HVDC Cable Systems Research Articles

Validation of a Qualification Procedure Applied to the Verification of Partial Discharge Analysers Used for HVDC or HVAC Networks

The insulation condition of HVDC grids consisting of cable systems, GIS, and converters should be monitored by partial discharge (PD) analysers using artificial intelligence (AI) tools for efficient insulation diagnosis. Although there are many experiences of PD monitoring solutions developed for the supervision of the insulation condition of HVAC grids using PD analysers, there are no standardised requirements for their qualification available yet. The international technical specification TS IEC 62478 provides general rules for PD measurements using electromagnetic methods but does not define performance requirements for qualification tests. HVDC and HVAC PD analysers must be tested by unambiguous test procedures. This paper compiles experiences of using PD analysers with HFCT sensors in HVAC grids (cable systems, GIS, and AIS) to define a qualification procedure for HVAC systems. This procedure is applicable to HVDC grids (cable systems, GIS, AIS, and converters) because the particularities related to the insulation behaviour under HVDC voltage are also considered. Representative PD sources are discussed in HVAC and HVDC positive and negative polarity. The PD pulse trend of representative insulation defects in HVDC cable systems is quite different from that of HVAC grids. Special attention should be paid to the acquisition of PD signals in HVDC grids since few pulses appear in solid insulations, mainly during voltage changes (polarity reversals or surges), but rarely in continuous operation with constant direct voltage. A synthetic PD simulator has been developed to reproduce trains of PD pulses or noise signals, similar to those that can appear in the power network. A set of three functionality tests has been developed for qualification of the diagnostic capabilities of PD analysers working up to 30 MHz addressed to HVDC or HVAC grids: (1) PD recognition test, (2) PD clustering test, and (3) PD location test. This qualification procedure has been validated by means of a round-robin test performed by five research institutes (RISE, FFII, TUDelft, TAU, and UPM) using commercial and in-development AI PD recognition and clustering tools to demonstrate its robustness and applicability. Applying this qualification procedure, two PD methods for electrical detection and prevention of insulation defects have been approved, one for HVAC and the other for HVDC grids.

Electrical Tree Initiation and Growth in LDPE Under Negative HVDC Superimposed With AC Ripples

Understanding the role of power quality in the aging of HVdc cable systems is critical to the reliable connection of offshore renewable energy sources, and hence global ambitions to reduce carbon emissions. This work investigates electrical treeing with a needle-plane geometry in low-density polyethylene (LDPE) under a high negative dc voltage superimposed with ac ripples (−60 kV dc ± 7 kV ac). Tree initiation showed behavior similar to that widely reported under pure power frequencies. Subsequent tree growth, however, was observed to be confined in a smaller area with limited length and width than seen under pure ac voltages, even after long periods of voltage application. Traditional 2-D imaging showed overlapped tree channels which developed to cover the whole area within the tree outline. A distinguishing tree shape resulted, which we have named a “slim bouquet” shape. The volume rendering from X-ray computed tomography (XCT) showed the structure had a 3-D fractal dimension greater than 2, considerably larger than its 2-D representation. PD signals during the growth had wing-like phase-resolved partial discharge (PRPD) clusters and signal phase concentrations between 10°–45° appeared after hours of growth. There was a comb-like appearance in maximum partial discharge (PD) magnitude variation, which is typical in pure high negative dc fields. Consideration is given to both space charge distribution controlled by high dc fields and continuous degradation by ac fields to explain the slim bouquet tree formation.

Improved Evaluation of the Life Lost by HVDC Extruded Cables due to Long TOVs

Previously the author developed a procedure for estimating the life lost by HVDC extruded cables under the long Temporary Over-voltages (TOVs) that may arise in VSC HVDC cable systems. As the topic is gaining interest, here the procedure is improved to target an upper conservative limit of the aging effects of such TOVs. This goal is achieved in two steps. First the line length effect on insulation volume is considered via the so-called enlargement law for HVDC cables, as the longer the cable, the greater the insulation volume, and the number and size of defects; this lowers insulation endurance to applied stresses and raises the fraction of life lost at every single TOV. Second, a life model developed for harmonics superimposed onto rated voltage is included in the procedure to better address the challenge set by the fast peak of long TOVs. The improved procedure is applied to 320-kV and 525-kV cable designs. The results highlight that, with long lengths and conservative values of insulation parameters, the improved procedure provides orders-of-magnitude higher loss-of-life fractions, which in the most pessimistic case might be non-negligible for severely-aged cables in service since long time.

Monitoring of Partial Discharges in HVDC Power Cables

This article describes the need for monitoring HVDC cable systems and presents a new approach for monitoring partial discharge at DC voltage on long-length, land-based HVDC cables containing multiple joints.

Electric Field Distribution in HVDC Cable Joint in Non-Stationary Conditions

Accessories such as joints and terminations represent weak points in HVDC cable systems. The DC field distribution is intimately dependent on the thermal conditions of the accessory and on material properties. Moreover, there is no available method to probe charge distribution in these conditions. In this work, the field distribution in non-stationary conditions, both thermally and electrically, is computed considering crosslinked polyethylene (XLPE) as cable insulation and different insulating materials (silicone, rubber, XLPE) for a 200 kV joint assembled in a same geometry. In the conditions used, i.e., temperatures up to 70 °C, and with the material properties considered, the dielectric time constant appears of the same order or longer than the thermal one and is of several hours. This indicates that both physical phenomena need to be considered for modelling the electric field distribution. Both the radial and the tangential field distributions are analysed, and focus is given on the field distribution under the stress cone on the ground side and near the central deflector on the high voltage side of the joint. We show that the position of the maximum field varies in time in a way that is not easy to anticipate. Under the cone, the smallest tangential field is obtained with the joint insulating material having the highest electrical conductivity. This results from a shift of the field towards the cable insulation in which the geometrical features produce a weaker axial component of the field. At the level of the central deflector, it is clear that the tangential field is higher when the mismatch between the conductivity of the two insulations is larger. In addition, the field grows as a function of time under stress. This work shows the need of precise data on materials conductivity and the need of probing field distribution in 3D.

Reliability Assessment of a Multi-State HVDC System by Combining Markov and Matrix-Based Methods

The purpose of this paper is to highlight that, in order to assess the availability of different HVDC cable transmission systems, a more detailed characterization of the cable management significantly affects the availability estimation since the cable represents one of the most critical elements of such systems. The analyzed case study consists of a multi-terminal direct current system based on both line commutated converter and voltage source converter technologies in different configurations, whose availability is computed for different transmitted power capacities. For these analyses, the matrix-based reliability estimation method is exploited together with the Monte Carlo approach and the Markov state space one. This paper shows how reliability analysis requires a deep knowledge of the real installation conditions. The impact of these conditions on the reliability evaluation and the involved benefits are also presented.

The Effects of Transient Overvoltages on the Reliability of HVDC Extruded Cables. Part 1: Long Temporary Overvoltages

Extensive simulations have been carried out so far of transient voltages superimposed onto the DC voltage of HVDC cable transmission systems. With the advent of VSC HVDC cable systems, attention has turned to long Temporary Over-voltages in such systems. Preliminary experiments on the possible adverse effects of these long TOVs on the insulation of VSC HVDC cables have started, but no simulation models for estimating their effect on cable life and reliability have been developed yet. This investigation fills this gap by carrying out the simulative evaluation of the life lost by HVDC extruded cables subjected to long TOVs (Part 1) and switching impulses applied during Type Tests (Part 2), so as to compare the severity of these two transient voltages superimposed onto the DC voltage of HVDC extruded cables. This Part 1-paper is focused on estimating the severity of TOVs on extruded cable insulation by: calculating the electric field during the TOV; comparing it with the field during Type Test load cycles; using Miner's law of cumulated damage to calculate the life lost during the TOV, thereby also locating the most severely stressed point of insulation. In Part 2-paper, a similar evaluation for switching impulses and an overall comparison are performed.

The Effects of Transient Overvoltages on the Reliability of HVDC Extruded Cables. Part 2: Superimposed Switching Impulses

In a previous paper (Part 1) the authors have performed the simulative evaluation of the life lost by HVDC extruded cables subjected to long TOVs occurring in VSC HVDC cable systems. The conclusions were that it cannot be completely ruled out that the field levels and the duration of such TOVs are a potential threat to certain cable designs, and tests other than Load Cycling Type Tests after CIGRÉ Technical Brochure 496 could assess quantitatively the potential threat associated with long TOVs. For these reasons, in this Part 2 superimposed switching impulses applied during Type Tests after TB 496 are analyzed in order to check if such impulses might be equivalent to long TOVs as for their electro-thermal stress effects on extruded insulation. The analysis is similar to that carried out for long TOVs in Part 1, i.e., it consists of: calculating the electric field during superimposed switching impulses; comparing it with the field during long TOVs; calculating with Miner's law of cumulated damage the life lost during superimposed switching impulses and comparing it with the loss of life during TOVs. A brief literature survey of potential high field effects during long TOVs is also reported.

An Improved Statistical Method for Calculating Lightning Overvoltages in HVDC Overhead Line/Cable Systems

HVDC cable systems connected to HVDC overhead lines are subject to fast front overvoltages emanating from the line when lightning strikes a shield wire (backflashover) or a pole conductor (shielding failure). Representative fast front overvoltage levels for HVDC cable systems are usually established without considering their statistical characteristics. A statistical method to determine overvoltages related to the acceptable mean time between failure (MTBF) for the cable system was developed previously. The method accounts for the statistical distribution of lightning current magnitudes as well as the attenuation of the overvoltage wave due to corona discharges on the line, since this effect dominates for system voltages up to about ±320 kV. To make the method suitable for higher system voltages as well, this article introduces an improved statistical method which also accounts for surge attenuation through resistive effects, soil ionization, and statistical treatment of overvoltages due to shielding failures. To illustrate the improved method, it is applied to a case study for a ±525 kV DC line.

The insulation of HVDC extruded cable system joints. Part 1: Review of materials, design and testing procedures

This position paper by the DEIS HVDC Cable Systems Technical Committee provides a review of existing diagnostic electrical and dielectric techniques for testing the insulation of polymeric extruded HVDC cable joints in the present Part 1. Here, the state of the art on the insulation of HVDC extruded cable system joints is covered with reference to types, design and testing techniques. This helps to identify routine tests as the first target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers' practices for quality control of the insulation of accessories during routine tests and VHF/UHF wireless sensors as the best tool for performing such measurements on joints in the noisy factory environment. Thereby, a novel protocol for the measurement of partial discharges using AC voltages and VHF/UHF sensors, for quality control during routine tests on such joints, is derived in the next Part 2. This protocol is the main novelty of this investigation.

The insulation of HVDC extruded cable system joints. Part 2: Proposal of a new AC voltage PD measurement protocol for quality control during routine tests

The review of materials, design and testing of joints for HVDC extruded cable systems provided in previous Part 1 paved the way to this Part 2 position paper by the DEIS HVDC Cable Systems Technical Committee, whose aim is to remedy the scarcity of existing standardized tests on joints. After a sound analysis, here routine tests are identified as the first practical target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers' experience for quality control of joints during routine tests and VHF/UHF wireless sensors as the best tool for such measurements in the noisy environment of factories. Thereby, a novel protocol for PD measurement using AC voltages and VHF/UHF electromagnetic sensors, for quality control during routine tests on HVDC extruded joints, is proposed.

The insulation of HVDC extruded cable system joints. Part 2: Proposal of a new AC voltage PD measurement protocol for quality control during routine tests

The review of materials, design and testing of joints for HVDC extruded cable systems provided in previous Part 1 paved the way to this Part 2 position paper by the DEIS HVDC Cable Systems Technical Committee, whose aim is to remedy the scarcity of existing standardized tests on joints. After a sound analysis, here routine tests are identified as the first practical target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers' experience for quality control of joints during routine tests and VHF/UHF wireless sensors as the best tool for such measurements in the noisy environment of factories. Thereby, a novel protocol for PD measurement using AC voltages and VHF/UHF electromagnetic sensors, for quality control during routine tests on HVDC extruded joints, is proposed.

Partial discharge detection on 320 kV VSC-HVDC XLPE cable with artificial defects under DC voltage

VSC-HVDC power transmission systems based on extruded cables are being commissioned with higher rated voltages and longer distances in recent years. In China, the rated voltage of commissioned VSC-HVDC cable projects has been increased from 160 to 200 kV, and then to 320 kV in the past 5 years. To investigate the feasibility of discovering insulation defects of an XLPE HVDC cable system using partial discharge detection under DC voltage, a test cable loop was built using the same batch of 320 kV HVDC XLPE cable used to construct the Xiamen ± 320 kV VSC-HVDC project. Artificial defects of insulation wound and semi-conductive layer tips were designed and constructed at the joint. The two types of defects were effectively detected by the HVDC partial discharge test, and the measured results show distinguished discharge characteristics. Suggestions for determining trigger level and partial discharge inception voltage under DC voltage for partial discharge detection of extruded HVDC cables are given.

Robustness Analysis of Classical High Voltage Joint Design Under High Voltage DC Stress

The demand for extruded HVDC cable systems for higher voltages has been increasing in different fields. This demand comes from the need for higher transmission capacity in applications such as integration of wind power, power system interconnectors and connection of offshore oil and gas platforms to land. Prefabricated joints are used to connect the cable delivery lengths. If badly designed, the possibility of breakdown in the joints is higher than in other parts of the system because of the number of interfaces and triple points between different materials. Due to the huge potential costs and the down-time imposed to the system in case of a joint failure, the robustness of the joints is of great importance. Two main approaches in the design of prefabricated joints for extruded cables exist in which the more recent one takes advantage of the nonlinear properties of modern field grading materials. In this work, we have examined the design challenges with focus on robustness issues in a 150 kV classical joint design without field grading material. The results show that with this approach, there are serious robustness issues which are challenging to overcome without field grading material.

Suppressing interface charge between LDPE and EPDM for HVDC cable accessory insulation

Space charges are likely to accumulate at the interface between insulation materials with different properties, which may lead to accelerated degradation of insulation systems. Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. Direct-fluorination can be applied to adjust the electrical properties of polymer insulation. Therefore, this paper intends to investigate the effects of direct-fluorination on the interface charge distribution between LDPE and EPDM composite insulation. The EPDM samples were fluorinated for 0, 15, 30 and 60 min, and LDPE for 0 and 30 min, respectively. The interface charge distribution were measured under 20 kV/mm. Obtained results show that the polarity of interface charge is the same with the voltage applied on EPDM sample, which can be verified by the Maxwell-Wagner-Sillars (MWS) theory. The fluorination for 30 min effectively suppresses the interface charge accumulation between LDPE and EPDM. However, excessive fluorination leads to significant accumulation of interface charge. Though fluorination of both dielectrics has the same effect with the single fluorinated EPDM on interface charge behaviors, more hetero-charges injection in the vicinity of cathode. The interface charge accumulation and dissipation process depends on not only the time constant but the surface state controlled by the fluorination time. As a consequence, the approximate direct fluorination can effectively suppress the interface charge accumulation of cable accessories and will make a potential application for the dc cable accessories.

Growth and partial discharge characteristics of electrical tree in XLPE under AC-DC composite voltage

The growth characteristics of electrical trees under DC and AC voltages are quite different. In order to investigate the influences of AC component in HVDC cable system on the electrical tree properties, the growth and discharge characteristics of electrical trees under AC-DC composite voltages were studied in this paper. The results showed that the AC component greatly accelerated the developing process of the electrical trees. The influences of the positive and negative DC bias voltages on the electrical tree growth properties were quite different. The growth rate under negative DC bias voltage was similar to that under pure AC voltage and pine-branch type electrical trees were more likely to form. In contrast, the growth rate of the electrical tree increased with the increase of the positive DC bias voltage and it was much faster than the negative DC biased one. More branch-like electrical trees would form under positive DC bias voltage. When the AC component decreased, the developing process of the electrical tree was fairly tough and it was easy to form bush-like electrical trees, which were the typical conducive electrical trees with fairly small discharges. Under the positive DC bias voltage, there was a fast re-growth process of electrical tree after the bush tree formed, and the positive DC bias voltage could promote the coming of the re-growth process. However, the detected partial discharge during this process was quite small. The test results indicated that it may pose a great threat to the safety of the cable insulation if there is a large AC component in the HVDC cable system.

Interface charge behaviors between LDPE and EPDM filled with carbon black nanoparticles

Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. The charges are likely to accumulate at the interface between insulation materials with different conductivity and permittivity, which may induce the occurrence of partial discharge and breakdown. Nanoparticles can be applied to adjust the interface charge behaviors. In this paper, carbon black (CB) nanoparticles were dispersed into EPDM with 0, 0.5, 1, 3 and 5 wt% respectively. The effects of nanoparticle doping on the interface charge distribution of LDPE and EPDM composite insulation were measured under −15 kV/mm. Furthermore, dielectric properties and DC conduction were introduced to discuss the suppression mechanism of carbon black nanoparticle doping. Obtained results show that the nanoparitcles are well dispersed in the EPDM. The permittivity increases with the nanoparticle fillgrade while the ac conductivity and tan(delta) at lower frequencies are lower than the base EPDM. The interface charge density can be suppressed to 0.5 C/m3 with 1 wt% carbon black nanoparticle doping, much less than the undoped and other doped groups. The mechanism of interface charge suppression is attributed to the decrease of conduction and permittivity mismatch by carbon black nanoparticle doping, which was proved by the dielectric spectroscopy and conduction current tests. The interface charge buildup and dissipation process depends on the time constant controlled by the doping proportion. As a consequence, the approximate 1 wt% carbon black nanoparticle doped EPDM can effectively suppress the accumulation of interface charge of cable accessories and will make a potential application for the dc cable accessories.

Qualification Test of a 80 kV 500 MW HTS DC Cable for Applying Into Real Grid

Until now, some countries including South Korea have made considerable progress in the development of HTS (High Tc Superconducting) power equipment. Superconducting power cable systems are the strongest candidate from the viewpoint of their use in the real grid owing to their high current capacity. Specially, in the future, high-current long-distance HTS DC cables will play an important role in power transmission systems because, in contrast to HTS AC cables, HTS DC cables experience nearly no loss. In this paper, the authors suggest a new testing method for the load cycle and suitable thermal cycles to meet the requirements for HTS DC cables to be applied to the real grid. A prototype 100 m/3.25 kA/80 kV HTS DC cable system was developed for the qualification test in South Korea. The qualification test was carried out, based on HTS experience, the international standard for HTS AC cables, and the standards for conventional HVDC cables. It was performed in the same manner as that for a conventional HVDC cable for 6 months in the KEPCO PT (Power Testing) Center. Since its successful qualification test, the 500 MW/80 kV HTS HVDC cable system will have been operated in the KEPCO real grid since 2015, to evaluate its practical requirements and confirm its technical feasibility. This paper presents the recommendations and results of the qualification test for HTS DC cables.

A protocol for space charge measurements in full-size HVDC extruded cables

This position paper, prepared by the IEEE DEIS HVDC Cable Systems Technical Committee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests.

Thermal-Electric Rating Method for Mass-Impregnated Paper-Insulated HVDC Cable Circuits

Compared to conventional ac cables, the current rating of HVDC cable circuits is more complicated because the permissible current is not only limited by the operating temperature, but also by the electrical stress within the insulation. Therefore the IEC60287 current rating standard is considered insufficient for some HVDC applications because it fails to include the stress limitation. This paper presents an analytical methodology to calculate an electrical stress-limited rating for HVDC cables, which considers the maximum dielectric stress as the current limiting factor. This paper represents the first part of a project which has comprehensively reviewed cable rating calculations for mass impregnated HVDC cable systems.