The Scaling of Machines for Renewable Energy ApplicationsRamzi Solomon

Energy Postgraduate Conference 2013

Introduction• Future generation from renewable sources will employ

rotating electrical machines as generators.• Constant & variable speed generators connected to the

grid at the sub-transmission and distribution level.• Generator performance and power system stability

studies are of interest.• Two questions:

1. Can a utility-scale IPP-type synchronous generator be scaled such that a laboratory-based equivalent system can be designed?

2. What is the impact of the connection of machines at the sub-transmission and distribution level on the national grid?

Project Aims• This project will scale, design,

analyse and then prototype a micromachine of a wound cylindrical rotor synchronous generator typical of many constant speed generator IPPs.

• A laboratory-based test bench will be created to quantify the impact of the integration of IPPs and in particular renewables on the South African grid.

Project Aims• Dimensional analysis is the

mathematical method that allows machines and systems to be down-scaled by establishing laws of similitude between the original and its scaled model.

• Conduct detailed testing of several PQ and grid integration issues on the laboratory-based system.

Different Scaling Methods

Laboratory setup

Defining the design process

Analytical design 5 kVA wound rotor

synchronous generator

Optimization

Design 5 kVA using FEA

Prototype micromachine

Convergence

Test micromachineunder steady-state

and dynamic conditions

Define scaling factors

Analytical pu design of utility-scaleIPP using scaling factors

Yes

No

Compare test results to industrial-size IPP

Convergene

Acquire dimensions and putest data of utility-scale IPP

Yes

No

Machine Design Challenge• Design a medium-voltage synchronous

machine of the order of 55MW that replicates the performance of Sasol’s compressor-driving synchronous motor.

• The rotor is cylindrical. • The machine is a fully enclosed self-

cooled machine with air-to-water heat exchangers.

Comparison in machine specification for two machines

Name Value

Number of phases 3

Real Power P 5 kW

Power Factor 1

Apparent Power Q 5 kVA

Line to line voltage 380 V

Stator current per phase

7.6 A

Synchronous speed 1500 rpm

Frequency 50 Hz

Number of poles 4

Number stator slots 36

Slots per pole per phase

3

Name Value

Number of phases 3

Real Power P 55 MW

Power Factor 1

Apparent Power Q 55 MVA

Line to line voltage 11,000 V

Stator current per phase

2919 A

Synchronous speed 1500 rpm

Frequency 50 Hz

Number of poles 4

Number stator slots 36

Slots per pole per phase

3

55 MVA 5 kVA

Sizing SpecificationSizing  

Stator bore D=0.796 m

Gross length of machine

L=6.8045 m

Specific magnetic loading

Bav=0.54

Specific electric loading

Ac=45,000

Current density J=3.2

Power coefficient Co=255.27

Winding factor Kw=0.955

Pole pitch 0.0747

Minimum teeth width 0.0226 m

Permissible slot width 0.0521 m

Sizing  

Stator bore D=0.12 m

Gross length of machine

L=0.1269 m

Specific magnetic loading

Bav=0.4

Specific electric loading

Ac=13000

Current density J=3.4

Power coefficient Co=54.7219

Winding factor Kw=0.9567

Pole pitch 0.0942

Minimum teeth width 0.0046 m

Permissible slot width 0.0132 m

55 MVA 5 kVA

Conclusion• Analytically designed two machines,

laboratory machine (5 kVA) and reference design (5 MVA).

• Verifying designs using FEA package, FLUX.• Establish equivalence between lab and field

machines • Prototype 5 kVA scaled design• Test 5 kVA in laboratory under various PQ and

transient conditions• Use software to predict behaviour under

extrapolated scenario and compare with prototype.