The September/October Issue of the Electrical Insulation Magazine has been released. Use the accordion headings below to explore this issue’s content, and visit the IEEE Xplore for full magazine access.
Material progress toward recyclable insulation of power cables. Part 1: Polyethylene-based thermoplastic materials
Novel EPR-insulated DC cables for future multi-terminal MVDC integration
HVDC and UHVDC polymeric cables: Feasibility and material development
Space charge accumulation in double-layer dielectric systems—measurement methods and quantum chemical calculations
Indispensable Insulated Conductors
Since its inception almost 140 years ago, the insulated conductor is still as important today as it was when it was invented and still represents the future of electrical energy transmission and distribution.
Climate change has introduced the world to major weather disturbances, including wildfires, wind storms, cyclones, hurricanes, typhoons, tornadoes, earthquakes, tsunamis, ice storms, floods and landslides. The ever-increasing numbers of these events is resulting in power system outages due to destruction of existing above ground power system structures. Politicians in Florida are looking to harden their regional power systems thus joining other utilities around the world such as Singapore Power which is 100% underground and Holland with 100% of its distribution system underground.
In the southern US, from August 2017 to September 2018, Hurricanes Michael and Irma collectively knocked out power for more than 6.5 million Floridians, some for more than two weeks. As a result, new legislation making its way through the Florida Senate seeks to prevent such outages in the future. In March 2019, State Senator Joe Gruters commented “You always think to yourself, why are we putting up those same poles?” According to Paul Griffin, Executive Director of Energy Fairness, “The numbers have shown when you have upfront investment in hardening the grid in a state like Florida you only get dividends on the back end.” The legislation would direct utility companies in Florida to develop a 30-year plan to “harden” the entire electrical grid in the State of Florida, meaning that a maximum of four percent of the grid would be converted to an underground system each year. The long-term nature of the plan would, in theory, prevent customers from seeing large rate increases.
Furthermore, in a recent study commissioned by CIGRE, WG B1.54 found that large disturbances can have a dramatic effect on both overhead and underground power systems resulting in major power interruptions. However, planning and design modifications prior to installation can minimize and even prevent disruptions to an underground power system. Underground power cables in most flood, fire and storm situations are subject to minimal damage due to their underground location. Normally cables are de-energized for safety reasons, inspected after a flood for example, and then re-energized without issue once the flood waters have receded. Seismic events, on the other hand, are possibly the most onerous for an underground cable system, but with seismic audits and implementation of mitigation measures during the planning stage, potential damage can be dramatically reduced.
Increased popularity of insulated power cables has also been associated with an interest in HVAC and HVDC submarine cables, particularly offshore windfarm applications. Major investments in offshore wind generation, mainly in Europe and Scandinavia, has sparked a major increase in the manufacture of submarine cable for windfarm export cables up to 230 kV and array or collector cables up to 66 kV. The first windfarm went in operation in Denmark in 1995 at Tuno Knob and since then 12 more windfarms have been installed with 554 wind turbines for a total capacity of 1,666.1 MW. There are four current projects underway and four more sites where permission has been granted.
Within the IEEE, ICC Working Group C11W is updating IEEE Standard 1120 “Guide for the Planning, Design, Installation, and Repair of Submarine Cable Systems”, to include a section on offshore submarine cable installation. In addition, IEC Standard 61400-3-1, “Wind Energy Generation Systems” will include hurricanes/tropical cyclones, floating ice and wave phenomena as well as an alternative offshore normal turbulence model. Furthermore, CIGRE TB 610 “Offshore Generation Cable Connections” specifically addresses offshore windfarm submarine cables and electrical systems.
In the US, an initiative is underway on offshore wind standards to develop a comprehensive set of consensus-based roadmaps to navigate the existing standards and guidelines to facilitate safe designs and an orderly deployment of US offshore wind energy. These standards will take into account the unique offshore conditions on the US Outer Continental Shelf and provide recommended practices that document industry “best practices” with a targeted completion for December 2020. Supporting these initiatives in the US are associations such as the AWEA (American Wind Energy Association), The Bureau of Ocean Energy Management, the DOE, The Business Network for Offshore Wind and NREL (National Renewable Energy Laboratory).
It is remarkable to think that the growth of insulated conductors is a result of Thomas Edison’s invention of almost 140 years ago. In his US Patent No. 251,552 dated December 27, 1881, Edison invented the underground “Street Pipes” asa means to connect two of his previous inventions, the generator and the incandescent lamp. And only 3 years later in 1884, the power system structure for New York City went underground when a law was passed to remove all overhead structures. The Edison “Street Pipes” operated at 110 Volts dc and consisted of jute-wrapped copper bars that were inserted into an iron tube and the interstices filled with a bituminous or wax compound. Present day power cables are amazingly similar in spite of the passage of time. However, considerable progress has been made with improved materials, allowing for cables to be produced more economically and for the development of both HVDC and HVAC power cables.
During the intervening years, the prominent designs of underground cables were fluid-impregnated-paper insulated cables that were perfected by Luigi Emanueli of the Pirelli Company in 1917, and cross-linked polyethylene (XLPE) insulation that was developed by the General Electric Company in 1963. In Japan, fluid-filled cables were the most prominent design until 1967, when the quantity of installed XLPE insulated cables exceeded that of fluid-filled cable because of the rapid progress made in polymer chemistry after World War II. By 1991 more than 90% of all cables installed in Japan were XLPE-insulated cables.
Just as importantly, in the latter half of the 19thcentury, the transmission and distribution of telegraph and telephone systems led to the development of rubber electrical insulation technology. In 1837, the telegraph was invented by Samuel Morse and the first overhead telegraph lines were installed between Washington DC and Baltimore in 1844. Installation of these lines for underground and underwater applications followed. Natural rubber provided the combination of ozone, moisture and temperature resistance whether the cables were placed overhead, buried or submerged in fresh or salt water. By 1868 over 200 rubber-insulated telegraph cables had been installed in the United States, Canada, Panama and Egypt. Alexander Graham Bell invented the telephone in 1876, so moisture resistant natural rubber cables were widely used. Later in 1882, Thomas Edison used rubber insulated cables on Pearl Street in New York. By 1889 rubber insulated cables were operating at the Seymour Electric Light Company at 1100 Volts and by 1898 a 1000kcmil submarine cable was laid across the Gowanus Canal by the Coney Island and Brooklyn Railway. By 1925 rubber-insulated cables were rated for voltages up to 7,500 Volts and by 1962 producers in Europe and the US were offering EPR insulated power cables that are now available up to 138 kV.
The main considerations in the industry-wide conversion from lead-covered, fluid-impregnated paper systems to polymeric insulated systems were:
- Environmental concerns with the lead sheaths used on fluid-impregnated paper insulated cables
- Reduced maintenance costs for polymer insulated cables dielectric insulation
- Loss of expertise required for installing fluid-impregnated paper insulated cables
- Reduced installation costs for polymeric insulated cables
- No fluid leaks to locate and repair
- Weight reduction (no lead sheath required), allowing for the installation of longer cable lengths
- Reduced risk of fire during earthquakes
- Reduced dielectric losses
Despite the technological success of underground power systems since the 1800’s, the major impediment to putting many systems around the world underground has been financial. The greater initial cost of underground systems was a big disincentive when compared to overhead technology. But recent advanced and improved installation methods for underground power systems have made the undergrounding of insulated conductors much more competitive. Many utilities worldwide are now showing increased interest in longer cable lengths for both AC and DC applications.
Additional future growth for the insulated conductor includes increased installation lengths, subsea substation applications, subsea cable repair, improved dielectric materials including nanocomposites leading to a more efficient and economical cable production and insulated cable systems resistant to major weather disturbances such as earthquakes.
Although the insulated conductor was invented in 1881, it was wholeheartedly adopted by New York City in 1884, and is now used around the world in an ever-increasing number of new applications, the insulated conductor shows no sign of diminishing importance. Plus, changing weather patterns and weather-related disturbances represent more opportunities for unprecedented growth of the insulated conductor, both in its traditional uses, as well as in yet-to-be discovered applications.
From The Editors
From the Editors’ Desk
Behavior and properties of materials for insulation of high voltage power cables get more and more attention as the operating voltage levels of cable systems steadily increase and the environmental constraints impact their design. We present in this issue four articles describing and discussing new trends related to this subject.
The first article entitled “Material Progress towards Recyclable Insulation of Power Cables: Part 1 – Polyethylene Based Thermoplastic Materials” is jointly authored by Xingyi Huang, Jun Zhang, Pingkai Jiangrepresenting Shanghai Jao Tong University in China, and Toshikatsu Tanakaof Waseda University in Japan. The article is dedicated to the eightieth birthday of Professor Toshikatsu Takada. This is the first of the authors’ two articles focusing on materials for recyclable power cable insulation. The driving forces behind the attempts to alternative replacements to the currently used cross-linked polyethylene (XLPE) based insulation are multiple. Firstly, this type of insulation has low recycling capability. Secondly, the cross-linking process undesirably affects cable production efficiency and, thirdly, the by-products of cross-linking reaction have an impact on cable performance, particularly important in the case of high voltage direct current (HVDC) cables. The article gives a comprehensive review on the development of polyethylene (PE) based thermoplastic cable insulation. After giving a brief historic introduction, the article summarizes the relationship between supramolecular structure and electrical properties of PE. Then the authors review various approaches developed in the past decades towards improving the electrical properties of PE, including use of new catalysis, copolymerization, chemical modification, multi-phase blending, and addition of various additives and nanoparticles. For each approach, the structure-property relationship of the corresponding PE materials is discussed. The article gives an emphasis on electrical property enhancement of PE by using nanotechnology (e.g., introducing nanoparticles). The authors also emphasize difficulties related to the introduction of nanotechnology because of dispersion problem in large scale production and the poor understanding on the enhancement mechanism.
The second article entitled “Novel EPR Insulated DC Cables for Future Multi-terminal MVDC Integration” is jointly authored by a team of researchers representing academic and industrial institutions from USA and Japan. It is authored by Mattewos Tefferi, Zongzi Li and Yang Cao from the University of Connecticut, Storrs, USA, Hiroaki Ueharafrom Kanto Gakuin University, Yokohama, Japan, and Qin Chen from GE Global Research Center, Niskayuna, NY, USA. The authors argue that despite the emerging opportunities, significant challenges persist for multi-terminal medium voltage DC cabling technology, mainly residing in the insulation due to performance deterioration by space charge accumulation. Potential risks of polarity reversal during the operation and appearance of fault transients are the factors considered in further development of space charge ageing prone insulation, greatly needed for enhanced performance and reliability. A novel high performance EPR is developed with superior space charge resistance to polarity reversal, well suited for multi-terminal DC grid for renewal integration, harsh environmental and marine electrifications. Characteristics of this novel DC EPR are presented along with the state of art in DC XLPE cabling. In addition to steady-state conduction, their space charge behaviors under thermal gradient are presented as obtained by using a modified pulsed-electroacoustic system. Moreover, the role of degassing process on DC characteristics of both XLPE and EPR is elucidated, which indicates a largely suppressed space charge in EPR under thermal gradient, and more importantly, during polarity reversal, when compared to DC XLPE. The relevance of the findings for applications to extruded MVDC cable insulation is also discussed.
The third article presents detail considerations on design criteria and material requirements for reaching high feasibility of ultra high voltage DC (UHVDC) cable links. It is entitled “HVDC and UHVDC polymeric cables: feasibility and material development” and authored by Gian Carlo Montanari and Paolo Seri of the University of Bologna, Italy. Gian Carlo Montanari is also affiliated with Texas University at Austin, USA. DC cables of very high ampacity for transmission of electric power should be technically feasible and as lossless as possible. The use of such cables is one of the major technical and economical drivers for the present and future energy contest and provides in many cases the preferred solution compared to overhead lines as well as the compulsory choice in case of submarine links. Regardless of the type of material used for electrical insulation (oil-paper or oil-paper-polypropylene or solid XLPE and PP), the trend to go towards higher voltage and power must match with cable feasibility which, in turn, might require the development of new insulating materials suitable to work at high fields and temperatures. The authors present an iterative algorithm to investigate the feasibility of the design of (U)HVDC cables, based on data that can be obtained from accelerated life tests as well as from space charge and conduction current measurements. It is shown that the algorithm can predict substandard feasibility that present materials (specifically cross-linked polyethylene) might not be capable to guarantee when going towards higher voltage and ampacity levels. The model is then capable to provide indication on how electrothermal insulation properties need to be modified for manufacturing polymeric cables for increased voltage and current levels, thus driving the development of existing and new materials.
The fourth article in this issue is entitled “Space Charge Accumulation in Double-layer Dielectric Systems- Measurement Methods and Quantum Chemical Calculations” and authored by Tatsuo Takada, Tsuyoshi Tohmine, and Yasuhiro Tanaka from Tokyo City University, Japan, and Jin Li from Tianjin University, China. It concentrates on elucidation of space charge behavior in a double-layer material system (EPDM-LDPE) that models the real situation in HVDC cable accessories (joints and terminations). The experiments are done by using the pulse electro-acoustic (PEA) method and the newly developed current integration method (Q(t)), the latter of which has a great potential for future industrial applications. The presented results indicate that the space charges accumulating at the interface between two dielectric materials originate from external semiconducting (SC) or metal (Al) electrodes, which is confirmed by quantum chemical calculations (QCC) that provide detailed information on carrier (electrons and holes) electron energy levels in LDPE and EPDM, height of barrier for carrier injection from the electrodes, trap depths in the materials’ bulk and formation of electric double layers at the interface. It is especially emphasized that the performed measurements and calculations point to the difference between hole and electron trap distributions as the responsible contributor to the polarity dependent charge injection and helps to better understand the mechanism of interface charge formation. It is also underlined that presented in the article investigations allow seeking new ideas on how to design cable accessory insulation systems.
News From Japan
Online Insulation Monitoring System for Rotating Machines
The necessity of diagnosis and monitoring of electrical insulation integrity has been steadily growing since many industries want to make the operational period of power apparatus as long as possible. Furthermore, natural renewable energy resources such as solar power and wind power are recently growing rapidly. However, the outputs from such renewable energy resources tend to fluctuate significantly. To balance such fluctuating outputs with the demands, thermal power generation is subjected to rapid load changes and frequent start-and-stop operations more often than before. This would cause severe degradation of electrical insulation in steam turbine generators. From these backgrounds, the importance of insulation monitoring has been increased significantly.
Partial discharges (PDs) are one of the major concerns to both manufacturers and users of high voltage powerequipment. In the “News from Japan” column of the May/June 2009 issue , it was reported that Mitsubishi Electric Corporation, Tokyo, developed a patch or microstrip antenna suitable for an online PD monitoring system for high voltage rotating machines .
The present short article introduces the installation of the antenna to an online monitoring system. The PD detection system using the microstrip antenna is the same as that reported in [1, 2], although several system modifications and PD criteria to trigger an alarm have been added.
Figure 1 shows a schematic diagram of the system, which consists of two antennas, a PD detector, and a monitoring personal computer (PC). The antennas have a narrow bandwidth around 1.8 GHz, which is a typical characteristic frequency for detecting PDs occurring in the vicinity of stator coils, so that the antennas can catch only PD signals selectively among various noises. The antennas are set on the inside surface of the generator enclosure, away from the edges of a stator coil where a high voltage is applied. Therefore, no bad effects are exerted on the insulation of the stator coil. Furthermore, the removal of a rotor is not needed to install the antenna system into an existing generator. When PD signals are detected by one of the two antennas, they are transported to the PD detector and noise is removed. Then, the signals are converted to digitized data to send them to the monitoring PC. When the PD intensityreaches a certain preset value, an alarm will be issued. Because the PD signals can be transmitted through the Internet, the insulation condition can be monitored in a remotesite.
Figure 2 shows a test bar, which simulates a real stator coil in a turbine generator. The arrangement of a PD detection system is also shown. Note that the length in Figure 2 is in mm. The specifications of the test bar, including the materials and dimensions of the coil conductor, mica tape, semi-conductive layer, and stress relief layer are the same as those of a real stator coil. Using this test bar, researchers in Mitsubishi did several tests to ensure that the antenna system worked as a PD monitoring tool. The tests include those to examine the effects of insulation damages on PD characteristics and those to know possiblelocations and the temperature dependence of PDs.
As an example, the test to examine the effect of abrasion of insulation surface was conducted as mentioned below. That is, stator coils in a turbine generator are always subjected to electromagnetic vibrational forces with a potential risk that their surfaces are abraded. Figure 3 shows a cross-section of the test bar used to examine the above point. Two metal plates were put on the test bar to simulate the stator core which is always at the ground potential during operation. Part of the main insulation under one of the two metal plates was abraded, as shown in Figure 3, to a depth of 6, 30, or 52% of the total thickness of the main insulation and the semi-conductive layer.
Figure 4 shows the result. In this case, the surface was abraded by 52% as a result of planar abrasion. An ac voltage of 10.5 kVrms was applied to the upper metal plate. The intensities and voltage phases of PDs were recorded for 5s. Figure4(a) shows the relation between the PD intensity and the PD phase. Here, the intensity is represented by a unit of dBm defined as 10log10P, where P is the quotient of the PD power received by the antenna, divided by 1 mW. On the other hand, the phase is defined as the angle 0 of the sinusoidal ac voltage, at which the PD was detected. Figure 4(b) shows the relation between the number of occurrences of PDs and their intensities.
It is indicated from Figure 4(a) that PDs occur in relatively narrow phase ranges, near 45° and 135°, where the applied voltage is approaching a positive or negative maximum, showing nearly square intensity-phase patterns. Figure 4(b) indicates that about 1.5×104 PDs with intensities around -90 dBm occurred in the measurement period of 5 s. In the monitoring system, the occurrences of 300 PDs in 5 s or 60 pps (PD pulses per second) was used as a judgment criterion. This means that PDs with an intensity of -72.1 dBm is critical. Such a critical intensity of PD is calculated automatically by software for each measurement, and the temporal change in critical intensity can be used as a measure for periodical maintenance of generators. Here, the above values are for 60 Hz ac voltages and the critical number of PD occurrences is to be changed in proportion to the frequency; 50 pps for 50 Hz ac in the above-mentioned case.
Figure 5 shows the dependence of the PD intensity (at 60 pps) on the depth of the planar abrasion. It is clearly indicated that PDs can be detected when the abrasion reaches 30% of the insulation thickness and that the PD intensity increasesabruptly when the abrasion becomes 52%. This means that the abrasion of the main insulator of a generator can be monitored by watching the intensity and the number of PDs.
The researchers in Mitsubishi conducted similar experiments using various potential PD sources like concave hollows and embedded metal plates in the main insulation. They also examined the positions where PDs occur. For this purpose, they replaced the upper electrode shown in Figure 3 by a transparent one, consisting of a glass plate with a vacuum-evaporated metal electrode. They observed weak luminescence induced by PDs using a digital camera with an image intensifier for 0.5 s.
Figure 6 shows the luminescence images observed when ac voltages of 8.4 and 10.5 kVrms were applied to the transparent electrode on a rectangular hollow. In this case, the depth of the hollow was 52% of the insulation thickness with the top semi-conductive layer inclusive. If the hollow is planar, the electric field in the hollow would be uniform. Therefore, PDs are induced almost uniformly, which is consistent with two figures in Figure 6.
Mitsubishi has installed 76 sets of the insulation monitoring systems up to January 2019 (Figure 7) in air-cooled and hydrogen-cooled turbine generators. The world’s first hydrogen-cooled 900 MVA turbine generator, which was introduced in this News from Japan column in the January/February 2018 issue is also equipped with this system.
This article was completed in cooperation of Mr. Shinichi Okada and Dr. Hirotaka Muto of Mitsubishi Electric Corporation.
- Ohki, “News from Japan”, IEEE Electr. Insul. Mag., Vol.25,No.3, pp.60-61,2009.
- Muto, Y. Kaneda, H. Aoki and O. Hamamoto, “On-line PD monitoring system for rotating machines using narrow band detection of EM wave in GHz range”, Int. Conf. Condition Monitor. Diagn., pp.1093-1096, Beijing, China, 2008.
- Ohki, “News from Japan–Development of a High-Performance Indirectly Hydrogen-Cooled Turbine Generator”, IEEE Electr. Insul. Mag., Vol.34, No.1, pp.61-63,2018.