eStorage D1.4 New Rotor Design Guidelines for both … · New Rotor Design Guidelines for both ......

European Commission – Directorate General for Research

S E V E N T H F R A M E W O R K P R O G R A M M E

T H E M E 5 - E N E R G Y

Project acronym : eStorage

Project full title : Solution for cost-effective integration of renewable intermittent

generation by demonstrating the feasibility of flexible large-scale energy

storage with innovative market and grid control approach.

Grant agreement no.: 295367

Collaborative project / Demonstration Project

Number of deliverable:

D1.4

Dissemination level (PU, PP, RE, CO, RUE, CUE, SUE) : PU

Date of preparation of the deliverable (latest version): 30/10/2016

Date of approval of the deliverable by the Commission: dd/mm/yyyy

New Rotor Design Guidelines for both Doubly

and Full-Fed Solutions –

Publishable Summary

ENERGY.2011.7.3-2- STORAGE AND BALANCING VARIABLE ELECTRICITY SUPPLY AND DEMAND / ESTORAGE

eStorage_D1.4_New Rotor Design Guidelines for both Doubly and Full-Fed Solutions - Publishable Summary fv

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R E V I S I O N C H A R T A N D H I S T O R Y L O G

V E R S I O N S

Version number When Organisation name Comments

v0.1 31/10/2016 GE (former AHF) First draft

V2.0 24/02/2017 GE (former AHF) Final version including reviewers remarks

V3.0 03/03/2017 ALG Quality check

Final version 06/03/2017 GE (former AHF) Final version sent to PO

D E L I V E R A B L E Q U A L I T Y R E V I E W

Quality check Status Date Comments

Reviewer 1 (AGR) 15/02/2017

Reviewer 2 (ICL) 20/02/2017

Quality Manager 03/03/2017

PC 06/03/2017



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T A B L E O F C O N T E N T S

1 INTRODUCTION ..................................................................................................................................... 7

BACKGROUND OF THE STUDY ........................................................................................................................ 7

OBJECTIVE ................................................................................................................................................. 8

2 VALIDATION OF THE ROTOR WINDING OVERHANG CONCEPT .......................................................................... 10

DESCRIPTION OF THE ROTOR DESIGN ............................................................................................................ 10

DEVELOPMENT NEED ................................................................................................................................. 11

DEVELOPMENT OF MEASUREMENT EQUIPMENT ............................................................................................. 11

LEARNING ................................................................................................................................................ 13

3 VARIABLE SPEED CONTROL SPMAX .......................................................................................................... 14

DEVELOPMENT PROCESS ............................................................................................................................ 14

3.1.1. Development ........................................................................................................................... 14

3.1.2. Three level validation tests with associated platforms ........................................................... 15

RECENT DEVELOPMENTS ............................................................................................................................ 15

3.2.1. Testing of primary frequency capability .................................................................................. 16

3.2.2. Primary frequency control capability – measurements .......................................................... 17

LEARNING ................................................................................................................................................ 18

4 HYDRAULIC DEVELOPMENTS FOR REVERSIBLE FULLY FED SYNCHRONOUS MACHINES .............................................. 19

SPECIFIC REQUIREMENTS FOR FULLY FED REVERSIBLE MACHINES ........................................................................ 19

4.1.1. Benefits of speed variation in pump and turbine mode ......................................................... 19

4.1.2. Specific hydraulic approach for variable speed ....................................................................... 21

DESIGN APPROACH .................................................................................................................................... 22

4.2.1. Overview of design approach .................................................................................................. 22

4.2.2. Improvements needed from the reference case..................................................................... 23

4.2.3. Runner design .......................................................................................................................... 23

4.2.4. Guide Vanes and stay vanes .................................................................................................... 24



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TEST CAMPAIGN ....................................................................................................................................... 25

4.3.1. Overview of test rig facility ...................................................................................................... 25

4.3.2. Test rig description .................................................................................................................. 25

4.3.3. Data Acquisition ....................................................................................................................... 25

RESULT AND ANALYSIS ............................................................................................................................... 26

4.4.1. Pump results ............................................................................................................................ 26

4.4.2. Optimisation of Power variation ............................................................................................. 26

CONCLUSIONS & RECOMMENDATIONS ......................................................................................................... 27

4.5.1. General Conclusions ................................................................................................................ 27

4.5.2. Suggestion for future research ................................................................................................ 28

5 INSULATION SYSTEM OF CONVERTER FED MACHINES ..................................................................................... 29

DEVELOPMENT FOR LARGE DOUBLE-FED MACHINES ........................................................................................ 29

RECENT DEVELOPMENTS ............................................................................................................................ 30

EQUIVALENT CIRCUITS FOR CORONA PROTECTION SYSTEM ............................................................................... 31

RESULTS / LEARNING ................................................................................................................................. 32



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T A B L E O F F I G U R E S

Figure 1: Cylindrical rotor of a double fed asynchronous machine................................................................. 10

Figure 2: Installed rotor winding overhangs .................................................................................................... 11

Figure 3: Measurement track on the winding overhang and on the rotor rim ............................................... 12

Figure 4: Measurement position on the rotor and laser displacement transducer ........................................ 12

Figure 5: Raw signal and evaluated «cold» / «warm» on driven and non-driven end ................................... 13

Figure 6: Structure of the GE variable speed control system ..................................................................... 14

Figure 7: Test rig facility for real time simulation in Grenoble ................................................................... 16

Figure 8: Test for primary control capability v.1.1 – SWISSGRID .................................................................... 16

Figure 9: Head variation with variable speed technology compared to fixed speed (left) and pump input

variation (right) ................................................................................................................................................ 20

Figure 10: Efficiency optimisation in turbine mode with variable speed ........................................................ 20

Figure 11: Link between power variation and cavitation constraint in pump mode ...................................... 22

Figure 12: Hydraulic components of the whole pump-turbine ....................................................................... 23

Figure 13: Flow paths and minimum pressure area (in purple) at the blade inlet in pump ........................... 24

Figure 14: Example of flow visualisation for the design optimization of Stay vanes and Guide vanes .......... 24

Figure 15: Photograph of the test rig used for this project ............................................................................. 25

Figure 16: Cavitation incipient limits : Comparison between CFD and experimental results ......................... 26

Figure 17 : Power variation obtained with DEV02 .......................................................................................... 27

Figure 18: VET testing of single bars at rated and testing of grading system on single bars .......................... 29

Figure 19: Sample bars for VET testing and Pulse voltage patterns ................................................................ 30

Figure 20: - IR-recording to check temperature increase on CPS ................................................................... 32



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P A R T I C I P A N T O R G A N I S A T I O N S

Participant organisation name Short name Country

GENERAL ELECTRICS (former ALSTOM HYDRO FRANCE) GE France

ELECTRICITE DE FRANCE S.A. EDF France

ELIA SYSTEM OPERATOR ELI Belgium

ALSTOM GRID SAS AGR France

IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE ICL United Kingdom

KEMA NEDERLAND BV KEM Netherlands

ALGOE ALG France



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1 INTRODUCTION

Background of the study

Upgrading a significant portion of existing European Pumped Storage Plants (PSPs) to more flexible variable

speed PSPs has economic and technical challenges that necessitate the development of innovative solutions.

Identifying and developing these solutions are the main goals of eStorage Work Package 1 (WP1).

The first step was to address these goals through the upgrade and conversion to variable speed of an existing

plant. Le Cheylas PSP (2 groups of 270 MVA, operated by EDF) had been selected as fully representative of

the core target of the European PSP fleet.

The studies performed during the three first years of the eStorage project regarding le Cheylas conversion to

variable speed proved that this conversion is technically feasible with performance in line with expectations

(i.e. deliverables D1.1 and D1.3). But, because the conversion has not been implemented, it is not possible

to have feedback on the work period or on the operating results. The key challenges with the conversion

were the greater volume as well as the complexity of the doubly fed induction machines. These machines

had lower rotation speeds which impacted the pump turbine efficiency. Furthermore, they did not have an

adequate protective relay for such units.

The second step aimed to go further taking into account the lessons learnt from the Le Cheylas upgrade

design and the stakes highlighted in the conclusions of eStorage Work Package 4 (Feasibility and technical

cost benchmark of PSP sites in Europe). Additional research & development has been performed to address

the key identified gaps:

• improve the doubly-fed asynchronous technology, increase cost-competitiveness and make

conversion feasible when available space in the existing plant is so limited that keeping the existing

stator is the only option. To properly tackle these needs, ventilation and mechanical design are key.

• develop a fully fed solution as an option for plants with an output below about 100 MW per unit.

Hydraulic design and insulation system need to be specifically targeted.



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Objective

This report will summarize the additional research undertaken after the Le Cheylas upgrade design,

addressing the design limitations and potential simplifications of large variable speed motor-generators.

The first two axes of investigation were related to doubly-fed asynchronous machines. The improvement of

the design guidelines were proposed through measurement campaigns performed on an existing variable

speed motor-generator under operation to reach breakthrough understanding of physical phenomena and

of the current limitation of the variable speed rotor technology. The Linthal 2015 variable speed PSP, supplied

by Alstom, represents an opportunity to collect valuable feedback for the doubly-fed asynchronous

technology as the first synchronization and the first operation trials started in the beginning of 2016.

• Rotor winding overhang concept

The rotor design, particularly the winding overhang support, is a sophisticated concept resulting in very

demanding requirements for the production and the assembly of the rotor bars and their supporting system.

Therefore, measurements of key parameters such as temperature and axial movement at different operation

points are needed to review the design space for potential optimization.

• Variable Speed control

Considering that the power variation capability of the variable speed units is not clearly defined in the

transmission code special tests in pump mode were planned during commissioning of the first unit in the

Linthal 2015 PSP.

During commissioning, a real time simulation was used for backup simulation in parallel to the progress on

site. Improved simulators with complete electrical and hydraulic models, including turbine hill chart and

detailed models of the generator excitation system, helped to predict the operational behaviour of the whole

control system.

The last two axes were specifically related to fully-fed synchronous machines. Indeed, the analysis of the

“Potential for conversion of classical PSP to variable speed units in EU15, Norway and Switzerland” (Work

Package 4, Deliverable D4.1) has highlighted a significant technical potential for variable speed units designed

with full size converters, particularly for refurbishment of horizontal machines the solution with full size

converters turns out to be the only feasible solution for conversion without constraining the unit power.



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• Hydraulic developments for reversible fully-fed synchronous machines

The hydraulic design represents the major technical barrier commonly identified for fully fed variable speed

PSP technology. The design should enable safe and efficient operation from 0 to 100% of load, especially in

pump mode. To address this need, GE undertook specific hydraulic developments during the course of the

project: first performing Computational Fluid Dynamic (CFD) design optimization (computer simulation), then

scale model testing (design, manufacturing a scale model and hydraulic test rig testing) of a fully fed pump

turbine to measure new turbine design performance and validate the computer simulations.

• Insulation system of converter fed machines

The insulation system of converter-fed machines must be able to withstand voltage generated by the

converter feeding the winding. Due to a lack of long term experience with large drives in the hydro industry,

further developments of the insulation and grading system of high voltage windings were needed. Therefore,

lab tests, in compliance with the latest international standards for converter fed windings, were conducted.

The tests validated the design criteria for the main insulation and corona protection system.



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2 VALIDATION OF THE ROTOR WINDING OVERHANG CONCEPT

The rotor design of double fed induction machines is significantly different from conventional synchronous

machines. Consequently, the design and industrialization of the motor-generator rotor parts have been

developed for various new applications.

The rotor of a double fed induction motor-generator has a three-phase rotor winding wound on a cylindrical

rotor. The winding overhang must be supported to avoid deformation and damage if the machine speed

comes approaches runaway speed during load rejection.

The trade-off between electrical optimization and mechanical design of the overhang support lead to a

sophisticated concept resulting in very demanding requirements for the production and the assembly of the

rotor bars and their supporting system.

Figure 1: Cylindrical rotor of a double fed asynchronous machine

Description of the rotor design

The entire winding overhang and its support elements have been calculated by 3D finite element method

(FEM), considering different load cases such as rated operation and load rejection of the unit. Based on all

relevant operation points, the stresses and deformations in each component have been investigated and

finally used for design validation and material definition. In addition, the 3D FEM calculations confirmed the

boundary conditions for the mechanical lab testing on single components.

The 3D FEM calculation provides data on deformations resulting from mechanical and thermal loads on

components. The combination of mechanical efforts and thermal expansion due to rotor currents is difficult

to reproduce and measure on a model test. Based on the first results, the retaining elements of the winding

overhang have been further optimized to minimize deformation of the supporting elements following

thermal expansion of the rotor bars.



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Development need

The design complexity of the winding overhang support system as well as the rotor leads is influenced by the

need to compensate for axial deformation. Consequently, accurate measurements provide valuable

information for design validation as well as potential design simplification and future maintenance

requirements of the rotor.

Figure 2: Installed rotor winding overhangs

Development of measurement equipment

The development focused on two main objectives:

– Measure thermal expansion of the winding overhang support at different operation points

– Review limitations of the design concept and identify potential for design simplification

The following challenges had to be resolved when defining the concept:

– Reliable measurements inside the machine during operation

– Very high circumference speed and many parts

– Potential impact of magnetic field and dust

– High voltage on terminals

To monitor the axial movement of the winding overhang system at different load conditions, a unique

measurement system had to be developed. The final system design is based on four high speed analogue

laser displacement transducers working with the triangulation measurement principle.

Due to the specific design requirements, some key points had to be considered in the early concept stage:



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– Positioning of the key-phasor on the rotating parts and sensors on the bearing brackets. The system

is designed to handle machine vibrations on the sensor fixation, as well as individual angular offsets

and ambient temperatures up to 60°C.

– Potential impact of the rotor angle on the trigger impulse due to the operation mode such as turbine

and pump mode. Software enable adjustment of data record of one revolution to keep positioning

of physical reference point on the rotor

– System working with very high sampling rate due to high circumference speed and number of

rotating parts

– Precision of measurement in axial direction 0.1 mm

– Design requirements: insensitive to pollution, lifting of the rotor, pressurized air to cool the sensors

Figure 3: Measurement track on the winding overhang and on the rotor rim

The development of the entire application requires specific know-how on sensor and software development

to fulfil requirements such as being insensitive to pollution, screening against electro-magnetic fields and

work at high speed respectively very high sampling rate.

Figure 4: Measurement position on the rotor and laser displacement transducer



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The sensors enable on-line measurements on the reference position of the stacked rotor rim and on the

overhang support on NDE and DE side based on the triangulation measurement principle.

Figure 5: Raw signal and evaluated «cold» / «warm» on driven and non-driven end

Learning

The axial expansion of the entire overhang support is working as designed. Thermal expansion is directly

linked to the cooling of the machine and therefore to the temperature of the rotor winding overhang.

The measured expansion provides valuable input for potential simplification. A supporting system with the

capability to allow thermal expansion seems clearly a benefit to compensate tolerances from assembly and

minimize deformation of the rotor bars.



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3 VARIABLE SPEED CONTROL SPMAX

The control system of the variable speed units is built in three levels and consists of the unit control system

(UCS), the variable speed controller (SPMAX), the turbine governor (T.SLG) and the inverter controller (IVC).

SPMAX is linking the T.SLG and the IVC to the unit control system. It handles all signals to be exchanged

between the unit and the unit control system. The variable speed controller defines the optimum speed

based on the current head and the desired power in normal operation.

Figure 6: Structure of the GE variable speed control system

Development process

The main development steps of the variable speed controller (SPMAX) have been described in previous

publications and are briefly summarized hereafter:

3.1.1. Development

– Integration of the electrical control into the overall control process together with the supplier of the VSI,

GE Power Conversion.

– Common simulation model for sequences, main parameters and regulation functions with a common tool

(Matlab).

– Integration of the improved simulation models into local test platforms with dedicated real time test tools

which also required development of hardware and software

– Detailed process simulation models:

o Pump/Turbine with hydraulic circuit and hill charts from model test

o Machine plus excitation with current and synchronization controllers

o Unit control system and auxiliaries’ sequences using realistic process time

o Hardware simulation of process and breakers with hardwired signals and interlocking



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3.1.2. Three level validation tests with associated platforms

Platform 1 Sub-system control platform dedicated to the validation of variable speed controller (SPMAX),

turbine governor (T.SLG) and inverter control (front end control).

Platform 2 SPMAX-T.SLG control platform dedicated to the detailed validation and final acceptance tests.

The platform was also used for the permanent support during commissioning.

Platform 3 Unit control system platform dedicated to the validation of the overall UCS-SPMAX control with

final sequences and signals. Common tests for interface adaptations. The platform was moved

to site for operator training and pre-commissioning tests with operators.

Recent Developments

In normal operation, low rotor current frequencies must be avoided due to the thermal constraints of

inverter power electronics. The control is designed to avoid unit operation with rotor current frequency

below 0.2 Hz. For a given head and grid frequency, the pumping power reference may give a resulting speed

inside the synchronous dead band. Since active power is directly linked to the speed, the system can be

controlled satisfying inverter limitation by adjusting the input power. Dynamic switchover between speed

control, when speed is close to sensitive area and active power control, when dead band is avoided allows a

smooth and performant management of the transition of the synchronous dead band. The fine tuning of this

control function is complex and had to be done during commissioning.

Very high head pump-turbines have several intrinsic technical challenges like transient behaviour, operation

stability and design for high pressure. To cope with the effects of high head in combination with- low specific

speed and the very demanding hydraulic circuit, special closing laws on the guide vanes and main inlet valves

were introduced and tested. During commissioning, new concepts and design solutions were validated by

measurements and finally used to optimize future plant operation. Specifically, the confirmation of the

speed-power range requires detailed measurements of the pump-turbine characteristics as well as the

dynamic response of the entire system.

Considering that the power variation capability of the variable speed units were not clearly defined in the

transmission code, special tests in pump mode, which were a new concept for all involved parties were done

during commissioning of the first unit.

During commissioning a real-time simulation on a test bench was used for backup simulation in parallel to

the progress on site. Improved simulators with complete electrical and hydraulic models, including turbine

hill chart and detailed models of the generator excitation system, helped to successfully predict the

operational behaviour of the whole control system. To prove compliance with the new grid code

requirements, comprehensive network simulations were performed followed by common acceptance tests

involving the plant owner and TSO.



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Figure 7: Test rig facility for real time simulation in Grenoble

3.2.1. Testing of primary frequency capability

During commissioning of the Linthal 2015 PSP the primary frequency control capability was tested following

the test procedure of Swissgrid, frequency ramp and required power threshold.

Figure 8: Test for primary control capability v.1.1 – SWISSGRID



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3.2.2. Primary frequency control capability – measurements

Turbine mode

– Pini = 150 MW

– Pfin = 198.5 MW

– Droop = 2%

– ωrini = 470 RPM

– ωrfin = 470 RPM

Turbine mode

– Pini = 150 MW

– Pfin = 198.5 MW

– Droop = 2%



Pump mode

– Pini = 200 MW

– Pfin = 180.6 MW

– Droop = 5%



140.000

150.000

160.000

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49.750

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50.000

50.050

0.000 20.000 40.000 60.000 80.000 100.000120.000140.000160.000

Frequency [Hz]

P measurement [MW]

465.000

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467.000

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469.000

470.000

471.000

472.000

473.000

474.000

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49.750

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50.050

0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000

Frequency [Hz]

Rotor speed [RPM]

140.000

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49.750

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50.050

0.000 20.000 40.000 60.000 80.000 100.000120.000140.000160.000

Frequency [Hz]

P measurement [MW]



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Pump mode

– Pini = 200 MW

– Pfin = 180.6 MW

– Droop = 5%



Learning

Given the decoupling between the power control (operated by the VSI) and the speed control (operated by

the governor), a variable speed machine can activate its primary frequency response without any initial delay,

unlike standard machines. Both the droop and the dead band are adjustable and can be adapted to the

specific need of the transmission system operator. In particular, a machine such as the Linthal PSP does not

have an intrinsic dead band, or frequency insensitivity, just the one that is imposed to it. Finally, this type of

technology can also perform primary frequency control while in pump mode, i.e. when operating as a motor

and withdrawing power from the grid.

Variable speed systems can respond more dynamically to changes in power set point values. The dynamic

VSI control offers many advantages but the fast response needs also require some “protection” such as

reverse power, which is independent of the mechanical speed and torque. Or, in other words, the

active/reactive power controlled by the electrical power could theoretically jump from generating rated

value to pumping rated value in a few milliseconds. The implementation of the fast control vs protection

incorporates several control and adapted protection functions to detect any potentially incorrect operation

for which careful parameter setting and testing at site is mandatory.

472.000

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482.000

484.000

49.750

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50.050

0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000

Frequency [Hz]

Rotor speed [RPM]



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4 HYDRAULIC DEVELOPMENTS FOR REVERSIBLE FULLY FED SYNCHRONOUS MACHINES

This section presents the improvements of hydraulic components to be perfectly suited to fully fed

technology. The case study is a typical European fully fed project with power lower than 100MW, about 200

meters of head and an expected power variation of 0-100% of maximum power.

Specific requirements for fully fed reversible machines

Contrary to conventional reversible machines operating at fixed speed, variable speed machines allow input

power regulation in pump mode. In a fixed speed machine, the input power depends on the gross head and

on the characteristic curve, i.e. the relation between the power and the head which is unique for a single

speed machine; therefore, power regulation is not possible.

For a variable speed machine, the characteristic curve varies with the speed and allows the power flexibility

needed for grid frequency regulation.

In addition to this benefit in pump mode, variable speed allows increasing the hydraulic efficiency in turbine

mode by shifting the operating point towards the best efficiency line.

4.1.1. Benefits of speed variation in pump and turbine mode

4.1.1.1. Pump mode

In pump mode (motor mode), the characteristic curve defining the relationship between the head and the

discharge or the power and the head (See Figure 9) is unique. The input power required to pump the water

from the lower reservoir to the upper one depends only on the levels of these two reservoirs. The head

operating range is obviously delimited and then the input power range is delimited.

The variable speed technology allows shifting this characteristic curve by adapting the speed. The pumping

delivered head can be increased (or decreased) by increasing (or decreasing) the speed of the machine.

Therefore, for given values of reservoir levels, the input power can be drastically and quickly changed by

varying the speed.

This is a huge advantage for power regulation but the drawback is that the hydraulic behaviour of the

machine is pushed to its limits. The machine needs to operate in hydraulic areas far from the optimum design

point. In such areas, hydraulic instabilities may appear as well as cavitation phenomenon on the runner blade

profiles. For low head levels (i.e. high power), cavitation may occur. For high head (i.e. low power) both

hydraulic instabilities and cavitation may occur.



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Figure 9: Head variation with variable speed technology compared to fixed speed (left) and pump

input variation (right)

4.1.1.2. Turbine mode

In turbine mode (generating mode), for given reservoirs levels, the power regulation is done by the governing

system, more precisely by changing the guide vane’s opening, meaning that power regulation is possible even

for a fixed speed machine. However, the operating range is not centred on the best hydraulic efficiency line

but is moved towards lower heads or higher speeds. The overall hydraulic efficiency is then not optimal.

Variable speed allows shifting the operating range towards the Best Efficiency Line (See Figure 10). The

hydraulic efficiency of each set point is then optimized. Furthermore, as the hydraulic point is closer to the

best hydraulic efficiency, all hydraulic instabilities are then decreased.

Figure 10: Efficiency optimisation in turbine mode with variable speed



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4.1.2. Specific hydraulic approach for variable speed

From a hydraulic point of view, variable speed brings challenges to increasing the power range of the

machine. To extend the hydraulic operating range of the machine in pump mode, a specific study had be

done to improve cavitation behaviour on the runner blade profiles as well as on hydraulic instabilities.

Cavitation phenomenon appears on the runner blade profile when the flow angle is strongly misaligned with

the geometric angle of the runner blade profile. This misalignment leads to a local pressure drop at the

vicinity of the blade profile and generates cavitation bubbles (gas bubbles) if the local pressure level is less

than the vapour pressure level of water.

The parameter used to define cavitation is the Thoma number. The Thoma number is a dimensionless term

indicating the conditions of cavitation under which the machine operates. It is expressed as the ratio of Net

Positive Suction Head NPSH (mWC) to the net Head (mWC) of the operating point. For pump mode, it is

common practice to use the NPSH value as parameter to define cavitation limit such as the incipient limit.

The incipient cavitation limit is defined when the value of NPSH is associated with the beginning of visible

runner cavitation usually detected by observation.

Two types of cavitation can be distinguished and

appear on both sides of the blade profile:

� pressure side for high head or low discharge

� suction side for low head or high discharge

As depicted on Figure 11, a decrease of the pump input will move the operating point towards the lowest

discharge area, therefore towards the suction side cavitation limit. Reciprocally, an increase of pump input

will move the operating point towards the highest discharge levels, therefore towards the pressure side

cavitation limit.

Therefore, variable speed pump-turbines with large power variation in pump mode will require a wider

cavitation free area than fixed speed machines. This implies to improve the hydraulic design for the two types

of cavitation, both for low and high discharges.



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Figure 11: Link between power variation and cavitation constraint in pump mode

In pump mode, instabilities can occur under high head level. This region is shown on the head versus

discharge characteristic on Figure 3 where the head increases with the discharge. In this region, the pump is

not under a stable point and the discharge may vary rapidly for a given head. This zone must be avoided

during pump operation and during start-up. With variable speed technology, if a lower input is requested

under high head, the speed decrease shifts the operating point closer to the pump hump zone as can be seen

on Figure 11.

Consequently, the hump zone location directly impacts and limits the input variation range of the variable

speed machine. Deleting or shifting the hump zone is a clear challenge for hydraulic designers and a specific

attention must be paid to it.

Design approach

4.2.1. Overview of design approach

To find the best hydraulic design for a given project, the principle is first to identify a reference design in the

GE portfolio with hydraulic characteristics close to those of the new project. Then, modifications of the

reference design are performed to optimize the hydraulic performance and adapt it to the project

requirements.

The work presented in this report is based on this methodology. The case study project is representative of

the European market and has the following technical specifications: two units with maximum output lower

than 100 MW and rated head of about 200 mWC.



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4.2.2. Improvements needed from the reference case

Before the start of the 3D design, projection tools are used to evaluate the impact of macroscopic changes

in the design. To ensure a sufficient margin towards the hump zone and the cavitation in pump, the main

objectives of the new design are:

� Improve by 3.5% in Head for the Hump zone

� Improve by 48% in cavitation width

These objectives are particularly challenging because the reference design is already optimized. To fulfil these

objectives, multiple designs are done and the best are tested on scale models.

For the runner, two developments have been planned to be tested (DEV01 and DEV02): The objectives are

cavitation and hump zone while maintaining a good level of hydraulic efficiency both in pump and turbine

modes.

As the hump zone is linked to instability of the flow between the guide vanes and the stay vanes, the design

of each of these two components must be improved.

Figure 12: Hydraulic components of the whole pump-turbine

4.2.3. Runner design

To improve the cavitation free area in pump mode, multiple CFD calculations were performed for different

discharges covering the expected operating range. The objective of the CFD computations was to analyse the

flow at the leading edge of the runner blade to reduce the pressure drop areas. Even if the blade angles are

optimized through an in-house optimization loop, the shape of the runner blade profile needs to be locally

adapted to reduce the pressure drop.



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Figure 13: Flow paths and minimum pressure area (in purple) at the blade inlet in pump

About thirty configurations were computed with CFD for each runner types. The cavitation limit width has

been widely increased for both low and high discharges.

The hump zone is a partial load instability that has been widely studied. However, the design keys and the

prediction of the control of this instability are still not well known. It is characterized by a flow separation

and backflow taking place in the guide vanes and the stay vanes profiles. One of the main root causes of this

hydraulic instability is linked to the quality of the flowfield delivered by the runner at the guide vane leading

edge. Consequently, a specific attention was paid to the runner’s hydraulic design to improve the hump zone.

4.2.4. Guide Vanes and stay vanes

Multiple guide vanes and stay vanes were numerically designed. The objective was the control of the hump

zone with a moderate impact on the hydraulic efficiency. An example of flow visualization is presented on

Figure 14.

Figure 14: Example of flow visualisation for the design optimization of Stay vanes and Guide vanes



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Test campaign

4.3.1. Overview of test rig facility

Every hydropower station is unique and is optimized for the unique site conditions such as head, flow

discharge and geodesic particularities. Thanks to a permanent research and development program, hydraulic

design teams at GE use state of the art tools to optimize turbine hydraulic designs. Among those tools, the

hydraulic laboratory in Grenoble, France is key: As an integrated step of the optimization process, each

turbine design is manufactured at model scale and validated by a series of tests in the hydraulic laboratory:

turbine efficiency and output, cavitation, stability, thrust, probing tests and flow observation. The complete

hydraulic behavior of the machine is measured and analyzed to ensure that the properties of the turbine

respond perfectly to the project requirements.

The Grenoble laboratory is equipped with six test-rigs and one hydrodynamic loop including the latest

measurement tools that allow complete testing of Francis, Kaplan, Bulb, Pump-Turbine and Tidal bulb

turbines. The model tests are performed in accordance with the international hydraulic standards IEC 60193.

The turbine models are fitted under the runner with transparent elements to allow visualization,

photography and video records.

4.3.2. Test rig description

The test rig used for this study is shown below:

Figure 15: Photograph of the test rig used for this project

4.3.3. Data Acquisition

The following basic measurements are necessary to determine the model performance: runner rotation

speed, mechanical torque on the shaft, water discharge through the model, Head which is the difference of

the total pressure between the spiral case inlet and the draft tube outlet. Complementary measurements

are also performed like absolute pressure and water temperature.



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Result and analysis

4.4.1. Pump results

The main results in pump mode are presented hereafter. For cavitation (Figure 8), the incipient limits defined

from model test are really close to the one predicted by CFD. This comparison between CFD and experimental

results proves that CFD calculations are quite reliable to estimate cavitation limit inception.

Figure 16: Cavitation incipient limits : Comparison between CFD and experimental results

Major improvements have been done to enlarge the operating area in pump mode compared to the

reference design. For the reference case, the cavitation and hump zone limit were problematic. For DEV02

the new working area shows a great improvement for cavitation. The initial objective of +48% was reached.

However, to meet the full operating range, additional improvement on the hump was still needed.

4.4.2. Optimisation of Power variation

The final power variation diagram obtained with DEV02 is summarized in Figure 17. The electrical power is

shown for different configurations with only one unit in operation and two units in operation together. With

a combination of the two units, the power variation is almost continuous. However, for high gross head in

pump, a power range is not reachable. In fact, it comes from the power limitation in pump mode due to the

generator maximum input.



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Figure 17 : Power variation obtained with DEV02

Conclusions & Recommendations

4.5.1. General Conclusions

Fully fed machines are particular in such that they need an extensive operating area in pump mode. The

extension of this operating area can only be feasible by mastering the hydraulic behaviour of the machine

especially where the machine is not stable. This requires both in-depth understanding of flow field physics in

pump-turbines and specific predictive tools to improve the design.

The work presented in this report starts from the understanding of the particularities of fully fed machines

and describes the whole process used to design a specific fully fed machine. A complete scale model has been

manufactured with two different runners, new guide vanes, stay vanes and spiral cases and tested in GE’s

Hydraulic Laboratory in Grenoble.

Major efforts have been deployed to challenge the standard design process of a fixed speed machines,

resulting in:

� Wide extension of the operating area in pump mode, especially the cavitation free area

compared to the reference design.

� Overall efficiency obtained for turbine and pump mode is very close to the reference case

under fixed speed. This extension has been done while controlling the trade-off between

range extension and efficiency both in turbine and pump mode.



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4.5.2. Suggestion for future research

The design requirements to improve the cavitation inception are now well known and this improvement can

be done without sacrificing the efficiency. This is not the case for the hump zone prediction. This instability

area is difficult to predict with the same numerical tools used for efficiency prediction. More accurate

numerical tools and with more embedded physics need to be developed and/or tested. Such tools require

also large computational capacities.



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5 INSULATION SYSTEM OF CONVERTER FED MACHINES

Development for large double-fed machines

Standard corona protection systems for large high voltage generators are optimized for 50 and 60 Hz,

therefore these systems must be adapted to converter applications. Due to this, the impact of the voltage

shape on the partial discharge inception was studied in the lab starting in 2012 and finally confirmed by

electrical measurements on the installed winding of the large-scale prototype motor-generator rotor for the

Linthal PSP project. The impact of the main parameters such as peak voltages in operation conditions as well

as air distances was studied to confirm the PD inception for different design solutions. The measurements

were done using different voltage patterns e.g. variation of frequency and voltage rise time. The

investigations on corona protection finally lead to the implementation of a system with grounded winding

caps, a new GE patented concept.

Per IEC 60034-18-42, the corona protection system of a bar must be tested with converter impulse voltage.

The test can be done by an impulse generator which reproduces equivalent pulse patterns. The test peak

voltage amplitude, slew rate and switching frequency on the specimen must be at least as severe as in the

severest operation mode. The bars must be mounted in a mock up to simulate the real operating

conditions. For example, a bar must be installed in a slot device with round-packing and any relevant

neighbouring elements around it.

The tests were done at room temperature and at maximum expected operation temperature with five

samples per relevant configuration. Per IEC 60034-18-42 the following acceptance criteria for the corona

protection system have been applied:

− No visible corona during test

− No visible damage after test by visual control and acceptable temperature rise

Figure 18: VET testing of single bars at rated and testing of grading system on single bars



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The first validation tests have identified the following elements as the most important criteria to be

considered in the detailed design and industrialization process:

− PD inception due to air distances

− Tolerances of rotor bars (CTQ)

− Rotor bar surface voltage (VSI pulse)

To mitigate the risk for partial discharges between rotor bars and overhang support a new grading system

with grounded insulation caps was developed. This new patented concept incorporated several new

developments such as high conductive tapes and insulation caps.

The lessons learnt from the assembly of the large-scale mock-up were implemented during the first

installation of the motor-generator rotor in the Linthal PSP in 2014. Nevertheless, several improvements on

tools and procedures were identified and adapted to the conditions at site. These lessons learnt are today

transferred to the engineering of other projects which are currently under manufacturing.

Recent Developments

The influence of different voltage waveforms and frequencies on time-to-failure was investigated with

comparative voltage endurance tests (VET) on standard test bars.

The tests revealed long lifetimes of the insulation system, which exceeded distinctly the expected equivalent

lifetimes for GE internal bar qualification, based on IEEE standard 1553-2002. The equivalent lifetimes

between the different voltage levels and frequencies are calculated according to the life model described in

IEC 60034-18-42. The pulse voltage does not lead to premature failures compared to the life time under sine-

wave voltage. The state of the art main insulation systems are therefore deemed suitable for applications

with pulse voltage and high frequency in the range of the actual test parameters.

Figure 19: Sample bars for VET testing and Pulse voltage patterns

Tests with frequencies higher than 50 Hz and pulse voltage stress showed increased heat generation on the

bars, especially at the grading (end winding corona protection, ECP). Elevated insulation temperatures can

influence the time to failure during VET and were therefore restricted to a maximum of 50°C in the middle

of each sample. Forced cooling and a special grading area with reduced capacitive currents were used on the

test bars and the bars were monitored by thermography.



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Voltage source converters generate harmonic frequency content by their switching for the output voltage

modulation (PWM). It results in a significant stress increase on the corona protection system (CPS) compared

to grid frequency sine-wave voltage. The main reasons are the pulse slopes and the frequent switching of the

converter.

The typical voltage pulse contains steep slopes (several 100V/μs to kVs/μs) that pass through the capacitive

main insulation easier than a 50Hz sine-wave due to their higher frequency content. The resulting capacitive

current increase through the main insulation causes higher ohmic losses in the corona protection system

before the current can reach ground potential in the slot. This can build up a significant voltage potential

already along the slot exit. The high capacitive current is not a concern for the main insulation itself because

it is almost free of losses.

The switching frequency of a converter is a multiple of the grid frequency. During each switching event, the

current driven by the pulse slopes stresses the corona protection system. It results in a multiple of stress

events compared to the grid frequency during the same time interval.

In the grading, the capacitive currents of the pulses also cause a shortened active grading length that

concentrates the voltage stress.

Equivalent circuits for corona protection system

Each corona protection part (grading, slot exit and slot) forms an RC-high pass filter (resistive-capacitive filter

of 1st order) together with the main insulation capacitance lying underneath it. When pulse voltage

components are high enough in frequency to reach the pass band of such a filter, the stress on the

corresponding corona protection part increases.

For 50Hz grid frequency only the grading is stressed, while the converter pulses can additionally also stress

the slot exit and to a lesser extent the slot part.

The smaller the impedance of the corresponding corona protection part, the higher up starts the pass band

of its RC-filter and the less stress is caused by the converter pulse voltage because the RC-filter dampens it.



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Figure 20: - IR-recording to check temperature increase on CPS

Results / Learning

Due to the (high frequency) harmonics the stress on the corona protection increases, therefore the slot exit

voltage will become significant. Considering the insulation system as an RC-filter (high pass), the impedance

of the main insulation Cins decreases with rising frequency, consequently the stress goes to the CPS.

The use of pulse voltage for validation tests of the CPS is necessary. Substitution with sine wave is not possible.

The pulse duration is significant whereas not specified in draft.

Depending of application optimization of CPS e.g. resistance is mandatory which requires development of

specific corona protection tapes or varnish. Thanks to the performed validation tests important learning is

made, further developments outside the eStorage project scope are needed and initiated.

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