial discharge testing of HV cable systems

This paper describes the application of an innovative partial discharge (PD) detection and analysis approach to the diagnosis of HV polymeric cable systems. The results of PD measurements performed off-line on cable systems, rated from 220 to 400 kV just after laying or after some time in operation, are presented. PD generating defect identification and location are discussed, also in relation to forensic observations

Fault current distribution in HV cable systems

An advanced analytical model is presented for the evaluation of ground return currents at substations fed by both three and single-core cable circuits. The circuit model includes the accurate treatment of an intermediate substation between the source and the faulted substation, unlike Engineering Recommendation EA S.34, which adopts a more simplified approach. Through detailed parametric studies, the differences between the simplified and the more accurate treatment are quantified. The effects that variations in the key parameters — cable length and type, substation earth impedances — have on the magnitude of the ground return current are identified and explained.

Discrimination between internal PD and other pulses using directional coupling sensors on HV cable systems

On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal PD and corona. The method works independently well on line voltage, resonance sources, oscillating voltages and 0.1 Hz cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany.