Rotating Electrical Machines

Study Committee A1 covers all aspects of rotating electrical machines associated with power generation ranging from research, development, design, manufacture and testing of new machines to commissioning, operation, condition assessment, maintenance, refurbishment, upgrades, conversion, storage, and de-commissioning.

The Study Committee has 24 Regular Members, 6 Additional Regular Members and 14 Observer Members representing 38 countries across all continents. They are complemented by over 150 experts from utilities, manufacturers, consultants, research centres and universities who all contribute to SC A1 activities including working groups, committee meetings, symposiums, colloquiums, tutorials, and webinars.

Further details of the organisational set-up, strategic directions and current activities are given below.

Highlights of 2021

It was another difficult year for all as the COVID pandemic continued to dominate and disrupt our daily lives and routines. This resulted in limited progress on many fronts particularly in the closure and report submission for working groups in the final stages of completion. In 2022 the focus will be to clear this backlog and move forward on newer topics.

There were still however some notable highlights:

2021 Virtual Centennial Session

Due to the prevailing travel restrictions, the 2021 session took place in the form of a Virtual Session broadcast from studios in the Palais des Congrès. The meetings were chaired by a small team of SC A1 members in Paris and a similar team remotely on-line. Professional companies were engaged to schedule the presentations and organise the studio and online participation.

The Study Committee A1 General Discussion Meeting comprised two four-hour sessions on 26th and 27th August 2021. The discussion was based on the prepared contributions received in response to the questions raised in the Special Report resulting from the technical papers presented in the 2020 e-session.

19 prepared contributions were presented, complimented by 2 presentations by the Cigre New Generation Network, 2 invited presentations on topics of interest to SC A1 members, 3 working group presentations, and 2 spontaneous presentations. All presentations and contributions were carefully thought out, well prepared and of high technical interest.

The success of the Virtual Centennial Session was testament to the tremendous effort, dedication and engagement by Cigre Central Office, technical support teams and CIGRE members. Thanks to all who supported this event under such challenging circumstances.

Ljubljana Symposium, Slovenia

The symposium took place from 21st to 24th November 2021, as a hybrid event, and was supported by SC A1 with 4 submitted papers, and a tutorial. Two of the papers were selected for presentation. SC A1 chaired session 15 remotely on 23rd November with SC members also present at the venue.

Tutorials

The following tutorials were presented at the two events mentioned above:

• Virtual Centennial Session, 18th August 2021: “Impact of Flexible Operation on Large Motors” presented by John Doyle and Tom Beckwith, based on the work of WG A1.54.
• Ljubljana Symposium, 21st November 2021: “Challenges in the Calculation and Design of Large Power Hydro Generators in the 21st Century” presented by Johnny Rocha.

Annual SC Meetings

A review of current working group status was held using GoToMeeting on 19th August 2021, and the Annual General Meeting followed on 24th August 2021. The meetings were well attended with useful discussions regarding working group progress, and more specifically on the alignment of the current SC organisational set-up and strategic directions with current power generation trends and priorities for 2022 (more details on this below). No new working groups were proposed this year – we agreed to focus on the progress and completion of those in work given the ongoing circumstances.

Future activities

The priorities for 2022 will be to close out the current working groups which are in the final stages of completion. This means submitting the final reports / technical brochures for publication although the actual publishing date will be determined by Central Office according to the Electra content schedule.

Additionally, the focus will be to consolidate and progress the work of the remaining working groups listed in Table 1. New working groups will be considered during the 2022 Annual General Meeting.

The organisational set-up and strategic directions will be reviewed in 2022 and aligned to better reflect the evolving role of rotating electrical machines in power generation and control.

The next Study Committee Meetings will take place during the Paris 2022 Session scheduled from 28th August to 2nd September 2022. The Preferential Subjects for the meeting in Paris will be as listed below. The Group Discussion will be based on the Special Report to be released in 2022 based on the new papers submitted for this session. We accepted 30 abstracts, the papers for which should be submitted soon for review. This year will see the introduction of a more formal review process based on the ConfTool platform.

PS 1: Generation Mix of the Future

• Impact and effect of increasing renewable power mix on new and existing generators, generator auxiliaries and motors
• Synchronous compensator and high inertia machine design and performance for supporting power generation networks
• Adaptation of international standards for electrical machine design & performance to current power grid requirements

PS 2: Asset Management of Electrical Machines

• Experience with refurbishment, replacement, conversions, power up-rating and efficiency improvement of generators
• Novel techniques to overcome known operational and design problems
• Optimised condition monitoring, diagnosis, prognosis, and maintenance practices to improve reliability and extend operational life of conventional plant and in new volatile grid conditions, including data handling and digital modelling

PS 3: Developments of Rotating Electrical Machines and Operational Experience

• Latest design, specification, materials, manufacture, maintenance and performance and efficiency improvements
• Operational experience: Failures, root cause analysis, recovery options, cost, and time reduction initiatives
• Evolution and trends in designs of rotating electrical machines for renewable generation (wind, hydro)

If you have any comments or suggestions or would like to know more about the activities of Study Committee A1, please do not hesitate to contact the Chair or Secretary.

Organisation and Strategic Directions of SC A1

SC A1 is organized into 4 technology groups based on the application of rotating electrical machines:

• Turbogenerators
• Hydro-Generators
• Large and High Efficiency Motors
• New Technologies

Each of these groups is concerned with the international exchange of information, knowledge, practice, and experience on rotating electrical machines.

Key focus areas within Study Committee A1 are embodied in the strategic directions which are regularly reviewed to ensure relevance to the changing power generation landscape.

• SD 1: Asset Management
• SD 2: Machine/System Interaction
• SD 3: Renewable Generation
• SD 4: Motors
• SD 5: Machine Monitoring, Diagnosis and Prognosis
• SD 6: Efficiency of Electrical Machines

There are specific activities, mostly in the form of working groups, addressing these strategic directions and forward-looking opportunities within the 4 technology groups. The full list of current working groups is given in Tables 1 to 3 at the end of this report.

In the 2021 Annual Study Committee meeting we discussed the relevance of the current organisational set-up and strategic directions in view of the ongoing evolution of the role of rotating machines in power generation. Areas identified which require better visibility and prominence are:

• Synchronous Condensers/Compensators have regained popularity in recent years due to the various advantages they can provide to grid support in the face of ever-increasing penetration of renewable generation and the challenges that brings to power system control
• The high level of introduction of wind turbines over the last few years across the world has meant that there is a huge installed base that is developing quickly both on- and off-shore. Whilst initial unit sizes were small, they have increased steadily and today have reached 10 MW for the latest offshore applications for large scale wind farm installation. Reliability and availability of equipment together with monitoring and maintenance strategies challenged by limited access, availability of suitable equipment, and weather constraints on the timing of maintenance activities makes this an important area of focus for SC A1.

Whilst these topics are already covered by the scope of SC A1 technology groups and strategic directions, the current lack of visibility is a concern and will be addressed in 2022.

The strategic directions are explained individually below.

SD 1: Asset Management

Asset management covers a broad range of aspects relating to the ownership, operation, condition assessment, maintenance and repair, upgrade, conversion, replacement, and storage of rotating electrical machines including strategic spares.

There are currently 10 active working groups associated with this Strategic Direction:

• WG A1.33 has developed a guide for the proper storage and cleanliness of turbogenerators and their components. The guide gives insight into good industry practice for the storage conditions, environmental control, type and frequency of testing/monitoring and cleanliness procedures put in place during the various storage and assembly periods to ensure a successful preservation of the equipment. The Technical Brochure/Guide has been submitted for publication.

• WG A1.42 seeks to achieve consensus between customer and supplier so that the requirements contained in the technical specification are not a mere continuation of practices originated in the past or even impractical requirements due to a lack of equipment knowledge but to incorporate the current state of the art in the field.

• WG A1.55 is carrying out a survey on split core stators: Site access limitations and the significant size of large hydro generators can make it impossible to transport them as integral units and make site installation very difficult. A split core stator is a practical compromise but is more critical than a single continuous stator core as a poor design can result in core vibrations, noise or shaft voltage and can even lead to core and/or winding damage, shortening the stator life.

• WG A1.56 has completed a survey on lap and wave windings and their consequences on maintenance and performance of hydro-generator stator windings. The focus of this WG has been to describe the different characteristics of lap and wave windings and give a guideline for the optimal choice.

• WG A1.59 has evaluated different techniques and solutions for repairing failed stator windings on a hydro-generator when it is not possible to immediately replace the faulted coil due to operational, commercial, or contractual reasons where the generator must be returned to service as soon as possible. In such circumstances the solution is often to cut out the faulted coil and to operate the generator with a voltage unbalance until a scheduled outage period in which the faulted coil can be replaced.

• WG A1.60 is compiling a guide on the economic evaluation for refurbishment or replacement decisions on hydro generators to assist asset management considerations. It will complement the guide for turbogenerators described in Technical Brochure 641.

• WG A1.62 investigates various problems, root cause analysis and operational limits on thrust bearings of hydroelectric units utilising tilting pads. Failures in the design of these complex systems may lead to failures resulting in major, costly outages.

• WG A1.63 will examine user experiences on large generator terminal bushings and bushing flexible connections. Terminal bushings are a vital part of each turbine generator with extremely high energy concentration, and which are assumed to operate for many years without any maintenance. This working group will restart in 2022 under a new convenor.

• WG A1.64 is developing a guide for evaluating the repair/replacement of standard efficiency motors considering the repairability of the original stator and rotor, the root-cause of the failure, repair cost, motor power rating and efficiency, load duty, environmental conditions, load factor, energy efficient motor purchase price, annual operating hours, electricity price, and economic analysis.

• WG A1.67 is tasked with developing a basic guide on the main state-of-the -art methods applied for end winding corona testing, experience regarding the techniques, the devices used and test conditions and, if possible, establish limit values for end winding corona test results.

SD 2: Machine/System Interaction

The behaviour of generators directly influences the behaviour of the system to which it is connected and vice versa. It is essential to clearly understand the effect both systems have on each other to enable effective management of a stable grid as well as to ensure prolonged life of generating equipment.

This strategic direction complements the work of study committee C4 ‘Power system technical performance’, the focus of SC-A1 being the impact on such interaction on the rotating machine and the machine properties and behaviour defined by its design which influence the power system response and reliability.

Since system performance is determined by the output and behaviour other generation sources on the same grid, then this strategic direction also has some overlap with SD 3 ‘Renewable Generation’. Similarly, the operational flexibility required by modern grids can influence the demands put on auxiliary components e.g., large motors (strategic direction 4).

 Currently there are 1 activity associated with this strategic direction in the form of a Joint Working Group (JWG) with Study Committee C4.

• JWG A1/C4.52 - As wind generation is becoming a significant component of the generation portfolio in many power systems, provision of frequency-active power control is increasingly required for this technology in many regions. This joint working group between A1 and C4 will document the state-of-art in developing such capabilities for wind turbine generators, as well as the system technical performance aspects of such controls and the impact of such controls on equipment design and performance.

SD 3: Renewable Generation

The retirement of large fossil-fuelled plant as part of decarbonisation initiatives around the world has led to a significant reduction in system inertia which impacts network frequency control capability and the ability to ride through system abnormalities and faults. This has in turn resulted in a resurgence in the installation of synchronous condensers/compensators at strategic locations in power distribution networks and the conversion of existing generators whose MW power output is no longer required.

• A1/C4.66 - A Joint Working Group between SCs A1 and C4, is preparing a guide on the assessment, specification, and design of synchronous condensers for power systems with high levels of renewable generation. The main objective of this WG is to produce an application guide which power network owners/operators and other parties can use to help determine network needs and thereby specify synchronous condenser design requirements to successfully maintain network inertia and other aspects necessary for a reliable power generation and distribution network.

SD 4: Motors

Power plants use motors extensively throughout the many diverse systems to drive compressors, fans and pumps, and many other equipment which are critical to successful and reliable power generation. The scope includes smaller rating, high efficiency motors used for dispersed generation in consideration of their large potential for reducing the energy consumed within a power plant (see Technical Brochures 724 and 729).

This strategic direction also considers the evolution of motor technology as applied in power generation, and the manufacturing and operational aspects of motors across their full lifecycle.

There are currently 5 active working groups addressing this strategic direction.

• WG A1.45 is compiling a guide for Determining the Health Index of Large Electric Motors. The guide will aid utilities in identifying the appropriate measurements that are necessary for statistical quantification of in-service failure risk for effectively planned predictive maintenance interventions.
• WG A1.53: As the penetration of VFDs in industry and in power stations has increased, several motor failures have been reported worldwide and it has become clear that manufacturers do not have common design criteria for inverter grade motors considering the different stresses imposed on motor insulation by the variety of available VFD technologies. This WG shall elaborate the design requirements of three phase induction motors for variable frequency drive (VFD) applications including a user guide for retrofitting of existing installations.
• WG A1.54 reviews the effects of power plant cycling operation on large induction motors used in power plants in terms the lifecycle impact and gives recommendations for users regarding specification, installation, and operation of such motors.
• WG A1.58 aims to compile a guide for the selection of copper versus aluminium for the rotor cages of squirrel cage induction motors used in power generation plant. The guide seeks to give a clearer understanding of the advantages and disadvantages of these material choices.
• WG A1.68 was established to compile a guide covering the best available practices on evaluating the quality performance of motor manufacturing and repair facilities. 

SD 5: Machine Monitoring, Diagnosis and Prognosis

Condition Monitoring is a vital instrument in any business as it can result in significant cost savings if done correctly and effectively. The long-term goal of any plant owner is to effectively operate machines to achieve maximal performance, reliability, and efficiency and to make intelligent maintenance decisions through understanding the behaviour and signs of deterioration. The system can also be designed to provide diagnosis of machine condition allowing the prediction of problems, optimize operational efficiency and improve plant productivity.

There are currently 9 active SC-A1 groups performing work in this field.

• WG A1.43 has completed an evaluation of state-of-the-art rotor temperature measurement systems. Some rotating machines today are equipped with temperature measurement instrumentation to detect local overheating of rotor windings or, in the case of hydro-generators, rotor structure. This topic is of high importance, not just for machine monitoring and diagnostics purposes, but also for the purposes of machine design and construction improvements.
• WG A1.44 is compiling a guideline on Testing of Turbo and Hydro-generators. This guide will complement the work published in TB 386 ‘Generator Maintenance, Inspection and Test Programs’ by providing guidance to plant personnel on test procedures and practices to ensure equipment integrity, including hydro-generators, as well as an overall guidance regarding safety precautions, industry references, acceptable ranges of results, and, where appropriate, actions to be taken should the results be outside acceptable ranges.
• WG A1.48 has completed a guide on the requirements for high-speed balancing / over-speed testing of turbine generator rotors following maintenance or repair. The requirement for testing a generator rotor following maintenance varies substantially across the industry, with different methodologies being applied depending on the nature of the repair work, the owner/operator, the original equipment manufacturer (OEM) and the geographical location of the equipment. This guide intends to simplify the decision making of whether to carry out high speed balancing on generator rotors following repair work.
• WG A1.61, has been tasked with filling the information gap in the predictive diagnosis of winding insulation in large motors (more than 800 kW and 1000 V). The technologies in this field have evolved fast and new measurement and diagnostic tools are now available, including new kinds of PD sensors and new signal processing tools for noise and disturbance suppression. Simultaneously, the industrial use of converter fed high-voltage machines is steadily growing. Particularly in motors, where the effects of motor drivers on the insulation aging process and in PD monitoring have a significant influence, there is still a lack of information, reference cases and standard procedures.

• The following 4 WGs related to hydro-generators are in the initial phase. WG A1.69 addresses measured v calculated excitation current anomalies, whilst WG A1.71 and A1.72 will carry out surveys on rotor damper winding concepts, and multi-turn stator coil construction respectively. WG A1.73 will assess customer requirements for the qualification of form wound stator insulation systems for hydro-generators.
• WG A1.70 will build on the excellent work done on the use of Dielectric Dissipation Factor (DDF) measurements in relation to the assessment of the condition and quality of stator insulation systems of rotating electrical machines (reported in Technical Brochure 769) by focussing on DDF measurements of complete stator windings.

SD 6: Efficiency of Electrical Machines

Generators and motors are the highest efficient components used in the power generation process, with power conversion efficiencies ranging from 98% to 99%. However, on a large turbo generator of 1000MWe output, a 0.1% improvement in efficiency can result 1MWe extra being available to the national grid, powering an additional 50 to 60 households, without any increase to operational costs.

Rotating machines are generally optimised at the design rating based on the assumption of base load operation, so prolonged operation at part load results in a decrease in the actual mean efficiency that can be achieved.

With a gradual change in the load duties of large power plant in support of the increasing penetration of renewable power sources, machines will operate more and more at part loads for relatively short and variable time frames. Therefore, considerations of efficiency may have to be adapted in future.

Currently there are no active Working Groups on this topic.

Currently Active Working Groups

There are currently 26 active working groups although 8 are in the final stages of completion and should conclude in 2022 (see next section Tables 2 and 3). The focus of SC A1 activities in 2022 will be to complete the Technical Brochures associated with these Working Groups and submit for publication.

Table 1 lists the remaining 18 Working Groups that will continue their work in 2022.

Proposals for new Working Groups will be reviewed during the 2022 CIGRE Session planned to take place in Paris.

Publications

No Technical Brochures were published in Electra in 2021 although one was submitted for publication. Several other Working Groups are in the final stages of preparing the submittal documents – these are listed below:

SC A1 publications planned to be submitted in 2022:

Contact

• Contact of the Chair and/or the Secretary of the Study Committee