Surface partial discharge in hydrogenerator stator windings: Causes, symptoms, and remedies
Nicolas Dehlinger; Greg Stone — Xplore Link
Fault localization and analysis for a damaged hydrogenerator and a proposal to improve the standard for generator commissioning tests
Asghar Akbari; Mohammad Rahimi; Peter Werle; Hossein Borsi — Xplore Link
Prospects for increasing supply voltage and design of electrical field rotating machine windings supplied from power electronics
Gian Carlo Montanari; Paolo Seri; Greg Stone — Xplore Link
Dielectric properties and partial discharge endurance of thermally aged nano-structured polyimide
Tao Han; Andrea Cavallini — Xplore Link
Greetings fellow DEIS members. It will be my honor to serve as your president during 2020. I want to thank the members of the Administrative Committee for putting their trust in me and I would also like to thank our past president Reimund Gerhard for his help and guidance over the last 2 years of my apprenticeship. I should also note and apologize for the fact that my initial presidential editorial is late; it should have been included in the January/February issue of the Magazine.
I want to take this opportunity to share with you the results of the recent revision to our Society’s strategic goals. All IEEE Societies are reviewed by the IEEE Technical Activities Board (TAB) every 5 years and our last review was completed in 2018. As part of that review we promised to update our existing strategy statement and, after considerable discussion, a revised set of goals was agreed to and approved by AdCom last year. The revised Strategic Goals statement is as follows:
DEIS Strategic Goals
(I) Within the Society’s Field of Interest, DEIS will be the prime source for dielectricians worldwide and will be internationally recognized as the world’s leading learned/technical authority.
(II) Within the IEEE, DEIS will be seen as the primary center of excellence for all activities directly related to our Field of Interest.
(III) DEIS will respond rapidly and effectively to the latest developments in areas encompassed in our Field of Interest.
(IV) DEIS will continually improve the quality of services and offers to our members and to the world-wide community of dielectricians.
(V) DEIS will maintain its commitment to diversity and inclusion with all its aspects, and to professional ethics standards of the highest level.
Actions Supporting Our Strategic Goals
- We will seek relationships with technical groups and learned societies all over the world with similar fields of interest.
- We will offer conferences, workshops, schools, discussion sessions, and other events worldwide within our Field of Interest.
- We will publish peer-reviewed original papers and survey articles of high quality and of significant interest to our worldwide community.
- We will remain embedded in the IEEE with a unique identity and will interact with other IEEE Organizational Units for mutual benefit.
- We will acknowledge and reward volunteers who actively participate in ensuring that the Society achieves its goals, as well as dielectricians who made seminal and significant contributions to our Field of Interest.
The previous version of our strategic goal statement was considered unfocused because it contained 11 vision statements, 6 overall aim statements, and 9 measure-of-success statements. We now have a simplified list of 5 goals and 5 actions to support those goals. Each administrative committee of the Society also has a list of recommended action items that directly support the overall plan, but I will spare you that level of detail. So, where do we want to go in the future and how does this goal statement guide us? What can you do to help the Society achieve these goals? Let’s examine each point separately.
(I) Becoming “internationally recognized as the world’s leading learned/technical authority” in the field of dielectrics is an admirable goal but it can only be achieved by supporting our members who do the work, publish the results in our Transactions, and present at our conferences. By serving the next generation of technical experts, we ensure our survival and relevance. We need to be inclusive and reach out to anyone who can be considered a dielectrician (engineers, physicists, chemists, etc.). You help us achieve this goal by actively participating in Society activities so I encourage you to take an active role. Please feel free to reach out to me directly if you are looking for volunteer opportunities.
(II) Being seen as a center of excellence within IEEE can be accomplished by actively interacting with other Societies and Councils to address their needs related to our field of interest. We already do much of this work through participation in our Sister Society group and membership in several Councils (Nanotechnology, Sensors, and Superconductivity). We are always looking for volunteers who want to more actively participate in Council activities. Let me know if you are interested.
(III) Keeping up-to-date on “the latest developments in areas encompassed in our Field of Interest” is a job that requires constant vigilance by everyone. Our Technical Committees have the primary responsibility to follow new work, but we do not have committees covering all aspects of dielectrics. We need to be made aware when something new comes to light that requires our attention. Then we can form new Technical Committees as needed. If you discover a topic within our Field of Interest that is not being adequately addressed, please let me or our VP-Technical, Davide Fabiani, ([email protected]) know so that we can take action.
(IV) Our society exists to serve our membership and humanity in general. Therefore, continually improving “the quality of services and offers to our members and to the worldwide community” is an obvious goal. We include this in the Strategic Goal statement because we want to be constantly reminded that providing these services is an ongoing process. We need to evolve because our constituents’ needs are constantly changing. Methods of communication and dissemination of information available today were unheard of only a few decades ago. What we do today may be totally inadequate tomorrow. When we are not meeting the needs of our members, we need you to let us know.
(V) The final goal item relates to diversity, inclusion, and ethics. These are areas where the IEEE has insisted that its organizational units adhere to a high set of standards that are posted on the IEEE website. Our Society will fully support these standards and I expect all of our members to enthusiastically support them. It is simply the right thing to do.
That is a brief overview of my thoughts relating to our Strategic Goals. I encourage all members to do their part in helping the society meet these goals. If you have any comments (or complaints) please do not hesitate to contact me directly. We are one of the smallest societies in the IEEE but our field of interest covers a vast technical area that is critical to the success of many other societies. I believe we can do great things when we all work together, so please consider taking an active part in the operations of the DEIS.
From The Editors
From the Editors’ Desk May-June 2020
This issue of the Magazine brings a series of articles dedicated to insulation systems of electrical machines.
The first article entitled “Surface Partial Discharge in Hydrogenerator Stator Windings – Causes, Symptoms and Remedies” is jointly authored by Nicolas Dehlinger and Greg Stone from Iris Power – Qualitrol. This paper reviews the mechanisms that may lead to surface partial discharges, discusses how they can be detected and addresses possible corrective actions. Surface discharges usually occur on hydrogenerator stator windings as well as other types of air-cooled machines on the semiconductive and grading coatings of line-end coils/bars, as well as in the endwinding, where spacing at phase breaks is inadequate. Their presence can be marked during visual inspections. On-line PD measurements and sometimes off-line tests (PD, corona probe, blackout) can also identify the presence of surface PD. Causes for surface PD are multiple, going generally from poor manufacturing to improper design or installation issues. While surface PD will attack and weaken the insulation and, if uncorrected, lead to a ground or phase-to-phase fault, the degradation process is usually very slow, usually developing for many years. Surface PD also creates ozone within the generator, which degrades most metallic and organic components of the generator and can lead to unhealthy ozone levels for plant staff. The article presents repair methods for semiconductive and grading damages as well as deterioration at phase breaks due to improper spacing.
The second article in this issue is on “Fault Localization and Analysis for a Damaged Hydro Generator and a Proposal to Improve the Standard for Generator Commissioning Tests”. It is authored by Asghar Akbari and Mohammad Rahimi fromK. N. Toosi University of Technology in Tehran, Iran together with Peter Werle from Leibniz University and Hossein Borsi from DESC Electrical Engineering, both in Hannover, Germany. This article reports on the process used in the root cause analysis of a short circuit fault in the stator of a generator at large hydroelectric power plant where, despite a prompt operation of the machine protection system at the early moments of the failure, losses and damages to the generator became very severe. A methodology allowing to recognize the initial point of the fault and to calculate transient electromagnetic forces caused by short circuit in the stator is introduced and the analyses show that in the first cycles after the start of the fault and before opening the generator breaker, a strong repulsive shock force caused by current passing through the bars in some slots, moved the upper bar into the air gap. This machine had two years earlier passed the commissioning tests for withstanding transient against three-phase short circuits at its terminals. Authors of the article argue that the recommendation of IEEE standard C50.12 for ensuring withstand of generators against transients is not appropriate for machines with multiple windings per phase and a new procedure for commissioning tests of large generators with such types of windings is required. A proposal of some modification is drafted with a call for more investigations on this topic.
The third article reports on “Prospects for increasing supply voltage and design electrical field in rotating machine windings supplied from power-electronics” and is jointly authored by Gian Carlo Montanari from Florida State University, USA, Paolo Seri from University of Bologna, Italy, and Greg Stone from Iris Power – Qualitrol, Canada. The increasing, widespread use of power electronic converters and inverters cause overstresses to electrical insulation and can cause accelerated aging, loss of reliability and, at the very end, premature failure of electrical asset components. IEC standards already provide guidance on how to design insulation systems and perform type, quality and acceptance tests on rotating machines controlled by a pulse-width modulation supply, dividing insulation in two families, Type I and Type II, and devising specific and different testing approaches for both families. Also, application limits for such standards are established, i.e. usually below 700 V for most machines using Type I and usually above 700 V for those using Type II insulation. Since technology evolution is more rapid than the time to develop standards, the distinction between Type I and Type II insulation, in terms of voltage limits for their application, is blurred by the need of increasing power density, thus voltage and electric field, in electric drives for electrified traction. The article shows that most likely partial discharges will become unavoidable in upcoming, power-dense drives and, therefore, there is not too much of a chance that present insulating materials and insulation system design can effectively address this technological challenge. Relevant calculations, evaluations and speculations are presented.
The fourth and last article of the issue is entitled “Dielectric Properties and Partial Discharge Endurance of Thermally Aged Nano-structured Polyimide”. It is authored jointly by Tao Han from Tianjin University, China together with Andrea Cavallini from University of Bologna, Italy. It presents results of endurance tests, which show that the film used in this work exhibits an outstanding tolerance to temperatures as high as 270°C. The permittivity, conductivity, and PD withstand show only moderate changes after over 800 hours of testing. Inverter-fed motors manufactured using such film can thus operate at elevated temperatures throughout their service life with minor effect in their ability to withstand PD activity. However, above 320°C, the partial discharge endurance decreases substantially, despite the PDIV does not change remarkably. SEM observations of the aged films suggest that structural changes inside the nanostructured layers of the material are the cause of this reduction. Clearly, PD endurance tests on virgin samples cannot replicate these modifications. Therefore, when establishing international standards aimed at ranking competing corona-resistant insulation systems, it would be important to require that partial endurance tests must be carried out on thermally aged insulation models.
News from Japan
Ocean Thermal Energy Conversion Development Center in Okinawa
Japan is an archipelagic country surrounded by oceans. This means that the good use of sea would provide great advantages to Japan. One possible candidate for this is the ocean thermal energy conversion (OTEC). About half of the energy reaching the earth from the sun is absorbed by the surface of the earth or its ocean and land. Because of that, the yearly average temperature of seawater in the shallow surface mixed layer (water depth: 30 to 100 m) in the tropical and subtropical areas reaches 26 to 29°C. In contrast, the temperature of seawater at a much deeper position, around 1000 m deep, is nearly constant at 4 to 6°C throughout the entire globe, as shown in Figure 1.
Therefore, if we can generate electric power using this difference in seawater temperature between the shallow sea and the deep sea, we can gain a tremendous amount of energy. This idea became well-known by Jules Gabriel Verne’s famous scientific fiction, “Vingt mille lieues sous les mers – Twenty thousand leagues under the sea”. That is, the Captain Nemo of Submarine Nautilus in that scientific fiction suggested the possibility of generation of electricity by the use of the temperature difference between two electrodes sunk at different depths under the sea . The idea has now developed to OTEC.
For OTEC, it is desirable that the seawater temperature in the surface mixed layer is high. Regarding this, major OTEC research facilities in Japan are in Okinawa Islands, which are in the southernmost region of Japan. In this short article, the status quo of one of these facilities is briefly introduced.
If we consider the meaning of OTEC widely, the idea of Jules Gabriel Verne that used the Seebeck effect, would be a part of OTEC. However, presently, a thermal engine is used in OTEC to generate the electricity. This concept of OTEC has already a long history. As shown in Figure 2 , the first boom of OTEC came in the second half of the 1970s, since the oil crisis happened in 1973 and 1979. Then, in accord with the subsequent decrease in oil price, research activities on OTEC became inactive. However, with the recent re-boost of oil prices, OTEC is now in its second boom. This second boom is supported mainly by the recent progress of the following two technologies. First, the technology to retrieve thermal energy effectively from two thermal sources with a small temperature difference has been advanced significantly. That is, technologies related to the geothermal power generation that uses low-temperature steam or low-temperature hot water have actively been developed. In other words, the use of low-temperature thermal waste has become active in many industrial plants. Secondly, technologies for the building, construction, and installation in deep sea have been developed significantly in relation to the undersea oil field mining.
In OTEC, the price of fuel, which is seawater, is free. In contrast, the construction and installation costs of a thermal cycle system are very high. Therefore, a small system would not be economically adequate. For systems of less than several hundred kW, the complex utilization of deep seawater is recommended. For much bigger systems with an output power of 100 MW or more, the energy harvesting cost would be around 10 US cents per kWh, which is cost-competitive with other renewables.
The electric power generation system that has been researched actively is a closed cycle system. Its principle of the power generation is shown in Figure 3. It is essentially the same as the principle used in a thermal power plant or a pressurized water reactor (PWR) in a nuclear power plant. That is, the system is composed of an evaporator, a turbine, a condenser, two water inlets and outlets for cold and warm water, pipelines that connect these devices, and several pumps. A generator is directly connected to the turbine. While the operating fluid is a water in a PWR, fluids with low boiling points such as ammonia and fluorine-based refrigerants such as HFCs or HFOs are used for OTEC.
Both an onshore plant and an offshore floating plant have been proposed for OTEC. Although the onshore type has various advantages such as the easy maintenance, its inlet pipeline for the cool water becomes inevitably very long. Therefore, due to the weight limit, we cannot use pipelines with large inner areas. For this reason, the onshore type is suitable to comparatively small generating plants with an output power less than 1 MW. In contrast, the offshore floating type is believed to be adequate to large plants above 1 MW.
In Japan, a 100-kW-class demonstration facility shown in Figure 4 was built in Kumejima Island, Okinawa. This island is located in the southernmost part in Japan as shown in Figure 5, which means that the island’s climate is subtropical and that the temperature difference between the shallow and the deep seas is suitable for OTEC . This facility can pump up 1.3×107 kg of deep seawater daily from a point of 612 m deep and 2,300 m offshore. The temperature of the pumped water is 9°C throughout a year.
One important thing of this facility is that the ocean’s thermal energy and the nature of deep seawater are comprehensively utilized not only for the energy conversion but also for other various purposes. Namely, in this “Kumejima model”, the abundance in nutrition and the cleanliness of deep seawater play a very important role. The sunlight cannot reach the deep sea. Therefore, the deep seawater does not contain phytoplankton, but contains abundant inorganic minerals. This clean and nutritious water is used for cultivation of prawn and sea grapes. As shown in Figure 6, the yearly financial total of products relating to this deep seawater project reached nearly 2.5 × 109 Japanese Yen, which is more than 22 million US dollars, and is almost the same as the total of agricultural products or that of fishery products in this island with a population of about 8,000.
This article was completed with the cooperation of Mr. Shin Okamura and Mr. Ben Martin of Xenesys Inc.
 Jules Gabriel Verne, “Vingt mille lieues sous les mers”, 1870.
 Based on the data retrieved from https://www.ngb.co.jp/ip_articles/detail/994.html
(Jan. 19, 2020) (in Japanese).
 Based on the data retrieved from https://www.nedo.go.jp/content/100881510.pdf (Jan. 19, 2020) (in Japanese).