Haitian Wang

Research on Overvoltage for XLPE Cable in a Modular Multilevel Converter HVDC Transmission System

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.

The effect of composition and processing on electric characteristics of XLPE in HVDC cable applications

Polyethylene exhibits many key characteristics including low dielectric loss, high breakdown strength and good processability. Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. The main advantage of XLPE is its excellent thermo-mechanical properties; it is relatively cheap and has low dielectric loss and low conductivity making it an ideal material for this application. Crosslinking enhances a number of thermo-mechanical properties such as deformation resistance at higher temperatures, tensile strength and creep properties. In comparison with lov density polyethylene (LDPE), the heat deformation characteristics of XLPE are superior and, for this reason, XLPE is currently the most common insulation material for power cables ranging from low to high voltages. This paper reports on an investigation into the development of a new XLPE formulation for use in high voltage direct current (HVDC) cable applications. Specifically, the electrical performance of two novel LDPE resins are compared with an industrial standard (reference) LDPE material. For crosslinking, dicumyl peroxide (DCP) was selected, as the decomposition temperature is high enough to prevent pre-curing during processing and to allow an efficient and rapid crosslinking at moderate temperatures. Moreover, the behavior of various systems is compared in terms of electrical breakdown performance and the influence of material composition and processing on these parameters is described.

The impacts of the temperature and electric field on the electrical characteristics in semicon-bonded XLPE insulation

HVDC cables play a vital role in the power transmission system for renewable energy and global power trade. Nowadays, the crosslinked polyethylene (XLPE) extruded cables have been widely applied in power industry due to the superior performance on the thermo-mechanical properties and dielectric properties. The low volume conductivity and the minimized space charge accumulation are the two key requirements for a reliable high voltage direct current (HVDC) cable insulation. This paper reports on the impact of temperature and electric field on the space charge behavior and DC conductivity in XLPE material for cable insulation. The samples were carefully prepared to simulate the real cable insulation structure. A layer of LDPE film mixed with DCP (dicumyl peroxide) was sandwiched between two layers of semicons (also contain crosslink agent) and then crosslinked at 200 °C to ensure the semicon layers were thermally bonded with the XLPE insulation. The crosslinked samples were then degassed in the vacuum oven with for 6 days at 80 °C. The space charge characteristics and the conductivity of the semicon-bonded XLPE samples were measured at room temperature and high temperature. The electric fields were kept at 20 kV/mm and 40 kV/mm and the influences of the electric field and the temperature on space charge dynamics and conductivity in the semicon-bonded XLPE samples are discussed.

Numerical modelling on partial discharge in HVDC XLPE cable

Purpose High voltage direct current (HVDC) cable is an important part in the electric power transmission and distribution systems. However, very little research has been carried out on partial discharge under DC conditions. Niemeyer’s model has been widely used under AC conditions. This paper intends to modify the Niemeyer’s model considering both electric field and charge dynamics under DC conditions, and therefore propose a numerical model describing partial discharge characteristics in HVDC cable. Design/methodology/approach This paper intends to understand partial discharge characteristics under DC conditions through numerical modelling. Niemeyer’s model that has been widely used under AC conditions has been modified, taking both electric field and charge dynamics under DC conditions into consideration. The effects of loading level or current through the conductor, cavity location and material properties on partial discharges have also been studied. Findings Electrical conductivity is important in det...

Numerical study of influential factors on partial discharges in HVDC XLPE cables

For the dramatically developed high voltage direct current (HVDC) power transmission, HVDC cables play a vital role in the power transmission across seas and connections with renewable power sources. However, the condition monitoring of HVDC cables is still a challenging research topic. This paper aims to understand the influence of external factors, namely, current, cavity location and material properties, on partial discharge (PD) characteristics in HVDC cable in a numerical way referring to the refined Niemeyer’s model.,The influences of the three external factors are studied by a proposed numerical model for DC PDs based on the modification of a conventional PD model for AC voltage via a finite element analysis method.,The external factors can influence the discharge magnitude and discharge repetition rate via affecting the electrical conductivity of the material: DC PD is more frequent and with higher discharge magnitude when the cavity is closer to the conductor or the current through the conductor is higher. Both DC PD repetition rate and average discharge magnitude in long-term aged material are lower than virgin material. The effect of discharge on insulation degradation becomes decreasingly significant.,The current work is based on the numerical modelling of DC PDs. Further experimental validations and comparisons are essential for improving the model.,The studies of the influence factors for PDs under HVDC voltage can benefit the research and practical power transmission on DC PDs, contributing the design and test of DC PDs in HVDC cables, exploring the understandings of the DC PDs’ mechanism.,This paper, to the best of author’s knowledge, first studies the influence factors on DC PDs based on the numerical modelling work.

The effect of electrode material and semicon bonding on space charge dynamics of XLPE

This study sets out to explore the space charge characteristics of high voltage direct current (HVDC) cable grade crosslinked polyethylene (XLPE) using different electrode materials and configurations, ranging from sputtered gold to thermally bonded semi-conducting materials. The existence of both homo and hetero charges in HVDC cables is well known; however, the role of semi-conducting materials and the formation of space charge is critical and required more attention. Space charge accumulation at high electric stress was monitored using the pulsed electroacoustic (PEA) technique. Experimental results have shown that charge injection processes take place in all cases and the amount of charge and polarity of the dominated injected charge has significant dependence on the electrode material under the same applied electric field.

Data recovery algorithm in space charge measurement by PEA method

Purpose – The pulsed electro-acoustic method is widely applied for space charge measurement in solid dielectrics. The signals, however, can be seriously distorted during transmission, especially in non-planar specimens. The purpose of this paper is to find an efficient algorithm to correctly recover the space charge profile for different types of specimens. Design/methodology/approach – The distortion can be associated with both geometry and material (attenuation and dispersion). Hence the recovery algorithm consists of two parts, respectively. The influences of geometries, causing the divergences of electric force and acoustic waveform, can be corrected by sets of factors. The attenuation and dispersion of the material can be suppressed based on the transfer function matrix in frequency domain, which could be obtained from calibration. Findings – A general algorithm applicable to three kinds of specimens (single-layer, multi-layer and coaxial-geometry dielectrics) has been proposed. Compared with the oth...