SC A1: 2014 Annual Report

Study Committee A1 (Rotating Electrical Machines)

by Nico W Smit, Chairman of SC A1

Introduction

The invention and development of rotating electrical machines has inspired a complete global technological explosion as these machines can be found in the front end and back end of virtually every process in all levels of industry, from generating 99% of global electrical power through to electrical motors driving virtually every production process in industry, consuming an estimated 43% to 46% of all generated energy1,2. Electrical machines therefor play a vital and key role in all sectors of modern society – for developing new and maintaining existing infrastructure in industry, service functions and in the domestic sector. The development has been fascinating and new applications and requirements still continue to expand technological boundaries.

The mission of SC A1 is to facilitate and promote the progress of engineering and the international exchange of information and knowledge in the field of electrical rotating machines by means of synthesizing state-of-the-art practices and developing new recommendations. The field of activities of study committee A1 covers research, development, design, manufacturing, operation, and de-commissioning of large rotating electrical machines. This includes the assessment of the condition of rotating machine components and elements, the maintenance, refurbishment, power upgrade, environmental aspects, asset management and long term health assessment. SC A1 is active in the areas of Turbo-Generators, Hydro-Generators and Large Motors. A dedicated Advisory Group deals with New Technologies. Each of these Groups is concerned with the international exchange of information, knowledge, practice and experience on design, construction, test and performance of rotating electrical machines.

Currently, SC A1 has 24 regular members, 14 observer members, 8 advisors, a chairman and secretary and around 290 experts from more than 40 countries following A1 activities and attending A1 meetings. The members of the SC come from Utilities, Manufacturers, Consultants, Research Centres and Universities.

Fig 1. Three Gorges Power Plant (22500 MW)

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Strategic Directions of SC A1

The activities of the Study Committee have been driven by the following six key main Strategic Directions:

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

SD 1: Asset Management

Asset management is a broad term that also includes other subjects such as: service and operating experience, availability, maintenance, failure modes, condition assessment, and all aspects of machine performance associated with asset ownership. Due to the importance of these themes on the machine behaviour, SC A1 has been conducting several works in this area. Among other Working Groups (WGs) we can mention the following:

WG A1.22 compiled a guide which focusses on global operating practices, ageing processes of generator windings, the consequences of different operating regimes and the way to monitor and detect fault appearance. The guide proposes condition assessment and maintenance practices according to different generator technologies. It was published as TB574 in the April 2014 Issue of Electra.

WG A1.14 published a guide on minimising damage to generator stator windings due to ground faults. This work was published as TB 573 also in the April 2014 issue of Electra.

Cleanliness of generator internals is an essential requirement for ensuring prolonged generator life as ingress of oil and other foreign matter can significantly reduce the insulation ability of generator stator windings and rotor coils. WG A1.25 published in August 2014 its work as TB 582 in which the results of global surveys of hydro-generator cleaning methods are presented.

Condition based assessments is vital for ensuring longevity of generators and therefore all available testing methods need to be considered in the aid of determining the condition of stator winding insulation. The TVA Probe (Corona Electromagnetic probe) is one instrument which is not used often due to the perceived danger of using it but which can give valuable insight into stator winding condition. WG A1.28 published a guideline on Corona Electromagnetic Probes Tests in the June 2014 issue of Electra in the form of TB581.

Asset management of power plants requires critical decisions to be made on different options available to improve asset reliability. Economic and technical decisions range from the repair of components, major refurbishment, or the total replacement of a generator. The design life of a generator is normally about 30 years for turbo-generators and over 50 years for hydro-generators. Turbo-generators built in the 80’s and hydro generators built in the 50’s and 60’s has already reached the end of their useful life and nearly all show various technical problems caused by ageing. Alternatives such as: partial repairs, components replacement to extend the life of existing generators or even to recommend their complete replacement with due consideration to the time in which the machine is out-of-service without generating income to the owner and, as much, as possible, using the existing foundations in order to exclude expenditures due to construction work and to keep the weight-overall characteristics of the power plant, are considered in the decision process.

The WG A1.05 is focused on the economic evaluation of generator refurbishment versus replacement, whose TB is foreseen to be published in 2015.

WG A1.41 is working on a TB with the aim of establishing an inventory of main maintenance interventions such as main components replacement to extend the life of existing generators (e.g., stator bars rewinding, core restacking) or even their complete replacement with due consideration to the time in which the machine is out-of service. The investigation is necessary

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to get a view of all main interventions done on the turbo-generators and duty cycles experienced before these interventions. This in depth investigation has also to identify the reasons of the required generator interventions, their duration and the way these machines were operated that leads to the required interventions. The final output of this work group is planned to be available for publication in March 2017.

Vibrations due to oscillations induced by the control systems (voltage and turbine governor) and hydraulic phenomena on the intake and draft tubes of hydro-generators, as well caused by mechanical problems can lead to damages, unplanned outage of machinery and the need for repair or larger refurbishment. WG A1.36 was created in 2012 with the aim to study and do a survey of the vibration and stability problems in hydro-generators and review the vibration limits given in different standards.

Generator life time is significantly related to the ageing of its windings. Several factors such as cyclic duty are contributing to reduce the design life. Four previous CIGRE questionnaires have already investigated the ageing processes. The TB of WG A1.22, which was published as TB 574 in the June 2014 Electra issue is an update to integrate more feedback based on the experience and latest developments in higher thermal class insulation and on-line monitoring.

SD 2: Machine/System Interaction

Generators are always connected to a load to enable power delivery. The behaviour of generators directly influences the behaviour of the connected load, and vice versa the load has a major effect and impact on generators. 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.

A Panel on generator-grid interaction issues previously held by SC A1 indicated that an inconsistency between generator standards and grid code requirements around the world exists. The WG A1.29 was created to compile the grid code requirements worldwide, identify extra requirements imposed by the grid codes with respect to IEC and IEEE/ANSI standards, and elaborate recommendations of consistent requirements on over/under voltage, over/under frequency, volts/hertz, harmonic loading, ceiling voltage, etc. The final work of this WG is expected for publication during 2015 in Electra.

Changes in the power generation industry due to deregulation as well as the introduction of vast amounts of renewable energy, is having a significant effect on the way we operate our generating plant. Particularly, hydroelectric generators require functioning under load cycling, from partial or no load to full load in a short period of time; in some cases they are used to supply peak loads, and therefore are required to be connected daily. This operation gives rise to both thermal and mechanical stress-cycling, resulting in wear of components and in worst cases may lead to failures. Also, the possibility of an out-of-step synchronization, loss of field and load rejection can cause damage to the machine. WG A1.35 was created in 2012, to study these effects on hydro-generators and formulate recommendations to improve their performance.

Development of a guide for on-line generator operation in inductive and capacitive mode, including investigations of generator limits for various operating modes with the main focus on the effects on stator core heating when operating in the capacitive mode. WG A1.38 compiled a very informative TB which will be published in 2015.

SD 3: Renewable Generation

Renewable energy technologies vary widely in their technical and economic maturity. Their common feature is that they produce little or no greenhouse gases, and rely on virtually inexhaustible natural

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sources for their ‘fuel’. The wind generation presented a great growth in the last decade. The installed wind capacity has more than doubled every third year, in the last decade. According to the National Energy Agency Technology Roadmap for Wind Energy, wind power can make up 18% of the total energy production by 2050.

Fig 2. Global Energy Mix by 2050 (High renewable scenario)

Due to this anticipated growth in wind power generation, it is essential to focus work groups in this area to ensure that we keep abreast the latest technologies available in the market with specific focus on efficiency, reliability and monitoring of wind turbine generators. As wind turbine generation is a relatively new technology in its present modern form, we need to influence the present and future technologies to ensure optimum reliability and availability. WG A1.24 is undertaking a survey on the latest technological developments in this form of power generation. WG A1.51 and WG A1.52 are new Work Groups that will focus on wind turbine generation. WGA1.51 will do a survey on the monitoring, reliability and availability of wind generators and JWG A1/C4.52 will focus on the Frequency-active power control of wind generation.

SD 4: Large Motors

Electrical motors perform almost all mechanical work in factories, offices, service centres, hospitals, households and schools. In auxiliary services of power plants and in industry motors are used for driving compressors, fans and pumps, machine tools, robots, transport equipment and many other equipment. Historically the SC was focused on motors with more than 800 kW and 1000 V for power stations but it was recently agreed by the SC to amend the scope of the Advisory Group on Large motors to allow the Advisory Group to focus not only on large motors with more than 800 kW and 1000 V for power stations, but also on smaller ratings high efficiency motors used for dispersed generation considering their large potential for energy saving. The significant potential that energy saving by electrical motors can have on future power generation requirements is noted and by encouraging work groups to study the potential and effect of large scale deployment of high efficiency motors, hopefully significant benefits for the total industry can be obtained.

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WG A1.27 “Adjustable speed drives and high-efficiency motors applications in power plants” concluded its work in 2014 with publication of its Technical Report in the February 2014 issue of Electra;

WG A1.45 is compiling a guide to determine the health index of large electric motors. It has as its main task to quantify the severity of known motor defects, as determined through condition monitoring and periodic testing and inspections, and link these defects to the likelihood/probability of motor functional failures.

In general, over 43% of the global electric energy demand is consumed by electric motor systems.2 They are used to drive pumps, fans, compressors, traction systems and industrial handling & processing equipment in the various fields such as industry, transport, large building environmental control and home appliances. Energy efficient electric motors represent one of the largest opportunities for cost-effective electric savings and the action plans for the reduction of greenhouse gas emissions. In order to gain fast and efficient access to the energy efficiency improvements of electric motor systems, regulations mandating the energy labelling of products for minimum energy performance standards (MEPS) have been widely applied to three-phase electric motors and the MEPS efficiency has resulted in higher efficiency levels such as IE3 - premium efficiency type motors. The world can save about 160 GW on power plants by improving 5% of motor efficiency. Based on these aspects, Study Committee A1 is launching a renewed focus on the use of energy efficient motors as well as the further improvements of motor efficiency. The following two work groups were therefor created in 2014:

WG A1.46 compiling a guide on the use of premium Efficiency motors and carbon credit claims;

WG A1.47 surveying the Technological feasibilities for IE4 (Super-Efficient) and IE5 (Ultra-Premium Efficient) motors.

SD 5: Machine Monitoring, Diagnosis and Prognosis

Condition Monitoring is becoming a vital instrument in any business as it can result in significant cost savings if done correctly and effectively. The field of condition monitoring is one of continuous evolution and development. 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. This is called Condition Based Maintenance, which is maintenance performed when it is optimally required. Short, medium and long-term risks can be evaluated with an effective condition monitoring system. The system is also designed to provide diagnosis of machine conditions allowing the prediction of problems, optimize operation efficiency and improve plant productivity. Some systems are based on expert systems and further development is anticipated. There is an important link between monitoring and diagnosis and the life management and life extension processes (prognosis). The following three work groups are performing work in this field:

WG A1.40 is conducting a survey on Hydro-generator instrumentation and condition monitoring systems.

WG A1.43 focusses on state of the art rotor temperature measurement systems WG A1.51 will conduct a study on monitoring, reliability and availability of wind Generators.

SD 6: Efficiency of Electrical Machines

The development of new materials, improving cooling and insulation in generators and motors is contributing to increase the efficiency of electrical machines. Higher efficient electric motors, due to its widespread use and location at the centres of consumption, have a great impact on the reduction of

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losses in power systems. The utilization of polymer nano-composite materials in electrical machines is a promising near-future HV electrical insulation by enamelling the conductors to be more resistant to partial discharges and improving their voltage and thermal endurance capacity contributing to an additional increase in efficiency.

As the scope of the Large Electric Motor Advisory group was amended to include smaller high efficiency motors, work group A1.46 was created in this field to compile a guide on the use of premium efficiency motors and carbon credit claims (shared with SD 4).

Currently Active Working Groups

Six WGs were disbanded in 2014, after completion of their activities, with publication of four Technical Brochures and two Technical Reports in Electra. Five new WGs were approved in 2014 and one WG disbanded due to a lack of participation on the topic. At present there are 23 WGs active in the SC with focus on the following topics:

Economic Evaluation of Generator Refurbishment/Replacement Literature Survey on Diagnostic Trends for Wind generators for Reliability Improvement Guide on Generator/Power System Inter-relationship issues Guide on New Generator-Grid Interaction Requirements State of the art of stator winding supports in slot area and winding overhang of Hydro-

Generators Guide for the Proper Storage and Cleanliness of Turbo-generators and Components Testing Voltage of Doubly-Fed Asynchronous Generator-Motor Rotor Windings for Pumped

Storage System Hydroelectric Generators Behaviour under Abnormal Operating Conditions Vibration and Stability Problems Met In New, Old and Refurbished Hydro Generators, Root

Causes and Consequences Turbo-generator Stator Winding Support System Experience Guide for Generator On-Line Over and Under Excitation Operating Issues Application of dielectric dissipation factor measurements on new stator coils and bars Survey on Hydro Generator Instrumentation and Monitoring Inventory of Main Maintenance Interventions on Turbo-generators Influence of Key Requirements on the Cost of Hydro-generators State of the Art of Rotor Temperature Measurement Guideline on Testing of Turbo and Hydro-generators Guide for determining the Health Index of Large Electric Motors Guide on Use of Premium Efficiency IE3 (IEC 60034-30) Motors Determining Benefits of

Green House Gas Emission Reduction Technological Feasibility Studies for Super and Ultra Premium Efficient Motors Guidance on the Requirements for High Speed Balancing / Overspeed Testing of Turbine

Generator Rotors Following Maintenance or Repair. Magnetic core dimensioning limits in Hydro-Generators Factory Quality Assurance Testing Requirements for Turbo-generator Components

Technical Meetings

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The SC A1 meetings in 2014 were held during the 45th CIGRE Session in Paris France, which took place from August 24th to August 29th.

The Technical Meeting was attended by 150 delegates, improving from the 126 delegates that attended the previous Technical meeting in Paris 2012, in spite of the current global financial situation.

The following Preferential Subjects was selected for the Technical Meeting:

Preferential Subject 1 - Developments in Electrical Machines Preferential Subject 2 - Life management of Generators Preferential Subject 3 - Rotating Machines for Dispersed Generation

In total 19 papers were selected by the Special Reporter on which 29 prepared contributions were received.

In 2015, the SC A1 Annual Meeting and a Colloquium on “Rotating Electrical Machines: Requirements, Operation & Maintenance”, will be held in Madrid, Spain, from September 6 th to September 11th.

The subjects for the Colloquium are:

Assessment of large electrical machines: diagnostic criteria, risk management, retrofit, replacement, power up-rating (repowering), on-line monitoring, efficiency improvement and methods for evaluation.

Quality assessment of new purchased and repairing machines: repairing options, inspections, technical purchase & repair specification issues, new designs.

Operation of electrical machines: grid codes and requirements on the machines, standards, limits of frequency, excitation limits, torsional electromechanical oscillations, etc.

Experiences of operation and maintenance on electrical machines with cycled operation: effects on generator & motors, change or intensification of maintenance programs or testing.

Wind & renewable generation: experience with testing, commissioning, operation, integration in the network and maintenance.

Tutorials and Workshops Tutorials are regarded as a valuable initiative to disseminate knowledge from work done by work groups within the study committee. A number of tutorials are annually presented by experts within the study committee at colloquiums, training seminars and technical meetings. The sharing of knowledge by experts in Study Committee A1 is encouraged and invitations from National Committees to present tutorials at regional meetings will be welcomed.

Future Activities

SC A1 is conducting studies and international surveys on new developments and technologies to improve the performance of electrical rotating machines in order to increase their availability and reliability for the supply of electrical power systems. The following subjects will guide the future activities of the SC:

Improvements in design, materials, insulation, cooling, bearings, availability, reliability, efficiency and maintenance of rotating electrical machines.

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Life management Large Generators and Motors Renewable generation Machine/System Interaction Machine monitoring, diagnosis and prognosis New developments in excitation systems Superconducting machines. Utilization of polymer nano-composites is promising as near-future HV electrical insulation in

rotating machines.

Publications:

Published in 2014

WG Title of Publication Type/Number Electra Issue/ Month

A1.22 Guide – Consideration of Duty on Windings TB574

Electra 273/ April

A1.27 Adjustable Speed Drives and High-Efficiency Motors applications in Power Plants

WRA1.27

Electra 272/ February

A1.14 Guide for Minimizing the Damage from Stator Winding Ground Faults in Hydro-generators.

TB573

Electra 273/April

A1.25 Survey on hydro generator cleaning TB582

Electra 275/August

A1.30 Usage of Magnetic Slot Wedges in Hydro Generators

WRA1.30

Electra 274/June

A1.28 Guide – Corona Electromagnetic Probe Tests (TVA)

TB581

Electra 274/ June

TB = Technical Brochure, WR = Working Group Report

To come in 2015

The following Technical Brochures and Report are programmed to be published in 2015:

WG Title Type Expected Publication date

A1.05 Generator Economic Evaluation of Generator Refurbishment / Replacement

TB Expected August 2015

WG A1.29

Guide on New Generator-Grid Interaction Requirements

TB Expected October 2015

WG A1.31

State of the art of stator winding supports in slot area and winding overhang of Hydro-Generators

TB Expected August 2015

WG A1.33

Guide for the Proper Storage and Cleanliness of Generators and Components

TB Expected October 2015

WG A1.34

Testing Voltage of Doubly-Fed Asynchronous Generator-Motor Rotor Windings for Pumped Storage Systems

TB Expected August 2015

WG A1.35

Hydroelectric Generators Behaviours under Abnormal Operating Conditions

TB Expected December 2015

WG Vibration and Stability Problems in new, TB Expected October 2015

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A1.36 Old and Refurbished Hydro-generators, Root Causes and Consequences

WG A1.38

Guide for Generator On-Line Over and Under Excitation Operation Issues

TB Expected June 2015

WG A1.41

Inventory of Main Maintenance Interventions on Turbo Generators

TB Expected December 2015

Latest Publications:

Guideline for Consideration of duty on windings – TB 574Generator life time is significantly related to the ageing of its windings. Several factors such as cyclic duty are contributing. Four previous CIGRE questionnaires have already investigated the ageing processes. This brochure is an update to integrate more feedback based on the experience and latest developments such as higher thermal class insulation or on-line monitoring.

Fig 3. Degradation process of stator bar insulation

This Guide reviews operating practices, ageing processes of generator windings, their consequences and the way to monitor and detect fault appearance, and proposes condition assessment and maintenance practices according to different generator technologies.

Technical Brochure 573 - Guide for Minimizing the Damage from Stator Winding Ground Faults in Hydro-generators

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Stator ground faults are the most common winding failure in generators, and this kind of fault occurs due to stator winding insulation breakdown and electrical contact between the active phase winding and the grounded stator core.

Protection relays have to trip the generator as soon as possible, tripping the main breaker, disconnecting the excitation supply and tripping the prime mover. However, the fault current will not disappear immediately, because of the finite time taken to discharge the stored energy in the field circuit. Following a stator ground fault and protection relay trip, the generator performance and the damage caused to the stator core, depend on how the stator winding is grounded.

This document is a natural extension of a previous work contained in Technical Brochure TB397 “Guide for minimizing the damage from stator winding grounds on turbo-generators”, and tries to analyse the different grounding schemes that could be founded on hydraulic power plants.

The purpose of this guide is to review the effect of phase-to-ground faults on generators and to analyse the different stator winding ground schemes and protection systems. Calculation criteria for stator winding grounding are also proposed in order to reduce damage from electrical faults.

Technical Brochure 582 - Survey on Hydro-Generator Cleaning

Hydro generators are exposed to contamination from the surroundings through its cooling system.Carbon dust from slipring brushes, oil leakage from bearings, dust from brakes and dust from outside are typical sources causing contamination. This contamination reduces insulation resistance and can lead to electric failure of the generators. The contamination can also lead to reduced cooling on parts of the generator.

In other cases flooding with oil or water or the presence of humidity can lead to damage or reduced insulation resistance. Cleaning and drying in order to regain acceptable insulation resistance values can be a very long process. Correct planning of generator cleaning can reduce unexpected down time and be a positive contributor to increased lifetime.

Non-severe damage

Fig. 4 Relationship between Phase-to-ground fault current, the fault duration and resulting Generator damages on an unprotected system

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900

80706040302010 50 100

Fault duration (Seconds)

5 Severe damage

Medium damage

Phase-to-ground fault current(Amperes)

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Oil from bear-ings 41%

Oil from brakes 2%

Carbon dust 36%

Dust from brakes 10%

Dust from outside 6%

Dirty water and mud 3%Humidity 2%

Fig 5. Contamination found in closed loop air cooled generators.

Effective and continuous maintenance of systems that are regarded as the largest contributors of contamination, in order to reduce the amount of contamination, is another important issue.

Typical sources of contamination and the respective cleaning criteria are discussed in the published brochure. Various methods used by generator owners for cleaning of generators, testing performed after cleaning and general health issues are important issues that are discussed.

Technical Brochure 581 - Guideline on Corona Electromagnetic Probe Tests

The Corona Electromagnetic Probe test (also known as TVA Prober test) method of detecting and determining the location of partial discharges at slot level in high voltage electric generators and motors has been used for many years but according to some reports it has led to several accidents and even resulted in death by electrocution of the operators of this equipment. As a result some companies are avoiding the use of this method. Incidents occurring while performing these tests are only caused by lack of respect of safety rules for high voltage work. This has slowed the development of this method for testing high voltage electrical machines.

In form-wound bars and coils rated greater than about 4 kV, partial discharges can occur within the groundwall insulation or between the surface of the coil or bar and stator coil.Groundwall insulation is the component that separates copper conductors from the grounded stator core. These partial discharges, which are sometimes colloquially (but incorrectly) called corona, are created by the high voltage stresses that occurs in the groundwall insulation.

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Fig 6. TVA probe test being performed on a 60MW Generator.

According to the IEEE Standard Dictionary 100-1996 [13] corona is a form of partial discharges but the term corona is reserved for the visible partial discharges that can occur on bare metal conductors operating at high voltage, which ionize the surrounding air. Since partial discharges within the groundwall insulation are not visible, they should not be termed corona.

If an air pocket (also called a void or a delamination) exists in the groundwall insulation, the high electric stress will break down the air, causing a spark. This spark will degrade the insulation and, if not corrected, repeated discharges may eventually erode a hole through the groundwall insulation, leading to failure. The off-line and on-line partial discharge tests indicate that partial discharges are occurring somewhere in the winding, thus, some stator winding insulation deterioration has occurred. However, these tests do not give any indication of where in the winding the problem is occurring. The TVA probe test can answer this question as it can detect localised partial discharges at the exact location where it occurs.

References:

1. Renewables 2014 Global Status Report; Ren212. Energy Efficiency Policy Opportunities for Electric Motor Driven systems; International

Energy Agency; Paul Waide, Conrad U. Brunner3. National Energy Agency Technology Roadmap for Wind Energy; IEA

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